Surface defects induced by acid etching for promoting Ni-rich cathode regeneration

Abstract

Direct regeneration has been considered as the promising strategy for the recycling of spent LiNi_xCo_yMn_zO₂ (NCM) cathode materials. The spent NCM suffers from the lithium deficiency in the interior and the phase transition on the surface. The phase transition on the surface suppresses the Li⁺ diffusion during the direct regeneration process. Surface acid etching is employed to eliminate by-products and degraded phases from spent NCM, aiming to mitigate the Li⁺ diffusion barrier during regeneration. However, the underlying mechanism of this surface engineering on defect formation and material regeneration remains unclear. Here, we systematically investigated the surface acid etching process and regeneration mechanism of spent NCM. We reveal that controlled surface dissolution of metal ions induces the formation of oxygen vacancies. This enhances Li⁺ adsorption on the spent NCM surface and facilitates Li⁺ transport during regeneration process, thus effectively restoring the layered structure and lithium deficiency. Consequently, the regenerated NCM exhibits a discharge capacity of 192.9 mA h g^-1 at 0.1 C, surpassing that of the regenerated NCM without etching (188.1 mA h g^-1). In addition, the regenerated NCM delivers a high-capacity retention of 92.6% after 100 cycles at 1 C, while that of the regenerated NCM without etching is only 68.8%. This finding provides mechanistic insight into the role of surface oxygen vacancies for promoting NCM regeneration.

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