b-Axis Phase Boundary Movement Induced (020) Plane Cracking in LiFePO4

Phase boundary movement accomplishing reversible LiFePO4/FePO4 biphasic transition is a fundamental Li-ion intercalation/deintercalation mechanism for LiFePO4 cathode. Phase boundary energetically favors crack nucleation and propagation; thus, postmortem observation on cracks becomes a feasible approach to investigate the phase-transition behavior and the Li-ion diffusion mechanism. The previously observed (200) plane cracks facilitate the "domino" diffusion model. Herein, our microscopic observations reveal another type of cracks along the (020) planes in a commercial LiFePO4 cathode cycled at moderate rates (0.1C, 0.33C, and 1C). Such (020) plane cracks are more detrimental to electrochemical performance because they can cut off the Li-ion diffusion pathway, causing inactive segments of LiFePO4. The (020) plane cracks indicate the LiFePO4/FePO4 phase boundary is along the (020) plane and moving along the b-axis during battery operation, which is a typical bulk diffusion-limited Li-ion diffusion behavior. Our observations stress that large LiFePO4 primary particle (>200 nm) not only aggravates cracking degradation but also switches the Li-ion diffusion mode to a slow bulk diffusion mechanism, plunging the overall battery performance.