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Batteries 2022, 8, 6 9 of 18 Table 1. Coulombic efficiency as a function of cycle and binder material. Cycle No./C-Rate Cycle1@0.1C Cycle2@0.1C Cycle3@0.1C Cycle4@0.1C Cycle5@0.1C Average in cycles 50–100 @ 1 C PVDF 147.7% 98.3% 99.4% 99.4% 98.4% 99.8 ± 0.8% Binder Material Na Alginate 248.9% 101.5% 99.0% 99.9% 99.8% 100.0 ± 0.2% TBA Alginate 157.2% 98.5% 99.6% 99.2% 99.9% 99.9 ± 0.6% 2.4.2. TBA Alginate Binder in Other Cell Chemistries In order to understand the extent to which the improvement of electrochemical proper- ties with TBA alginate binder can be beneficial to other cell chemistries, PVDF, Na alginate, and TBA alginate electrodes were measured in two other cells—SIB half-cell without SEI-stabilising additive added to the electrolyte and LIB half-cell with LiFePO4 cathode. As the electrolyte chemistries used in Na-ion batteries are more diverse than in LIBs, it is important to note that the measurements reported so far have been performed in NaClO4 electrolyte (1 M NaClO4 salt in propylene carbonate (PC)) containing 5 wt.% fluoroethylene carbonate (FEC) additive. FEC is known to stabilise the electrode–electrolyte interface and prevent the dissolution of PVDF, functioning as an additional source of fluorine that forms NaF interfacial layer [49]. Na alginate can extend the cycle life of electrodes due to uniformly coating the electrode surfaces [11] and preventing parasitic reactions between the electrolyte and electrode. Na alginate is in general not as vulnerable to decomposition as PVDF [50]. To clarify where TBA alginate binder lies in this spectrum, long-term cycling tests were performed in electrochemical cells with 1 M NaClO4 electrolyte in PC without FEC additive. In such conditions, all electrodes display significantly worse cycle life than measured with the FEC-containing electrolyte (Supplementary Information, Chapter S5, Figure S5). The capacities of PVDF, TBA alginate, and Na alginate-based electrodes decrease below 80% of the initial value after 27, 16, and 288 cycles respectively. Since NaF is known to be a crucial part of the SEI [49] and TBA alginate does not contain fluorine, the results are perhaps not too surprising. The obtained data again outline the importance of a stable SEI layer and indicate that the TBA alginate does not provide as strong of an effect on stabilising the SEI layer as Na alginate, where a protective particle coating effect has been suggested [51]. As at no point does the capacity of TBA alginate-based electrode exceed that of PVDF-based, it is likely that the rapid decomposition of the electrode leaves a notable mark already during the first full charge-discharge cycle. Both initial Coulombic efficiencies (CEs) as well as stabilised CEs are significantly lower than those measured in 1 M NaClO4 electrolyte with 5 wt.% FEC additive (Supple- mentary Information, Table S1 in Chapter S5). TBA alginate sees the lowest CE, pointing to the most intense parasitic reactions. Na alginate, on the other hand, has the highest CE, consistent with the relatively good long-term cycling results. The results put TBA and sodium salts of alginic acid in stark contrast and again indicate that there are notable functional differences between both alginate binders. We also carried out tests with the LiFePO4 cathode for Li-ion batteries and found the results to be very similar (Supplementary Information, Chapter S6). PVDF outperforms TBA alginate binder in the LIB cell chemistry with 1M LiPF6 electrolyte salt in the ethylene carbonate (EC)–dimethyl carbonate (DMC) mixture. Although the reasons for this require further investigation, we hypothesize that this behaviour could be due to instability of TBA alginate in the electrolyte used in LIB cell (1 M LiPF6 in EC/DMC), resulting in the reduced mechanical integrity of the electrode and subsequent loss of electric contact between the current collector and active particles.PDF Image | Na-Ion Batteries Tetrabutylammonium Alginate Binder
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