Electrochemical Characteristics And Modification Of Sulfur Rich Pyrrhotite For Lithium Ion Batteries

Lithium-ion battery has been widely used as power sources for portable electronic equipments for many years and has been considered as one of promising power batteries due to its advantages such as high energy density,energy conversion efficiency and long cycle life,etc.However,its curent performance can not meet the practical application for power batteries,which is mainly caused by the capacity limitation of traditional insertion electrodes due to their structure characteristics.Especially for graphite anode,its low lithium-ion intercalation potential?0.02⁰.2V vs Li/Li+?tends to cause short circuit,even safety issues such as battery explosion etc.Therefore,it has become domestic and international research hotspots by seeking new generation of electrode materials to improve performance of lithium-ion batteries.Recently,electrode materials based on conversional reaction,including various transition-metal compounds that can be used as anode materials,have attracted much attention due to their high theoretical capacity,whose discharge/charge process were achieved by displacement reaction between Li+ and transition metals and thus have a higher lithium-ion storage capacity than traditional insertion electrodes.Among transition metal compounds,iron sulfides can exhibit stable disaharging/charging potential plateau,and thus has a good application prospect.However,as one of main natrual forms,pyrrhotite,especially sulfur rich pyrrhotite?Fe₇S₈?lacks research reportes.In this work,sulfur rich pyrrhotite as anode material for lithium-ion batteries has been systematically studied,and its discharge/charge reaction mechanism was expounded by analyzing the electrochemical behavior and lithium storage characteristics.Based on the influencing factors of the performance,effective modification treatments have been proposed.By the electrochemical performance comparison with traditional graphite anode and FeS/CNTs material,Fe₇S₈ materials with modification treatment exhibits excellent performance and application prospect.1.Studies on the discharge/charge machanism of Fe₇S₈ material and influencing factors on the performance.Firstly,according to the characteristic of large particle size for natural pyrrhotite,micron-sized Fe₇S₈ bulk material with amorphous state and hexagonal structure were investigated,the results show that Fe₇S₈ hexagonal structure exhibited better cycle stability indicating that initial structure has an important effect on the performance.Secondly,by analyzing the discharge/charge behaviour and products formed during reation precoess,the discharge/charge machanism was deduced.The results show that due to the excess S^2-in Fe₇S₈,the initial disharge/charge behavior is different from that of FeS reported,there are two discharging potencial potential plateaues.When the voltage range was extended from 2.5-0.9V to 2.5-0.05 V,Fe₇S₈ electrode deliveried a high reversible capacity of 604.1 mAh/g?the theoretical capacity is 660mAh/g?.Lastly,by comparing the peroformance under different conditions,the main influencing factors were concluded and corresponding improvements were proposed.The analytic results show that the electrochemical peroformance of Fe₇S₈ material is influenced by many aspects of factors,including the catalytic activity decline of Fe0 due to the increase of Fe0 particle size,volume expansion effect,and dissolution of by-product Li₂S_x?2<x<8?into electrolyte,etc.According to above factors,some effective improvements were proposed,such as setting reasonable discharge/charge voltage range,setting “rapid-discharge/slow-charge” current conditions,and necessary surface coating treatment,etc.2.Studies on the influence of carbon coating treatment on the electrochemical behaviors and performance of Fe₇S₈ nanomaterials.Spherical Fe₇S₈ nanoparticles were prepared by a solvothermal method,followed by carbon coating treatment through pyrolysis of organic carbon source,and the effect of the different carbon contents on Fe₇S₈ performance has been investigated.The results shows that the carbon-coating treatment can significantly improve the performance of Fe₇S₈ electrodes by enhancing the conductivity,decreasing the direct contact area between electrode materials and electrolyte,and stabilizing the SEI film,thus avoid the unnecessary capacity loss and thus improvethe performance.Increasing the thickness of carbon layer can improve the coulumbic efficiency and the cyclic stablility,but overmuch carbon contents can hinder the diffusion of Li+,and thus leading to a low reversible capacity.Especially when the carbon content increases to 30 wt.%,the discharge/charge behavior also changes.In this paper,the optimal carbon content is 15 wt.%,which can maintain a reversible capacity of 689.5 mAh/g at 0.1 C after 50 cycles,whereas that of pristine Fe₇S₈ electrode is only 181.2 mAh/g.3.Preparing carbon coated VGCF/Fe₇S₈ composites,improving the practical application of Fe₇S₈ material.Due to the excess S^2-in Fe₇S₈,the conductivity declines seriously after lithiation process.VGCF was used as carrier by loading Fe₇S₈ particles to improve the electrochemical performance,which can take advantages of high conductivity of VGCF.The results show that by loading Fe₇S₈ nanoparticles on its surface,VGCF can better display its high conductivity and more effectively decrease the Li2 S accumulation,and thus improve the cyclic stability and rate capacity,the reversible capacity can maintain 494.2 m Ah/g at 5C.Under the same conditions,traditional graphite anode and FeS material has a only 29.9 mAh/g and 213 mAh/g capacity,respectively.4.The primary cause to actualize the reversibility of electrochemical reaction lies on the catalytic activity of Fe0 produced during reduction process,however,there are no reports on how to improve the catalytic activity of Fe0.In view of the widespread phenomenon of isomorphism-phase replacement in natural pyrrhotite,we first proposed doping Fe₇S₈ material through Co²⁺ and Ni²⁺ replacing Fe²⁺ and the effects of different doping amount on the performance were investigated.The results show that doped samples Fe?7-x?MxS8?M=Co ?Ni?exhibit better performance than pure Fe₇S₈ material,the optimal doping amount is x=1.Co²⁺ and Ni²⁺ changes the cell parameters and crystal grain growth activation energy,and thus influences the electrical property and decreases grain size.The doped samples exhibit better reversibility of discharge/charge process than pure Fe₇S₈ material,thus it can be concluded that “Fe-Co” and “Fe-Ni” nano alloy has a higher catalytic activity.