Oxygen redox cathodes, such as Li_1.2Ni_0.13Co_0.13Mn_0.54O₂, deliver higher energy densities than those based on transition metal redox alone. However, they commonly exhibit voltage fade, a gradually diminishing discharge voltage on extended cycling. Recent research has shown that, on the first charge, oxidation of O²⁻ ions forms O₂ molecules trapped in nano-sized voids within the structure, which can be fully reduced to O²⁻ on the subsequent discharge. Here we show that the loss of O-redox capacity on cycling and therefore voltage fade arises from a combination of a reduction in the reversibility of the O²⁻/O₂ redox process and O₂ loss. The closed voids that trap O₂ grow on cycling, rendering more of the trapped O₂ electrochemically inactive. The size and density of voids leads to cracking of the particles and open voids at the surfaces, releasing O₂. Our findings implicate the thermodynamic driving force to form O₂ as the root cause of transition metal migration, void formation and consequently voltage fade in Li-rich cathodes.

氧氧化还原阴极，例如Li _1.2 Ni _0.13 Co _0.13 Mn _0.54 O ₂，​​比单独基于过渡金属氧化还原的阴极提供更高的能量密度。然而，它们通常会表现出电压衰减，即在延长的循环过程中放电电压逐渐降低。最近的研究表明，在第一次充电时，O ²⁻离子的氧化形成了被困在结构内纳米尺寸空隙中的 O ₂分子，在随后的放电中可以完全还原为 O ^{2− 。在这里，我们表明，循环时O-氧化还原能力的损失以及因此电压衰减是由O 2−} /O ₂氧化还原过程可逆性的降低和O ₂损失的组合引起的。_{捕获O 2}的封闭空隙在循环过程中不断生长，使得更多捕获的O ₂电化学不活跃。空隙的尺寸和密度导致颗粒破裂并在表面形成开放空隙，释放O ₂。我们的研究结果表明，形成O _{2 的}热力学驱动力是富锂正极中过渡金属迁移、空隙形成以及电压衰减的根本原因。