Synthesis And Lithium Storage Properties Of Black Phosphorus,Phosphorene And Its Composites

With the development of the pure electric vehicle industry and the rise of large-scale energy storage,society has put forward higher requirements for the energy and power density of lithium-ion batteries（LIBs）.Developing the LIBs with a higher specific capacity,safety performance and longer cycle life is the hotspot and critical direction in the new energy industry.The electrode material is a vital component of the battery,which has a decisive influence on the energy density and power density of the battery.Therefore,developing high-performance electrode materials has become an effective solution to improve battery energy density and power density.Black phosphorus（BP）is a new two-dimensional material in recent years.Due to the high theoretical specific capacity(2596 m Ah g^-1,forming Li₃P)and suitable charge/discharge potential（～0.7 V vs.Li⁺/Li）,BP was considered as an ideal anode material for the fast-charging LIBs.However,BP still cannot achieve the high energy density and fast-charging property because of its relatively low electrical conductivity,significant volume change（～300%）during lithiation/delithiation,and poor Li⁺diffusion rate.In view of the above problems,the multi-phase and multi-scale BP-based composites were constructed in this paper.On the one hand,the multi-phase and multi-scale structure can relieve the volume change of BP during the charge/discharge cycling.On the other hand,introducing materials with high ionic conductivity can enhance the ionic conductance of the composite,thus improve the electrochemical performance of BP-based anode material.Secondly,a new method was proposed for large-scale synthesizing BP and few layers BP.The primary study contents are as follows:Firstly,three different ball milling techniques（planetary milling,shake milling and plasma-assisted ball milling）were applied to synthesize BP samples with red P as raw material.The XRD patterns of ball-milled products show that the shake milling with higher mechanical is conducive to BP formation and can reduce the milling time and improve the crystallinity of the synthesized BP.TEM analysis shows that the（020）crystal plane is preferentially formed at the surface of two adjacent amorphous nanoparticles,and gradually evolves into the interior of the amorphous particles with the ball milling process.After combined with expanded graphite（EG）,the BP-EG composite demonstrates a higher initial coulombic efficiency and reversible charge/discharge specific capacities.Furthermore,the“zero-strain”material Ti O₂ was introduced into the BP-EG matrix to construct a multi-phase and multi-scale BP-Ti O₂-C composite.The electrochemical performance results show that compared with the BP-C composite,the cycle performance and rate performance of BP-Ti O₂-C composite have been improved.Analysis of SEM,XPS,EIS reveals three reasons for the performance improvement:1)The introduction of Ti O₂ can buffer the stress caused by the volume change of BP during charge/discharge.2)Ti O₂ with a rapid Li⁺migration（along the（001）crystal plane）can also improve the ionic conductivity of the composite.3)The chemical covalent bonds of Ti-O-P,P-C and P-O-C can enhance the connection between the components,contributing to the transfer of interfacial electron and stabilize the structural integrity.Furthermore,the Ti O₂has a relatively high Li⁺insertion/extraction potential（1.75/2.05 V vs.Li⁺/Li）and low specific capacity,which will reduce the energy density of the BP-C composite.Then,the Ti P₂ with high specific capacity,low lithiation/delithiation potential,low volume change and rapid ion extraction was selected as the third phase to construct the BP@Ti P₂-C multi-phase multi-scale composite.By adjusting the atomic ratio of Ti to P,Ti P₂ nanocrystals can be in-situ introduced into the BP matrix by one-step ball milling process,and then combine with EG to prepare the BP@Ti P₂-C composite.XPS analysis shows that Ti-C and P-C chemical covalent bonds can be formed between Ti P₂,BP and C,enhancing the connection among the three components.The electrochemical performance test demonstrates that the BP@Ti P₂-C composite has a high specific capacity,excellent cycle stability and rate performance.The ex-situ TEM results show that the BP@Ti P₂-C composite converts to Li₃P and cubic phase Li_yTi P₄during the discharge process.At the same time,excess Ti nanocrystals were in-situ extruded during the initial discharge process.CV,EIS and GITT results demonstrate that the stable cubic Li_yTi P₄ phase upon lithiation can reduce the Li⁺diffusion energy barrier and accelerate the Li⁺extraction from Li P₃ upon delithiation.Finally,a ball milling process was used to exfoliate bulk BP into few layer black phosphorene（FLBP）.Urea-assisted ball milling was first used to exfoliate bulk BP.It was found that the bulk BP can be effectively exfoliated into FLBP with an average thickness of 4-5 nm.Using low molten mixtures（LMMs）as grinding agent,the lateral dimension of exfoliated FLBP has increased,and the average thickness has not changed.Besides,the exfoliating mechanism of ball milling was also discussed with TEM analysis.It was found that the exfoliating process is that the BP layer slips and exfoliated or curls and exfoliated under the action of ball milling shearing force and friction force.