Design Of TiO₂ Anode Materials For High Performance Sodium Storage

The key issue to develop practical sodium ion batteries?SIBs?is to find suitable host materials due to the larger sodium ion radius compared lithium ion.Titanium dioxide?TiO₂?is considered to be the most promising anode material for sodium ion batteries owing to its abundant resources,stable structure and suitable sodium storage potential.However,the sodium insertion performance of titanium dioxide is limited by its poor electronic conductivity and low sodium ions diffusion rate.In this paper,the Na-ion storage performance of TiO₂-based materials is comprehensively improved by the design of tunable morphology and structure,heterogeneous atom doping,and ion group doping accompanying with electrolyte optimization.The research content of this paper is divided into the following three parts.Co²⁺-doped TiO₂ nanodisks were firstly synthesized through the thermal treatment of metal organic frameworks precursors of MIL-125?Ti?-Co as a superior anode material for SIBs.The Co²⁺-doped TiO₂ possesses uniform nanodisk morphology,a large surface area and mesoporous structure with narrow pore distribution.The reversible capacity,Coulombic Efficiency?CE?and rate capability can be improved by Co²⁺doping in mesoporous TiO₂ anode.Co²⁺-doped mesoporous TiO₂ nanodisks exhibited a high reversible capacity of 232.0 mAh g^-1 at 0.1 A g^-1,good rate capability and cycling stability with a stable capacity of about 140.0 mAh g^-1 at 0.5 A g^-1 after 500 cycles.The enhanced Na-ion storage performance could be due to the increased electrical conductivity revealed by Kelvin probe force microscopy measurements.S-doped TiO₂ nanosheets with porous and layered structures were designed and utilized as anodes to fabricate hybrid sodium-ion capacitors in an ether electrolyte with high performance,especially through dramatically improved Initial Coulombic Efficiency?ICE?.For Na-ion half cell tests in a DME electrolyte,S-doped TiO₂ nanosheets display record high ICE of 88.6%,excellent rate capability and good cycling performance.We also find that TiO₂ material with a large surface area is helpful for reducing the first irreversible capacity in an ether electrolyte,which is different from that observed using a traditional ester-based electrolyte.This could be due to excellent electronic conductivity with charge resistance of 1.0?through the construction of an ultrathin solid electrolyte interphase?SEI?layer.Coupling with an Na₃V₂?PO₄?₃ cathode,we verify a successful Na-ion hybrid capacitor,delivering high energy and power density values of 158.0 Wh kg^-1 and 1075.0 W kg^-1,respectively,at room temperature.Moreover,it also exhibits satisfactory performance of 82.0 Wh kg^-1 at-20 ^oC and outstanding cycling performance with over 95.0%retention after 800 cycles even at 1.0 A g^-1 charge and discharge rate.Heteroatom doping is an effective mean to modulate local structure of TiO₂-based material enabling enhanced electrochemical performance.However,current studies generally adopt a single atom doping strategy,while ionic group doping is rarely achieved and a distinctly bigger challenge.Herein,we prepared phosphate group doped TiO₂?B?as an anode material by skillfully utilizing the defects induced during the dehydration and topology transformation process of H-titanate precursor.Blue TiO₂?B?is created after doping,which possess remarkably sodium ion storage performance.B-TiO₂?B?-P has an average reversible specific capacity of 124.0 mAh g^-1 at a current density of 50.0 A g^-1,and its capacity remains 90.2 mAh g^-1 after 5000 cycles of 10.0 A g^-1.Moreover,the cycle performance was tested at a current density of 2.0 A g^-1 at-20°C,and the capacity retention rate was as high as 96.0%after 1000 cycles.We also assemble a full cell which delivers a maximum energy density of 170.0 Wh kg^-1 and a maximum power density of5000.0 W kg^-1.The enhanced electrochemical performance may be due to the improved conductivity with much reduced bandgap from 3.0 eV to 2.6 eV.