Advanced TexSy-C Nanocomposites for High-Performance Lithium Ion Batteries

This study is dedicated to expand the family of lithium-tellurium sulfide batteries, which have been recognized as a promising choice for future energy storage systems. Herein, a novel electrochemical method has been applied to engineer micro-nano TexSy material, and it is found that TexSy phases combined with multi-walled carbon nanotubes endow the as-constructed lithium-ion batteries excellent cycling stability and high rate performance. In the process of material synthesis, the sulfur was successfully embedded into the tellurium matrix, which improved the overall capacity performance. TexSy was characterized and verified as a micro-nano-structured material with less Te and more S. Compared with the original pure Te particles, the capacity is greatly improved, and the volume expansion change is effectively inhibited. After the assembly of Li-TexSy battery, the stable electrical contact and rapid transport capacity of lithium ions, as well as significant electrochemical performance are verified.


INTRODUCTION
Lithium-tellurium (Li-Te) batteries have attracted increasing attention owing to their high theoretical volume capacity (Liu et al., 2014;Ding et al., 2015;Li et al., 2017;Li G. et al., 2018;Yin et al., 2018;Wenjie Han et al., 2021), excellent electronic conductivity (He et al., 2017), and relieved shuttle effects compared to Li-sulfur, Li-selenium batteries (Yang et al., 2013;Eftekhari, 2017;Fan et al., 2019;Wang et al., 2020;Yu et al., 2020;Dai et al., 2021;Sun et al., 2021;Xiao et al., 2021). However, the huge volume expansion of Te severely deteriorates its practical applications towards the newly emerged battery systems. Therefore, how to alleviate or eliminate the volume variation is of great importance to fulfill the promising properties of Te. Since our first introduction of Li-Te x S y battery , it seems there is a hope to light a new path to conquer the volume expansion challenge by the incorporation of sulfur elements inside tellurium lattice. Although our prepared Li-Te x S y cathode materials were not perfectly composed of pure Te x S y phase, it has been demonstrated such Te x S y phase is surprisingly stable in terms of in situ TEM observation, which can be survived during the repetitive cycling without obvious volume variation.
Many related works have tried to map the phase diagram of Te x S y , such as Te 0.92 S 0.08 , Te 0.04 S 0.96 , Te-n-S (where n represents the mass ratio) (Xu et al., 2018;Li et al., 2019a;Li et al., 2019b;Ge and Yin, 2019;Lee et al., 2019;Zhang et al., 2020). Sulfur incorporation leads to lattice distortion and d-spacing enlargement of Te phase, rendering the composited Te x S y with a fast transport of ions and electrons, as well as excellent structural stability during lithiation/delithiation processes . Together with the superior electronic conductivity and enhanced reaction kinetics derived from Te, Li-Te x S y batteries exhibit extraordinary energy storage performance and foreseeable bright future for next-generation battery systems.
In this work, we have attempted to design new types of Te x S y phases and fill some blank in Te x S y phase diagram by applying different kinds of sulfur sources during the nonlinear electrochemical synthesis of Te x S y (Li et al., 2019a). The experimental results suggested that different sulfur sources give rise to distinguished lattice distortions of Te, and thus different types of Te x S y phases, among which, Na 2 S-derived Te x S y ball milled with multi-walled carbon nanotubes endows Li-Te x S y batteries profound volumetric capacity performance and high cycling stability.

Synthesis of Te x S y Micro-nano Materials
Sodium sulfide (Na 2 S·9H 2 O), tellurium ingot (Te) and sodium hydroxide (NaOH) were all purchased from Aladdin. The sintered Te rod, platinum wire and calomel electrode (Hg/ HgCl 2 ) was used as the working, counter and reference electrode, respectively. The three-electrode system was employed in an equilateral triangle manner with a distance of 1.8 cm. Before experiments, the working and counter electrodes were cleaned with ultrasonic cleaner (Branson 1510, United States) for 10 min, and then rinsed with distilled water. The temperature of reaction cell was maintained at 25.0°C. The electrochemical synthesis experiments were carried out at the CHI 660e Electrochemical Workstation (Shanghai Chenhua). A typical solution preparation is to dissolve 0.5 mol L −1 NaOH first, and then add 0.5 mol L −1 Na 2 S·9H 2 O (other sulfur sources with different concentrations were specified) to the solution to get a clear solution. The voltage window of 0-1.5 V was set by cyclic voltammetry (CV) with a scan rate of 0.1 mV s −1 , and the electrochemical reaction was carried out by 3 CV cycles. The black solid products were finally collected, cleaned and centrifuged, which was later identified as Te x S y micro-nano materials.

