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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-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 (NaPF6) 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 degrees 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 degrees 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.

In selecting electrolytes for Na-ion batteries, simply importing the analogue of common lithium-ion battery electrolytes to sodium-ion batteries does not address safety concerns like toxicity and flammability. Electrolytes based on sodium bis(oxalato)borate (NaBOB) in organophosphates like triethyl phosphate (TEP) largely alleviate these specific safety concerns. However, it may be beneficial to obtain solutions with higher ionic conductivities than NaBOB in TEP, and compare the performance in Na-ion batteries with high mass loading electrodes. Here, we have shown that N,N-dimethyl formamide (DMF), N,N-dimethyl acetamide (DMAc), and N-methyl pyrrolidone (NMP) cosolvents almost double the ionic conductivity (from similar to 3.5 mS/cm to similar to 7 mS/cm) without sacrificing the flame-retarding properties of the base NaBOB in TEP electrolyte. The physical properties of these cosolvent electrolyte mixtures are investigated, along with the electrochemical performance of these electrolytes full-cells based on hard carbon anodes and Prussian white cathodes with near-commercial areal capacity (similar to 2 mAh/cm(2)).

For sodium-ion batteries, two pressing issues concerning electrolytes are flammability and compatibility with hard carbon anode materials. Non-flammable electrolytes that are sufficiently stable against hard carbon have—to the authors’ knowledge—previously only been obtained by either the use of high salt concentrations or additives. Herein, the authors present a simple, fluorine-free, and flame-retardant electrolyte which is compatible with hard carbon: 0.38 m sodium bis(oxalato)borate (NaBOB) in triethyl phosphate (TEP). A variety of techniques are employed to characterize the physical properties of the electrolyte, and to evaluate the electrochemical performance in full-cell sodium-ion batteries. The results reveal that the conductivity is sufficient for battery operation, no significant self-discharge occurs, and a satisfactory passivation is enabled by the electrolyte. In fact, a mean discharge capacity of 107 ± 4 mAh g−1 is achieved at the 1005th cycle, using Prussian white cathodes and hard carbon anodes. Hence, the studied electrolyte is a promising candidate for use in sodium-ion batteries.

Metallic bismuth is here studied as an anode material for sodium-ion batteries. The details of electrochemical redox reactions, rate performance and cycled life were investigated using relatively high mass loading electrodes in two- and three-electrode full-cells. It demonstrated that the rate capability of bismuth anodes with high mass loading are not as good as indicated in previous literatures where low mass loading electrodes were used. It also indicated that the resistances causing a faltering rate performance may be connected to a loss in particle contact during desodiation. Efforts were also made to study the different electrochemical processes that occur during early cycles. Less advantageous characteristics of bismuth electrodes are also discussed. For example, several different electrolyte solutions were tested for compatibility with the bismuth system, where only glyme-based solutions seemed to facilitate robust cycling.

Anodic dissolution of aluminum (commonly called aluminum corrosion) is a potential issue in sodium-ion batteries. Herein, it is demonstrated how different sodium-ion battery electrolyte solutions affect this phenomenon. The type of electrolyte was critical for the presence of anodic dissolution, while the solvent appeared to alter the dissolution process.

Electrolytes based on sodium bis(oxolato)borate (NaBOB) in organophosphates (trimethyl phosphate and triethyl phosphate (TEP)) have shown promise in sodium-ion batteries when compared to conventional electrolytes in that they are fire resistant, fluorine-free and are of lower toxicity. However, these electrolytes tend to exhibit low initial Coulombic efficiency and high overpotentials. We have here demonstrated that NaBOB in TEP can be used in cells with near-commercial capacity loadings. Furthermore, we have shown that formation cycle conditions have a significant positive effect on the cell performance in these higher mass loading cells, and that modification of the formation cycle conditions can be used to increase the capacity retention, lower the overpotentials, and as such increase the rate capability. The viability of optimized formation protocols was also demonstrated in scaled up prototype cells. Formation cycling: In this article we present a solution to the poor cyclability of non-flammable sodium bis(oxolato)borate (NaBOB) in triethyl phosphate (TEP) electrolyte. By developing a formation cycle specific to this NaBOB in TEP electrolyte, we are able to cycle a 4.5 Ah full cell with high mass loading electrodes to beyond 900 cycles before reaching 80 % state of health.image