Reactivity of lithium battery electrode materials toward non-aqueous electrolytes: spontaneous reactions at the electrode–electrolyte interface investigated by FTIR
Introduction
The existence of chemical reactions at the electrode–electrolyte interface in lithium batteries (LB), leading to decomposition of electrolyte components is known since late 1960s [1]. Later, it was suggested that the products of these reactions create a film on the surface of lithium metal [2] and graphite [3] electrodes. This surface film, often called a “passivating layer” or a “solid electrolyte interphase” (SEI), was found to play an important role in the electrochemical processes occurring during LB cycling and has, therefore, attracted great scientific interest. Since pioneering work [2], [3], the mechanisms of formation, the composition and the different characteristics of passivating layer were intensively studied by various techniques, and the amount of relevant publications is continuously growing [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16].
The SEI formation is usually believed to take place only at the negative electrodes, such as metallic lithium or graphite compounds, whereas interfacial effects on the positive electrodes have rarely been discussed. Only recently [17], [18], the formation of passivation film (or even more exactly — appearance of new surface species) has been detected for the positive electrodes based on LiNiO2 and LiMn2O4. It is also generally accepted that only in the case of alkali-metal electrodes (due to their extremely high reactivity) a SEI may appear just at contact with an electrolyte, without applied potential [4], [5]. For all other commonly used electrode materials, formation of the passivating layer is believed to start upon cycling, and no signs of reduction of the electrolyte or electrode material solely due to the contact between them are usually observed. In our recent study [19], however, we found that new compounds and/or functional groups appear on the surface of LiNi0.75Co0.25O2-based electrode during storage in non-aqueous organic electrolytes even without an applied potential. Moreover, the type of these new compounds depends on the electrolyte used, and it was, therefore, suggested that the spontaneous reactions at the electrode surface occurs due to decomposition of the electrolyte components caused by chemical reactions between electrode active material and electrolyte.
In this work, we report the results of an extended spectroscopic investigation of the behavior of several typical electrode materials in contact with organic non-aqueous electrolytes. Different battery electrodes were stored in various electrolyte solutions over times varying from 2 to 6 weeks and thereafter examined by IR diffuse reflectance spectroscopy. In order to further discuss the mechanisms of the spontaneous surface reactions, the influence of different parameters, such as degree of lithiation of active material, specific surface area, etc. were also investigated.
Section snippets
Materials and samples preparation
The LiNi0.8Co0.2O2 and LiMn2O4 cathodic material powders were obtained from Merck. The electrodic membranes were prepared using poly(vinylidenefluoride-co-hexafluoropropilene) PVdF-HFP, Solvay or poly(ethylene-co-propylene-co-5-methyl-2-norbornene), EPDM-70, Aldrich as binder and Super P (M.M.M. Carbon Belgium) as carbon. The weight ratio of the components was (93–95) wt.% active material, (3–5) wt.% carbon, 2 wt.% binder.
After dissolving PVdF-HFP in 1-methyl-2-pyrrolidinone (Aldrich) or EPDM in
Results and discussion
The products of spontaneous interfacial reactions observed for different electrode–electrolyte combinations are summarized in Table 1. The detailed description of the infrared spectra along with identification of the surface species appearing upon electrode storage is presented below.
Conclusion
The main result of the present study is the observation of the occurrence of spontaneous electrode–electrolyte reactions for some commonly used lithium ion cathode materials in contact with typical organic non-aqueous electrolytes. In the absence of applied potential, the main type of reactions is the reduction of the solvent molecules and salt anions, the main cause suggested to be a spontaneous lithium deintercalation from the electrode’s active material. The rate and the type of these
Acknowledgements
The authors wish to thank Prof. M. Armand (Canada) for helpful discussion and providing some of the samples. D.O. acknowledges MISTRA program (Sweden) for financial support. B.S. and F.R. acknowledge CNR, PF MSTA II for financial support, contract no. 99.01869.PF34.
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