Contribution of Cation Addition to MnO2 Nanosheets on Stable Co3O4 Nanowires for Aqueous Zinc-Ion Battery

Zinc-based electrochemistry attracts significant attention for practical energy storage owing to its uniqueness in terms of low cost and high safety. In this work, we propose a 2.0-V high-voltage Zn–MnO₂ battery with core@shell Co₃O₄@MnO₂ on carbon cloth as a cathode, an optimized aqueous ZnSO₄ electrolyte with Mn²⁺ additive, and a Zn metal anode. Benefitting from the architecture engineering of growing Co₃O₄ nanorods on carbon cloth and subsequently deposited MnO₂ on Co₃O₄ with a two-step hydrothermal method, the binder-free zinc-ion battery delivers a high power of 2384.7 W kg⁻¹, a high capacity of 245.6 mAh g⁻¹ at 0.5 A g⁻¹, and a high energy density of 212.8 Wh kg⁻¹. It is found that the Mn²⁺ cations are in situ converted to Mn₃O₄ during electrochemical operations followed by a phase transition into electroactive MnO₂ in our battery system. The charge-storage mechanism of the MnO₂-based cathode is Zn²⁺/Zn and H⁺ insertion/extraction. This work shines light on designing multivalent cation-based battery devices with high output voltage, safety, and remarkable electrochemical performances.

Introduction

The ever-growing demands for electrical energy storage have stimulated the pursuit of advanced energy sources [batteries (Zhang H. et al., 2019), supercapacitors (Gan et al., 2020), photoelectrocatalysis (Wang et al., 2016; Shu et al., 2017)] with high energy density and high power density as well as high cycle lifetime (Pan et al., 2016; Ming et al., 2019; Zhang N. et al., 2019; Blanc et al., 2020). Li-ion batteries (LIBs) have become a protagonist due to their excellent comprehensive performances. However, lithium would present a long-term risk of shortage, since it is not quite abundant and even predicted to be regarded as the gold in the coming century (Li et al., 2019; Li C. et al., 2020). Zn-ion batteries (ZIBs), as a pronounced alternative, are receiving increased attention due to their merits of low cost, high safety, and high eco-efficiency. The ZIB device, in particular, consists of a zinc-metal anode, an aqueous electrolyte in majority and a cathode for accommodation of Zn ions. Among these components, the zinc anode has intrinsic merits of high theoretical capacity of 820 mAh g⁻¹ and a low redox potential of −0.76 V vs. a standard hydrogen electrode (SHE). The aqueous electrolyte not only is safe and cheap but also has typically two times higher conductivity than their organic counterparts and hence may satisfy the high capacity and high power demands (Chae et al., 2017; Zhang et al., 2018; Yu et al., 2019). Multivalent aqueous ZIBs allow multiple electron transfer during electrochemical reactions instead of single electron transfer in LIBs (Fang et al., 2018; Verma et al., 2019).

Similar to LIBs, transition-metal oxides [Mn-based materials (Zhang H. et al., 2020; Zhang M. et al., 2020), V-based materials (Fang et al., 2019; Zhang L. et al., 2020), and Mo-based materials (He et al., 2019)], Prussian blue analogs (Trocoli and La Mantia, 2015), and organic materials (Zhang H. et al., 2020) are attractive cathode-material candidates in ZIBs, particularly manganese dioxide (MnO₂), owing to its low cost and low toxicity. MnO₂ exists in various crystallographic polymorphs, i.e., the α-, β-, γ-, δ-, λ-, and ε-phase, which are composed of MnO₆ octahedral units, where each Mn⁴⁺ cation is coordinated by six oxygen atoms (Song et al., 2018; Han et al., 2020). These fundamental structural units are interconnected to each other by edges and/or corners, forming various crystalline structures that correspond to these different polymorphs. Among them, layered-type δ-MnO₂ demonstrates decent electrochemical performances vs. zinc, due to its relatively large interlayer distance (~7.0 Å) (Hou et al., 2020). The Kim's group presented a δ-MnO₂ ZIB with 252 mAh g⁻¹ high capacity at 83 mA g⁻¹, which maintained 112 mAh g⁻¹ capacity after cycling for 100 cycles. Increasing the current density to 666 mA g⁻¹ and 1,333 mA g⁻¹, the capacities become 92 and 30 mAh g⁻¹ (Alfaruqi et al., 2015). Zhi's group revealed a Na⁺ and H₂O pre-intercalated δ-MnO₂ ZIB with a high capacity of 278 mAh g⁻¹ at 1 C, which maintained a capacity of 106 mAh g⁻¹ when increasing the current to 1 C, and this ZIB keeps 98% capacity after cycling for 10,000 cycles (Wang et al., 2019). Guo et al. showed an ultrathin δ-MnO₂ nanosheet ZIB, exhibiting a reversible capacity of 133 mAh g⁻¹ at the current density of 100 mA g⁻¹, which maintained 86 mAh g⁻¹ when increasing the current density to 500 mA g⁻¹ (Guo C. et al., 2019).

