Preparation of Ni-Sn Alloy Nanorods with Composition Gradient, and Its Effect on Li-Ion Battery Anode Performance
Authors: Chowdhury Ridwanur Rahman , Hoffman Lance Raymond , Kaule Joseph Matthew , Mukaibo Hitomi 
Published: February 2020

The development of Li-ion batteries (LIBs) with high capacity, fast charge/discharge rates and long lifecycle is indispensable for the advancement of high power portable devices, plug-in hybrid and electric vehicles, and renewable energies. Novel anode materials that offer higher specific capacity than that of conventional carbon-based materials are considered a key factor in this development. Numerous Li-alloying materials are being investigated, but they all experience enormous volume changes during the lithiation/delithiation process, leading directly to electrode degradation and short battery life.

Tamura et al. have demonstrated that a concentration gradient at the interface between the active material and the current collector can enhance the interface strength and lead to longer lifecycle [1]. Our research involves investigating how this anode degradation may be avoided by creating a gradient of stress withinthe anode. We hypothesize that allowing the volume change of anodes to occur gradually within its structure will result in less pulverization and longer cycle life.

In this presentation, we report our studies on this approach using an array of Ni-Sn alloy nanorods as the model system. Sn is one of the most promising anode materials, with high theoretical specific capacity, low price, high abundance and high electrical conductivity. The Li-inactive Ni acts as a matrix to buffer the volume expansion during the alloying process [2]. The nanorod array structure allows us to easily incorporate a 1-dimensional stress gradient into the structure by changing the Ni/Sn ratio of the nanorod along its length, making it an ideal simplified platform to study the stress gradient effect.

In our work, we use the well-known template synthesis method to construct the Ni-Sn alloy nanorod array. Briefly, the array was prepared by electrodepositing Ni-Sn alloy into the pores of commercially available polycarbonate (PC) filter membranes, and later exposing the deposits by removal of the PC membrane. Electrodeposition allows control of the composition of the Ni-Sn deposits via the current density [3]. Table 1 shows how the composition changed when Ni-Sn alloy was deposited on flat Cu substrates (with no PC membrane). We found that a higher current density (10mA/cm2) resulted in a lower Sn content than a lower current density (1.25mA/cm2).

Fig. 1a shows the FE-SEM image of the surface of the PC membrane used. The pore diameters were ~0.1μm, as indicated by the manufacturer. To make a gradient of the composition within the nanorods, the Ni-Sn alloy was deposited while varying current density. Fig. 1b shows a representative image of such rods. Here, we varied the current density between two values: 10mA/cm2 for the first 12 minutes, and then at 1.25mA/cm2 for the next 80 minutes. We can confirm that the nanorod size is consistent with the pore diameter of the PC membrane.

Furthermore, we will discuss the elemental distribution and crystal structure of the Ni-Sn nanorods using TEM-EDX mapping and XRD. We will also present the electrochemical properties of these nanorods with various composition gradients, through cyclic voltammetry and cycle performance studies.

Table 1Energy dispersive x-ray spectroscopy (EDX) results of thin film Ni-Sn alloy samples prepared under two different current densities.

Current Density
Sn
Atomic %
Ni
Atomic %

10mA/cm2

62.69 %
37.31 %

1.25mA/cm2

67.01 %
32.99 %

References

[1] N. Tamura, R. Ohshita, M. Fujimoto, S. Fujitani, M. Kamino,J. Power Sources 107 (2002) 48-55.

[2] H. Mukaibo, T. Sumi, T. Yokoshima, T. Osaka, Electrochem. Solid-State Lett. 6 (2003) A218.

[3] V.D Joviæ, U. Laènjevac, B.M. Joviæ, L. Karanoviæ, Int. J. Hydrogen Energy 37 (2012) 17882-17891.