High purity Mn₅O₈nanoparticles with a high overpotential to gas evolution reactions for high voltage aqueous sodium-ion electrochemical storage

Abstract

Developing electrodes with high specific energy by using inexpensive manganese oxides is of great importance for aqueous electrochemical energy storage (EES) using non-Li charge carriers such as Na-or K-ions. However, the energy density of aqueous EES devices is generally limited by their narrow thermodynamic potential window (~1.23 V). In this paper, the synthesis of high purity layered Mn₅O₈ nanoparticles through solid state thermal treatment of Mn₃O₄ spinel nanoparticles, resulting in a chemical formula of [Mn²⁺₂ ][Mn⁴⁺₃ O₈²⁻], evidenced by Rietveld refinement of synchrotron-based X-ray diffraction, has been reported. The electro-kinetic analyses obtained from cyclic voltammetry measurements in half-cells have demonstrated that Mn₅O₈ electrode has a large overpotential (~ 0.6 V) towards gas evolution reactions, resulting in a stable potential window of 2.5 V in an aqueous electrolyte in half-cell measurements. Symmetric full-cells fabricated using Mn₅O₈ electrodes can be operated within a stable 3.0 V potential window for 5000 galvanostatic cycles, exhibiting a stable electrode capacity of about 103 mAh/g at a C-rate of 95 with nearly 100% coulombic efficiency and 96% energy efficiency.

Keywords

manganese oxides Mn₅O₈ / high voltage / aqueous Na-ion storage

Introduction

Electrochemical energy storage (EES) devices such as batteries and electrochemical capacitors have been of increasing importance for portable electronics, electrical vehicles and stationary energy storage adjunct with renewable energy sources such as wind turbine and solar panel. Rechargeable batteries, especially lithium-ion batteries with a specific energy of 200–250 Wh/kg, have wide applications in transportation and grid storage. However, non-aqueous lithium-ion batteries currently have the problems of high cost, and short lifetime which is more stringent than energy density, particularly for the grid applications [1,2]. Aqueous-based EES devices, especially aqueous sodium-ion (Na-ion) storage, potentially become a more sustainable technology with the advantages of high safety, low cost, high power performance and long cycle life compared with non-aqueous lithium-ion batteries. Moreover, Na-ion storage is particularly attractive due to the fact that Na and Li share similar redox potentials while Na has a much higher abundance than Li in the earth crust [2]. Notwithstanding the advantages, the challenges still remain for the aqueous Na-ion EES. For example, the deterioration of electrode materials is often more severe upon the insertion and extraction of Na-ions since the ionic radius of Na is much larger than that of Li [3]. Also, the energy density of aqueous EES devices is largely limited by the thermodynamic potential window of ~1.23 V, beyond which water decomposition occurs. Therefore, it is of great importance to design novel electrode materials with extended potential window for aqueous energy storage, as well as high stability toward Na-ion storage, and thus enhanced energy density and cycling life.

Manganese is an earth abundant element with a low toxicity and superior safety, and its oxides with various oxidation states from 2⁺ to 4⁺ have been widely used as promising electrode materials for EES applications. In addition, manganese oxides have different structures including tunneled, layered and amorphous structures which provide rich varieties in the structural and electronic properties hardly found in other three-dimensional (3D) transition metal oxides. Ceder’s group have reported that the O3-type layered NaMnO₂ shows a high discharge capacity of 185 mAh/g at a 0.1 C rate (1st cycle) and improved cyclability relative to O3-LiMnO₂ which suffers from a rapid capacity decay due to the structural transformation from layer to spinel during cycling. However, the capacity of NaMnO₂ only remains to be 132 mAh/g after 20 cycles, indicating that the stability and cycle life are still not satisfactory[4]. In addition to layered structure, 3D tunnel-structured Na_0.44MnO₂ has been widely investigated as positive electrode material in a hybrid device by coupling with negative electrode such as activated carbon, where large tunnel size is suitable for the accommodation of Na-ions [5, 6]. Similarly, Whitarce’s group have reported that reversible Na-ion storage which occurs within 0.22<x<0.44 for the Na_xMnO₂ electrode in 1 M Na₂SO₄ aqueous electrolyte shows a stable cycling life of up to 1000 cycles, but a rather low specific capacity (45 mAh/g) due to the narrow potential window (~ 0.6 V). Though many advances above provide new opportunities for the study of manganese oxides in Na-ion storage, manganese oxides electrodes still suffer from the poor cycling performance and the low electrode capacity, especially compared with their non-aqueous counterparts. Recently, Wang and Xu’s groups have reported an aqueous Li-ion battery with a stable potential window of around 3.0 V by using a “water-in-salt” electrolyte, obtained by dissolving lithium bis(trifluoromethanesulfonyl)imide at extremely high concentration in water [7, 8]. The resulting aqueous Li-ion batteries have shown an impressive energy density of around 100 Wh/kg, which possibly offers a pathway for enhancing the energy density of aqueous energy storage. However, the aqueous electrolyte with a high concentration of Li salts is still costly for large scale applications. To the best of the authors’ knowledge, few studies have been conducted to develop electrodes which show sluggish kinetics towards hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) at the anode and cathode, respectively. By using the electrode materials having a high overpotential toward gas evolution, a large potential window will be sustained in aqueous electrolyte, and therefore, the energy density of aqueous Na-ion EES can be significantly enhanced, so that the aqueous EES may become promising alternatives to non-aqueous Li-ion batteries.

