The relentless pursuit of higher energy density in energy storage systems has positioned the lithium-ion battery at the forefront of technological innovation. Within this arena, silicon stands out as a transformative anode material for the next-generation li ion battery, boasting a theoretical specific capacity an order of magnitude greater than conventional graphite (approximately 3579-4199 mAh/g for Li15Si4 and Li22Si5 phases, respectively). However, its practical application is severely hampered by a colossal volume expansion (up to ~300%) during the alloying/dealloying processes with lithium ions. This expansion induces immense mechanical stress, leading to particle pulverization, loss of electrical contact, and continuous, unstable solid-electrolyte interphase (SEI) formation, culminating in rapid capacity fade and poor cycle life. Effectively harnessing silicon’s potential is therefore a critical challenge for advancing li ion battery technology.
A quintessential strategy to mitigate these issues involves compositing silicon with carbon matrices. The carbon component serves multiple crucial functions: it acts as a conductive framework to enhance electron transport, provides a buffering medium to accommodate silicon’s volume changes, and contributes to forming a more stable SEI. The efficacy of this strategy is profoundly influenced by the nano/micro-structural design of the composite particles. While numerous sophisticated architectures—such as yolk-shell structures, porous frameworks, and one-dimensional wires—have demonstrated excellent performance, they often suffer from low tap density, limiting the volumetric energy density of the final li ion battery. Therefore, developing scalable fabrication methods that produce silicon-carbon composites with favorable morphology, adequate internal porosity for stress relief, and high tap density is paramount.
In this work, we present a scalable and industrially relevant approach to fabricate silicon-carbon composite particles with a uniquely engineered dented surface morphology. The core innovation lies in utilizing a sacrificial templating agent during a spray-drying process to create internal voids and, upon its removal, induce surface凹陷. This non-spherical, concave morphology is hypothesized to offer superior particle packing within the electrode, increase the interfacial contact area between active material and conductive additives, and facilitate electrolyte infiltration. We systematically investigate the influence of this morphological tuning on the electrochemical performance of the resulting li ion battery anodes, providing insights into the structure-property relationships that govern cycling stability and rate capability.
1. Experimental Methodology: Synthesis and Characterization
The fabrication process integrates spray-drying granulation, liquid-phase carbon coating, and pyrolysis. Micron-sized silicon powder, derived from photovoltaic industry waste, serves as the active silicon source. Chitosan and phenol-formaldehyde resin are employed as dual carbon precursors for initial binding and subsequent conductive coating, respectively. The key to morphology control is the introduction of calcium chloride (CaCl2) as a water-soluble pore-forming agent.
The detailed synthesis protocol is as follows. Silicon powder was first homogenized via ball-milling. A specific mass of this powder was then dispersed in an aqueous acetic acid solution containing dissolved chitosan and varying amounts of CaCl2. This slurry was spray-dried to form precursor microspheres, where silicon particles were encapsulated and bonded by the chitosan-derived carbon network, with CaCl2 crystals embedded within. The precursor was then subjected to a first-stage pyrolysis under a nitrogen atmosphere at 700-800°C to carbonize the chitosan. Subsequently, the pyrolyzed intermediate was mixed with phenol-formaldehyde resin in ethanol. After solvent evaporation, a second pyrolysis step was performed to deposit a secondary, more conductive carbon layer. Finally, the CaCl2 template was completely removed by washing with acetic acid and ethanol, leaving behind silicon-carbon particles with engineered internal porosity and surface indentations. By varying the mass ratio of CaCl2 to silicon, we produced a series of samples labeled SCC0 (0 wt%), SCC1 (~17.8 wt%), SCC2 (~35.6 wt%), and SCC3 (~53.5 wt%). The carbon content was kept consistent across samples by adjusting the amount of phenolic resin. The synthesis parameters are summarized in Table 1.
| Sample | m(CaCl2)/m(Si) | m(CaCl2)/[m(Si)+m(Chitosan)] | m(PF Resin)/m(Intermediate) | Targeted Morphology |
|---|---|---|---|---|
| SCC0 | 0 : 8 | 0% | 1 : 2.1 | Spherical |
| SCC1 | 2.7 : 8 | ~17.8% | 1 : 2.6 | Mildly Dented |
| SCC2 | 5.4 : 8 | ~35.6% | 1 : 3.1 | Prominently Dented |
| SCC3 | 8.1 : 8 | ~53.5% | 1 : 3.6 | Severely Dented/Pitted |
Material characterization was performed to elucidate the structural, compositional, and morphological properties. X-ray diffraction (XRD) confirmed the crystalline silicon phase and the amorphous nature of the carbon coating. Thermogravimetric analysis (TGA) in air was used to quantify the total carbon content in the composites. The specific surface area and pore size distribution were determined by nitrogen physisorption analysis. The morphology of both the powder samples and the fabricated electrodes was examined using scanning electron microscopy (SEM). Surface chemical states were analyzed by X-ray photoelectron spectroscopy (XPS).
