TMC) based material, are very promising candidates as an anode for SIB due to their higher theoretical capacity (≈494.36 mAh g −1 for CoSe 2 ) than graphite (≈35 mAh g −1 ) [7,8] and low synthesis cost. [9,10] Nevertheless, severe volume changes during sodiation/desodiation process, slow transport of charges, and diffusion of Na + along with high intrinsic resistance resulted in capacity fading and hindered the practical application of transition metal diselenides. [9,11] Until now, a lot of efforts have been devoted to overcome these issues by synthesizing transition metal diselenides based nanostructures with various shapes, morphologies, and particle sizes. [10][11][12] Although creating voids, hollow, or porous nanostructures have improved the gravimetric capacity due to their ability to accommodate the volume expansion, shorten the diffusion path for charge transportation, optimized the electrode and electrolyte contact area, etc. [13,14] However, downsizing the materials at the nanoscale is not always in favor of rechargeable batteries for practical applications. [15] Because the nanomaterials possess low tap density that results in low areal mass loading (<1 mg cm −2 ), consequently lowering the areal and volumetric capacities which are very important metrics in practical applications. [16,17] Traditional high areal mass loadings of active material bring high electrode thicknesses which severely limits the in-depth accessibility of active materials ultimately fading the reversible capacity. [15,18] 3D electrode design is normally opted for multiple advantages, however, instability/detachment of active material causes the pulverization and degradation of the structure, and ultimately poor cyclic stability. Here, a dually protected, highly compressible, and freestanding anode is presented for sodium-ion batteries, where 3D carbon nanotube (CNT) sponge is decorated with homogeneously dispersed CoSe 2 nanoparticles (NPs) which are protected under carbon overcoat (CNT/CoSe 2 /C). The 3D CNT sponge delivers enough space for high mass loading while providing high mechanical strength and faster conduction pathway among the NPs. The outer amorphous carbon overcoat controls the formation of solid electrolyte interphase film by avoiding direct contact of CoSe 2 with electrolyte, accommodates large volume changes, and ultimately enhances the overall conductivity of cell and assists in transmitting electron to an external circuit. Moreover, the hybrid can be densified up to 11-fold without affecting its microstructure that results in ultrahigh areal mass loading of 17.4 mg cm −2 and an areal capacity of 7.03 mAh cm −2 along with a high gravimetric capacity of 531 mAh g −1 at 100 mA g −1 . Thus, compact and smart devices can be realized by this new electrode design for heavy-duty commercial applications.

Nontrivial topological structures offer a rich playground in condensed matters and promise alternative device configurations for post-Moore electronics. While recently a number of polar topologies have been discovered in confined ferroelectric PbTiO3 within artificially engineered PbTiO3/SrTiO3 superlattices, little attention was paid to possible topological polar structures in SrTiO3. Here we successfully create previously unrealized polar antivortices within the SrTiO3 of PbTiO3/SrTiO3 superlattices, accomplished by carefully engineering their thicknesses guided by phase-field simulation. Field- and thermal-induced Kosterlitz–Thouless-like topological phase transitions have also been demonstrated, and it was discovered that the driving force for antivortex formation is electrostatic instead of elastic. This work completes an important missing link in polar topologies, expands the reaches of topological structures, and offers insight into searching and manipulating polar textures.

Ferroelectric vortices formed through complex lattice–charge interactions have great potential in applications for future nanoelectronics such as memories. For practical applications, it is crucial to manipulate these topological states under external stimuli. Here, we apply mechanical loads to locally manipulate the vortices in a PbTiO3/SrTiO3 superlattice via atomically resolved in-situ scanning transmission electron microscopy. The vortices undergo a transition to the a-domain with in-plane polarization under external compressive stress and spontaneously recover after removal of the stress. We reveal the detailed transition process at the atomic scale and reproduce this numerically using phase-field simulations. These findings provide new pathways to control the exotic topological ferroelectric structures for future nanoelectronics and also valuable insights into understanding of lattice-charge interactions at nanoscale.

Recently, several captivating topological structures of electric dipole moments (e.g., vortex, flux closure) have been reported in ferroelectrics with reduced size/dimensions. However, accurate polarization distribution of these topological ferroelectric structures has never been experimentally obtained. We precisely measure the polarization distribution of an individual ferroelectric vortex in PbTiO3/SrTiO3 superlattices at the subunit cell level by using the atomically resolved integrated differential phase contrast imaging in an aberration-corrected scanning transmission electron microscope. We find, in vortices, that out-of-plane polarization is larger than in-plane polarization, and that downward polarization is larger than upward polarization. The polarization magnitude is closely related to tetragonality. Moreover, the contribution of the Pb─O bond to total polarization is highly inhomogeneous in vortices. Our precise measurement at the subunit cell scale provides a sound foundation for mechanistic understanding of the structure and properties of a ferroelectric vortex and lattice-charge coupling phenomena in these topological ferroelectric structures.

No abstract