A class of liquid anode for rechargeable batteries with ultralong cycle life

Low cost, highly efficient and safe devices for energy storage have long been desired in our society. Among these devices, electrochemical batteries with alkali metal anodes have attracted worldwide attention. However, the practical application of such systems is limited by dendrite formation and low cycling efficiency of alkali metals. Here we report a class of liquid anodes fabricated by dissolving sodium metal into a mixed solution of biphenyl and ethers. Such liquid anodes are highly safe and have a low redox potential of 0.09 V versus sodium, exhibiting a high conductivity of 1.2 × 10−2 S cm−1. When coupled with polysulfides dissolved in dimethyl sulfoxide as the cathode, a battery is demonstrated to sustain over 3,500 cycles without measureable capacity loss at room temperature. This work provides a base for exploring a family of liquid anodes for rechargeable batteries that potentially meet the requirements for grid-scale electrical energy storage.

impedance spectrum of 1 M 0.5Na-BP-DME (cell constant K=1.05) (Note that the 57 0.5Na-BP-DME refers to 0.5 mol Na removal from 1 mol Na-BP-DME.). b, 58 Electrochemical impedance spectrum of 1 M 1.5Na-BP-DME (cell constant K=1.05) 59 (Note that the 1.5Na-BP-DME refers to 0.5 mol Na uptake into 1 mol Na-BP-DME.). solutions. c, Photograph of Na 2 S 4 dissolve into DMSO solutions. Na 2 S 8 , Na 2 S 4 and 77 Na 2 S 3 solutions were prepared by dissolving Na 2 S and S into DMSO at molar ratio of 78 1:7, 1:3, 1:2 respectively, and the concentrations of Na 2 S 8 , Na 2 S 4 and Na 2 S 3 were 1 M 79 respectively. For 1 M Na 2 S 3 system, one can clearly see that there is precipitation 80 on the bottom, indicating that the solubility of Na 2 S 3 in DMSO is less than 1 M. In 81 contrast, we do not observe any precipitation for 1 M Na 2 S 4 and Na 2 S 8 system, which  which nickel foam was used as the Na-BP-DME anode current collector and carbon 98 felt as the Na 2 S 8 cathode current collector. c, Schematic of the Na 2 S 8 |BASE| 99 Na-BP-DME cell with Na metal insertion into Na saturated BP-DME hybrid anode.

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impedance spectra of Na-BP dissolved in different ether solvents: DME, DEGDME, 142 TRGDME, and TEGDME. There conductivities at room temperature are calculated to 143 be 1.2x10 -2 S cm -1 , 7x10 -3 S cm -1 , 3.5x10 -3 S cm -1 , 2.5x10 -3 S cm -1 , respectively (cell's     3 M Na-BP-TEGDME anolyte. Note that the polarization is larger than that of cylinder 212 battery as shown in Fig. 4, which is mainly due to the design of the flow battery: on 213 one hand, the thickness of the BASE plate in the flow battery is 2 mm, however, the 214 thickness of the BASE tube used in the cylinder battery is 1 mm; on the other hand, 215 the thickness of the electrode in the flow battery is 10 mm while it is only 2 mm in 216 the cylinder battery. We believe that the performance can be further improved by 217 optimizing the system and engineering the cell structure, for instance, using new 218 catholyte system with higher energy density, a thinner Na-β"-Al 2 O 3 electrolyte or 219 new electrolyte with higher ionic conductivity, and a highly porous current collector.   Raw material cost of Na 2 S 8 |BASE|Na-BP-DME(4 M) cell is 11.8＄/kWh.

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All of these raw materials are non-toxic and environmentally-friendly.

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The solubility of Na in the BP-DME solution was determined by a chemical titration.

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In detail, an excessive Na metal was added into 5 M BP-DME solutions for several 356 days to ensure that the solution was saturated (named as Na saturated BP-DME 357 solution). Then, 1 mL Na saturated BP-DME solution was pipetted to react with 1 358 mL distill water (note that NaOH was produced in this reaction, which can be titrated 359 by an acid). After complete reaction, the solution was titrated by 0.225 mL HCl

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(Aldrich, 38% with density of 1.18 g cm -3 : 12.3 mol L -1 ). From this result, we can 361 calculate the solubility of Na in BP-DME is 5 M.