Preparation And Electrochemical Properties Of Heteroatom-Doped Carbon Materials

Lithium-ion batteries?LIBs?have been playing a vital role in portable electronic devices?mobile phones,cameras,etc.?and new energy vehicles for the last couple of decades thanks to their light weight,high energy density and longer cycle life.However,the limited availability in the crust and uneven distribution of lithium reserves have hampered the large-scale applications of LIBs severely.Therefore,it is crucial to develop renewable and efficient energy storage devices based on earth-abundant elements and high energy storage capabilities as alternatives to LIBs.Fortunately,the potassium element,belonging to IA of lithium,has rapidly attracted researchers'considerable attention as a result of its wide abundance or distribution and low cost.In addition,potassium has a lower standard electrode potential than sodium so that the potassium ion batteries?PIBs?have a wider operating voltage window.And the weaker Lewis acidity of K⁺can lead to the smaller solvation radius,thus K⁺exhibits a faster diffusion rate.Given these forementioned advantanges,PIBs are expected to be a prospective alternative to LIBs.Carbon materials are the most principal anode materials in ion batteries owing to their good conductivity,easy chemical modification and adjustable structure characteristics.It has been proven that graphite can be applied to PIBs with the theoretical capacity up to 279 mAh g^-1 by generating stable potassium intercalated graphite?KC₈?compound.However,the larger radius of potassium ions could bring about intense volume expansion during potassiation/depotassiation process,resulting in fast capacity decay.Heteroatom doping is an effective method for improving carbon materials as anode materials for PIBs.The reason is that the incorporation of heteroatoms can produce more defect sites in the carbon lattice and improve the conductivity of the material,thereby enhancing electrochemical performance significantly.But at present the synthetic steps for heteroatom-doped carbon materials are still cumbersome and toxic,so exploring relatively simple,green and environmentally-friendly synthetic routes is particularly essential for the further development of carbon materials in PIBs.This paper aims to explore facile and efficient fabrications of novel nano-carbon materials and to probe into their electrochemical properties as anode materials for PIBs.And the specific work was summarized as follows:?1?By a simple high temperature solid phase method,the nitrogen-doped 3D porous carbon?NPC?was obtained directly during which sodium polyacrylate acts as carbon source and ammonium chloride as nitrogen source.Sodium chloride?NaCl?template was in-situ formed during the reaction process and porous structure was retained after the removal of NaCl template rinsed with water.The well-developed pore structure and high nitrogen doping level of the NPC sample introduce abundant defects that facilitate the full penetration of the electrolyte,and shorten the ion diffusion path,ultimately improving the cycling and rate performance of PIBs.?2?The nitrogen-sulfur codoped porous carbon?NSPC?material was synthesized by a facile high-temperature solid phase method using sodium polyacrylate as carbon source,thiourea as both nitrogen and sulfur source,and sodium chloride as the template.The synergistic effect of nitrogen-sulfur codoping can expand the interlayer spacing,make redistribution of charge density and spin density,and further improve the conductive activity of the carbon material,finally displaying good potassium storage performance.?3?The phosphorus/carbon nanotube hybrid?P/CNT?was fabricated by a simple and efficient ball-milling method.During the long ball-milling process,the red phosphorus particle grew smaller gradually and was distributed in the conductive carbon matrix uniformly.In addition,the stable P-C and P-O-C bonds in the P/C can endow intimate electrical contact of phosphorus and carbon,help to alleviate the drastic volume expansion of phosphorus in the charge and discharge process and keep the integrity or stability of the structure.