Chloride Ion Batteries Based on Metal Oxychloride/Metal Electrode Systems

Abstract

Rechargeable batteries are receiving particular attention in diverse areas of portable electronic devices, electric vehicles (EV) and other energy storage systems. We previously reported the proof-of-principle of a new concept of rechargeable batteries based on chloride shuttle, i.e., chloride ion batteries, by means of charge and discharge test, cyclic voltammetry, X-ray diffraction, and X-ray photoelectron spectroscopy. This battery system includes a metal chloride/metal electrochemical couple and an electrolyte composed of binary ionic liquids allowing chloride ion transfer at room temperature. The reactions for this battery could be expressed as:Cathode: McCln ↔ Mc + nCl- (1)Anode: Ma + mCl- ↔ MaClm (2)Cell: mMcCln + nMa ↔ mMc + nMaClm (3)where Mc is the metal element in the cathode, Mais the metal used in the anode, and m or n is the number of chloride ions. Lithium may be included as reaction partner but not necessarily, depending on the choice of the electrochemical couple. The concept has the advantage of a broad variety of potential electrochemical couples with high theoretical energy density up to values of 2500 Wh/L, which is close to the theoretical energy density of the Li/S battery. Moreover, chloride ion batteries can be built from abundant material resources, due to the lack of Li they can be safer, and have environmentally friendly features. These attributes could make the chloride ion battery a potential alternative in the field of rechargeable batteries. A key challenge is to suppress the dissolution of cathode materials mainly composed of transition metal chlorides, which are Lewis acid and can react with a Lewis base containing chloride ion, resulting in the formation of soluble complex ions. Metal oxychlorides are regarded to be stable in the ionic liquid electrolyte of chloride ion batteries. Moreover, metal oxychloride/metal systems could also show a large Gibbs free energy change yielding a high electromotoric force (EMF) during the chloride ion transfer. For instance, the VOCl2/Li electrochemical couple possesses an EMF of 2.78 V and a theoretical energy density of 984.2 Wh g-1. The overall reaction for this battery could be expressed as:mMcOCln + nMa ↔ mMcO + nMaClm (4) The electrochemical performance and the reaction mechanisms of the BiOCl and FeOCl cathode were investigated, where Li was used as anode. In first experiments, the BiOCl cathode showed a reversible capacity of about 60% of the theoretical capacity, which is mainly derived from the conversion reaction. An intercalation reaction during cycling is also observed. In the discharge stage, the BiOCl loses the chloride ion and transforms to Bi metal and amorphous Bi2O3, with a considerable morphology change from needle-like to granular. The return of chloride ion during charge results in the recovery of BiOCl phase at the cathode side. The FeOCl cathode also showed reversible reactions based on the chloride shuttle. A discharge capacity of 158 mAh g-1 was measured in the first cycle and a stable discharge capacity of 60 mAh g-1after 30 cycles. These results suggest that metal oxychlorides are a promising cathode materials for chloride ion batteries. A key advantage of chloride ion batteries is the use abundant materials such as Mg, La, Ca and Na as anode materials. We found that Mg is promising as anode material based on our new results. For instance, the BiOCl cathode (BiOCl/Mg) showed a discharge capacity of 70 mAh g-1, i.e., 68% of theoretical capacity at the second cycle. The electrochemical performance of metal oxychloride/Mg systems was investigated including single electron or multi-electron cathode. Moreover, a new approach was tested using multi-electron metal oxychloride cathode and Mg/MgCl2 composite anode.