Phase Engineering Improves the Electrochemical Stability of Lithium-rich Cobalt-free Layered Oxides for Lithium-ion Batteries

Abstract

The expanding electric vehicle market is largely dependent on LiNixMnyCozO2 (NMC) as positive electrode material for lithium-ion batteries (LIBs). The resulting increased demand for cobalt is a pressing concern, since 65% of its global mining is concentrated in a single region. Also driven by its high cost and toxicity, there is a consensus to diminish cobalt usage in LIBs. This has led academia and industry to prioritize developing innovative positive electrode materials with reduced cobalt content or cobalt-free alternatives, such as lithium-rich cobalt-free layered oxides (xLiMO2*(1-x)Li2MO3 (M = Mn, Ni, etc.)). With both rhombohedral and monoclinic crystal phases, this materials class has theoretical capacities around 250 mAh g-1, has improved thermal stability, and is cost-effective because of the manganese-rich composition. Still, the practical application of these materials is hindered because of the pronounced voltage decay during electrochemical cycling, which is caused by the transition from a layered to a disordered spinel-type structure. Research efforts have focused on mitigating this voltage fade, for instance by including dopants to stabilize the crystal structure. For further optimization, it is crucial to quantify the degree in which doping affects the change in crystal structure during electrochemical cycling.

In this study, various post-mortem characterization techniques were used to investigate the impact of aluminum doping on the electrochemical stability of lithium-rich, cobalt-free Li1.26Ni0.15Mn0.61O2. Using statistical analysis based on spectroscopic data, the extent to which Al doping reduces disordered spinel phase formation was quantitatively investigated. The active materials were synthesized via spray pyrolysis followed by calcination. Aluminum doping effectively mitigated voltage fade, enhancing capacity retention from 46% to 67% over 250 cycles at 0.2 C. Structural analysis revealed that doping has a significant effect on the crystalline properties of the materials: the undoped material has a monoclinic crystal structure, but doping increases the rhombohedral character of the layered oxides. As shown by electron microscopy, the more stable rhombohedral phase is present as a shell around monoclinic core particles. This effectively shields the monoclinic phase from the electrolyte, avoiding irreversible oxygen redox, the formation of Mn3+, and the degradation into a disordered spinel phase. As such, the doping procedure results in the formation of a stabilizing rhombohedral interface. Over the course of galvanostatic cycling, the rhombohedral content of the doped materials increases, further contributing to the overall electrochemical stability. As such, this study provides insight into the role that aluminum doping plays in phase engineering and in the improvement of the electrochemical stability of lithium-rich cobalt-free layered oxides.

This work was supported by the HORIZON 2020 project COBRA (H2020-EU.3.4.−875568).

REFERENCES [1] D. De Sloovere, S. K. Mylavarapu, J. D’Haen, T. Thersleff, A. Jaworski, J. Grins, G. Svensson, R. Stoyanova, L. O. Jøsang, K. R. Prakasha, M. Merlo, E. Martínez, M. N. Pascual, J. Jacas Biendicho, M. K. Van Bael, A. Hardy. Phase Engineering via Aluminum Doping Enhances the Electrochemical Stability of Lithium-Rich Cobalt-Free Layered Oxides for Lithium-Ion Batteries. Small, 2400876, 2024.