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Batteries 2022, 8, 6 Batteries 2022, 8, x FOR PEER REVIEW 6 of 18 3.5 3.0 2.5 3.5 3.5 4.0 3.5 3.0 2.5 2.0 180 140 0.1 C When using0.1TCBA alginatCehabrgein/ disechra,rgwe ceapacritye able to eliminate the exposure to water while 160 0.2 C PVDF 120 100 (a) reasonable accuracy, the peak is not associated with Na2CO3 or NaHCO3. The connection (b) (c) to NaOH or its hydrates, however, cannot be ruled out. 140 0.5 C TBA alginate with TBA alginate binder display 6% higher gravim100etric capacity, up to 164 mAh/g8,0 with (f) Figure 3. SEM of Na0.67MnO2 electrodes prepared with (a,d) PVDF; (b,e) Na alginate and (c,f) TBA (d) (e) Figure 3. SEM of Na0.67MnO2 electrodes prepared with (a,d) PVDF; (b,e) Na alginate and (c,f) TBA alginate binders. The length of the scale bar is 10 μm for images in the top row and 3 μm for images alginate binders. The length of the scale bar is 10 μm for images in the top row and 3 μm for images in the bottom row. in the bottom row. 2.4. Electrochemical Properties and Cycling Behaviour 2.4. Electrochemical Properties and Cycling Behaviour 22.4.4.1.1..EElelecctrtroocchheemicicaallPrroopeerrttiieessiinSttaandarrdSIIBHallff--Cellls First, a baseline is established by assessing the electrodes prepared with PVDF and First, a baseline is established by assessing the electrodes prepared with PVDF and Na alginate binders. While PVDF-based Na MnO displays charge capacity of roughly Na alginate binders. While PVDF-based Na0..6677MnO2 2displays charge capacity of roughly 155 mAh/g, as shown in Figure 4a, the capacity decreases to 120 mAh/g if Na alginate 155 mAh/g, as shown in Figure 4a, the capacity decreases to 120 mAh/g if Na alginate binder is used (Figure 4b). This is a result of the aqueous preparation route, as Na MnO binder is used (Figure 4b). This is a result of the aqueous preparation route, as Na0.67MnO22 is known to be unstable in water and humidity [24]. We find that the water-unstable is known to be unstable in water and humidity [24]. We find that the water-unstable Na MnOhasbeenpartiallydecomposedbyexchangingN+a+with+H+andultimately Na0.06.76M7 nO2 h2as been partially decomposed by exchanging Na with H and ultimately in- intercalating H O between the Mn-O layers, as confirmed by XRD in Figure 5. While one tercalating H2O2between the Mn-O layers, as confirmed by XRD in Figure 5. While one can speculate that some of the intercalated hydrogen can later be displaced with Na again, can speculate that some of the intercalated hydrogen can later be displaced with Na again, it has been shown that air-exposed electrodes display decreased rate capability, gravimetric it has been shown that air-exposed electrodes display decreased rate capability, gravimet- capacity, and worse cyclability. This is indeed what we see in this study (Figure 4), as the ric capacity, and worse cyclability. This is indeed what we see in this study (Figure 4), as gravimetric capacity of Na alginate-based electrodes is significantly reduced. the gravimetric capacity of Na alginate-based electrodes is significantly reduced. The phase associated with water-exposure of NaxMO2 (M – transition metal) is bir- 4.0 nessite,and4i.t0hasanincreasedinterlayerdistanceco4.m0paredtoP2-typeNa0.67MnO2[43]. PVDF Na Alginate TBA Alginate Further intercalation of water is associated with formation of the buserite phase. Despite the hydrated (birnessite) phase being electrochemically active and capable of storing sodium with0.1aCgravimetric charge of at least 84 m0A.1hC/g [44], the value is significantly low0e.r1 Cthan 0.2 C the 0r.o5 Cughly 160 mAh/g observed for pristine P2-type Na MnO . As shown in Figure 5, 0.2 C 0.2 C 1 C 3.0 0.5 C 3.0 0.67 2 0.5 C 2C 5C 2C 2 C 1C 1 C we found that within 4 h of water exposure (4 h is the time needed to ensure proper mixing of electrode slurry), birnessite impurities have already become apparent. If water exposure 10 C 2.5 5 C 2.5 5 C 10 C is co20nCtinued, after 24 h almost none of the original structure is retained, and most peaks 20 C can be attributed to birnessite or buserite phases. A third phase also appears. Although the 2.00 20 40 60 80 100 120 140 160 2.00 20 40 60 80 100 120 2.00 20 40 60 80 100 120 140 160 lack of additional peaks means that we are not able to pinpoint the impurity phase with a Gravimetric Capacity (mAh/g) Gravimetric Capacity (mAh/g) Gravimetric Capacity (mAh/g) still using a salt of alginic acid as a biNnadalgeinra.teResults in Figure 4c show that electrodes prepared 120 1C 2C good rate capability when compared to PVDF and Na alginate binders (Figure 4e). In 100 5 C 80 60 general, the results indicate a good application potential of TBA alginate as a binder for Na- PVDF 80 10 C 60 ionbatteries.Judgedfromthecharge-dischargecurvesat0.1C(Figure4d),theoverv4o0ltage Na Alginate 60 20 C 40 CE / Discharge capacity TBA Alginate andthusinternalresistancesofPVDFandTBAalginate-basedNPVaDF MnO electrodesare 40 0.67 20 Na Alginate 2 similar, while binders with Na alginate display noticeably higher overvoltage and less 20 TBA Alginate sharp voltage plateaus, likely a consequence of the partially transformed crystal structure. 0 20 40 60 80 100 120 140 160 00 5 10 15 20 25 30 35 40 0 100 200 300 400 500 We are able to observe all voltage plateaus characteristic to Na MnO for both TBA Gravimetric Capacity (mAh/g) Cycle No. Cy0c.6le7No. 2 alginate and PVDF-based electrodes (Figure 4d). (d) (e) (f) Figure 4. Electrochemical properties of Na0.67MnO2 electrodes prepared with PVDF, Na alginate and TBA alginate binders: charge-discharge curves of Na0.67MnO2 electrodes with (a) PVDF binder, (b) Na alginate binder, (c) TBA alginate binder; (d) charge-discharge curves at 0.1 C (17.5 mA/g); (e) 20 00 6 of 19 10 C Voltage (V) Voltage (V) Gravimetric Capacity (mAh/g) Voltage (V) Gravimetric Capacity (mAh/g) Voltage (V) Coulombic Efficiency (%)PDF Image | Na-Ion Batteries Tetrabutylammonium Alginate Binder
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