| The rapid development of society and tremendous progresses in science and technologies,has resulted in the significant increase of both demand and use of the fossil energy,while challenges raising in inevitable environmental pollution and resultant saftety concern of energy structure behind the convenience of living.Therefore,developing clean renewable energy to support sustainable development,holds great significance and application value to manage the energy structure and improve the environmental quality.Recently,lithium/sodium-ion batteries(LIBs,NIBs),as an important part of conversion and storage of the clean energy,are urgently required to further improve their energy density and power density to meet the demand of high-quality development.Of which,anode material is one of the most important components determining the battery performance.Among the anode materials,titanium-based compounds based on a(de)insertion mechanism(such as TiO2,Li4Ti5O12,NaTi2(PO4)3,etc.)are proposed as a promising anode candidate for advancing LIBs or NIBs,due to their advantages including low volumetric expansion,good cycling stability,high safety and environmental friendliness.However,there are two major issues of these materials that severely restrict their further development,i.e.,(1)poor electrical and ionic conductivity;(2)low specific capacity under high current density.To address the above issues,in this paper,we have rationaly designed and constructed a series of oxygendeficient,pomegranate-structured titanium-based composites as anode materials,where well-designed composite materials composed of N-doped mesoporous carbon host and embedded titanium-based compounds nanoparticles were prepared by confined in-situ growth.Such a design affords multiple advantages:(1)the three-dimensional(3D)carbon networks with heteroatom doping can effectively increase the electron conduction of the composite;(2)the mesoporous carbon functions as a robust hard template and host to in situ grow Ti-based compounds as well as a strong matrix to support the whole structure,in which the mesoporous cavities regulate the primary particle size of Ti-based compounds and thereby shortening the Li+/Na+ diffusion distance;(3)the open porous structure ensures fast mass transfer kinetic process;(4)the carbothermic reduction process in situ implants abundant oxygen vacancies,modulating the electron sturctures and electrical conductivity to facilitate the Li+redox,which does not require any other reductants or post-treatment.Based on the above design,three parts of this paper were carried out as the following:1.Preparation of oxygen-deficient TiO2x/carbon pomegranate-structured anode materials and its lithium storage performance studyOxygen-deficient TiO2-x and N-doped carbon composites with a pomegranate nanostructure were prepared by a confined in-situ synthesis strategy.The size of TiO2 nanoparticles as well as the cavity voids was adjusted by varying the massloading to optimize the lithium storage performance.Typically,the excess electrons produced by the O-deficient TiO2-x,can be localized or de-localized by the surrounding Ti cations,which modulate the surface electron structure to improve the conductivity and promote the lithium adsorption.Among the five as-prepared anode materials,NMC-M(exhibiting a moderate particle size in a 22 nm cavity)showed the best electrochemical performance,which delivered a high lithiation capacity of 421 mAh g-1 after 350 cycles at 500 mA g-1 and maintained a high reversible capacity of 203 mAh g-1 with a high Coulomb efficiency of 99.8%after 2000 cycles at 1 A g-1.Moreover,the morphological changes of TiO2-x upon the lithiation process were analyzed,the modulation of oxygen vacancy concentration and factors were also analyzed,and finally the relationship between the oxygen vacancy and lithium storage performance was discussed.2.Preparation of oxygen-deficient Li4Ti5O12-x/carbon pomegranatestructured anode materials and study of lithium storage performanceMeanwhile,oxygen-deficient Li4Ti5O12-x/N-doped carbon pomegranate-structured composites as a fast-charging anode,were prepared by a similar confined in-situ growth method.The Li4Ti5O12 massloading and calcination temperature were systematically adjusted to regulate the mesoporous void size and vacancy concentration,and the relationship between the vacancy of the composites and their electrochemical properties was investigated.The optimal sample LTO@NMC-800 anode material(calcined at 800℃)showed significantly enhanced cycling performance,achieving a high capacity of 300 m Ah g-1 after 350 cycles at 350 mA g-1,and maintaining a high reversible capacity of 143 mAh g-1 with a high Coulomb efficiency of 99.8%after 3000 cycles at 3.5 A g-1.Electrochemical tests were also carried out at multiple temperatures,in which the obtained composties demonstrated good lithium storage performance at both low(-20℃)and high temperatures(50℃).Postmortem analysis reveals the Li+ storage mechanism of Li4Ti5O12-x and demonstrates the stability of surface oxygen defects and the structural integrity during cycling.3.Preparation of oxygen-deficient NaTi2(PO4-x)3/carbon pomegranate-structured anode materials and study of lithium storage performanceThere are rare studies reported the defective sodium titanium phosphates as an anode material for SIBs.In this section,oxygen-deficient NaTi2(PO4-x)3 and N-doped carbon pomegranate-structured composites were prepared by confined in-situ growth.Oxygen vacancy was introduced into NaTi2(PO4)3 nanocrystals during carbothermic reduction,which increases the electrical conductivity and improve the sodium storage.The compound material NMC@NTP12-x anode material(calcined at 800℃)showed significantly enhanced cycling performance,achieving a high capacity of 260 mAh g-1 after 100 cycles at 143 mA g1,and maintaining a high reversible capacity of 97.8 mAh g-1 with a high Coulomb efficiency of 99.8%after 2000 cycles at 1.3 A g-1. |
PDF Full Text Request