Atomic Resolution Characterization Of Structure Modification Of Sodium-ion Battery Using Aberration-corrector Scanning Transmission Electron Microscopy

The energy is consistent with the development of society nowadays.The most common used energy storage devices in our daily life are lithiumion batteries(LIBs).However,the LIBs are handicapped by its neartotal dependence on Li sources and the fact of the reduction of lithium content on Earth requires nextgeneration energy storage devices.Sodiumion batteries(SIBs),which has the similar chemical properties with LIBs,is the promising candidate for the solution of energy problem.In material sciences,the structure and performance are interrelated and closely linked.In our study,we used the highresolution electron microscopy to reveal the structures and the related performance of two kinds of SIBs made by special process.We are focusing on the improvements of the structure of the layered oxide cathodes at atomiclevel scale,to find out the means of promoting the properties of SIBs.The study mainly includes the following two parts.In the first part,we present the studies on a novel NaP3/Li2MnO3 layered composite cathode with an alternating heteropitaxial nanostructure fabricated by an insitu composition modulation route.By using aberrationcorrected HAADF/ABFSTEM,we understand the structure evolution accompanying Li substitution.The formation of Li2MnO3(LiO'3)changes the crystallographic and chemical features of the neighboring NaP3 layered significantly and leads to the alternating NaP3/LiO'3 heteropitaxial nanostructure.The special structure delivers an extremely high reversible capacity of210 mAhg'1 between 1.5 and 4.5 V vs Na/Na+,much improved cycling stability and excellent electrode kinetics.Its enhanced electrochemical performance can be ascribed to the effective suppression of the P3P3" phase transition and subsequent amorphization upon cycling to 4.5 V.In the second part,we report the ordered and disordered polymorphs of Na3Ni2RuO6 honeycombordered cathodes for SIBs,and it is determined that the ordered sample exhibits beteer electrochemical performance,with a specific capacity of 117 mAh·g1 between 2 and 3.8 V(vs Na/Na+)with minimal capacity fade during extended cycling.HAADFSTEM was employed to show that the specific cationic ordering in transition slabs.Using electron diffraction,we checked the cathodes in different state of dis/charge and all the diffraction patterns showed the super lattice spots.When compared with disordered samples,the ordered one can maintain structural integrity of layered oxide while the disordered one shows cracks in the particles and ion mixing at the surface.It can be believed that the Ru ions can be fixed to the structure and such honeycomb improves the cycling stability.