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Advanced materials for batteries

Final Report Summary - MAT4BAT (Advanced materials for batteries)

Executive Summary:
MAT4BAT project was aiming at an increase of the competitiveness, sustainability and longevity of future Li-ion batteries for electric vehicles.
One target of MAT4BAT project was to develop 3 successive generations of cells with improved materials. First GEN1 materials from commercial suppliers and MAT4BAT partners (active materials, binders, conductor additives) were successfully implemented in 18 Ah NMC/Graphite GEN1 cells with standard liquid electrolyte. Advanced Li-rich cathode material for GEN2 and GEN3 was scaled-up to 10 kg during the project. Large efforts were dedicated to water-based formulation optimization of both electrodes with innovative PVDF-based binders. These components were then integrated in GEN2 prismatic cells (Li-rich/Graphite, 12 Ah). GEN3 technology was designed with new polymer electrolyte implementation and first trials were made at lab scale.
New composite hard casings were also developed on Li-ion cells to improve safety performances while minimizing the impact on energy density. Two processes were successfully used to prepare such overpackagings, namely resin transfer molding and vacuum infusion. Safety and aging tests showed that these new overpackagings have no detrimental effect on cell performances.

MAT4BAT project also offered the opportunity to gain better understanding of battery ageing mechanisms using relevant cell designs for EV application. A large experimental test plan involving 6 experimenters was performed on 76 state-of-the art Li-ion cells (NMC/Graphite, 16 Ah) as well as on 38 GEN1 cells. Several parameters were varied such as ageing mode (either calendar or cycling), SOC window, ambient temperature and charge C-rate. The best sets of operating conditions were identified in order to optimize the batteries lifetime.
A large arsenal of physico-chemical methods available within the consortium was the key for enhanced understanding. Classical ageing mechanisms related to anode for cycling and calendar aging were further analyzed and a new ageing mechanism based on the decomposition of the additive biphenyl in reference cells was reported. Collected data helped for the development of an empirical law for calendar and cycling aging of NMC/Graphite cells. Models were developed to simulate aging (capacity decrease and resistance increase) for different usage scenarios. Finally extensive evaluation of aging tests and Post-Mortem results gave very useful recommendations for battery management strategies.

As a complete project on battery value chain MAT4BAT also enabled to develop cost, market and ecological assessment methods on model batteries for EV application. For example the green value chain analysis revealed that the environmental profile is mainly dominated by the positive electrode materials, but the impact of the cell production is positively influenced by changing the binder solvent of the positive electrode from NMP to water and by the change from NMC to Li-rich.
Standardization requirements were also highlighted during the project. A collection of current regulations and standards enabled to detail the good practice for battery handling and labelling for example. Requirements for future test standardization were also collected.

MAT4BAT project was a success regarding knowledge dissemination with 25 scientific peer-reviewed publications issued by the partners which represent a significant means to consolidate all generated data on battery ageing. The results could be hopefully exploited by partners and stakeholders to strengthen market business on advanced materials for batteries or implement ageing models in smart battery management systems for example. All gathered knowledge could be useful for future projects focusing at battery module or pack as well as at final battery system integration.

Project Context and Objectives:
Li-ion technologies initiated in the 90’ at a fast development pace thanks mainly to emerging ICTs with more than 20 GWh sold in 2010. Soon, it appeared as a credible technology for electrical vehicles as it could provide average energy densities of about 140 Wh/kg. However and since then, major breakthroughs have been expected to reach higher storage levels of 250 Wh/kg on battery system level with an acceptable lifetime of 3000 cycles in order to develop an affordable economical business plan for car batteries.

MAT4BAT was built as an EV battery strategy project on advanced materials and pilot line processes, proposing three novel concepts of cells initiating from a state-of-the art combination of cell materials (NMC/Carbonate liquid electrolyte/Graphite). MAT4BAT addressed all critical ageing mechanisms associated to this technology and having direct impacts on product lifetime and safety by implementing two work programs for Battery Assessment (#1) and Battery Technologies (#2).

Program #1 (WP1 and WP2) set a framework to define critical charging modalities for a battery system during practical use and associated testing tools and methods for relevant functional performance and lifetime assessment. Simulation and post-mortem analysis were implemented to generate extensive knowledge on failure roots and a better understanding of critical ageing mechanisms in order to propose behavioural and predictive models for battery performance and life time under various practical operating conditions (charge, discharge, rest time). This framework served as an enabling technology for the recommendation of advanced materials suitable for emerging technologies of ageing resistant batteries.

Within the framework defined in program #1, the program #2 (WP3 and WP4) implemented three generations of cells with a focus on electrolytes which were steadily transformed from Liquid to Gel to All-Solid state electrolytes in order to promote substantial gain in cell lifetime and safety by preventing degradations and hazards (e.g. internal short-circuit caused by dendrites formation) and by improving energy density with a separator-free cell (all-solid state electrolyte). MAT4BAT addressed most of the experienced based and acknowledged ageing mechanisms by proposing advanced materials solutions stepwise through the 3 generations of cells. This allowed tests and an in-depth interpretation of the overall performance of cells and this allowed drawing conclusions on the added value of each improvement. The targets and success indicators for MAT4BAT cells were an increase of the energy density from 150Wh/kg to 250Wh/kg during the successive generations combined with an improvement of the lifetime from 2000 to 4000 cycles in standard charging and from 1500 to 3000 cycles in fast charging.

The project methodology included a first phase during which a hundred state-of-the-art commercial cells were assessed to define normal and critical charge/discharge conditions of testing with appropriate testing protocols. Besides, materials increments were screened out on coin-cells prior a benchmarking of most promising materials at full cells level. Eventually, 10 to 40 A.h prototypes had to be produced to validate MAT4BAT best technologies against quantified objectives.

To achieve these ambitious goals, MAT4BAT gathered 17 European partners for a 42 month long project. The well-balanced consortium incuded academics and RTOs, industrials and one consultant for management (9 countries represented), all covering the key aspects from material development to Li-ion cell testing and modelling with a high level of expertise.

Project Results:
Work Package 1: Battery ageing assessment – methods & tools

Task 1.1: Provisioning SoA commercial cells

Based on the advantages and drawbacks of several preselected Li-ion cells manufacturers, Task 1.1 leads to the selection of the Li-ion cells reference “Kokam SLPB78205130H” as the reference representative of the state-of-the-art (SOA) at the beginning of the project (end of 2013), presenting the following characteristics:
• Chemistry: NMC / Graphite
• Nominal capacity: 16Ah
• Measured capacity at C/2-C/2, 25°C: 16.2Ah +/- 0.1Ah
• Measured resistance at SOC=0.5 25°C: 0.87mΩ +/- 0.02mΩ
• Nominal specific energy: 148Wh/kg (energy type cells compatible with EV application)
• Measured specific energy: 150.1Wh/kg 281.3Wh/L
• Possibility to open cells for post-mortem analyses
Renault provisioned 100 Kokam cells for the ageing study, distributed among 7 experimenters (CEA, CIDETEC, EIGSI, KIT, UNEW, VITO, ZSW, respectively).

Task 1.2: Definition of accelerated testing protocols for battery ageing

The Task 1.2 leads to the definition of an extensive ageing experimental plan, both in calendar and in cycling, and the definition of periodical electrical tests divided into Extended Check-Up (ECU) and Short Check-Up (SCU), all performed at 25°C whatever the ageing conditions. ECU included 2 capacity tests at 1C-1C, a Dynamic Stress Test (DST) discharge, 1 cycle at C/25-C/25, several discharges at different discharge C-rates, pulses at 1C – 30s according to 8 SOC, and for some of experimenters, additional Electrochemical Impedance Measurements. SCU were part of ECU, including only the 2 capacity tests at 1C, the Dynamic Stress Test (DST) discharge, and pulses at 1C – 30s according to 5 SOC.
The table 1 presents the calendar and cycling ageing conditions performed on Kokam cells, together with the number of cells tested and the experimenter.

Task 1.3: Embedded sensors strategy for in operando recording

The Task 1.3 leads to the study of 3 kinds of sensors, namely temperature sensors, reference electrodes, and strain gauges, respectively.
Regarding temperature sensors, 2 approaches have been studied by ZSW:
• external temperature distribution by thermographic imaging, on pouch and cylindrical cells;
• internal temperature sensor inside some 1.5Ah home-made cells.
The main results can be summarized as follow:
(i) It is possible to build pouch cells with internal temperature sensor and to conduct operando measurements during cycling. These results were published in [J. Electrochem. Soc. 162 (2015) A921].
(ii) Measurements showed that in cylindrical cells, radial temperature gradients are stronger compared to temperature gradients on the cell surface. In contrast, for pouch cells, the through-plane temperature gradients are small compared to in-plane temperature gradients on the cell surface, as illustrated by the Figure 1.
(iii) After 20 months of calendar ageing at 25°C, the temperature sensor was still usable. In similarity to the fresh pouch cell the temperature on the cell surface and inside the electrode stack was very similar during discharging.

