【作者】 李丹；

【导师】 李庆余；

【作者基本信息】 广西师范大学 ， 应用化学， 2023， 硕士

【摘要】 锂离子电池由于其具有高能量密度、长循环寿命和环境友好等优点,在便携式电子设备和混合动力汽车中得到了广泛的应用。近年来,随着人们对便携式设备需求的日益增长以及混合动力汽车的迅猛发展,锂离子电池的能量密度面临着更高的要求。作为锂离子电池的重要组成部分,负极材料的电化学性能的提升对锂离子电池整体的能量密度的提高起到很大的作用。然而,目前商用的石墨负极材料仅有372 mA h g^-1的理论比容量,这无法满足人们对于高能量密度锂离子电池的需求。因此,寻找一种高容量的负极材料成为当务之急。硅具有超高的理论容量(4200 mA h g^-1),是最具吸引力的负极材料之一。不幸的是,由于硅负极存在一些问题,包括低的导电子能力、较差的Li⁺扩散速率以及锂化/脱锂化过程中巨大的体积膨胀（～400%）。因此,硅负极通常表现出较差的电化学性能,包括倍率性能和循环稳定性,使得硅基材料在锂离子电池中的实际应用受到了很大的限制。本文针对硅基负极材料在循环过程中存在容量衰减过快,以及如何改善硅基电极倍率性能差的问题开展研究。研究结果如下:设计了一种新型多重结构的Si/Fe_xSi_y@NC/CNTs复合材料。制备的Si/Fe_xSi_y@NC/CNTs复合材料由氮掺杂碳层包覆Si/Fe_xSi_y纳米颗粒和交联碳纳米管（CNTs）网络构成。这种Si/Fe_xSi_y@NC/CNTs复合材料作为锂离子电池负极材料具有多种优点:（1）引入的Fe_xSi_y不仅能有效适应Si在充放电过程中的体积膨胀,还能提高导电性。（2）氮掺杂碳层（NC）可以进一步提高材料的电导率,适应Si的体积膨胀,并避免电解质与Si的直接接触。（3）交联CNTs网络可以为电极提供长程导电性。制备的Si/Fe_xSi_y@NC/CNTs-1在半电池中表现出优异的循环性能(在1.0 A g^-1电流密度下循环600次后的可逆容量为994.4 mA h g^-1)和优异的倍率性能(在5.0 A g^-1电流密度下能获得441.7 mA h g^-1的比容量)。此外,组装的Si/Fe_xSi_y@NC/CNTs-1//LiFe PO₄全电池在1 C下循环90次后可获得141.6 mA h g^-1的比容量。合理设计了一种带纳米间隙的Si/Bi/NC复合材料。引入超细的金属纳米铋修饰硅纳米颗粒表面,既能提高锂离子和电子传输速率,同时在碳包覆设计的间隙可以给硅的体积膨胀提供缓冲空间。得益于以上独特的结构的设计,Si/Bi/NC复合材料表现出优异的电化学性能,在0.5 A g^-1电流密度下循环450次后,提供了955.8 mA h g^-1的比容量。此外,以预锂化后的Si/Bi/NC为负极,LiFe PO₄为正极组装全电池,在1C的倍率下测试了全电池的循环性能,循环90次后可提供138.8 mA h g^-1的比容量,循环过程中库伦效率均接近100.0%。通过自组装法和简单的热还原法制备了Si/Sn/NC复合材料。先利用自组装法,使Sn²⁺和三聚氰胺在纳米硅表面分布均匀,然后通过简单的热还原法,在纳米硅表面成功包覆一层金属Sn与碳均匀交错分布的保护层。该保护层既可以提高硅基材料的导电子和离子的能力,同时避免了硅与电解液的直接接触。得益于以上结构的设计,Si/Sn/NC-1复合材料表现出优异的电化学性能,Si/Sn/NC-1在1.0 A g^-1电流密度下循环600次后,提供了1039.0 mA h g^-1的高比容量。此外将预锂化的Si/Sn/NC-1与LiFe PO₄组装全电池,在5.0 C大倍率下,全电池循环1000次后比容量可达75.9 mA h g^-1（容量保持率为65.5%）,实现了硅基材料快速充放电。更多还原

