English

• 0.   Introduction

  To mitigate the environmental problems resulting from the escalating consumption of fossil fuels, highly efficient and environmentally friendly energy storage and conversion devices have been extensively and deeply researched^[1-3]. Supercapacitors have obtained widespread attention owing to their good eco-friendliness, excellent capacitance performance, good power density, and fast charge-discharge speed. However, the relatively low energy density, poor rate capability, and insufficient cycle stability have constrained their further practical application^[4-7]. Therefore, developing novel electrode materials with excellent electrochemical properties is essential^[8-10].

  Transition metal sulfides have received considerable attention and deep investigation for their electrochemical activity^[11-12]. Especially, bimetallic sulfides, such as NiCo₂S₄, ZnCo₂S₄, and MnCo₂S₄, as supercapacitor electrodes, exhibit much better electrochemical performance than the corresponding monometallic sulfides^[13-15], due to the introduction of heterogeneous transition metal ions and the existing of S element, leading to the modification of the electronic structure and the enhancement of charge transfer rate, thus resulting in the improved electrochemical performance^[16-19]. However, the path to practical application for transition metal sulfides is still fraught with challenges, including volume expansion during charge and discharge processes, limiting their structural stability and cycle life, slow kinetics, and possible side reactions, negatively influencing their specific capacitance^[20]. Modern micromorphology and microstructure engineering have been shown as a useful method to develop transition bimetal sulfide electrode materials with excellent specific capacitance^[21-22]. For example, the nickel cobalt sulfide (NiCoS) nanosheet array (NiCoS/CC)^[23] growing on carbon cloth (CC) with a rough surface and porous microstructure, exhibited an excellent specific capacitance of 206.6 mAh·g^-1 (1 A·g^-1). The core-shell NiCo₂S₄^[24] microspheres were prepared by the anion-exchange method, providing a good specific capacitance of 129.5 mAh·g^-1 (1 A·g^-1). Moreover, the novel amorphous Co-Mo-S microspheres^[25] were proved to have a high specific capacitance of 284.6 mAh·g^-1 (1 A·g^-1) and good rate capability. The superior electrochemical performance of these materials can be attributed to the unique chemical composition, the increased specific surface area, and pronounced porosity, which provide abundant reactive sites, effective channels for ion transport and electron transfer, allowing for easy access of electrolyte ions to the active material and effectively prevents the aggregation of active material during electrochemical reactions^[26]. Although electrode materials with good electrochemical performance have been obtained, the overall performance is still unattractive. That is, with high specific capacitance, it does not have high power density, high energy density, and good stability at the same time, seriously hindering the application prospect in the future. Hence, it is highly essential to design and prepare electrode materials with excellent overall performance.

  Based on the above point of view, we prepared a novel electrode material using a simple synthetical method, which shows excellent overall performance due to the unique microstructure and chemical composition. Specifically, the CoMn-glycerate precursor microspheres were synthesized by a simple solvothermal method, then sulfurized using thioacetamide (TAA) into the MnCo₂S₄ with a porous spherical structure. Since cobalt has a higher oxidation potential than manganese, therefore the introduction of manganese can facilitate the electron transfer and modify the surface properties, further influencing the electrochemical performance, resulting in greatly improved specific capacitance^[27]. In addition, the surface of the porous spherical structure provides enough active sites for electrochemical reactions. This feature endows the MnCo₂S₄ electrode material with excellent electrochemical capabilities with a specific capacity high up to 190.8 mAh·g^-1 at 1 A·g^-1, and a specific capacitance retention of 75.1% at 20 A·g^-1. Furthermore, the constructed MnCo₂S₄||porous carbon (PC) hybrid supercapacitor (HSC) showed high energy density (76.88 Wh·kg^-1 at 374.5 W·kg^-1), high power density (25.84 Wh·kg^-1 at 15 kW·kg^-1), and excellent cycling stability performance (86.8% capacity retention) after 10 000 charge-discharge cycles at 5 A·g^-1, and outstanding Coulombic efficiency of 99.7%.

