Kinetic Origin of Planar Gliding in Single-Crystalline Ni-Rich Cathodes.

Single-crystalline Ni-rich cathodes with high capacity have drawn much attention for mitigating cycling and safety crisis of their polycrystalline analogues. However, planar gliding and intragranular cracking tend to occur in single crystals with cycling, which undermine cathode integrity and therefore cause capacity degradation. Herein, we intensively investigate the origin and evolution of the gliding phenomenon in single-crystalline Ni-rich cathodes. Discrete or continuous gliding forms are revealed with new surface exposure including the gliding plane (003) and reconstructed (-108) under surface energy drive. It is also demonstrated that the gliding process is the in-plane migration of transition metal ions, and reducing oxygen vacancies will increase the migration energy barrier by which gliding and microcracking can be restrained. The designed cathode with less oxygen deficiency exhibits outstanding cycling performance with an 80.8% capacity retention after 1000 cycles in pouch cells. Our findings provide an insight into the relationship between defect control and chemomechanical properties of single-crystalline Ni-rich cathodes.

Unified View of the Local Cation-Ordered State in Inverse Spinel Oxides.

Cation ordering/disordering in spinel oxides plays an essential role in the rich physical and chemical properties which are hallmarks of the structural archetype. A variety of cation-ordering motifs have been reported for spinel oxides with multiple cations residing on the octahedral site (or B-site). This has attracted tremendous attention from both experimental and theoretical communities in the last few decades. However, no unified view has been reached, presumably due to the richness of cation species and corresponding complex arrangements emergent in this large family of compounds. In this report, local cation-ordered ground states of (inverse) spinel oxides with two different cations on the octahedral site have been thoroughly investigated using neutron and X-ray total scattering, and a comprehensive theory has been proposed to explain the commonly observed cation-ordered polymorphs. It is found that a cation-zigzag-ordered structure (space group P4122) is the ground state for inverse spinel oxides with a pure or strong ionic lattice, while a cation-linear-ordered arrangement (space group Imma) emerges when one of the B-site cations forms very strong directional covalent bonds with lattice oxygen. The degree and length scale of cation ordering is strongly correlated with the charge and ionic radius difference between the two octahedral site cations. More complicated cation ordering schemes can be formed when there is a concomitant charge and orbital ordering which fall on a similar energy scale. This can lead to the formation of orbital-driven cation clusters or the broad concept of "molecules" in solid- state compounds. It is expected these findings will help to better understand the observed physical properties of spinel oxides and thus facilitate design strategies for improved functional materials.

Lattice Dynamics and Contraction of Energy Bandgap in Photoexcited Semiconducting Boron Nitride Nanotubes.

Structural dynamics and changes in electronic structures driven by photoexcited carriers are critical issues in both semiconducting and optoelectronic nanodevices. Herein, a phase diagram for the transient states and relevant dynamic processes in multiwalled boron nitride nanotubes (BNNTs) has been extensively studied for a full reversible cycle after a fs-laser excitation in ultrafast TEMs, and the significant structural features and evolution of electronic natures have been investigated using pulsed electron diffraction and femtosecond-resolved electron energy-loss spectroscopy (EELS). It is revealed that nonthermal anisotropic alterations of the lattice apparently precede the phonon-driven thermal transients along the radial and axial directions. Ab initio calculations support these findings and show that electrons excited from the π to π* orbitals in the BN nanotubes weaken the intralayer bonds while strengthening the interlayer bonds along the radial direction. Importantly, time-resolved EELS measurements show contraction of the energy bandgap after fs-laser excitation associated with nonthermal structural transients. This fact verifies that laser-induced bandgap renormalization in semiconductors can essentially be correlated with both the rapid processes of excited carriers and nonthermal lattice evolution.

Interfacial Mechanism in Lithium-Sulfur Batteries: How Salts Mediate the Structure Evolution and Dynamics.

Lithium-sulfur batteries possess favorable potential for energy-storage applications because of their high specific capacity and the low cost of sulfur. Intensive understanding of the interfacial mechanism, especially the polysulfide formation and transformation under complex electrochemical environment, is crucial for the buildup of advanced batteries. Here, we report the direct visualization of interfacial evolution and dynamic transformation of the sulfides mediated by the lithium salts via real-time atomic force microscopy monitoring inside a working battery. The observations indicate that the lithium salts influence the structures and processes of sulfide deposition/decomposition during discharge/charge. Moreover, the distinct ion interaction and the diffusion in electrolytes manipulate the interfacial reactions determining the kinetics of the sulfide transformation. Our findings provide deep insights into surface dynamics of lithium-sulfur reactions revealing the salt-mediated mechanisms at nanoscale, which contribute to the profound understanding of the interfacial processes for the optimized design of lithium-sulfur batteries.