Department of Materials Science & Engineering , McMaster University , Hamilton , ON L8S 4L7 , Canada.
Department of Chemistry and Chemical Biology , McMaster University , Hamilton , ON L8S 1A8 , Canada.
ACS Nano. 2018 Mar 27;12(3):2708-2718. doi: 10.1021/acsnano.7b08945. Epub 2018 Mar 12.
Lithium-rich layered oxides are promising cathode candidates because of their exceptional high capacity. The commercial application of these high-energy cathodes, however, is thwarted by the undesired rapid performance decay during cycling. Surface degradation has been widely considered to correlate with the performance decay of the cathodes, whereas, in this work, we demonstrate that the degradation of Li-rich high-energy LiNiMnCoO (HENMC) cathode material not only takes place at surfaces but also proceeds from its internal structure. In addition to demonstrating the surface reconstruction and the formation of a cathode-electrolyte interphase (CEI) layer of cycled HENMC cathode, this study uncovers the irreversible bulk phase transition from a Li-excess monoclinic ( C2/ m) solid solution into a conventional "layered" ( R3̅ m) phase, accompanied by complete loss of Li from the TM layers during cycling. Furthermore, the internal grains of HENMC bear lattice distortions, leading to the formation of "nano-defect" domains, which could limit the Li diffusion inside the grains. More prominently, the layered-to-spinel transition in the form of large spinel grains ( Fd3̅ m), hundreds of nanometers across, is discovered, and their detailed atomic arrangement is studied. The findings suggest that, instead of attributing the overall capacity fade to the surface degradation, these drastic bulk evolutions would be the main degradation mechanisms at the source of the rapid failure of Li-rich cathodes.
富锂层状氧化物因其高容量而成为有前途的正极候选材料。然而，这些高能正极的商业应用受到循环过程中性能快速衰减的阻碍。表面降解被广泛认为与正极的性能衰减有关，然而，在这项工作中，我们证明了富锂高能 LiNiMnCoO（HENMC）正极材料的降解不仅发生在表面，而且还从其内部结构开始。除了证明循环 HENMC 正极的表面重构和形成正极-电解质界面（CEI）层外，本研究还揭示了从过剩锂的单斜（C2/m）固溶体到常规“层状”（R3̅m）相的不可逆体相转变，伴随着 TM 层在循环过程中完全失去 Li。此外，HENMC 的内部晶粒承受晶格畸变，导致形成“纳米缺陷”畴，这可能限制 Li 在晶粒内部的扩散。更突出的是，发现了以大尖晶石晶粒（Fd3̅m）形式的层状到尖晶石的转变，其直径达数百纳米，并研究了其详细的原子排列。研究结果表明，与其将整体容量衰减归因于表面降解，这些剧烈的体相演变将是富锂正极快速失效的根源的主要降解机制。
