Research On The Performance Of Solid Electrolyte Coated Separator

Lithium-ion batteries have been widely used in different kinds of applications due to their excellent properties(high energy density and long cycling performance).Cathode,anode,electrolyte and separator are the most important components of lithium-ion batteries.The separator plays a key role in separating anode and cathode,insulating electrons and conducting lithium ions.The separators used in commercial lithium-ion batteries are either polyolefine separators or modified separators based on polyolefine separators.Polyolefine separators have good mechanical properties,stable electrochemical and chemical properties,good mass processability and low price.However,due to the melting of polyolefine PP/PE at high temperature,the positive and negative electrodes may contact directly,resulting in a large-area short circuit and causing safety accidents.The current main solution is to coat a layer of heat-resistant ceramic material(such as Al₂O₃,Si O₂,Mg O,boehmite)on the PP/PE separator.The ceramic-coated separator greatly improves the safety of lithium ion batteries,but cannot solve the inherent safety problems of lithium-ion batteries.On the other hand,the research on solid-state batteries and solid-state electrolyte materials is highly valued.This is because solid-state batteries and solid-state electrolytes are expected to essentially solve the safety problems of lithium batteries,and at the same time make it possible to use metallic lithium as a negative electrode,thereby further improve energy density of batteries.However,solid electrolyte materials still have problems such as poor mechanical properties,poor processing properties,unstable chemical properties,and poor compatibility with positive and negative materials,which cannot be applied on a large scale.During the transition from traditional polyolefine separators with liquid electrolytes to final all-solid electrolytes,there may be a transition state.This thesis combines the existing separator technology and cutting-edge research on solid electrolyte materials,proposes a separator with solid electrolytes coating,and systematically studies its related properties.In chapter two,we focus on the solid electrolyte material Li_1.5Al_0.5Ti_1.5(PO₄)₃(LATP),which has high lithium ion conductivity,stable chemical properties,low potential cost,and is easy to scale up as the research object.We focus on the impact of particle size of solid electrolyte LATP(300 nm,500 nm and 700 nm),coating thickness,and single-sided or double-sided coating on the performance of separators.The performances of separators include basic properties(surface quality,air permeability,moisture),mechanics performances(tensile strength,acupuncture strength,peel strength),thermal performances(heat shrinkage),electrochemical performances(first charge-discharge cycle,rate performance,DC internal resistance,AC internal resistance,self-discharge rate).After research and analysis,the following conclusions were drawn:the tensile strength and needle punch strength of the separator are only related to the thickness of the base separator,but not to the coating particle size,thickness,single or double sides of the coating.The thermal stability of the separator is closely related to the coating thickness of the separator and the single or double sides.The 300 nm LATP-coated separator has higher moisture which causes high self-discharge rate.Meanwhile,the peel strength of the 300 nm LATP-coated separator is very low.300 nm LATP-coated separator has good kinetic performance.700 nm LATP-coated separator has the best thermal stability,otherwise,its kinetic performance was poor.Overall consideration,the 500 nm LATP-coated separator was chosed to further study.In chapter three,we investigated the influence of solid electrolyte coating separator on the suppression of uneven deposition of metallic lithium.Solid electrolyte LATP and Li_0.5La_0.5Ti O₃(LLTO)coated separator were prepared.In the test of Li-Cu cell and ternary NCM811-Cu cell,LATP-coated PE separator and LLTO-coated PE separator both showed better electrochemical performance,and LATP-coated PE separator performed best.Further AC impedance spectroscopy shows that the internal resistance of the Al₂O₃coated PE separator battery rises rapidly after ten cycles.Both the LATP-coated PE separator and the LLTO-coated PE separator show a tendency to decrease the internal resistance first.Further analysis and characterization showed that the LATP-coated separator and LLTO-coated separator formed a dense and uniform layer on the surface of the separator after several weeks of cycling.This dense and uniform layer may make the current distribution more uniform and induce uniform deposition of metallic lithium.In chapter four,we investigated the effect of solid electrolyte coating separator on inhibiting the deposition of transition metal ions on the anode side.Conventional Al₂O₃-coated separators and solid electrolyte LATP-coated separators were prepared,the full cell of the LCO-SOC400 and LMO-SOC650 using the solid electrolyte LATP-coated separators showed better cycle performance,compared to the cell using conventional Al₂O₃-coated separators.The capacity retention rate of the LCO-SOC400 full battery with LATP-coated separators after 50 weeks of cycling is83.23%,while the capacity retention rate using traditional separators is only 59.57%.After a certain number of cycles of the cell,we performed AC impedance characterization,and found that the internal resistance of the battery using the solid electrolyte LATP-coated separator was significantly lower than that of the Al₂O₃-coated separator.The SIMS characterization showed that on the anode surface of the battery after cycling,the content of transition metal elements in the separator coated with solid electrolyte LATP is significantly lower than that of conventional Al₂O₃coating.On the surface of the solid electrolyte LATP coating,a dense intermediate layer is formed.It may be the existence of this dense intermediate layer that prevents transition metal ions from drifting to the negative electrode layer,thereby improving the battery cycling performance.In summary,we have more comprehensively studied the performance of solid electrolyte-coated separator.The study shows that the use of solid electrolyte LATP coated separator has certain advantages over the traditional Al₂O₃-coated separator.The use of solid electrolyte coated separator can better control the lithium ion deposition behavior,inhibit the formation of lithium dendrites and improve the utilization efficiency of lithium ion.At the same time,the solid electrolyte coated separator can inhibit the migration of transition metal ions to the anode layer.Since the preparation process of solid electrolyte LATP is simple,easy to engineer and scale-up,low-cost,and the coating process is completely similar to that of alumina coating process,compatible with the existing process,large-scale production of solid electrolyte coated separator is promising.