Research On Improvement Of Cycling Stability In Lithium Ion Battery At Elevated Temperature

Lithium ion batteries have been widely used in laptops, mobilephones, camera and other portable electronic devices after many years’development due to the long cycle life, high energy density, non-toxicity,and environmental friendliness. However, it is still a long way to go forlithium ion batteries to be used as the power sources for electric vehicles（EVs） and hybrid electric vehicles （HEVs） because of their poor cycleperformance at elevated temperature. The performance of lithium ionbatteries at elevated temperature is dependent on the choice of electrolytesystem to a large extent. In our work, two different methods includingdevelopment of new functional electrolyte additive into the conventionalliquid electrolyte and preparation of novel polymer electrolyte wereinvestigated to improve the cycling stability of lithium ion batteries atelevated temperature.Firstly, it is well known that the conventional liquid electrolyte isprone to cause side reaction especially at elevated temperature. In order to solve this problem, we used tris（trimethylsilyl） borate （TMSB） asfunctional electrolyte additive into the conventional liquid electrolyte（1M LiPF₆into EC-DMC1:1） to improve the cycle performance oflithium ion batteries at elevated temperature. The mechanism of TMSBwas investigated by AC impedance, cycle performance, coulombefficiency, X-ray photoelectron spectroscopy （XPS）, scanning electronmicroscope （SEM）, and X-Ray diffraction （XRD）. The battery withoutTMSB displayed about18%capacity loss after the80th cycle at55°C,while the battery containing0.5wt%TMSB additive showed less than3%capacity loss at the same cycle number. The testing results indicatedthat TMSB participated in the formation of solid electrolyte interface（SEI） on the surface of the electrode, which protected LiMn₂O₄fromdamage and restricted the increase of interface impedance, and finallyimproved the cycle performance.Secondly, in order to solve the problem that the conventional liquidelectrolyte is prone to cause side reaction especially at elevatedtemperature, we used perfluorinated ion exchange membrane （PFSA-Limembrane） as both electrolyte and separator in LiMn₂O₄/Li battery.Polymer electrolyte （PFSA-Li membrane） was prepared after theprocesses of solution cast film, salification, dehydration, and organicsolvents swelling. We tested the conductivities of different IEC ofPFSA-Li swollen with different kinds of organic solvents. The conductivity of EP-PFSA-Li membrane（EC:PC=1:1, v:v） reached1.46×10^-3S cm-1at room temperature （25°C）, which was drasticallygreater than the conductivity of the conventional solid polymerelectrolyte. Cyclic voltammetry curves showed that the EP-PFSA-Li filmhad stable wide electrochemical window. The capacity fading of thebattery using the conventional liquid electrolyte （LiMn₂O₄/LiPF₆-EC-DMC/Li） was48.3%of the initial capacity after50cycles,while the battery using the membranes as electrolyte（LiMn₂O₄/EP-PFSA-Li/Li） showed high capacity retention as70.7%ofthe initial capacity even after100cycles. The LiMn₂O₄/EP-PFSA-Li/Libattery showed very good cycling stability at60°C.Finally, it is well known that the stability of LiMn₂O₄is poor due tothe Jahn-Teller distortion at elevated temperature. To further improve thecycling stability of lithium ion battery at elevated temperature, we usedLiFePO4to take the place of LiMn₂O₄in lithium ion battery. At55°C, thecapacity of battery using conventional liquid electrolyte （LiFePO4/LiPF₆-EC-DMC/Li） decreased to76%of the initial capacity after80cycles, however, polymer electrolyte battery （LiFePO4/EP-PFSA-Li/Li）showed less than3%discharge capacity loss over120cycles. Besides,the discharge capacity of polymer electrolyte battery （LiFePO4/EP-PFSA-Li/Li） at6C was60mAh/g.