To construct a more advanced zinc-ion battery device, optimizing its structure of both the cathode electroactive materials and the electrolyte is necessary (Cai et al., 2018; Guo S. et al., 2019; Pan et al., 2019; Tan et al., 2020; Wang et al., 2020). Highly conductive and high-surface-area Co₃O₄ nanowires were firstly grown on carbon cloth, followed by growing a layered structure of δ-MnO₂ nanosheets on Co₃O₄ by a two-step hydrothermal method. Mn²⁺ and Co²⁺ additives were added into an aqueous ZnSO₄ electrolyte to optimize the cation storage within δ-MnO₂. The aqueous zinc ion battery demonstrates outstanding electrochemical performances, and the charge storage mechanism and the effects of the electrolyte additive contribution were systemically investigated.

Experimental

Material

Cobalt chloride hexahydrate (CoCl₂·H₂O, >99.5%), urea [CO(NH₂)₂, >99.5%], potassium permanganate (KMnO₄, >99.0%), carbon cloth (CC), zinc sulfate heptahydrate (ZnSO₄·7H₂O, 99.0%), cobalt sulfate heptahydrate (CoSO₄·7H₂O, >99.0%), and manganese sulfate monohydrate (MnSO₄·H₂O, >99.99%) were purchased from Sinopharm Chemical Reagent. All chemicals were used as received.

Synthesis

Preparation of Co₃O₄ Nanowires on Carbon Cloth

Co₃O₄ nanowires were synthesized by a hydrothermal method on the surface of carbon cloth, which is used as substrate for growing all cathode materials throughout this manuscript. As for electrochemical measurements, carbon cloth serves as a flexible current collector. In a typical synthesis process, 9 mmol CoCl₂·6H₂O and 4 mmol urea were dissolved into 60 ml of deionized (DI) water by stirring for 2 h. Then, the solution was transferred into a 100-ml Teflon liner with a 3 × 4 cm carbon cloth substrate. After hydrothermal reaction at 120°C for 10 h, the Co₃O₄ nanowire precursors were formed on carbon cloth, which was cleaned with DI water, dried in an oven at 70°C, and then annealed in a tubular furnace at 350°C for 2 h with a heating rate of 2°C min⁻¹.

Deposition of δ-MnO₂ Nanosheets on the Surface of Co₃O₄ Nanowires

The as-synthesized Co₃O₄ nanowire/CC was transferred into a Teflon liner with 60 ml of 5 mM KMnO₄ solution. After a second hydrothermal reaction at 160°C for 4 h, the δ-MnO₂ nanosheets were deposited on the surface of Co₃O₄ nanowires, forming core-shell Co₃O₄@δ-MnO₂ on CC substrate, which was then washed with DI water and dried at 70°C before electrochemical measurements.

Characterizations

X-ray diffraction patterns were collected with a Bruker D8 diffractometer with Cu Kα radiation. SEM characterization was accomplished with a JEOL JSM-7100F, and TEM analysis was performed using a JEOL ARM 200F microscope equipped with a cold field emission electron source, an image Cs corrector (CEOS GmbH), and a Gatan image filter operated at 200 kV. X-ray photoelectron spectroscopy (XPS, Thermo Escalab) was used to identify elements and valence states, to determine the relative content of elements, to understand the interaction between central ions and coordination atoms and charge distribution, and to analyze the structure of compounds.

