Organofluorine borate-involved cathode interphase enabling high-rate and high-voltage NCM batteries

Abstract

High-voltage and Ni-rich cathode materials are highly attractive due to their high specific energy. However, cathode-electrolyte interphase (CEI) layer often fails to adequately protect cathode materials from side reactions with electrolyte and degradation induced at the surface. In this study, we introduce an approach to construct a CEI layer that spontaneously passivate cathode surface and facilitate Li-ion transfer at the interface. This was realized by using an organofluorine borate, tris(2,2-difluoroethyl) borate (B(OCH₂CHF₂)₃), as an electrolyte additive to facilitate CEI formation. Spectroscopic and microscopic studies demonstrated that the fluorinated borates provided superior passivation compared to traditional borates, enabling stable operation at high voltages of up to 4.5 V and elevated temperatures of 60 °C. Moreover, fluorinated borates enhanced Li-ion transfer through O-Li-F interactions, unlike conventional interface protection methods that typically cause increased interfacial resistance. A 4.5 V, 410 Wh kg^-1 anode-less LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) pouch cell achieved a capacity retention of 80.1% over 120 cycles. Our findings highlight a promising strategy for spontaneous passivation of the cathode interface and enhanced Li-ion transfer for batteries with high specific energies.