Improving the electrochemical cyclability of lithium manganese orthosilicate through the pillaring effects of gradient Na substitution

Lithium manganese orthosilicate is an attractive cathode material providing high theoretical specific capacity (ca. 330 mAhg(-1)) and reasonably high potential; however, it suffers from rapid capacity/voltage decay upon cycling. The origin of the poor cyclability is closely related to the structural instability of Li2MnSiO4 polymorphs, including layer exfoliation and pulverization during extended cycling. To address these problems, a gradient Na substitution method was developed to prepare Li2MnSiO4 cathode materials with a Na+-enriched surface pillaring layer and a moderately Na+-substituted core material. The results shows that the pillaring layer can effectively suppress the occurrence of layer exfoliation/collapse during delithiation/lithiation and prevent particle pulverization upon extended cycling. This corresponds to a high initial Coulombic efficiency (89.8%) and improved cyclability with a capacity retention (81.3%) after 200 cycles in Na-substituted materials. The gradient Na substitution process also results in improved Li+ diffusivity and rate performance in Na-substituted materials by shortening Li-Li distances. This gradient Na-doping method can be further applied to other structure-unstable polyanion-type cathode materials, such as phosphates, fluorophosphates and borates.(C) 2017 Elsevier B.V. All rights reserved.