Abstract
Antisite defects are a type of point defect ubiquitously present in intercalation compounds for energy storage applications. While they are often considered a deleterious feature, here we elucidate a mechanism of antisite defects enhancing lithium intercalation kinetics in LiFePO4 by accelerating the FePO4 → LiFePO4 phase transformation. Although FeLi antisites block Li movement along the [010] migration channels in LiFePO4, phase-field modeling reveals that their ability to enhance Li diffusion in other directions significantly increases the active surface area for Li intercalation in the surface-reaction-limited kinetic regime, which results in order-of-magnitude improvement in the phase transformation rate compared to defect-free particles. Antisite defects also promote a more uniform reaction flux on (010) surface and prevent the formation of current hotspots under galvanostatic (dis)charging conditions. We analyze the scaling relation between the phase boundary speed, Li diffusivity and particle dimensions and derive the criteria for the co-optimization of defect content and particle geometry. A surprising prediction is that (100)-oriented LiFePO4 plates could potentially deliver better performance than (010)-oriented plates when the Li intercalation process is surface-reaction-limited. Our work suggests tailoring antisite defects as a general strategy to improve the rate performance of phase-changing battery compounds with strong diffusion anisotropy.
Highlights
Point, line and/or planar defects are ubiquitously present in all materials and frequently have beneficial effects on material properties
Driven first-order phase transformations in battery electrode materials upon ionintercalation are subject to the kinetic control of various rate-limiting steps
We previously show9 that the competition between Li diffusion and surface reaction can give rise to three distinct phase transformation modes in LiFePO4, i.e. bulk-diffusion-limited (BDL), SRL and an intermediate hybrid mode, in which phase boundary migration is BDL or SRL in different directions
Summary
Line and/or planar defects are ubiquitously present in all materials and frequently have beneficial effects on material properties. Recent studies find that antisite defects, which are common in battery compounds, can promote Li transport and enhance rate performance by opening up alternative diffusion channels with lowered migration energies in numerous lithium-ion battery electrode materials. Such phenomena have been reported in Li1.211Mo0.467Cr0.3O2,1 Li2(Mn, Fe)P2O7,2 α-LiMn1−xFexPO4 We present a computational study that reveals a new mechanism of antisite defects enhancing the rate capability of intercalation compounds by accelerating surface-reaction-limited (SRL) phase transformation during battery charge/discharge. While we demonstrate the possibility of using antisite defects to accelerate phase transitions in LiFePO4, it may have general applicability to other phase-changing battery materials that exhibit ion diffusion anisotropy
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