due to heat conduction, the temperature of the surrounding composite material increases. The binder material for tape-cast electrodes (~5 wt%) is PVDF which has a low decomposition temperature in the range of 250–350° C [5]
. Therefore, the PVDF
binder matrix spontaneously evaporates and active particles are removed from the laser beam interaction zone.
With ns-laser radiation, structure widths of about 40-55 µm can be achieved (Fig. 5a & 5b). The current collector for cathodes are made of aluminium with a thickness of 20 µm and for
anodes they consist of copper with a thickness of 10 µm. Laser structuring with a ns-laser can be realized without damage of the current collector (Fig. 5a). Laser structuring can be realized even for double-side coated aluminium substrates, which is a required processing step for process up-scale for manufacturing of lithium-ion cells with high capacities [4]
.
Nanosecond laser ablation is not appropriate for each type of electrode material. For example, ns laser structuring of LFP electrodes always leads to melt formation and therefore to an undesired modification of the active material. Furthermore, the ablation efficiency of LFP increases by a factor of 3 by using femto- or pico-second laser ablation in comparison to ns-laser ablation [6]
. Another aspect is the loss of active material due
to the ablation process. For the application of structured foils in batteries, it is important to reduce the amount of ablated material which in turn means that small capillary widths and high aspect ratios are preferred. By using ultrafast laser ablation it could be shown that the aspect ratio could be significantly increased (Fig. 5c & 5d) and that the loss of active material can be reduced from 20 percent down to values below 5 percent [7]
.
Figure 5. Capillary structures in NMC electrodes. Cross section and SEM top view of ns- (a & b) and fs- (c & d) laser structured NMC (pitch of capillary structures: 200 µm, pulse lengths: 200 ns, 350 fs)
Capacity retention and cell life-time can be illustrated by plotting the cell voltage as function of discharge capacity for different cycle numbers. For the lithium-ion cell with the structured NMC electrode, the 80 percent capacity limit of the initial discharge capacity is reached after 2290 cycles (Fig. 6). While the cell life-time for the lithium-ion cell with unstructured electrodes is reached after 141 cycles. Furthermore, the discharge capacity of the cell with the laser-structured NMC electrode reaches a value of 108 mAh/g after 2290 cycles indicating that efficient liquid electrolyte transport due to micro capillary structures
Figure 6. Cell voltage versus discharge capacity for pouch cells with laser-structured (right) and unstructured (left) NMC electrodes and without storage [4] 14 LIATODAY FOCUS: LASERS IN MANUFACTURING MARCH/APRIL 2016
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