Intercalation compounds, used as electrodes in Li-ion batteries, are a fascinating class of materials that exhibit a wide variety of electronic, crystallographic, thermodynamic, and kinetic properties. With open structures that allow for the easy insertion and removal of Li ions, the properties of these materials strongly depend on the interplay of the host chemistry and crystal structure, the Li concentration, and electrode particle morphology. The large variations in Li concentration within electrodes during each charge and discharge cycle of a Li battery are often accompanied by phase transformations. These transformations include order-disorder transitions, two-phase reactions that require the passage of an interface through the electrode particles, and structural phase transitions, in which the host undergoes a crystallographic change. Although the chemistry of an electrode material determines the voltage range in which it is electrochemically active, the crystal structure of the compound often plays a crucial role in determining the shape of the voltage profile as a function of Li concentration. While the relationship between the voltage profile and crystal structure of transition metal oxide and sulfide intercalation compounds is well characterized, far less is known about the kinetic behavior of these materials. For example, because these processes are especially difficult to isolate experimentally, solid-state Li diffusion, phase transformation mechanisms, and interface reactions remain poorly understood. In this respect, first-principles statistical mechanical approaches can elucidate the effect of chemistry and crystal structure on kinetic properties. In this Account, we review the key factors that govern Li diffusion in intercalation compounds and illustrate how the complexity of Li diffusion mechanisms correlates with the crystal structure of the compound. A variety of important diffusion mechanisms and associated migration barriers are sensitive to the overall Li concentration, resulting in diffusion coefficients that can vary by several orders of magnitude with changes in the lithium content. Vacancy clusters, groupings of vacancies within the crystal lattice, provide a common mechanism that mediates Li diffusion in important intercalation compounds. This mechanism emerges from specific crystallographic features of the host and results in a strong decrease of the Li diffusion coefficient as Li is added to an already Li rich host. Other crystallographic and electronic factors, such as the proximity of transition metal ions to activated states of hops and the occurrence of electronically induced distortions, can result in a strong dependence of the Li mobility on the overall Li concentration. The insights obtained from fundamental studies of ionic diffusion in electrode materials will be instrumental for physical chemists, chemical engineers, synthetic chemists, and materials and device designers who are developing these technologies.

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The choice of cathode material critically influences the performance, cost and energy density in a Li ion battery. LiMn 2 O 4 (LMO) cathode is an attractive cathode material because of its high rate capability, low cost, safety and non-toxicity. However, low cycle life or capacity fading with cycling is a big hurdle in the practical application of LMO cathodes. The dominant cause of the capacity fading, Mn dissolution from LMO, is still a challenge to overcome. In this review, we attempt to emphasize the loopholes in the understanding related to the Mn dissolution phenomenon in LMO cathodes in the presence of both aqueous and non-aqueous electrolytes. The underlying mechanism behind the dissolution process is often explained with the help of ionic charge disproportionation argument. Nevertheless, there are experimental and theoretical evidence in the literature to counter the simplistic view of dissolution due to charge disproportionation. Moreover, the correlation between the metal dissolution and the capacity fading is not causally established yet in the literature which hinders the formulation for a systematic strategy to prevent the capacity loss. In the current article we discuss all the relevant investigations related to the understanding of the cause, effect and prevention strategies of Mn-dissolution. We attempt to point out the gaps and contradictions in the literature and motivate further targeted studies in this seminally important material for better Li-ion batteries having higher rate capability and capacity retention.

We report on a first-principles study of non-dilute Li diffusion in spinel Li x TiS 2 with the aim of elucidating the role of crystal structure and chemistry on Li mobility in intercalation compounds used as electrodes in Li-ion batteries. In contrast to transition metal oxide spinels, where Li ions occupy tetrahedral interstitial sites, Li ions in spinel Li x TiS 2 preferentially occupy octahedral sites. This makes spinel Li x TiS 2 a useful model system to explore diffusion mechanisms in threedimensional intercalation compounds with octahedral Li occupancy. Elementary Li hops between neighboring octahedral sites pass through intermediate tetrahedral sites. High coordination of these intermediate tetrahedral sites by octahedral sites causes the migration barrier to be sensitive to the local Li concentration and configuration. Kinetic Monte Carlo simulations predict diffusion mechanisms mediated by triple vacancies and divacancies, which 2 leads to a strong concentration dependence of the chemical diffusion coefficient. Insights from this study combined with those gathered in past first-principles studies of layered intercalation compounds indicate that crystal structures with activated states that are highly coordinated by Li sites will result in diffusion mechanisms mediated by vacancy clusters, producing a chemical diffusion coefficient that decreases with increasing Li composition.

We present an asteroseismic study of the solar-like stars KIC 11395018, KIC 10273246, KIC 10920273, KIC 10339342, and KIC 11234888 using short-cadence time series of more than eight months from the Kepler satellite. For four of these stars, we derive atmospheric parameters from spectra acquired with the Nordic Optical Telescope. The global seismic quantities (average large frequency separation and frequency of maximum power), combined with the atmospheric parameters, yield the mean density and surface gravity with precisions of 2% and ∼0.03 dex, respectively. We also determine the radius, mass, and age with precisions of 2-5%, 7-11%, and ∼35%, respectively, using grid-based analyses. Coupling the stellar parameters with photometric data yields an asteroseismic distance with a precision better than 10%. A v sin i measurement provides a rotational period-inclination correlation, and using the rotational periods from the recent literature, we constrain the stellar inclination for three of the stars. An Li abundance analysis yields an independent estimate of the age, but this is inconsistent with the asteroseismically determined age for one of the stars. We assess the performance of five grid-based analysis methods and find them all to provide consistent values of the surface gravity to ∼0.03 dex when both atmospheric and seismic constraints are at hand. The different grid-based analyses all yield fitted values of radius and mass to within 2.4σ, and taking the mean of these results reduces it to 1.5σ. The absence of a metallicity constraint when the average large frequency separation is measured with a precision of 1% biases the fitted radius and mass for the stars with non-solar metallicity (metal-rich KIC 11395018 and metal-poor KIC 10273246), while including a metallicity constraint reduces the uncertainties in both of these parameters by almost a factor of two. We found that including the average small frequency separation improves the determination of the age only for KIC 11395018 and KIC 11234888, and for the latter this improvement was due to the lack of strong atmospheric constraints.

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. In this paper, we investigate the relative structural stability of normal and inverse structures of the 3d transition metal oxide spinels with first-principles DFT calculations. We have considered ternary spinel oxides LixM2O4 with M = Ti, V, Cr, Mn, Fe, Co and Ni in both lithiated (x = 1) and delithiated (x = 0) conditions. 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. We have also calculated the average voltage of lithiation for these spinels. The calculated voltages agree well with the previously measured and calculated values, wherever available. For the yet to be characterized spinels, our calculation provides voltage values which can motivate further experimental attention. Lastly, we observe that all the normal spinel oxides of the 3d transition metal series have a driving force for a transformation to the non-spinel structure upon delithiation.