Short communicationSEI film formation on highly crystalline graphitic materials in lithium-ion batteries
Introduction
In lithium-ion batteries with either liquid or gelled polymer electrolytes, a passivating layer called the solid electrolyte interphase (SEI) [1], [2] is formed during the first charge. The SEI layer suppresses, if the film forming process is optimised, any further electrolyte decomposition and avoids the exfoliation of the graphite structure [1], [2], [3], [4]. At the same time, it allows the passage lithium ions. Thus, the SEI is the key component in the negative electrode determining the electrochemical performance and safety of the whole lithium-ion battery. However, the mechanism of its formation is rather complex and not yet completely understood.
A thorough understanding of the chemistry and morphology of this interphase layer is crucial for an improvement of the cell performance. In recent years, there have been numerous investigations of SEI-related phenomena and this field is an ongoing topic of research. Besenhard et al. [5] proposed that the SEI film is formed through the co-intercalation of solvent molecules, along with the Li+ ions, into the graphite host. However, an alternative formation mechanism of SEI formation was also proposed. According to this model, the SEI is formed by the decomposition of electrolytes on the graphite surface [6], [7]. For the formation of an effective SEI layer, both, the electrolyte composition and the material bulk and surface properties of the graphite material play an important role [5], [6], [7], [8], [9], [10], [11], [12]. It is necessary to understand the individual influences of structural defects (rhombohedral stacking faults) and surface defects on the irreversible charge capacity (often called “charge loss”). The identification of the material parameters that influence the SEI formation will be an important achievement for facilitating further improvements of carbon negative electrode materials for lithium-ion batteries [13], [14], [15], [16], [17], [18], [19], [20].
In commonly used electrolytes such as 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC), the irreversible charge loss occurring during the first reduction depends linearly on the specific BET surface area of the graphite material used [2], [18]. On the other hand, many graphite-based negative materials are not able to intercalate lithium-ions reversibly in propylene carbonate (PC)-based electrolytes. It is known that most graphitic materials with high crystallinity show exfoliation during the first electrochemical insertion of lithium in propylene carbonate. This graphite exfoliation results in an enhanced irreversible charge loss and reduced cycling stability [21], [22], [23] and, thus, in battery failure. The graphite exfoliation can be avoided if propylene carbonate is replaced by its non-substituted cyclic carbonate homologue, ethylene carbonate [4], [10], [11]. So, in the EC/PC, LiPF6 mixture, striking differences can be observed in the SEI film formation depending on the type of graphite used. One way to avoid the exfoliation of graphite is the use of electrolyte additives. Among them, vinylene carbonate (VC) is a well investigated substance that is able to form a stable polymer film at the graphite surface prior to the electrochemical reduction of the main solvents [24].
The complex interaction between the graphite electrode and the electrolyte makes the analysis of the SEI layer highly challenging. We are confident that by combining various in situ and ex situ techniques, a sufficient basic knowledge to understand these processes can be acquired. We employed different techniques as in situ DEMS and post mortem SEM along with classical electrochemical charge/discharge tests.
Section snippets
Experimental
As active anode material, either TIMREX® SLX50 (TIMCAL Ltd., Bodio, Switzerland) graphite (specific BET surface area: 4 m2 g−1) powder, or an experimental graphite powder which is further denoted graphite A (specific BET surface area: 5 m2 g−1) was used as received. Electrodes were prepared by blade-coating the graphite on a copper foil. 10 wt.% polyvinylidene difluoride (SOLEF 1015, Solvay SA) was used as binder. The electrolyte solvents EC, PC and DMC, as well as the conducting salts LiPF6 and
Results and discussion
Fig. 1 shows the different electrochemical response in the first galvanostatic lithium insertion/de-insertion in TIMREX® SLX50 graphite in 1 M LiPF6, EC/DMC 1/1 (w/w) and 1 M LiPF6, EC/PC 1/1 (w/w) as electrolyte system. In EC/DMC, SLX50 graphite shows the typical insertion properties expected for a highly crystalline graphite material. The short plateau or shoulder at ca. 0.8 V versus Li/Li+ (see arrow in Fig. 1, bottom) corresponds to the SEI film formation process. A reversible capacity of about
Conclusion
The SEI formation on graphite is a complex electrochemical surface reaction which can be influenced by many factors. The differences in electrolyte decomposition and passivation mechanisms of graphite in different electrolytes indicate that the nature of the electrolyte has an essential impact on the formation and composition of the SEI layer. In EC/PC-based electrolytes, electrochemical exfoliation can be observed. When VC is used as an additive, the SEI formation is completed prior to the
Acknowledgements
The authors wish to thank the Swiss National Science Foundation and the Swiss Federal Office of Education and Science for financial support.
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