Investigation of TiO2 and Other Electrode Materials in Lithium-ion Batteries: Improving Capacity and Cycling Performance

Two promising areas of research with respect to lithium ion batteries (LIB’s) include (a) transition metal oxides, and (b) other high capacity Li-containing compounds that can serve as conversion reactions. Recent studies show that utilizing conversion reactions leads to greater capacities, and nano-porous carbons (NPCs) impregnated by materials such as metal oxides have yielded high reversible lithium storage capacities. A capacity of 600 mAh/g has been achieved using TiO2 particles formed inside NPCs and lithium foil as a counter electrode. The sample was cycled at a constant current of 0.5 mA between .1 V to 4 V. Ordered mesoporous carbon-TiO2 (OMCT65) was prepared by a basic polymerization method utilizing self-assembly and pyrolysis. Light metal hydrides are also attractive as conversion materials and may improve these results when used instead of metal oxides, as they have exhibited high lithium mobility. Ordered mesoporous carbon containing LiBH4 particles inside the pores has been synthesized using a similar method, and results are pending. Introduction Renewable sources of energy are in high demand for applications such as hybrid electric vehicles (HEV’s). Lithium-ion batteries (LIB’s) are promising, but limited by the properties of the electrode materials. Due to volume expansion and contraction during lithiation and extraction, the electrode experiences degradation, resulting in severe capacity fading, and buildup of insoluble reaction products. This compromises the electrical contact and fascile diffusion of lithium and leads to poor cycle life. Interestingly, metal fluorides have proved favorable compounds for conversion reactions of the following type. MnXn + nLi + ne <-> nLiX + M The M-F bond is extremely ionic, which should theoretically allow for high lithium storage capabilities and high voltage range. However, metal fluorides experience poor electrical conductivity due to their large band gap, and are also limited by theoretical capacities to a few hundred mAh/g. Metal oxides have also been investigated as potential electrode materials in order to take advantage of their multiple oxidation states. Titanium Oxide nanostructures exhibit high structural stability and have recently drawn attention. When incorporated into the pores of ordered mesoporous carbons (OMC’s), the material’s conductivity improves and ordered mesoporous carbon-TiO2 is formed (OMCT). The OMC’s provide a conductive matrix of amourphous carbon that directs and separates the TiO2 nanostructures in addition to providing large pore volumes for lithium ion diffusion. The high surface area and variable pore size of the OMC’s make them a good option for electrode materials with other types of compounds to be incorporated into the pores. Furthermore, the high porosity of the material allows the electrolyte to easily diffuse, which further maximizes active-particle contact with the electrolyte. The framework also helps keep the integrity of the electrode microstructure intact. Some light metal hydrides in particular show very high lithium mobility. Since the crystal structures of many light metal hydrides are known, first-principles studies can be conducted for investigation of possible lithium kinetics with the compounds. Phase diagrams for the conversion compounds (i) LiBH4, (ii) LiNH2, and (iii) Mg(BH4)2 have been investigated by Mason and Majzoub, and show high lithium capacities. Incorporating these light metal hydrides into the pores of OMC’s may allow for control of volume expansion and better electron transfer during conversion reactions, and may yield high specific capacities. TiO2 nanoparticles were inserted into OMC’s, synthesized during the microstructure selfassembly. This is similar to the method utilized by Chang, Huang, and Doong. The material was tested against a lithium foil counter electrode. The results show promising capacities of 600 mAh/g for the electrode materials. LiBH4 particles inside of OMC’s were synthesized using similar methods. With light metal hydrides in place of metal oxides we may see high lithium mobility and therefore high capacity of the LIB. Experimental OMCT with a weight percentage of 65% (OMCT65) was synthesized via a three-step process. The first step consisted of synthesis of a 20 wt% resol ethanolic solution. 6.1 grams of phenol 99% reagent was added to a shenk flask containing a stir bar. The flask was then placed in a 40C water bath until the phenol was fully melted. 