Solid-state batteries use both solid electrodes and electrolytes. They serve to be a potential alternative to conventional lithium-ion batteries, which use liquid or polymer electrolytes.
In a conventional battery design—most commonly lithium-ion—two solid metal electrodes are used with a liquid lithium salt acting as an electrolyte. Ionic particles move from one electrode (the cathode) to the other (the anode) as the battery charges, and in reverse as it discharges. The liquid lithium salt electrolyte is the medium that allows that movement. If you’ve ever seen a battery corrode or get punctured, the “battery acid” that oozes (or sometimes explodes) out is the liquid electrolyte.
In a solid-state battery, both the positive and negative electrodes and the electrolyte between them are solid pieces of metal, alloy, or some other synthetic material. The term “solid-state” might remind you of SSD drives, and that’s not a coincidence. Solid-state storage drives use flash memory, which doesn’t move, as opposed to a standard hard drive, which stores data on a spinning magnetic disc powered by a tiny motor.
Though the idea of solid-state batteries has been around for decades, advances in their development are just beginning, currently spurred on by investment from electronics companies, car makers, and general industrial providers.However, solid-state batteries are an emerging trend for next-generation traction batteries, as they offer high performance and safety at low cost. Additionally, they have low flammability, higher electrochemical stability, higher potential cathodes, and higher energy density as compared to liquid electrolyte batteries.
A solid-state battery is a superiority over others
Solid-state electrolyte enables the integration of better performed materials such as lithium metal and high-voltage cathode materials. However, it has been observed that the early-generation solid-state batteries may contain similar types of active electrode materials, with the liquid electrolyte being replaced by solid-state electrolyte. In this case, solid-state batteries have no obvious advantage over liquid-based lithium-ion batteries in terms of energy density.
No matter what, solid-state batteries still provide values in this case. As both the electrodes and the electrolyte are solid state, the solid electrolyte also behaves as the separator, allowing volume and weight reduction due to the elimination of certain components (e.g. separator and casing). They allow more compact arrangement of cells in the battery pack. For instance, bipolar arrangement enables higher voltage and capacity at cell level. The simplified connection provides extra space in the battery pack for more cells.
In addition, the removal of flammable liquid electrolytes can be an avenue for safer, long-lasting batteries as they are more resistant to changes in temperature and physical damages occurred during usage. Solid-state batteries can handle more charge/discharge cycles before degradation, promising a longer lifetime. Better safety means less safety monitoring electronics in the battery modules/packs.
Therefore, even the initial generations of solid-state batteries may have similar, or even less energy density than conventional lithium-ion batteries, the energy available in the battery pack can be comparable or even higher than the latter.
With the larger electrochemical window that the solid electrolytes can provide, high voltage cathode materials can be used. In addition, high-energy-density lithium metal anode can further push the energy density beyond 1,000 WH/L. These features can further make solid-state battery a game changer.
Uptake of Electric Vehicles in the nearest future
Electric vehicles are expected to account for 10–12% of the total automotive sales by 2030, according to industry expectations. Multiple factors that are driving the uptake of EVs include favorable regulations in Europe, China, and India; product developments made by major OEMs such as Honda and Volkswagen; and technological advancements in batteries. According to Goldman Sachs, electric vehicles will reach an inflation point by 2025 that might lead to the adoption of electric vehicles.
Technological advancements in batteries are one of the major driving factors for the adoption of electric vehicles. Currently, the battery cost is around US $220/kWh, which is expected to reduce over the next decade. Conventional liquid lithium-ion based batteries account for the majority of the batteries used in electric vehicles. In order to reduce the cost of the battery in electric vehicles, either the technology should improve to have a high energy density, or lithium prices should decline over the period. However, according to industry experts, prices of lithium-ion batteries are expected to remain high, which is the main hurdle in lowering the battery cost. To overcome these hurdles, new technologies such as metal-air, solid-state batteries are expected to gain significant share in the next few years. The solid-state battery is the most promising technology available; it is on the verge of mass adoption in electric vehicles.
