Lithium insertion into carbonaceous anode materials prepared by electrolysis of molten Li-K-Na carbonates

Carbon powders were prepared by electroreduction of molten lithium-sodium-potassium carbonates at 450°C. The influence of the potential and the heat-treatment after washing in HCl solution on their electrochemical performances when they are used as anode in Li-ions battery were studied. Correlation between the presence of surface disordering (Raman spectroscopy) and the presence H, Li, K, and Na on the outermost layer on the powder surface (secondary ion mass spectroscopy), and their electrochemical performances was pointed out: samples having both the higher surface disordering and the lower H, Li, K, and Na content on their surface exhibit the lowest electrochemical performances. The best results were obtained for carbon deposited at -2.4 V vs. CO₂-O₂ and heat-treated at 400°C: the reversible capacity obtained in 1 M LiPF₆-ethylene carbonate:diethyl carbonate:dimethyl carbonate is 1080 mAh g^-1 (composition of Li_2.9C₆). This value is 2.9 times higher than the theoretical one observed with graphite (372 mAh g^-1, composition of LiC₆). The potential profile obtained in galvanostatic mode is intermediate between that usually observed for graphite and amorphous carbon, because in the potential range 1.5-0.3 V vs. Li/Li⁺, the potential profile shows rather continuous charge-discharge curves sloping, and between 0.3 and 0.02 V vs. Li/Li⁺, phase transformations between different stages occur successively as in the case of pure graphite.