Amorphous Heterostructure Derived from Divalent Manganese Borate for Ultrastable and Ultrafast Aqueous Zinc Ion Storage

Abstract Aqueous zinc‐manganese (Zn–Mn) batteries have promising potential in large‐scale energy storage applications since they are highly safe, environment‐friendly, and low‐cost. However, the practicality of Mn‐based materials is plagued by their structural collapse and uncertain energy storage mechanism upon cycling. Herein, this work designs an amorphous manganese borate (a‐MnBO x ) material via disordered coordination to alleviate the above issues and improve the electrochemical performance of Zn–Mn batteries. The unique physicochemical characteristic of a‐MnBO x enables the inner a‐MnBO x to serve as a robust framework in the initial energy storage process. Additionally, the amorphous manganese dioxide, amorphous Zn x MnO(OH)2, and Zn4SO4(OH)6·4H2O active components form on the surface of a‐MnBO x during the charge/discharge process. The detailed in situ/ex situ characterization demonstrates that the heterostructure of the inner a‐MnBO x and surface multicomponent phases endows two energy storage modes (Zn2+/H+ intercalation/deintercalation process and reversible conversion mechanism between the Zn x MnO(OH)2 and Zn4SO4(OH)6·4H2O) phases). Therefore, the obtained Zn//a‐MnBO x battery exhibits a high specific capacity of 360.4 mAh g−1, a high energy density of 484.2 Wh kg−1, and impressive cycling stability (97.0% capacity retention after 10 000 cycles). This finding on a‐MnBO x with a dual‐energy storage mechanism provides new opportunities for developing high‐performance aqueous Zn–Mn batteries.


Dalian University of Technology
Dalian 116024, China
The Whatman GF/D (Glass Microfiber Filters) separators were purchased from Shanghai root biological technology limited company. Graphite paper (thickness: 0.1 mm), and zinc foils (thickness: 0.08 mm) were from commercial supplies.

Synthesis of amorphous manganese borate (a-MnBO x )
In detail, 0.5 g manganese chloride tetrahydrate (MnCl 2 ·4H 2 O) and 0.3 g sodium borohydride (NaBH 4 ) powder were respectively added to the round-bottom flask, followed by adding 100 mL deionized water. The reaction flask was placed in an ice bath to carry out the reaction and stirred for 2 hours. The round bottom flask was then removed from the ice bath condition and standed for 10 minutes. After that, the obtained precipitates were washed several times with deionized water, and finally dried at 60°C for 2 h under a vacuum oven.

Preparation of the positive electrodes
The a-MnBO x electrodes were fabricated by homogenously mixing the active material powders, acetylene black, and PTFE binder with a mass ratio of 8:1:1. The prepared slurry was mixed uniformly to form a disk electrode (diameter: 12 mm), which was then dried in a vacuum oven overnight. The loading mass of the active material is about 1.6 mg.

The assembly of the Zn//a-MnBO x battery
The Zn//a-MnBO x battery was assembled in the CR2032-type coin cell in the open-air environment, in which the a-MnBO x electrode was used as the cathode, graphite paper was used as the current collector, Zn foil was used as the anode, 1 M ZnSO 4 with 0.1 M MnSO 4 aqueous solution were used as the electrolyte and Whatman GF/D (glass microfiber filters) was used as the separator. In addition, the amount of electrolyte was 120 μL in all cases for all the cells.

Electrochemical measurements
Galvanostatic charge/discharge (GCD) and cycling stability tests were conducted on a Land Battery Test System, and the current densities ranged from 0.3 to 20.0 A g -1 . Cyclic voltammetry (CV) tests were measured on an electrochemical workstation (CHI660E, shanghai Chenhua) between 0.8 to 1.9 V under different scan rates changed from 0.1 to 50 mV s -1 . The electrochemical impedance spectroscopy (EIS) was carried out at a frequency ranging from 0.01 to 10 5 Hz at an open circuit potential with an amplitude of 5 mV. The calculation of current density and specific capacity was based on the mass of the cathode active material. The specific energy density (E) and specific capacity (C) were read directly from the Land Battery Test System. The specific power density (P) of the cells was obtained from the following equations: where E (Wh kg -1 ) is the energy density, C (mAh g -1 ) is the specific capacity, P (W kg -1 ) is the specific power density, and Δt (h) is the discharging time.

