Enhancing the cycling performance of manganese oxides through pre-sodiation for aqueous Zn-ion batteries

Abstract

Although Li-ion batteries have dominated the portable electronics market due to their high energy density and long cycle-life, it is essential to find alternative energy storage devices that can be cost-effective and environmentally friendly because of the low abundance and high cost of Li. In this regard, aqueous Zn-ion batteries are promising, because of their high abundance, low cost, high gravimetric capacity of the Zn anode and the environmental friendliness of aqueous electrolytes compared to flammable and costly organic electrolytes. Manganese oxides are the preferred cathode materials for aqueous Zn-ion batteries because of their specific capacity above 200 mA h g⁻¹, low cost and environmental friendliness. However, they suffer from capacity fading due to manganese dissolution and structural transformation upon cycling. In this study, pre-sodiated manganese oxide Na_0.6MnO₂ is synthesized by a hydrothermal method, followed by annealing at 900 °C, and its performance was tested for Zn-ion batteries by means of cyclic voltammetry, galvanostatic charge–discharge cycling and electrochemical impedance spectroscopy. Interestingly, Na_0.6MnO₂ delivers an initial specific capacity of 165 mA h g⁻¹, showing approximately 77% capacity retention over 150 cycles when cycled at 0.2 A g⁻¹ in the voltage domain of 1.0–2.0 V vs. Zn in 1.0 M ZnSO₄ + 0.1 M MnSO₄. On the other hand, Mn₂O₃ (without pre-sodiation) exhibits a high specific capacity above 200 mA h g⁻¹; however, it undergoes severe capacity fading and retains only 26% capacity after 150 cycles. The ex situ XRD analysis shows the formation of a major spinel ZnMn₂O₄ phase along with a ZnMn₃O₇ phase, thus confirming the intercalation of the zinc-ion during the discharge process. Thus, this study enlightens the importance of pre-sodiation of manganese oxides in aqueous Zn-ion batteries for maintaining a stable specific capacity upon prolonged cycling.

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1. Introduction

Owing to the increasing energy demand and environmental pollution from the use of fossil fuels, the harvesting and storage of renewable energies, such as solar and wind energy, by large-scale energy storage devices have become the need of the hour.^1,2 Although Li-ion batteries (LIBs) have dominated the portable electronic market for the past few decades due to their high energy density, long cycle-life, and compactness; low Li reserves, high cost, and flammability issues of organic electrolytes challenge the usage of LIBs for large-scale energy storage applications.^3–5 Hence, alternative energy storage devices, such as rechargeable aqueous metal-ion batteries utilizing earth-abundant metal ions (Na⁺, K⁺, Mg²⁺, Zn²⁺, Ca²⁺) and non-flammable aqueous electrolytes, can potentially be good alternatives for LIBs in large-scale grid energy storage applications. Among them, aqueous Zn-ion batteries (AZIB) have advantages, such as high Zn abundance in the earth's crust (70 ppm), low reduction potential of Zn (−0.76 V vs. SHE), high theoretical capacity of Zn (820 mA h g⁻¹), compatibility of the Zn anode with aqueous electrolytes and environmental friendliness.^6,7 Aqueous electrolytes possess two orders of high ionic conductivity (10⁻¹–1 S cm⁻¹) compared to that of organic electrolytes (10⁻³–10⁻² S cm⁻¹) and have no flammability issues, which makes them safer for grid energy storage applications.^8,9 Despite these advantages of AZIB, the practical difficulty comes from designing a suitable cathode material for hosting reversible Zn²⁺ storage.

Various cathode materials have been reported for AZIB, which include Mn-based oxides (MnO, MnO₂,¹⁰ Mn₂O₃,^11,12 Mn₃O₄¹³), V-based oxides (V₂O₅,¹⁴ ZnV₂O₄¹⁵), Mo-based sulfides (MoS₂¹⁶), Prussian-blue analogues (FeFe(CN)₆,¹⁷ KCuFe(CN)₆,¹⁸ ZnHCF¹⁹), polyanionic compounds (Na₃V₂(PO₄)₃),²⁰ and Chevrel phase compounds (Mo₆S₈). However, most of these materials possess inherent difficulties, such as vanadium-based materials, which display poor structural stability and limited operational potential (vs. Zn) despite its high capacity. Conversely, Prussian-blue analogues show very low specific discharge capacity (∼50–60 mA h g⁻¹) and aging issues.^11,21,22 By comparison, Mn-based oxides have several advantages such as high theoretical capacity, high Mn reserve in the earth's crust (∼900 ppm), low cost, eco-friendliness, and satisfactory operational voltage (vs. Zn), making them a potential choice as a cathode material in AZIB. Mn-based oxides exist in various crystallographic forms, such as α, β, γ, δ, and λ-MnO₂, possessing either a tunnel or layered structure.^23,24 Based on the MnO₆ linkage, α-MnO₂ has one dimensional (1 × 1) (2 × 2) tunnel structures, β-MnO₂ has (1 × 1) tunnels, and γ-MnO₂ has (1 × 1) (1 × 2) tunnels, while δ-MnO₂ has a two-dimensional layered structure, and λ-MnO₂ has a three-dimensional spinel structure.^25–28 Among these forms, α-MnO₂ possessing larger tunnels and δ-MnO₂ possessing a large interlayer spacing exhibit high specific capacity for AZIB application. However, the underlying problems of Mn-based oxides are Mn²⁺ dissolution, faster capacity fading, structural transformation during the charge/discharge process, and poor conductivity.²⁹ Various strategies have been followed to alleviate these issues, such as defect introduction, electrolyte additive, compositing with a conducting material, and nano-structuring.³⁰ Pan et al. reported the use of a MnSO₄ additive in an electrolyte to prevent Mn²⁺ dissolution and observed a high specific capacity of ∼260 mA h g⁻¹ at 1C rate and 92% capacity retention after 5000 cycles at 5C, whereas only ∼100 mA h g⁻¹ was observed in the absence of the additive.³¹ During charging, the Mn²⁺ is electrochemically oxidised to a higher oxidation state (Mn²⁺/MnO₂ transformation), thus boosting the capacity.³² Wu et al. prepared graphene scroll-coated α-MnO₂ nanowires, which showed a high specific capacity of ∼301 mA h g⁻¹ at 0.1 A g⁻¹ current density with 94% capacity retention after 3000 cycles when cycled at 3 A g⁻¹, compared to the non-graphene coated nanowire (∼242 mA h g⁻¹ at 0.1 A g⁻¹).³³ Zhai et al. demonstrated that the sodium ion/crystal water co-intercalated cathode (Na_0.55Mn₂O₄·0.57H₂O) shows a higher specific capacity of ∼285 mA h g⁻¹ compared to δ-MnO₂ (194 mA h g⁻¹) at 0.5 A g⁻¹, highlighting the vital role of alkali metal ion (Na⁺) doping and crystal water, which enhances the interlayer spacing for reversible Zn²⁺ insertion/extraction.²³ Another manganese-based oxide, Mn₂O₃ cathode has been widely investigated due to its high specific discharge capacity and easy synthesis. However, they display Mn dissolution and volume changes during the charge–discharge process, which has been tackled by using various techniques like the synthesis of a metal oxide using MOF sacrificial precursors,³⁴ Ni²⁺ doping³⁵ and carbon compositing.^36,37

