Yin-Wen Tsai

Effect of Al-substitution on the stability of LiMn2O4 spinel, synthesized by citric acid sol–gel method

Abstract Spinel LiMn2O4 and LiAlxMn2−xO4 (x=0.05, 0.15) are synthesized by a sol–gel method using citric acid as a chelating agent. The effect of calcination temperature on the purity of the Al-substituted spinel is examined by X-ray diffraction measurements which suggests that pure material is obtained by calcination at 800°C. Cyclic voltammetry is employed to characterize the reactions of lithium insertion into and extraction from the spinel materials. Surface morphology changes are observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicate that Al-substitution enhances sintering of the spinel LiMn2O4. Charge–discharge cycling studies show that Al-substitution improves substantially the capacity retention of the spinel LiMn2O4.

Influence of Mn content on the morphology and electrochemical performance of LiNi1−x−yCoxMnyO2 cathode materials

LiNi1−x−yCoxMnyO2 cathode materials were successfully synthesized by a sol–gel method. X-Ray diffraction patterns show that these materials have a typical layered structure with Rm space group. The effect of Mn content and sintering temperature on the surface morphology has been carefully examined by scanning electron microscopy. Among the substituted materials synthesized, LiNi0.65Co0.25Mn0.1O2 sintered at 850 °C has shown the best characteristics in terms of initial capacity (198 mA h g−1) and capacity retention (92%). However, excessive Mn substitution (y > 0.3) leads to a significant decrease in both initial capacity and capacity retention. Preliminary X-ray absorption near edge structure (XANES) results for LiNi1−x−yCoxMnyO2 cathode materials revealed that the oxidation state of Ni is decreased when the amount of Mn substituted for Ni is increased.

LiMn2 O 4 Core Surrounded by LiCo x Mn2 − x O 4 Shell Material for Rechargeable Lithium Batteries Synthesis and Characterization

We demonstrate a novel method for the synthesis of LiMn 2 O 4 core surrounded by LiCo 3 Mn 2-x O 4 shell material for rechargeable lithium batteries. Surface and composition analysis by electron spectroscopy for chemical analysis and inductively coupled plasma method, indicated that cobalt mostly distributed on the surface of the LiMn 2 O 4 particles. X-ray diffraction studies revealed that cobalt was doped into the lattice of the LiMn 2 O 4 particles. X-ray absorption fine structure analysis further demonstrated that cobalt was doped mainly on the surface of the LiMn 2 O 4 particles, LiCo x Mn 2-x O 4 solid solution thus formed on the surface of the LiMn 2 O 4 particles greatly enhanced the initial capacity, capacity retention, and rate capability of the spinel LiMn 2 O 4 . The enhanced initial capacity of the LiMn 2 O 4 core/LiCo x Mn 2-x O 4 shell material may be due to the involvement of Co in the charging/discharging behavior and the improved capacity retention and the rate capability were most probably due to less cation disordering during charge/discharge cycling experiments at low as well as at high C-rates.

Structure stabilization of LiMn2O4 cathode material by bimetal dopants

The structural changes of spinel Li 1.02 Mn 2 O 4 and Li 1 0.2 Co 0.1 Ni 0.04 Mn 1 85O 4 cathode materials have been studied by synchrotron powder X-ray diffraction and differential scanning calorimetry (DSC) measurements. The results show that spinel Li 1.02 Mn 2 O 4 undergoes a phase transition from cubic (Fd3m) to orthorhombic symmetry (Fddd) at T = 285 K. However, substitution of a small amount of Co 3+ and Ni 3+ ions suppresses phase transition and the cubic phase is maintained at low temperature due to a decrease in the concentration of Jahn-Teller active Mn 3+ ions.

In-situ X-ray absorption spectroscopy investigations of a layered LiNi0.65Co0.25Mn0.1O2 cathode material for rechargeable lithium batteries

In-situ X-ray absorption spectroscopy investigations have been carried out to evaluate the oxidation states and the local structure at the Mn, Co and Ni K-edges for LiNi0.65Co0.25Mn0.1O2 during a charging and discharging process in this study. It was found that only the Ni atom in LiNi0.65Co0.25Mn0.1O2 is electroactive from the evolution of XANES spectra and the bond length variation of Ni–O. We observed that the NiII/NiIV and NiIII/NiIV redox pairs exist and the oxidation states of Mn and Co remain as MnIV and CoIII, respectively, in LiNi0.65Co0.25Mn0.1O2 on a charge and discharge cycle. In addition, the irreversible capacity at the first cycle derives mainly from the appearance of inactive Ni during the first discharging, which is consistent with the energy shift E–E0 of the absorption edge for the Ni absorber and the bond length change of Ni–O. A decrease/increase of Debye–Waller factor of the Ni–O contribution results from a decrease/increase of Jahn–Teller active NiIII concentration on the charge and discharge cycle. The charge ordering between NiII and MnIV may take place in this compound, resulting from the significant difference of the bond length and the Debye–Waller factor for the second shell Mn–M and Ni–M contributions between the starting and fully charged states. A balance of the repulsive force and size effect in the layered compound plays an important role in the inhibition of structure distortion during the delithiation and lithiation cycle.

Local structure transformation of nano-sized Al-doped LiMn2O4 sintered at different temperatures

LiMn2O4 and LiAl0.15Mn1.85O4 were synthesized via the sol–gel process using citric acid as the chelating agent, followed by sintering at various temperatures. The electronic and atomic structures of LiMn2O4 and LiAl0.15Mn1.85O4 powders were probed by means of Mn K-edge X-ray absorption spectroscopy (XAS). Al-doping was found to promote the sintering of spinel LiMn2O4 so that the degree of structural disorder around Mn atoms in LiAl0.15Mn1.85O4 becomes lower than that of LiMn2O4, leading to an excellent capacity retention of this cathode material for lithium battery in charge–discharge cycle. # 2003 Published by Elsevier Science B.V.

In Situ X-Ray Absorption Spectroscopy Studies of Nanosized LiAl0.15Mn1.85 O 4 Cathode Material in an Aqueous Solution

We have performed in situ Mn K-edge X-ray absorption spectroscopy (XAS) studies on nanosized LiAl 0.15 Mn 1.85 O 4 cathode material at various charging and discharging potentials in an aqueous solution (9.0 M LiNO 3 ). The associated changes in the electronic and local atomic structures were investigated during the course of charging and discharging. The B1 and B2 peaks appeared in the Mn K-edge X-ray absorption near-edge structure spectra of LiAl 0.15 Mn 1.85 O 4 were due to pronounced multiple scattering effect. Two types of Mn-O bonds, namely Mn-O(4) and Mn-O(2), were observed in the first coordination shell. It was found that the Mn-O(4) and the second shell Mn-Mn/Al bond distances as well as the Debye-Waller factors decrease on charging and increase on discharging. In contrast, the Mn-O(2) bonds show an elongation on charging and contraction on discharging. The Debye-Waller factor of Mn-O(2) does not change significantly during both charging and discharging. Thus, we infer that the ordering of Mn-O 6 octahedra is mainly due to the contribution from Mn-O(4) bonds.