Elsevier

Current Applied Physics

Volume 26, June 2021, Pages 72-77
Current Applied Physics

Trimethoxymethylsilane as a solid-electrolyte interphases improver for graphite anode

https://doi.org/10.1016/j.cap.2021.03.011Get rights and content

Highlights

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    Trimethoxymethylsilane is proposed to improve cycling retention of graphite anode.

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    Mechanism for formation of SEI layers is clarified by first principal calculation.

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    Silyl ether-functionalized SEI layers improve surface stability of graphite.

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    The cell cycled with trimethoxymethylsilane exhibits improved cycling retention.

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    Electrolyte decomposition is well suppressed on the surface of graphite.

Abstract

To improve the cycling performance of graphite anode materials, we propose a functional electrolyte additive, trimethoxymethylsilane (TMSi), which contains a silyl ether functional group as part of its molecular structure. First principal calculation studies, in addition to ex situ analyses, demonstrated that electrochemical reduction of ethylene carbonate (EC) gives an anionic reduced EC product. Subsequent chemical reaction with TMSi then generates solid-electrolyte interphase (SEI) layers of Si–O and Si–C functionalized carbonate on the surface of the graphite anode, which prolongs and stabilizes the cycling performance of the cells. As a result, the cell cycled with TMSi-controlled electrolyte exhibits a cycling retention of 89.5%, whereas the cell cycled with standard electrolyte suffers from poor cycling retention (84.3%) after 100 cycles.

Introduction

For the past few decades, carbon materials have received considerable attention as electrode materials for lithium-ion batteries (LIBs) [[1], [2], [3]]. Among many carbon materials, graphite (C6), composed of layered graphene structure, has served as the main anode material for conventional LIBs. However, the electrochemical redox potential of lithiation/de-lithiation into graphite anode materials is lower [4,5] than stability limit of conventional carbonate-based electrolytes [6,7], implying that the intercalation reaction of Li+ that normally occurs at the graphite/electrolyte interphases may be seriously hindered by a continuous electrolyte decomposition during cathodic polarization. In this regard, SEI (solid-electrolyte interphase) layers play an extremely important role in achieving stable cycling performance of graphite anode materials [[8], [9], [10]].

The SEI layers, which can be defined as ionic conductors but electronic insulators that develop on the surfaces of electrodes, could be formed on the surface of graphite anode by the use of functional additives in the cell. Once the electrochemical reaction of a functional additive creates SEI layers on the surface of an electrode, the resulting SEI prohibits electron transfer reactions but still allows Li+ migration between the electrode/electrolyte interphases during electrochemical charging/discharging processes. Because electrolyte decomposition is triggered by electron transfer reactions at electrode/electrolyte interphases [11,12], electrolyte decomposition could be effectively suppressed once the SEI layers have firmly formed on the surface of the electrodes. This strongly suggests that the use of a functional additive is advantageous for achieving prolonged cycling performance of graphite anode materials, which have quite low electrochemical potentials compared to conventional electrolytes. In this regard, many attempts have been made to find useful functional additives, since the graphite anode is the main anode material used in LIBs. Among the many potential candidates, vinylene carbonate (VC) has been recognized as a good candidate for graphite anodes because it forms quite extensive and solid SEI layers on the surfaces of graphite anodes. These layers provide remarkably improved cycling performance of the graphite anode [13,14]; therefore, VC is now an indispensable component of conventional electrolytes.

In this work, we propose an alternative functional additive, trimethoxymethylsilane (TMSi), as a surface modifier that can improve cycling performance of graphite anode materials (Fig. 1a). Compared to VC additives, the TMSi additive has many reactive sites that allow participation in the formation of SEI layers. TMSi can participate in the formation of SEI layers with the assistance of carbonate-based solvents: once a carbonate-based solvent (i.e., ethylene carbonate, EC) is decomposed by electrochemical reduction in the cell, it provides nucleophilic (anionic) EC intermediates, and it subsequently reacts with the electrophilic TMSi additive via chemical reactions in the cell. For this reason, understanding the underlying chemistry of the TMSi additive would provide informative clues for the development of additional functional additives for graphite anode materials. In this paper, we elucidate the chemical/electrochemical reaction pathways of the TMSi additive by atom-level analyses using first principal calculations and we evaluate the additive's electrochemical behaviors.

Section snippets

Experimental

LSV was measured by three-electrode cells assembled with a graphite working electrode and a Li metal counter and reference electrodes. The cells were scanned from 2.8 V (vs. Li/Li+) to 0.05 V (vs. Li/Li+) at a rate of 1 mV s−1. The cycling performance of the cells was evaluated with graphite anodes prepared as follows. A mixture of graphite, styrene butadiene rubber (Aldrich), carboxymethyl cellulose (Aldrich), and a carbon conducting agent (Super P) was finely dispersed in water at a

Results and discussion

Fig. 1a shows the 2D-structure of the TMSi additive, which is functionalized by a task-specific Si–O chemical moiety on the molecular structure of the TMSi additive. The TMSi-controlled electrolytes exhibit lower ionic conductivity as the amount of TMSi increases in the standard electrolyte (standard electrolyte: 8.97 mS cm−1, 0.25 TMSi: 8.86 mS cm−1, and 1.00 TMSi: 8.76 mS cm−1); however, it can still be used in conventional LIBs without any trade-off effect [19]. As already mentioned, the

Conclusions

The use of a silyl ether functionalized electrolyte additive, TMSi, was proposed to improve the cycling performance of graphite anode materials. Our LSV and XPS results indicate that the chemical and electrochemical reactions of the TMSi additive allowed the formation of SEI layers, which were composed of Si–O and Si–C functional groups, on the surface of the graphite anode. Quantum chemical simulations predicted that TMSi is less likely to be reduced than EC electrolyte, but reduced EC anion

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work was supported by the Technology Innovation Program (20011905) funded by the Ministry of Trade, Industry and Energy (MOTIE, Korea) and the Research Assistance Program (2019) in the Incheon National University.

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