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Mg-doping for improved long-term cyclability of layered Na-ion cathode materials – The example of P2-type NaxMg0.11Mn0.89O2
Journal of Power Sources 282 (2015) 581e585
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Mg-doping for improved long-term cyclability of layered Na-ion cathode materials e The example of P2-type NaxMg0.11Mn0.89O2 Daniel Buchholz a, b, c, Christoph Vaalma b, c, Luciana Gomes Chagas a, b, c, Stefano Passerini b, c, * a b c
University of Muenster, Institute of Physical Chemistry, Corrensstraße 46, 48149 Muenster, Germany Helmholtz Institute Ulm (HIU), Helmholtzstrasse 11, 89081 Ulm, Germany Karlsruhe Institute of Technology (KIT), P.O. Box 3640, 76021 Karlsruhe, Germany
h i g h l i g h t s Mg-doping as strategy for improving the electrochemical behaviour of P-type oxides. Synthesis of NaxMg0.11Mn0.89O2 via co-precipitation method and solid state reaction. NaxMg0.11Mn0.89O2 has P2-type structure and consists of m-sized particles. Promising long-term cycling performance for 200 cycles. 125 mAh g1 delivered capacity and 93.8% capacity retention (10the100th cycle).
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 December 2014 Received in revised form 5 February 2015 Accepted 13 February 2015 Available online 18 February 2015
Sodium-ion batteries (SIBs) are establishing themselves as a low-cost alternative to the widespread lithium-ion technology, a trend that is exemplified by the use of aluminium as anode current collector. In order to be in line with this philosophy, environmentally friendly, abundant and cheap materials need to be used in order to provide a complementary rather than competing battery technology other than lithium-ion. With the same scope in mind, herein we present the structural and electrochemical characterization of P2-type NaxMg0.11Mn0.89O2 material to demonstrate the effectiveness of Mg-doping for the development of future layered cathode materials. Of particular interest is the effect on the long-term cyclability (200 cycles), which has not been reported, yet. As shown in the manuscript, a Mg content as low as 11% in the MO2 layer leads to a smoothing of the potential profile, very high coulombic efficiencies exceeding 99.5% at 12 mA g1 and a stable long-term cycling behaviour. © 2015 Elsevier B.V. All rights reserved.
Keywords: Sodium ion battery Cathode Doping Magnesium Layered structure Long-term cycling
1. Introduction In the last few years, the academic community has expressed a strong interest on sodium-ion batteries (SIBs), as shown by the increasing number of publications since 2009 [1]. Listed below lithium in the periodic table, sodium provides a comparably high potential and enables large low-cost advantages, such as the use of aluminium (instead of pricey copper) as anode current collector [1,2]. Moreover, superior electrochemical performance to other
* Corresponding author. Helmholtz Institute Ulm (HIU), Helmholtzstrasse 11, 89081 Ulm, Germany. E-mail address: [email protected] (S. Passerini). http://dx.doi.org/10.1016/j.jpowsour.2015.02.069 0378-7753/© 2015 Elsevier B.V. All rights reserved.
secondary batteries (with the exception of the lithium-ion technology) has already been demonstrated by several research works [3]. This competitive advantage to other battery chemistries leads the assumption that SIBs might be a suitable technology for applications where price is an issue, e.g. stationary energy storage. However, in order to enter the market, SIBs should follow their own philosophy, e.g., using environmentally friendly, abundant, nontoxic and cheap materials, in accordance with the economical concept of diversification. SIBs might appear as a young research field but, in fact, lithiumion and sodium-ion chemistries have been studied in parallel for decades. In the past, the main motivation behind research in the sodium-ion based chemistry was to find sodium precursors that could be used to synthesize the lithium analogues via ion-exchange
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reactions [4]. A great variety of materials have already been structurally but not electrochemically characterized, in terms of their sodium-ion chemistry. Investigations on sodium ion battery materials started in the mid-1970s with the studies of Parant et al. on the physical properties of Na0.44MnO2 [5]. As an analogue to the very prominent LiCoO2 [6], Delmas et al. investigated NaCoO2, already setting the focus specifically on sodium intercalation in 1981 [7]. Following the characterization of layered transition metal oxides comprising only one transition metal, such as nickel, cobalt or manganese for the sodium- and lithium-based systems, strategies were developed to solve the specific problems observed in the parent materials and reduce material costs [8]. With the commercialization of the portable devices powered with lithium-ion batteries by Sony in 1991, the main focus of battery research was set on the optimization of the existing lithium technology. This led to the intermixing of transition metals like cobalt, nickel and manganese, as a way to improve the poor cyclability observed for LiNiO2 and LiMnO2 and to reduce the content of the expensive cobalt, which finally led to the commercially famous LiNi1/3Co1/3Mn1/3O2 [8]. A similar trend of intermixing transition metals in order to combine their positive properties was also observed in sodium-ion cathodes, and several compounds similar to the lithium analogues were proposed, revealing good electrochemical performances [9e11]. Recently, several groups showed an interest in using environmentally friendly and low cost materials like Fe and Mg as dopant or redox active species in sodium-based layered oxides [12e14]. Magnesium in particular, an abundant and cheap element, represents a very interesting component for P-type layered transition metal oxides in SIBs, and for this reason a series of Na0.67Mn1xMgxO2 (0 x 0.3) materials has already been synthesized and structurally characterized. Billaud et al. reported the electrochemical properties of a series of Mg-doped Na0.67Mn1xMgxO2 (0 x 0.2) materials [13]. The effect of quenching in the solid state synthesis and an investigation on the galvanostatic cycling and the (de-)sodiation process was performed. The doping Mg content of x ¼ 0.05 led to smooth potential profiles, while higher Mg concentrations (e.g., x ¼ 0.2) resulted in improved cycling stability but also lower discharge capacities. Consequently, an intermediate Mg content appears to be a reasonable compromise. Most recently, Komaba et al. reported the structural and electrochemical characterization of P2-type Na0.67Mn0.72Mg0.28O2, exhibiting a very high specific capacity of 220 mAh g1, unfortunately accompanied by a high capacity fade per cycle [14]. Herein, we present a study on the structural and electrochemical properties of P2-type NaxMg0.11Mn0.89O2 with a particular focus on the long-term cycling performance, not reported for Ptype layered oxides, only containing manganese and magnesium in the MO2-layer. We believe this work to provide very useful information about this material class and to highlight the importance of Mg doping for the development of next-generation layered cathode materials for SIBs.
