Revealing the voltage decay of LiMn0.7Fe0.3PO4 cathodes over cycling

Mn-rich olivine phosphates (LiMnxFe1−xPO4, x>0.6) are promising cathode candidates for Li-ion batteries (LIBs) with high energy density and low cost. However, it suffers voltage decay which causes severe energy loss and is considered as a crucial barrier on the way to application. Here, using LiMn0.7Fe0.3PO4 as a model, we reveal that the increasing non-synchronous reaction among particles is an important cause for the voltage decay. Specifically, during lithiation, the inhomogeneity between particles is autoaccelerating and accumulating with cycling. While the inhomogeneity between particles may be overwhelmed by fast kinetics during delithiation, causing little change in the charge curves. The deteriorated kinetic of Li+ diffusion in the Mn2+↔Mn3+ plateau postpones the response of Mn0.7Fe0.3PO4 (MFP) phase and causes a different evolution of heterogeneity during lithium-ion insertion/extraction. In addition, by applying an electrode additive to enhance electron transfer to each LiMn0.7Fe0.3PO4 particle, the voltage decay and energy decay of the LiMn0.7Fe0.3PO4 electrode can be greatly suppressed. This study pinpoints a new insight into the degradation of LiMn0.7Fe0.3PO4 electrode, and discloses effective designs of electrode architecture to achieve long-life batteries.