Regarding reference electrodes, 2 kinds of reference electrodes have been tested by CEA:
• LTO reference electrodes;
• Li metal reference electrodes.

Best results were obtained with LTO reference electrodes, with a stability ~1 month compared to only ~1 day with Li metal ones.
Furthermore, some reference electrodes LTO-based have been introduced directly in some Kokam 16Ah cells, which is a real step forward versus the state of the art. MAT4BAT project shown hence that it is possible to introduce a reference electrode in a large commercial cell (16Ah) without any significant perturbation, giving then a direct measurement of each potential of electrodes in real operating conditions. In this way, it constitutes a promising investigation tool for research aspects (since it is an intrusive measure), for modeling and mechanisms understanding (see Figure 2).

Finally, some Kokam cells in ageing have been equipped by CEA with strain gauges, used to measure the deformation in 3 directions with a resolution around one micrometer/meter.
Main results have shown that it is possible to follow the “breath” of a NMC/graphite cells in soft casing. Indeed, during a charge, the cell is expanding, while it is contracting during the discharge.
Besides, the evolution of the amplitude during charging as function of SOH presents rather small variation and with erratic behavior: high dispersion has been observed, attributed to gas emission / soft packaging. Thus, this kind of sensor could be more relevant for hard casing cells (see Figure 3).

The table 2 gives an overview of results obtained from the different sensors.

Task 1.4: Battery performance / ageing testing and data collection

The figure 4 presents the main results obtained on Kokam 16Ah in calendar ageing.

First, we can underline that the initial end of tests criteria in calendar ageing (SOH=80% or 18 months) have been overpassed, reaching 28 months for some conditions, or 55% of SOH for some others. We observe here as expected an increase of the ageing with the increase of temperature and SOC. Furthermore, it has been demonstrated here over 18 months that there is no ageing at low temperature, whatever the SOC. Besides, a correlation has been observed between the SOH decrease and the R_DC increase, even if we did not have exploitable R_DC data at 60°C. The very fast degradation observed at this temperature (corresponding to the temperature limit of cells specified by the manufacturer) could explain this high dispersion.
Finally, a new ageing mechanism has been observed at high temperature (T) and high SOC, and explained thanks to WP2 post-mortem analyses and modeling, namely the formation of dried zones, leading to a local deposition of Li metal during charge phases consecutive to such calendar phases (see Figure5). These results were published in [J. Electrochem. Soc. 164 (2017) A1089-A1097].

The figure 6 presents the main results obtained on Kokam 16Ah in cycling ageing:

As for calendar ageing, the initial end of tests criteria (SOH=80% or 4000 cycles) have been overpassed for the cycling ageing, reaching near 6000 cycles for some of the tested ageing conditions, and a SOH=20% for some others. We can underline here that the lifetime target (4000 cycles with DOD=80%) is only reached at 45°C for the SOC window ΔSOC=10-90. We observed that the higher the SOCmax, the faster the ageing, while the charge C-rate effect is significant only for SOCmax=100%. Besides, we observed as expected a lower ageing at 25°C compared to 45°C, while another behavior (explained by another ageing mechanism in the WP2) is observed at 5°C: in a first step, we observed a fast capacity fade, and then no more evolution (but also very low cumulated Ah charged/discharged at low temperature). Furthermore, at 5°C, the higher the charge C-rate, the faster the capacity fade. Finally, a sudden capacity fade is observed from around SOH = 60-70%.
Contrary to calendar ageing, no clear relationship is observed between SOH and R_DC; it are rather the upper and lower cut-off voltages that determine the increase of the cell resistance.
Hence, the MAT4BAT project brought new knowledge about the impact of charge conditions (charge C-rate / DSOC / temperature) on the ageing of batteries, and two distinguished behaviors have been put into evidence between low temperature (here at 5°C) and higher ones (here 25°C and 45°C).
The table 3 gives an overview of results obtained from ageing tests on Kokam cells.

In addition to this extensive ageing study performed on Kokam cells, 43 cells were assembled by CEGASA, denoted as MAT4BAT GEN#1 cells (graphite/NMC, 16 Ah, 157Wh/kg). These cells were tested with a similar protocol than for the Kokam cells, simply reduced to cover 6 calendar conditions and 13 cycling conditions, as mentioned in the table 4.

On these cells, up to 18 months of ageing results were collected. The results obtained according to these conditions can be summarised by the graphs in Figure 7.

We observed a very poor reproducibility of GEN # 1 cells aged in calendar mode. This poor result can be explained by the fact that the GEN # 1 cells were divided into 2 batches, a first batch with lower initial performances and which were used for calendar aging, and a second batch with higher initial performances which have been used for aging in cycling.
Good reproducibility has been overserved in cycling ageing mode. As for Kokam cells, we observed a clear bad impact of high SOC values, an acceleration of the ageing at 45°C compared to 25°C. However, contrary to Kokam cells, no bad effect of SOC=0-80 has been observed, and the bad impact of low temperature (5°C) is here clearly combined with the charge C-rate, 1C appearing as the upper limit of these cells.
The table 5 presents a synthesis of MAT4BAT GEN#1 cells, which are in good agreement with the initial objectives of the MAT4BAT proposal. Only the fast charging performance are limited to 2C instead of 3C in order to reach the target of 1500 cycles.

Task 1.5: Battery lifetime assessment & recommendations

According to the battery lifetime assessment done in the WP1, together with mechanisms understanding brought by post-mortem analyses performed in the WP2, 3 different ageing mechanisms has been observed, each time from the anode:
• the “SEI (Solid Electrolyte Interphase) growth”, both in calendar and in cycling, at high temperature and high SOC;
• the “Li plating”, in charge at low temperature and high charge C-rate;
• the “local Li depositon”, in charge, after calendar periods at high SOC and temperature.

This summary leads hence to the following recommendations regarding the operating conditions in order to avoid premature aging, namely:
• temperature in cycling: not too low to avoid Li plating, and not too high to avoid SEI growth and/or biphenyl decomposition and subsequent local Li deposition (heating and cooling required). It is also highly recommended to keep cells and packs temperature as homogeneous as possible;
• temperature in storage: the lower the better;
• charging C-rates: high C-rate at the beginning, lower at the end, and low C-rate for low temperatures;
• SOC-levels in cycling: not too high to avoid SEI growth and Li plating;
• SOC under storage conditions: mean SOC.

Furthermore, we can underline that the ageing mechanism of “Li plating” on graphite anodes leads to rapid capacity fade and has therefore to be prevented. In T2.2 ZSW showed that Li deposition can be suppressed by three main operating conditions and their interplay (see Figure 8):
• Charging C-rate
• Temperature
• End-of-charge voltage

It turned out that not only the single parameters but the combination of these parameters is very important. E.g. high charging C-rates can lead to Li deposition in combination with high end-of-charge voltages. However, lowering the charging C-rate at higher end-of-charge voltages can suppress Li deposition and therefore enhance the cycle-life significantly. The results are in agreement with cycling ageing results by KIT, EIGSI, CIDETEC, VITO, and CEA from T1.4 and were published as a collaborative paper: [J. Electrochem. Soc. 163 (2016) A1232-A1238].

Finally, it has been shown in MAT4BAT project for the very first time that high temperatures lead to decomposition of biphenyl, as found by CEA and published in the paper: [J. Electrochem. Soc. 164 (2017) A1089-A1097]. Post-Mortem analyses in WP2 by CIC, CEA, and ZSW showed that in this case, the ageing mechanism is “local Li deposition”. These results suggest operating Li-ion cells in a moderate temperature range (~25°C) by cooling or heating in case of high or low ambient temperatures, respectively. In case of too low temperatures, the upper SOC limit and/or the charging C-rate must be limited.

Work Package 2: Understanding of ageing mechanisms – simulation & modelling

Task 2.1 Simulation of battery aging behaviour

In this task, the aging results from WP1 were modelled by mathematical functions for commercial 16Ah cells and for GEN1 cells developed in the project. There are different sets of functions for calendar and cycling aging, which is consistent with the different aging mechanisms found in T2.3 after Post-Mortem analysis performed in T2.2. For the modelling, large data sets with cell capacity, impedance, and resistance including reproduced cells (statistical analysis) were taken into account.

Both, the loss of capacity and the increase of cell resistance can be described by semi-empirical models.
For calendar aging, the capacity loss can be described by the Figure 9.
The developed models allow modelling of conditions relevant for vehicles, i.e. variations of SOC and temperature with time.

