Direct evidence for the influence of lithium ion vacancies on polaron transport in nanoscale LiFePO4

Improving the electronic conductivity in lithium-based compounds can considerably impact the design of rechargeable batteries. Here, we explore the influence of lithium ion vacancies on the electronic conductivity of LiFePO4, an active cathode material, by varying the crystallite sizes. We find that about 17% lithium ion vacancy concentration leads to an enhancement in electronic conductivity of about two orders of magnitude at 313 K with respect to our initial crystallite size. We attribute the enhanced electronic conductivity to the lithium ion vacancy concentration in addition to the reduced hopping length. The lithium ion vacancies are the source of polarons in LiFePO4, which increases with decreasing crystallite size due to the surface energy kinetics. The substantial increase in the polaronic sites (Fe3+) at a lower crystallite size leads to a reduction in lattice parameters including the unit cell volume. The analysis of temperature dependent dc conductivity within the framework of the Mott model of polaron conduction enables us to quantify the various physical parameters associated with polaron hopping in LiFePO4.