Electrochemical Testing

Punch Type Cell Packaging and Testing

The as-synthesized Co₃O₄@δ-MnO₂ on CC was sliced into a disk with a diameter of 1.2 cm and used as the cathode, 2 M ZnSO₄ aqueous solution was used as the electrolyte, and a zinc foil (~0.07 mm thickness) was used as the anode, to encapsulate a punch type ZIB cell. In addition, MnSO₄ or CoSO₄ additive was added into the electrolyte with the cation (Mn²⁺ or Co²⁺) concentration of 0.2 M. The mass loading of the cathode was about 2.5 mg, including Co₃O₄, MnO₂, and carbon cloth. Cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), cycle life, and other electrochemical performance were tested by CHI760E electrochemical workstation and Neware battery test system. All electrochemical tests were performed at room temperature.

Results and Discussions

The formation of core@shell Co₃O₄@MnO₂ on carbon cloth is illustrated in Figure 1A. Alkaline cobalt carbonate nanowires were firstly grown on carbon cloth by a hydrothermal method, which were converted into Co₃O₄ nanowire arrays by a following thermal treatment. δ-MnO₂ nanosheets were deposited on the surface of Co₃O₄ by a second hydrothermal reaction to form core@shell Co₃O₄@MnO₂. SEM imaging was used to monitor its morphology evolution after hydrothermal reactions. Figures S1A,B show pristine Co₃O₄ grown on carbon cloth and demonstrate its needle shape. After coping with MnO₂, core-shell nanowires were obtained, as shown in the low- and high-magnification SEM images of Figures 1B,C, respectively. TEM imaging was used to characterize the microstructures of as-synthesized Co₃O₄@MnO₂, as shown in Figure 1D, which indicates that MnO₂ are thin, with nanosheet shape and uniformly covering the surface of Co₃O₄. Figure 1E is a high-resolution TEM (HRTEM) image of a surface area of Co₃O₄@MnO₂, in which the 0.47 nm lattice can be assigned to the (111) plane of Co₃O₄ and the 0.25 nm lattice is assigned to the (−201) plane of δ-MnO₂. In addition, we can see some cross-section areas of the MnO₂ nanosheets with a characteristic (001) lattice distance of 0.69 nm, which also proves that MnO₂ crystallizes perpendicular to its (001) direction and its thickness is <10 nm. A detail lattice analysis of δ-MnO₂, as the marked area in blue color of Figure 1E, is shown in Figure S1D. STEM-EDX mapping was used to characterize the elemental distribution of a single Co₃O₄@MnO₂ nanowire, and its core@shell structure can be clearly confirmed, as shown in Figures 1F–I.

FIGURE 1

Figure 1. (A) Formation mechanism of Co₃O₄ nanowires@MnO₂ nanosheets/CC. (B,C) Low-magnification and high-magnification SEM images of Co₃O₄@MnO₂/CC. (D,E) Low-magnification and high-magnification TEM images of Co₃O₄@MnO₂. (F–I) STEM-EDX elemental mapping of a single Co₃O₄@MnO₂ nanowire.

To investigate the crystalline structure of Co₃O₄@MnO₂ and its surface elemental valence states, the XRD and XPS techniques were used, as shown in Figure 2. The diffraction peaks at 19.0, 31.3, 37.0, 44.8, 59.4, and 65.4° match well with the crystal planes of (111), (220), (311), (400), (511), and (440) of Co₃O₄ (JCPDS 43-1003). An only rest XRD peak at 12.5° can be assigned to δ-MnO₂ (JCPDS 80-1098), to confirm that we prepared MnO₂ nanosheets (see SEM image of Figure S1C) on carbon cloth without pre-growing of Co₃O₄ nanowires. The corresponding XRD spectrum of MnO₂ on carbon cloth is also shown in Figure 2A. The diffraction peaks at 12.5, 37.0, and 65.4° belong to (001), (−111), and (020) lattices of δ-MnO₂ with layered structure, which is consistent with the HRTEM results. High-resolution XPS spectra of Mn 2p, Co 2p, and O 1s of as-synthesized Co₃O₄@MnO₂ are shown in Figures 2B–D, respectively. The Mn 2p spectrum of Figure 2B displays two peaks at the binding energies of 642.3 and 654.0 eV, corresponding to Mn 2p 3/2 and Mn 2p 1/2 with a spin-energy separation of 11.7 eV, which indicates that Mn has 4+ valence state. The Co peaks can be assigned to Co³⁺ and Co⁴⁺ based on XPS analysis, which is consistent with the formation of Co₃O₄. The noisy XPS spectrum of Co is due to weak signal and Co is in the bulk of Co₃O₄@MnO₂ but XPS is a surface technique. The O 1s spectrum in Figure 2D shows three oxygen peaks, where the Mn–O bond is at 529.8 eV, –OH is at 530.1 eV, and oxygen defect peak is at 531.3 eV. The XPS spectra of Co₃O₄@MnO₂ show the characteristic photoelectron peaks of Mn 2p, Co 2p, and O 1s, and no impurities were detected.