Task 2.2 Post-Mortem Analysis

In this task, the cells aged in WP1 were disassembled and the aging mechanisms were analysed with a variety of physico-chemical methods available at the different partners (Post-Mortem analysis).
• A literature research on Post-Mortem analysis was conducted to homogenize the procedures in the labs at the different partners. The results were published as a collaborative review paper [1].
• A special benefit in MAT4BAT were the capabilities of physico-chemical analysis methods of all involved project partners altogether. The deep insights gained into aging mechanisms of Li-ion batteries in MAT4BAT were strongly enhanced by the collaboration of different labs. In particular, it was possible to analyze all components of the cells, including anodes, cathodes, separator, and electrolyte and their impact on aging. Since each physico-chemical analysis method is not able to detect all aging phenomena, in the MAT4BAT project, Post-Mortem analysis was carried out by a variety of methods available at CEA, CIC, KIT, and ZSW. The applied methods included GC-MS, FIB, ion beam cutting, TOF-SIMS, XPS, Auger spectroscopy, EIS in reconstructed symmetrical cells, in-situ XRD, NMR, SEM, EDX, electrochemical tests in reconstructed coin half cells and in pouch full cells with reference electrodes, CT, XPS mapping, ICP-OES, GD-OES, and thermal analysis. The results of WP2 were further improved by cross-connections to the other WPs of MAT4BAT. For example, ZSW developed a method for depth profiling of anodes in WP3, which was also very useful for characterization of aged anodes in WP2 [2,3].
• Post-Mortem analysis tests were conducted with commercial 16Ah cells with graphite anodes and NMC cathodes [4,3,5][7] and with 17Ah GEN1 cells with graphite anodes and NMC cathodes developed within the project. Additionally, different types of 18650 cells with graphite anodes and NCA, NMC/LMO blend, and LFP cathodes were tested [3,6].
• For cycling aging three main aging mechanisms were found, for calendar aging, there is in most cases only one aging mechanism (see T2.3). The results were published as peer-reviewed papers [2–5][7] as well as on international conferences (see report on WP6).

Task 2.3 Fundamental study and modelling of aging mechanisms

In this task, the results from the Post-Mortem analysis in T2.2 were evaluated to gain insights on the whole picture of the aging mechanisms in Li-ion cells.
• In cells with graphite anodes and NMC cathodes, aging happens mainly on the anode side [3,5,2][7]. The cathode side is in most cases not strongly aged, besides reversible Li loss [5][7]. Two main aging mechanisms were found: i) Cyclable Li is mainly trapped inside SEI films grown on the anode surface by decomposition of electrolyte [5][7]. ii) Cyclable Metallic Li is deposited on the graphite surface [4,3,5,2][7]. The cyclable Li which is trapped on the anode surface cannot be intercalated into the cathode, leading to capacity loss and resistance increase of the cells in both cases [5][7].
• For a given cell design and chemistry, the aging mechanism depends strongly on the operating conditions [4]. In particular, the combination of charging C-rate, ambient temperature, and end-of-charge voltage were investigated by long-term aging tests, Post-Mortem analysis and by measurements of anode potentials [4,3,5,2].
• Low temperatures (5°C) during cycling lead to a mostly homogeneous deposition of Li (so called ‘Li plating’) on graphite anodes. Li deposition happens during charging when the anode potential gets negative vs. Li/Li+. Li plating is mostly located on the anode surface, i.e. at the anode | separator interface [3,2]. Li plating reacts with electrolyte and it can be electrically disconnected and therefore leads to capacity fade. It also turned out that Li plating can lead to a decrease of safety behavior in accelerating rate calorimetry (ARC) tests [6]. This decrease of safety is not connected to Li dendrites and internal short circuits but to exothermic reactions of Li metal. In cylindrical cells where temperature gradients are present, Li plating was found to be thicker near the cell housing where the mean temperature was lower [3].
• High temperatures (45°C) and high charging C-rates (3C) lead to local Li deposition on graphite anodes [3]. These localized depositions should not be called ‘Li plating’ since they are more inhomogeneous (island-like distributed on the anode surface). Like ‘Li plating’ created to low temperatures, the localized Li deposition is mostly located on the anode surface (at the anode | separator interface).
• Temperatures around room temperature (25°C) during cycling in combination with limited end-of-charge voltage (upper SOC limit: 90%) lead to mainly SEI growth on the graphite anodes. Like Li deposition, the SEI is also mostly located on the anode surface (between anode and separator).
• For calendar aging in the range of 5°C-60°C at 100%SOC, Arrhenius analysis of capacity fade data shows only one line, i.e. only one main aging mechanism, which is SEI growth [5][7]. However, for cells with 10% biphenyl in the electrolyte, it was found that this component is decomposed at high 100% SOC and above 45°C [5][7]. Therefore, at 45°C and 60°C, this lead to H2 gas bubbles between anode and cathode [5][7]. Around the gas bubbles, the local current density during charging in capacity check-ups was strongly increased as shown by simulations. This lead to localized negative anode potentials and local Li deposition around the gas bubbles. Calendar aging at 60°C without capacity check-ups showed that this can prevent the local Li deposition [5][7]. However, in contrast to cycling aging, the main aging mechanism remained SEI growth for calendar aging, since the capacity check-ups were performed rarely. However, the influence of periodic capacity check-ups should be considered in further projects [5][7].
• Simulations were successfully conducted to explain voltage curves, aging gradients inside Li-ion cells and the impact of gas bubbles during aging at high temperatures. Furthermore, simulations were employed to take aging mechanisms into account and for diagnosis of aging mechanisms from experimental data.

[1] T. Waldmann, A. Iturrondobeitia, M. Kasper, N. Ghanbari, F. Aguesse, E. Bekaert, L. Daniel, S. Genies, I.J. Gordon, M.W. Löble, E. De Vito, M. Wohlfahrt-Mehrens, Review—Post-Mortem Analysis of Aged Lithium-Ion Batteries: Disassembly Methodology and Physico-Chemical Analysis Techniques, Journal of The Electrochemical Society. 163 (2016) A2149–A2164. doi:10.1149/2.1211609jes.
[2] N. Ghanbari, T. Waldmann, M. Kasper, P. Axmann, M. Wohlfahrt-Mehrens, Detection of Li Deposition by Glow Discharge Optical Emission Spectroscopy in Post-Mortem Analysis, ECS Electrochemistry Letters. 4 (2015) A100–A102. doi:10.1149/2.0041509eel.
[3] N. Ghanbari, T. Waldmann, M. Kasper, P. Axmann, M. Wohlfahrt-Mehrens, Inhomogeneous Degradation of Graphite Anodes in Li-Ion Cells: A Postmortem Study Using Glow Discharge Optical Emission Spectroscopy (GD-OES), The Journal of Physical Chemistry C. 120 (2016) 22225–22234. doi:10.1021/acs.jpcc.6b07117.
[4] T. Waldmann, B.-I. Hogg, M. Kasper, S. Grolleau, C.G. Couceiro, K. Trad, B.P. Matadi, M. Wohlfahrt-Mehrens, Interplay of Operational Parameters on Lithium Deposition in Lithium-Ion Cells: Systematic Measurements with Reconstructed 3-Electrode Pouch Full Cells, Journal of The Electrochemical Society. 163 (2016) A1232–A1238. doi:10.1149/2.0591607jes.
[5] B.P. Matadi, S. Geniès, A. Delaille, T. Waldmann, M. Kasper, M. Wohlfahrt-Mehrens, F. Aguesse, E. Bekaert, I. Jiménez-Gordon, L. Daniel, X. Fleury, M. Bardet, J.-F. Martin, Y. Bultel, Effects of Biphenyl Polymerization on Lithium Deposition in Commercial Graphite/NMC Lithium-Ion Pouch-Cells during Calendar Aging at High Temperature, Journal of The Electrochemical Society. 164 (2017) A1089–A1097. doi:10.1149/2.0631706jes.
[6] T. Waldmann, M. Wohlfahrt-Mehrens, Effects of rest time after Li plating on safety behavior—ARC tests with commercial high-energy 18650 Li-ion cells, Electrochimica Acta. 230 (2017) 454–460. doi:10.1016/j.electacta.2017.02.036.
[7] A. Iturrondobeitia Ellacuria, F. Aguesse, S. Genies, T. Waldmann, M. Kasper,
N.Ghanbari M. Wohlfahrt-Mehrens, E. Bekaert, Post-mortem analysis of calendar
aged 16 Ah NMC/graphite pouch cells for EV application, submitted to J. Phys.
Chem. C, 2017.

Work Package 3: Advanced materials for durable and safe cell development

WP3 has been dedicated to the selection and development of the cell components (positive electrode, negative electrode, electrolyte, separator) for the 3 generations of cells that were to be developed. Generation 1 cells have been built employing SoA commercial materials that were carefully selected amongst a panel of materials with different characteristics, on the basis of their performance in half and full coin cells. Materials for generation 2 cells have been developed, optimized and upscaled within this work package. Preliminary electrode formulations to be further scaled up in WP4 were developed using the materials prepared within this WP. Below is a summary of the major accomplishments obtained in this WP. Materials for generation 3 cells were developed although require further optimization before being upscaled and prototyped in order to achieve the ambitious targets that were set in MAT4BAT.

