Unveiling the mechanism of dense cathode‒electrolyte interphase formation in lithium-ion batteries using cyclophosphamide additive

Abstract

High-voltage lithium-ion batteries (LIBs) have attracted increasing attention for their high energy density. However, at high voltages, cathode degradation and electrolyte decomposition trigger parasitic side reactions that deteriorate battery cycle performance. These issues have been addressed through various studies on cathode‒electrolyte interphase (CEI)-forming additives. In particular, 2-ethylmethylamino-1,3,2-dioxaphospholane 2-oxide (EMPA), a cyclophosphamide (CPA) CEI-forming additive, has shown excellent capacity retention and battery cycle performance at high voltages when added at only 0.5 vol % in LIB systems. However, the molecular-level understanding of CPA additives remains limited. Here, our first-principles calculations reveal that EMPA oxidizes before the solvent in the electrolyte while also scavenging HF and H₂O. Specifically, calculations of the dimerization of asymmetric EMPA trimers, represented by two identical [(EMPA)₃OH] species forming a [(EMPA)₃OH]₂ dimer, imply that after oxidation these two identical EMPA polymers bind very strongly and in very close proximity. This was due to the favorable electrostatic interactions with the more widely distributed polar surface in EMPA, in addition to the small number of carbons in the alkyl groups of the amine moiety in CPA. We suggest that the asymmetry in the alkyl groups of the amine moiety in CPA is closely related to the excellent CEI formation observed in the experimental results.