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Aluminium substituted β–type NaMn1-xAlxO2: A stable and enhanced electrochemical kinetic sodium-ion battery cathode
Journal of Power Sources 438 (2019) 227025
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Aluminium substituted β–type NaMn1-xAlxO2: A stable and enhanced electrochemical kinetic sodium-ion battery cathode Debasis Nayak a, *, Pawan Kumar Jha a, Sudipto Ghosh a, Venimadhav Adyam b a b
Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur, 721302, India Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur, 721302, India
H I G H L I G H T S
� We report successful doping of Al (4 and 11%) at the Mn site of β-NaMnO2. � First time single phase β type NaMn0.96Al0.04O2 to be synthesized and reported. � β type NaMn0.96Al0.04O2 shows superior electrochemical and kinetics performance. � Al doping increases the lattice volume, thus provide free access to Naþ ions. � Al doping alleviates Jahn Teller distortion during Mn3þ/4þ redox reaction process. A R T I C L E I N F O
A B S T R A C T
Keywords: Zigzag β type structure Aluminium doping Cathode material Na diffusion coefficient Sodium-ion batteries
Sodium-ion batteries are generating considerable interest for large-scale energy storage systems and electric vehicles. However, high energy density cathodes often show low power due to poor kinetics and rate capability. The primary cause is large ionic radius of Naþ ion that provide high kinetic barrier to Naþ ion transport. The β NaMnO2 show high discharge capacity but fails to retain long range order during sodium extraction that prevents it from fast charging. Here, we report successful aluminium doping of 4 and 11% to the parent structure to mitigate this problem. The β NaMnO2 shows capacity retention of only 41% after 35 cycles when cycled at 1C rate. On the contrary, aluminium doping of 4 and 11% shows capacity retention of about 76% and 80.4%, respectively after 50 cycles. The designed material, β NaMn0.96Al0.04O2, can deliver a discharge capacity of 139 mAh g 1. These values demonstrate an excellent electrochemical reversibility at high rate. The density functional theory based analysis shows that aluminium doping increases bond length of Mn O and a decrease bond length of Al O. This increases the overall volume of the structure and facilitates free access to the sodium ions without significant distortion.
1. Introduction Room-temperature sodium-ion batteries (SIBs) have fascinated re searchers to substitute high-cost lithium-ion batteries (LIBs), which can be used in electric vehicles (EVs), large-scale energy storage (ESS) ap plications for renewable energy and smart grids. The most alluring factors of SIBs are abundant sodium resources, low cost and environ ment friendliness [1]. In recent past, a significant amount of work has been done on SIB electrodes towards achieving higher volumetric reversible capacity [2,3]. From application point of view the next decade is likely to witness a considerable rise in SIBs and other varieties of batteries [4–6].
A considerable amount of search has been done to search appropriate cathode material for SIBs [7–10]. The most studied class of cathode materials for electrochemical studies are sodium transition metal oxides NaxMO2 (M ¼ Mn, Co, Cr, Fe and V) [11–14] and Nax[M1xM2(1-x)]O2 (M1 and M2 are different transition metals) [8,15–17]. Layered sodium transition metal oxides can be classified into O3, P2, and P3 type [18]. In O3, P2 and P3 the letters O and P refers to octahedral prismatic sites where sodium resides and the numbers “3” and “2” refers to three-layered or two layered respectively [19]. The presence of sodium at prismatic sites provide high diffusion coefficient [20]. However, the P2 type cathode materials suffer from a complex phase transition during sodium ion removal due to the complicated interactions between
* Corresponding author. E-mail address: [email protected] (D. Nayak). https://doi.org/10.1016/j.jpowsour.2019.227025 Received 11 April 2019; Received in revised form 7 August 2019; Accepted 15 August 2019 Available online 19 August 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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Na ion ordering, charge ordering, magnetic ordering and cooperative Jahn–Teller distortions [21,22]. Layered NaMnO2 has been observed to exist in two polymorphic phases namely α, and β phase. The α phase corresponds monoclinic structure (C2/m space group) while β phase is orthorhombic (Pmnm space group). In α NaMnO2, the MnO2 slabs remain in layered planes with sodium ions sandwiched in them while in β NaMnO2, the MnO2 slabs form a zigzag pattern [23]. In both the polymorphs sodium reside at the octahedral sites of the slabs. However, electrochemical properties of α NaMnO2 are mainly affected by the Jahn-Teller distortion effect during Mn3þO6 transferring to Mn4þO6 octahedron, leading to the greater structural changes limiting extraction of more Na atoms [24]. Bruce et al. [25] found that zigzag nature of β NaMnO2 provides a significant reduction in Jahn Teller distortion. They found discharge capacity up to 190 mAh g 1 at a current rate of C/20. They also obtained discharge capacity of 142 mAh g 1 at a discharge rate of 2C, while charging rate was C/4. However, they also found through room tem perature NMR (Nuclear Magnetic Resonance) spectra that β NaMnO2 changes partially to α NaMnO2 upon cycling. Besides, β NaMnO2 show unique characteristics like high reversible capacity and reduced Jahn–Teller distortion. Despite immense possibilities of β type cathode materials for prac tical application a very few studies have been done to harness its true potential. It appears that magnetically frustrated antiferromagnetic Mn O Mn bonds prohibit it from fast charging and retaining the initial structure [23,25]. It is observed that, magnetically frustrated antiferromagnetism in LixNi1/3Co1/3Mn1/3O2 hinders diffusion of lithium during charge/discharge process [26]. On a similar note, the zig-zag nature of β NaMnO2 facilitates triangular lattice of Mn atoms. Thus it promotes complex magnetic configurations that release sym metry restrictions for the coexistence of ferroelectric and magnetic or ders and hinders diffusion of sodium during charge/discharge process [27]. Therefore, it brings limitation to Na kinetics/rate capability. These problems could be sorted out through selective doping to reduce effect of distorted magnetic ordering. Consequently, loss of long range order during charging can be reduced. Attempts have been made in order to stabilize β NaMnO2 with different doping elements. In a major advance in 2018, Komaba et al. [28] proposed that β NaMnO2 can be stabilized/doped by Cu cations. However, other transition elements (Ti, Sc, V, Cr, Mn, Fe, Co, Ni, and Zn) and elements such as Al and Mg partially tend to stabilize α phase during the synthesis. They employed DFT based studies to establish a correlation between experimental observations and found analogous results that complement the findings. However, other methods of doping these elements need to be explored. Herein, we report successful doping of Al (4 and 11%) at the Mn site of β-NaMnO2 with enhanced electrochemical performance and diffusion kinetics. To the best of our knowledge, this is the first time that single phase β type NaMn0.96Al0.04O2 to be synthesized. The β NaMn0.96Al0.04O2 can deliver a high specific capacity up to 139 mA h g 1 at 1C rate in the voltage range of 2–4.0 V versus Naþ/Na as cathode material for SIB. Moreover, it owns excellent rate capabilities and good cycling performances. After 50 cycles, the capacity capacity retention of about 76% and 80.4% were observed at 1C for Al doping of 4 and 11%, respectively. Cyclic voltammetry (CV) results suggests that, Al doping elevates average discharge potential and also increases diffusion coefficient (DNa) value. Density functional theory (DFT) based computations were also carried out to apprehend these obtained results. Al doping increases the lattice volume and bond length of Mn O and Na O in the cathode material, thereby providing free access to Naþ ions. Similarly, Al doping alleviates Jahn Teller distortion that arises during Mn3þ/4þ oxidation process.
