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Batteries

Overview


Li-ion batteries are ubiquitous in portable electronics, and battery makers are now looking to transportation and the electric grid for new markets.  To grow these markets, battery makers must improve energy, power, lifespan, cost, and safety.  These performance metrics are determined by materials properties, by chemistry, and by materials chemistry, most notably at the electrode particles surfaces. Our research focuses on a variety of solid inorganic materials used in Li-ion batteries, including materials for anodes, cathodes, cathode coatings, and solid electrolytes.  We employ first principles DFT calculations, Monte Carlo methods, and high-throughput database techniques.  Our work is funded by the Center for Electrical Energy Storage (CEES), an Energy Frontier Research Center (EFRC) based out of Argonne National Lab, by The Dow Chemical Company, and by Ford Motor Company.

Cathodes


Besides the commercial cathode materials such as layered-LiMO2, spinel-LiMn2O4, and olivine-LiMPOtype compounds, many new positive electrodes are being developed, such as the core-shell, concentration gradient and Li2MnO3 stabilized cathode materials. We address the outstanding questions about fundamental properties of these materials using first-principles calculations, with a particular focus on the mechanism of lithiation, Li diffusion, phase stability and electronic structure.

Layered cathodes

Layered cathodes such as LiCoO2LiNiO2, 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.

Spinel cathodes


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


Cathode particle surfaces can be tailored with coating materials including oxides, phosphates, and fluorides.  These cathode coatings range in thickness from <1 to >100 nanometers.  These coatings are designed to improve lifespan, power, safety, and to increase voltage limits.  A central challenge for most Li-ion cathode materials is stability during cycling and aging.  Degradation often occurs at the electrode-electrolyte interface and is exacerbated by the high voltages, which can drive electrolyte oxidation and dioxygen evolution.  This surface degradation reduces battery lifespan, slows Li diffusion, and increases the risk of thermal runaway. Today's electric vehicles use voltage limits and cell packaging to ensure safety, but this approach increases battery weight. To achieve a major break-through in battery performance, the cathode/electrolyte interface must be stabilized at the atomistic scale.


Designing cathode coating materials from first-principles by materials screening

The extent of protection a cathode coating can provide depends on many variables, both thermodynamic and kinetic. Using first-principles calculations, we have recently screened many prospective cathode coating materials based on their thermodynamic attributes, such as enthalpies of protective chemical reactions and equilibrium lithiation voltages. Besides the agreement with experimental observations, we were able to predict new promising coating materials.

Further details: 


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.


Further reading