Vanadium disulfide flakes with nanolayered titanium disulfide coating as cathode materials in lithium-ion batteries

Unlike the vast majority of transition metal dichalcogenides which are semiconductors, vanadium disulfide is metallic and conductive. This makes it particularly promising as an electrode material in lithium-ion batteries. However, vanadium disulfide exhibits poor stability due to large Peierls distortion during cycling. Here we report that vanadium disulfide flakes can be rendered stable in the electrochemical environment of a lithium-ion battery by conformally coating them with a ~2.5 nm thick titanium disulfide layer. Density functional theory calculations indicate that the titanium disulfide coating is far less susceptible to Peierls distortion during the lithiation-delithiation process, enabling it to stabilize the underlying vanadium disulfide material. The titanium disulfide coated vanadium disulfide cathode exhibits an operating voltage of ~2 V, high specific capacity (~180 mAh g−1 @200 mA g−1 current density) and rate capability (~70 mAh g−1 @1000 mA g−1), while achieving capacity retention close to 100% after 400 charge−discharge steps.

= R × ℎ ℎ where, R is calculated from the current-voltage curves and the aspect ratio range of the flakes is about 1 to 3. Rs is used to evaluate and compare the conductivity of the VS2 and VS2-TiS2 flakes. Rs of the VS2 flake is about 200-900 Ω ⎕ −1 while that of the VS2-TiS2 flake is about 500-2400 Ω ⎕ −1 . After TiS2 deposition, the conductivity of the VS2 flake has decreased, but is still comparable to pure (i.e., uncoated) VS2. Scale bar = 20 μm.

Supplementary Tables
Supplementary Table 1. Raman spectroscopy data for VS2 flakes from several references. The literature indicates that there is large scatter in the Raman data for VS2.

Supplementary Notes
There are three main reasons for the large scatter reported in Supplementary Table 1: 1.
The first reason is the manufacturing process. Hydrothermal and chemical vapor deposition (CVD) methods have different nucleation processes. Hydrothermal method for 2D materials synthesis usually starts from small nuclei that grow into nanosheets by following the oriented attachment mechanism 10,12,13 . On the other hand, the CVD process is usually governed by the layer-by-layer (LBL) growth model 14,15 . In case of VS2 synthesis, these different growth mechanisms could introduce different stacking arrangements. Also, excess V atoms are expected to reside as intercalators between CVD grown VS2 layers 4 . Such structural distortions can interfere with the in-plane and out-of-plane vibration, causing the peak positions to shift. Note that the Raman peaks that we report are comparable 4, 5 to other CVD grown VS2.

2.
It has been claimed in literature that the flake curvature influences the Raman response. In particular, the intense peak of the in-plane mode (282 cm −1 ) has been attributed to the curvature of VS2 flakes 9 . Simulation studies also indicate that the relative intensity between the E1g and A1g modes of 2D transition metal dichalcogenides are very sensitive to the laser set-up (polarization set-up) and could be tuned from 0 to infinity 16 . Based on the above studies, the lower intensity of the in-plane mode (~263 cm −1 ) for our VS2 as compared to the out-of-plane mode (~379 cm −1 ) can be attributed to the low curvature of our VS2 flakes as well as the polarization angle of the laser in our testing. Further in-depth experiments will be necessary to quantify the effects of structural changes and laser polarization on the relative intensities of the Raman peaks.

3.
Another important reason for the large variation in Raman observation on VS2 flakes among different published works is that VS2 flakes are sensitive to the laser intensity and may decompose or react with oxygen if the characterization is conducted in atmosphere. This has been observed by both us (Supplementary Figure 7) and other scientists 5 . For this particular study, a vacuum optical-cryostat was used for our Raman characterization to minimize such effects.