Insight and regulation of interfacial coordination chemistry of high-voltage LiCoPO4 cathode via functionalized carbon layer anchoring for robust surface passivation

Abstract

Lithium cobalt phosphate (LiCoPO₄, LCP) is a high-voltage (high-V) polyanionic compound, of critical importance for high energy density. However, LCP’s application has been hampered by inferior Coulombic efficiency and capacity retention owing to cathode degradation triggered by the elevated cut-off voltage of 5.0 V vs. Li/Li⁺. Namely, such high-V leads to severe parasitic side reactions at the cathode-electrolyte interface involving complex phenomena, the understanding of which holds the key to designing robust high-V cathodes. Herein, post-mortem analysis of cycled LCP electrodes in different aging conditions revealed fundamental capacity fade mechanism at high-V interfaces. Detrimental degradation occurs through redox metal (RM) dissolution, caused by undesired electrochemical and chemical reactions with electrolyte. This process induces cation vacancies, leading to lattice collapse and progressive structural ingress. To combat this, we successfully engineered sucrose-derived functionalized carbon layer (FCL) anchoring strategy to regulate interfacial coordination chemistry. This conductive FCL layer serves a dual role: (i) expediting charge-transfer kinetics by establishing an efficient electronic network, and (ii) anchoring surface cations, thereby promoting a protective inorganic-enriched cathode-electrolyte interphase (CEI) layer characterized by Li-F and Co-F species while suppressing organic ligand-induced dissolution. This passivation suppresses structural ingress and preserves olivine structure, resulting in significantly enhanced rate capability and cycling stability.