The Structure Design Of Ultrathick Dense Electrodes For Compact Lithium/Sodium/Potassium Ion Batteries With High Volumetric And Areal Capacities

In recent decades,developing energy storage systems with high energy density to meet the practical application requirments of energy supply in the fields of communication equipment,high energy consuming electronic equipment,smart grid and electric vehicles is of great significance.Nevertheless,due to the low tap density and loose electrode structure of traditional electrode materials,the energy storage system usually needs a large space volume,which makes it difficult to design more sophisticated power consuming equipment,thus reducing the portability of electronic devices.At the same time,the low mass loading of the active materials makes it difficult to achieve the long-term life span of the energy storage system.As for the alloying anode materials(such as Si and Sb),the serious volume expansion(>300%)easily causes the peeling off from the collector,thus limiting their large-scale application.Designing an electrode structure to make the efficient use of electrode space for realizing the ultrathick and dense energy storage of the electrode system,could achieve the maximum volumetric capacity and areal capacity of the electrode system.However,the problems of slow diffusion kinetics caused by the increase of electrode thickness and the active materials peeling off from the current collector must be overcome.Therefore,improving the volumetric and areal capacity of the energy storage system and realizing the long-term life span of the electrode system has become the research focus in the field of energy storage.Ti₃C₂T_x MXene,as a newly two-dimensional material similar to graphene,is gradually applied in the field of energy storage due to its metal like properties,wide layer spacing,low diffusion energy barrier,and good scalability.Based on the above research hotspots,this thesis mainly focous on the structural design of the ultra-thick dense electrode engineering.Taking the advantage of graphene,which can make the electrode material self-assembled into three-dimensional(3D)structure in liquid phase,and further make the as-obtained 3D structured electrode material shrunk and densified,a series of ultrathick dense electrodes with graphene encapsulated structures are prepared.The as-obtained ultrathick dense electrodes are applied in lithium/sodium/potassium ion batteries,and their kinetic behaviors are systematically explored.In situ Transmission electron microscopy(In-situ TEM),Ex situ scanning electron microscopy(ex-situ SEM),and various electrochemical mechanism characterization methods were used to reveal the structure-activity relationship between electrochemical performance and ultrathick dense electrode structure.The main research contents are as follows:(1)Taking the cathode material Na₃V₂(PO₄)₃ of sodium ion battery as the research object.A highly dense three-dimensional graphene encapsulated Na₃V₂(PO₄)₃ conductive network monolith(HD-NVP@G)with high-density(2.7 g cm^-3)and high conductivity(313S m^-1)was obtaind.Besides,a series of HD-NVP@G electrodes with ultrathick and dense structure were prepared.Even the mass loading of HD-NVP@G electrode is as high as 12.2mg cm^-2,its volumetric capacity can reach up to 250.1 mAh cm^-3.Notably,the thickest HD-NVP@G electrode owns the highest thickness of 492?m and endows the areal capacity of 9.3 mAh cm^-2.Meanwhile,the kinetic behaviors of ion diffusion and electron conduction of dense ultrathick electrode are explored to reveal its energy storage mechanism.It is revealed that the excellent electrochemical performance of the ultrathick dense electrode originate from the 3D high-conductivity graphene encapsulated NVP network structure,which maintains good electron/ion diffusion kinetic and the stability of the electrode structure.The ultrathick dense electrode structure also endows the electrode with high volumetric and areal capacity.(2)The commercial Lithium iron phosphate(LiFePO₄)and Graphite were selected to prove the universality of ultrathick electrode engeniering,which could be used in lithium ion batteries for practical application.The highly dense graphene encapsulated LiFePO₄(HD-LFP@G)and graphene encapsulated Graphite(HD-Graphite@G)were obtained.The highest thickness of dense ultrathick HD-LFP@G electrode can reach up to 623?m(mass loading:152 mg cm^-2)and exhibits the highest areal capacity of 21 mAh cm^-2.Besides,the HD-LFP@G//HD-Graphite@G full battery realizes compact energy storage,even the HD-LFP@G as cathode at the loading of 75 mg cm^-2,the areal capacity of full battery could reach to 9.4 mAh cm^-2,which proves that the ultrathick dense electrode can realize the high volumetric capacity and areal capacity for the LIB full battery.(3)The commercial Si anode material was selected as a research object.The 2D Ti₃C₂T_xMXene and graphene were used to double encapsulate Si for bufferring the serious volume expansion and increasing the electronic conductivity.By introducing a small amount of graphene as self-assembly agent,a highly dense HD-Si@Ti₃C₂T_x@G monolith with the density of 1.6 g cm^-3 was obtained.Moreover,the density of compact HD-Si@Ti₃C₂T_x@G electrode can up to 2 g cm^-3,and exhibited excellent volumetric capacity and areal capacity.Notably,the HD-Si@Ti₃C₂T_x@G electrode can reach a high volumetric capacity of 5206mAh cm^-3 at a current density of 0.1 A g^-1,even at the mass loading of 14 mg cm^-2,it endows a high areal capacity of 17.9 mAh cm^-2.In-situ TEM,ex-situ SEM and systematical kinetic analysis were conducted to reveal the excellent lithium storage performance of ultrathick dense electrode,which originate from the double encapsulated Si structure composed of 3D conductive and elastic network formed by Ti₃C₂T_x and graphene,which can provide the rapid electron and ion transfer and the effective buffering volume change.Ensuring the ultrathick electrode keeps good electrolyte permeability and electrode structure stability,and endows a large volumetric capacity and areal capacity for the battery system.(4)In the research field of potassium ion battery,metallic Sb is selected as the research object.By using the in-situ growth of Sb nanoparticles on the surface of 2D Ti₃C₂T_x MXene,a small amount of graphene was introduced as self assembly agent to obtain a highly dense Ti₃C₂T_x MXene and graphene doubly encapsulated Sb monolith(HD-Sb@Ti₃C₂T_x),which owns the density of 3.1 g cm^-3.Besides,a series of ultrathick and compact HD-Sb@Ti₃C₂T_xelectrode with different loading were obtained,and they exhibits excellent volumetric capacity and areal capacity.The compact HD-Sb@Ti₃C₂T_x electrode exhibits a high volumetric capacity of 1978 mAh cm^-3 at 0.1 A g^-1,and the capacity retention rate is up to89.2%after 100 cycles.Even at a current density of 0.5 A g^-1for 500 cycles,it still has a high capacity retention rate of 82%.Moreover,HD-Sb@Ti₃C₂T_x electrode with the highest loading of 31 mg cm^-2,exhibiting a high areal capacity of 8.6 mAh cm^-2.At the same time,In-situ TEM,ex-situ SEM and diffusion kinetic analysis were used to reveal the reason of ultrathick dense HD-Sb@Ti₃C₂T_x electrode shows the excellent potassium storage with high volumetric capacity and areal capacity.Benefiting from the double encapsulation of graphene and Ti₃C₂T_x,the HD-Sb@Ti₃C₂T_x electrode brings good electrolyte penetration,ensures the rapid ion diffusion and electron conduction,and the stable electrode structure effectively buffers the volume expansion of Sb,which endows the ultrathick and dense HD-Sb@Ti₃C₂T_x electrode with high volumetric capacity and areal capacity.