On the capacity fading of LiCoO2 intercalation electrodes:: the effect of cycling, storage, temperature, and surface film forming additives
Introduction
One of the greatest successes of modern electrochemistry is the development and commercialization of Li-ion batteries. These batteries are rechargeable and possess the highest energy density available today for secondary battery systems (>150 W h/Kg) [1]. The anodes in these batteries are carbonaceous materials that can reversibly and electrochemically insert lithium. The most commonly used anodes are graphitic carbons with a capacity approaching a theoretical value of 372 mA h/g (LiC6) [2]. The electrolyte solutions usually comprise mixtures of ethylene carbonate (EC) with dimethyl carbonate (DMC), diethyl carbonate (DEC), or ethyl-methyl carbonate (EMC), and LiPF6 as the salt [3]. These components of the electrolyte solutions for Li-ion batteries were selected after 30 years of R&D related to secondary Li battery systems. The alkyl-carbonates have a wide electrochemical window (>4.5 V) [4], and are apparently stable with lithium, lithiated carbons, and all relevant cathode materials (in the potential range of 2.0–4.5 V vs. Li/Li+). It should be noted that on a thermodynamic basis, all the alkyl carbonate solvents should be reduced by lithium or lithiated carbons to form organic and inorganic carbonates (ROCO2Li, Li2CO3). Lithium or lithiated carbons are apparently stable in these solvents only due to passivation phenomena [5]. The reduction products of these solvents precipitate as surface films that cover the electrodes and block a further electron transfer from Li or Li–C to solution species. Fortunately, due to the intrinsic properties of Li salts when precipitated as thin surface films, Li-ion transport is possible through these passivating surface layers when an electric field is applied [6]. LiPF6 was selected as the preferred salt for Li-ion batteries because its solutions are highly conductive, its thermal stability seems to be better than other relevant Li salts such as LiClO4. Moreover, LiPF6 is much more environmentally friendly than Li salts such as LiAsF6, and it can be distributed at a reasonable price. However, this salt is also reactive with lithium and lithiated carbon, and can be reduced to LiF and LixPFy species [7]. It also decomposes to LiF and PF5 [8]. The latter species react readily with protic species or groups to form HF, and hence, a LiPF6 solution always contains HF contamination. However, in spite of these problems, LiPF6 still rivals other relevant salts as the most commonly used electrolyte for Li-ion batteries.
The cathode materials for Li-ion batteries are usually lithiated transition metal oxides, which undergo reversible electrochemical Li intercalation–deintercalation processes in polar aprotic Li salt solutions. The most commonly used cathode material for commercial Li-ion batteries is LiCoO2. Completed delithiation–lithiation processes of this compound involve 270 mA h/g [9].
The major practical process occurs at 4 V and involves the exchange of Li per CoO2 unit via a first order phase transition [10]. Another exchange of Li per CoO2 unit (Li0.5CoO2⇌CoO2+e−+Li+) occurs at potentials between 4.0 and 4.2 V (Li/Li+) via the formation of a solid solution [11]. Although there are reports in the literature on Li-ion batteries that deliver many hundreds of full depth of discharge (DOD) charge–discharge cycles, many studies of practical Li-ion batteries comprising LiCoO2 cathodes show that these batteries suffer from capacity fading upon cycling, especially at elevated temperatures.
The present work involves a study of the electrochemical behavior of LiCoO2 electrodes at different temperatures and storage conditions in a selected standard solution of practical importance, namely, EC–EMC/LiPF6. The goal of this study was to better understand possible capacity fading mechanisms of the LiCoO2 electrodes upon cycling and during storage. We explored the effect of two film-forming additives in solutions, namely, vinylene carbonate (VC) and an organo-borate complex (denoted as Merck's AD25). The tools for this study included XRD and SEM for structural and morphological analysis, XPS and FTIR for surface analysis, and electrochemical techniques, such as cyclic voltammetry, chronopotentiometry, impedance spectroscopy, and electrochemical quartz crystal microbalance (EQCM).
Section snippets
Experimental
LiCoO2 electrodes comprising active mass (95%), a PVdF binder (2.5%) and carbon black Super P (a conductive additive, 2.5%), on both sides of an aluminum foil (0.03 mm) current collector were obtained from LG Chem. The thickness of the electrodes was around 120–150 μm, their mass and geometric surface area exposed to the electrolyte solution were 41–42 mg and 1.80 cm2, respectively. LiCoO2 electrodes were also prepared from Merck's (KGaA) LiCoO2, PVdF, synthetic graphite particles and aluminum
Results and discussion
Fig. 1, Fig. 2 show cyclic voltammograms of lithiated cobalt-oxide electrodes measured in standard solutions and solutions containing VC or AD25 (indicated) at 25 and 60 °C, respectively (potential range of 3.25–4.25 V vs. Li/Li+). These figures present voltammograms measured at ν=0.5 and 0.05 mV/s, as indicated. The voltammograms measured at 0.05 mV/s at 25 °C (Fig. 1) are characterized by two corresponding redox peaks, which clearly reflect the delithiation–lithiation processes. These
Conclusion
LiCoO2 electrodes stored in an excess of LiPF6 solutions or cycled at elevated temperatures show an apparent deterioration in their performance. When cycled at similar rates, practical LiCoO2 electrodes stored at elevated temperatures, and even at room temperature for prolonged periods (weeks), show apparent capacity fading. Analysis of the solutions revealed that cobalt dissolves in the solutions and can be deposited as metallic cobalt on the counter electrodes (Li or Li–C anodes). However, it
Acknowledgements
Partial support for this work was obtained from the BMBF, the German Ministry of Science, in the framework of the DIP program for Collaboration between Israeli and German Scientists.
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