Abstract:

Lithium-oxygen (Li-O₂) batteries are considered as the most promising new generation energy storage devices because of their high theoretical energy density and environmental friendliness. However, large overpotential, low practical energy density and poor cycle life are the huge obstacles to their practical application. Since the insoluble and insulating discharge products (Li₂O₂) gradually form on the surface or in the pores of cathode during the discharging process, these products will block the diffusion channel of O₂/electrolyte and cover the catalytic active sites. In addition, the sluggish kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) require noble-metal-based materials as catalyst to reduce the overpotential and achieve high electrochemcial efficiency. However, high cost and scarcity of the noble-metal catalysts hinder their large scale applications. Development of economically viable and highly efficient noble-metal-free electrocatalysts is extremely urgent, especially for heteroatom-doped carbons as the most promising candidates. In this paper, a novel Co, N and S co-doped three-dimensional self-standing porous carbon (wd-NSC) was prepared as cathode for Li-O₂ battery using melamine and thiourea as nitrogen and sulfur sources via one-step pyrolysis of paulownia wood in NaCl/CoCl₂ mixed molten salt medium. X-ray diffraction (XRD) and scanning electron microscopy (SEM) results found that the wd-NSC inherited the original vertical channel structure of paulownia wood, and formed porous carbon fiber network on the surface and inside the channels of wd-NSC. Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) confirmed that N and S were successfully doped into the C skeleton, while Co might exist in the form of single atom or clusters. Nitrogen adsorption/desorption isotherms showed that the wd-NSC possess hierarchical porous structure with pore sizes ranging from micropore to macropore. Raman spectra certified that the introduction of S increased the defect degree of wd-NSC. At a current density of 0.05mA/cm², the discharge specific capacity of wd-NSC reached to 12.83mA·h/cm², and exhibited a good rate capability and cycling stability (at a current density of 0.1mA/cm² and a limited capacity of 0.5mA·h/cm², the cell can maintain 125 cycles). The excellent electrochemical performance of wd-NSC in the Li-O₂ battery could be attributed to the synergistic effect of three-dimensional hierarchical porous structure and Co, N, S heteroatoms co-doping. The above results demonstrated that this study provided a promising strategy to produce efficiency and low-cost cathodes for practical application in Li-O₂ batteries.

Key words: lithium-oxygen (Li-O₂) battery, air electrode, three-dimensional self-standing, molten salt, doped, electrochemistry, nanostructure, preparation

摘要：

正极是锂-氧气（Li-O₂）电池的电化学反应场所，其直接决定了电池性能。本研究采用泡桐木为原材料，在NaCl/CoCl₂混合熔融盐介质中，以三聚氰胺和硫脲为氮源和硫源，一步热解炭化制备出Co、N、S共掺杂的三维自支撑多孔炭材料（wd-NSC），并直接用作Li-O₂电池正极。利用X射线衍射、扫描电镜、透射电镜、氮气吸脱附、拉曼光谱、X射线光电子能谱等对所获三维自支撑多孔炭材料的形貌、晶体结构与化学成分进行了表征。研究结果表明，Co、N、S共掺杂的三维自支撑多孔炭材料表现出高比容量（在0.05mA/cm²的电流密度下，放电比容量可达12.83mA·h/cm²）和出色的循环稳定性（在电流密度为0.1mA/cm²和限定容量为0.5mA·h/cm²下，循环寿命可达125次），优异的电池性能可归因于三维分级多孔结构及Co、N、S共掺杂的协同作用。

关键词: 锂-氧气电池, 空气电极, 三维自支撑, 熔融盐, 掺杂, 电化学, 纳米结构, 制备