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Batteries 2022, 8, 6 13 of 18 electrode. In general, the difference between SEI formed on PVDF-based electrode and SEI formed on TBA alginate-based electrode is not large. On the other hand, Na alginate electrodes have less NaF in the SEI and generally undergo smaller compositional changes upon cycling. 3. Discussion The set of results from structural, morphological, and interfacial characterisations combined with an array of electrochemical tests in different systems provide the first insight into the working mechanisms of TBA alginate binder. Qualitatively, there are more functional similarities between TBA alginate and PVDF than between TBA and Na alginate binders. Electrodes based on PVDF and TBA alginate binders both follow a similar capacity decay pattern during the long-term charge-discharge cycling. Internal electrode resistances measured with EIS increase slightly during cycling, and electrodes based on both binders have a considerable amount of Na-F bonding in the SEI after cycling in 1 M NaClO4 electrolyte with 5 wt.% FEC additive. XPS results indicate that the SEI grown on both TBA alginate- and PVDF-based electrodes contain considerable amounts of Na-F and RCH2ONa compounds. Perhaps the most notable difference between PVDF and TBA alginate-based electrodes is the porosity of the SEI, as evident in the SEM images. This is likely the reason why the observed SEI resistance measured by EIS is comparably largest for PVDF-based electrodes. Given the similar chemical composition of SEIs and good integrity of electrodes evidenced before and after long-term cycling experiments, it is likely that the improvement of the electrochemical properties of TBA alginate-based Na0.67MnO2 electrodes comes from an improved homogeneity of the SEI. Interestingly, considerable functional differences exist between TBA and Na alginate electrodes. As evident from the XPS spectra (especially C, O and Na), Na alginate undergoes considerably less surface change upon ageing, pointing to an already-stable protective layer before cycling. The SEI layer seems to get richer in fluorine, with the SEI composition not limited only to NaF, and with it the resistance of the electrode reduces, as evidenced by EIS. Furthermore, when the Na alginate-based electrode is cycled in the electrolyte without FEC additive, the lack of NaF interfacial layer does not seem to affect the cycle life as much as that of the PVDF and TBA alginate-based electrodes. It can be hypothesized that Na alginate forms a more homogenous protective coating on the surface of the electrodes and thus prevents some of the undesirable decomposition reactions. The reason behind the radically different interfacial behaviours between TBA and Na alginate-based electrodes, however, remains a potentially interesting topic for future studies. Naturally, challenges remain in optimising the process of mixing and coating the electrode slurries. In the present study, it took considerably longer to obtain a homogenous TBA alginate-based slurry than a comparable PVDF-based one. An interesting subject of practical future studies could be the performance of TBA alginate binder in conjunction with active materials that have an inherently better cycle life and do not suffer from Mn2+ dissolution and oxygen evolution as much as Na0.67MnO2 does. If work on Na0.67MnO2 is to be continued, several pathways could be considered for increasing cycle life of the cath- ode, as currently it falls below 50% of initial after 500 cycles. These pathways would most likely have to deal with mitigating the P2-P2′ and P2-OP4 phase transitions in the active material that have been shown to have a significant impact on the cycle life [21,29]. Ap- proaches to stabilize the structure by modifying the crystal lattice [31] or via the doping or substitution of Mn or other elements have provided good results [46,58]. Further alterations of electrode formulations, i.e., electron-conducting additives [59], protective coatings [60], or the formation of various nanostructured materials, e.g., nanocomposites [61] or nanos- tructures with high specific surface area [62], could also be considered. Our study also opens up potential to further investigation and optimisation of the electrode slurries. From a practical point of view, the use of TBA alginate binder instead of PVDF is appealing, as this research demonstrates the superior electrochemical performance ofPDF Image | Na-Ion Batteries Tetrabutylammonium Alginate Binder
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