1 Introduction

With the growing energy crisis and population explosion, clean alternative energy storage systems remain a significant challenge to attaining carbon neutrality and environmental protection [1,2,3,4,5]. Next-generation energy storage devices, such as aqueous rechargeable batteries [6], lithium ion batteries [7,8,9], lithium-oxygen batteries [10, 11], and supercapacitors [12,13,14,15,16,17], have attracted significant research interest. Aqueous rechargeable zinc ion batteries (ZIBs) are a promising technology due to their environmental friendliness, intrinsic safety, low cost, high specific capacity, and high energy density [18]; however, they show restricted capacity and limited cycle life [19, 20]. Currently, ZIB performance is mainly limited by cathode materials, which need to be further developed to achieve stable cycle life and suitable crystalline structures [21].

Among cathode materials, Co3O4 displays low cost, excellent stability, high theoretical capacity, and excellent electrochemical performance; however, it suffers from low practical capacity due to its intrinsic electronic conductivity [7, 22,23,24,25,26,27,28,29]. The electronic transport in Co3O4 has been improved by two strategies, i.e., by doping ions and by incorporating carbon-based materials as “express channels” [30,31,32,33,34,35,36,37,38]. Compared with monometallic oxides, binary metal oxides possess a more complicated chemical composition and show a decreased band gap; all these factors act synergistically and improve electronic conductivity and electrochemical performance [39, 40]. For instance, Mo-doped Co3O4 electrodes exhibit superior electrochemical performance due to the synergy between the Co2+/Co3+/Co4+ and Mo6+/Mo4+ redox couples during the electrochemical processes [41]. Hence, designing promising Mo-doped Co3O4 cathode materials with high electrochemical performance can help achieve high specific capacity and long cycling performance of MoCo-Zn batteries.

Herein, we report the fabrication of hierarchical porous Mo-Co3O4-CNTc composites as cathode materials for aqueous rechargeable ZIBs. The construction process of Mo-Co3O4-CNTc composites involves the oil-in-water (O/W) emulsion system, which is a facile method and involves industrial manufacturing equipment. A series of interconnected CNTs act as “express channels” and are connected to the Mo-Co3O4 nanosheets by a sol-gel method, leading to 3D conductive networks with improved electronic conductivity. The obtained Mo-Co3O4-CNTc cathode material exhibited a specific capacity of 152.9 mAh g−1 at 0.5 A g−1, and showed excellent cycling performance, with a 80.3% capacity retention even after 4000 charge-discharge cycles at 25 A g−1. Notably, the assembled MoCo-Zn batteries also exhibited excellent electrochemical performance. Additionally, the Zn2+ ion storage mechanism of the Mo-Co3O4-CNTc cathode was further investigated via ex situ XRD patterns, Raman spectra, and XPS measurements to characterize structural evolution at certain voltages to further investigate MoCo-Zn batteries.

2 Experimental section

2.1 Materials

All reagents used in this work are of analytical grade. Co(NO3)2·6H2O (> 98.5%), Na2MoO4·2H2O (> 99.0%), CH2Cl2, polyethylene glycol (PEG, molecular weight 6000), ammonia water (25~28%), KOH (> 85.0%), acetylene black, polyvinylidene difluoride (PVDF), and N-methyl-2-pyrrolidone (NMP, 99.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd. The carboxylic CNT (CNTc) was bought from Nanjing/Jiangsu XFNANO Materials Tech Co., Ltd.

2.2 Preparation of hierarchical porous Mo-Co3O4-CNTc composites

The hierarchical porous Mo-Co3O4-CNTc composites were synthesized in an O/W emulsion system by a sol-gel method. The polyethylene glycol (PEG, 6 g, molecular weight 6000) was dissolved in dichloromethane (CH2Cl2, 30 mL), and the CNTc (acid-modified CNT, 0.12 g) was dispersed in water (130 mL) under sonication. Then, PEG/CH2Cl2 was added to CNTc/water under high magnetic stirring. Co(NO3)2·6H2O (0.582 g, 2 mmol) and Na2MoO4·2H2O (0.0484 g, 0.2 mmol) were dissolved in water (20 mL), and then added to the mixture dropwise. With the volatilization atmosphere of 8 mL of ammonia water, the system of the emulsion was covered in a beaker and further reacted for 12 h to obtain the Mo-Co(OH)x-CNTc composites as precursors. Then, the precipitates were ultrasonically washed and dried at 60 °C in a vacuum oven. Finally, the Mo-Co3O4-CNTc composites were obtained by an annealing process at 250 °C in a muffle furnace for 2 h. Moreover, a series of Mo-Co3O4 electrode materials were further prepared with different Mo-Co molar ratios of 1:10, 5:10, and 10:10 for comparison with pure Co3O4 and Mo-Co3O4-CNTc composites.

