Electrochemical kinetics of thin film vanadium pentoxide cathodes for lithium batteries

Abstract

Electrochemical experiments were performed to investigate the processing-property-performance relations of thin film vanadium pentoxide cathodes used in lithium batteries. Variations in microstructures were achieved via sputtering and anneal treatments, resulting in films with different morphologies, grain size distributions, and orientations. Key findings included (1) grain size distributions largely did not affect the current rate performance of the cathodes. Rather, the film orientation and the ability to undergo rapid phase transformation were more vital to improving performance; (2) interfacial resistance and ohmic polarization were also dominant at the high current rates used (> 600 [mu]A/cm²) in addition to solid diffusion; and (3) optimization of thin film batteries requires that film thickness be < 500 nm to avoid diminishing returns in power and energy densities. Kinetic parameters including the transfer coefficient ([alpha] = 0.90± 0.05) and standard rate constant (k⁰ [approx.] 2 x 10⁻⁶ cm/s) for vanadium pentoxide films were quantified using slow scan DC cyclic voltammetry and AC cyclic voltammetry. The reaction rate was found to be potentially limiting at moderate to high current rates (> 200 [mu]A/cm²).

(cont.) An analysis of the wide variation in current-rate performance for different V₂0₅ architectures (including composite, nanofiber, and thin film) shows a convergence in results when the area of active material has been factored into the metric. This convergence suggests that either the reaction rate or interfacial resistance is limiting in V₂0₅ as opposed to diffusion.