Batteries | Free Full-Text | Bubble Wrap-like Carbon-Coated Rattle-Type silica@silicon Nanoparticles as Hybrid Anode Materials for Lithium-Ion Batteries via Surface-Protected Etching
3.1. Role of APTES in the Synthesis of SiO2@Si Shells
The choice of APTES and TEOS for synthesizing SiO2 shells is guided by the unique properties and functionalities introduced by APTES into the SiO2 matrix. In the conventional Stöber process, TEOS undergoes slow hydrolysis and condensation, necessitating extended reaction times for SiO2 formation. Additionally, the condensation of TEOS into siloxane networks requires a potent base catalyst. Furthermore, employing TEOS with a base catalyst offers limited control over particle size, growth, and morphology. Factors such as temperature, rotations per minute, and pH level influence reaction conditions. Achieving uniform particle size with TEOS is challenging, as incomplete control over synthesis conditions may yield a broad distribution of particle sizes.
It is important to highlight that while pristine Si nanoparticles inherently acquire a native oxide layer due to unavoidable surface oxidation during manufacturing, treating them with piranha solution is generally preferred. This treatment optimizes the number of surface hydroxyl groups, facilitating bond formation with the silanol groups of APTES and TEOS. The –OH groups resulting from piranha pre-treatment on Si–OH energetically promote siloxane bond formation in a condensation reaction, leading to a well-ordered silane layer on the Si surface within APTES/TEOS, even in the absence of alkali catalysts.
3.2. Multifaceted Effects of Proposed Modified Stöber via Hydrothermal Treatment
Ensuring the uniform coating of the Si active material with a thin carbon layer is imperative to create a protective barrier against direct electrolyte contact. Given the anticipated Si volume expansion, effective etching of the SiO2 sacrificial layer during template removal is essential to facilitate the formation of an internal void space. Additionally, the rigid and amorphous nature of SiO2 poses specific challenges. The yolk shell structure’s void space serves not only to absorb Si volume expansion but also plays a crucial role in averting crack formation on the carbon shell due to repetitive volume fluctuations. Furthermore, the carbon coating must endure the etching process without compromising its mechanical structure. Addressing these conditions necessitates a three-fold strategy involving (1) the creation of robust and uniform SiO2 shells through APTES/TEOS dual precursor, (2) utilizing PVP K30 polymer for surface protection during etching, adding flexibility to the rigid SiO2 matrix, and (3) implementing a conformal PDA carbon coating with PEI crosslinking through the proposed hydrothermal treatment. A control sample was fabricated following identical procedures at room temperature.
Opting for PVP K30 polymer over the previously utilized PVP K15 in the existing literature presents several advantages in this study. PVP K30, characterized by a higher molecular weight compared to PVP K15, offers improved properties such as enhanced viscosity, solubility, and the capacity to form more stable complexes with other substances. Typically employed as a stabilizing agent, PVP aids in preventing particle agglomeration and fortifying the stability of particles or in the preparation of polymeric films and coatings. In the current investigation, PVP K30 polymer was specifically chosen to afford surface protection to SiO2 shells during rigorous etching conditions. The elongated polymer chains resulting from the greater molecular weight of PVP K30, in contrast to PVP K15, were deemed essential to establish a robust protective layer, ensuring the steric stabilization of SiO2 particles. Moreover, the relatively higher molecular weight of PVP K30 played a critical role in preventing the collapse of the outer carbon coating and selectively etched inner SiO2 layer, preserving the integrity of the yolk structure.
3.3. Significance of PVP K30 Surface Protection during NaOH Etching
The reaction mechanism between the chosen PVP K30 molecules and the synthesized SiO2 shells involves a combination of physical adsorption and chemical bonding. PVP, a water-soluble polymer, typically undergoes adsorption on the SiO2 surface through hydrogen bonding and Van der Waals forces in an aqueous environment. The oxygen atoms in the pyrrolidone ring of PVP K30 readily form hydrogen bonds with the hydroxyl groups on piranha-treated Si–OH, subsequently coated with SiO2 from APTES and TEOS condensation. PVP K30 proves advantageous in preventing Ostwald ripening in high-surface energy SiO2 nanoparticles. The hydrothermal treatment employed in this study promotes chemical bonding between PVP K30 and SiO2 shells, as verified by FT-IR results, where oxygen atoms of the pyrrolidone ring form coordination bonds with the surface silanol groups of APTES/TEOS–SiO2@Si.
