ECS Electrochemistry Letters, 3 (4) A23-A25 (2014) A23 2162-8726/2014/3(4)/A23/3/$31.00 © The Electrochemical Society Na 2/3 Ni 1/3 Ti 2/3 O 2 : “Bi-Functional” Electrode Materials for Na-Ion Batteries Rengarajan Shanmugam and Wei Lai z Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, Michigan 48824, USA In this paper, we demonstrate the electrochemical properties of a P2-type layered oxide, Na x (Ni 2+ ) 1/3 (Ti 4+ ) 2/3 O 2 (x = 2/3), as “bi-functional” electrode material for room temperature, non-aqueous Na-ion batteries. Making use of the high-voltage redox couple Ni 2+ /Ni 3+ or the low-voltage redox couple Ti 4+ /Ti 3+ , we substantiate Na 2/3 Ni 1/3 Ti 2/3 O 2 can function either as a cathode with an average voltage of 3.7 V and 75 mAh/g at C/20 or a anode with an average voltage of 0.7 V and 75 mAh/g at C/20. The cathodic Na 2/3 Ni 1/3 Ti 2/3 O 2 displays reversible sodium insertion/extraction but has a lower rate capability compared with anodic Na 2/3 Ni 1/3 Ti 2/3 O 2 . © 2014 The Electrochemical Society. [DOI: 10.1149/2.007404eel] All rights reserved. Manuscript submitted November 5, 2013; revised manuscript received January 24, 2014. Published February 6, 2014. Geographical limitations and uncertainties about the availability of sufficient lithium resources have led researchers to develop alter- native battery chemistries, such as Na-ion batteries, based on cheaper and more widely available elements, especially for bulk energy stor- age applications. 1–6 Developing good electrode materials that can re- versibly host sodium ions is crucial for further technological advance- ment and commercialization of Na-ion batteries. One of the prototypic sodium electrode materials is Na x CoO 2 (x∼0.7) 7 with a hexagonal layered structure (space group P6 3 /mmc), also denoted as P2-type by Delmas et al. 8 Na 0.7 CoO 2 was able to sustain sodium insertion/extraction at a potential range of 2–3.8 V. 9 However, this material exhibited complex phase transformation in this window, possibly due to the interplay of Na + /vacancy ordering at Na sites and charge ordering at Co sites. 10,11 The application of two or more transition metals with different valences is likely to interfere with vacancy/charge ordering and lead a more solid-solution behav- ior. Good electrochemical behaviors were reported for mixed valence P2-type oxides such as Na x Ni 1/3 Mn 2/3 O 2 , 12,13 Na x Fe 1/2 Mn 1/2 O 2 , 14 Na 0.45 Ni 0.22 Co 0.11 Mn 0.66 O 2 , 15 Na 0.85 (Li 0.17 Ni 0.21 Mn 0.64 )O 2 , 16 etc, al- though some plateaus/steps were still visible suggesting phase trans- formation. Na 3 V 2 (PO 4 ) 3 has been shown to work as “bi-functional” electrode using V 3+/4+ (E o = 3.40 V) and V 2+/3+ (E o = 1.63 V) electro-active redox couples. 17 However, the toxicity of vanadium can potentially become an issue for building large-scale, commercial devices. The excellent ionic conductivity of another mixed valence P2- type material Na x (Ni 2+ ) 1/3 (Ti 4+ ) 2/3 O 2 (x = 2/3), was reported previously. 18,19 By selectively activating the high-voltage redox cou- ple Ni 2+ /Ni 3+ or the low-voltage redox couple Ti 4+ /Ti 3+ , we suppose it can function either as a cathode with 90 mAh/g (x from 2/3 to 1/3) or as an anode with the same 90 mAh/g (x from 2/3 to 1). In this letter, we report for the first time electrochemical properties of Na 2/3 Ni 1/3 Ti 2/3 O 2 (SNTL), as an alternative bi-functional electrode either at the high or low voltage windows using two different redox active species in the same material. Experimental Starting materials (all from Sigma-Aldrich) for