以纤维素为前驱体构筑硬炭基复合电极材料及其储锂机制研究

The theoretical capacity, rate capacity and low-temperature performance of hard carbon is much better than graphite. But hard carbon has defects of large irreversible capacity and hysteresis voltage when charge and discharge as anode material of lithium ion batteries. These are the main barriers of industrialization of it. The emphases and difficulties to solve the problems include the removal of surface impurities, reduction of lithium ion consumption for SEI and reduction of H/C atomic ratio. Cellulose with multi-level three-dimensional network structure is the research object of this project. The purpose is to improve the lithium storage performance of its derivative hard carbon. First, optimize the pyrolyzation process to remove the surface impurity and then discuss its carbonation mechanism. Second, coat coal pitch and PVDF on the surface to form the high conductive carbon layer and then reveal the transmission mode of electron and Li+. Third, connect non-metallic heteroatoms on surface of hard carbon by Van der Waals' force to increase the reactivity point, then clarify the influence mechanism of this hetero-structure to hard carbon. Finally, disperse graphene oxide among cellulose as “bridge” to construct metallic oxide/hard carbon composite. The oxygen-containing groups are used to form covalent bond with metallic ions. This project aims at designing hard carbon structure rationally to fill the gap of lithium storage, acquiring the inner link between composite structure and reaction activity/stability, and finally promoting the application of hard carbon based composite in the filed of lithium ion power battery.

硬炭的理论容量、倍率性能与低温性能均优于石墨，但较低的首次库伦效率和电压滞后现象限制了其应用。去除表面杂质组分、减少SEI膜对Li+的消耗以及降低H/C原子比是解决问题的关键。项目以多层级三维网络结构的纤维素为前驱体，深入研究其热解硬炭的储锂性能与机制：控制炭化反应以去除杂原子及调控微观结构，探讨其炭化机制；包覆软沥青或PVDF以形成高导电碳层，揭示电子与Li+在硬炭网络中的传输模式；将非金属元素修饰在硬炭表面以增加反应活性点，阐明此异质结构对于功能化炭材料的作用机理；以氧化石墨烯为桥梁，利用其丰富的含氧基团与金属离子形成共价键以构筑金属氧化物/硬炭复合材料，分析组分之间的界面交互作用，探明其作为锂离子电池负极的储锂反应机理。本项目旨在合理设计材料结构与组分以提高硬炭的储锂能力，揭示其结构、组分、储锂机制及电化学稳定性的内在联系，进而推动此类硬炭基复合电极其在锂离子动力电池领域的应用。

硬炭的理论容量、倍率性能与低温性能均优于石墨，但较低的首次库伦效率和电压滞后现象限制了其应用。本项目以纤维素为硬炭前驱体，深入研究和探讨了其热解硬炭及其复合材料的储锂性能与机制，研究结论有以下几点：.(1)在800-1000℃热解的纤维素，C-O与C=O的比例明显降低，O原子基本被去除，微观结构发生变化并重新排列。微孔数量远大于中孔，减少了锂离子在中孔中的不可逆沉积。.(2)对纤维素热解硬炭进行碳包覆后，材料具有丰富的孔隙，粒径更细小，有更多的锂存储位置，锂离子与表面官能团的不可逆副反应减少。200mA/g电流密度下容量约为400mAh/g，100次循环后其容量保持率为95.9%。.(3)硼原子在硬炭中的掺杂可以占据缺陷空位，由于硼原子的缺电子特性和快速的电子传输速度，形成了有序且面积较大的碳层。掺杂后的硬炭负极在30、1500mA/g的电流密度下，比容量分别为526.8 和196.6 mAh/g。.(4)在PVDF包覆硬炭的基础上再与SnO2复合成为金属氧化物/硬炭复合电极材料。PVDF衍生的碳网络有助于形成紧密堆积的复合材料纳米颗粒，这使得在电化学循环过程中对体积变化的抵抗力更强。此外，纳米结构增加了Sn/Li2O界面之间的互扩散动力学，从而增加了电化学反应的可逆性，改善了循环性能。.(5)探讨了粘结剂类型对电池充放电性能的影响，结果表明海藻酸钠粘结剂与SnO2@硬炭复合物的相容性更好。这应是源于海藻酸钠中富含羧基，羧基可能与SnO2形成络合物，能够形成更稳定的SEI，使得充放电过程更加稳定。.(6)通过超声辅助与低温热处理制备了SnO2@Ti3C2Tx复合电极材料，证明了SnO2纳米点可在这些储能活性点上原位生长。MXene纳米片作为导电骨架加速电子和离子的迁移，并作为模板阻止SnO2纳米粒子的聚集和体积膨胀。.本项目对于硬炭的电化学性能改性进行了系统的研究与分析，对于推进其实用化进程具有重要作用。