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Applications of silicon as a high-performance anode material has been impeded by its low intrinsic conductivity and huge volume expansion (> 300%) during lithiation. To address these problems, nano-Si particles along with conductive coatings and engineered voids are often employed, but this results in high cost anodes. Here, we report a scalable synthesis method that can realize high specific capacity (~800 mAh g-1), ultrafast charge/discharge (at 8 A g-1 Si) and high initial Coulombic efficiency (~90%) with long cycle life (1000 cycles) at the same time. To achieve 1000 cycle stability, micron-sized Si particles are subjected to high-energy ball milling to create nanostructured Si building blocks with nano-channel shaped voids encapsulated inside a nitrogen (N)-doped carbon shell (termed as Si micro-reactor). The nano-channel voids inside a Si micro-reactor not only offer the space to accommodate the volume expansion of Si, but also provide fast pathways for Li ion diffusion into the center of the nanostructured Si core and thus ultrafast charge/discharge capability. The porous N-doped carbon shell helps to improve the conductivity while allowing fast Li ion transport and confining the volume expansion within the Si micro-reactor. Submicron-sized Si micro-reactors with limited specific surface area (35 m2 g-1) afford sufficient electrode/electrolyte interfacial area for fast lithiation/delithiation, leading to the specific capacity ranging from ~800 to 420 mAh g-1 under ultrafast charging conditions (8 A g-1), but not too much interfacial area for surface side reactions and thus high initial coulombic efficiency (~90%). Since Si micro-reactors with superior electrochemical properties are synthesized via an industrially scalable and eco-friendly method, they have the potential for practical applications in the future.
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On the Specific Capacity and Cycle Stability of Si@void@C Anodes: Effects of Particle Size and Charge/Discharge Protocol
Silicon has the potential to be a high-performance anode material, but its practical application is impeded by huge volume expansion during lithiation. Many studies have revealed that the huge volume expansion problem can be mitigated by introducing engineered voids into Si/C core–shell structures. In this study, a Si/C core/shell structure with engineered voids, termed Si@void@C, is investigated for its specific capacity and cycle stability as a function of particle size and charge/discharge protocol. The study shows that finer Si@void@C particles result in higher specific capacities, but with little impact on the cycle stability. Further, lower and upper cutoff voltages in charge/discharge have a profound impact on the specific capacity and cycle stability. Importantly, cutoff voltages in formation cycles have long-lasting effects on the cycle stability, indicating the critical role of forming a robust solid electrolyte interphase (SEI) layer during formation cycles. Using a constant current charge followed by potentiostatic hold charge can further improve the cycle stability and minimize the sharp capacity decay in the first 20–40 cycles. With proper choices of charge/discharge protocols, the specific capacities of Si@void@C anodes at the electrode level are 66.8%, 38.2% and 22.7% higher than those of graphite anodes at the 1st, 300th and 500th cycles, respectively, proving that Si@void@C has promising potential to replace graphite anodes for practical applications in the future.
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- Award ID(s):
- 1918991
- PAR ID:
- 10427325
- Date Published:
- Journal Name:
- Batteries
- Volume:
- 8
- Issue:
- 10
- ISSN:
- 2313-0105
- Page Range / eLocation ID:
- 154
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Silicon/graphite (Si/Gr) nanocomposites with controlled void spaces and encapsulated by a carbon shell (Si/Gr@void@C) are synthesized by utilizing high-energy ball milling to reduce micron-sized particles to nanoscale, followed by carbonization of polydopamine (PODA) to form a carbon shell, and finally partial etching of the nanostructured Si core by NaOH solution at elevated temperatures. In particular, the effects of ball milling time and NaOH etching temperature on the electrochemical properties of Si/Gr@void@C are investigated. Increasing the ball milling time results in the improved specific capacity of Si-based anodes. Carbon coating further enhances the specific capacity and capacity retention over charge/discharge cycles. The best cycle stability is achieved after partial etching of the Si core inside Si/Gr@void@C particles at either 70 or 80 C, leading to little or no capacity decay over 130 cycles. However, it is found that both carbon coating and NaOH etching processes cause some surface oxidation of the nanostructured Si particles derived from high-energy ball milling. The surface oxidation of the nanostructured Si results in decreases in specific capacity and should be minimized in future studies. The mechanistic understanding developed in this study paves the way to further improve the electrochemical performance of Si/Gr@void@C nanocomposites in future.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3390/batteries8100154
