Correlation between surface chemistry, morphology, cycling efficiency and interfacial properties of Li electrodes in solutions containing different Li salts

The influence of the Li salt used on the behaviour of Li electrodes in tetrahydrofurane (THF) and propylene carbonate (PC) solutions was investigated. The salts studied included Li halides (LiBr, LiI), LiBF4, LiPF6, LiSO3CF3 and LiN(SO2CF3)2. The correlation between the electrochemical properties, surface chemistry and morphology of Li electrodes in the above systems was studied using impedance spectroscopy, surface sensitive in situ and ex situ FTIR, X-ray microanalysis, electron microscopy and standard experiments of charge discharge cycling. It was found that all the salt anions explored have strong effects on all the above aspects, eg they strongly affect Li surface chemistry in solutions and participate in the build-up of surface films. The electrical properties of the Li-solution interphase formed in the different salt solutions are remarkably dependent on the salt anion. Consequently, the morphology and Li utility in repeated charge-discharge cycling are also strongly influenced by the salt used. Except for the Li halides, all the salts studied seem to be more reactive to lithium than LiClO4 and LiAsF6. They are worse than the commonly used LiAsF6 for rechargeable Li battery systems because their strong involvement in the Li surface chemistry adversely affects Li cycling efficiency.