2.3 Material characterization

The Mo-Co3O4-CNTc composites were characterized by X-ray diffraction (XRD, Bruker, Germany), thermogravimetric analysis (TGA, SDT-Q600), X-ray photoelectron spectroscopy (XPS, ESCALAB-250), scanning electron microscopy (SEM, HITACHI S-4800), transmission electron microscopy (TEM, Philips Tecnai-12), high-resolution TEM (HRTEM), and Raman spectroscopy (LabRAM HR Evolution). The surface areas and pore volume were confirmed by the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods (BSD-660 equipment), respectively.

2.4 Assembly of zinc ion batteries

The MoCo-Zn ZIBs were fabricated with a Zn metal foil as anode and 1 mg of hierarchically porous Mo-Co3O4-CNTc composite as cathode, deposited on a Ni foam current collector (1 × 1 cm2), and 6 M KOH with 0.2 M zinc acetate as electrolyte. The MoCo-Zn batteries were prepared based on a nickel foam (4 × 4 cm2) current collector, with a loading of 16 mg of Mo-Co3O4-CNTc and Zn metal foil in a soft package of polyethylene (PE).

2.5 Electrochemical characterization

The electrochemical properties of Mo-Co3O4-CNTc (with Hg/HgO as reference electrode) and MoCo-Zn batteries were characterized by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS), with an electrochemical workstation (CHI660e and SLAN-CT2001A). The specific capacities of the Mo-Co3O4-CNTc electrodes and MoCo-Zn batteries were obtained according to the following equation:

$${C}^{*}=It/3.6m$$
(1)

wherein C*, I, t, and m refer to the capacity (mAh g−1), discharge current (A), discharging time (s), and the mass of active mass on working electrode (g), respectively. Furthermore, the energy density (E, W h kg−1) and power density (P, W kg−1) were calculated according to equations:

$$E=\int IVdt/m$$
(2)
$$P=E/t$$
(3)

wherein V is the discharging voltage (V).

2.6 Computational methods

All first-principles calculations were conducted using density functional theory (DFT) as implemented in the Vienna ab initio simulation package (VASP) [42, 43] code interfaced with the MedeA software. The frozen-core projector augmented wave (PAW) [44] technique of Blöchl was employed to represent the inner core potentials. The generalized gradient approximation, as described by Perdew-Burke-Ernzerhof (GGA-PBE) [45], was adopted to treat the exchange-correlation energy. The kinetic energy cut-off of 400 eV and Gaussian smearing [46] of 0.1 eV were set for all calculations. The sampling of the Brillouin zone was obtained from the Monkhorst-Pack [47] k-point grid, with a resolution of 0.2 Å−1. A vacuum layer of about 15 Å was used to avoid interactions between the adjacent layers. Spurious slab-to-slab dipole interactions were also corrected (IDIPOL = 3). The van der Waals (vdW) effects were described using a dispersion-corrected DFT-D3 scheme [48] with Becke-Johnson damping. Hubbard correction [49], with Ueff = 4 eV for Co and Ueff = 1 eV for Mo, was selected to describe the strong correlation effect. Geometries were fully optimized until the energy converged to 1.0 × 10−6 and the force converged to 0.005 eV/Å.