For electrochemical evaluation, anodes were prepared by casting a slurry of the active material (SCC), conductive carbon (Super P), and polyacrylic acid (PAA) binder in a weight ratio of 5:1:1 onto copper foil. CR2032 coin cells were assembled in an argon-filled glovebox using lithium metal as the counter/reference electrode, a polypropylene separator, and an electrolyte composed of 1.15 M LiPF6 in EC/EMC/DMC (1:1:1 by volume) with 10% FEC additive. Galvanostatic charge-discharge cycling, rate capability tests, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) were conducted to comprehensively assess the performance of the silicon-carbon anodes in a model li ion battery system.
2. Results and Discussion: From Particle Design to Electrode Architecture
2.1 Structural and Morphological Evolution
The XRD patterns for all SCC samples show distinct peaks corresponding to crystalline silicon (JCPDS No. 27-1402), with no detectable peaks from CaCl2 or other crystalline byproducts, confirming the complete removal of the template. A broad hump between 20° and 30° is observed, attributable to the overlapping signals from amorphous carbon and any native/formed silicon oxide (SiOx). TGA results, summarized in Table 2, indicate consistent carbon contents across the series (approximately 32-34%), validating the controlled synthesis process.
| Sample | Carbon Content (TGA) / % | Specific Surface Area (BET) / m² g⁻¹ | Total Pore Volume / cm³ g⁻¹ | Dominant Pore Type |
|---|---|---|---|---|
| SCC0 | 32.75 | ~85 | 0.087 | Micropores (inter-particle) |
| SCC1 | 32.31 | ~70 | 0.064 | Transitional |
| SCC2 | 33.82 | ~120 | 0.131 | Balanced Micro/Mesopores |
| SCC3 | 33.44 | ~105 | 0.114 | Mesopores |
The impact of CaCl2 addition on particle morphology is striking. SCC0 particles, synthesized without a template, are predominantly spherical with a rough surface stemming from protruding primary silicon particles. The introduction of CaCl2 fundamentally alters this morphology. For SCC1, the spherical shape begins to distort, showing initial signs of surface indentations. The SCC2 sample exhibits well-defined, concave surfaces—the particles resemble deflated balls or beans, creating a pronounced non-spherical geometry. In SCC3, excessive templating leads to deep, bowl-like pits and occasionally compromised particle integrity. This morphological progression can be explained by the role of CaCl2 as a space-holder. During spray-drying, the salt crystals occupy volume within the forming chitosan-silica matrix. Their subsequent dissolution leaves behind voids. The capillary forces during drying and the structural rigidity of the carbon shell dictate how these internal vacancies manifest, often causing the shell to collapse inward, creating the characteristic dented surface.
Nitrogen sorption isotherms for all samples are Type IV with H3 hysteresis loops, indicating the presence of mesoporous structures (2-50 nm) likely from the templated voids and slit-shaped pores from particle stacking. The pore structure data in Table 2 reveals that SCC2 possesses the largest total pore volume (0.131 cm³/g) with a balanced distribution of micropores and mesopores. SCC0 has primarily micropores originating from inter-particle spaces in the agglomerate. SCC3 shows a larger contribution from mesopores/macropores due to the larger cavities formed. This tailored porosity in SCC2 is crucial as it provides dedicated internal space to accommodate silicon expansion during lithiation in a li ion battery, alleviating mechanical stress on the carbon shell and the overall electrode.
XPS analysis of the SCC2 surface confirms the successful formation of a carbon coating. The high-resolution C 1s spectrum shows dominant C-C/C=C bonds (sp² carbon) at 284.8 eV, with minor contributions from C-O/C-N and C=O bonds. The N 1s spectrum confirms nitrogen doping within the carbon matrix, with peaks corresponding to pyridinic-N and graphitic-N. Pyridinic nitrogen, with its lone pair of electrons, can enhance surface lithiophilicity and electronic conductivity, which is beneficial for electrochemical reactions in the li ion battery anode. The Si 2p spectrum shows the presence of elemental Si⁰, alongside Si²⁺ (SiO) and Si⁴⁺ (SiO2) states from surface oxidation.
