1 Introduction

The research and development of Li-ion secondary batteries have intensified because of its safety, high specific capacity and long cycle life. At present, LiCoO2, LiMn2O4, and LiFePO4 are dominating in the commercial cathode materials. In order to improve their power density and cycle stability, the novel derivatives of the above cathode materials, such as LiNi0.8Co0.2O2 and LiNi1/3Co1/3Mn1/3O2 etc., are focused by researchers, which can be applied in plug-in/hybrid electric vehicles [1]. However, the commercial cathode materials in Li-ion batteries have some disadvantages such as high cost, toxicity and limited sources of Cobalt or Nickel, which restrict their development. Thus, it is urgent to develop the novel cathode materials in Li-ion secondary batteries with natural abundance, environmental friendliness, low cost and excellent electrochemical performance. The Li/MnO2 battery has high discharge voltage (3 V), high energy density (230 Wh kg−1) and long storage life. Electrolytic manganese dioxide (EMD) with high purity, low cost and strong electrochemical activity etc. is the critical component of the cathode material in commercial Li/MnO2 primary battery [2,3,4], and it has a decisive influence on performance of cells. Furthermore, manganese oxide is inexpensive (less than 1% of the cost of Co) [5,6,7]. Developing and improving the cycle stability of EMD materials have been studied by researchers, but they show poor rechargeable ability [8,9,10].

EMD is a multiphase material incorporating several polymorphs of MnO2, and it possesses intergrowth, microtwinning, and phase mixture, which have been investigated [5, 11, 12]. The structure and morphology of EMD have been strongly influenced by various prepared parameters such as electrolyte, current density, electrodes (anode and cathode), bath temperature, current type and the pH of the electrolyte [4]. By decreasing the temperature and pH, the microtwinning EMD including α-MnO2 and γ-MnO2 has been prepared and extensively investigated as a cathode for Li-ion primary batteries [13]. The formation of α-MnO2 in EMD at low pH is attributed to a template or structure-directing effect due to H3O+ [13].

As known to all, α-MnO2 and γ-MnO2 compounds with one-dimensional tunnel structure are favorable candidates for Li-ion secondary batteries. The structure of α-MnO2 (Fig. 1a) is made up of double chains of edge-sharing octahedra which can share corners to form (2 × 2) and (1 × 1) channels, and the (2 × 2) channels have suitable size for the insertion/extraction of Li+ [14]. Single-phase α-MnO2 with high degree of crystallinity limits its use as insertion/extraction electrode in rechargeable Li-ion battery due to an instability of the large (2 × 2) channels of the α-MnO2 framework to lithiation and an inability to electrochemically extract lithium below 3.8 V [5, 15]. γ-MnO2 (Fig. 1b) with smaller (2 × 1) channels can provide greater stability than α-MnO2, but the anisotropic expansion and contraction of the unit cell during lithium insertion/extraction cause structural damage and loss of capacity during cycling [16]. According to the study of Thackeray [17], the α-MnO2 structure is inherently unstable. It could be stabilized by connecting to MnO2 structures with smaller dimension channels to withstand the repeated insertion and extraction of lithium during cycling, thus the co-electrodeposited α∙γ-MnO2 exhibits good electrochemical performance. As depicted in Fig. 1c, it is expected that α∙γ-MnO2 composite with interconnected α-MnO2 and γ-MnO2 can make its structure more stable than single-phase α-MnO2 or γ-MnO2 during lithium insertion/extraction reactions [16].

Fig. 1
figure 1

The idealized MnO2 structures for a α-MnO2, b γ-MnO2 and c the interconnected α∙γ-MnO2 composite

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At present, MnO2 can be prepared by chemical and electrochemical methods, but it is difficult to prepare the intergrowth of α-MnO2 and γ-MnO2 with exact ratio of α-/γ-MnO2 in the composite. In this paper, we introduce a facile electrodeposition method to prepare EMD particles on a large scale, and α∙γ-MnO2 composite with exact ratio can be obtained by controlling the electrolytic temperature and pH. The microstructure of α∙γ-MnO2 can composite the synergetic effects of α-MnO2 and γ-MnO2 and exhibit excellent cycle stability and rate capability. The promising electrochemical performance of α∙γ-MnO2 can make researchers focus again on using metal oxide as the cathode materials in the rechargeable Li-ion batteries.

