Abstract
Li-ion battery cathodes based on LiFePO4 are fabricated by a layer-by-layer spray printing method with a continuous through thickness gradient of active material, conductive carbon, and binder. Compared with cathodes with the more usual homogeneous distribution, but with the same average composition, both C-rate and capacity degradation performance of the graded electrodes are significantly improved. For example at 2C, graded cathodes with an optimized material distribution have 15% and 31% higher discharge capacities than sprayed uniform or conventional slurry cast uniform cathodes, and capacity degradation rates are 40–50% slower than uniform cathodes at 2C. The improved performance of graded electrodes is shown to derive from a lower charge transfer resistance and reduced polarization at high C-rates, which suggests a more spatially homogeneous distribution of over-potential that leads to a thinner solid electrolyte interphase formation during cycling and sustains improved C-rate and long-term cycling performance.
Highlights
Improvements in the power, energy density, and lifetime of lithium ion batteries are critical for their greater penetration of electric vehicle markets [1]
AC indicates that the weight ratio of active material decreases continuously from 100% at the electrode/separator interface to 0% at the electrode/Al interface; while the carbon increases from 0% to 50% (Fig. 2a, green line)
Graded composition cathodes based on LiFePO4 that had a continuous micro-scale variation in the weight ratios of active material:carbon:binder across the electrode thickness were fabricated using a layer-by-layer spray printing technique
Summary
Improvements in the power, energy density, and lifetime of lithium ion batteries are critical for their greater penetration of electric vehicle markets [1]. While much effort continues to be made in searching for new materials, or the chemical modification/decoration of existing materials [2,3], less attention has been paid to radical approaches to electrode engineering that includes more careful control of the electrode structure [4] Research into these structured electrodes has strengthened in recent years, including detailed optimization of the fraction of each electrode constituent, layered electrodes, and optimized and/or directional porosity distributions [5,6,7]. In this paper we use a large area, layer-by-layer spray printing approach [7,23,24,25,26] to fabricate compositionally graded lithium-ion battery cathodes based on LiFePO4 that have a micro-scale continuous gradation through the electrode thickness of active material, carbon electron conductive additive, and polymer binder fraction. All electrodes are fabricated with an identical average composition ratio of active material:carbon:binder of 80:10:10 (wt.%), and had similar thickness, overall porosity, and active material loading per area in order to isolate the effect of material distribution only on electrode performance
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