Electrochemical Analysis of Na0.7Co1-xNbxO2(x = 0, 0.05) as Cathode Materials in Sodium-ion Batteries

Content Provider	Semantic Scholar
Author	Pati, Jayashree Chandra, Mahesh Dhaka, Rajendra S.
Copyright Year	2019
Abstract	Sodium-ion batteries (SIBs) have received significant attention as promising alternative for energy storage applications owing to the large availability and low cost of sodium. In this paper we study the electrochemical behavior of Na0.7Co1-xNbxO2 (x = 0 and 0.05 samples), synthesized via solid-state reaction. The Rietveld refinement of x-ray diffraction pattern reveals the hexagonal crystal symmetry with P63/mmc space group. The Na0.7Co0.95Nb0.05O2 cathode exhibits a specific capacity of about 91 mAhg at a current density of 6mAg, whereas Na0.7CoO2 exhibits comparatively low specific capacity (70 mAhg at a current density of 6mAg). The cyclic voltammetry (CV) and electron impedance spectroscopy (EIS) were performed to determine the diffusion coefficient of Na, which found to be in the range of 10!! − 10!!" cms. INTRODUCTION Sustainable energy alternatives have become a global need in order to satisfy growing energy demands in recent years. Although recent developments in Li-ion battery (LIB) technology have created a benchmark in the energy requirement for large scale energy storage systems, the inadequacy of Liion in the earth’s crust and high cost have led to a search for low cost battery system. In this direction, sodium-ion batteries (SIBs) have been considered as a promising alternative for energy storage devices owing to the large abundance and low cost of sodium [1]. However, larger ionic size of Na-ion (1.02Å) than Li-ion (0.76Å), is a major problem to explore efficient electrodes for high rate capability SIBs. Therefore, developing an efficient cathode material with high specific capacity and long cycle life is a bottleneck process. Layered metal oxides are considered as one of the highly reversible cathode materials due to their stable cycling performances, rich chemistry and capability to tolerate the high stress originated due to structural changes. These layered oxides are classified into two main groups, O3 type and P2 type, where “O” and “P” refers to the Na coordination (P-Prismatic, O-Octahedral) and “2” or “3” to the number of MO2(M-transition metals) slabs in that particular cell [1]. In case of P2-NaxCoO2, there are two distinct prismatic sites; one is sharing only faces whereas the second one is sharing only edges with CoO6 octahedra [2]. Usui et al. proposed Nb-doped rutile TiO2 as an anode for SIBs, where Nb-doping significantly boosts the Naion storage performance due to the enhanced conductivity and broadened ionic channels [3]. These effects of Nbdoping are also expected for Na0.7CoO2 cathode material, which has never been reported to the best of our knowledge. Here, we have prepared P2Na0.7Co1-xNbxO2 (x=0, 0.05) by a facile solid-state route. We observe improvement in the specific capacity and diffusion coefficient, which can be due to the widened Na-ion migration channels and much reduced charge transfer resistance with Nb substitution in Na0.7CoO2. EXPERIMENTAL DETAILS We have synthesized polycrystalline Na0.7CoO2 (NCO) and Na0.7Co0.95Nb0.05O2 (NCNO) through solid-state reaction using percursors Na2CO3 (sodium carbonate), Co3O4 (cobalt oxide) and Nb2O5 (niobium pentoxide, for x=0.05 sample). Initially, these two materials with proper stoichometric amount were grinded for 6-7 hours and then sintered directly at 800°C in a preheated furnace to obtain the final product. The crystal structure of the samples were obtained by x-ray diffraction (XRD) using CuKα radiation (λ =1.5406 Å) from Panalytical x-ray diffractometer in the 2θ range of 10–70°. The electrode was fabricated by casting the slurry on bare aluminium foil. We used doctor blade method to prepare the slurry, where active material, carbon and binder (Polyvinylidene fluoride, PVDF) are mixed with a initial weight ratio 8:1:1. The 2016 type coin-cells were assembled using a crimping machine (MTI Corp.) inside a nitrogen filled glove box (Jacomex). The electrolyte used was 1M NaClO4 dissolved as the solute in 1:1ethylene carbonate (EC) and diethyl carbonate (DEC). We have used potentiostat (Palmsens) for CV, battery cycler (Bio-Logic-VMP3) for EIS and galvanostatic charging discharging using cycler from Neware. RESULTS AND DISCUSSION The Reitveld refinement of room temperature XRD patterns of both the smaples confirm the hexagonal crystal structure with space group: P63/mmc (no. 194), as shown in Figs. 1(a) and 1(b). We found the lattice parameters of a=b=2.823Å and c=10.933Å for NCO sample and a=b=2.825Å and c= 10.954Å for NCNO sample. The convergence factor (χ) is about 1.4, which confirms the good quality of fitting. The XRD results show that the Nb substitution slightly enhances the volume of the unit cell. The scanning electron microscopy (SEM) images in Figs. 1(c) and (d) reveal the platelet like morphology and confirm that the particles are typically in the μm range. FIGURE 1: Room temperature XRD pattern (red) and Rietveld refinement (black) of as-prepared (a) NCO and (b) NCNO, and SEM images of the (c) NCO (d) NCNO samples. Figures 2(a) and 2(b) depict the Nyquist plots of freshly fabricated Na-NCO and Na-NCNO half cells, respectively where Na metal is used as counter electrode. Based on the nature of the Nyquist plots, we here design equivalent circuits for the EIS spectra, as shown in the insets of Figs. 2(a) and 2(b). The R1 represents the sum of Ohmic resistances (resistances of working electrode, electrolyte and current collector). In Fig. 2(a) inset Q1//R2 and Q2//R3 correspond to the capacitance and resistance in the high frequency (HF) and low frequency (LF) regions, respectively. Here, constant phase element (Q) is used instead of capacitor to show the non-ideal behaviour of the electrode due to rough/porous surface [4]. The straight line in the LF region is fitted by using a constant phase element, Q3 instead of warburg impedance. Warburg impedance is used to show the diffusion of the ions within the cathode. In Fig. 2(b), R2 is the charge transfer resistance, which reflects the direct charge transfer to and from the electrode surface and R3 refers the charge transfer resistance at the electrolyte/immobilised substances [5]. We can calculate the diffusion coefficient D from the LF region data of EIS using the following equation: D = ! ! !! !"# !" !" ! (1) where, V! is the molar mass of the active material, A is the area of the electrode, dE/dx is the slope of the cell voltage to the Na ion concentration, which is procured from the plot in Fig. 2(i), F is the Faraday constant (96486 C 10 20 30 40 50 60 70 Yobs Ycalc Yobs Ycalc Bragg Position (1 06 ) (1 10 ) (1 12 ) (1 03 ) (1 02 )
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