2. Experimental 2.1. Synthesis of cathode materials The β-NaAlxMn1-x02 (x ¼ 0, 0.04, 0.11) samples were synthesized by the solid-state reaction which involves homogeneous mixing of Na2CO3 (>99%, Sigma-Aldrich) with Mn2O3 and Al2O3 using mortar and pestle in a stoichiometric ratio. To compensate evaporation of Na2O during firing, 10 wt % of excess Na was used. The mixture was pressed into a pellet and heated under oxygen flowing condition, in two steps. In the first step, the sample was heated at the rate of 1 � C min 1 from room temp to 950 � C and kept at that temperature for 24 h and then quenched in liquid Nitrogen. The second step involves heating at the rate of 5 � C min 1 from room temperature to 950 � C and kept at that temperature for 24 h followed by quenching in liquid Nitrogen. 2.2. Sample characterization Structural characterization of the samples were performed using Xray diffraction (XRD, Bruker D8 Advance X-ray diffrac-tometer, Cu Kα) at room temperature. The morphology of the samples were examined using a field emission scanning electron microscope (FE-SEM, ZEISS EVO 60) and high-resolution transmission electron microscopy (HRTEM, JEOL, JEM 2100F). TG analysis was carried out using Per kinElmer Pyris Diamond TG-DTA-DTG (PerkinElmer, USA) to monitor the weight loss of the samples as a function of temperature. X-ray photoelectron spectroscopy (XPS) analysis at room temperature were done using a PHI 5000 Versa Probe II system with monochromatic Al Kα (1486.6 eV) irradiation. The binding energies obtained were calibrated using the C1s (284.8 eV) spectrum of hydrocarbon that remained in the XPS analysis chamber as a contaminant. 2.3. Electrochemical characterization The electrodes were made by preparing slurry of cathode active material, acetylene black, and polyvinylidene fluoride (PVDF) in the weight ratio of 80:10:10. The PVDF was dispersed in N-methyl-2-pyr rolidone (94:6:: N-methyl-2-pyrrolidone:PVDF). The slurry was coated onto Al foil and drying at 120 � C for overnight. All electrochemical ex periments were done by using 2032 coin cells. The cells were assembled inside the argon-filled glovebox (Unilab Plus, MBraun, Germany) with a controlled moisture and oxygen atmosphere, which were below the level of 1 ppm. The half-cells were made by taking the prepared working electrodes and sodium metals as counter electrode, respectively. A glass fibre filter (GB-100R, ADVANTEC) soaked in 1 M NaClO4 in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 wt %) was used as a separator. 3. Computational methods All the density functional theory (DFT) based calculations were performed using the medeA-VASP within the generalized gradient approximation by Perdew Burke Ernzherof (GGA PBE) [29–31]. The electron-ion potential is described by the projected augmented wave (PAW) potential with kinetic energy cutoff of 520 eV. The Brillouin zone was sampled using Γ centered 4 � 4 � 4 k-point mesh and the elec tronic states were smeared using the Methfessel Paxton scheme keep ing a broadening width of 0.2 eV [32,33]. The Na ion migration pathways are calculated by the nudged elastic band (NEB) method. Prior to the calculation of electronic structures, full relaxation of lattice pa rameters and the internal atomic coordinates was done until the forces on each atom is less than 0.01 eV/Å. All the calculations were performed with spin polarization. We replicated the initial unit cell of NaMnO2 to 2 � 2 � 3 supercell (24 Na, 24 Mn, and 48 O). Doping of aluminium was done by replacing one and three Mn atom with Al atoms resulting in 4.16 and 12.5% doping, which is fairly close to doping of 4% and 11% in the 2
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Aluminium doping induces significant difference in the relative in tensities between ð102Þ and ð012Þ reflections that could be attributed due to slight change in lattice parameter values. Peaks of orthorhombic β Na0.91MnO2 (space group: Pmmn, JCPDF card No. 00-038-0965) are also observed for samples NaMnO2 and NaMn0.89Al0.11O2. It can thus be suggested that the involvement of high temperature during sample preparation might have led to difference in concentration and presence of stalking fault as proposed by Grey et al. [34]. The corresponding lattice parameters (with Pmmn, settings 2) are a � 4.7763 Å, b � 2.8570 Å and c � 6.3047 Å for NaMnO2, a � 4.7767 Å, b � 2.8481 Å and c � 6.3142 Å for NaMn0.96Al0.04O2, and a � 4.7862 Å, b � 2.8582 Å and c � 6.3147 Å for NaMn0.89Al0.11O2. An overall increase in volume up to 0.46% is observed for Al doping of 0.11%. It is interesting to note that no impurity phase of α NaMnO2 was found. 4.2. Morphology and structure of the obtained materials Fig. 2(a, b and c) compares the morphologies of NaMn1-xAlxO2 (x ¼ 0, 0.04 and 0.11). Rod-like morphologies of particles could be observed for all the samples. Such morphology can be ascribed due to growth of particles along ð200Þ plane as hypothized in the previous paragraph. The Elemental mapping done by EDS (Fig. S1) suggests that as-prepared NaMnO2 consists of 21.4% Na and 49.3% Mn in weight, giving an atomic ratio of Na and Mn to be � 1:1. In addition, the EDS elementary mapping done by STEM techniques (Fig. 2) reveals that Na, Mn, Al and O elements are uniformly distributed in the sample NaM n0.96Al0.04O2. Fig. S2 illustrates presence of both β NaMnO2 and β Na0.91MnO2 in the HRTEM images. Selected area electron diffraction (SAED) (inset Fig. 2e and g) further confirms slight presence of β Na0.91MnO2 in NaMnO2 and NaMn0.89Al0.11O2. The corresponding HRTEM images verify the lattice to be ð001Þ. Similarly, SAED pattern reaffirms successful synthesis of single phase β NaMn0.96Al0.04O2 (Fig. 2f). The SAED pattern is indexed as ½0 2 1� zone axis and diffraction spots as planes ofð200Þ,ð012Þ, and ð212Þ respectively. Lattice fringes
Fig. 1. XRD pattern of NaMnO2, NaMn0.96Al0.04O2 and NaMn0.89Al0.11O2.
experimental study. 4. Result and discussion 4.1. XRD characterization Fig. 1 shows XRD patterns of NaMn1-xAlxO2 (x ¼ 0, 0.04 and 0.11). Reference pattern of β NaMnO2 (JCPDF card No. 04-010-1810) has been appended below in the figure. These XRD results show successful synthesis of desired β phase. However, the absence of the ð101Þ reflection at 23.3� indicates the presence of stacking fault in the struc ture [25]. All the samples show relatively higher intensity of ð200Þ plane compared to the reference pattern signify elongated shape, which is confirmed through micrographs elucidated in the further section.
Fig. 2. FESEM and HRTEM images of NaMn1xAlxO2 (x ¼ 0, 0.04 and 0.11): (a, b, and c) FESEM of x ¼ 0, 0.04 and 0.11, respectively (scale is 10 μm), Advanced STEM elemental mapping: (d1) presence of O in red, (d2) Na in green, (d3) Al in yellow and (d4) Mn in magenta for the sample NaMn0.96Al0.04O2, (e, f, and g) HRTEM and cor responding SAED (insert) of x ¼ 0, 0.04 and 0.11, respectively (scale is 1 nm). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 3. TG-DTA-DTG curves of the NaMnO2 precursor in an argon atmosphere (after 1st heating to show temperature for beta phase formation).
Fig. 4. X-ray photoelectron spectroscopy of Mn2p and Al2p of NaMn0.89Al0.11O2.
ofð200Þ, and ð012Þ can be seen in the HRTEM image (Fig. 2f) with spacing of 0.241 and 0.216 nm respectively. These findings provide additional support for XRD results.
major peaks for manganese are Mn 2p3/2 and Mn 2p1/2 at 642.012 and 653.412 eV, respectively. The broader area in Mn 2p3/2 is because of aligned spin and angular momentum, and anti-aligned for Mn 2p1/2 [35]. The Al 2p3/2 peak can be observed at 73.38 eV, which share sim ilarity with AlFe3 (2p3/2 at 73.4) signifying aluminium in 3 þ state in NaMn0.89Al0.11O2 [36]. The XPS analysis confirmed atomic ratio of Mn and Al in NaMn0.89Al0.11O2 are 0.89:0.11, which is in good agreement with the desired stoichiometry.