3 Results and discussion

The synthesis process to obtain hierarchical porous Mo-Co3O4-CNTc composites is schematically depicted in Fig. 1a. The Mo-Co(OH)x-CNTc composites, as precursors, were prepared in the O/W emulsion. Remarkably, the Mo-Co3O4-CNTc composites can be obtained in the annealing process, in which the CNTc content was confirmed to be about 21.8% from TGA curves (Fig. S1). The crystal structure of the Mo-Co3O4-CNTc composites was confirmed to be the cubic phase (JCPDS no. 42-1467) by the XRD patterns (Fig. 1b), which shows diffraction peaks for the (111), (220), (311), (222), (400), (422), (511), and (440) planes. Furthermore, the chemical composition and state were determined via XPS (Fig. S2a). The Co 2p spectra (with shake-up satellites (“Sat.”) at 787.6 and 804.2 eV) were detected for Co3+ and Co2+ (Fig. 1c), thus indicating that the fitting peaks at band energy of 780.5 and 795.5 eV can be assigned to Co3+, and the fitting peaks at 782.0 and 797.0 eV can be ascribed to Co2+ [50, 51]. The Mo 3d spectrum (Fig. 1d) can be assigned to Mo 3d3/2 at 235.3 eV and Mo 3d5/2 at 232.2 eV, thus indicating the existence of Mo6+ with a width of 3.1 eV in the Mo-Co3O4-CNTc composites [32, 52]. Furthermore, the spectrum of O1s can be resolved as the lattice oxide ions O2− at 530.3 eV, defective oxide ions Ox− at 531.5 eV, and adsorbed surface water at 533.5 eV in Fig. 1e. The two peaks at 284.8 and 286.2 eV can be attributed to C–C/C = C and C–O–C, respectively (Fig. S2b), thus confirming the successful preparation of the Mo-Co3O4-CNTc composites.

Fig. 1
figure 1

a The schematic illustration of hierarchical porous Mo-Co3O4-CNTc composites. b XRD patterns. c–e The Co 2p, Mo 3d, and O 1 s spectra of Mo-Co3O4-CNTc

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The detailed morphologies of the obtained Mo-Co3O4-CNTc composites can be observed from the SEM images (Fig. 2a-b). Compared with the Mo-Co3O4 electrode materials (Fig. S3), the Mo-Co3O4-CNTc composites are composed of intertwisted and crinkly nanosheets to form hierarchically porous structures. Meanwhile, the CNTs were uniformly entangled and inserted into the Mo-Co3O4 nanosheets as express electron transport channels (Fig. S4). The detailed morphology of the hierarchically porous structures can be identified by TEM (Fig. 2c-f). The cross-linked and doped CNTs were combined with the Mo-Co3O4 nanosheets to form an interconnected electric network to facilitate the transfer of electrons. Notably, the interlaced ultrathin nanosheets reveal a thickness of 2–4 nm and substantial mesoporous scale holes, as shown in Fig. 2d. Meanwhile, the hierarchical mesoporous structures of Mo-Co3O4 nanosheets, combined with CNTs as an electric network, are beneficial for the rapid electrolyte ion diffusion and fast electrons transport with low resistance. Moreover, the nitrogen adsorption–desorption analysis (as Langmuir type IV, Fig. S5) [26, 53] indicated a BET surface area of 168.73 m2 g−1, BJH pore size distribution of ~3.9 nm, and pore volume 0.55 mL g−1, respectively. Accordingly, the HRTEM image shown in Fig. 2f (inset) presents lattice spaces of 0.28, 0.23, and 0.20 nm, corresponding to the (220), (222), and (400) planes of Mo-Co3O4, indicating high crystallinity and the polycrystalline nature of the Mo-Co3O4 nanoparticles. Meanwhile, a lattice space of 0.34 nm was detected from the (002) plane of the CNTs in the Mo-Co3O4-CNTc composites. Additionally, the EDS pattern (inset Fig. 2d) shows the presence of Co, Mo, O, and C, thus further indicating the successful preparation of Mo-Co3O4-CNTc composites. The elemental mapping shows that these elements are distributed homogeneously on the entire Mo-Co3O4 nanosheets, as shown in Fig. 2g-k, which is consistent with the XPS results.

