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Layer-by-layer synthesis of Zn-doped MnO2 nanocrystals as cathode materials for aqueous zinc-ion battery

This work first described the new oxidation-reduction route for the synthesis of Zn-doped MnO2 nanocrystals via layer-by-layer method as cathode material for an aqueous zinc-ion battery. The obtained nanolayers were characterized by SEM, XRD, XPS and FTIR spectroscopy. The results show the synthesized nanolayers were formed from two-dimensional nanocrystals Zn0.3MnO2 the thickness of about 3–8 nm and the morphology of “nanosheets” with the birnessite-like crystal structure. Benefiting from the aqueous 2M ZnSO4 electrolyte and Zn0.3MnO2 nanocrystals-based cathode, the zinc-ion battery delivers a high specific capacity (216 mAh/g at 1 A/g) and excellent cycling stability (95% capacity retention after 1000 charge-discharge cycles). The obtained results demonstrate the manganese oxide-based aqueous zinc-ion battery is a promising technology for powering next-generation electronics.


Introduction
In recent years, zinc-ion batteries are receiving much attention as low-cost and safe energy storage technology for emerging applications in flexible and safe devices [1]. However, deficiency of high energy-density electrode materials is now the main limit for obtaining high-performance zinc-ion batteries. Thus, the development of highperformance electrode materials is the key to improving the capacity and increase the commercial application of zinc-ion batteries [2][3][4].
The main function of a zinc-ion battery cathode is to provide a stable structure in which Zn 2+ ions can be chemically bonded with low binding energy and fast kinetics [5]. Thus, the cathode must have a crystal structure with suitable crystallographic sites for Zn 2+ accommodation, low-energy paths for ion diffusion, and high electronic conductivity for charge transfer [6]. To meet these requirements, layered and tunnel structured compounds with redox components are often chosen. In this case, the cathode must also have sufficient porosity to allow the penetration of the electrolyte solution, thereby increasing the number of active sites for the maximum charge transfer rate [7]. The materials that meet the presented requirements include analogs of Prussian blue, manganese and cobalt oxides, vanadium oxides, as well as organic compounds [8][9][10]. Among these materials, manganese oxides are of the greatest interest from the point of view of their further commercialization, since they have a high specific capacity and a wide working potential window, while they can be synthesized from relatively inexpensive precursors [11,12]. However, these cathode materials have low electronic conductivity, which does not allow them to approach the theoretical values of the specific capacitance.
The solution to this problem may consist of obtaining 2D nanocrystals with a graphene-like morphology of the so-called "nanosheets", which is due to a set of their unique physical and chemical properties. The flat 2D structure of such materials provides a sufficient number of active adsorption centers. Another key characteristic of 2D materials is their ultra-small thickness, on the order of a few nanometers, so charge carriers can travel extremely short distances from the volume to the surface, while significantly improving electronic conductivity [13][14][15].
New possibilities of manganese oxides nanocrystals synthesis rose after the development of Layer-by-Layer (LbL) synthesis techniques that make it possible to deposit MnO 2 layers on the surface of samples with an irregular shape, providing an exact thickness of the layer [16]. The purpose of the present study was to explore the feasibility of synthesis of the MnO 2 nanolayers by the Successive Ionic Layer Deposition (SILD) method [17], also called the Successive Ionic Layer Adsorption and Reaction (SILAR) [18]. The method is based on multiple and successive treatments of a substrate by solutions of reagents which enter into reaction at its surface and form a layer of poorly soluble substance. The distinctive features of this method are its capability of deposition of the layers of controlled thickness on the surface of parts of any shape, which are exactly the requirements to the methods of synthesis of the layers on the surface of electrodes for the zinc-ion battery.
In the present paper, we report a new simple oxidation-reduction route for the synthesis of Zn-doped MnO 2 nanolayers via the SILD method. The obtained nanolayers consist of MnO 2 nanocrystals, including zinc atoms, with nanosheets-like morphology. We also describe their properties as electroactive cathode materials for an aqueous zincion battery.

Experimental
As a substrate for the synthesis of Zn-doped MnO 2 nanolayers 0.3 × 5 × 25 mm polycrystalline Ni foam (NF) (110 ppi) plates were used, on which electrochemical experiments were performed, and also 0.35 × 10 × 25 mm single-crystal Si plates with 100 orientation, were used for physical characterization. Extra pure water (Milli-Q) was used in all experiments. Si substrates were cleaned in an ultrasonic bath filled with acetone for 10 min. Then plates of Si were sequentially treated for 10 min in concentrated HF, water, 70% HNO 3 , water, 0.1 M KOH and then flushed out by water. Ni plates were treated according to the technique described in [19] for 15 min in 6 M HCl solution, then several times rinsed by water and dried on air at 120 • C for 30 min.
A solution of mixed salts manganese and zinc was prepared by dissolving dry analytical grade salts MnSO 4 and ZnSO 4 (C = 0.01 M) in deionized water. An aqueous solution of NaClO (C = 0.01 M) was used as an oxidizer. The pH of this solution was 10.5 and adjusted by the addition of 1M NaOH solution. For the synthesis of Zn-doped MnO 2 nanolayers, substrate plates were sequentially immersed for 30 seconds into a solution of mixed manganese and zinc salt, washed in water, and then it was immersed for 30 seconds into the NaClO solutionand again washed in water. The sequence corresponds to one SILD cycle, which is repeated 30 times to obtain desired film thickness. Then samples were calcined on air at 150 • C for 10 min at a heating rate of 5 • C/min.
The morphology and composition of synthesized films were investigated by scanning electron microscope (SEM) at accelerating voltage 1-10 kV on Zeiss Merlin microscope and X-ray photoelectron spectroscopy (XPS) was obtained used ESCALAB 250Xi electron spectrometer, with Al Kα radiation (14866 eV). XRD patterns were obtained using a Rigaku Miniflex II X-ray diffractometer with CuKα radiation (λ = 0.154056 nm). FT-IR transmission spectra of synthesized films on silicon surface were registered by FCM-2201 spectrophotometer using a different technique with respect to spectra of bare silicon plate.
The electrochemical measurements of NF electrodes with the synthesized nanolayers were carried out in a twoelectrode electrochemical cell using Elins P-45X potentiostat. The cathode was prepared by deposition Zn-doped MnO 2 nanolayers on NF surface as a result of 30 treatment cycles by the SILD method. Zinc foil serves as an anode. Electrochemical characterization of the films was made by cyclic voltammetry (CV) and galvanostatic chargedischarge (CD) techniques.
The specific capacitance C (mAh/g) as an electrode for alkaline battery at different current densities can be calculated via eq (1): where I (mA) is a galvanostatic current, dt(h) is the discharge time of a cycle and m(g) is the mass of the active material in the film electrode [20]. The electroactive mass of Zn-doped MnO 2 for the cathode was measured using an OHAUS Pioneer T M PA54C microbalance.

