Ni/Mg Dual Concentration-Gradient Surface Modification to Enhance Structural Stability and Electrochemical Performance of Li-Rich Layered Oxides

Abstract

Li-rich layered oxides (LRLOs), with the advantages of high specific capacity and low cost, are considered as candidates for the next-generation cathode of lithium-ion batteries (LIBs). Unfortunately, sluggish kinetics and interfacial degradation lead to capacity loss and voltage decay of the material during cycling. To address these issues, we propose a Ni/Mg dual concentration-gradient modification strategy for LRLOs. From the center to the surface of the modified materials, the contents of Ni and Mg are gradually increased while the content of Mn is decreased. The high Ni content on the surface increases the proportion of cationic redox, elevating the operating voltage and accelerating reaction kinetics. Moreover, the doped Mg on the surface of the material acting as a stabilizing pillar suppresses the migration of transition metals, stabilizing the layered structure. Therefore, the material with the Ni/Mg dual concentration-gradients delivers a superior electrochemical performance, exhibiting a suppressed voltage decay of 2.8 mV per cycle during 200 cycles (1 C, 2−4.8 V) and an excellent rate capability of 94.84 mAh/g at 7C. This study demonstrates a synergic design to construct high-performance LRLO cathode materials for LIBs.

Modification Strategies of High-Energy Li-Rich Mn-Based Cathodes for Li-Ion Batteries: A Review

“…Yu et al [43] employed atomic resolution STEM to provide convincing evidence for the coexistence of a rhombic LiMO 2 structure and a monoclinic Li 2 MnO 3 structure in the Li 1.2 Mn 0.567 Ni 0.166 Co 0.067 O 2 material. More precisely, the microregions containing these two phases are separated by the (001)rh/(103)mon plane along the [001]rh/ [103]mon direction, proving the local structural variations of the LRMO cathode material, which is not simply a single solid solution (Figure 2c). phase and Li2MnO3 phase.…”

“…Yu et al [43] employed atomic resolution STEM to provide convincing evidence for the coexistence of a rhombic LiMO2 structure and a monoclinic Li2MnO3 structure in the Li1.2Mn0.567Ni0.166Co0.067O2 material. More precisely, the microregions containing these two phases are separated by the (001)rh/(103)mon plane along the [001]rh/ [103]mon direction, proving the local structural variations of the LRMO cathode material, which is not simply a single solid solution (Figure 2c). In addition to the similarity and complexity of the structures of the two phases, LiTMO2 and Li2MnO3, the factors that make it difficult to determine the crystal structure of LRMO are also related to the different material compositions and synthesis conditions, such as the lithium content, calcination conditions, and cooling rate [8,44].…”

“…Therefore, the structural stability of Cr-doped Li 1.2 Ni 0.2 Mn 0.6 O 2 at high delithiation is significantly enhanced, resulting in longer cycle life. Cong et al [ 103 ] used a Ni/Mg double concentration-gradient-repair doping strategy, where the Ni/Mg content gradually decreases from the surface to the center of the material and the content of Mn gradually increases. The high Ni on the surface increased the ratio of cationic redox, increased the working voltage, and accelerated the reaction kinetics.…”