Layered sodiated transition metal oxides (P2 and O3 type), such as Na_2/3MnO₂, Na_2/3Mn_2/3Ni_1/3O₂, and NaMn_1/2Ni_1/2O₂, have been the most studied cathode materials for sodium-ion batteries (SIB).³⁸ Especially, P2-type oxide cathodes have more advantages, because of the trigonal prismatic sites being occupied by Na ions and the AB BA stacking of the transition metal oxide layers (TMO), whereas Na ions occupy octahedral sites and TMO shows AB CA BC stacking in O3-type oxide. P2-type oxides exhibit high capacity as the Na has to cross less energy barrier, thus making its jump more feasible from one trigonal prismatic site to another without structural collapse, which is not the case for the O3 type.^39,40 However, P2-type oxides are rarely investigated as positive electrode materials for aqueous Zn-ion batteries. Hence, it is essential to examine the performance of P2-Na_0.6MnO₂ as a cathode material for AZIB. It should be noted that while manganese oxides are usually discharged first in the 1st cycle, this sodiated metal oxide material can be charged in order to extract Na⁺ ions in the 1st cycle, in a principle similar to Li or Na-ion batteries. This work highlights the importance of a pre-sodiation strategy to tackle the capacity fading issue and boost the capacity retention of Mn-based oxide materials.

In this work, we have synthesized pre-sodiated P2-Na_0.6MnO₂ and Mn₂O₃ (without pre-sodiation) by a hydrothermal method, followed by annealing, as reported in our previous publication,³⁹ and studied the impact of the pre-sodiation strategy for aqueous Zn-ion batteries. From the galvanostatic charge–discharge (GCD) cycling studies, a significant enhancement of capacity retention (around 77%) is achieved via a simple pre-sodiation strategy compared to the low capacity retention (26%) of Mn₂O₃ after 150 cycles. The performance of hydrothermally synthesized Na_0.6MnO₂ is compared with that of manganese oxides reported previously in the literature and represented here in Table 1.

Table 1 Comparison of the aqueous Zn||Na_0.6MnO₂ battery with other previously reported aqueous rechargeable batteries

Cathode materials	Synthesis method	Morphology	Electrolyte	Potential window (V)	Specific discharge capacity (mA h g^−1)	Capacity retention (%)	Ref.
α-MnO2	Hydrothermal	Nanorods	1 M ZnSO4	0.7 to 2.0	210 at 1C	—	10
Na pre-intercalated Mn2O3@graphene	Molten salt	Vertical Mn2O3 nanosheets on graphene	3 M ZnSO4 + 0.1 M MnSO4	0.8 to 1.9	320 at 0.6 A g^−1	∼71% after 5000 cycles at 7 A g^−1	11
MOF derived α-Mn2O3	Microwave	Microrods with nanoparticles	2 M ZnSO4 + 0.2 M MnSO4	1.0 to 1.85	225 at 0.05 A g^−1	42% after 1600 cycles at 2 A g^−1	12
MCM@Mn3O4	Precipitation	Spherical particles	2 M ZnSO4 + 0.1 M MnSO4	0.8 to 1.9	220 at 0.2 A g^−1	80% after 1000 cycles at 0.6 A g^−1	13
Ni-doped Mn2O3	Co-precipitation	Nanospheres	2 M ZnSO4	0.8 to 1.8	252 at 0.1 A g^−1	85.6% after 2500 cycles at 1 A g^−1	21
MnO2–Mn2O3	Molten salt	Nanobelt + nanoparticle	2 M ZnSO4 + 0.2 M MnSO4	1.0 to 1.85	289.8 at 0.2 A g^−1	96.7% after 1000 cycles at 1 A g^−1	22
Graphene scroll coated α-MnO2	Hydrothermal	Nanowire with rGO sheets	2 M ZnSO4 + 0.2 M MnSO4	1.0 to 1.85	301.2 at 0.1 A g^−1	94% after 3000 cycles at 3.0 A g^−1	33
Na0.55Mn2O4·0.57H2O	Selective etching	Nanoflakes	2 M ZnSO4 + 0.2 M MnSO4	0.8 to 1.9	340.7 at 0.2 A g^−1	67% after 400 cycles at 0.5 A g^−1	23
Ni-doped ZnMn2O4/Mn2O3	Pulsed potential electrodeposition	Fine particles	2 M ZnSO4 + 0.2 M MnSO4	0.8 to 1.9	237.2 at 0.2 A g^−1	91.3% after 3000 cycles at 2 A g^−1	35
Na0.44MnO2	High temperature with cell grinding	Shorter nanorods	3 M ZnSO4 + 0.1 M MnSO4	1.0 to 1.8	301.3 at 0.1 A g^−1	69.3% after 800 cycles at 1.0 A g^−1	41
D-Mn3O4	Solvothermal	Flake-like with nanoparticles	1 M ZnSO4 + 0.1 M MnSO4	1.0 to 1.9	270 at 0.3 A g^−1	56% after 3000 cycles at 2.0 A g^−1	42
Na0.44MnO2	Sol–gel	Micro-rods	6 M NaOH	1.1 to 2.0	74.8 at 1C	73% after 1000 cycles at 10C	43
Na0.6MnO2	Hydrothermal	Hexagonal sheets	1 M ZnSO4 + 0.1 M MnSO4	1.0 to 2.0	165.2 at 0.2 A g^−1	77% after 150 cycles at 0.2 A g^−1	This work
Mn2O3	Hydrothermal	Agglomerated particles	1 M ZnSO4 + 0.1 M MnSO4	1.0 to 2.0	208.4 at 0.2 A g^−1	26% after 150 cycles at 0.2 A g^−1	—

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2. Experimental section

2.1. Synthesis of Na_0.6MnO₂ and Mn₂O₃

P2-Na_0.6MnO₂ was synthesized using a hydrothermal method according to our previous study.³⁹ Briefly, 1.02 g of sodium nitrate (12 mM) (5% excess added to compensate the loss of Na at high temperatures), 5.02 g of manganese nitrate (20 mM), and 1.92 g of urea (32 mM) were separately dissolved in 20 mL of de-ionized water through stirring. Afterward, the sodium nitrate solution was added to the manganese nitrate solution with vigorous stirring, and then the urea solution was added dropwise to the mixture. After continuous stirring for 2 h, the solution was transferred to a Teflon-lined stainless steel autoclave, and kept in an oven at 150 °C for 15 h. After completion of the reaction, the autoclave was cooled down to room temperature. The resulting brown solution was then evaporated at 80 °C to obtain a dry mass, which was annealed in two steps. The initial step involves annealing the sample at 500 °C (heating rate of 5 °C min⁻¹) for 3 h under air atmosphere. The sample was then ground to a powder form with a mortar-pestle, and then annealed at 900 °C for 12 h (Fig. 1a). The resulting Na_0.6MnO₂ black powder was stored in an air-tight container. For comparison, Mn₂O₃ (without pre-sodiation) was prepared in a similar process without the addition of sodium nitrate, with final annealing at 400 °C for 6 h, which resulted in a black powder material.

