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Interfaces of Fluid Electrodes: New Conceptual Explorations

Final Report Summary - INFLUENCE (Interfaces of Fluid Electrodes: New Conceptual Explorations)

Executive Summary:
The FP7 project INFLUENCE aimed to improve the fundamental understanding and control of interfaces of a battery type based on Li-ion and Na-ion active materials and redox flow battery systems: the semi solid flow battery (SSFB). The fact that the case study was SSFB set-up instead of a conventional Li-ion battery constitutes an asset, given that the methods and techniques developed are generic and could as well be implemented for conventional Li- and Na-ion systems (except the techniques addressing flow aspects).
The goal of the scientific activities of INFLUENCE was therefore to gain better insights into interface processes relevant to secondary batteries, taking as particular case study SSFBs. This entailed the examination of the following aspects:
• Formation of interfaces between the electrolyte and the Electro Active Particles (EAPs) in fluid electrodes.
• Colloidal interactions between EAPs and Carbon Nano Particles (CNPs), including the cross-interactions, as a function of electrode composition and state of charge.
• Interactions between fluid electrode and current collector, and their behaviour in the flow channels.
A lab scale prototype SSFB (ca.0.7 cm² surface area) was proposed and validated for the study these interactions. A variety of materials were evaluated as components for fluid electrodes; some of them were not commercial and have been optimized within the project (e.g. Na-ion EAPs, ionic liquid based electrolytes). Substantial efforts were dedicated to the optimisation of electrode formulation and its preparation.
Main project outcomes:
The scientific work provided the basis for the core technological goal of the project: to develop specific characterization tools for a reliable evaluation of interface processes (some of them generically applicable to batteries and supercapacitors):
• Battery analysis techniques were implemented to study the afore mentioned interfaces: EIS (Electrochemical Impedance Spectroscopy) and GITT (Galvanostatic Intermittent Titration Technique) were used to assess the effect of several parameters on the state of health of a SSFB.
• The safety of the system was evaluated by means of TGA-MS (Thermo-Gravimetric Analysis coupled with Mass Spectrometry).
• The rheological behaviour of the particles in fluid electrodes was first examined from the perspective of colloidal interactions between EAPs and CNPs.
• Rheological and electrical properties of the fluid electrode were investigated by means of electrochemical cycling experiments (charge/discharge) in a rheo-impedance set-up.
The project realised the first proof of concept for a non-aqueous SSFB based on the Na-ion chemistry, using P2-type Na0.45Ni0.22Co0.11Mn0.66O2 (NaNMC) and NaTi2(PO4)3 (NTP) as positive and negative electrode, respectively. This concept opens the door for developing a new low-cost type of non-aqueous semi-solid flow batteries based on the rich chemistry of Na-ion intercalating compounds.
Experimental investigations have been complemented with computational modelling. At molecular level, a detailed kinetic mechanism was developed and validated for four material systems. At macroscopic level, the modelling of electrochemical cells in static and dynamic flow conditions was performed using COMSOL Multiphysics® various scales: laboratory cell (mono-channel), laboratory stack (single and multi-channel configurations) and larger electrochemical systems.
Conclusions from the modelling were crucial input for the design of a SSFB mini-stack (consisting of three cells connected in series, ca 70 cm² surface area), which was delivered and validated in the last months of the project, bringing SSFB energy storage systems to TRL 4.
Lastly, it was concluded from calculations that SSFBs electrochemical stack systems with power input/output up to 1 kW with a reasonable size (1.5 m x 1.5 m) could be feasible for energy storage and production at an household level.

