Study of the Reaction Mechanism of Aqueous Li-O2 Batteries

Li-O₂ battery is one of the most expected electrochemical energy storage systems beyond lithium ion batteries, because of its high theoretical energy density.  Most of the groups are working on the non-aqueous Li-O₂ battery system whose discharge reaction is described in the equation (1).

2Li⁺ + O₂ + 2e ➔ Li₂O₂                           (1)

Even though recent studies proved that the reversibility of the reaction, there still remains a lot of fundamental problems before the development of a practical Li-O₂ battery.  Especially the overpotential of the air electrode during the charging process is an essential problem considering the energy efficiency of the battery system, hence a lot of research groups have been focusing on finding a new catalyst to reduce the overpotential. 

Besides these studies for the non-aqueous system, our group has been focusing on an aqueous Li-O₂ battery system based on the reaction (2), because overpotential of the charge-discharge reaction is much lower than that of the non-aqueous one. 　

 4Li ⁺+ O₂ + 2H₂O + 4e ➔ 4LiOH            (2)

Since the reaction is well known as the cathode reaction of alkaline fuel cell, the detail of the reaction mechanism of aqueous Li-O₂ battery has never been investigated recently.  Previously we reported that the discharging product remaining at the surface of the air-electrode was NOT LiOH but Li₂O₂.  We suspect that the saturated electrolyte solution hindered the hydrolysis of the Li₂O₂ formed during the discharging process resulting in the stabilization of the Li₂O₂ even in the aqueous electrolyte solution. 

In the present study we performed further investigation for the reaction mechanism of the aqueous Li-O₂ battery, using various analytical tools such as XRD, SEM-EDX, FTIR and so forth. 

The electrochemical studies were carried out using a three-electrode cell consists of a gas-diffusion electrode as a working electrode, a Ag/AgCl reference electrode and a Pt/Pt-black counter electrode.  The gas-diffusion electrode was prepared by mixing carbon black and PTFE powder and layered with a carbon paper.  The electrolyte solution was 10 LiCl and saturated LiOH aqueous solution.  The electrochemical tests were performed under pure O₂. 

 

SEM images for an air-electrode after the discharging process showed flake-like deposits suggesting the formation of crystalline Li₂O_2.  The deposits were mostly disappeared after the charging process.  Furthermore, an XRD pattern for a charged electrode also showed that the Li₂O₂ completely disappeared after the charging process. 

However some side-reactions could have been taken place during the charging process, because small diffraction peaks corresponding to LiOH·H₂O and Li₂CO₃ were observed.  The possible side reactions for these products could be 1. a hydrolysis of Li₂O₂ for LiOH·H₂O and 2. a decomposition of carbon electrode. 

In addition, the electrode potential observed in the charging-discharging test indicated that the actual electrochemical reaction is not based on oxygen-water couple but on oxygen-peroxide couple.  

Detailed discussions concerning the reaction mechanism will be held in the meeting.