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FP7

INFLUENCE Report Summary

Project ID: 608621
Funded under: FP7-ENERGY
Country: Belgium

Periodic Report Summary 1 - INFLUENCE (Interfaces of Fluid Electrodes: New Conceptual Explorations)

Project Context and Objectives:
The FP7 project InFluENCE aims to improve the fundamental understanding and control of interfaces of a battery type based on Li-ion and Na-ion active materials: semi solid flow batteries (SSFB). In this novel type of battery, proposed by MIT only few years ago, the active materials are pumped through the cell in a slurry. The fact that the case study is a SSFB set-up instead of classic lithium ion batteries containing porous electrodes is an asset: the methods and techniques developed are generic and could as well be implemented for conventional Li- and Na-ion systems for other aspects not related to flow.
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. Therefore, the distinct features of SSFBs are:
• Flow cell architecture.
• Flowable semi-solid ‘fuels’ defined by an electrochemical composites or slurry that contains: electroactive particles or true electrodes, ion carriers, electrolyte carrier and conductive additives defined for anodic and cathodic compartments.
• Characteristic viscosities that are generated in the slurry which enable to keep the active materials dispersed.
• Current collector and packaging.
A main goal of Influence is the investigation and optimization of the interfaces developing between the electrolyte and the electrochemically active material particles in fluid electrodes. The acquired knowledge would allow 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 warrant stable interfaces.
A second goal is the understanding and control the mechanical and conductive behaviours of the slurries. For this, it is necessary to determine the role of shape anisotropy and the overall 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.
The experimental work is accompanied by thorough modelling to understand the physical phenomena occurring at the microscopic scale, to derive scaling rules towards macro-scale and to enable design recommendations leading to optimal interface behaviour (size of anodic and cathodic compartments, geometry of collectors, etc.).
The general objectives of the project and in which part of the work programme are they addressed are summarised below:
• Objective 1: to investigate and optimize of interfaces developing between the electrolyte and the electrochemically active material particles in fluid electrodes. The acquired knowledge would allow 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 warrant stable interfaces. This objective will be pursued intensively in WP3; some activities in the frame of WP2 and WP4 will also have a remarkable contribution to the achievement of this objective.
• Objective 2: to improve understanding and control the mechanical and conductive behaviour of the slurries. For this, it is necessary to determine the role of shape anisotropy and the overall 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. WP4 is the main contributor here.
• Objective 3: to complement experimental data by means of implementing multi-scale and multi-physics computational modelling tools, in order to further improve the understanding of physical and electrochemical phenomena occurring at the interfaces at the microscopic scale, and derive scaling rules towards macroscopic scale, as well as to enable design recommendations leading to optimal interface behaviour (size of anodic and cathodic compartments, geometry of collectors, etc.). This is addressed in WP5.
• Objective 4: to establish and maintain active exchange of information with top class research groups from third countries, leading to synergistic collaboration between European and third countries research projects in the field of interface characterisation and optimisation. WP6 will facilitate the means to initiate such a network.