FIGURE 2

Figure 2. (A) XRD patterns of as-synthesized core-shell Co₃O₄@MnO₂ and MnO₂ on carbon cloth, respectively, and standard XRD spectra of Co₃O₄ and MnO₂ as a reference. (B–D) High-resolution XPS spectra of Mn 2p, Co 2p, and O 1s of as-obtained Co₃O₄@MnO₂, respectively.

We constructed a Zn ion battery device with as-synthesized Co₃O₄@MnO₂ as the cathode, Zn metal as the anode, and 2.0 M ZnSO₄ as the electrolyte, namely, Co₃O₄@MnO₂//Zn. Besides, 0.2 M MnSO₄ or CoSO₄ is used as additive to the electrolyte to optimize the electrochemical performance of the device. Figure 3A shows CV curves of Co₃O₄@MnO₂//Zn in the electrolyte with/without a cation additive. When an additive (Mn²⁺ or Co²⁺) is present, a new reduction peak appears at 1.8 V, and a new oxidation peak appears around 2.2 V, results in enlarged area of CV curves, and thus enhances energy storage capacities as compared to that without additive. We believe that it related to the electronegativity of these metal ions. As we know, Zn²⁺ will bind six water molecules in aqueous solution to form Zn[(H₂O)₆]²⁺ (Lee et al., 2015; Wang et al., 2018). This strong interaction disassociates H₂O, so the pH of 2 M ZnSO₄ solution is 4.57. When Co²⁺ or Mn²⁺ is added, disassociation of H₂O becomes more apparent (Li G. et al., 2020). When 0.2 M CoSO₄ or MnSO₄ is mixed with 2 M ZnSO₄, the solution pH value would decrease to 3.84 and 3.48, respectively. Figure 3B shows the charge and discharge curves of the Co₃O₄@MnO₂ at a current density of 0.5 A g⁻¹. CV curves of single pieces of MnO₂/CC, Co₃O₄/CC, and Co₃O₄@MnO₂/CC electrolyte were measured, and the results are shown in Figure S2. Co₃O₄ exhibits good electrical conductivity, indicating that it can be used as a suitable channel for electron transmission. There is a concave at around 1.6 V during charging process when the electrolyte is with Mn²⁺ additive. We experimentally found that in the Mn²⁺-containing electrolyte, the CV curve of the device became stable after the second cycle, which means Mn²⁺ can also enhance the stability of our battery device, as shown in Figure 3C. As a comparison, the CV curve was not stable after four CV cycles in the Co²⁺-containing electrolyte (Figure 3C, inset). To reveal the working mechanism of Mn²⁺ additive in our battery system, we construct a battery device of CC//Zn and performed SEM and XRD analyses of the cathode material after CV cycling in the electrolyte with and without Mn²⁺ additive, as shown in Figures S3A–C. We found that in the Mn²⁺-containing electrolyte, Mn₃O₄ can be detected on CC cathode by XRD analysis, as well as surface deposition happens by SEM images. Then, we can logically draw a conclusion that in the Co₃O₄@MnO₂//Zn device, Mn²⁺ cations in the electrolyte can be in situ deposited on the cathode as Mn₃O₄ during electrochemical operations, which serve as electroactive cathode materials and enhance both the capacity and stability of our device. To investigate the influence of MnSO₄ additive, electrochemical impedance spectra (EIS) measurements of Co₃O₄@MnO₂ without electrolyte additives and with MnSO₄ additive were also taken. As is shown in Nyquist plots in Figure S4, the behaviors of two EISs in the different electrolyte circumstance are quite similar. However, after examination of the EISs, we found that the charge transfer resistance of Co₃O₄@MnO₂ in the Mn²⁺-doped ZnSO₄ electrolyte (33.47 Ω) is smaller than that in the pure ZnSO₄ electrolyte (37.32 Ω), illustrating a lower charge transfer resistance of Co₃O₄@MnO₂ in the Mn²⁺-doped ZnSO₄ electrolyte. This may account for higher ionic conductivity of the Mn²⁺-doped ZnSO₄ electrolyte. The specific capacity and cycling stability of Co₃O₄@MnO₂//Zn in three different electrolytes at different current densities from 0.5 to 1.5 A g⁻¹ are shown in Figures 3D,E, respectively. We can see that the Co₃O₄@MnO₂//Zn battery device has higher capacity and higher stability in the Mn²⁺ additive electrolyte. The energy density of Co₃O₄@MnO₂/CC in the Mn²⁺ additive electrolyte is as high as 212.8 ± 8.2 W h kg⁻¹, when the power density is 313.3 ± 1.8 W kg⁻¹. The energy density is still 82.1 ± 0.3 W h kg⁻¹, when the power density is 2384.7 ± 3.6 W kg⁻¹. With these advantages, the energy density and power density of zinc ion batteries in this system can be much higher than that of cathode materials in reported systems. For comparison, the properties of CuHCF (Trocoli and La Mantia, 2015), ZnHCF (Zhang et al., 2015), todorokite-type MnO₂ (Lee et al., 2013), PANI (Guerfi et al., 2014), Na₃V₂(PO₄)₃ (Hu et al., 2019), VS₂ (He et al., 2017), and Zn₃V₂O₇(OH)₂·2H₂O (Xia et al., 2018) are illustrated in Figure 3F. However, the Co²⁺ additive in the electrolyte does not have performance enhancement properties as compared to Mn²⁺, as shown in Figure S5.

