Lithium-ion batteries are prevalent in the power source market and are enjoying deeper market penetration within some of the mobility sectors, in particular micro-mobility and light to medium-duty personal and commercial vehicles. Lithium-ion batteries have relatively high energy density compared to legacy nickel-metal hydride and alkaline batteries. However, their energy density is considerably lower than fossil or sustainable liquid or gaseous fuels. Furthermore, because the latest generation of lithium-ion batteries utilises flammable organic liquid electrolytes, there are severe safety concerns regarding their future use since thermal runaway events can occur if battery cell or pack integrity is compromised.

All-Solid-State Batteries (ASSBs) use solid electrolytes instead of liquid electrolytes and provide several benefits including improved safety (compared to liquid electrolyte batteries), higher energy density (potentially over twice as much as the latest generation lithium-ion batteries), and longer lifecycles. Furthermore, due to the elimination of the liquid electrolyte, batteries can be compact with small footprints, therefore allowing more efficient packaging. However, whilst there are many benefits, ion conductivity through solid materials is lower compared to liquid electrolytes consequently reducing the power output. The manufacturing of ASSBs at a large scale is also particularly complex and cost-prohibitive at present.

To improve the market penetration of ASSBs, the most suitable combination of materials for the anode, cathode, and solid electrolyte must be established. Lithium and silicon tend to be popular contenders for the anode material in an ASSB due to their high specific energy densities. However, both exhibit shortfalls (lithium dendrite formation and silicon ‘swelling’) that need to be addressed before ASSBs can proliferate. Materials of great interest for the solid electrolyte are sulphide, oxide and polymer-based compounds. The benefits and shortfalls of these are discussed here along with recent innovations in solid electrolytes that will help deepen market penetration.

Comparing different types of solid electrolytes

Sulphide-based solid electrolytes are amongst the most promising materials for high energy density ASSBs and have become steadily available in recent years. They exhibit ion conductivity in the region of 10-2S/cm which is similar to the conductivity of contemporary liquid electrolyte lithium-ion batteries. This is partly due to their soft material properties (and therefore high modulus) and hence strong wettability against anodes manufactured from lithium. However, sulphide electrolytes have several drawbacks including their sensitivity to humid air and instabilities when coupled with certain electrode materials.

Oxide-based solid electrolytes have reached ion conductivity parity with sulphide-based electrolytes in recent years. They offer greater tolerance against air and moisture compared to sulphide-based electrolytes and exhibit high chemical and thermal stability in addition to offering a wide electrochemical operating window. This enables it to be deployed with a lithium-metal anode and a high-voltage cathode. Shortfalls of these inorganic electrolytes include the requirement for thick composite electrodes that involve high sintering temperatures and therefore expensive manufacturing techniques.

Polymer-based solid electrolytes for ASSBs have the advantages of low flammability, good flexibility, outstanding thermal stability, and high safety. They are also relatively easy to manufacture compared to sulphide and oxide-based solid electrolytes. Solid polymer electrolytes, including polyethylene oxide, polycarbonate, and polysiloxane have been extensively investigated. However, the ionic conductivity and mechanical strength of these electrolytes are still not ideal, which is the major obstacle that limits their market penetration and narrows their field of application.

Innovations in Solid-State Battery Electrolytes

In late 2021, Solid Power, USA, declared that third-party safety testing demonstrated that its battery cells, which employ a sulphide-based solid electrolyte, were safer than current liquid electrolyte lithium-ion batteries. Testing involving punctures to the battery pack and overloading to 200% were undertaken to simulate generalised abuse and degradation. After continuous testing, the only degradation established was a minor increase in cell temperature – there were no thermal runaway events or gas ejections reported. Solid Power claim that their sulphide-based ASSBs can deliver a specific energy of 350Wh/kg and can withstand 750 charge/discharge cycles with 80% capacity retention. These specifications therefore make this battery type suitable for use in all transportation sectors as they are typically 30% more energy-dense than the latest lithium-ion batteries. The potential of this battery technology meant that in 2022, Solid Power partnered with Ford and BMW for in-field testing, with BMW targeting large-scale testing of sulphide-based ASSBs by 2025 and a production-ready version by 2030.