Fig. 1 (a) and (b) Schematic for the hydrothermal synthesis of Na_0.6MnO₂ and Mn₂O₃.

2.2. Material characterization

The X-ray diffraction (XRD) patterns of the synthesized Na_0.6MnO₂ and Mn₂O₃ were measured in 2θ range of 10° to 80° using Bruker, USA, D8 Advance, Davinci, which utilizes Cu Kα radiation (λ = 1.54 Å). The morphology of both samples was examined using a high-resolution scanning electron microscope (HRSEM) from ThermoScientific Apreo S. High-resolution transmission electron microscope (HRTEM) images, along with energy dispersive X-ray spectra (EDS), were recorded using JEM 2100 Plus, JEOL Japan. X-ray photoelectron spectroscopy (XPS) measurements were obtained using PHI VersaProbe III to study the elemental composition and the oxidation state of Mn. Electrochemical impedance spectroscopy (EIS) was conducted with an ac amplitude of 10 mV using Biologic/VSP 300, in the frequency range of 100 kHz to 10 mHz.

2.3. Electrode preparation and electrochemical measurements

The electrochemical measurements were performed using the CR2032 coin cell. The slurry was prepared by grinding 75 wt% of Na_0.6MnO₂, 15 wt% of super P carbon, and 10 wt% of poly(vinyl difluoride) (PVDF), and adding a few drops of NMP (N-methyl pyrrolidone) using a mortar and pestle for 30 min. Then, the homogeneous slurry was hand-painted onto circular graphite electrodes with 14 mm diameter. The electrodes were dried at 100 °C under vacuum overnight to remove the NMP. The geometric area of the electrode was 1.54 cm² and the mass loading of the active material is ∼1.1 mg cm⁻². The CR2032 coin cells were assembled using a Whatman GF/D glass fibre filter membrane as the separator, Na_0.6MnO₂ or Mn₂O₃ as the cathode, Zn foil as the anode and an aqueous solution of 1 M ZnSO₄ + 0.1 M MnSO₄ as the electrolyte. All cells were assembled and tested at a temperature of 25 °C. The specific capacity values of Na_0.6MnO₂ and Mn₂O₃ are reported after testing the performance of at least two cells for the reproducibility of data with 6% error. The cyclic voltammetry (CV) experiments were conducted with various scan rates ranging from 0.5 to 5.0 mV s⁻¹. Galvanostatic charge–discharge (GCD) studies were carried out at various specific currents within a voltage range of 1.0–2.0 V by employing Autolab potentiostat/galvanostat (Metrohm, Netherlands). The galvanostatic intermittent titration technique (GITT) experiments were carried out by using a pulse current of 50 mA g⁻¹ for 15 min with a relaxation time of 40 min for evaluation of diffusion coefficients at various voltages. The electrochemical impedance spectra (EIS) were measured at an open circuit potential (OCP) with an AC amplitude of 10 mV in the frequency range of 10 kHz–0.01 Hz using the ZIVE SP1 potentiostat/galvanostat.

3. Results and discussion

The crystal structures and phase purity of the synthesized Na_0.6MnO₂ and Mn₂O₃ were examined with XRD patterns. The XRD peaks with 2θ values of 15.84°, 32.13°, 36.02°, 39.67°, 43.80°, 49.07°, 62.39°, and 64.77° are well indexed as hexagonal Na_0.6MnO₂ (JCPDS: 027-0751, space group: P6₃/mmc), which corresponds to the (002), (004), (100), (102), (103), (104), (106), and (110) planes, respectively (Fig. 2a).³⁹ The intense sharp peaks indicate the good crystallinity of the synthesized Na_0.6MnO₂. The diffraction peaks at around 23.13°, 32.95°, 35.68°, 38.23°, 45.16°, 49.35°, 55.18°, 60.62°, 64.09°, and 65.92° are indexed as cubic Mn₂O₃ (JCPDS: 98-005-1463, space group: Ia3), corresponding to the (112), (222), (123), (004), (233), (134), (044), (116), (145), and (226) planes, respectively (Fig. 2b).

Fig. 2 XRD patterns of the hydrothermally synthesized (a) Na_0.6MnO₂ and (b) Mn₂O₃.

The morphology and particle size were investigated by HRSEM and HRTEM. The HRSEM image of Na_0.6MnO₂ features a hexagonal sheet-like morphology with a particle size of ∼966 nm (Fig. 3a and Fig. S1, ESI†).³⁹Fig. 3b shows the HRTEM image of Na_0.6MnO₂, where it exhibits a thin sheet-like nature and the continuous lattice from Fig. 3d. This indicates the good crystalline nature of Na_0.6MnO₂ with a lattice fringe spacing of 0.549 nm, which corresponds to the (002) plane of hexagonal Na_0.6MnO₂. The selected area electron diffraction (SAED) pattern with distinct spots arranged in a hexagonal array fashion, further confirmed the crystalline nature of Na_0.6MnO₂. The presence of (100) and (103) planes of Na_0.6MnO₂ can be ascribed to XRD peaks appearing around 36.02° and 43.8° (Fig. 2a). These results verify the layered structure of Na_0.6MnO₂, which could facilitate the insertion/extraction of Zn²⁺ compared to Mn₂O₃, which is reflected in the subsequent electrochemical performance. Fig. 3i shows the EDX elemental analysis, where (atom%) of (0.7 : 1 for Na:Mn) further confirms the formation of Na_0.6MnO₂. HRSEM and HRTEM images of Mn₂O₃ displayed agglomerated particles morphology (Fig. 3e and f) with varying particle sizes in the range of ∼20–70 nm. The SAED pattern of Mn₂O₃ shows the presence of (044) plane, which can be ascribed to an XRD peak around 55.18° (Fig. 3g), and the presence of concentric circles confirms its polycrystalline nature. Fig. 3h shows the interplanar lattice fringe spacing of 0.278 nm, which matches to the interplanar distance of (222) plane, corresponding to the most intense XRD peak of Mn₂O₃ appearing at 32.95°.

Fig. 3 HRSEM images of (a) Na_0.6MnO₂ and (e) Mn₂O₃; (b) and (f) HRTEM images of Na_0.6MnO₂ and Mn₂O₃; (c) and (g) SAED patterns of Na_0.6MnO₂ and Mn₂O₃; (d) and (h) lattice fringes of Na_0.6MnO₂ and Mn₂O₃; (i) EDX elemental analysis of Na_0.6MnO₂.