FIGURE 3

Figure 3. Electrochemical performance of Co₃O₄@MnO₂ in 2 M ZnSO₄ electrolyte and with CoSO₄ or MnSO₄ additive. (A) CV curves. (B) GCD curves. (C) The first four CV cycles in MnSO₄ contained electrolyte, and within CoSO₄ contained electrolyte (inset). (D) Rate performance at different current densities from 0.5 to 1.5 A g⁻¹. (E) Cycling performance of Co₃O₄@MnO₂ at 3 A g⁻¹. (F) Ragone plots of Zn ion battery and results from literature.

To investigate the charge storage mechanism and dynamics of our device, we carried out electrochemical property characterization as shown in Figure 4. Figure 4A shows the CV curves of the Co₃O₄@MnO₂ cathode at different scanning rates between 0.5 and 10.0 mV s⁻¹. Two pairs of redox peaks in the CV curves can be clearly seen, which are consistent with the charging and discharging platforms in GCD curves. The capacitance of the battery system can be calculated by Equations (1, 2) (Tang et al., 2020):

$\begin{array}{ll}I=a{v}^b& (1)\end{array}$

$\begin{array}{ll}logI=b\times logv+loga& (2)\end{array}$

FIGURE 4

Figure 4. (A) CV behavior of Co₃O₄@MnO₂ in the Mn²⁺-containing ZnSO₄ electrolyte at different scan rates of 0.5, 1.0, 2.0, 3.0, 5.0, 6.0, 7.0, and 10.0 mV s⁻¹, respectively. (B) Plots of log i vs. log v of the oxidation peak of P1, and reduction peak of P2, as marked in (A). (C) GITT curves at 0.5 A g⁻¹ current density. (D) The diffusion coefficients calculated from GITT curve.

where I is the current density, v is the scanning rate, and a and b are adjustable parameters. When the b-value approaches 1.0, a capacitive response is observed. According to the graph of log I and log v, the b-values of peaks one and two are 0.8567 and 0.8647, respectively, indicating that the charge storage process is controlled cooperatively by the capacitance and diffusion behavior, which leads to fast Zn²⁺ diffusion dynamics (Figure 4B). The contribution of the capacitor can be separated by quantifying the current (I) at a fixed potential (V), the capacitance effect (k₁v), and the diffusion-controlled insertion (k₂v^1/2), according to the following Equation (3)