Material Characterizations
The structure of the prepared sample was examined by the X-ray powder diffraction (XRD, Brüker D8) measurement with Cu Kα radiation. Of note, the range of in-situ XRD testing was 10-30° with a scan rate of 4°/min. Scanning electron microscopy (SEM, SU-8010), transmission electron microscopy (TEM, FEI Tecnai G2 F30), and high-resolution transmission electron microscope (HRTEM) were employed to reveal the microstructure of the prepared samples. Elemental mapping images were collected from energy dispersive X-ray (EDX) spectroscopy. X-ray photoelectron spectroscopy (XPS, ESCALAB 250X i ) spectra were employed to investigate the surface chemical species of pristine samples and calibrated to the C 1s peak binding energy of 284.8 eV. Furthermore, in the XPS etching test, argon ions were used for etching, and the etching depth was 20 nm. Of note, ex-situ tests including XRD, FESEM, HRTEM, and XPS were utilized to analyze the reaction mechanism of the a-MnBO x cathode at the selected states of the 5 th discharge/charge cycle. Raman spectra were performed from a Micro-Raman spectrometer (Brüker Senterra spectrometer). To investigate the water content, Thermogravimetric analysis (TGA) (NETZSCH STA 449F3) was performed from room temperature to 1100°C at a ramping rate of 20°C min −1 in the nitrogen atmosphere. Nitrogen adsorption/desorption isotherms were measured on a Micromeritics ASAP 2460 3.00 volumetric adsorption analyzer at 77.3 K.

Density functional theory (DFT) calculations
To simulate the formation energy on β-MnO 2 and MnB 4 O 7 , Density functional theory (DFT) calculations were performed using the Vienna Ab initio Simulation Package (VASP) [S1,S2] based on the pseudopotential plane wave (PPW) method. The perdew-Bueke-Ernzerhof (PBE) functional [S3] was used to describe the exchange-correlation effects of electrons. We have chosen the projected augmented wave (PAW) potentials [S4,S5] to describe the ionic cores and take valence electrons into account using a plane wave basis set with a kinetic energy cutoff of 500 eV. To simulate the surface of β-MnO 2 and MnB 4 O 7 , slab models were built by slicing the (010) and (021) planes for β-MnO 2 and MnB 4 O 7 , respectively. An extra vacuum zone of 15 A was added to both models to avoid possible interaction. The structures were first relaxed until the force on every atom was less than 0.02 eV/A. The formation energy was calculated by the following equation: Where E(A) and E(B) represent the energy of elemental material while E(A x B y ) represents the energy of the compound. Figure S1. Schematic diagram of the preparation for the a-MnBO x sample.   The TGA analysis of the a-MnBO x sample was investigated in the N 2 atmosphere from room temperature to 1100℃ , respectively. It can be observed that the weight loss of a-MnBO x at 400°C is 16.05%. While there is about 6.20% and 9.85% weight loss up to 100℃ and 100 -400℃ for a-MnBO x , which are attributed to the loss of the physically absorbed water and crystal water within the a-MnBO x sample, respectively. All the XPS tests were calibrated form C 1s, and the peak position of the calibrated C1s was 284.8 eV ( Figure S5b). From the XPS wide spectrum in Figure S5a  The area integration of the fine XPS spectra of Mn 2p, B 1s, and O 1s was carried out respectively.

Supplementary Figures and Tables
The relative area obtained was divided by the respective sensitivity factors, and therefore, the   The calculation of the b value is mainly through the following formula [S6, S7] : where i is the peak current, v is the sweep speed, a and b are constants.
The area ratio of diffusion control and capacitive-controlled capacitance is calculated by the following Formula S5: where k 1 v represents the capacitive contribution to the total current and k 2 v 1/2 is the diffusion control current. As shown in Figure S8a, the b value of the reduction peak is 0.53, and the b value of the corresponding oxidation peak is 0.57. It indicates a solid diffusion-controlled kinetic of a-MnBO x during the charge/discharge process. At 0.1 mV s -1 , the capacitive contribution is 42% in Figure S8b.
As a conclusion, the diffusion contribution is 57.9%, 56.6%, 43.5%, and 29.8% at 0.1, 0.2, 0.5, and 1.0 mV s -1 , respectively.                  The fine XPS spectra of B were tested for the a-MnBO x electrode in different states. As shown in Figure S27a, the content of element B in the electrode is relatively high in the initial state, which gradually decreases with the continuous charging and discharging ( Figure S27b). Finally, when the battery fails, the content of element B in the outer layer and the inner layer is relatively small, which indicates that the electrode will gradually dissolve in the process of charging and discharging ( Figure   S27c-d). At the current density of 20A g -1 , SEM tests were conducted on the a-MnBO x electrode sheets before and after the battery failure. The surface of the material is loose and porous before cycling tests, but there is a lot of agglomeration after the battery failure ( Figure S28). At the current density of 20A g -1 , SEM images show of the Zn foil anode before and after 10,000 cycles. The surface of zinc foil is smooth before cycling, but the surface is covered by a large amount of zinc dendrites after cycling ( Figure S29). Table S1. The pH value of the different electrolytes. Table S2. Reaction mechanism of the a-MnBO x cathode during the charge-discharge process. References