Research On Ketjen Black And CeO₂ Modified Separator For Lithium Sulfur Batteries

Since 1990,lithium ions battery has been regarded as a common and stable power source for various electronic devices.However,with the development of electronic vehicles caused by the environmental problems and energy crisis and the ever increasing need of high energy density power source for modern smart devices,many scholars started to investigate new battery systems with high energy density.Lithium-sulfur batteries which chose sulfur as cathode and lithium as anode have a very high theoretical energy density of 2600 Wh kg-1.Nevertheless,there are some inner obstacles which hinder the large scale commercialization of lithium-sulfur battery,including lower ions and electrons conductivity of sulfur and the end discharge products,the serious volume change of cathode material during redox process,the dissolution of discharge products?lithium polysulfides?in the organic electrolyte and the mitigation of these polysulfides.In order to deal with the low usage of active material and serious polarization caused by the shuttle phenomenon of polysulfides during the discharge/charge process,the researches of separator modification have been done here.This paper includes the following two parts research works.1)The research on commercial ketjen black?KB?modified separator for lithium sulfur batteries.The effect and mechanism of KB modification is investigated through comparison with routine separator.We can get some conclusions through the analysis of test data.The KB layer consists of nanocarbon particles and shows porous structure.This sponge-like layer can adsorb polysulfides effectively and restrain shuttle phenomenon.The excellent conductivity of KB layer can re-active the “dead” cathode material which migrates into this layer during the redox process.This KB layer can also work as an upper current collector promoting the transtion of electrons/ions.Then,the modified cells own better cyclicity and rate capability.The first discharge capacity at 0.1 C is 1318 mAh g-1 and the discharge capacity reduces from 946 mAh g-1 to 815 mAh g-1 at 1 C after 100 cycyles.Besides,the rate capability is also improved and discharge capacity at 2 C can reach 934 mAh g-1.When the current density turns back to 0.1 C,the reversible discharge capacity is 1173 mAh g-1,which is much higher than cells equipped with routine separator.2)The research on cerium oxide?CeO₂?modified separator for lithium sulfur batteries.Hollow CeO₂ is synthesized by Spray granulation and employed to modify the separator.The effect and mechanism of CeO₂ modification is investigated through comparison with routine and Super-P separator.Through the analysis of test data,we can get some conclusions.The CeO₂ layer can not only trap polysulfides by physical adsorption but also form weak chemical bonding with polysulfides.The reduce process of polysulfides restrained by this layer can be catalyzed by CeO₂.The special arrangement of its outer nuclear electrons makes the exchange between Ce³⁺ and Ce⁴⁺ possible,which can improve the conductivity of active material trapped by CeO₂ layer.All of these indicate the excellent cyclicity of cells equipped with CeO₂ modified separator.The CeO₂ cells show a high initial discharge capacity of 1004 mAh g-1 and have an excellent reversible capacity of 625 mAh g-1 after 500 cycles at 1 C.CeO₂ cells can bear high current density even at 2 C and keep good discharge capacity of 760 mAh g-1.When the current turns back to 0.5 C,the capacity of CeO₂ cells is 960 mAh g-1,which is almost the same of first discharge capacity at 0.5 C.This proves the excellent rate capability.The electrochemical test data of CV and STEM can further demonstrate that the CeO₂ cells can restrain the polysulfides shuttle phenomenon more effectively and own much better electrochemical performance than Super-P cells.