钴-氮共掺杂空心碳球提升硒正极电化学性能研究

Enhancement of Electrochemical Performance of Selenium Cathode by Cobalt and Nitrogen Codoped Hollow Carbon Spheres

为了解决传统的硒基复合正极材料导电性差,循环寿命短,倍率性能差,活性物质大量流失的问题。本文采用SiO_(2)纳米球为模板,N,N'-二水杨醛乙二胺钴为钴源,盐酸多巴胺为氮源和碳源,合成了一种新型的钴-氮共掺杂空心碳球(Co-N-C),并将其作为硒正极宿主材料复合得到Co-N-C/Se复合正极。在0.5C充放电电流密度下,Co-N-C/Se复合正极材料循环200圈后,比容量仍高达238 mAh·g^(-1),且经过阶梯放电,在40圈后,2.0C大电流充放电密度下,比容量仍能维持在222 mAh·g^(-1)。利用导电碳骨架、多样性的化学吸附位点和催化活性位点协同作用成功获得高比容量、大倍率性能及长循环寿命的Li-Se电池。

With the exhaustion of fossil energy and the increasingly severe environmental problems,it is urgent to change the energy structure from fossil energy to clean renewable energy.High performance and low-cost secondary batteries play an important role in modern new energy system.Lithium selenium(Li-Se)battery,as a representative of a new type of secondary battery,uses elemental selenium as the cathode material,with ultra-high volume specific capacity,which is of great practical significance for the development needs of mobile devices and electric vehicles where battery volume is strictly limited at present.However,the selenium cathode is still facing a series of problems.For examples,the bad conductivity of the selenium cathode,the great volume expansion during discharge process,and the serious“shuttle phenomenon”that caused by the active polyselenides dissolved in the ether-based electrolyte and so on.In order to solve the problems of poor electrical conductivity,short cycle life,poor rate performance and large amount of active material loss for traditional selenium-based cathode materials.A series of strategies have been developed by researchers,such as encapsulating the element sulfur into the carbon skeleton,inserting the interlayers,developing the binders,modifying the separators,optimizing the electrolyte as well as protecting the lithium anode.Inspired by the previous work,in this work,a novel cobalt and nitrogencodoped with hollow carbon sphere(Co-N-C)was synthesized by using SiO_(2) nanospheres as template,N,N'-disalicylaldehyde ethylenediamine cobalt as cobalt source,and dopamine hydrochloride as nitrogen source and carbon source.The obtained Co-N-C materials were conducted by a series of materials characterizations,including X-ray photoelectron spectroscopy(XPS),X-ray diffraction(XRD),scanning electron microscope(SEM),N_(2) adsorption/desorption,in order to verify its structures and components.The characterization results proved that the cobalt and nitrogen were successfully doped in the skeleton of the materials and Co-N-C illustrated a hollow structure with a specific surface area of 843.75 m^(2)·g^(-1)and high volume of 0.93 cm^(3)·g^(-1).After encapsulating the element selenium into Co-N-C,the Co-N-C/Se composite positive electrode was synthesized.As expected,the synthesized Co-N-C/Se demonstrated enhanced electrochemical performances compared to the selenium cathode.At a charge and discharge current density of 0.5C,the specific capacity of Co-N-C/Se composite cathode could maintain a reversible capacity of 238 mAh·g^(-1)after 200 cycles,and the specific capacity could still keep as high as 222 mAh·g^(-1)at a high charge and discharge current density of 2.0C.However,the element selenium cathode only illustrated a reversible capacity of 62.1 mAh·g^(-1)after 200 cycles at 0.5C and it could only reach a specific capacity of 75.3 mAh·g^(-1)at 2.0C.Therefore,the Li-Se cells with high specific capacity,high-rate performance and long cycle life were successfully obtained through the synergistic action of conductive carbon skeleton,diversified chemisorption sites and catalytic active sites of the Co-N-C.This work provided a feasible technical path for the realization of higher specific energy density,longer cycle life selenium cathodes.