| dc.description.abstract | Presently, LiCoO2 is the most widely used cathode material in commercially
available Li-ion batteries, due to its high energy density and good cycle life
performance. However, the phase transformation from a hexagonal to monoclinic
phase, occurring between 4.1 and 4.2 V, induces a nonuniform volume change along
the c direction (~2 % expansion). This change eventually induces strains and extended
defects between and within the particles, leading to the disconnection of electrical
contact between particles and increased cell capacity fading. To overcome this
problem, the LiCoO2 cathode material was surface treated with the Li4M5O12 (M=Ti,
Mn) particles by a simple mechano-thermal process.
The Li4M5O12 (M=Ti, Mn) material possesses enhanced electrochemical activity,
good reversibility, zero-strain insertion, a very flat discharge-charge plateau and high
cycle stability during the charge–discharge process. The advantages of this compound
led us to focus on preparing Li4M5O12 (M=Ti, Mn) material as a coating material on
commercial LiCoO2 particles by a simple mechano-thermal process and studying its
electrochemical cell performance when charged at higher voltages.
A mixed metal oxide formed as a compact coating over the LiCoO2 cathode
particle to suppress the capacity fading caused by reactions with the electrolyte. The
Li4Ti5O12 and Li4Mn5O12 coated LiCoO2 cathode delivered excellent cyclability for
148 and 125 cycles, respectively, at a 0.2 C-rate between 4.40 and 2.75 V with charge
retention to 80 % of FMC-LiCoO2.
ESCA results revealed that the titanium and manganese ions of coating materials
could be observed on the LiCoO2 surface. The XPS spectra showed the coating
materials would react with LiCoO2 to form the LiMyCo1-yO2 (M=Ti, Mn) mixed metal
oxide. The DSC results showed that the coated LiCoO2 significantly depressed
exothermic activity and reduced heat generation at a highly delithiated state. In
addition, Li4M5O12 (M=Ti, Mn) coated LiCoO2 has better thermal safety
characteristics compared to the pristine LiCoO2 cathode material. The cobalt amounts
dissolved in the electrolyte of the Li4M5O12 (M=Ti, Mn) coated LiCoO2 were less than
the pristine one. Cyclic voltammetry revealed that the
hexagonal-monoclinic-hexagonal phase transformations were retained for the coated
cathode materials upon continuous cycling. Impedance spectra showed the electrolyte
resistance of the coated cathode decreased ( Is this right, wouldn’t a film increase
resistance) because the coating materials would form a thin-film on the cathode
surface to protect the cathode from reacting with the electrolyte. | en_US |