ИСТИНА

Li-O2 batteries promise extraordinary high specific energy that makes them interesting for the next generation power technologies [1]. Unfortunately, at the moment many obstacles hinder the development and practical application of such type of batteries. One of the issues is poor cycle life associated with side reactions, which involve the major discharge product lithium peroxide (Li2O2) and/or discharge intermediate lithium superoxide (LiO2), electrode materials and electrolytes [2]. As a result of side reactions electrode surface is passivated by side products and Coulombic efficiency of the battery drops down. Currently many research efforts are focused at studying the chemical stability of electrolyte components – both solvents and salts - under Li-O2 battery operation conditions. It was found that most of solvents that have ever been used as Li-O2 battery electrolyte’s component – alkylcarbonates, ethers, amides, dimethylsulfoxide (DMSO) – are to a different extent unstable and can be oxidized by LiO2 or/and Li2O2. At the same time, acetonitrile (MeCN) is considered to be among relatively stable solvents [3]. Unfortunately, it is difficult to experimentally trace possible chemical reactions of MeCN in Li-O2 battery due to its reactivity with metallic lithium and high volatility. For these reasons only few works have been devoted to the evaluation of MeCN chemical stability in Li-O2 batteries up to now. The data reported earlier using XPS [4] and cycling voltammetry [5] imply that acetonitrile solutions are considered to be a good choice for Li-O2 batteries. Nevertheless, the data obtained ex situ XPS often includes certain artifacts related to surface contaminations and sample transfer. Direct in situ observations of products that can be formed during Li-O2 battery discharge in presence of MeCN are currently missing. Here we employ near ambient pressure X-ray photoelectron spectroscopy (NAP XPS) to characterize MeCN behavior in Li-O2 battery. We elaborated the electrochemical cell, which contains the graphene electrode and Li-conductive solid electrolyte. Solvent vapor was admitted in the gas phase. To trace MeCN reactivity towards Li2O2, we discharged the cell in O2 and then the electrode covered by Li2O2 MeCN vapor. The discharge was then continued in O2+MeCN mixture to evaluate possible reactions of MeCN with short-living discharge reaction intermediate LiO2. We demonstrated that in both cases MeCN was oxidized yielding species that are weakly bonded to the surface and can be easily desorbed in vacuum. XPS of clean peroxide and superoxide, which were synthesized chemically in ultra-high vacuum chamber and further exposed to MeCN vapors, fortunately evidences for low oxidation reaction rate.