It is noteworthy that the Si–O–Si band intensity exhibited a declining trend from sol-gel coating to etching. The initial decrease, upon the addition of PVP K30, was attributed to the formation of PVP-treated SiO2 shells. Subsequent reduction in peak intensity occurred with the introduction of PDA–PEI, coinciding with the emergence of a robust C=C peak. Thermal treatment at 800 °C contributed to increased SiO2 stability, evidenced by a slight increase in Si–O–Si band intensity. Eventually, a significant reduction in Si–O–Si band intensity ensued after NaOH etching, signifying the dissolution of the SiO2 template.
3.4. Characterization of Representative Core Shell and Yolk Shell Composites
3.5. Electrochemical Performances of Representative Core Shell and Yolk Shell Composites
The observed phenomenon of low ICE followed by a significant increase in the second cycle for Si-based anodes is linked to the formation and stabilization of the SEI layer. This behavior, known as SEI activation, involves an irreversible and necessary consumption of Li+ during the initial cycles. In the first discharge cycle, Li+ is intercalated into the Si electrode structure, resulting in the formation of lithiated precipitates. The expansion and contraction of the Si volume induce mechanical stress, causing morphological pulverization and SEI layer breakdown. Cracks in the Si morphology lead to the construction of a new SEI layer, consuming additional Li+ and contributing to reduced reversible Li+ availability during subsequent charging cycles. As the lithiation and delithiation cycles progress, the SEI undergoes stabilization, becoming more robust and protective. After SEI layer stabilization, the Si anode attains enhanced stability, facilitating the more effective storage and release of Li+, thereby improving CE values in subsequent cycles.
The yolk shell structures performed better than its core shell counterparts in terms of cycling performance and CE stability at low-current density conditions for 100 cycles. Comparing the yolk shell PDA–PEI@SiO2@Si composite sample without PVP K30 and the representative composite PDA–PEI@PVP–SiO2@Si sample, the PDA–PEI@PVP–SiO2@Si sample was able to maintain 539.44 mAh g−1, only slightly higher than the 531.25 mAh g−1 of yolk shell PDA–PEI@SiO2@Si after 100 cycles.
Meanwhile, even with the help of PVP K30 surface protection, the core shell PDA–PEI@PVP–SiO2@Si composite sample demonstrated inferior cycling performance with a capacity of only 339.62 mAh g−1 after 100 cycles. This result highlights that the role of void spaces in yolk shell structures is sufficient in absorbing the internal volume changes of Si and stabilizing cycling performance. This result concludes that the PVP K30 polymer significantly affects the electrochemical performance of core shell samples at low-current density testing and yolk shell samples at high-rate loading.
The variations in cycling performance and CE between PDA–PEI@SiO2@Si and PDA@SiO2@Si composites, both featuring core shell structures, result from improved electronic conductivity in the former. This enhancement is attributed to PEI crosslinking throughout the electrode, establishing continuous pathways for rapid electron and ion transport.
While the yolk shell PDA–PEI@SiO2@Si composite exhibited slightly superior electrochemical performance compared to the representative yolk shell composite, a sudden capacity increase from 456.32 mAh g−1 to 476.41 at 5 A g−1 suggested a short circuit due to Li dendritic formations, common at high current densities. Similar abrupt capacity increases were observed for core shell PDA–PEI@PVP–SiO2@Si and core shell PDA–PEI@SiO2@Si composites. It is noteworthy that slight capacity increases in other sample composites stabilized upon reducing the current density to 0.1 A g−1, indicating satisfactory recovery after high-rate tests.
The representative composite displayed the smallest diameter in the high-frequency semicircle, indicating the lowest RSEI value (6.30 Ω). This reduction was attributed to the formation of a mechanically stable SEI layer facilitated by the PDA coating, preventing excessive electrolyte decomposition. Yolk shell PDA–PEI@SiO2@Si also exhibited a relatively lower RSEI value (8.13 Ω) compared to its core shell counterparts, underscoring the significance of the yolk shell structure. However, a larger RCT value for yolk shell PDA–PEI@SiO2@Si (19.28 Ω) compared to the representative composite (9.71 Ω) emphasized the importance of constructing yolk shell structures with PVP K30 surface protection. Core shell composites PDA–PEI@PVP–SiO2@Si (9.51 Ω) with PVP K30 demonstrated lower RSEI values than the PDA–PEI@SiO2@Si (10.77 Ω) sample, highlighting the efficacy of PVP K30 in enhancing the electrochemical performance.