XPS analysis was performed to explore the valence state of individual elements. The high-resolution Mn 2p spectrum of Na_0.6MnO₂ exhibits two peaks at around 642.3 eV and 653.9 eV, corresponding to Mn 2p_3/2 and Mn 2p_1/2, respectively (Fig. 4a), with a ΔE value of 11.6 eV. On fitting, the Mn 2p_3/2 peak is divided into two peaks, Mn³⁺ (642.2 eV) and Mn⁴⁺ (644.1 eV), and the 2p_1/2 peak is divided into two peaks Mn³⁺ (653.8 eV) and Mn⁴⁺ (654.6 eV) (Fig. 4a).^4,23 The Mn 3s peaks are broad with multiple splitting caused by the coupling of 3s with 3d electrons. The peak difference denotes the valence state of Mn in the sample. In the case of Na_0.6MnO₂, two peaks were centred at 84.35 eV and 89.26 eV with a peak difference of 4.91 eV, indicating the presence of Mn⁴⁺.^27,44 The average oxidation state of Mn (n) was calculated to be 3.49 using eqn 1, which shows the mixed oxidation state of Mn³⁺ and Mn⁴⁺ in the Na_0.6MnO₂ sample.

E = 7.88 − 0.85n (1)

where, E is the energy difference between the two peaks of Mn 3s; and n is the average oxidation state of Mn. The Mn 2p and Mn 3s spectra show the co-existence of Mn³⁺ and Mn⁴⁺, which highlights the mixed oxidation state of Mn due to the presence of sodium ions in Na_0.6MnO₂.²³ In the case of Mn₂O₃, due to the absence of sodium ions, the Mn 2p peaks shift to lower binding energy, with a peak separation value of ΔE of 11.7 eV between Mn 2p_3/2 (at 641.82 eV) and Mn 2p_1/2 (653.52 eV), corresponding to the Mn³⁺ state and confirming Mn₂O₃ formation (Fig. 4d).¹¹ The Mn 3s spectrum shows two peaks centred at 82.88 eV and 88.47 eV with a peak difference of 5.59 eV, which further confirms the presence of the Mn³⁺ state and the formation of the Mn₂O₃ phase.⁴⁵ The presence of sodium ion in Na_0.6MnO₂ is confirmed from the high-resolution Na 1s spectrum, exhibiting a peak around 1071.45 eV (Fig. 4b).^11,23 The Mn–O–M, Mn–O–H, and H–O–H bonds in Na_0.6MnO₂ can be confirmed from the high-resolution O 1s spectrum (Fig. 4c), which displays three peaks at 529.54, 531.20, and 535.54 eV.^22,23,35 Meanwhile, only two peaks appear at 529.38 eV and 530.93 eV in the O 1s spectrum of Mn₂O₃, which corresponds to the Mn–O–M and Mn–O–H bonds (Fig. 4d). Based on the above results from the XRD, HRSEM, HRTEM, EDX elemental analysis and XPS, the formation of layered Na_0.6MnO₂via simple hydrothermal method had been confirmed.

Fig. 4 High-resolution XPS spectra of Na_0.6MnO₂ and Mn₂O₃ samples; (a)–(d) Mn 2p, Mn 3s, Na 1s and O 1s of Na_0.6MnO₂; (e)–(g) Mn 2p, Mn 3s and O 1s of Mn₂O₃.

The electrochemical performance of Na_0.6MnO₂ and Mn₂O₃ for zinc-ion storage was investigated using a CR2032 coin cell with Na_0.6MnO₂ or Mn₂O₃ as the cathode, and Zn foil as the anode in an aqueous electrolyte of 1 M ZnSO₄ + 0.1MnSO₄. The assembled half-cells were left overnight for proper electrolyte percolation. Zn||Na_0.6MnO₂ revealed an open circuit potential (OCP) of ∼1.41 V, whereas Zn||Mn₂O₃ showed an OCP of ∼1.35 V. Fig. 5a shows the CV curves of the first five cycles of the Na_0.6MnO₂ cathode at a scan rate of 0.5 mV s⁻¹ in the potential range of 1.0 to 2.0 V (vs. Zn/Zn²⁺). The first anodic peak at 1.64 V shows the extraction of Na⁺ along with the exchange of Zn²⁺ ions (as explained in the mechanism through XRD analysis) and oxidation of manganese ions.²³ The cathodic scan reveals two reduction peaks, one centered at 1.31 V and the other at 1.13 V, attributed to H⁺ insertion and Zn²⁺-ion insertion, along with manganese ion reduction, respectively.⁴¹ However, the second reduction peak at 1.13 V vanishes after the initial cycle, indicating the reversible intercalation of small hydrated H⁺ ions (1.0 Å). Yuan et al. had studied the performance of Na_0.44MnO₂ in 6.0 M NaOH, and provided the existence of various redox pairs in the CV data at 1.20/1.18 V, 1.41/1.38 V, 1.51/1.48 V, and 1.71/1.70 (vs. Zn²⁺/Zn), providing clear evidence for the reversible insertion and extraction of sodium ions.⁴³ However, our CV analysis does not reveal such redox peaks corresponding to the intricate phase transitions in Na_0.6MnO₂, suggesting the reversible intercalation and extraction of Zn²⁺, rather than intercalation/extraction of Na⁺ ions.

Fig. 5 (a) and (d) CV curves of Na_0.6MnO₂ and Mn₂O₃ cathodes at a scan rate of 0.5 mV s⁻¹ in 1 M ZnSO₄ + 0.1 M MnSO₄; (b) and (e) CV curves of Na_0.6MnO₂ and Mn₂O₃ at various scan rates of 0.5, 1.0, 2.0, 5.0 mV s⁻¹; (e) and (f) power law dependence of the peak current on the scan rate for Na_0.6MnO₂ and Mn₂O₃ cathodes.

In the case of Mn₂O₃, the first anodic peak appears at 1.60 V with a shoulder peak around 1.56 V due to the oxidation of manganese ions (Mn³⁺ to Mn⁴⁺) at 0.5 mV s⁻¹ scan rate (Fig. 5d). The corresponding cathodic scan shows two distinct reduction peaks at 1.32 and 1.13 V, which are due to the co-intercalation of H⁺ and Zn²⁺, accompanied by the reduction of Mn⁴⁺ to Mn³⁺ and Mn³⁺ to Mn²⁺.^22,33 In the subsequent cycles, the appearance of the oxidation peak, along with a prominent shoulder peak, can result from the extraction of Zn²⁺ and H⁺.⁴² The peak current of the first reduction peak occurring at a higher potential shows an increase in the peak current in further cycles due to the gradual activation process,³³ while the second reduction peak overlaps in all five cycles at 0.5 mV s⁻¹. However, the oxidation peaks show a decrease in the peak currents in further cycles, which may be due to the irreversibility of the Mn²⁺/Mn⁴⁺ reaction. To better understand the reaction, the CV of Na_0.6MnO₂ was carried out at different scan rates of 0.5, 1.0, 2.0, 5.0 mV s⁻¹. With an increase in the scan rate, the peak currents increased, indicating a diffusion controlled process, which was further confirmed by the plot of I_pvs. ν^1/2 (Fig. S2, ESI†). A similar trend of an increase in current with scan rate was observed in the case of Mn₂O₃, which reaches a maximum anodic peak current of 7.73 mA and cathodic peak current of −5.66 mA at 5.0 mV s⁻¹ scan rate, respectively (Fig. 5e). From Fig. 5e, we can clearly see two pairs of oxidation–reduction peaks at low scan rates (0.5 mV s⁻¹ and 1.0 mV s⁻¹), which is attributed to H⁺ and Zn²⁺ intercalation, respectively.^21,46 When the scan rate increases to 5.0 mV s⁻¹, only one pair of oxidation–reduction peaks is observed. This is due to intercalation of small hydrated H⁺ ions (1.0 Å).^12,47 Thus, this study reveals that the CV patterns are well-maintained for Na_0.6MnO₂ at all scan rates compared to Mn₂O₃, indicating better structural stability of Na_0.6MnO₂, which in turn helps in better cycling performance of Na_0.6MnO₂ (Fig. 6d) with capacity retention (77%) over 150 cycles.

