Ni-rich cathodes are the most promising candidates for realizing high-energy-density Li-ion batteries. However, the high-valence Ni4+ ions formed in highly delithiated states are prone to reduction to lower valence states, such as Ni3+ and Ni2+ , which may cause lattice oxygen loss, cation mixing, and Ni ion dissolution. Further, LiPF6 , a key salt in commercialized electrolytes, undergoes hydrolysis to produce acidic compounds, which accelerate Ni-ion dissolution and the interfacial deterioration of the Ni-rich cathode. Dissolved Ni ions migrate and deposit on the surface of the graphite anode, causing continuous electrolyte decomposition and threatening battery safety by forming Li dendrites on the anode. Herein, 1,2-bis(diphenylphosphino)ethane (DPPE) chelates Ni ions dissolved from the Ni-rich cathode using bidentate phosphine moieties and alleviates LiPF6 hydrolysis via complexation with PF5 . Further, DPPE reduces the generation of corrosive HF and HPO2 F2 substantially compared to the amounts observed using trimethyl phosphite and tris(trimethylsilyl) phosphite, which are HF-scavenging additives. Li-ion cells with Ni-rich cathodes and graphite anodes containing DPPE exhibit remarkable discharge capacity retentions of 83.4%, with high Coulombic efficiencies of >99.99% after 300 cycles at 45 °C. The results of this study will promote the development of electrolyte additives.
Keywords: chelating agents; electrolyte additives; lithium-ion batteries; nickel-rich cathodes; transition metal dissolution.
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