Lithium-ion batteries have significantly increased the capability of electric vehicles, with high energy storage density and efficiency. They can currently achieve specific energy and power densities of 260 Wh/kg and 340 W/kg, respectively, with a life of over 1,000 cycles. In recent years, most developments in lithium-ion batteries have focused on cathode chemistry, with battery technologies named according to their cathode. Dominant technologies include:

• NMC, which uses lithium, nickel, manganese, and cobalt oxide cathode.
• LMO, which uses a lithium manganese oxide cathode.
• LFP, which uses a lithium iron phosphate cathode.

Cathode chemistries are now close to their theoretical limits, and further developments are expected to come from other components of lithium-ion batteries, as well as entirely new battery chemistries. Lithium-ion batteries are now focused on anode materials and moving from a liquid electrolyte to a solid-state battery.

Current lithium-ion batteries use graphite anodes, which have limited energy storage potential. There has been interest in the use of lithium or silicon anodes, which have significant energy storage capability and could increase the cell’s specific energy by 20% to 40%. Challenges include expansion leading to cracking as silicon anodes absorb electrons and poor stability combined with high cost for lithium anodes. Aluminum has also been used as an anode material with limited success. Silicon anodes are emerging as the next step for commercial EV batteries and are expected to enter serial production within the next few years. Graphene and nanowire enhancements in both the anode and the cathode could increase power density by increasing the surface area of the electrodes in contact with the electrolyte, enabling a more rapid transfer of charge.

The electrolyte in the production of lithium-ion batteries is a solution of lithium salt. The solvent used is typically not aqueous but is instead a mixture of organic solvents such as ethylene carbonate and dimethyl carbonate. The solution is not a pool of liquid, like the acid in a lead-acid battery. Instead, the electrolyte solution is a thin film applied to a separator sheet between the anode and cathode metal foils. At high rates of charge and discharge, the solvent can evaporate, leading to the battery off-gassing, shortening its life, while also potentially leading to thermal runaway and ultimately a flammable explosion. Developments in solid-state batteries seek to replace the electrolyte solution with a polymer or ceramic-based electrolyte. This will remove the issues of solvent evaporation, increase cycle life and safety, enable more rapid charging and power density, and enable more energy-dense chemistries. Solid-state batteries are now moving from the lab into industrial development. They are expected to be introduced into high-end applications such as supercars within the next five years and volume models in around ten years.

Regarding solid-state batteries, it is also significant to note that although commercial EV cells achieve 260 Wh/kg, the specific energy at the pack level is only around 150 Wh/kg due to the structure and cooling system of the pack, which adds weight without storing any additional energy. Because a solid-state battery could operate at higher temperatures, the parasitic weight of the cooling system could be reduced. They could, therefore, achieve much higher specific energy at the pack level.

In addition to incremental improvements in lithium-ion batteries, entirely new battery chemistries are also being developed. Emerging technologies include sodium-ion, lithium-sulfur, and aluminum-ion. Lithium-air has the greatest potential energy density, but many fundamental challenges remain in realizing this technology. While the developments discussed above could together take lithium-ion batteries close to their theoretical limit of 400 Wh/kg and achieve charge rates of 6C, alternative chemistries offer even more potential. Sodium-ion could realistically reach 760 Wh/kg, while in theory, lithium-sulfur could achieve 2,500 Wh/kg [1, 2], and lithium-air as much as 13,000 Wh/kg. At this level of performance, battery-electric long-haul aircraft would become feasible. Pre-production aluminum-ion/graphene batteries have already been produced, which offer relatively modest specific energy of 200 Wh/kg, but with capacitor-like extremely high charge rates and lives of 360C and 250,000 cycles. This could allow a full charge in just ten seconds and virtually unlimited battery life. With this kind of performance combined with frequent wireless charging points, ultra-light vehicles with much smaller batteries could become desirable.

It is unlikely that production batteries using these new chemistries will come close to the theoretical performance figures in the foreseeable future. However, lithium-sulfur batteries can already give higher specific energy than lithium-ion, although with relatively short cycle lives. Sodium-ion is currently somewhat expensive and low performance but uses abundant materials and could rival lithium-ion for some applications within the next few years. Over the next few decades, alternative chemistries will likely supplant lithium-ion in many more applications.

References

1. Zhu, K. et al., How Far Away Are Lithium-Sulfur Batteries From Commercialization? Frontiers in Energy Research, 2019. 7(123).
2. Cao, W., J. Zhang, and H. Li, Batteries with high theoretical energy densities. Energy Storage Materials, 2020. 26: p. 46-55.