Task 3.1 Positive electrode development & characterization

Task 3.1.1: benchmark for Generation 1 development (M01-M12)
In this first task six different samples of lithium nickel manganese cobalt oxides with general formula LiNixMnyCozO2 (NMC) were obtained and benchmarked. The samples were characterized from the chemical, structural, morphological and electrochemical point of view. The material that exhibited the best capacity retention in the rate capability tests, the best cycle stability and the best Mn dissolution properties was selected as the Generation 1 cathode material (LiNi1/3Mn1/3Co1/3O2, NMC111).

Task 3.1.2: synthesis and characterization of materials for Generation 2 and 3 (M1-M18)
Li-rich oxides were selected as cathode materials for Generations 2 and 3. These overlithiated materials represent a gain in capacity able to reach the targeted cell performances for these generations of cells. These materials were developed by MAT4BAT partners in this task. Several compositions were prepared in order to obtain the best performing materials. Doping strategies were explored both for the Li site and the metal site (Li1.2-xAxNi0.2Mn0.6O2 and Li1.2MnwNixM1yM2zO2) and several synthesis methods leading to different microstructures were tested (Co-precipitation, ceramic, sol-gel). Also coatings with graphene prepared by DIRECTA+ were explored in order to improve the electronic conductivity of the materials. The best results were obtained with Li1.2Ni0.2Mn0.6O2 prepared by co-precipitation and this was the material selected for prototyping.
Water processing of the selected material was thoroughly evaluated using complementary chemical, structural and electrochemical characterization techniques so that the water based PVdF binders developed in task 3.3 could be used in the formulation of the positive electrodes of generations 2 and 3. The results obtained indicated that the material does not suffer any transformation during the slurry preparation and its electrochemical performance is equivalent to that of an organic processed electrode.

Task 3.1.3: upscale for Generation 2 and 3 development (M12-M30)
10 kg of were successfully upscaled and the prepared material was characterised in half-cells to validate the synthesis, delivering 250 mhA/g at a rate of C/10 with less than 5% dispersion.

Task 3.2 Negative electrode development & characterization

Actilion 1 graphite produced at Imerys Graphite & Carbon was selected as reference negative electrode materials to be used in Generation 1. Surface-modified synthetic graphites have been developed for generations 2 and 3 and thoroughly characterized using the water based PVdF binders developed in task 3.3. The graphite materials synthesized during the project outperform the commercial material used as reference and >360 mAh/g were obtained in half-cells at 1C/2C rate, which is close to the theoretical capacity of graphite (372 mAh/g).
DIRECTA+ developed graphene-based carbon materials to be used as additives in negative electrodes and an improvement of the capacity is obtained, particularly at high C-rates, without compromising the first cycle irreversible capacity.

Task 3.3 Active binder development & characterization

PVDF-based water dispersions (latexes) for both cathode and anode electrode formulations with different crystallinity and melting temperature were developed by Solvay both at lab scale and pilot scale. The proper processing conditions were identified and specific process guidelines for slurry mixing procedure and post-treatment steps were selected and shared with partners. These binders were tested in tasks 3.1 3.2 and 3.4 and the binders for cathode and anode preparation for cell generations 2 and 3 were selected according to processability, stability, rheological behaviour in a 50 g batch slurry, morphology, electronic conductivity, and adhesion of the electrode layer; and electrochemical performance of the electrode in half coin cell.

Task 3.4 Electrode formulation scale up
This task was dedicated to the development and scaling up of waterborne formulations of negative and positive electrodes for the three cell generations using the binders developed in T3.3. For each of the different materials selected as positive and negative electrode materials, the following activities were carried out to improve slurry preparation and electrode coating processes:
• Benchmark and selection of the most appropriate PVdF latex to be used with NMC material, according to Solvay recommendations;
• Selection of CMC and latex ratios to prepare slurries with appropriate rheological properties and electrodes with good mechanical properties;
• Optimization of slurry preparation, regarding slurry stability and particle dispersion state (pH correction, etc.).
A working process chain for electrode production, namely mixing, coating, drying, calendaring and cell assembly was established and the impact of drying boundary conditions on electrode properties was investigated for which it was found that the adhesive force decreases significantly with increasing temperatures, underlining the importance of the drying process. The development and upscaling of formulations is summarized below.

-Generation 1 (Graphite / NMC)
Negative electrode waterborne formulation based on ACTILION 1 graphite (Imerys Graphite & Carbon) and water based PVdF binder (SOLVAY) were developed and scaled. An aqueous cathode formulation based on NMC111 and water based PVdF binder has been elaborated and upscaled too. Several technical issues were solved by adaptation of mixing procedures, sequence of materials addition, mixing schedule and intensity, fine tuning of formulations for each type of slurry. NMC111 exhibits an alkaline behavior when mixed with water so pH control was introduced as an effective way to deal with this issue. Optimization of density values was also carried out with the aim to increase energy density. As result, the prepared electrodes have demonstrated a very good homogeneity and mechanical stability. Both electrodes had very good compressibility and optimal densities of 3.5 g/cm3 and 1.6 g/cm3 for the cathode and the anode respectively were selected according to the electrochemical performance of the electrodes.
Both electrodes demonstrated a very good electrochemical behavior and long term cyclability in full coin cells and pouch cells (80% of capacity retention at cycle 3200 when cycled at 100% depth of discharge at 25ºC), satisfying the targets claimed in project DOW (part B, p.8 Table 4).

-Generations 2 & 3 (Graphite / Li-rich)
Several negative electrode formulations based on water based PVdF binder were developed and scaled up to 500 g batch size. The best positive electrode formulation has been successfully scaled up to 100 g batch size. During development, the importance of using graphene based nanoplatelets; increasing carbon black content, longer and more intensive slurry mixing procedure on the electrochemical performance of Li-rich positive electrodes was demonstrated (generation 3). The acquired knowledge has been applied during positive electrode manufacturing stage (WP4).
Finally, both formulations have successfully been validated in half and generation 2 full coin cells using 1.2M LiPF6 dissolved in EC/EMC (3/7 wt.) mixture with addition of 5 wt. % FEC and 0.5 wt. % LiBOB, was used in the both of cases (see T3.5). The obtained discharge capacity is about 200 mAh/gLirich with an initial coulombic efficiency about 77.0% and a capacity retention of more than 250 cycles at 100% depth of discharge. Such relatively short cycle life is expected to be improved by further optimization of cell design, separator choice, cell formation procedure and electrolyte in WP4.

Task 3.5 Advanced liquid Electrolyte development & characterization

Different electrolytes were formulated by Solvionic and were shipped to partners for evaluation. These included different mixtures of Li conducting salts (LiPF6, LiTFSI), ionic liquids (ILs, PYR14TFSI, PYR14FSI), organic solvents (DMC, EC, EMC) and additives (VC, FEC, and F2EC from Solvay).

Since the reference electrolyte formulation used for Generation 1 cell cannot be used for Generation 2 cell due to the high operation voltage of Li rich oxides, more than twenty different electrolytes have been prepared and tested in order to stabilize the Generation 2 cell capacity fade. In summary, it has been found noticeable positive effect of EMC solvent, LiBOB additive, and control of quantity of the electrolyte. The best capacity retention has demonstrated with 1.2M LiPF6 dissolved in EC/EMC 3/7 wt. mixture with addition of 5wt.% of FEC and 0.5wt. % of LiBOB. Therefore, this electrolyte has been defined as standard one for the manufacturing of pouch Generation 2 cells.

With the aim of improving the safety of organic liquid electrolytes ionic liquids, which are non-flammable, were tested as electrolyte. However the use of purely ionic liquids was found to hamper the formation of a stable SEI and therefore several formulations were prepared mixing 1M LiPF6 in EC:DMC 1:1wt. with 5% FEC with different amounts of ionic liquids (LiTFSI (0.3M or 1.0M) and PYR14TFSI). These electrolytes were tested with generation 1 materials in coin cell and pouch cells. Unexpectedly, the cells filled by electrolytes with higher IL percent (45%) showed better performance than the cells with electrolytes containing less IL (10%) independently of LiTFSI molarity in the electrolytes. Similar results were obtained in pouch cells: the cells with high IL fraction outperformed those with a low one. Adding VC drastically improved the ageing behavior. It was also found that removing the LiPF6 had a quite negative effect on the two cells with low IL fraction (10%) and 0.3M LiTFSI probably due to Al corrosion of the cathode current collector. Normally the LiPF6 would form a protective passive layer on top of the current collector, but as LiPF6 is absent the Al gets oxidized during the charging of the cells. The best way to implement ILs (PYR14TFSI) as Li-ion battery electrolytes was found to be by mixing them with standard organic compounds (DMC and EC) in a ratio of 45:55 and use LiTFSI (1M) as conductive salt and adding 5% VC, 5% FEC and 2% LiPF6 to protect the anode. With this formulation results close to that of the reference cell were obtained. However these IL-electrolytes were not used in generation 2 and generation 3 cells due to the still lower capacity compared to the reference cell and the low reproducibility of the cycling behavior.
The flash point of these electrolytes was measured. It was found that the influence of mixing the PYR14TFSI and FEC to the carbonate-based electrolyte is rather small (<13°C). Therefore, it is questionable that ionic liquids with these amounts have a significant influence on the safety properties on cell level and larger amounts (detrimental to the electrochemical properties) would be required. The reason is most likely that the vapour pressure of electrolyte mixtures is dominated by the most volatile compound which is the carbonate component.

