리튬이온전지와 소듐이온전지의 전극 및 활물질의 표면개선을 통한 전기화학적 특성의 향상

Abstract

Lithium Ion Batteries have widely applied to portable electronic devices such as cell phones, laptop computers due to their outstanding energy and power capability. The properties of cathode materials mainly determine the performance and application of batteries. The spinel structured LiNi0.5Mn1.5O4, one of cathode active materials in lithium ion batteries, has a theoretical capacity of about 140 mAh/g and high operation voltage of about 4.8 V. So it has a high energy density and high rate power. However, the side reaction between cathode and electrolyte is occurred due to the high operation voltage. Thereby, it has severe capacity degradation, especially at elevated temperature. Researches have been carried out to resolve the problems mentioned above by coating of LiNi0.5Mn1.5O4 particles with some metal oxides. In Part I, The 5 V Spinel LiNi0.5Mn1.5O4 cathodes have been surface-modified with ZnAl2O4 by a sol-gel method. Although the pristine electrode experienced the prominent degradation after the storage test at 60 oC in the intervals of cycling test at room temperature, the ZnAl2O4-coated LiNi0.5Mn1.5O4 cathode exhibited the significant capacity retention even after storing at elevated temperatures. The X-ray photoelectron spectroscopy data reveals that the improved electrochemical performances of surface-coated cathode are mostly due to the suppressed side reviaction between the cathode and the electrolyte especially at the high-temperature environment. Differential scanning calorimetry showed that the decreased heat evolution could be found with the surface-modified cathode. Our experimental findings suggest a direction to the further development of cathode materials which are endurable to the highly oxidized state and high-temperature environment. In Part II, Tin oxide coating by employing electron cyclotron resonance metal–organic chemical vapor deposition was performed on 5 V-class spinel LiNi0.5Mn1.5O4 cathode electrodes prepared by a conventional tape-casting method. The pristine and SnO2-deposited LiNi0.5Mn1.5O4 electrodes were characterized by X-ray diffraction, field-emission electron probe microanalyzer, field-emission scanning electron microscopy, Auger electron spectroscopy, charge-discharge measurements, and electrochemical impedance spectroscopy were evaluated in lithium cells using the fabricated electrodes. The SnO2-deposited LiNi0.5Mn1.5O4 electrodes exhibited better rate capability at room temperature and superior electrochemical performance during the storage test evaluated at 60 °C in a fully charged state than the pristine LiNi0.5Mn1.5O4 electrode. Also, surface modification of the LiNi0.5Mn1.5O4 electrode was found by impedance analyses to be effective for suppressing the increase of the charge transfer resistance during the storage test at elevated temperatures. Recently, sodium ion batteries, which are promising alternative to lithium ion batteries, have been developed for market-friendly expenses, their abundant resources and physically and chemically similar properties to lithium. Among the limited number of anode materials of sodium ion batteries, spinel Li4Ti5O12, which has been attracted attention as a promising anode material in lithium ion batteries, has been applied recently. However, spinel Li4Ti5O12 anode materials exhibit low electrical conductivity, which results in poor cyclability and rate capability of electrodes because of Ti4+ with electron configuration of [Ar]3d0. In Part III, we focus on surface modification of Li4Ti5O12 materials with carbon coating technique to enhance the electrical conductivity and understand the important factors in sodium ion battery application. The carbon-coated Li4Ti5O12 was synthesized by using the citric acid sol-gel method. Carbon-coated Li4Ti5O12 exhibits superior cycle performance as well as the rate capability in comparison to the pristine Li4Ti5O12. Electrochemical impedance spectroscopy analyses also reveal that surface modification with carbon suppresses the increase in resistance concerning charge transfer reaction as well as solid electrolyte interface layer formation during cycle test.