Article pubs.acs.org/JPCC
Analysis of the Deterioration Mechanism of Si Electrode as a Li-Ion Battery Anode Using Raman Microspectroscopy Masahiro Shimizu,†,‡ Hiroyuki Usui,†,‡ Takahiro Suzumura,† and Hiroki Sakaguchi*,†,‡ †
Department of Chemistry and Biotechnology, Graduate School of Engineering, and ‡Center for Research on Green Sustainable Chemistry, Tottori University, 4-101 minami, Koyama-cho, Tottori 680-8552, Japan S Supporting Information *
ABSTRACT: Propylene carbonate (PC) and an ionic liquid consisting of 1-[(2-methoxyethoxy)methyl]-1-methylpiperidinium (PP1MEM) and bis(trifluoromethanesulfonyl)amide (TFSA) were used as electrolyte solvents for Li-ion batteries, and the anode properties of Si electrodes were investigated using a thick film prepared by gas deposition without any binder or conductive additive. The Si electrode in PP1MEMTFSA exhibited good cycle performance with a reversible capacity of 1050 mA h g−1 even at the 100th cycle, whereas the Si electrode in PC showed a capacity of only 110 mA h g−1. It is noteworthy that the electrode performance was significantly enhanced just by changing the electrolyte solvent to an ionic liquid even with the same Si used as the active material. Raman mapping analyses of the Si anodes after cycling were conducted to clarify the deterioration mechanisms of the electrodes. It was revealed that, in the case of PC, crystalline Si locally remained in the electrode after cycling, and Li−Si alloying and dealloying reactions occurred in limited regions. This led to the generation of intensive stress accumulation due to the extreme volume changes of Si in the regions inside the electrode, causing severe disintegration of the Si electrode. Consequently, the anode property of the Si electrode in PC resulted in very poor performance. In contrast to the behavior in the organic electrolyte, Li−Si reactions uniformly took place over the entire electrode in PP1MEM-TFSA, which relatively avoided any stress accumulation that could lead to electrode disintegration. This is considered to be the reason for the significant improvement in the cycle performance of the Si electrode using the ionic liquid instead of the conventional electrolyte.
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INTRODUCTION Li-ion batteries are one of the most popular energy storage devices because of their high energy densities. In light of applications in large-scale systems such as power supplies for electric vehicles and stationary batteries, a further increase in their energy density is required. Silicon is a promising material for replacing the currently used graphite anode because of its high theoretical capacity of 3580 mA h g−1 (Li15Si4).1,2 There is, however, the critical issue that Si undergoes severe volume expansion and contraction during alloying and dealloying reactions with Li. The volumetric change per Si atom from Si to Li15Si4 corresponds to 380%, which results in the generation of high stresses and large strains in the active material.3 The strains accumulated upon repeated charge−discharge cycling cause disintegration of the Si electrode, leading to rapid capacity fading. This is the main reason hindering the practical application of Si electrodes. On the other hand, the electrolyte is one of the key factors determining battery performance. Room-temperature ionic liquids have received much attention as an alternative to a conventional organic electrolytes consisting of carbonate-based solvents because of their excellent physicochemical properties of high thermal stability, negligible vapor pressure, and wide electrochemical window.4−7 Nevertheless, only about 30 © XXXX American Chemical Society
reports on Si-based electrodes in ionic-liquid electrolytes have been published.8−11 Song et al. prepared a SiO1.3 film on a stainless steel substrate by pulsed laser deposition and studied its anode properties in an ionic-liquid electrolyte composed of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and Nmethyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)amide (Py13-TFSA).12 The SiO1.3 electrode in the ionic-liquid electrolyte achieved a reversible capacity of 930 mA h g−1 at the 200th cycle with a good capacity retention of 88%, whereas the capacity in an organic electrolyte of 1.0 M lithium hexafluorophosphate (LiPF6) dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) faded rapidly within the first 50 cycles. They investigated the characteristics of the surface layers formed on their anodes after cycling and concluded that the reason for the higher cycle performance obtained from Py13-TFSA was that the surface layer covering the SiO1.3 electrode was stable and thinner than that in the organic electrolyte.13 We also demonstrated that the cycle stability of a Si electrode can be markedly improved by using the ionic liquid 1-[(2-methoxyethoxy)methyl]-1-methylpiperReceived: December 8, 2014 Revised: January 19, 2015