2 Experimental section

2.1 Synthesis and characterization of electrolytic MnO2

All chemicals were analytical grade and were used without further purification. MnSO4∙H2O and concentrated sulfuric acid were supplied by Tianjin Chemical Reagent Company (P. R. China). Distilled water was used as solvent.

The EMD was prepared by constant current electrodeposition method in 5L electrolyte with current density of 300 A m−2 under 30 °C. Prior to electrodeposition, the lead plate with 10 cm × 13 cm was used as the working electrode after polishing and washing with detergent and distilled water. Copper plate was used as the counter electrode. The electrolyte was composed of 0.34 mol L−1 MnSO4 and 0.47 mol L−1 H2SO4 (pH ≈ 0.03). The as-prepared EMD was purified by filtration, washing with distilled water and ethanol. The yield of as-prepared EMD was controlled to be 0.028 g cm−2 h−1.

Scanning electron microscope (SEM) images were collected using JEOL JSM-6610LV microscope operated at the accelerating voltage of 30 kV. The phase structure of the as-prepared sample was identified using Rigaku D/max 2500 X-ray diffractometer (Cu, Kα radiation, λ = 1.5418 Å) at a voltage of 40 kV and a current of 250 mA with a scan rate of 0.02° s−1 (5° < 2θ < 90°) at room temperature. The BET (Brunauer–Emmett–Teller) surface area and the BJH (Barrett–Joyner–Halenda) pore distribution of the as-prepared sample were measured by the N2 adsorption/desorption method (TriStarII3020, micromeritics, USA).

2.2 Electrochemical measurements

The electrochemical measurements were carried out using the CR 2016 coin-type cells. Test electrodes were prepared by mixing as-prepared EMD sample (80 wt%), acetylene black (10 wt%), and poly (vinylidene fluoride) (10 wt%). An amount of 3–5 mg of the mixture was coated on aluminum foil. The electrodes were dried at 60 °C in a vacuum drying oven for 24 h before use, and Li foil used as counter electrode. The coin-type cells were fitted together in a glovebox under an argon atmosphere. The electrolyte was a solution of 1 M LiPF6 in ethylene carbonate (EC)-dimethyl carbonate (DMC) (volume ratio of 1:1). The constant current charge–discharge experiment was carried out on a NEWARE battery testing system (NEWARE BTS-5 V/5 mA) with a series of current densities of 46 mA g−1 (0.06 mA cm−2), 151 mA g−1 (0.2 mA cm−2), 350 mA g−1 (0.5 mA cm−2) and 755 mA g−1 (1 mA cm−2), the cells were charged and discharged between 4.2 and 1.8 V (vs. Li/Li+). Cyclic voltammetry was carried out using a CHI660C electrochemical workstation (Shanghai, China) at a scan rate of 0.1 mV s−1, with the operating voltage range consistent with the charge–discharge experiment. Electrochemical Impedance Spectroscopy (EIS) was carried out using a CHI660C Corrosion Electrochemical Measurement System (Shanghai, China) at a frequency range between 0.01 and 105 Hz.

3 Results and discussion

Figure 2 shows the typical SEM images of as-prepared EMD material. The uniform EMD particles can be found in Fig. 2a, the diameters of EMD particles are about 350–500 nm. As shown in Fig. 2b, part of EMD particles are aggregated with each other which can be ascribed to their high surface energy during prepared process. To our knowledge, the EMD not only provides the enhancement in the Li+ effective insertion pathways, but also enhances diffusion kinetics due to the increase of the specific surface area, which can improve the rate capacity of electrode.

