Titanium Sulfides As Intercalation-Type Cathode

Research Article www.acsami.org Titanium Sulfides as Intercalation-Type Cathode Materials for Rechargeable Aluminum Batteries † ‡ † § ‡ § Linxiao Geng, Jan P. Scheifers, Chengyin Fu, Jian Zhang, Boniface P. T. Fokwa, , † § and Juchen Guo*, , † Department of Chemical and Environmental Engineering, University of California, Riverside, California 92521, United States ‡ Department of Chemistry, University of California, Riverside, California 92521, United States § Materials Science and Engineering Program, University of California, Riverside, California 92521, United States *S Supporting Information ABSTRACT: We report the electrochemical intercalation−extraction of aluminum (Al) in the layered TiS2 and spinel-based cubic Cu0.31Ti2S4 as the potential cathode materials for rechargeable Al-ion batteries. The electrochemical characterizations demonstrate the feasibility of reversible Al intercalation in both titanium sulfides with layered TiS2 showing better properties. The crystallographic study sheds light on the possible Al intercalation sites in the titanium sulfides, while the results from galvanostatic intermittent titration indicate that the low Al3+ diffusion coefficients in the sulfide crystal structures are the primary obstacle to facile Al intercalation−extraction. KEYWORDS: aluminum-ion battery, aluminum intercalation, multivalent ion battery, titanium sulfides, ionic liquid electrolyte 17 fi ■ INTRODUCTION tion in Mo6S8. Our selection of transition metal sul des in The rechargeable aluminum-ion (Al-ion) battery is an place of oxides as Al-ion cathode materials is crucial: Due to the strong Coulombic effect, the energy barrier for multivalent ion intriguing electrochemical energy storage technology based 18 on the most abundant metal in the earth’s crust. It can be a transport in the oxide crystal structure is high. Therefore, a potentially interruptive technology for large scale energy more polarizable (softer) anionic framework is needed for facile 3+ − storage applications.1 However, reversible electrochemical Al ion intercalation extraction, making transition metal fi processes involving Al are inherently difficult. One challenge sul des promising candidates as Al-ion cathode materials. is the lack of feasible electrolytes for Al deposition−dissolution Although Mo6S8 showed unambiguous electrochemical Al − fi at the anode. The most commonly adopted Al electrolytes to intercalation extraction properties, its speci c capacity was date are Lewis acidic mixtures of aluminum halides (AlX ,X= not ideal due to its high molecular mass. We herein report the 3 − fi Cl and Br) and halide-containing ionic liquids (ILs), such as 1- Al intercalation extraction properties of two titanium sul des, butylpyridinium chloride and 1-ethyl-3-methylimidazolium layered TiS2 and spinel-based, cubic Cu0.31Ti2S4, as potential chloride, with molar ratio > 1.2,3 Halide-free IL electrolyte4 intercalation-type cathode materials for Al-ion batteries. 5,6 and aqueous AlCl3 electrolytes have also been reported. Another challenge comes from the lack of understanding of ■ EXPERIMENTAL SECTION cathode materials. With increasing interest, a few potential Synthesis of Layered TiS . Layered TiS was synthesized via solid cathode materials have been proposed based on various 2 2 7−17 state reaction by heating stoichiometric mixture of the elements (Ti reaction mechnisms. Transition metal oxides, particularly and S).19 Stoichiometric amounts of Ti powder (Alfa Aesar, 325 mesh, vanadium(V) oxide (V O ), were studied as intercalation-type − − 2 5 99.5%) and S powder (Sigma-Aldrich 99.5 100.5%) were thoroughly cathode materials.10 14 Lin and co-workers proposed graphitic mixed and then sealed in an evacuated quartz tube, which was carbon as a possible cathode material based on an anion subsequently heated in a muffle furnace. The temperature was first − intercalation mechnism.15 Archer’s and Wang’s groups also ramped up to 450 °C at a rate of 0.3 °C min 1 and then held at 450 °C presented the possibility of sulfur as a cathode material based for 24 h, after which the temperature was ramped up to 640 °Cin24h ° on a redox conversion reaction.8,9 and held at 640 C for 3 days. The synthesized TiS2 was collected in We previously reported the Chevrel phase molybdenum fi fi sul de (Mo6S8) as an intercalation-type metal sul de cathode Received: March 23, 2017 material.16 An independent investigation by Lee and co-workers Accepted: June 1, 2017 further confirmed the reversible electrochemical Al intercala- Published: June 1, 2017 © 2017 American Chemical Society 21251 DOI: 10.1021/acsami.7b04161 ACS Appl. Mater. Interfaces 2017, 9, 21251−21257 ACS Applied Materials & Interfaces Research Article fi Figure 1. XRD patterns and Rietveld re nements of layered TiS2 (a) and cubic Cu0.31Ti2S4 (b); SEM images of layered TiS2 (c) and cubic Cu0.31Ti2S4 (d). fi an argon- lled glovebox to prevent oxidization and hydrolysis. was determined to be Cu0.31Ti2S4 via XRD analysis and Rietveld Mechanical ball milling was employed to reduce the