NASICON (sodium (Na) superionic conductor) compounds have attracted considerable attention as promising solid electrolyte materials for advanced Na-based batteries. In this study, we investigated the improvement in ionic conductivities of von-Alpen-type NASICON (vA-NASICON) ceramic electrolytes by introducing a magnesium ion (Mg²⁺) as a heterogeneous element. The optimal Mg-doped vA-NASICON exhibited a high ionic conductivity of 3.64×10⁻³ S·cm⁻¹, which was almost 80% higher than that of un-doped vA-NASICON. The changes in physicochemical properties of the vA-NASICONs through the Mg introduction were systematically analyzed, and their effects on the ionic conductivities of the vA-NASICON were studied in detail. When the optimal ratio of Mg²⁺ was used in a synthetic process, the relative density (96.6%) and grain boundary ionic conductivity (σ_gb) were maximized, which improved the total ionic conductivity (σ_t) of the vA-NASICON. However, when Mg²⁺ was introduced in excess, the ionic conductivity decreased because of the formation of an undesired sodium magnesium phosphate (Na_xMg_yPO₄) secondary phase. The results of this study are expected to be effectively applied in the development of advanced sodium-based solid electrolytes with high ionic conductivities.

Herein, hierarchically structured SnO₂ microspheres are designed and synthesized as an efficient anode material for lithium-ion batteries using hollow SnO₂ nanoplates. Three-dimensionally ordered macroporous (3-DOM) SnO_x-C microspheres synthesized by spray pyrolysis are transformed into hierarchically structured SnO₂ microspheres by a two-step post-treatment process. Sulfidation produces hierarchically structured SnS-SnS₂-C microspheres comprising tin sulfide nanoplate and carbon building blocks. A subsequent oxidation process produces SnO₂ microspheres from hollow SnO₂ nanoplate building blocks, which are formed by Kirkendall diffusion. The discharge capacity of the hierarchically structured SnO₂ microspheres at a current density of 5 A·g^-1 for the 600^th cycle is 404 mA·h·g^-1. The hierarchically structured SnO₂ microspheres have reversible discharge capacities of 609 and 158 mA·h·g^-1 at current densities of 0.5 and 30 A·g^-1, respectively. The ultrafine nanosheets contain empty voids that allow excellent lithium-ion storage performance, even at high current densities.

Multicomponent metal sulfide materials with a yolk–shell structure and a single phase were studied for the first time as anode materials for sodium-ion batteries. Yolk–shell-structured Fe–Ni–O powders with a molar ratio of iron and nickel components of 1/1 were prepared via one-pot spray pyrolysis. The prepared Fe–Ni–O powders were transformed into yolk–shell-structured (Fe_0.5Ni_0.5)₉S₈ solid-solution powders via a sulfidation process. The initial discharge and charge capacities of the (Fe_0.5Ni_0.5)₉S₈ powders at a current density of 1 A·g⁻¹ were 601 and 504 mA·h·g⁻¹, respectively. The discharge capacities of the (Fe_0.5Ni_0.5)₉S₈ powders for the 2^nd and 100^th cycle were 530 and 527 mA·h·g⁻¹, respectively, and their corresponding capacity retention measured from the 2^nd cycle was 99%. The (Fe_0.5Ni_0.5)₉S₈ powders had high initial discharge and charge capacities at a low current density of 0.1 A·g⁻¹, and the reversible discharge capacity decreased slightly from 568 to 465 mA·h·g⁻¹ as the current density increased from 0.1 to 5.0 A·g⁻¹. The synergetic effect of the novel yolk–shell structure and the multicomponent sulfide composition of the (Fe_0.5Ni_0.5)₉S₈ powders resulted in excellent sodium-ion storage performance.

Porous FeS nanofibers with numerous nanovoids for use as anode materials for sodium-ion batteries were prepared by electrospinning and subsequent sulfidation. The post-treatment of the as-spun Fe(acac)₃-polyacrylonitrile composite nanofibers in an air atmosphere yielded hollow Fe₂O₃ nanofibers due to Ostwald ripening. The ultrafine Fe₂O₃ nanocrystals formed at the center of the fiber diffused toward the outside of the fiber via Ostwald ripening. On sulfidation, the Fe₂O₃ hollow nanofibers were transformed into porous FeS nanofibers, which contained numerous nanovoids. The formation of porosity in the FeS nanofibers was driven by nanoscale Kirkendall diffusion. The porous FeS nanofibers were very structurally stable and had superior sodium-ion storage properties compared with the hollow Fe₂O₃ nanofibers. The discharge capacities of the porous FeS nanofibers for the 1^st and 150^th cycles at a current density of 500 mA·g^–1 were 561 and 592 mA·h·g^–1, respectively. The FeS nanofibers had final discharge capacities of 456, 437, 413, 394, 380, and 353 mA·h·g^–1 at current densities of 0.2, 0.5, 1.0, 2.0, 3.0, and 5.0 A·g^–1, respectively.

