Synthesis And Performance Of Titanium Dioxide Composites As Negative Electrode Material For Lithium Storage

Transition metal oxides have drawn research attention for applications in the field of lithium-ion batteries and supercapacitors due to its superior electrochemical performances. Titanium dioxide, one of the promising materials, has become the focus of recent researches due to its advantages such as high cycling ability, low cost, environment friendly, and safety. However, Ti O2 material has poor electrical conductivity and slow diffusion rate for lithiumion. Thus, researches have been dedicated to synthesis of nano-sized Ti O2 by combining with conductive carbon, metal, or metal oxide material to increase its conductivity. In this work, we use carbon nano-tube(CNT) as a framework to create sandwich structured composites of C/Mo O2@Ti O2@CNT where the amorphous carbon or Mo O2 serve as a conductive shell. The composites were further characterized with SEM, TEM, and XRD techniques. The electrochemical performances of C/Mo O2@Ti O2@CNT have been evaluated by CV, constant current charging/discharging, and EIS methods.1. Through hydrolysis of isopropyl titanate mixed with CNT, Ti O2 particles are formed with CNT covered on surface. After optimization, the mixture of 1 mmol of isopropyl titanate and 25 mg of CNT is used to synthesize a well coated Ti O2@CNT material. Using glucose as a carbon source, the synthesized Ti O2@CNT is rested in a 0.1 M glucose solution. After pyrolysis, the C@Ti O2@CNT with a 5 nm thick and 5.26 %wt. of carbon layer is created. This sandwich structure material shows great electrochemical performance. After 100 cycles under a rate of C/5, the carbon coated material maintains a high capacity of 176 m Ah/g compared to 137 m Ah/g of uncoated material, while having a coulombic efficiency of 98%.2. Through hydrolysis of sodium molybdate in an acidic environment, Ti O2@CNT is synthesized with a layer of Mo O3 covered on surface, then this layer is reduced to Mo O2 under Ar/H2 environment, creating a Mo O2@Ti O2@CNT composite material. By investigating the thickness effect of Mo O2 layer on the composite structure, a mixture of 0.5 mmol of sodium molybdate and a 100 mg of Ti O2@CNT are mixed to synthesize an uniformly coated Mo O2@Ti O2@CNT composite. The Mo O2 layer has a 10 nm thickness and takes 24.07 %wt. of the composite. After 100 cycles under a rate of C/5, this composite material maintains a capacity of 253 m Ah/g with a coulombic efficiency of 98%, only losing 9.6% of its original capacity. After 5000 cycles under a high rate of 30 C, this material is able to keep a capacity of 60 m Ah/g, demonstrating enhanced electrochemical performance.3. Constant current charging/discharging and cyclic voltammetry are used to test the capacitance performance of Mo O2@Ti O2@CNT composite and actived carbon in aqueous Na2SO4 electrolytes, the AC/Mo O2@Ti O2@CNT sodium ion hybrid supercapacitor has been assembled and tested. Using saturated calomel as reference electrode, Pt as counter electrode, and the composite material as working electrode, capacitance performance of Mo O2@Ti O2@CNT is tested in a 1 M Na2SO4 solution. In the first cycle, it has a specific capacitance of 119.3 F/g, and after 2000 cycles 83.5 F/g remains. This high retention ratio of 70% demonstrates its great cycling ability. Using the same system of three electrodes, capacitance performance of AC is tested in a 1 M Na2SO4 solution. At the same current density, the specific capacitance of Mo O2@Ti O2@CNT material is 1.6~1.8 times of AC, so the optimized mass ratio of AC:Mo O2@Ti O2@CNT was 2:1 for the positive and negative electrodes. The sodium ion hybrid capacitor shows an initial discharge capacity of 101 F/g at the current density of 200 m A/g. After 2500 cycles, the hybrid capacitor maintains a capacity of 73 F/g with a capacity retention rate of 72.3%, showing excellent cycle stability and rate performance.