Improvement of High-Rate Performance of LiFePO4 Cathode with through-Holed LiFePO4/Activated Carbon Hybrid Electrode Structure Fabricated with a Pico-Second Pulsed Laser

Abstract

In order to improve high-rate performance of lithium iron phosphate (LiFePO4, LFP) cathodes, the LFP and activated carbon (AC) layers were coated on one side and the other side, respectively, of an aluminum (Al) current collector (producing the LFP/AC cathode), and then through-holes with the average diameter of 18-23 mm and opening rate of 0.44-0.64 % were formed in the LFP/AC cathode (producing the through-holed LFP/AC cathode). The cell fabricated with one through-holed LFP/AC cathode and two lithium (Li) metal anodes exhibited a significantly improved high-rate performance in the C-rate region of 50 to 100 C. The through-holed LFP/AC cathode was found to be superior in high-rate performance to a through-holed LFP/LFP cathode, where the LFP layers were coated on the both sides of an Al current collector, and a LFP/AC cathode having no through-holes. From the results obtained, it was found that, in a high-rate discharging process, AC layer is at first discharged because of its higher high-rate performance than the LFP layer and then energy (electrons) is (are) transferred by the difference in the electrode potential between the AC and LFP layers from the charged LFP layer to the discharged AC layer with the transfer of Li+ ions via through-holes from the AC layer to the LFP layer (i.e., the AC layer is charged again) and a series of such processes are repeated (Fig. 1). Forming through-holes in the LFP/AC cathode results in an improvement in its charging/discharging capacity and high-rate performance. Fig. 1 Schematic drawing of charging/discharging processes of the through-holed LFP/AC cathode. (A) Fully discharged condition, (B) fully charged condition, (C) first discharging of AC at high rate, (D) first open-circuit condition and electron and lithium transfer to charge the AC, (E) second discharging of AC at high rate, (F) second open-circuit condition and electron and lithium transfer to charge the AC. Figure 1