First Atomic-Scale Insight into Degradation in Lithium Iron Phosphate Cathodes by Transmission Electron Microscopy

The capacity-voltage fade phenomenon in lithium iron phosphate (LiFePO₄) lithium ion battery cathodes is not understood. We provide its first atomic-scale description, employing advanced transmission electron microscopy combined with electroanalysis and first-principles simulations. Cycling causes near-surface (∼30 nm) amorphization of the Olivine crystal structure, with isolated amorphous regions also being present deeper in the bulk crystal. Within this amorphous shell, some of the Fe²⁺ is transformed into Fe³⁺. Simulations predict that amorphization significantly impedes ion diffusion in LiFePO₄ and even more severely in FePO₄. The most significant barrier for ion transfer will be in the partially delithiated state due to the presence of FePO₄, resulting in the inability to extract the remaining Li⁺ and the observed capacity fade. The pyrrole coating suppresses the dissolution of Fe and allows for extended retention of the Olivine structure. It also reduces the level of crossover of iron to the metal anode and stabilizes its solid electrolyte interphase, thus also contributing to the half-cell cycling stability.