CHEMSUSCHEM COMMUNICATIONS DOI: 10.1002/cssc.201402138
A High-Capacity, Low-Cost Layered Sodium Manganese Oxide Material as Cathode for Sodium-Ion Batteries Shaohua Guo,[a, b] Haijun Yu,*[a] Zelang Jian,[a] Pan Liu,[c] Yanbei Zhu,[d] Xianwei Guo,[c] Mingwei Chen,[c] Masayoshi Ishida,[b] and Haoshen Zhou*[a, b, e] A layered sodium manganese oxide material (NaMn3O5) is introduced as a novel cathode materials for sodium-ion batteries. Structural characterizations reveal a typical Birnessite structure with lamellar stacking of the synthetic nanosheets. Electrochemical tests reveal a particularly large discharge capacity of 219 mAh g1 in the voltage rang of 1.5–4.7 V vs. Na/Na + . With an average potential of 2.75 V versus sodium metal, layered NaMn3O5 exhibits a high energy density of 602 Wh kg1, and also presents good rate capability. Furthermore, the diffusion coefficient of sodium ions in the layered NaMn3O5 electrode is investigated by using the galvanostatic intermittent titration technique. The results greatly contribute to the development of room-temperature sodium-ion batteries based on earthabundant elements.
Lithium-ion batteries (LIBs) have been applied in many fields because they offer both a long service lifetime and a high energy density, useful for portable electronic devices.[1–4] However, worldwide demand for these batteries will grow very rapidly if they are adopted in large-scale applications such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and smart grids. The limited availability and high cost of lithium will not satisfy such market demands.[5–7] Similar to lithium, sodium can [a] S. Guo, Dr. H. Yu, Dr. Z. Jian, Prof. H. Zhou Energy Technology Research Institute National Institute of Advanced industrial Science and Technology (AIST) AIST Tsukuba Central 2, Tsukuba, Ibaraki 305-8568 (Japan) Fax: (+ 81) 29-861-3489 E-mail: [email protected] [email protected] [b] S. Guo, Prof. M. Ishida, Prof. H. Zhou Graduate School of System and Information Engineering University of Tsukuba Tennoudai 1-1-1, Tsukuba, 305-8573 (Japan) [c] Dr. P. Liu, Dr. X. Guo, Prof. M. Chen WPI Advanced Institute for Materials Research Tohoku University Sendai 980-8577 (Japan) [d] Dr. Y. Zhu National Metrology Institute of Japan National Institute of Advanced Industrial Science and Technology Umezono 1-1-1, Tsukuba, 305-8568 (Japan) [e] Prof. H. Zhou National Laboratory of Solid State Microstructures & Department of Energy Science and Engineering Nanjing University Nanjing 210093 (PR China) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201402138.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
also be used to develop so-called sodium-ion batteries (SIBs), operated at room temperature. Compared to lithium, sodium is very abundant (2.64 wt % on Earth), and widely available in different forms (e.g., sea salt, rock salt). Thus, SIBs are considered a promising alternative to LIBs for the next generation of rechargeable batteries.[8–12] Recently, the deintercalation and intercalation of sodium in layered NaxTMO2 materials (where the transition metal (TM) was Ti, V, Cr, Mn, Fe, Co, Ni) was reported.[13–18] The oxides have high theoretical capacities (up to 250 mAh g1) with a singleelectron redox reaction, during which the valence of the transition metals varies between + 3 and + 4. However, the high theoretical capacities have yet to be fully achieved experimentally. The O3-type compounds NaFeO2 NaCoO2, NaCrO2, and NaVO2 are electrochemically active, but their available reversible capacity is limited to 121 mAh g1; less than half of their theoretical capacities. Furthermore, several compounds of sodium with mixed transition metals[19–28] of layered structure, such as NaTi0.5Ni0.5O2,[24] NaxNi0.6Co0.4O2,[25] Na2/3Ni1/3Mn2/3O2,[26] Na[Ni1/3Fe1/3Mn1/3]O2,[29] and Na0.85Li0.17Ni0.21Mn0.64O2[28] attract much attention. Our group recently reported that among these materials, NaTi0.5Ni0.5O2 shows a suitable capacity of 120 mAh g1, a long service life, and good rate capability.[24] Based on the abundance of their constituent elements in the Earth’s crust and their large specific capacity, layered NaMnO systems are very interesting for SIB applications. So far, O3-NaMnO2, and P2-Na0.7MnO2.25 and P2-Na0.6MnO2 as well as tunnel Na0.44MnO2 have been developed as cathodes for SIBs.[16, 30–33] P2-Na0.6MnO2 delivers a high capacity of about 150 mAh g1 in the first cycle, but its capacity quickly decays to about 70 mAh g1 over 10 cycles.[30] The monoclinic O3NaMnO2 also shows extremely weak capacity retention, despite a large capacity of 197 mAh g1.[16] Although Na0.44MnO2 with tunnel structure possesses good cycle performance and rate capability, its capacity is limited to only about 130 mAh g1.[31, 33, 34] That is, the identification of high-performance, low-cost (nontoxic) sodium manganese oxide cathodes with large capacity remains a challenge. Herein, a layered NaMn3O5 material with a Birnessite structure[35] is prepared by a simple redox reaction followed by hydrothermal treatment.[36] The electrochemical properties of this material serving as cathode in a sodium-ion battery at room temperature are reported for the first time. The layered NaMn3O5 electrode delivers a large capacity of 219 mAh g1, which is about 72.5 % of its theoretical capacity, and exhibits a good rate performance (115 mAh g1 at 5C rate) and a suitable capacity retention. The diffusion coefficient of sodium ChemSusChem 2014, 7, 2115 – 2121
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CHEMSUSCHEM COMMUNICATIONS ions in the layered NaMn3O5 cathode is investigated by galvanostatic intermittent titration. The structures of layered NaMn3O5·nH2O and NaMn3O5 were determined by powder X-ray diffraction (XRD) crystallography, and are shown in Figure 1. The black curve in Figure 1 a shows the XRD pattern of the synthesized NaMn3O5·n H2O precursor.