Energy fulfilment of EVs has become the holy grail of solid-state batteries
Various restraining factors affecting the uptake of electric vehicles can be overcome by the use of solid-state batteries, as they fulfill the energy and technical requirements of electric vehicles.
• Energy density can be increased per kg as solid-state batteries are 80–90% thinner, and the decomposition voltage is high as compared to lithium batteries. Enhanced energy density would lead to high power output, and a vehicle’s driving range would increase significantly, thereby solving frequent charging requirement as well as the need for a large number of charging stations.
• Safety issues are critically resolved while using solid-state batteries. Liquid electrolytes are generally flammable, and any leakage of these would lead to safety concerns of batteries and overall vehicles. There are safeguards used in liquid batteries; however, solid-state batteries eradicate the need for them and provide overall safety. As solid-state batteries use flame retardant electrolyte, there is very less risk of fire and flammability. In addition, the operating range of solid-state batteries is higher as compared to lithium-ion batteries.
• Fast charging: Solid-state batteries do not contain a liquid electrolyte, which gets heated due to fast charging, and thus, solid-state batteries provide high safety as compared to liquid lithium-ion batteries. The fast charging feature of solid-state batteries is one of the most significant factors leading to the higher uptake of electric vehicles powered by solid-state batteries in the near future.
• Low cost: Conventional liquid lithium-ion batteries are costly, and the current cost is approximately US $220/kWh. This cost is expected to reduce over a period of time; however, it is dependent on scarce materials such as cobalt. Research & development activities in solid-state batteries will contribute to the development of advanced batteries at an affordable cost. Lowering the cost of batteries in electric vehicles would make them an attractive option against gasoline-based vehicles.
However, solid- state batteries offer many advantages: They cannot leak, are generally less toxic, and in case of failure, the components generally cannot burn or explode. Most importantly, solid electrolytes allow the use of electrode materials with voltages in excess of 5 volts because of their wider electrochemical window as well as the use of lithium metal as anode because of their chemical compatibility with lithium. All of these advantages mean an increase in energy density while maintaining the inherent safety of the battery.
But it’s still a long way to go
It’s because investment in various solid-state battery companies reflected the huge potential of solid-state batteries. However, solid-state battery is not based on only a single technology. Instead, there are multiple technology approaches available in the industry. Solid-state electrolytes can be roughly segmented into three categories: organic types, inorganic types, and composite.
Within the inorganic category, LISICON-like, argyrodites, garnet, NASICON-like, Perovskite, LiPON, Li-Hydride and Li-Halide are considered as 8 popular types. LISICON-like and argyrodites belong to sulfide system, while garnet, NASICON-like, Perovskite and LiPON are based on oxide system.
The race between polymer, oxide and sulphide systems is unclear so far and it is common to see battery companies trying multiple approaches. Polymer systems are easy to process and they are closest to commercialization, while the relatively high operating temperature, low anti-oxide potential and worse stability indicate challenges. Sulfide electrolytes have advantages of high ionic conductivity, low processing temperature, wide electrochemical stability window, etc.
Many features make them appealing, being considered by many as the ultimate option. However, the difficulty of manufacturing and the toxic by-product that can be generated in the process make the commercialization relatively slow.
Conclusion
Conventional lithium-ion batteries are inching towards saturation level in terms of technology advancements. There is a need to develop an alternative solution addressing all restraining factors for the uptake of electric vehicles. Solid-state batteries offer multiple advantages, such as high energy density and safety over conventional liquid lithium-ion batteries. Technological advancements in solid-state batteries are expected to provide improved products in terms of the overall cost of production and performance.
Major OEMs such as Toyota, BMW, Honda, and Hyundai are investing in technology development by collaborating with R&D institutes, battery material manufacturing companies, and battery manufacturers. However, the fully commercialized solid-state battery-based electric vehicles are expected to be launched by 2025.