Fig. 6 (a) Comparison of the rate capabilities of Na_0.6MnO₂ and Mn₂O₃; (b) and (c) the voltage–capacity curves of Na_0.6MnO₂ and Mn₂O₃ at 0.2 A g⁻¹ cycling; (d) comparison of the cycling performances of Na_0.6MnO₂ and Mn₂O₃ at 0.2 A g⁻¹.

The charge storage kinetics of both Na_0.6MnO₂ and Mn₂O₃ was analysed using power law equation, which relates the peak current (i) with the scan rate (ν) according to the following equation (eqn (2)):

i = aν^b (2)

where i represents the peak current (mA), ν represents the corresponding scan rate (mV s⁻¹), and a and b are adjustable parameters. The b value is obtained from the slope of log i vs. log ν plot, which signifies a diffusion-controlled process if the b value is close to 0.5, and a capacitive dominant process if it is close to 1.0.^36,48 From Fig. 5c and f, the calculated b values are found to be 0.455 and 0.598, respectively, for the cathodic peaks of Na_0.6MnO₂ and Mn₂O₃. A value of b close to 0.5 for Na_0.6MnO₂ and Mn₂O₃ indicates a diffusion-controlled process in both electrodes.

The rate performance for both oxide cathodes was evaluated by GCD cycling at different specific currents (Fig. 6a). GCD was performed with increasing specific currents from 0.1 A g⁻¹ to 1.0 A g⁻¹ with four cycles at each current, and finally returning back to cycling at the low current of 0.1 A g⁻¹. On increasing the specific current from 0.1 A g⁻¹ to 1.0 A g⁻¹, Na_0.6MnO₂ exhibits reversible discharge capacities of 188.0, 154.0, 104.4, 90.5, 77.0, and 67.2 mA h g⁻¹ at the specific currents of 0.1, 0.2, 0.4, 0.6, 0.8 and 1.0 A g⁻¹, respectively. Interestingly, on returning to cycling at the lower current of 0.1 A g⁻¹, the Na_0.6MnO₂ cathode delivered a specific capacity of 181.5 mA h g⁻¹, thus displaying its stable capacity. Generally, manganese oxide materials show high specific capacity compared to pre-sodiated manganese oxides (Na_0.6MnO₂). At different specific currents of 0.1 A g⁻¹ to 1.0 A g⁻¹, the Mn₂O₃ cathode exhibited a comparatively higher discharge capacity of 220.0, 181.0, 148.7, 130.0, 115.0, and 101.6 mA h g⁻¹ at the specific currents of 0.1, 0.2, 0.4, 0.6, 0.8 and 1.0 A g⁻¹, respectively. On returning back to cycling at 0.1 A g⁻¹, the discharge capacity was much higher than the initial capacity at 0.1 A g⁻¹ in the case of Mn₂O₃ (∼281.5 mA h g⁻¹). This may be attributed to the gradual electrode activation process, which is clearly seen in the cycling data, where Mn₂O₃ reaches a maximum capacity of 235.5 mA h g⁻¹ at 0.2 A g⁻¹ after 25 cycles (from the initial capacity of 208.4 mA h g⁻¹), followed by capacity fading (Fig. 6d).²² The discharge capacities of the first four cycles of Na_0.6MnO₂ at the specific current of 0.1 A g⁻¹ are 188.0, 194.5, 194.7, 199.7 mA h g⁻¹, and that for Mn₂O₃ are 220.0, 219.0, 232.0, and 242.0 mA h g⁻¹ at 0.1 A g⁻¹ (Fig. 6a). This shows the stable capacity of Na_0.6MnO₂, whereas there is an increase in capacity upon the cycling of Mn₂O₃. Such an increase in the specific capacity can be ascribed to the gradual activation of the electrode with the electrolyte percolation, as reported previously.^11,35,49 Thus, this result highlights the importance of the pre-sodiation of manganese oxides for the reversible insertion and extraction of Zn²⁺ ions.

The cycling stabilities of both Na_0.6MnO₂ and Mn₂O₃ are examined by GCD at 0.2 A g⁻¹ for 150 cycles (Fig. 6d). As shown in Fig. 6(b and d), Na_0.6MnO₂ delivers a stable capacity for the first 100 cycles when cycled at 0.2 A g⁻¹, followed by a gradual capacity fading, whereas the Mn₂O₃ cathode exhibits a stable capacity for only the first 50 cycles. The voltage vs. specific capacity curves of Na_0.6MnO₂ and Mn₂O₃ at 0.2 A g⁻¹ shows the plateaus at 1.6/1.3 V and 1.5/1.3 V, corresponding to the oxidation/reduction process in CV (Fig. 6b and c). The cycling stability study displays discharge capacities of 165.2, 166.4, 180.1, 145.4, and 128.0 mA h g⁻¹ at the 1st, 2nd, 50th, 100th, and 150th cycle, respectively, for Na_0.6MnO₂, thus exhibiting around 77% capacity retention after 150 cycles (Fig. 6b and d) (Table 2). Conversely, the discharge capacities of Mn₂O₃ at 0.2 A g⁻¹ are 208.4, 213.3, 197.3, 99.6, and 55.5 mA h g⁻¹ at the 1st, 2nd, 50th, 100th, and 150th cycle, respectively (Fig. 6c). Thus, the GCD studies conclude that the pre-sodiated Na_0.6MnO₂ cathode exhibits 77% capacity retention (Fig. 6d) compared to 26% for Mn₂O₃ after 150 cycles.

