Degraded charge / discharge capacity cells in speed performance tests of lithium iron phosphate (LFP) positive electrodes with different loads of active material on both sides of the stacked current collectors were not actually observed at low C rates. did. However, rate performance data obtained at high C rates showed that imbalances in the loading of active positive electrode materials on both sides of the Al current collector caused significant volume reduction. It was found that using picoseconds to laser drill the positive electrode with a micrometer-sized grid pattern can prevent the capacitance loss observed at high C rates. Wired Co., Ltd.