The Design And Performance Of Polymer Electrolyte In Solid-state Lithium Metal Batteries

The safety risk of lithium ion batteries（LIBs）would increase sharply with the development of high energy density batteries.Replacing the liquid electrolyte and separator with solid electrolyte is considered to be an effective strategy to guarantee the safety of LIBs even at extreme conditions.Polymer electrolytes are soft,flexible,and generally can keep excellent contact with electrodes in LIBs.However,the transport of ions in solid polymer electrolytes lags behind the transport in liquid electrolyte,which result in that the ionic conductivity is far from the requirement in LIBs.For example,the ionic conductivity of poly（ethylene oxide）at room temperature is only 10^-6 S cm^-1.Therefore,it is necessary to promote the ionic conductivities of the traditional solid polymer electrolytes,or develop new polymer electrolytes with high ionic conductivities.In this thesis,firstly,the silicon-based anode materials with high energy density were studied,and then,inspired by the SEI layers in the alloying reaction of silicon,an artificial solid electrolyte interface with high ionic conductivity was designed constructed in the poly（ethylene oxide）matrix.The ionic conductivity of novel alloy-polymer composite electrolyte with special artificial layer is about an order of magnitude higher than poly（ethylene oxide）.However,this result still does not meet the requirement of batteries.Therefore,vinylene carbonate,a component of liquid electrolyte,was polymerized into solid electrolyte matrix,and the new poly（vinylene carbonate）with high ionic conductivity,wide electrochemical window and long-cycling stability was designed and constructed.The main work content of this thesis can be summarized as the followed:Firstly,cheap coarse silicon was refined rapidly by plasma milling with the carbides as the micro abrasive particles.Among these carbides,our experiment demonstrates that Cr₃C₂carbides are the most suitable one owing to the significant refining effect and less introduced impurities while milling.The XRD and TEM results show that the crystalline size of the silicon was smaller than 30 nm after plasma milling and,the internal stress caused by volume change was alleviated in the process of alloying reaction.At this situation,the transformation between crystalline Li₁₅Si₄ and amorphous Li_xSi was suppressed successfully.It is found that the suppression of the phase transformation during alloying-dealloying reaction improves the cycle stability and reaction kinetics of silicon anode significantly.At the current density of 0.2 A g^-1,the capacity of the Si-Cr₃C₂@FLG is more than 1200 m A h g^-1 after 100 cycles,and at 1 A g^-1,this anode could maintain 881.8 m A h g^-1 even after 300 cycles.Secondly,inspired by the SEI layer formed on the surface of Li-Si alloy during the alloying reaction of silicon anode,the Li-Si alloy particles were soaked in the DOL-DME solvent with poly（ethylene oxide）to trigger the ring opening polymerization on the surface of themselves.While the DOL-DME solvent volatilized completely,the special artificial solid electrolyte interface for rapid lithium ion transport was solidified in the polymer matrix.The FT-IR and XPS results show that the composition of the fast ion conductive layer is highly consistent with that of SEI,and the width of the special layer is about 50～75nm.This was the first time to observe and measure the special interface between polymer matrix and filler.Besides,according to the percolation model,it was found that the ionic conductivity of the fast ion conduction layer is as high as 5×10^-4 S cm^-1,which is about 2 orders of magnitude higher than that of poly（ethylene oxide）matrix.The construction of special layer on Lialloy particles embedded in polymer matrix provides a fast transport path for lithium ions in the electrolyte,and increase the ionic conductivity of polyethylene oxide electrolyte significantly.The ionic conductivity of the alloy-polymer composite electrolyte is 3.9×10^-5 S cm^-1 at 30℃,which is about 1 order of magnitude higher than poly（ethylene oxide）.The all-solid-state lithium metal batteries assembled with this electrolyte can still maintain capacity of 130 m A h g^-1 after 100 cycles at30℃,0.2 C and 110 m A h g^-1 after 200 cycles at 45℃,0.5 C.Finally,to design novel polymer electrolyte with higher ionic conductivity,vinylene carbonate,one of the liquid electrolyte addictive,was polymerized to poly（vinylene carbonate）.After compounding with lithium salt by solution pouring method,the new polymer electrolyte based on poly（vinylene carbonate）was prepared.By FT-IR,Raman spectroscopy and molecular dynamics model,it was found that the degree of dissociation of LiTFSI is significantly improved in the“polymer in salt”solid electrolyte with the interaction between Li⁺and C=O from poly（vinylene carbonate）and residual solvent.The higher dissociation of LiTFSI significantly increases the content of Li⁺transporting in the electrolyte.Thus,the ionic conductivity of poly（vinylene carbonate）electrolyte could reach to 0.82 m S cm^-1 at 30℃,and the voltage window could reach to 4.53 V.The LiFe PO₄|Lisolid-state batteries could exhibit the capacity of 116.6 m A h g^-1 after 2400 cycles at 30℃,1 C and 118.1 m A h g^-1 after 100cycles at 0℃,0.2 C.Even at the high voltage of 4.5 V,the NCM523|Lisolid-state batteries could maintain stable capacity of 156.4 m A h g^-1 after 100 cycles,and the capacity retention is97.5%,showing excellent prospect for application.