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, Li2Sen, 4≤n≤8) in the ether electrolytes and the sluggish one‐step solid‐solid conversion between Se and Li2Se 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 Li2Se, 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 gSe 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. The discovery of a new low‐barrier two‐step solid reaction pathway for Li‐Se chemistry is reported. The finding has enabled a highly compact Se electrode (2.35 gSe cm−3) with a record‐breaking Se content (80 wt %), and high Se loading (8 mg cm−2), as well as a superhigh volumetric energy density of up to 2502 Wh L−1, surpassing that of LiCoO2.