Sodium-ion batteries based on Prussian blue analogues (PBAs) are ideal for large-scale energy storage applications due to the ability to meet the huge volumes and low costs required. For Na 2−x Fe[Fe(CN) 6 ] 1−y •zH 2 O, realizing its commercial potential means fine control of the concentration of sodium, Fe(CN) 6 vacancies, and water content. To date, there is a huge variation in the literature of composition leading to variable electrochemical performance. In this work, we break down the synthesis of PBAs into three steps for controlling the sodium, vacancy, and water content via an inexpensive, scalable synthesis method. We produce rhombohedral Prussian white Na 1.88( 5) Fe[Fe-(CN) 6 ]•0.18(9)H 2 O with an initial capacity of 158 mAh/g retaining 90% capacity after 50 cycles. Subsequent characterization revealed that the increased polarization on the 3 V plateau is coincident with a phase transition and reduced utilization of the high-spin Fe(III)/Fe(II) redox couple. This reveals a clear target for subsequent improvements of the material to boost longterm cycling stability. These results will be of great interest for the myriad of applications of PBAs, such as catalysis, magnetism, electrochromics, and gas sorption.

Sodium-ion batteries based on all-naturally abundant elements, in which no cobalt, nickel, copper, and fluorine is used, can lead to a major breakthrough in making batteries more sustainable. Safety aspectsin particular, flammability of electro-lytesin the state-of-the-art battery technology is another important concern, especially for applications in which large numbers of cells are employed. Nonflammable battery electrolytes studied so far are based on highly fluorinated compounds or high salt concentrations, which suffer from high cost and toxicity. We here propose an electrolyte based on a single solvent and low-cost and fluorine-free salt at a lower range of "standard" concentrations. Our results showfor the first timethat sodium bis(oxalato)borate (NaBOB) is soluble in the nonflammable solvent trimethyl phosphate (TMP). This finding enables a nonflammable electrolyte with high ionic conductivity and promising electrochemical performance in full-cell sodium-ion batteries. An electrolyte of 0.5 M NaBOB in TMP provides an ionic conductivity of 5 mS cm −1 at room temperature, which is comparable to the commonly used electrolytes based on sodium hexafluorophosphate (NaPF 6 ) and organic carbonate solvents. The proposed electrolyte shows a Coulombic efficiency of above 80% in the first cycle, which increased to about 97% from the second cycle in sodium-ion battery full-cells consisting of a hard carbon anode and a Prussian white cathode. This work opens up opportunities to design safe electrolytes which can further be optimized with electrolyte additives such as vinylene carbonate for industrial applications.

Common battery electrolytes comprise organic carbonate solvents and fluorinated salts based on hexafluorophosphate (PF6 –) anions. However, these electrolytes suffer from high flammability, limited operating temperature window, and high cost. To address those issues, we here propose a fluorine-free electrolyte based on sodium bis­(oxalate)­borate (NaBOB). Although lithium bis­(oxalate)­borate (LiBOB) has previously been investigated for lithium-ion batteries, NaBOB was considered too insoluble in organic solvents to be used in practice. Here, we show that NaBOB can be dissolved in mixtures of N-methyl-2-pyrrolidone (NMP) and trimethyl phosphate (TMP) and in each sole solvent. NMP provides higher solubility of NaBOB with a concentration of almost 0.7 M, resulting in an ionic conductivity up to 8.83 mS cm–1 at room temperature. The physical and electrochemical properties of electrolytes based on NaBOB salt dissolved in NMP and TMP solvents and their binary mixtures are here investigated. The results include the thermal behavior of the sole solvents and their mixtures, flammability tests, NaBOB solubility, and ionic conductivity measurements of the electrolyte mixtures. Full-cell sodium-ion batteries based on hard carbon anodes and Prussian white cathodes were evaluated at room temperature and 55 °C using the aforementioned electrolytes. The results show a much improved performance compared to conventional electrolytes of 1 M NaPF6 in carbonate solvents at high currents and elevated temperatures. The proposed electrolytes provide a high ionic conductivity at a wide temperature range from room temperature to −60 °C as NMP–TMP mixtures have low freezing points. The flammability tests indicate that NaBOB in NMP–TMP electrolytes are nonflammable when the electrolyte contains more than 30 vol % TMP.

Non‐aqueous carbonate solvents have been the main choice for the development of lithium‐ion batteries, and similarly most research on sodium‐ion batteries have been performed using carbonate‐based solvents. However, the differences between sodium and lithium batteries – in term chemistry/electrochemistry properties as well as electrode materials used – open up opportunities to have a new look at solvents that have attracted little attention as electrolyte solvent. This work investigates properties of a wide range of different solvent classes in the context of sodium‐ion battery electrolytes and compares them to the performance of propylene carbonate. The thirteen solvents studied here include one or several members of glymes, carbonates, lactones, esters, pyrrolidones, sulfones, and alkyl phosphates. Out of those, five outperforming solvents of γ‐butyrolactone (GBL), γ‐valerolactone (GVL), N‐methyl‐2‐pyrrolidone (NMP), propylene carbonate (PC), and trimethyl phosphate (TMP) were further investigated using additives of ethylene sulfite (ES), vinylene carbonate (VC), fluoroethylene carbonate (FEC), prop‐1‐ene‐1,3‐sultone (PES), sulfolane (TMS), tris(trimethylsilyl) phosphite (TTSPI), and sodium bis(oxalato)borate (NaBOB). The solvents TMS and tetraethylene glycol dimethyl ether (TEGDME) were tested in 1 : 1 mixtures by volume with the co‐solvents; NMP, dimethoxyethane (DME), and TMP. All electrolytes used NaPF6 as the salt. Primary evaluation relied on electrochemical cycling of full‐cell sodium‐ion batteries consisting of Prussian white cathodes and hard‐carbon anodes. Galvanostatic cycling was performed using both two‐ and three‐electrode cells, in addition, cyclic and linear sweep voltammetry was used to further evaluate the electrolyte formulations. Moreover, the resistance was measured on the anode and cathode, using Intermittent current interruption (ICI) technique.

Sodium-ion batteries based on all-naturally-abundant elements, in which no cobalt, nickel, copper, and fluorine is used, can lead to a major breakthrough in making batteries more sustainable. Safety aspects -in particular flammability of electrolytes- in the state-of-theart battery technology is another important concern, especially for applications in which large numbers of cells are employed. Non-flammable battery electrolytes studied so far are based on highly fluorinated compounds or high salt concentrations, which suffer from high cost and toxicity. We here propose an electrolyte based on a single solvent and lowcost and fluorine-free salt at the lower range of “standard” concentrations. Our results show -for the first time- that sodium bis(oxalato)borate (NaBOB) is soluble in the nonflammable solvent trimethyl phosphate (TMP). This finding enables a non-flammable electrolyte with high ionic conductivity and promising electrochemical performance in fullcell sodium-ion batteries. An electrolyte of 0.5 M NaBOB in TMP provides ionic conductivity of 5 mS cm-1 at room temperature, which is comparable to commonly used electrolytes based on sodium hexafluorophosphate (NaPF6) and organic carbonate solvents. The proposed electrolyte shows the Coulombic efficiency of above 80% in the first cycle, which increased to about 97% from the second cycle in sodium-ion battery fullcells consisting of a hard carbon anode and Prussian white cathode. This work opens up new opportunities to design safe electrolytes which can further be optimized with electrolyte additives such as vinylene carbonate for industrial applications.<br>