High capacity rechargeable battery electrode based on mesoporous stacked Mn₃O₄ nanosheets

Abstract

Novel stacked nanosheets of Mn₃O₄ thin films were synthesized on a large scale by a facile and efficient low-temperature chemical bath deposition (CBD) route, without templates or surfactants. The aligned nanosheets have a high surface area and a mesoporous structure, which were expected to help to improve the electrochemical property in Li⁺ batteries. This synthetic procedure is straightforward, inexpensive and thus facilitates mass production of Mn₃O₄ stacked nanosheets.

---

The automobile market is presently aimed toward the development of low emission cars, such as hybrid electric vehicles (HEVs) and plug-in hybrid electric vehicles (PHEVs), and of zero emission, full electric vehicles (EVs).¹ Making these sustainable vehicles a reality still depends on the availability of suitable energy storage systems such as high energy lithium ion batteries. Great efforts have been devoted to develop different types of materials with high reversible capacity, long cycle life, and low cost.² Electrochemically active metal oxides such as SnO₂, Fe₂O₃, Co₃O₄, MnO₂ and NiO have long been considered as promising anode materials for lithium ion batteries because of their higher theoretical capacities and high energy densities than those of conventional graphite anodes (372 mAh g⁻¹).³ However, these materials suffer from poor stability due to the pulverization process, although efforts have been taken to improve the cyclability and specific capacity through nanostructuring transition metal oxides.⁴

Among different nanostructured transition metal oxides, manganese oxides are some of the most attractive materials for battery electrode due to their low cost, great environmental compatibility and good specific capacity.⁵ There are different polymorphs of manganese oxide (MnO, Mn₃O₄, MnO₂), among them Mn₃O₄ undergoes relatively little research. However, it plays important roles in energy storage, catalysts, soft magnetic materials etc., and has recently drawn growing attention.⁶ As far as we know, only a few reports were related to the Li-ion battery properties of Mn₃O₄ material. Surprisingly, previous reports of Mn₃O₄ have suggested that the material has poor lithiation activity, despite being isostructural with Co₃O₄. In one report, pure Mn₃O₄ was shown to have a reversible capacity of just 200 mAh g⁻¹, whereas a cobalt-doped sample of Mn₃O₄ exhibited a stable reversible capacity of 400 mAh g⁻¹ with a first coulomb efficiency of 45%.⁷ The activity of electrode materials is primarily determined by the microstructure of active species. Various nanostructures of Mn₃O₄ such as nanorods/nanowires, mesoporous/hollow spheres, nanofibers have been synthesized by different routes, such as solid state reaction, chemical bath deposition and γ-ray irradiation.⁸ Especially, CBD method as a typical solution-based approach has been proven to be an effective and convenient process in preparing various inorganic materials with diverse controllable morphologies and architectures directly on the substrates.⁹

Herein, a mild, simple and scalable strategy has been developed to realize the synthesis of self-assembly of stacked layers of Mn₃O₄ nanosheets (2D). The size and crystalline nature of the stacked nanosheets layers can be tuned easily by changing the concentration of complexing agent. The results indicate that this type of Mn₃O₄ exhibits a high initial capacity of 824 mAh g⁻¹. The Li storage performance of Mn₃O₄ electrodes was much better than those reported for manganese oxide based anode materials. High surface area and a mesoporous structure of well-arranged stacked nanosheets help to improve the electrochemical property in Li⁺ batteries. Although the specific capacity reported here is lower than the previously reported values for Mn₃O₄, this material can be used as new and promising electrode material in battery application due to better stability and high rate capability.

Synthesis of stacked nanosheets of Mn₃O₄ thin films by CBD method is based on immersing the titanium substrates into an aqueous solution of manganese sulfate complexed with hexamethylenetetramine (HMT). Firstly, solutions of 0.1 M manganese sulfate as a source of manganese with two different concentrations of HMT (0.05 and 0.1 M) were prepared. Well cleaned titanium substrates were immersed in these above prepared two baths placed at temperature of 343 K. When the bath attained the temperature of 343 K, the brownish precipitation started in the bath. During precipitation, a heterogeneous reaction occurred and Mn₃O₄ was deposited on the substrates. The deposition time for Mn₃O₄ sample placed at 343 K is kept constant at 3 h for both baths. The films were annealed at 473 K for 2 h, in order to remove hydroxide and to improve the crystallinity of deposited films. The films obtained after the deposition period of 3 h for the HMT concentration of 0.05 and 0.1 M are hereafter symbolized as M1 and M2, respectively. For experimental set up see ESI, Fig. S1.†

