The Lithium Storing Properties Of Li2FeSiO₄ At High/Low Temperture And Its Performance In Lithium-Sulfur Batteries

At present,new devices with energy storage and conversion capabilities-secondary batteries?such as lithium-ion batteries?LIB?,lithium-sulfur?Li-S?batteries,etc.?with the advantages of high energy conversion efficiency,light weight,environmentally friendly and pollution-free,have become the hot spot of research at the moment.However,commercialized secondary batteries today are difficult to meet the power requirements of high-energy electric vehicles because the positive electrode material is difficult to provide a higher energy density.Although Li₂FeSiO₄ cathode material has many advantages?such as ultra-high theoretical specific capacity of³20mA/g,stable crystal structure,safety and low cost?,which attracts a lot of attention,but Li₂FeSiO₄ still has some inherent defects,such as low conductivity,low ion diffusion coefficient,and its actual specific capacity are largely related to the operating temperature.So it need further improvements.In this paper,the experiment was based on the inherent advantages and disadvantages of Li₂FeSiO₄ material,and the temperature-performance relationship of Li₂FeSiO₄ material was explored by using advanced in-situ XRD technology.At the same time,owing to crystal structure stability and surface polarity of Li₂FeSiO₄,the application of Li₂FeSiO₄ in Li-S battery cathode material was realized for the first time.The synthesized Li₂FeSiO₄/C-S showed excellent performance.The specific work of the study is as follows:?1?The high and low temperature lithium storage properties of Li₂FeSiO₄ were studied.Firstly,the precursor of Li₂FeSiO₄ was synthesized by sol-gel method.Then,the carbon coated Li₂FeSiO₄ nanoparticles were successfully prepared by high temperature calcination of the precursor.A half-cell of LIB was assembled using Li₂FeSiO₄ as a positive electrode material to test its actual performance at different temperatures.The advanced in-situ XRD test method was used to monitor the changes in the phase structure during the cycle in real time.The test results shown that Li₂FeSiO₄ has excellent lithium storage capacity at high temperatures.When the half-cell based Li₂FeSiO₄ circulated at a lower temperature?293 K?,only a small part of Li⁺participated in the reaction,which corresponded to the mutual conversion between Li₂FeSiO₄ and Li_?2-x?FeSiO₄?0<x?1?.As the temperature increased,more Li⁺participated in the electrochemical reaction.When charging and discharging at 333 K,the Li₂FeSiO₄ electrode material underwent a three-phase reaction among Li₂FeSiO₄,LiFeSiO₄ and Li_?1-x?FeSiO₄.?2?Li₂FeSiO₄ was first applied to lithium-sulfur?Li-S?batteries.Li₂FeSiO₄/C and elementary substance S were composited to obtain Li₂FeSiO₄/C-S nanosheet material.When applied as a Li-S battery active material,Li₂FeSiO₄/C-S exhibited excellent electrochemical performance.Compared with C-S,Li₂FeSiO₄/C-S had lower electrochemical impedance,and it was particularly advantageous for transport of charge and Li⁺.At an operating current density of 160 mA g^-1,the battery exhibited an ultra-high capacity of 1260 mAh g^-1.After 200 cycles,the specific capacity of the battery was maintained at 450 mAh g^-1,demonstrating a stable cycle capability.Combined with various characterization methods and test methods,it can be found that Li₂FeSiO₄/C-S has good performance,not only because unique nanosheet structure of Li₂FeSiO₄/C-S provides a large specific surface area,but also it is closely related to the synergy between Li₂FeSiO₄,C and S.Among them,the existence of carbon network optimizes the electrical conductivity of materials to a large extent,and also effectively shortens the transmission distance of electron and Li⁺.Li₂FeSiO₄ has a stable crystal structure,and its surface polarity can reduce the"shuttle effect"of active substances,and alleviate the structural damage of the electrode material during operation,ensuring that the battery can cycle stably.S,as the main active substance,is the most important contributor to battery energy.