“碳中和”和“碳达峰”重要战略目标的提出以及电动汽车的快速发展使得提高锂离子电池的能量密度成为研究的热点。锂离子电池的主要组成部分包括正极材料、负极材料、电解液。提高负极材料的比容量是提高能量密度的主要途径之一，而负极材料按照其反应机理可分类为以商业化的石墨为代表的插层型，硅为代表的合金型，以及以金属化合物为代表的转换型。金属氧/磷化物具有理论比容量高、储量丰富等优点，是替代商业化石墨负极的备选之一。然而大多数金属氧/磷化物的低电导率以及充放电循环中体积膨胀等现象严重限制了其开发应用，成为在锂离子电池应用中亟待解决的问题。本工作选择钴氧化物和磷化物为研究对象，对其作为锂离子电池负极材料的电化学性能进行优化。所研究的优化策略包括对钴氧化物和磷化物进行结构调控和组分调整，采用导电性良好的碳材料和二维过渡金属碳化物（Ti₃C₂ MXene）与其复合发挥协同作用，利用其与MXene所形成的异质界面促进电子迁移，构筑柔性自支撑膜电极等；并将实验结果与第一性原理计算（DFT）相结合，深入揭示优异电化学性能的原因，为优化钴氧/磷化物负极材料电化学性能提供思路。具体研究内容和成果如下：

（1）围绕CoO导电性不佳和循环过程中的体积膨胀问题，采用水热法和聚多巴胺（PDA）包覆制备前驱体，通过热解可控地制备了氮掺杂碳包覆空心球状结构Co/CoO@NC。Co/CoO@NC为纳米棒取向排布而成的空心球状多级结构，能充分暴露电化学活性表面，有效缩短离子/电子的迁移路径。在0.2 A g^-1时经过200次循环后能提供919.3 mAh g^-1的可逆比容量，在1 A g^-1大电流密度下，循环1000次后具有587.4 mAh g^-1的可逆比容量。系统探究并揭示电极容量衰减的原因，主要包括活性材料结构坍塌，无法参与锂离子的存储；固液相界面膜（SEI）过厚，增大了锂离子的传质阻力，导致电化学反应速率减小。采用真空抽滤法制备了无粘结剂的柔性自支撑CoO和碳纳米管（CNT）复合膜电极（f-CoO/CNT），三维CNT导电网络有效抑制了充放电循环过程中CoO的结构崩塌并提高导电性，在1 A g^-1的电流密度下循环400次后仍具有500.9 mAh g^-1可逆比容量。组装的f-CoO/CNT||LiFePO₄软包电池在不同弯曲程度时均可点亮LED灯。这种简单方法制备的柔性自支撑电极为柔性器件的应用提供新选择。

（2）进一步开展镍钴双金属氧化物的研究：采用苯甲酸根（BAˉ）插层的镍钴层状双金属氢氧化物作为前驱体（BAˉ-NiCo LDH），通过热解制备镍钴双金属氧化物碳复合材料（NiCo/NiCoO₂@C）。LDH层板与BAˉ之间的静电作用使得金属氧化物粒子与BAˉ碳化的碳在层板厚度尺度下结合，与未经BAˉ插层的LDH热解获得的NiCoO₂相比，碳源的引入有效地抑制了产物的聚集，大的比表面积和多孔结构缩短电子/离子传输路径。所形成的7~10 nm薄层碳壳不仅提高材料的结构稳定性，还能增强材料的导电性，加快反应动力学。用作锂离子电池负极时，在0.5 A g^-1电流密度下循环500次后具有高达927.5 mAh g^-1的比容量。组装的NiCo/NiCoO₂@C||LiFePO₄全电池在0.2 C的电流密度下，经过50次充放电循环后能表现出148.4 mAh g^-1比容量，容量保持率达97.8%。这种合成方法可推广至其他小粒径双金属氧化物碳复合电极材料的制备。

（3）选用锂离子扩散势垒更低的MXene代替碳进一步优化镍钴双金属氧化物的性能：以二甲基咪唑钴（ZIF-67）为模板，利用化学刻蚀ZIF-67制备的空心NiCo-LDH纳米笼通过静电作用诱导MXene纳米片卷曲并包覆于表面，再经过煅烧构筑MXene包覆空心镍钴氧化物三维复合材料（CoO/NiCo₂O₄@MXene）。相比于CoO/NiCo₂O₄，导电性良好的MXene与CoO/NiCo₂O₄形成的三维复合材料具有多级孔结构和大的比表面积，提高材料导电性并有效缓解电化学过程中体积变化。用作锂离子电池负极时，在1 A g^-1大电流密度下经过400个循环后，可表现出619.6 mAh g^-1的高可逆比容量。XPS以及DFT计算表明，MXene与氧化物通过强耦合作用构筑的异质界面引起电子重构，促进电子/离子的传输，加快反应动力学。与LiFePO₄组装的锂离子全电池在0.2 C的电流密度下经过50次循环后，可逆比容量为136.2 mAh g^-1，容量保持率达91.7%。该LDH诱导法为构筑金属氧化物与MXene三维复合材料提供了新思路。