Shan et al. have recently reported a facile synthesis of a new type of manganese oxide Mn₅O₈ material composing of 80% of Mn₅O₈ and 20% of Mn₃O₄, and the resulting materials shows a high resistance to the gas evolution reaction in an aqueous electrolyte, resulting in stable potential windows of 2.5 V in half-cell tests and 3.0 V in full-cell tests [9]. Although these results are encouraging, the influence of Mn cations on the gas evolution reactions remains vague because of the existence of wide range of mixed valences of Mn components such as 2⁺ (from Mn₅O₈ and Mn₃O₄), 3⁺ (from Mn₃O₄) and 4⁺ (from Mn₅O₈). In this paper, comprehensive studies of temperature-dependent phase evolution of manganese oxides via X-ray diffraction (XRD) analyses have been reported. For the first time, high purity Mn₅O₈ nanoparticles are able to be synthesized by the oxidation of Mn₃O₄ spinel in the open air at 300°C for 12 hours. Binary manganese oxide Mn₅O₈, expressed as [Mn²⁺₂][Mn⁴⁺₃O^2-₈], having a layered structure with inter-/intra-layer defects in Mn cationic sites, is demonstrated to be superior electrodes for high-voltage sodium-ion energy storage. Electro-kinetic analyses demonstrate that Mn₅O₈ electrode has a large overpotential (~ 0.6 V) and high TAFEL slope values towards HER and OER. Mn₅O₈ electrodes exhibit a charge storage specific capacity of 103 mAh/g at a scan rate of 5 mV/s in half-cells and show a stable electrode capacity of about 103 mAh/g at a C-rate of 95 C with nearly 100% coulombic efficiency and 96% energy efficiency after 5000 galvanostatic cycles in 3.0 V symmetric full-cells.

Materials and methods

Material synthesis and characterizations

Manganese oxide nanomaterial was synthesized with the solution-phase method. In a typical synthesis, MnCl₂·4H₂O (0.7 g, AlfaAesar, 99% metals basis) were dissolved in deionized water (156 mL, 18.2 MW; Millipore, Inc.) in a 500 mL flask. The solution was magnetically-stirred for 30 min at a rate of 500 r/min at room temperature under an open air environment. Then, 0.124 g/mL of NaOH (Alfa Aesar, 99.98% metals basis) was injected at a constant rate of 0.167 mL/min for 50 min by a syringe (24 mL, HSW Inc.). The dark brown precipitate was formed and the reaction continued for 30 min. The final product was then thoroughly washed by deionized water and ethanol. The resulting powder was vacuum-dried before being heated at 300 °C for 12 hours in air. Transmission electron microscopy (TEM) images were collected on Zeiss/LEO 922 Omega TEM at the University of New Hampshire. The X-ray photoelectron spectroscopy (XPS) was measured on Axis HS XPS using the standard Mg source (1253.6 eV) at 10^‒9 torr with imaging down to the 30 µm level at the University of New Hampshire. The thermogravimetric analysis (TGA) of manganese oxide was measured on Mettler-Toledo instrument under air flow (40 mL/min). The temperature was ramping from 25°C to 100°C at a rate of 7.5°C/min, and then holding at 100°C for 50 min. Afterwards, the temperature was controlled as follows: ramping from 100°C to 200°C at a rate of 10°C min^‒1 and holding at 200°C for 50 min, ramping from 200°C to 300°C (10°C/min) and holding at 300°C for 50 min, ramping from 300°C to 400°C (10°C/min) and holding at 400°C for 50 min, ramping from 400°C to 500°C (10°C/min) and holding at 500°C for 50 min, ramping 500°C to 600°C (10°C/min) and holding at 600°C for 50 min, ramping from 600°C to 700°C (10°C/min) and holding at 700°C for 50 min. The X-ray diffraction was conducted at 17-BM at the Advanced Photon Source at the Argonne National Laboratory with a wavelength ofl = 0.72768 Å. The GSAS-II software was used for crystallography structure analysis [10].

Electrochemical half-cell measurements

The cyclic voltammetry (CV) measurements were conducted via a three-electrode half-cell using a CHI 660d single channel electrochemical workstation. The three-electrode system consisted of a glassy carbon rotating disc electrode (Pine Instruments) as the working electrode, platinum wire and silver-silver chloride electrode as counter and reference electrodes, respectively. The ink solution was first prepared by mixing 7 mg of active material (manganese oxide) with carbon black (3mg, Alfa Aesar>99.9%) and then the mixture was dissolved into the deionized water with a concentration of 0.5 mg/mL. The resulting solution was subsequently sonicated until the materials were homogeneously dispersed. Then, 10mL of suspension was drop-cast onto the glassy carbon electrode (0.5 cm in diameter). The working electrode material was vacuum-dried with the loading of manganese oxide (3.5mg) and carbon black (1.5 mg). The CV measurements were conducted in a 250 mL flat bottom flask containing 100 mL of argon-purged Na₂SO₄ aqueous electrolyte (0.1 M) at a rotating rate of 500 r/min with an applied potential window range (from -1.25 V to 1.25 V vs Ag/AgCl) for 3 cycles. All the half-cell calculations were provided in the Appendix.

