Hierarchical porous Mo-doped Co3O4 nanosheets for aqueous rechargeable zinc ion batteries with ultralong life

doi:10.21203/rs.3.rs-2241583/v1

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Hierarchical porous Mo-doped Co3O4 nanosheets for aqueous rechargeable zinc ion batteries with ultralong life

https://doi.org/10.21203/rs.3.rs-2241583/v1

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published 21 Apr, 2023

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The aqueous rechargeable zinc ion batteries (ZIBs) have attracted many attentions because of their remarkable energy density, good reliability, and high safety as promising next-generation energy storage devices. However, the ZIBs also suffer from the restricted capacity and limited cycle life for the potential energy storage-conversion applications. In this work, the advanced Mo ions dopant in Co₃O₄-CNTc composites (as Mo-Co₃O₄-CNTc) with hierarchical porous ultrathin nanosheets have been prepared via the sol-gel method in the emulsion system as industrial manufacturing technologies and investigated as the promising cathode materials for aqueous rechargeable ZIBs. Owing to experimental electrochemical data and density-functional first-principles calculations, the as-prepared Mo-Co₃O₄-CNTc composites represent superior electrochemical performance than pure Co₃O₄ and Mo-Co₃O₄ electrode materials. Furthermore, the as-prepared MoCo-Zn batteries with zinc metal foil anode and Mo-Co₃O₄-CNTc cathode exhibited specific capacity of 195.7 mAh g^–1 at 0.5 A g^–1, energy density of 237.6 Wh kg^–1 at 1692.4 W kg^–1, and remarkably ultralong cycling life over 10000 cycles with 85.1% capacity retention according to the hierarchical porous structures with opened space as “ion-buffering reservoirs”. The MoCo-Zn batteries were systematically further investigated in the zinc ions storage mechanism via the ex-situ XRD patterns, Raman spectra, and XPS measurements. Hence, this work may promote the further development of innovative strategy to synthesize the carbon modification composites with hierarchical porous nanosheets as cathode materials for the ultra-long cycle-life aqueous rechargeable ZIBs towards potential applications.

Hierarchical porous Mo-Co3O4 nanosheets

Zinc ion batteries

Ultralong cycle life

Zinc ions storage mechanism

Density-functional first-principles calculations

With the soaring of energy crisis and population explosion, the clean alternative energy storage systems are also still significant challenge to complete the strategy of “carbon neutrality” and environmental protection [1−5]. Remarkably, aqueous rechargeable zinc ion batteries (ZIBs) have triggered intensively great attentions due to their environmental friendliness, intrinsic safety, low cost, specific capacity, and high energy density [6, 7]. However, the ZIBs also suffer from the restricted capacity and limited cycle life for the potential energy storage-conversion applications [8−10]. Currently, the development of ZIBs is still mainly restricted by the cathode materials with stable cycle life and suitable crystalline structure [11].

In the most cathode materials, although the Co₃O₄ electrode materials display low cost, excellent stability, high theoretical capacity, and excellent electrochemical performance, they also suffer from practical low capacity due to their intrinsic electronic conductivity [12−14]. Interestingly, two elegant strategies can be employed to improve the electron transport ability of Co₃O₄ electrode materials by doping ions and incorporating carbon-based materials as “express channels” [15−17]. Compared with monometallic oxides, the binary metal oxides possess more complicated chemical composition and decreased band gap, all these factors are beneficial to improve the electrical conductivity and electrochemical reactivity by synergistic effect [18, 19]. Mo-doped Co₃O₄ electrode materials can synergistically exhibit superior electrochemical performances based on redox couples Co²⁺/Co³⁺/Co⁴⁺ and Mo⁶⁺/Mo⁴⁺ during the electrochemical processes [20]. Hence, designing the promising Mo-doped Co₃O₄ cathode materials with high electrochemical performance is also a significant challenge to improve the high specific capacity and ideal long cycling performance of MoCo-Zn batteries.