dispersed in an aqueous solution of sodium hydroxide (0.76 eq of NaOH per mole of Mg0.11Mn0.89(OH)2). Water was slowly removed by rotary evaporation. After drying and grinding, the material was pre-annealed in an open-air muffle furnace at 500 C for 5 h (heating rate: 10 C min1). Following, the reddish pre-annealed material was ground and then, as a powder, subjected to the high temperature annealing at 900 C for 6 h (heating rate: 5 C min1), using an open-air muffle oven, resulting in dark brownish powder. Finally, about 1 g of the pristine material was stirred in 20 mL of distilled water at 25 C for 5 min. The suspension was then filtered, washed with 80 mL of distilled water and dried at 100 C in air for 24 h. Afterwards, the material was ground, screened over a 45 mm sieve and finally stored under inert atmosphere.
2. Experimental part
The stoichiometry of the NaxMg0.11Mn0.89O2 material was verified via ICP-OES which revealed a Na: Mg: Mn ratio of 27.8: 11.1: 88.9. These metal values are in a very good agreement with the targeted stoichiometry. The lower sodium content, with respect to the 0.76 eq mol1 added during the synthesis is associated to sodium evaporation during the annealing process, sodium loss during the mixing process and the water treatment, which is leading to the dissolution of impurities (e.g. sodium carbonate) formed during the solid state synthesis in air, but also a partial chemical desodiation. Although the water treatment led to an improved electrochemical performance for analogous P-type layered oxides, [9,10] it also has to be considered that a sodium content below x 0.33 results in a
2.1. Synthesis of NaxMg0.11Mn0.89O2 The material was synthesized via a two-step solid-state reaction with sodium hydroxide (NaOH, Aldrich >98%) and a mixed manganeseemagnesium hydroxide precursor. The precursor was prepared by co-precipitating the aqueous solution of the two metal acetate tetrahydrate salts (MnAc2$4 H2O and MgAc2$4 H2O; Aldrich >98%) in a stoichiometric ratio of 11:89 with sodium hydroxide (50% excess). After extensive rinsing with distilled water, the precipitate was dried at 100 C overnight. The dried material was then
2.2. Material characterization The composition of the synthesized materials, in terms of sodium content and magnesium to manganese ratio, was determined by inductively coupled plasma optical emission spectrometry (ICPOES) with a ULTIMA 2 (Horiba, Kyoto, Japan) instrument with axial plasma viewing. The crystalline structure was characterized by Xray diffraction (XRD) using the Cu Ka radiation on the Bruker D8 Advance (Germany) in the 2q range from 10 to 90 . Lattice parameters were determined by Rietveld refinement with TOPAS software. The particle size distribution and particle morphology was evaluated with the help of a high-resolution scanning electron microscope (FE-SEM, Zeiss Auriga). 2.3. Electrode preparation & cell assembly Electrodes were made from slurries, which dry composition was 85 wt.% active material, 10 wt.% carbon black Super C65 (TIMCAL), and 5 wt.% polyvinylidene fluoride (PVDF e Kynar Flex 761A, Arkema Group), using N-methyl-2-pyrrolidone (NMP) as solvent. The slurries were homogenized via ball milling and, afterwards, casted on aluminium foil and dried at 80 C overnight. Disc electrodes with 12 mm diameter were cut, pressed (6.67 tons cm2 for 10 s), and finally dried at 120 C in a glass oven (Büchi B585) under vacuum for 24 h. The active material mass loading in the electrodes was about 2 mg cm2. Electrodes were assembled into threeelectrode Swagelok® cells with a glass fibre separator (Whatman) soaked with 1 M NaPF6 (99%, Alfa Aesar) in propylene carbonate (PC e UBE, Japan) and sodium metal as counter and reference electrodes. The sodium was cut from sodium chunks (99.8%, Acros Organics), rolled, pressed and finally punched on the current collector. Cells were cycled galvanostatically at different constant currents from 12 mA g1 to 610 mA g1 between 4.4 and 1.5 V vs. Na/Naþ, or 4.6e1.5 V vs. Na/Naþ, at 20 ± 2 C using battery tester (Maccor series 4000, USA). All potentials reported in this work refer to the Na/Naþ couple. 3. Results and discussion
D. Buchholz et al. / Journal of Power Sources 282 (2015) 581e585
higher sensitivity towards water [15]. The structural and morphological properties of the NaxMg0.11Mn0.89O2 material were investigated by X-ray diffraction and Scanning electron microscopy (Fig. 1). The diffractogram reveals, that NaxMg0.11Mn0.89O2 can be indexed as a P2-type layered material, with space group P63/mmc. The corresponding lattice parameters obtained via Rietveld refinement are a ¼ 2.8713(8) and c ¼ 11.237(7). Details of the Rietveld refinement can be found in the Supplementary Information (SI-1). The very weak reflection at 22.1 2q originates from the ordering of the magnesium and manganese cations in the transition metal layer. The low intensity of this reflections is in accordance with Billaud et al. for NaxMn1xMgxO2 (0 x<¼0.2) materials, slowly cooling down to room temperature after the solid state reaction [13]. The presence of the hydrated phase is evidenced by the diffraction peaks at 12.6 and 25.4 , corresponding to the (002) and (004) reflections of the hydrated phase. The SEM image confirms the hexagonal layered structure and reveals NaxMg0.11Mn0.89O2 of being composed by flake-like particles with an average size of 0.5e2 mm, which is typical for P2-type layered materials. Summarizing, the successful synthesis of P2-type layered NaxMg0.11Mn0.89O2 in terms of composition, structure and morphology is confirmed by ICP-OES, XRD and SEM. The differential capacity plots of NaxMg0.11Mn0.89O2, recorded at 12 mA g1 reveals more information about the (de-)sodiation process and the electrochemical performance (Fig. 2). Four peaks, located at about 2.2 V, 3.0 V, 3.5 V and 4.2 V, are observed during charge. During the consecutive cycling the peak at 2.2 V decreases in intensity and shifts to higher potentials. In addition, also the peak at 4.2 V decreases while all other peaks remain constant (3.0 V) or even increase in intensity (3.5 V). Similarly, during the sodiation four corresponding peaks at about 2.1 V, 2.9 V, 3.4 V and 3.9 V (very weak) are observed. The peak at the lowest potential decreases in intensity and shifts to higher potentials. The very broad peak at 2.9 V and the very weak peak at 3.9 V remain constant, while interestingly, the small peak at 3.4 V increases upon cycling. It is important to mention that the peak couples at higher potentials, e.g. 3.5 V/3.4 V and 4.2 V/3.9 V, nicely demonstrate the sequence of the structural changes upon the (de-) sodiation process. The peak at 3.5 V is known to be caused by the phase transition from the P2 to a stacked fault OP4 structure [12,13,16,17]. It is interesting to mention, that the peak related to the final O2 phase transition decreases in intensity upon cycling
Fig. 1. X-ray diffraction pattern, corresponding Rietveld refinement and SEM image of pristine NaxMg0.11Mn0.89O2 material. Asterisks in the diffractogram mark the diffraction peaks of the hydrated phase. The reflection originating from the superlattice ordering of Mg2þ/Mn4þ is indicated by the dashed black box.
