The redox processes of
Li2Sn false(6⩽n⩽12false)
at a glassy carbon electrode in THF was studied by programmed cyclic voltammetry in the range of +1300 to −2000 mV (vs. polysulfide reference electrode) at sweep rates of 2–200 mV/s. One anodic and up to three cathodic peaks were detected. The anodic peak seems to result from the oxidation of all PS's through the same intermediate to elemental sulfur. The first cathodic peak is caused by the reduction of all PS
false(n>6false)
to
Li2S6
in a diffusion controlled reaction. The second reduction peak most likely arises from the reduction of
S62−
to
S52−
. This is apparently preceded by a chemical step. The third reduction peak is caused by the reduction of
S52−
to
S22−
or S2− or a mixture of both in a diffusion‐controlled reaction. The high Tafel slope of the third peak apparently results from passivation of the electrode by the precipitation of
Li2S
and
Li2S2
.
The correlation between the electrochemical properties of Li carbon intercalation electrodes and their surface chemistry in solutions was investigated. The carbons investigated were primarily graphite and petroleum coke, and the solvent systems included methyl formate (MF), propylene and ethylene carbonates, ethers and their mixtures. The surface chemistry of the electrodes was studied using mainly diffuse reflectance Fourier transform infrared spectroscopy. The following aspects were studied: (i) the effect of temperature on the buildup of the surface films; (it) the effect of additives (e.g., CO2, crown ethers), (iii) the behavior when the passive layer is built in one solution followed by cycling in another; and (iv) the
The electrochemical behavior of Li-graphite intercalation anodes .in ethylene and diethyl carbonates (EC-DEC) solutions was studied using surface sensitive Fourier transform infrared spectroscopy (FTIR) and impedance spectroscopy in conjunction with standard electrochemical techniques. Three different solvent combinations, four different salts: LiBF4, LiPF6, LiC104, and LiAsF6, and the influence of the presence of CO2 were investigated. Graphite electrodes could be cycled hundreds of times obtaining a reasonable reversible capacity. The best electrolyte was found to be LiAsF6 and the presence of CO2 in solutions considerably increased the reversible capacity upon cycling. This improved performance is due to precipitation of the ethylene carbonate reduction product, (CH2OCO2Li)2, which is an excellent passivating agent, on the electrode surface. Aging processes of these surface films and their influence on the electrode properties are discussed.
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