particle size of refinement and was confirmed by EDS elemental mapping. The SEM TiS2 with a Fritsch Pulverisette 23 mini-Mill. The entire ball milling images and the XRD patterns of CuTi2S4 before and after ball milling process was performed in the argon-filled glovebox. In a typical ball are shown in the Supporting Information (Figure S3) with the EDS milling process, two 10 mm diameter and 18 5 mm diameter tempered element analysis of the synthesized Cu0.31Ti2S4 (Figure S4 and Table steel grinding balls along with 0.346 g of TiS2 (weight ratio 50:1) were S2). put into a tempered steel grinding bowl with 3 mL of anhydrous Electrochemical Analysis. CR2016 coin cells were assembled in toluene. To prevent overheating, the milling was performed in 10 an argon-filled glovebox using Al foil (0.2 mm thickness, Alfa Aesar, sessions of 5 min, each separated by 10 min of idling. After the ball 99.9999%) as the anode. The cathode was fabricated by coating TiS2 milling, the TiS2 particles were washed with anhydrous acetonitrile 3 or Cu0.31Ti2S4 slurry onto a carbon paper current collector times and then collected with a centrifuge. The scanning electron (Spectracarb 2050A-0550, Fuel Cell Store). The carbon paper current microscopy (SEM) image and the powder X-ray diffraction (XRD) collector was demonstrated to be electrochemically inert in the applied pattern of the pristine TiS2 are shown in the Supporting Information potential window as shown in the Supporting Information (Figure S5). (Figure S1) with the energy dispersive X-ray spectroscopy (EDS) The slurry was made by mixing 80 wt % TiS2 or Cu0.31Ti2S4,10wt% elemental mapping of the TiS2 after ball milling (Figure S2 and Table carbon black, and 10 wt % polystyrene in N-methyl-2-pyrrolidone via a S1). mechanical mixer for 5 min. A single Whitman glass fiber filter was fi Synthesis of Cubic Cu0.31Ti2S4. Thiospinel CuTi2S4 was rst used as the separator. The electrolyte was prepared by slowly adding synthesized using a solid state method similar to that described anhydrous AlCl3 (Sigma-Aldrich, 99.99%) into 1-butyl-3-methylimi- above.20 Stoichiometric amounts of Cu (Sigma-Aldrich, 14−25 μm, dazolium chloride (Sigma-Aldrich, 98%) with a molar ratio of 1.5:1 99%), Ti, and S powders were thoroughly mixed and sealed in an while stirring in the argon-filled glovebox. It is worth noting that evacuated quartz tube. The quartz tube was heated in a muffle furnace polystyrene was selected as the binder due to its inertness in the Lewis at 700 °C (ramp rate, 0.3 °C min−1) for 3 days. The synthesized acidic ionic liquid electrolyte compared to conventional binders such fi fl CuTi2S4 was collected in the argon- lled glovebox to prevent as poly(vinylidene uoride) (Figure S6 in the Supporting oxidization and hydrolysis. The particle size of CuTi2S4 was reduced Information). To prevent corrosion from the acidic electrolyte, via mechanical ball milling as described above. Cu was leached from titanium foil (Strem, 0.025 mm thickness, 99.6%) and pyrolytic the CuTi2S4 product by reaction with bromine. In a typical leaching graphite sheet (MTI, 0.017 mm thickness, 99.90%) were punched into process, 300 mg of CuTi2S4 was suspended in 100 mL of anhydrous discs of an appropriate size to be used as linings in the stainless-steel acetonitrile. Then, 94 μL of pure bromine was added to the coin cell case. The schematic of the coin cell construction is shown in suspension. The mixture was stirred for 24 h. After the reaction, the Supporting Information (Figure S7). Cyclic voltammetry (CV) with a product was washed twice with both acetonitrile and carbon disulfide scan rate of 0.1 mV s−1 was performed with a Gamry potentiostat and collected with a centrifuge. The chemical formula of the product Interface 1000. Galvanostatic discharging and constant-current− 21252 DOI: 10.1021/acsami.7b04161 ACS Appl. Mater. Interfaces 2017, 9, 21251−21257 ACS Applied Materials & Interfaces Research Article Figure 2. First, second, 10th, and 20th galvanostatic discharge and CCCV charge curves of layered TiS2 (a and b) and cubic Cu0.31Ti2S4 (c and d) at RT and 50 °C. constant-voltage (CCCV) charging and galvanostatic intermittent chemically leached CuTi2S4 had a formula of Cu0.31Ti2S4. The titration technique (GITT) were performed on an Arbin battery test particle size of TiS2 (diameter of the plate-like particle) and station. In GITT, the batteries were discharged and charged at 10 mA −1 Cu0.31Ti2S4 obtained from SEM images (Figure 1c,d) was g for 15 min and rested for 2 h. The current pulse was repeated until μ ff ff ffi 3+ mostly smaller than 1 m. the potential reached the cuto limit. The di usion coe cient of Al − was calculated according to the following equation: The electrochemical Al intercalation extraction in these two titanium sulfides were characterized with discharge−charge ⎡ ⎤2 ° ⎛ ⎞2 dE experiments at both room temperature (RT) and 50 C 4 IV ⎢ ()dx ⎥ D = ⎜ M ⎟ ⎢ ⎥ considering the slow intercalation kinetics observed in the ⎝ ⎠ dE 16,22 τ ZFSA ⎢ ⎥ Chevrel phase Mo S at RT.

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