How Solid state batteries differ from modern Li-ion batteries?
If we understand how modern Li-ion batteries function then we can easily understand solid state batteries as well. All modern Li-ion batteries need two electrodes e.g. anode and cathode, and an electrolyte. Positive and negative electrodes are needed to host lithium ions while liquid based electrolyte is needed to enable migration of lithium ions back and forth between positive and negative electrodes as the cells are charged and discharged.
Even though modern Li-ion batteries boast superior energy density and power density compared to various well-established and mature battery technologies such as e.g. lead-acid, Ni-Cd, Ni-MH, etc. Li-ion batteries have some inherent safety issues mainly due to the use of highly flammable organic liquid electrolytes. These organic liquid electrolytes also limit the possibility of using high voltage electrode materials to further improve the performance of Li-ion batteries.
How solid state batteries promise step change for batteries?
Solid state batteries solve these problems by replacing organic liquid electrolytes of Li-ion batteries with Li-ion conducting solid electrolyte. This improved battery architecture introduced by solid state batteries can bring host of benefits including: possibility of having high gravimetric and volumetric energy densities, high voltage and improved thermal stability of the system. On the electrodes side, solid state batteries can have conventional intercalation electrode materials, currently used in the conventional Li-ion batteries or can have completely new materials like thin layer of metal lithium serving as an anode.
Why Solid state batteries?
Improved safety (no flammable, toxic or corrosive electrolyte present).
Possibility to have higher gravimetric and volumetric energy densities, i.e. up to 700 Wh/kg and 1400 Wh/l (thanks to high specific capacity of Li metal anode).
Ability to have a wider operating cell voltage window because many solid electrolytes are chemically more inert and more stable over a larger potential window, enabling also the use of high-voltage cathode materials).
Higher power density (related to wider operating cell voltage window as well as to higher thermal conductivity) enabling fast charging.
Broader operational temperature range, also due the ability to operate at elevated temperatures up to >100°C.
Longer shelf life (low self-discharge rate)
Possibility to have more robust, flexible, custom cell and battery designs, including flexible and wearable architectures.
What properties Solid state electrolyte must have?
The most critical and game changing component of solid state batteries is solid state electrolyte. Solid state electrolytes need to satisfy host of requirements to be suitable for demanding applications i.e. mobility and stationary storage. These Requirements include:
To be chemically and electrochemically stable in contact with Li metal and cathode materials in a broad range of operating temperatures and cell voltages.
To exhibit sufficient conductivity for Li+ (at least >10-4 S/cm and better >10-3 S/cm), while being non-conductive electronically <10-12 S/cm (i.e. Li transference number as close as possible to 1).
To have adequate mechanical properties to withstand mechanical stress a.o. at interfaces caused by insertion/de-insertion of Li ions into the electrodes during battery cycling, and also to enable their incorporation into the solid state batteries cells.
To show good charge transfer kinetics and low interfacial resistance between electrodes and ionic conductor phase.
To be ideally insensitive to water vapour and CO2 in the air to facilitate their manufacturing and handling.
Why Li metal anode important for solid state batteries?
Solid state batteries can have higher gravimetric and volumetric energy densities only if they can manage to incorporate high-capacity electrode materials inside the cell. Metallic Lithium is an ideal anode material for rechargeable batteries due to its high theoretical specific capacity (3,860 mAh/g), low density (0.5 g/cm3) and the lowest negative electrochemical potential (-3.04 V vs. standard hydrogen electrode), all of these characteristics can enable solid state batteries to have higher gravimetric and volumetric energy densities, i.e. up to 700 Wh/kg and 1400 Wh/l.
Main challenges of working with Li metal anodes:
Fabrication and handling of thin lithium metal layers (foils and/or Li deposits onto an anode current collector).
Integration of (protected) Li metal anodes into an all solid state batteries with Li metal anodes cell ensuring controlled anode/solid electrolyte interface.
High lithium plating rates (C-rate for charging) without formation of dendrites.
On ground applications of solid state batteries
Bolloré is the only company who has introduced solid state batteries with lithium metal anode to the mobility market and stationary energy storage applications.
Bolloré Li Metal Polymer (LMP) batteries use polymer based solid electrolyte, LiFePO4 (LFP) cathode. The use of LiFePO4 (LFP) cathode operating at a low voltage of 3.6 V explains why LMP battery has a relatively low energy density of 120 Wh/kg at the pack level and 240 Wh/kg at the cell. LMP battery also require higher temperatures i.e. 60-80°C to operate because of the limited ionic conductivity of polymer based solid electrolyte at room temperature. Therefore there is a need to find ways to incorporate high-capacity electrode materials with well-functioning solid state electrolytes to unlock full potential of solid state batteries.
Recycling of solid state batteries
Recycling of solid state batteries with lithium metal anode will also need to be addressed to ensure these batteries are sustainable. Since many of the solid state batteries with lithium metal anode concepts plan to utilise conventional cathodes currently used in contemporary Li-ion batteries, a challenge would be to handle and recycle lithium metal anodes and solid-state electrolytes in a safe and economically-viable way.
Technology readiness level (TRL)
Solid state batteries definitely promise step change for batteries but still not ready for the prime time because they have many engineering and manufacturing challenges that need to overcome before wide commercialisation of these batteries is possible. It is commonly agreed that a noticeable market uptake of the solid state batteries with lithium metal cannot be expected before 2030, especially in electrification of transport and stationary energy.
Solid state batteries companies/startups to watch:
Solid Power (USA), QuantumScape (USA), Ionic Materials (USA), Solid Energy (USA),
Seeo (USA), Sakti3 (USA), ProLogium (Taiwan), Polyplus (USA), Sion Power (USA), Hydro-Québec (CA).
References:
Batteries Technology Development Report, technical report by the Joint Research Centre (JRC), the European Commission’s science and knowledge service.
Lithium ion battery value chain and related opportunities for Europe, Science for Policy report by the Joint Research Centre (JRC), the European Commission’s science and knowledge service.
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