Project Results:
In WP2, the required specifications for integrating SSFB storage system into electrical grid and the dimensions of the system based on those specifications were defined. A modular electrochemical cell design was proposed by IREC for the investigation of slurry components (e.g. active material, electrolyte, carbon content, etc), as well as for the study of interfaces in WP3 and WP4. The prototypes of half-cell and full-cell were fabricated and validated using semi-solid fluid electrodes. For the full cell, reversible capacity values of ca. 100 mAh g-1 were obtained with main charge and discharge potentials of ca. 2.8 V and 1.7 V, respectively.
Several candidate materials were evaluated at IREC as active electrode particles for SSFBs, some of which were patners’ developments. Electrochemical reversibility, in terms of coulombic efficiency (%CE), was the main criterion for selection. Both ionic liquids (SOLVIONIC) and classic alkyl carbonate solvents were considered as electrolyte system.. ZnO, TiO2, Li4Ti5O12, LiFePO4, LiNMC and LiCoO2 were selected as active materials for Li-ion chemistry, whereas TiO2 and NaNMC were selected for Na-ion chemistry. The proof-of concept of Na-based SSFB using NTP-NaNMC was demonstrated as well (collaboration IREC–KIT). %CE were typically higher that 80%, and conversions beyond 20%.
Activities in WP3 in the period mainly focused on the study of non-commercial materials: NaNCM (cathode material for the sodium based system, developed at KIT), as well as Al flakes and ZnO particles (ECKART). The active materials were optimized and characterized at KIT via the use of various analytical techniques, simultaneously proving their capability for the ex-situ characterization of the active material electrolyte interface, and provided to the project partners upon request. The NaNCM and anatase materials revealed a very good electrochemcial performance, superior to most of related materials in literature. The cycling performance of anatase reveals one of the best high-rate long-term cycling performance reported so far. Electronic conductivity was identified as limiting factor for the performance of these fluid electrodes, revealing that an optimization of electrode composition is crucial to enable improved performances and is the focus of current activities.
WP4 deals with the rheological and conductive behaviour of candidate slurries. Two particle components are needed in the slurries for SSFB: Electro Active Particles (EAPs) for storage and release of electrochemical energy, and Carbon Nano Particles (CNPs) for conducting the electrons to and from the current collectors. In the period M1-18, progress was made along several lines. ECKART developed slurries of Al and ZnO particles. The slurry based on ZnO (plus CNP) in ionic liquid has been selected for further development. U TWENTE studied the colloidal interactions of CB, TiO2 and NaNCM and found that while Carbon Black (CB) forms strong gels, the EAPs show weak or no aggregation, even in presence of 1 M salt. As a result, both rheology and percolation are dominated by CB. Networks of the latter break up in shear flow, causing lower viscosity and higher Ohmic resistance. Addition of EAPs appears to lower the connectivity of the CB network. Visualization experiments on CB suspensions indicated structural heterogeneities on length scales larger than a few microns.
Experimental investigations have been complemented with computational modelling in WP5. At molecular level, diffusion coefficients, density, viscosity were calculated by IMPERIAL for various conventional carbonate solvents, ionic species (Li+, Na+) and particle volume fractions. In order to reproduce experimental kinetic measurements detailed kinetic mechanisms were developed by 6TMIC for four electrode systems: anatase TiO2, ZnO (ECKART), LTO, NaNCM (KIT). In terms of rheology, the behaviour of suspensions of solid particles was studied using the Lattice-Boltzmann model for particle volume fractions of 0; 5; 10; 20 and 25%. At macroscopic level, the modelling of electrochemical cells in non-flowing and flowing conditions was performed by VITO and 6TMIC using modeling software (COMSOL Multiphysics®). Two different SSFB modelling approaches have been implemented. In the first approach, rheology (non-Newtonian laminar flow) was coupled with electrochemistry (secondary current distribution), using kinetic data validated for Anatase TiO2 and NaNCM electrodes. In the second approach masss transfer (laminar flow) was coupled to electrochemistry (concentrated solution theory) and solid-state diffusion within the EAPs was simultaneously solved, based on data from literature.

Potential Impact:
The RTD activities carried out in the frame of the Influence project will result in an advancement of the fundamental knowledge on Li-ion and Na-ion batteries regarding the formation of the (artificial) Solid Electrolyte Interface (SEI) layer. By overcharging and heat release experiments the safety aspects are explored.
In addition to the study of these aspects, of paramount importance for all rechargeable batteries, the investigation of semi-solid electrodes leads to a more thorough understanding of interface aspects that cannot 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. However, in fluid electrodes, this combination of properties must be different since the electronic conductive pathways are not fixed as in conventional, solid-state electrodes, but rather dynamic.
• To ensure conductivity within the electrode slurry, ultrafine graphite and graphene dispersions as well as silicon particles are added in the electrolyte medium. The percolation behaviour is studied under flow conditions. The project leads to insight in the rheology of the individual components and the particle interactions behaviour (aggregation, network formation, segregation).
Further SSFB offer a great potential concerning control of interfaces, the longevity and reliability. A remarkable advantage of SSFBs versus conventional batteries is the possibility of adjusting the chemistry of the system, during its operative life, by simply adding the necessary chemical into the electrode flow. This has the potential of largely improving both the operative life and the reliability of the battery system. It should be noted that rather simple techniques such as conductivity, viscosity and density, which can be easily performed in-line, might supply crucial information to monitor the state of health of the system.
A remarkable characteristic of 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. Due to their similarity, redox flow batteries and semi-solid flow batteries (SSFB) are suitable for similar applications. The main difference is that SSFB offer higher energy density, and therefore occupy smaller volumes to store the same amount of energy, leading to a more compact system. 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.
In a wider perspective the Influence project is envisaged to have a positive impact on the implementation of the industry-led European Industrial Initiatives of the SET-Plan:
• Two of the companies in the consortium are active in the field of specialised materials and are already in the position to valorise the new materials developed in the project.
• Thework on computational modelling is coordinated by a SME, which gains a leading position in the field.
• The safety aspects of the knowledge 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 implementation of this project will boost the development of new energy conversion technologies and their commercial deployment, paving the way towards a low-carbon economy.

List of Websites:

www.fp7-influence.eu

Contact

Yolanda Alvarez Gallego, (Project Manager)
Tel.: +32 14336941
Fax: +32 14321186
E-mail
Record Number: 171827 / Last updated on: 2015-10-13
Information source: SESAM
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