Project Context and Objectives:
The understanding and control of the SEI (solid electrode-electrolyte interface) in rechargeable batteries is of paramount importance from the point of view of performance and of safety. The physical and chemical phaenomena occurring at these interfaces determine the kinetics of battery processes, and are key to the state-of-health and failure modes. Being able to monitor changes in SEI is therefore of particular relevance in order to control battery processes.
The FP7 project INFLUENCE aimed to improve the fundamental understanding and control of interfaces of a battery type based on Li-ion and Na-ion active materials and redox flow battery systems: the semi solid flow battery (SSFB). The fact that the case study was SSFB set-up instead of a conventional Li-ion battery constitutes an asset, given that the methods and techniques developed are generic and could as well be implemented for conventional Li- and Na-ion systems (except the techniques addressing flow aspects).
A flow battery is an energy system in which, at least, one of the electrolyte is pumped between the electrolyte tank and the electrochemical cell. The first redox flow batteries (RFB) were developed in the late 70s. Since then, a variety of flow battery systems have been proposed; among them, the all-vanadium redox battery (VRB) is an energy storage and conversion system currently used to store intermittent renewable energy as well as for stationary applications. Nevertheless, it suffers from low specific energy due to the limited solubility of vanadium in the used electrolytes. In 2011 researchers at MIT reported a new energy storage system named semi solid flow battery (SSFB).The SSFB is a promising energy storage technology that combines the flexibility in configuration and operation as well as independent scalability of energy and power capabilities of redox flow batteries with the high energy density of Li-ion battery materials
The operational principle of SSFB is based on RFB, but anolyte and catholyte consist of flowable suspensions of solid active materials instead of dissolved redox species, i.e. fluid electrodes. The chemistry of most reported SSFB relies on the chemistry of well-investigated Li-ion battery materials. A key aspect in both Li-ion and Na-ion batteries is the formation of the SEI when electrodes are operated outside the electrochemical stability window, due to reductive or oxidative decomposition of the carbonate based electrolyte. The formation of a stable solid electrolyte interphase (SEI), which is electrically insulating, at the negative electrode of conventional Li ion batteries has a positive impact in the functioning of the battery, making it possible to use active materials operating at very cathodic potentials, e.g. graphite at 0.1 V vs. Li/Li+. However, the fluid electrodes do not necessarily behave as conventional solid electrodes. There is a major difference between the SEI in Li-ion batteries and in SSFBs, namely: the location of the SEI. In classic Li-ion batteries, the SEI is formed around the ‘‘static’’ solid electrodes, allowing many contact points between particle/particle and particle/current collector to remain mostly ‘‘uncovered’’ for facile electron transport across the entire solid electrode. Thus, the SEI in Li-ion batteries is mostly located at the interface between electrode and electrolyte. On the other hand, in SSFBs the active particles are in continuous motion. The contacts for electron transfer between current collector and particles are severed and re-established continuously, which allows the SEI to cover the entire current collector. Thus, the SEI in SSFBs is mostly located between the current collector and the fluid electrode. Once the SEI is formed in SSFBs, the electrons must cross this electrically insulating barrier on their way from the current collector to the active material and vice versa.Whereas the properties of the SEI in SSFBs may be very similar to those of SEI in a Li ion battery based on the same active material and electrolyte solution, the formation of SEI has a specific detrimental effect in the fluid electrodes of SSFBs since it hinders the electrical connection between the current collector and single particles dispersed in the electrolyte.
From an application point of view, the main differences and advantages of SSFBs with respect to conventional flow batteries are based on the fact that energy is stored in suspensions of solid storage formulations (the fluid electrodes). Charge transfer in the electrode is realised via dilute percolating networks of nanoscale conductors. This looks close to classic Li-ion and Na-ion batteries, except that the active materials flow through the cell. As for any of the secondary batteries known to date, fundamental insight in the interfacial processes is needed. Many processes in SSFBs are similar to ‘classic’ Li-ion batteries, e.g. the formation of a passive layer called SEI on the surface of e.g. anodic graphite. The addition of small graphite particles to the cathode material is needed at the cathode to enable electric pathways from the active material to the current collector; the use of the same type of carrier fluids (electrolyte) are used and finally, the active material particles which undergo the same type of stresses during the (de-) lithiation intercalation processes. In addition to interfacial aspects, there are also new phenomena to be understood:
• particles have to stay in suspension;
• the electric pathway connection is dynamic (breaking and formation of percolating structures);
• the current collector for lithium-ion chemistries it cannot be the graphite material (as it is in RFBs).
The primary goals of the project were:
• To study and optimize of interfaces developing between the electrolyte and the electrochemically active material particles in fluid electrodes. The acquired knowledge was subsequently applied to the chemical and morphological optimization of active materials as well as the design of optimized interfacial layers (also called artificial Solid Electrolyte Interfaces, art-SEI) capable of warranting stable interfaces.
• To improve the understanding and control the mechanical and conductive behaviours of the slurries. For this, it was a prerequisite to investigate the role of shape anisotropy and the nature (attractive or repulsive) of the short ranged interactions of the active materials besides the strength of the attractive forces for conductive nano-particles. The cross interaction should allow intimate contact between active material and the conductive particles.
• To complement experimental data with computational modelling in order to further improve the understanding of physical and electrochemical phaenomena occurring at the interfaces at the microscopic scale, and to derive scaling rules towards mascroscopic scale. This makes it possible to provide design recommendations leading to optimal interface behaviour (size of anodic and cathodic compartments, geometry of collectors, etc.).
One implicit objective for the period for the second half of the project was to implement the knowledge gained in the first half of the project for the design and realization of a SSFB stack by the end of the project.
The work was structured in four main areas of development: definition and design of the system, study of interface electrolyte/active material particles, study of interface current collector/slurry and modelling. The specific objectives for each of them is given in the corresponding subsections of the section S&T results/foregrounds.
Project Results:
See section 3 in the document in attachment "Influence 608621 Final publishable summary report v2.0.pdf"
Potential Impact:
