Elsevier

Thin Solid Films

Volume 546, 1 November 2013, Pages 410-413
Thin Solid Films

Si film electrodes prepared on discontinuous current collector

https://doi.org/10.1016/j.tsf.2013.05.047Get rights and content

Highlights

  • Si film electrodes with various distances between discontinuous lines were fabricated.

  • The discontinuous electrode improved coulombic efficiency and cycleability.

  • The discontinuous line in the Si film electrode enhanced the structural stability.

Abstract

Discontinuous Si film electrodes with 400, 800, and 1700 μm discontinuous lines (break lines) were fabricated by a simple masking and etching process. The structural and electrochemical properties of continuous and discontinuous Si film electrodes were investigated by means of optical microscopy, field emission scanning electron microscopy, X-ray diffraction, and charge–discharge tests. Although all electrodes showed similar first-charge capacities in the range of 210–230 μAh/g, the discontinuous electrode exhibited improved coulombic efficiency and cyclability when compared to the continuous electrode. Up to 100 cycles, the discontinuous electrode with the shortest line distance of 400 μm demonstrated the highest efficiency (95.2%) and capacity retention (89%). Observation of the cycled Si film electrodes revealed that discontinuity enhanced the structural stability of the electrode during the charge–discharge process.

Introduction

In recent studies, the available capacity of graphitic carbon anodes in commercial lithium (Li) ion batteries has approached the theoretical upper limit (~ 370 mAh/g). Among the many candidates for anode materials (Si, Sn, Ge, and Al) that could replace graphitic carbon, Si is promising due to its high theoretical capacity of 3579 mAh/g (based on the formation of the Li15Si4 phase) and low working potential (~ 0.5 V Li/Li+) [1], [2]. Its capacity is approximately 9.7 times that of graphitic carbon and is the highest for any of the Li alloys studied to date. Such characteristics allow Si electrodes to be used in the form of thin films [3], [4], [5].

Although deposited Si film electrodes show a high capacity during initial cycles, this fades rapidly with an increase in cycle number because of large volume expansion and shrinkage during the charge–discharge (lithiation–delithiation) process. The volume change of Si (~ 310%) causes surface cracking and pulverization during cycles and results in the loss of electric path. The poor electrochemical performance and structural instability are ultimately caused by repetitive mechanical stress cycles (tensile stress during charge and compressive stress during discharge) [6], [7]. Thus, the key to developing Si film electrodes with long cycle lives is the accommodation of stress generated during cycles.

Until now, many attempts have been made to improve the cycle life of Si film electrodes [8], [9], [10], [11], [12], [13], [14]. Research mainly focused on enhancing the adhesion between the Si film and the current collector (substrate) because enhanced adhesion limited the reaction of Li with Si and leads to the reduction of generated stress. However, this is insufficient for practical applications due to the high irreversible capacity loss during the initial cycle. Additional development is required to advance a Si film electrode.

In our previous work [15], we found that patterned Si film electrodes accommodated volume change and released stress generated during the charge–discharge process because the vacant space between neighboring Si structures acts as a structural buffer, preventing the agglomeration of Si. However, the large amount of vacant space in the patterned electrode reduced its capacity and an electrochemical reaction with the exposed current collector induced a large irreversible capacity loss.

In the current study, microsized breaks in the electrode resulted in a pattern of discontinuous space which was expected to minimize the capacity loss as well as accommodate the stress generated by volume change during the charge–discharge process. In addition to this, Si film electrodes with various distances between discontinuous lines were prepared to investigate the effects of the vacant space on the electrode's electrochemical properties.

Section snippets

Experimental procedure

Discontinuous Si film electrodes were fabricated in five steps: cleaning current collector, printing ink (masking), etching current collector, removing printed ink, and Si film deposition. A 25-μm-thick Cu foil was cleaned to remove impurities from the surface and used as a current collector. Ink containing iron oxide and styrene acrylate copolymer was printed on the clean Cu foil with a laser printer (HP7000, Hewlett-Packard, USA). The printed ink played the role of a mask during the etching

Structural features of discontinuous Si film electrodes

Fig. 1(a)–(c) shows OM images of the fabricated discontinuous Cu current collectors with distances between discontinuous lines (D) of 400, 800, and 1700 μm. Discontinuous lines with a zigzag shape were formed with a width of about 5 μm, where the number of lines increases with a decrease in distance between discontinuous lines. Fig. 1(d)–(f) shows the discontinuous Si film electrodes (1 cm in diameter) for the coin cell test. The electrodes were obtained after Si film deposition on the

Conclusion

Discontinuous Si film electrodes with various distances between break lines were fabricated using a simple masking and etching process. The coulombic efficiency of the first cycle and the cyclability of the Si film electrode were remarkably improved by the formation of vacant spaces in the electrode. The Si film electrode with the shortest distance between discontinuous lines exhibits the best electrochemical properties.

Acknowledgments

This research was supported by the Pioneer Research Center Program and the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (2011–0024767 and 2012015831).

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  • A review of current collectors for lithium-ion batteries

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    Citation Excerpt :

    Selective etching on Cu current collectors can effectively alleviate the volume change of Si anodes during cycling. Cho et al. selectively etched Cu foils with discontinuous lines at regular spacings and subsequently deposited Si on the etched Cu current collectors to obtain discontinuous Si anodes [105]. The discontinuous Si anode exhibits better cycle stability than continuous Si anodes, i.e. Si deposited on a conventional Cu foil current collector.

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