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Structure plays a crucial role in determining the mechanical behavior of materials. When subjected to external stimuli such as thermal expansion, solid and hollow spheres exhibit significant differences in stress relaxation behavior. Solid spheres lacking cavities have limited internal adjustability, which usually leads to localized stress concentrations and rapid rupture. On the contrary, hollow spheres are able to effectively release stresses during the expansion process, and thus outperform solid spheres in this regard. This difference is further amplified when the material is reduced to the nanoscale. Therefore, the complex design of hollow nanostructures has always been a central topic in optimizing stress release in deformable materials in a wide range of applications.
A typical scenario is the design of silicon nanostructures used as high-capacity electrodes for lithium-ion batteries (LIBs), where silicon undergoes huge volume changes of up to 300% during lithiation and delithiation. As a result, silicon pulverization becomes a key challenge that hinders its practical application. A great deal of research has been conducted on the design of silicon nanostructures, and as mentioned earlier, hollow structures offer advantages in terms of mechanical stability. As shown in Fig. 1a, the core-shell structure of silicon/carbon (Si/C) is an effective mode for mitigating side reactions at the electrode/electrolyte interface, improving electronic conductivity, and enhancing structural stability. However, due to the relatively weak mechanical strength of the carbon coating, the large radial compressive stress in the core may induce debonding at the core-shell interface, while the tangential tensile stress at the interface may lead to shell fracture. To address these issues, a yolk-shell structure characterized by the preservation of internal voids has emerged, thereby greatly improving its mechanical stability. However, the limited core-shell contact area of this design leads to a significant reduction in electron and Li+ transport efficiency, which is a great challenge.
The main point of this paper
Challenges of porous core-shell structures:
Internal pores may lead to stress concentrations that affect long-term cycling stability.
Shell-shell silicon/carbon (SS-Si/C) design:
A novel SS-Si/C structure is proposed, aiming to solve the problems of stress concentration and interfacial contact.
Limitations of existing preparation methods:
Existing methods (e.g., sacrificial element method, metal-assisted etching method, silicon oxide reduction method, chemical deposition method) do not fully meet the requirements for the construction of SS-Si/C structures.
Fast, straightforward and scalable preparation methods:
Graphene was grown on silicon nanoparticles by chemical vapor deposition (CVD) to form precursors for SS-Si/C structures.
Rapid heating to above 1550°C in an inert gas environment releases the silicon core into the gas phase, leaving the inner shell tightly attached to the carbon shell.
Finite element modeling results:
The synergistic effect of the external carbon shell and the internal hollow structure promotes the inward expansion of the silicon shell.
Electrochemical performance:
The reversible capacity of the SS-Si/C anode was about 1690.3 mA h g-1 at 0.5 A g-1 current density.
The capacity of 1055.6 mA h g-1 was maintained after 1000 cycles at a high current density of 8 A g-1.
Implications of the self-templated preparation strategy:
Demonstrates the potential to synthesize a variety of nanostructures to accelerate the development of practical methods for high-capacity and fast-charging lithium-ion battery (LIB) electrode materials.
Challenges of silicon-carbon composites:
During the lithiation process, silicon-carbon composites face problems with internal stress release and inter-composite contact, leading to material degradation and rapid capacity loss.
Innovations in shell-shell silicon-carbon (SS-Si/C) composites:
A novel SS-Si/C composite, consisting of a carbon shell tightly wrapped around a silicon shell, is reported, effectively solving the internal stress and contact problems.
Mechanical Analysis:
The dominant inward expansion of the silicon shell is realized by the synergistic effect of the external carbon shell and the internal hollow structure.
Performance advantages of SS-Si/C anode:
High specific capacity (1690.3 mA h g-1 after 550 cycles at 0.5 mA g-1).
High specific capacity (2.05 mA h cm-2 after more than 400 cycles at 0.5 mA cm-2).
Long cycle life (1055.6 mA h g-1 after 1000 cycles at 8 A g-1).
Full battery performance:
Full cells assembled with lithium cobaltate (LCO) or lithium iron phosphate (LFP) cathodes exhibit good rate capability and cycle stability.
At high 6 C multiplicity (170 mA g-1 for LFP and 270 mA g-1 for LCO), the full-cell specific capacities reached 79.5 mA h g-1 and 64.9 mA h g-1, respectively.
Synthesis method:
A simple and safe synthesis method was used to rationally design the hollow structure with unique properties.
Practical application potential:
SS-Si/C anode shows great potential in practical applications, especially the excellent performance at high multiplicity.
In this study, shell-shell structured silicon-carbon (SS-Si/C) composites were synthesized by the self-templating method. Carbon-encapsulated silicon nanoparticles synthesized by CVD undergo an FJH process to rapidly release silicon nuclei, and the remaining silicon is tightly and uniformly attached to the carbon shell to form SS-Si/C. Unlike conventional synthesis methods involving complex template construction and removal, our method implements a self-templating approach that facilitates the simple preparation of the shell-shell structure by utilizing the transformation of physical states.
In addition, our electrochemical-mechanical modeling shows that the shell-shell structure reduces the lithiation-induced stress level mainly through inward deformation. The outer rigid carbon shell and the inner hollow structure play an important role in promoting this inward deformation. The inward deformation promotes the formation of a stable SEI layer, which improves the capacity retention and cycle life compared to the previously reported hollow nanospheres and porous Si particles. In conclusion, SS-Si/C demonstrates great potential as a high-performance anode. For practical applications, future work should focus on improving magnetic field loading, cathode/anode pairing, and electrolyte through integrated design.
