A Halogen‐Free and Flame‐Retardant Sodium Electrolyte Compatible with Hard Carbon Anodes

Abstract

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.

Recycling of Lithium‐Ion Batteries—Current State of the Art, Circular Economy, and Next Generation Recycling

“…Such designs are ideal conditions in design for recycling, but the overall electrochemical capacity and stability is still missing significant improvements to properly address the needs of the market. Some of these limitations are being addressed by the design of new sodium battery chemistries that are more environmentally benign from the start such as: new fluorine free electrolytes, [307] and biomass based hard carbons [308] among other innovations. Nevertheless, the constantly changing battery compositions are a major limitation for direct recycling, because the reactivation of cathode materials requires a singletype material input.…”

On the compatibility of high mass loading bismuth anodes for full-cell sodium-ion batteries

“…7). 26 This revealed bismuth has a significantly lower resistance than hard carbon electrodes, 5,27 despite the relatively high mass-loading of the bismuth anode. There was also a clear connection between the polarization during the rate test, with the resistances expressed within the corresponding regions since there is a clear increase of resistance at capacities about 75 mAh g −1 , see Fig.…”

Fluorine‐Free Electrolytes for Lithium and Sodium Batteries

“…In fact, full cells based on hard carbon and Prussian white have retained 57 % capacity at 1000 cycles using TMP solvent and 2000 cycles with 80 % retention using TEP, although the power fade was quite significant in the latter case. [69,114,115] Based on cyclic voltammetry performed on carbon-coated aluminum substrates (Figure 13), the voltage stability of NaBOB in TMP and NMP is rather low until passivation has occurred. Reduction occurs as early as 1.5 V vs. Na + /Na while the oxidation occurs at ca.…”

“…Another aspect of the NaBOB salt is that it appears to passivate the aluminum cathode current collector, and this too probably occurs in a similar reaction to LiBOB, [70] although this mechanism has yet to be investigated. In terms of practical application of NaBOB, it is currently restricted to use with amides such as N-Methyl-2-pyrrolidone (NMP) [114] and alkyl phosphates such as trimethyl phosphate (TMP) [69] and triethyl phosphate (TEP), [115] as these solvent classes often show much higher solubility of NaBOB than carbonates or ethers. The highest ionic conductivity (8.83 mS cm À 1 ) is attained by NaBOB in the solvent NMP at saturation concentration which corresponds to 0.66 M. The literature on NaBOB in batteries is somewhat limited thus far, especially in terms of how well it can handle higher-voltage chemistries but it has been shown to provide decent initial coulombic efficiency (up to 82 %) with hard carbon anodes.…”