Understanding reactions at the SEI (solid electrode-electrolyte interface) in rechargeable batteries is essential to developing strategies to enhance cycle life and safety of rechargeable batteries. Despite past research efforts, there is still limited understanding by what means different components are formed at the SEI and how they influence SEI layer properties and behavior. The RTD activities carried out in the frame of the INFLUENCE project led to a better understanding of interfaces in Li-ion and Na-ion batteries, particularly the formation of the (artificial) SEI layer. In addition to the study of this aspect, of paramount importance for all rechargeable batteries, the investigation of interfaces in fluid electrodes led to a more thorough understanding of interface aspects that could not be obtained from the study of conventional batteries.
In conventional Li-ion battery anodes (graphite based) a good SEI is electronically insulating and ionically conductive to electrically passivate the surface and avoid further decomposition, which enables safe operation beyond the stability window of the electrolyte. However, in fluid electrodes, the active particles are in continuous motion and the electronic conductive pathways are rather dynamic. The contacts for electron transfer between current collector and particles are severed and re-established continuously, which allows the SEI to cover the entire current collector. Thus, the SEI in SSFBs is mostly located between the current collector and the fluid electrode. Once the SEI is formed in SSFBs, the electrons must cross this electrically insulating barrier on their way from the current collector to the active material and vice versa. The electrically insulating character of the SEI turns from a beneficial feature in classic LIBs to a detrimental one in SSFBs. A non-electrically insulating SEI does not prevent the electrolyte decomposition and the potentials beyond the SEI formation are not accessible. Either way, operating outside the stability window of the electrolyte solutions does not seem possible in SSFBs.
The project led to specific insights in the rheology of electrochemically active slurries, in the particle interactions (aggregation, network formation, segregation) and in the behaviour of their individual components and of the.To ensure electrical conductivity within the electrode slurry, conductive particles were added to the dispersions in the electrolyte medium. The percolation behaviour was studied under flow conditions. Rheological and electrical properties were measured in conjunction, also in combination with (dis)charging currents. This combination of techniques and scope of application is novel. The rheological and conductive properties of the fluid electrodes at a given composition were found to depend on the mechanical formulation. The contact resistance between current collector and electrode slurry has been identified as the main factor limiting the electrochemical performance of the SSFB. Results point out that the conductive particles determine to a large extent this contact resistance.
The enhancement of the energy density in SSFBs requires either thus the search and development of novel active materials operating at 1.2–0.8 V vs. Li/Li+ or the replacement of carbonate-based electrolyte solution by others which are more stable at very cathodic potentials such as some ionic liquids. Until now, materials operating at 1.2–0.8 V vs. Li/Li+ were of little interest for the battery community due to their lower energy density. Now, materials such as Sb, ZnSb, Bi, black phosphorous or metal phosphides (e.g. NiP2), operating above 0.5 V vs. Li/Li+ may deserve the attention of the battery community for the next generation of SSFBs.
Sodium-ion batteries are attracting increasing attention. Nowadays, they are considered as a future alternative to Li-ion batteries for large scale energy storage applications, because they possess a cost advantage with respect to Li-ion batteries due to the lower costs of sodium based raw materials and the replacement of the copper anode current collector with cheaper aluminum. Energy densities up to 200 Wh kg-1 may be achievable. Na-ion battery technology is still considered to be in its infancy, and new active materials are developed rapidly. The set-up and working principle of both battery technologies are similar, which facilitates the transfer of Na-ion batteries into application. A part of the RTD activities within INFLUENCE focussed on Na-ion battery technology. Novel NaNMC layered Na-based oxides were synthesised, optimisation an characterised, revealing very promising behaviour as positive electrode for Na-ion batteries. NaNMC materials were also tested in different battery systems, both conventional solid Na-ion batteries and SSFBs. These activities resulted in an advancement of the knowledge on Na-ion batteries and their state of development.
4.1 Socio-economic impact and wider societal impact of the project
In a wider perspective the INFLUENCE project makes a contribution to the implementation of the SET-Plan. The availability of storage technologies suitable for integration with RES is key to ensure an increase of the share of RES in the energy mix. There is a need for a next-generation storage systems, which should allow absorbing energy when the generated power exceeds the need and releasing energy when the demand is higher, stabilizing then the intermittent renewable energy. This constitutes a key to large-scale deployment of renewable energy and absorption of distributed power in the grids, crucial in order to reach 2020 and 2050 energy and climate targets. SSFB, the battery technology studied within INFLUENCE is an energy storage technology highly suited for integration with RES.
For grid-scale storage based on conventional Li-ion batteries, thousands of cells are assembled since the capacity of Li-ion cells is not very high.This has a negative impact on costs (due to the assembly of and the monitoring electronics for each cell), the safety (every individual cell can fail) and reliability (if one cell ages in a string whole this string cannot operate anymore). Conversely, these systems deliver power performances better than actually needed. In a conventional solid Li-ion or Na-ion battery, power and energy are intrinsically linked since the active materials are inside the battery. RFB technology offer a solution to the limitations of Li-ion batteries. In RFB batteries the active materials are stored in containers and pumped through cell stacks that deliver the power. A remarkable characteristic of RFBs (including SSFBs) is the fact that power and energy are decoupled. This is not only important for safety, but also for reliability: a vast amount of energy to be stored does not imply the need for huge amount of cells (high statistical opportunity of failure). Since the active materials are stored in tanks, more energy can be stored just by increasing the amount of active materials. The performance of conventional RFBs does not allow grid-scale distribution and storage because the individual cells do not deliver enough power and the energy density is too low. Both aspects would lead to a gigantic storage size. SSFB technology, the system in which INFLUENCE studies focused, combines RFB technology with Li (Na)-ion active materials. In consequence, with SSFB technology power and energy are decoupled while power performance derives from Li-ion active materials and is thus notably improved in comparison with conventional RFBs.
RFBs and SSFBs have an analogous way of operation and are suitable for similar applications. The main difference is that in SSFBs the active materials are largely the same as used in Li-ion and Na-ion batteries, offering higher energy density than regular RFBs, and therefore occupying smaller volumes to store the same amount of energy. Consequently, SSFB seems very suitable for applications urbanized areas. SSFB can be deployed for integration of variable distributed generation, commercial and industrial energy/power management for which power requirements are in the range of 100-1000 kW and duration time of 2-10 hours. It must be clear however that the strength of SSFBs is on high-energy applications instead of high-power. Still, the power is much higher compared to conventional RFBs.