$\begin{array}{ll}I=k_{1}v+k_2{v}^{1/2}& (3)\end{array}$

Figure S6A depicts a typical 10 mV s⁻¹ capacitive current (blue area) compared to the total current. About 95.4% of the total charge comes from the capacitive contribution, which is closely related to the battery's rate performance. We further quantified the non-diffusion control capability of the Co₃O₄@MnO₂ cathode, as shown in Figure S6B. When the scanning rate was 0.5, 1, 2, 3, 5, 6, 7, and 10 mV s⁻¹, the contribution rate of the capacitor control is 39.4, 53.6, 63.0, 70.3, 77.0, 80.7, 82.9, and 95.4%, respectively. This is why the Co₃O₄@MnO₂/CC punch type cell has such a good rate performance and a high power density along with a high energy density. According to the result of galvanostatic intermittent titration technique (GITT) in Figure 4C, the diffusion coefficient (D) is obtained from the following Equation (4) (Yang and Rogach, 2019):

$\begin{array}{ll}D=\frac{4}{\pi \tau }{(\frac{m_B{V}_M}{M_{B}S})}^2{(\frac{△E_S}{△E_{\tau }})}^2& (4)\end{array}$

where τ is a constant current pulse time; m_B, V_M, S, and M_B are the mass loading, the molar volume, the electrochemical active area, and the molar weight, respectively; and E_S and E_τ are the voltage changes in the rest and dis-/charge steps. On the basis of GITT results, we calculated the diffusion coefficients at each point as is shown in Figure 4D. The diffusion coefficient values are all <6 × 10⁻⁷ cm² s⁻¹.

The typical chemistry of zinc ion batteries is Zn²⁺ insertion/extraction into/out of the cathode materials, accompanied by Zn²⁺ ↔ Zn Faradic reaction at the anode. Among which, the cathode part is critical but rarely studied, meanwhile, cation additives of electrolytes could enhance system complexity. We proposed a working mechanism of our zinc ion battery with the Co₃O₄@MnO₂/CC cathode and the Mn²⁺-containing electrolyte, as shown in Figure 5A. To clarify that, we use XRD and XPS to analyze MnO₂/CC cathode at different charge/discharge states, due to MnO₂ serving as electroactive materials in our system and its noisy or confusing XRD spectrum when Co₃O₄ is present (see Figure 2A). Figure 5B shows that the XRD spectra of MnO₂/CC started at 0.4 V (bottom curve) and then charged into concave one (~1.6 V) and 2.0 V, followed by a discharge process to concave two (~1.6 V) and 0.4 V. The XRD peaks are mainly assigned to ZnMn₂O₄ and MnO₂. During a charging process from bottom 0.4 V to 2.0 V, Zn²⁺ cations were gradually extracted out of ZnMn₂O₄, resulting in a decay of ZnMn₂O₄ peaks and an increase of MnO₂ peaks, as shown in Equation (5).

$\begin{array}{ll}\text{Z}{\text{n}}_{\text{x}}\text{Mn}{\text{O}}_2\to \text{Mn}{\text{O}}_2+\text{xZ}{\text{n}}^{2+}+2\text{x}{\text{e}}^{-}& (5)\end{array}$

The XRD peaks for ZnMn₂O₄ is thus the weakest at 2.0 V of the blue XRD spectrum. Following a discharge process, Zn²⁺ cations were inserted into the cathode material, forming the Zn_xMnO₂ phase, as shown in Equation (6).

$\begin{array}{ll}\text{Mn}{\text{O}}_2+\text{xZ}{\text{n}}^{2+}+2\text{x}{\text{e}}^{-}\to \text{Z}{\text{n}}_{\text{x}}\text{Mn}{\text{O}}_2& (6)\end{array}$

The XRD peaks for ZnMn₂O₄ is then recovered when discharged to 0.4 V again, as compared with black XRD spectra, which demonstrates high reversibility of Zn²⁺ extraction/insertion of the cathode materials during charging and discharging. Figure 5C shows high-resolution Mn XPS spectra of MnO₂/CC at different charge/discharge states, revealing the change of Mn valence from mainly 4+ to mainly 3+ and then back to mainly 4+. During charging, Mn₃O₄ was deposited on the electrode surface following Equation (7), resulting in 3+ dominated XPS peak (Hao et al., 2018).

$\begin{array}{ll}3\text{M}{\text{n}}^{2+}+4{\text{H}}_2\text{O}\to \text{M}{\text{n}}_3{\text{O}}_4+8{\text{H}}^{+}+2{\text{e}}^{-}& (7)\end{array}$

After discharging, Mn₃O₄ is oxidated into MnO₂ according to Equation.

