| Lithium-ion batteries have been widely used in portable electronic devices and electric vehicles.They have also a strong tendency to be applied in large-scale energy storage systems.However,a shortage of resources and distribution imbalance of lithium become the increasingly prominent problems.Sodium and potassium as abundant elements are worldwide available,and their similar chemical properties make them possible alternatives for lithium.The key to the development of high-performance potassium ion batteries(KIB s)is to find suitable electrode materials.Layered transition metal oxides have attracted much attention in lithium-ion batteries because of their advantages such as high specific capacity,high energy density,and high ionic conductivity.However,the large ionic radius of K+with strong K+-K+ electrostatic repulsive makes it difficult for K+to insert into the layered structure,and the voltage and specific capacity are significantly decreased.Therefore,it is necessary to design new structures for the layered materials.In this thesis,we focus on screening the electrostatic repulsive between K+ions in the layered structure,including the introduction of polyanionic groups and interlayer pillars.In addition,we design a series of samples with special morphologies to prevent the layered materials from exfoliation,and simultaneously to achieve in-situ carbon coating and carbon nanotube compounding of the materials.In Chapter 1,a general introduction of the working mechanism and key components of KIBs are given.Then a detailed research progress of typical electrode materials and electrolytes for KIBs is presented.Additionally,a brief introduction of the development status and modification methods of layered materials in KIBs is expounded.In Chapter 2,the experimental reagents used in the course of the thesis experiments are listed.The equipments used to prepare and characterize the samples are introduced.A detailed procedure to fabricate coin-cells and measure electrochemical performances for KIBs is also presented.In Chapter 3,the polyanionic groups C2O42-and HPO42-are introduced into the layered KVO cathode material by a hydrothermal method.The as-obtained layered metal-organic-framework compound K2[(VO)2(HPO4)2(C2O4)]consists of submicron porous particles by controlling the concentration of the reactants,which delivers a reversible capacity of 81 mAh g-1 and a high voltage of 3.6 V.In Chapter 4,the polyanionic groups in Chapter 3 are replaced by a high-stability group PO43-so that a layered KVOPO4 with a smaller molecular weight is obtained.Through the tuning of hydrothermal conditions,we have obtained three products with different morphologies.Among them,the KVOPO4 nanosheets deliver the optimal electrochemical performances,including an average discharge voltage of 3.65 V(higher than the reported KVO materials),a reversible capacity of 115 mAh g-1,and a high energy density of 420 Wh kg-1(higher than most of polyanionic materials reported in the literature).In Chapter 5,we design a novel carbon coating method for VPO4,via the isobutanol intercalation to layered VOPO4ยท2H2O combined with exfoliation and thermal reduction.The well-constructed flower-like VPO4 delivers a high reversible capacity of 400 mAh g-1 with a long cycle-life,as the special structure is suitable to accommodate the large volume expansion during the electrochemical charge/discharge process.The VPO4 sample with uniform carbon coating also manifests excellent rate capability.Moreover,we synthesize KVPO4F with VPO4 as the raw material,and explore the effects of carbon content,morphology,and sintering time on F volatilization and electrochemical performance.For the first time,we have assembled K-ion full cells based on KVPO4F and VPO4.In Chapter 6,we design a polyaniline intercalated titanate(HTO-PANI)via ion exchange,the intercalation and polymerization of aniline.Polyaniline plays critical roles in stabilizing the layered titanate structure,enlarging the interlayer spacing and enhancing the electronic conductivity.As a result,the HTO-PANI delivers a reversible capacity of 258 and 219 mAh g-1 at a current density of 50 mA g-1 in sodium and potassium-ion batteries,respectively.It also exhibits ultra-stable cycling stability with no obvious capacity fade after 2500 cycles.And the fully reversible structural changes of HTO-PANI during sodium and potassium ions insertion/extraction are demonstrated.In Chapter 7,we introduce a novel method of preparing dipotassium terephthalate(K2TP)nanosheets,via slowly hydrolyzing dimethyl terephthalate in alcoholic KOH solution.This method realizes the nanosizing of K2TP,which can also be combined with conductive carbon nanotubes.The composite delivers a high specific capacity of 250 mAh g-1,and an excellent rate capability at a high current of 12.5 A g-1.In Chapter 8,a brief summary of the main innovative work and deficiencies of this thesis are presented.Some expectations and suggestions for the further research are also given.Finally,a study of Prussian blue analogs and electrolytes for KIBs is involved in this thesis,which is described in the appendix. |
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