In early 2022, a research team in the Department of Chemical and Biomolecular Engineering at the Korea Advanced Institute of Science and Technology, Korea, developed an organic polymer-based solid electrolyte utilising lithium-metal as the anode material. By three-dimensionally connecting crystalline plastics, an elastomeric solid electrolyte exhibiting excellent elasticity and ion-conductivity properties was developed. The electrolyte exhibits ion conductivity similar to that of the most advanced lithium-ion liquid electrolyte batteries with an energy density of more than 410Wh/kg. With a 50% increase in energy density (over lithium-ion batteries), these batteries would result in an electric vehicle range of approximately 750km, up from 500km for an equivalent sized liquid electrolyte lithium-ion battery. They are also likely to find favour in short-haul electric aircrafts and drones.

In early 2020, Samsung Advanced Institute of Technology (SAIT) and Samsung R&D Institute, Japan presented a study on an ASSB with a sulphide-based electrolyte. It chose sulphide for the electrolyte because of its strengths in production scale expansion and charging speed compared to polymer-based or oxide-based electrolytes. Researchers suggested that the prototype pouch had an energy density greater than 900Wh/L and could be cycled up to 1000 times without significant degradation. This technology was stated to provide an electric vehicle range of up to 800km on a single charge. The lithium-metal anodes that are frequently used in ASSBs are prone to dendrite growth that reduce battery lifespan and safety. To mitigate this, a silver-carbon composite layer was utilised as the anode. This led to improved electrochemical cyclability and robustness compared to ASSBs that utilise lithium for the anode.

Murata Manufacturing Co., Ltd headquartered in Japan, has developed a non-combustible, heat-resistant solid-state battery that employs a ceramic oxide electrolyte instead of the electrolytic solution used in conventional lithium-ion batteries. The high degree of safety and durability of the new battery is ideal for Internet of Things (IoT) technologies and can be scaled to battery sizes suitable for the micro-mobility transportation sector due to its high volumetric energy density of approximately 400Wh/L. Murata’s engineers worked on three elements to obtain excellent characteristics: 1) solid electrolyte material with high ion conductivity, 2) technologies for producing accurate, thin electrolyte layers, and 3) techniques for increasing the adhesion of electrode active materials and electrolytes.

A study at Stanford University, USA in late 2020 used machine learning to identify materials suitable for solid electrolytes that form part of an ASSB. After screening more than 12,000 lithium-containing compounds, the algorithm identified 20 promising materials including four little-known compounds, all of which contained lithium, boron, and sulphur. Whilst purely theoretical at present, it is predicted that these lithium-boron-sulphur solid electrolytes could be more stable than current leading solid electrolytes and could be designed for use in the light to medium-duty mobility markets. Furthermore, since lithium, boron, and sulphur are abundant, they are cheap materials, making solid-state battery production potentially cost-effective, provided a suitable manufacturing technique could be devised.

What Next?

The global ASSB market is predicted to increase to US$314 million by 2028 from US$58 million in 2022. The rising need for ASSBs to fulfil electromobility needs, the growing trend towards miniaturisation of consumer devices, and increased research and development activities by major organisations are key drivers of this emerging market.

The true benefits and shortfalls of polymer, oxide, and sulphide-based solid electrolytes are uncertain with battery technology companies deploying many experiments to consider all eventualities. Polymer systems are simple to produce and are the most commercially viable, but the relatively high operating temperature, low anti-oxide potential, and poor stability provide difficulties. High ionic conductivity, low processing temperature, and a wide electrochemical stability window are all advantages of sulphide electrolytes. Many aspects make them desirable, and many supporters consider them to be the best alternative. However, due to the difficulties of production and the possibility of harmful by-products, commercialisation is taking a long time. While oxide systems are stable and safe, the greater interface resistance and high processing temperatures present some challenges.