4.3. Thermo-gravimetric analysis (TG-DTA-DTG) In-order to understand the beta phase formation in NaMnO2, the TGDTA-DTG was carried out after 1st sintering and liquid nitrogen quenching of the pellet. The experiment is carried out at a heating rate of 5 � C min 1 from room temperature to 800 � C in an argon atmosphere. As shown in Fig. 3, the 1st peak of weight loss around 100 � C possibly due to dehydration reaction. Similarly, the 2nd peak of weight loss at 562 � C is due to phase transformation from α type to β type by inducing twinning in the α-phase. It was observed that mass change is less than 3% during the process of phase change due to absence of any chemical reaction. It could be implied that liquid nitrogen quenching of the pellet helps in retaining the β-phase effectively. Our results establish evidence to the arguments (drawn from DFT calulations) put forwarded by Grey et al. [34] that, frustrated antiferromagnetic β NaMnO2 is favoured at 0 K while α phase at room temperature due to entropic effects.
5. Electrochemical properties 5.1. Charge-discharge and rate capability Fig. 5(a, b and c) shows galvanostatic charge-discharge tests of all there electrode materials (NaMn1-xAlxO2, x ¼ 0, 0.04 and 0.11) done at 1C rate in a potential window of 2.0–4.0 V. The 1st cycle charge/ discharge capacities are 119.3/144.3, 164.9/141.1, and 131.3/74.25 mAh g 1 for NaMnO2, NaMn0.96Al0.04O2, and NaMn0.89Al0.11O2, respectively. An increase in discharge capacity (161.1 mAh g 1) is observed for NaMn0.96Al0.04O2. Similarly, the 10th cycle charge/ discharge capacities are 88.2/89.8, 140.9/120.3, and 101.9/91.93 mAh g 1 for the aforementioned order of electrodes. During charging for NaMnO2 and NaMn0.96Al0.04O2 two plateaus are observed around 2.7 and 3.5 V. Similarly, during sodium insertion less pronounced plateaus are observed from 4 to 2.7 V while a prominent plateau is observed around 2.5 V. In case of NaMn0.89Al0.11O2, the charge plateaus are less noticeable while discharge plateau around 2.5 V is also observed as
4.4. X-ray photoelectron spectroscopy (XPS) The oxidation state of manganese and aluminium of NaM n0.89Al0.11O2 was investigated by XPS are shown in Fig. 4. The XPS spectra were deconvoluted and analyzed with XPSPEAK41 software with Lorentzian-Gaussian functions and a linear background. The two 4
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Fig. 5. Charge-discharge profiles of (a) NaMnO2, (b) NaMn0.96Al0.04O2 and (c) NaMn0.89Al0.11O2 done at 1C rate, (d) corresponding cycle performance done at 1C rate, and (e) rate-capability of all samples.
other two cathodes. These values correlate fairly well with previous electrochemical studies [23,25]. These plateaus resemble with peaks in the cyclic voltammogram (CV) profiles which are discussed in detail below. Fig. 5d shows the corresponding capacity retention of all the samples upon cycling. Clearly it can be seen aluminium doping increases cycling stability. NaMnO2 shows capacity retention of only 41% after 35 cycles. On the other hand, aluminium doping of 4 and 11% shows capacity retention of about 76% and 80.4%, respectively even after 50 cycles. These values demonstrate an excellent electrochemical reversibility at high rate. However, the NaMn0.89Al0.11O2 electrode displays a lower specific capacity of 74.25 mAh g 1 at the first discharge process. The reasons for obtaining these set of results can be explained as follows. The Al doping in the structure reduces degeneracy during Mn3þ/Mn4þ redox reaction process. Therefore, it brings stability during charge discharge cycling process. However, with increase in aluminium doping redox active manganese concentration decreases. This results in decrease in
discharge capacity though keeping cycling stability intact. Besides, replacing Mn3þ with Al3þ decreases the Jahn Teller distortions which improve the capacity retention [37]. Fig. 5e shows rate capability study of all the cathode materials. Aluminium doping of 11% provides excellent rate capability features. At C/5 rate the discharge capacities with the increasing order of Al doping are 131, 178 and 104 mAh g 1, respectively. At 5C the discharge ca pacity of NaMn0.96Al0.04O2 sustains to be 44.1 mAh g 1. Fig. S3 provides the corresponding voltage profiles of the electrodes at C/5 and 2C. In the same vein, Table S1 gives comparative electrochemical properties analysis of previously reported sodium-ion battery cathodes with NaMn0.96Al0.04O2. 5.2. Cyclic voltammetry The cyclic voltammogram measurement is an essential electro chemical characterization technique that provides insight into Na-ion 5
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Fig. 6. Cyclic voltammogram (CV) curves of (a) NaMnO2, (b) NaMn0.96Al0.04O2 and (c) NaMn0.89Al0.11O2 done at 0.01 mV s 1, inserts (ai, bi, and ci) CV done at 0.01, 0.02, 0.05 mV s 1, and (aii, bii, and cii) the corresponding relationship between the square root of the scan rate v1/2 and peak current ip.
2.70 � 10 11 cm2 s 1 and 1.75 � 10 11 cm2 s 1. Again, for NaM n0.89Al0.11O2 cathodic and anodic diffusion constants are 1.41 � 10 11 cm2 s 1 and 1.36 � 10 11 cm2 s 1, respectively.
diffusion behaviour. Fig. 5(a)–(c) compares the first two CV curves of the samples NaMn1-xAlxO2 (x ¼ 0, 0.04 and 0.11) at a scan rate of 0.01 mV s 1 for potential window of 2 V and 4 V. The pair of oxidation/ reduction peaks appeared at 2.87/2.48, 2.72/2.53, and 2.82/2.51 V, respectively for samples in the increasing order of Al doping. These redox peaks are ascribed to Mn3þ/Mn4þ transition. Besides, few other oxidation peaks are obtained due to preferential extraction of sodium to form β Na0.57MnO2 followed by Na0.49MnO2 and Na0.39MnO2 during charging [25]. This is due to loss of long range order and follows staging extraction of sodium. These effects can be complemented with the fact that sodium in β NaMn1-xAlxO2 resides at ideal β like environment and at the twin boundary [34]. However, these peaks in CV plots merge to form one peak at high scan rate. On the other hand, NaMn0.89Al0.11O2 show diminished second peak and a long single peak which intimidate that loss of long range order mitigated to a certain extent. Fig. 6 (ai, bi, and ci) shows CV curves of the cells done at different scan rate of 0.01, 0.02 and 0.05 mV s 1. The Naþ diffusion coefficients of the cathodes opposite to sodium metal as an anode are calculated ac cording to Randles–Sevcik equation [38]. � �12 F 1=2 ip ¼ 0:4463 n3=2 ADNa Cν1=2 RT