  1.   Experimental

  1.1   Materials

  Mn(NO₃)₂·4H₂O, Co(NO₃)₂·6H₂O, isopropanol, glycerol, and TAA (CH₃CSNH₂) were bought from Aladdin Biochemistry Technology Company Ltd. All reagents are analytical grade.

  1.2   Synthetic methods

  0.5 mmol Co(NO₃)₂·6H₂O and 0.25 mmol Mn(NO₃)₂·4H₂O were added to a mixed solution prepared from 15 mL of glycerol and 45 mL of isopropanol and then the mixture was stirred for 20 min to achieve a pink homogeneous solution. The mixture was subsequently transferred to a reactor with a 100 mL capacity and kept at 180 ℃ for 6 h. After cooling down to room temperature, the supernatant of the as-obtained solution was discarded and the remainder was washed by centrifugation (8 000 r·min^-1, 5 min) with deionized water and ethanol alternatively for six times, then dried at 50 ℃ for 12 h in vacuum. The product was recorded as a CoMn-glycerate precursor. 90 mg of CoMn-glycerate precursor and 150 mg of TAA were added into 60 mL of ethanol and then the mixture was agitated for 20 min until all the CoMn-glycerate precursor and TAA were dissolved homogeneously. Subsequently, the mixture was transferred to a 100 mL reactor and kept at 160 ℃ for 6 h, the resulting black precipitate was centrifugally washed and dried at the same condition as the precursor. The obtained sample was named MnCo₂S₄. For comparison, the same preparation method was used to synthesize Co₃S₄ and MnS samples.

  1.3   Material characterization

  The crystal structure, physical phase composition, and valence state of the samples were analyzed using X-ray diffraction (XRD, Rigaku SmartLab SE, Cu Kα radiation, λ=0.154 nm, scanning speed of 5 (°)·min^-1, scanning interval of 10°-90°, working voltage of 40 kV, working current of 100 mA) and X-ray photoelectron spectrophotometer (XPS, Thermo Scientific Nexsa, Al Kα). The morphology, microstructure, and elemental composition of the obtained samples were further inspected with field emission scanning electron microscopy (FESEM, ZEISS, Sigma 300, accelerated voltage 5 kV), transmission electron microscopy (TEM, JEOL JEM-F200, accelerated voltage 200 kV), high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectrometry (EDS).

  1.4   Electrochemical measurement

  Ni foam with a density of (420±25) g·m^-2 was employed as the collector. The active material, the conductive agent (Ketjenblack), and the polytetrafluoroethylene (PTFE, 60%) were blended in a mass ratio of 7∶2∶1 and thoroughly mixed with ethanol to obtain a homogeneous paste. The prepared paste was evenly spread onto the nickel foam and maintained in a vacuum oven at 60 ℃ until completely dry and then pressed for 5 s at 10 MPa to ensure effective contact between the nickel foam and the active material. The mass loading of active substances in the electrodes was about 1.0 mg·cm^-2. The electrochemical evaluations were conducted using a CHI 760E electrochemical workstation, employing a three-electrode system with a platinum electrode serving as the counter electrode, and an Hg/HgO electrode served as the reference electrode. Electrochemical measurements, including cyclic voltammetry (CV) within the voltage range of 0 to 0.55 V, galvanostatic charge-discharge (GCD) tests from 0 to 0.5 V, and electrochemical impedance spectroscopy (EIS) assessments from 0.01 to 100 000 Hz, were conducted on the electrochemical workstation. The specific capacitance (C) was calculated according to the following equation.

  $ C=\frac{I \Delta t}{m \Delta V} $

  (1)

  where I represents the discharge current, Δt denotes the discharge time, m indicates the mass of the active substance, and ΔV is the voltage interval.

  HSCs were prepared by assembling a MnCo₂S₄ cathode and a PC anode in a two-electrode button cell using NKK diaphragm (TF40, Nippon Kodoshi Corporation) and 2 mol·L^-1 KOH solution as electrolyte. Applying the sequence of pole piece/diaphragm/pole piece, the HSC were assembled, and then sealed. The electrochemical performance was investigated after 1 h of resting. To ensure a balance between positive and negative electrode capacity, the following charge balance equation was utilized to calculate the mass loading of the cathode and anode.

