Multishelled Ni-Rich Li(NixCoyMnz)O2 Hollow Fibers with Low Cation Mixing as High-Performance Cathode Materials for Li-Ion Batteries

Layered Li(NixCoyMnz)O2 is one of the most promising cathode materials for lithium ion batteries (LIBs), due to its stable structure, compositional flexibility, thermal stability, low cost, and relatively high reversible capacity.1, 2 In particular, the Ni‐rich oxides such as Li(NixCoyMnz)O2 (x ≥ 0.5) have attracted intense research attention,3 since they provide very high specific capacity, ≈212 mAh g−1 for x = 0.8 and 220 mAh g−1 for x = 0.86. However, unlike low Ni content oxides which generally exhibit outstanding stabilities,2 Ni‐rich oxides suffer inherent drawbacks including poor cycle life and rate performance due to low Li diffusion rates caused by cation mixing. This manifests as Ni2+ ions occupying 3b Li sites in the Li slab, whilst Li+ ions also occupy sites in the transition metal (TM) layers. This cation mixing leads to a higher activation energy barrier for Li diffusion because of the smaller separations between the TM layers, and also leads to structural instability during electrochemical charge/discharge cycles.4 Many approaches have been adopted to address the cation mixing and disorder in Ni‐rich materials. Most approaches focus on adjusting the synthesis conditions to reduce cation migration from the TM site to Li site by controlling the lithiation temperature5 or the Li/TM ratio.6 However, in these methods, an excess of Li is necessary to produce highly ordered Ni‐rich oxides. Residual Li remains on the surface of the active materials and reacts with air to form LiOH and Li2CO3, leading to undesirable side reactions with the electrolyte. The undesirable LiOH and Li2CO3 species also impede the diffusion of Li+ ions due to their insulating properties, and thus deteriorate the electrochemical cycle performance.5 In addition, traditional synthesis methods, which usually involve coprecipitation and annealing at high temperature, do not allow controllable synthesis of Ni‐rich nanostructures needed to address the sluggish Li+ ion diffusion.6, 7 Therefore, excellent rate performance and high specific capacity are hard to achieve.