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DOI: 10.1021/jp5121965 J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
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idinium bis(trifluoromethanesulfonyl)amide (PP1MEMTFSA).14 The ether functional group in the PP1MEM cation reduces the interaction between Li ions and the TFSA anions compared to a piperidinium-based ionic liquid with a cation having an alkyl side chain, which can promote Li insertion into the Si electrode and increase its reversible capacity. PP1MEMTFSA delivered a comparatively high capacity of 1050 mA h g−1 even at the 100th cycle, whereas the capacity in an organic electrolyte of 1.0 M LiTFSA dissolved in propylene carbonate (PC) decreased dramatically to 110 mA h g−1. As is wellknown, during the first charge (Li-insertion) process, electrolytes based on organic solvents are generally decomposed to form a surface layer consisting of organic and inorganic compounds on a negative electrode.15,16 Additionally, in the case of Si-based electrodes, drastic volume changes accompany Li−Si alloying and dealloying reactions, generating cracks on the electrode surface and a partial breakup of the Si electrode.17 This causes damage to the surface layer and thereby exposes a new surface of the negative electrode, and as a result, a renewed surface layer is reconstructed by decomposition of the electrolyte. The continuous formation of the surface layer makes itself thicker, which leads to an increase in the resistance to Li insertion into the Si electrode.18 Because of these reasons, the Si electrode is partially passivated, and intact (unreacted) regions remain after cycling. In other words, the utilization of the active material becomes lower, which is one of the causes of the reduction of the cycling performance of Si electrodes in organic electrolyte systems. In contrast, it has been reported that ionic-liquid-derived surface layers are stable and thinner.12,13 Meanwhile, the reason for the better cycling performances of Si electrodes in ionic-liquid electrolytes is still unclear. Understanding this phenomenon will contribute to the development of high-performance electrode materials and electrolytes. Raman spectroscopy is a powerful tool for identifying the crystallinity of active materials. Crystalline Si (c-Si) reacts electrochemically with Li to form c-Li15Si4 at room temperature.1,2 The c-Si then undergoes amorphization after the Liextraction process to transform into amorphous Si (a-Si), and the Raman band position of 520 cm−1 for c-Si typically shifts to 490 cm−1, indicating a-Si.19−21 By taking advantage of the nature of this Raman scattering, Si that has reacted with Li can be distinguished from unreacted Si, which allows the Liinsertion distribution in the Si electrode surface to be clarified. A gas-deposition method is a suitable technique for forming thick films and does not require any binder or conductive additive for the preparation of thick-film electrodes. The advantages of this method are as follows: (1) strong adhesion between the active material particles, as well as between the particles and the substrate, and (2) nearly unchanged composition in a thick film formed without atomization (e.g., vaporization) of the particles.22,23 It is thus possible to directly observe an electrochemical reaction between pure Si and electrolytes. To the best of our knowledge, this is the first report that addresses the elucidation of the deterioration mechanism of Si electrodes by Raman imaging analysis. In this study, we investigated the origin of the improved cycling performance of Si electrodes in an ionic-liquid electrolyte using PP1MEM-TFSA by analyzing the deterioration mechanisms for Si electrodes in organic and ionic-liquid electrolytes from the viewpoint of the Li-insertion distributions.
Article
EXPERIMENTAL SECTION
Preparation of Electrolytes and Electrodes. The organic electrolyte of 1.0 M LiTFSA dissolved in propylene carbonate (PC, C4H6O3; Kishida Chemical Co., Ltd.) was purchased and used without further purification (water content of