Figure 1. (a) XRD patterns of layered NaMn3O5·n H2O and NaMn3O5. The insets show the structures of the samples, in which the white and black circle represent lattice water and sodium ions, respectively. (b) TG/DTA curves of layered NaMn3O5·n H2O with Birnessite structure.
www.chemsuschem.org to be Na0.36Mn1.1O1.8 (simplified as NaMn3O5) by inductively coupled plasma (ICP) measurements. The molar ratio of oxide to manganese is less than 2:1, which agrees with results published by Givanoli et al. and Manceau et al.[38–41] Thermogravimetric/differential thermal analysis (TG/DTA) profiles are shown in the Figure 1 b. The profiles can be divided into three stages. Two obvious weight loss steps occur below 200 8C. The first occurs at around 110 8C, and corresponds to the removal of surface water.[37] The second weight loss step, with a steep slope, continues from 110 8C to 150 8C. No further weight loss is obvious until 500 8C. The weight loss between 110 8C and 150 8C can be assigned to the removal of the lattice water, and it is reasonable to consider that almost all lattice water is eliminated by the thermal treatment at 200 8C. The weight loss around 500 8C is due to a phase transition, from a layered structure to spinel Mn3O4.[37] The TG/DTA results agree well with earlier results.[37] Therefore, it is reasonable to assume that the thermal treatment at 200 8C for 2 h eliminates all of the lattice water occupying the sites in between the sheets of MnO6 octahedra. This elimination of lattice water leads to much improved electrochemical properties. The detailed crystal structure of the layered NaMn3O5 material was further investigated by scanning transmission electron microscopy (STEM) and selected-area electron diffraction (SAED). Figure 2 a shows that NaMn3O5 clearly displays a sheetlike morphology, composed of lamellar stacks of the synthetic nanosheets (circled). The inset of Figure 2 a shows the corresponding SAED pattern. The two diffraction rings indicate a polycrystalline structure and can be indexed to the (111) and (310) crystal planes of layered material with a Birnessite structure, which agrees well with the characterization by XRD. In addition, HRTEM characterization (Figure 2 b, c) further suggests the typical Birnessite structure, and the interlayer spacings of 0.71 nm and 0.24 nm respond to (001) and (111) fringes, respectively. The presence of sodium as well as manganese and oxygen was also confirmed by electron energy loss spectroscopy (EELS) mappings (Figure 2 d, e, g; Supporting Information, Figure S1). Electrochemical properties of the samples were characterized by galvanostatic charge and discharge tests in the voltage range 1.5–4.7 V. Figure 3 shows typical charge–discharge curves of layered NaMn3O5 at 0.1C rate (defining 1C as
All peaks of this material match well to the Birnessite structure (space group C2m), showing a basal spacing of 0.72 nm along the c axis, where crystal water and sodium ions locate in the sheets formed by MnO6 octahedra.[35] In order to minimize the effect of water on the electrochemical performance in sodium-ion batteries,[37] the pristine material was dehydrated by thermal treatment at 200 8C for 2 h. The peaks of the dehydrated layered material are similar to those of the hydrated sample, showing only a minor change towards higher angles with a decreased basal spacing of 0.71 nm Figure 2. (a) TEM images of layered NaMn3O5. Inset: corresponding SAED. (b, c) HRTEM bright-field images of with (Figure 1 a, red curve). Its chemi- different interlayer spacing. (d) TEM image of layered NaMn3O5, and corresponding STEM-EELS chemical maps for cal composition was determined (e) Na, (f) Mn, and (g) O. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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CHEMSUSCHEM COMMUNICATIONS 200 mA g1). The reversible capacity reaches over 200 mAh g1 in the first few cycles (Figure 3 a). Although the charge capacity is only 103 mAh g1, the largest discharge capacity is 219 mAh g1 for the initial cycle (Supporting Information, Figure S2). This allows to conclude that using a partially inserted sodium compound as anode material is an acceptable choice for the design of full sodium ion batteries. The discharge capacity of 219 mAh g1 is much higher than values obtained with other layered sodium manganese oxides that have P2 and O3 structures. The result indicates that more sodium ions can be reversibly inserted into/out of the layered NaMn3O5 during charge–discharge processes. As far as we know, this is the largest discharge capacity obtained for SIB cathodes prepared by chemical methods.