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Fig. 2. Differential capacity plots for the 3rd, 4th, 5th, 10th, 20th, 50th and 100th cycle of NaxMg0.11Mn0.89O2 galvanostatically cycled at 12 mA g1 in the potentials range of 4.4e1.5 V vs. Na/Naþ. Counter and reference electrode: Na. Electrolyte: 1 M NaPF6 in PC. Temperature: 20 ± 2 C.
while the peak related to the P2-OP4 phase transition simultaneously increases. The appearance of several peaks in the potential range of 1.5 Ve3.3 V might confuse as only manganese remains as redox active species but their presence can be explained with the general structural properties of P2-type materials. In P2-type structures two different sodium sites of different energy are present, although all sodium cations are prismatically coordinated [4]. In the energetically stabilized site the sodium cation is superimposed by additional two oxygen anions of the next coordination sphere. Therefore, this site is nearly exclusively occupied at lower sodium contents [18]. Instead, in the second site the sodium cation is superimposed by a metal cation (in this case manganese or magnesium) of the MO2 layer, leading to additional coulombic repulsion. Therefore, the (de-)intercalation of sodium cations from these two different sites occurs at different potentials. Another consequence is the specific distribution and arrangement of the sodium cations and vacant positions within the sodium layer due to the coulombic repulsion of the sodium cations. Consequently, the appearance of several peaks in, e.g., a cyclic voltammogram or differential capacity plot can be considered a specific intrinsic property of a P2-type material, independently of the composition of the transition metal layer [9,12,18e21]. An overview about the electrochemical performance of NaxMg0.11Mn0.89O2 is given in Fig. 3. Despite the presence of the hydrated phase, NaxMg0.11Mn0.89O2 reveals an overall promising electrochemical performance, additionally highlighted by the high average coulombic efficiency (99.6%) and the good long-term cycling stability. As for other P2type materials a marked initial capacity fade is observed as evidenced by the capacity decreasing from 174 mAh g1 to 151 mAh g1 and 129 mAh g1 in the 1st, 2nd and 10th cycles, respectively. In more detail, about 45 mAh g1, or 25.7% of the initial capacity are lost within the first ten cycles. This phenomenon is mostly related with the presence of the hydrated phase although also a higher initial capacity fading was observed in analogues P2type materials like NaxNi0.22Co0.11Mn0.66O2 or NaxNi0.23Fe0.13Mn0.63O2 but of smaller extent [9,10]. However, a very stable subsequent cycling is observed from cycle 10 onwards, which leads to discharge capacities of 121 mAh g1 and 96.8 mAh g1 after 100 and 200 cycles, respectively. This corresponds to a rather high capacity retention of 93.8% and 75.0% (vs. 10th cycle), respectively, which is even more remarkable when considering the stress caused by the two intermediate current rate tests. Interestingly, these latter tests reveal the material to exhibit good rate capability
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Fig. 3. a) Galvanostatic cycling at 12 mA g1 for NaxMg0.11Mn0.89O2 material. Cycles 21e45 and 65e90 illustrate the electrochemical behaviour for five cycles each at increased currents of 24 mA g1, 61 mA g1, 122 mA g1, 244 mA g1 and 610 mA g1 b) Potential profile of the initial charge and discharge process at 12 mA g1. Cut-off limits: 4.4e1.5 V vs. Na/Naþ. Counter and reference electrode: Na. Electrolyte: 1 M NaPF6 in PC. Temperature: 20 ± 2 C.
although the highly conductive metals like cobalt or nickel are absent. In more detail, discharge capacities of 116 mAh g1, 103 mAh g1, 93 mAh g1, 82 mAh g1 and 63 mAh g1 were obtained at current rates of 24 mA g1, 61 mA g1, 122 mA g1, 244 mA g1 and 610 mA g1, respectively. The low initial charge capacity of 55 mAh g1 (0.20 eq of Naþ removal) is in accordance with the detected sodium content via ICP-OES and was also observed by Billaud et al. [13]. The initial discharge capacity for NaxMg0.11Mn0.89O2 corresponds to the intercalation of 0.64 eq of Naþ per mole. The potential profiles of the first cycle are similar to those of P2-type Na0.67Mn0.72Mg0.28O2, but no distinct plateau at 4.2 V is observed upon charge [14]. This plateau is probably related to the oxidation of oxygen anions and is essential for the access of capacities of up to 220 mAh g1. The observed lower initial discharge capacity of 174 mAh g1 for NaxMg0.11Mn0.89O2, thus, is in accordance with the absence of the high potential plateau, probably only occurring at higher magnesium contents. The potential profiles of some selected cycles during the galvanostatic cycling at low currents and during the current rate test are depicted in Fig. 4. Interestingly, the features typically observed in the potential profiles of P2-type materials are not observed in the rather smooth low-rate potential profiles of NaxMg0.11Mn0.89O2. This clearly reveals one effect of the Mg-doping, as the (de-)sodiation appears to occur as a solid solution process. The smooth profile might further offer advantages regarding the coupling of a P2-type layered oxide
Fig. 4. a) Charge and discharge potential profiles recorded during the galvanostatic cycling for a) the 2nd, 5th, 10th, 50th, 100th and 200th cycle at 12 mA g1 and for b) 12 mA g1 (18th cycle), 24 mA g1 (23rd cycle), 61 mA g1 (27th cycle), 122 mA g1 (32nd cycle), 244 mA g1 (37th cycle) and 610 mA g1 (42nd cycle) of NaxMg0.11Mn0.89O2 material. Cut-off limits: 4.4e1.5 V (vs. Na/Naþ). Reference and counter electrodes: Na. Electrolyte: 1 M NaPF6 in PC. Temperature: 20 ± 2 C.