  $ \frac{m_{+}}{m_{-}}=\frac{C_{-} \Delta V_{-}}{C_{+} \Delta V_{+}} $

  (2)

  where, m represents the mass of the active material in the electrode material, C denotes the specific capacity of the electrode material, and ΔV is the voltage range of the electrode material. The subscripts + and - represent the cathode and anode, respectively.

  The specific capacity of the HSC was calculated using Eq.1. The Eq.3 and 4 were used to calculate the energy density (E, Wh·kg^-1) and power density (P, W·kg^-1) of the device, respectively.

  $ E=\frac{C_{\mathrm{H}}(\Delta V)^2}{2 \times 3.6} $

  (3)

  $ P=\frac{3\;600 E}{\Delta t} $

  (4)

  where, C_H (F·g^-1) is the specific capacity of HSC, ΔV (V) is the voltage range of the device, Δt (s) is the discharging time of the GCD curve of the device.

  2.   Results and discussion

  2.1   Characterizations

  Fig. 1 illustrates the synthesis route for the MnCo₂S₄ material. Initially, Mn(NO₃)₂·4H₂O, Co(NO₃)₂·6H₂O, and glycerol were first mixed and homogeneously dissolved in isopropanol, then heated up to 180 ℃ and kept for 6 h. During the solvothermal reaction, the glycerol decomposed to form glycerate ions, which gradually bind with Co and Mn ions to form CoMn-glycerate. Due to the homogeneous nucleation mechanism^[28], the CoMn-glycerate exhibited microsphere-like morphology with diameters of about 500 nm. Secondly, sulfur-negative ions were generated through the decomposition of TAA during the vulcanization, then reacted with the metal cations in the CoMn-glycerate, and according to the mechanism of the ion-exchange reaction, with the increase of vulcanization, a certain core-shell structure may appear to form MnCo₂S₄ porous microspheres.

  Figure 1

  

  Figure 1.  Synthesis roadmap of MnCo₂S₄

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  XRD characterization results of MnCo₂S₄, MnS, and Co₃S₄ are shown in Fig. 2a. As seen, the XRD patterns from MnS and Co₃S₄ well match the standard diffraction patterns for MnS (PDF No.72-1534) and Co₃S₄ (PDF No.73-1703), respectively. It can be seen that the peaks of MnCo₂S₄ were broad and weak, which may be caused by the low crystallinity of the substance^[29]. The XRD peaks of MnCo₂S₄ correspond well to the Co₃S₄ pattern (PDF No.73-1703), especially at the peaks of MnCo₂S₄ at 31.4°, 37.9°, 50.1°, and 55.3°, indicating that substitution of Co ions with Mn ions did not alter the crystal structure significantly, and the lattice parameter changed only slightly, which is in agreement with some reports in the literature^[30-32].

  Figure 2

  

  Figure 2.  (a) XRD patterns of MnS, Co₃S₄, and MnCo₂S₄; (b) Mn2p, (c) Co2p, and (d) S2p XPS spectra, (e) N₂ adsorption-desorption isotherm, (f) pore-size distribution diagram, and (g) CA images for MnCo₂S₄

