Interphase design of LiNi0.6Mn0.2Co0.2O2 as positive active material for lithium ion batteries via Al2O3 coatings using magnetron sputtering for improved performance and stability

To enhance the performance and stability of NMC622, an alumina coating is applied and optimized as protective interphase, using a specifically developed RF-magnetron sputtering technique. The uniform coating significantly improves capacity retention compared to pristine uncoated NMC622 originating from less structural degradation and reduced particle cracking, which also enhances thermal stability.

Abstract

LiNi _x Mn _y Co _z O₂ (x+y+z=1) is one of the most present and versatile positive active materials for lithium ion batteries due to comparatively high specific capacity and high operating potential. However, NMC materials are prone to various degradation effects including moisture uptake, formation of impurities at the particle surface and transition metal dissolution during charge/discharge cycling and/or at elevated temperatures. Beyond that, cation mixing can lead to phase transformation, oxygen evolution, particle cracking and particle disintegration. Therefore, an alumina coating was applied and optimized as protective interphase on LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622) powders, using a specifically in-house developed RF-magnetron sputtering technique. The alumina coated NMC622 showed a 13 % improvement in capacity retention after 200 charge/discharge cycles in lab-scale cells, compared to pristine uncoated NMC622. Using electrochemical impedance spectroscopy, the interfacial/interphasial resistance of pristine and alumina coated NCM622 based electrodes were explored to study the impact of the coating on lithium ion transport. Furthermore, the structural and thermal stability of cyclic aged NMC622 were analyzed via TEM, EELS and TGA. Therein, alumina coated samples demonstrated enhanced thermal stability, less structural degradation, and reduced particle cracking.

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