4.1.1 Contribution to the international standardization framework
Standards are under development for large Li-ion battery systems and for the use of energy storage in a broader sense in electricity grids. Safety and reliability are important aspects that have to be covered by standards. Fundamental understanding of battery behaviour is mandatory for this. VITO is member of IEC SC21A WG5 that prepares a standard for storage based on large Li-ion cells. The knowledge developed in the InFluENCE project will be brought to the standardization committee.
4.1.2 Contribution to rational and sustainable use of resources
An impact on rational use of resources is the application of Na-ion materials since sodium is much more abundantly available than lithium. This said, lithium or sodium only contributes a small fraction of the total material usage in the active materials. Therefore, recycling remains important.
Recycling is crucial for the market penetration of energy storage systems. The EU battery directive (2006/66/EC) implies that the only option for (H)EV or stationary batteries is second life or recycling. Removing the active materials is easier in a flow battery set-up than in a classic battery configuration.
4.1.3 Increased competitiveness of European industry. Impact for industrial partners of the project
Storage is important for the European electricity grid and electrochemically based storage has a clear position in it, as explained in the section on smart grids above. Most of the batteries are made nowadays in Asia (Japan, China, Korea). Fundamental understanding of battery behaviour leads to increasing the competitiveness of the European industry. Two aspects of the project reinforce this:
• Solvionic, an SME active in the field of specialised materials are already valorizing some of the developmnents from the project.
• The work on computational modelling was coordinated by 6TMIC, a SME, which gained a leading position in the field.
• The knowledge gained in safety aspects can be used in standardisation activities.
• The semi-solid flow battery is both a better concept for storing renewable energy in the European electricity grids than Li-ion batteries or redox flow batteries alone.
In summary, the outcomes of this project bring a contribution to development of new energy conversion technologies and their commercial deployment, paving the way towards a low-carbon economy.
4.2 Dissemination activities and exploitation of the results
4.2.1 Dissemination
Non IP-sensitive project results were disseminated through various channels, the most frequent being SCI publications and communications in scientific conferences. A total of 13 SCI publications related to the project have been realized; additionally three publications dealing the latest developments on the project are under review/preparation and will be published in the coming months. Out of these 13 publications, 10 are open access. Project results were also communicated to scientific conferences (28 communications). A detailed list of SCI publications and all other dissemination activities are given in Template A1 and Template A2, respectively.
A project website was designed and launched beginning of December 2013. While the login area of the website was intended for internal project purposes, the public part of the website is intended as dissemination tool for the wider public. Non confidential documents are made accessible to the general public under the menu “downloads”, e.g. a general presentation about the project or links to SCI papers published open access. Organisation of workshops
KIT organized in November 2014 a topical workshop on active materials in conventional lithium-ion and sodium-ion batteries in the Helmholtz Institute in Ulm. This workshop makes part of the dissemination activities planned in the frame of the EU-FP7 project Influence to favour dissemination of knowledge within and outside project.. Two out of the six presentations will be delivered by speakers external to the project, from internationally renowned research groups in the field (namely Prof. Roberto Marassi and Prof Vito di Noto). The focus of this first workshop was on the interfaces of active materials in conventional metal-ion batteries (e.g.: Li, Na, Mg). Some recent results originating from the project were presented, specifically concerning Na based active materials. One presentation on conventional redox flow batteries was included as well, as their state of the art is most relevant for the developments in the frame of the Influence project.
A 2 day course on electrochemical engineering and modelling (with special regards to batteries) was organized by VITO. The course took place on 27 and 28 August 2015 in the VITO satellite offices in Antwerp (Belgium). Four out of the six lectures were delivered by speakers external to the project. The external speakers were Prof. Digby Macdonald (UC Berkeley, USA), Prof . Mirna Urquidi (Penn State University, USA), Prof. Theo Tzedakis (Univ Toulouse III, France) and Prof. Philippe Barboux (Chimie Paristech, France). The lectures by each of the external speakers consisted of 3 blocks of 45 min (2h30 minutes lecture per speaker with short breaks included). The lectures by the 2 speakers from the Influence consortium, Dr. Xochitl Domínguez (VITO, Belgium) and Dr. Remy Lacroix (6TMIC Ingenieries, France), consisted of 2 blocks of 45 min (1h30 minutes total per speaker). The topics covered in the lectures offered interesting insights for junior R&D professionals working in the field of battery research. In order to facilitate the interaction between the speakers and the attendees, the size of the group was kept small (. No participation fee was requested (participants must however to arrange their travel and accommodation expenses). Further details can be found in D6.32.
IREC organized a topical workshop on rheology and system design of flow batteries, which makes part of the dissemination activities planned in the frame of the EU-FP7 project Influence. The duration of the workshop was half a day. Seven lectures of ca. 25 minutes each were delivered by speakers specialised in areas relevant to the topic. The workshop counted 47 attendees, out of which 34 external to the Influence Consortium. The focus of this last workshop was on flow batteries and analogous devices. Some recent results originating from the project were presented, specifically concerning rheological investigations on fluid electrodes. One presentation on battery standarisation was included as well, as cross-cutting issue in battery research. The external speakers were Dr. Jens Burfeind (Fraunhofer UMSICHT, Germany), Dr. Carlos Ponce de Leon (University of Southampton, United Kingdom), Dr. Belabbes Merzougui (Qatar Energy & Environment Research Institute, Qatar) and Juhan Lee (INM - Leibniz Institute for New Materials, Germany). Further details can be found in D6.33. Target Groups
The INFLUENCE project aimed to engage a number of stakeholders at different dissemination levels. The target audience could be classified in groups as follows:
• Scientific Community was widely addressed via the usual means: SCI publications, communications in scientific conferences, workshops organised in the frame of the project.
• Industry: manufacturers of components and whole batteries; enterprises active in innovative energy technologies, as they also attend conferences dedicated to battery technology.
• Wider Public¬ has access to information about the project and its progress via the public website