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  XPS was applied to further examine the ionic valence state and elemental composition of the MnCo₂S₄ material. As observed in Fig. 2b, the peaks of Mn2p at 642.1 and 653.8 eV are attributed to Mn²⁺, while the peaks at 643.4 and 654.9 eV belong to Mn^3+[33]. In Fig. 2c, the peaks of Co2p at 780.7 and 797.1 eV can be ascribed to Co²⁺, while the peaks at 778.1 and 793.2 eV correspond to Co^3+[8]. Fig. 2d shows the S2p fine spectrum. As seen, the peaks at 162.1 and 163.1 eV are assigned to the S2p_3/2 and S2p_1/2, belonging to S^2- and metal-sulfur bonds (Mn—S and Co—S), respectively. The peak at 169.5 eV indicates the sulfur oxidized to a higher valence state on the surface^[34]. In summary, the XPS analysis indicates the co-presence of Mn³⁺, Mn²⁺, Co³⁺, Co²⁺, and S^2- in MnCo₂S₄, which can provide abundant Faraday pseudo-capacitance^[35]. As can be seen from Fig. 2e, the N₂ adsorption-desorption isotherm of MnCo₂S₄ is type Ⅳ isotherm with a clear hysteresis loop, indicating that the material had a mesoporous structure and a specific surface area of 84 m·g^-1. From Fig. 2f, it is evident that the pore size was mainly distributed in 3-10 nm, providing an abundant porous structure, which is advantageous for the transport and diffusion of ions in the electrolyte^[36]. The hydrophilicity of the material can be reflected from the contact angle (CA). Fig. 2g shows the CA graph of MnCo₂S₄ material measured on a CA tester, from which it was observed that the CA value was 9.2° and the contact time was rapidly less than 1 s, indicating that it is superhydrophobic, which facilitates the penetration of the electrolyte and increases the contact area of the active sites.

  FESEM, TEM, and HRTEM were used to examine the micro-morphological and micro-structural characteristics of prepared samples. As observed in Fig. 3a-3c, the CoMn-glycerate precursor displayed homogeneously dispersed microspheres with diameters of about 500 nm which are composed of numerous interconnected nanosheets. The images of MnCo₂S₄ (Fig. 3d-3f) demonstrated a similar microsphere structure to the precursor, but not any longer with nanosheets. From the TEM images of MnCo₂S₄ (Fig. 3g-3h), it can be found that the middle of the microsphere was much darker (inner core) than the edge (outer shell), indicating a certain core-shell structure may have been formed during the introduction of the sulfur element. This core-shell structure facilitates the transfer of ions and electrons and greatly promotes redox reactions^[37]. As shown in the HRTEM image (Fig. 3i), two lattice spacings of 0.27 and 0.30 nm can be attributed to the (511) and (440) crystal planes of MnCo₂S₄, respectively, well consistent with the reports in the literature^[38-40]. The elemental mapping images obtained through EDS reveal a uniform distribution of Mn, Co, and S elements in the MnCo₂S₄ microspheres.

  Figure 3

  

  Figure 3.  (a-c) SEM images of CoMn-glycerate precursor; (d-f) SEM images, (g, h) TEM images, (i) HRTEM image, and (j) elemental mapping images of MnCo₂S₄

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  2.2   Electrochemical properties

  The electrochemical properties of MnS, Co₃S₄, and MnCo₂S₄ electrodes were tested with a standard three-electrode system in a 2 mol·L^-1 KOH solution. All the CV curves of Co₃S₄, MnS, and MnCo₂S₄ in Fig. 4a exhibited distinct oxidation-reduction peaks at 50 mV·s^-1, indicating typical battery-type capacitive behavior^[41]. Notably, the MnCo₂S₄ electrode displayed the largest CV curve area, implying its highest specific capacity among all the electrodes. Fig. 4b demonstrates all the CV curves of MnCo₂S₄ electrodes had clear redox peaks and without distinct deformation at the scan rates ranging from 5 to 100 mV·s^-1 in the potential ranges of 0-0.55 V, which originates from the reversible redox reactions of Co²⁺/Co³⁺and Mn²⁺/Mn³⁺ pairs, further implying the typical battery capacitive behavior and quick charge/discharge properties of the material^[42]. The reversible redox reactions can be expressed as follows^[43]:

  $ \mathrm{CoS}+\mathrm{OH}^{-} \rightleftharpoons \mathrm{CoSOH}+\mathrm{e}^{-} $

  (5)

  $ \mathrm{CoSOH}+\mathrm{OH}^{-} \rightleftharpoons \mathrm{CoSO}+\mathrm{H}_2 \mathrm{O}+\mathrm{e}^{-} $

  (6)

  $ \mathrm{MnS}+\mathrm{OH}^{-} \rightleftharpoons \mathrm{MnSOH}+\mathrm{e}^{-} $