cathode with a suitable anode in a sodium ion battery as the cell voltage would change less rapid compared to other P2-type layered oxides [13]. Upon charge, only a very weak feature at about 4.0 V is observed, indicating the phase transition to O2-type to occur only in minor extent, which is in contrast to other P2-type layered oxides like NaxNi0.22Co0.11Mn0.66O2. At higher currents of about 122 mA g1, the plateau fully vanishes as the two phase reaction is kinetically slow. However, the major capacity fade is clearly related to the continuous shrinkage of the smooth potential plateau between 2.1 and 3.9 V. This might indicate the continuous loss of the redox active species, i.e. the ongoing active material degradation process most likely due to manganese dissolution into the electrolytic solution as a result of the disproportion reaction associated with the Jahn-Teller distorted Mn3þ cation [20]. The optimization of the electrolytic solution to prevent the dissolution or the modification of the active material (e.g., applying carbonaceous or metal oxide coatings) might be strategies to improve the cycling performance. To investigate if higher capacities can be obtained, the upper cut-off voltage was increased to 4.6 V (Fig. 5). The comparison illustrates that, indeed, higher initial discharge capacities are accessible as the material delivers 178 mAh g1 between 4.4 V and 1.5 V and 190 mAh g1 between 4.6 V and 1.5 V. However, the higher initial discharge capacity is also accompanied by a higher capacity fade, a worse performance at increased current rates, and lower coulombic efficiencies (about 99.2% at 12 mA g1). These features are certainly correlated to a more pronounced active
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material by application of coatings. The successful utilization of magnesium-doped layered oxides in Na-ion batteries also necessitates an increased initial sodium content in the material and an improved average discharge potential, e.g., via the implementation of other suitable transition metals like nickel. Acknowledgements L. G. C. acknowledges the Conselho Nacional de Desenvolvigico (CsF-CNPq, Brazil) for the financial mento Científico e Tecnolo support. TIMCAL is acknowledged for kindly providing SuperC 65. Appendix A. Supplementary data Fig. 5. Galvanostatic cycling at 12 mA g1 for NaxMg0.11Mn0.89O2 material. Cycles 21e45 and 65e90 illustrate the electrochemical behaviour during increased currents of 24 mA g1, 61 mA g1, 122 mA g1, 244 mA g1, and 610 mA g1. Cut-off limits: 4.6e1.5 V or 4.4e1.5 V vs. Na/Naþ. Counter and reference electrode: Na. Electrolyte: 1 M NaPF6 in PC. Temperature: 20 ± 2 C.
material degradation process as the electrolyte was found to be rather stable in this potential range [22]. As a consequence, already after 20 cycles NaxMg0.11Mn0.89O2 exhibits a poorer electrochemical performance, when cycled between 4.6 V and 1.5 V. 4. Conclusions P2-type NaxMg0.11Mn0.89O2 was successfully synthesized. Formula, structure and morphology were confirmed via ICP-OES, XRD and SEM, respectively. The electrochemical characterization focused on the investigation of the long-term cyclability of P2-type NaxMg0.11Mn0.89O2 and revealed high cycling stability for the galvanostatic cycling between 4.4 and 1.5 V. In more detail, a high capacity retention of 93.8% (from 10th to 100th cycle) and 75.0% (from 10th to 200th cycle) as well as very high coulombic efficiencies, exceeding 99.6%, were obtained. Reversible specific capacities of about 125 mAh g1 were obtained at low specific currents (12 mA g1) and, surprisingly, the material additionally exhibited good rate capability (e.g. 82 mAh g1 at 244 mA g1), despite the absence of transition metals like nickel or cobalt. Attempts to access higher reversible capacities by increasing the upper cut-off potential (4.6 V vs. Na/Naþ) succeeded, but led to worse cycling stability and coulombic efficiency. The continuous shortening of the potential profiles between 2.1 and 3.2 V upon long-term cycling indicates for a continuous active material degradation process, most likely due to manganese dissolution. Future work should be dedicated to counter this issue via optimization of the electrolytic solution and modification of the active
Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.02.069. References [1] M. Dahbi, N. Yabuuchi, K. Kubota, K. Tokiwa, S. Komaba, Phys. Chem. Chem. Phys. 16 (2014) 15007. lez, [2] V. Palomares, P. Serras, I. Villaluenga, K.B. Hueso, J. Carretero-Gonza T. Rojo, Energy Environ. Sci. 5 (2012) 5884. [3] M.D. Slater, D. Kim, E. Lee, C.S. Johnson, Adv. Funct. Mater. 23 (2013) 947. [4] J.M. Paulsen, R.A. Donaberger, J.R. Dahn, Chem. Mater. 12 (2000) 2257. [5] J. Parant, R. Olazcuaga, M. Devalette, C. Fouassier, P. Hagenmuller, J. Solid State Chem. 3 (1971) 1. [6] K. Mizushima, P.C. Jones, P.J. Wiseman, J.B. Goodenough, Mater. Res. Bull. 15 (1980) 783. [7] C. Delmas, J. Braconnier, C. Fouassier, P. Hagenmuller, Solid State Ionics 3e4 (1981) 165. [8] M.S. Whittingham, Chem. Rev. 104 (2004) 4271. [9] I. Hasa, D. Buchholz, S. Passerini, B. Scrosati, J. Hassoun, Adv. Energy Mater. 4 (2014), http://dx.doi.org/10.1002/aenm.201400083. [10] D. Buchholz, A. Moretti, R. Kloepsch, S. Nowak, V. Siozios, M. Winter, S. Passerini, Chem. Mater. 25 (2013) 142. [11] K. Kubota, N. Yabuuchi, H. Yoshida, M. Dahbi, S. Komaba, MRS Bull. 39 (2014) 416. [12] N. Yabuuchi, M. Kajiyama, J. Iwatate, H. Nishikawa, S. Hitomi, R. Okuyama, R. Usui, Y. Yamada, S. Komaba, Nat. Mater. 11 (2012) 512. [13] J. Billaud, G. Singh, A.R. Armstrong, E. Gonzalo, V. Roddatis, M. Armand, T. Rojo, P.G. Bruce, Energy Environ. Sci. 7 (2014) 1387. [14] N. Yabuuchi, R. Hara, K. Kubota, J. Paulsen, S. Kumakura, S. Komaba, J. Mater. Chem. A 2 (2014) 16851. [15] D. Buchholz, L.G. Chagas, C. Vaalma, L. Wu, S. Passerini, J. Mater. Chem. A 2 (2014) 13415. [16] Z. Lu, J.R. Dahn, J. Electrochem. Soc. 148 (2001) A1225. [17] Z. Lu, J.R. Dahn, J. Electrochem. Soc. 148 (2001) A237. [18] R. Berthelot, D. Carlier, C. Delmas, Nat. Mater. 10 (2011) 74. [19] D. Buchholz, L.G. Chagas, M. Winter, S. Passerini, Electrochimica Acta 110 (2013) 208. [20] L.G. Chagas, D. Buchholz, L. Wu, B. Vortmann, S. Passerini, J. Power Sources 247 (2014) 377. [21] L.G. Chagas, D. Buchholz, C. Vaalma, L. Wu, S. Passerini, J. Mater. Chem. A 2 (2014) 20263. [22] A. Ponrouch, E. Marchante, M. Courty, J.-M. Tarascon, M.R. Palacín, Energy & Environ. Sci. 5 (2012) 8572.
