The role of phase transformation in the rate performance limited Lix̳ V₂ O₅ battery cathode

Abstract

It has recently been reported that the rate performance of Lix̳ V₂O₅, a widely studied candidate Li-ion battery cathode material, can be significantly improved through a variety of particle size reduction techniques, (e.g. nano templating). It is widely believed that the microscale mechanism responsible for this improvement in rate performance is a reduction in the Li+ diffusion path length. Yet, the experimentally observed discharge performance of Lix̳ V₂ O₅ cathode films comprised of active material particles of varying sizes (between films) and subject to variable rates of discharge deviates sharply from results predicted by Fickian scaling laws. In a crystalline Li+ insertion host the incorporation of ionic volume commensurate with electrochemical discharge often leads to phase transformation. While the consequent phase coexistence is largely responsible for the high energy densities reported in many crystalline insertion hosts, its effect upon rate performance, (or power density), is not well understood. Recently, researchers identified facilitated phase boundary motion as the mechanism responsible for improved high-rate performance in one nanoscaled insertion compound. The preservation of a coherent phase boundary between differentially lithiated, coexistent end-member phases that would normally relax the interfacial strain associated with biphasic volumetric mismatch by forming incoherent phase boundaries, they reasoned, lead to the observed improvement in high-rate performance. A number of discrete structural and electrochemical signatures have subsequently been identified that are believed to correlate with facilitated phase-boundary-motion in nanoscaled insertion hosts. These equilibrium signatures, which include; enhanced Li+ solubility in end-member phases, decreased volumetric mismatch between coexistent end-member phases, increased interfacial strain between coexistent end-member phases, and reduced cycling hysteresis, have been identified in the dimensionally graded Lix̳ V₂ O₅ system, suggesting that rate performance in this system may, in fact, also be gated by sluggish phase boundary motion.

(cont.) Finally, a non-equilibrium experimental technique, (modified GITT), designed to identify regimes in which phase-boundary motion is rate-limiting, is described. This modification entails the suppression of several assumptions governing the conventional application of the GITT technique to kinetic parameter extraction, namely, the use of "small" current pulses and the violation of the implicit monophasic constraint. It is observed that this variable rate-technique can identify regimes of phase boundary motion control in dimensionally graded Lix̳ V₂ O₅. Further, it is proposed that this technique might allow the microscale phenomenology of rate-limiting phase evolution to be modeled.