The Preparation Of Gelatinous Composite Electrolytes And The Application In Lithium-ion Batteries

With the rapid development of Li-ion and energy storage industry in recent years,Li-ion/Li-ion batteries have received wide attention and application in high-power power,portable household and power storage.As an important part of the cell in the battery,the safety and innovation of the separator determines the reliability and multiple charge/discharge sustainability of the whole battery under different operating conditions.At present,in order to improve the high temperature dimensional stability and electrolyte wettability of conventional polyolefin microporous separator,the industry commonly uses inorganic ceramic powder for single-sided or double-sided coating,but this inorganic ceramic coating will undoubtedly aggravate the damage to the polyolefin base membrane when subjected to external extrusion,so the design of flexible coatings in gel form and the corresponding application of coated separators are promising options.In addition,with the increase of battery safety requirements,the development of solid electrolytes and flexible gel electrolytes has become a hot spot for domestic and international research.In this thesis,firstly,based on the already commercialized alumina ceramic coating composite separator,a silicone modified acrylic acrylic emulsion with flexible buffering effect is introduced into the aqueous coating slurry,the organic-inorganic composite separator coated with surface flexible gel electrolyte is prepared by immersion method to explore the surface morphology of the composite separator,the electrochemical performance of the assembled battery,and the surface morphology of the separator after cycle punching in the simulated battery destruction environment and the corresponding electrochemical performance.On this basis,a PVDF-based flexible gel electrolyte is further prepared,polyurethane（PU）is introduced to overcome the electrochemical stability problem caused by the strong polarization effect of PVDFβ-crystals,the crystal shape modulation of the flexible composite gel electrolyte and the corresponding electrochemical performance are explored.The main work and conclusions of this thesis are as follows.1)Aqueous silicone modified acrylic emulsions are compounded with nano-alumina powder and coated on both sides of polyethylene microporous separator,and the coated separators are tested for fatigue damage and electrochemical properties of the separators by extreme punching simulation.The results show that the coated separators show a gel state after wetting by liquid electrolyte,the PEAE-5 coated separator coated with 5%concentration shows the lowest solid electrolyte impedance(R_SEI)and charge transfer impedance(R_ct)compared to the separators coated with 0%and 10%concentration of silicone modified acrylic emulsion,and the electrochemical stability window is improved to 5.3 V,the problem of discharge capacity drop in the cycle charge-discharge test is improved.The PEAE-5 coated separator exhibits the highest resistance to deformation load（111.6 N）in the subsequent fatigue failure of the separator simulated by extreme punching,and after extreme punching the PEAE-5 coated separator still exhibits an ionic conductivity of 6.89 x 10^-4 S cm^-1 and an electrochemical oxidation limit close to 5.3 V,while maintaining a low steady-state current between 0 and 5 V.PEAE-5 separator has no internal soft short circuit inside the battery.In addition,the PEAE-5 separator retains nearly 80%of its pre-punching cycle charge/discharge performance after extreme punching,thus the use of a gelatinous flexible coating effectively prevents damage to the battery separator and provides an effective way to prevent internal short circuiting leading to failure of the battery.2)Polyvinylidene fluoride（PVDF）and polyurethane（PU）are solution blended and multi-stage porous polymer membranes containing polarβ-phase are prepared by solvent-induced phase separation,followed by swelling activation in liquid electrolyte to obtain PVDF-PU composite gelatinous polymer electrolytes（GPEs）,the crystalline and electrochemical properties of the composite electrolytes are characterized.The results indicate that PU has no effect on the crystalline morphology of PVDF in the polymer composite membranes,and the flexibility is greatly improved while the liquid absorption rate and swelling are effectively improved.Among them,the apparent activation energy of PVDF_0.8PU_0.2 composite gel electrolyte is 18.9 kJ mol^-1 which is in line with the most desirable apparent activation energy range of polymer electrolyte(16-24 kJ mol^-1),the steady-state oxidation limit of 4.35 V ensures the normal operation of the atmospheric cell in the voltage range of 0-4.2 V.Most significantly,the PVDF_0.8PU_0.2 composite gel electrolytes with more than 99%Coulomb efficiency and 97.1%capacity retention at 100 discharges are attributed to the depolarizing effect of PU.Additionally,the excellent Li⁺ion mobility number（0.845）and excellent ionic conductivity(8.81×10^-4 S cm^-1)indicate that the PVDF-PU composite polymer electrolytes are expected to replace conventional polyolefin microporous separators.