Structural identification of Mn₃O₄ films were carried out using a Rigaku Rotalflex RU-200B diffractometer using a Cu-Kα (λ = 1.5418 Å) source. The microstructures of films were examined with a field emission scanning electron microscope (FESEM JEOL JSM-7500F). TEM and HRTEM observations were conducted using JEOL JEM-2100 operated at 200 kV. Electrochemical measurements were conducted in coin cells. The Mn₃O₄ electrode film was directly deposited on to titanium current collector with CBD method. It was cut into circular electrodes. The coin cells were assembled by employing a composite electrode with metallic lithium foil and 1 M LiPF₆ dissolved in a solution of ethylene carbonate/dimethyl carbonate as an electrolyte in a glove box filled with argon. The cell was galvanostatically cycled between 0.01 and 3.0 V vs. Li/Li⁺ at various current densities.

In CBD, when the ionic product of anion and cation exceeds the solubility product, precipitation occurs and ions combine on the substrate and in the solution to form nuclei. Solid phase formation from the solution involves two steps as nucleation and particle growth. Nucleation implies that the clusters of molecules formed undergo rapid decomposition and particles combine to grow up to a certain thickness of the film. Mn₃O₄ thin film depositions take place on titanium substrates via slow hydrolysis of manganese sulphate solution. This can be represented as follows: Here, it is thought that HMT serves as a source of a weak reducing agent. Formaldehyde and ammonia are produced by the hydrolysis of HMT at an elevated temperature¹⁰ as shown in following equations,

C₆H₁₂N₄ + 6H₂O → 6HCHO + 4NH₃ (1)

NH₃ + H₂O ⇔ NH₄OH (2)

Slowly released free Mn²⁺ cations from the stable precursor form ammine complexes of Mn(NH₃)_n²⁺ (n = 1–4) with NH₃ (aq) in moderately basic media (pH ∼8).

MnSO₄ + 2NH₄OH → Mn(OH)₂ + (NH₄)₂SO₄ (3)

Mn²⁺ + nNH₃ ↔ Mn(NH₃)_n²⁺ (4)

6Mn²⁺ + O₂ + 12OH⁻ → 2Mn₃O₄ + 6H₂O (5)

Coordination compound of manganese ammoniate might play a role in controlling not only release velocity of Mn²⁺ ions, but also growth direction of Mn₃O₄ nanocrystal. Here excess dissolved NH₃ acts as a ligand adsorbed on two surfaces of a nanoparticle reduces growth rate of these two planes. As a result, the edge of nanosheets covered with NH₃ molecules grow fastest and stacked nanosheets were formed.

The X-ray diffraction study was carried out for the determination of crystal structure along with structural changes and identification of phases of prepared Mn₃O₄ thin films. Fig. 1 shows XRD patterns of polycrystalline Mn₃O₄ stacked nanosheets prepared at different HMT concentrations (M1 and M2) on titanium substrate. The planes corresponding to (112), (202), (220), (301), (321), (400), (105), (332), (305) and (602) are in good agreement with the standard values of tetragonal hausmannite Mn₃O₄ structure. The calculated lattice constants, a = 5.48 Å, b = 5.48 Å and c = 9.45 Å, are in good agreement with standard lattice constants (a = 5.7621 Å, b = 5.7621 and c = 9.4696 Å, JCPDS card no. 01-75-1560). Also from Fig. 1, it is clearly seen that as the concentration of HMT increases the crystallinity of Mn₃O₄ structure is also increased. The above results showed that the obtained Mn₃O₄ films are phase-pure and well-crystalline material. Mn₃O₄ is well known to have the normal spinel structure in which the Mn²⁺ ions occupy the tetrahedral sites while the Mn³⁺ ions occupy the octahedral sites. Mn₃O₄ also has a stable tetrahedral structure in which the oxygen octahedron is tetragonally distorted due to the Jahn–Teller effect on the Mn³⁺ ion.¹¹

Fig. 1 XRD patterns of Mn₃O₄ samples at two different concentrations of HMT on titanium substrate.

