Electrochemical Reaction on Plane Carbon As Cathode Electrode of Lithium Oxygen Battery
New mobility with low CO2 emission is strongly desired to realize a sustainable society. Before now, a lot of attempts such as the reduction in size and weight of vehicle and the improvement of fuel efficiency of engine have been done.
The cruising range of PHV or EV is not enough, because the range is closely related with the energy density of batteries and it is not so high at present. In order to widely spread electrical vehicles such as EV or PHV, the energy density of rechargeable batteries must be highly increased.
Metal air battery is a promising candidate of innovative batteries. Especially, much attention has been devoted to lithium-air battery because of high cell voltage and high theoretical capacity of anode. During discharging, Li metal is easily dissolved on an anode and the dissolved Li ion reacts with O2 gas on a cathode, forming lithium oxide as a discharged product. On the contrary, a charging process as the reverse reaction has been considered to be very difficult, because the discharged product is insulative. Recently, some groups have succeeded in discharging and recharging of the batteries 1-4). Our research group also showed that ionic liquid was good for controlling cathode reaction process against side reaction 5-7).
Additional to mention, plane electrode is suitable for analysis of discharge deposit morphology in cathode reaction 8-10).
HOPG was commercialized one as a plane carbon electrode. And HOPG cathode electrode was set parallel to anode electrode in our battery cell. Electrolyte was PP13/TFSA ionic liquid including Li salt, and discharge test was carried out in temperature of 60 degree. Before and after discharge battery test, surface of HOPG was observed with ex-situ SPM measurement.
3. Results and Discussion
Rough SPM image of HOPG surface before battery test was shown in Fig.1. Flat surface and gaps were observed. Future careful observation was carried out on flat surface. As a result, there are many small gaps lower than 1 nm height (Fig.2,3), and gaps seemed to be distributed in mesh shape. Next, HOPG surface after discharge test was observed. As shown in Fig.4. At the beginning of discharge process, distribution of discharge deposit was mesh shaped. From the results of SPM, we found that very small gap (less than 1nm) seemed to be starting point of discharge deposit. Result of electrochemical or outcomes of XPS measurement will be presented in detail.
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