Fig. 2
figure 2

The characterization of Mo-Co3O4-CNTc composites. a, b Different magnification SEM images. c–f Low- and high-magnification TEM images, EDS pattern, and HRTEM image (inset). g–k Co-K, Mo-K, O-K, and Cl-K

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The electrochemical performance of the as-prepared Mo-Co3O4-CNTc composites was systematically evaluated in the three-electrode configuration. Compared with the pure Mo-Co3O4 electrode materials, the Mo-Co3O4-CNTc composites exhibited superior electrochemical properties, as demonstrated by the CV curves at 50 mV s−1 (Fig. 3a), GCD curves at 0.5 A g−1 (Fig. 3b), and average capacity (four samples) at various current densities (Fig. 3c). Moreover, the pure Co3O4 (as 107.3 mAh g−1 at 0.5 A g−1) and a series of Mo-Co3O4 electrode materials with different Mo-Co molar ratios of 1:10, 5:10, and 10:10 were evaluated for comparison (Fig. 3d), indicating the superior electrochemical properties obtained at 1:10 as 112.0 mAh g−1 at 0.5 A g−1. The CV curves of the Mo-Co3O4-CNTc cathode materials are exhibited in Fig. 3e with obvious battery-type features at multiple scan rates from 0.5 to 50 mV s−1. The oxidative peaks shift toward more positive values and reductive peaks shift toward more negative values with the increase in scan rates due to the polarization effect and more reversible redox reactions. Furthermore, Fig. 3f shows the log i and log v plots at peak current values, and the b-values were determined to be 0.775 and 0.845 (in the range of 0.5–1.0) by the Dunn method [54], according to Eq. (4). Consequently, the as-prepared Mo-Co3O4-CNTc cathode materials represent both battery-type and pseudocapacitive-type characteristics.

Fig. 3
figure 3

The electrochemical properties of Co3O4, Mo-Co3O4, and Mo-Co3O4-CNTc for comparison: a CV curves, b GCD curves, c rate performance, and d rate performance with different Mo-Co molar ratios. The electrochemical properties of Mo-Co3O4-CNTc composites (with the Mo-Co molar ratio of 1:10): e CV curves, f b value, g capacitive contribution for the total current at 1 mV s−1, h capacitive and diffusion-controlled proportions at various scan rates, i GCD curves, j rate performance, k cycling performance, and l Nyquist plots and equivalent circuit (inset)

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$$\mathrm{log}i=b\mathrm{log}v+\mathrm{log}a$$
(4)

The capacitive contribution for the total current at 1 mV s−1 is shown in Fig. 3g. The contribution ratio of the capacitive and diffusion-controlled capacity at various scan rates (Fig. 3h) can be calculated by the following equations [55, 56]:

$$I={I}_{\mathrm{cap}}+{I}_{\mathrm{diff}}={\mathrm{av}}^{b}$$
(5)

wherein Icap and Idiff are the surface capacitance-led and diffusion-controlled current densities, respectively. The capacitive-controlled processes are 46.7%, 49.9%, 53.3%, 55.8%, 59.5%, 65.5%, 72.2%, 79.3%, and 91.3% at 0.5, 1, 2, 3, 5, 10, 20, 30, and 50 mV s−1, respectively. Additionally, the typical GCD profiles at various current densities deliver a remarkable specific capacity of 152.9 mAh g−1 at 0.5 A g−1 and 82.7 mAh g−1 at 40 A g−1, reaching 54.1% capacity retention, as shown in Fig. 3i. Compared with pure Co3O4 and Mo-Co3O4 electrode materials, the rate performance of Mo-Co3O4-CNTc showed enhanced specific capacity at the lower current density of 0.5 A g−1 in the initial 5 cycles. Thus, the last 15 cycles indicate good structural stability, as shown in Fig. 3j. Furthermore, Fig. 3k shows that excellent cycling performance was obtained, with 80.3% capacity retention after over 4000 GCD cycles at 25 A g−1 and a high Coulombic efficiency of 99.6%. Additionally, the Nyquist plots (Fig. 3l) of Mo-Co3O4-CNTc show a lower electrochemical resistance (Rs) of around 0.43 Ω and a lower charge transfer resistance (Rct) of 0.53 Ω, compared to both pure Co3O4 and Mo-Co3O4 electrode materials (Fig. S6). The superior properties of the Mo-Co3O4-CNTc composites can be attributed to the following factors: (i) the Mo-Co3O4, as a binary metal oxide, possesses higher electrical conductivity and electrochemical reactivity during the electrochemical processes due to the synergistic effect between the Co2+/Co3+/Co4+ and Mo6+/Mo4+ redox couples; (ii) the hierarchical porous structures provide open spaces for ion-buffering reservoirs, filled with electrolyte ions during the charge-discharge process, and substantial mesoporous structures in the ultrathin nanosheets exhibited short ion-diffusion channels from the external electrolyte to the interior of the Mo-Co3O4 nanosheets, thus leading to long cycling life and low internal resistance, respectively; and (iii) the Mo-Co3O4-CNTc composites with interpenetrating CNTs forming 3D conductive networks led to “express channels” through the hierarchical porous Mo-Co3O4 electrode materials to further synergistically improve the electron transport and electrochemical performance.

