|$B3X0LO@J8BjL\(J||Synthesis, Structure and Electrochemical Characteristics of
Double Transition Metal Oxide Electrode Materials for Li-ion Secondary Batteries|
$B!!(JHomogeneity of the starting material is critical point to determine purity of final product. If not, undesired compounds may be formed as impurities. It is well known that recalcinations and subsequent repeated regrindings were significantly necessary to prepare high purity transition metal oxides. From these reasons, soft chemistry routes, such as emulsion drying and hydrothermal methods, were employed to synthesize high purity double transition metal oxide electrode materials, i.e., LiAlxCo1-xO2, LiAlxMn2-xO4, LiNi0.5Mn1.5O4, LiCoxMn1-xO2 and so on, for lithium-ion battery.
$B!!(JCommercially available lithium-ion battery adopts LiCoO2 ceramic powder as the cathode material. The material shows usually different electrochemical properties, especially capacity. That is, smaller particle-sized powders deliver much higher capacity than larger ones. In the present study, small particles having high crystallinity were successfully prepared by employing the emulsion drying method, and the obtained powders showed somewhat enhanced capacity. Even though the material possessed higher capacity, it was difficult to prevent dissolution of Co derived from the drastic variation in the c-axis at higher potential. Fortunately, incorporation of the non-transition element, Al, which gives stronger bonds in all oxide matrix, was effective to suppress the drastic change in the c-axis, leading finally to increase in structural integrity and Li+ diffusion.
$B!!(JEven though LiCoO2 is now commercially available, Co is hazardous element to human body, sometimes causing lung and kidney cancers. In order to substitute the risky material, spinel type of LiMn2O4 was investigated. Basically, Mn is cheap, abundant, environmentally benign comparing to Co. LiAlxMn2-xO4 powders having spinel structure have been synthesized from homogeneous emulsion precursor at 850 $B!n(J in this study. Rietveld refinements of their neutron diffraction data revealed that Al was substituted selectively for Mn at the 16d site and that Li occupied only the 8a site. As-synthesized powders were sub-micron single crystalline oxide particles. The homogeneous mixing among cations was confirmed by the TEM observation. The initial discharge capacity was higher due mainly to the improved nature of prepared powder by the emulsion drying method. For the case of the doped amount of 0.3 $B!e(J x $B!e(J 0.5 in LiAlxMn2-xO4, the two potential plateaus disappeared due to the strong repulsive interaction between lithium positive-positive ions in the oxide lattice. The structural integrity was greatly enhanced by the Al substitution of the 16d manganese site. And, this led to decrease in the chemical diffusivity with increasing Al content. SEM and TEM illustrated that particle disruption observed in Al-free LiMn2O4 was also suppressed by Al substitution in LiMn2O4. The increased structural integrity and reduced strain during repeated de-/intercalation of lithium by Al doping resulted in excellent cyclablility with relatively higher discharge capacity retention at both 25 $B!n(J and 45 $B!n(J.
$B!!(JIn order to achieve much higher power, spinel type of LiNi0.5Mn1.5O4 was designated as use for 5 V applications. When the emulsion-dried precursor was calcined at 750 $B!n(J for 24 h, the observed particle size by TEM was about 50 nm in diameter. The spinel phase showed a two-phase reaction from the composition of $B&D(J= 0.2 in Li$B&D(JNi0.5Mn1.5O4 during initial deintercalation by ex-situ XRD examination. This composition exhibited relatively higher capacity upon cycling.
$B!!(JOrthorhombic LiMnO2 (hereinafter referred as to o-LiMnO2) having a zig-zag $B&B(J-NaMnO2 layered structure is an attractive alternative to the conventional spinel LiMn2O4 oxide as a cathode material for lithium-ion secondary batteries. Because the LiMn2O4 has been suffered by abrupt structural changes from cubic to tetragonal phase derived from the collective Jahn-Teller distortion on 3 V region. Orthorhombic LiMnO2 phase transforms to spinel LiMn2O4 phase by electrochemical cycling. Obviously, there exists the Jahn-Teller distortion when it cycles on 3 V region, the cycle-induced spinel phase from o-LiMnO2 can maintain high capacity on 3 V region with help of the formation of ferroelastic nano-structured antiphase boundary domains. However, main concerns of this material are difficulty of preparation, because it needs a very careful control of average oxidation state of Mn to be 3+. Provided not, undesired secondary phase should appear.
$B!!(JThose difficulties have been overcome by employing emulsion drying method and hydrothermal method in the present study. First, The optimum synthesis condition to crystallize into zigzag layer $B&B(J-NaMnO2 system was to calcine the emulsion-dried precursor at 925 $B!n(J in an Ar atmosphere. Li/LiMnO2 cell showed the capacity of 173 mAh g-1 and excellent capacity retention upon cycling with help of cycle-induced spinel like phase, more than 155 mAh g-1 over 300 cycles at 25 $B!n(J.
$B!!(JSecond, LiCoxMn1-xO2 powders having orthorhombic structure have been synthesized from the reaction between (CoxMn1-x)3O4 as a hydrothermal precursor and aqueous lithium hydroxide solution by hydrothermal treatment at a temperature as low as 170 $B!n(J. The substitution of Mn3+ by Co element leads to the enhanced structural integrity. The maximum discharge capacities were 135 and 182 mAh (g-oxide)-1 for undoped and Co doped electrode during cycling, respectively. For the case of o-LiCo0.1Mn0.9O2 by field emission scanning electron microscopy (FE-SEM) observation, the particle shapes after 100 cycles were maintained upon extensive cycling, although those were not maintained for undoped o-LiMnO2. The higher capacity for o-LiCo0.1Mn0.9O2 was due to the improved structural integrity and electrical conductivity.
$B!!(JLayered LiMnxCr1-xO2 oxides with the layered $B&A(J-NaFeO2 structure have been readily synthesized by the emulsion drying method. The solid solution range was greatly enlarged with help of emulsion. LiMn0.6Cr0.4O2 compound showed reversible structural changes during cycling by in situ XRD technique, and the reversibility based on electroactive redox of Mn3+/4+ and Cr3+/6+ couples was also confirmed by X-ray absorption nearest edge spectroscopy (XANES). In the composition, the charge and discharge curves showed S-shaped potential variation, indicating dilution of the Jahn-Teller distortion.
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