Should Harvard’s experimental “sandwich-like” solid state lithium batteries gain market viability, not only will they maintain thousands of charge/discharge cycles and longer lifespans, but they will also enable EVs to be charged from zero to full within 10 minutes.
Li-ion batteries have excellent capacities and densities, which make them perfect for storing energy in phones, laptops, and other consumer electronics products. In addition, these batteries have enabled an industry-wide migration from ICE vehicles to EVs and served as the bedrock of BESS (battery energy storage systems), which are indispensable for the development of renewable energy. However, they are not without flaws: Xin Li, associate professor of materials science at the Harvard John A. Paulson School of Engineering and Applied Science (SEAS), indicates that they have poor stability.
Li-ion batteries are composed of a graphite anode (negative electrode) and a lithium metal cathode (positive electrode), between which lithium ions are transferred via liquid electrolytes. However, as the number of charge/discharge cycles increases, sharp, tendril-like dendrites can form around the electrodes, which eventually grow long enough to “pierce the barrier separating the anode and cathode, causing the battery to short or even catch fire”, according to Harvard.
At the moment, scientists are hard at work to find a suitable solid state electrolyte that can replace the mainstream liquid electrolytes, which are highly volatile and highly corrosive. One such promising substitute is the “BLT sandwich battery” design from Harvard researchers.
The Harvard Gazette describes the sandwich structure as such:
First comes the bread — the lithium metal anode — followed by lettuce — a coating of graphite. Next, a layer of tomatoes — the first electrolyte — and a layer of bacon — the second electrolyte. Finish it off with another layer of tomatoes and the last piece of bread.
As for the reason for surrounding the second electrolyte layer with the first layer much like how tomatoes sandwich bacon in a BLT, the research team indicates that these two electrolytes differ in terms of chemical composition. Dendrite can easily pierce the first electrolytic layer, which is relatively stable with lithium metals, while the opposite is true for the second electrolytic layer, which is unstable with lithium metals and relatively unaffected by dendrites.
Although this new battery design cannot prevent the growth of dendrites, it can safely and stably control dendrites, as well as prevent them from piercing the “bacon” layer. Furthermore, through a process known as “backfilling” in which this design can fill in gaps left by dendrite growth, this new design is self-repairing. Experiments indicate that even after 10,000 charge/discharge cycles, the new battery can still maintain an 82% efficiency. Moreover, the high electric current density of the battery allows for EVs equipped with these batteries to be fully charged between 10 to 20 minutes.
Luhan Ye, author of the research documenting the new battery design, indicates that this multi-layer design can be used to guide and control the growth of dendrites. Li also adds that the new design indicates that solid state lithium batteries are competitive with commercial Li-ion batteries. With the flexibility and multipurpose advantages derived from its multi-layer design, solid state lithium batteries are more likely than ever to be featured in current mass production technologies for batteries.