Investigation of Conversion Positive Electrode Active Materials for the Aluminum Rechargeable Battery

Introduction

 As a post lithium ion battery, new rechargeable batteries with metal negative electrodes which form multivalent cations during discharging are actively being investigated because they are expected to have high specific power, high energy density and low cost. Mg is the most popular materials for the multivalent cation battery because it shows higher energy density (3837 mAh cm^-3) than Li (2062 mAh cm^-3). Al is two times as large in energy density (8043 mAh cm^-3) as Mg. Thus Al rechargeable battery is a promising next generation power source.

 To construct the Al rechargeable battery, investigation of the positive electrode material is necessary. Preciously, we reported the Al rechargeable battery using V­­₂O₅ positive electrode and its specific capacity was about 200 mAh g^-1, however this battery can only be charged/discharged with 1/10 ~ 1/40 C rate and it is too low charging/discharging rate for practical application.1)

 In this study, we have developed the Al rechargeable battery using CuF₂ as the positive electrode to increase the high rate dischargeability. This battery can be operated with high charging/discharging rate, however, XRD data suggested that in the process of conversion reaction, CuCl and CuCl₂ was produced by substitution of F^- with Cl^-supplied from the electrolyte. So, we also investigated transition chlorides (CuCl, CuCl₂ CoCl₂, FeCl₂) as the conversion positive electrode active material for the Al rechargeable battery to increase specific power and energy density.

Experimental

 All electrochemical measurements were performed with a glass cell. The electrolyte was a mixture of aluminum chloride, dipropylsulfone and toluene with the mole ratio of 1:10:5. An Al plate with diameter of 14 mm was used as counter and reference electrodes for all experiments. A glass fiber filter was used as separator.

 44 mg of transition Chlorides and 5 mg of Ketjen black powders were mixed, and 1-Methyl-2-pyrrolidone and 4 mg of polyimide were added to the mixture to prepare a paste. The positive electrode was prepared by casting the paste onto a molybdenum current collector and drying at 230 °C.

Results and discussions

 Figure 1 shows CVs of CuF₂, CuCl and CuCl₂ positive electrodes. Almost the same oxidation and reduction peaks were observed. This suggests that CuF₂ was converted to CuCl or CuCl₂ by substitution of F^- with Cl^-during charge-discharge reactions.

 An Al rechargeable battery using CuCl and CuCl₂ as cathode material was fabricated and subjected to charge-discharge tests (Figure. 2). For the initial 15 cycles, discharge capacity was higher than 150 mAh g^-1, suggesting CuCl and CuCl₂ was a new positive electrode material for Al rechargeable battery.

 CoCl₂ and FeCl₂ positive electrodes showed oxidation and reduction peaks. However, their capacities were less than that of CuCl and CuCl₂.

Reference

1) M. Chiku et al. 224^thECS Meeting # 521