[42, 43] The corresponding dQ/dV curves, also given in Figure 3 b, suggest a good reversibility of layered NaMn3O5 in the charge and discharge processes.[34] Figure 3 c shows the discharge profiles of the layered NaMn3O5 at different discharge rates after charging at 0.1C rate. The specific discharge capacities (and corresponding capacity retention) at 0.2C, 0.5C, 1C, 2C, and 5C discharge rate are 210 (95.9 %), 197 (90 %), 165 (75.3 %), and 115 (52.5 %) mAh g1, respectively. Notably, a large discharge capacity of 115 mAh g1could be obtained at the high rate of 5C, indicating excellent rate capability OK. The capacity retention of the layered NaMn3O5 is shown in Figure 3 d, and it reveals a suitable capacity retention of 70 % after 20 cycles. The capacity fading may be caused by disproportionation of Mn3 + into Mn4 + and dissolved Mn2 + .[16, 30] The capacity curve of the layered NaMn3O5 becomes less steep with increasing cycle number, showing (to our best knowledge) a better cycle stability than other reported layered NaMnO-system electrode materials. Moreover, except for the initial activation process, the Coulombic efficiency of the cathode material is near to 100 % in the charge–discharge process (Figure 3 d).
www.chemsuschem.org Tests according to the galvanostatic intermittent titration technique (GITT) were carried out to evaluate Na + diffusion in the layered NaMn3O5 material after the initial activation process. Figure 4 a shows the GITT curves of layered NaMn3O5 during the second charge process between 1.5 V and 4.7 V. The chemical diffusion coefficient of Na + (DNa + ) was calculated according to Equation (1), derived by Weppner and Huggins as follows:[42, 44, 45] DNaþ ¼
2 4 mB V M 2 DEs pffiffiffi ðt L2 Þ p MB S tðdEt =d tÞ
ð1Þ
where VM is the molar volume of the compound, and MB and mB are the molecular weight and the mass of dehydrated layered NaMn3O5, respectively. S is the interface area between the active material and electrolyte. L is radius of the active particle. Figure 4 b shows a typical t versus E profile for a single titration. Because E versus t1/2 shows straight-line behavior over the entire period of current flux, as shown in Figure 4 c, Equation (1) can be further simplified to: DNaþ ¼
4 mB VM 2 DEs 2 p MB S DEt
ð2Þ
Based on Equation (2) and the GITT measurements, the DNa + at varied voltages during the whole charge process could be determined. These results are shown in Figure 4 d. Obviously, the Na + diffusion coefficient of sodium layered manganese oxide trends towards decrease during the whole charge process. This trend can be divided into three stages: the diffusion coefficient is about 1013 cm2 s1 at the beginning of charge process (before 3.06 V), and decreases to around 1014 cm2 s1 until 3.64 V, and then continues to decrease to about 1015 cm2 s1 near the end of the charge process. The variation of sodium ion diffusion during the whole charge process may be due to complex dynamics of electrochemical desodiation. This can also be inferred from cyclic voltammetry (CV) curves (Figure 3 b): there are many cathodic peaks in the whole charge process, including many extremely complex sodium-insertion processes. These not only include Na + diffusion but also possible structural transformations. Their further investigation is an ongoing work and will be detailed in future publications. The available capacity and voltage variations of the layered cathode materials for SIBs are summarized in Figure 5. The available reversible capacity of the layered NaMn3O5 reaches Figure 3. (a) Charge–discharge profile of layered NaMn3O5, and (b) corresponding dQ/dV plots. (c) Discharge pro219 mAh g1 with an average files at rates of 0.1C, 0.2C, 0.5C, 1C, 2C, and 5C after charging at 0.1C. (d) Cycle profiles of layered NaMn3O5 at voltage of 2.75 V vs. Na/Na + . a charge–discharge rate of 0.1C. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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www.chemsuschem.org other manganese-based layered materials. The energy density of the material is ca. 602 Wh kg1, presenting the highest value among all reported layered transitional metal oxides. Thus, it is expected that the layered NaMn3O5 material is excellently suited for application in sodiumion battery cathodes. Further investigations towards the reaction mechanism and improvement of the cycle performance by doping and coating are in progress.