Task 3.6 Separator selection and Gel electrolyte development & characterization

Task 3.6.1: Benchmark for Generation 1 development (M01-M12) (KIT, IMC)
The goal of task 3.6.1 was to find a benchmark separator for the MAT4BAT Gen 1 cell with respect to usage in electric vehicles. Several manufacturers of separators were approached but only Celgard was willing to sell a separator free of restrictions. A trilayer separator was acquired and its handling and processability were investigated. Based on that, processes for drying, filling, wetting, soaking and formatting cells in a small laboratory format were developed.

Task 3.6.2: Gel electrolyte synthesis (M01-M24)
A gel electrolyte membrane was developed using two PVdF-HFP grades from Solvay (PvdF-HFP SOLEF®21510 and PvdF-HFP SOLEF®21216) as baseline components. Two types of fillers were added to the formulation in order to improve the mechanical properties of the membranes: Tixosil® inorganic silica from Solvay and beech flour powder from LIGNEX (organic filler). Among all formulations studied the one with Solef® 21510 and 4 wt.% of Tixosil exhibited quite interesting performances close to the reference test with Celgard separator.

Phase inversion process was also evaluated as another way for manufacturing PVdF-HFP membrane. With this technique the polymer is dissolved in a mixture of good solvent (acetone)/non solvent (water or ethanol) which leads to phase inversion and creation of pores upon evaporation. Separators obtained were homogeneous with quite uniform porosity size distribution.
A technique to determine more accurately the membrane thickness was also developed. Getting thickness values of polymer membranes is rather complicated because they are thin (10 µm range) and the samples have soft nature and are likely to stick on the substrate. Two techniques were compared, ie. profilometry and near-infrared spectroscopy. Trials on several samples led to good matches between these two techniques.

Task 3.6.3: Gel electrolyte development (M01-M24) (CEA, INSA, Solvay)
The scaling up of the membrane synthesized was carried out from lab scale (using bar coater) to pre-pilot scale (using dynamic coater bench). . To avoid mechanical issues related to handling of self-standing membranes as well as to get better interface it was decided to directly cast the polymer solution on one or both electrodes of the cell. Lab scale deposits on both generation 2 electrodes were evaluated by using small pouch cells. The best cell design leading to no internal short-cuts and thin membrane was obtained with the coated graphite anode and bare cathode. With this configuration acceptable capacity fade was observed over more than 200 cycles. However, further development using dynamic coater bench would have been necessary to validate the manufacturing process for generation 2 prototypes and therefore the liquid electrolyte developed in T3.5 was used (1.2M LiPF6 dissolved in EC/EMC 3/7 wt. mixture with addition of 5wt.% of FEC and 0.5wt. % of LiBOB).

Task 3.7. All solid state Electrolyte development & characterization (from M01 to M36)

Task 3.7.1: Polymer electrolyte development & characterization
Several polymerizable ionic liquids (PIL) with different chemical structures were synthesized and characterized. Monomers with vinylimidazolium and vinylbenzyl functional groups were purified and analysed by elemental analysis and NMR. Polymerized ILs did not exhibit satisfactory electrochemical properties so novel polymer electrolyte formulations were developed based on copolymers of vinylphosphonic acid and are being characterized.

Generation 3 electrolytes still require further optimization and therefore generation 3 cells were not prototyped within the duration of the project.

Task 3.7.2: SEI (solid electrolyte interface) artificial development & characterization
A novel method for SEI characterization of solid electrolyte interface (SEI) was developed. This method is based on glow discharge optical emission spectroscopy (GD-OES) depth profiling. The method was calibrated using lab-coated reference samples with known composition. This method was applied to anodes from fresh commercial 16Ah-Kokam cells disassembled MAT4BAT in WP2.

Work-Package 4: Innovative battery assembly, packaging and prototyping

Task 4.1. Electrode fabrication and cell assembly

This task was dedicated to the manufacturing of Li-ion demonstration cells with the implementation of materials and formulations innovations from WP3.

Scale-up and manufacturing of the first generation of cells (NMC-Graphite, GEN#1) was made by CEGASA. Pilot-scale cell assembly equipment at CEGASA has been adapted to the number of electrodes established in the cell design. Electrode rolls were cut in the high-precision die-notching machine to obtain the individual anodes and cathodes in the size of the cell design. These cut electrodes were then loaded into the stacking-winding unit, together with the separator roll, to manufacture the cell stack. The automatic machine comprises an electrode pick-and-place system to position anode and cathode electrodes alternatively onto the separator roll which goes through a lamination step (low pressure and temperature) that provides a physical adhesion between electrode and separator membrane. This step provides an improved contact (lower resistance) between layers and fixed positioning of components for the prismatic winding to manufacture the internal cell stack. The electrode tabs were connected in parallel (Cu-Cu in the anode and Al-Al in the cathode) and to the cell terminal tabs by ultrasonic welding. The cell stacks were finally enclosed in the pouch bag (aluminium multilayer laminated foil) by thermal sealing of edges. 50 dry cells were finally assembled and supplied to KIT for electrolyte activation (see Figure 10). High capacity cells of almost 17 Ah were produced by the partners. After formation the energy density value of 150 Wh/kg target was achieved, hence validating Milestone MS6 “Cell fabricated”.

For GEN#2 cells the emphasis was put on the cell energy density target of the Description of Work (200 Wh/kg) with the introduction of Li-rich as cathode material.
EN#2A cells were manufactured with standard separator and liquid electrolyte (similar design as GEN#1 cells) whereas GEN#2B cells were planned with PVdF-HFP membrane swelled by electrolyte. Like previous Reference (Kokam) cells and GEN#1 cells, prismatic cells with soft packaging was selected, but electrodes and separator assemblies were spirally wound. Electrodes loadings were increased based on WP3 developments to be able to reach the energy density target.

Main technical issue was related to anode mechanical properties which were not sufficient enough to be able to increase electrode loading in the cell and bend the electrodes properly during assembly. CEA worked on the electrode coating and processing optimization to finally suceed in manufacturing meter-long electrodes for further processing. Electrodes were then calendered and slitted to the appropriate dimensions, and winded with commercial microporous separator to get the final wound assemblies (Figure 11a). Then the cells were put into the soft packaging before activation. The selected electrolyte was defined based on the trials in WP3 and the up-scaled mixture was prepared and supplied by SOLVIONIC. Finally 10 GEN#2A cells were successfully assembled and activated (Figure 11b).

The estimated cell energy density after formation was around 190 Wh/kg and very close to the DoW target for Gen#2 cells (200 Wh/kg). Cycling tests were performed at 25°C and 1C CCCV charge - 1C discharge rates but poor capacity retention was obtained with more than 20% capacity loss after 50 cycles (Figure 12). These results were in accordance with the ones obtained in WP3 for lower capacity cells (coin and pouch cells) and they are related to the poor cycle life of the Li-rich cathode material. Thus there is a clear need for optimized Li-rich material and electrolyte combination to get this technology more competitive.
GEN#2B cells were designed to integrate PVdF-HFP membrane in replacement of standard micropourous separator. The polymeric membrane was developped in WP3 with promising results compared to standard design. The challenging part of this development was related to the up-scaling of the membrane casting. Thin (< 20 µm ideally) and homogenous membrane has to prepared on a meter-long scale and this has never been done before by the partners. CEA decided to directly cast the membrane on the graphite anode which has the benefit to avoid producing self-standing and fragile polymer membrane. Preliminary casting trials on GEN#2 anode were realized with roll-to-roll process but the coating was not homogeneous at all with lot of bubbles generated at the surface. Casting process should be completely optimized in next projects to be able to produce large capacity cells.

GEN#3 cells suffered from huge accumulated delay and technical issues (material design and selection in WP3, innovative processes to develop) so they could not reach the manufacturing step. Instead development of GEN#3 technology remained at lab scale in WP3 with polymer and electrolyte materials provided by IMC and formulations developped accordingly by the partners.

