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Nitriles have been successfully used as electrolyte additives for performance improvement of commercialized lithium-ion batteries based on the LiCoO 2 cathode, but the underlying mechanism is unclear. In this work, we present an insight into the contribution of nitriles via experimental and theoretical investigations, taking for example succinonitrile. It is found that succinonitrile can be oxidized together with PF 6 − preferentially on LiCoO 2 compared to the solvents in the electrolyte, making it possible to avoid the formation of hydrogen fluoride from the electrolyte oxidation decomposition, which is detrimental to the LiCoO 2 cathode. Additionally, inorganic LiF and −NH group-containing polymers are formed from the preferential oxidation of succinonitrile, constructing a protective interphase on LiCoO 2 , which suppresses electrolyte oxidation decomposition and prevents LiCoO 2 from structural deterioration. Consequently, the LiCoO 2 cathode presents excellent stability under cycling and storing at high voltages.

A Ni-rich cathode is one of the representatives for lithium-ion batteries that can deliver a higher energy density with lower cost than any other counterparts, but its cyclic stability is unsatisfactory. 1,3,5-Trimethyl-1,3,5-tris­(3,3,3-trifluoropropyl) cyclotrisiloxane (D3F) as an electrolyte additive is effective for performance improvement of the Ni-rich cathode. Evaluated in a cell based on the Ni-rich cathode (LiNi0.8Co0.1Mn0.1O2, NCM811) and a graphite anode in 1 M LiPF6-EMC/EC/DEC (3/5/2 in weight) between 3.0 and 4.35 V, the application of 5% D3F without any co-contribution of other electrolyte additives enhances the capacity retention of the cell from 38 to 76% after 300 cycles at 1C. The underlying mechanism is understood by combining theoretical calculation with physical characterization and electrochemical measurements. D3F can be electrochemically oxidized and reduced preferentially compared to the electrolyte components, creating robust interphases simultaneously on the cathode and anode. Additionally, it can scavenge HF and eliminates its detrimental effect on battery performances. This understanding helps to further enhance the performance of Ni-rich/graphite cells via designing electrolyte additives.

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