The formation of a stabilized SEI film in core shell PDA–PEI@PVP–SiO2@Si can be elucidated by the influence of PVP K30 polymer chains when loaded into amorphous SiO2 shells. In the event of crack formation in the PDA carbon coating due to the expansion of lithiated Si and SiO2 components, the embedded PVP K30 polymer chains within SiO2 shells act as a secondary barrier, preventing direct contact with Si active materials. Furthermore, the flexibility of PVP K30 polymer chains contributes to the stable formation of SEI by serving as a buffer against the rigid and dense SiO2 layer, susceptible to crack formation during repetitive volume fluctuations. The incorporation of PVP K30 polymer chains within SiO2 also enhances the conductivity of amorphous SiO2 seeds, resulting in a slight improvement in the RCT of core shell PDA–PEI@PVP–SiO2@Si (23.32 Ω) compared to core shell PDA–PEI@SiO2@Si (28.01 Ω). Conversely, the absence of the PEI component in core shell PDA@SiO2@Si led to a higher RSEI (10.84 Ω) coupled with thick SiO2 shells, obstructing Li+ migration and increasing tortuosity (32.50 Ω).
As representative yolk shell composite electrodes, specifically yolk shell PDA–PEI@SiO2@Si and yolk shell PDA–PEI@PVP–SiO2@Si displayed the highest initial discharge capacities of 853.94 and 767.07 mAh g−1, respectively. After 200 cycles, both electrodes exhibited stable cycling performance, retaining reversible capacities of 523.50 and 512.76 mAh g−1, respectively, showing minimal capacity losses. Although yolk shell PDA–PEI@SiO2@Si initially demonstrated a slightly higher discharge capacity, its cycling stability gradually declined after approximately 170 lithiation/delithiation cycles. In contrast, yolk shell PDA–PEI@PVP–SiO2@Si demonstrated a superior cycling performance, maintaining a relatively stable capacity retention rate during extended high-density cycling. This divergence in long-cycling performance was attributed to the significant influence of PVP K30 acting as a protective barrier between the PDA–PEI coating layer and SiO2 shells during etching. Additionally, embedded PVP K30 polymer chains within SiO2 shells contributed to a flexible silica structure, mitigating particle pulverization.
The capacity contribution for each component (i.e., Si, SiO2, APTES, TEOS, PVP K30, PDA, and PEI) can be summarized as follows. Si, due to its excellent theoretical specific capacity, was used to boost the energy density of typical graphite-based commercial anodes. SiO2 was fabricated from APTES and TEOS dual template strategy to design a yolk shell structure to provide void spaces to buffer inevitable Si volume fluctuations. The APTES was used as a structure to regulate TEOS to facilitate monodispersed SiO2 synthesis without a base catalyst and as a precursor to amino-functionalized SiO2. The PVP K30 polymers provided surface protection to prevent crack formation on the carbon coating and acted as a barrier that controls the rate of SiO2 dissolution during the etching process. The PVP K30 polymers embedded within the SiO2 shells also allowed for flexibility and conductivity to the rather amorphous and rigid SiO2 shells. The PDA coating layer was designed to encapsulate the SiO2-coated Si active material and prevent direct electrolyte contact while mitigating the low conductivity of Si. Lastly, the crosslinking reaction between PDA and PEI contributed to the construction of a 3D, bubble wrap-like, interconnected porous matrix with a thermal stability reaching up to 700 °C. Each component in the representative yolk shell composite exhibited synergistic effects that resulted in a stable cycling performance with minimal capacity loss even after 200 cycles.
The variation in specific surface areas among the investigated composites offers additional insights into the observed differences in electrochemical performance during prolonged cycling at high-rate loading. A higher specific surface area provides more active sites for the interaction between the electrode material and the electrolyte. The recorded SBET values for the studied composites in ascending order were 224.56, 226.40, 409.05, 589.83, and 654.63 m2 g−1 for the core shell PDA@SiO2@Si, core shell PDA–PEI@SiO2@Si, core shell PDA–PEI@PVP–SiO2@Si, yolk shell PDA–PEI@SiO2@Si, and yolk shell PDA–PEI@PVP–SiO2@Si, respectively.
The BET analysis results indicate that the representative yolk shell PDA–PEI@PVP–SiO2@Si composite, characterized by the lowest pore volume (0.12 cm3 g−1), highest SBET value, and greatest specific surface area attributed to mesopores (570.96 m2 g−1), exhibited the most stable electrochemical performance in terms of cycling and rate stability. The notable increase in the contact area enhanced the electrode–electrolyte interface, facilitating efficient ion transfer during both charging and discharging cycles. The high specific surface area contributed to improved ion diffusion, allowing Li+ ions to traverse the electrode structure with reduced diffusion path lengths. Moreover, the extensive surface area of mesopores within the electrode structure helped distribute Si volume fluctuations effectively, thereby minimizing mechanical stress and mitigating issues related to electrode degradation over multiple cycles.
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