Symmetric full-cell measurements

Symmetric two-electrode full-cells with manganese oxide electrodes were assembled and measured to characterize energy, power performance and long cycle stability as well. Electrodes were made by drop casting ~ 5 mg of Mn₅O₈ and 1.25 mg of carbon black with a mass ratio of 4:1 on Toray carbon paper (E-Tek, Inc., 1.5 cm in diameter). An accurate mass loading of active material was weighed after vacuum-dried overnight. Two symmetric electrodes were separated by cellulose-based filter paper (Whatman), and 150mL of Na₂SO₄ aqueous solution (1 M) was used as electrolyte. The cell stack of electrodes and separator was tightened by stainless plate and compression spring to ensure good electrical contact, and then assembled in a split test cell (model EQ-STC, MTI Corp.). Galvanostatic charge and discharge measurements of symmetric full-cell were conducted on the battery analyser (model B-TG, Arbin Instruments) within a 3.0 V potential window for 5000 cycle numbers at various current densities (5, 10 and 15 A/g). All the symmetric full-cell calculations were provided in the Appendix.

Results and discussion

Synthesis and structural characterizations

Mn₅O₈ is manganese oxide with a chemical compositional formula of [Mn²⁺₂ ][Mn⁴⁺₃‚O₈²⁻], whose crystal structure is isostructural to monoclinic Cd₂Mn₃O₈ which is first determined by Oswald et al. [11]. Mn₅O₈ can be obtained by either the oxidation of Mn₃O₄ with suitable particles or the decomposition of b- or g-MnOOH, which results in Mn₅O₈ nanoparticles or nanorods, respectively [11,12]. However, the synthesis of pure Mn₅O₈ phase poses great challenge due to its nature of metastable property, and also, lack of systemic study of Mn₅O₈ nanomaterials, especially in the application for battery or other energy storage devices. In this paper, to synthesize high purity Mn₅O₈ nanoparticles, Mn₃O₄ spinel nanoparticles were first synthesized at room temperature by using the solution-phase method as reported previously [13]. The Mn₃O₄ nanoparticles were annealed for 12 hours in the open air at various temperatures, ranging from 100°C to 700°C. The resulting manganese oxides were characterized by X-ray Diffraction (XRD) measurements and the corresponding crystalline structures were investigated by Rietveld refinement, which provides the fraction of crystalline phases and oxygen content (Fig. 1, Fig. A1 and Table A1). The Rietveld refinement showed that after 100°C treatment of Mn₃O₄, the resulting materials remained the same spinel phase with an oxygen ratio of 27.95%. As the annealing temperature increased to 200°C, a large portion of Mn₃O₄ was converted into the Mn₅O₈ material with a weight percentage of 80.6%, possibly through the following oxidation reaction:

(1){\mathrm{5 Mn}}_3{\mathrm{O}}_4 +{\mathrm{ 2 O}}_2\to {\mathrm{3 Mn}}_5{\mathrm{O}}_8.

It has been pointed out that the above reaction formula (Eq. (1)) is also evidenced by an increased oxygen ratio from 27.95% to 31.02%. After being annealed at 300°C for 12 hours, nearly all the Mn₃O₄ nanoparticles were converted into high purity Mn₅O₈ material with a 96.6% calculated weight of Mn₅O₈ phase and a 3.4% calculated weight of Mn₃O₄ phase. Meanwhile, the ratio of oxygen was calculated to be ~ 31.63%, which was very close to that of completely pure Mn₅O₈ (31.76%). When the annealing temperature increased to 400°C, the resulting manganese oxide showed a decreased oxygen ratio of 31.47%, and the XRD analysis indicated a slightly decreased Mn₅O₈ phase percentage, accompanied by an increased phase fraction of Mn₃O₄ and the generation of a trace portion of a-MnO₂ phase (~ 1.5%). The decreased oxygen ratio and increased amount of Mn₃O₄ was likely attributed to the high temperature decomposing of Mn₅O₈through the following reactions [12, 14]:

(2){\mathrm{Mn}}_5{\mathrm{O}}_8\to {\mathrm{ 5 Mn}}_3{\mathrm{O}}_4+{\mathrm{ 2 O}}_2.

Meanwhile, a-MnO₂ tetragonal with a tunnel structure could probably result from a disproportionation of Mn₅O₈ through the following reaction:

(3){\mathrm{Mn}}_5{\mathrm{O}}_8\to {\mathrm{ Mn}}_3{\mathrm{O}}_4+\mathrm{ 2}a-{\mathrm{MnO}}_2.

Since the disproportionation of Mn₅O₈ did not cause any change to oxygen content, the observed slight decreased oxygen ratio (from 31.63% to 31.47% as temperature increased from 300°C to 400°C) clearly indicated that both reactions (Eq. (2) and Eq. (3)) occurred. When the temperature increased to 500°C, the phase fractions ofa-MnO₂ and Mn₃O₄ increased, while the oxygen ratio remained constant, strongly suggesting that the disproportionation of Mn₅O₈ (Eq. (3)) was the dominant process. As the temperature further increased to 600°C or higher, only highly crystalline a-Mn₂O₃ was observed. In this work, high purity Mn₅O₈ has been synthesized by annealing Mn₃O₄ spinel at a temperature of 300°C for 12 hours under an open-air environment.