Herein, we have constructed the hierarchical porous Mo-Co₃O₄-CNTc composites as cathode materials for aqueous rechargeable ZIBs. Based on the typical construction process of Mo-Co₃O₄-CNTc composites in the oil in water (O/W) emulsion system as the facile method and industrial manufacturing technologies, a series of CNTs as “express channels” were interpenetrated in and connected to the Mo-Co₃O₄ nanosheets by a sol-gel method, thus exhibiting the 3D conductive networks to improve the electronic conductivity. The Mo-Co₃O₄-CNTc cathode materials exhibit specific capacity of 152.9 mAh g^–1 at 0.5 A g^–1, and excellent cycling performance with 80.3% capacity retention even over 4000 charge-discharge cycles at 25 A g^–1. Notably, the assembled MoCo-Zn batteries exhibit excellent electrochemical performance. Additionally, we have explored the zinc ions storage mechanism of Mo-Co₃O₄-CNTc cathode via the ex-situ XRD patterns, Raman spectra, and XPS measurements to display the structural evolution at certain voltages to further investigate the MoCo-Zn batteries.

2. 1 Materials

All reagents used in this work are 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 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 emulsion system (oil in water) by the sol-gel method. The polyethylene glycol (PEG, 6 g, molecular weight 6000) was dissolved in CH₂Cl₂ (30 mL), and the carboxylic CNT (acid-modified CNTc, 0.12 g) was dispersed in water (130 mL) under sonication, then PEG/CH₂Cl₂ was added CNTc/water with 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), then added to the mixture by drop. The system was in the volatilization atmosphere of 8 mL ammonia water under the beaker for 12 h. Then the precipitates were ultrasonically washed and dried at 60°C in a vacuum oven. Finally, the Mo-Co₃O₄-CNTc composites were obtained with an annealing process at 250°C in a muffle furnace for 2 h. Moreover, a series of Mo-Co₃O₄ electrode materials were further prpared with different Mo-Co molar ratio of 1:10, 5:10, and 10:10 for comparison with pure Co₃O₄ and Mo-Co₃O₄-CNTc composites, similarly.

2.3 Characterizations

The Mo-Co₃O₄-CNTc composites were characterized by X-ray diffraction (XRD, Bruker, Germany), thermogravimetric analysis (TGA, SDT-Q600), X-ray photoelectron spectroscopy spectra (XPS, ESCALAB-250), scanning electron microscopy (SEM, HITACHI S-4800), transmission electron microscopy (TEM, Philips Tecnai-12) and high-resolution TEM (HRTEM), Raman spectroscopy (LabRAM HR Evolution), respectively.

2.4 Preparation of zinc ion batteries

The MoCo-Zn ZIBs have been fabricated with Zn metal foil and 1 mg of hierarchically porous Mo-Co₃O₄-CNTc composites as cathode electrode materials on the current collectorof nickel foam (1×1 cm²), and 6 M KOH electrolyte with 0.2 M zinc acetate. Furthermore, the MoCo-Zn batteries were prepared based on the nickel foam (4×4 cm²) with loading 16 mg of Mo-Co₃O₄-CNTc and Zn metal foil in a soft package of polyethylene (PE) to form the MoCo-Zn batteries devices for energy storage application.

2.5 Electrochemical characterization

The electrochemical properties of Mo-Co₃O₄-CNTc (with Hg/HgO as reference electrode) and MoCo-Zn batteries were characterized by the cyclic voltammograms (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) with electrochemical workstation (CHI660e and SLAN-CT2001A). The specific capacities of the Mo-Co₃O₄-CNTc electrodes and MoCo-Zn batteries were obtained according to C*=It/3.6m, 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 E=∫IVdt/m (wherein V was discharging voltage (V)) and P = E/t, respectively.

2.6 Computational methods

All first-principles calculations were conducted using the density functional theory (DFT) as implemented in Vienna ab initio simulation package (VASP) [21, 22] code interfaced with MedeA software. The frozen-core projector augmented wave (PAW) [23] technique of Blöchl were employed to represent the inner core potentials. The generalized gradient approximation as described by Perdew-Burke-Ernzerhof (GGA-PBE) [24] was adopted to treat the exchange-correlation energy. The kinetic energy cut-off of 400 eV and Gaussian smearing [25] of 0.1 eV was set for all calculations. The samplings of the Brillouin zone were obtained from the Monkhorst-Pack [26] k-point grid with resolution of 0.2 Å^–1. A vacuum layer of about 15 Å was used to avoid the interaction between the adjacent layers. Spurious slab-to-slab dipole interactions were also corrected (IDIPOL = 3). The van der Waals (vdW) effects are described by employing dispersion-corrected DFT-D3 scheme [27] with Becke-Jonson damping. Hubbard correction [28] with U_eff = 4 eV for Co and U_eff = 1 eV for Mo was selected to describe the strong correlation effect. Geometries were fully optimized until the energy was converged to 1.0×10^–6 and the force was converged to 0.005 eV/Å.

