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Batteries 2022, 8, 6 2 of 18 make polyvinylidene fluoride (PVDF) one of the most popular binder choices for LIBs, so PVDF is also often used in SIBs. However, PVDF-based electrodes cannot tolerate the large volume changes that some of the current electrode materials undergo upon charg- ing and discharging [5]. Moreover, in SIBs, PVDF is shown to undergo defluorination if no additional source of fluorine is available [6], so alternative binders are starting to emerge. Organic compounds, such as carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR) and alginate compounds have been reported [7]. Due to their improved mechanical properties and chemical stability, these new binders often offer better cycle life, higher capacity, and rate capability. Among alginate compounds, sodium alginate [8], the sodium salt of alginic acid, a compound that is obtained from the cell walls of brown algae, is by far the most studied. It can extend the cycle life of many battery materials due to its higher mechanical strength (as shown in studies on Si anode for LIBs [9,10]), as well as by forming a more uniform protec- tive coating on the particles of the active material [11–13]. An added benefit is Na alginate’s solubility in water, enabling preparation of aqueous electrode slurries and thus ensuring environmentally friendlier preparation of electrodes. Unfortunately, organic solvents are not able to dissolve Na alginate. This makes the Na alginate binder incompatible with many of the SIB electrodes that decompose rapidly if exposed to air or water, promoting the search for a compound that would combine the favourable properties of Na alginate while also enabling the preparation of anhydrous electrode slurries. The instability in water is perhaps best exemplified by layered metal oxide cathodes, such as P2-type Na0.67MnO2 [14]. Capacity-wise, P2-type Na0.67MnO2 is a very competitive SIB cathode, with high practical gravimetric capacity in the range of 160–200 mAh/g [15]. At the average discharge potential of 2.75 V vs. Na/Na+, this results in material energy density of up to 550 Wh/kg. Actual measured capacities are a subject to the chosen voltage range, which is in turn often a compromise between specific charge capacity and cycle life. First investigated in 1999 by Paulsen et al. [16], it has received increased attention from the scientific community over the last five years. Luo et al. showed a capacity of roughly 160 mAh/g and cycle life decrease of roughly 50% over 500 full charge-discharge cycles over a narrow 2.0–3.8 V vs. Na/Na+ range as well as improvement when substituting some Mn with Fe and Ni [17]. Lyu et al. demonstrated that the cycle life of Na0.67MnO2 can be significantly improved when switching to solid electrolyte [18]. More recently, materials engineered on nanoscale, such as nanoplatelets [19] and other nanostructures [20,21], have displayed increased rate capability, albeit with only marginal improvements in cycle life and overall capacity. On average, Na0.67MnO2 lost around 15% of its capacity over first 100 charge-discharge cycles. Doping has yielded improvements regarding cycle life and stability in air. For example, doping with Co [22], Ni, and Mg [23] has been somewhat effective. Creating higher charge-discharge potentials by substituting Mn with Ni, Cu, or Fe [24] have yielded promising results. A more detailed review on recent progress in development of cathodes for SIBs can be found in references [25,26]. The underlying reason why Mn in Na0.67MnO2 is often partly replaced by Ni, Fe, Ti, Al, Mg, and other metals [21,23,27–30] is the attempt to improve its cycle life. The cycle life of pristine P2-type Na0.67MnO2 is known to be relatively bad due to the gliding and reordering of crystalographic planes at the highly charged states, that lead to P2-P2′, P2-O2, or P2-OP4 phase transitions [21,29]. Doping and Mn substitution lead to altered interlayer spacing and thus altered O-O repulsion and O-Na attraction forces that lead to changes in the structural stability of Na0.67MnO2 [31]. The same effect of changing interlayer spacing take place upon exposing Na0.67MnO2 to water. However, the structural changes happen to such an extreme degree that the P2-type Na0.67MnO2 structure eventually disintegrates. Upon exposure to water, Na+ in Na0.67MnO2 is displaced by H+, changing the interlayer spacing, causing crystallographic planes to glide and leading to the phase transformation of Na0.67MnO2 [24,32]. Ultimately, proton intercalation increases interlayer distance to the point where water molecules can be inserted in the interlayer space [24,33], leading to decomposition of the original P2-typePDF Image | Na-Ion Batteries Tetrabutylammonium Alginate Binder
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