Structure Design And Electrochemistry Storage Properties Of NASICON-Structured Electrode Materials

To achieve carbon peak before 2030 and carbon neutrality before 2060,it is imperative to build a smart grid mainly powered by renewable energy.To solve the problem of mismatch between renewable energy and power demand in space and time distribution,it is a general trend to convert renewable energy into secondary energy source such as electric.Among all energy storage systems（EESs）,electrochemical energy storage device is one of the most promising choices to store electricity due to the properties of good flexibility,low cost,simple maintenance,and high conversion efficiency.However,the lithium-ion batteries（LIBs）,which is the most successful commercial battery,are limited by the resources,thus it can’t match the needs of EESs.Sodium and potassium in the same main group of Li possesses similar physical and chemical properties.Also,sodium and potassium are abundant and cost effective.Therefore,with the similar electrochemical properties to LIBs,sodium-ion batteries（SIBs）and potassium-ion batteries（KIBs）are ideal substitutes,especially with great potential for EESs.However,due to the extremely large radii of sodium and potassium,it is hard to find suitable host materials.Among all electrode materials,sodium super ionic conductor structural compounds（NASICON）equipped with high redox potential,high thermal stability,and stable frame structure,which are considered to be promising materials for SIBs and KIBs.However,the low intrinsic electronic conductivity and limited theoretical specific capacity greatly restrict their commercial applications.In view of the above problems,a variety of NASICON structural materials with excellent electrochemical properties are designed and prepared through molecular design and micro regulation,and their application prospects in EESs are demonstrated.The main research contents are summarized as follows:1.To overcome the disadvantage of low electronic conductivity,we proposed a double carbon modification strategy by introducing flexible reduced graphene oxide（rGO）and design Na₄MnCr（PO₄）₃@C@rGO with excellent rate performance.The carbon layer coated on the material surface effectively improves the transmission speed of electrons on the particle surface;The introduction of reduced graphene oxide（rGO）inhibits the agglomeration of material particles and improves its electrochemical reaction kinetics.At the same time,flexible rGO effectively constructs an excellent conductive network between particles and current collector,and improves the electronic conductivity of the electrode.Therefore,based on the redox pair of Mn²⁺/Mn⁴⁺,the double carbon modified electrode material has achieved excellent rate performance(35mAhg^-1 at 50 C),and a capacity retention of 73.5%after 500 cycles at 10 C current density.The sodium ion battery based on this material further proves its good application prospects.2.To achieve high theoretical capacity,we adopted the polymetallic synergy.Through crystal structure design,we synthesized Na₄Mn Cr_0.9Al_0.1（PO₄）₃@C by a simple sol-gel method.By regulating the voltage range,three redox pairs of Mn²⁺/Mn³⁺,Mn³⁺/Mn⁴⁺and Cr³⁺/Cr⁴⁺are realized,and the capacity of 150 mAhg^-1 is achieved,which successfully improves the specific capacity.At the same time,the introduction of Al³⁺with smaller radius strengthens the M-O bond in the structure,strengthens the crystal structure and improves the cycle stability of the material.In addition,ex-situ XRD indicates the evolution of the material during the sodium storage process.Reversible structural changes provide the possibility to achieve excellent cyclic properties.This research provides a new research idea for developing high specific energy NASICON structure electrode materials.3.To overcome the poor dynamic properties and huge volume changes during potassiation process,we synthesized hierarchical Ca_0.5Ti₂（PO₄）₃@C microspheres with excellent rate performance and cycle life by electrospray method.Benefiting from the rich vacancies in the crystal structure and rational nanostructural design,the Ca_0.5Ti₂（PO₄）₃@C electrode delivers a high reversible capacity(239 mAhg^-1)and superior rate performance(63 mAhg^-1 at 5 A g^-1).Moreover,the hybrid potassium-ion capacitor affords a high energy/power density(80 W h kg^-1 and 5144 W kg^-1)in a potential window of 1.0–4.0 V,as well as a long lifespan of over 4000 cycles.In addition,in situ XRD is used to unravel the structural transition in Ca_0.5Ti₂（PO₄）₃,suggesting a two-phase transition above 0.5 V during the initial discharge and solid solution processes during the subsequent K⁺insertion/extraction.The present study demonstrates a low-cost potassium-based energy storage device with high energy/power densities and a long lifespan.In this thesis,we propose several effective solutions to the issues of NASICON structure.The research results provide new ideas for the design strategy and research direction of NASICON electrode materials.And the energy storage devices established in this thesis show great possibility in applications of EESs.