Figure 4. (a) GITT curves of layered NaMn3O5 for the second charge process between 1.5 V and 4.7 V (current density: 20 mA g1 corresponding to 0.1C rate, time interval: 4 h). (b) t vs E profile of layered NaMn3O5 for a single GITT titration. (c) Linear behavior of E vs t1/2. (d) Diffusion coefficients of Na + in layered NaMn3O5 at different charge states.
Figure 5. Comparison of reversible capacity and operating voltage ranges of the layered sodium insertion materials. The energy density was calculated on the basis of the voltage versus metallic sodium for simplicity. Na0.44MnO2 is also shown for comparison.
The energy density is estimated to be 602 Wh kg1, which is not only much higher than the value of the reported cathode materials for SIBs, but also far exceeds some commercial cathode materials for LIBs such as LiFePO4 (about 530 Wh kg1) and LiMn2O4 (about 450 Wh kg1).[46, 47] These characteristics are highly beneficial to increase the energy density of the electrode materials in SIBs. Summarizing, we report the synthesis and application of a layered NaMn3O5 material with Birnessite structure. When used as cathode in a sodium-ion battery (SIB), the layered NaMn3O5 material shows excellent electrochemical performance, especially in terms of a large discharge capacity (219 mAh g1) and good rate capability (a discharge capacity of 115 mAh g1at a high rate of 5C). These values far surpass 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Experimental Section
Synthesis of layered NaMn3O5·n H2O and NaMn3O5 : The layered NaMn3O5·n H2O with Birnessite structure was synthesized by a simple redox reaction and subsequent hydrothermal treatment, as documented in an earlier publication.[36] Briefly, a mixed solution of 0.6 m NaOH and 2 m H2O2 was quickly poured into a 0.3 m Mn(NO3)2 solution and strongly stirred for 30 min. The obtained precipitates were transferred, together with an appropriate amount of 2 m NaOH solution, to a 120 mL Teflon-lined stainless steel autoclave, and kept in an oven at 150 8C for 16 h. The resulting product was thoroughly filtered with deionized water until the pH of the filtrate was equal to 7, and fully dried at 110 8C for 24 h. In this way we obtained as-prepared layered NaMn3O5·n H2O. To exclude the effect of lattice water on the battery performance, a certain amount of as-prepared NaMn3O5·n H2O was thermally treated at 200 8C for 2 h with a temperature ramp of 2 8C min1. The layered NaMn3O5 material was quickly sealed to avoid water absorption in air until use. Characterization: X-ray powder diffraction (XRD) measurements were performed on a Bruker D8 Advance Diffractometer instrument using CuKa radiation (40 kV and 40 mA) with a scanning speed of 1.0 min1 in steps of 0.028. The samples were also characterized by Cs-corrected transmission electron microscopy (TEM, JEOL, JEM-2010, and JEM-2100F), N2 adsorption/desorption (TriStar 3000, SHIMADA), and thermogravimetric analysis (TG-DTA2010SAG4H, BRUKER). Electrochemical Measurement: The charge–discharge tests were carried by using CR2032 coin-type cells, consisting of a cathode and sodium metal anode separated by a glass fiber film. The cathode electrodes consisted of active material, teflonized acetylene black (AB), and polytetrafluoroethylene (PTFE) in a weight ratio of 80:15:5. Pellets were pressed in the form of disks approximately 3 mg in mass and 7 mm in diameter (the loading mass was around 1.95 mg cm2), then dried under vacuum at about 110 8C for 5 h before cell assembly. The cells were assembled in a glove-box filled with dried argon gas. The electrolyte was 1 m NaPF6 in ethylene carbonate/diethyl carbonate (EC/DEC, 1:1 (v/v)). The galvanostatic charge–discharge tests were performed by using a Hokuto Denko HJ1001SD8 battery tester at different current densities within a cut-off voltage window of 1.5–4.7 V vs. Na/Na + at 25 8C after a rest for 12 h. In the rate test, a 1C rate corresponded to about 0.39 mA cm2 (200 mA g1). ChemSusChem 2014, 7, 2115 – 2121
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CHEMSUSCHEM COMMUNICATIONS Acknowledgements This work was supported by the Innovative Basic Research toward Creation of High-performance Battery in Funding Program for World-leading Innovative R&D on Science and Technology. P.L., X.G., and M.C. are sponsored by JST-CREST “Phase Interface Science for Highly Efficient Energy Utilization”, JST (Japan). Keywords: cathodes · electrochemistry · energy storage · layered compounds · sodium-ion batteries [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
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Received: March 5, 2014 Published online on June 11, 2014
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A High-Capacity, Low-Cost Layered Sodium Manganese Oxide Material as Cathode for Sodium-Ion Batteries Shaohua Guo,[a, b] Haijun Yu,*[a] Zelang Jian,[a] Pan Liu,[c] Yanbei Zhu,[d] Xianwei Guo,[c] Mingwei Chen,[c] Masayoshi Ishida,[b] and Haoshen Zhou*[a, b, e] A layered sodium manganese oxide material (NaMn3O5) is introduced as a novel cathode materials for sodium-ion batteries. Structural characterizations reveal a typical Birnessite structure with lamellar stacking of the synthetic nanosheets. Electrochemical tests reveal a particularly large discharge capacity of 219 mAh g1 in the voltage rang of 1.5–4.7 V vs. Na/Na + . With an average potential of 2.75 V versus sodium metal, layered NaMn3O5 exhibits a high energy density of 602 Wh kg1, and also presents good rate capability. Furthermore, the diffusion coefficient of sodium ions in the layered NaMn3O5 electrode is investigated by using the galvanostatic intermittent titration technique. The results greatly contribute to the development of room-temperature sodium-ion batteries based on earthabundant elements.
