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The new formula electrolyte Kurun efficiency of 98%
Jul 21, 2017

Potash secondary batteries are recognized as promising candidates for future energy storage technologies due to their resource availability, low cost and sufficient battery voltage. The Yiying Wu team at Ohio State University reported a K-O2 battery with a specific energy of 935Wh / kg in 2013. Since then, people have begun to study by reducing the K metal electrode on the side effects, thereby enhancing its cycle stability.


It has been reported that different potassium secondary battery positive and negative materials, compared with carbonaceous materials, alloys or embedded compounds, the direct use of potassium as a negative electrode can provide a higher specific capacity. However, K metal has high reactivity to the electrolyte, and reversible deposition and stripping of K in the electrolyte is still a major challenge and requires a strongly passivated solid electrolyte interface (SEI) to stabilize the potassium metal surface to achieve the above aims.


The Yiying Wu team at Ohio State University for the first time demonstrated that long and highly reversible K deposition / stripping can be achieved in potassium bisfluorosulfonylimide (KFSI) - dimethyl ether (DME) electrolyte. In the absence of any surface coating or membrane modification, the K metal negative electrode was deposited and peeled at a high coulombic efficiency (~ 99%) in 200 cycles.


Further characterization confirmed (XPS and NMR), homogeneous SEI was mainly composed of the reaction product between K metal and KFSI, as well as formate and acetate from DME decomposition. Finally, the electrochemical window of the KFSI-DME electrolyte can be extended to 5V (vs K / K +) by adjusting its concentration.


The reversibility of the K deposition / dissolution process in various electrolytes was evaluated by conducting a thermostatic cycle experiment. With K as the electrode, pure copper as the working electrode and Celgard diaphragm assembly button cell. In the different electrolytes tested, KFSI-DME is the only formulation that can be used for long-term reversible K deposition and stripping.


1M KPF6-DME, 1M KTFSI-DME and 0.8M KPF6-EC / DEC are unable to perform reversible K deposition / stripping and fail due to low capacity decay and low coulomb efficiency, resulting in failure of the battery in 10-20 cycles. In contrast, KFSI-DME electrolytes using dilution (molar ratio = 0.1) and concentration (molar ratio = 0.5) can achieve 99% high coulombic efficiency in more than 100 cycles.


Figures 1b and c are the constant current charge-discharge curves and cycling performance of the K / Cu half-cell in the KFSI / DME = 0.1 electrolyte. Due to the initial side reaction between the KFSI-DME and the highly reducible K metal, the Coulomb efficiency was less than 90% in the first five cycles. However, the formation of SEI film to prevent the continuous consumption of electrolytes, with the progress of the cycle, Coulomb efficiency continues to improve, and ultimately 99%.


At 4 mA / cm2, once the remaining K accumulates on the Cu electrode side, the charging curve stops bending (a straight line), which results in the behavior of the K / K symmetrical battery. The high Coulomb efficiency of KFSI / DME = 0.5 electrolyte in the initial cycle may be due to the fact that fewer free DME molecules are reduced by potassium metal.

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