Study On Anode Performance Of TiO₂ Of Sodium Ion Battery Based On Heterogeneous Structure And Composite Strategy

Lithium-ion batteries and sodium-ion batteries（SIBs）are among the most promising types of clean and renewable energy storage systems.Sodium（Na）is extremely abundant in the earth,have low cost,and is environmentally friendly.Na is expected to replace the scarce lithium（Li）in large-scale energy storage.Since the radius of Na⁺ions are greater than that of Li⁺ions.Therefore,proper choice and design of an electrode material which can effectively host and transport Na⁺ions,while presenting a stable structure and excellent rate performance The anatase phase（Ti O₂）is one of the ideal SIBs anode materials,with a stable structure,low cost and moderate specific capacity.However,low conductivity and high ion diffusion barrier in Ti O₂ limit its application in electrode materials.Heteroatomic doping,building heterostructures and introducing advanced conductive agents of active material Ti O₂can effectively improve conductivity and ion mobility,and bring excellent charge transfer kinetics and energy storage performance.In particular,electrodes based on heterostructures have received increasing attention in recent years.Researchers often assume that the built-in electric field at the heterogeneous interface can improve ion mobility.However,this assumption is not currently supported by direct evidence.In addition,specific analyses of the direction and intensity of the built-in electric field,and the state of the heterogeneous interface are missing in these studies.Firstly,design a suitable heterostructure in the energy storage electrode and analyze the electron concentration and built-in electric field at the heterogeneous interface.Designed heterogeneous structure in the electrode,its built-in electric field should meet the following requirements:the electric field direction points to the side of the electrode material which hosts Na⁺and Li⁺ions;the strength of the electric field should be large enough to significantly reduce the diffusion barrier.The concentration of electrons and the strength of the built-in electric field at the heterogeneous interface are deduced by using semiconductor theory.In order to meet the above requirements,a titanium carbide（Ti C）material was selected to construct a Ti C/Ti O₂ heterogeneous structure because the work function of Ti C is lower than the Ti O₂ electron affinity.The theoretical value of electron concentration at the interface of the Ti C/Ti O₂ heterostructure is as high as 1.46×10²⁰ cm^-3,resulting in a built-in electric field with an intensity of 1.03×107 V cm^-1.Therefore,the diffusion barrier at the heterointerface is 1.75 times lower than that of bulk Ti O₂ due to a high strength of the built-in electric field,which effectively improving the Na⁺ion diffusion rate.Secondly,non-rectifying titanium carbon nitride/nitrogen-doped titanium dioxide(Ti C_xN_1-x/N-Ti O₂)heterostructure with a perfect interface was constructed by plasma approach.Such complex processes of heterostructure construction,nitrogen doping and Ti C_xN_1-x epitaxial growth were realized within 5 min,which greatly improved the conductivity and Na⁺ion mobility of the material.Particularly,the extra-fine size of Ti C_xN_1-x epitaxial growth produces a near-perfect heterogeneous interface,effectively avoiding the interface state and Fermi energy level pinning.The Ti C_xN_1-x/N-Ti O₂ electrode exhibited excellent electrochemical properties,with reversible specific capacities of312.3 and 173.7 m Ah g^-1 at current densities of 0.1 and 10 C,respectively.The specific capacity at10 C can retain 98.7%after 600 charge-discharge cycles.The high electron concentration and strong electrostatic fields have thus been induced at the heterointerface,the Na⁺ion diffusion barrier is reduced by a factor of 1.7 compared with that of pure Ti O₂ electrodes.Since the experimental results matched with the theoretical predictions,it is proved that heterostructure can significantly reduce the Na⁺ion diffusion barrier and improve the electrochemical properties of the material.Thirdly,the Ti O₂/RGO composite was successfully prepared using hydrothermal synthesis.During the experiment,acetic acid was used to adjust the hydrolysis rate and alleviate the agglomeration of Ti O₂ particles.The nano-sized Ti O₂ particles and the flaky RGO synergistically improve the conductivity and promote charge transfer of the Ti O₂/RGO material.The Ti O₂/RGO electrode provides a reversible capacity of 207.94 m Ah g^-1 at 0.1 C,23%more than the pure Ti O₂electrode.In summary,this work provides detailed guidelines for the design of heterostructure in electrodes.The direction of the built-in electric field should be directed to the active material;the strength of the built-in electric field should be large enough to significantly reduce the ion diffusion barrier;the heterogeneous interface did not introduce defective states and related negative effects.The Ti C_xN_1-x/N-Ti O₂ heterogeneous epitaxy was quickly obtained by plasma induction method,which satisfies the above three-point design requirements.The experimental results are confirmed by theoretical calculations,which prove that the built-in electric field of its heterogeneous structure can significantly reduce the Na⁺diffusion barrier.The study not only reveals the specific mechanism of diffusion barrier reduction by heterostructures,but also provides practical guidelines for the design and synthesis of sodium-storage heterostructures based on Ti O₂ and other materials.