Synthesis And Electrochemical Properties Of Surface-Functionalized Boron Nanoplates

As a traditional intercalation host material,graphite can not meet the increasing demand of high energy density power supply due to its relatively few active centers for lithium and sodium ion storage.Therefore,the alloying reaction of materials has attracted much attention in the battery industry.Among the alloyed electrode materials studied,boron has attracted lots attention because it can form a series of alloy phases with lithium.Once Li-rich alloy li₅B can be formed,an unprecedented theoretical capacity of 12395 m Ah g^-1 will be obtained.Although boron based lithium-ion batteries with ultra-high specific capacity are predicted by simulation,boron only shows limited electrochemical performance in practical tests and the mechanism of its lithiation process is still under investigation.Two dimensional boron materials are considered as an ideal model to study such process.However,most of the reported two-dimensional boron materials can be easily oxidized at room temperature,which is not convenience for further study.In order to solve these problems,we propose a method of simultaneous etching and in-situ functionalization to prepare two-dimensional boron nanosheets with single atomic boron layer,which can maintain stability at room temperature and have strong oxidation resistance.This provides a model for the study of the lithium process of boron.Simultaneously,comprehensive experiments and theoretical calculations were carried out to study the lithiation process of boron and explore the main reason for the limited capacity of boron anode.The results are shown as follows:（1）The Mg layer in MgB₂ crystals was selectively etched with a solution containing methanol,iodine and hydrochloric acid to prepare methyl functionalized boron nanosheets.At the same time,the mixed etchant can react to generate iodomethane,slowly to realize the methylation of boron layers,which were further sonicated to yield isolated nanosheets.The characterization results verify that the nanosheet structure is the planar sp² configuration with functionalized methyl groups attached to the surface.The size distribution of nanosheets ranges from less than 100 nm to a few microns.Thermogravimetric analysis showed that the nanosheets remain stable up to 250℃.（2）At the current density of 20 mA g^-1,the capacity of the anode decreases sharply after the first discharge,and the reversible value is 392 m Ah g^-1.The X-ray diffraction results show that Li₃B₁₄and LiB₃ are formed after the first discharge,and remain in the anode for the rest of the cycles.the lithium content is less than 25%.The dynamic stability of Li_xB_y alloy was studied based on density functional theory,and the relative ground state enthalpies of formation of different Li_xB_y phases with respect to the elemental crystals of Li and B were calculated.It shows that Li₃B₁₄ and LiB₃ are more stable in the calculated Li_xB_y phase.Therefore,it is unlikely to transform the phases with higher Li content such as Li B and Li₅B at room temperature.To reveal whether unwanted electrochemical events occurred between the SEI and the boron electrode,a galvanostatic intermittent titration technique（GITT）analysis was employed.Since boron is an electron-deficient,it is easy to coordinate with nucleophilic functional groups such as carbonyl groups,the side reaction may be caused by the interaction between boron electrode and organic compounds such as（CH₂OCO₂Li）₂ in SEI film,which is usually reduced from vinyl carbonate in electrolyte,and will lead to the change of chemical state of boron on the electrode surface.In summary,we found that reducing the enthalpy of formation and choosing non nucleophilic electrolyte is the key to prepare high performance boron anode materials.（3）The solvothermal decomposition and explosion of DMSO adsorbed on the layered precursor MgB₂ can induce layer exfoliation of these particles.The simultaneously formed organic sulfides in the solution can easily bond with the B sites on the surface after the Mg removal.Hence,environmentally stable,exfoliated and surface functionalized boron nanoplates are obtained.The size of nanosheets ranges from several hundred nanometers to several microns,and the thickness is about 20 nanometers.Then their electrochemical properties were studied.the reversible capacity is280 m Ah g^-1 after 100 cycles at the current density of 500 mA g^-1.