Solid-State Batteries

A Possible Replacement for Conventional Liquid Lithium-ion Batteries in Electric Vehicles

Introduction

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.

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.

History

Solid electrolytes

Currently, there are more than 25 types of solid-state electrolytes, such as oxides, sulfides, phosphates, polyethers, polyesters, nitrile-based, polysiloxane, polyurethane, etc.

Major difference between inorganic/ceramic and polymer solid electrolytes is the mechanical property. High elastic moduli of ceramics make them more suitable for rigid battery systems, while low elastic moduli of polymers make them more suitable for flexible devices. Polymers are easier to process than ceramics, which reduces fabrication costs. Ceramics are more suitable for harsh environmental conditions, such as high temperatures.

Performance of the battery depends on the electrolyte. Polyether and Lithium Phosphorus Oxy-nitride (LiPON)-based electrolytes are commonly used in solid-state batteries.

Selection criteria for solid polymer electrolytes

Application of solid-state batteries

Solid-state batteries find application across multiple industries, such as automotive, energy storage, consumer electronics, industrial, and aerospace.

The adoption of solid-state batteries depends on the value addition, as are increasingly used for various applications as compared to conventional liquid lithium-ion batteries. Safety is the main driving factor for the development of solid-state batteries across various industries.

Advantages and challenges in solid-state batteries

Solid-state batteries are available in small sizes as compared to liquid lithium-ion batteries. They do not produce hydrogen gas due to the absence of flammable materials, thereby strengthening the operational safety.

Solid-electrolyte Interfacial Layer (SEI) is not formed in case of solid-state batteries, which results in very low self-discharge rates allowing multi-year power storage with minimal loss. These batteries are expected to operate 50–100 times longer than conventional liquid lithium-ion batteries.

Currently available solid-state batteries face a challenge in terms of operating life, which is approximately three years. Research & development activities help extend the operating life of solid-state batteries for more than three years so that they can be fully commercialized for electric vehicles.

Uptake of Electric Vehicles

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.

Factors affecting the uptake of electric vehicles

The adoption of electric vehicles has various restraining factors, which include:

• Long time in charging: Current lithium batteries need longer charging times to get fully charged. This is the major unmet need that is inhibiting the full adoption of electric vehicles.
• Few EV charging stations and short driving range: As the energy density of the current batteries, including lithium-ion batteries is less, it cannot offer large driving range. This results in the need for frequent charging and more EV charging stations.
• Expensive (batteries have the major cost contribution): The price of electric vehicles is higher as compared to the internal combustion engine-based vehicles. According to the cost structure of production, the contribution of the batteries is 42–48%, which leads to the high cost of electric vehicles, thereby impacting their uptake.
• Safety concerns: Batteries are the major component in electric vehicles. Though passive and active safeguards are designed in battery structure, there is a significant risk in terms of safety, including thermal stability of active materials at high temperature and occurrence of internal short-circuits that may cause thermal runaway.

How would solid-state batteries satisfy the energy and technical requirements in EVs?

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.

Investment on solid-state batteries by major automotive OEMs and future scenario

Major material companies, OEMs, and research institutes are increasingly investing in R&D of solid-state batteries. OEMs are collaborating closely with various stakeholders in the battery manufacturing space to ensure that solid-state batteries are commercialized soon. More than 120 players are involved in the development of the technology at material, battery pack, and vehicle levels.

Investments made by OEMs in solid-state battery development:

• Volkswagen invested US$ 100 million in QuantumScape, a start-up working on the development of solid-state batteries. Volkswagen has set a target production of 1 million electric vehicles by 2025, including solid-state battery cell powered vehicles.
• BMW has invested US$ 20 million in Solid Power to scale up the production of solid-state batteries. BMW is expecting the production capacity to be operational by the end of 2019, and the launch of electric vehicles is expected to be by 2025 with 12 different models.
• Toyota is expected to launch solid-state battery-based electric vehicles by 2020; however, this is only for pilot projects, and the company is expected to have fully commercialized electric vehicles by 2030. Toyota and Panasonic formed a joint venture to produce next-generation solid-state batteries for electric vehicles.
• Hyundai invested in a US-based start-up, Ionic Materials, which is involved in solid-state electrolyte material development. The Korea-based OEM is expected to have solid-state battery-based electric vehicles on the road by 2025.

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.