Separator Design And Structure Control In High Performance Lithium Ion Battery

Lithium ion batteries have attracted much attention due to their high energy density,high volume density and reusability.At present,polyolefin membrane is widely used in lithium-ion batteries because of its excellent mechanical strength,uniform and stable pore structure and low cost.But there is a very obvious disadvantage:poor wettability to liquid electrolyte.In order to solve this problem and improve the electrochemical performance of the battery,we can modify the polyolefin membrane to improve the contact performance of the electrode/electrolyte interface.Meanwhile,the liquid organic electrolyte batteryhas a disadvantage that it is easy to become invalid or even explode when short circuit or local overheatingoccurs.Therefore,the research and development of asafer and higher performance electrolyte system has become a new hotspot of lithium-ion battery separator in recent years.Among them,the gel electrolyte with dual chemical characteristics of solid electrolyte and liquid electrolyte has been widely concerned in the application of lithium-ion batteries due to its high safety and high ionic conductivity.To solve these above problems,the following three aspects are studied:（1）In this paper,MnO₂@PP membrane was prepared by modifying commercial PP membrane with MnO₂.The preparation method is:PP separatorswere immersed intoanacidic KMnO₄aqueous solution for in situ growth of MnO₂layers.The method can effectively improve the wettability of liquid electrolyte to PP separator.Compared with the traditional commercial PP separator（69%）,MnO₂@PP separator has a higher electrolyte uptake of 112%.In addition,with a slight increase in the thickness of MnO₂modified layer,MnO₂@PP separatorcontributes an extra specific capacity.At0.2 C,the battery with the MnO₂@PP separator can obtain a high initial specific capacity of 202.5 m Ah/g,which is about 50 m Ah/g higher than that obtained bythe PP membrane（151.7 m Ah/g）.（2）A kind of high performance polymer gel electrolyte was prepared by mixing carboxycellulose nanofibers（CNF）and ball milling treated coniferous lignocellulosic（BWF）with PEO.The PEO-based gel electrolyte is composed of environmentally friendly and renewable cellulose materials,which is consistent with the development of green energy advocated at present.It is found that the cellulose enhanced PEO-based gel electrolyte exhibit sexcellent electrochemical properties.The specific discharge capacity is about 133 m Ah/g at 1 C,and the capacity retention rate is up to97%after 100 cycles of charging and discharging.Besides,it shows a high ionic conductivity at room temperature,6.12×10^-4S/cm,as measured by electrochemical impedance spectroscopy（EIS）.The polymer gel electrolyte also plays a role in inhibiting the formation of lithium dendrites.When the current density is 5 m A/cm²,the growing lithium dendrites pierced the separatorused in the liquid electrolyte battery with a polarization voltage of 200 m V after 10 hours,while the gel electrolyte battery changed slightly after 235 hours,and the polarization voltage is only about 20mV.（3）A polymer gel electrolyte with high liquid uptake,high liquid holdup and excellent electrochemical properties has been prepared.The（PEO/PAA）@BWF composite gel electrolyte membrane was prepared by mixing polyacrylic acid（PAA）with PEO solution and the BWF after ball milling.The main function of BWF isto maintain the dimensional stability of the gel electrolyte membrane.It is found that the gel electrolyte membrane had a high electrolyte uptake（281.2%）and maintained anelectrolyte uptake of about 120%after 24 h at room temperature.The polymer gel electrolyte battery exhibits excellent electrochemical performance.The special capacity of the cells with the polymer gel electrolyte at 1 C is as high as 141 m Ah/g.The polymer gel electrolyte obviously inhibited the lithium dendrite.When the current density is 5 m A/cm²,the lithium dendrite of the liquid electrolyte battery has punctured the diaphragm in about 10 hours,and the polarization voltage is up to 200m V.When the current density is 118 hours,the battery has completely failed to work.The gel electrolyte battery circulate 400 hours at a current density of 5 m A/cm²,and its polarization voltage starts at a minimum of about 20 mV.