  (7)

  Figure 4

  

  Figure 4.  (a) CV curves of different samples at 50 mV·s^-1; (b) CV curves of MnCo₂S₄ at different scan rates; (c) GCD plots of samples at 1 A·g^-1; (d) GCD plots of MnCo₂S₄; (e) Rate capabilities of samples; (f) EIS plots of different samples

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  It is seen in Fig. 4c that the GCD curve of MnCo₂S₄ exhibited the longest discharge time and the broadest voltage plateau at 1 A·g^-1 than the Co₃S₄ and MnS electrodes, demonstrating the superior capacitive performance of the MnCo₂S₄ material. Fig. 4d shows the GCD curves of MnCo₂S₄ at 1-20 A·g^-1. As calculated, the specific capacity of MnCo₂S₄ at 1 A·g^-1 was up to 190.8 mAh·g^-1, outperforming MnS (31.7 mAh·g^-1) and Co₃S₄ (86.7 mAh·g^-1). Additionally, as deduced from Fig. 4e, the retention rate of specific capacitance for MnCo₂S₄ after the current density rising from 1 to 20 A·g^-1 was up to 75.1%, better than those of MnS (71.9%) and Co₃S₄ (53.7%), further certifying the efficient synergetic effect of cobalt and manganese bimetals.

  Impedance analysis was carried out to assess the electrical conductivity and ion diffusion rates of MnS, Co₃S₄, and MnCo₂S₄ electrodes, the results are depicted in Fig. 4f. As shown, all the EIS curves consist of a semicircular curve and a real-axis intercept in the high-frequency region, along with an oblique line in the low-frequency region. The real-axis intercept and semicircle diameter correspond to the equivalent interface resistance (R_s) and the charge transfer resistance (R_ct), respectively, while the oblique line is related to the ion diffusion velocity^[44], the inset in Fig. 4f is an equivalent circuit diagram composed of R_ct, R_s, constant phase element (CPE) and Warburg element (Z_W). The MnCo₂S₄ electrode (Fig. 4f) had a R_s of 0.53 Ω and a R_ct of 0.65 Ω, showing a better impedance performance and a superior electronic conductivity than MnS (R_s=0.59 Ω, R_ct=0.84 Ω) and Co₃S₄ (R_s=0.55 Ω, R_ct=1.19 Ω). In addition, it is also observed from Fig. 4f that the MnCo₂S₄ electrode had the steepest slope of the oblique line in the low-frequency region, implying the best ion diffusion performance^[45].

  The electrochemical measurement results indicate the MnCo₂S₄ electrode had the highest specific capacity, good rate capability, and extremely low electrochemical impedance. This can be ascribed to the synergistic effect between cobalt and manganese, providing ample redox reactions, and the porous spherical structure, offering sufficient active sites and facilitating charge transfer and ion diffusion^[46].

  To determine the contribution to the total capacitance of surface redox reaction and diffusion-controlled redox reaction in the MnCo₂S₄ electrode, a semi-quantitative analysis based on Eq.8 and 9^[47-48].

  $ i_{\mathrm{p}}=a v^b $

  (8)

  $ \lg i_{\mathrm{p}}=b \lg v+\lg a $

  (9)

  where i_p is the average peak current, v is the scan rate, and a and b are adjustable parameters, the value can be fitted from the correlation between lg i_p and lg v. Typically, the b-value of 0.5 implies a diffusion-controlled process, and the b-value of 1.0 indicates a surface-controlled process^[49]. As explicitly shown in Fig. 5a, the b-value of the MnCo₂S₄ electrode was 0.77, signifying a mixed charge storage behavior manipulated by both surface-redox captive behavior and diffusion process. The larger value of the electrode indicates a larger contribution from the surface capacitance process. This result is further demonstrated by quantitative calculations of the surface capacitive charge and diffusion-controlled charge contributions based on the following equations^[50].