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Mg-doping for improved long-term cyclability of layered Na-ion cathode materials e The example of P2-type NaxMg0.11Mn0.89O2 Daniel Buchholz a, b, c, Christoph Vaalma b, c, Luciana Gomes Chagas a, b, c, Stefano Passerini b, c, * a b c
University of Muenster, Institute of Physical Chemistry, Corrensstraße 46, 48149 Muenster, Germany Helmholtz Institute Ulm (HIU), Helmholtzstrasse 11, 89081 Ulm, Germany Karlsruhe Institute of Technology (KIT), P.O. Box 3640, 76021 Karlsruhe, Germany
h i g h l i g h t s Mg-doping as strategy for improving the electrochemical behaviour of P-type oxides. Synthesis of NaxMg0.11Mn0.89O2 via co-precipitation method and solid state reaction. NaxMg0.11Mn0.89O2 has P2-type structure and consists of m-sized particles. Promising long-term cycling performance for 200 cycles. 125 mAh g1 delivered capacity and 93.8% capacity retention (10the100th cycle).
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 December 2014 Received in revised form 5 February 2015 Accepted 13 February 2015 Available online 18 February 2015
Sodium-ion batteries (SIBs) are establishing themselves as a low-cost alternative to the widespread lithium-ion technology, a trend that is exemplified by the use of aluminium as anode current collector. In order to be in line with this philosophy, environmentally friendly, abundant and cheap materials need to be used in order to provide a complementary rather than competing battery technology other than lithium-ion. With the same scope in mind, herein we present the structural and electrochemical characterization of P2-type NaxMg0.11Mn0.89O2 material to demonstrate the effectiveness of Mg-doping for the development of future layered cathode materials. Of particular interest is the effect on the long-term cyclability (200 cycles), which has not been reported, yet. As shown in the manuscript, a Mg content as low as 11% in the MO2 layer leads to a smoothing of the potential profile, very high coulombic efficiencies exceeding 99.5% at 12 mA g1 and a stable long-term cycling behaviour. © 2015 Elsevier B.V. All rights reserved.
Keywords: Sodium ion battery Cathode Doping Magnesium Layered structure Long-term cycling
1. Introduction In the last few years, the academic community has expressed a strong interest on sodium-ion batteries (SIBs), as shown by the increasing number of publications since 2009 [1]. Listed below lithium in the periodic table, sodium provides a comparably high potential and enables large low-cost advantages, such as the use of aluminium (instead of pricey copper) as anode current collector [1,2]. Moreover, superior electrochemical performance to other
* Corresponding author. Helmholtz Institute Ulm (HIU), Helmholtzstrasse 11, 89081 Ulm, Germany. E-mail address: [email protected] (S. Passerini). http://dx.doi.org/10.1016/j.jpowsour.2015.02.069 0378-7753/© 2015 Elsevier B.V. All rights reserved.
