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Figure 42: Comparison of a symmetric cell with and without interlayer. Plating/stripping experiments at 60 °C. For the experiment 100 µA/cm² were applied for 2 h in each direction.
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work, they will be discussed below. It is pointed out that all specific capacities are given with respect to the mass of the CAM.
Figure 43: Comparison of the discharge capacities of ASSBs with interlayers with different thicknesses. As precise thicknesses for short deposition times cannot be given, the deposition time of P3N5 is given instead. The batteries with interlayer have a smaller discharge capacity than the battery without interlayer. The interlayer decreases the capacity fading during the first 40 charge/discharge cycles. A dependence of the capacity retention on the interlayer thickness cannot be seen.
The batteries consist of an NCM611:LPS composite cathode with a mass ratio of 70:30, LPS solid electrolyte and a lithium metal anode. Battery cycling was performed between an upper cutoff potential of 4.3 V vs. Li+/Li and a lower cutoff potential of 2.6 V vs. Li+/Li by applying a constant current of ± 140 µA (200 µA/cm²). The battery without interlayer between electrolyte and anode has a specific discharge capacity of 110 mAh/g (Figure 43). The discharge capacity decreases strongly during the first 20 cycles. The capacity loss is around 50 mAh/g or 45 % of the initial discharge capacity. The capacity fading continues but slows down after 40 cycles. At that time the discharge capacity is only 40 mAh/g. The difference in the degradation rate is probably caused by different degradation mechanisms. During the first charge/discharge cycles a reaction between both electrodes and the electrolyte takes place, leading to the formation of an SEI at both electrode|electrolyte interfaces. This SEI is passivating the interface but increasing the interface resistance, leading to an impeded transport of lithium ions, which reduces the capacity. A second contribution to the capacity
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loss, is the loss of CAM. Upon subsequent cycling, the volume change of the CAM leads to a contact loss between the CAM and the electrolyte and less CAM can be addressed. The volume change and contact loss will also occur during the first 20 cycles but its effect is superimposed by the SEI formation.
The batteries with interlayer have a lower initial discharge capacity of only 80 mAh/g to 90 mAh/g. The smaller discharge capacity may be due to overpotentials caused by the modified interface. In the impedance spectra of the assembled cells before cycling, a higher interfacial resistance can be seen than in the case of the cells without interlayer (cf. Appendix I). The cause may be the interlayer that need to be converted into Li3N and Li3P in the first couple of cycles before providing a protective effect and a decreased interfacial resistance. A smaller cathode utilization can be excluded as the composite cathode is not in contact with the interlayer.
Although the initial capacity of the batteries with interlayer is smaller than that of the batteries with unmodified interface, the capacity loss is also smaller. The discharge capacity decreases with prolonged cycling of the battery but it decreases steadily. After 40 cycles, the slope of the capacity fading is the same for the batteries without interlayer and batteries with interlayer, indicating that after 40 cycles the capacity fading is not caused by the formation of an SEI but by the degradation of the composite cathode.
The coulombic efficiency – the ratio between the electric charge that is retrieved during the discharge step and the charge that is needed for the charging step – is in the range of 98 % – 99 % for the batteries without interlayer but also shows some fluctuations (Figure 44). For the batteries with interlayer, the coulombic efficiency is also in the range of 99 % but slightly higher. Although the difference is only 99.8 % for the cells with interlayer compared to 99.2 % for the cells without interlayer, the energy loss during each cycle decreases by almost an order of magnitude. In addition, the cells with interlayer reach such a high coulombic efficiency after around 10 cycles, whereas the cells without interlayer need more than 40 cycles to reach a coulombic efficiency of > 99 %. After 40 cycles, the coulombic efficiencies of the cells with and without interlayer are virtually the same. The interlayer reduces the loss of charge carriers during the first 40 battery cycles. We suggest that the increased coulombic efficiency is due to the prevention of the electrolyte decomposition in contact with lithium metal. This observation indicates that the drastic capacity loss during
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the first 60 charge/discharge cycles is caused by the interphase formation between the anode and the electrolyte whereas the capacity loss during subsequent cycling is only due to mechanical issues of the cathode side.
Figure 44: Coulombic efficiency of an ASSB with P3N5 interlayer (blue) and without interlayer (yellow). Batteries with an interlayer exhibit a higher coulombic efficiency during the first 60 charge/discharge cycles. After 60 cycles the efficiencies are similar.
The variation of the thickness of the P3N5 interlayer does not have a significant influence on the performance of the battery. The variation of the properties of batteries prepared with the same material under the same conditions (around 10 cells) is just as big as the difference in the discharge capacity of batteries with different interlayer thicknesses. The reasons for these differences are plentifold: They can originate from differences of the particle distributions in the composite cathode after manual mixing and grinding of the powders, from the size of the CAM primary and secondary particles, from differences in the glovebox atmosphere during cell fabrication, especially during the handling of lithium metal, and due to differences of the pressure that is applied during cycling. This pressure changes during cycling; during each cycle the pressure changes according to the volume change of the CAM and the lithium anode and it also changes when during subsequent cycling the utilization of the CAM decreases. Aging of the cell housing and frames may also alter the battery performance. An aged housing may not be fully gastight anymore and may cause a reaction between the battery materials and the atmosphere. The surface of the stamps, which are used
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to press the powders, influences the roughness of the electrolyte surface and can also cause different current densities during cycling. As can be seen in Figure 45, these differences in the discharge capacity for cells that were prepared the same day from the same powder can already be as big as 15 mAh/g after 30 cycles (dark blue and light blue data). For a profound evaluation of the influence of a protective layer on the battery properties, a standardized cell manufacturing procedure and a statistic evaluation with an increased number of cells is necessary. A statistical approach is of outmost importance because also the preparation of the interlayer can lead to varying results depending on the state of the deposition chamber.
Humidity in the chamber atmosphere can cause impurities in the interlayer.
Figure 45: Discharge capacities of batteries with and without interlayer. The initial discharge capacity of a battery without interlayer is around 20 mAh/g higher than that of batteries with interlayer. Two batteries with an interlayer of the same thickness can exhibit a difference in the discharge capacities by 10 mAh/g and a different capacity retention due to the complex fabrication.
In ASSBs, there is no clear dependence of the battery performance on the thickness of the interlayer. For a thickness of the interlayer between 1 nm and 4 nm (2 min – 8 min deposition time) the initial discharge capacity varies between 80 mAh/g and 90 mAh/g. The reason that the thickness of the interlayer does not have a clear influence on the battery performance may be due to the fact that the pressed electrolyte powder is rough and the interlayer is not thick enough to fully cover the electrolyte surface. In that case, parts of the surface are covered and protected from decomposition, while other parts are still exposed to
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lithium metal and undergo a reaction. To cover a rough surface, a thicker interlayer may be necessary but it may come along with an increased resistance if the layer becomes too thick.
The differences in the capacity retention of the cells with different interlayers may also be caused by an inhomogeneous interlayer thickness. Although the thickness should not exhibit large lateral differences, even slight differences (e.g. caused by randomly ablated bigger particles) can cause lateral potential distributions across the interface (cf. chapter 2.4) and lead to a locally higher driving force for decomposition.
Further evidence of the protective effect of the interlayer is given by the impedance spectra of the batteries with and without interlayer.
Although the procedure is not optimized, the positive influence of the interlayer on the ASSBs is visible. A more effective protection may be achieved by using a more conformal deposition method, e.g. ALD.