The schematically synthetic processes of hierarchical porous Mo-Co₃O₄-CNTc composites were depicted in Fig. 1a in the oil/water emulsion via a typical and simple sol-gel method, in which the Mo-Co(OH)_x-CNTc composites as precursors were obtained and the Mo-Co₃O₄-CNTc composites in the annealing process corresponding to the CNTc content of 21.8% (Fig. S1). To further confirm the crystal structure of the Mo-Co₃O₄-CNTc composites to the cubic phase (JCPDS no. 42-1467), the XRD patterns (Fig. 1b) represent the diffraction peaks (111), (220), (311), (222), (400), (422), (511) and (440) planes, respectively. Furthermore, the chemical composition and state were determined via XPS (Fig. S2a). Co 2p spectra (with shake-up satellites (“Sat.”) at 787.6 and 804.2 eV) can be detected for Co³⁺ and Co²⁺ (Fig. 1c), thus indicating that the fitting peaks at banding energy of 780.5 and 795.5 eV are assigned to Co³⁺, and the fitting peaks at 782.0 and 797.0 eV can be ascribed to Co²⁺ [29, 30]. The Mo 3d spectrum (Fig. 1d) can be assigned to Mo 3d_3/2 at 235.3 eV and Mo 3d_5/2 at 232.2 eV, thus indicating the existence of Mo⁶⁺ with a width of 3.1 eV in the Mo-Co₃O₄-CNTc composites [31, 32]. Besides, the spectrum of O1s can be resolved as the lattice oxide ions O^2– at 530.3 eV, defective oxide ions O^x– at 531.5 eV, and adsorbed surface water at 533.5 eV in Fig. 1e, respectively. The two peaks at 284.8 and 286.2 eV should be respectively divided into C–C/C = C and C–O–C (Fig. S2b). So these can further confirm the successful preparation of the Mo-Co₃O₄-CNTc composites.

The detailed morphologies of obtained Mo-Co₃O₄-CNTc composites can be observed from SEM imagines (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 the hierarchically porous structures. Meanwhile, the CNTs can be uniformly mingled and inserted into the Mo-Co₃O₄ nanosheets as express electron transport channels (Fig. S4). Moreover, the detailed morphology of the hierarchically porous structures can be identified by TEM (Fig. 2c−f). The cross-linking and doping CNTs are combined with the Mo-Co₃O₄ nanosheets as interconnected electric network to facilitate the transfer of electron. Notably, the interlaced ultrathin nanosheets reveal 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 electric network are beneficial for the rapid electrolyte ion diffusion and fast electrons transport with low resistance, respectively. Accordingly, the HRTEM image Fig. 2f (inset) delivers the 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, respectively. Meanwhile, lattice spaces of 0.34 nm can be detected as the (002) planes of CNTs in the Mo-Co₃O₄-CNTc composites. Additionally, the EDS pattern (inset Fig. 2d) represents Co, Mo, O, and C elements, thus further indicating the successful preparation of Mo-Co₃O₄-CNTc composites. The elemental mapping analysis can directly display that these elements are distributed homogeneously on the entire Mo-Co₃O₄ nanosheets as shown in Fig. 2g−k, which is consistent with the observation from XPS results.

Due to such excellent typical structures, the electrochemical performance of 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 exhibit superior electrochemical properties based on 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), respectively. Moreover, the pure Co₃O₄ and a series of Mo-Co₃O₄ electrode materials with different Mo-Co molar ratio of 1:10, 5:10, and 10:10 were evaluated for comparison (Fig. 3d), indicating the superior electrochemical properties molar ratio of 1:10. The CV curves of Mo-Co₃O₄-CNTc cathode materials are clearly exhibited in Fig. 3e with obvious battery-type features at multiple scan rates from 0.5 to 50 mV s^–1, in which the oxidative peaks shift toward more positively and reductive peaks shift toward more negative values with the increasing scan rates due to polarization effect and more reversible redox reactions. Furthermore, Fig. 3f shows the log i and log v plots at peak currents, and the b-values can be calculated determined to 0.775 and 0.845 (in the range of 0.5–1.0) by the Dunn method [33] according to the Eq. (1). Consequently, the as-prepared Mo-Co₃O₄-CNTc cathode materials represent both battery-type and pseudocapacitive-type characteristics.

