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Mapping the Inhomogeneous Electrochemical Reaction Through Porous LiFePO4 Electrodes in a Standard Coin Cell Battery

DOI: 10.1021/cm504317a DOI Help

Authors: Fiona Strobridge (University of Cambridge) , Bernardo Orvananos (University of Michigan) , Mark Croft (Rutgers University) , Hui-chia Yu (University of Michigan) , Rosa Robert (University of Cambridge) , Hao Liu (University of Cambridge) , Zhong Zhong (Brookhaven National Laboratory) , Thomas Connolley (Diamond Light Source) , Michael Drakopoulos (Diamond Light Source) , Katsuyo Thornton (University of Michigan) , Clare P. Grey (University of Cambridge)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Chemistry of Materials , VOL 27 (7) , PAGES 2374 - 2386

State: Published (Approved)
Published: April 2015
Diamond Proposal Number(s): 8385

Open Access Open Access

Abstract: Nanosized, carbon-coated LiFePO4 (LFP) is a promising cathode for Li-ion batteries. However, nano-particles are problematic for electrode design, optimized electrodes requiring high tap densities, good electronic wiring, and a low tortuosity for efficient Li diffusion in the electrolyte in between the solid particles, conditions that are difficult to achieve simultaneously. Using in situ energy-dispersive X-ray diffraction, we map the evolution of the inhomogeneous electrochemical reaction in LFP-electrodes. On the first cycle, the dynamics are limited by Li diffusion in the electrolyte at a cycle rate of C/7. On the second cycle, there appear to be two rate-limiting processes: Li diffusion in the electrolyte and electronic conductivity through the electrode. Three-dimensional modeling based on porous electrode theory shows that this change in dynamics can be reproduced by reducing the electronic conductivity of the composite electrode by a factor of 8 compared to the first cycle. The poorer electronic wiring could result from the expansion and contraction of the particles upon cycling and/or the formation of a solid-electrolyte interphase layer. A lag was also observed perpendicular to the direction of the current: the LFP particles at the edges of the cathode reacted preferentially to those in the middle, owing to the closer proximity to the electrolyte source. Simulations show that, at low charge rates, the reaction becomes more uniformly distributed across the electrode as the porosity or the width of the particle-size distribution is increased. However, at higher rates, the reaction becomes less uniform and independent of the particle-size distribution.

Subject Areas: Chemistry, Energy

Instruments: I12-JEEP: Joint Engineering, Environmental and Processing