Article pubs.acs.org/cm
Tetragonal Tungsten Bronze Framework as Potential Anode for NaIon Batteries Yanlin Han,†,‡ Minghui Yang,§ Ye Zhang,‡ Junjie Xie,‡ Dongguang Yin,*,† and Chilin Li*,‡ †
School of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, China State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Ding Xi Road, Shanghai 200050, China § Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China Chem. Mater. 2016.28:3139-3147. Downloaded from pubs.acs.org by UNIV OF NEW ENGLAND on 01/13/19. For personal use only.
‡
S Supporting Information *
ABSTRACT: Na-ion batteries (NIBs) are becoming more promising owing to potentially better rate performance than Li-ion batteries apart from resource abundance. However, activation of Na-storage electrochemistry remarkably depends on the modification or expansion of existing structure prototypes by adjusting the substituent and linkage of their ligand moieties or the redox transition metals. Recently the natural existence of mineral phases has inspired us to explore more plentiful artificial analogues, most of which possess open framework properties. Here for the first time we propose a novel bronze phase (tetragonal tungsten bronze, TTB) of oxyfluoride (KNb2O5F) as a potential NIB anode. This tunnel-type open framework is achieved by equimolar KF doping into T-Nb2O5 with K and F as channel supporter and ligand substituent, respectively. The improvement of intrinsic conductivity and near-zero strain sodiation enable a highly reversible capacity even by employing undecorated samples of less surface defects. Reduction of Nb5+ may cause a uniform precipitation of fine NbO2 nanoparticles around larger solidated KNb2O5F.
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INTRODUCTION Room temperature sodium ion batteries (NIBs) are attracting increasing attention in view of the abundant reserve and low cost of sodium resource.1,2 Owing to a 34% larger ion radius (1.02 Å) of Na+ than that (0.76 Å) of Li+,1 Na diffusion is usually sluggish in many electrode structures and often drives the generation of staged or intermediate phases during sodiation/desodiation (e.g., in spinel Li 4Ti5O12, olivine FePO4).3,4 At the NIB anode side, alloying (Sn, Sb) and conversion (FeOx, VN) materials have been widely investigated due to their high theoretical capacities.5−8 In view of the promising application of NIBs in large-scale stationary energy storage, structural stability is also an important concern.9 To match with current insertion-type cathodes for NIBs dominated by layered oxides, polyanionic compounds and Prussian Blue analogues with a moderate capacity (99%, United copper foils (Huizhou) Ltd.) and then dried in a vacuum oven at 80 °C for 12 h. The loading mass of active material in the electrode slurry is ∼2 mg/cm2. The electrolyte was 1 M NaClO4 dissolved in ethylene carbonate (EC) and propylene carbonate (PC) with a volume ratio of 1:1 or 0.5 M sodium trifluomethanesulfonate (NaTFSI) dissolved in tetraethylene glycol dimethyl ether (TEGDME). Glass fibers (GF/B) from Whatman and pure sodium metal foil were used as separators and counter electrode, respectively. The coin cells were assembled in an argon filled glovebox (H2O, O2 < 0.1 ppm, Mbraun, Germany). Charge−discharge measurements were performed at room temperature under different rates from 0.1C to 5C
EXPERIMENTAL SECTION
KNb2O5F of tetragonal tungsten bronze structure was prepared by high temperature solid state reaction. The mixture of niobium pentaoxide (99.9%, Aladdin) and potassium fluoride (99%, Alfa Aesar) with a molar ratio of 1:1 was ground in a mortar for 1 h under argon atmosphere. After grinding the mixture was put into a sealed container and was heated in a tube furnace at 900 °C for 12 h. Then the sample was naturally cooled to room temperature. Structure and crystallinity of pristine and cycled samples were analyzed by X-ray diffraction (XRD, D8 Discover, Bruker) in a 2-theta range of 10−80° at a scan rate of 0.02°/s using Cu Kα radiation. Crystal structure of pristine powder sample was confirmed by powder 3140
DOI: 10.1021/acs.chemmater.6b00729 Chem. Mater. 2016, 28, 3139−3147
Article
Chemistry of Materials
Figure 2. Galvanostatic charge/discharge curves of (a) KNb2O5F/NaClO4 and (b) KNb2O5F/NaTFSI cells during the early cycles at 0.1C. Insets: corresponding first discharge curves. (c) Cycle and (d) rate performance of KNb2O5F anodes in both electrolytes at different current densities from 0.1C to 5C. In a voltage range of 0.1−3.0 V, both cells display sloped charge/discharge curves with good symmetry and without distinct plateaus during the following cycles. KNb2O5F/NaClO4 shows better cycling stability and rate performance especially above 2C, whereas higher reversible capacity is achieved in KNb2O5F/NaTFSI. (1C denotes the current density to theoretically achieve one-electron transfer within 1 h for KNb2O5F or Nb2O5) in a voltage range of 0.1 V−3.0 V (vs Na+/Na) on the Land multichannel battery testing system (CT2001A). Cyclic voltammetry (CV) was carried out at a series of scan rates from 2 to 20 mV/s in a voltage range of 0.1−3.0 V by electrochemical workstation (700D, CHI Instrument).