【Abstract】 Lithium-ion batteries have been widely used in portable electronic devices and hybrid electric vehicles due to its high energy density,long cycle life and environmental friendliness.Recently,with the increasing demand for portable devices and the rapid development of hybrid electric vehicles,the energy density of lithium-ion batteries has been demanded higher requirements.As an important part of lithium ion battery,anode material’s electrochemical performance is related closely to the overall energy density of lithium ion battery.However,the current commercial graphite anode material has a low theoretical specific capacity(372 mA h g^-1),which cannot meet the pursuit of high energy density lithium-ion batteries.Therefore,it is urgent to find a high capacity anode material.Silicon has an ultra-high theoretical capacity(4200 mA h g^-1)and is one of the most attractive anode materials.Unfortunately,silicon anodes have a number of problems due to low electron conductivity,poor Li⁺diffusion rate,and large volume expansion（～400%）during lithium/delithium.Therefore,silicon anodes usually exhibit poor electrochemical performance,including rate performance and cycle stability,which greatly limits the practical application of silicon-based materials in lithium-ion battery.In this work,the capacity decay of silicon-based anode materials is too fast in the cycle process,and how to improve the poor magnification performance of silicon-based electrodes is studied.The results are as follows:（1）We reported a Si/Fe_xSi_y@NC/CNTs composite via a simple one-step method.The as-prepared Si/Fe_xSi_y@NC/CNTs composite is made of Si/Fe_xSi_ynanoparticles coated with an N-doped carbon layer and a cross-linked CNTs network.Such a multiple Si/Fe_xSi_y@NC/CNTs composite as anodes have various advantages:（1）the introduced Fe_xSi_y can not only effectively adapt to the volume expansion of Si during the charge/discharge process,but also improve the conductivity.（2）the N-doped carbon（NC）layer may further improve the conductivity,and accommodate the volume expansion of Si,as well as inhibit the direct contract between electrolyte and Si.（3）the cross-linked CNTs network may help establish a long-range conductivity for the overall electrode.As a result,the Si/Fe_xSi_y@NC/CNTs composite exhibited impressive electrochemical performance,including outstanding cycling stability(994.4 mA h g^-1 after 600 cycles at 1.0 A g^-1)and rate capability(441.7 mA h g^-1 at 5.0 A g^-1).In addition,the Si/Fe_xSi_y@NC/CNT-1//LiFe PO₄full cell obtained a specific capacity of 141.6 mA h g^-1 after 90 cycles at 1 C.（2）A Si/Bi/NC composite with nanovoids was designed.Ultrafine metal bismuth nanoparticles with lithium storage activity were introduced to modify the surface of silicon nanoparticles,improving the lithium-ion and electron transport rates.The nanovoids of carbon coating provide buffer space for the volume expansion of silicon during the process of charge/discharge.Benefiting from the unique structural design,Si/Bi/NC composite exhibited excellent electrochemical performance,providing a high capacity of 955.8 mA h g^-1 after 450 cycles at 0.5 A g^-1 current density.In addition,the full battery was assembled with pre-lithium Si/Bi/NC as the anode electrode and LiFe PO₄ as the cathode electrode.The cycle performance of the full battery was tested at 1 C.After 90 cycles,the capacity of 138.8mA h g^-1 was provided,and the coulombic efficiency was close to 100.0%during the cycles.（3）Si/Sn/NC composites were prepared by self-assembly method and simple thermal reduction method.Firstly,Sn²⁺and melamine were evenly distributed on the surface of the nano-silicon by the self-assembly method.Then,a protective layer of metal Sn and carbon was successfully coated on the surface of the nano-silicon by a simple thermal reduction method.The protective layer can not only improve the conductance of electrons and ions of silicon-based materials,but also avoid direct contact between silicon and electrolyte.Benefiting from the unique structural design,the Si/Sn/NC-1 composite exhibited excellent electrochemical performance,with Si/Sn/NC-1 providing a high capacity of 1039.0 mA h g^-1 after 600 cycles at 1.0 A g^-1 current density.In addition,the pre-lithium Si/Sn/NC-1 and LiFe PO₄ are assembled with a full cell.At a 5.0 C high rate current,the full battery can provided a high capacity of 75.9 mA h g^-1（capacity retention rate is 65.5%）after 1000 cycles,realizing the rapid charge and discharge of silicon-based materials.更多还原