Table 2 Comparison of the discharge capacities of Na_0.6MnO₂ and Mn₂O₃ at 0.2 A g⁻¹ over 150 cycles

Cycle number	Discharge capacity (mA h g^−1) of Na0.6MnO2 at 0.2 A g^−1	Discharge capacity (mA h g^−1) of Mn2O3 at 0.2 A g^−1
1	165.2	208.4
2	166.4	213.3
50	180.1	197.3
100	145.4	99.6
150	128.0	55.6

---

Ex situ XRD was conducted to find the phase transformation occurring during the charge–discharge process. In Fig. 7a, three different voltage states of the Na_0.6MnO₂ cathode are marked, where state B corresponds to the partially charged state with a potential of 1.5 V, state C corresponds to the fully charged state with a potential of 2.0 V, and state D corresponds to a fully discharged state with a potential of 1.0 V in the first cycle. Fig. 7b shows the comparison of the XRD patterns recorded at states B and C, along with state A being the pristine electrode (before assembling the cell). The XRD pattern of state A shows the diffraction peak of the P2-type Na_0.6MnO₂ phase appearing at 15.8° due to the (002) plane with high intensity, which belongs to the hexagonal crystal system, space group: P6₃/mmc (JCPDS: 00-027-0751). (i) Once the cathode is assembled in the CR2032 coin-cell against the Zn anode using 1 M ZnSO₄ + 0.1 M MnSO₄ electrolyte, some Zn²⁺ ions may get inserted into the cathode during the partial charging potential (1.5 V vs. Zn²⁺/Zn from OCP), forming the ZnMn₂O₄ spinel phase. Hence, at state B, the peaks around 32.5° and 36.0° correspond to the (103) and (211) planes of the ZnMn₂O₄ spinel phase (JCPDS: 01-077-0470, space group: I4₁/amd), respectively. This type of zinc ion-exchange mechanism taking place even before starting the galvanostatic charge–discharge cycle has been reported recently in the MnO₂ cathode.⁵⁰ (ii) At state C (fully charged state 2.0 V), sodium ions are extracted from the cathode, leading to the formation of the tetragonal α-MnO₂ phase, which can be confirmed by peaks appearing at 13.0° (110), 17.5° (200) (JCPDS: 00-044-0141, space group: I₄/m). Also, peaks at 32.5° and 36.0° corresponding to the ZnMn₂O₄ spinel phase are seen even at state C due to the irreversible nature of the spinel phase. Fig. 7c shows the XRD comparison of the pristine electrode and fully discharged state D. (iii) At the fully discharged state D (1.0 V), the spinel ZnMn₂O₄, tunnel γ-Zn_xMnO₂, and layered Zn_yMnO₂ phases are formed as expected due to the insertion of the Zn²⁺ ions into the manganese oxide structure. The peaks around 32.5° and 36.0° correspond to the (103) and (211) planes of the ZnMn₂O₄ spinel phase, as previously discussed in state C. The tetragonal ZnMn₃O₇ phase can be indexed to peaks present at 18.2°, 27.9°, and 57.6°, corresponding to planes (002), (003) and (404) (JCPDS: 00-047-1825), respectively. The tunnel γ-Zn_xMnO₂ phase can be confirmed by peaks appearing at 2θ values of 41.9°, 44.1° and 77°, which correspond to planes (300), (002) and (450) belonging to the orthorhombic phase (JCPDS: 00-014-0644), respectively. The layered Zn_yMnO₂ phase can be confirmed by peaks appearing at 2θ values of 48.3° and 59.2°, corresponds to planes (202) and (203) belonging to monoclinic phase (JCPDS: 00-042-1317), respectively.^51,52

Fig. 7 (a) Charge–discharge curve of the Na_0.6MnO₂ electrode showing different states of charge during the first cycle; (b) XRD patterns of the Na_0.6MnO₂ electrode at the selected states: A (pristine), B (partially charged to 1.5 V) and C (fully charged state at 2.0 V) during the first cycle charging; (c) XRD patterns of Na_0.6MnO₂ at selected states: A (pristine) and D (fully discharged to 1.0 V) electrode.

Apart from these diffraction peaks discussed above, highly sharp & highly intense peaks seen at 26.42°, 54.51° and 86.90° appear in all states (A–D) as it arises from the graphite sheet current collector used in our study, and can be indexed to the planes (002), (004) and (006), respectively, of graphite (JCPDS: 00-012-0212, space group: P6₃/mmc).

All XRD data correlate with the SEM images taken at different voltages, which show the morphology change at different potentials, signifying the phase transformation occurring in the pre-sodiated manganese oxide cathode during the charge–discharge cycle.

Based on the XRD analysis, the charge storage mechanism can be proposed as follows.

During the partial charged state (1.5 V):

Na_0.6MnO₂ + xZn²⁺ → ZnxMn₂O₄ + xNa⁺(ion-exchange)

During the fully charged state (2.0 V):

ZnxMn₂O₄ → xZn²⁺ + 2MnO₂

During the fully discharged state (1.0 V):

Zn²⁺ + 2MnO₂ → ZnMn₂O₄ + L-ZnMnO₂ + T-ZnMnO₂

The possible charge storage mechanism can be explained from the information obtained from XRD analysis at different potentials. During the partially charged state (1.5 V), the sodium ions are exchanged with Zn²⁺ ions from the electrolyte, as reported recently by Cui et al.⁵⁰ At the fully charged state (2.0 V), the remaining sodium ions and Zn²⁺ ions are completely extracted, forming manganese oxide (MnO₂). During the fully discharged state (1.0 V), MnO₂ undergoes Zn²⁺ intercalation, forming possible Zn-Manganese oxide phases such as ZnMn₂O₄ spinel phase, T-γ-ZnMnO₂ (Tunnel) and L-ZnMnO₂ (layered). On further cycling, Zn²⁺ extraction/intercalation reversibly takes places, resulting in the enhanced cycling stability of the pre-sodiated manganese oxide cathode.

In order to study the morphological changes occurring at the electrode at different potentials, ex situ SEM was performed. Fig. 8 shows the changes in the morphology during charge–discharge in the first cycle of the Na_0.6MnO₂ cathode. Three points (point 1 to point 3) marked in Fig. 8a indicate the three different states of the electrode in the first cycle. Fig. 8b shows the agglomerated particle-like morphology of the pristine electrode. Considering that sodium extraction in the first cycle occurs at the voltage of ∼1.56 V to 1.6 V, the electrode was partially charged to 1.5 V (point 1) from OCP, which shows the morphology of Na_0.6MnO₂ during the partial charging process. At point 1 (Fig. 8c), smaller nanoflakes were formed with a size of ∼1 μm. The pristine electrode morphology was mainly transformed to smaller nanoflakes, along with the appearance of some agglomerated particles similar to that of the pristine electrode. Fig. 8d shows the fully charged state 2.0 V (point 2), where smaller nanosheets transformed into larger nanosheets of size 4–5 μm, along with the emergence of a new flower-like morphology. Considering point 1 and point 2 as states before and after sodium extraction, the morphology has significantly changed from smaller nanosheets to larger nanosheets, accompanied by flower-like morphology, which may be due to the nanosheet-like MnO₂ phase formation on sodium extraction. The nanosheet-like formation could be due to excess crystal water content present in the interlayer spacing during charging, as reported in the previous literature.⁵³ When the electrode was discharged to 1.0 V (point 3), all of the nanosheet and flower-like structures disappeared, resulting in large-size agglomerated/irregular particles. This serves as evidence for Zn²⁺ insertion during discharge, forming the ZnMn₂O₄ and Zn_xMnO₂ phases. At point 3, the particles are large in size (>5 μm) compared to pristine electrode (∼5 μm). The EDS measurements reveal the presence of Zn, Mn, and O, and the absence of the sodium element. This further confirms the reversible insertion/extraction of the Zn²⁺ species, rather than the reversible insertion/extraction of sodium ions.⁵⁴ The appearance of no cracks and flake formation during the discharged state ensures the reversible Zn²⁺ insertion/extraction with the higher cycling stability of the pre-sodiated manganese oxide electrode. The XRD serves as evidence along with SEM images, proving the phase transformation occurring in the manganese oxide-based cathodes.