As illustrated in Fig. 4a, the MoCo-Zn batteries were assembled with the hierarchically porous Mo-Co3O4-CNTc composites as the advanced cathode materials and zinc metal as the anode, in the 6 M KOH aqueous electrolyte with 0.2 M zinc acetate. The CV curves of the MoCo-Zn batteries exhibited similar shapes and redox peaks, with the increasing scan rate from 0.5 to 50 mV s−1 (Fig. 4b). The b values of the MoCo-Zn batteries were calculated as 0.786 and 0.746 by the Dunn methods [54], as shown in Fig. 4c, thus revealing the coexistence of battery-type and pseudocapacitive-type characteristics. Moreover, the contribution ratio of capacitive and diffusion-controlled reactions is exhibited in Fig. 4d as 48.0%, 51.5%, 53.8%, 55.3%, 56.6%, 57.6%, 59.4%, 61.1%, 65.3%, 69.0%, 75.1%, 81.6%, and 86.8% at various scan rates of 1, 2, 3, 4, 5, 6, 8, 10, 15, 20, 30, 40, and 50 mV s−1, respectively. Furthermore, the GCD curves with an average discharge platform of around 1.68 V represent the voltage window of 1.93 V from 1 to 30 A g−1, as shown in Fig. 4e, and deliver the specific capacity of 195.7 mAh g−1 at 0.5 A g−1 and 97.6 mAh g−1 at 30 A g−1 (with capacity utilization of 49.9%), respectively. The MoCo-Zn batteries display outstanding rate performance and Coulombic efficiency, as shown in Fig. 4f, thus demonstrating good structural stability. Meanwhile, the energy density and power density can be evaluated from the Ragone plots (Fig. 4g) as 237.6 Wh kg−1 at 1692.4 W kg−1 and 162.7 Wh kg−1 at 50,032.0 W kg−1, respectively. Compared with the Mo-Co-based supercapacitors and other aqueous rechargeable ZIBs, the as-prepared MoCo-Zn batteries exhibited a superior energy density, such as CoMoO4–x//AC 62.3 Wh kg−1 at 800 W kg−1 [57], ZnCo2O4@CoMoO4//AC 29.24 Wh kg−1 at 884.57 W kg−1 [58], CoMoO4@Ni(OH)2//AC 62.5 Wh kg−1 at 776 W kg−1 [59], NiMoO4/CoMoO4//AC 33.1 Wh kg−1 at 199.6 W kg−1 [60], Zn//Co3O4 241 Wh kg−1 at 1487.7 W kg−1 [61], Zn//NiCo 210.1 Wh kg−1 at 11600 W kg−1 [62], Zn//core-shell Co3O4@δ-MnO2/CC 212.8 Wh kg−1 at 313.3 W kg−1 [63], Zn//MnO2 254 Wh kg−1 at 197 W kg−1 [64], Zn//P-MoO3–x@Al2O3 240 Wh kg−1 at 931.3 W kg−1 [65], and Zn//LiVPO4F-CNTs@PPy 235.6 Wh kg−1 at 320.8 W kg−1 [66]. The MoCo-Zn batteries exhibited excellent cycling performance, with 85.1% capacity retention over 10,000 cycles at 25 A g−1, and there was no decay at the initial 2000 cycles (Fig. 4h). Meanwhile, the Mo-Co3O4-CNTc composites also possessed hierarchical porous structures with opened space functioning as “ion-buffering reservoirs” [67,68,69], which outperformed most aqueous rechargeable ZIBs. Furthermore, the Coulombic efficiency of the MoCo-Zn batteries was nearly 100%. The inset (Fig. 4h) displays GCD curves at different cycles from 1st to 10,000th, thus indicating the changes in the GCD curves during long-term cycling life, including capacity decay, electrode polarization, stabilization of Coulombic efficiency, and displacement of the discharge platform. Finally, the LEDs (2.2 V, 0.06 W) could be lit up by a series of MoCo-Zn devices, as demonstrated in Fig. 4h (inset image), verifying their potential for practical applications.

