Improvement of Performance of LiFePO4 Cathode and Graphite Anode By Forming Micrometer-Sized through-Holed Electrode Structures with a Pico-Second Pulsed Laser

Abstract

Holing of lithium iron phosphate (LiFePO4, LFP) cathodes with a pico-second pulsed laser, in which the average hole diameter and hole opening rate were 20-30 mm and 1-2%, respectively, enabled to retain the high-rate discharging performance even in the LFP cathodes composed of the having the LFP layer with the thickness of over 40 mm on an aluminum current collector. The conventional and flat LFP cathode exhibited the degradation of discharge retention at the high-rate discharge because of the low utilization of LFP materials in the case of the thick cathode layer. On the other hand, in the case of “through-holed” and “non-through-holed” LFP cathodes, there can be a more efficient insertion/de-insertion of Li+ ions to/from the LFP materials through the holes formed in the LFP layer, resulting in retaining the high-rate charging/discharging performance even in thick LFP cathodes. The electrochemical impedance spectroscopy analysis confirmed that the formation of through-holes in the thick LFP layer is significantly effective to improve the high-rate discharging performance as a result of the decreased charge-transfer resistance of the LFP discharge process. The decrease in the charge-transfer resistance results from the increase in the area available in the LFP discharge process because the sidewalls of the holes can also take part in the Li+ ion transfer during the discharge process.In addition, in order to actually compensate “an irreversible capacity (i.e., an active lithium loss) ˮ usually observed at the 1st charging/discharging cycle of lithium ion batteries (LIBs), e.g., caused by a solid-electrolyte interphase (SEI) formation, a so-called pre-lithiation was applied to the graphite anodes of the laminated cell composed of three graphite anodes and three LFP (lithium iron phosphate) cathodes which were through-holed with the hole diameter of 20, 100 or 200 mm like the squares of a “Go-board”. The pre-lithiation was carried out (keeping an appropriate pre-lithiation charge amount) firstly by directly contacting the anodes with Li metal, which was previously introduced in the cell, to intercalate Li+ ions to them and then conducting the capacity balancing among the Li+-intercalated anodes by contacting them for a various period of time. It was found that in the cell composed of the through-holed anodes and cathodes with a given hole diameter the irreversible capacity can be significantly reduced by appropriately choosing the pre-lithiation charge amount and the capacity balancing time. Typically, in the anodes (the hole diameter: 20 mm) with the total capacity of 35 mAh, when the pre-lithiation charge amount was 3 mAh and the capacity balancing time was 12 h, no irreversible capacity could be observed actually, i.e., the 1st charge capacity was close to 100% and the expected stable charge/discharge capacities were obtained with the capacity retention of almost 100% in the continuous charge/discharge cycle. The results obtained demonstrate that the pre-lithiation of LIBs can be significantly improved using through-holed anodes and cathodes.