Structural Characterization of Amorphous VS4 and Its Amorphous Phase Transformation during Liinsertion
Authors: Shimoda Keiji , Koganei Kazuto , Takeuchi Tomonari , Matsunaga Toshiyuki , Murakami Miwa , Sakaebe Hikari , Abe Takeshi 
Published: May 2020

Vanadium sulfide (VS4) is one of the promising positive electrode materials for high capacity lithium ion secondary batteries. In this study, we prepared an amorphous VS4 (a-VS4) by mechanical treatment of the crystalline VS4 (c-VS4) and characterize its local structure. Furthermore, its lithium insertion/extraction behavior was investigated using solid-state nuclear magnetic resonance (NMR), X-ray pair distribution function (PDF), and Raman spectroscopy analyses.
Crystalline VS4 was synthesized from V2S3 (99%, Kojundo Chemical Lab.) and S (99.9%, Wako).1 The mixture was heated in a sealed glass tube under vacuum at 400 °C for 12 h. Then, the amorphous VS4 was prepared from the crystalline sample by mechanical milling for 40 h at 270 rpm. A working electrode was prepared from a mixture of the active material (a-VS4), conductive carbon, and Teflon binder in a weight ratio of 59:29:12. Coin-type or laminate-type cells were assembled in an Ar-filled glove box. The cells were cycled between 1.5 and 2.6 V at a constant current density of 59.8 mA g–1 using a TOSCAT-3100 battery-testing system (Toyo System). They were then carefully disassembled at selected discharge/charge states in the glove box and rinsed with dimethyl carbonate (DMC) to remove the residual electrolyte solution.
Local structures of c-VS4 and a-VS4 were characterized and the structural changes of the electrode samples were examined by NMR spectroscopy, X-ray PDF analyses, and Raman spectroscopy. Operando 7Li NMR measurements were performed to study the Li insertion/extraction process under electrochemical operation.2

It is known that c-VS4 has a unique structure, in which the Peierls-distorted one-dimensional chains of V-V bonds along the c-axis are loosely connected to each other through van der Waals interactions. We confirmed that a-VS4 shows a chain structure similar to that of c-VS4. The chain structure is found to change drastically during the Li insertion process to form amorphous Li3VS4, where the V ions become tetrahedrally coordinated by S ions. The valence states of V and S ions simultaneously change from V4+ and S– (a-VS4) to V5+ and S2– (a-Li3VS4), respectively. When the Li insertion proceeds further (i.e., a-Li3+d
VS4), the valence state of V ion is reduced, and a broad 7Li NMR peak shifts to higher frequencies (Fig. 1). After the 1st cycle, the amorphous VS4 recovers the chain-like structure although it is highly disordered. It is also found that the conversion reaction to metallic V and Li2S are observed when the cell is discharged to 0.2 V.
This study indicates that the Li insertion/extraction process of the amorphous sulfur-rich transition metal (TM) sulfides such as a-VS4 and a-TiS4
3 involves the amorphous-amorphous phase transformation, which is related to the changes in the bonding nature of sulfide species and the coordination number of TM ions. The amorphous host structure can accommodate these modifications, which is clearly different from the Li insertion/extraction mechanism in crystalline electrode materials.

References

1. Koganei et al., Solid State Ionics, 323, 32-36 (2018).
2. Shimoda et al., RSC adv. 9, 23979-23985 (2019).
3. Sakuda et al., J. Am. Chem. Soc., 139, 8796-8799 (2017).

Figure 1