Formation of Si Hollow Structures as Promising Anode Materials through Reduction of Silica in AlCl₃–NaCl Molten Salt

Abstract

Hollow nanostructures are attractive for energy storage and conversion, drug delivery, and catalysis applications. Although these hollow nanostructures of compounds can be generated through the processes involving the well-established Kirkendall effect or ion exchange method, a similar process for the synthesis of the pure-substance one (e.g., Si) remains elusive. Inspired by the above two methods, we introduce a continuous ultrathin carbon layer on the silica nano/microstructures (Stober spheres, diatom frustules, sphere in sphere) as the stable reaction interface. With the layer as the diffusion mediator of the reactants, silica structures are successfully reduced into their porous silicon hollow counterparts with metal Al powder in AlCl 3 −NaCl molten salt. The structures are composed of silicon nanocrystallites with sizes of 15−25 nm. The formation mechanism can be explained as an etching−reduction/nucleation−growth process. When used as the anode material, the silicon hollow structure from diatom frustules delivers specific capacities of 2179, 1988, 1798, 1505, 1240, and 974 mA h g −1 at 0.5, 1, 2, 4, 6, and 8 A g −1 , respectively. After being prelithiated, it retains 80% of the initial capacity after 1100 cycles at 8 A g −1 . This work provides a general way to synthesize versatile silicon hollow structures for high-performance lithium ion batteries due to the existence of ample silica reactants and can be extended to the synthesis of hollow structures of other materials.

Catalytic Growth of Graphitic Carbon‐Coated Silicon as High‐Performance Anodes for Lithium Storage

“…The spectrum confirms that the sample contains Si, C, O elements (Figure d) . Moreover, the high‐resolution spectrum of Si is shown in Figure e; a Si 2p peak is observed around at 99.6 eV, and the lower binding energy at 103.5 eV is assigned to the presence of SiO x on the Si surface, while a weak peak at 101.8 eV belonging to Si‐C is observed. The C 1s peaks (Figure f) could be split into four peaks at 288.82, 286.54, 284.32, and 283.25 eV, corresponding to O—C=O, C=O, C—C, and C—Si bonds, respectively.…”

Top‐Down Synthesis of Silicon/Carbon Composite Anode Materials for Lithium‐Ion Batteries: Mechanical Milling and Etching

“…Si nanostructures such as nanoparticles, nanowires, nanotubes and nanosheets are designed to relieve mechanical strain. Porous Si, hollow spheres, yolk–shell structures, and their carbon composites are also designed to provide appropriate space for Si expansion. For the Si/C composite, Si particles are distributed in a conductive carbon matrix to ease Si volume variation and keep the mechanical integrity of the whole composite electrode.…”

Template‐Free Electrochemical Formation of Silicon Nanotubes from Silica

“…In addition, the initial current efficiency (ICE) of Si-NT/Ag (81%, Figure S12b, Supporting Information) is much higher than that of Si-NW/Ag (62%, Figure S13a, Supporting Information), revealing that the hollow structure of Si-NT/Ag with more exposed specific surfaces can promote the swift formation of stable SEI film. [2,4,5] The capacity of Si-NT@Ag is further improved with the aid of the buffer Ag layer, as manifested in Figures S12b and S13c, Supporting Information. After 100 cycle at 1000 mA g −1 , the capacity of Si-NT@Ag still maintains a high value of 1780 mAh g −1 , much higher than that of the Si-NT/Ag (1351 mAh g −1 ) counterpart.…”

“…[3,4,10] The free empty space inside the tube can accommodate more volume changes, provide extra electrolyte-accessible inner surfaces, and guarantee shortened diffusion length for lithium ions. [2,4,5] In addition, the 1D character of Si-NT can facilitate axial charge transfer and shorten the radial Li-ion diffusion distance. [11] Coating a highconductivity surface layer can further improve the performance of Si-NT as anode materials because such a surface layer can not only maintain the structure integrity of Si nanostructures by resisting the volume variation during the lithiation/delithiation process, but also increase the conductivity of Si.…”

“…[11,41] Specially, the Si-NT@Ag also shows comparable/superior performance with Si nanostructures prepared by some other methods. [1,2,8,10,12,19,22,26] Even so, substantial improvement for the battery performance is needed before evaluation for a full cell. The present study mainly focuses on increasing the fabrication efficiency of Si-NT in molten salts.…”