Sn Negative Electrode Consists of Flexible 3D Structures for Sodium Ion Secondary Batteries
Authors: Okamoto Naoki , Morita Koki , Saito Takeyasu 
Published: February 2020

Tin(Sn) and its alloys have been attracting attentions as a negative electrode material for lithium-ion and sodium-ion secondary batteries with high theoretical capacity (Li22Sn5, ca. 990 mAh･g
−

1) and high electromotiveforce. There still remains the issue as regards the discharge capacity decrease with increasing the number of cycles. In order to improve cycle performance, there are many studies such as using Sn-Ni alloy, macroporous patterns and enhancing adhesiveness of active materials. However, most of these studies are using Sn based alloy as negative electrode materials and it suffer from the disadvantage of lowering of discharge capacity.

In this study, Sn film which has nano-sized or amorphous grain to alleviation volume expansion with using electrodeposition was developed. In addition, 3D-nano flexible structure was used with collector for creating space to alleviate volume expansion.

The effect of additives on the surface morphology and microstructure of Sn film was investigated. Furthermore, we evaluated the effect of 3D-nano flexible structure into the Sn film on cycle performance of the Sn negative electrode.

The Sn film was formed by electrodeposition using aqueous bath at low temperature. Cu foil with a thickness of 35 μm was used for the substrates We used 5 types of Sn deposition bath. Tin sulfate (SnSO4) and tin chloride (SnCl2) were used as metal resource reagents. Phosphinic acid (H2PO2), boric acid (H3BO3) and potassium sulfate (K2SO4) were used as additives. In addition, conventional Sn deposition bath, which consists of 30 g L−
1 (0.14 M) tin sulfate (SnSO4), 96 g L−
1 sulfuric acid (H2SO4) and 100 ppm polyethylene glycol (PEG, Mw: 4000), was used for comparison.

The electrochemical properties of Sn electrodes were investigated using double pole cell (tomcell) consist of Sn circle film (φ16 mm) as working electrode and Na metal film as counter electrode. The electrolyte consist of a solution of 1M NaPF6 in a 1:2 (vol.%) mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) (Kishida). The whole assemble was processed in an argon-filled glove box. The Charge-Discharge test was galvanostatically examined with a current density of 200 μm. Potential range of the Charge-Discharge test was set between 0.01 to 1.0 V (vs. Na/Na+). The cyclic voltammetry (CV) was performed in the potential range of 0.01-1.0 V (vs. Na/Na+) at a scan rate 0.2 mV/s.

The surface morphologies of the electrodes were examined by scanning electron microscope (SEM, JEOL JSM-6700F), and phase identification was investigated by X-ray diffraction (Rigaku, Smart Lab).

Both amorphous and crystalline structure was observed in the deposited Sn film. In contrast to conventional Sn electrodeposited film, this unique Sn film has a good cycle characteristic (over 50 cycles) and discharge capacity (over 400 mAh･g-1). Furthermore, in the case of using the bath which includes phosphinic acid (H2PO2) in composition discharge capacity after first cycle approached over 700 mAh g-1 and that of after twenties cycle was over 400 mAh g-1. Amorphous structure in the Sn film showed a microscopic effect on the volume change by lithiation and delithiation.

Sn particles became smaller and formed amorphous structure by using pulse current.

Sn electrode deposited on 3D-nano flexible structure showed good cycle characteristic (over 50 cycles) and discharge capacity (840 mAh/g (the C-rate of 2)). The Charge-Discharge curves indicated Na-Sn alloy process as follows: β-Sn, NaSn5, NaSn, Na9Sn4.