Influence of oxygen and hydrogen milling atmospheres on the electrochemical properties of ballmilled graphite

The effect of the grinding oxygen or hydrogen atmosphere on the physical/electrochemical properties of ballmilled graphite was studied. These properties strongly depend on the atmosphere and pressure of the milling container of the oxygen and the hydrogen contents of the samples. For instance, grinding under a 10 bars of H-2 atmosphere leads to a highly disordered carbon, while 10 bars under O-2 gives a heterogeneous carbon having a well and poorly crystallized graphite component. In both cases, the reversible (x(rev)) and irreversible (x(irrev)) capacities and the double-layer capacitance increase with milling time. The graphite ground 10 h under a p(O2) = 10 bars strongly differs from the other samples. We ascribed such behavior to the presence of a large amount of oxygenated functional groups in relation with the formation of nanoporosity and/or the predominance of edge carbon upon grinding. In contrast, under p(O2) = 0.2 bar or p(H2) = 10 bars, the variation of the Brunauer-Emmett-Teller (BET) surface area with milling time is correlated with the irreversible capacity. Differences in the voltage composition curves are also noticed with a low oxygen pressure (p(O2) = 10(-6) mbar) strongly reducing the polarization of the charge/discharge cycling curve while leading to x(rev) = 1.7 Li for LixC6. Finally, 10 h of milling (R = 24) under p(O2) = 0.2 bar were shown to produce a carbon with an x(rev) = 1.6 Li and x(irrev) = 0.5 Li for LixC6, while 80 h (R = 8) under p(O2) = 10 bars led to a carbon having a double-layer capacitance of 57 F/g in Et4N BF4. (C) 2001 The Electrochemical Society.