Metal-oxide coatings are a favoured strategy for mitigating surface degradation problems in state-of-the-art lithium-ion battery Ni-rich layered positive electrode materials. Despite their extensive use, a full, fundamental understanding of the role of coatings in reducing degradation and extending cycling lifetimes is currently lacking. In this work, the interactions between an atomic layer deposited (ALD) alumina coating on polycrystalline LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) and a carbonate-based battery electrolyte are studied. Solid-state nuclear magnetic resonance (ssNMR) heteronuclear experiments show that the Al₂O₃ coating transforms by reacting with electrolyte species present before and during electrochemical cycling, scavenging protic and acidic species. Density-functional theory calculations highlight the additional chemical effect of the coating in locally stabilising the structure of the NMC811, limiting oxidation of the oxygen atoms coordinated to both Al and Ni, thereby limiting the surface reconstruction process and improving the electrochemical performance. Improved NMC811 surface stability is confirmed by monitoring gaseous degradation species by online electrochemical mass-spectrometry and via X-ray spectroscopic analysis of the electrochemically aged samples to examine changes in Ni and O oxidation state and local structure. The combination of this experimental and theoretical analysis suggests that Al₂O₃ coatings have a dual role: as a protective barrier against attack from chemical species in the electrolyte, and as an artificial passivating layer hindering oxygen loss and surface phase transformations. This holistic approach, which provides a fundamental understanding of how the surface stability is improved by the coating, will aid the design of the state-of-the-art and future positive electrode materials.

It is of significant technological interest to have an understanding of cooperativity in spin-crossover phenomena observed in metalorganic polymeric complexes. Microscopic origin of this cooperativity has been discussed in literature primarily in terms of elastic interactions. Through Monte Carlo simulation of the model Hamiltonian we show that the superexchange interactions between localized magnetic moments at spin-crossover sites can play a crucial role in cooperativity, depending on the nature of the elastic interactions. While in the case of the effective elastic interaction of ferroelastic nature, inclusion of the above-mentioned exchange interaction produces quantitative changes in the hysteresis loop, for effective elastic interaction of antiferroelastic nature, the existence of hysteresis is found to be dictated entirely by the magnetic exchange interaction. Considering the example of a real material, namely Fe-triazole, employing the material-specific density functional theory calculation, we show the later scenario to be true for these systems. Our study should motivate further investigation on the role of magnetic exchanges in metalorganic polymers.

Metal organic complexes consisting of transition metal centers linked by organic ligands, may show bistability which enables the system to be observed in two different electronic states depending on external condition. One of the spectacular examples of molecular bistability is the spin-crossover phenomena. Spin-Crossover (SCO) describes the phenomena in which the transition metal ion in the complex under the influence of external stimuli may show a crossover between a low-spin and high-spin state. For applications in memory devices, it is desirable to make the SCO phenomena cooperative, which may happen with associated hysteresis effect. In this respect, compounds with extended solid state structures containing metal ions connected by organic spacer linkers like linear polymers, coordination network solids are preferred candidates over isolated molecules or molecular assemblies. The microscopic understanding, design and control of mechanism driving cooperativity, however, are challenging. In this review we discuss the recent theoretical progress in this direction.

Motivated by the puzzling report of the observation of a ferromagnetic insulating state in ${\mathrm{LaMnO}}_3/{\mathrm{SrTiO}}_3$ heterostructures, we calculate the electronic and magnetic state of ${\mathrm{LaMnO}}_3$, coherently matched to a ${\mathrm{SrTiO}}_3$ square substrate within a “strained-bulk” geometry. We employ three different density functional theory based computational approaches: (a) density functional theory (DFT) supplemented with Hubbard $U\left(\mathrm{DFT}+U\right)$, (b) DFT $+$ dynamical mean-field theory (DMFT), and (c) a hybrid functional treatment of the exchange-correlation functional. While the first two approaches include local correlations and exchange at Mn sites, treated in a static and dynamic manner, respectively, the last one takes into account the effect of nonlocal exchange at all sites. We find in all three approaches that the compressive strain induced by the square substrate of ${\mathrm{SrTiO}}_3$ turns ${\mathrm{LaMnO}}_3$, from an antiferromagnet with sizable orbital polarization, to a ferromagnet with suppressed Jahn-Teller distortion, in agreement with experiment. However, while both $\mathrm{DFT}+U$ and $\mathrm{DFT}+\mathrm{DMFT}$ provide a metallic solution, only the hybrid calculations result in an insulating solution, as observed in experiment. This insulating behavior is found to originate from an electronic charge disproportionation. Our conclusions remain valid when we investigate ${\mathrm{LaMnO}}_3/{\mathrm{SrTiO}}_3$ within the experimental setup of a superlattice geometry using $\mathrm{DFT}+U$ and hybrid calculations.