Alkali metal anodes for next generation, high-energy density rechargeable batteries

In order to progress beyond the state-of-the-art lithium-ion batteries (LIBs), it is necessary to replace the graphitic electrode with an anode of higher energy density. Lithium metal’s ultra-high gravimetric capacity (3860 mAh g⁻¹), coupled with currently used metal oxide cathode materials could increase the gravimetric energy density of LIBs by approximately 35%. However, the well-known problem of lithium’s poor plating/stripping efficiency and dendrite formation has stalled its commercialization. The low cycling efficiency and formation of dendritic deposits is largely a consequence of the solid electrolyte interphase (SEI). This SEI forms promptly when the highly reducing lithium metal contacts the liquid organic electrolyte (LOE). It is composed of organic and inorganic reaction products. This SEI can be engineered by meticulously selecting electrolyte components (salts, solvents, and additives) which yield passivating and dendrite inhibiting constituents. A series of electrolyte compositions was surveyed and it was found that a concentrated LiNO₃ electrolyte could yield dendrite-free lithium deposits. Further optimization of this electrolyte yielded a much higher capacity retention and retained the dendrite-free deposition behavior in the visualization cell. The scarcity of lithium in the earth’s crust brings to question its sustainability as the ultimate anode choice for high-energy density, rechargeable batteries. Akin to lithium, sodium metal anodes have a high gravimetric charge capacity. However, the deposition of sodium is poorly understood compared to lithium. Sodium electrodeposits from ethylene carbonate (EC), diethyl carbonate (DEC), and propylene carbonate (PC) electrolytes were shown to generate large volumes of gas and yielded fragile, porous dendrites. The use of fluoroethylene carbonate (FEC) was shown to improve SEI passivation and mitigate gas evolution. Formation of sodium dendrites would be avoided by electrodepositing on the molten metal anode; however, this requires an operating temperature higher than the melting point of sodium. Formation of a room temperature sodium-potassium (NaK) liquid alloy has recently been proposed as a solution to address the operating temperature and dendrite challenges. However, visualization cell electrodepositions revealed that the NaK alloy deposits dendritically akin to solid sodium anodes in LOEs. However, when sodium was deposited on a potassium-rich alloy, dendrite formation was reduced but not completely avoided.