Fabrication of a New Tin Anode for a Lithium-Ion Battery Using a Three-Dimensional Copper Nanostructure

Introduction

Lithium-ion batteries (LIBs) are one of the most important energy storage and conversion devices and are widely used in portable electronic devices such as mobile phones and laptops. However, LIBs cannot satisfy the ever-growing needs of high-power applications due to the low energy density of commercial graphite. Tin and lithium form reversible alloys such as Li_4.4Sn with maximum composition　and have a specific capacity of 994 mA h g^-1, almost three times higher than the theoretical value for a conventional graphite anode (372 mA h g^-1). However, there are huge variations in the volume occupied by the atoms within the alloy structure because of aggregation of Sn during the electrochemical alloying-dealloying process. These volume variations result in considerable mechanical stress, which leads to rapid capacity fade (short cycle life) due to pulverization of the material. To address this issue, several strategies have been proposed to improve the cyclability of tin materials by decreasing the particle size and using porous thin films of tin. We have developed a simple method for fabricating a three-dimensional (3D) copper nanostructure using electrodeposition.

In this study, the microstructure of a tin-coated anode was analyzed and the charge/discharge characteristics of the anode were evaluated following the direct electrodeposition of a pure tin layer on the 3D copper nanostructure.

Experimental

An acidic copper sulfate bath (0.85 M CuSO₄∙5H₂O + 0.55 M H₂SO₄ + 3.0×10^-4 M polyacrylic acid M.W=5000)¹⁾ was used to fabricate the 3D copper nanostructure. Electrodeposition was conducted under galvanostatic conditions (1 A dm^-2) at 25°C without agitation. A tin alloy plating bath containing 1 M K₄P₂O₇ + 0.25 M Sn₂P₂O₇ + 0.002 M PEG +0.005 M HCHO was prepared. Electroplating was carried out under galvanostatic conditions at 25°C without agitation.

The morphology and structure of the obtained samples were examined by field emission-scanning electron microscopy (FE-SEM) and energy-dispersive X-ray spectrometry (EDX).

A cross-section polisher was used to generate cross-sections of the samples. Electrochemical measurements were obtained using coin-type cells assembled in an argon-filled glove box. LiPF₆ (1 M) in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 vol%) was used as the electrolyte solution. Cycling tests were performed between 0.02-1.5V (vs. Li/Li⁺) at a constant temperature of 25°C.

Results and Discussion

Figure 1 (a) shows a surface SEM image of the new tin anode and demonstrates the homogeneous plating of 3D copper nanostructures on the tin film.

Figure 1 (b) shows a cross-sectional SEM image of the new tin anode, revealing that the inside of the 3D copper nanostructure is also plated uniformly.

This new tin anode exhibits both high reversible capacity and improved cyclability.

Charge/discharge characteristics of this anode will be presented in detail at the meeting.

References

1)       S.Arai and T.Kitamura, ECS Electrochemistry Letters. 2014, 3 (5), D7-D9.