Lithium-ion batteries (LIBs) have been applied in many fields because they offer both a long service lifetime and a high energy density, useful for portable electronic devices.[1–4] However, worldwide demand for these batteries will grow very rapidly if they are adopted in large-scale applications such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and smart grids. The limited availability and high cost of lithium will not satisfy such market demands.[5–7] Similar to lithium, sodium can [a] S. Guo, Dr. H. Yu, Dr. Z. Jian, Prof. H. Zhou Energy Technology Research Institute National Institute of Advanced industrial Science and Technology (AIST) AIST Tsukuba Central 2, Tsukuba, Ibaraki 305-8568 (Japan) Fax: (+ 81) 29-861-3489 E-mail: [email protected] [email protected] [b] S. Guo, Prof. M. Ishida, Prof. H. Zhou Graduate School of System and Information Engineering University of Tsukuba Tennoudai 1-1-1, Tsukuba, 305-8573 (Japan) [c] Dr. P. Liu, Dr. X. Guo, Prof. M. Chen WPI Advanced Institute for Materials Research Tohoku University Sendai 980-8577 (Japan) [d] Dr. Y. Zhu National Metrology Institute of Japan National Institute of Advanced Industrial Science and Technology Umezono 1-1-1, Tsukuba, 305-8568 (Japan) [e] Prof. H. Zhou National Laboratory of Solid State Microstructures & Department of Energy Science and Engineering Nanjing University Nanjing 210093 (PR China) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201402138.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
also be used to develop so-called sodium-ion batteries (SIBs), operated at room temperature. Compared to lithium, sodium is very abundant (2.64 wt % on Earth), and widely available in different forms (e.g., sea salt, rock salt). Thus, SIBs are considered a promising alternative to LIBs for the next generation of rechargeable batteries.[8–12] Recently, the deintercalation and intercalation of sodium in layered NaxTMO2 materials (where the transition metal (TM) was Ti, V, Cr, Mn, Fe, Co, Ni) was reported.[13–18] The oxides have high theoretical capacities (up to 250 mAh g1) with a singleelectron redox reaction, during which the valence of the transition metals varies between + 3 and + 4. However, the high theoretical capacities have yet to be fully achieved experimentally. The O3-type compounds NaFeO2 NaCoO2, NaCrO2, and NaVO2 are electrochemically active, but their available reversible capacity is limited to 121 mAh g1; less than half of their theoretical capacities. Furthermore, several compounds of sodium with mixed transition metals[19–28] of layered structure, such as NaTi0.5Ni0.5O2,[24] NaxNi0.6Co0.4O2,[25] Na2/3Ni1/3Mn2/3O2,[26] Na[Ni1/3Fe1/3Mn1/3]O2,[29] and Na0.85Li0.17Ni0.21Mn0.64O2[28] attract much attention. Our group recently reported that among these materials, NaTi0.5Ni0.5O2 shows a suitable capacity of 120 mAh g1, a long service life, and good rate capability.[24] Based on the abundance of their constituent elements in the Earth’s crust and their large specific capacity, layered NaMnO systems are very interesting for SIB applications. So far, O3-NaMnO2, and P2-Na0.7MnO2.25 and P2-Na0.6MnO2 as well as tunnel Na0.44MnO2 have been developed as cathodes for SIBs.[16, 30–33] P2-Na0.6MnO2 delivers a high capacity of about 150 mAh g1 in the first cycle, but its capacity quickly decays to about 70 mAh g1 over 10 cycles.[30] The monoclinic O3NaMnO2 also shows extremely weak capacity retention, despite a large capacity of 197 mAh g1.[16] Although Na0.44MnO2 with tunnel structure possesses good cycle performance and rate capability, its capacity is limited to only about 130 mAh g1.[31, 33, 34] That is, the identification of high-performance, low-cost (nontoxic) sodium manganese oxide cathodes with large capacity remains a challenge. Herein, a layered NaMn3O5 material with a Birnessite structure[35] is prepared by a simple redox reaction followed by hydrothermal treatment.[36] The electrochemical properties of this material serving as cathode in a sodium-ion battery at room temperature are reported for the first time. The layered NaMn3O5 electrode delivers a large capacity of 219 mAh g1, which is about 72.5 % of its theoretical capacity, and exhibits a good rate performance (115 mAh g1 at 5C rate) and a suitable capacity retention. The diffusion coefficient of sodium ChemSusChem 2014, 7, 2115 – 2121
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CHEMSUSCHEM COMMUNICATIONS ions in the layered NaMn3O5 cathode is investigated by galvanostatic intermittent titration. The structures of layered NaMn3O5·nH2O and NaMn3O5 were determined by powder X-ray diffraction (XRD) crystallography, and are shown in Figure 1. The black curve in Figure 1 a shows the XRD pattern of the synthesized NaMn3O5·n H2O precursor.