For detailed information about dissemination activities see document in attachment "Influence 608621 PUDF (dissemination part) v2.0".

4.3 Exploitation
An FTO analysis was made at the beginning of the project regarding the existing pending patent applications on SSFB technology, in the name of Massachusetts Institute of technology (MIT) and/or 24M Technologies Inc, or others. The existing patents did not seem critical for the areas of commercial interest of the industrial partners.

Some of the project foreground is being exploited, directly or indirectly, by SME partners (Solvionic and 6TMIC). Since the start of the project, one of the industrial partners - Solvionic- devoted their activities in the project to the development of ionic liquids for application in batteries. The work being carried out in INFLUENCE gave Solvionic the opportunity to select, improve and understand their materials for Li-ion and Na-ion (semisolid flow) batteries.Solvionic is a young player for electrolyte systems, with a material that is nowadays not used in commercial batteries: ionic liquids.Their formulations for the slurries are key to the stability of the suspensions. For them it may open a new market, since commercial Li-ion batteries use nowadays no ionic liquids. The work program of INFLUENCE contributed to the development of ILs based electrolytes for application in electrochemical energy storage (EES) systems and help Solvionic and the European industry to integrate the supply chain of electrolytes for EES systems including new semi solid flow batteries, but also Li-ion or Na-ion battery. 6TMIC is an engineering and technology transfer company active in electrochemical engineering and numerical modelling. They have a strong background in optimizing the design of electrochemical systems to improve their performance. The exploitation goal of 6TMIC is to offer engineering services The main path for exploitation of project foreground by RTO/university partners is the execution of new research projects, where the knowledge and experience gained in INFLUENCE can be applied and advanced further. Multiphysics modelling of battery processes and systems is the main ground of exploitation for VITO. In the case of KIT, the focus of exploitation is on optimised materials and formulations for Na-ion batteries. Universiteit Twente focus their exploitation actions on novel method for electrochemical charging of slurries inside a rheo-impedance setup. For IREC the main ground for exploitation is on formulation and operation of SSFBs. Also, in some cases the knowledge is implemented in university courses or lectures in master programmes.