Synthesis of Te x S y -C Nanocomposites
The above as-prepared Te x S y materials were mixed with certain mass ratio of multi-walled carbon nanotubes (purchased from XFNANO, 50 µm in length, 8-15 nm in diameter, purity>95%) by using a ball milling machine (QM-3C, Nanjing University). After fully mixing for 20 h, the composites of Te x S y -multi-walled carbon nanotubes (Te x S y -C) were obtained.

Characterization
Scanning electron microscopy (SEM) was performed on a Nova Nanosem 200 system with an acceleration voltage of 15 kV. Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) were conducted on JEM-2100F. Energy dispersive X-ray energy spectrum (EDX) and TEM measurements were performed simultaneously. Raman spectroscopy (INVIA, Renishaw, United Kingdom) was carried out at an ambient temperature with a 514 nm laser excitation. X-ray photoelectron spectroscopy (XPS) was performed in the spectrometer from Kratos axis Ultradld, using Mono Al Ka radiation power of 120 W (8 mA, 15 kV). X-ray diffraction (XRD) was tested by using a Cu-Ka radiation (A 0.15406 nm) on the Bruker D8 Advanced Diffractometer with a data acquisition range of 10°-80°and sweep rate of 0.02°s −1 . Thermogravimetric analysis (TGA) was performed on Perkin-Elmer PRIS1 TGA/Clarus SQ 8T at a heating rate of 5°C min −1 .

Electrochemical Test
The electrochemical properties of Te x S y -C nanocomposites were studied by using the 2025 coin battery on the Neware-battery testing system. The working electrode was prepared by pasted a mixture of 70 wt% Te x S y -C nanocomposites, 15 wt% acetylene black and 15 wt% polyvinylidene fluoride (PVDF) on the aluminum foil. The mass loading of the active material on the electrode was 1-2 mg cm −2 , and the lithium metal wafer was used as the counter electrode. The electrolyte was containing 1 mol L −1 lithium bis(trifluoromethane)sulfonimide (LiTFSI) electrolyte, 2% LiNO 3 , and 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (volume ratio 1:1). The battery was assembled in a glove box filled with pure argon gas.

