Defect-Enhanced Phase Transformation Kinetics in Intercalation Compounds
Authors: Yang Kaiqi , Hong Liang , Tang Ming 
Published: February 2020

Point, line and/or planar defects are ubiquitously present in all materials and intentional introduction and control of defects plays a key role in the development of advanced materials with better performance and new functionality. Well-known examples include doping semiconductors to modify the band structure and using phase or grain boundaries to strengthen alloys. Like other materials, battery intercalation compounds contain various types of defects, and “defect engineering” is a promising strategy for this class of materials, which nonetheless has not yet been widely explored. In particular, recent studies find that antisite defects in battery compounds, can promote Li transport by opening up alternative diffusion channels with lower migration energies in numerous lithium-ion battery electrode materials, which enhances the rate performance of batteries. Antisite defects are also reported to improve the stability and cyclability of cubic LixTi2O4. These examples demonstrate that the rational tailoring of antisite defects provides a potentially general approach to improving battery electrode properties.
Here 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. Antisite defects are generated when the sites of intercalating ions are occupied by other cations that are usually less mobile. A prominent effect of antisite defects in battery electrodes is to block the existing paths of intercalating ions and generate new migration pathways at the same time. For intercalation compounds with strong anisotropic transport properties, such effect usually leads to the reduction in the diffusion anisotropy of intercalating ions, which has already been observed in practically synthesized lithium iron phosphate olivine (LiFePO4).
In this work, we discovered that although antisite defects in LiFePO4 impede Li movement in the fast [010] diffusion direction, their ability to enhance transport along other slower diffusion directions leads to an unexpected increase in the phase transition rate when it is kinetically limited by surface reaction. Figure 1. compares the discharge simulations of two LiFePO4 particles with different antisite defect concentrations. Orders-of-magnitude improvement in SRL phase transformation kinetics is observed in “defect-rich” particle due to the increase of active surface area for Li intercalation. More transparent understanding of such mechanism was obtained by solving 1D depth-average model numerically and analytically. Meanwhile, it is revealed that the presence of defects causes Li intercalation time to increase less pronouncedly with particle sizes, and can reduce the risk of electrochemical shock under galvanostatic (dis)charge conditions by homogenizing the reaction flux. Due to the kinetic competition between surface reaction and bulk diffusion, an optimal defect concentration is predicted to exist for a given particle geometry to maximize the Li (de)intercalation rate. Criteria for the co-design of defect content and particle morphology are proposed. Counter-intuitively, we find that (100)-oriented LiFePO4 platelike particles may exhibit even better rate performance than (010)-oriented plate particles in the SRL kinetic regime, which are commonly viewed as the most desirable LiFePO4 particle morphology. While we demonstrate the possibility of using antisite defects to accelerate phase transitions in LiFePO4, it has general applicability to other phase-changing battery materials that exhibit ion diffusion anisotropy.

Figure 1