6. DFT based study Many experts argue that rather than adding a Hubbard correction parameter (i.e. PBE þ U) it might be more useful to use only PBE functional. The U parameter corrects selfinteraction (i.e. Coulombic interaction of spin-up and spin-down electrons) errors in the functional. However, magnitude of U paremeter is highly dependent on degree of sodiation. Moreover, plane wave basis set being used that eliminates core level Coulombic interaction potential with a pseudopotential and considers only chemically active valence electrons. Since, Mn3þ ((t2g)3(eg)1) or Mn4þ ((t2g)3) does not contain spin down electrons it is highly probable that U parameter would overestimate formation energy values. On a similar note Dixit et al. [39] found that electronic structure of aluminium doped LiNi0.5Co0.2Mn0.3O2 obtained by PBE functional matches with experimentally obtained values. Therefore, keeping these factors in view we expect it is more appropriate to use only PBE func tional for this class of material. Fig. 7a, b and c shows the DOS versus energy plots of NaMn1-xAlxO2 (x ¼ 0, 0.04 and 0.125). The Mn ions are located in the octahedral site surrounded by 6 oxygen ions, therefore 3d bands of Mn split into t2g and eg bands. Upon assessing the electronic structure of NaMnO2, a negative charge transfer gap is observed so it can be called as a charge transfer insulator. The valence band maximum (VBM) lies at a distance from the Fermi level. There is absence of down spin state in the valence band due to (t2g)3(eg)1 of Mn3þ. Aluminium doping replaces Mn3þ ions with same trivalent oxidation state (Fig. 1b, b’, c and c’). It can be observed in Fig. 1b and c that VBM reaches Fermi level with a positive charge
(1)
where ip, n, A, C and ν are the peak current (A), the number of exchanged electrons, the surface area (cm2), the concentration of sodium inserted in NaMn1-xAlxO2 (x ¼ 0, 0.04 and 0.11) (mol cm 3), the sweep rate (mV s 1). DNa is the diffusion coefficient (cm2 s 1) measured by CV. Simi larly. Fig. 6 (aii, bii, and cii) shows linear relationship between peak current and square root of the scan rate. The cathodic and anodic diffusion constants are calculated to be 1.47 � 10 11 cm2 s 1 and 1.4 � 10 11 cm2 s 1, respectively for NaMnO2. Similarly, for NaM n0.96Al0.04O2, cathodic and anodic diffusion constants are 6
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Fig. 7. Density of states (DOS) plot and corresponding geometries of (a, aʹ) NaMnO2, (b, bʹ) NaMn0.96Al0.04O2, and (c, cʹ) NaMn0.875Al0.125O2. Table 1 Lattice-parameter of NaMn1-xAlxO2 (x ¼ 0, 0.04 and 0.125) as obtained from DFT calculations (Pmmn, setting 4) and comparison with experimental equiv alents (Pmmn, settings 2). Samples NaMnO2 NaMn0.96Al0.04O2 NaMn0.875Al0.125O2 (Exp: NaMn0.89Al0.11O2)
DFT Experiment DFT Experiment DFT Experiment
a (Å)
b (Å)
c (Å)
Vol (Å3)
2.9272 4.7763 2.8814 4.7767 2.8730 4.7862
4.2971 2.8570 4.8363 2.8481 4.88133 2.8582
6.31489 6.3047 6.3702 6.3142 6.3589 6.3147
79.43 86.03 88.77 86.04 89.17 86.38
transfer gap. Eventually Al doping reduces the band gap value. Further Al doping promotes a small contribution of down spin state to t2g of Mn 3d orbital. Besides, an increase in the density of states of eg and decease in the density of states of t2g in Al-doped samples signify that d electrons participate and contribute more in bonding bands than nonbonding bands. Thus, more d electrons participate in the 3d–2p bonding, which corresponds to a higher Mn valence state [40–42]. To further our research we intend to study structural modification to this class of ma terial due to different dopants through experiment and DFT based simulations.
Fig. 8. Na diffusion path in NaMn0.875Al0.125O2, (a) top view, and (b) front view.
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Table 1 shows lattice parameter values of NaMn1-xAlxO2 (x ¼ 0, 0.04 and 0.125) as obtained from DFT calculations are in accordance with experimentally obtained values. With Al doping lattice parameter a decreases slightly while b and c increases. This is observed due to decrease in bond length of Al O. Consequently an overall increase in volume is observed which facilitates free access to the sodium ions without significant distortion. However, the increase in volume is due to increase in bond length of Mn O. Fig. 8 shows the Na diffusion path in NaMn0.875Al0.125O2, where sodium moves to the vacancy site (from position 1 to 2).
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7. Conclusions In summary, we successfully synthesized β type NaMn1-xAlxO2 (x ¼ 0, 0.04, and 0.11) as cathode materials for SIBs. Single phase β NaMn0.96Al0.04O2 has been confirmed through XRD, HRTEM and SAED pattern. The β NaMnO2 showed high discharge capacity for high rate charging. However, this material looses long range order during charging. Therefore during fast charging show less capacity, low rate capability and fading. Aluminium doping at the manganese site has been proven to be an effective strategy to overcome adverse effects and provide good capacity retention and rate capability. At 1C rate, β NaMnO2 delivered discharge capacity of 144 mAh g 1 with capacity retention of only 41% after 35 cycles. However, upon aluminium doping of 4 and 11%, capacity retained up to about 76% and 80.4%, respec tively after 50 cycles. The β NaMn0.96Al0.04O2 showed an initial discharge capacity of 141 mAh g 1 at 1C rate. Our approach could be applied to dope other transition or non-transition materials to this class of cathode material to achieve high electrochemical performance. We hope that our research will serve as a base for future studies on under standing cause of stability and retaining long range order. Acknowledgement The authors would like to acknowledge that the financial support for this work came from Ministry of Human Resource Development, Gov ernment of India through the initiative of IMPACTING RESEARCH INNOVATION AND TECHNOLOGY (IMPRINT), grant number 7911. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227025. References [1] N. Yabuuchi, K. Kubota, M. Dahbi, S. Komaba, Research development on sodiumion batteries, Chem. Rev. 114 (2014) 11636–11682, https://doi.org/10.1021/ cr500192f. [2] J. Deng, W. Bin Luo, S.L. Chou, H.K. Liu, S.X. Dou, Sodium-ion batteries: from academic research to practical commercialization, Adv. Energy Mater. (2017) 1–17, https://doi.org/10.1002/aenm.201701428, 1701428. [3] J.-Y. Hwang, S.-T. Myung, Y.-K. Sun, Sodium-ion batteries: present and future, Chem. Soc. Rev. 46 (2017) 3529–3614, https://doi.org/10.1039/C6CS00776G. [4] M.I. Jamesh, A.S. Prakash, Advancement of technology towards developing Na-ion batteries, J. Power Sources 378 (2018) 268–300, https://doi.org/10.1016/j. jpowsour.2017.12.053. [5] R. Zhang, J. Zhao, L. Guo, H. Qin, W. Shi, Z. Lu, First-principles investigation of a β-MnO2 and graphene composite as a promising cathode material for rechargeable Li-ion batteries, RSC Adv. 7 (2017) 29821–29826, https://doi.org/10.1039/ C7RA04837H. [6] S. Leng, X. Sun, Y. Yang, R. Zhang, Borophene as an anode material for Zn-ion batteries: a first-principles investigation, Mater. Res. Express 6 (2019), https://doi. org/10.1088/2053-1591/ab1a88, 085504. [7] Y. Noguchi, E. Kobayashi, L.S. Plashnitsa, S. Okada, J. Yamaki, Fabrication and performances of all solid-state symmetric sodium battery based on NASICONrelated compounds, Electrochim. Acta 101 (2013) 59–65, https://doi.org/ 10.1016/j.electacta.2012.11.038. [8] H. Pan, Y.-S. Hu, L. Chen, Room-temperature stationary sodium-ion batteries for large-scale electric energy storage, Energy Environ. Sci. 6 (2013) 2338, https://doi. org/10.1039/c3ee40847g.
