Topics in Catalysis https://doi.org/10.1007/s11244-018-1072-5 ORIGINAL ARTICLE Notable Reactivity of Acetonitrile Towards Li2O2/LiO2 Probed by NAP XPS During Li–O2 Battery Discharge Tatiana K. Zakharchenko1 · Alina I. Belova1 · Alexander S. Frolov1 · Olesya O. Kapitanova1 · Juan‑Jesus Velasco‑Velez2 · Axel Knop‑Gericke2,5 · Denis Vyalikh3,4 · Daniil M. Itkis1 · Lada V. Yashina1 © Springer Science+Business Media, LLC, part of Springer Nature 2018 Abstract One of the key factors responsible for the poor cycleability of Li–O2 batteries is a formation of byproducts from irreversible reactions between electrolyte and discharge product Li 2O2 and/or intermediate LiO2. Among many solvents that are used as electrolyte component for Li–O2 batteries, acetonitrile (MeCN) is believed to be relatively stable towards oxidation. Using near ambient pressure X-ray photoemission spectroscopy (NAP XPS), we characterized the reactivity of MeCN in the Li–O2 battery. For this purpose, we designed the model electrochemical cell assembled with solid electrolyte. We discharged it first in O2 and then exposed to MeCN vapor. Further, the discharge was carried out in O2 + MeCN mixture. We have dem- onstrated that being in contact with Li–O2 discharge products, MeCN oxidizes. This yields species that are weakly bonded to the surface and can be easily desorbed. That’s why they cannot be detected by the conventional XPS. Our results suggest that the respective chemical process most probably does not give rise to electrode passivation but can decrease considerably the Coulombic efficiency of the battery. Keywords Li–O2 battery · In situ NAP XPS · Acetonitrile · Side reactions 1 Introduction Li–O2 batteries promise extraordinary high specific energy that makes them interesting for the next generation power technologies [1, 2]. Unfortunately, at the moment many obstacles hinder the development and practical applica- Electronic supplementary material The online version of this tion of such type of batteries [3, 4]. One of such issues is article (https ://doi.org/10.1007/s1124 4-018-1072-5) contains supplementary material, which is available to authorized users. poor cycle life associated with side reactions that involve the major discharge product lithium peroxide (Li 2O2) and/ * Lada V. Yashina or discharge intermediate lithium superoxide (LiO 2), elec- [email protected] trode materials [5–7] and electrolytes [8, 9]. As a result of 1 Lomonosov Moscow State University, Leninskie gory, side reactions, Coulombic efficiency of the battery drops Moscow, Russia 119991 down, and electrode surface is passivated by side products. 2 Depatment of Inorganic Chemistry, Fritz-Haber-Institut der Currently many research efforts are focused at studying the Max-Planck-Gesellschaft, Faradayweg 4-6, 1495 Berlin, chemical stability of electrolyte components—both sol- Germany vents and salts—under Li–O2 battery operation conditions 3 IKERBASQUE, Basque Foundation for Science, [10–12]. Reactions of the electrolytes with commercially 48011 Bilbao, Spain available and synthesized in-house Li2O2 [13, 14] and KO2 4 Departamento de Fisica de Materiales and CFM-MPC [15], which is used instead of unstable LiO 2 [16], are also UPV/EHU, Donostia International Physics Center (DIPC), often investigated. Several publications report the theoreti- 20080 San Sebastian, Spain cal simulations of electrolyte reactivity towards oxidation 5 Department of Heterogeneous Reactions, in Li–air batteries [17–19]. It was found that most of sol- Max-Planck-Institute for Chemical Energy Conversion, vents that have ever been used as Li–O battery electrolyte’s Stiftstraße 34,36, 45413 Mülheim, Germany 2 Vol.:(0123456789)1 3 Topics in Catalysis component—alkyl carbonates [20, 21], ethers [22, 23], during Li–O2 battery discharge in presence of MeCN are amides [24, 25], dimethylsulfoxide (DMSO) [14, 26]—are to currently missing. a different extent unstable towards oxidation by LiO2 or/and Here we employ near ambient pressure X-ray photoelec- Li2O2. At the same time, acetonitrile (MeCN) is considered tron spectroscopy (NAP XPS) to characterize MeCN behav- to be among relatively stable solvents [8]. ior in Li–O2 battery. We designed a model electrochemical The commonly acknowledged reaction pathway of the cell containing graphene electrode and Li-conductive solid electrolyte solvent degradation includes proton abstrac- electrolyte, while MeCN vapor was introduced in the gas tion of α-CH2 group by strong nucleophile—superoxide phase. To trace the MeCN reactivity towards Li2O2 in situ − anion (O2 ). It leads to oxidation [25] and, in addition, to we discharged the cell in O2 followed by exposure to MeCN polymerization [23] of the solvent molecules. Therefore, vapor. To evaluate possible reactions of MeCN with LiO 2 solvent tendency to oxidize is characterized by C–H disso- the discharge was continued in O2 + MeCN mixture. We ciation constant pKa, with the