In addition to the XRD analysis, the thermogravimetric analysis (TGA) was conducted to study the conversion of Mn₃O₄ to the Mn₅O₈ (Fig. A2). The Mn₃O₄ nanoparticle first showed a weight loss due to water desorption at a temperature of up to 200°C, and then exhibited a weight gain at around 300°C due to the oxidation of Mn₃O₄ into Mn₅O_8. No distinct weight loss was observed at the temperature ranging from 400°C to 500°C. This is because only a very small portion of Mn₅O₈ decomposes into Mn₃O₄ and therefore, only causes a very small variation of weight at this temperature range. Besides, the thermal decomposition of Mn₅O₈ into Mn₃O₄ and α-MnO₂ (disproportionation of Mn₅O₈ as shown in Eq. (3)) around 500°C does not cause any change to the sample weight which is evidently seen from Eq. (3). Finally, a distinct weigh loss resulted from the phase change intoα-Mn₂O₃ was observed when the temperature exceeded 600°C. Overall, the TGA result with weight variation at various temperatures is well collaborated with the XRD analysis for the phase changes of manganese oxides.

The transmission electron microscopy (TEM) showed that Mn₅O₈ had the morphology of nanoparticles with a calculated average particle size of ~ 18 nm as shown in Fig. A3. The Rietveld structural refinement pointed out the formation of Mn₅O₈ with a monoclinic structure (space group C 2/m) with lattice constants a = 10.399 Å, b = 5.729 Å, c = 4.876 Å and β = 109.6°, which was in good agreement with published data (16956-ICSD) (Fig. 1(c) and Table A2). The resulting Mn₅O₈ materials showed a layered structure containing binary Mn²⁺ and Mn⁴⁺ sites with a crystalline size of ~ 15 nm, comparable to the averaged particle size calculated from TEM. The XRD refinement showed that one third of the cationic sites in the main octahedral sheets [Mn₃⁴⁺O₈]⁴⁻ was not fully occupied (Fig. 1(c)), and that above and below the empty Mn⁴⁺ sites were Mn²⁺ cationic sites. Although with certain amount of structural defects, the Mn²⁺cationic sites are coordinated with six oxygen atoms, of which three oxygen atoms are from the upper octahedral layer and three from the bottom layer.

Electrochemical characterizations

Electrochemical characterizations of high purity Mn₅O₈ nanoparticles that contained 96.6 % of Mn₅O₈ and only 3.4 % of Mn₃O₄ were conducted in both half-cells and symmetric full-cells. Cyclic voltammetry (CV) measurements of the electrodes with 70% of Mn₅O₈ and 30% of carbon black by weight were first conducted in a three-electrode half-cell with silver-silver chloride as reference electrode in 0.1 M Na₂SO₄ aqueous electrolyte. CV scans in Fig. 2(a) showed a large stable potential window from –1.25 V to 1.25 V (vs. Ag/AgCl) at various scan rates from 5 to 200 mV/s, indicating a high resistance towards water electrolysis with a high overpotential of 0.6 V towards HER and OER. The CV curves exhibited reversible and distinct redox peaks during the anodic and cathodic scans, a strong indication of a pseudo capacitive process during redox reaction. The CVs of commercial Mn₃O₄ were also conducted, which shows much more distinct HER and OER features without obvious redox peaks compared with those of Mn₅O₈(Fig. A4). In addition, the low purity Mn₅O₈ material composing of 80% of Mn₅O₈ and 20% of Mn₃O₄ showed similar CV curves with redox features relative to high purity Mn₅O₈, indicating that the Na-ion charge storage mechanism in low purity Mn₅O₈ material could be dominant by Mn₅O₈ phase (Fig. A5(a)). In addition, the CVs of carbon electrode were conducted without any active materials loaded as shown in Fig. A6. The result demonstrated that different from Mn₃O₄, carbon electrode showed a high overpotential to HER and OER as well. However, the current signal of the carbon electrode was much smaller than that of Mn₃O₄ and Mn₅O₈, due to the fact that the current of carbon electrode only resulted from the surface adsorption/desorption of Na-ion in CV without the redox process. The charge storage specific capacities of Mn₅O₈, Mn₃O₄, and low purity Mn₅O₈ material were all calculated as a function of scan rates and depicted in Fig. 2(b). The Mn₅O₈ material had a capacity of 103 mAh/g at a scan rate of 5 mV/s, which is twice as large as that of Mn₃O₄. Moreover, the Mn₅O₈ material retained a capacity of 46 mAh/g at the higher scan rate of 200 mV/s which was 50% higher than that of low purity Mn₅O₈ material, though both materials had similar capacities at 5 mV/s, demonstrating excellent rate capability of Mn₅O₈.

It has been pointed out that the redox peaks usually shift to higher potentials as the scan rate increases during the anodic scan, and vice versa for cathodic scans, a typical phenomenon indicating that a higher overpotential is required to transport charge carriers at higher rates [15]. In the CVs of Mn₅O₈, only minor peak separations were observed for anodic (from 0.84 V to 0.975 V vs. Ag/AgCl) and for cathodic (from–0.037 V to –0.107 V vs. Ag/AgCl) when the scan rates were increased from 5 to 200mV/s, unambiguously, suggesting fast ionic transport in Mn₅O₈ electrode for Na-ion storage possibly due to rich inter-/intra-layered defects as demonstrated from Rietveld refinement (Fig. 1(c) and Table A2). In addition, the Mn₅O₈ materials were charged at 1.25 V (vs. Ag/AgCl) for 2 hours before being analysed by XPS, along with the pristine Mn₅O₈ (Fig. A7). Both the pristine and charged Mn₅O₈ showed the elements of manganese, oxygen and carbon without other residuals on the surfaces. Besides, the manganese had chemical oxidation states of Mn⁴⁺ and Mn²⁺ as indicated in the Mn 2p spectrums. The XPS spectrums of Mn 2p was fitted, and the pristine Mn₅O₈ had a molar ratio of Mn⁴⁺ to Mn²⁺ of 1.65 : 1, which was close to the theoretical ratio of 1.5 : 1 for Mn₅O₈ with a chemical formula of [Mn²⁺₂][Mn⁴⁺₃O^2-₈]. Moreover, the charged Mn₅O₈ material had a higher molar ratio of Mn⁴⁺ to Mn²⁺of 1.8 : 1 compared to that of pristine Mn₅O₈, while the increased averaged oxidation states of manganese for charged Mn₅O₈ material indicated the possible two-electron charge transfer reaction occurred in half-cell with a redox couple of Mn⁴⁺/Mn²⁺. Similar Mn⁴⁺/Mn²⁺ redox couple in manganese oxides were also observed in the low purity Mn₅O₈ material and recently reported LiF-MnO system, which was verified by surface-sensitive soft X-ray spectroscopy (sXAS) [9, 16] .

