Construction of Lithium-Ion Battery Tin Anode Utilizing Cu/CNT Composite Plating Method
Authors: Kobayashi Masaki , Shimizu Masahiro , Arai Susumu 
Published: February 2020

Introduction
Lithium-ion batteries are compact and lightweight secondary batteries with high capacity. They have already been put into practical use in electric and hybrid vehicles, but the development of lithium-ion batteries with higher capacity has advanced even further. Although graphite (theoretical capacity, 372 mAh g-1) is typically employed as the negative electrode active material, the development of a new, higher capacity negative electrode material using tin (theoretical capacity, 991 mAh g-1) as the active material has been investigated.1) However, the volume of tin changes significantly during charge-discharge cycles, and as a result, the tin slips down from the current collector, lowering the lifetime of the lithium-ion battery. Anchoring at the interface between the tin layer and the underlying copper layer to improve the adhesion to the fibrous material is considered to be an effective method of resolving this problem.2) In this study, an anode structure in which carbon nanotubes (CNTs) anchor the tin layer and the copper current collector was fabricated, and its charge-discharge characteristics were evaluated.

Experimental
An EDTA copper plating bath was used as the basic plating bath. To prepare the plating bath for Cu/CNT composites, CNTs, sodium dodecyl sulfate (SDS), and hydroxypropyl cellulose (HPC) were added to the basic bath. The latter two were used as dispersing agents for the CNTs. The CNTs employed (Baytubes C70P, Bayer Material Science Co., Ltd.) were multi-walled (MWCNTs), with a diameter of 13–16 nm and a length of 10–20 μm. The pH was adjusted to 12 with KOH.3) The tin plating bath used was a pyrophosphoric acid tin plating bath. To prepare it, polyethylene glycol (PEG) and formaldehyde (HCHO) were added to a pyrophosphoric acid tin plating bath. PEG was used as a smoothing agent and HCHO was used as a reducing agent. Electroplating was performed under current regulation conditions, using a pure copper anode and a cathode. Reverse electrolysis was carried out to exposing CNTs in Cu/CNT composite plating films. The microstructure of the samples was observed using field-emission scanning electron microscopy (FE-SEM), and the phase structure of the deposited material was analyzed using X-ray diffraction (XRD). Finally, electrochemical studies on the prepared plating films were carried out with coin cells that were assembled in an Ar-filled glove box. Each coin cell consisted of a lithium foil as the counter electrode and the prepared plating films as the working electrode. The electrolyte was 1 M LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 vol%). Cycling tests were performed in the range of 0.02–1.5 V (vs. Li/Li+).

Results and Discussion
Cu/CNT composite plating films containing many CNTs were fabricated by electrodeposition. CNTs in the produced Cu/CNT composite plating film was incorporated with little agglomeration. The morphology of the electrodeposited tin on the composite films was found to depend on the current density. When the current density was low, tin was deposited primarily on the copper underlayer (see Fig. 1a). In contrast, when the current density was high, tin was deposited not only on the copper underlayer, but also on the CNTs (see Fig. 1b). The results of charge-discharge testing and electrochemical measurements on the new anode material will be presented at the meeting.

References

1) Dongjoon Ann, Xingcheng Xiao, Yawen Li, Anil. K, Sachdev, Hey Woong Park, Aiping Yu, Zhongwei Chen ; Journal of Power Sources 212, p.66-72 (2012)

2) S.Arai R. Fukuoka, J. Appl. Electrochem.,46 331 (2016).

3) K. Kirihata, T. Mano, S. Arai, M. Ueshima, K. Hirota ; The Surface Finishing Society of Japan 131nd lecture tournament Abstracts, p.7 (2015)

Figure 1