Study On Structure Regulation And Sodium Storage Performance Of Titanate Anode Materials

Titanates are considered as appealing anode materials for high-performance sodium-ion capacitors?SICs?due to their high capacity,low sodiated potential,and good structural stability.To relieve the issues of low conductivity,sluggish Na⁺diffusion,irreversible Na⁺trapping,and lattice distortion during cycling,titanates with different compositions and crystal structures were synthesized by defects modification,electronic structure regulation,and sodium storage mechanism optimization.Moreover,the correlation among composition,crystal structure,electronic structure and electrochemical properties of titanates was studied.Free-standing sodium titanate?Na₂Ti₂O₅?H₂O?nanowire arrays were prepared on Ti foil as precursors by hydrothermal method,Ti³⁺and oxygen vacancies?OVs?doped Na₂Ti₂O_5-x nanowire arrays were further prepared by thermal reduction,and the influence mechanism of the defects on electrochemical properties was studied.It was found that the doping of low-valent ions and OVs could endow Na₂Ti₂O_5-x with high conductivity,intercalation pseudocapacitance,and enhanced rate capability and superior cycle performance.It delivered capacities of 210,200,174,159,131,114,100,80 and 72 mAh g^-1 at current rates of 0.1,0.2,0.5,1.0,2.0,3.0,5.0,8.0 and10.0 A g^-1,and maintained 92%capacity retention after 1000 cycles at 1.0 A g^-1,almost 100%capacity retention after 20000 cycles at 5.0 A g^-1.When assembled as flexible SICs with rGO/AC cathode,a high energy density of 70 Wh kg^-1 and a high power density of 3600 W kg^-1 were achieved,82.5%capacity retention was maintained after 5000 cycles.Based on Na₂Ti₂O₅?H₂O precursors?NTO?,protonated titanates?HTO?nanowire arrays were prepared by ion-exchange,and the influence of protonation on the crystal structure,Na⁺diffusion,and electrochemical properties was studied.It was found that protonation could expand the interlayer spacing,from 0.83 to 0.89nm,and the expanding interlayer spacing can not only effectively facilitate the Na⁺migration,but also suppress the side reaction and inhibit the irreversible trapping of Na-ions in the crystal framework,leading to fast Na⁺diffusion kinetics.Compared with NTO,HTO exhibited better rate and cycle properties.It delivered capacities of199,75 and 50 mAh g^-1 at 0.1,5.0 and 10 A g^-1,and maintained 98%capacity retention after 1000 cycles at 2 A g^-1,85%capacity retention after 8000 cycles at 5.0A g^-1.However,NTO cannot function well at high current density,the capacity rapidly decayed to 0.5 mAh g^-1 when the current density was greater than 2.0 A g^-1.When assembled as SICs with rGO/AC cathode,a high energy density of 94 Wh kg^-1and a high power density of 3750 W kg^-1 were achieved.Interlayer-defect modified titanates?Q-LT?were designed based on the protonated layered titanates,the crystal structure evolution and the formation mechanism of interlayer defects during the dehydration process were analyzed,and the influence of interlayer defects on the electronic structure and electrochemical properties of Q-LT was explored.It was found that Q-LT exhibited lower band gap,smaller polarization,and faster Na⁺diffusion coefficient.At current rates of 0.1,0.2,0.5,1.0,2.0,3.0,5.0,8.0 and 10.0 A g^-1,it delivered capacities of 252,233,209,191,164,151,130,109 and 101 mAh g^-1,and maintained 97%capacity retention after 10000 cycles at 5.0 A g^-1.Through in-situ XRD and theoretical calculation,it was found that the special layered structure of Q-LT was attributed to the structure rearrangement caused by the release of bonding-OH during the dehydration process,and the electronic structure of Q-LT had also been adjusted,then led to the lower band gap.Based on the density functional theory?DFT?and bond valence sum maps,it was found that Q-LT had more continuous Na⁺diffusion network and lower diffusion barrier,which was beneficial to alleviate the problems of intrinsic Na⁺diffusion kinetics and irreversible trapping of Na-ions.Applied in the high-working voltage?1.5-4.5 V?SICs as additive anodes,high energy density(124 Wh kg^-1)and competitive cycle stability?88%capacity retained after 5000 cycles?were achieved.The interlayer-confined Sn²⁺modified titanates?S-LT?were prepared by double ion-exchange and low-temperature thermal treatment,and the effect of the interlayer-limited Sn²⁺on the crystal structure,valence state,electronic structure,Na⁺diffusion kinetics and electrochemical properties of S-LT was investigated.It was found that the Sn-ions were located between the Ti-O galleries,resulting in the reduction of interlayer spacing,and a small number of Sn-ions were grown on the surface of S-LT nanowires in the form of SnO nanoparticles.S-LT delivered higher reversible capacities than Q-LT at various current densities,exhibiting 396,331,289,265 and 227 mAh g^-1 at 0.1,0.2,0.5,1.0 and 2.0 A g^-1,respectively,and maintained90%capacity retention after 950 cycles at 1.0 A g^-1.Based on DFT calculation,the chemical formula of S-LT was H_1.0Sn_0.5Ti₄O₉,the average activation barrier of Na hopping was around 258 meV,and the Na⁺diffusivity was around 1.14×10^-8 cm² s^-1.When assembled as SICs with rGO/AC cathode,a high energy density of 132.8 Wh kg^-1 and a high power density of 12500 W kg^-1 were achieved,81%capacity retention was maintained after 5000 cycles.