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Batteries 2022, 8, 6 11 of 18 Batteries 2022, 8, x FOR PEER REVIEW 11 of 19 (a) (d) (b) (c) (e) (f) Figure 7. SEM of Na0.67MnO2 electrodes cycled at 1 C for 500 cycles, prepared with (a,d) PVDF; (b,e) Figure 7. SEM of Na0.67MnO2 electrodes cycled at 1 C for 500 cycles, prepared with (a,d) PVDF; Na alginate and (c,f) TBA alginate binders. The length of the scale bar is 10 μm for images in the top (b,e) Na alginate and (c,f) TBA alginate binders. The length of the scale bar is 10 μm for images in the row and 3 μm for images in the bottom row. top row and 3 μm for images in the bottom row. Surfaces of cycled (500 cycles at 1 C) and uncycled electrodes were analysed with XPS Surfaces of cycled (500 cycles at 1 C) and uncycled electrodes were analysed with (Figure 8). Typically, upon cycling, the solid–electrolyte interface (SEI) starts to grow, con- XPS (Figure 8). Typically, upon cycling, the solid–electrolyte interface (SEI) starts to grow, suming some of the sodium and often increasing the internal resistance. The SEI is typi- consuming some of the sodium and often increasing the internal resistance. The SEI is cally composed of NaF, Na2O, Na2CO3, and other products of electrolyte and electrode typically composed of NaF, Na2O, Na2CO3, and other products of electrolyte and elec- decomposition. trode decomposition. NaF is a prominent part of the SEI. The XPS peak corresponding to Na-F bonding at 684.3 eV is observed for all the cycled electrodes. This is the result of the desirable decomposition of FEC electrolyte additive. The peak for C-F bonding (CF2 in PVDF binder) at 688.2 eV is observed in fresh PVDF-based electrodes. Depth profiles of F indicate a considerable amount of F trapped in the SEI. The disappearance of C-F peak at 688.2 eV in the aged PVDF-based electrode again confirms the considerable thickness of the SEI, while the depth distribution of fluorine indicates a relatively higher fluorine content in TBA alginate and PVDF-based electrodes compared to Na alginate-based electrodes. C1s peak can be deconvoluted into several peaks. Located at 284.8 eV is the peak associated with the C–C bonds of adventitious carbon and acetylene black. The peak at approximately 285.8 eV corresponds to hydrocarbon bonding (C-H), while peaks at 287.2, 289.2 and 290.7 eV are associated with C–O/C=O, Na2CO3 and R–CO3/CFx (x ≥ 2), respectively [49,56]. The shape and intensity of PVDF electrode changes significantly upon cycling, signalling a significant SEI growth, while that of Na alginate-based electrode remains virtually unchanged. The intensity of the TBA alginate electrode’s peaks is lower for the aged electrode, but the shape remains unchanged and signals presence of mostly C–C and C–O/C=O bonds. Mn signal measured for most electrodes is weaker in aged electrodes. This points to the formation of a SEI where Mn is not present. An exception to this is the PVDF-based electrodes, where the Mn signal in cycled electrode intensifies. The dissolution of Mn was previously observed In Na0.67MnO2 [57] and is likely more prevalent due to a more inhomogeneous SEI. This is in agreement with Zou et al. who have also reported formation of MnF2 on the surface of a PVDF-based electrodes [21].PDF Image | Na-Ion Batteries Tetrabutylammonium Alginate Binder
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