The corresponding morphologies and detailed structures were characterized by both FESEM and TEM analysis, as shown in Fig. 2. Fig. 2(a) shows the SEM image of the products synthesized at low HMT concentration (M1). The products look like dense stacked nanosheets assembled nearly perpendicular to the substrate. Most of the nanosheets are aggregated together (ESI, Fig. S2†) Furthermore, the stacked small nanosheets create abundant space (Fig. 2(b)), which ensures an easier electrolyte ion transport and more superficial electroactive species. When the concentration of HMT was increased, uniform stacking of Mn₃O₄ nanosheets (M2) can be seen in Fig. 2(c). The stacked nanosheets in sample M2 possess a larger size than that of sample M1 due to the increased HMT concentration. As the HMT concentration further increases, the stacked nanosheets grow further with increased thickness and assemble nearly perpendicular to surface (sample M2, Fig. 2(d)). The SEM image suggests that sample M2 possesses a relatively better alignment of stacked nanosheets on the surface than sample M1. However, on the basis of the observations from the TEM images (Fig. 2(e, f)), the products are actually confirmed to be loose and very thin nanosheet which is in good agreement with the XRD results. The well-resolved lattice fringes give an interplanar spacing of 0.42 nm, which is in good agreement with the distance of the (220) plane. It is reasonable that the stacked nanosheets become larger with better crystallinity when the HMT concentration increases, hence resulting in more thick stacked nanostructures.

Fig. 2 SEM images of (a–b) sample M1 and (c–d) sample M2 on titanium substrate, (e and f) TEM and HRTEM images of samples M1, respectively.

A possible growth process and corresponding schematic illustration is proposed in Scheme 1. Initially, the partial hydrolysis of HMT produces formaldehyde and ammonia.¹² Ammonia can form a complex with Mn²⁺, which decreases the free Mn²⁺ ion concentration and reduces the rate of crystal growth. As the reaction proceeds, more HMT hydrolyze leading to the increase of the concentration of OH⁻ in the solution, which is favorable for the formation of thin Mn₃O₄ nanosheets.¹³ Here, HMT decomposes slowly to provide a gradual and controlled supply of ammonia. At the same time, the unreacted HMT may selectively adsorb on certain facets of Mn₃O₄ nanoparticles due to their high binding capacity¹⁴ resulting in the formation of nanosheets. From a thermodynamics perspective, the surface energy of an individual nanosheet is high and therefore its tendency to aggregate perpendicularly to the surface planes to decrease the surface energy is high.¹⁵ Consequently, as the reaction proceeds further, the thin sheets would self-aggregate to form Mn₃O₄ stacked sheet-like nanostructures for minimizing the overall surface energy. Thus as the concentration of HMT increases more thick stacked nanosheets are produced.

Scheme 1 Schematic depiction of the selective formation of Mn₃O₄ stacked nanosheets on the substrate.

In order to investigate the oxidation state of Mn in Mn₃O₄, the XPS analyses were carried out. Fig. 3a displays the full XPS spectrum of Mn₃O₄ stacked nanosheets. As seen from Fig. 3a, the Mn2p_3/2 peak observed at 641.48 eV and the Mn2p_1/2 peak at 653.3 eV indicate that the element Mn is in +2 oxidation state (Mn²⁺).¹⁶ In the present case the observed spin–orbit splitting is 11.82 eV. Similar types of results are reported elsewhere.¹⁷ The O1s spectrum of Mn₃O₄ was analyzed by curve fitting (ESI, Fig. S3†). The spectrum is fitted with three components observed at 529.7 eV, 531.2 eV and 532.5 eV which are related to Mn–O–Mn for the tetravalent oxide, Mn–OH for hydrated trivalent oxide and H–O–H for residual water, respectively. Fig. 3b shows the nitrogen adsorption and desorption isotherms of samples and the pore size distributions of Mn₃O₄ sample (M1). The specific surface areas were calculated by employing the Brunauer–Emmett–Teller (BET) method and the pore size distributions (PSD) were obtained by means of the Barrett–Joyner–Halenda (BJH) equation using the adsorption isotherm branch. The adsorption behaviors of the two samples indicated typical mesoporous materials. The BET specific surface area of samples M1 and M2 were found to be 104 and 82 m² g⁻¹, respectively. The average pore diameter of Mn₃O₄ nanosheets samples are found to be in the mesoporous region, with bimodal pore size distributions. However, the pore size distribution maxima of the samples are centered at ∼3.97 and 14 nm, and 3.75 and 13.97 nm for samples M1 and M2, respectively. These results show that mesopores of different sizes originate from the spaces in interlayers and intralayers of nanosheets. Thus the mesoporous structure of stacked Mn₃O₄ nanosheets provides easy access for the ions in the electrolyte as well as a short diffusion path.¹⁸

Fig. 3 (a) Full XPS spectrum of well-ordered Mn₃O₄ nanosheets (sample M1). (b) Nitrogen adsorption–desorption isotherms and corresponding pore size distribution of M1 and M2 samples.