widening of channel size and decrease of transport dimensionality from 2D to 1D but also potential modification of intrinsic electric properties. The galvanostatic charge−discharge process of KNb2O5F as NIB anode was performed at 25 °C in two typical electrolytes, i.e., 1 M NaClO4 dissolved in ethylene carbonate (EC) and propylene carbonate (PC) with a volume ratio of 1:1 (termed KNb2O5F/NaClO4 cell) and 0.5 M sodium trifluomethanesulfonate (NaTFSI) dissolved in tetraethylene glycol dimethyl ether (TEGDME) (termed KNb2O5F/NaTFSI cell) as shown in Figure 2. The electrochemistry of conductive carbon was also measured under the conditions of similar carbon load and current density to those of composite electrodes in order to resolve its capacity contribution (Figure S3). Considering the mass ratio of KNb2O5F and C (5:3), the charge capacity of carbon is as small as ∼20 mAh/g at 0.1C. Therefore, the capacity value is estimated based on the mass of KNb2O5F in the following discussion. In both the cases, the first discharge capacities are ∼600 mAh/g for KNb2O5F/NaClO4 and ∼400 mAh/g for KNb2O5F/NaTFSI at 0.1C from 2 to 0.1 V. They exceed the theoretical value of 165.5 mAh/g based on one electron transfer by reducing Nb5+ to Nb4+ (even though removing the capacity contribution of carbon). Since it is difficult to electrochemically synthesize Nb2+ species and Nb0 metal (as also demonstrated by XPS later), the excess capacity stems from the formation of solid electrolyte interphase (SEI) during the initial sodiation as discussed below.25 The nature and component of SEI influence the ultimate capacity released in both the electrolytes. The reversible charge capacity at 0.1C in KNb2O5F/NaClO4 is preserved at 80 mAh/g for at least 140 cycles (from 90−95 mAh/g in the first charge) without serious capacity fluctuation, whereas that in KNb2O5F/NaTFSI degrades gradually from a higher 170−180 mAh/g in the first charge to 100−110 mAh/g after 140 cycles (Figure 2c,d). KNb2O5F/NaClO4 presents better capacity retention at higher
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RESULTS AND DISCUSSION Figure 1a shows the XRD pattern and its Rietveld refinement of KNb2O5F prepared by high-temperature solid state reaction.29 The XRD pattern is well fitted by Rietveld refinement according to TTB structure model (Rwp = 14.7%).28 The lattice parameters as summarized in Table S1 are refined to be a, b = 12.6269(3) Å, c = 3.9478(1) Å (cell volume = 629.43 (5) Å3) with tetragonal symmetry in space group P4/mbm. In this TTB phase (Figure 1b), Nb atoms are 6-fold-coordinated by O and F atoms with a number ratio of 5:1. NbO5F octahedral chains are connected by sharing vertex O/F atoms, leading to the formation of tetragonal (A1-site) and pentagonal (A2-site) cavities stacking along [001] axis. These cavities are partially (for A1-site) or fully (for A2-site) occupied by K-ions. Cavity cations are either 15-fold coordinated in the tunnel with a pentagonal section or 12-fold coordinated in the surviving perovskite cage. The channel with a triangular section (C-site) is left empty, since it is too small to host K-cations. High temperature synthesis results in coarsing of sample grains with size ranging from 100 to 400 nm as shown in the SEM image of Figure 1c. These grains are tightly contacted with each other to form distinct grain boundaries as usually observed in ceramic samples. In contrast to white powder of commercially available T-Nb2O5 (Figure S1), KNb2O5F powder is dark green (Figure 1d), indicating bandgap narrowing and conductivity improvement after partially displacing coordinational O by F. An equimolar doping of KF into dense T-Nb2O5 (see the crystalline structure in Figure S2) triggers not only the 3141
DOI: 10.1021/acs.chemmater.6b00729 Chem. Mater. 2016, 28, 3139−3147
Article
Chemistry of Materials
Figure 3. CV curves of (a) KNb2O5F/NaClO4 and (b) KNb2O5F/NaTFSI cells at various scan rates from 2 to 20 mV/s between 0.1 and 3.0 V. Insets: power law dependence of measured current on scan rate at corresponding peak potentials (eq 2). CV curves of (c) KNb2O5F/NaClO4 and (d) KNb2O5F/NaTFSI at a scan rate of 4 mV/s with capacitive currents (k1ν) being plotted at certain potentials. The capacitive contribution of charge storage is shown in the green area. Column graphs of rate-dependent stored charge comparison from both capacitive and intercalated process respectively for (e) KNb2O5F/NaClO4 and (f) KNb2O5F/NaTFSI.
