Identification of electrochemical reaction products in lithium-oxygen cells with Li-7 nutation spectroscopy

In the Li-O-2 battery system, it is has been shown to be challenging to differentiate the discharge products or determine the electrolyte stability with direct Li-7 NMR. Defined Li-7 quadrupole lineshapes are not observed for cycled cathodes. Here, Li-7 nutation NMR is demonstrated to be an effective method for the identification of Li2O2 in cycled cathodes. The Li-7 quadrupole interaction of Li2O2 (35 kHz) and Li2CO3 (120 kHz) are of similar magnitude to typically radiofrequency fields (ranging from 40 to 60 kHz). The Li-7 nutation frequency will therefore be influenced by both interactions. The discharge products of the cycled cathodes were determined by comparing the Li-7 nutation frequencies of the cycled cathodes to the Li-7 nutation frequency of the pristine materials when the applied radiofrequency field was 30 kHz. Li2CO3 was determined to be the main discharge product in the propylene carbonate/dimethyl carbonate and trimethyl phosphate electrolyte systems, since the Li-7 nutation frequencies of the cathodes corresponded to the Li-7 nutation frequency of pristine Li2CO3. The Li-7 nutation frequency of the tetraethylene glycol dimethyl ether cathode was between the Li-7 nutation frequencies of both pristine Li2O2 and pristine Li2CO3, indicating that both Li2O2 and Li2CO3 were discharge products influencing the observed nutation frequency. From Li-7 nutation NMR the novel trimethyl phosphate electrolyte was determined to be an unsuitable Li-O-2 electrolyte, as the fast Li-7 nutation frequency indicated that Li2O2 was not a primary discharge species. With O-17 NMR, Li2CO3 was confirmed to be a main discharge product formed with the trimethyl phosphate electrolyte.