Elsevier

Electrochimica Acta

Volume 51, Issue 19, 20 May 2006, Pages 3890-3894
Electrochimica Acta

Improving the performance of soft carbon for lithium-ion batteries

https://doi.org/10.1016/j.electacta.2005.11.004Get rights and content

Abstract

A novel technique for designing a robust solid electrolyte interface (SEI) on the negative electrodes of lithium-ion batteries has been developed using a silane coating. Two silane compounds, 3,3,3-trifluoropropyltrimethoxysilane (TFPTMS) and dimethoxybis(2-(2-(2-mothoxyethoxy)ethoxy)ethoxy)silane (1ND3(MeO)), have been investigated with respect to improving the capacity retention of lithium manganese oxide spinel/soft carbon cells. The impact of the silane coating on the soft carbon electrode will be attributed to (1) changes in surface functional groups, (2) compositional change of the SEI, and (3) changes in the kinetics of manganese deposition. The impact of the upper cutoff voltage on the capacity retention of the cell was also discussed.

Introduction

Lithium-ion batteries have been widely used in portable electronics. Recently, high-power lithium-ion batteries were also proposed for the development of hybrid electric vehicles (HEV) and electric vehicles (EV) because of their high energy densities [1], [2], [3]. LiCoO2 is dominant positive electrode material in the current lithium-ion battery market. However, the depletion and price hiking of cobalt have motivated the research and development of lithium-ion batteries based on lithium manganese oxide spinel materials, which are much cheaper, safer, and more environmentally friendly than Co- or Ni-containing positive electrode materials.

Although, spinel-based lithium-ion batteries are promising for high-power applications, their calendar life and cycling life are significant limiting issues, especially for EV or HEV applications, because batteries consisting of a spinel cathode and a carbonaceous anode suffer from a fast capacity fade, particularly at elevated temperatures [4], [5], [6], [7], [8], [9]. It is widely accepted that dissolution of Mn2+ from the positive electrode and the deposition of Mn on the surface of the negative electrodes are responsible for the fast capacity fading of the cells; the manganese migration problem becomes more severe at elevated temperatures. Several strategies have been proposed to improve the capacity retention of spinel/carbon or spinel/Li cells. One solution has involved placing a metal oxide coating on the lithium manganese oxide particles to suppress the dissolution of Mn2+ from the positive electrode [8], [10], and it has been reported that the doped lithium manganese oxide has improved capacity retention [9]. It was also proposed that HF, which is one of the products of LiPF6 decomposition with the presence of moisture, plays a key role in dissolving Mn2+ from the charged spinel material (λ-MnO2). Therefore, a non-fluorinated lithium salt (LiBOB) was proposed to replace LiPF6, and hence suppress manganese dissolution [11]. Alternatively, lithium-rich spinel materials (Li1+xMn2−xO4) were specially formulated to stabilize the charged spinel materials [11], [12].

It has also been reported that the metallic manganese deposited on the negative electrode surface is responsible for the fast capacity fading of the negative electrode. It is also believed that the deposited manganese accelerates some side reactions at the negative electrode surface [4]. However, very little work on the anode side has been reported to suppress such a negative impact. In this work, the critical importance of the surface chemistry of carbonaceous materials was addressed. A silane coating technique was developed to modify the surface of carbonaceous materials, thereby helping to form a robust solid electrolyte interface (SEI) layer on the anode surface. The experimental results clearly demonstrate that the surface coating on soft carbon significantly improves the cycling performance of spinel/carbon cells, even at elevated temperatures. The diversity of the silane compounds provides researchers with flexibility in designing the surface chemistries of carbonaceous materials.

Section snippets

Preparation of negative electrodes

The carbonaceous material investigated was a soft carbon anode. The silane compounds used were 3,3,3-trifluoropropyltrimethoxysilane (TFPTMS) and dimethoxybis(2-(2-(2-mothoxyethoxy)ethoxy)ethoxy)silane (1ND3(MeO)) (see Fig. 1). The TFPTMS was purchased from Sigma–Aldrich, and the 1ND3(MeO) was supplied by Dr. Robert C. West at the University of Wisconsin-Madison. The carbonaceous material was pre-mixed with poly(vinylidene fluoride) (PVDF) in N-methyl-2-pyrrolidinone (NMP, Aldrich) in a 92:8

Impact of silane coating on soft carbon half cells

The soft carbon was investigated with great interest because of its promising application in high-power lithium-ion batteries. A recently developed soft carbon has an improved initial irreversible capacity loss, which is currently similar to that of mesocarbon microbeads (MCMB). However, the disordered structure of soft carbon was believed to provide more pathways for lithium transportation, leading to a better power capability. In this work, this material was chosen to investigate the impact

Conclusion

A novel technique based on silane coating has been developed to improve the cycling performance of lithium-ion cells consisting of a lithium manganese oxide spinel (LiMn2O4) cathode and a carbonaceous anode. It has been clearly demonstrated that a silane compound containing a poly(ethoxy) terminal group is highly desired to assist lithium-ion transportation through the surface film on soft carbon. The PEO-based group was also shown to be important in maximizing the capacity retention of cells,

Acknowledgements

The authors acknowledge the financial support of the U.S. Department of Energy, FreedomCAR & Vehicle Technologies Program, under contract No. W-31-109-Eng-38. Also, the authors are very grateful for the continued support of their DOE sponsor, Mr. Tien Duong. The authors additionally warmly thank Dr. Robert C. West at the University of Wisconsin-Madison for supplying the 1ND3(MeO) sample.

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The submitted manuscript has been created by the University of Chicago as Operator of Argonne National Laboratory (“Argonne”) under Contract No. W-31-109-ENG-38 with the U.S. Department of Energy. The U.S. Government retains for itself, and others acting on its behalf, a paid-up, nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.

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