SEI-free Layered Ti-based Na~+ Anode Material

Energy is the core part of social and economic progress.However,due to the excessive exploitation of non-renewable energy,the utilization of renewable energy has attracted people's attention.However,renewable energy,such as wind,solar which is instableand intermittent,will bring great impact on the power grid when the power transport directly into the power grid.Therefore,modulation and storage of this energy source have become an important research problem.Among all the energy storage system,Lithium-ion battery has high energy-conversion efficiency and long cycle life,which is the best choice for energy storage.Sodium,in the same group with lithium,is earth abundant,low-cost.Moreover,sodium-ion battery has similar operating principle with lithium-ion battery.Based on these,sodium-ion battery is regarded as an potential candidate for large application.Compared with positive electrode materials,it is more difficult to explore negative electrode materials for SIBs.Based on these thoughts,we have designed a SEI-free Ti-based negative electrode material for sodium-ion batteries.In this paper,a kind of SEI-free Ti-based sodium ion-layered oxide NaFe_0.5Ti_0.5O₂ was designed and obtained.The reversible capacity of the layered oxide reached 106 mAh g^-1 between1-3 V,and the average potential was about 1.8 V,corresponding to the reversible transformation of Ti⁴⁺/Ti³⁺redox.When negative electrode is discharged below 1 V,SEI film generates easily,which leads to lower Coulombic efficiency and poorer cycle life.Therefore,the most attractive characteristic of the proposed negative electrode material in this paper is the high working potential?1.8 V v.s Na⁺/Na?which gives a SEI-free negative electrode.Moreover,Na,Fe and Ti elements,used in this electrode material,are abundant and widely distributed.Thus,it is believed that NaFe_0.5Ti_0.5O₂ has great potential in the future large-scale applications.A number of electrochemical tests were performed on these materials,for example:long cycle,rate capability,etc.It is found that the material displayed poor rate performance,and only had a capacity of 25 mAh g^-1 at the 1C rate.To improve the rate capability,replacement of partial Fe²⁺and Ti⁴⁺in the material with other elements is also explored.