Synergistic Performance Boosts of Dopamine‐Derived Carbon Shell Over Bi‐metallic Sulfide: A Promising Advancement for High‐Performance Lithium‐Ion Battery Anodes

Abstract A CoMoS composite is synthesized to combine the benefits of cobalt and molybdenum sulfides as an anodic material for advanced lithium‐ion batteries (LIBs). The synthesis is accomplished using a simple two‐step hydrothermal method and the resulting CoMoS nanocomposites are subsequently encapsulated in a carbonized polydopamine shell. The synthesis procedure exploited the self‐polymerization ability of dopamine to create nitrogen‐doped carbon‐coated cobalt molybdenum sulfide, denoted as CoMoS@NC. Notably, the de‐lithiation capacity of CoMoS and CoMoS@NC is 420 and 709 mAh g⁻1, respectively, even after 100 lithiation/de‐lithiation cycles at a current density of 200 mA g⁻1. Furthermore, excellent capacity retention ability is observed for CoMoS@NC as it withstood 600 consecutive lithiation/de‐lithiation cycles with 94% capacity retention. Moreover, a LIB full‐cell assembly incorporating the CoMoS@NC anode and an NMC‐532 cathode is subjected to comprehensive electrochemical and practical tests to evaluate the performance of the anode. In addition, the density functional theory showcases the increased lithium adsorption for CoMoS@NC, supporting the experimental findings. Hence, the use of dopamine as a nitrogen‐doped carbon shell enhanced the performance of the CoMoS nanocomposites in experimental and theoretical tests, positioning the material as a strong candidate for LIB anode.


Materials and Method
All chemicals used in the process were of research-grade and were used as they arrived without further purification.Cobalt nitrate (Co(NO3)2.6H2O)and sodium molybdate (Na2MoO4.2H2O)were purchased from Junsei Chemical Co., Ltd., Japan.Thioacetamide (C2H5NS) was purchased from Wako pure chemical, Japan.Dopamine hydrochloride was purchased from Sigma Aldrich, Germany.Tris buffer was purchased from Alfa Aesar, China.Ethanol (C2H5OH) was purchased from Daejung Chemicals & Metals Co., Ltd., South Korea.Extra pure deionized (DI) water was used throughout the experiment.JAC ULTRASONIC-4020 series was used for the ultrasonication work.

Preparation of CoMoO4
In a typical synthesis process, 1 mmol of Co(NO3)2.6H2Oand 1 mmol of Na2MoO4.2H2O were added in 80 mL solution of solvents composed of water, ethanol and diethyl glycol in the volumetric ratio of 2:1:2.The solution was stirred in magnetic stirrer for 1 h then kept in 100 ml Teflon-lined steel autoclave and heated at 180 ℃ for 6 h.The final solution was obtained as a pink segregate in a clear solution.The obtained solution was washed several times with ethanol and DI water and kept in a vacuum oven at 80 ℃ for 8 h to dry.Dark violet (color code: #9400d3) powder was obtained after vacuum drying.This powder is believed to be the cobalt molybdenum hydroxide.The product obtained from vacuum drying was then kept in a high-temperature furnace at 400 ℃ at the rate of 1 ℃ per minute for 3 h for the final annealing process.Dark blue magenta (color code: #685580) powder was obtained after the final calcination process.The final product was named CoMoO4.

Preparation of CoMoS
CoMoS composite was synthesized through the second step hydrothermal treatment of CoMoO4 using thioacetamide as the sulfur source.In a typical process, CoMoO4 powder (100 mg) and thioacetamide (300 mg) were mixed for 30 min in 40 ml DI water.The mixed solution was then put in an oven at 120 ℃ for 5 h in the hydrothermal vessel.The final product obtained in the form of a black suspension was centrifuged and washed several times with DI water and ethanol and dried in a vacuum oven at 80 ℃ overnight.The final product was ground again to make the fine powder for further use and named CoMoS-3.For ease of literature writing and understanding, the sample names CoMoS and CoMoS-3 are used interchangeably and hold the same meanings.As a control sample, the cobalt molybdenum sulfide was prepared using different CoMoO4 and thioacetamide powder weight concentrations such as 1:1, 1:2, and 1:4 and the samples obtained were named CoMoS-1, CoMoS-2, and CoMoS-4 respectively.Furthermore, a solid-state synthesis method was also employed to synthesize the cobalt molybdenum sulfide, where, a 1:3 weight ratio of CoMoO4 and thioacetamide was ground together and calcined at 500 degrees to get the cobalt molybdenum sulfide.Due to its morphological non-homogeneity (Figure S1), the solid-state synthesized cobalt molybdenum sulfide was not used for further processes.Furthermore, as a control sample, the CoMoS was also prepared with a one-step hydrothermal approach, where Co, Mo, S precursors (1 mmol Co(NO3)2.6H2O, 1 mmol of Na2MoO4.2H2O, and 2mmol of thioacetamide in 80 mL solution of solvents composed of water, ethanol and diethyl glycol in the volumetric ratio of 2:1:2) were mixed in a single Teflon lined hydrothermal vessel and heated at 180 ℃ for 6 h.The final product obtained after consecutive washing and drying is named CoMoS-H.

Preparation of CoMoS@NC
We implemented the self-polymerization ability of dopamine to form a very thin carbon film on CoMoS nanoparticles.In a typical process, 200 mg of CoMoS was dispersed on tris buffer solution (pH 8.5) and left to sonicate for 1 h.After the sonication, 100 mg dopamine hydrochloride was added to the solution and left to stir overnight in a magnetic stirrer.The dark black solution obtained after stirring was washed with DI water a few times and then with ethanol.The final black compound was kept in a vacuum oven at 80 ℃ for drying.The vacuum-dried black compound was kept in a furnace at 500 ℃ for 4 h at 3 ℃ /min under an argon atmosphere for calcination.
The final black powder obtained was named CoMoS@NC.A similar process was followed to obtain the CoMoS@NC-H, which is nitrogen-doped carbon coated CoMoS-H.

Physical characterization
The nanocomposites obtained were characterized using different approaches like X-ray diffraction (XRD) analysis by using the PANanalytical's Empyrean XRD with Cu Kα (λ = 0.15405 nm) radiation in the scan range (2θ) of 10° to 80°.The structural morphologies of the samples were observed with a field emission scanning electron microscope (FE-SEM) (TESCAN, MIRA3), and chemical compositions were evaluated by energy-dispersive spectroscopy (EDS) measurements and element mapping (TESCAN, MIRA) measurements at 15 kV.High-resolution (HR) brightfield imaging and combined high-angle annular dark-field (HAADF) scanning were also performed with Thermo Scientific Talos F200X G2 (Jeju National university, South Korea).Xray photoelectron spectroscopy (XPS) was conducted using a Theta Probe K-ALPHA+XPS

Fig
Fig. S1.FE-SEM images of high-temperature solid-state sulfurization under different magnifications

Fig. S8 .
Fig. S8.EIS equivalent electrical circuit of a) CoMoS and b) CoMoS@NC with relevant equivalent electrical parameters

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Fig. S10 (a) Discharge capacity of CoMoS@NC and NMC-532, and (b) Discharge capacity of a full cell over different anode to cathode AM mass ratios.