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[38] C. Deng, S. Zhang, Y. Wu, Hydrothermal-assisted synthesis of the Na7V4(P2O7)4(PO4)/C nanorod and its fast sodium intercalation chemistry in aqueous rechargeable sodium batteries, Nanoscale 7 (2014) 487–491, https://doi. org/10.1039/c4nr05175k. [39] M. Dixit, B. Markovsky, D. Aurbach, D.T. Major, Unraveling the effects of Al doping on the electrochemical properties of LiNi0.5Co0.2Mn0.3O2 using first principles, J. Electrochem. Soc. 164 (2017) A6359–A6365, https://doi.org/10.1149/ 2.0561701jes. [40] M.S. Islam, C.A.J. Fisher, Lithium and sodium battery cathode materials: computational insights into voltage, diffusion and nanostructural properties, Chem. Soc. Rev. (2014), https://doi.org/10.1039/c3cs60199d. [41] J. Su, Y. Pei, Z. Yang, X. Wang, First-principles investigation on the structural, electronic properties and diffusion barriers of Mg/Al doped NaCoO2 as the cathode material of rechargeable sodium batteries, RSC Adv. 5 (2015) 27229–27234, https://doi.org/10.1039/C5RA01895A. [42] N. Tapia-Ruiz, W.M. Dose, N. Sharma, H. Chen, J. Heath, J.W. Somerville, et al., High voltage structural evolution and enhanced Na-ion diffusion in P2-Na2/3Ni1/ 3 xMgxMn2/3O2 (0 � x � 0.2) cathodes from diffraction, electrochemical and ab initio studies, Energy Environ. Sci. 11 (2018) 1470–1479, https://doi.org/10.1039/ C7EE02995K.
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Aluminium substituted β–type NaMn1-xAlxO2: A stable and enhanced electrochemical kinetic sodium-ion battery cathode Debasis Nayak a, *, Pawan Kumar Jha a, Sudipto Ghosh a, Venimadhav Adyam b a b
Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur, 721302, India Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur, 721302, India
H I G H L I G H T S
� We report successful doping of Al (4 and 11%) at the Mn site of β-NaMnO2. � First time single phase β type NaMn0.96Al0.04O2 to be synthesized and reported. � β type NaMn0.96Al0.04O2 shows superior electrochemical and kinetics performance. � Al doping increases the lattice volume, thus provide free access to Naþ ions. � Al doping alleviates Jahn Teller distortion during Mn3þ/4þ redox reaction process. A R T I C L E I N F O
A B S T R A C T
Keywords: Zigzag β type structure Aluminium doping Cathode material Na diffusion coefficient Sodium-ion batteries
Sodium-ion batteries are generating considerable interest for large-scale energy storage systems and electric vehicles. However, high energy density cathodes often show low power due to poor kinetics and rate capability. The primary cause is large ionic radius of Naþ ion that provide high kinetic barrier to Naþ ion transport. The β NaMnO2 show high discharge capacity but fails to retain long range order during sodium extraction that prevents it from fast charging. Here, we report successful aluminium doping of 4 and 11% to the parent structure to mitigate this problem. The β NaMnO2 shows capacity retention of only 41% after 35 cycles when cycled at 1C rate. On the contrary, aluminium doping of 4 and 11% shows capacity retention of about 76% and 80.4%, respectively after 50 cycles. The designed material, β NaMn0.96Al0.04O2, can deliver a discharge capacity of 139 mAh g 1. These values demonstrate an excellent electrochemical reversibility at high rate. The density functional theory based analysis shows that aluminium doping increases bond length of Mn O and a decrease bond length of Al O. This increases the overall volume of the structure and facilitates free access to the sodium ions without significant distortion.
1. Introduction Room-temperature sodium-ion batteries (SIBs) have fascinated re searchers to substitute high-cost lithium-ion batteries (LIBs), which can be used in electric vehicles (EVs), large-scale energy storage (ESS) ap plications for renewable energy and smart grids. The most alluring factors of SIBs are abundant sodium resources, low cost and environ ment friendliness [1]. In recent past, a significant amount of work has been done on SIB electrodes towards achieving higher volumetric reversible capacity [2,3]. From application point of view the next decade is likely to witness a considerable rise in SIBs and other varieties of batteries [4–6].
A considerable amount of search has been done to search appropriate cathode material for SIBs [7–10]. The most studied class of cathode materials for electrochemical studies are sodium transition metal oxides NaxMO2 (M ¼ Mn, Co, Cr, Fe and V) [11–14] and Nax[M1xM2(1-x)]O2 (M1 and M2 are different transition metals) [8,15–17]. Layered sodium transition metal oxides can be classified into O3, P2, and P3 type [18]. In O3, P2 and P3 the letters O and P refers to octahedral prismatic sites where sodium resides and the numbers “3” and “2” refers to three-layered or two layered respectively [19]. The presence of sodium at prismatic sites provide high diffusion coefficient [20]. However, the P2 type cathode materials suffer from a complex phase transition during sodium ion removal due to the complicated interactions between
* Corresponding author. E-mail address: [email protected] (D. Nayak). https://doi.org/10.1016/j.jpowsour.2019.227025 Received 11 April 2019; Received in revised form 7 August 2019; Accepted 15 August 2019 Available online 19 August 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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Na ion ordering, charge ordering, magnetic ordering and cooperative Jahn–Teller distortions [21,22]. Layered NaMnO2 has been observed to exist in two polymorphic phases namely α, and β phase. The α phase corresponds monoclinic structure (C2/m space group) while β phase is orthorhombic (Pmnm space group). In α NaMnO2, the MnO2 slabs remain in layered planes with sodium ions sandwiched in them while in β NaMnO2, the MnO2 slabs form a zigzag pattern [23]. In both the polymorphs sodium reside at the octahedral sites of the slabs. However, electrochemical properties of α NaMnO2 are mainly affected by the Jahn-Teller distortion effect during Mn3þO6 transferring to Mn4þO6 octahedron, leading to the greater structural changes limiting extraction of more Na atoms [24]. Bruce et al. [25] found that zigzag nature of β NaMnO2 provides a significant reduction in Jahn Teller distortion. They found discharge capacity up to 190 mAh g 1 at a current rate of C/20. They also obtained discharge capacity of 142 mAh g 1 at a discharge rate of 2C, while charging rate was C/4. However, they also found through room tem perature NMR (Nuclear Magnetic Resonance) spectra that β NaMnO2 changes partially to α NaMnO2 upon cycling. Besides, β NaMnO2 show unique characteristics like high reversible capacity and reduced Jahn–Teller distortion. Despite immense possibilities of β type cathode materials for prac tical application a very few studies have been done to harness its true potential. It appears that magnetically frustrated antiferromagnetic Mn O Mn bonds prohibit it from fast charging and retaining the initial structure [23,25]. It is observed that, magnetically frustrated antiferromagnetism in LixNi1/3Co1/3Mn1/3O2 hinders diffusion of lithium during charge/discharge process [26]. On a similar note, the zig-zag nature of β NaMnO2 facilitates triangular lattice of Mn atoms. Thus it promotes complex magnetic configurations that release sym metry restrictions for the coexistence of ferroelectric and magnetic or ders and hinders diffusion of sodium during charge/discharge process [27]. Therefore, it brings limitation to Na kinetics/rate capability. These problems could be sorted out through selective doping to reduce effect of distorted magnetic ordering. Consequently, loss of long range order during charging can be reduced. Attempts have been made in order to stabilize β NaMnO2 with different doping elements. In a major advance in 2018, Komaba et al. [28] proposed that β NaMnO2 can be stabilized/doped by Cu cations. However, other transition elements (Ti, Sc, V, Cr, Mn, Fe, Co, Ni, and Zn) and elements such as Al and Mg partially tend to stabilize α phase during the synthesis. They employed DFT based studies to establish a correlation between experimental observations and found analogous results that complement the findings. However, other methods of doping these elements need to be explored. Herein, we report successful doping of Al (4 and 11%) at the Mn site of β-NaMnO2 with enhanced electrochemical performance and diffusion kinetics. To the best of our knowledge, this is the first time that single phase β type NaMn0.96Al0.04O2 to be synthesized. The β NaMn0.96Al0.04O2 can deliver a high specific capacity up to 139 mA h g 1 at 1C rate in the voltage range of 2–4.0 V versus Naþ/Na as cathode material for SIB. Moreover, it owns excellent rate capabilities and good cycling performances. After 50 cycles, the capacity capacity retention of about 76% and 80.4% were observed at 1C for Al doping of 4 and 11%, respectively. Cyclic voltammetry (CV) results suggests that, Al doping elevates average discharge potential and also increases diffusion coefficient (DNa) value. Density functional theory (DFT) based computations were also carried out to apprehend these obtained results. Al doping increases the lattice volume and bond length of Mn O and Na O in the cathode material, thereby providing free access to Naþ ions. Similarly, Al doping alleviates Jahn Teller distortion that arises during Mn3þ/4þ oxidation process.
