Storage and Effective Migration of Li-Ion for Defected β-LiFePO4 Phase Nanocrystals.

Lithium iron phosphate, a widely used cathode material, crystallizes typically in olivine-type phase, alpha-LiFePO4 (alpha LFP). However, the new phase beta-LiFePO4 (beta LFP), which can be transformed from aLFP under high temperature and pressure, is originally almost electrochemically inactive with no capacity for Li-ion battery, because the Li-ions are stored in the tetrahedral [LiO4] with very high activation barrier for migration and the one-dimensional (1D) migration channels for Li-ion diffusion in alpha LFP disappear, while the Fe ions in the beta-phase are oriented similar to the 1D arrangement instead. In this work, using experimental studies combined with density functional theory calculations, we demonstrate that beta LFP can be activated with creation of effective paths of Li-ion migration by optimized disordering. Thus, the new phase of beta LFP cathode achieved a capacity of 128 mAh g(-1) at a rate of 0.1 C (1C = 170 mA g(-1)) with extraordinary cycling performance that 94.5% of the initial capacity retains after 1000 cycles at 1 C. The activation mechanism can be attributed to that the induced disorder (such as FeLiLiFe antisite defects, crystal distortion, and amorphous domains) creates new lithium migration passages, which free the captive stored lithium atoms and facilitate their intercalation/deintercalation from the cathode. Such materials activated by disorder are promising candidate cathodes for lithium batteries, and the related mechanism of storage and effective migration of Li-ions also provides new clues for future design of disordered-electrode materials with high capacity and high energy density.