the synthesis of Na 2/3 Ni 1/3 Ti 2/3 O 2 (SNTL) powders were stoichiometric amount of Na 2 CO 3 (≥99.5%), NiO (micron powder, 99%; < 50 nm powder, 99.8%), and TiO 2 (micron powder, 99%; 21 nm powder, ≥99.5%). For a typical synthesis of 10 g SNTL powder, 4.6 g of Na 2 CO 3 , 2.5 g of NiO, and 5.4 g of TiO 2 were used. Na 2 CO 3 of 10 wt% excess were added to compensate the sodium oxide evaporation during high-temperature processing. Both micron- and nano-sized NiO and TiO 2 were employed to compare the effect of particle size on the ease of synthesis and the electrochemical properties. Micron-sized z E-mail: laiwei@msu.edu precursors were subjected to dry-milling using SPEX SamplePrep 8000 M mixer/mill. Nano-sized precursors were mixed in a jar mill with 2-propanol as solvent. After uniform mixing and solvent removal at 120 ◦ C, the powders were fired at 900 ◦ C in air within a box furnace. The synthesized powders were made into a slurry which con- tained 80 wt% active powders, 10 wt% PVDF binder in N-methyl- 2-Pyrrolidone (NMP), and 10 wt% Timcal Super-P as conductive additives. Powders from nano-sized precursors were used unless stated otherwise. The slurry was cast into a composite film on an alu- minum foil. The loading of active material was around 4 mg/cm 2 . The Swagelok cell assembly was carried out inside the glove box with sodium metal serving as counter and reference electrodes. 0.5 M NaPF 6 in 50/50 vol% of ethylene carbonate (EC)/diethyl carbonate (DEC) was used as the electrolyte for testing. Electrochemical testing was performed with a Bio-logic VSP300 workstation. Galvanostatic charging was carried out in a voltage window of 2.0–4.2/4.5 V for the cathode and 0.2–3.0 V for the anode. The cell was disassembled inside the glove box and the film was washed with dimethyl carbonate solvent to remove residual salts and eventually the solvents were dried in the glove box. Powder X-ray diffraction (XRD) measurements were performed using a Bruker D8 ADVANCE diffractometer using Cu-Kα X-rays. Both pristine powders and composite films were studied. The powder samples were also imaged in a scanning electron microscope (JEOL- JSM 7500-F). The BET surface area of the powders was measured using Micromeritics ASAP 2020 instrument with Krypton as the ad- sorbate gas. Results and Discussion Powder characterization.— It was found that micron-sized pre- cursors yielded phase-pure samples at 900 ◦ C when fired for 12 hours while nano-precursors yielded pure powders in just 2 hours, as ob- served in XRD in Figure 1a. Below 2 hours, impurity peak probably due to NiO appeared. Micron-sized and nano-precursor powders were 5–10 μm and less than 5 μm, respectively, as evident from SEM micrographs in Figure 1b. The powders made from micron-sized pre- cursors and nano-precursors had a BET surface area of 0.58 m 2 /g and 1.30 m 2 /g, respectively. SNTL as a cathode.— When cycled between 2 V and 4.2 V at a rate of C/50 (1.8 mA/g), SNTL had a sloping voltage profile, as shown in Figure 2a, suggesting a solid-solution behavior. This is further substan- tiated by ex-situ XRD, in Figure 2b, where no additional phases were identified upon sodium removal at 40% and 80% of state-of-charge (SOC). The large irreversible capacity observed in the first few cycle might be due to parasitic side-reactions such as aluminum passivation and/or electrolyte degradation. After the first charge, good reversibil- ity at an average voltage of ∼3.7 V and ∼90 mAh/g (x from 2/3 to 1/3) was obtained for 5 cycles. 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