Fig. 4
figure 4

a Schematic illustration of the MoCo-Zn batteries. Electrochemical performance of the batteries: b CV curves, c b value in CV curves, d capacitive and diffusion-controlled proportions at various scan rates, e GCD curves, f rate performance, g Ragone plots, and h cycling performance, the inset shows GCD curves at different cycles and the photographs of red-light emitting diodes (LEDs) lighted by MoCo-Zn batteries in series

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To further investigate the MoCo-Zn batteries, the summary of Zn2+ ion storage mechanism of Mo-Co3O4-CNTc was explored via ex situ XRD patterns, Raman spectra, and XPS measurements to characterize the structural evolution at certain voltages. Figure 5a represents the schematic illustration of the charge-discharge process with Zn2+ intercalation/de-intercalation. Figure 5b represents the different states in the charge-discharge process, labeled C0, C1, C2, C3, C4, D2, D1, and D0 (where C and D represent charge and discharge, respectively). As shown in ex situ XRD patterns (Fig. 5c), the diffraction peaks shift at around 2θ = 20°, corresponding to the (111) planes of Co3O4 after the intercalation/de-intercalation of Zn2+ during the charge-discharge process. Simultaneously, the new diffraction peaks appeared in the range of 11–13°, thus signifying a new layer of α-Co(OH)2 on the surface of the C3, C4, D2, and D1 states. Moreover, the ex situ Raman spectra (Fig. 5d) show that the peaks shifted to a higher value at around 666 cm−1 in the states labeled C2, C3, C4, D2, and D1, according to the Zn2+ ingress/egress. Additionally, more detailed information of the chemical composition and states can be further investigated by ex-XPS measurements (Fig. S7). Compared with the state of C0 without the Zn 2p region, the Zn 2p spectrum (Fig. 5e) can be detected as the absorbed Zn2+ at Zn 2p3/2 at 1022.0 eV and Zn 2p1/2 at 1045.1 eV, and the intercalated Zn2+ at Zn 2p3/2 at 1021.4 eV and Zn 2p1/2 at 1044.5 eV [70]. Thus, during the discharge/charge process, the Zn2+ intercalation/de-intercalation can be further demonstrated, with the intensity of intercalated Zn2+ peaks increasing in the C1, C2, C3, and C4 states and decreasing of intercalated Zn2+ peaks in the D2, D1, and D0 states. Furthermore, the Mo 3d spectrum (Fig. 5f) of the Mo-Co3O4-CNTc cathode materials could be deconvoluted as Mo 3d5/2 and Mo 3d3/2, corresponding to Mo6+ at 232.2 eV, Mo4+ at 231.7 eV, Mo6+ at 235.3 eV, and Mo4+ at 234.9 eV, respectively, thus indicating the electrochemical reaction between the redox couple Mo6+/Mo4+ during the charge-discharge processes. Similarly, the Co 2p spectra can be identified as the fitting peaks at 780.5 and 795.5 eV for Co3+, and 782.0 and 797.0 eV for Co2+ [71, 72], respectively. Remarkably, the peaks of C3, C4, and D2 shift to lower binding energy due to more electrochemical oxidation of Co3+, as shown in Fig. 5g. Furthermore, the C4 charge state of the Mo-Co3O4-CNTc composites maintained the hierarchically porous structures with nanosheets and the interpenetrating conductive networks of CNTc in the TEM images (Fig. 6a-c). Moreover, the EDS elemental pattern of the C4 charge state (Fig. 6d) indicates a homogeneous distribution of Zn, O, Co, Mo, and C, thus further indicating the Zn2+ intercalation/de-intercalation in the entire Mo-Co3O4-CNTc composite and a good agreement with XPS measurements.