2. Experimental 2.1. Synthesis of cathode materials The β-NaAlxMn1-x02 (x ¼ 0, 0.04, 0.11) samples were synthesized by the solid-state reaction which involves homogeneous mixing of Na2CO3 (>99%, Sigma-Aldrich) with Mn2O3 and Al2O3 using mortar and pestle in a stoichiometric ratio. To compensate evaporation of Na2O during firing, 10 wt % of excess Na was used. The mixture was pressed into a pellet and heated under oxygen flowing condition, in two steps. In the first step, the sample was heated at the rate of 1 � C min 1 from room temp to 950 � C and kept at that temperature for 24 h and then quenched in liquid Nitrogen. The second step involves heating at the rate of 5 � C min 1 from room temperature to 950 � C and kept at that temperature for 24 h followed by quenching in liquid Nitrogen. 2.2. Sample characterization Structural characterization of the samples were performed using Xray diffraction (XRD, Bruker D8 Advance X-ray diffrac-tometer, Cu Kα) at room temperature. The morphology of the samples were examined using a field emission scanning electron microscope (FE-SEM, ZEISS EVO 60) and high-resolution transmission electron microscopy (HRTEM, JEOL, JEM 2100F). TG analysis was carried out using Per kinElmer Pyris Diamond TG-DTA-DTG (PerkinElmer, USA) to monitor the weight loss of the samples as a function of temperature. X-ray photoelectron spectroscopy (XPS) analysis at room temperature were done using a PHI 5000 Versa Probe II system with monochromatic Al Kα (1486.6 eV) irradiation. The binding energies obtained were calibrated using the C1s (284.8 eV) spectrum of hydrocarbon that remained in the XPS analysis chamber as a contaminant. 2.3. Electrochemical characterization The electrodes were made by preparing slurry of cathode active material, acetylene black, and polyvinylidene fluoride (PVDF) in the weight ratio of 80:10:10. The PVDF was dispersed in N-methyl-2-pyr rolidone (94:6:: N-methyl-2-pyrrolidone:PVDF). The slurry was coated onto Al foil and drying at 120 � C for overnight. All electrochemical ex periments were done by using 2032 coin cells. The cells were assembled inside the argon-filled glovebox (Unilab Plus, MBraun, Germany) with a controlled moisture and oxygen atmosphere, which were below the level of 1 ppm. The half-cells were made by taking the prepared working electrodes and sodium metals as counter electrode, respectively. A glass fibre filter (GB-100R, ADVANTEC) soaked in 1 M NaClO4 in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 wt %) was used as a separator. 3. Computational methods All the density functional theory (DFT) based calculations were performed using the medeA-VASP within the generalized gradient approximation by Perdew Burke Ernzherof (GGA PBE) [29–31]. The electron-ion potential is described by the projected augmented wave (PAW) potential with kinetic energy cutoff of 520 eV. The Brillouin zone was sampled using Γ centered 4 � 4 � 4 k-point mesh and the elec tronic states were smeared using the Methfessel Paxton scheme keep ing a broadening width of 0.2 eV [32,33]. The Na ion migration pathways are calculated by the nudged elastic band (NEB) method. Prior to the calculation of electronic structures, full relaxation of lattice pa rameters and the internal atomic coordinates was done until the forces on each atom is less than 0.01 eV/Å. All the calculations were performed with spin polarization. We replicated the initial unit cell of NaMnO2 to 2 � 2 � 3 supercell (24 Na, 24 Mn, and 48 O). Doping of aluminium was done by replacing one and three Mn atom with Al atoms resulting in 4.16 and 12.5% doping, which is fairly close to doping of 4% and 11% in the 2
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Aluminium doping induces significant difference in the relative in tensities between ð102Þ and ð012Þ reflections that could be attributed due to slight change in lattice parameter values. Peaks of orthorhombic β Na0.91MnO2 (space group: Pmmn, JCPDF card No. 00-038-0965) are also observed for samples NaMnO2 and NaMn0.89Al0.11O2. It can thus be suggested that the involvement of high temperature during sample preparation might have led to difference in concentration and presence of stalking fault as proposed by Grey et al. [34]. The corresponding lattice parameters (with Pmmn, settings 2) are a � 4.7763 Å, b � 2.8570 Å and c � 6.3047 Å for NaMnO2, a � 4.7767 Å, b � 2.8481 Å and c � 6.3142 Å for NaMn0.96Al0.04O2, and a � 4.7862 Å, b � 2.8582 Å and c � 6.3147 Å for NaMn0.89Al0.11O2. An overall increase in volume up to 0.46% is observed for Al doping of 0.11%. It is interesting to note that no impurity phase of α NaMnO2 was found. 4.2. Morphology and structure of the obtained materials Fig. 2(a, b and c) compares the morphologies of NaMn1-xAlxO2 (x ¼ 0, 0.04 and 0.11). Rod-like morphologies of particles could be observed for all the samples. Such morphology can be ascribed due to growth of particles along ð200Þ plane as hypothized in the previous paragraph. The Elemental mapping done by EDS (Fig. S1) suggests that as-prepared NaMnO2 consists of 21.4% Na and 49.3% Mn in weight, giving an atomic ratio of Na and Mn to be � 1:1. In addition, the EDS elementary mapping done by STEM techniques (Fig. 2) reveals that Na, Mn, Al and O elements are uniformly distributed in the sample NaM n0.96Al0.04O2. Fig. S2 illustrates presence of both β NaMnO2 and β Na0.91MnO2 in the HRTEM images. Selected area electron diffraction (SAED) (inset Fig. 2e and g) further confirms slight presence of β Na0.91MnO2 in NaMnO2 and NaMn0.89Al0.11O2. The corresponding HRTEM images verify the lattice to be ð001Þ. Similarly, SAED pattern reaffirms successful synthesis of single phase β NaMn0.96Al0.04O2 (Fig. 2f). The SAED pattern is indexed as ½0 2 1� zone axis and diffraction spots as planes ofð200Þ,ð012Þ, and ð212Þ respectively. Lattice fringes
Fig. 1. XRD pattern of NaMnO2, NaMn0.96Al0.04O2 and NaMn0.89Al0.11O2.
experimental study. 4. Result and discussion 4.1. XRD characterization Fig. 1 shows XRD patterns of NaMn1-xAlxO2 (x ¼ 0, 0.04 and 0.11). Reference pattern of β NaMnO2 (JCPDF card No. 04-010-1810) has been appended below in the figure. These XRD results show successful synthesis of desired β phase. However, the absence of the ð101Þ reflection at 23.3� indicates the presence of stacking fault in the struc ture [25]. All the samples show relatively higher intensity of ð200Þ plane compared to the reference pattern signify elongated shape, which is confirmed through micrographs elucidated in the further section.
Fig. 2. FESEM and HRTEM images of NaMn1xAlxO2 (x ¼ 0, 0.04 and 0.11): (a, b, and c) FESEM of x ¼ 0, 0.04 and 0.11, respectively (scale is 10 μm), Advanced STEM elemental mapping: (d1) presence of O in red, (d2) Na in green, (d3) Al in yellow and (d4) Mn in magenta for the sample NaMn0.96Al0.04O2, (e, f, and g) HRTEM and cor responding SAED (insert) of x ¼ 0, 0.04 and 0.11, respectively (scale is 1 nm). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 3. TG-DTA-DTG curves of the NaMnO2 precursor in an argon atmosphere (after 1st heating to show temperature for beta phase formation).
