Lithium-ion batteries are now reaching the energy density limits set by their electrode materials, requiring new paradigms for Li(+) and electron hosting in solid-state electrodes. Reversible oxygen redox in the solid state in particular has the potential to enable high energy density as it can deliver excess capacity beyond the theoretical transition-metal redox-capacity at a high voltage. Nevertheless, the structural and chemical origin of the process is not understood, preventing the rational design of better cathode materials. Here, we demonstrate how very specific local Li-excess environments around oxygen atoms necessarily lead to labile oxygen electrons that can be more easily extracted and participate in the practical capacity of cathodes. The identification of the local structural components that create oxygen redox sets a new direction for the design of high-energy-density cathode materials.

The rapidly expanding field of nonaqueous multivalent intercalation batteries offers a promising way to overcome safety, cost, and energy density limitations of state-of-the-art Li-ion battery technology. We present a critical and rigorous analysis of the increasing volume of multivalent battery research, focusing on a wide range of intercalation cathode materials and the mechanisms of multivalent ion insertion and migration within those frameworks. The present analysis covers a wide variety of material chemistries, including chalcogenides, oxides, and polyanions, highlighting merits and challenges of each class of materials as multivalent cathodes. The review underscores the overlap of experiments and theory, ranging from charting the design metrics useful for developing the next generation of MV-cathodes to targeted in-depth studies rationalizing complex experimental results. On the basis of our critical review of the literature, we provide suggestions for future multivalent cathode studies, including a strong emphasis on the unambiguous characterization of the intercalation mechanisms.

Diffusion constants are typically considered to be independent of particle size with the benefit of nanosizing materials arising solely from shortened transport paths. We show that for materials with one-dimensional atomic migration channels, the diffusion constant depends on particle size with diffusion in bulk being much slower than in nanoparticles. This model accounts for conflicting data on LiFePO(4), an important material for rechargeable lithium batteries, specifically explaining why it functions exclusively on the nanoscale.

The diffusion of ions in solid materials plays an important role in many aspects of materials science such as the geological evolution of minerals, materials synthesis, and in device performance across several technologies. For example, the realization of multivalent (MV) batteries, which offer a realistic route to superseding the electrochemical performance of Li-ion batteries, hinges on the discovery of host materials that possess adequate mobility of the MV intercalant to support reasonable charge and discharge times. This has proven especially challenging, motivating the current investigation of ion mobility (Li + , Mg 2+ , Zn 2+ , Ca 2+ , and Al 3+ ) in spinel Mn 2 O 4 , olivine FePO 4 , layered NiO 2 , and orthorhombic δ-V 2 O 5 . In this study, we not only quantitatively assess these structures as candidate cathode materials, but also isolate the chemical and structural descriptors that govern MV diffusion. Our finding that matching the intercalant site preference to the diffusion path topology of the host structure controls mobility more than any other factor leads to practical and implementable guidelines to find fast-diffusing MV ion conductors.

contributed equally to this work. AbstractBatteries that shuttle multi-valent ions such as Mg 2+ and Ca 2+ ions are promising candidates for achieving higher energy density than available with current Li-ion technology. Finding electrode materials that reversibly store and release these multi-valent cations is considered a major challenge for enabling such multi-valent battery technology. In this paper, we use recent advances in high-throughput firstprinciples calculations to systematically evaluate the performance of compounds with the spinel structure as multivalent intercalation cathode materials, spanning a matrix of five different intercalating ions and seven transition metal redox active cation. We estimate the insertion voltage, capacity, thermodynamic stability of charged and discharged states, as well as the intercalating ion mobility and use these properties to evaluate promising directions. Our calculations indicate that the Mn2O4 spinel phase based on Mg and Ca are feasible cathode materials. In general, we find that multivalent cathodes exhibit lower voltages compared to Li cathodes; 2 the voltages of Ca spinels are ~ 0.2V higher than those of Mg compounds (versus their corresponding metals), and the voltages of Mg compounds are ~1.4 V higher than Zn compounds; consequently, Ca and Mg spinels exhibit the highest energy densities amongst all the multivalent cation species. The activation barrier for the Al 3+ ion migration in the Mn2O4 spinel is very high (~1400 meV for Al 3+ in the dilute limit); thus, the use of an Al based Mn spinel intercalation cathode is unlikely.Amongst the choice of transition metals, Mn-based spinel structures rank highest when balancing all the considered properties. Broader ContextThe high price and limited volumetric capacity of the lithium ion battery (LIB) challenges its application in electric vehicles and portable electronics. Multivalent batteries, such as those utilizing Mg 2+ or Ca 2+ as the working ions, are promising candidates for beyond LIB technology due to the increase in volumetric capacity and reduced cost. In the present work, we use first-principles calculations to systematically evaluate the theoretical performance of the spinel structure host with the general formula AB2O4 across a matrix of chemical compositions spanning A={Al, Y, Mg, Ca, Zn} and B={Ti, V, Cr, Mn, Fe, Co, Ni} for multivalent battery applications. The evaluation incorporates screening on voltage, capacity, thermodynamic structural and thermal stability as well as ion mobility and discusses the results in the context of available host structure sites, preference of the intercalating cation, and the oxidation state of the redox-active cation. Overall, the Mn2O4 spinel phases paired with Mg 2+ or Ca 2+ emerge as the most promising multivalent cathode materials. As the first comprehensive screening of multivalent intercalation compounds across size, valence, and redox-states of the involved cations, our work is intended to