Kinetics of non-equilibrium lithium incorporation in LiFePO4

Lithium-ion batteries are a key technology for multiple clean energy applications. Their energy and power density is largely determined by the cathode materials, which store Li by incorporation into their crystal structure. Most commercialized cathode materials, such as LiCoO2 (ref. 1), LiMn2O4 (ref. 2), Li(Ni; Co; Al)O-2 or Li(Ni; Co; Mn)O-2 (ref. 3), form solid solutions over a large concentration range, with occasional weak first-order transitions as a result of ordering(1) of Li or electronic effects(4). An exception is LiFePO4, which stores Li through a two-phase transformation between FePO4 and LiFePO4 (refs 5-8). Notwithstanding having to overcome extra kinetic barriers, such as nucleation of the second phase and growth through interface motion, the observed rate capability of LiFePO4 has become remarkably high(9-11). In particular, once transport limitations at the electrode level are removed through carbon addition and particle size reduction, the innate rate capability of LiFePO4 is revealed to be very high. We demonstrate that the reason LiFePO4 functions as a cathode at reasonable rate is the availability of a single-phase transformation path at very low overpotential, allowing the system to bypass nucleation and growth of a second phase. The LixFePO4 system is an example where the kinetic transformation path between LiFePO4 and FePO4 is fundamentally different from the path deduced from its equilibrium phase diagram.