Figure 1. (a) XRD patterns of layered NaMn3O5·n H2O and NaMn3O5. The insets show the structures of the samples, in which the white and black circle represent lattice water and sodium ions, respectively. (b) TG/DTA curves of layered NaMn3O5·n H2O with Birnessite structure.
www.chemsuschem.org to be Na0.36Mn1.1O1.8 (simplified as NaMn3O5) by inductively coupled plasma (ICP) measurements. The molar ratio of oxide to manganese is less than 2:1, which agrees with results published by Givanoli et al. and Manceau et al.[38–41] Thermogravimetric/differential thermal analysis (TG/DTA) profiles are shown in the Figure 1 b. The profiles can be divided into three stages. Two obvious weight loss steps occur below 200 8C. The first occurs at around 110 8C, and corresponds to the removal of surface water.[37] The second weight loss step, with a steep slope, continues from 110 8C to 150 8C. No further weight loss is obvious until 500 8C. The weight loss between 110 8C and 150 8C can be assigned to the removal of the lattice water, and it is reasonable to consider that almost all lattice water is eliminated by the thermal treatment at 200 8C. The weight loss around 500 8C is due to a phase transition, from a layered structure to spinel Mn3O4.[37] The TG/DTA results agree well with earlier results.[37] Therefore, it is reasonable to assume that the thermal treatment at 200 8C for 2 h eliminates all of the lattice water occupying the sites in between the sheets of MnO6 octahedra. This elimination of lattice water leads to much improved electrochemical properties. The detailed crystal structure of the layered NaMn3O5 material was further investigated by scanning transmission electron microscopy (STEM) and selected-area electron diffraction (SAED). Figure 2 a shows that NaMn3O5 clearly displays a sheetlike morphology, composed of lamellar stacks of the synthetic nanosheets (circled). The inset of Figure 2 a shows the corresponding SAED pattern. The two diffraction rings indicate a polycrystalline structure and can be indexed to the (111) and (310) crystal planes of layered material with a Birnessite structure, which agrees well with the characterization by XRD. In addition, HRTEM characterization (Figure 2 b, c) further suggests the typical Birnessite structure, and the interlayer spacings of 0.71 nm and 0.24 nm respond to (001) and (111) fringes, respectively. The presence of sodium as well as manganese and oxygen was also confirmed by electron energy loss spectroscopy (EELS) mappings (Figure 2 d, e, g; Supporting Information, Figure S1). Electrochemical properties of the samples were characterized by galvanostatic charge and discharge tests in the voltage range 1.5–4.7 V. Figure 3 shows typical charge–discharge curves of layered NaMn3O5 at 0.1C rate (defining 1C as
All peaks of this material match well to the Birnessite structure (space group C2m), showing a basal spacing of 0.72 nm along the c axis, where crystal water and sodium ions locate in the sheets formed by MnO6 octahedra.[35] In order to minimize the effect of water on the electrochemical performance in sodium-ion batteries,[37] the pristine material was dehydrated by thermal treatment at 200 8C for 2 h. The peaks of the dehydrated layered material are similar to those of the hydrated sample, showing only a minor change towards higher angles with a decreased basal spacing of 0.71 nm Figure 2. (a) TEM images of layered NaMn3O5. Inset: corresponding SAED. (b, c) HRTEM bright-field images of with (Figure 1 a, red curve). Its chemi- different interlayer spacing. (d) TEM image of layered NaMn3O5, and corresponding STEM-EELS chemical maps for cal composition was determined (e) Na, (f) Mn, and (g) O. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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CHEMSUSCHEM COMMUNICATIONS 200 mA g1). The reversible capacity reaches over 200 mAh g1 in the first few cycles (Figure 3 a). Although the charge capacity is only 103 mAh g1, the largest discharge capacity is 219 mAh g1 for the initial cycle (Supporting Information, Figure S2). This allows to conclude that using a partially inserted sodium compound as anode material is an acceptable choice for the design of full sodium ion batteries. The discharge capacity of 219 mAh g1 is much higher than values obtained with other layered sodium manganese oxides that have P2 and O3 structures. The result indicates that more sodium ions can be reversibly inserted into/out of the layered NaMn3O5 during charge–discharge processes. As far as we know, this is the largest discharge capacity obtained for SIB cathodes prepared by chemical methods.