(Invited) Effects of LiPF6 Salt Concentration on the Cycling Performance of NMC532 Cell Containing Organic Carbonate Solvents

Abstract

Pouch-type cells with NMC532 cathode, MCMB anode (A/C loading ratio of 1.1) and electrolytes containing various LiPF6 concentrations in EC/EMC (3:7 by volume) + 2wt% VC were prepared for investigating the effects of lithium salt concentration on the cycling performance of NMC532 cells. These cells were cycled at 0.5 C rate within 3.0 and 4.2 V until their residual capacity become lower than 70% of their initial capacities, respectively, as shown in Figure 1. After end of life, these cells were disassembled for post-mortem study. The cycled electrodes were used to assemble into coin-type cells with lithium metal for estimating their losses of lithium inventory (LLI), losses of cathode material (LAMC) and losses of anode material (LAMA) by comparing their specific capacities of first and second cycles of the coin-cells with those of the coin-cells prepared with fresh electrodes, respectively. XRD, SEM, HRTEM and XPS were also used to reveal the phase evolution, structure and morphology changes of the electrode materials after cycling. Results suggest that the fast capacity fade of the cell prepared with the electrolyte containing 1 M LiPF6 salt concentration can be attributed to the carbonate solvent oxidation via dehydrogenation on the cathode surface at voltages higher than 3.8 V as reported by Zhang et al.(1) The oxidation can not only accelerate the increase of impedances at anode side, the losses of lithium inventory and anode material by the SEI formation from the reactions between the oxidation products and lithium ions, but also cause the loss of cathode active material and resistance growth at the cathode side, though they are less significant. They can be caused by the phase evolution of spinel and rock-salt phases on the surface of cathode particles, which had been observed from the HRTEM and SAD studies of the cathode particle collected from the cycled cell prepared with 1 M LiPF6 in EC/EMC electrolyte after 600 cycles. It was also found that the amounts of spinel and rock-salt phases formed on the NMC particle surface, rates of capacity fading and resistance growth of the aged cells decreased with the increased LiPF6 salt concentration contained in the electrolytes of the cells. It can be due to the higher salt concentration can induce the less free solvent for being oxidized on the cathode particle surface at voltages higher than 3.8 V. Whereas the layered NMC phase only can be found on the surface of NMC particle collected from the cathode of the cycled cell prepared with 3 M LiPF6 electrolyte after 1100 cycles. This cell shows lower LLI, LAMC, LAMA and impedances growth after 1100 cycles than those of the cell with electrolyte of 1 M concentration after 600 cycles.Reference: Zhang, Y. Katayama, R. Tatara, L. Giordano, Y. Yu, D. Fraggedakis, J. G. Sun, F. Maglia, R. Jung, M. Z. Bazant and Yang Shao-Horn, Energy Environ. Sci., 2020, 13, 183-199.