Lithium-ion batteries are everywhere: You see them in consumer electronics, electric vehicles, robots, and power-grid storage. Largely driven by falling prices and widespread electrification, global Li-ion demand has grown from a modest 19 GWh in 2010 to 285 GWh in 2019, with Bloomberg forecasting 8,000 GWh by the end of the decade.
Although falling prices have a central role to play in the increase in demand, advances in technologies that support cost reduction and improve battery performance are equally important. Whilst prices (at a pack level) dropped by 89 percent from above $1,100 per kWh in 2010 to $137 per kWh in 2020, improvements in technology have seen the energy density almost triple over the same period.
What is more, a move to silicon-rich anode materials has also provided a stepwise improvement in energy density, as witnessed by StoreDot’s Extreme Fast Charging (XFC) battery’s gravimetric energy density of 300 Wh/kg.
However, with commercial lithium-ion batteries based on graphite anodes hitting a plateau that limits performance, lithium metal is seen as a possible next step. The lightest metal in the periodic table and with a theoretical capacity of 3862 mAh/g, Lithium Metal Batteries (LMBs) could deliver an energy density upwards of 500 Wh/kg and 1000 Wh/l at a cell level.
Despite their advantage over lithium-ion batteries, LMBs have not been considered a viable option to power electric vehicles because of their limited cycle life and potential safety hazards, largely as a result of uneven lithium deposition that can lead to dendrite growth, resulting in internal shorts of the battery.
However, when studying the behavior of LMBs during charging researchers and technologists have noted that subjecting the LMB cells to pressure during battery cycling increases performance and stability.
Can pressure unlock the promise of lithium metal batteries?
In a study published in Nature Energy in October 2021, a team of American materials scientists and chemists described how, using characterization and imaging techniques they were able to study LMB morphology and quantify an LMB’s performance when subjected to a range of pressures.
The team that included researchers from the University of California San Diego, and the General Motors Research and Development Center, found that higher pressure levels forced lithium particles to deposit in neatly arrayed columns, with no porous spaces in between. This was achieved with a pressure of 350 kPa. At lower pressure levels the anode remained porous with lithium particles depositing in a random fashion, which left room for dendrites to grow.
Researchers also observed that increasing pressure to 350 kPa did not affect the solid electrolyte interphase (SEI) structure of the battery’s electrolytes. This is important as the SEI allows certain lithium ions to pass through but limits unwanted chemical reactions that reduce battery performance and accelerate cell failure. A primary objective of research into LMB performance has been to reduce unwanted side reactions between the electrolyte and the lithium metal to encourage vital chemical reactions while restraining unwanted ones.
Whilst the above study deals with the benefits of applying external pressure to LMB cells, a paper published on Electrochimica Acta sets out to determine whether increasing the external mechanical force on a silicon alloy lithium-ion cell offers any benefits.
The research, by G Berckman et al. (2019), was conducted on pouch cell batteries with silicon-alloy/graphite anodes and nickel-rich lithium nickel manganese cobalt oxide (NMC 622) cathodes. By applying an external force of 75 N researchers recorded a capacity increase of 19 percent with the discharge ohmic resistance decreasing by 50 percent.
Furthermore, with the non-uniform distribution of SEI and plated lithium often observed in larger format cylindrical cells, suspected of being responsible for non-uniform aging of supposedly identical cells, could pressure inhibit the rate at which the battery ages?
External pressure increases cyclability in pouch cells
To assess the influence of pressure on battery life in Berckman’s study, eight pouch cells were cycled with an applied external force of 35 N, 75 N, 112.5 N, and 150 N until the batteries reached ‘End of Life’ (80 percent capacity retention). While the results would seem to suggest that the amount of pressure exerted on the cell by applying an external force has a limited effect on capacity retention and thus State of Health (SoH), a small improvement of 2 to 3 percent was noted during the first 100 cycles at 35 N and 75 N.
However, another study, published by Abdilbari Shifa Mussa et al. into the effects of external compression on the performance and aging of NMC/Graphite single-layer Li-ion pouch cells, concluded that at a pressure of around 1,300 kPa lithium loss was significantly reduced.
The research conducted at external pressures of 660, 990, 1,320, and 1,980 kPa, measured the change in impedance at the given pressure levels. The cells were also analyzed for capacity fade and impedance rise at the cell and electrode level. The effect of pressure distribution in large-format cells or battery packs was simulated using parallel connected cells.
The results thus obtained confirmed a higher capacity fade in cells subjected to lower stack pressure. Results from an Electrochemical Impedance Spectroscopy test also indicated an increase in ohmic resistance for high stack pressure cells, while low stack pressure cells exhibited an increase in charge transfer resistance.
Whilst research would seem to indicate that applying an external force or pressure to certain types and forms of battery cell could improve performance and safety and extend the battery’s life, there remain many challenges, not least of all how to increase the pressures required, in particular with LMBs, without adding to the complexity and weight of the battery pack.
Moreover, with the industry focused on developing solid-state electrolytes to deal with the dendrite growth in LMBs, researchers have to grapple with the fact that solid to solid interfaces require even higher pressure to reduce the resistance. Although there has been significant progress in the technology, it remains an ongoing R&D process that will take time to mature, with the distinct possibility that the very high cell pressures will remain a requierment for proper battery function.
So, even as solid-state technology is set to offer a longer term solution, StoreDot’s silicon dominant anode cells already provide a much needed balance between high energy density, liferspan and safety at significantly lower pressures. What is more, the silicon dominant anodes also support extreme fast charging, allowing a battery to charge from 0 to 80 percent in under 10 minutes.