Identification of electrochemical reaction products in lithium–oxygen cells with 7Li 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 7 Li NMR. Defined 7 Li quadrupole lineshapes are not observed for cycled cathodes. Here, 7 Li nutation NMR is demonstrated to be an effective method for the identification of Li 2 O 2 in cycled cathodes. The 7 Li quadrupole interaction of Li 2 O 2 (35 kHz) and Li 2 CO 3 (120 kHz) are of similar magnitude to typically radiofrequency fields (ranging from 40 to 60 kHz). The 7 Li nutation frequency will therefore be influenced by both interactions. The discharge products of the cycled cathodes were determined by comparing the 7 Li nutation frequencies of the cycled cathodes to the 7 Li nutation frequency of the pristine materials when the applied radiofrequency field was 30 kHz. Li 2 CO 3 was determined to be the main discharge product in the propylene carbonate/dimethyl carbonate and trimethyl phosphate electrolyte systems, since the 7 Li nutation frequencies of the cathodes corresponded to the 7 Li nutation frequency of pristine Li 2 CO 3 . The 7 Li nutation frequency of the tetraethylene glycol dimethyl ether cathode was between the 7 Li nutation frequencies of both pristine Li 2 O 2 and pristine Li 2 CO 3 , indicating that both Li 2 O 2 and Li 2 CO 3 were discharge products influencing the observed nutation frequency. From 7 Li nutation NMR the novel trimethyl phosphate electrolyte was determined to be an unsuitable Li–O 2 electrolyte, as the fast 7 Li nutation frequency indicated that Li 2 O 2 was not a primary discharge species. With 17 O NMR, Li 2 CO 3 was confirmed to be a main discharge product formed with the trimethyl phosphate electrolyte.