3D yolk–shell Si@void@CNF nanostructured electrodes with improved electrochemical performance for lithium-ion batteries

Electrochemical stability of electrospun silicon/carbon nanofiber anode materials: a review

Silicon (Si) is regarded as a promising anode material owing to its high specific capacity and low lithiation potential. The large volume change and the pulverization of silicon during the lithiation/delithiation process hinder its direct energy storage application. This review focuses on the electrospun silicon/carbon (Si/C) nanofiber anode materials for lithium-ion batteries for long-term stable energy storage. Silicon is completely embedded in electrospinning-based carbon nanofibers to form electrospun Si/C nanofibers. It not only creates pore space to buffer silicon volume expansion, but also prevents direct contact between silicon and the electrolyte, consequently forming a stable solid electrolyte interface film. The electrospun Si/C nanofibers solve the pulverization issue of silicon to achieve improved cycling stability. Furthermore, the electrospun carbon nanofibers form a flexible conductive network for surrounding silicon by facilely introducing sacrificial polymers or template agents. The electrospun Si/C nanofibers ultimately promote the lithium-ion transport to achieve rate stability. The silicon source selection and microstructure regulation of the electrospun Si/C nanofibers are overviewed. The silicon sources include the direct utilization of silicon or silicon oxide particles as well as the indirect conversion of silicon-based precursors. The cycling stability regulation of various metal- and metal oxide-modified silicon composites and heterogeneous carbon material-decorated electrospun Si/C nanofibers is summarized. In addition, the microstructure designs of the electrospun Si/C nanofibers associated with the improvement of long-term capacity retention are overviewed. The main challenges in the electrospun Si/C nanofiber anode materials are summarized, and the future perspectives are also proposed.

Highly reversible Li-ion full batteries using a Mg-doped Li-rich Li1.2Ni0.28Mn0.468Mg0.052O2 cathode and carbon-decorated Mn3O4 anode with hierarchical microsphere structures

Microsphere structured Mg-doped Li-rich Li1.2Ni0.28Mn0.468Mg0.052O2 cathode and carbon-decorated Mn3O4 anode materials were prepared for application to lithium-ion full batteries. As-assembled lithium-ion full batteries exhibited enhanced electrochemical performances like high charge/discharge capacity, and long-term capacity retention.

3D Yolk-Shell Structured Si/void/rGO Free-Standing Electrode for Lithium-Ion Battery

In this study, we have successfully prepared a free-standing Si/void/rGO yolk–shell structured electrode via the electrostatic self-assembly using protonated chitosan. When graphene oxide (GO) is dispersed in water, its carboxyl and hydroxyl groups on the surface are ionized, resulting in the high electronegativity of GO. Meanwhile, chitosan monomer contains -NH₂ and -OH groups, forming highly electropositive protonated chitosan in acidic medium. During the electrostatic interaction between GO and chitosan, which results in a rapid coagulation phenomenon, Si/SiO₂ nanoparticles dispersed in GO can be uniformly encapsulated between GO sheets. The free-standing Si/void/rGO film can be obtained by freeze-drying, high-pressure compression, thermal reduction and HF etching technology. Our investigation shows that after 200 charge/discharge cycles at the current density of 200 mA·g⁻¹, the specific discharge capacity of the free-standing electrode remains at 1129.2 mAh·g⁻¹. When the current density is increased to 4000 mA·g⁻¹, the electrode still has a specific capacity of 469.2 mAh·g⁻¹, showing good rate performance. This free-standing electrode with a yolk–shell structure shows potential applications in the field of flexible lithium-ion batteries.

