Reduction-oxidation (redox) reactions that transfer conduction electrons from the anode to the cathode are the fundamental processes responsible for generating power in Li-ion batteries. Electronic and microstructural features of the cathode material are controlled by the nature of the redox orbitals and how they respond to Li intercalation. Thus, redox orbitals play a key role in performance of the battery and its degradation with cycling. We unravel spectroscopic descriptors that can be used to gain an atomic-scale handle on the redox mechanisms underlying Li-ion batteries. Our focus is on X-ray Compton Scattering and Positron Annihilation spectroscopies and the related computational approaches for the purpose of identifying orbitals involved in electrochemical transformations in the cathode. This review provides insight into the workings of lithium-ion batteries and opens a pathway for rational design of next-generation battery materials.

Selective separation of rare earth elements (REEs) from solutions of mixed heavy and light metals by solid adsorbents is an important challenge in the fields of water treatment and metal recovery. The main challenge is water instability of many adsorbents, specifically metal–organic frameworks (MOFs), and their low selectivity. Grafting particular organophosphorus compounds (OPCs) on the MIL-101(Cr) MOF can provide both stability and selectivity. When the tributyl phosphate (TBP), bis(2-ethylhexyl) hydrogen phosphate (D2EHPA or HDEHP) and bis(2,4,4-trimethylpentyl) phosphinic acid (Cyanex®-272) OPCs are grafted and applied to mixed-metal aqueous solutions containing Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Nd³⁺, Gd³⁺ and Er³⁺, MIL-101(Cr) offers high selectivity towards the Nd³⁺, Gd³⁺ and Er³⁺ REEs (with stronger affinity towards Er³⁺). However, the underlying chemistry is unknown and the factors leading to the selectivity remain poorly understood. To uncover the key molecular-level factors, we performed state-of-the-art computational simulations using a combination of high-level density functional theory (DFT), semi-empirical calculations, and configurational sampling of the metal ion-MOF binding modes in aqueous solutions. Our simulation study reproduced the available experimental results, in addition to determining the contributing intermolecular interactions, uptake modes and the most significant structural features for improving selectivity towards the REEs. Therefore, our most important result is rationalization of the mechanism of REE separation by OPC-grafted MOFs using quantum mechanical and electrostatic principles. The results provide guidelines for synthesis of OPC-grafted MIL-101(Cr) structures with enhanced selectivity and stability. Moreover, an efficient computational framework is proposed to facilitate comprehensive modeling of similar systems.

Previous studies have shown that positron-annihilation spectroscopy is a highly sensitive probe of the electronic structure and surface composition of ligand-capped semiconductor quantum dots (QDs) embedded in thin films. The nature of the associated positron state, however, whether the positron is confined inside the QDs or localized at their surfaces, has so far remained unresolved. Our positron-annihilation lifetime spectroscopy studies of CdSe QDs reveal the presence of a strong lifetime component in the narrow range of 358-371 ps, indicating abundant trapping and annihilation of positrons at the surfaces of the QDs. Furthermore, our ab initio calculations of the positron wave function and lifetime employing a recent formulation of the weighted density approximation demonstrate the presence of a positron surface state and predict positron lifetimes close to experimental values. Our study thus resolves the long-standing question regarding the nature of the positron state in semiconductor QDs and opens the way to extract quantitative information on surface composition and ligand-surface interactions of colloidal semiconductor QDs through highly sensitive positron-annihilation techniques.