rates (>2C, e.g., 45 and ∼30 mAh/g at 2C and 5C, respectively) than KNb2O5F/NaTFSI, which however displays higher capacity at moderate rates (e.g., ∼90 and 60 mAh/g at 0.5C and 1C, respectively). In a voltage window of 0.1−3.0 V, both the cells display sloped charge/discharge curves with good symmetry and without distinct plateaus during the following cycles at different rates (Figure 2a,b and Figure S4), indicating a potential contribution of pseudocapacitance effect. We also performed T-Nb2O5/NaClO4 cell as a comparison (Figure S5). Its reversible charge capacity quickly decays from 225 mAh/g in the first cycle to 50 mAh/g after 140 cycles at 0.1 C. The better capacity performance of KNb2O5F than T-Nb2O5 indicates the advantage of channel expansion for favoring Na-storage after KF doping. CV measurement (Figure 3a,b) at various scan rates is performed to quantitatively distinguish the pseudocapacitance contribution from insertion reaction.22,30 In accordance with charge/discharge curves, the redox peaks are broad and lie at ∼0.5 V with small overpotential in both the electrolytes. By analyzing the relationship between current (i) and scan rate (v) from CV data, we can deduce the type of electrochemical process:
i(V ) = avb
(1)
log i(V ) = b log v + log a
(2)
In view of overpotential change with scan rate, the independent variable potential is slightly adjusted based on equidistribution principle. The b-value can be determined from the slope of the plots of log i vs log ν (eq 2, insets of Figure 3a,b at corresponding peak positions). If the process is diffusioncontrolled (i.e., faradaic intercalation), the b-value approaches 0.5, whereas b is close to 1.0 if the process is capacitancedominated.31,32 It is obvious that b-values lie between 0.5 and 1 at lower scan rates (≤4 mV/s) and approach 0.5 with increasing scan rate (≥6 mV/s). It indicates a conversion from capacitance- (fast kinetics) to diffusion-control (slow kinetics) with the increase of scan rate. The similar b-value change was also often seen in other nonaqueous Li- and Na-ion capacitors.19,24 By further analyzing the current response at a fixed potential as a combination of capacitive effect and intercalation process, we can calculate their respective contributions: i(V ) = k1v + k 2ν1/2
(3)
i(V )/ν1/2 = k1ν1/2 + k 2
(4)
1/2
where k1v and k2v denote the current contributions from capacitive effect and intercalation process, respectively. Both the parameters k1 and k2 are determined from the linear relationship of iv−1/2 and ν1/2 (eq 4, also see Figure S6). Figure 3c,d displays the CV curves at 4 mV/s with the plotted capacitive current (k1ν) distinguished from the total current (k1ν + k2ν1/2). The ratio of stored charge from capacitive
where i as a function of potential (V) obeys a power law relationship with ν and a and b are the adjustable parameters. 3142
DOI: 10.1021/acs.chemmater.6b00729 Chem. Mater. 2016, 28, 3139−3147
Article
Chemistry of Materials
Figure 4. (a) Ex situ XRD patterns of KNb2O5F powder, pristine electrode, electrodes discharged to 0.1 V and recharged to 3.0 V in NaTFSITEGDME. (b) Magnified XRD patterns of the corresponding KNb2O5F samples from 2θ = 25° to 30°. The cycled samples show similar diffraction peaks as pristine sample without remarkable evolution and shift, indicating a topotactic solid-solution (de)sodiation with negligible cell volume change. The near-zero strain characteristic benefits from the robust open framework of TTB phase.
from the distribution of SAED spots (Figure 6c). The tetragonal symmetry is recoverable after recharging to 3 V (Figure 6f). At that time the SEI layer generated during discharge is almost removed (Figure 6d,e). The SEI (reversible) evolution is also indicated by XPS as discussed below. Figure 7 shows that the Nb 3d spectra of the pristine sample have two characteristic peaks at 208.5 and 211.2 eV, which should be assigned to Nb (V) 3d5/2 and Nb (V) 3d3/2, respectively. The 1 eV more positive shift of binding energy than for Nb2O5 (207.4 and 210.2 eV for Nb 3d5/2 and 3d3/2) is likely caused by fluorination of Nb−O units into NbO5F octahedra, as well as polyhedral linkage change from the coexistence of edge- and corner-sharing manners in Nb2O5 to exclusive corner-sharing way with alleviation of polyhedral distortion in KNb2O5F.34,35 Complete fluorination even leads to a Nb(V) 3d5/2 peak with more positive binding energy at 209.4 eV as observed in K2NbF7.36 After discharge to 0.1 V, KNb2O5F/NaTFSI has a more negative shift of binding energy for Nb 3d peaks (206.9 and 209.6 eV for Nb 3d5/2 and 3d3/2) than KNb2O5F/NaClO4 (207.9 and 210.6 eV for Nb 3d5/2 and 3d3/2), indicating a deeper reduction of Nb5+ for the former. This is in good agreement with a larger reversible capacity for the former. As opposed to mesoporous Nb2O5, the generation of Nb2+ or NbO can be ruled out in view of the absence of XPS peaks at lower binding energy (
