The Effect of Metal Doping on the Anode Characteristics of Ge Thin Films in Lithium-Ion Secondary Batteries

1. Introduction
Ge has garnered attention as a next-generation anode material due to its high specific capacity and fast lithium diffusion rate^[1]. However, the low capacity retention rate resulting from damage to the film caused by volumetric expansion has posed a significant challenge. In this context, various studies have reported improvements in properties through the addition of metal (M) to suppress volumetric expansion^[2]. In this study, we investigated the anode properties of Yb_xGe_1-x, which had not previously been reported. Remarkably, we discovered a dramatic improvement in these properties. Furthermore, we systematically examined the impact of adding various elements on the anode characteristics, confirming a trend where the larger atomic size of M leads to enhanced anode performance of Ge.

2. Experimental Procedures
A layer of M_xGe_1-x (500 nm thick), where M represents Al, Ag, Cu, Ni, Ta, W, and Yb, was sputter-deposited on a Mo substrate. Subsequently, a two-electrode cell was fabricated by placing metallic Li as the counter electrode and using an electrolyte composed of 1M LiPF₆ in EC/DEC (1:1 v/v). Charge-discharge tests were conducted at a current density of 500 mA g⁻¹. The composition of the Ge_1-xM_x anode was evaluated using Energy Dispersive X-ray Spectroscopy. To assess the damage to the Ge_1-xM_x anode caused by charge-discharge cycling, Scanning Electron Microscope images were acquired post cycling.

3. Results and Discussion
[i] Yb addition effects on Ge
From the charge-discharge characteristics of Ge and Yb_0.1Ge_0.9, it was confirmed that the addition of Yb improves capacity retention. Subsequently, charge-discharge tests on Yb_xGe_1-x anodes showed that the initial capacity remained almost constant for x < 3% but exhibited a decreasing trend for x > 10%. Regarding anode lifespan, Ge demonstrated a lifespan of only about 60 cycles, whereas the addition of Yb resulted in stable anode performance for approximately 150-240 cycles.
To evaluate the damage to the Yb_xGe_1-x anode caused by charge-discharge cycling, SEM images of the anode post-cycling were acquired. Low-magnification images revealed that the shape of the anode deformed into a rectangular form with an increase in Yb composition. High-magnification images indicated the formation of unique nanoscale structures corresponding to the varying Yb compositions. These shape changes are presumed to result from alterations in the mechanical strength of the Ge thin film due to the addition of Yb.
[ii] Impact of M on Anode Characteristics
Next, we fixed x at approximately 3% and examined the impact of varying the type of M on the anode characteristics of M_xGe_1-x. The graph indicates that the anode capacity varies depending on the type of M. We then quantitatively summarized the effect of the atomic size of M on anode performance. While the initial capacity was about 1.3 Ah g⁻¹ and almost unchanged regardless of the type of M, it became clear that the larger the atomic size of M, the longer the lifespan of the M_xGe_1-x anode.
It was previously unknown that the doped metal element's size affects Ge anodes' lifespan. The findings of this study significantly contribute to the improvement of anode characteristics in secondary batteries.

References
[1] X. Li et al., ACS Nano 9, 1858(2015).
[2] K. Wang et al., J. Coll. Int. Sci. 571, 387(2020).