Na₂Ti₃O₇ as an anode material for Na-ion batteries

Abstract

Nowadays, topics relating to alternative energy sources and electrochemical storage devices, including rechargeable batteries, are very popular in our society. Sodium-ion batteries have been extensively studied in recent years due to the lower cost compared to Lithium-ion batteries, the most common electrochemical device used in electronic devices and electric vehicles. This thesis explores the fundamental properties of Na₂Ti₃O₇, aiming at improving the electrochemical performance as an anode material in Na-ion batteries. Na₂Ti₃O₇ is a promising anode candidate for Na-ion batteries due to the low operating voltage (0.3 V vs Na⁺ /Na), structural and chemical stability. However, it exhibits low electronic conductivity, resulting in a low specific capacity when tested at high rates and rapid capacity decay. In order to improve the performance, various strategies were approached, such as investigating the incorporation of graphene nanoribbons in the electrode formulation, carbon coating, creation of surficial oxygen vacancies and Nb-doping. This work demonstrates that the incorporation of graphene nanoribbons in the electrode formulation, results in superior performance, in terms of rate capability and cycling stability, compared to carbon coated Na₂Ti₃O₇. In addition, the introduction of lattice and surficial defects in Na₂Ti₃O₇ such as oxygen vacancies and Ti³⁺ -sites enables considerable improvement of the electrochemical performance in terms of specific capacity and rate capability. More importantly, the method used to create oxygen vacancies on the surface of Na₂Ti₃O₇ avoids the use of the highly explosive and expensive H₂. Finally, the incorporation of Nb into the Na₂Ti₃O₇ lattice, by replacing Ti⁴⁺ -sites, has resulted in the improvement of the rate capability due to an expansion of the interlayer distance and generation of Ti³⁺ -sites. The enhanced properties contributed to improving the suitability of the electrodes used here towards their application as anode materials for Na-ion batteries. Consequently, when tested in half-coin cells, they exhibited superior properties compared to various reported literature. Furthermore, many of the principles used in this work can be extended to other metal oxide materials for various applications where the electronic and ionic conductivity are essential requirements.