A Novel Nano-Structured Sn-SnO2-TiO2/Cu5Sn5 Composite Film Electrodeposited on Cu As High Capacity Anode Materials for Lithium Ion Batteries

Abstract

Introduction. Lithium-ion batteries (LIB) have been widely used as power sources in many electric/electronic products such as cellular phones, digital cameras, portable computers, etc. Recently, considering on the applications in automobile industry for electric vehicles (EVs) and plug-in hybrid vehicles (PHVs), it is highly desirable to explore new electrode materials with higher power density and satisfied lifetime cycles to meet expanding needs for lithium-ion battery industry. Among various anode materials, Sn-based materials have attracted great attention as promising candidate materials for the replacement of the conventional carbonaceous anode materials of Li-ion batteries, owing to that the Sn materials, with good electrical conductivity, possess higher theoretical capacitance (Li4.4Sn: 994 mAh g-1) than graphite (LiC6: 372 mAh g-1). However, it is known that pure Sn material delivers substantial volume changes during alloying/de-alloying process with Li. During the charge-discharge cycles, volume change of Sn will cause the deterioration and peeling of the film electrode from the Cu electric collectors, thus resulting in rapid capacity fade and significantly affecting the cyclic lifetime of the batteries. In this study, we propose a novel nanoporous, bi-layered Sn-SnO2-TiO2/Cu6Sn5 (SSTC) composite film that directly electrodeposited on Cu sheets as binder-free and conductor-free anode material for Li-ion rechargeable batteries. Here, the nanoporous structure can provide space for the volume change of Sn material, and the TiO2 and Cu6Sn5 alloy can meditate the volume change inside the Sn-based composite film and at the film/Cu interface during the lithium insertion/extraction. Experimental. Various Sn-SnO2-based composite films were fabricated on Cu sheets by a hybrid electroplating method, i.e., combining electro-deposition with electrophoric deposition, using a mixed EtOH-H2O bath mainly containing SnCl2 and TiCl3, followed by annealing at 473 K for 2 h. The effects of ratios of [Sn2+]/[Ti3+] and EtOH/H2O, and operating conditions on the formation and microstructures of the plating films were investigated. The morphology, chemical composition, chemical state, and crystalline structure of the electrodeposited films before and after heat treatment were investigated by FE-SEM, EDX, TEM (FIB), XRD, and XPS. Moreover, electrochemical charge-discharge performances of the SSTC composite films on Cu were investigated with home-made pouch cells which assembled in an Ar-filled glove box. The cathode was utilized with a lithium metal foil, and the electrolyte solution was made of 1M LiPF6 in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) with a volumetric ratio of 1:1:1, respectively. The cells were charged and discharged at 0.1 A g-1 by a battery test system, and the cut-off voltage range was 0.01 – 2.0 V or 0.01 – 3.0 V (vs. Li+/Li) at 50 – 100 mA g-1 and 303 K. Results and Discussion. Figs. 1a-b show the surface FESEM images of as-deposited Sn-TiO2 composite films on Cu sheets, which were fabricated in plating baths with different [Sn2+]/[Ti3+] ratios. It can be seen that the films composed of nano-flakes or nano-needles, thus making a networked nanoporous structure. The morphologies of the electrodeposited films were depended predominantly on ratios of [Sn2+]/[Ti3+] and EtOH/H2O in electroplating baths, while only slightly affected by operating conditions. According to EDX and XPS analysis, the electrodeposited films consisted of Sn, SnO2, and a small amount of TiO2 in 1–10 at%, in proportional to the [Sn2+]/[Ti3+] ratio. A representative cross-sectional TEM image in Fig.1c disclosed that the electrodeposited specimen after annealing at 473 K for 2h consisted in a nanoporous top-layer with scaly Sn nano-crystals and a dense under-layer of Cu-Sn alloy on Cu substrate. The corresponding XRD pattern of a SSTC composite film on Cu after annealed was illustrated in Fig.1d. The Sn electrodeposits had a polycrystalline structure with a preferential in Sn (200) facet. In addition, an intermetallic compound of Sn and Cu in form of h-Cu6Sn5 was also detected, which can be attributed to the alloying reaction at the interface between the Sn-TiO2 deposited film and Cu substrate during annealing. Moreover, galvanostatic charge/discharge tests revealed that the SSTC composite films delivered a high initial specific discharge capacities of 1042 mAh g-1, with a maximum value of 1227 mAh g-1, and a high Coulombic efficiency of more than 90%, indicating a promising future as binder-free and conductor-free anode materials for LIBs. Figure 1