The charge-storage mechanism of Mn₅O₈ nanoparticles was further studied by analysing the peak currents at various scan rates. Assuming the peak current,i, obeys a power relationship with the scan rate, v, thus it can be expressed [17] by

(4)i = a{v}^b\mathrm{ or} \log (i) =\mathrm{ log}(a) + b\mathrm{log }(v).

where a is constant. As b equals to 0.5, it indicates that the charge transfer was dominated by a diffusion-controlled redox process. However, it is dominated by a surface-controlled capacitive process asb is 1. The peak currents at various scan rates obtained from CVs were shown in Fig. 2(a), and the calculated slopes of log(i) versus log(v) provided the values of b of anodic and cathodic peaks (Fig. 2(c)). The Mn₅O₈ material showed a b of 0.76 for anodic scans, and 0.71 for cathodic scans. The values of b residing between 0.5 and 1.0 demonstrated that both capacitive and diffusion-limited redox processes contributed to the overall current throughout the CV measurements. Moreover, the similar values ofb calculated from anodic and cathodic currents indicated a highly reversible Na-ion charge-transfer process for the redox reactions in Mn₅O₈ electrode. The calculated values of b of anodic and cathodic currents for the low purity Mn₅O₈material was 0.64 and 0.59 respectively, which was lower than those of Mn₅O₈ and was much closer to 0.5 (Fig. A5(b)). Thus, Mn₅O₈ has more current contribution from capacitive process during Na-ion storage compared with the low purity Mn₅O₈ material, which was also reflected by the results that larger charge storage specific capacities could be maintained especially at the high rates (Fig. 2(b)). Therefore, electro-kinetic analysis from CVs in half-cells showed that the Mn₅O₈ nanomaterials had the highly reversible and high-rate performance for aqueous Na-ion storage within a wide voltage window of 2.5 V.

The symmetric two-electrode full-cells of Mn₅O₈ nanomaterials were assembled and measured to characterize the energy/power performance and the long cycle stability. Each electrode contained 4 mg of Mn₅O₈ and 1 mg of carbon black in average on the Toray carbon paper current collector (1.77 cm² in surface area). In addition, 150 mL of aqueous electrolyte Na₂SO₄(1M) were loaded on the cellulosed-based separator during the assembling of button-cells. Figure 3(a) showed the discharge-profile of Mn₅O₈ electrodes within a potential range of 3.0 V. The nearly linear curve of potential versus electrode capacities at various current densities indicated the pseudo capacitive charge storage mechanism without phase change. As the current density increased from 5 A/g to 15 A/g, the discharge electrode capacity decreased from 104 mAh/g to 54 mAh/g, and the discharge time decreased from 38 s to 6.9 s, corresponding to the C-rate of 95 and 514, respectively. The Mn₅O₈ materials exhibited excellent cycling stability without capacity fade at various C-rates from 95 C to 514 C upon 5000 long-term cycles (Fig. 3(b)).

It has been pointed out that at all current densities (5, 10, 15 A/g), the discharge electrode capacity increased continuously within the first 2000 cycles, and then reached very stable values for the rest of the 3000 cycles. These similar electrode behaviours have also been reported by other energy storage systems, such as the LiFe_0.9P_0.95O_4-d electrode in the Li-ion battery system, the Na_0.50Ni_1/6Co_1/6Mn_2/3O₂ electrode in the Na-ion battery system, and the carbon nanofiber electrode in the Na-S battery system [18–20], from which such prolonged conditioning behaviours of initial increase and sequential decrease of electrode capacity was attributed to a slow building up of ionic interface between the electrode and the electrolyte and/or membrane. This mechanism is also supported by the results with the calculated electrical cell resistances fromi-R drop of discharge process during the galvanostatic cycling (Fig. A8). The electrical cell resistance initially decreased dramatically and then became constant for the rest of the cycles, indicating a process for the optimization of electrodes by facilitating ionic transport. Therefore, the resulting electrode capacity had a continuous increase at initial cycles and then reached a very stable electrochemical performance for the rest of the charge and discharge processes.

