Electrochemical Performance of Nickel Silicide Electrodes for Lithium-Ion Batteries in an Ionic-Liquid Electrolyte
Authors: Ando Takumi , Domi Yasuhiro , Usui Hiroyuki , Sakaguchi Hiroki 
Published: December 2020

Developing lithium-ion batteries (LIBs) with a high energy density, long cycle, and high safety is essential for establishing a sustainable society because LIBs are used as electric vehicle batteries and stationary batteries for utilizing renewables. Silicon (Si) has great potential as an anode active material for next-generation LIBs due to its high theoretical capacity (3580 mA h g-
1 for Li15Si4). However, Si electrodes show poor cycle stability, which is mainly caused by a large volume change in Si during the lithiation (charge) and delithiation (discharge) reactions. Additionally, Si has other disadvantages, such as high electrical resistivity and a low Li+ diffusion coefficient.

We have previously investigated the lithiation and delithiation properties of pure binary silicide (M
ySiz, M : transition metal) electrodes. Some electrodes maintained a high reversible capacity in an ionic-liquid electrolyte, whereas they showed a poor cycling performance in a conventional organic-liquid electrolyte. In contrast, the effect of difference in crystal phase of silicides composed of the same elements on the lithiation and delithiation properties has not yet been clarified. Herein, we synthesized various pure nickel silicide (Ni-Si) powders. and investigated their lithiation and delithiation properties in an ionic-liquid electrolyte.

Ni-Si powders (NiSi2, NiSi, Ni2Si, and Ni3Si) were synthesized by a mechanical alloying (MA) method. The obtained powders were characterized by X-ray diffraction (XRD) and Raman spectroscopy. XRD patterns were identified compared with patterns in the Inorganic Crystal Structure Database (ICSD).

Each silicide electrode was prepared by a gas deposition (GD) method, which does not require a binder or conductive agent. We assembled 2032-type coin cells, which consisted of the silicide electrode as the working electrode, Li metal foil as the counter electrode and a glass fiber filter as the separator. 1 mol dm-
3 (M) lithium bis(fluorosulfonyl)amide (LiFSA) in N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)amide (Py13-FSA) was used as an ionic-liquid electrolyte. A galvanostatic charge-discharge test was carried out in the potential range between 0.005 and 2.000 V vs. Li+/Li at 303 K. The current density was set at 50 mA g-
1. To discuss the charge density of each element in Li
x
Ni
y
Si
z
, a first-principle calculation based on density functional theory (DFT) was performed using the projector augmented wave (PAW) method as implemented in the plane wave code of the Vienna Ab initio Simulation Package (VASP). A generalized gradient approximation (GGA) was used as the term exchange correlation with a kinetic cutoff of 350 eV. Brillouin zone sampling was performed with an 8×8×8 k point mesh within a Gamma point centered mesh scheme.

We synthesized various Ni-Si powders by a MA method. Comparing the XRD pattern of the prepared samples and the corresponding ICSD pattern, all peaks were assigned to pure Ni-Si phase. Additionally, there were no peaks arising from the raw materials (Ni and Si). In Raman spectra, peaks assigned to crystalline Si (c-Si) and amorphous Si (a-Si) appear at approximately 520 and 490 cm-
1, respectively. Each silicide also gives no Raman peaks of c-Si and/or a-Si which indicates that neither c-Si nor a-Si is included in the synthesized powders. Therefore, the MA treatment successfully produced a pure Ni-Si phase.

Fig.1 shows the dependence of the gravimetric discharge capacity of various nickel silicide electrodes on the cycle number in 1 M LiFSA/Py13-FSA.The NiSi2 electrode exhibited the highest initial capacity of approximately 800 mA h g-
1, and the Ni3Si electrode showed the second highest capacity. The results demonstrated that there was no correlation between initial capacity and silicon content in Ni-Si.

To reveal the difference in the initial capacity, we determined the charge density of each element in Li
x
Ni
y
Si
z
based on computational chemistry. Li and Si have positive charges in all silicides, whereas Ni has a negative charge in Li
x
Ni
y
Si
z
. These results indicate that Li has a high affinity with Ni in Li
x
Ni
y
Si
z
. In addition, it is suggested that the higher the affinity of Ni with Li is, the higher the initial capacity is. Furthermore, we investigated the distance between Li and close atoms (Ni or Si) in Li
x
Ni
y
Si
z
. The initial capacity increased with an increase in the distance. Therefore, both the affinity of Ni with Li and the distance between Li and neighboring atom should influence on the initial capacity.

Figure 1