Maowen Xu†
*ab,
Yubin Niu†ab,
Yutao Lic,
Shujuan Baoab and
Chang Ming Liab
aInstitute for Clean Energy & Advanced Materials, Faculty of Materials and Energy, Southwest University, Chongqing 400715, P. R. China. E-mail: xumaowen@swu.edu.cn; Fax: +86-23-68254969; Tel: +86-23-68254969
bChongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies, Chongqing 400715, P. R. China
cTexas Materials Institute, University of Texas Austin, Texas 78712, USA
First published on 30th June 2014
Sodium manganese oxides (SMO) with different crystal structures have been synthesized by high-temperature solid-state reaction (HTSSR), in which both morphology and crystal structure of SMO can be well-controlled by synergistic effects of both the ratio of sodium-to-manganese and the heat-treatment temperature. The material is further used as the cathode in Li/Na-ion batteries, and the experimental results indicated that the performance of SMO as the electrode material mainly depends on its crystal structure, while its morphology only plays an important role in the initial stage. SMO with a 3-D tunnel structure (Na0.44MnO2) and a layer structure (Na0.67MnO2 and Na0.91MnO2) delivers better battery performance.
To overcome these problems, other kinds of electrochemically active manganese oxides based on Na–Mn–O system9–12 have drawn significant attention in recent years, which has a variety of crystal structures, such as P2 and O3 structure. Up to present, many researchers have made us aware of the conclusion that depending on the sodium contents and reaction temperature, either tunnel compounds13 or layered compounds14 can be obtained during synthesis in the Na–Mn–O system. They are often used as sodium-ion battery materials; however, all of these materials undergo at least one or more phase transformations leading to several voltage steps in their electrochemical profiles. These transformations represent major practical issues for Na-ion batteries because they greatly shorten cycle life and reduce rate capabilities and energy density.15 Therefore, some authors used these phases as precursor for the synthesis of lithiated layered manganese oxides via an ion-exchange procedure.16,17 As the electrode material of lithium ion battery, the contained Na ions as pillaring species between MnO2 layers has been proven to limit the magnitude of the structural response of the host lattice during the Li insertion extraction reaction.18 Although there are too many methods to synthesize sodium manganese oxides or other Na compounds (NaxMnyAz, A = transition metal), such as solid-state route,12,19 glycine–nitrate combustion,20 hydrothermal synthesis,21,22 thermo-chemical conversion,23 polymer-pyrolysis,24 coprecipitation technique15 and molten salt synthesis.25 To our knowledge, however, there is no systematically study about the influence of the precursor's morphologies, sodium-to-manganese molar ratio (R) and temperature on the Na–Mn–O system.
In this work, cathode materials NaxMnO2 were synthesized by high temperature solid state reaction (HTSSR), and their morphologies and crystal structures could be tailored by controlling the sintering temperature and R. SMO with 3-D tunnel structure (Na0.44MnO2) and layer structure (Na0.67MnO2 and Na0.91MnO2), which have better battery performance, could be obtained. The metal segregation phenomenon was verified as well.
Mn2O3 spheres were prepared in a procedure described by Li's group with amelioration.27 15 mL ethanol (AR) was added into 150 mL MnSO4 solution with the concentration of 0.0143 mol L−1 under stirring. 150 mL NH4HCO3 solution with concentration of 0.143 mol L−1 was directly put into the mixture solution mentioned above under vigorous stirring. The obtained suspension was maintained at room temperature for 12 h. Finally, the as-obtained precursors were subjected to the same process as described above.
NaxMnO2 was prepared from high-temperature solid-state reaction (HTSSR) by mixing thoroughly NaOH and the as-prepared various morphologies of Mn2O3 in distilled water with R of 0.45, 0.50, 0.70 and 1.05, respectively. After rotary evaporation, the resulting powders were then heated to 600 °C (500 °C, 650 °C and 800 °C) in air at a rate of 2 °C min−1 and held at 600 °C for 10 h followed by cooling. The samples with different morphologies, temperatures and R were obtained. The samples NaxMnO2 with rods, cubics, ovals and spheres were referred as R-SMO, C-SMO, O-SMO and S-SMO, respectively.
Fig. 1 Schematic illustrations of the crystal structures of tunnel compounds (a) and layered compounds (b) in the Na–Mn–O system. |
As shown in Fig. 2, the morphology of precursor has no obviously influence on the original structure of sodium manganese oxide. The sodium manganese oxides with tunnel structure could be mainly obtained and simultaneously accompanied by the formation of layer structure compound. Morphologies and sizes of the as-prepared precursors and the corresponding Na0.50MnO2 were examined using SEM (Fig. 3). As can be seen from the figure, the morphologies of the sample have been maintained to some extent after the reaction. It must be pointed out that the surfaces of all products become rough, and a lot of debris is produced, especially for R-SMO and O-SMO after HTSSR.