[42, 43] The corresponding dQ/dV curves, also given in Figure 3 b, suggest a good reversibility of layered NaMn3O5 in the charge and discharge processes.[34] Figure 3 c shows the discharge profiles of the layered NaMn3O5 at different discharge rates after charging at 0.1C rate. The specific discharge capacities (and corresponding capacity retention) at 0.2C, 0.5C, 1C, 2C, and 5C discharge rate are 210 (95.9 %), 197 (90 %), 165 (75.3 %), and 115 (52.5 %) mAh g1, respectively. Notably, a large discharge capacity of 115 mAh g1could be obtained at the high rate of 5C, indicating excellent rate capability OK. The capacity retention of the layered NaMn3O5 is shown in Figure 3 d, and it reveals a suitable capacity retention of 70 % after 20 cycles. The capacity fading may be caused by disproportionation of Mn3 + into Mn4 + and dissolved Mn2 + .[16, 30] The capacity curve of the layered NaMn3O5 becomes less steep with increasing cycle number, showing (to our best knowledge) a better cycle stability than other reported layered NaMnO-system electrode materials. Moreover, except for the initial activation process, the Coulombic efficiency of the cathode material is near to 100 % in the charge–discharge process (Figure 3 d).
www.chemsuschem.org Tests according to the galvanostatic intermittent titration technique (GITT) were carried out to evaluate Na + diffusion in the layered NaMn3O5 material after the initial activation process. Figure 4 a shows the GITT curves of layered NaMn3O5 during the second charge process between 1.5 V and 4.7 V. The chemical diffusion coefficient of Na + (DNa + ) was calculated according to Equation (1), derived by Weppner and Huggins as follows:[42, 44, 45] DNaþ ¼
2 4 mB V M 2 DEs pffiffiffi ðt L2 Þ p MB S tðdEt =d tÞ
ð1Þ
where VM is the molar volume of the compound, and MB and mB are the molecular weight and the mass of dehydrated layered NaMn3O5, respectively. S is the interface area between the active material and electrolyte. L is radius of the active particle. Figure 4 b shows a typical t versus E profile for a single titration. Because E versus t1/2 shows straight-line behavior over the entire period of current flux, as shown in Figure 4 c, Equation (1) can be further simplified to: DNaþ ¼
4 mB VM 2 DEs 2 p MB S DEt
ð2Þ
Based on Equation (2) and the GITT measurements, the DNa + at varied voltages during the whole charge process could be determined. These results are shown in Figure 4 d. Obviously, the Na + diffusion coefficient of sodium layered manganese oxide trends towards decrease during the whole charge process. This trend can be divided into three stages: the diffusion coefficient is about 1013 cm2 s1 at the beginning of charge process (before 3.06 V), and decreases to around 1014 cm2 s1 until 3.64 V, and then continues to decrease to about 1015 cm2 s1 near the end of the charge process. The variation of sodium ion diffusion during the whole charge process may be due to complex dynamics of electrochemical desodiation. This can also be inferred from cyclic voltammetry (CV) curves (Figure 3 b): there are many cathodic peaks in the whole charge process, including many extremely complex sodium-insertion processes. These not only include Na + diffusion but also possible structural transformations. Their further investigation is an ongoing work and will be detailed in future publications. The available capacity and voltage variations of the layered cathode materials for SIBs are summarized in Figure 5. The available reversible capacity of the layered NaMn3O5 reaches Figure 3. (a) Charge–discharge profile of layered NaMn3O5, and (b) corresponding dQ/dV plots. (c) Discharge pro219 mAh g1 with an average files at rates of 0.1C, 0.2C, 0.5C, 1C, 2C, and 5C after charging at 0.1C. (d) Cycle profiles of layered NaMn3O5 at voltage of 2.75 V vs. Na/Na + . a charge–discharge rate of 0.1C. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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www.chemsuschem.org other manganese-based layered materials. The energy density of the material is ca. 602 Wh kg1, presenting the highest value among all reported layered transitional metal oxides. Thus, it is expected that the layered NaMn3O5 material is excellently suited for application in sodiumion battery cathodes. Further investigations towards the reaction mechanism and improvement of the cycle performance by doping and coating are in progress.
