Both enhanced Li diffusivity and high discharge capacities make Ni-rich LiNi_1-x-yCo_xMn_yO₂ (NCM) cathodes attractive for application in high energy density Li-ion batteries with good power [1]. However, further improvement in power density requires a thorough understanding of the mechanisms underlying the Li diffusion kinetics. Here, we investigate changes in Li diffusion coefficient (D_Li) during charging of NCM/Li cells and link the evolution of D_Li to the atomic structure and material resistivity.

Results from galvanostatic intermittent titration technique show that Li diffusivity in NCM cathodes evolves non-monotonically during charge and discharge. The most significant changes are observed in the potential range from 3.6 to 4.0 V with respect to Li/Li⁺. Electrochemical impedance spectroscopy suggests that the lithium intercalation kinetics is primarily controlled by the charge transfer resistance at the cathode/electrolyte interface, while in operando X-ray diffraction indicates that the spacing along the c-axis increases, which we attribute to an increase of the repulsion between the oxygen slabs in the absence of screening Li. Furthermore, we observe a drastic decrease in the interplane spacing between 4.0 and 4.3 V, and a gradual decrease of the in-plane spacing (a,b-axis) during Li-deintercalation. The latter indicates shortage of the O-Me bonds with oxidation of Ni. Overall, our research data demonstrate that Li diffusion in Ni-rich LiNi_1-x-yCo_xMn_yO₂ is strongly controlled by the interplane spacing of the host structure, and the overall kinetics appears to be limited by high interfacial charge transfer resistance at the beginning of charge.

References

[1] Wei, Y.; Zheng, J.; Cui, S.; Song, X.; Su, Y.; Deng, W.; Wu, Z.; Wang, X.; Wang, W.; Rao, M.; Lin, Y.; Wang, C.; Amine, K. & Pan, F. Kinetics Tuning of Li-Ion Diffusion in Layered Li(Ni_xMn_yCo_z)O₂, Journal of the American Chemical Society, 2015, 137, 8364-8367.