  $ i=k_1 v+k_2 v^{1 / 2} $

  (10)

  $ i / v^{1 / 2}=k_1 v^{1 / 2}+k_2 $

  (11)

  Figure 5

  

  Figure 5.  (a) lg i_p vs lg v fitting curves at characteristic peak currents; (b) Contributions of surface and diffusion-controlled capacity of MnCo₂S₄ at 100 mV·s^-1; (c) Contributions of surface and diffusion-controlled capacities at different scan rates

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  This k₁v represents the surface capacitive charge and k₂v^1/2 is the diffusion control charge, the slope (k₁) and intercept (k₂) are determined by a linear fitting between i and v^1/2. As shown in Fig. 5b and 5c, the surface capacitive contribution of the MnCo₂S₄ electrode increased with increasing the scan rates, showing a lower contribution rate of 34% at 5 mV·s^-1, and a higher one at 100 mV·s^-1 up to 70.4%, indicating fast charge-discharge performance at higher current densities.

  2.3   Electrochemical properties of MnCo₂S₄||PC HSC

  To thoroughly research the practical application ability of MnCo₂S₄ electrode materials, an HSC device was fabricated employing 2 mol·L^-1 KOH solution as the electrolyte, MnCo₂S₄ as the cathode, and PC as the anode, and tested with a two-electrode system. Based on Eq.2, the ideal mass ratio of the cathode to anode was calculated to be 1∶2.4. In consideration of the voltage range of 0 to 0.5 V and -1 to 0 V for the MnCo₂S₄ electrode and PC electrode respectively (Fig. 6a), it can be deduced that the voltage window of the assembled HSC can be broadened to 0-1.5 V. Fig. 6b displays the CV curves of the MnCo₂S₄||PC HSC at various scan speeds (2-200 mV·s^-1). As observed, when the scanning rate was elevated to 100 mV·s^-1, the CV shape remained almost unchanged, which suggests that the capacitor has excellent rate performance^[51], when the scan rate was increased to 200 mV·s^-1, the curve closure at 1.5 V became sharp, which is due to the slower transfer of ions and electrons relative to the scanning rate, resulting in polarization^[52]. As displayed in Fig. 6c, all the GCD curves of MnCo₂S₄||PC HSC at 0.5-20 A·g^-1 showed similarly symmetrical charge/discharge behaviors, suggesting the device possessed excellent electrochemical reversibility. The specific capacitances of the HSC at different current densities from 0.5-20 A·g^-1 were calculated based on the GCD curves in Fig. 6c and the results are plotted in Fig. 6d, revealing an excellent capacity performance of MnCo₂S₄||PC HSC with a high specific capacitance of 226.6 F·g^-1 at 1 A·g^-1, and even 82.7 F·g^-1 at 20 A·g^-1.

  Figure 6

  

  Figure 6.  (a) CV curves of PC and MnCo₂S₄ scanned at 10 mV·s^-1; (b) CV curves of the HSC at different scanning rates; (c) GCD curves of MnCo₂S₄||PC HSC at different current densities; (d) Specific capacity schematic of MnCo₂S₄||PC HSC at different current densities

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  Fig. 7a displays the Ragone plots of MnCo₂S₄||PC HSC. As seen from it, a highest energy density of 76.88 Wh·kg^-1 came up at the power density of 374.5 W·kg^-1, when the power density rising highly up to 15 kW·kg^-1, the energy density was still considerable with the value 25.84 Wh·kg^-1, outperforming previously reported supercapacitors with transitional metal sulfides as electrode material, such as NiCo₂S₄@MnS/CC||AC (725 W·kg^-1, 23.3 Wh·kg^-1)^[53], MCS/rGO/NF-3h||rGO (850.2 W·kg^-1, 45.4 Wh·kg^-1)^[54], CMS-8||AC (524.5 W·kg^-1, 48.5 Wh·kg^-1)^[55], CoNi₂S₄||YS-CS ACS (640 W·kg^-1, 35 Wh·kg^-1)^[56], C/NiCoS-4||AC (1 160 W·kg^-1, 34.1 Wh·kg^-1)^[57], NiCoMn-S-1.5||RGO HSC (11.3 kW·kg^-1, 14.5 Wh·kg^-1)^[58], Ni-Mn-S||RGO (775 W·kg^-1, 36 Wh·kg^-1)^[59], and NiCo₂S₄/Co₉S₈||AC (800 W·kg^-1, 36.7 Wh·kg^-1)^[60]. Additionally, the electrochemical impedance of the device was analyzed and the result was shown in Fig. 7b. It can be fitted that R_s and R_ct were 2.85 and 2.91 Ω, respectively, implying a low electrochemical resistance of the hybrid capacitor. Cycle stability is a critical parameter for assessing the actual usability of the equipment. Therefore, the MnCo₂S₄||PC HSC was subjected to constant current charge-discharge for 10 000 cycles at 5 A·g^-1 and the results are shown in Fig. 7c. As demonstrated, the device exhibited a capacity retention of about 86.8% and a Coulombic efficiency of 99.7%, indicating that the device possesses excellent cycling stability and further confirms the promising future application.