Fig. 4a shows first discharge–charge profiles of both Mn₃O₄ samples (M1 and M2) at 0.1 C-rate in 1.0 M LiPF₆ EC/DEC (1 : 1 by volume). The general features of the discharge–charge profiles are consistent with other transition metal oxide anodes that undergo conversion reactions, especially other manganese oxides.¹⁹ For both the samples (M1 and M2), in the first discharge curve, the potential decreases steeply to approximately 1.5 V followed by a weak slope corresponding to lithium insertion into the crystal structure of Mn₃O₄. When the potential decreases to about ∼0.3 V, an obvious potential plateau is observed that extends to approximately 746 mAh g⁻¹ (sample M1) which is very near to the theoretical capacity of 937 mAh g⁻¹ for the conversion reaction to Mn and Li₂O as follows: Mn₃O₄ + 8Li⁺ + 8e⁻ → 3Mn(0) + 4 Li₂O. The sloping region at ∼1.3 V during the charge process indicates the reversible nature of this electrochemical reaction. The overall discharge capacity reaches 824 mAh g⁻¹ and 709 mAh g⁻¹ for samples M1 and M2, respectively, with the overall discharge capacity approximately consistent with the sloping voltage below 0.30 V. These results reveal that the morphology plays a significant role in the discharge performance, in terms of the lithium intercalation in the crystal structure of Mn₃O_4. When the surface area is high, the lithium ion intercalation capacity and affinity will be greatly enhanced, since the diffusion lengths of the lithium ions are greatly shortened.²⁰ Therefore, materials with small size and high surface area always yield high discharge capacities. As has been reported for a number of transition metal oxides, the second discharge profile is different from the first one. Only one voltage plateau was observed around 0.7 V, which was distinctly higher and more sloped than the main feature of the first discharge (see Fig. 4b). This may indicate that the lithiation reaction of the second cycle is easier and is often the feature of a single phase reaction. The results showed that the Mn₃O₄ stacked nanosheets can serve as a potential anode material for lithium-ion batteries. This could result from the different preparation method that we developed, which gives rise to a different morphology. Although we also observed a significant irreversible capacity loss after the first cycle, such behavior is common to virtually all systems based on conversion reactions. Fig. 4c shows the discharge–charge profiles of stacked Mn₃O₄ nanosheets at different C-rates. It is seen that Mn₃O₄ shows 298 mAh g⁻¹ capacity at high C-rate which is attributed to stacked layers of nanosheets.

Fig. 4 (a) Discharge–charge profiles of two Mn₃O₄ samples. (b) First and second discharge–charge profiles of sample M1 at current rate of 0.1 C (c) Discharge–charge profiles of sample M1 at different C-rates. (c) Cycle performance of Mn₃O₄ at a current rate of 0.1 C in 1.0 M LiPF₆ EC/DEC (1 : 1 by volume).

Fig. 4d presents the changes in discharge–charge capacity of Mn₃O₄ with cycling at a rate of 0.1 C. It can be seen that the cycling is quite stable. The reversible capacity exhibits almost no significant fading after 60 cycles. Better electrochemical performance of the synthesized Mn₃O₄ could be attributed to its special stacked architecture. The nanoscale provides large surface area, which improves the utilization of active material, and the loose porous nanosheets-like structure allows lithium ions to transfer easily in and out. The material is also able to accommodate the strain induced by possible volume change during discharge–charge process and maintain the integrity of the electrode. This may be responsible for the good cycling stability.

Conclusions

In conclusion, simple and scalable CBD method has been developed to synthesize well-arranged stacked Mn₃O₄ nanosheets with different sizes directly on to titanium substrate. Based on these stacked nanostructures, the relationships between the specific area and the electrochemical performances have been discussed in our work. Among two samples, the sample synthesized at low concentration of HMT, with an especially high specific area (104 m² g⁻¹), uniform pore size distribution and poor crystallinity, demonstrated the good specific capacity as well as cycling stability. Electrochemical measurements indicate that the well-ordered stacked Mn₃O₄ nanosheets shows a high initial capacity 824 mAh g⁻¹ and a stabilized reversible capacity of around 400 mAh g⁻¹ upon cycling. The electrochemical reaction becomes highly reversible after the first discharge, and the reversible capacity exhibits almost no significant fading after 60 cycles. The excellent electrochemical performance of this specific Mn₃O₄ could be attributed to its special stacked architecture.

Acknowledgements

One of the authors (D.P.D.) appreciates the award of a Humboldt Fellowship of the Alexander von Humboldt Foundation (AvH), Germany.

