Influence of Charge Transportation in Nitroxyl-Radical Polymer Gel on Charging Characteristics for Organic Radical Batteries

Introduction

Organic-Radical battery (ORB) is a rechargeable battery based on a radical polymer cathode. Nitroxyl-radical polymers such as poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl methacrylate) (PTMA) are used as a cathode material of the ORB.^1,2 PTMA exists in gel state in the battery because it can absorb and swell in organic electrolyte. The polymer gel electrode is considered to be suitable as a component for flexible and bendable battery. Our previous studies showed that the ORB with PTMA cathode can charge /discharge at large current with high capacity retention². Furthermore, it was also found that the high charge/discharge rate capability is attributed to the fast charge transportation of PTMA gel brought by high mobility of the electrolyte anion³. In this study the dependence of electrolyte salts on charge transportation in PTMA gel and the influence of the transportation on charging characteristics were investigated.

Experiments

1.0 M of Lithium hexafluorophosphate(LiPF₆) and 1.0 M Lithium bis(fluorosulfonyl)imide(LiFSI) in ethylene carbonate (EC)/dimethyl carbonate (DMC) are used as the electrolyte with the ionic conductivity of 11.0 mS/cm and 11.7 mS/cm, respectively, to investigate the performance of organic radical battery. Electrolyte swelled PTMA gels were prepared by dispersing PTMA powder into electrolyte. Diffusion coefficient of PTMA gels were measured from potential step chronoamperometry. The PTMA cathode containing 70-wt.% PTMA and 21-wt.% carbon as a conductive additive was used for ORB. The capacity of the ORBs cell was controlled as 2.5 mAh and measured under a charging current range from 2.5 mA(1C) to 300 mA(120C) at 20°C.

Results and discussion

The ORB showed an excellent rate capability that the 20C-charge capacity retained 90% and 95% capacity, respectively, in 1.0 M LiPF₆- and LiFSI-based electrolyte compare with 1C-charge capacity. However, the capability of much higher rate was drastically deteriorated such as less than 40% capacity retention for a 120C-charge in both LiPF₆- and LiFSI-based electrolyte. On the other hand, the use of higher concentration LiFSI (1.5 M) can improve the retention from 40% to 60% in 120C. It is considered the use of electrolyte with high concentration could solve the problem of lack of anion at the cathode surface. In contrast with LiFSI, the 120C-charge capacity of the ORB decreased to below 20% capacity in 1.5 M LiPF₆ electrolyte. It is concluded that the rate performance in 120C depends on the anion species in high concentration (1.5M) electrolyte. This is because the diffusion coefficient of PTMA gel in 1.5 M LiFSI was 4.8 x 10-9 cm2/s from the measurement of potential step chronoamperometry is almost two times higher than that of PTMA gel even in 1.0 M LiPF₆. It is suggested high charge transportation of the PTMA gel with LiFSI can be used for ultra-high rate organic radical batteries.

References

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