Two major strategies are currently pursued to improve the energy density of lithium-ion batteries using LiNi_xCo_yMn_zO₂ (NCM) cathode materials. One is to increase the fraction of redox active Ni (≥80%), which allows larger amounts of Li to be extracted at a given cutoff voltage (U_max). The other is to increase U_max, in particular for medium-Ni content NCM materials. However, the accompanying lattice changes ultimately lead to capacity fading in both cases. Here the structural changes occurring in Li_1.02Ni_xCo_yMn_zO₂ (with x = ¹/₃, 0.5, 0.6, 0.7, 0.8 and 0.85) during cycling operation in the voltage range between 3.0 and 4.6 V vs Li are quantified by means of operando X-ray diffraction combined with detailed Rietveld analysis. All samples show a large decrease in unit cell volume upon charging, ranging from 2.4% for NCM111 (33% Ni) to 8.0% for NCM851005 (85% Ni). To make a fair comparison of the structural stability of the different NCM materials, energy densities as a function of U_max are estimated and correlated with X-ray diffraction results. It is shown that NCMs with a lower Ni content allow for specific energies similar to that of, e.g., Ni-rich NCM811 (80% Ni) when operated at sufficiently high U_max, but still undergo less pronounced changes in structure. Nevertheless, as indicated by charge/discharge tests, the capacity retention of low- and medium-Ni content NCMs cycled to high U_max is also strongly affected by factors other than stability of the layered crystal lattice (electrolyte decomposition etc.). Overall, it is demonstrated that the complexity of the degradation processes needs to be better understood to identify optimal cycling conditions for specific cathode compositions.

中文翻译：

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在Scylla和Charybdis之间：用于高级锂离子电池的层状NCM阴极材料的结构稳定性和能量密度之间的平衡

当前正在采用两种主要策略来改善使用LiNi _x Co _y Mn _z O ₂（NCM）阴极材料的锂离子电池的能量密度。一种是增加氧化还原活性Ni的比例（≥80％），这允许在给定的截止电压（U _max）下提取更多的Li 。另一个是增加U _max，特别是对于中等Ni含量的NCM材料。但是，伴随的晶格变化最终导致两种情况下的容量衰减。在这里，Li _1.02 Ni _x Co _y Mn _z O ₂（与X = ¹ / ₃，0.5，0.6，0.7，3.0和4.6V下对Li之间的电压范围内操作循环借助被量化期间0.8和0.85）operando X射线衍射用详述Rietveld分析相结合。所有样品在充电后均显示出单位电池体积的大幅降低，范围从NCM111（33％Ni）的2.4％到NCM851005（85％Ni）的8.0％。为了公平地比较不同NCM材料的结构稳定性，估算了能量密度与U _max的关系，并将其与X射线衍射结果相关联。结果表明，当在足够高的U下运行时，具有较低Ni含量的NCM允许的比能类似于富Ni的NCM811（80％Ni）的比能。_max，但结构上的变化仍然不太明显。然而，如充电/放电测试所示，循环到高U _max的低和中Ni含量的NCM的容量保持率也受到层状晶格稳定性（电解质分解等）以外的因素的强烈影响。总的来说，表明需要更好地理解降解过程的复杂性，以识别特定阴极组合物的最佳循环条件。