References

1. (a) Advances in Lithium-ion Batteries, ed. W. A. van Schalkwijk and B. Scrosati, Kluwer Academic/Plenum Publishers, New York, 2002; (b) J. Hassoun, K. S. Lee, Y. K. Sun and B. Scrosati, J. Am. Chem. Soc., 2011, 133, 3139.
2. (a) P. Poizot, S. Laruelle, S. Grugeon, L. Dupont and J. M. Tarascon, Nature, 2000, 407, 496; (b) C. Wang, Y. Zhou, M. Y. Ge, X. B. Xu, Z. L. Zhang and J. Z. Jiang, J. Am. Chem. Soc., 2010, 132, 46.
3. (a) C. Kim, M. Noh, M. Choi, J. Cho and B. Park, Chem. Mater., 2005, 17, 3297; (b) N. Li, G. Liu, C. Zhen, F. Li, L. Zhang and H.-M. Cheng, Adv. Funct. Mater., 2011, 21, 1717.
4. C. Kim, M. Noh, M. Choi, J. Cho and B. Park, Chem. Mater., 2005, 17, 3297.
5. (a) B. Li, G. Rong, Y. Xie, L. Huang and C. Feng, Inorg. Chem., 2006, 45, 6404; (b) C. D. Lokhande, D. P. Dubal and O. S. Zoo, Curr. Appl. Phys., 2011, 11, 255.
6. (a) L. Ji, A. Medford and X. Zhang, J. Mater. Chem., 2009, 19, 5593; (b) Y. G. Wang, L. Chen, F. Li, H. M. Xiong and Y. Y. Xia, Chem. Mater., 2007, 19, 2095.
7. D. Pasero, N. Reeves and A. R. West, J. Power Sources, 2005, 141, 156.
8. (a) P. Li, C. Y. Nan, Z. Wei, J. Lu, Q. Peng and Y. D. Li, Chem. Mater., 2010, 22, 4232; (b) M. Fang, X. L. Tan, M. Liu, S. H. Kang, X. Y. Hu and L. D. Zhang, CrystEngComm, 2011, 13, 4915.
9. (a) V. R. Shinde, H. S. Shim, T. P. Gujar, H. J. Kim and W. B. Kim, Adv. Mater., 2008, 20, 1008; (b) D. P. Dubal, V. J. Fulari and C. D. Lokhande, Microporous Mesoporous Mater., 2012, 51, 511.
10. X. Gao, X. Li and W. Yu, J. Phys. Chem. B, 2005, 109, 1155.
11. (a) H. Y. Xu, S. L. Xu, X. D. Li, H. Wang and H. Yan, Appl. Surf. Sci., 2006, 252, 4091; (b) A. S. Fritsch, J. Sarrias, A. Rousset and G. U. Kulkarni, Mater. Res. Bull., 1998, 33, 1185.
12. (a) L. E. Greene, B. D. Yuhas, M. Law, D. Zitoun and P. D. Yang, Inorg. Chem., 2006, 45, 7535; (b) H. Wang, J. Q. Xu and Q. Y. Pan, CrystEngComm, 2010, 12, 1280.
13. C. Z. Yuan, X. G. Zhang, L. H. Su, B. Gao and L. F. Shen, J. Mater. Chem., 2009, 19, 5772.
14. (a) F. Chen, R. J. Zhou, L. G. Yang, N. Liu, M. Wang and H. Z. Chen, J. Phys. Chem. C, 2008, 112, 1001; (b) X. D. Gao, X. M. Li and W. D. Yu, J. Phys. Chem. B, 2005, 109, 1155.
15. L. X. Yang, Y. J. Zhu, H. Tong, Z. H. Liang and W. W. Wang, Cryst. Growth Des., 2007, 7, 2716.
16. P. S. Shah, J. D. Holmes, K. P. Johnston and B. A. Korgel, J. Phys. Chem., 2002, 106, 2545.
17. R. J. Iwanowski, M. H. Heinonen and E. Janik, Chem. Phys. Lett., 2004, 387, 110.
18. M. W. Xu, L. B. Kong, W. J. Zhou and H. L. Li, J. Phys. Chem. C, 2007, 111, 19141.
19. (a) Q. Fan and M. S. Whittingham, Electrochem. Solid-State Lett., 2007, 10, A48; (b) J. Zhao, Z. Tao, J. Liang and J. Chen, Cryst. Growth Des., 2008, 8, 2799; (c) K. Zhong, X. Xia, B. Zhang, H. Li, Z. Wang and L. Chen, J. Power Sources, 2010, 195, 3300.
20. H. Wang, L.-F. Cui, Y. Yang, H. Sanchez Casalongue, J. T. Robinson, Y. Liang, Y. Cui and H. Dai, J. Am. Chem. Soc., 2010, 132, 13978.

---

Footnote

† Electronic supplementary information (ESI) available: Experimental set up, SEM, XPS spectrum. See DOI: 10.1039/c2ra21806b

---