secondary batteries (with the exception of the lithium-ion technology) has already been demonstrated by several research works [3]. This competitive advantage to other battery chemistries leads the assumption that SIBs might be a suitable technology for applications where price is an issue, e.g. stationary energy storage. However, in order to enter the market, SIBs should follow their own philosophy, e.g., using environmentally friendly, abundant, nontoxic and cheap materials, in accordance with the economical concept of diversification. SIBs might appear as a young research field but, in fact, lithiumion and sodium-ion chemistries have been studied in parallel for decades. In the past, the main motivation behind research in the sodium-ion based chemistry was to find sodium precursors that could be used to synthesize the lithium analogues via ion-exchange
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reactions [4]. A great variety of materials have already been structurally but not electrochemically characterized, in terms of their sodium-ion chemistry. Investigations on sodium ion battery materials started in the mid-1970s with the studies of Parant et al. on the physical properties of Na0.44MnO2 [5]. As an analogue to the very prominent LiCoO2 [6], Delmas et al. investigated NaCoO2, already setting the focus specifically on sodium intercalation in 1981 [7]. Following the characterization of layered transition metal oxides comprising only one transition metal, such as nickel, cobalt or manganese for the sodium- and lithium-based systems, strategies were developed to solve the specific problems observed in the parent materials and reduce material costs [8]. With the commercialization of the portable devices powered with lithium-ion batteries by Sony in 1991, the main focus of battery research was set on the optimization of the existing lithium technology. This led to the intermixing of transition metals like cobalt, nickel and manganese, as a way to improve the poor cyclability observed for LiNiO2 and LiMnO2 and to reduce the content of the expensive cobalt, which finally led to the commercially famous LiNi1/3Co1/3Mn1/3O2 [8]. A similar trend of intermixing transition metals in order to combine their positive properties was also observed in sodium-ion cathodes, and several compounds similar to the lithium analogues were proposed, revealing good electrochemical performances [9e11]. Recently, several groups showed an interest in using environmentally friendly and low cost materials like Fe and Mg as dopant or redox active species in sodium-based layered oxides [12e14]. Magnesium in particular, an abundant and cheap element, represents a very interesting component for P-type layered transition metal oxides in SIBs, and for this reason a series of Na0.67Mn1xMgxO2 (0 x 0.3) materials has already been synthesized and structurally characterized. Billaud et al. reported the electrochemical properties of a series of Mg-doped Na0.67Mn1xMgxO2 (0 x 0.2) materials [13]. The effect of quenching in the solid state synthesis and an investigation on the galvanostatic cycling and the (de-)sodiation process was performed. The doping Mg content of x ¼ 0.05 led to smooth potential profiles, while higher Mg concentrations (e.g., x ¼ 0.2) resulted in improved cycling stability but also lower discharge capacities. Consequently, an intermediate Mg content appears to be a reasonable compromise. Most recently, Komaba et al. reported the structural and electrochemical characterization of P2-type Na0.67Mn0.72Mg0.28O2, exhibiting a very high specific capacity of 220 mAh g1, unfortunately accompanied by a high capacity fade per cycle [14]. Herein, we present a study on the structural and electrochemical properties of P2-type NaxMg0.11Mn0.89O2 with a particular focus on the long-term cycling performance, not reported for Ptype layered oxides, only containing manganese and magnesium in the MO2-layer. We believe this work to provide very useful information about this material class and to highlight the importance of Mg doping for the development of next-generation layered cathode materials for SIBs.
dispersed in an aqueous solution of sodium hydroxide (0.76 eq of NaOH per mole of Mg0.11Mn0.89(OH)2). Water was slowly removed by rotary evaporation. After drying and grinding, the material was pre-annealed in an open-air muffle furnace at 500 C for 5 h (heating rate: 10 C min1). Following, the reddish pre-annealed material was ground and then, as a powder, subjected to the high temperature annealing at 900 C for 6 h (heating rate: 5 C min1), using an open-air muffle oven, resulting in dark brownish powder. Finally, about 1 g of the pristine material was stirred in 20 mL of distilled water at 25 C for 5 min. The suspension was then filtered, washed with 80 mL of distilled water and dried at 100 C in air for 24 h. Afterwards, the material was ground, screened over a 45 mm sieve and finally stored under inert atmosphere.
2. Experimental part
The stoichiometry of the NaxMg0.11Mn0.89O2 material was verified via ICP-OES which revealed a Na: Mg: Mn ratio of 27.8: 11.1: 88.9. These metal values are in a very good agreement with the targeted stoichiometry. The lower sodium content, with respect to the 0.76 eq mol1 added during the synthesis is associated to sodium evaporation during the annealing process, sodium loss during the mixing process and the water treatment, which is leading to the dissolution of impurities (e.g. sodium carbonate) formed during the solid state synthesis in air, but also a partial chemical desodiation. Although the water treatment led to an improved electrochemical performance for analogous P-type layered oxides, [9,10] it also has to be considered that a sodium content below x 0.33 results in a
2.1. Synthesis of NaxMg0.11Mn0.89O2 The material was synthesized via a two-step solid-state reaction with sodium hydroxide (NaOH, Aldrich >98%) and a mixed manganeseemagnesium hydroxide precursor. The precursor was prepared by co-precipitating the aqueous solution of the two metal acetate tetrahydrate salts (MnAc2$4 H2O and MgAc2$4 H2O; Aldrich >98%) in a stoichiometric ratio of 11:89 with sodium hydroxide (50% excess). After extensive rinsing with distilled water, the precipitate was dried at 100 C overnight. The dried material was then
2.2. Material characterization The composition of the synthesized materials, in terms of sodium content and magnesium to manganese ratio, was determined by inductively coupled plasma optical emission spectrometry (ICPOES) with a ULTIMA 2 (Horiba, Kyoto, Japan) instrument with axial plasma viewing. The crystalline structure was characterized by Xray diffraction (XRD) using the Cu Ka radiation on the Bruker D8 Advance (Germany) in the 2q range from 10 to 90 . Lattice parameters were determined by Rietveld refinement with TOPAS software. The particle size distribution and particle morphology was evaluated with the help of a high-resolution scanning electron microscope (FE-SEM, Zeiss Auriga). 2.3. Electrode preparation & cell assembly Electrodes were made from slurries, which dry composition was 85 wt.% active material, 10 wt.% carbon black Super C65 (TIMCAL), and 5 wt.% polyvinylidene fluoride (PVDF e Kynar Flex 761A, Arkema Group), using N-methyl-2-pyrrolidone (NMP) as solvent. The slurries were homogenized via ball milling and, afterwards, casted on aluminium foil and dried at 80 C overnight. Disc electrodes with 12 mm diameter were cut, pressed (6.67 tons cm2 for 10 s), and finally dried at 120 C in a glass oven (Büchi B585) under vacuum for 24 h. The active material mass loading in the electrodes was about 2 mg cm2. Electrodes were assembled into threeelectrode Swagelok® cells with a glass fibre separator (Whatman) soaked with 1 M NaPF6 (99%, Alfa Aesar) in propylene carbonate (PC e UBE, Japan) and sodium metal as counter and reference electrodes. The sodium was cut from sodium chunks (99.8%, Acros Organics), rolled, pressed and finally punched on the current collector. Cells were cycled galvanostatically at different constant currents from 12 mA g1 to 610 mA g1 between 4.4 and 1.5 V vs. Na/Naþ, or 4.6e1.5 V vs. Na/Naþ, at 20 ± 2 C using battery tester (Maccor series 4000, USA). All potentials reported in this work refer to the Na/Naþ couple. 3. Results and discussion
