Solid State Lithium-Ion batteries to Eliminate Range Anxiety

The widespread introduction of affordable, energy-efficient electrified vehicles (including electric and hybrid cars) is essential for meeting ambitious government CO2 emissions targets. And, ultimately, it will create a pathway to a transportation system not based on carbon fuels.

The maximum driving range of an electric vehicle is determined by the amount of energy that’s contained in the individual lithium-ion battery cells of the car’s battery pack. These cells are connected in series and parallel arrangements to provide the high currents and high voltage needed to power the electric engine. 

While the energy density of the Li-ion battery cell has more than tripled since its market introduction by Sony in 1991, today’s best Li-ions containing a liquid electrolyte have an energy density of little over 700 watt hours per liter (Wh/l), resulting in a maximum driving range of about 500 km. 

Introduction of new active cathode materials and the gradual addition of silicon to the graphite anode have provided some energy density increases, but researchers say that the energy density of Li-ion cells is likely to reach a practical limit of around 800 Wh/l. To achieve a driving range of 800 km, cells are required with an energy density of about 1000 Wh/l (or 500 Wh/kg)

Solid-state batteries

Simply put, solid-state batteries use a solid electrolyte as opposed to the liquid or polymer gel one found in current lithium-ion batteries, and it can take the form of ceramics, glass, sulphites or solid polymers. Solid electrolyte aside, solid-state batteries function much like those in lithium-ion batteries, in that they contain electrodes (cathodes and anodes) separated by an electrolyte that allows charged ions to pass through it. 

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.

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. Oxide system are stable and safe, while the higher interface resistance and high processing temperature show some difficulties in general.

How do solid state batteries work?

Much the same way as a normal battery. The flow of ions trigger a chemical reaction between the battery’s materials called ‘Redox’ where, when discharging power, oxidation occurs at the anode to create compounds with free electrons, which deliver electric energy, and reduction at the cathode which sees compounds gain electrons and thus store power. When a battery is charged the process is reversed.

Much like lithium-ion batteries, when delivering power in solid-state batteries, aka discharging, positively charged ions travel through the electrolyte from the negative electrode (anode) to the positive one (cathode). This leads to a build up of positive charge in the cathode which attracts electrons from the anode. But as the electrons can’t travel through the electrolyte they have to travel across a circuit and thus deliver power to whatever it’s connected to, say an electric motor. 

During charging, the opposite happens with ions flowing to the anode building up a charge that sees electrons pulled to it across a circuit from the cathode. When no more ions will flow to the negative electrode, the battery is considered fully charged.  

Solid-state batteries have been around for a while, but are only used for small electronic devices like RFID tags and pacemakers and in their current state as non-rechargeable. As such, work is being done to allow them to power larger devices and be recharged. 

Advantages of Solid State Batteries

The best way to understand why solid state batteries are so exciting is to look at the problems caused by liquid electrolyte in lithium ion batteries on the market today. Much of the bulk found in lithium ion batteries is due to separator systems and safety precautions required to deal with the catastrophic failure modes of lithium batteries. Let's take a look at some of the more pressing problems scientists hope this technology will be able to solve.

No Electrolyte Leakage

The most obvious advantage of solid state batteries is the avoidance of electrolyte leakage. If you've ever had to deal with the messy aftermath of some old AA batteries left behind in an old toy, you're already somewhat familiar with the problem. In order to function, the battery needs a medium through which ions can be transferred during discharge and charge. If a cell dries out, due to exposure, rupture the battery will no longer be able to function. In higher rate applications electrolyte leakage can be devastating, creating a fire hazard, providing paths for electrical shorts and other problems. Using a solid electrolyte inherently avoids this failure mode. Solid state batteries can help manufacturers by removing the need for advanced sealants, pressurizing electrolyte and including flame retardant failsafes.

No Thermal Runaway

In batteries, a thermal runaway reaction is a series of cascading exothermic reactions that are accelerated by an increase in temperature that occurs when a cell rapidly discharges its stored energy. The consequences of this reaction are rising internal cell temperature, rising pressure, venting of flammable gases in the liquid electrolyte and the risk of explosion and shrapnel. The liquid electrolyte in lithium ion cells is highly flammable, and leakage due to rupture can lead to disastrous consequences especially in scenarios like an automobile crash. Replacing the flammable liquid with a solid electrolyte can prevent thermal runaway from occurring.

No Dendrite Formation

Cycle life or the total number of charge/discharge cycles a battery can perform is the main metric used by the industry to judge the operating life of a battery. A key limiter on conventional liquid electrolyte batteries is the tendency for metal deposits to form within the battery during charge. These deposits can form dendrites which penetrate through separator material and potentially cause a short. On a fundamental level, liquid electrolytes are also attacking the electrodes within the battery themselves. The metals will slowly dissociate into the surrounding liquid medium over time, with the ebb and flow of electrolyte during cycling. The more cycles a cell experiences, the more deposits will inevitably form within the cell leading to a short. A solid electrolyte avoids this problem entirely allowing the cells to survive hundreds of thousands of cycles.

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 players

• VW has been collaborating with QuantumScape since 2012, and claims to be the company's largest automotive investor, with a previous investment of $100 million. In 2018, the two companies formed a joint venture to spur production of solid-state batteries for VW, beginning with a pilot factory. That will be a step toward VW's plan for automotive use of solid-state batteries in some way around 2025. The automaker wants its batteries to last the life of the car, so any development in the solid-state realm will have to measure up to that.
• Researchers at the Samsung Advanced Institute of Technology (SAIT) and the Samsung R&D Institute Japan (SRJ) decided to remove the lithium metal anodes used in solid-state batteries and replace them with a thin silver-carbon layer. It’s those lithium-metal anodes that cause issues with the batteries. They grow dendrites (tiny crystal spikes) that bore through the electrolyte and cause a short circuit during charging. Hence the low life expectancy of a solid-state battery.
• BMW has invested US$ 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 2021, 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 end; 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.
• IBM and Daimler are working together to better understand battery technology. 'We need to find a fundamentally different chemistry to create the batteries of the future,' Katie Pizzolato, director of applications research at IBM, says. 'Quantum computing could let us effectively peer inside the batteries chemical reactions, to better understand the materials and reactions that will give the world those better batteries.'
• Vacuum and other air-blowing tech maker Dyson had planned to make an electric car powered by solid-state batteries by 2021. But it killed off its car plans last Autumn, though it aims to keep working on the battery tech.