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J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al 1.4. Prussian blue and analogues William R Brant and Ronnie Mogensen Department of Chemistry—Ångström Laboratory, Uppsala University, SE-75121 Uppsala, Sweden Status Prussian blue analogues (PBAs), with the general formula NaxM[M′(CN)6]1−y.zG (where M and M′ are transition metals, most commonly Mn, Fe, or Ni, y is the number of [M′(CN)6]n− vacancies, and G is a neutral guest, such as H2O), are highly porous positive-electrode materials for NIBs (top of figure 6). Since PBAs are comprised of abundant elements and can be produced at low cost, they provide the best price-to-performance ratio [39] for a positive electrode. Thus, PBAs capitalise on the key advantages of NIBs as cheap, sustainable, and safer alternatives to lithium-ion technology. They have become viable cathode materials for sodium batteries within the last decade. During this time, the greatest challenge facing the implementation of PBAs as positive electrodes has been the ability to produce highly-sodiated compounds that are structurally stable during cycling. The structural stability and maximum capacity of the material are severely reduced by the presence of the vacancies (y) introduced during synthesis. Due to the loss of a reducible cation (M′) and the necessary charge compensation required to offset the negative charge of [M′(CN)6]n−, 1 mol% of vacancies can lead to a 3–4 mol% reduction in the maximum sodium content. Resolving this issue requires alternate synthetic approaches to the more traditionally applied co-precipitation reactions. For some M cations, in particular, Fe, co-precipitation results in a high vacancy concentration. In 2014, You et al reported a way of producing a low-vacancy PBA from a single precursor via an acid decomposition synthesis route for the first time [40]. This approach was subsequently used to produce the best-performing PBA to date, which delivered 120 mAh g−1 for over 800 cycles of charging at 0.5 C (75 mA g−1) and discharging at 2 C (300 mA g−1) [41]. Since then, extensive effort has been invested in the exploration of different M combinations, particle sizes, morphologies, and coatings, and improvements have been seen for either initial capacity or total cycling life, but not both [42]. Ultimately, the goal for PBAs is to achieve capacities of more than 160 mAh g−1 which are stable for up to 10 000 cycles. Eight thousand cycles have been demonstrated using aqueous electrolytes (figure 6 top) [43]; however, the resulting capacity is frequently limited to 60–80 mAh g−1. Conversely, higher capacities (>80 mAh g−1) can be achieved using non-aqueous electrolytes due to their wider potential stability window although, the number of cycles obtained is often an order of magnitude lower. Current and future challenges Two connected issues are plaguing the development of PBAs: moisture sensitivity and limited reversibility when cycling more than one atom of sodium per formula unit. Capacity fading in PBAs has generally been attributed to limited electronic conductivity, the presence of water, and phase transitions of the highly sodiated compounds. Several methods have been employed to overcome their limited electronic conductivities, such as growing PBAs directly on carbon nanotubes, enabling stable cycling down to −25 ◦C [45]. Structural stability has seemingly been a greater challenge to overcome, since phase transitions in sodium-rich PBAs are heavily influenced by their water content. Wang et al [41] first demonstrated that, when completely dehydrated, Fe-based PBAs adopt a rhombohedral structure with a unit cell volume that decreases by ≈18%. This work was followed up by Rudola et al [44], who demonstrated that obtaining a moisture-free PBA depends heavily on the temperature and pressure used for drying and that, if exposed to air, a hydrated structure forms within minutes (bottom of figure 6). Further, a 16% volume change between cubic Fe[Fe(CN)6] and rhombohedral Na2Fe[Fe(CN)6] leads to a lower cycling stability compared to the hydrated compound, if the capacity and voltage window are limited in the hydrated case. Water also increases the average voltage output [46], which may be perceived as an advantage. However, the high voltage plateau may lie beyond the upper stability limit of water (3.9 V vs. Na+/Na). Finally, it has been shown that desodiated PBAs have a lower affinity for water [47]. Consequently, charging the cell runs an additional risk of releasing water into the electrolyte. Thus, the influence of water on electrochemical performance is complicated. The presence of water is beneficial for ensuring structural stability by minimising volume contraction during cycling. However, it limits the maximum capacity that can be utilised, as water is either released from the structure and reacts with the electrolyte, or is oxidised inside the material at high potential. Solving this challenge will require further consideration of the role of the electrolyte. Electrolyte–PBA interactions will be the greatest hurdle to overcome in the future, as all synthetic design choices (figure 7) will have to determine the optimum electrolyte combination. The synthetic possibility space for PBAs is large, and so one must take a holistic view when designing a cell based on this material. 15PDF Image | 2021 roadmap for sodium-ion batteries
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