Fig. 2
figure 2

a SEM image of as-prepared EMD material, b the higher magnification SEM image of as-prepared EMD material

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The N2 adsorption/desorption isotherms and the pore size distribution (inset) of as-prepared EMD material are presented in Fig. 3. The BET surface area of the EMD material is 66 m2 g−1, which can be calculated from N2 adsorption. As shown in Fig. 3, the adsorption/desorption isotherms are identified as type II, which is characteristic of mesoporous materials [18,19,20]. It can be known from the inset of Fig. 3 that the pore size is centered at 10 nm with a narrow distribution. Mesoporous structure is beneficial for the migration of Li+ and transfer of electron, and corresponds to the superior electrochemical performance of the MnO2 samples [19, 20]. In a word, mesoporous structure and high specific surface area can further improve the electrochemical performance of the EMD.

Fig. 3
figure 3

N2 adsorption/desorption isotherms and pore size distribution (inset) of as-prepared EMD material

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Figure 4 shows the XRD pattern and the fitting result of as-prepared EMD material. All diffraction peaks can be indexed to α-MnO2 (JCPDS NO. 72-1982) and γ-MnO2 (JCPDS NO. 43-1455), which are in good agreement with the standard values, indicating that the intergrowth of two crystalline can be produced by controlling electrolytic process. The fitting result using the software of Jade 6.0 matches well with the tested diffraction peak, indicating a better fitting. The mass fraction of α-MnO2 in the composite can be calculated to be 55 wt%. The result shows that it is simple and facile to prepare the EMD with intergrowth of α∙γ-MnO2 compared with the chemically-produced material [11].

Fig. 4
figure 4

XRD pattern and the fitting result of as-prepared EMD material

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Reversibility of as-prepared EMD cathode is investigated by cyclic voltammetry. As shown in Fig. 5, the reduction peak (R) of the 2nd cycle is significantly different from the 1st cycle. The potential of reduction peak shifts from 2.53 to 2.61 V, and current of reduction peak decreases significantly, indicating that an irreversible phase conversion can occur during the first cycle. The oxidation peak (O) gradually shifts to high potential, indicating that the extraction of Li+ is becoming difficult with the increasing reduction depth. Furthermore, the reduction peak slightly shifts to opposite direction from 3rd to 5th cycle, reaching 2.53 V in the 5th cycle. The peak potential difference between reduction and oxidation gradually stabilizes during cycling. This result suggests that an irreversible phase conversion and the electrochemical activation can be gradually completed in several cycles. This result is in accordance with the cycle performance.

Fig. 5
figure 5

Cyclic voltammogram of as-prepared EMD cathode at the scan rate of 0.1 mV s−1 between 4.2 and 1.8 V

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Figure 6a shows the charge–discharge behaviors of as-prepared EMD cathode in rechargeable Li/MnO2 battery at current density of 46 mA g−1. From Fig. 6a, the first discharge curve of the EMD cathode shows a flat plateau in the voltage range between 3.0 and 2.4 V, and the discharge capacity reaches 202 mAh g−1, indicating that a desirable lithium inserted amount can be obtained at the low current density. This value is much higher than that of the related materials [21]. The shapes of charge–discharge curves are analogous. The discharge capacity of EMD [20] cathode still delivers 166 mAh g−1 at the 100th cycle. The capacity retention ratio is 99% compared with 2nd cycle (167 mAh g−1), exhibiting the excellent cycle stability. It can be seen that the discharge voltage plateau slightly increases from 1st to 5th cycle, which can be stable after 5 cycles. The result is in accordance with cyclic voltammetry, indicating that the irreversible phase conversion occurs in the first cycle and gradually finishes in several cycles. Figure 6b shows the XRD pattern of as-prepared EMD and EMD cathode after 1st discharge. From Fig. 6b, there is no obvious LixMnO2 peak after 1st discharge and the structure of α∙γ-MnO2 may be maintained, but the peak at 21.06° belonging to γ-MnO2 become wider and shifts to the left, indicating that the interplanar spacing become larger, which may be caused by the contraction/deformation of the large (2 × 2) channels of the α-MnO2 or the expansion of the (2 × 1) channels of the γ-MnO2 after insertion of Li+. During the first few cycles, the deformed channels may make it difficult to remove lithium, leading to the gradual rise of the charging platform and capacity fading. After the first few cycles, the stable microstructure of α∙γ-MnO2 can make better charge and discharge efficiency and cycle performance in the subsequent cycle, which is consistent with the charge–discharge curves and CVs.

