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Laser cutting of prelithiated graphite-silicon anodes

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Prelithiation of electrodes is a new process step to avoid the nearly 20% lithium loss during the SEI formation. A thin lithium layer is applied to the electrode surface as a top coating via a physical vapor deposition process. This sacrificial lithium layer is designed to reduce the lithium loss during the first formation cycle of the SEI. However, this change in the electrode composition influences every subsequent processing step. One processing step that is particularly impeded is the separation of the electrodes.
Due to the fact that lithium has a strong adhesive character, a conventional separation process- such as a punching process – is not suitable since lithium residues adhere to the cutting tool leading to its deterioration. An alternative solution to separate the electrodes is the use of laser cutting as a contactless process. Nevertheless, the thermal energy input of the laser leads to a heat influence on the electrode. This energy input could cause the applied lithium layer to react with the gas components of the ambient atmosphere or to be directly ablated, thus losing its sacrificial benefit.
In this research, the processing of prelithiated electrodes as well as the influence of the laser cutting on prelithiated electrodes was investigated. Graphite-Silicon anodes were coated with different lithium layer thicknesses and then cut by a ND:YAG laser. The influence of varying laser parameters in terms of frequency and cutting velocity was investigated as the cutting quality was examined by measuring the length of the heat affected zone. Finally, the influence of the cutting edges on the electrochemical performance was investigated.
The mass of the applied lithium layer shows a dependence on the used ampere currents as well as the coating time. With increasing ampere currents and longer coating times, the lithium masses deposited on the electrode increase significantly. Thus, the resulting prelithiation reaches a higher level of up to 38%. The subsequent cutting process revealed that the heat affected zone is strongly dependent on the prelithiation level and less on the pulse repetion frequency or cutting velocity, respectively. The size of the heat affected zone is within a satisfactory range of 200-400 µm. For the prelithiation level between 10-17%, however, an increase of the range can be observed, which is probably due to a deposited lithium layer thickness of 2 µm and therefore a change in laser energy coupling behavior. For the electrochemical performance can be stated, that the investigated prelithiation levels lead to lower initial capacities (7-15%). For the prelithiation level of 16%, the fading of capacities is stronger for the laser cut electrodes than for the mechanical separated electrodes. For the prelithiation level of 38%, the capacities seem to be in a similar range, concerning the standard deviation.
Overall the results suggest that laser cutting as an alternative compared to a mechanical separation process can be used to cut prelithiated electrodes. However, a loss in capacity can occur. This loss may seems to be linked to the prelithierung level. Further investigations of higher prelithiation levels and therefore the change in the laser material interaction are necessary, as significant increases in capacity can be achieved. Additionally, cutting experiments with a variation of pulse width modulation in the range of picoseconds have to be conducted in order to achieve a cold ablation and therefore a reduction of thermal stress.

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