（4）进一步探究了具有比钴氧化物更高导电性以及更低极化电位的钴磷化物的电化学性能，并选用MXene和碳进行优化：在MXene表面原位生长ZIF-67，并将ZIF-67作为牺牲模板，通过聚多巴胺包覆、刻蚀、碳化和磷化过程构建了MXene支撑钴磷化物碳复合材料（CoP/MXene@NC）。在缓冲体系中，多巴胺在ZIF-67表面聚合同时将ZIF-67溶解释放Co²⁺，经过碳化和磷化转为CoP。MXene和碳层提供导电网络，碳包覆结构有效缓解CoP充放电过程中的体积效应。这种独特的结构使得CoP/MXene@NC用作锂离子电池负极时，在0.2 A g^-1的电流密度下经过140次充放电循环后，能提供599.1 mAh g^-1可逆比容量。交流阻抗表明MXene的引入提高了导电性，促进锂离子扩散。DFT计算揭示MXene与CoP之间形成良好的界面耦合，二者间强的相互作用诱导界面电荷重排，提高导电性，进而加快反应动力。CoP/MXene@NC||LiFePO₄全电池在0.4 C电流密度下，经过50次循环后具有109.3 mAh g^-1的可逆比容量。本工作为设计高比容量、长循环寿命的钴磷化物负极提供参考。

The rapid development of electric vehicles makes it a hot topic to develop lithium-ion batteries (LIBs) with high energy density, which plays a critical role in achieving the goal of carbon neutrality and emission peak. Electrode materials and electrolyte are the main parts of lithium-ion batteries. Developing anodes with high specific capacity is one of the promising ways to obtain high-energy-density LIBs. Various kinds of anode materials have been explored and based on the mechanism of lithium storage, among them are intercalation-type carbon materials, alloying-type silicon-based composites, and the conversion-type transition metal compounds. As the promising anode material, transition metal oxides/phosphides are preferred because of the high theoretical capacities and abundant reserves, which are alternatives to commercial graphite anode. Nevertheless, transition metal oxides/phosphides-based anodes are suffered from low conductivity and large volume change during discharge-charge process, which are the main barriers that need to be overcome urgently for the application of LIBs. Herein, aiming at improving the electrochemical performance of transition metal oxides/phosphides-based anodes, a series of strategies are adopted including constructure optimization and component regulation, constructing composites with carbon and Ti₃C₂ MXene based on the synergistic effect by providing heterogeneous interface and improved lithium-ion transport, designing flexible free-standing electrodes. Combined with the experiments results and DFT calculations, the relationship between the structure and lithium storage performance is revealed. The main contents are listed below:

(1) CoO electrodes suffer from low conductivity and huge volume expansion during cycling process. Herein, N-doped carbon coated cobalt oxide composites of hollow Co/CoO@NC spheres are prepared by a facile hydrothermal reaction, polydopamine coating and a temperature-controlled post-annealing treatment of precursor. Co/CoO@NC composites are hollow hierarchical spheres assembled by orientated nanorods, which can make full use of active materials by exposing both ends of each nanorod into electrolyte and providing short Li⁺ diffusion path. When evaluated as an anode for LIBs, Co/CoO@NC electrode can exhibit a high reversible specific capacity of 919.3 mAh g^-1 at 0.2 A g^-1 after 200 cycles and a high specific capacity of 587.4 mAh g^-1 can be obtained over 1000 cycles at 1 A g^-1. The capacity loss behavior of metal oxide electrode is explored and two promising fading mechanisms are proposed: the collapses of active materials lead to inferior contact with substrate, making it a tardy conversion reaction; the slow electrochemical reactions that active material are blocked by the thick solid-electrolyte interface (SEI) film that accumulated on the surface of electrode, which can increase the mass transfer resistance. Binder free flexible free-standing f-CoO/CNT electrode is prepared by a simple route of vacuum filtration. CoO particles are embedded among interconnected CNT skeleton, which realize the direct connect of active material and the substrate. The 3D conductive network of CNT can buffer volume change during the repeated charge/discharge process. As a result, the flexible free-standing f-CoO/CNT electrode delivers extraordinary Li⁺ storage ability with high reversible capacity of 500.9 mAh g^-1 at 1 A g^-1after 400 cycles. Furthermore, f-CoO/CNT||LiFePO₄ pouch cell can light up a light-emitting diode bulb under flat and at successive bending states. The facile method provides a new way to fabricate flexible self-supported electrode and hold a great promise for the application of flexible energy storage devices.

