To improve the battery performance of lithium-ion batteries (LIBs), modifying the anodes and cathodes of LIBs using laser beams to prepare through-holes, nonthrough-holes or ditches in the arrangements of grid and line patterns has been proposed by many researchers and engineers. In this study, a laser system with a roll-to-roll machine, which realizes this modification without large changes in the present mass-production system, was developed. The laser system apparatus comprises a roll-to-roll system in which a hollow cylindrical body with openings on a circumferential surface is the main part and laser equipment is attached. A foil of thin electrode for LIBs is wound around the cylindrical body in the longitudinal direction of the thin electrode. The pulsed beam reflected from a central axis of the cylindrical body can continuously open a large number of through-holes in the thin electrode. The through-holes were formed at a rate of 100000 holes per second on the lithium iron phosphate (LiFePO4, LFP) cathodes and graphite anodes used in this study. The through-holed cathode and anode prepared with this system exhibited higher C-rate performance than nonholed (nontreated) anodes and cathodes.
To move the performance of lithium-ion batteries into the next stage, the modification of the structure of cells is the only choice except for the development of materials exhibiting higher performance. In this review paper, the employment of through-holing structures of anodes and cathodes prepared with a picosecond pulsed laser has been proposed. The laser system and the structure for improving the battery performance were introduced. The performance of laminated cells constructed with through-holed anodes and cathodes was reviewed from the viewpoints of the improvement of high-rate performance and energy density, removal of unbalanced capacities on both sides of the current collector, even greater high-rate performance by hybridizing cathode materials and removal of irreversible capacity. In conclusion, the points that should be examined and the problem for the through-holed structure to be in practical use are summarized.
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 μm like the squares of a “Go-board” (Fig. 1). 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 (Fig. 2). 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 μm) 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.
Figure 1
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