E. Effect of Heat Treatment for BC2N on the Electrochemical Behavior [33]

In Fig. 35, CV curves for BQ>N treated at different temperatures are compared with each other. The BC2N(1500°C) electrode showed almost the same CV curve as that for the BC2N(1000°C) electrode, whereas the BC2N(2000°C) electrode gave only low current during anodic and cathodic scans. This is probably a consequence of the major transformation of the graphitic BQN phase into other phases during the heat treatment. Figure 36 shows the potential-time curves of the BC2N electrodes during the charge-discharge cycles. The heat treatment of the BQN samples significantly affected the charge-discharge characteristics. The electrode using BC2N(1500°C) showed the lowest potential difference be­ tween the charge (intercalation) and discharge (deintercalation) processes. The electrode of BQNpOOCFC) revealed a relatively short time for the discharge, suggesting that some side reactions occur during the charge and/or discharge processes. It is notable that there were only slight variations in the chargedischarge curves with repeated cycles. This is probably because of the slight changes in the crystal structure after the electrochemical Li intercalation (Fig. 32 and Table 4). Figure 37 shows the variation of the efficiency with the cycle number. The efficiency for BQN^OOO or 1500°C) was about 95% or higher under this cycling condition, and the electrodes did not show any degradation after repeated cycles. However, BQN^OOCFC) showed efficiencies less than

Figure 35 Cyclic voltammograms of BC2N electrodes in PC containing 1 mol dm 3 LiC104. Scan rate 1 mV s~\ Heat treated at (a) 1000°C, (b) 1500°C, and (c) 2000°C. (From Ref. 33.)

70%. These characteristics correspond to the results on the cyclic voltammetry of BQN heat treated at different temperatures (Fig. 35).