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higher sensitivity towards water [15]. The structural and morphological properties of the NaxMg0.11Mn0.89O2 material were investigated by X-ray diffraction and Scanning electron microscopy (Fig. 1). The diffractogram reveals, that NaxMg0.11Mn0.89O2 can be indexed as a P2-type layered material, with space group P63/mmc. The corresponding lattice parameters obtained via Rietveld refinement are a ¼ 2.8713(8) and c ¼ 11.237(7). Details of the Rietveld refinement can be found in the Supplementary Information (SI-1). The very weak reflection at 22.1 2q originates from the ordering of the magnesium and manganese cations in the transition metal layer. The low intensity of this reflections is in accordance with Billaud et al. for NaxMn1xMgxO2 (0 x<¼0.2) materials, slowly cooling down to room temperature after the solid state reaction [13]. The presence of the hydrated phase is evidenced by the diffraction peaks at 12.6 and 25.4 , corresponding to the (002) and (004) reflections of the hydrated phase. The SEM image confirms the hexagonal layered structure and reveals NaxMg0.11Mn0.89O2 of being composed by flake-like particles with an average size of 0.5e2 mm, which is typical for P2-type layered materials. Summarizing, the successful synthesis of P2-type layered NaxMg0.11Mn0.89O2 in terms of composition, structure and morphology is confirmed by ICP-OES, XRD and SEM. The differential capacity plots of NaxMg0.11Mn0.89O2, recorded at 12 mA g1 reveals more information about the (de-)sodiation process and the electrochemical performance (Fig. 2). Four peaks, located at about 2.2 V, 3.0 V, 3.5 V and 4.2 V, are observed during charge. During the consecutive cycling the peak at 2.2 V decreases in intensity and shifts to higher potentials. In addition, also the peak at 4.2 V decreases while all other peaks remain constant (3.0 V) or even increase in intensity (3.5 V). Similarly, during the sodiation four corresponding peaks at about 2.1 V, 2.9 V, 3.4 V and 3.9 V (very weak) are observed. The peak at the lowest potential decreases in intensity and shifts to higher potentials. The very broad peak at 2.9 V and the very weak peak at 3.9 V remain constant, while interestingly, the small peak at 3.4 V increases upon cycling. It is important to mention that the peak couples at higher potentials, e.g. 3.5 V/3.4 V and 4.2 V/3.9 V, nicely demonstrate the sequence of the structural changes upon the (de-) sodiation process. The peak at 3.5 V is known to be caused by the phase transition from the P2 to a stacked fault OP4 structure [12,13,16,17]. It is interesting to mention, that the peak related to the final O2 phase transition decreases in intensity upon cycling
Fig. 1. X-ray diffraction pattern, corresponding Rietveld refinement and SEM image of pristine NaxMg0.11Mn0.89O2 material. Asterisks in the diffractogram mark the diffraction peaks of the hydrated phase. The reflection originating from the superlattice ordering of Mg2þ/Mn4þ is indicated by the dashed black box.
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Fig. 2. Differential capacity plots for the 3rd, 4th, 5th, 10th, 20th, 50th and 100th cycle of NaxMg0.11Mn0.89O2 galvanostatically cycled at 12 mA g1 in the potentials range of 4.4e1.5 V vs. Na/Naþ. Counter and reference electrode: Na. Electrolyte: 1 M NaPF6 in PC. Temperature: 20 ± 2 C.
while the peak related to the P2-OP4 phase transition simultaneously increases. The appearance of several peaks in the potential range of 1.5 Ve3.3 V might confuse as only manganese remains as redox active species but their presence can be explained with the general structural properties of P2-type materials. In P2-type structures two different sodium sites of different energy are present, although all sodium cations are prismatically coordinated [4]. In the energetically stabilized site the sodium cation is superimposed by additional two oxygen anions of the next coordination sphere. Therefore, this site is nearly exclusively occupied at lower sodium contents [18]. Instead, in the second site the sodium cation is superimposed by a metal cation (in this case manganese or magnesium) of the MO2 layer, leading to additional coulombic repulsion. Therefore, the (de-)intercalation of sodium cations from these two different sites occurs at different potentials. Another consequence is the specific distribution and arrangement of the sodium cations and vacant positions within the sodium layer due to the coulombic repulsion of the sodium cations. Consequently, the appearance of several peaks in, e.g., a cyclic voltammogram or differential capacity plot can be considered a specific intrinsic property of a P2-type material, independently of the composition of the transition metal layer [9,12,18e21]. An overview about the electrochemical performance of NaxMg0.11Mn0.89O2 is given in Fig. 3. Despite the presence of the hydrated phase, NaxMg0.11Mn0.89O2 reveals an overall promising electrochemical performance, additionally highlighted by the high average coulombic efficiency (99.6%) and the good long-term cycling stability. As for other P2type materials a marked initial capacity fade is observed as evidenced by the capacity decreasing from 174 mAh g1 to 151 mAh g1 and 129 mAh g1 in the 1st, 2nd and 10th cycles, respectively. In more detail, about 45 mAh g1, or 25.7% of the initial capacity are lost within the first ten cycles. This phenomenon is mostly related with the presence of the hydrated phase although also a higher initial capacity fading was observed in analogues P2type materials like NaxNi0.22Co0.11Mn0.66O2 or NaxNi0.23Fe0.13Mn0.63O2 but of smaller extent [9,10]. However, a very stable subsequent cycling is observed from cycle 10 onwards, which leads to discharge capacities of 121 mAh g1 and 96.8 mAh g1 after 100 and 200 cycles, respectively. This corresponds to a rather high capacity retention of 93.8% and 75.0% (vs. 10th cycle), respectively, which is even more remarkable when considering the stress caused by the two intermediate current rate tests. Interestingly, these latter tests reveal the material to exhibit good rate capability
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Fig. 3. a) Galvanostatic cycling at 12 mA g1 for NaxMg0.11Mn0.89O2 material. Cycles 21e45 and 65e90 illustrate the electrochemical behaviour for five cycles each at increased currents of 24 mA g1, 61 mA g1, 122 mA g1, 244 mA g1 and 610 mA g1 b) Potential profile of the initial charge and discharge process at 12 mA g1. Cut-off limits: 4.4e1.5 V vs. Na/Naþ. Counter and reference electrode: Na. Electrolyte: 1 M NaPF6 in PC. Temperature: 20 ± 2 C.
