How to make lithium iron phosphate better: a review exploring classical modification approaches in-depth and proposing future optimization methods
LiFePO4 is still a promising cathode, which is inexpensive, nontoxic, environmentally benign, and most importantly safe. However, LiFePO4 suffers from low conductivity and sluggish diffusion of lithium ions. Surface decoration, nanocrystallization and lattice substitution (doping) are modification approaches widely employed to promote the conductivity of electrons and the diffusion of lithium ions in the crystal lattices of LiFePO4. This review focuses on discussing the functional mechanisms of these optimization methods from the extent of electron and lithium ion migration and the features of LiFePO4, namely, its structure and phase transformation reactions. At the interface of LiFePO4 and the electrolyte, decoration layers not only ensure the stability of LiFePO4 by excluding HF corrosion and surface degradation, but also reduce charge transfer resistances for the surface reactions with fast lithium ions and electrons. When it comes to the lattices of LiFePO4, nanocrystallization unblocks the diffusion path, as well as shortens the diffusion length of lithium ions. Decoration layers in the inner surface avoid slowing down the diffusion of lithium ions in the lattices throughout the reactions and maximize the utilization of LiFePO4. Lattice substitutions, which increase the electronic conductivity by decreasing the band gap, interrupt the major advantage of LiFePO4, the structural stability, which guarantees the safety as well as the cycling and rate performances. To make the electrochemical performance of LiFePO4 better and overcome the contradiction about the miscibility gaps, -oriented LiFePO4 nanoflakes/nanomeshes/nanoplates, -oriented or -oriented nanorod/nanowire structures and nanowires/nanorods/nanotubes with a carbon/LiFePO4/carbon coaxial structure (graphically shown in the text) can be developed in the future.