A few-layered MoS₂-C composite material is studied as a supporting material for silicon nanopowder. Microspheres of the few-layered MoS₂-C composite embedded with 30 wt.% Si nanopowder are prepared by one-pot spray pyrolysis. The Si nanopowder particles with high capacity are completely surrounded by the few-layered MoS₂-C composite matrix. The discharge capacities of the MoS₂-C composite microspheres with and without 30 wt.% Si nanopowder after 100 cycles are 1, 020 and 718 mAh·g^-1 at a current density of 1, 000 mA·g^-1, respectively. The spherical morphology of the MoS₂-C composite microspheres embedded with Si nanopowder is preserved even after 100 cycles because of their high structural stability during cycling. The MoS₂-C composite layer prevents the formation of unstable solid-electrolyte interface (SEI) layers on the Si nanopowder. Furthermore, as the MoS₂-C composite matrix exhibits high capacity and excellent cycling performance, these characteristics are also reflected in the MoS₂-C composite microspheres embedded with 30 wt.% Si nanopowder.

SnS-C composite powders were prepared through one-pot spray pyrolysis for use as anode materials for Na-ion batteries. C microspheres with uniformly attached cubic-like SnS nanocrystals, which have an amorphous C coating layer, were formed at a preparation temperature of 900 ℃. The initial discharge capacities of the bare SnS and SnS-C composite powders at a current density of 500 mA·g^-1 were 695 and 740 mA·h·g^-1, respectively. The discharge capacities after 50 cycles and the capacity retentions measured from the second cycle of the bare SnS and SnS-C composite powders were 25 and 433 mA·h·g^-1 and 5 and 89%, respectively. The prepared SnS-C composite powders with high reversible capacities and good cycle performance can be used as Na-ion battery anode materials.

The use of new three-dimensional (3D) porous graphene-metal oxide composite microspheres as an anode material for Li-ion batteries (LIBs) is first introduced here. 3D graphene microspheres are aggregates of individual hollow graphene nanospheres composed of graphene sheets. Metal oxide nanocrystals are uniformly distributed over the graphene surface of the microspheres. The 3D porous graphene-SnO₂ microspheres are selected as the first target material for investigation because of their superior electrochemical properties. The 3D porous graphene-SnO₂ and graphene microspheres and bare SnO₂ powders deliver discharge capacities of 1, 009, 196, and 52 mAh·g^-1, respectively, after 500 cycles at a current density of 2 A·g^-1. The 3D porous graphene-SnO₂ microspheres exhibit uniquely low charge transfer resistances and high Li-ion diffusivities before and after cycling.

In this study, for the first time, polymeric precursors have been used in the preparation of yolk-shell powders using a large-scale spray drying process. An esterification reaction between the carboxyl group of citric acid and the hydroxyl group of ethylene glycol inside the droplet produced organic polymers during the drying process of the droplet. During the spray drying process, the polymeric precursors enabled the formation of multi-shell cobalt oxide yolk-shell powders with superior electrochemical properties. The maximum number of shells of the particles in the yolk-shell powders post-treated at 300, 400, and 500 ℃ were six, five, and four, respectively. The initial discharge capacities of the cobalt oxide yolk-shell powders post-treated at 300, 400, and 500 ℃ were 1, 188, 1, 331, and 1, 110 mAh·g^-1, and their initial charge capacities were 868, 1, 005, and 798 mAh·g^-1, respectively. The discharge capacities of the powders post-treated at 300, 400, and 500 ℃ after 100 cycles were 815, 958, and 670 mAh·g^-1, respectively, and their corresponding capacity retentions measured after the first cycles were 92%, 93%, and 82%, respectively. The pure phase Co₃O₄ yolk-shell powders post-treated at 400 ℃ had low charge transfer resistance and high lithium-ion diffusion rate.