Figure 4. (a) GITT curves of layered NaMn3O5 for the second charge process between 1.5 V and 4.7 V (current density: 20 mA g1 corresponding to 0.1C rate, time interval: 4 h). (b) t vs E profile of layered NaMn3O5 for a single GITT titration. (c) Linear behavior of E vs t1/2. (d) Diffusion coefficients of Na + in layered NaMn3O5 at different charge states.
Figure 5. Comparison of reversible capacity and operating voltage ranges of the layered sodium insertion materials. The energy density was calculated on the basis of the voltage versus metallic sodium for simplicity. Na0.44MnO2 is also shown for comparison.
The energy density is estimated to be 602 Wh kg1, which is not only much higher than the value of the reported cathode materials for SIBs, but also far exceeds some commercial cathode materials for LIBs such as LiFePO4 (about 530 Wh kg1) and LiMn2O4 (about 450 Wh kg1).[46, 47] These characteristics are highly beneficial to increase the energy density of the electrode materials in SIBs. Summarizing, we report the synthesis and application of a layered NaMn3O5 material with Birnessite structure. When used as cathode in a sodium-ion battery (SIB), the layered NaMn3O5 material shows excellent electrochemical performance, especially in terms of a large discharge capacity (219 mAh g1) and good rate capability (a discharge capacity of 115 mAh g1at a high rate of 5C). These values far surpass 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Experimental Section
Synthesis of layered NaMn3O5·n H2O and NaMn3O5 : The layered NaMn3O5·n H2O with Birnessite structure was synthesized by a simple redox reaction and subsequent hydrothermal treatment, as documented in an earlier publication.[36] Briefly, a mixed solution of 0.6 m NaOH and 2 m H2O2 was quickly poured into a 0.3 m Mn(NO3)2 solution and strongly stirred for 30 min. The obtained precipitates were transferred, together with an appropriate amount of 2 m NaOH solution, to a 120 mL Teflon-lined stainless steel autoclave, and kept in an oven at 150 8C for 16 h. The resulting product was thoroughly filtered with deionized water until the pH of the filtrate was equal to 7, and fully dried at 110 8C for 24 h. In this way we obtained as-prepared layered NaMn3O5·n H2O. To exclude the effect of lattice water on the battery performance, a certain amount of as-prepared NaMn3O5·n H2O was thermally treated at 200 8C for 2 h with a temperature ramp of 2 8C min1. The layered NaMn3O5 material was quickly sealed to avoid water absorption in air until use. Characterization: X-ray powder diffraction (XRD) measurements were performed on a Bruker D8 Advance Diffractometer instrument using CuKa radiation (40 kV and 40 mA) with a scanning speed of 1.0 min1 in steps of 0.028. The samples were also characterized by Cs-corrected transmission electron microscopy (TEM, JEOL, JEM-2010, and JEM-2100F), N2 adsorption/desorption (TriStar 3000, SHIMADA), and thermogravimetric analysis (TG-DTA2010SAG4H, BRUKER). Electrochemical Measurement: The charge–discharge tests were carried by using CR2032 coin-type cells, consisting of a cathode and sodium metal anode separated by a glass fiber film. The cathode electrodes consisted of active material, teflonized acetylene black (AB), and polytetrafluoroethylene (PTFE) in a weight ratio of 80:15:5. Pellets were pressed in the form of disks approximately 3 mg in mass and 7 mm in diameter (the loading mass was around 1.95 mg cm2), then dried under vacuum at about 110 8C for 5 h before cell assembly. The cells were assembled in a glove-box filled with dried argon gas. The electrolyte was 1 m NaPF6 in ethylene carbonate/diethyl carbonate (EC/DEC, 1:1 (v/v)). The galvanostatic charge–discharge tests were performed by using a Hokuto Denko HJ1001SD8 battery tester at different current densities within a cut-off voltage window of 1.5–4.7 V vs. Na/Na + at 25 8C after a rest for 12 h. In the rate test, a 1C rate corresponded to about 0.39 mA cm2 (200 mA g1). ChemSusChem 2014, 7, 2115 – 2121
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CHEMSUSCHEM COMMUNICATIONS Acknowledgements This work was supported by the Innovative Basic Research toward Creation of High-performance Battery in Funding Program for World-leading Innovative R&D on Science and Technology. P.L., X.G., and M.C. are sponsored by JST-CREST “Phase Interface Science for Highly Efficient Energy Utilization”, JST (Japan). Keywords: cathodes · electrochemistry · energy storage · layered compounds · sodium-ion batteries [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
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Received: March 5, 2014 Published online on June 11, 2014
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