Aiswarya Bhaskar

Synthesis, Characterization, and Comparison of Electrochemical Properties of LiM0.5Mn1.5O4 (M = Fe , Co, Ni) at Different Temperatures

LiM 0.5 Mn 1.5 O 4 (M = Fe, Co, Ni) normal spinel oxides were prepared by a citric acid assisted Pechini synthesis with different thermal treatments and compared with respect to their electrochemical performance as cathodes in lithium-ion batteries. Characterization methods include X-ray diffraction, neutron diffraction, inductively coupled plasma optical emission spectroscopy analysis, and scanning electron microscopy. While LiM 0.5 Mn 1.5 O 4 samples crystallize for M = Fe and Co with the 3d cation-disordered cubic spinel-like structure (Fd3m space group), the 600°C annealed LiM 0.5 Mn 1.5 O 4 shows a partially ordered structure (belonging to the P4 3 32 space group). The absolute discharge capacity is slightly higher for the Ni-doped samples in comparison with the Co-and Fe-doped spinels. 1000°C annealed samples show an improved cyclability in comparison with the 600°C annealed samples. At elevated temperatures, Co- and Fe-doped samples show much faster degradation in comparison with the Ni-doped sample. The responsible mechanisms are discussed.

Thermal stability of Li1−ΔM0.5Mn1.5O4 (M = Fe, Co, Ni) cathodes in different states of delithiation Δ

The thermal stability of sol–gel synthesized Li1−ΔM0.5Mn1.5O4 (M = Fe, Co, Ni) electrodes with different degrees of delithiation were analyzed with TG-DSC and in situ synchrotron diffraction under an Ar atmosphere and compared. The onset temperatures for structural degradation are dependent on the amount of lithium 1−Δ in the sample. The Li1−ΔFe0.5Mn1.5O4 electrode exhibited the highest thermal stability among the three materials with different dopant M. The reason for this difference is discussed with respect to the oxidation states of the transition metals. The mechanism of degradation for M = Fe, Co was found to be through gas evolution, mainly CO2 and O2, and the carbon conductive additive was found to play a major role in the thermal degradation process. For delithiated Li1−ΔNi0.5Mn1.5O4 the temperature induced degradation includes phase separation into Mn3O4 with spinel structure and LixNi1−xO with rock-salt structure together with oxygen and carbon dioxide release.

Coexistence of conversion and intercalation mechanisms in lithium ion batteries: Consequences for microstructure and interaction between the active material and electrolyte

Abstract Conversion-type lithium ion batteries experience severe and partly irreversible phase transitions during operation. Such phase transitions reduce the crystallite size and therefore enhance the exchange of the Li ions. Concurrently, the irreversible nature of the phase transitions may deteriorate the cycling stability and the long-term capacity of conversion-type batteries. In this contribution, the observed correlations between the crystal structures of compounds which are employed as anodes in conversion-type Li ion cells, the capacity and the long-term stability of these cells are discussed. The central characteristics affecting the performance of conversion-type Li ion cells seem to be the similarity of crystal structures of intermediately forming phases during the charge/discharge process, which facilitates strong local preferred orientation of nanocrystallites of neighboring phases and for the formation of local strain fields at partially coherent phase boundaries. The effect of the above-mentioned phenomena on capacity and cycle stability is argued from the point of view of a possibly impeded ion exchange. Equilibrium open circuit potentials are calculated using the CALPHAD method. However, it is shown that in order to better reproduce the experimentally determined plateau voltages, thermodynamic descriptions of the non-equilibrium intermediate phases have to be included. In addition, the stabilization of the conversion reaction by the electrolyte is pointed out.