Fig. 6
figure 6

a Charge–discharge curves of as-prepared EMD cathode at 1st, 2nd, 5th, 10th, 20th, 50th and 100th cycles, b XRD pattern of as-prepared EMD and EMD cathode after 1st discharge

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Figure 7 shows the cycle performance and rate capability of as-prepared EMD cathode between 4.2 and 1.8 V. From Fig. 7a, the discharge capacity of EMD cathode retains 84 mAh g−1 over 400 cycles with high current density of 755 mA g−1 (1 mA cm−2). The capacity retention ratio is 66% after 400 cycles compared with 2nd cycle (128 mAh g−1). According to research of Haibo Tan [21], the capacity retention ratio of EMD materials at the 100th cycle is 54% compared with 2nd cycle (0.1 mA cm−2). The capacity loss is primarily attributed to the crystal structure transformation by capture of lithium ions during cycling [21, 22]. As far as we know, the key of improving electrochemical performance of EMD is to control the microstructure of the intergrowth of α-MnO2 and γ-MnO2. The interconnected α-MnO2 and γ-MnO2 can make its structure more stable during lithium insertion/extraction reactions. To further evaluate the rate capability of the EMD cathode, current densities from 46 to as high as 755 mA g−1 are applied to the half-cells. Figure 7b shows the discharge capacities at four different current densities. The reversible capacities retains 166, 152, 127 and 114 mAh g−1 of 100th cycle at current densities of 46, 151, 350 and 755 mA g−1, and the reversible capacity retention ratios of 100th cycle are 82, 83, 68 and 68% compared with 1st cycle, respectively. The results suggest that the capacities at each current density are quite stable without notable fading. The excellent rate capability can result from high surface area, mesoporous structure [19] of the EMD cathode and the stable microstructure of α∙γ-MnO2 [16].

Fig. 7
figure 7

a The cycle performance of as-prepared EMD cathode at current density of 755 mA g−1, b the rate capability of as-prepared EMD cathode at different current densities of 46, 151, 350 and 755 mA g−1

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To investigate more information about the kinetic behavior of the Li+ insertion process in the as-prepared EMD cathode, an electrochemical impedance spectroscopy (EIS) test was performed. Figure 8 is Nyquist plots of the EMD cathode and the corresponding simulation result. Inset is the equivalent circuit. The Nyquist plots consist of one semicircle in the high frequency region, which is attributed to the charge transfer impedance (Rct), and an inclined line in the low frequency region is related to Warburg impedance (Zw). The Rct and Zw of the as-prepared EMD cathode is 142 and 143 Ω cm−2, respectively. The low Rct and Zw imply that the as-prepared EMD has the low Li+ diffusion activation energy and high diffusion rate of Li+, resulting in the fast redox reaction.

Fig. 8
figure 8

Nyquist plots of the EMD cathode and the corresponding simulation result (Inset: equivalent circuit)

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4 Conclusions

In summary, EMD (α∙γ-MnO2) with intergrowth of α-MnO2 and γ-MnO2 is prepared by constant current electrodeposition method. The prepared method is simple, which is suitable for large-scale application. The intergrowth of α-MnO2 and γ-MnO2 with interconnected microstructure can inhibit their crystal structure transformation during Li-ion insertion/extraction reactions. The EMD with mesoporous structure has high specific surface area, which can effectively improve the rate capability. In a word, the EMD delivers high initial specific capacity, excellent cycle stability and rate capability, which can be attributed to the stable microstructure of α∙γ-MnO2, high specific surface area, mesoporous structure and smaller particles.