Fig. 3 SEM images of different morphologies of the precursors (a, c, e and g) and corresponds to SMO samples (b, d, f and h) obtained from the HTSSR with R of 0.50 at 600 °C. |
Both the morphologies and crystal structures of SMO samples are different with changed reaction temperature and sodium-to-manganese molar ratio (R). Fig. 4d and 5m, n, o and p show XRD patterns and SEM images of the samples obtained with different R at 500 °C, 600 °C, 650 °C and 800 °C, respectively. With increasing temperature, morphologies and crystal structures are certainly dissimilarity with different R. In our system, when the reaction temperature is less than 650 °C, the PSP did not occur during the R range (0.45–1.05), and morphologies can also keep well. When the reaction temperature is above 650 °C, the PSP phenomena could be observed, but also the morphologies of the samples transform partially into rods. The larger the R value, the more rods of SMO. When the R values is greater than or equal to 0.50, there are not only no PSP, but also SMO samples can maintain the morphologies, which is possibly caused by sodium component volatilization or segregation phenomenon at higher sintering temperature.30,31 In order to verify the hypothesis, we analyses the content of sodium and manganese in the SMO samples obtained under different temperatures with R of 0.70, as shown in Table 1. As can be seen from those data, more sodium are lost in SMO samples sintered at high temperatures and the reaction products change toward the low sodium component phase. This is why the phases obtained with high (1.05) and low (0.50) ratios at high temperature are different from those obtained at low temperature. The phase transformation is much easier when the R value is close to 0.50 with the sintering temperature at 650 °C. According to the present analytic results, the following schematic diagram will intuitively show us the influence of sintering temperature and R on the crystal structures of the SMO system (Scheme 1).
wt% | Room temperature | 500 °C | 600 °C | 650 °C | 800 °C |
---|---|---|---|---|---|
Mn | 75.7 | 75.4 | 75.8 | 79.9 | 79.8 |
Na | 24.3 | 24.6 | 24.2 | 20.1 | 20.2 |
To elucidate the dependence of rate capability of the SMO samples, we applied 10, 25, 50, 125 and 250 for charge and discharge currents across the SMO electrode in the voltage range from 2.0 to 4.3 V. The discharge capacity obtained as a function of the cycle number for different discharge–charge rates is shown in Fig. 6c. The SMO samples with different morphologies exhibit analogous rate capability with discharge capacity. The performance of SMO mainly depends on the crystal structure, while the morphology plays an important role in the initial stage. The impurities make the long-term cycling unstable and lead to the capacity fading. In order to obtain the SMO with good electrochemical performance by the HTSSR, crystal structure must be first considered. The tunnel structure Na0.44MnO2 can be obtained for SMO samples sintered at high temperature with R close to 0.5 while monoclinic romanechite Na0.44MnO2 with bad electrochemical performance; the layer structure (P2 phase) Na0.67MnO2 can be maintained for SMO samples with R above 0.67 and the initial discharge capacity of 140 or more obtained at a current density of 10 in the sodium ion batteries, but it is not stable and the capacity decreases rapidly. For this, we have investigated cycle performance and charge–discharge profiles for sodium-ion battery, as shown in Fig. 6e and f. As can be seen from this information, SMO undergo at least one or more phase transformations leading to several voltage steps in their electrochemical profiles and not quite stable under cycling.
In order to further explore the reason why the stability of the material is not good, and decay rapidly, we examined voltammetry and electrochemical impedance spectra of the C-SMO system before and after cycles, respectively. Cyclic voltammograms were recorded for the electrodes in the voltage range of 2.0–4.3 V at a scan rate of 50 μV s−1. Voltammograms for the sample is shown in Fig. 6d. Before cycles, one couple of peaks in cathodic sweep and anodic sweep can be seen. It has an anodic peak at 3.35 V with a corresponding cathodic peak at 2.98 V; these peaks are assigned to a Mn3+/Mn4+ redox couple.32 However, there are two couples of peaks in cathodic sweep and anodic sweep for SMO samples after cycling, another hump anodic peak at 4.15 V with a corresponding cathodic peak at 3.93 V suggests that the transition from layer to spinel-like structure occurs during the charge–discharge process. This behavior is well consistent with the result reported by Bach et al.14 The peak potential difference between the cathodic peak and anodic peak are bigger for samples after cycling, indicating that the increasing irreversibility with the cycle of the electrode.
Meanwhile, the electrochemical impedance measurements were performed in the frequency range of 0.01–100 kHz, and the complex plane impedance plots are shown in Fig. 6b (insert image). Each impedance spectrum consists of two arcs at high and middle frequency and a tail inclined at approximately 45°and 60° to the real axis in the low frequency range. The high frequency semicircle is related to lithium ion migration through the SEI layer covered on the electrode materials, and the middle frequency semicircle is ascribed to charge-transfer through the electrode/electrolyte interface, and the low frequency line is assigned to solid state diffusion of the lithium ion in the materials matrix, namely the Warburg impedance. As can be seen from Fig. 6b, the internal resistance of the electrode diffusion impedance increased after cycling, with the charge and discharge repeatedly, the kinetics of system is almost entirely limited by the rate of chemical diffusion process of Li+ in the host material. The HTSSR is undoubtedly a very rude synthesis method and make sodium ions enter into the matrix lattice by a very overbearing process, which explains why the reaction products obtained covered broken crumbs. The structure becomes unstable even collapses when the sodium ions are extracted in the subsequent test cycle, which hindered the diffusion path of lithium ions.
Footnote |
† These authors contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2014 |