Environmental Molecular Science Laboratory, Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, Washington 99352, USA.
ACS Nano. 2013 Jan 22;7(1):760-7. doi: 10.1021/nn305065u. Epub 2012 Dec 18.
Pristine Li-rich layered cathodes, such as Li(1.2)Ni(0.2)Mn(0.6)O(2) and Li(1.2)Ni(0.1)Mn(0.525)Co(0.175)O(2), were identified to exist in two different structures: LiMO(2)R3[overline]m and Li(2)MO(3)C2/m phases. Upon 300 cycles of charge/discharge, both phases gradually transform to the spinel structure. The transition from LiMO(2)R3[overline]m to spinel is accomplished through the migration of transition metal ions to the Li site without breaking down the lattice, leading to the formation of mosaic structured spinel grains within the parent particle. In contrast, transition from Li(2)MO(3)C2/m to spinel involves removal of Li(+) and O(2-), which produces large lattice strain and leads to the breakdown of the parent lattice. The newly formed spinel grains show random orientation within the same particle. Cracks and pores were also noticed within some layered nanoparticles after cycling, which is believed to be the consequence of the lattice breakdown and vacancy condensation upon removal of lithium ions. The AlF(3)-coating can partially relieve the spinel formation in the layered structure during cycling, resulting in a slower capacity decay. However, the AlF(3)-coating on the layered structure cannot ultimately stop the spinel formation. The observation of structure transition characteristics discussed in this paper provides direct explanation for the observed gradual capacity loss and poor rate performance of the layered composite. It also provides clues about how to improve the materials structure in order to improve electrochemical performance.
原始富锂层状阴极，如 Li(1.2)Ni(0.2)Mn(0.6)O(2) 和 Li(1.2)Ni(0.1)Mn(0.525)Co(0.175)O(2)，被确定存在于两种不同的结构中：LiMO(2)R3[overline]m 和 Li(2)MO(3)C2/m 相。在 300 次充放电循环后，这两种相逐渐转变为尖晶石结构。从 LiMO(2)R3[overline]m 到尖晶石的转变是通过过渡金属离子迁移到 Li 位而不破坏晶格来完成的，导致在母体颗粒内形成镶嵌结构的尖晶石颗粒。相比之下，从 Li(2)MO(3)C2/m 到尖晶石的转变涉及到 Li(+)和 O(2-)的去除，这会产生大的晶格应变，并导致母体晶格的破坏。新形成的尖晶石颗粒在同一颗粒内显示出随机取向。在循环后，一些层状纳米颗粒内也注意到了裂纹和孔隙，这被认为是由于锂离子的去除导致晶格破坏和空位凝聚的结果。在循环过程中，AlF(3)涂层可以部分缓解层状结构中尖晶石的形成，从而导致容量衰减较慢。然而，AlF(3)涂层并不能最终阻止尖晶石在层状结构中的形成。本文讨论的结构转变特征的观察结果为观察到的层状复合材料逐渐容量损失和较差的倍率性能提供了直接解释。它还提供了关于如何改进材料结构以提高电化学性能的线索。
