Reaction mechanisms for the limited reversibility of Li-O_2 chemistry in organic carbonate electrolytes

摘要

The Li-O_2 chemistry in nonaqueous liquid carbonate electrolytes and the underlying reason for its limited reversibility was systematically investigated. X-ray diffraction data showed that regardless of discharge depth lithium alkylcarbonates (lithium propylenedicarbonate (LPDC), or lithium ethylenedicarbonate (LEDC), with other related derivatives) and lithium carbonate (Li_2CO_3) are constantly the main discharge products, while lithium peroxide (Li_2O_2) or lithium oxide (Li_2O) is hardly detected. These lithium alkylcarbonates are generated from the reductive decomposition of the corresponding carbonate solvents initiated by the attack of superoxide radical anions. More significantly, m situ gas chromatography/mass spectroscopy analysis revealed that Li_2CO_3 and Li_2O cannot be oxidized even when charged to 4.6V vs. Li/Li*, while LPDC, LEDC and Li_2O_2 are readily oxidized, with CO_2 and CO released from LPDC and LEDC and O_2 evolved from Li_2O_2. Therefore, the apparent reversibility of Li-O_2 chemistry in an organic carbonate-based electrolyte is actually an unsustainable process that consists of (1) the formation of lithium alkylcarbonates through the reductive decomposition of carbonate solvents during discharging and (2) the subsequent oxidation of these same alkylcarbonates during charging. Therefore, a stable electrolyte that does not lead to an irreversible by-product formation during discharging and charging is necessary for truly rechargeable Li-O_2 batteries.