Research On Lithium-ion Battery Based On Boron-carbon Materials

Graphite is a carbon allotrope that is stable under various environmental conditions.It has a typical layered crystal structure,a semi-metallic electronic band structure,and extremely high chemical stability and heat resistance.Technology applications.Its well-known intercalation compounds,especially intercalation compounds of alkali metals and alkaline earth metals,are of particular interest in energy storage applications（for example,as anode materials in lithium-ion batteries）.For the past 50 years,people have been studying the replacement of carbon in graphite lattice with heteroatom dopants（especially adjacent elements boron and nitrogen）as a way to adjust the physical and electrochemical properties of graphite.In particular,boron can be regarded as a simple chemical method to introduce electron deficiency into graphite,which can reduce the Fermi level in p-doped semiconductors and increase the ability to embed alkali metal ions.Among the known B-C compounds synthesized,BC₃ and LiBC materials are especially favored by researchers.BC₃ is a graphite-like material with good lithium and hydrogen storage properties.It is a promising electrode material.But so far,there are no reports of synthetic methods of highly crystallized BC₃ samples.Finding new synthetic methods is the bottleneck that BC₃ materials must face in the future.LiBC material is also a potential anode material for lithium ion batteries.Studies have shown that the actual specific capacity of LIBC materials synthesized with acetylene black as a carbon source can reach 500 m Ah/g,far exceeding the current commercial graphite.However,there are no related reports on the reasons affecting the capacity of LiBC materials.LiB₂is a material that has never been reported.Theoretical research shows that it is a semiconductor at normal temperature and has a higher ionic conductivity at high temperature.At the same time,the crystal structure of LiB₂ material is similar to Ca B₆.The octahedron is connected to form two different channels parallel to the c-axis.In a plane,the small channel is composed of three interconnected boron octahedra,and the large channel is composed of six.Consisting of octahedrons of interconnected boron atoms,it is a potential material for solid electrolytes.Based on the predecessors,this paper conducts in-depth research on the structure that affects LiBC materials,and also makes a preliminary exploration of LiB₂ materials.The results show that LiBC materials that can be charged and discharged have different structures from previous ones.The structural differences are related to the Li content.The smaller the Li content,the larger the structural difference and the higher the capacity.When the Li content of the LiB₂ material changes,its electrical conductivity changes greatly,and it cannot be used as a solid electrolyte,but when it is used as a negative electrode material,it shows good electrochemical performance.The synthesis method in this paper adopts the solid-phase method uniformly.All processes except sintering are performed in a glove box.A special stainless steel container is used.The tube furnace is sintered at a constant temperature of 800℃for10 hours in an argon atmosphere.It is concluded that the structure of the BC layer of the lithium-like LiBC material has changed,resulting in a difference in the capacity of the material.During the synthesis process,we adjusted the ratio and source of Li source to synthesize different LiBC materials for comparison.The results show that the specific capacity of LiBC samples of lithium-rich materials is only 40 m Ah/g.The reversible chemical equation is LiBC?Li_0.96BC+0.04Li,while LiBC(Li_1-δBC)of lithium-deficient materials can reach 220 m Ah/g,and the reversible chemical equation is Li_（1-δ）BC?Li_{（1-δ-0.22）}BC+0.22Li.At the same time,we successfully increased the capacity of Li1-δBC through secondary sintering and sintering with aluminum foil.Finally,we determined that the heat treatment method of 600℃for 10h constant temperature secondary sintering will increase the material capacity to 330 m Ah/g.XRD,Raman and SSNMR techniques were used to analyze the two samples of LiBC and Li_（1-δ）BC,and the structural changes that caused the difference in capacity were studied.The results showed that the XRD pattern of Li_（1-δ）BC relative to LiBC material In the two,the main peaks of the two are basically the same,but the former peak position of Li_（1-δ）BC is shifted to the left,indicating that the lattice of the material does not change but the interlayer distance becomes larger.The change in the proportion of the peak position in the Raman spectrum is presumed to be related to the change in the Li content contained in the material.Finally,in solid nuclear magnetic resonance,the peak positions of the B and C spectra of Li_（1-δ）BC appear at two positions,and The LiBC material has only one main peak,which further proves the structural difference between the two materials.The application of LiB₂ as a solid electrolyte and electrode material in lithium-ion batteries has been studied.The results show that the conductivity of LiB₂ materials will change abruptly with the increase of Li content,and it is not suitable as a solid electrolyte.We rolled the film by dry method and made it into pole pieces,and found that at a current density of 10μA,its reversible capacity can reach 300 m Ah/g,and its capacity changes greatly when charging and discharging at a large rate.