ABSTRACT
A nanometer sized solid solution of NiO and Al2O3 was synthesized by calcination of Ni-Al layered double hydroxides (LDHs). The crystal structure of the obtained compound was determined by XRD and XAFS analyses: Ni2+ and Al3+ ions are located at the metal ion site of the rock salt structure and a certain amount of cation vacancies are also introduced for charge compensation. The electrochemical properties of the Ni-Al binary metal oxide as an anode material for lithium ion batteries were examined by the constant current charge-discharge test. Ni-Al oxide showed higher charging capacity in comparison with pristine NiO. In particular, the capacity in the lower voltage region (below 1.5 V), the limited capacity in this region is the weak point of the conversion anode, was improved to 540 mA h g-1 that is about twice that of pristine NiO. This improvement in the capacity in the lower voltage region is concluded to be due to the redox activity of Al3+ ions during the charge-discharge on the basis of the results of electrochemical measurements and ex situ XAFS measurements at the Ni and Al edge. The reaction mechanism of this compound is investigated using ex situ XRD and XAFS methods. For the charge (reduction) in the higher voltage region (OCV-1.0 V), lithium ion intercalation into the cation vacancy sites and/or lithium ion adsorption on the surface of particles are proceeding. For the charge in the lower voltage region (1.0-0.03 V), conversion reaction occurs by the reduction of Ni2+ and Al3+ ions to metal particles with surface electrolyte interface (SEI) layer formation. For the discharge in the lower voltage region (0.03-1.5 V), only Al metal particles are oxidized to Al3+ ions and some intermediate complexes are formed. For the discharge in the higher voltage region (1.5-3.0 V), the lattice of the Ni-Al binary oxide solid solution is reconstructed with the oxidation of Ni to Ni2+.
ABSTRACT
Surface and bulk structural changes of LiNi(0.5)Mn(0.5)O(2) were investigated during electrochemical reaction using synchrotron X-ray scattering and a restricted reaction plane consisting of two-dimensional epitaxial-film electrodes. The changes in bulk structure confirmed lithium diffusion through the (110) surface, which was perpendicular to the two-dimensional (2D) edges of the layered structure. No (de)intercalation reaction was observed through the (003) surface at voltages of 3.0-5.0 V. However, intercalation did proceed through the (003) plane below 3.0 V, indicating unusual three-dimensional (3D) lithium diffusion in the over-lithiated 2D structure. During the electrochemical process, the surface of the electrode showed different structure changes from those of the bulk structure. The reaction mechanism of the intercalation electrodes for lithium batteries is discussed on the basis of surface and bulk structural changes.
ABSTRACT
The rechargeable lithium-ion cell is an advanced energy-storage system. However, high cost, safety hazards, and chemical instability prohibit its use in large-scale applications. An alternative cathode material, LiFePO(4), solves these problems, but has a kinetic problem involving strong electron/hole localization. One reason for this is believed to be the limited carrier density in the fixed monovalent Fe(3+)PO(4)/LiFe(2+)PO(4) two-phase electrode reaction in LixFePO4. Here, we provide experimental evidence that LixFePO4, at room temperature, can be described as a mixture of the Fe(3+)/Fe(2+) mixed-valent intermediate LialphaFePO4 and Li1-betaFePO4 phases. Using powder neutron diffraction, the site occupancy numbers for lithium in each phase were refined to be alpha=0.05 and 1-beta=0.89. The corresponding solid solution ranges outside the miscibility gap (0