Fig. 8 (a) Charge–discharge curve of the Na_0.6MnO₂ electrode during the first cycle, SEM images for the (b) pristine electrode; (c) charged to 1.5 V; (d) fully charged state (2.0 V); (e) fully discharged state at 1.0 V.

In order to measure the Zn-ion diffusion coefficients at particular voltages, a GITT experiment was performed. Before performing the GITT experiment, the cell was charged and discharged at 100 mA g⁻¹ for two cycles. During the GITT procedure, a current pulse of 50 mA g⁻¹ was applied for a time of 15 min and the cell was relaxed for 40 min, which was then repeated for the entire charge/discharge process. The diffusion coefficients of Zn²⁺ ions in the pre-sodiated (Na_0.6MnO₂) and non-pre sodiated (Mn₂O₃) cathode were calculated from the GITT data using the formula:

where, τ is the constant current pulse time (s), m_A is the mass of the active material (g), ρ_A is the density of the active material (g cm⁻³), S is the total contact area of the electrode with the electrolyte (cm²), ΔE_s is the change in the steady state voltage (E_s − E₀), and ΔE_τ is the total change in the cell voltage during the constant current pulse time τ of a single-step GITT experiment regardless of the IR drop (E_τ − E₀).

Fig. 9a and b shows the GITT curves, and the calculated diffusion coefficient (D_Zn²⁺) of the Na_0.6MnO₂ and Mn₂O₃ cathodes during the charge and discharge process are shown in Fig. 9(c and d). The calculated D_Zn²⁺ for the Na_0.6MnO₂ cathode is found in the range of 10⁻⁹ to 10⁻¹¹ cm² s⁻¹ orders of magnitude, and is in the range of 10⁻⁸ to 10⁻¹³ cm² s⁻¹ for the Mn₂O₃ cathode.^46,55 This indicates that the good conductivity and high diffusion of the Zn²⁺ ions in the Na_0.6MnO₂ cathode are due to its hexagonal sheet-like structure, which facilitates faster reaction kinetics, electron transfer and Zn-ion diffusion. Conversely, the diffusion coefficient of the Zn²⁺ ions in Mn₂O₃ is 2 orders of magnitude lower compared to that of Na_0.6MnO₂. During discharge, the H⁺ diffusion occurring around 1.3 V shows a higher diffusion coefficient (10⁻⁹ cm² s⁻¹) compared to the Zn²⁺ diffusion occurring around 1.1 V (10⁻¹⁰ cm² s⁻¹) in Na_0.6MnO₂. This is because of the large size of the hydrated Zn²⁺ ion (4.7 Å).

Fig. 9 Single GITT curves of (a) Na_0.6MnO₂; (b) Mn₂O₃; (c) diffusion coefficient values of Na_0.6MnO₂ during charge–discharge; (d) diffusion coefficient values of Mn₂O₃ during charge–discharge.

In order to evaluate the electrode kinetics parameters such as charge transfer resistance (R_ct) and diffusion coefficient of Zn²⁺ onto the active mass, the electrochemical impedance spectra (EIS) were measured on the Na_0.6MnO₂ and Mn₂O₃ cathodes in the frequency range of 100 kHz to 10 mHz. Two semicircles are observed for both samples when analyzed after completing 5 cycles at the discharged state (Fig. 10a). The semicircle at the high-frequency region corresponds to the parallel combination of the surface film resistance (R_sf) and capacitance, whereas the diameter of the 2nd semicircle represents the value of R_ct.^12,35 The appearance of a semicircle at the high-frequency range indicates the presence of a surface precipitation layer on both cathode surfaces due to the pH change during battery cycling.⁴² The R_sf values (diameter of 1st semicircle) are found to be 68 Ω and 44 Ω, respectively, and the R_ct values are found to be 378 Ω and 223 Ω, respectively, for Na_0.6MnO₂ and Mn₂O₃ (Fig. 10a). These cell impedance values can be well correlated with their initial specific capacity differences. Thus, the Mn₂O₃ cathode exhibits a high specific capacity (∼208 mA h g⁻¹) due to its small R_ct (223 Ω) compared to that of the Na_0.6MnO₂ cathode (165 mA h g⁻¹) with a R_ct value of 378 Ω, when cycled at 0.2 A g⁻¹ (Fig. 6a and b).

Fig. 10 (a) EIS spectra of Na_0.6MnO₂ and Mn₂O₃ after 5 cycles; (b) and (c) comparison of EIS of Na_0.6MnO₂ and Mn₂O₃ after 5 cycles and upon completion of 150 cycles at 0.2 A g⁻¹.

The Nyquist plots in Fig. 10b show the comparative impedance response of Na_0.6MnO₂ recorded after 5 cycles and after completing the 150th cycle at 0.2 A g⁻¹. The values of R_ct (the diameter of the second semicircle) are found to increase from 378 Ω to 945 Ω after 150 cycles, indicating the increase in the charge-transfer resistance during cycling (Table 3). Fig. 10c shows the impedance response of Mn₂O₃ recorded after 5 cycles and after completing 150 cycles at 0.2 A g⁻¹. The linear spike at the low frequency region after 5 cycles indicates the diffusion of zinc-ions onto the active mass, whereas after 150 cycles, there is a transition from the Warburg diffusion to the capacitive behaviour (less diffusion). Thus, the Mn₂O₃ cathode exhibits a high initial specific capacity (∼208 mA h g⁻¹) and suffers from a drastic capacity fading of 153 mA h g⁻¹ (finally ∼55 mA h g⁻¹), retaining only 26% capacity retention after 150 cycles. Hence, the EIS data support the GCD cycling performance, where Na_0.6MnO₂ shows less capacity fading after 150 cycles (Fig. 6d), supporting the enhanced cycling stability of the pre-sodiated material during extensive cycling.

Table 3 Comparison of the R_s, R_sf and R_ct values from the Nyquist plots of Na_0.6MnO₂ and Mn₂O₃

Sample	R s (Ω)	R sf (Ω)	R ct (Ω)
Na0.6MnO2	9.50	67.58	378.29
Mn2O3	14.73	44.17	223.26

---

The diffusion coefficient (D) can be calculated from EIS using the following equations (eqn (3) and (4)):³⁵

Z′ = R₁ + R_ct + σω^(−1/2) (3)

where Z′ is the real part of the resistance (Ω), R₁ is the solution resistance, R_ct is the charge transfer resistance, ω is the angular frequency, R is the gas constant (J mol⁻¹ K⁻¹), T is the absolute temperature (K), A is the electrode surface area (cm²), F is the Faraday constant (As mol⁻¹), C is the molar concentration of the Zn²⁺ ions (mol cm⁻³), σ is the Warburg diffusion factor, and D_Zn²⁺ is the diffusion coefficient of Zn²⁺.

