Voltage fade is a major problem in battery applications for high-energy lithium- and manganese-rich (LMR) layered materials. As a result of the complexity of the LMR structure, the voltage fade mechanism is not well understood. Here we conduct both in situ and ex situ studies on a typical LMR material (Li_1.2Ni_0.15Co_0.1Mn_0.55O₂) during charge–discharge cycling, using multi-length-scale X-ray spectroscopic and three-dimensional electron microscopic imaging techniques. Through probing from the surface to the bulk, and from individual to whole ensembles of particles, we show that the average valence state of each type of transition metal cation is continuously reduced, which is attributed to oxygen release from the LMR material. Such reductions activate the lower-voltage Mn³⁺/Mn⁴⁺ and Co²⁺/Co³⁺ redox couples in addition to the original redox couples including Ni²⁺/Ni³⁺, Ni³⁺/Ni⁴⁺ and O²⁻/O⁻, directly leading to the voltage fade. We also show that the oxygen release causes microstructural defects such as the formation of large pores within particles, which also contributes to the voltage fade. Surface coating and modification methods are suggested to be effective in suppressing the voltage fade through reducing the oxygen release.

对于高能富锂和锰（LMR）层状材料，电压衰减是电池应用中的主要问题。由于LMR结构的复杂性，电压衰减机制尚未得到很好的理解。在这里，我们对典型的LMR材料（Li _1.2 Ni _0.15 Co _0.1 Mn _0.55 O ₂）在充放电循环中，使用多尺度X射线光谱和三维电子显微镜成像技术。通过探测从表面到整体，以及从单个粒子到整个整体，我们发现每种过渡金属阳离子的平均价态不断降低，这归因于从LMR材料中释放的氧气。除了包括Ni ²⁺ / Ni ³⁺，Ni ³⁺ / Ni ⁴⁺和O ²的原始氧化还原对之外，这种还原还激活了较低电压的Mn ³⁺ / Mn ⁴⁺和Co ²⁺ / Co ³⁺氧化还原对。^- / O ^-，直接导致电压衰减。我们还表明，氧气释放会引起微观结构缺陷，例如在颗粒内形成大孔，这也有助于电压衰减。建议使用表面涂层和改性方法来通过减少氧气释放来有效抑制电压衰减。