$$\text{l}\text{o}\text{g}i=b\text{l}\text{o}\text{g}v+\text{l}\text{o}\text{g}a$$

Moreover, the capacitive contribution for the total current at 1 mV s^− 1 was shown in Fig. 3g. The contribution ratio of the capacitive and diffusion-controlled capacity at various scan rates (Fig. 3h) can be calculated as the following equations [34, 35]:

$$I={I}_{\text{c}\text{a}\text{p}}+ {I}_{diff}={av}^{b}$$

Wherein, I_cap and I_diff are the surface capacitance-led and diffusion-controlled in the instant current, respectively. The capacitive-controlled process 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 the 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 in Fig. 3i. Compared with the pure Co₃O₄ and a series of Mo-Co₃O₄ electrode materials, the rate performance displays enhanced specific capacity at the lower current density of 0.5 A g^–1 in the initial 5 cycles, and so that in the last 15 cycles corresponding to the good structural stability in Fig. 3j. Furthermore, Fig. 3k delivers excellent cycling performance with 80.3% capacity retention even over 4000 GCD cycles at 25 A g^–1 and high columbic efficiency of 99.6%. Additionally, the Nyquist plots (Fig. 3l) of Mo-Co₃O₄-CNTc deliver lower electrochemical resistance (R_s) around 0.43 Ω and charge transfer resistance (R_ct) 0.53 Ω than pure Co₃O₄ and Mo-Co₃O₄ electrode materials for comparison due to the typical 3D interpenetrating CNTs conductive networks as the “express channels” (Fig. S5), respectively.

As illustrated in Fig. 4a, the MoCo-Zn batteries were assembled with the hierarchically porous Mo-Co₃O₄-CNTc composites as advanced cathode materials and zinc anode in the electrolyte of 6 M KOH aqueous with 0.2 M zinc acetate. The CV curves of the MoCo-Zn batteries exhibit similar shapes with obvious redox peaks at the increasing scan rate from 0.5 to 50 mV s^–1 (Fig. 4b). The b values of the MoCo-Zn batteries can be calculated as 0.786 and 0.746 by Dunn methods [33] 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 were 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 columbic efficiency in Fig. 4f, thus demonstrating the good structural stability. Meanwhile, the energy density and power density can be evaluated in the Ragone plots (Fig. 4g) as 237.6 Wh kg^–1 at 1692.4 W kg^–1 and 162.7 Wh kg^–1 at 50032.0 W kg^–1, respectively. Compared with the Mo-Co based supercapacitors and other aqueous rechargeable ZIBs, the as-prpared MoCo-Zn batteries exhibited a superior level in energy density, such as CoMoO_4–x//AC 62.3 Wh kg^–1 at 800 W kg^–1 [36], ZnCo₂O₄@CoMoO₄//AC 29.24 Wh kg^–1 at 884.57 W kg^–1 [37], CoMoO₄@Ni(OH)₂//AC 62.5 Wh kg^–1 at 776 W kg^–1 [38], NiMoO₄/CoMoO₄//AC 33.1 Wh kg^–1 at 199.6 W kg^–1 [39], Zn//Co₃O₄ 241 Wh kg^–1 at 1487.7 W kg^–1 [40], Zn//NiCo 210.1 Wh kg^–1 at 11600 W kg^–1 [41], Zn//core-shell Co₃O₄@δ-MnO₂/CC 212.8 Wh kg^–1 at 313.3 W kg^–1 [42], Zn//MnO₂ 254 Wh kg^–1 at 197 W kg^–1 [43], Zn//P-MoO_3–x@Al₂O₃ 240 Wh kg^–1 at 931.3 W kg^–1 [44], Zn//LiVPO₄F-CNTs@PPy 235.6 Wh kg^–1 at 320.8 W kg^–1 [45]. More strikingly, the MoCo-Zn batteries exhibited excellent cycling performance with 85.1% capacity retention over 10000 cycles at 25 A g^–1 and the capacity also represented any decay at the initial 2000 cycles (Fig. 4h). Meanwhile, the Mo-Co₃O₄-CNTc composites also possess hierarchical porous structures with opened space as “ion-buffering reservoirs” [46−48], which impressively outperformed most aqueous rechargeable zinc ion batteries. Furthermore, the coulombic efficiency of MoCo-Zn batteries is nearly 100%. The inset displays GCD curves at different cycles from 1st to 10000th, thus indicating the changes of GCD curves in the long cycle life including capacity decay, electrode polarization, stabilization of coulombic efficiency and displacement of discharge platform. Finally, the LEDs (2.2 V, 0.06 W) could be lit up by the series of MoCo-Zn devices for potential applications as demonstrated (Fig. 4h the inset image).