most stable molecules having have demonstrated that in both cases MeCN is oxidized. the highest pKa [19, 27]. Khetan et al. also show that pKa This yields species that are weakly bonded to the surface and value characterizes H-abstraction from solvent molecule at can be easily desorbed. That is why they cannot be detected Li2O2 surface [28]. Due to the low pKa value MeCN might by conventional ex situ XPS of clean Li 2O2 and KO2 sur- be unstable to some extent towards oxidation by LiO2 and faces exposed to MeCN vapors. Our results suggest that the Li2O2. respective chemical process probably does not give rise to Value of pKa is not, however, the only factor comprehen- electrode passivation but can decrease considerably the Cou- sively describing the reactivity of the solvent with strong lombic efficiency of the battery. nucleophiles. Various functional groups can be also attacked by superoxide species. For instance, in case of DMSO − O2 attacks −S=O group thus yielding –SO2 [24]. As for 2 Experimental MeCN, it is well-known that along with other nitriles of R-CN type it reacts with peroxide and superoxide anions 2.1 Preparation of Li2O2 and KO2 Layers and Their in alkaline solutions yielding the corresponding amides Exposure to MeCN Vapor RCONH2 [29]. It was also found that MeCN can undergo self-condensation in contact with KO2 containing KOH Thin layers of Li2O2 and KO2 free from contaminations were impurities, which results in 3-aminocrotonitrile [18]. Based prepared by deposition of metallic Li or K on clean graphene on these data one can suppose MeCN can reveal certain substrate under oxygen (99.995% purity, Air liquid) pressure reactivity towards both peroxides and superoxide in aprotic of 8 × 10−5 mbar in the preliminary baked preparation cham- media also. ber of photoelectron spectrometer (base pressure 1–2 × 10−9 Unfortunately, it is difficult to experimentally trace pos- mbar) at Russian-German beamline at BESSY II (Hemholtz sible chemical reactions of MeCN in Li–O2 battery. First Zentrum Berlin). As a substrate, we used commercial sin- reason is that metallic Li anode reacts with MeCN form- gle layer graphene on Cu foil (Donghuk University, South ing lithium cyanide and, therefore, Li–O2 test cells should Korea) grown by CVD [35]. It was preliminary heated at either comprise a solid electrolyte membrane, which isolates 600 °C for 40 min to obtain clean surface; the cleanness was lithium electrode [30, 31], or employ another counter elec- confirmed by the absence of O 1s signal in XPS and by the trode [32]. In addition, MeCN cannot be used as electrolyte typical shape of C-K edge NEXAFS spectra. Layers were for Li–air battery opened to the atmosphere because of its deposited from Li or K dispensers (SAES). The absence of high volatility [19]. For these reasons, few works only have carbon contaminations from the Li and K source was pre- been devoted to the evaluation of MeCN chemical stability liminary checked by the deposition of same films on a clean in Li–O2 batteries up to now. In some reports, ex situ X-ray WO3 surface and further recording C 1s spectra. photoelectron spectroscopy (XPS) showed that MeCN is Further Li2O2 and KO 2 layers were exposed to MeCN stable both in contact with Li 2O2 [33] and under Li–O2 bat- vapor with pressure of 1 mbar for 30 min. For this a vacuum- tery operation conditions [10]. It is also supported by cyclic compatible flask was filled with MeCN (anhydrous, 99.8%, voltammetry results [19]. Theoretical modelling also pre- Sigma-Aldrich) in an Ar-filled glovebox (M. Braun). The dicts negligible reactivity and strong molecular adsorption flask was attached to the preparation chamber without expo- (1010̄ ) of solvent at Li2O2 surface [34]. All in all, the data sure to air. The solvent in the flask was degassed by several reported earlier imply that acetonitrile solutions are consid- freezing and evacuation cycles and then dosed to the cham- ered to be a good choice for Li–O2 batteries. Nevertheless, ber through a leak valve. the data obtained by ex situ XPS often include certain arti- C 1s, O 1s, N 1s and Li 1s core level spectra of the clean facts related to surface contaminations and sample transfer. graphene, Li2O2 and KO2 films before and after their expo- Direct in situ observations of products that can be formed sure to MeCN were acquired using a SPECS Phoibos 150 1 3 Topics in Catalysis electron energy analyzer at a photoelectron kinetic energy theory. To verify the transition state, a frequency analysis of 200 eV. Photon energy was calibrated using 2nd order was performed. The variation of chemical shifts in XP-spec- reflection of the plane grating. All spectra were fitted by tra are calculated as a difference in electrostatic potential at Gaussian/Lorentzian convolution functions using Unifit centre of each atom of interest between reagent and reaction 2014 software. Spectral background was optimized using a product [38].