Impacts of residual electrolyte components of spent lithium-ion batteries on the physical/electrochemical properties of resynthesized cathode active materials

Lithium-ion batteries (LIBs) with excellent energy density are used as energy storage devices and power supply sources for various portable electronic devices including electric vehicles. We investigate the effects of electrolyte components of LIBs on the resynthesized LiNi0.8Mn0.1Co0.1O2 (NMC811) performance. Inspired by our findings on the presence of F in the actual industrial leachate of spent LIBs, various PxFy-containing NMC is prepared via co-precipitation method with different PF6− sources including NH4PF6, LiPF6, and a commercial electrolyte (1 M LiPF6 dissolved in organic solvents) at an impurity level of F. The physical and electrochemical properties of the respective NMC samples, which are labelled as NMCN, NMCL, and NMCE, are compared with pristine NMC. The PF6− anion is incorporated into the precursor in the form of POxFyz− or F− rather than intact PF6−. The resulting formation of LixPOyFz and LiF on the surface of PxFy-containing NMC is beneficial to the LIB cyclability. The suppressed dissolution of transition metals from NMC is confirmed by a post-mortem elemental analysis using full-cells and an electrochemical impedance spectroscopy. The improved rate performance of PxFy-containing NMC is also observed and attributed to the widened lithium interslab thickness and the resulting high Li+ diffusion coefficients. Meanwhile, NMCE had a relatively small particle size and a rough surface compared to the other samples. In summary, the possible inflow of LIB electrolyte components into the resynthesis of cathode active materials have very contrasting aspects in their LIB performance. Whereas organic solvents affect the morphology of cathode active materials and the initial discharge capacity adversely, the PF6− anion improves the cyclability and the rate performance.