Research Of Lini_0.5Mn_1.5O₄ Cathode Materialfor Lithium Ion Batteries And Piperidineionic Liquid Based Mixed Electrolyte

Spinel LiNi_0.5Mn_1.5O₄ cathode material has a broad prospect for Li-ion battery used in electric vehicles because of its high voltage, high energy density,long life and no harm to environment. However,the material has some defects in practical application,such as the impurity and the poor cycle performance,improve and without the suitable high-voltage electrolyte. Therefore,in this paper the preparation and modification of LiNi_0.5Mn_1.5O₄ cathode material were studied and the piperidine ionic liquids based mixed electrolyte were developed.LiNi_0.5Mn_1.5O₄ cathode material was prepared by rheological method with Mn（CH₃COO）₂·4H₂O, Ni（CH₃COO）₂·4H2O and CH₃COOLi·2H₂O. the calcining temperature, sintering time and lithium content were optimized. The structure, morphology and electrochemical properties of the LiNi_0.5Mn_1.5O₄ cathode material were characterized by X-ray diffraction, scanning electron microscope and charge-discharge test. The results showed that the material with 5% excess of Lithium content, calcined at 850℃ about 6 hours was confirmed to the best in view of electrochemical performance. Its specific discharge capacity was 127.1 mAh/g at 0.1 C discharge rate, and after 50 cycles the capacity retention was 95.4%.Li Ni_0.5-xAl_2xMn_1.5-xO₄ cathode material was prepared by spray-drying method with Mn（CH₃COO）₂·4H₂O, Ni（CH₃COO）₂·4H₂O, CH₃COOLi·2H₂O and Al（NO₃）3·9H₂O. The structure, morphology and electrochemical properties of LiNi_0.5-xAl_2xMn_1.5-xO₄ cathode material were characterized by XRD, SEM and charge-discharge tests. The results showed that the LiNi_0.5-xAl_2xMn_1.5-xO₄（x=0.05）sample had the best electrochemical performance, Its specific discharge capacity was 123.5 mAh/g at 0.1 C discharge rate, and after 50 cycles the capacity retention was 98.6%,and showed good cycling ability.Room temperature ionic liquids（RTILs） of N-methyl-N-ethyl（propyl,butyl） piperidinium bis（ trifluo romethanesulfonyl） imide（PP12（3,4）TFSI） were synthesized with two step constant reflux method and then mixed electrolyte 1 mol/ L LiPF6（PP12（ 3,4） TFSI） + ethylene carbonate（EC）+ dimethylene carbonate（DMC） with organic solvent EC,DMC. The results from Nuclear magnetic resonance（NMR） and infrared spectroscopic analysis（IR） show that the ionic liquids is very pure. The results from thermogravimetric（ TG） indicated that the adding of ionic liquid restrained the volatilization of organic solvent, at the same temperature,increasing the volume content of ionic liquid, the weight loss of the mixed electrolyte will be less. When cycled with 150mAh/g in 3.0-4.35 V, after 200 cycles the capacity retention was higher than the pure organic electrolyte,which Li/ LiCoO₂ cell used 1 mol/L LiPF₆ / δPP12（3,4）-TFSI +（ 1-δ） EC/DMC（ 1:1, vol） mixed electrolyte, the cycle performance was improved, and the effect was relatively best when the content of PP13 TFSI was 10%.