摘 要

单斜型的磷酸钒锂（Li₃V₂(PO₄)₃，简写为LVP）作为新一代正极材料，其优良的离子电导率、较高的可操作电压平台和高比容量使其成为相当有应用前景的正极材料。然而，磷酸钒锂自身电导率较低，在锂离子脱嵌过程中结构不能长期保持稳定，使得其实际电化学效率较理论值有一定距离。具有一维结构的碳纳米管（CNTs）有着较高的电导率，其能够为磷酸钒锂颗粒内部的锂离子和电子的嵌入脱出提供传输通道。因此，使磷酸钒锂原位生长于碳纳米管上，而无定形碳包裹在整个磷酸钒锂/碳纳米管体系上，制备出Li₃V₂(PO₄)₃/(CNTs C)复合材料，不仅可以有效的提高材料的导电率，还会极大的提高材料的比容量和循环稳定性。

本论文采用溶胶凝胶法制备出了Li₃V₂(PO₄)₃/(CNTs C)纳米复合材料，通过XRD对LVP/(CNTs C)复合材料进行了晶相结构分析，采用SEM、TEM等分析所制备复合材料的表面形貌及内部微观组织，用TG/DSC确定出了材料中所包裹的无定形碳在材料中所占的质量百分数，采用LAND电池测试系统对所组装出的纽扣电池进行了恒电流充放电电化学性能测试，从而得到了不同CNTs含量该正极材料的首次充放电比容量和循环特性。在电化学工作站（Autolab Potentiostat 30系统）上对所组装的纽扣电池进行了线性循环伏安（CV）和交流阻抗谱（EIS）测试，得到了CV曲线和阻抗等电化学性能参数，通过以上实验分析，得到了以下结论：

经过多种电化学手段的测试发现，当CNTs含量为11.6 %时，本实验制备的LVP/(CNTs C)作为正极材料其首次放电比容量可达到129.7 mAh g^-1，高达理论比容量133 mAh g^-1的97.5 %，在50次循环后仍可以达到125.4 mAh g^-1，其容量保留率高达96.7 %。且该材料的电导率更高，结晶度更好，颗粒尺寸更小，晶格更加稳定。以该材料为正极材料组装出的纽扣电池可逆性更好，循环稳定性更好。

关键词：磷酸钒锂；碳纳米管；原位生长；碳包覆

Abstract

As a new generation of cathode material, the monoclinic Li₃V₂(PO4)₃ (LVP) has a considerable application prospect because of its excellent ionic mobility, relatively high operating voltage and high specific capacity. However, the problems that the intrinsic poor electrical conductivity and the structure prone to be degraded in the process of lithium-ion insertion/extraction limit its further application in lithium ion batteries as a cathode material. The carbon nanotubes (CNTs) with one dimensional structure has high electrical conductivity, and it can provide a transmission channel for the insertion/extraction of electrons and the lithium-ions inside of Li₃V₂(PO4)₃ particles. Therefore, the process that the Li₃V₂(PO4)₃ in-situ grows on the CNTs as well as the amorphous carbon enwrap the Li₃V₂(PO4)₃/CNTs to synthesize the Li₃V₂(PO₄)₃/(CNTs C) composite materials not only can effectively increase the conductivity of the material, but also will increase the specific capacity and the cycling stability of the material greatly.

The Li₃V₂(PO₄)₃/(CNTs C) nanocomposites were prepared by sol-gel method. The crystal structure of the LVP/(CNTs C) composites were characterized by XRD. The surface morphology and the internal microstructure of the prepared materials were tested by SEM and TEM analysis. The mass percentage of amorphous carbon in materials was tested by TG/DSC measurement in air atmosphere. The initial charge-discharge specific capacity and cycling property of the cathode materials with different CNTs content were obtained by Galvanostatic charge-discharge measurement which was performed on a LAND battery testing system. The electrochemical performance parameters of CV curves and impedance were obtained by the cyclic voltammetry (CV) and the electrochemical impedance spectrum (EIS) measurement which were carried out on the electrochemical workstation (Autolab Potentiostat 30 system). Through the above experimental analysis, the following conclusions can be obtained:

Through a variety of electrochemical methods of tests, we found that when the content of CNTs is 11.6 %, the initial discharge capacity of the prepared LVP/(CNTs C) can reach 129.7 mAh g^-1, up to 97.5 % of the theoretical capacity of 133 mAh g^-1, and it can still reached 125.4 mAh g^-1 after 50 cycles, the capacity retention rate up to 96.7 %. And the electrical conductivity of LVP/(CNTs _{11.6 wt.%} C) is higher, with better crystallinity, smaller particle size, and its lattice is more stable. The button batteries assembled with this cathode material has better reversibility, and its cycle stability is better.

Keywords: lithium vanadium phosphate; carbon nanotubes; in-situ growth; carbon coating

目 录

摘 要 ………………………………………………………………………………...I

Abstract ……………………………………………………………………………....II

目录 ……………………………………………………………………………..…III

1. 绪 论 …………………………………………………………………………1

1.1 引言 ..…………………………………………………………………….……1

1.2 锂离子电池 ....………………………………………………………………...1

1.2.1 锂离子电池的工作原理 ...….……………………………………………1

1.2.2 锂离子电池的特性 ..……………………………..………………………2

1.2.3 锂离子电池的应用前景 ..……………………………..…………………2

1.3 锂离子电池正极材料 ..…………………………………………………..…...3

1.3.1锂离子电池正极材料的研究进展 ..…………………………………..….3

1.3.2 Li₃V₂(PO₄)₃正极材料的研究进展 ..…………………………………...…5

1.3.2.1 Li₃V₂(PO₄)₃的结构 ..…………………………………………………5

1.3.2.2 Li₃V₂(PO₄)₃的制备方法概述 ..………………………………………6

1.4 本论文研究工作概述 ………………………………………………………...8

1.4.1 本论文研究体系的提出 …………………………………………………8

1.4.2 本论文研究内容概述 ………………………………………………..…..9

第2章 复合材料的制备与表征 …………………………………………………..10

2.1 实验药品与实验仪器 ………………………………………………...……..10

2.2 LVP/(CNTs C)纳米复合材料的制备 ..………………………..………….....10

2.3 LVP/(CNTs C)正极复合材料纽扣电池的组装 ..………………………..….11

2.4 结构和性能表征方法 ……………………………………………………….12

2.4.1 结构表征 ………………………………………………………………..12

2.4.2 电化学性能测试 ………………………………………………………..12

第3章 LVP/(CNTs C)纳米复合材料结构分析 ………………………...………..14

3.1 热重-差热分析 ….………………………………………………………..….14

3.2 物相结构分析 .………………………………...…………………………….15

3.3 表面形貌分析 ..………………………………………………...……………15

3.4 材料内部微观结构分析 ..……………………….…………………………..16