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structural degradation.175 Figure 4 presents that V, Mn, and Fe are the widely used TMs in Na polyanions cathode materi- als. Fe/V-content is mainly used in the range of 0.5–1 thanks to their great feasibility to reside in the different polyanionic frameworks. Among them, V-based polyanions can enable high capacity and long-term cycling. While Mn, Ni, and Co dominate layered oxides research, their presence in polyanions is quite modest. This could be explained by their specific chemical and crystallochemical properties where Mn, Ni, and Co can hardly be incorporated in the polyanionic framework. Except few struc- tures such as olivine (LiMPO4), Mn, Ni, and Co can only exist in polyanionic frameworks at a minor portion (less than 0.5) thanks to the presence of other structural stabilizers. Some impor- tant examples of these compounds are NaxMV(PO4)3 (M = Mn, Ni,).176,177 The energy density of polyanions can be engineered by modulating the operating voltage by varying the nature of the electroactive ions or the counterpart anions. For example, incorporating fluorine (the most electronegative element) into some polyanion structures178,179 allowed an operating voltage up to 4 V vs Na+/Na with high energy densities. Despite this feature, the development of polyanion materials with low-cost and earth- abundant elements is necessary and is still an ongoing research challenge.180–183 The third category of sodium cathodes is PBAs with the gen- eral formula of AxMy[M′(CN)6]z nG. PBAs structures include one or more transition metal ions (in M and M′ sites) coordi- nated by CN− ions to form hexacyano complex. The connection between [M′ (CN)6]n− units results in opened channels allowing a fast ionic diffusion process inside the structure. Their crystal structure can be indexed in the Fm-3m, R-3, or P21/n space group depending on the degree of distortion induced by the amount of Na+, water molecules, and the nature of the elec- troactive center. Most PBAs utilize abundant elements such as Mn and Fe, making this class of cathode materials one of the best price-to-performance ratios reported to data.103,184 Mn- and Fe-based PBAs can provide a wide range of energy densi- ties (200–600 Wh kg−1) and capacity retentions over relatively high number of cycles in non-aqueous electrolytes (Fig. 4). The accessible specific capacity strongly depends on the stoi- chiometry of the structure and the initial Na+ concentration. Generally, the electrochemical cycling of the PBAs is catego- rized into two main classes: (i) only hexacyanometallate active group, and (ii) active M site TM as well as the hexacyanometal- late active group.103 The class (ii) with higher electron transfer reactions is more favorable in practical batteries by enabling higher specific capacity. Manganese hexacyanoferrate is the most well-known and commonly used PBA cathode material with two active sites of transition metals offering two electron transfers. Recently, cobalt hexacyanoferrate is also introduced as another type of the PBA cathode material with this prop- erty,185,186 yet low yield of synthesis considering the high cost of cobalt suppresses the large-scale applications. The number of the electrochemical active TM also affects the operating potentials. For example, an active polarized M site TM can tune the inductive effect on M′(CN)6 leading to higher operat- ing potentials. These cathodes with two electron transfers are considered among the highest energy density cathode materi- als with more than 150 mAh g−1 specific capacities in above 3 V operating voltages in NIBs. One of the limiting factors in TM selection in PBAs is origi- nated from the synthesis procedure to obtain a stable and insol- uble PBA material. Most common bulk synthesis methods are from the reaction of a transition metal salt (Mm+) with a simple cyanide (CN−) or with a hexacyanometallate salt ([M′(CN)6]n−). Although using the simple cyanide results in high yield produc- tion, but it limits the material to only one type of the transition metal resulting in lower specific capacity. On the other hand, utilization of hexacyanometallate salt is considered as a flexible and high yield method and is the main method used in the most patent documents using PBA cathode materials. One of the well- studied materials with this method is manganese hexacyanofer- rate with sodium-rich initial composition enabling full two elec- tron transfers.187 Another synthesis method for PBAs is through the decomposition products of a hexacynometallate salt.188,189 This method leads to a highly crystalline and fine primary crystal grain size; however, it is not a suitable method for large batch production and scalability due to the required high tempera- ture or pH and large quantity of HCN as the by-product.190,191 Moreover, the long cycle-life of PBA materials is mainly limited by the electronic conductivity in the bulk of the active materi- als due to the limited reversible sodium intercalation into the bulk structure.192 This point highlights the importance of the size and morphology control in this type of materials. Many on- going researches have focused on the employment of multi-elec- troactive centers to enhance the stability and the electrochemical performance of this class of materials. For example, Moritomo et al.193 showed lower capacity loss by partial substitution of the Mn with Fe or Ni in manganese hexacyanoferrate. In general, the energy density of polyanions and PBAs are not as high as those of layered oxides, which is a penalty of the high weight of the anionic part. However, the cyclability of polyanions and PBAs are greatly higher than oxides with the average number of cycles for oxides, PBA, and polyanions are 93, 257, and 686, respectively. The robustness of polyanions and PBAs helps them to find their place in applications where energy density is not the critical criterion, such as large-scale applications in grid storage or aqueous batteries. Yet, the cyclability of NIBs is inferior to LIBs at the moment. This can be due to several factors such as (i) less advancement in design and structure of the sodium cathode materials, (ii) more sensitivity of the cathode materials to moisture and carbonates which results in more limitations in preparing and handling of the cathode materials, and (iii) limited electrolyte study and development in NIBs. Majority of the studies in NIBs are still in discovery stage with the focus on the synthesis and development of materials with different compositions and their electrochemi- cal performance in limited time and conditions. Moreover, there is a very limited attention to the long cycling performance and the mitigation of the degradation mechanisms. In overall, as sug- gested, development of next generation of NIBs for large-scale production, require a comprehensive investigation with more than thousands of cycles. 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