Ni-rich layered oxide cathodes can deliver higher energy density batteries, but uncertainties remain over their charge compensation mechanisms and the degradation processes that limit cycle life. Trapped molecular O₂ has been identified within LiNiO₂ at high states of charge, as seen for Li-rich cathodes where excess capacity is associated with reversible oxygen redox. Here we show that bulk redox in LiNiO₂ occurs by Ni–O rehybridization, lowering the electron density on O sites, but importantly without the involvement of molecular O₂. Instead, trapped O₂ is related to degradation at surfaces in contact with the electrolyte, and is accompanied by Ni reduction. O₂ is removed on discharge, but excess Ni²⁺ persists forming a reduced surface layer, associated with impeded Li transport. This implicates the instability of delithiated LiNiO₂ in contact with the electrolyte in surface degradation through O₂ formation and Ni reduction, highlighting the importance of surface stabilisation strategies in suppressing LiNiO₂ degradation.

中文翻译：

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区分锂镍氧化物阴极中的本体氧化还原和近表面降解


富镍层状氧化物阴极可以提供更高能量密度的电池，但其电荷补偿机制和限制循环寿命的退化过程仍然存在不确定性。在高电荷态的 LiNiO₂ 中已鉴定出被捕获的分子 O₂，如富锂阴极所见，其中过剩的容量与可逆氧氧化还原有关。在这里，我们表明 LiNiO₂ 中的体氧化还原是通过 Ni-O 再混合发生的，降低了 O 位点的电子密度，但重要的是没有分子 O₂ 的参与。相反，被捕获的 O₂ 与与电解质接触的表面的降解有关，并伴随着 Ni 还原。O₂ 在放电时被去除，但过量的 Ni²⁺ 持续形成减少的表层，与阻碍 Li 传输有关。这意味着脱锂化的 LiNiO₂ 在与电解质接触时不稳定性，通过 O₂ 形成和 Ni 还原进行表面降解，突出了表面稳定策略在抑制 LiNiO₂ 降解中的重要性。