Design And Performance Studies Of Layered Titanate Hollow Spheres In Li-S Batteries And Na-Ion Batteries

Traditional Li-ion batteries（LIBs）have achieved widespread commercialization,but they are still limited by low energy density(150-260Wh kg^-1)and high raw material costs,so it is of great significance to develop ion energy storage systems with high energy density and low cost.Li-S batteries have ultra-high theoretical energy density(2600 Wh kg^-1)and specific capacity(1675 m Ah g^-1),and S is cheap and environmentally friendly.Therefore,the development of Li-S batteries is very promising and commercially useful.Compared with Li,Na is abundant and inexpensive in the earth,and Na-ion batteries（NIBs）have great potential in the field of large-scale energy storage.However,Li-S batteries and NIBs face problems of slow kinetic process which caused by limited ion diffusion.S and lithium polysulfides（Li PSs）have poor conductivity,and Li PSs are easily soluble in electrolytes which leads to"shuttle effect"."Shuttle effect"causes problems such as limited diffusion of Li⁺and low S utilization.When the S loading increasing,the"shuttle effect"is further exacerbated,resulting in a sharp decrease in areal capacity.For NIBs,the radius of Na⁺is larger than that of Li⁺,and the process of insertion/extraction of Na⁺in the electrode material is limited.As a result,it leads to problems such as low specific capacity and poor rate performance of NIBs.Layered titanate materials with hollow structure could solve the problems of slow ion diffusion.First,layered materials have large layer spacing,which accelerates the insertion/extraction process of ions.Secondly,titanate materials have a low voltage platform and good electrochemical activity,and are widely used in electrode materials.However,titanate materials have poor electrical conductivity.The composite of layered titanic acid hollow structure and conductive carbon network can improve its conductivity and give full play to the electrochemical activity of titanic acid materials.The conductive network facilitates the transport of electrons/ions,while the hollow structure of titanic acid provides better electrochemical activity.Compared with the hollow structure of single shell,the multi-shell structure not only has a larger specific surface area,but also constructs a unique temporally-spatially ordered structure with multiple shells arranged from outside to inside.This temporally-spatially ordered structure makes the insertion/extraction of ions more controllable.Based on the problems we mentioned,this thesis tries to solve the problem of limited ion diffusion by improving the micro-nanostructure of titanate materials in multiple dimensions,so as to optimize the kinetic process and improve the electrochemical performance of the batteries.Firstly,an ultra-thick electrode was developed for Li-S batteries,which was comprised by H₂Ti₃O₇electrocatalyst and hollow structure carbon foam with vertical channels（HTO/HCF）.The ultra-thick free-standing electrode was an excellent candidate for Li-S batteries to realize high areal capacity without the addition of extra current collector,binder or additives.Vertically aligned channels offset the defects of sluggish ions/electrons transport in thick electrodes.In addition,hollow structure could accommodate more S and HTO could chemically adsorb Li PSs for high loading and utilization.As expected,the Li-S batteries using HTO/HCF/S cathode achieved a remarkable areal capacity of 6.68 m Ah cm^-2with the S loading of 13.4 mg cm^-2.Thus,this work provided a versatile method to fabricate free-standing electrodes using hollow structure for high areal capacity batteries.Furthermore,we designed multi-shell H₂Ti₂O₅·H₂O（nS-HTO）as an electrode material for NIBs.Multi-shell hollow nanomaterials had high specific surface area and abundant reactive sites,which were conducive to optimizing electrochemical reactions.The shells could support each other,which improved the mechanical properties.The interspace between shells can mitigate the volume change of the electrode material during charging and discharging.The merits improved the cycle stability of n S-HTO.The short distance between different shells accelerated the diffusion of electrons/ions,optimized the electrochemical process,and improved the rate performance of the electrode material.The material had a high porosity and facilitated good wetting between the electrode material and the electrolyte.Therefore,3S-HTO showed good rate performance in NIBs.The specific discharge capacity reached to 84.4 m Ah g^-1at current density of 4 A g^-1.Further,3S-Ca Ti₂O₅·H₂O（3S-Ca TO）was synthesized by ions exchange.The Ca²⁺could effectively expand the layer spacing to accelerate the diffusion of Na⁺and increase reactive sites.When 3S-Ca TO was applied to NIBs,the discharge specific capacity was 120.7mAh g^-1after 700 cycles at current density of 0.1A g^-1.