Nanoporous Amorphous Silicon Thin Films Fabricated with Helium Plasma for Lithium Ion Battery Application
Authors: Kajita Shin , Uchida Giichiro , Tanaka Hirohiko , Tabata Kiho , Yamamoto Yuta , Ohno Noriyasu 
Published: December 2024

Lithium-ion batteries (LIBs) have the advantage of being compact and lightweight, and significant advancement has been made commercially and acadmically in recent several decades. It is essential to develop a new anode material that can replace graphite to further increase capacity, and various materials are being investigated including silicon (Si). Among them, Si is being studied as a candidate for a new anode material because its theoretical capacity per weight is about ten times higher than that of graphite and it is abundant and inexpensive. Among them, thin film approach has achived a great success and good electrochemical performance in terms of specific capacity and cycle life. One of the challenging issues with the thin film approach is that thick films, typically 500 nm, cannot maintain the performance identified in thinner film samples. In this study, we introduce a novel method using helium (He) plasma to form porous Si thin film. Different magnetron sputtering, in the present plasma-assisted method deposit Si atoms on the surface while the surface is exposed to the He plasma. It is shown that plasma assisted method can be another candidate to form nanoporous amourphous Si thin film for high performance LIB application.

Silicon thin film was formed in the linear plasma device called Co-NAGDIS. The plasma is produced by a direct current arc discharge using a meander lanthanum hexaboride cathode heated with a Joule current. The magnetic field by two solenoidal coils formed a cylindrical plasma. Helium gas was used for the discharge gas, and the He flux was measured with a reciprocating electrostatic probe. Difference from the magnetron sputtering, which also uses plasma and has been widely used to fabricate Si layer for LIB application, is in the fact that particles (He ions) of high-density plasma, typically ~1018 m-3, are also implanted together with Si atoms on the substrate. Copper (Cu) substrate is equipped to an air-cooled sample stage, of which temperature can be controlled by changing the flow rate of the air. Silicon sputtering plate is installed 1 cm away from the substrate and biased to ~-100 V to induce sputtering. The sputtered Si atoms deposit to the substrate together with the He ions. The deposition time was 90 min for all the samples. The porosities of the two samples shown in this work (Si1 and Si2) was ~0.5, and the thickness was ~1.5 micrometer. The sample tempearture during deposition was 523 K (Si1) and 573 K (Si2).

Figure shows the discharge capacities of the two samples (Si1 and Si2). Charging and discharging were repeated with C rate of 0.01C for times 1-3, 0.02C for times 4-6, 0.05C for times 7-9, and 0.1C for times >9. They had an initial capacity of approximately 3000 mAh/g, and they retained higher than 1000 mAh/g even after 20-30 cycles. For Si1, the number of charge-discharge cycles was extended to 250 cycles, and the performance of Si1 gradually decreased but still remained 1800 mAh/g after 100 cycles and ~1200 mAh/g at 250 cycles. The surface was observed by scanning electron microscope (SEM), the surface of Si1 and Si2 was almost flat. From cross section observation by a transmission electron microscope (TEM) after a focus ion beam miiling, it was found that there is no clear evidence of He bubbles in the Si deposition layer, but the layer is mainly composed of 100-200 nm diameter clusters, and the density between the clusters is lower. Thermal desorption spectroscopy (TDS) analysis was performed to analyze the behavior of implanted He atoms in Si layer. It was found that He atoms are implanted to the thin film layer. From the Raman spectroscopy, it was found that amorphous Si was formed at 523 K, but it becomes crystal structure at 573 K. The importance of the amorphous structure is well recognized, and it can be said that Si1 showed better performance than Si2 due to its amorphous structure. In the Figure, evolutions of coulombic efficiency (CE) of Si1 and Si2 is also shown. The CE increases in the initial 5-10 cycles and almost constant after then. The averaged CE after 50 cycles for Si1 and Si2 were 99.51 and 98.72%, respectively. The results showed that the plasma implanting method can be a novel method to form amorphous porous thin film for lithium ion battery.

Figure 1