RESULTS AND DISCUSSION
In this work, we have tried various sulfur sources to fabricate different types of Te x S y phases via a nonlinear electrochemical approach. By changing the actively reducing sulfur species such as sodium dimethyldithiocarbamate (C 3 H 6 NNaS 2 ) and thiourea (CH 4 N 2 S), sodium hydrogen sulfide (NaHS) and sodium sulfide (Na 2 S) for the production of Te x S y phases, distinctive micro-nano structured Te x S y materials were engineered in Figure 1 via the control of nonlinear electrochemical dynamics in Supplementary Figure S2. The scanning electron microscope (SEM) images indicated that the presence of Na 2 S could lead to a distinguished morphology (flakes) compared to that of other products (rods). More importantly, Raman spectra in Supplementary Figure S3 revealed that the sulfur content was maximized in Te@Na 2 S, denoted as Te x S y phases prepared by Na 2 S. The optimal concentration of Na 2 S for the construction of nano-flaked Te x S y phases was determined as 0.5 mol L −1 . As the concentration of Na 2 S was increased, the nano-flaked Te x S y phases were broken into randomly downsized nano-particles, as shown in Supplementary Figure S4. Surprisingly, the increasing concentration of Na 2 S also led to an overwhelmed Raman peak intensity of sulfur than tellurium in Supplementary Figure S5.
Transmission electron microscopy (TEM) characterization of Te x S y material was obtained when the concentration of Na 2 S was set to 2.0 mol L −1 . It can be found that the downsized nanoparticles have a very poor crystallinity from Figures  2A,B, in which a typical lattice parameter is emerged in a typically selected area, with a d-spacing of 0.334 nm representing Te (011)  as-obtained materials are not composed of pure Te x S y phases. Another evidence is from XRD analysis in Supplementary Figure  S5B, where the XRD peaks are broadened as the concentration of Na 2 S is increased, indicating the incorporation of sulfur in Te crystalline causes the poor crystallinity. Furthermore, Te x S y phases are dominated in the as-prepared materials, strongly supported by the homogeneous distribution of Te and S elements in Figure 2C.
In order to further study the composition of Te x S y component and the formation mechanism of Te-S bond, the product synthesized  Frontiers in Chemistry | www.frontiersin.org May 2021 | Volume 9 | Article 687392 3 by 2.0 mol L −1 Na 2 S solution was selected for XPS characterization (Figure 3). Figure 3A presented a XPS survey image of Te x S y components possessing the main elements of Te and S. Figure 3B  , which are then chemically reduced by different organic or inorganic sulfides in this study to form Te x S y . The overall reaction mechanism is followed by the electrochemical-chemical (EC) reaction pathway, similar as the first two steps of our previous study . The distinguished reducibility of organic and inorganic sulfides enabled the self-assembly of Te x S y with different nano-micro morphologies and chemical compositions, rendering Te x S y with varied physicochemical properties for seeking the promising electrochemical performance.
However, compared to our previous study , the asprepared Te x S y phases without any confinements from carbon hosts were failed to contribute a promising electrochemical performance towards lithium ion batteries. As shown in Supplementary Figure S6, high charge transfer resistance, poor cycling stability and rate performance seems a total failure. Therefore, in this work we applied multi-walled carbon nanotubes (MWCNTs) as a carbon host to confine the Te x S y phases. Astonishingly, the ball milling of Te x S y phases with MWCNTs rendered the Te x S y (Na 2 S)/MWCNT with a strange thermal degradation feature, that is, Te x S y phases actually reacted with MWCNTs, and resulted in a two-stage thermal degradation of Te x S y in Supplementary Figure S8B. In comparison with Te x S y , the Te x S y -C possessed a distinctive thermal degradation kinetics, where the less mass ratio of Te x S y :C such as 5:5 and 3:7 could lead to the lower decomposition temperatures (∼630°C as shown in Supplementary Table S1) than that of pure Te x S y and the high mass ratio of Te x S y :C 7:3 (∼700°C), indicating the rearrangement of Te x S y phases was occurred in the presence of appropriate amounts of MWCNTs.
Afterwards, the Te x S y (Na 2 S)/MWCNT composited materials with different mixing ratios were further evaluated by battery performance test, as shown in Figure 4. Figure 4A showed a diagram of the rate performance of Te x S y (Na 2 S)/MWCNT composites with different mixing ratios. It is worth noting that the materials with composite ratio of 5:5 have relatively better rate performance than the other two materials. Electrochemical impedance spectroscopy (EIS) of Te x S y (Na 2 S)/MWCNT composited batteries with different mixing ratios were also tested in Figure 4B. As it can be seen, the increased amount of multi-walled carbon nanotubes would worsen the charge transfer, suggesting an integrated interfacial effects between Te x S y and MWCNTs. In addition, Figure 4C showed the charge-discharge curves of lithium ion battery with a mixture ratio of 5:5 at different current densities. The first charge-discharge curves suggest a relatively high initial Coulombic efficiency. Figures 4D-F presented that the compound ratio of 5:5 Te x S y (Na 2 S)/MWCNT composited battery exhibits a promising cycling behavior at 1.0, 2.0, 5.0 A g −1 , in which a larger current density would lead to a better cycling stability, indicating a potential fast-charging application in rechargeable batteries. When the current density was set to 5.0 A g −1 , the first specific capacity was obtained as 406.56 mAh g −1 , and capacity retention remained as 45.03% after 500 cycles. While the current density was set less than 5.0 A g −1 , the rate performance behaved much worse than that at 5.0 A g −1 , demonstrating Te x S y (Na 2 S)/ MWCNT composite material is more suitable for high rate performance in lithium ion batteries. The as-prepared lithiumtellurium sulfide battery may potentially tackle the cons of low rate performance in lithium-sulfur battery, and high volume expansion and low capacity performance in lithium-tellurium battery.

CONCLUSION
In summary, we designed a promising electrochemical method to control the synthesis of Te x S y micro-nano structured composites, verified the formation mechanism and qualitatively evaluated the influence of chemical composition on the battery performance. The morphology and composition ratio of Te x S y (Na 2 S)/MWCNT were controlled by the types of sulfur sources, concentration and synthetic voltage. In addition, MWCNTs as an ideal carbon host were used for the confinement of the dissolution of tellurium and sulfur, which significantly improved the electrochemical performance of the Te x S y (Na 2 S)/MWCNT composited battery. The nonlinear electrochemical synthetic method and ball milling aftertreatments provide a new way for the sustainable development of high-performance Li battery manufacturing.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.

AUTHOR CONTRIBUTIONS
JL, HJ, and SW designed the experiments. GL and CY performed the material synthesis, characterization and battery tests. GL analyzed the data and drafted the manuscript. JL made the major revision. All authors participated in discussions.

FUNDING
This work was supported by the National Natural Science Foundation of China (51872209, 51972239, and 52072273), the Zhejiang Provincial Natural Science Foundation of China (Z21E020002) and Natural Sciences and Engineering Research Council of Canada (NSERC).