Two-Plateau Li-Se Chemistry for High Volumetric Capacity Se Cathodes

For Li-Se batteries, ether- and carbonate-based electrolytes are commonly used. However, because of the "shuttle effect" of the highly dissoluble long-chain lithium polyselenides (LPSes, Li₂ Se_n , 4≤n≤8) in the ether electrolytes and the sluggish one-step solid-solid conversion between Se and Li₂ Se in the carbonate electrolytes, a large amount of porous carbon (>40 wt % in the electrode) is always needed for the Se cathodes, which seriously counteracts the advantage of Se electrodes in terms of volumetric capacity. Herein an acetonitrile-based electrolyte is introduced for the Li-Se system, and a two-plateau conversion mechanism is proposed. This new Li-Se chemistry not only avoids the shuttle effect but also facilitates the conversion between Se and Li₂ Se, enabling an efficient Se cathode with high Se utilization (97 %) and enhanced Coulombic efficiency. Moreover, with such a designed electrolyte, a highly compact Se electrode (2.35 g_Se  cm^-3 ) with a record-breaking Se content (80 wt %) and high Se loading (8 mg cm^-2 ) is demonstrated to have a superhigh volumetric energy density of up to 2502 Wh L^-1 , surpassing that of LiCoO₂ .