Mn₅O₈ electrodes in symmetric full-cells not only exhibited a decent electrode capacity of ~ 104 mAh/g and a good retention of capacity after 5000 galvanostatic cycles, but also had nearly 100% coulombic efficiencies and extremely high energy efficiencies from 96% to 77% at current densities of 5, 10 and 15 A/g (Fig.3(c)). The profiles of charge and discharge electrode capacities after 5000 cycles were plotted and remained a linear relationship with respective to the potential at various current densities, indicating stable single-phase electrode reaction during charge-discharge even after long-term cycling (Fig.S9). As the current density increased from 5 A/g to 15 A/g, the discharge voltage of Mn₅O₈ button-cell decreased (Fig. 3(a) and Fig.S9). The increased i-R drop, which was attributed to the polarization of electrode materials, indicated that a large overpotential was required to deliver Na-ion storage at high rates and thus the energy required to fully charge the electrode material was much larger than the energy from discharge. Overall, high coulombic and energy efficiencies with reversible and stable electrode reactions for long-term cycles all suggested that Mn₅O₈ was a kind of superior electrode material for aqueous Na-ion storage. In addition, the Mn₅O₈ electrodes were also measured at low-rate with a current density of 2 A/g, which showed a maximum discharge electrode capacity of ~ 123 mAh/g at a cycle number of 650 and then decreased rapidly to 68 mAh/g after 2000 cycles (Fig. A10). The exact reason for the rapid fading of capacity at low-rate is still unclear. However, it should be mentioned that water decomposition at lower rate would occur as longer exposure of aqueous electrolyte to extreme potentials during cycling and the resulting products might deteriorate the Mn₅O₈ electrodes in button-cells.

The gravimetric specific energy and power of symmetric Mn₅O₈ full-cells after 5000 galvanostatic cycles were plotted and compared with other energy storage systems (Fig. A11). As the current density increased from 5 to 15 A/g, the Mn₅O₈ electrodes exhibited a specific energy from 39.5 Wh/kg to 21.4 Wh/kg, and the specific power from 3750 W/kg to 11250 W/kg. The values are much higher than those of listed several commercial aqueous or non-aqueous energy storage devices such as C/PbO₂, Ness 4600 C/C ultracapacitors and Panasonic (17500) Li-ion battery (data collected less than 5 cycles) [21, 22]. The Mn₅O₈ electrodes also showed a higher specific energy and power compared with those of recently reported NaTi₂(PO₄)₃/Na_0.44MnO₂ aqueous sodium-ion battery and V₂O₅ or Nb₂O₅ based electrodes in non-aqueous devices[23–25]. Therefore, the Mn₅O₈ materials have a competitive electrode performance at high-rates for high-voltage aqueous energy storage.

Electro-kinetics analyses for HER and OER

To understand the underlying mechanism of high-voltage performance of Mn₅O₈ nanomaterials, the electro-kinetics of HER and OER were studied in detail by using TAFEL analyses of CVs from the half-cell measurements in 0.1 M Na₂SO₄ with Eq. (5) (see details in Appendix).

(5)h = A\mathrm{ ln}(i/i_{\mathrm{0}}).

The normalized current densities (i/i₀) of pure Mn₅O₈, commercial Mn₃O₄, as well as the previously reported Mn₅O₈ material with a low purity consisting of 80% of Mn₅O₈ and 20% of Mn₃O₄ were shown as a function of overpotential at a scan rate of 5 mV/s in Fig. 4(a), and the calculated TAFEL slopes as a function of scan rates from Tafel analyses (Fig. A13) were plotted in Fig. 4(b) (for HER) and Fig. 4(c) (for OER), respectively. Mn₅O₈ showed very large TAFEL slopes for HER and OER, indicating its inactive characteristics for generating H₂ or O₂ gas even at an overpotential of 0.6 V in aqueous neutral solution. Relative to the Mn₅O₈, the Mn₃O₄ material was much more active towards HER and OER. Though the low purity Mn₅O₈ material containing 20% of Mn₃O₄ was observed to have a similar HER activity compared with Mn₅O₈, the TAFEL slopes of OER were lower at all scan rates and the enhanced OER activity could be caused by a non-ignorable portion of Mn₃O₄ phase (~ 20% by weight) in the mixture, which resulted in more obvious OER activities. Therefore, the high purity Mn₅O₈ appeared to be a better electrode material that can offer a stable and wide potential window in aqueous solution for its high overpotential towards HER and OER. To further demonstrate its superiority to suppress HER and OER, Mn₅O₈ was also compared with various commercial 3D transition metal oxides (Co ₃O₄, NiO, Fe₃O₄, V₂O₅, TiO₂), which were also investigated and analysed in the same electrochemical conditions (Fig. A14 and A15). The results showed that Mn₅O₈ exhibited high TAFEL slopes towards HER and OER relative to other metal oxides. An effective potential window about 2.5 V, beyond the thermodynamic limit for water electrolysis (~ 1.23 V), can be extended in aqueous electrolyte, and thus Mn₅O₈ has a competitive performance compared with electrode materials such as Co₃O₄, NiO with a high theoretical specific capacitance but very limited potential window (~ 0.6 V).

In addition to TAFEL slope analysis, the onset potentials for HER and OER were calculated as shown in Fig. A12 (g–i). The results showed that the high purity Mn₅O₈ material had the lowest HER onset potential and highest OER onset potential at a scan rate of 5 mV/s, indicating its sluggish kinetics towards gas evolution reaction compared with the low purity Mn₅O₈ (80% Mn₅O₈ and 20% Mn₃O₄ by weight) and Mn₃O₄. However, it has been pointed out that the onset potential analyses of high purity Mn₅O₈, low purity Mn₅O₈ and Mn₃O₄ may not be completely concurrent with the results obtained from TAFEL slope. This is largely due to the existence of current contribution from Na-ion de-intercalation during anodic scan at the high potential range and/or Na-ion intercalation during cathodic scan at the low potential range.

