Investigation Of Novel Ti-based Anode Materials For Sodium Ion Batteries

Sodium-ion batteries(SIBs)have been expected to be the most promising alternative to the traditional lithium ion batteries(LIBs),since sodium is the sixth rich element and widely distributed in the crust,and sodium ions have similar physical and c hemical properties to lithium ions.Unfortunately,because of the larger diameter of sodium ions than that of lithium ions,the insertion of sodium ions in the electrode materials is more difficult than lithium ions,and it is difficult to find appropriate negative materials with excellent comprehensive performances like the role of graphite anode in commercial LIBs.So the anode material has been an important bottleneck restricting the development of sodium ion battery technology.Considering the wide abundance,high security,environmental friendliness,the insertion-type titanium-based anodes have been widely researched and regarded as a very promising choice for sodium-ion anode.However,the current reported Ti-based anodes commonly possess compact crystal structure,slow ion diffusion,limited sodium sites and poor electronical conductivity,which lead to the unsatisfactory electrochemical performance.In this paper,in response to the shortcomings of low electrochemical activity of titanium-based anode materials for sodium ion batteries,we have carried out the following work by designing crystal structure in order to improve sodium storage capacity and sodium ion diffusion:(1)The layered O3-Na0.73Li0.36Ti0.73O2 is synthesized by a solid-state method and used as an anode material for sodium-ion batteries.This material exhibits great rate performance and cycle stability for the facilitated ion diffusion in the layered structure.And the air stability of layered Na0.73Li0.36Ti0.73O2 is further investigated.Compared with P2 type layered anode,O3-Na0.73Li0.36Ti0.73O2 shows better stability against H2 O molecule.The results show that O3-Na0.73Li0.36Ti0.73O2 not only exhibits excellent electrochemical performance but also good air stability as a new anode.(2)Extending the titanium-based anodes from the Na-Ti-O system to K-Ti-O system,we synthesis the ultrafine K2Ti6O13 nanowires which holds an analogous structure to Na2Ti6O13 by a facile hydrothermal method and use it as an anode material for sodium-ion batteries.Compared with Na2Ti6O13,K2Ti6O13 possesses larger cell parameters and larger channel structure,and the ultrafine nanowires grow perpendicularly to the tunnel direction,thus can greatly shorten the diffusion path length of Na+ in the structure.The nanostructure designing and three-dimensional channel endow K2Ti6O13 excellent electrochemical activity with enhanced capacity and good rate capability for sodium storage.(3)The carbon coated hollandite Kx TiO2 with large(2×2)tunnels is synthesized by a facile carbothermal reduction method,and its formation mechanism is investigated.Due to the larger channel structure with more sodium storage sites and faster sodium ion diffusion,and the enhanced electronic conductivity for the presence of carbon coating and oxygen vacancies with low valence titanium,the Kx TiO2 shows higher capacity and better rate capability than electrode with(1×1)tunnels as well as excellent stability when served as an anode for sodium ion battery.And the carbothermal reduction method is believed to be an effective and facile way to develop novel high-performance Ti-based anodes.(4)Based on the study of half cell performance of the titanium-based anodes,we further investigate the application of the titanium-based anode in the full cell.We develop a new aqueous rechargeable sodium-ion battery based on all NASICON structured electrodes of NaTi2(PO4)3 anode and Na3V2(PO4)3 cathode,then investigate the electrochemcial performance of full cell and analysis the cause to capac ity decay at different mass ratio of anode to cathode.The results show that NTP/NVP full cell demonstrates better electrochemcial performance when the anode is excess.The high output working voltage(1.2 V)and superior rate performance of NTP/NVP full cell make it a promising alternative in low cost,high power and high safety large scale energy storage application.