Task 4.2 - Packaging for safety & high energy density
CEA and INSA de Lyon developed a new artificial hard casing (overpackaging) in order to improve the safety performance of cells while minimizing its impact on the energy density of the final encapsulated cell. This could be achieved by processing the casings by resin transfer molding (RTM) and vacuum infusion process. Thermosetting composites filled with high fiber-to-resin ratio and therefore outstanding mechanical properties has been processed using these techniques.

• Resin transfer molding versus vacuum infusion process
Resin transfer molding is a multi-staged closed mould process of high quality composites fiber-reinforced thermosetting polymer. RTM involves placing a fiber shape in a matched mould tool, and then injecting a thermosetting resin matrix with an initial viscosity of around 500 mPa.s into the mould cavity under pressure up to 5 bars. The use of mould leads to a smooth surface of the composite part. However, during the processing of casing, directly on the cell, the high injection pressure could be a safety issue regarding the sealing of cell. Therefore RTM packaging development has been done on 7.5 A.h Kokam cells.

In the other hand, vacuum infusion process is more flexible and does not require the use of a mould, a bagging film is often sufficient. This technique is usually used to manufacture large composite parts such as boat hulls and wind turbine blades. The vacuum is used to drive resin into a stack of fiber plies. Materials are laid at dry state into a mold or a bagging film if the shape of final part is simple. The vacuum is then applied before resin is introduced. Once a complete vacuum is achieved, resin is sucked into the laminate via carefully placed spiral tubing. Another advantage of vacuum infusion compared to RTM is that the use of vacuum only causes less safety issue regarding the sealing of cells. However, the quality of composite parts is a bit lower than for RTM, with a lower fiber-to-resin ratio, a thickness deviation if the vacuum is not well controlled and a rough surface at finished state. Also resins with low viscosity (~300 mPa.s) and a longer gel time are required in order to get correct composite parts.

• Composite formulation
The final properties of composite parts are also greatly dependent on the type of fibers, the fabric construction, and the resin employed (epoxide, polyester etc....). The identification of the relevant properties of the material to the safety performance is crucial and allowed choosing the best combination. From their former experiences and knowledge in this field, partners indicate that stiffness of casing in the tensile mode is the most relevant parameter unlike to the impact resistance. Other composite parameters regarding the safety performance are a good thermal stability and fire resistance, electrical insulating and easy shaping process.

o Resin selection
In both cases, the selected resins should be suitable to RTM as well as the vacuum infusion processes. In terms of fire resistance and thermal stability, the phenolic resins are known to be the best and used in solid propellant rocket. However, the processing of such resins is difficult and low temperature curing will lead to “green” part of phenolic resin with a significant amount of water due to the initial water content in the resin and also to the condensation water. Thus, the phenolic resins are excluded. From the ignition time versus irradiance of the most common resins, epoxy resin is the best one compare to polyester and vinylester resins. Moreover, cured epoxy prepolymer exhibit better mechanical properties compared to polyester, vinylester and phenolic resins. Then, epoxy prepolymer has been selected for the casing matrix.

o Fiber content and fabric construction
For safety concerns and functional issues of cell, the casing has to exhibit a high stiffness and tensile strength in tensile mode, and have to be insulated to avoid short-circuit. In a composite, stiffness in tensile mode is brought by the fibers. Carbon fibers exhibit the highest mechanical properties, followed by bore and Kevlar fibers, the lowest properties being obtained with the glass fibers. Bore fibers are excluded since the development focuses on common commercial fibers. Carbon fibers have been excluded for short-circuit of cells reason.
Different couple of matrix and fibers have been characterized by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) in order to obtain glass temperature, degradation temperature, degradation kinetic measurements and their enthalpie. Furthermore, viscosity and mechanical properties (static properties) have been performed in order to select the matrix and fibers. Couple of materials selected are shown in Table 6 and Table 7.

• Hard casing development by RTM process on 7.5 A.h Kokam cells
Partners validated a thickness of overpackaging of 1mm, which lead 3 plies of flax 200g/m2 and a specific mold have been developed for the overpackaging of Kokam cells. The RTM over-packaging have been performed with resin 1800 and this hardener associated (1805). The Nanotom 180S-PHOENIX have been used to perform X-rays tomography characterization. These characterizations have been performed in order to validate the interface between fibers and matrix. A good interface between fibers and matrix have been shown. Concerning the overpackaging by RTM process, a specific protocol has been developed and permit to obtain a hard-casing without external defect and a none overpackaged current tabs (see Figure 13) which is mandatory for the following safety and performance tests. Sensor has been integrate on the pouch foil and on the external layer of overpackaging, see Figure 14, with specific procedure.

• Hard casing development by infusion process on dummy cells
Several dummy cells have been over-packaged using the infusion tools at INSA Lyon. Over packaging were made with carbon, glass or flax fibers (see Figure 15). Aramid was not used because of its machining issues. Three different methods of draping were developped taking into account the shape and the size of the final pouch cells (See Deliverable 4.3).

Over-packagings were also successfully processed by vacuum infusion process with an embedded sensor (Figure 16). The over-packaging was processed using glass-fiber fabrics in order to get a good optical transparency of the composite and be able to monitor the position of the sensor after the processing. The gauge did not move during the processing and the gauge integrity has been tested using a Wheatstone bridge. The processing of the packaging does not cause any damage on the strain gauges.

Task 4.3. Performance and safety acceptance of prototypes
Today Li-ion technology is used for energy storage in a variety of applications such as smartphones, smartwatches, tablet/laptop computers, mp3 players, power tools, stationary energy storage, unmanned aerial vehicles (UAV), space flight applications, light electric vehicles (LEV), as well as electric cars (EV, HEV, PHEV). During the whole life of a battery, the safety is most important. In the past, failure of Li-ion cells lead to product recalls which are very expensive for the manufacturers and they alienate costumers, even if it happens only in very few cases (ppm range). Therefore, safety tests were performed in WP4.3 of the MAT4BAT project.

Firstly, all types of available safety tests were evaluated and four types of tests were chosen: nail penetration, blunt rod, overcharge, and accelerating rate calorimetry (ARC) tests. The tested cells were conducted with fully charged cells since they represent the worst case. Tested cells included 17Ah pouch cells produced in the project, as well as commercial 7.5Ah and 16Ah pouch cells, and 3.25Ah 18650-type cells. The results were classified according to the EUCAR hazard levels. CEA and ZSW worked on safety evaluation of overpackaging. ZSW performed nail penetration and blunt rod tests on 7.5Ah cells with overpackaging developed in WP4.2. CEA tested cells with overpackaging in ARC tests for thermal runaway measurments. By comparison with cells without overpackaging, it turned out that the overpackaging has only a minor effect on the safety behaviour in these tests.

For the 3.25Ah cells, ARC tests were performed on aged cells with Li plating (cycled at 0°C). The experiments showed that the rest time between aging and the ARC tests is important for the result and for its reproducibility [1]. For short rest times (1.5h) after Li plating, the safety performance was drastically reduced, i.e. the onset of self-heating was shifted to low temperatures and the degree of destruction was stronger (ejection of jelly roll from cell housing) [1]. The reason are most likely stronger exothermic reactions of the deposited Li metal with electrolyte. For longer rest times (8d) after Li plating, a partial recovery of the safety properties and of cell capacity was observed. This was discussed in terms of chemical intercalation of metallic Li into adjacent graphite particles lowering the amount of Li metal [1]. However, also in case of the partial recovery of the safety properties after longer waiting time, the degree of destruction was stronger for cells with Li plating in comparison with cells without Li plating. Therefore, it is clearly indicated that the aging mechanism of Li plating has to be prevented in Li-ion cells. Further work in this direction is under progress by ZSW and CEA.

Published results from T4.3:
[1] T. Waldmann, M. Wohlfahrt-Mehrens, Effects of rest time after Li plating on safety behavior—ARC tests with commercial high-energy 18650 Li-ion cells, Electrochimica Acta. 230 (2017) 454–460. doi:10.1016/j.electacta.2017.02.036.

Work Package 5: Product and Market Acceptances
This work package provided an economic and ecological assessment of the three MAT4BAT cell generations, as well as a comparison of the cell materials and developed testing procedures with regulations and standards.