The resistance to HER and OER on Mn₅O₈ surface demonstrated by TAFEL analyses was further discussed with the mechanistic understanding below. The previous results showed that the surface of Mn₅O₈ was terminated by Mn²⁺, which can strongly interact with water to form a highly ordered hydroxylated interphase [9]. In the present paper, similar surface structure on Mn₅O₈ could be maintained during electrode reactions since a highly pure Mn₅O₈ phase has been synthesized, confirmed by XPS measurements shown in Fig. 5(a), (b). Nesbitt and Banerjee found that the lattice O^2-, hydroxyl group OH, and water H₂O peaks in XPS spectrum of O 1s were located at 529.6 eV, 530.8 eV and 532.3 eV, respectively [26]. Hence, it was assigned the same de-convoluted peaks for the fitting of O 1s spectrum of pristine and charged Mn₅O₈ material, showing that hydroxyl OH group and H₂O components were strongly bonded on surface of cycled Mn₅O₈. On the other hand, for the pristine Mn₅O₈ without electrochemical treatment, only lattice O^2- was observed without the contribution from water or hydroxyl group. It has been suggested previously, that gas evolution reactions on hydroxylated Mn₅O₈ could be strongly suppressed due to the large activation energy of the first intermediate step of water dissociation (Fig. 5(c), (d))[9,27].

It has been reported that the accumulated Mn³⁺ on the surface of Mn₅O₈ or d-MnO₂ during the OER process could contribute to OER at the anode, showing a decreased OER overpotential for these materials in a near neutral solution [28–30]. The results in this paper confirmed that the Mn³⁺species found in low purity Mn₅O₈ could result in a higher catalytic activity for OER relative to the high purity Mn₅O₈ where only Mn²⁺ and Mn⁴⁺ exist. Therefore, the synthesis of Mn₅O₈ with high purity is of great significance to optimize the electrode for high-voltage aqueous devices. Moreover, since hydroxylated Mn₅O₈ would largely inhibit the water dissociation, the first intermediate step of the OER process, it is proposed that the generation of surface intermediate Mn³⁺during OER be suppressed as indicated in Fig. 5(d), which, in return, also lead to sluggish kinetics for O₂ gas evolution.

Conclusions

The studies show that Mn₅O₈ electrode enables a high-voltage (~ 3.0 V in full-cell) for Na-ion aqueous energy storage, and provide further discussion about the underlying mechanism of its inert characteristics for water electrolysis. Detailed analyses of XRD results indicate that highly pure Mn₅O₈ nanoparticles can be achieved by a facile solid state thermal treatment method using Mn₃O₄ nanoparticles as the precursor, and the Mn₅O₈ electrode is very promising for a large-scale synthesis with a simpler and cost-effective method of direct oxidation of Mn₃O₄ in air at 270°C. The resulting Mn₅O₈ electrode material is very promising for applications of high voltage aqueous energy storage, showing competitive energy density compared with other energy storage systems. HER and OER have been strongly suppressed on the surface of Mn₅O₈ with a large overpotential (~ 0.6 V). It is worthwhile for further study that a protective interphase such as hydroxylated surface, similar to solid-electrolyte-interphase (SEI) in non-aqueous Li-ion batteries, can be applied to other metal oxides or electrode materials in aqueous energy storage, and thus the energy density can be achieved by an enlarged potential window in aqueous but without a severe sacrifice of conductivity.

References

[1] Chu S, Cui  Y, Liu N . The path towards sustainable energy. Nature Materials, 2016, 16(1): 16–22

[2] Grey C P, Tarascon  J M. Sustainability and in situ monitoring in battery development. Nature Materials, 2016, 16(1): 45–56

[3] Kim S W, Seo  D H, Ma  X, Ceder G ,  Kang K. Electrode materials for rechargeable sodium-ion batteries: potential alternatives to current lithium-ion batteries. Advanced Energy Materials, 2012, 2(7): 710–721

[4] Ma X, Chen  H, Ceder G . Electrochemical properties of monoclinic NaMnO₂. Journal of the Electrochemical Society, 2011, 158(12): A1307–A1312

[5] Sauvage F, Laffont  L, Tarascon J M ,  Baudrin E . Study of the insertion/deinsertion mechanism of sodium into Na_0.44MnO₂. Inorganic Chemistry, 2007, 46(8): 3289–3294

[6] Whitacre J F, Tevar  A, Sharma S . Na₄Mn₉O₁₈ as a positive electrode material for an aqueous electrolyte sodium-ion energy storage device. Electrochemistry Communications, 2010, 12(3): 463–466

[7] Suo L M, Borodin  O, Gao T ,  Olguin M ,  Ho J, Fan  X L, Luo  C, Wang C S ,  Xu K. “Water-in-salt” electrolyte enables high-voltage aqueous lithium-ion chemistries. Science, 2015, 350(6263): 938–943

[8] Xu K, Wang  C S. Batteries: widening voltage windows. Nature Energy, 2016, 1: 16161

[9] Shan X, Charles  D S, Lei  Y, Qiao R ,  Wang G, Yang  W, Feygenson M ,  Su D, Teng  X. Bivalence Mn₅O₈ with hydroxylated interphase for high-voltage aqueous sodium-ion storage. Nature Communications, 2016, 7: 13370

[10] Toby B H, Von Dreele  R B. GSAS-II: the genesis of a modern open-source all purpose crystallography software package. Journal of Applied Crystallography, 2013, 46(2): 544–549

