Solid-state batteries are considered the next big step in battery development. The first concepts of solid electrolyte cells date back to the seventies and yet there is still no mass production of the cells today. This article gives an overview of the current state of solid-state batteries.
A solid-state battery is basically a concept in which the electrolyte of a cell consists of a material in a solid state of aggregation. This distinguishes the solid-state battery from today’s Li-ion batteries, as these usually have a liquid or gel-like electrolyte. The other structure of a solid-state battery is otherwise not necessarily different from a lithium-ion battery:
The solid-state battery consists of a cathode, an anode, two current collectors and the solid-state electrolyte. Figure 1 shows the general structure of a solid-state battery.
Figure 1 : General structure of a solid-state battery, own representation
In principle, the same material can be used for the cathode as for Li-ions (e.g. NMC nickel-manganese-cobalt oxide). For the electrolyte, there are various variants, which essentially differ in oxides, sulfides and polymers (more information about the electrolytes can be found here).
Li metal is often used as anode material. This material is difficult to use for Li-ion batteries because it forms chemical bonds with the liquid electrolyte. Li-metal has a much higher energy density than today’s anodes, which is why it is expected that the energy density will increase considerably. More information about the general structure and the individual components can be found here.
Charging and discharging process of a solid-state battery
Figure 2: Charging and discharging process of a solid-state battery (a). Charging (b) Discharging, Own illustration
The charging process of a solid-state battery essentially works like that of a lithium-ion battery. Figure 2 shows how the charging and discharging process takes place in a battery. To charge a battery, a voltage is applied to the cell. This potential causes electrons to travel across the conductor and voltage source to the anode. The Li+ ions migrate through the solid electrolyte to the anode side where they deposit as metallic lithium. The deposition causes the volume of the anode to grow. This volume growth is still one of the challenges to be solved for the commercialization of solid-state batteries because it leads to large mechanical stresses within the cell.
During discharge, the process is reversed. Electrons migrate back to the cathode via a connected external load and the ions also migrate back via the electrolyte. The volume of the anode thus shrinks back to its original size.
Advantages and challenges of solid-state batteries
The solid-state battery is said to have great potential to be superior to today’s Li-ion battery. It is becoming apparent that the energy density of solid-state batteries will increase significantly. This is due in particular to the switch to Li metal as the anode material. In practice, energy densities are expected to be twice as high as those of today’s Li-ion batteries. Whether an increase in performance can also be achieved is not yet clear today. More information on the energy and power density of solid-state batteries can be found here.
Another frequently cited advantage of solid-state batteries is their intrinsic safety. Since no (flammable) liquid electrolytes are incorporated, the battery is supposed to be safe. However, this is only true to a limited extent. Although an electrolyte fire with solid electrolyte is almost impossible, there are other dangers, such as the formation of dendrites with resulting short circuits. A detailed analysis of the possible damage reactions of the solid-state battery can be found here.
In the long term, solid-state batteries should also be cheaper to manufacture. Due to the higher energy density, fewer cells are needed for the same capacity, and partial improvements in safety could possibly reduce cost-intensive safety measures. More information on costs can be found here.
Commercialization approaches
It is well known that practically all major car manufacturers are working on the commercialization of solid-state batteries. The companies usually work together with various start-ups that were founded in the field of solid-state battery development [1]. The concepts presented differ considerably and range from systems with Li anodes [2] to graphite and silicon anodes [3]. In terms of electrolytes, oxide, sulfide, and polymer variants can be found, with polymer variants being the most common due to ease of fabrication [4].High-energy cathodes such as NMC are preferred as cathode material, but there are also concepts that work with LFP cells [5].
It will still take several years before solid-state batteries are ready for (large-scale) production. There are also indications that hybrid systems, which have a solid electrolyte and an additional small amount of liquid electrolyte, could reach series production readiness as early as the next few years [6]. Classical solid-state batteries are not expected before 2025. Meanwhile, broad market entry is not expected until the second half of the twenties.
Sources
[1] IDTEchEx, Xiaoxi He: Solid State Batteries for EV Applications, June 2022, Conference Battery Show Stuttgart
[2] Quantumscape: Delivering on the promise of solid-state technology, 2023, https://www.quantumscape.com/technology
[3] SolidPower: All-Solid-State Battery Cell Technology, 2023, All-Solid-State Batteries – Solid Power (solidpowerbattery.com).
[4] Mercedes Benz: eCitaro, Battery Technology, 2023, https://www.mercedes-benz-bus.com/de_DE/models/ecitaro/technology/battery-technology.html
[5] Blue-Solutions: Battery Technology, 2023, Blue Solutions – Battery technology (blue-solutions.com)
[6] Wood Mackenzie: Will semi-solid battery technology render solid-state batteries redundant?, https://www.woodmac.com/news/opinion/will-semi-solid-battery-technology-render-solid-state-batteries-redundant/
