静电自组装制备三维石墨烯包覆红磷用于锂离子电池负极材料

Electrostatic Self-Assembly Preparation of Three-Dimensional Graphene Coated Red Phosphorus for Lithium-Ion Battery Anode

红磷具有低成本、比容量高等优点,但由于其本征电导率低,在脱嵌锂过程中体积变化大,导致其电化学性能稳定性差,严重制约了其商业应用。通过静电自组装的方法,将红磷包覆在高导电性的石墨烯中,采用扫描电镜(SEM)、X射线衍射(XRD)等测试手段对其形貌、组分进行了分析,将其作为锂离子电池负极材料,并进行了相关的电化学测试。结果表明,相比于红磷粉末电极材料,石墨烯包覆红磷电极材料,具有更好的电化学稳定性,在循环100圈后,仍能保持933 mAh·g^(-1)的比容量,远高于红磷粉末电极材料。这可以归结于高导电性的石墨烯可以提供有效的电子/离子传输,同时石墨烯包覆有助于抑制红磷颗粒的体积膨胀,保证了结构的稳定性。

Red phosphorus has the advantages of low cost and high specific capacity,but due to its low intrinsic conductivity and large volume change in the process of deintercalation of lithium,and its poor electrochemical performance stability,which has seriously restricted its commercial application.The composite of red phosphorus and conductive carbon materials,such as carbon nanotubes,has been a feasible and important research direction.The carbon material with high conductivity can effectively improve the conductivity of the composite material and the cycle performance of the battery.Meanwhile,the carbon material can effectively buffer the volume expansion of the electrode material in the process of charge and discharge,improve the structural stability of the electrode material and prolong the cycle life of the battery.However,the existing methods to improve the stability of phosphor-carbon composites are complex,and the step-by-step operation increases the complexity of the experiment and the uncertainty of the red phosphorus content in the composites.In this study,a positively charged polymer was modified on the surface of red phosphorus.By taking advantage of the electronegative property of the functional groups on graphene oxide(GO),red phosphorus particles were coated in graphene sheets with extremely high elastic modulus through electrostatic adsorption,so as to improve the electrical conductivity of the composite material and inhibit the volume expansion of red phosphorus,thus improving the electrochemical performance.500 mg raw red P powder was ultrasonically dispersed in 500 ml 1%poly(diallyldimethylammonium chloride)solution.After magnetic stirring for 6 h,the powder was pumped and washed with deionized water.After freeze drying,the powder was ultrasonically dispersed into 40 ml deionized water.The three-dimensional graphene-coated red phosphorus(P@rGO)composite was prepared by electrostatic self-assembly method after being added into 100 ml 2 mg·ml-1GO aqueous solution drop by drop under stirring condition,and then stirred evenly and hydroheated at 120℃for 12 h.The surface and internal morphology of the composites were observed by field emission scanning electron microscopy(SEM).The distribution of carbon,phosphorus and oxygen was observed by X-ray energy spectrum analysis(EDS).The crystal structure and phase of the samples were characterized by X-ray diffractometer(XRD).A thermogravimetric analyzer was used to determine the percentage content of red phosphorus in the composite.The results showed that the hydrothermal reduction of rGO presented a three-dimensional porous structure.After the introduction of red phosphorus particles,the three-dimensional porous structure of graphene in P@rGO was basically unchanged,and the red phosphorus particles were well wrapped in the graphene sheet,and the size of red phosphorus particles was about 100 nm.EDS elemental surface scanning analysis showed that the red phosphorus was uniformly distributed in the graphene network.The red phosphorus had a wide peak and was amorphous.The peak position of the red phosphorus in P@rGO remained unchanged after coating the graphene.The new peak at 2θ=26.5°was the(002)crystal plane of the graphene.The percentage of P in P@rGO composite material was 59.8%.Using lithium metal sheet as counterelectrode,polypropylene film(Celgard 2400)as membrane,1 mol·L-1of LiPF6(solvent volume ratio of 1∶1∶1 of vinyl carbonate(EC),dimethyl carbonate(DMC)and methyl ethyl carbonate(EMC))were used as the electrolyte,and 2%vinyl carbonate(VC)was used as the additive.The CR2016 button cell was constructed,and the related electrochemical tests were carried out.When P@rGO was used as the negative electrode of lithium-ion battery,the discharge specific capacity of the second cycle was as high as 1725.5 mAh·g^(-1)at the current density of 50 mA·g^(-1).After 100 cycles,the discharge specific capacity was as high as 933 mAh·g^(-1),and the capacity retention rate was54.1%.Meanwhile,the raw red P electrode has a specific discharge capacity of 1327.4 mAh·g^(-1)in the second cycle,but only289.3 mAh·g^(-1)in 100 cycles.After 100 cycles,the electrolyte diffusion impedance of P@rGO electrode and raw red P electrode was about 5Ω,while the charge transfer resistance of P@rGO electrode(114Ω)was much lower than that of raw red P(946Ω),and the diffusion of lithium ions was stable.Graphene coated the red phosphorus particles well,which effectively improved the electrical conductivity of the composite electrode material,inhibited the volume expansion of the red phosphorus particles,and improved the cyclic stability of the material.Its electrochemical stability was obviously better than that of red phosphorus powder electrode material.This could be attributed to the efficient electron/ion transport provided by the highly conductive graphene,while the graphene coating helped to inhibit the volume expansion of the red phosphorus particles,ensuring the stability of the structure.