[11] Oswald H R, Feitknecht  W, Wampetich M J . Crystal data of Mn₅O₈ and Cd₂Mn₃O₈. Nature, 1965, 207(4992): 72

[12] Gao T, Norby  P, Krumeich F ,  Okamoto H ,  Nesper R ,  Fjellvag H . Synthesis and properties of layered-structured Mn₅O₈ nanorods. Journal of Physical Chemistry C, 2010, 114(2): 922–928

[13] Yeager M P, Du  W, Wang Q ,  Deskins N A ,  Sullivan M ,  Bishop B ,  Su D, Xu  W, Senanayake S D ,  Si R, Hanson  J, Teng X . Pseudocapacitive hausmannite nanoparticles with (101) facets: synthesis, characterization, and charge-transfer mechanism. ChemSusChem, 2013, 6(10): 1983–1992

[14] Dhaouadi H, Ghodbane  O, Hosni F ,  Touati F . Nanoparticles: synthesis, characterization, and dielectric properties. ISRN Spectroscopy, 2012, 67(4): 1152–1153

[15] Augustyn V, Come  J, Lowe M A ,  Kim J W ,  Taberna P L ,  Tolbert S H ,  Abruña H D ,  Simon P ,  Dunn B. High-rate electrochemical energy storage through Li⁺ intercalation pseudocapacitance. Nature Materials, 2013, 12(6): 518–522

[16] Jung S-K, Kim  H, Cho M G ,  Cho S-P ,  Lee B, Kim  H, Park Y-U ,  Hong J, Park  K-Y, Yoon G ,  Seong W M ,  Cho Y, Oh  M H, Kim  H, Gwon H ,  HwangI, Hyeon  T, Yoon W-S ,  Kang K. Lithium-free transition metal monoxides for positive electrodes in lithium-ion batteries. Nature Energy,2017, 2, 16208

[17] Lindström H, Södergren  S, Solbrand A ,  Rensmo H ,  Hjelm J ,  Hagfeldt A ,  Lindquist S E . Li⁺ ion insertion in TiO₂ (anatase). 2. Voltammetry on nanoporous films. Journal of Physical Chemistry B, 1997, 101(39): 7717–7722

[18] Kang B, Ceder  G. Battery materials for ultrafast charging and discharging. Nature, 2009, 458(7235): 190–193

[19] Yu X W, Manthiram  A. Performance enhancement and mechanistic studies of room-temperature sodium-sulfur batteries with a carbon-coated functional nafion separator and a Na₂S/activated carbon nanofiber cathode. Chemistry of Materials, 2016, 28(3): 896–905

[20] Zheng J M, Yan  P F, Kan  W H, Wang  C M, Manthiram  A. A spinel-integrated P2-type layered composite: high-rate cathode for sodium-ion batteries. Journal of the Electrochemical Society, 2016, 163(3): A584–A591

[21] Burke A. The present and projected performance and cost of double-layer and pseudo-capacitive ultracapacitors for hybrid vehicle applications. In: 2005 IEEE Vehicle Power and Propulsion Conference Chicago, 2005, 356–366

[22] Nagasubramanian G, Jungst  R G, Doughty  D H. Impedance, power, energy, and pulse performance characteristics of small commercial Li-ion cells. Journal of Power Sources, 1999, 83(1–2): 193–203

[23] Li Z, Young  D, Xiang K ,  Carter W C ,  Chiang Y M . Towards high power high energy aqueous sodium-ion batteries: the NaTi₂(PO₄)₃/Na_0.44MnO₂ system. Advanced Energy Materials, 2013, 3(3): 290–294

[24] Chen Z, Augustyn  V, Jia X L ,  Xiao Q F ,  Dunn B, Lu  Y F. High-performance sodium-ion pseudocapacitors based on hierarchically porous nanowire composites. ACS Nano, 2012, 6(5): 4319–4327

[25] Wang X L, Li  G, Chen Z ,  Augustyn V ,  Ma X M ,  Wang G, Dunn  B, Lu Y F . High-performance supercapacitors based on nanocomposites of Nb₂O₅ nanocrystals and carbon nanotubes. Advanced Energy Materials, 2011, 1(6): 1089–1093

[26] Nesbitt H W, Banerjee  D. Interpretation of XPS Mn(2p) spectra of Mn oxyhydroxides and constraints on the mechanism of MnO₂ precipitation. American Mineralogist, 1998, 83(3-4): 305–315

[27] Zhuang Z, Sheng  W, Yan Y . Synthesis of monodispere Au@Co₃O₄core-shell nanocrystals and their enhanced catalytic activity for oxygen evolution reaction. Advanced Materials, 2014, 26(23): 3950–3955

[28] Jeong D, Jin  K, Jerng S E ,  Seo H, Kim  D, Nahm S H ,  Kim S H ,  Nam K T . Mn₅O₈ nanoparticles as efficient water oxidation catalysts at neutral pH. ACS Catalysis, 2015, 5(8): 4624–4628

[29] Takashima T, Hashimoto  K, Nakamura R . Mechanisms of pH-dependent activity for water oxidation to molecular oxygen by MnO₂ electrocatalysts. Journal of the American Chemical Society, 2012, 134(3): 1519–1527

[30] Takashima T, Hashimoto  K, Nakamura R . Inhibition of charge disproportionation of MnO₂ electrocatalysts for efficient water oxidation under neutral conditions. Journal of the American Chemical Society, 2012, 134(44): 18153–18156

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