Task 5.1 Green value chain management

The green value chain analysis provided results on the environmental impact of the MAT4BAT developments. Therefore, the ecological impact and criticality of the cell generations 1 to 3 were quantified and compared to a reference cell. The most significant results are (Figure 17):
• The environmental profile of the cell production is dominated by the positive electrode materials.
• The impact of the cell production is positively influenced by changing the binder solvent of the positive electrode from NMP to water (reference → Gen 1).
• Gen 2 uses a Li-rich cathode and a commercial separator. The impact of cell production is positively influenced by the change from NMC to Li-rich (Gen 1 → Gen 2). For the production and end-of-life treatment, generation 2 has the lowest environmental profile.
• The use of tetrahydrofuran for the processing of Gen 3 slurries has a negative impact on the environmental profile. Together with the reference cell, the Gen 3 cell has the highest environmental profile in cell production and end-of-life treatment. Gen 3 ranks last if a 100% NMP recovery rate is assumed for the reference cell.
Furthermore, benchmarking the cell production to the total battery life cycle shows that the use phase (not shown in Figure 1) accounts for around 90% of the reference cell’s environmental profile of the complete life cycle. Although the use phase’s impact depends on the electricity mix used for battery charging, this importance does not change even under optimistic assumptions.

The raw material supply risk and criticality is the highest for the reference cell, and is reduced with each cell generation. It can be lowered significantly by removing LiPF6 from the electrolyte and switching to Li-rich which does not contain cobalt (Figure 18).

The foreground was disseminated by VITO during a presentation at the MAT4BAT summer school.

Task 5.2. Cost Assessment

This task provided an assessment of the high volume manufacturing cell costs forecasted for the year 2020 based on the Battery Performance Cost Model published by Argonne National Labs. The three MAT4BAT cell generations were analysed and compared with a reference cell. For new materials and processes, input data for the cost assessment such as forecasted prices was obtained from the consortium partners.
The Gen 1 cell uses water for processing the cathode slurries and a special PVdF coated separator. Compared to the reference cell using NMP on the cathode side, the 2020 Gen 1 cell price decreases from 172 to 167 €/kWh (Figure 19). If the Gen 1 cell would use the same separator as the reference cell, a lower price of 157 €/kWh could be reached.
Due to higher active material capacities and lower material prices of Li-rich, the 2020 cell price for the Gen 2 cell decreases further to 144 €/kWh.
With the development of a polymer electrolyte for Gen 3 the 2020 price is 115 €/kWh. If the handling of binder solvent tetrahydrofuran could be avoided, another price reduction of 4% could be reached. The manufacturing process for Gen 3 is uncertain, an additional coating line for the membrane casting would increase the 2020 cell price from 118 €/kWh to 115 €/kWh.

Task 5.3. Business Study

This task evaluated the economic implications of the MAT4BAT developments for electric vehicles and stationary storage applications. The economic impact was quantified for both the Belgian and German market and compared for the different MAT4BAT cell generations. The business case analyses for mobile applications included grid congestion management and portfolio management, whereas multipurpose MW-scale batteries for wind farms and residential batteries for roof-top photovoltaic systems were selected as stationary applications.

The revenues a distribution system operator can achieve by using EVs for congestion management are 11-21 €/EV/year for then Gen 1 to 3 cells. It is thus unlikely that smart charging will be employed on a voluntary basis.

The benefits obtained by offering EV flexibility as balancing energy range between 347 for Gen 1 in Belgium (only grid to vehicle) to 2,026 €/EV/year for Gen 3 in Germany (including vehicle to grid) in the case of minimum flexibility, and 382 for Gen 1 in Belgium (only G2V) to 13,951 €/EV/year for Gen 3 in Germany (including V2G) in the case of maximum flexibility (Figure 20).
For the stationary application, the added value for battery systems installed in 2020 was analysed. Therefore, battery system cost were assumed to reach 2.55 million € for a 5MW / 5MWh battery system and 2,200 € for a 4kW / 4kWh battery system. In Germany, all cell generations have a positive added value in both applications. The calculation of an exemplary 5 MWh battery system connected to a 50 MW wind farm results in an added value of between 21 million € for a battery system based on the reference cell to 27 million € for a system based on the Gen 3 cell. In combination with residential roof-top PV systems, the end user’s added value reaches up to 3680 € per system and household.

In Belgium, batteries for wind farms seem not to be profitable for any cell generation. When used together with residential PV systems, batteries become profitable with the developments of Gen 2 and 3. The battery added value increased to up to 372 € per system and household (Figure 21).

Task 5.4. Regulations, Recommendations, Standards
Following a review of the current regulations and standards, this task investigated the impact of current standards and legislation on the materials and cell architectures developed in MAT4BAT and analysed the potential for improvements of those.

No issues were found regarding the legislation. The analysis of standards was based on a survey of the project partners. This task concluded that:
• Slow discharge and charge tests (• For coin (half) cell tests, standardisation may lead to better comparability of the test results obtained from laboratories for the development phase of new batteries.
• The ageing tests applied in MAT4BAT are different from those in standards. A more generic ageing approach would enable to calibrate aging models for simulations.
• Regarding abuse, reliability, safety and protection tests, it is recommended to review the external and internal short circuit tests and to standardize ARC tests.

The deliverables of task 5.4 i.e. the analysis of regulation and standardisation (D.5.1) and the recommendations (D5.6) are public deliverables and can thus be found at the project’s website and at Zenodo. Also, a website with all battery-related standards is available:

Potential Impact:
The activities performed in the scope of the MAT4BAT project led to results with high potential impact summarized below by Work-package:

WP1: Battery ageing assessment – methods & tools

WP1 jointly with WP2 have led to a better understanding of the aging mechanisms of NMC/C Li-ion batteries, with two articles already published, [J. Electrochem. Soc. 163 (2016) A1232-A1238] and [J. Electrochem. Soc. 164 (2017) A1089-A1097], and a third being drafted and that will be submitted soon, about the premature loss of capacity observed in cycling at low temperature.

This work made it possible to demonstrate a novel aging mechanism, corresponding to a local Li deposition following high-SOC / high-temperature storage conditions.

All of this knowledge will enable the Li-ion to be better managed under real operating conditions, to improve the lifetime of batteries and thus reduce their total cost of ownership.

In addition, this will contribute to the improvement of materials, as already experienced in WP3 and WP4. In particular, the WP1 and WP2 of the MAT4BAT project made it possible to demonstrate that biphenyl, an additive in the electrolyte, is not suitable as soon as batteries have to operate at high temperature and at high SOC. Subsequent work could therefore consist in replacing biphenyl, so as to have the advantages (safety at high temperature) without the disadvantages (premature capacity loss).

WP2: Understanding of ageing mechanisms – simulation & modelling

Li-ion cells are prone to aging effects such as capacity fade and resistance increase, which translate to a limitation of driving range and acceleration power in case of an electric car. Extended aging tests of a large number of Li-ion cells in the MAT4BAT project revealed the combinations of operating parameters and their trends that can increase the life-time of Li-ion cells. Additionally, Post-Mortem analysis of Li-ion cells revealed the understanding of main aging mechanisms which is useful to prevent them on a knowledge-based approach. One critical aging mechanism became apparent in MAT4BAT is Li deposition on anodes since it leads to fast degradation of Li-ion cells and it decreases their safety behaviour. This aging mechanism has not been considered to this extend before this project. A clear result of MAT4BAT is that the aging mechanism of Li deposition/plating has to be prevented to enhance safety and cycle-life of Li-ion batteries.
The project contributed to longer life-times and safer operating for products with Li-ion cells. Therefore, the project contributes to cost reduction, an increase in sustainability and in consumer acceptance.

The results of WP2 were published as peer-reviewed papers, see PUDK and list below and on international conferences as posters and presentations (e.g. ABAA, ECS meetings). Further publications of project results are currently in progress (after the end of the project). The publications were listed on the project website and in annual reports by different partners. By this strategy, the results of MAT4BAT were made available to a large audience including industry, stake-holders, scientists, media and the public. E.g. the collaborative review paper on Post-Mortem analysis was downloaded more than 1000 times within the first three month after publication. The results of MAT4BAT gained very valuable insights into aging mechanisms of Li-ion cells and gave hints how to prevent them. The collaborative work in MAT4BAT has enhanced the knowledge and skills of all involved project partners and motivates for further projects. Furthermore, the results from MAT4BAT are used in lectures for university students and in workshops with industry representatives to enhance knowledge in general.