In order to further investigate the MoCo-Zn batteries, we have explored the zinc ions storage mechanism of Mo-Co₃O₄-CNTc cathode materials via the ex-situ XRD patterns, Raman spectra, and XPS measurements to display the structural evolution at some certain voltages. Figure 5a represents the schematic illustration of charge-discharge process with Zn²⁺ intercalation/de-intercalation. More concretely, Fig. 5b represents the selecting different labeled states of C0, C1, C2, C3, C4, D2, D1, and D0 (C represents for charge and D for discharge) in the charge-discharge process. As shown in ex-situ XRD patterns (Fig. 5c), the diffraction peaks shift at around 2θ = 20^o corresponding to the (111) planes of Co₃O₄ after the intercalation/deintercalation of Zn²⁺ during the charge-discharge process. Simultaneously, the new diffraction peaks appear in the range of 11–13^o, thus signifying new layered of α-Co(OH)₂ on the surface of C3, C4, D2, and D1 states, respectively. Moreover, the ex-situ Raman spectra (Fig. 5d) show that the peaks shift to higher at around 666 cm^–1 in the states of C2, C3, C4, D2, and D1 according to the Zn²⁺ ingress/egress. Additionally, more detailed information of chemical composition and states can be further investigated in the ex-XPS measurements (Fig. S6). Compared with the state of C0 without Zn 2p region, Zn 2p spectrum (Fig. 5e) can be detected as the absorbed Zn²⁺ Zn 2p_3/2 at 1022.0 eV and Zn 2p_1/2 at 1045.1 eV, and intercalated Zn²⁺ at Zn 2p_3/2 at 1021.4 eV and Zn 2p_1/2 at 1044.5 eV [49], respectively. During the discharge/charge process, it remarkably indicates the Zn²⁺ intercalation/de-intercalation with increasing peaks of intercalated Zn²⁺ in the states of C1, C2, C3, C4 and decreasing peaks of intercalated Zn²⁺ in the states of D2, D1, D0, respectively. Furthermore, the Mo 3d spectrum (Fig. 5f) of Mo-Co₃O₄-CNTc cathode materials can be deconvoluted as Mo 3d_5/2 and Mo 3d_3/2, 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 couples of Mo⁶⁺/Mo⁴⁺ during the charge-discharge processes. Similarly, 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⁺ [50, 51], 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 Mo-Co₃O₄-CNTc composites can keep the hierarchically porous structures with nanosheets and the interpenetrating conductive networks of CNTc in the TEM imagines (Fig. 6a−c). Moreover, the EDS elemental pattern of C4 charge state (Fig. 6d) represents the homogeneous distribution of Zn, O, Co, Mo, and C elements, thus further indicating the Zn²⁺ intercalation/de-intercalation in the whole Mo-Co₃O₄-CNTc composites and a good agreement with XPS measurements.

