Joining Of In-situ Grown Lithium Battery Cathode Material And Metal Substrate

As a green and efficient power conversion system,lithium ion secondary battery is the focus of current research in the energy field.LiCoO₂ and LiFePO₄ are lithium battery cathode materials with high application value,of which LiCoO₂ is widely used in commercial lithium batteries owing to its high charge and discharge capacity and excellent low temperature performance,and LiFePO₄ has been becoming a kind of lithium battery cathode material with rapid development in the commercial field due to its stable thermal and chemical properties and safety.According to the current preparation process of a lithium battery,the cathode active material and current collector are connected by glue bonding method,which leads to a large contact resistance at the connection interface and reduces the performance of the lithium battery.To solve this problem,this topic proposes that LiCoO₂ and LiFePO₄ cathode materials are directly grown on the surface of the metal substrate through in-situ growth to achieve atomic bonding between the cathode active material and the metal substrate,thereby improving the performance of the lithium battery.LiCoO₂ crystal layer was grown on the surface of Pt substrate by the flux method and their interface was characterized.Through the growth tests on different substrate surfaces,it was determined that Pt is the best choice of substrate material.The LiCoO₂ crystal layer prepared by the fiux method has a nanometer-scale disc structure with a thickness of about 2 ?m.Then,the influence of the processing parameters such as the holding temperature,the holding time,and the annealing temperature is investigated,and the scanning electron microscope?SEM?analysis,X-ray diffraction?XRD?analysis,transmission electron microscopy?TEM?analysis,energy spectrum?EDS?analysis and other analytical methods are used,which determine the best preparation process at 500°C for 3 hours and the optimal annealing process at 700°C for 1 hour.Using focused ion beam?FIB?for sample preparation,TEM and super energy spectrum analysis,it was determined that LiCoO₂ crystals is grown directly on the Pt substrate,and the seamless connection between the two materials is achieved by the in situ growing process.The analysis of the growth orientation of the LiCoO₂ discotic crystals shows that the sides of the crystals are mainly composed of crystal planes parallel to the a-axis,and the top and bottom surfaces of the crystals are mainly composed of crystal planes parallel to the b-axis.This manner of LiCoO₂ crystal oriented growth is advantageous for the rapid embedding of Li+.In addition,to explore the influence of different crystal morphology on the electrochemical properties of LiCoO₂,nanorod-like LiCoO₂ crystals were successfully prepared by the flux method.The LiFePO₄ crystal layer was grown on the surface of SUS316 stainless steel using the flux method and their interface was characterized.The LiFePO₄ crystal grown on the surface of the substrate is a polyhedral bulk structure.The size of the crystal in the length direction is about 700 nm,and the thickness of the crystal layer is about 8 ?m.Then the process parameters such as holding temperature,holding time and solute concentration are studied for the growth of LiFePO₄ crystal.Through a variety of analytical test methods,it was determined that the optimum preparation process parameters were kept at 800° C for 0 h at a concentration of 50 mol %.Through the tape peeling test and SEM observation,it was initially found tha the LiFePO₄ crystal layer was directly grown on the surface of the stainless steel substrate,and a seamless connection between the two materials was realized.The surface of the LiFePO₄ crystal is mainly composed of {100},{110},and {201} crystal planes analyzed by face angle measurement and XRD analysis,and the crystal has a certain orientation.In addition,nanorod-like LiFePO₄ crystals were successfully prepared by the flux method to explore the influence of different crystal morphology on the electrochemical properties of LiFePO₄ cathode materials.The electrochemical properties of LiCoO₂ and LiFePO₄ cathode materials were characterized.Lithium-ion half-cells were assembled using disk-like LiCoO₂ powder and the pole pieces,nano-rod LiCoO₂ powder,bulk LiFePO₄ powder and the pole pieces,and spherical LiFePO₄ powder as positive electrodes,respectively.Comparing and analyzing the electrochemical performance between the in-situ grown LiCoO₂ positive electrode and the traditionally manufactured LiCoO₂ positive electrode,it was found that additive-free positive electrode has higher rate performance and cycle stability,mainly because of the connection interface.Reliable connection increases the electrical conductivity at the interface and the stability of multiple cycles.By comparing the electrochemical performances of the discotic LiCoO₂ powder cathode material and the nanorod-shaped LiCoO₂ powder cathode material,it was found that the nanorod-like material has better rate performance and cycle stability.This is mainly because the nanorod-like LiCoO₂ crystal is composed of smaller size nanoparticles,so that the material has a larger specific surface area.The in-situ grown LiFePO₄ positive plate cannot be charged and discharged by now.Comparing and analyzing the electrochemical performances of bulk LiFePO₄ cathode materials and spherical LiFePO₄ cathode,it is found that there are certain polarizations in both materials.The actual specific capacity of the spherical LiFePO₄ cathode at various magnifications is relatively stable,and its cycle stability is better than that of bulk LiFePO₄ cathode.