(2) Electrochemical performance of nickel/cobalt bimetallic oxide is investigated. Herein, a simple method is devised to synthesize nickel/cobalt bimetallic oxide/carbon composites (NiCo/NiCoO₂@C) by annealing benzoate ion (BA⁻) intercalated nickel/cobalt layered double metal hydroxide (BAˉ-NiCo LDH). The electrostatic interaction between laminates and BA⁻ can achieve the combination of carbon source and metal oxide at atomic scale. Compared with the LDH-derived NiCoO₂, the introduction of carbon can effectively prevent the aggregation of nanoparticles. The large specific surface area and abundant pores shorten the electron/ion transmission path. Thin carbon shell with the thickness of 7~10 nm can not only improve the structural stability, but also enhance the conductivity making it fast kinetics. As a result, NiCo/NiCoO₂@C electrode shows enhanced lithium storage performance with a high reversible capacity of 927.5 mAh g^-1 after 500 cycles at 0.5 A g^-1. Moreover, NiCo/NiCoO₂@C||LiFePO₄ full LIBs can deliver good cycling performance with a specific capacity of 148.4 mA h g^-1 at 0.2 C after 50 cycles and a capacity retention rate of 97.8%. This method may be extended to design carbon coated metal oxide with small size and advanced architectures.

(3) Compared with carbon materials, MXene possess lower lithium diffusion barrier and is used to optimize the electrochemical performance of nickel/cobalt bimetallic oxide-based electrode. Hollow NiCo-LDH nanocages are controllably synthesized by dissociating zeolitic-imidazolate-frameworks-67 (ZIF-67) and a NiCo-LDH-induced strategy is carried to crinkle MXene nanosheets and construct a cross-linked three-dimensional (3D) conductive network. The 3D CoO/NiCo₂O₄@MXene composites are obtained by a post-annealing treatment. Compared with CoO/NiCo₂O₄ nanocages, the 3D CoO/NiCo₂O₄@MXene with high surface area and conductivity can accelerate lithium diffusion and alleviate the volume expansion during cycling. As a result, the electrode can deliver a super specific capacity of 619.6 mAh g^-1 at after 400 cycles at 1 A g^-1. XPS and DFT calculations reveal that the heterojunction interface constructed by MXene and metal oxide through strong coupling can induce the charge reconstruction and accelerate the electrons/ions diffusion. The CoO/NiCo₂O₄@MXene||LiFePO₄ full LIBs can provide a reversible specific capacity of 136.2 mAh g^-1 with capacity retention rate of 91.7% after 50 cycles at 0.2 C. The LDH-induced strategy provides a new way to synthesize 3D metal oxide/MXene composites.

(4) Electrochemical performance of cobalt phosphide with higher conductivity and lower polarization potential than cobalt oxide is investigated by rationally designing MXene/carbon-based composites. ZIF-67 crystals that grown on MXene are used as the sacrifice template to construct MXene supported N-doped carbon coated CoP composites (CoP/MXene@NC) via PDA coating, etching, carbonizing and phosphating. Polymerization of dopamine on ZIF-67 surface in Tris–HCl solution and the intermediates are synthesized by dissolving ZIF-67, which can be converted to N-doped carbon coated CoP polyhedron. MXene nanosheets and N-doped carbon shell are served as the skeleton by providing the conductive network, and the carbon layer can effectively alleviate the volume expansion during the charge and discharge process. Benefiting from the unique architectures, CoP/MXene@NC anode can deliver a high reversible specific capacity of 599.1 mAh g^-1 at 0.2 A g^-1 after 140 cycles. The improved conductivity and lithium-ion diffusion kinetics are pioneeringly illustrated by electrochemical impedance spectroscopy. DFT results reveal that the heterojunction interface constructed by MXene and CoP by strong interaction can induce the charge rearrangement. The synergistic effect accelerates the electron/ion transport and electrochemical kinetics, leading to excellent cycling performance. CoP/MXene@NC||LiFePO₄ full LIBs can deliver a specific capacity of 109.3 mAh g^-1 at 0.4 C after 50 cycles. It may inspire the design of cobalt phosphide anodes with high specific capacity and long cycle life.