To gain deep insight for the interaction between CNT and Mo-Co₃O₄, the structural and electronic properties of Mo-Co₃O₄-CNT system were investigated by density-functional first-principles calculations [52–54]. The optimized structures and corresponding plane-averaged electrostatic potentials of Co₃O₄, Mo-Co₃O₄, and Mo-Co₃O₄-CNT have been calculated and displayed in Fig. 7a−c. There are no chemical bonds 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 was formed at the interface, which is beneficial to charge transfer [55, 56]. In addition, the calculated work function of the Mo-Co₃O₄-CNT (as 4.6 eV) is lower than that of the Co₃O₄ surface (as 6.1 eV) and Mo-Co₃O₄ surface (as 5.4 eV). The smaller work function means less loss when electrons escape to the surface for electron emission. It suggests the Mo-Co₃O₄-CNT composite is more beneficial to realize high electrical conductivity. The charge density difference and plane-averaged charge density difference of Co₃O₄, Mo-Co₃O₄, and Mo-Co₃O₄-CNT are plotted in Fig. 7d−f. The positive (yellow region) and negative (cyan region) values indicate the charge accumulation and depletion, respectively. The Bader charge analysis shows that 0.06 e per supercell has been transferred from CNT to Mo-Co₃O₄. It suggests that forming Mo-Co₃O₄-CNT interfaces may improve the electron transport performance of Mo-Co₃O₄ surface. To further study the interfacial contact properties, the atom-projected density of states (DOS) were analyzed in Fig. 7g−i. It can be seen that the increase in the density of states around the Fermi level that will result in increased conduction at elevated energies. CNT can alter the density of states and therefore alter the conductivity at the interface without damaging the significant characteristics of Mo-Co₃O₄ surface [57]. It is in good agreement with the EIS measurements.

Generally, we have synthesized and investigated the hierarchical porous Mo-Co₃O₄-CNTc composites as cathode materials for aqueous rechargeable ZIBs. Impressively, the interpenetrating CNTs as the “express channels” can construct the typical 3D conductive networks to improve the conductivity. The hierarchical porous Mo-Co₃O₄-CNTc composites represent superior electrochemical properties than pure Mo-Co₃O₄ electrode materials by experimental electrochemical data and density-functional first-principles calculations. Furthermore, the assembled MoCo-Zn batteries exhibited the 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 85.1% capacity retention over 10000 cycles, respectively. We further investigate the zinc ions storage mechanism of Mo-Co₃O₄-CNTc cathode to display the structural evolution at certain voltages. Significantly, we also provide an innovative strategy of constructing the M’-doped metal oxides composites modified with carbon materials as M’-MOx/Carbon by typical sol-gel emulsion method to promote the application of aqueous rechargeable batteries for energy storage and conversion.

Supplementary information

The online version contains supplementary material available at https://doi.org/xxxxx.

Authors’ Contributions

Changwei Lai carried out the experimental parts of materials synthesis and characterizations, and wrote the main manuscript text. Xiaoxiao Qu prepared Figure 1a. Yao Guo wrote the part 2.6 Computational methods and prepared Figure 7. Miaomiao Li prepared some data curation. Haixiang Song and Kwan Lee contributed the writing-review and editing, funding acquisition. All authors reviewed the manuscript.

Funding

This work was supported by the Science and Technology Project of Henan Province (no. 222102240103 and 192102210048); the Natural Science Foundation of Henan Province (No. 212300410099); the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2020R1I1A3070962); the Ph. D. Research Start-up Fund of Anyang Institute of Technology (no. BSJ2019033).

Conflict of interest The authors declare no competing interests.

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No competing interests reported.

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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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Hierarchical porous Mo-doped Co3O4 nanosheets for aqueous rechargeable zinc ion batteries with ultralong life

Status:

Journal Publication

Version 1

Abstract

Figures

1. Introduction

2. Experimental Section

2. 1 Materials

2.2 Preparation of hierarchical porous Mo-Co₃O₄-CNTc composites

2.3 Characterizations

2.4 Preparation of zinc ion batteries

2.5 Electrochemical characterization

2.6 Computational methods

3. Results And Discussion

4. Conclusions

Declarations

References

Additional Declarations

Supplementary Files

Status:

Journal Publication

Version 1

Hierarchical porous Mo-doped Co3O4 nanosheets for aqueous rechargeable zinc ion batteries with ultralong life

Status:

Journal Publication

Version 1

Abstract

Figures

1. Introduction

2. Experimental Section

2. 1 Materials

2.2 Preparation of hierarchical porous Mo-Co3O4-CNTc composites

2.3 Characterizations

2.4 Preparation of zinc ion batteries

2.5 Electrochemical characterization

2.6 Computational methods

3. Results And Discussion

4. Conclusions

Declarations

References

Additional Declarations

Supplementary Files

Status:

Journal Publication

Version 1

2.2 Preparation of hierarchical porous Mo-Co₃O₄-CNTc composites