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Impact of Dissolved Ni2+ on Solid Electrolyte Interphase of Graphite Anode.

Dissolution of transition metal ions, such as Ni 2+ , Mn 2+ , from layered-structured Ni-rich cathode can migrate to the anode side and aggravate the electrochemical polarization and capacity degradation of lithium-ion batteries (LIBs). The investigations of the impact and distribution of Ni species on the solid electrolyte interphase (SEI) on anode are crucial to understand the failure mechanism of LIBs in this scenario, yet few works have been reported due to the complicated structure of SEI. Herein, we used Time-of-flight secondary ion mass spectroscopy (TOF-SIMS) technique coupled with multivariate data analysis (MVA) to intuitively characterize the distribution of Ni species in SEI. We find that, in EC-based electrolyte, SEI on the graphite electrode exhibits a multi-stratum structure. In contrary to the conventional view, the outer and intermediate stratums are dominated by the decomposition products related with LiPF 6 salt, such as phosphates, fluorophosphates, and LiF, during the formation cycles. After accelerated aging the full cell of LiNi 0.88 Co 0.08 Mn 0.04 O 2 (NCM88)/graphite for 50 cycles, the concentration of Ni dissolved in electrolyte reaches ca. 690 ppm. With multivariate curve resolution analysis of TOF-SIMS 3D data, a strong correlation between the dissolved-Ni and organic species in SEI on graphite is illustrated. The ion exchange reaction between Ni 2+ and Li + in SEI is demonstrated to be the main reason for the increase of SEI resistivity.