1.3 grams of 20% NaOH in H2O was then added to the flask. The mixture was then stirred for ten minutes, and 10.5 grams of 37% formaldehyde solution in H2O was subsequently added. The water bath temperature was raised to 70C and allowed to sit for 80 minutes. The flask was then removed and allowed to cool to room temperature. The pH of the flask was then balanced to between 7 and 7.5 by adding 6.5 grams of 1N HCl solution. Initially, 5 grams is added and the remainder is added immediately afterwards drop-wise to ensure the desired pH level. The remaining drops must be added immediately to ensure that no undesired reaction products are produced. The solution was then placed in a 45C water bath and a vacuum pump was attached to remove excess water from the solution. The second step consisted of preparation of a 16.26 wt% TiCL3 solution. In a separate shenk flask, 12.03 g TiCl3 was added to 62 mL of deionized water. The mixture was then stirred at room temperature for 30 minutes. The third step consisted of synthesizing the OMCT65. 150 mL of 200-proof ethanol was measured into a separate flask and 15 g of F127 polymer was then added. The mixture was then heated to 40C to allow the polymer to dissolve. In another flask, the 20 wt% resol ethanolic solution and the 16.26 wt% TiCl3 solution, synthesized in steps 1 and 2 respectively, were mixed and stirred at room temperature for one hour. The mixture was then transferred into petri dishes at an even thickness and dried in a vacuum oven at 40C for 24 hours. The dishes were then baked at 100C for 24 hours. After baking, the materials were allowed to cool to room temperature and calcinated in a tubular furnace from 600C to 1200C under N2 at a rate of 1C per minute. Electrode slurry was prepared in an argon-filled glovebox using a solution of 0.8 wt% PVDF in DMSO. The OMCT65 powder was transferred into a weigh-boat and PVDF/DMSO solution was added drop-wise. The slurry was then spread by hand onto copper sheets and allowed to dry overnight. The sheets were then baked in a vacuum oven to 110C and allowed to cool to room temperature under vacuum. Electrochemical cells were assembled in an Argon-filled glovebox using lithium foil, copper coated electrodes, and a 1 M solution of LiPF6 in a solution of (1:1, by weight) DEC:EC. A Celgard separator of 25μm thickness was used. Prepared electrodes were assembled into appropriate configuration using in-house machined parts. Swagelock coin cells were used for airtightness. Capacity tests were carried out using a Maccor model 4300 constant current battery tester. Batteries were cycled between .1 V to 4 V at a constant current of .5 mA for 25 cycles. Results and Discussion A capacity of 600 mAh/g was achieved on the first charge cycle due to formation of the solid electrolyte interphase (SEI) layer. Subsequent cycles showed consistent behavior, but dropping capacity in the neighborhood of 200 mAh/g. (See Figure 1) Figure 1. Cycling data for the OMCT65 electrode. The left graph shows charging, and the right graph shows discharge curves. The initial large capacity on the first cycle is attributed to the formation of the solid electrolyte interphase (SEI) layer. We believe that the capacity of our electrodes will increase with improved electrode assembly techniques. Obtaining a proper balance of electrical conductivity and compaction in the assembled electrodes will require further study. New electrodes using OMCT65 will be synthesized using the same method previously outlined in this article. 10 wt% Carbon Black (CB) will be added to the OMCT65 powder during electrode synthesis. This has been shown to enhance electrical conductivity (4). The consistency of the powder also improves and spreads onto the copper sheets more evenly with the addition of CB. We hope this will improve cyclability of the electrode. OMC’s that have been infiltrated by LiBH4 (OMCLBH) particles have been synthesized using a similar method as OMCT65. We will assemble LIB’s with this material using the same electrode preparation method previously outlined. One electrode has currently been assembled into a LIB and shows a stable initial voltage of 2.8 V. This cell will be cycled on the Maccor model 4300 according to the procedure previously outlined. For future electrodes, 10 wt% CB will also be added to the OMCLBH to improve conductivity and consistency. A powder electrode will also be prepared using a pellet press and OMCLBH with PVDF, as well as several other light metal hydrides. We expect that this material will exhibit good capacity and lithium storage properties. Acknowledgements The authors acknowledge the financial support from the NASA Missouri Space Grant Consortium, and the UMSL Department of Physics and Astronomy. Special thanks to Dr. Stephen Holmes, Dr. E.H. Majzoub, and David Peaslee.