[1] T. Waldmann, A. Iturrondobeitia, M. Kasper, N. Ghanbari, F. Aguesse, E. Bekaert, L. Daniel, S. Genies, I.J. Gordon, M.W. Löble, E. De Vito, M. Wohlfahrt-Mehrens, Review—Post-Mortem Analysis of Aged Lithium-Ion Batteries: Disassembly Methodology and Physico-Chemical Analysis Techniques, Journal of The Electrochemical Society. 163 (2016) A2149–A2164. doi:10.1149/2.1211609jes.
[2] N. Ghanbari, T. Waldmann, M. Kasper, P. Axmann, M. Wohlfahrt-Mehrens, Detection of Li Deposition by Glow Discharge Optical Emission Spectroscopy in Post-Mortem Analysis, ECS Electrochemistry Letters. 4 (2015) A100–A102. doi:10.1149/2.0041509eel.
[3] N. Ghanbari, T. Waldmann, M. Kasper, P. Axmann, M. Wohlfahrt-Mehrens, Inhomogeneous Degradation of Graphite Anodes in Li-Ion Cells: A Postmortem Study Using Glow Discharge Optical Emission Spectroscopy (GD-OES), The Journal of Physical Chemistry C. 120 (2016) 22225–22234. doi:10.1021/acs.jpcc.6b07117.
[4] T. Waldmann, B.-I. Hogg, M. Kasper, S. Grolleau, C.G. Couceiro, K. Trad, B.P. Matadi, M. Wohlfahrt-Mehrens, Interplay of Operational Parameters on Lithium Deposition in Lithium-Ion Cells: Systematic Measurements with Reconstructed 3-Electrode Pouch Full Cells, Journal of The Electrochemical Society. 163 (2016) A1232–A1238. doi:10.1149/2.0591607jes.
[5] B.P. Matadi, S. Geniès, A. Delaille, T. Waldmann, M. Kasper, M. Wohlfahrt-Mehrens, F. Aguesse, E. Bekaert, I. Jiménez-Gordon, L. Daniel, X. Fleury, M. Bardet, J.-F. Martin, Y. Bultel, Effects of Biphenyl Polymerization on Lithium Deposition in Commercial Graphite/NMC Lithium-Ion Pouch-Cells during Calendar Aging at High Temperature, Journal of The Electrochemical Society. 164 (2017) A1089–A1097. doi:10.1149/2.0631706jes.
[6] T. Waldmann, M. Wohlfahrt-Mehrens, Effects of rest time after Li plating on safety behavior—ARC tests with commercial high-energy 18650 Li-ion cells, Electrochimica Acta. 230 (2017) 454–460. doi:10.1016/j.electacta.2017.02.036.
[7] A. Iturrondobeitia Ellacuria, F. Aguesse, S. Genies, T. Waldmann, M. Kasper, N.Ghanbari M. Wohlfahrt-Mehrens, E. Bekaert, Post-mortem analysis of calendar
aged 16 Ah NMC/graphite pouch cells for EV application, submitted to J. Phys. Chem. C, 2017.

WP3: Advanced materials for durable & safe cell development

MAT4BAT has generated a huge amount of know how in battery materials (electrodes, additives, electrolytes, separator, binders, etc.). The critical parameters affecting battery performance and cyclability have been identified for each of the studied materials and have been addressed through different types of strategies.

New and improved battery materials for the next generation of lithium-ion cells have been developed and, after optimization, have been characterized at the prototype level. In particular, a patent regarding amorphous carbon coating of carbonaceous particles has been filed. Materials scalability issues were successfully solved and are ready to be transferred to the industry. New electrode formulations with water based slurries as an alternative to organic media were developed, both for the positive and negative electrode, which is expected to have an impact both at the cost level (is cheaper, allows reducing the binder content, tolerates for faster drying speeds in the electrode fabrication process) and for the environment (non-toxic, recyclability of LIBs based on waterborne electrodes is expected to be much more efficient).

The ambitious energy density target for generation 2 cells has been achieved although further work is required to improve the lifetime of current cells with such an increase of energy density that requires operation at higher voltages that damage organic-based electrolytes. However, the critical cell components have been identified for their further improvement and the realisation higher energy age-resistant Li-ion cells of appears feasible with further research devoted to the development of more stable, safer electrolytes.

Such achievements will generate substantial added value for battery products and hence competitive advantages directly exploitable for all RTDs stakeholders in the Li-ion batteries value chain.

WP4: Innovative battery assembly, packaging & prototyping

CEA is a well known research center for its high quality research activities in the field of Li-ion batteries development by the Laboratory for Innovation in New Energy Technologies and Nanomaterials. As a Research and Technology Organisation (RTO) one important mission of CEA is to help French and European companies to increase their competitiveness by developing innovative technologies and transferring these technologies and the know-how to industrial partners.

In the Work Package 4 of MAT4BAT project, CEA has been involved in several technical areas:
- development of electrode manufacturing,
- manufacturing of large capacity cells for demonstration purpose,
- packaging process,
- and materials development.

CEA plans to valorize the main results obtained in the MAT4BAT project by transferring its know how to French and European batteries manufacturing and overpackaging company. This work package enabled to increase knowledge of drap, materials, process development and performance evaluation of cells thanks to implementation of sensors deposited on cells.
CEA plans also to disseminate the scientific results by participating to conferences focused on manufacturing process in order to update the best practices to increase efficiency and battery performances.

The knowledge gained from the project will also be used to identify new research challenges for the future research activities and in current developments by promoting for instance new bio sourced polymers and bio-fillers for batteries manufacturing for example not only for electric vehicles but for a broader range of applications.

For CIDETEC the developed GEN1 and GEN2 cells with water processed electrodes have demonstrated similar durability with organic processed ones. Thus, reducing of negative environmental impact of battery manufacturing by using water based PVdF binders will cause a positive socio-economic impact which may help to improve European battery production capacities.

Concerning ZSW and their works based on ageing mechanisms, one paper has been submitted in Electrochim. Acta. (T. Waldmann & M. Wohlfahrt-Mehrens). Moreover the studies developed in work package 4 of MAT4BAT project motivated further work on the impact of aging mechanisms on safety.

Finally for INSA de Lyon, the MAT4BAT project permitted to create one fixed term contract and to develop an engineer course based on vacuum infusion process.

WP5: Product and Market Acceptances

The green value chain analysis shows that in the environmental impact of the cell production and the end-of-life treatment can be positively influenced by avoiding NMP or THF as processing materials for electrode slurries and by the shift to Li-rich. From a supply risk perspective, the usage of Li-rich and the removal of LiPF6 from the electrolyte are recommended. However, safety and lifetime aspects should not be neglected. As around 90% of a cell’s life cycle environmental impact can be allocated to the use phase, current and future developments of new cell materials should always aim on at least maintaining or even increasing the cell durability. If the goal is to decrease the environmental impact of lithium ion cells over their total life cycle, effort should be made to improve the cell lifetime. Preliminary results of the green value chain analysis were disseminated by VITO during a presentation at the MAT4BAT summer school in La Rochelle.

The economic impact of the project developments was assessed by the dimensions production cell cost and profitability in EV and stationary applications. Compared to the state of the art, the potential for cost reduction is up to 35% with the change to Li-rich and polymer electrolyte membranes. Both cost and durability of the developed cell generations are crucial from environmental and economic perspective. Since long-term degradation tests for the cell generations 2 and 3 were not available until the project end, the business cases were analysed based on the target lifetime according to the description of work. Under the assumption that the cell cost decreases and lifetime increases with each cell generation, the results suggest a positive business case for offering EV flexibility as balancing energy in both Belgium and Germany. Revenues increase with increasing flexibility and by offering vehicle to grid services. The revenues potential in Germany is currently much higher than in Belgium for both mobile and stationary applications. In stationary applications and in a 2020 context, all analysed cell generations showed a positive value added when adding a battery system to either a wind farm or a residential PV system in Germany. In Belgium, the first batteries for residential PV systems become profitable with the developments of the generation 2 and 3 cells. There is no business case for batteries connected to wind farms. The revenues from an operator perspective increase with each cell generations. The second-life application of automotive batteries in stationary battery systems could be promising both from environmental and economic perspective. Knowledge on the cell degradation behaviour beyond 80% state of health could provide valuable information for an analysis of such business cases. Intermediate results for the stationary applications were presented in 2015 at the Advanced Battery Power Conference in Aachen and the International Conference on the European Energy Market [1, 2].

The project developments proved to be conform to current standard and regulations. However, the test approaches used in MAT4BAT differ from those applied in standards. A feed-back to the respective working groups is planned. A first step was made when VITO presented the results at the workshop “Putting Science into Standards, Driving Towards Decarbonisation of Transport: Safety, Performance, Second life and Recycling of Automotive Batteries for e-Vehicles" in September 2016. The workshop was organised by the Joint Research Centre (JRC) together with the European Committee for Standardization (CEN), the European Committee for Electrotechnical Standardization (CENELEC), and the European Commission Directorate-General for Internal Market, Industry, Entrepreneurship and SMEs.

[1] S. Ried, M. Reuter-Oppermann, P. Jochem, W. Fichtner. Dispatch of a wind farm with a battery storage. Operations Research Proceedings 2014: Selected Papers of the Annual International Conference of the German Operations Research Society (GOR), RWTH Aachen University, Germany, September 2-5, 2014, 473-479, 2016.
[2] S. Ried, P. Jochem, W. Fichtner. Profitability of photovoltaic battery systems considering temporal resolution. 2015 12th International Conference on the European Energy Market (EEM), Lisbon, Portugal, 19–22 May 2015, 1–5, 2015.
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