LETTER pubs.acs.org/JPCL
Amorphous TiO2 Nanotube Anode for Rechargeable Sodium Ion Batteries Hui Xiong,† Michael D. Slater,‡ Mahalingam Balasubramanian,§ Christopher S. Johnson,*,‡ and Tijana Rajh*,† †
Center for Nanoscale Materials, ‡Chemical Sciences and Engineering, and §Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States
bS Supporting Information ABSTRACT: Sodium ion batteries are an attractive alternative to lithium ion batteries that alleviate problems with lithium availability and cost. Despite several studies of cathode materials for sodium ion batteries involving layered oxide materials, there are few low-voltage metal oxide anodes capable of operating sodium ion reversibly at room temperature. We have synthesized amorphous titanium dioxide nanotube (TiO2NT) electrodes directly grown on current collectors without binders and additives to use as an anode for sodium ion batteries. We find that only amorphous large diameter nanotubes (>80 nm I.D.) can support electrochemical cycling with sodium ions. These electrodes maximize their capacity in operando and reach reversible capacity of 150 mAh/g in 15 cycles. We also demonstrate for the first time a full cell all-oxide Na ion battery using TiO2NT anode coupled to a Na1.0Li0.2Ni0.25Mn0.75Oδ cathode at room temperature exhibiting good rate capability. Energy Conversion and Storage
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urrent energy thrust and demand for green energy technologies have intensified the pursuit of high-performance and cost-effective battery systems, which have zero carbon footprints. Whereas Li ion batteries offer the highest energy density among present battery technologies,1 there are still challenges remaining to be solved such as limited Li sources, cost, and safety.1,2 Alternative rechargeable battery systems with transporting ions other than Li ion have attracted growing interests in recent years.3,4 Sodium is a cheap, nontoxic and abundant element that is uniformly distributed around the world and therefore would be ideal as a transporting ion for alternative rechargeable batteries. However, to date, no low-voltage metal oxide anodes capable of operating with sodium ion at room temperature have been reported.2 The reason for this is likely the prohibitively large ionic radius of Na ion (1.02 Å) compared with the size of Li ion (0.76 Å);5 therefore, insertion of Na ion requires large distortion of the metal oxide lattice, which would require unacceptably elevated temperatures not realistic for operation of batteries. In this study, we utilized amorphous electrochemically synthesized 1D titanium dioxide nanotube (TiO2NT) as an anode for Na-ion batteries and demonstrated reversible self-improving specific capacity of ∼150 mAh/g. In addition, we demonstrated for the first time all-oxide reversible Na-ion batteries using TiO2NT anode and a lithium-substituted sodium-layered transition-metal oxide cathode6 operating at room temperature. TiO2 is a versatile material used in numerous important and diverse technological areas including environmental remediation,7 dye-sensitized solar cells,8 and nanotherapeutics.9,10 As an exceptionally stable, inexpensive, nontoxic, and abundant material, TiO2 has garnered significant attention in energy-storage applications, in r 2011 American Chemical Society
particular, in lithium-ion batteries.11 TiO2 is one of a few transitionmetal oxide materials that intercalates Li ions at reasonably low voltage (∼ 1.5 V vs Li/Li+) with comparable capacities to the dominant graphite anodes. Although a variety of metal oxide materials have been identified as sodium-ion cathode materials,4,1215 there are very few materials reported to be suitable for anode materials. With the success of TiO2 as anode materials for Li-ion batteries,11,16 it is interesting to investigate its utilization for Na-ion batteries. In our approach, we have chosen nanoscale amorphous TiO2 electrode, expecting enhanced diffusion of transporting ions. Nanostructured materials have shown improved power and energy densities compared with their bulk counterparts due to enhanced kinetics and large surface area.17 Recently, it has been shown that increased concentration of interfacial regions in amorphous materials may form percolation pathways to facilitate the diffusion of ions.18,19 On the basis of these reasons, we have investigated amorphous TiO2NT electrode as anode materials for Na-ion batteries. Using electrochemical anodization of Ti foil,20,21 we have synthesized densely packed, vertically oriented amorphous TiO2NTs that are electronically connected to the current collector (Figure 1a). This electrode design facilities efficient electron transport while eliminating the need for conductive carbon additives and binders typically used in electrodes that can alter their long-term stability.22 In our previous work (unpublished results), we had found that amorphous TiO2NT electrode underwent irreversible phase Received: September 1, 2011 Accepted: September 26, 2011 Published: September 26, 2011 2560
dx.doi.org/10.1021/jz2012066 | J. Phys. Chem. Lett. 2011, 2, 2560–2565
The Journal of Physical Chemistry Letters
LETTER
Figure 1. SEM top-view images of TiO2NT electrodes: amorphous TiO2NT (a) before and (b) after cycling in Na system.
Figure 2. Electrochemical characterization of TiO2NT in Na half-cell. (a) Charge/discharge galvanostatic curves of amorphous 80 nm I.D. TiO2NT in Na half cell (red for discharge and black for charge) cycled between 2.5 and 0.9 V versus Na/Na+ at 0.05A/g (C/3, discharge the electrode in 3 h). (b) Specific capacities as a function of cycle number measured at a current density of 0.05A/g in Na system: red solid squares represent discharge and black open squares represent charge; and at 3 A/g in Li system: red solid circles represent discharge and black open circles represent charge.
transition by self-organization upon electrochemical cycling with Li+. The newly formed cubic phase exhibits self-improving specific capacity as high as 310 mAh/g in a Li full cell (theoretical value of 335 mAh/g for one Li per TiO2). Facile diffusion on the nanoscale with amorphous framework allows high concentration of Li+ near TiO2NT to initiate the phase transition of high-performance TiO2 electrode. When amorphous TiO2NTs are used as electrodes in a sodium half-cell, we find that cycling of narrow NTs of amorphous TiO2 (80 nm I.D. (wall thickness >15 nm), we observe cycling with relatively low specific capacity initially that self-improves as cycling proceeds. Starting at a value of 75 mAh/g (mass based exclusively on TiO2NT) in the first cycle, the specific capacity almost doubles to reach 150 mAh/g after only 15 cycles (Figure 2a). This self-improving of the specific capacity upon cycling occurs at a slow rate of 0.05 A/g (Figure 2b) compared with observed self-improving of the specific capacity for a Li/TiO2NT system that occurs only at fast cycling of 3 A/g (Figure 2b). The question that arises is why Na ions do not intercalate into the smaller diameter tube (
