output. [2] Li metal has been regarded as an ideal anode material because of its ultrahigh theoretical specific capacity (3860 mAh g −1 ) and very low electrochemical redox potential (−3.040 V vs standard hydrogen electrode). [3] However, the practical utilization of Li metal anode is greatly hampered by the formation of Li dendrites during repeated charge/discharge cycles and the low Coulombic efficiency. The most serious concern is the direct contact of Li metal with the cathode material after Li penetrates the battery separator layer, which causes short circuiting and can lead to serious safety hazards. [4] Therefore, significant efforts have been devoted to detect, understand, and prevent the Li dendrite formation processes within liquid electrolytes. [5] The use of solid-state electrolytes (SSEs) in Li metal batteries (LMBs) is considered attractive as most SSEs are not volatile or flammable. [6] SSEs are also thought to be less susceptible to the growth of Li dendrites. However, recent studies have shown that Li dendrites can form in the voids and grain boundaries of Li 7 La 3 Zr 2 O 12 (LLZO) based SSEs. [7] Once the dendrites penetrate through the SSEs, the interactions between Li metal and cathode materials set off a chain of highly energetic reactions. Mechanistic pathways of such far-fromequilibrium processes are not understood and approaches to their control and/or mitigation are missing.Overlithiation can also occur when the LIBs are overdischarged or during synthesizing Li-rich cathodes. For example, even though some Li-rich cathode materials (e.g., Li[LiNi 1/3 Mn 1/3 Co 1/3 ]O 2 (Li-rich NMC)) have shown greatly increased reversible capacity, [8] Li x CoO 2 (LCO), the most widely used cathode material, does not have a stable Li-rich (x > 1) form. [9] Overlithiation in LCO causes degradation in structural integrity and device performance. It has been reported that LCO will eventually be converted to Co metal during the overdischarge process, and various reaction intermediates, including Li 1+x CoO 2−y , Co 3 O 4 , and CoO have also been proposed through ex situ studies. [10] Recent in operando X-ray absorption spectroscopy (XAS) and spectroscopic transmission X-ray microscopy (TXM) studies reveal a core-shell conversion pathway from LCO to Li 2 O and Co metal during overlithiation of the cathode. [11] However, the atomic-scale structural and chemical evolution during the overlithiation reaction is still unclear.Nonuniform and highly localized Li dendrites are known to cause deleterious and, in many cases, catastrophic effects on the performance of rechargeable Li batteries. However, the mechanisms of cathode failures upon contact with Li metal are far from clear. In this study, using in situ transmission electron microscopy, the interaction of Li metal with well-defined, epitaxial thin films of LiCoO 2 , the most widely used cathode material, is directly visualized at an atomic scale. It is shown that a spontaneous and prompt chemical reaction is triggered once Li contact is made, leading to e...

The magnetic susceptibility of synthesized magnetite (FeO) microspheres was found to decline after the growth of a metal-organic framework (MOF) shell on the magnetite core. Detailed structural analysis of the core-shell particles using scanning electron microscopy, transmission electron microscopy, atom probe tomography, andFe-Mössbauer spectroscopy suggests that the distribution of MOF precursors inside the magnetic core resulted in the oxidation of the iron oxide core.

Understanding the interaction of water with compositionally tuned metal oxides is central to exploiting their unique catalytic and magnetic properties. However, processes such as hydroxylation, wetting, and resulting changes in electronic structure at ambient conditions are challenging to probe in situ. Here, we examine the hydroxylation and wetting of Fe (3−x) Ti x O 4 (001)-oriented epitaxial films directly using ambient pressure X-ray photoelectron spectroscopy under controlled relative humidity. Fe 2+ formation promoted by Ti 4+ substitution for Fe 3+ increases with hydroxylation, commensurate with a decrease in the surface work function or change in the surface dipole. The incorporation of small amounts of Ti (x = 0.25) as a bulk dopant dramatically impacts hydroxylation, in part due to surface segregation, leading to coverages closer to that of TiO 2 than Fe 3 O 4 . However, the Fe (3−x) Ti x O 4 compositional series shows a similar affinity for water physisorption, which begins at notably lower relative humidity than on TiO 2 . The findings suggest that relative humidity rather than surface hydroxyl density controls wettability. Studies of this kind directly relate to rational design of doped magnetite into more active catalysts for UV/Fenton degradation, the adsorption of contaminants, and the development of spin filters.

Double perovskites of the form R 2 BB′O6 (where R is a rare earth cation and B and B′ are chemically distinct transition metal cations with half-filled and empty e g orbitals, respectively) are of significant interest for their magnetoelectric properties. La2MnNiO6 is particularly attractive because of its large expected ferromagnetic moment per formula unit (5 μB f.u.–1) and its semiconducting character. If the ideal structure nucleates, superexchange coupling can take place via the BOB′ bonds that form, and the moment per formula unit can attain its maximum theoretical value. However, we show that even in the case of layer-by-layer deposition via molecular beam epitaxy, the system can follow multiple reaction pathways that lead to deviations from the double perovskite structure. In particular, we observe a spatially extended phase in which B-site cation disorder occurs, resulting in MnOMn and NiONi antiferromagnetic domains, as well as the formation of quasi-epitaxial, antiferromagnetic NiO nanoscale inclusions, surrounded by a Mn-rich double perovskite. The coexistence of the double perovskite and secondary phases in oxygen deficient conditions is supported by first-principles modeling. However, extended annealing in air restores long-range B-site order and begins to dissolve the NiO inclusions, yielding an ideal structure and an enhanced ferromagnetic moment. This study reveals fundamental structure–property relationships that may not be apparent during the design phase of a multielement crystalline solid and illustrates how to engineer a synthetic path to a desired product.