Fig. 4. X-ray photoelectron spectroscopy of Mn2p and Al2p of NaMn0.89Al0.11O2.
ofð200Þ, and ð012Þ can be seen in the HRTEM image (Fig. 2f) with spacing of 0.241 and 0.216 nm respectively. These findings provide additional support for XRD results.
major peaks for manganese are Mn 2p3/2 and Mn 2p1/2 at 642.012 and 653.412 eV, respectively. The broader area in Mn 2p3/2 is because of aligned spin and angular momentum, and anti-aligned for Mn 2p1/2 [35]. The Al 2p3/2 peak can be observed at 73.38 eV, which share sim ilarity with AlFe3 (2p3/2 at 73.4) signifying aluminium in 3 þ state in NaMn0.89Al0.11O2 [36]. The XPS analysis confirmed atomic ratio of Mn and Al in NaMn0.89Al0.11O2 are 0.89:0.11, which is in good agreement with the desired stoichiometry.
4.3. Thermo-gravimetric analysis (TG-DTA-DTG) In-order to understand the beta phase formation in NaMnO2, the TGDTA-DTG was carried out after 1st sintering and liquid nitrogen quenching of the pellet. The experiment is carried out at a heating rate of 5 � C min 1 from room temperature to 800 � C in an argon atmosphere. As shown in Fig. 3, the 1st peak of weight loss around 100 � C possibly due to dehydration reaction. Similarly, the 2nd peak of weight loss at 562 � C is due to phase transformation from α type to β type by inducing twinning in the α-phase. It was observed that mass change is less than 3% during the process of phase change due to absence of any chemical reaction. It could be implied that liquid nitrogen quenching of the pellet helps in retaining the β-phase effectively. Our results establish evidence to the arguments (drawn from DFT calulations) put forwarded by Grey et al. [34] that, frustrated antiferromagnetic β NaMnO2 is favoured at 0 K while α phase at room temperature due to entropic effects.
5. Electrochemical properties 5.1. Charge-discharge and rate capability Fig. 5(a, b and c) shows galvanostatic charge-discharge tests of all there electrode materials (NaMn1-xAlxO2, x ¼ 0, 0.04 and 0.11) done at 1C rate in a potential window of 2.0–4.0 V. The 1st cycle charge/ discharge capacities are 119.3/144.3, 164.9/141.1, and 131.3/74.25 mAh g 1 for NaMnO2, NaMn0.96Al0.04O2, and NaMn0.89Al0.11O2, respectively. An increase in discharge capacity (161.1 mAh g 1) is observed for NaMn0.96Al0.04O2. Similarly, the 10th cycle charge/ discharge capacities are 88.2/89.8, 140.9/120.3, and 101.9/91.93 mAh g 1 for the aforementioned order of electrodes. During charging for NaMnO2 and NaMn0.96Al0.04O2 two plateaus are observed around 2.7 and 3.5 V. Similarly, during sodium insertion less pronounced plateaus are observed from 4 to 2.7 V while a prominent plateau is observed around 2.5 V. In case of NaMn0.89Al0.11O2, the charge plateaus are less noticeable while discharge plateau around 2.5 V is also observed as
4.4. X-ray photoelectron spectroscopy (XPS) The oxidation state of manganese and aluminium of NaM n0.89Al0.11O2 was investigated by XPS are shown in Fig. 4. The XPS spectra were deconvoluted and analyzed with XPSPEAK41 software with Lorentzian-Gaussian functions and a linear background. The two 4
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Fig. 5. Charge-discharge profiles of (a) NaMnO2, (b) NaMn0.96Al0.04O2 and (c) NaMn0.89Al0.11O2 done at 1C rate, (d) corresponding cycle performance done at 1C rate, and (e) rate-capability of all samples.
other two cathodes. These values correlate fairly well with previous electrochemical studies [23,25]. These plateaus resemble with peaks in the cyclic voltammogram (CV) profiles which are discussed in detail below. Fig. 5d shows the corresponding capacity retention of all the samples upon cycling. Clearly it can be seen aluminium doping increases cycling stability. NaMnO2 shows capacity retention of only 41% after 35 cycles. On the other hand, aluminium doping of 4 and 11% shows capacity retention of about 76% and 80.4%, respectively even after 50 cycles. These values demonstrate an excellent electrochemical reversibility at high rate. However, the NaMn0.89Al0.11O2 electrode displays a lower specific capacity of 74.25 mAh g 1 at the first discharge process. The reasons for obtaining these set of results can be explained as follows. The Al doping in the structure reduces degeneracy during Mn3þ/Mn4þ redox reaction process. Therefore, it brings stability during charge discharge cycling process. However, with increase in aluminium doping redox active manganese concentration decreases. This results in decrease in
discharge capacity though keeping cycling stability intact. Besides, replacing Mn3þ with Al3þ decreases the Jahn Teller distortions which improve the capacity retention [37]. Fig. 5e shows rate capability study of all the cathode materials. Aluminium doping of 11% provides excellent rate capability features. At C/5 rate the discharge capacities with the increasing order of Al doping are 131, 178 and 104 mAh g 1, respectively. At 5C the discharge ca pacity of NaMn0.96Al0.04O2 sustains to be 44.1 mAh g 1. Fig. S3 provides the corresponding voltage profiles of the electrodes at C/5 and 2C. In the same vein, Table S1 gives comparative electrochemical properties analysis of previously reported sodium-ion battery cathodes with NaMn0.96Al0.04O2. 5.2. Cyclic voltammetry The cyclic voltammogram measurement is an essential electro chemical characterization technique that provides insight into Na-ion 5
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Fig. 6. Cyclic voltammogram (CV) curves of (a) NaMnO2, (b) NaMn0.96Al0.04O2 and (c) NaMn0.89Al0.11O2 done at 0.01 mV s 1, inserts (ai, bi, and ci) CV done at 0.01, 0.02, 0.05 mV s 1, and (aii, bii, and cii) the corresponding relationship between the square root of the scan rate v1/2 and peak current ip.
2.70 � 10 11 cm2 s 1 and 1.75 � 10 11 cm2 s 1. Again, for NaM n0.89Al0.11O2 cathodic and anodic diffusion constants are 1.41 � 10 11 cm2 s 1 and 1.36 � 10 11 cm2 s 1, respectively.