An overview of exploitable project foreground is given in template B2 (confidential information).

List of Websites:
Project website:

Project Coordinator:
Boeretang 200
2400 MOL
Contact: Dr. Yolanda Alvarez-Gallego
Tel. +32 (0) 14335612

Project participants
Helmholtz Institut Ulm (HIU)
Helmholtzstraße 11
89081 Ulm
Contact: Prof. Dr. Stefano Passerini
Tel. +49 (731) 50 34101

Dept. Science and Technology
PO Box 217
7500 AE Enschede
The Netherlands
Contact: Dr. Michel H.G. Duits
Tel. + 31 53 4893097

Jardins de les Dones de Negre 1, 2ª pl.
08930 Sant Adrià de Besòs (Barcelona)
Contact: Prof. Dr. Joan Ramón Morante
Tel. + 34 933 562 615

Guntersthal 4
91235 , Hartenstein
Contact: Angela Hullin

Site Bioparc Sanofi
195, route d'Espagne -BP1169
31036 Toulouse cedex 1
Contact: Sebastien Fantini, Ph. D.
Tel. +33(0)5 34 63 35 35

51 rue Ampère - Bat Stratège A
31 670 Labège
Contact: Dr. Serge Da Silva
Tel. +33(0) 5 34 43 63 39

Trinity Lane The Old Schools
CB2 1TN Cambridge
United Kingdom
Contact: Prof. Dr. Daan Frenkel
Tel. +44(1223)336377