Helmholtz Institute Ulm for Electrochemical Energy Storage (HIU) , Helmholtzstr. 11 , 89081 Ulm , Germany.
Institute of Theoretical Chemistry , Ulm University , 89069 Ulm , Germany.
ACS Appl Mater Interfaces. 2019 Oct 30;11(43):39848-39858. doi: 10.1021/acsami.9b12566. Epub 2019 Oct 17.
Disordered rock-salt compounds are becoming increasingly important due to their potential as high-capacity positive electrode materials for lithium-ion batteries. Thereby, a significant number of studies have focused on increasing the accessible Li capacity, but studies to manipulate the electrochemical potential are limited. This work explores the effect of transition-metal substitution on the electrochemistry of ternary disordered rock-salt-type compounds with LiMVO stoichiometry (M = Mn, Fe, Co) directly synthesized through mechanochemistry. Rietveld refinements of synchrotron X-ray diffraction patterns confirm the disordered rock-salt structures. First-principles density functional theory study is used to predict the impact of the cation substitution on the expected average voltage and the electronic structures of these materials are used to analyze the underlying redox processes. For LiMVO (M = Mn, Fe, Co), discharge voltages increase in the order of Mn < Fe < Co with 2.28, 2.41, and 2.51 V, exhibiting discharge capacities of 219, 207, and 234 mAh g, respectively. In comparison, for the disordered rock-salt LiVO an average discharge voltage of ∼2.2 V with V redox couple has been reported. However, detrimental electrode-electrolyte interactions manifested as transition-metal dissolution has been found to result in severe capacity fading. Thereto, the use of a concentrated 5.5 M LiFSI increased the cycling stability significantly, effectively reducing transition-metal dissolution. The underlying reasons for the capacity fading of disordered rock salts are yet unclear. We stress the importance of cathode-electrolyte interactions, thus opening new directions for the improvement of cation-disordered materials.
无序岩盐化合物因其作为锂离子电池高容量正极材料的潜力而变得越来越重要。因此，大量的研究集中在提高可用的 Li 容量上，但操纵电化学势的研究有限。这项工作通过机械化学直接合成具有 LiMVO 化学计量比（M = Mn、Fe、Co）的三元无序岩盐型化合物，探索了过渡金属取代对其电化学性能的影响。同步加速器 X 射线衍射图谱的 Rietveld 精修证实了无序岩盐结构。第一性原理密度泛函理论研究用于预测阳离子取代对预期平均电压的影响，并用于分析这些材料的电子结构，以分析潜在的氧化还原过程。对于 LiMVO（M = Mn、Fe、Co），放电电压顺序为 Mn < Fe < Co，分别为 2.28、2.41 和 2.51 V，放电容量分别为 219、207 和 234 mAh g。相比之下，对于无序岩盐 LiVO，已经报道了具有 V 氧化还原对的平均放电电压约为 2.2 V。然而，有害的电极-电解质相互作用表现为过渡金属溶解，导致严重的容量衰减。因此，使用浓度为 5.5 M 的 LiFSI 可显著提高循环稳定性，有效减少过渡金属溶解。无序岩盐容量衰减的根本原因尚不清楚。我们强调了阴极-电解质相互作用的重要性，从而为改善阳离子无序材料开辟了新的方向。
