A Multimodal Investigation of Controlled Oxygen-Vacancy Formation to Prevent the Degradation in Ni-Rich NMC811 Cathode

Ni-rich LiNi_xMn_yCo_zO₂ (conventionally known as NMC, where x+y+z=1 and x&gt;y; x&gt;z) is one of the most used cathode materials in electric vehicles due to its low cobalt concentration and high capacity¹. One of the significant degradation mechanisms of this material has been attributed to intragranular crack formation and propagation that can lead to mechanical failure of cathode particles^2–4. Due to cracking, the electrolyte can attack the newly exposed surfaces, releasing oxygen species such as radicals, ions, and molecules^5,6.<br/>To mitigate crack propagation to the surface, we can synthesize a heterogeneous particle with a dislocation-dense (oxygen-deficient) layer^3,7–9. This surface layer can improve the stability of the cathode particles by shielding the core from electrolyte and annihilating the cracks generated in its core. Furthermore, the controlled oxygen vacancy (OV) layer on the surface of different transition metal cathodes has improved high C-rate capacity retention and cyclability³. Hence, it is crucial to investigate the control parameters for creating a dense OV layer on the surface of Ni-rich NMC material and associated lattice and oxidation change.<br/>Herein, we explore a simple method for creating OV on the surface of NMC811 (LiNi₈Mn₁Co₁O₂) under a controlled atmosphere at elevated temperatures. The effect of different annealing temperatures has been analyzed using the in-situ X-ray diffraction (XRD) measurement. The intensity ratio of planes (003)/(104) decreases for oxygen-reduced structure, indicating a disordered cation layer with increased Oxygen loss³. Oxygen occupancy reduces from 0.99 to 0.975, indicating the presence of ∼1.52% oxygen vacancy in the structure. Ni occupancy in the Li layer rises from 0.17 to 0.217, indicating increased Ni²⁺/Li⁺ mixing. Increased concentration of Ni²⁺ induces more cation mixing⁴. Hence, the oxidation state of modified NMCs has been investigated with X-ray absorption near-edge spectroscopy (XANES). The coordination structure was further examined through extended XAS fine structure (EXAFS)^10,11. A first-principal calculation-based computational model¹² was employed to calculate oxygen vacancy formation energy and a Quantum Espresso-based Xspectra^13–15 package was used to reproduce the experimental metal K-edge XANES data to analyze further the optimum vacancy concentration required for the higher oxidation state of Ni (3+,4+). Finally, an ex-situ transmission electron microscopy study of the modified structure reveals the crack propagation in surface layers and the effect of the dense dislocation layer in crack annihilation.<br/><br/><b>References: </b><br/><b>1</b> Manthiram, A. et al. Adv Energy Mater 6, (2016)<br/><b>2</b> Su, Y. et al. Journal of Energy Chemistry 65, 236–253 (2022)<br/><b>3 </b>Su, Y. et al. ACS Appl Mater Interfaces 12, 37208–37217 (2020)<br/><b>4</b> Jiang, M. et al. Advanced Energy Materials 11, (2021)<br/><b>5</b> Gan, Q. et al. EnergyChem (2023)<br/><b>6</b> Bak, S. M. et al. ACS Appl Mater Interfaces 6, 22594–22601 (2014)<br/><b>7</b> Gu, Y. J. et al. Int J Electrochem Sci 12, 9523–9532 (2017)<br/><b>8</b> Chen, B. et al. J Mater Sci Technol 35, 994–1002 (2019)<br/><b>9</b> Song, J. et al. Chemistry of Materials 24, 3101–3109 (2012)<br/><b>10</b> Alamgir, F. M. et al. in 79–108 (2015)<br/><b>11</b> Yu, Y. et al. ACS Appl Mater Interfaces 12, 55865–55875 (2020)<br/><b>12</b> Cui, S. et al. Adv Energy Mater 6, (2016)<br/><b>13</b> Bunau, O. et al. Phys Rev B Condens Matter Mater Phys 87, (2013)<br/><b>14</b> Gougoussis, C. et al. Phys Rev B Condens Matter Mater Phys 80, (2009)<br/><b>15</b> Taillefumier, M. et al. Phys Rev B Condens Matter Mater Phys 66, 1–8 (2002)