diffusion behaviour. Fig. 5(a)–(c) compares the first two CV curves of the samples NaMn1-xAlxO2 (x ¼ 0, 0.04 and 0.11) at a scan rate of 0.01 mV s 1 for potential window of 2 V and 4 V. The pair of oxidation/ reduction peaks appeared at 2.87/2.48, 2.72/2.53, and 2.82/2.51 V, respectively for samples in the increasing order of Al doping. These redox peaks are ascribed to Mn3þ/Mn4þ transition. Besides, few other oxidation peaks are obtained due to preferential extraction of sodium to form β Na0.57MnO2 followed by Na0.49MnO2 and Na0.39MnO2 during charging [25]. This is due to loss of long range order and follows staging extraction of sodium. These effects can be complemented with the fact that sodium in β NaMn1-xAlxO2 resides at ideal β like environment and at the twin boundary [34]. However, these peaks in CV plots merge to form one peak at high scan rate. On the other hand, NaMn0.89Al0.11O2 show diminished second peak and a long single peak which intimidate that loss of long range order mitigated to a certain extent. Fig. 6 (ai, bi, and ci) shows CV curves of the cells done at different scan rate of 0.01, 0.02 and 0.05 mV s 1. The Naþ diffusion coefficients of the cathodes opposite to sodium metal as an anode are calculated ac cording to Randles–Sevcik equation [38]. � �12 F 1=2 ip ¼ 0:4463 n3=2 ADNa Cν1=2 RT
6. DFT based study Many experts argue that rather than adding a Hubbard correction parameter (i.e. PBE þ U) it might be more useful to use only PBE functional. The U parameter corrects selfinteraction (i.e. Coulombic interaction of spin-up and spin-down electrons) errors in the functional. However, magnitude of U paremeter is highly dependent on degree of sodiation. Moreover, plane wave basis set being used that eliminates core level Coulombic interaction potential with a pseudopotential and considers only chemically active valence electrons. Since, Mn3þ ((t2g)3(eg)1) or Mn4þ ((t2g)3) does not contain spin down electrons it is highly probable that U parameter would overestimate formation energy values. On a similar note Dixit et al. [39] found that electronic structure of aluminium doped LiNi0.5Co0.2Mn0.3O2 obtained by PBE functional matches with experimentally obtained values. Therefore, keeping these factors in view we expect it is more appropriate to use only PBE func tional for this class of material. Fig. 7a, b and c shows the DOS versus energy plots of NaMn1-xAlxO2 (x ¼ 0, 0.04 and 0.125). The Mn ions are located in the octahedral site surrounded by 6 oxygen ions, therefore 3d bands of Mn split into t2g and eg bands. Upon assessing the electronic structure of NaMnO2, a negative charge transfer gap is observed so it can be called as a charge transfer insulator. The valence band maximum (VBM) lies at a distance from the Fermi level. There is absence of down spin state in the valence band due to (t2g)3(eg)1 of Mn3þ. Aluminium doping replaces Mn3þ ions with same trivalent oxidation state (Fig. 1b, b’, c and c’). It can be observed in Fig. 1b and c that VBM reaches Fermi level with a positive charge
(1)
where ip, n, A, C and ν are the peak current (A), the number of exchanged electrons, the surface area (cm2), the concentration of sodium inserted in NaMn1-xAlxO2 (x ¼ 0, 0.04 and 0.11) (mol cm 3), the sweep rate (mV s 1). DNa is the diffusion coefficient (cm2 s 1) measured by CV. Simi larly. Fig. 6 (aii, bii, and cii) shows linear relationship between peak current and square root of the scan rate. The cathodic and anodic diffusion constants are calculated to be 1.47 � 10 11 cm2 s 1 and 1.4 � 10 11 cm2 s 1, respectively for NaMnO2. Similarly, for NaM n0.96Al0.04O2, cathodic and anodic diffusion constants are 6
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Fig. 7. Density of states (DOS) plot and corresponding geometries of (a, aʹ) NaMnO2, (b, bʹ) NaMn0.96Al0.04O2, and (c, cʹ) NaMn0.875Al0.125O2. Table 1 Lattice-parameter of NaMn1-xAlxO2 (x ¼ 0, 0.04 and 0.125) as obtained from DFT calculations (Pmmn, setting 4) and comparison with experimental equiv alents (Pmmn, settings 2). Samples NaMnO2 NaMn0.96Al0.04O2 NaMn0.875Al0.125O2 (Exp: NaMn0.89Al0.11O2)
DFT Experiment DFT Experiment DFT Experiment
a (Å)
b (Å)
c (Å)
Vol (Å3)
2.9272 4.7763 2.8814 4.7767 2.8730 4.7862
4.2971 2.8570 4.8363 2.8481 4.88133 2.8582
6.31489 6.3047 6.3702 6.3142 6.3589 6.3147
79.43 86.03 88.77 86.04 89.17 86.38
transfer gap. Eventually Al doping reduces the band gap value. Further Al doping promotes a small contribution of down spin state to t2g of Mn 3d orbital. Besides, an increase in the density of states of eg and decease in the density of states of t2g in Al-doped samples signify that d electrons participate and contribute more in bonding bands than nonbonding bands. Thus, more d electrons participate in the 3d–2p bonding, which corresponds to a higher Mn valence state [40–42]. To further our research we intend to study structural modification to this class of ma terial due to different dopants through experiment and DFT based simulations.
Fig. 8. Na diffusion path in NaMn0.875Al0.125O2, (a) top view, and (b) front view.
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Table 1 shows lattice parameter values of NaMn1-xAlxO2 (x ¼ 0, 0.04 and 0.125) as obtained from DFT calculations are in accordance with experimentally obtained values. With Al doping lattice parameter a decreases slightly while b and c increases. This is observed due to decrease in bond length of Al O. Consequently an overall increase in volume is observed which facilitates free access to the sodium ions without significant distortion. However, the increase in volume is due to increase in bond length of Mn O. Fig. 8 shows the Na diffusion path in NaMn0.875Al0.125O2, where sodium moves to the vacancy site (from position 1 to 2).
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7. Conclusions In summary, we successfully synthesized β type NaMn1-xAlxO2 (x ¼ 0, 0.04, and 0.11) as cathode materials for SIBs. Single phase β NaMn0.96Al0.04O2 has been confirmed through XRD, HRTEM and SAED pattern. The β NaMnO2 showed high discharge capacity for high rate charging. However, this material looses long range order during charging. Therefore during fast charging show less capacity, low rate capability and fading. Aluminium doping at the manganese site has been proven to be an effective strategy to overcome adverse effects and provide good capacity retention and rate capability. At 1C rate, β NaMnO2 delivered discharge capacity of 144 mAh g 1 with capacity retention of only 41% after 35 cycles. However, upon aluminium doping of 4 and 11%, capacity retained up to about 76% and 80.4%, respec tively after 50 cycles. The β NaMn0.96Al0.04O2 showed an initial discharge capacity of 141 mAh g 1 at 1C rate. Our approach could be applied to dope other transition or non-transition materials to this class of cathode material to achieve high electrochemical performance. We hope that our research will serve as a base for future studies on under standing cause of stability and retaining long range order. Acknowledgement The authors would like to acknowledge that the financial support for this work came from Ministry of Human Resource Development, Gov ernment of India through the initiative of IMPACTING RESEARCH INNOVATION AND TECHNOLOGY (IMPRINT), grant number 7911. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227025. References [1] N. Yabuuchi, K. Kubota, M. Dahbi, S. Komaba, Research development on sodiumion batteries, Chem. Rev. 114 (2014) 11636–11682, https://doi.org/10.1021/ cr500192f. [2] J. Deng, W. Bin Luo, S.L. Chou, H.K. Liu, S.X. Dou, Sodium-ion batteries: from academic research to practical commercialization, Adv. Energy Mater. (2017) 1–17, https://doi.org/10.1002/aenm.201701428, 1701428. [3] J.-Y. Hwang, S.-T. Myung, Y.-K. Sun, Sodium-ion batteries: present and future, Chem. Soc. Rev. 46 (2017) 3529–3614, https://doi.org/10.1039/C6CS00776G. [4] M.I. Jamesh, A.S. Prakash, Advancement of technology towards developing Na-ion batteries, J. Power Sources 378 (2018) 268–300, https://doi.org/10.1016/j. jpowsour.2017.12.053. [5] R. Zhang, J. Zhao, L. Guo, H. Qin, W. Shi, Z. Lu, First-principles investigation of a β-MnO2 and graphene composite as a promising cathode material for rechargeable Li-ion batteries, RSC Adv. 7 (2017) 29821–29826, https://doi.org/10.1039/ C7RA04837H. [6] S. Leng, X. Sun, Y. Yang, R. Zhang, Borophene as an anode material for Zn-ion batteries: a first-principles investigation, Mater. Res. Express 6 (2019), https://doi. org/10.1088/2053-1591/ab1a88, 085504. [7] Y. Noguchi, E. Kobayashi, L.S. Plashnitsa, S. Okada, J. Yamaki, Fabrication and performances of all solid-state symmetric sodium battery based on NASICONrelated compounds, Electrochim. Acta 101 (2013) 59–65, https://doi.org/ 10.1016/j.electacta.2012.11.038. [8] H. Pan, Y.-S. Hu, L. Chen, Room-temperature stationary sodium-ion batteries for large-scale electric energy storage, Energy Environ. Sci. 6 (2013) 2338, https://doi. org/10.1039/c3ee40847g.
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