Layered cathodes such as LiCoO2, LiNiO2, LiNi0.5Mn0.5O2 and LiNi0.33Mn0.33Co0.33O2 attract a lot of attention both experimentally and computationally. Recently, to achieve higher capacity and stability, new advanced layered cathode materials with improved electrochemical properties have been developed, such as Li-rich layered cathode and concentration gradient layered cathode. In our group, in addition to identifying structure and stability of phases at ground state, we study the further details of atomic arrangements in advanced cathode materials also at higher temperatures combining the first-principles calculations with methods such as cluster expansion, SQS and Monte Carlo simulation.
Vacancy Order-Disorder and Intercalation Battery Voltages in LixCoO2
We present first-principles technique, cluster expansion and Monte Carlo simulations for predicting the ordered vacancy ground states, intercalation voltage profiles, and voltage-temperature phase diagrams of Li intercalation battery electrodes. Application to the LixCoO2 system yields correctly the observed ordered vacancy phases. We further predict the existence of additional ordered phases, their thermodynamic stability ranges, and their intercalation voltages in LixCoO2/Li battery cells.
|Relative stability of normal vs. inverse spinel for 3d transition metal oxides as lithium intercalation cathodes|
Spinel oxides represent an important class of cathode materials for Li-ion batteries. Two major variants of the spinel crystal structure are normal and inverse. The relative stability of normal and inverse ordering at different stages of lithiation has important consequences in lithium diffusivity, voltage, capacity retention and battery life. We investigate the relative structural stability of normal and inverse structures of the 3d transition metal oxide spinels with first-principles DFT calculations. We find that for all lithiated spinels, the normal structure is preferred regardless of the metal. We observe that the normal structure for all these oxides has a lower size mismatch between octahedral cations compared to the inverse structure. With delithiation, many of the oxides undergo a change in stability with vanadium in particular, showing a tendency to occupy tetrahedral sites. We find that in the delithiated oxide, only vanadium ions can access a +5 oxidation state which prefers tetrahedral coordination.
Cathode coatings can improve battery power by offering high Li diffusivity, stable surface chemistries and preventing undesirable passivation layers. Coatings can allow for charging to high potentials by protecting the electrolyte from oxidation; this feature is enabling for layered-layered Li2MnO3-LiMO2 materials and high voltage LiNi0.5Mn1.5O4 spinel materials. For LiMn2O4 spinel materials, Mn dissolution causes capacity fade, and cathode coatings must be selected to minimize Mn dissolution while maintaining fast Li diffusion. For all cathode coatings, high performance design is enabled by control of defects, Li+ and e- diffusivity, strain, phase diagrams and reactivity during processing, cycling, and aging.
|Lithium diffusion in electrode coatings|
As an example, we examine the Li diffusivity in a typical metal oxide (Al2O3) and metal fluoride (AlF3). We use methods that combine first principles density functional theory calculations and statistical mechanics to investigate Li transport in amorphous Al2O3 and AlF3. Because of unfavorable Li binding sites and relatively high diffusion barriers, the Li diffusivities are found to be very low. The diffusivities are also much lower than those in benchmark materials, Li-β-alumina and LiFePO4, which have open channel structures. This work is one part of a framework for understanding the battery performance improvement associated with coatings and should aid in future discovering of coating materials.
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