Molecular Design of Asymmetric Cyclophosphamide as Electrolyte Additive for High-Voltage Lithium-Ion Batteries

Abstract

Elevating the charging voltage could greatly promote the energy density of lithium-ion batteries (LIBs) with LiNi_xMn_yCo_zO₂ cathodes, although challenges arise from severe parasitic reactions and rapid capacity decay at high voltage, especially for nickel-rich cathodes. Herein, by incorporating various useful functionalities into one single molecule, we rationally design and synthesize a new class of five-membered asymmetric cyclophosphamides as electrolyte additives to enable stable cycling of high-voltage LIBs. It is demonstrated that the strong Lewis-base feature of the P–N bond can effectively scavenge the detrimental HF and H₂O in the electrolyte. Meanwhile, the five-membered ring with an asymmetric amine moiety undergoes ring-opening polymerization to generate a highly robust and thin polymeric cathode–electrolyte interphase. Benefiting from the above merits, the asymmetric cyclophosphamide additive significantly suppresses decomposition of the electrolyte, dissolution of the transitional metals, and structural damage to the cathode, thus markedly improving the cycling stability and Coulombic efficiency of both high-voltage coin and pouch LIBs (up to 4.6 V).