  Figure 7

  

  Figure 7.  (a) Ragone plots of MnCo₂S₄||PC HSC; (b) EIS plots of MnCo₂S₄||PC HSC (Inset: the magnification plots of the high-frequency region); (c) Cycle stability of MnCo₂S₄||PC HSC

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  3.   Conclusions

  In this work, microsphere precursor was firstly prepared by solvothermal method. Subsequently, the MnCo₂S₄ with porous spherical micromorphology was prepared through solvothermal vulcanization using the CoMn-glycerate as raw material. The MnCo₂S₄ showed an impressive specific capacity (190.8 mAh·g^-1 at 1 A·g^-1), good rate capability (75.1% capacity retention at 20 A·g^-1), and excellent cycling stability. The HSC device fabricated with MnCo₂S₄ and PC exhibited a high energy density of 76.88 Wh·kg^-1 at the power density of 374.5 W·kg^-1, an excellent capacity retention of 86.8% after 10 000 cycles at 5 A·g^-1, and a remarkable Coulombic efficiency of 99.7%. These excellent electrochemical performances are originally attributed to the suitable combinations of the two transitional metals in MnCo₂S₄, as well as the unique porous spherical structure of the MnCo₂S₄. This work is of great reference to enhance the specific capacitance and energy density by simultaneously tuning chemical element composition and microstructure.

  
  
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  Figure 1  Synthesis roadmap of MnCo₂S₄

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  Figure 2  (a) XRD patterns of MnS, Co₃S₄, and MnCo₂S₄; (b) Mn2p, (c) Co2p, and (d) S2p XPS spectra, (e) N₂ adsorption-desorption isotherm, (f) pore-size distribution diagram, and (g) CA images for MnCo₂S₄

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  Figure 3  (a-c) SEM images of CoMn-glycerate precursor; (d-f) SEM images, (g, h) TEM images, (i) HRTEM image, and (j) elemental mapping images of MnCo₂S₄

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  Figure 4  (a) CV curves of different samples at 50 mV·s^-1; (b) CV curves of MnCo₂S₄ at different scan rates; (c) GCD plots of samples at 1 A·g^-1; (d) GCD plots of MnCo₂S₄; (e) Rate capabilities of samples; (f) EIS plots of different samples

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  Figure 5  (a) lg i_p vs lg v fitting curves at characteristic peak currents; (b) Contributions of surface and diffusion-controlled capacity of MnCo₂S₄ at 100 mV·s^-1; (c) Contributions of surface and diffusion-controlled capacities at different scan rates

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  Figure 6  (a) CV curves of PC and MnCo₂S₄ scanned at 10 mV·s^-1; (b) CV curves of the HSC at different scanning rates; (c) GCD curves of MnCo₂S₄||PC HSC at different current densities; (d) Specific capacity schematic of MnCo₂S₄||PC HSC at different current densities

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  Figure 7  (a) Ragone plots of MnCo₂S₄||PC HSC; (b) EIS plots of MnCo₂S₄||PC HSC (Inset: the magnification plots of the high-frequency region); (c) Cycle stability of MnCo₂S₄||PC HSC

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