$\begin{array}{ll}\text{M}{\text{n}}_3{\text{O}}_4+\text{x}{\text{H}}_2\text{O}\to 2\text{Mn}{\text{O}}_2·\text{x}{\text{H}}_2\text{O}+\text{M}{\text{n}}^{2+}+2{\text{e}}^{-}& (8)\end{array}$

FIGURE 5

Figure 5. (A) Illustration of the working mechanism of the Zn ion battery device. (B) XRD curves of MnO₂/CC at different charge/discharge states. (C–E) XPS spectra of Mn 2p, Zn 2p, and Co 2p at different charge/discharge states. (F) Summary of the electrolyte contribution to an as-synthesized Co₃O₄@MnO₂ cathode in our battery system and the electrochemical energy storage mechanism of the cathode.

Figures S7A,B show SEM-EDS results of single Co₃O₄@MnO₂ nanorods after firstly being discharged to 0.4 V and then followed by being charged to 2.0 V, respectively. The change of the Zn content in Figures S7A,B and the change of XPS peak intensity of Zn in Figure 5D prove the inserting/extracting Zn²⁺. XPS results of Figure 5E show that the binding energy of Co does not change much, which indicates that Co₃O₄ does not take part in electrochemical reactions. Based on the above analysis, the working mechanism of our cathode can be summarized in Figure 5F (Cheng et al., 2016; Shi et al., 2020). Many studies have also found that Mn₃O₄ has a tendency of phase transition to a layered MnO₂ in the cycling process (Jabeen et al., 2017; Hao et al., 2018). In this paper, after discharge, the surface of Mn₃O₄ will undergo a phase transition. Mn²⁺ will continue to dissolve into the electrolyte, and Mn³⁺ oxidizes to Mn⁴⁺, becoming layered MnO₂. The resulting layered manganese dioxide and the previously coated manganese dioxide jointly participate in the insertion and extraction of zinc ions, and the 0.69-nm layer spacing enables a good insertion process of Zn ions.

Conclusions

In conclusion, a core@shell Co₃O₄@MnO₂ nanowire has been synthesized on carbon cloth by a two-step hydrothermal method. The as-synthesized MnO₂ is a layered δ-MnO₂ phase and consists of nanosheets. When used as a cathode of an aqueous Zn ion battery with Mn²⁺ additive in a ZnSO₄ electrolyte, the core@shell Co₃O₄@MnO₂ battery presents a high energy density of 212.8 ± 8.2 Wh kg⁻¹ at a power density of 313.3 ± 1.8 W kg⁻¹ and maintains 82.1 ± 0.3 Wh kg⁻¹ at a power density of 2384.7 ± 3.6 W kg⁻¹, which is much higher than that for a pristine ZnSO₄ electrolyte or for the electrolyte with a Co²⁺ additive. In addition, a ZIB with a high output voltage of 2.0 V is presented. The performance enhancement mechanism of the Mn²⁺ additive in the electrolyte is that Mn²⁺ can be electrochemically oxidized to Mn₃O₄ on the cathode and further converted to electroactive MnO₂. The electrochemistry of our Zn ion battery reveals reversible Zn cation and proton insertion/extraction into/out of layered δ-MnO₂. This work demonstrates a promising cathode for Zn ion battery and other electrochemical energy storage devices.

Data Availability Statement

All datasets generated for this study are included in the article/Supplementary Material.

Author Contributions

NW and GY carried out the experimental parts of materials synthesis and characterizations. NW wrote the manuscript. GY, CW, QT, JJ, and XZ assisted the experiments and took part in data analysis and discussions on the results. XC, HaoW, JZ, XP, and Hwan were involved in designing and supervising this project and they have made a great contribution to the discussions. XC, YW, and PA carried out TEM characterization, analyzed TEM data, and played an important role in interpreting the results. HanW supervised the project and contributed to writing the paper, conceived and designed the battery device, and analyzed all the experiment data. All co-authors contributed to commenting on, editing the manuscript and SI. All authors contributed to the article, and approved the submitted version.

Funding

This work was supported by the National Natural Science Foundation of China (No. 11574077), the Science and Technology Department of Hubei Province (No. 2019AAA038), and the Wuhan Yellow Crane Talent Program (No. 2017-02). This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement no. 823717–ESTEEM3.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.2020.00793/full#supplementary-material

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