Fig. 11(a and b) shows the linear fitting results according to eqn (2), which is taken in the low-frequency region of the EIS spectrum of the Na_0.6MnO₂ cathode after 5 cycles and after 150 cycles. The linear fitted line gives the slope values of σ = 42.80 and 104.10 after 5 cycles and 150 cycles at 0.2 A g⁻¹, respectively. The diffusion coefficient values are calculated as per eqn (3), and found to be 8.17 × 10⁻⁶ after 5 cycles and 1.38 × 10⁻⁶ cm² s⁻¹ after 150 cycles, which clearly shows that the diffusion of the Zn²⁺ ions is maintained even after 150 cycles. Similarly, the linear fitted line of Mn₂O₃ gives the slope value of σ = 31.10 and 58.90 after 5 cycles and 150 cycles, respectively. The corresponding diffusion coefficient values are calculated as per eqn (3), and found to be 1.55 × 10⁻⁵ and 4.31 × 10⁻⁶ cm² s⁻¹, respectively, after 5 and 150 cycles (Table 4). Thus, there is a decrease in the diffusion coefficient of the Zn²⁺ ions into the Mn₂O₃ cathode upon prolonged cycling compared to that after 5 cycles. This may be due to the structural changes of Mn₂O₃ upon prolonged cycling.^12,21 The Zn²⁺ diffusion coefficients of Na_0.6MnO₂ and Mn₂O₃ were calculated using the Randles–Sevcik equation, and are 1.15 × 10⁻⁵ cm² s⁻¹ and 4.96 × 10⁻⁵ cm² s⁻¹, respectively (Fig. S2, ESI†). This data is comparable to the D_Zn²⁺ value calculated from EIS, as shown in Table 4. Thus, Mn₂O₃ cathode exhibits a high initial specific capacity (∼208 mA h g⁻¹) due to the high D_Zn²⁺ (1.55 × 10⁻⁵ cm² s⁻¹) compared to the Na_0.6MnO₂ cathode (165 mA h g⁻¹) with a low D_Zn²⁺ (8.17 × 10⁻⁶ cm² s⁻¹) when cycled at 0.2 A g⁻¹ (Fig. 6a and b).

Fig. 11 Plots of Z′ vs. ω^−1/2 of the Na_0.6MnO₂ cathode (a) and (b) and that of Mn₂O₃ (c) and (d) after 5 cycles and 150 cycles, respectively.

Table 4 Comparison of D_Zn²⁺ of Na_0.6MnO₂ and Mn₂O₃ from EIS, and that from CV using the Randles–Sevcik equation

	D Zn^2+ calculated from EIS after 5 cycles (cm^2 s^−1)	D Zn^2+ calculated from Randles Sevcik equation (cm^2 s^−1)
Na0.6MnO2	0.82 × 10^−5	1.15 × 10^−5
Mn2O3	1.55 × 10^−5	4.96 × 10^−5

---

To study the morphology changes before and after cycling, the coin-cell was disassembled after performing 150 charge–discharge cycles at 0.2 A g⁻¹, and the Na_0.6MnO₂ electrode was washed with deionized water and dried under vacuum. Fig. 12(a and b) shows the surface morphology of the pristine and cycled electrodes (150 cycles) of Na_0.6MnO₂ using HRSEM images. Only a minimal change in the surface morphology is observed upon the cycling of the Na_0.6MnO₂ electrode. This structural integrity helps in explaining the high capacity retention of the Na_0.6MnO₂ electrode, as seen in the GCD cycling data (Fig. 6d). Fig. 12(c and d) shows the morphology of the pristine and cycled Mn₂O₃ electrode, where a significant change in the morphology was observed after 150 cycles. This change in morphology can be the cause of the capacity fading observed in the case of the Mn₂O₃ cathode (Fig. 6d) due to the decrease in the zinc-ion diffusion and electrolyte percolation. Based on the post-mortem analysis, it highlights the importance of pre-sodiation in the manganese oxide-based cathode for AZIBs, which effectively alleviates the capacity fading issue predominantly observed in the case of manganese oxide-based cathodes. Thus, this work might help future researchers in studying various other pre-sodiated manganese oxide cathodes for AZIB application.

Fig. 12 Post-mortem analysis of the Na_0.6MnO₂ & Mn₂O₃ electrodes; (a) and (b) HRSEM images of the pristine Na_0.6MnO₂ and cycled electrode; (c) and (d) HRSEM images of the pristine and cycled electrode of Mn₂O₃.

In order to demonstrate the practical application, an aqueous Zn||Na_0.6MnO₂ coin-cell was charged to 1.9 V and then connected to a red LED. Fig. 13 shows that the aqueous Zn||Na_0.6MnO₂ battery can power a red LED for 30 min, which indeed shows its practical applicability. Hence, AZIBs are a promising candidate for large-scale stationary energy storage applications in future. This brings us one step closer towards the development of sustainable energy storage devices based on abundant, low-cost, and environmentally friendly pre-sodiated manganese oxides and aqueous electrolytes.

Fig. 13 Digital image of the Zn||Na_0.6MnO₂ battery powering a red LED. Inset: LED without connection to the Zn||Na_0.6MnO₂ battery.

4. Conclusions

The P2-Na_0.6MnO₂ material is synthesized by a simple hydrothermal method, followed by annealing at 900 °C, and its electrochemical performance for AZIB was studied. HRSEM and HRTEM show the morphology of Mn₂O₃ to be agglomerated particles, whereas that of Na_0.6MnO₂ was hexagonal sheets. HRTEM and XRD analysis confirmed the formation of different crystalline Mn₂O₃ and Na_0.6MnO₂. Although Mn₂O₃ exhibited an initial high specific capacity of 208 mA h g⁻¹, it can retain only 26% capacity, whereas Na_0.6MnO₂ can retain up to 77% capacity after 150 cycles. In addition, the EIS data support the high diffusion coefficient of Zn²⁺ and less charge transfer resistance of the Na_0.6MnO₂ cathode. The post-mortem analysis by SEM shows that Na_0.6MnO₂ retains its morphology even after 150 cycles, which supports its long-term cycling stability. While Mn₂O₃ possesses an initial high diffusion coefficient, there is a drastic change upon prolonged cycling. This may be due to the structural change that results in a large capacity fading. Therefore, the pre-sodiation strategy is found to be beneficial and imparts new insights into developing alkali metal cation (Na⁺ in particular)-inserted Mn-oxide-based advanced cathodes for sustainable rechargeable AZIBs.

Author contributions

Anjeline W. and P. K. Nayak conceptualized the work. Anjeline synthesized the sample, performed the experiments, and wrote the original draft, which was reviewed and edited by P. K. Nayak.

Data availability

The data supporting this article have been included in the main text and ESI.†

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

The authors would like to thank the SRM Institute of Science and Technology (SRMIST) for providing all the research facilities, including SRM-SCIF and Nanotechnology Research Centre (NRC) for structural measurements. P. K. Nayak is grateful for the funding support within Start-up Research Grant (SRG/2022/001178) by SERB, India.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ma00824c

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