Mechanochemically tuned structural annealing: a new pathway to enhancing Li-ion intercalation activity in nanosized βII Li2FeSiO4

Abstract

In this work, a relatively inactive cathode material, β_II orthorhombic Li₂FeSiO₄ with the Pmn2₁ space group, is shown to undergo crystal structural changes when subjected to nanosizing by high-energy milling that have a pronounced activation impact on Li-ion intercalation electrochemistry. In particular, we present evidence of mechanically induced structural annealing including lattice expansion, reduction of anti-site defects and preferred crystal orientation, opposite of what expected typically from nanosizing. As a result of this type of unreported structural activation for orthosilicates, a threefold increase of the accessible discharge (=intercalation) capacity of the high-energy milled Pmn2₁ material is attained, much higher than what would be expected solely from surface area increase due to nanosizing and surface area increase. Such mechanochemically induced structural annealing activates Li-ion diffusion (one order of magnitude enhancement in D_Li) into less accessible sites via the combined effects of anti-site defect removal and modulation of the Fe–O coordination adjustment. Structural activation of intercalation materials via controlled mechanochemistry opens new possibilities in nanoengineering high energy density Li-ion storage electrodes.