although the highly conductive metals like cobalt or nickel are absent. In more detail, discharge capacities of 116 mAh g1, 103 mAh g1, 93 mAh g1, 82 mAh g1 and 63 mAh g1 were obtained at current rates of 24 mA g1, 61 mA g1, 122 mA g1, 244 mA g1 and 610 mA g1, respectively. The low initial charge capacity of 55 mAh g1 (0.20 eq of Naþ removal) is in accordance with the detected sodium content via ICP-OES and was also observed by Billaud et al. [13]. The initial discharge capacity for NaxMg0.11Mn0.89O2 corresponds to the intercalation of 0.64 eq of Naþ per mole. The potential profiles of the first cycle are similar to those of P2-type Na0.67Mn0.72Mg0.28O2, but no distinct plateau at 4.2 V is observed upon charge [14]. This plateau is probably related to the oxidation of oxygen anions and is essential for the access of capacities of up to 220 mAh g1. The observed lower initial discharge capacity of 174 mAh g1 for NaxMg0.11Mn0.89O2, thus, is in accordance with the absence of the high potential plateau, probably only occurring at higher magnesium contents. The potential profiles of some selected cycles during the galvanostatic cycling at low currents and during the current rate test are depicted in Fig. 4. Interestingly, the features typically observed in the potential profiles of P2-type materials are not observed in the rather smooth low-rate potential profiles of NaxMg0.11Mn0.89O2. This clearly reveals one effect of the Mg-doping, as the (de-)sodiation appears to occur as a solid solution process. The smooth profile might further offer advantages regarding the coupling of a P2-type layered oxide
Fig. 4. a) Charge and discharge potential profiles recorded during the galvanostatic cycling for a) the 2nd, 5th, 10th, 50th, 100th and 200th cycle at 12 mA g1 and for b) 12 mA g1 (18th cycle), 24 mA g1 (23rd cycle), 61 mA g1 (27th cycle), 122 mA g1 (32nd cycle), 244 mA g1 (37th cycle) and 610 mA g1 (42nd cycle) of NaxMg0.11Mn0.89O2 material. Cut-off limits: 4.4e1.5 V (vs. Na/Naþ). Reference and counter electrodes: Na. Electrolyte: 1 M NaPF6 in PC. Temperature: 20 ± 2 C.
cathode with a suitable anode in a sodium ion battery as the cell voltage would change less rapid compared to other P2-type layered oxides [13]. Upon charge, only a very weak feature at about 4.0 V is observed, indicating the phase transition to O2-type to occur only in minor extent, which is in contrast to other P2-type layered oxides like NaxNi0.22Co0.11Mn0.66O2. At higher currents of about 122 mA g1, the plateau fully vanishes as the two phase reaction is kinetically slow. However, the major capacity fade is clearly related to the continuous shrinkage of the smooth potential plateau between 2.1 and 3.9 V. This might indicate the continuous loss of the redox active species, i.e. the ongoing active material degradation process most likely due to manganese dissolution into the electrolytic solution as a result of the disproportion reaction associated with the Jahn-Teller distorted Mn3þ cation [20]. The optimization of the electrolytic solution to prevent the dissolution or the modification of the active material (e.g., applying carbonaceous or metal oxide coatings) might be strategies to improve the cycling performance. To investigate if higher capacities can be obtained, the upper cut-off voltage was increased to 4.6 V (Fig. 5). The comparison illustrates that, indeed, higher initial discharge capacities are accessible as the material delivers 178 mAh g1 between 4.4 V and 1.5 V and 190 mAh g1 between 4.6 V and 1.5 V. However, the higher initial discharge capacity is also accompanied by a higher capacity fade, a worse performance at increased current rates, and lower coulombic efficiencies (about 99.2% at 12 mA g1). These features are certainly correlated to a more pronounced active
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material by application of coatings. The successful utilization of magnesium-doped layered oxides in Na-ion batteries also necessitates an increased initial sodium content in the material and an improved average discharge potential, e.g., via the implementation of other suitable transition metals like nickel. Acknowledgements L. G. C. acknowledges the Conselho Nacional de Desenvolvigico (CsF-CNPq, Brazil) for the financial mento Científico e Tecnolo support. TIMCAL is acknowledged for kindly providing SuperC 65. Appendix A. Supplementary data Fig. 5. Galvanostatic cycling at 12 mA g1 for NaxMg0.11Mn0.89O2 material. Cycles 21e45 and 65e90 illustrate the electrochemical behaviour during increased currents of 24 mA g1, 61 mA g1, 122 mA g1, 244 mA g1, and 610 mA g1. Cut-off limits: 4.6e1.5 V or 4.4e1.5 V vs. Na/Naþ. Counter and reference electrode: Na. Electrolyte: 1 M NaPF6 in PC. Temperature: 20 ± 2 C.
material degradation process as the electrolyte was found to be rather stable in this potential range [22]. As a consequence, already after 20 cycles NaxMg0.11Mn0.89O2 exhibits a poorer electrochemical performance, when cycled between 4.6 V and 1.5 V. 4. Conclusions P2-type NaxMg0.11Mn0.89O2 was successfully synthesized. Formula, structure and morphology were confirmed via ICP-OES, XRD and SEM, respectively. The electrochemical characterization focused on the investigation of the long-term cyclability of P2-type NaxMg0.11Mn0.89O2 and revealed high cycling stability for the galvanostatic cycling between 4.4 and 1.5 V. In more detail, a high capacity retention of 93.8% (from 10th to 100th cycle) and 75.0% (from 10th to 200th cycle) as well as very high coulombic efficiencies, exceeding 99.6%, were obtained. Reversible specific capacities of about 125 mAh g1 were obtained at low specific currents (12 mA g1) and, surprisingly, the material additionally exhibited good rate capability (e.g. 82 mAh g1 at 244 mA g1), despite the absence of transition metals like nickel or cobalt. Attempts to access higher reversible capacities by increasing the upper cut-off potential (4.6 V vs. Na/Naþ) succeeded, but led to worse cycling stability and coulombic efficiency. The continuous shortening of the potential profiles between 2.1 and 3.2 V upon long-term cycling indicates for a continuous active material degradation process, most likely due to manganese dissolution. Future work should be dedicated to counter this issue via optimization of the electrolytic solution and modification of the active
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