Fig. 5
figure 5

a The schematic illustration of charge-discharge process of Mo-Co3O4-CNTc cathode materials. The characterization of different labeled states from C0 to D0 during the charge-discharge process: b GCD curve, c ex situ XRD patterns, d ex situ Raman spectra, e–g Zn 2p, Mo 3d, and Co 2p XPS spectra

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Fig. 6
figure 6

The morphology of C4 charge state: a–c low- and high-magnification TEM images and d the EDS elemental mapping analysis of Zn, O, Mo, Co, and C

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To gain deep insight into the interaction between CNT and Mo-Co3O4, the structural and electronic properties of the Mo-Co3O4-CNT system were investigated by first-principles DFT calculations [73,74,75]. The optimized structures and corresponding plane-averaged electrostatic potentials of Co3O4, Mo-Co3O4, and Mo-Co3O4-CNT were calculated, as shown in Fig. 7a-c. No chemical bonds were formed at the interface, indicating a typical Van der Waals (vdW) interaction between CNT and Mo-Co3O4. Owing to the potential difference, an internal electric field formed at the interface, which is beneficial to charge transfer [76, 77]. In addition, the calculated work function of the Mo-Co3O4-CNT (4.6 eV) was lower than that of the Co3O4 surface (6.1 eV) and Mo-Co3O4 surface (5.4 eV). The smaller work function means less loss when electrons escape to the surface for electron emission. This suggests that the Mo-Co3O4-CNT composite is beneficial for achieving high electronic conductivity. The differences in charge density and plane-averaged charge density of Co3O4, Mo-Co3O4, and Mo-Co3O4-CNT are plotted in Fig. 7d-f. The positive (yellow region) and negative (cyan region) values indicate charge accumulation and depletion, respectively. The Bader charge analysis shows that 0.06 e− per supercell was transferred from CNT to Mo-Co3O4. This indicates that Mo-Co3O4-CNT interfaces improve electron transport at the Mo-Co3O4 surface. To further study the interfacial contact properties, the atom-projected density of states (DOS) were analyzed, as shown in Fig. 7g-i. The increase in the density of states around the Fermi level resulted in increased conduction at elevated energies. CNT could alter the density of states and, therefore, alter the conductivity at the interface without damaging the significant characteristics of the Mo-Co3O4 surface [78]. This result is in good agreement with the EIS measurements.

Fig. 7
figure 7

Density-functional first-principles calculations of Co3O4, Mo-Co3O4, and Mo-Co3O4-CNT composites for comparison: a–c optimized structure and plane-averaged electrostatic potential, d–f calculated charge density difference, and plane-averaged charge density difference, g–i atom-projected density of states (DOS). Color scheme: Co, blue; Mo, purple; O, red; C, brown. TOC: The cathode materials of Mo-doped in hierarchical porous Mo-Co3O4-CNTc composites were fabricated for aqueous rechargeable zinc ions batteries with ultra-long cycle life

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4 Conclusions

In summary, we report the synthesis and investigation of hierarchical porous Mo-Co3O4-CNTc composites as cathode materials for aqueous rechargeable ZIBs. The interpenetrating CNTs act as “express channels,” leading to 3D conductive networks that improve electronic conductivity. Experimental electrochemical data and first-principles DFT calculations demonstrated that hierarchical porous Mo-Co3O4-CNTc composites showed superior electrochemical properties compared to pure Mo-Co3O4 electrode materials. Furthermore, the assembled MoCo-Zn batteries exhibited a specific capacity of 195.7 mAh g−1 at 0.5 A g−1, 237.6 Wh kg−1 at 1692.4 W kg−1, and ultralong cycling performance, with a 85.1% capacity retention over 10,000 cycles. The Zn2+ ion storage mechanism in the Mo-Co3O4-CNTc cathode was further investigated to study the structural evolution at certain voltages. Therefore, in this study, we provide an innovative strategy for constructing the M’-doped metal oxide composites modified with carbon materials (M’-MOx/carbon), by a typical sol-gel emulsion method to help develop next-generation aqueous rechargeable batteries for energy storage and conversion.