Effects of Electrolyte Salts on the Performance of Li-O2 Batteries

The effects of lithium salts on the performance of Li-O-2 batteries and the stability of salt anions in the O-2 atmosphere during discharge/charge processes were systematically investigated by studying seven common lithium salts in tetraglyme as electrolytes for Li-O-2 batteries. The discharge products of Li-O-2 reactions were analyzed by X-ray diffraction, X-ray photoelectron spectroscopy, and nuclear magnetic resonance spectroscopy. The performance of Li-O-2 batteries was strongly affected by the salt used in the electrolyte. Lithium tetrafluoroborate (LiBF4) and lithium bis(oxalato)borate (LiBOB) decomposed and formed LiF and lithium oxalate, respectively, as well as lithium borates during discharge of Li-O-2 batteries. In the case of other salts, including lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTf), lithium hexafluorophosphate (LiPF6), lithium perchlorate (LiClO4), and lithium bromide (LiBr), the discharge products mainly consisted of Li2O2 and carbonates with minor signs of decomposition of LiTFSI, LiTf, and LiPF6. LiBr and LiClO4 showed the best stability during the discharge process. For the cycling performance, LiTf and LiTFSI were the best among the studied salts. In addition to the instability of lithium salts, decomposition of tetraglyme solvent was a more significant factor contributing to the limited cycling stability. Thus, a more stable nonaqueous electrolyte including organic solvent and lithium salt still needs to be further developed to reach a fully reversible Li-O-2 battery.