Structural Design,Preparation And Performance Research Of Iron-based Anode Material For Sodium Ion Batteries

Lithium-ion batteries are widely used in fields such as electric vehicles and consumer electronics products due to their high specific energy and long cycle life.However,the shortage of lithium resources limits its application in large-scale energy storage and power system.Sodium-ion batteries are expected to form a good complementarity with lithium-ion batteries in low-speed electric vehicles and energy storage owing to their rich resources and low cost.Therefore,the development of highperformance anode materials is one of effective ways to improve the relatively low energy density of sodium-ion batteries.Considering the relatively poor rate performance and stability of hard carbon,this paper designs a series of low-cost,highperformance iron-based transition metal compounds and nitrogen-doped carbon composites.The sodium storage mechanism and electrochemical properties for sodiumion batteries are systematically studied.Meanwhile,we demonstrate the mechanism of capacity improvement during cycling.The specific research content is as follows:(1)A coral-like FeP@NC composite is designed and prepared,and the electrochemical performance and energy storage mechanism for sodium/lithium-ion batteries are studied.The coral-like FeP@NC composite can keep a capacity retention of 82.0%after 10000 cycles at 10 A g-1 for sodium-ion batteries,and maintain a capacity retention of 90.3%after 5000 cycles at 10 A g-1 as an anode for lithium-ion batteries.Such a performance presentation is far better than that of similar materials reported currently.In response to the gradual increase of capacity,we propose a mechanism of particle refinement that can increase the graphitization degree and interface magnetization,accordingly improving the capacity during cycling through electrochemical tests and the in-situ TEM.This capacity increment effect is more significant in small currents.Moreover,the capacity increment is mostly related to the size of the nanoparticle and the content of nanoparticle.This study holds a new insight for the mechanism of the enhanced capacity induced by particle refinement during cycling and also offers a feasible solution to design high performance anode material for sodium-ion batteries.(2)Series of nitrogen-doped transition metal sulfide composites(FeS2@NC,CoS2@NC,NiS2@NC,Cu9S5@NC,and ZnO-ZnS@NC)are designed and prepared,and the electrochemical properties and energy storage mechanisms during cycling for sodium-ion batteries are systematically studied.Of which,the FeS2@NC anode can reach a capacity of 482.9 mAh g-1 after 10000 cycles at 10 A g-1,and retain a specific capacity of 563 mAh g-1 after 5000 cycles at 2 A g-1 for sodium-ion batteries,showing excellent rate performance and long cycle stability.For the common phenomenon of electrochemical changes dominantly from TMS→TM+Na2S to TMS→S+TM0 for the transition metal sulfide during cycling,we demonstrate that single-atom iron generated during cycling can reduce the reaction energy barrier and cloud density of Na2S,and then promote the decomposition of Na2S and transform it into S instead of FeS2,which eventually leads to the continuous cumulative conversion of electrode materials into S through the electrochemical tests,in situ/in situ spectra,cryo-environmental TEM,HAADF-STEM,XAFS and theoretical calculations.This study not only provides more accurate and deeper understanding of the transition metal sulfide for sodium-ion batteries,but also offers an important guidance for the design and research of highperformance transition metal sulfide.(3)Fe7S8-FeP@NC composites with heterogeneous structure are designed and prepared.The electrochemical properties and energy storage mechanisms for sodiumion batteries are systematically studied.The Fe7S8-FeP@NC composite can display an average specific capacity of 179.9 mAh g-1 at the rate of 10 A g-1,and can provide a high reversible discharge capacity of 217 mAh g-1 after 3000 cycles at a large current of 3 A g-1,showing good rate and cycle stability.Through the in situ XRD,ex situ TEM,and ex situ EPR,it is clarified that the heterogeneous structure can provide abundant active sites and shorten the transmission path of Na+,thus effectivcly improving reaction dynamics and stability performance.This study clarifies the unique stability of heterojunction structure,providing a feasible synthesis method for preparing metal phosphorus vulcanized compounds with heterojunction structure.(4)Fe2O3-Se@NC composite with an open structure and O vacancy defect is designed and prepared.And its electrochemical performance and performance improvement mechanism for sodium-ion batteries are systematically studied.The Fe2O3-Se@NC composite shows an outstanding rate performance and cycle stability as the anode of sodium-ion batteries.Fe2O3-Se@NC composite presents an excellent rate performance of 218.6 mAh g-1 at 20 A g-1 and a highly stable cycling performance of 215.3 mAh g-1 after 2310 cycles with a high-capacity retention rate of 97.1%.Through the in situ XRD,ex situ XPS,ex situ FTIR and DFT calculation,it is clarified that the existence of Na-O bonds formed between O atoms of Fe2O3 and Na atoms of Na2Sex in Fe2O3-Se@NC composite and the existence of oxygen vacancy in the two-phase Fe2O3 can effectively suppress the shuttle of the selenium.At the same time,it can catalyze conversion efficiency of Na2Sex,thereby improving the cycle stability and rate performance.This study clarifies the unique chemical properties of metal oxides,especially analogs with plentiful O vacancies,and provides a practical way for the structural design of high stability selenium-based sodium ion batteries.