Facile synthesis of core-shell structured Si@graphene balls as a high-performance anode for lithium-ion batteries

Encapsulating silicon (Si) nanoparticles with graphene nanosheets in a microspherical structure is proposed to increase electrical conductivity and solve stability issues when using Si as an anode material in lithium-ion batteries (LIBs). Currently the main strategies to produce high-quality Si-graphene (Si@Gra) electrodes are (1) chemical vapor deposition (CVD) of graphene grown in situ on Si by hydrocarbon precursors and (2) encapsulating Si with a graphene oxide followed by postannealing. However, both methods require a high-temperature and are costly and time-consuming procedures, which hinders their mass scalability and practical utilization. Herein, we report a Si@Gra composite with a ball-like structure that is assembled by a facile spray drying process without a postannealing treatment. The graphene sheets are synthesized by an electrochemical exfoliation method from natural graphite. The resulting Si@Gra composite exhibits a unique core-shell structure, from which the ball-like morphology and the number of graphene layers in the Si@Gra composites are found to affect both the electric conductivity and ionic conductivity. The Si@Gra composites are found to increase the capacity of the anode and provide excellent cycling stability, which is attributed to the high electrical conductivity and mechanical flexibility of the layered graphene; additionally, a void space in the core-shelled ball structure inside the Si@Gra compensates for the Si volume expansion. As a result, the Si@few-layer graphene ball anode exhibits a high initial discharge capacity of 2882.3 mA h g-1 and a high initial coulombic efficiency of 86.9% at 0.2 A g-1. The combination of few-layer graphene sheets and the spray drying process can effectively be applied for large-scale production of core-shell structured Si@Gra composites as promising anode materials for use in high-performance LIBs.

Porous SnO2 nanostructure with a high specific surface area for improved electrochemical performance

Tin oxide (SnO₂) has been attractive as an alternative to carbon-based anode materials because of its fairly high theoretical capacity during cycling. However, SnO₂ has critical drawbacks, such as poor cycle stability caused by a large volumetric variation during the alloying/de-alloying reaction and low capacity at a high current density due to its low electrical conductivity. In this study, we synthesized a porous SnO₂ nanostructure (n-SnO₂) that has a high specific surface area as an anode active material using the Adams fusion method. From the Brunauer–Emmett–Teller analysis and transmission electron microscopy, the as-prepared SnO₂ sample was found to have a mesoporous structure with a fairly high surface area of 122 m² g⁻¹ consisting of highly-crystalline nanoparticles with an average particle size of 5.5 nm. Compared to a commercial SnO₂, n-SnO₂ showed significantly improved electrochemical performance because of its increased specific surface area and short Li⁺ ion pathway. Furthermore, during 50 cycles at a high current density of 800 mA g⁻¹, n-SnO₂ exhibited a high initial capacity of 1024 mA h g⁻¹ and enhanced retention of 53.6% compared to c-SnO₂ (496 mA h g⁻¹ and 23.5%).

A porous SnO₂ nanostructure as an anode active material showed significantly improved electrochemical performance.

Facile one-pot synthesis of Ge/TiO2 nanocomposite structures with improved electrochemical performance

Germanium (Ge) as an alternative to graphite exhibits a fairly high theoretical energy density and improved Li⁺ ion diffusivity. However, the seriously deteriorated electrochemical performance of Ge during cycling and the difficulty in the preparation of Ge-based nanostructures can hinder the utilization of Ge as an anode. Thus, in this study, a nanocomposite structure with Ge and TiO₂ (Ge/TiO₂) was synthesized using a facile one-pot method with different ratios of a Ge source with a dominant GeO₂ phase and titanium isopropoxide. From X-ray diffraction, electron microscopy, and X-ray photoelectron spectroscopy, the Ge/TiO₂ nanocomposites were found to be spherical structures homogeneously consisting of the reduced Ge as an active material and amorphous TiO₂ as a matrix. In particular, the Ge/TiO₂ nanocomposite with an appropriate amount of TiO₂ exhibited improved electrochemical properties, i.e., a coulombic efficiency of 97% and a retention of 61% for 100 cycles, compared to commercial Ge (a coulombic efficiency of 82% and a retention of 16%). This demonstrates that the amorphous TiO₂ matrix could relieve a volumetric expansion of the Ge active material in the nanocomposite electrode generated during the cycling process.