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Batteries 2022, 8, 6 8 of 18 other types of materials for SIBs with higher stability due to doping or other structural modifications can further increase the cycling stability and gravimetric capacity. The cycling performance of all electrodes, shown in Figure 4f, indicate similarities between electrodes prepared with PVDF and TBA-alginate binders, although the measured absolute gravimetric charge capacity values are higher for TBA alginate-based electrodes. The capacity decreased to 80% of the initial value at cycle 162 for the PVDF-based electrode and cycle 197 for the TBA alginate-based electrode. There is also an initial increase of capacity for both PVDF and TBA alginate-based electrodes, which is higher for TBA alginate. This type of behaviour during first cycles is not uncommon [17,47,48], and is likely related to the formation of a stable SEI on the surface of the electrode, as it has previously been shown to be electrolyte-dependent [48]. As shown in Supplementary Figure S3, the evolution of charge-discharge curves follows a similar pattern for both PVDF and TBA alginate-based electrodes, where the plateaus and other features clearly visible in the initial cycles become less distinct over the cycling and transform somewhat already within the first 20 cycles. This is likely due to irreversible structural changes, induced by discharging in the range of 2.2–2.0 V [15]. The charge-discharge curves of Na alginate-based electrodes are slightly more washed-out. Somewhat surprisingly, we see the cycling behaviour of Na alginate-based electrode to be significantly improved over PVDF and TBA alginate-based electrodes. The capacity rises form initial 81 mAh/g to 96 mAh/g at 1 C. After 160 cycles the capacity of Na alginate-based electrode surpasses that of TBA alginate-based electrode. After 500 cycles, the capacity of Na alginate-based electrode is still maintained at 82.8 mAh/g or 102% of the initial value. It has been shown that Na alginate can effectively coat the particles of active material, thus preventing several processes: cracking of electrodes, increase in charge-transfer resistance (due to a more stable SEI) and the detachment of electrode from the current collector [11]. It is likely that these are also the reasons behind the improved cycle life of Na alginate-based electrodes in our study. What is somewhat surprising is that this behaviour is not mirrored in TBA alginate-based electrodes, indicating functional differences between TBA alginate and Na alginate. In order to rule out the improved cyclability of Na alginate-based electrodes coming from the water exposure, we prepared a PVDF-based electrode where Na0.67MnO2 had first been exposed to water in a similar way as while preparing the water-based slurry with Na alginate binder. The electrochemical measurements do not indicate improved cycling performance (see Chapter S4 and Figure S4 in Supplementary Information). Hence, it can be concluded that improvements in long term cycling observed for Na alginate- based electrodes are indeed a consequence of the binder and not the water-exposure of the active material. The initial Coulombic efficiency is an important parameter for evaluating the perfor- mance of electrodes. Measured Coulombic efficiencies are summarised in Table 1. The large values of the initial Coulombic efficiency are not surprising, as Na0.67MnO2 is initially sodium deficient. Assuming that 0.67 parts of Na can be extracted from the electrode, while one full part of Na is inserted in the discharge part of the cycle, the initial Coulombic efficiency should indeed be 149%, in good agreement with the values measured for PVDF and TBA alginate-based electrodes. The initial Coulombic efficiency of Na alginate is somewhat higher, which is consistent with the expected Na loss during aqueous process- ing of Na0.67MnO2 [24]. Estimated from the observed Coulombic efficiency, the initial stoichiometry of active material in alginate-based electrode is around Na0.4MnO2. For both alginate-based electrodes, the stabilised Coulombic efficiency is very close to 100. Coulombic efficiency of PVDF-based electrodes also stabilises above 99%, although the determined values are slightly lower than those for alginate electrodes.PDF Image | Na-Ion Batteries Tetrabutylammonium Alginate Binder
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