Results and discussion
Morphological characteristics of the sample are shown in the SEM images in Fig. 1. The layer was synthesized on the nickel foam via the SILD method by 30 cycles of treatment. It is clear from the image that the layers formed of nanocrystals with nanosheets-like morphology with a thickness of 3-8 nm. Synthesized nanolayer has a uniform dark brown color.
In the FTIR spectrum (Fig. 3), one can observe absorption bands with maximums at 3400 and 1620 cm −1 referring to valence oscillations of O-H groups in the molecules of adsorbed water [22]. The band with a maximum of about 440 cm −1 results from valence oscillations of manganese-oxygen bonds in MnO 2 [23].
The X-ray photoelectron spectra of the synthesized sample are shown in Fig. 4. Following the results of Ref. [24] the peaks at 642.1 eV (Mn 2p 3/2 ) and 653.8 eV (Mn 2p 1/2 ) in these spectra for the Mn 2p electrons with an energy separation ∆E of 11.7 eV (Fig. 4a) and energy separation of Mn 3s (∆E = 5.1 eV Fig. 4b) may be assigned to manganese atoms in the 4+ oxidation state [25]. The peak at 529.5 eV (Fig. 4c) corresponding to atoms of oxygen in the manganese oxide crystalline lattice, respectively [26]. The XPS spectrum also indicates the presence of Zn and Mn elements with an atomic ratio of 0.3/1.0. We assume that such structure can possess interesting and practical electrochemical performance as cathode materials of aqueous zinc-ion battery. The cyclic voltammograms of the NF electrode with Zn-doped MnO 2 nanolayer were recorded in a potential window from 500 to 2000 mV vs. Zn/Zn 2+ electrode in 2M ZnSO 4 electrolyte at scanning rates of 5, 10 and 15 mV/s (Fig. 5). At a scan rate of 5 mV/s, two electrochemical processes take place in the layer, including the intercalation of zinc ions at 1780 mV and their de-intercalation at 1230 mV. The proportionality of currents to scan rate provides information that the film is thick enough, and the charge transfer rate is limited by diffusion of charge carriers in the film. The specific capacitance of the Zn-doped MnO 2 NF electrode is calculated from charge-discharge curves (Fig. 6) by eq (1) to be 216 mAh/g at the current densities of 1 A/g. The high value of specific capacity can be explaining by the good conductivity of MnO 2 nanosheets and also the significant influence of zinc atoms on the crystal structure of the sample.
FIG. 6. Galvanostatic charge-discharge curve (at the current densities of 1 A/g) of the electrode with Zn-doped MnO 2 nanolayers Cyclic stability is also an important property for electroactive materials. The capacity retention of NF electrode with Zn-doped MnO 2 nanolayer after 1000 charge-discharge cycles at a current density of 1 A/g maintained 95% of its initial capacity, which shows good cycling stability of this material. High cycling stability can be explained feature morphology of ultrathin nanocrystals of MnO 2 which provide fast diffusion of zinc ions and while not being destroyed.
The high value of specific capacitance for the electrodes synthesized by the SILD method can be explained, in our opinion, by the relatively smaller size of oxide nanocrystals formed during the layer deposition, which enlarges the effective surface area of the electrode,and consequently, results in the higher specific capacitance. Additionally, the smaller size of nanocrystals brings about good intercalation/de-intercalation of zinc ions on the electrode surface and hence affects positively the electrochemical stability. We believe that these electrochemical capacity characteristics of Zn-doped MnO 2 nanocrystals, synthesized via the SILD method, can be improved through a new scheme of their synthesis, including nanocomposite with carbon materials (CNT, graphene) formation that has been obtained after a new sequence of reagent treatment. However, these experiments fall beyond the scope of the present paper.

Conclusion
In summary, the possibility was shown forobtaining Zn-doped MnO 2 nanolayers via the SILD method used mixed manganese and zinc salts aqueous solution and NaClO as an oxidant. The results show the synthesized nanolayers were formed nanocrystals of the thickness of about 3-8 nm with the nanosheets morphology and the monoclinic birnessite-like crystal structure of MnO 2 . The electrochemical study of Zn-doped MnO 2 nanolayers-modified nickel foam electrodes, prepared by 30 SILD cycles, demonstrates that the specific capacitance is 216 mAh/g at a current density of 1 A/g. Repeated cycling for 1000 charge-discharge cycles demonstrates a relatively small 5% capacitance fade. Thus this material can be potentially applied as an effective electroactive material for an aqueous zinc-ion battery.