An Improved High-Rate Discharging Performance of “Unbalanced” LiFePO4 Cathodes with Different LiFePO4 Loadingsby a Grid-Patterned Micrometer Size-Holed Electrode Structuring

The degradation of charging/discharging capacities in the rate-performance test of lithium iron phosphate (LFP) cathodes with different loading amounts of an active material on both sides of a current collector (i.e., "unbalanced" LFP/LFP cathodes) in a laminated cell (typically composed of anode/separator/unbalanced cathodes/separator/anode) was not observed actually at low C-rates (e.g., 0.1 C). However, the rate-performance data obtained at high C-rates (e.g., > 5 C) indicated that the imbalance of the loading amounts of an active cathode material on both sides of an Al current collector causes a significant capacity degradation. We have found that it is possible to prevent the capacity degradation observed at high C-rates by holing the unbalanced LFP/LFP cathodes in a micrometer-sized grid-patterned way (the percentages of the holed area are typically several %) using a pico-second pulsed laser: The non-holed unbalanced LFP/LFP cathodes exhibited a considerable capacity degradation at C-rates which are, for example, larger than 5C, while the holed ones showed no degradation in capacity even at high C-rates (e.g., 5-20 C). Forming micrometer-sized grid-patterned holes in the LFP/LFP cathodes leads to an improved capacity and high-rate performance of their charging/discharging processes. The improvement in the discharging performance at the holed cathodes at high C-rates can be considered to result from a facilitated transfer of Li+ ions in the discharging of the thick layer as well as the thin one through both the surface and the sidewalls (produced by forming the holes) of the LFP layer (or the LFP/current collector layers). In order to keep a high discharge capacity at high C-rates even in thick LFP layers, resulting in an improvement of the battery performance of LIBs, the optimization of the diameter of holes formed and the percentage of holed areas (i.e., opening rate) is examined now.Fig. 1 Schematic description of high-rate discharging processes on (A) non-holed, (B) through-holed and (C) non-through-holed LFP/LFP cathodes with loading ratios of 3 at low- and high-rate discharging (e.g., at 0.1 and 10 C).Figure 1