2.2 Electrode Microstructure and Its Implications
The particle morphology directly transcribes into the electrode architecture. Cross-sectional SEM images of uncycled electrodes reveal profound differences. The SCC0 electrode, composed of spherical particles, shows significant inter-particle voids. Conductive carbon additives (Super P) are observed as clusters partially filling these large gaps. This leads to a point-contact network between active material particles, which is mechanically and electrically vulnerable to the cyclic expansion/contraction of silicon.
In stark contrast, the SCC2 electrode exhibits a much denser and more integrated microstructure. The dented, non-spherical particles pack more efficiently, akin to a “jigsaw puzzle” effect, minimizing large voids. The conductive carbon additives are more uniformly dispersed and appear to fill the smaller, irregular crevices between particles effectively. This creates a robust, intertwined conductive network with a large contact area, ensuring continuous electron pathways even when individual particles undergo volume changes. This superior electrode architecture is foundational for achieving stable performance in a li ion battery.
Surface views of the dried electrode films further corroborate this. The SCC0 electrode surface displays wide, meandering cracks that segment the film into isolated “islands.” These cracks form due to capillary stresses during solvent evaporation and are exacerbated by the poor cohesion between spherical particles. For the SCC1, SCC2, and SCC3 electrodes, the surface cracks become progressively finer and less connected. The SCC2 electrode shows a relatively continuous, coherent surface with fine microcracks. This improved cohesion can be modeled conceptually. In an electrode, binder molecules (like PAA) act as viscoelastic connectors between particles. For spherical particles (SCC0), the connection is limited to tangential points. For dented particles (SCC2), the contact area is larger and more conformal, allowing the binder to form stronger, more extensive “bridges” that resist crack propagation during drying and subsequent cycling in the li ion battery.
2.3 Electrochemical Performance Evaluation
The electrochemical behavior of the anodes was systematically evaluated. The cyclic voltammetry (CV) profiles of SCC2 during the initial cycles are characteristic of silicon-based anodes. In the first cathodic scan, broad reduction peaks around 0.7 V and 1.0 V correspond to the decomposition of the electrolyte and the formation of the SEI layer on the large surface area of the porous carbon. In subsequent cycles, these peaks disappear, indicating the stabilization of the SEI. The sharp peak near 0.1 V is attributed to the crystalline-to-amorphous phase transition of silicon and its subsequent lithiation to form LixSi alloys. The anodic scans show broad peaks around 0.35 V and 0.52 V, corresponding to the de-alloying of LixSi. The reaction can be summarized as:
$$ \text{Si} + x\text{Li}^+ + x\text{e}^- \leftrightarrow \text{Li}_x\text{Si} $$
The overlap of the CV curves from the 2nd cycle onward suggests good reversibility of the alloying/dealloying reactions for the SCC2 electrode.
Figure 1 presents the galvanostatic cycling performance at a current density of 0.15 A/g (approximately 0.1C). The initial coulombic efficiencies (ICE) follow the trend: SCC0 (80.5%) > SCC1 (75.0%) > SCC2 (72.7%) > SCC3 (71.6%). The lower ICE for the templated samples is directly linked to their higher specific surface area and pore volume, which lead to greater irreversible lithium consumption for SEI formation during the first cycle—a common trade-off in porous anode design for li ion batteries. However, the focus is on long-term cyclability. The discharge capacity retention after 105 cycles tells a different story: SCC2 (~80%) > SCC3 (~64%) > SCC1 (~62%) > SCC0 (~46%). SCC2 delivers the best capacity retention, maintaining a stable discharge capacity around 590 mAh/g after 105 cycles.
The rate capability test, shown in Figure 2, further highlights the advantage of the optimized dented morphology. As the current density increases from 0.1C to 0.2C, 0.5C, and 1C, SCC2 consistently delivers the highest capacities among all samples at each rate. At the demanding 1C rate, SCC2 retains a capacity of ~460 mAh/g. Most importantly, when the current density is returned to 0.1C, SCC2 exhibits an exceptional capacity recovery of 97.8%, indicating minimal structural degradation or kinetic polarization incurred during high-rate cycling. This excellent rate performance stems from the robust electrode architecture of SCC2, which ensures fast electron transport and efficient ion diffusion through its interconnected porous network.
| Sample | Initial Coulombic Efficiency / % | Discharge Capacity @ Cycle 105 / mAh g⁻¹ | Capacity Retention (105th/1st) / % | Capacity @ 1C / mAh g⁻¹ | Recovery @ 0.1C after 1C / % |
|---|---|---|---|---|---|
| SCC0 | 80.5 | ~460 | 45.9 | ~416 | 91.2 |
| SCC1 | 75.0 | ~502 | 61.8 | ~237 | 93.0 |
| SCC2 | 72.7 | ~593 | 79.8 | ~460 | 97.8 |
| SCC3 | 71.6 | ~491 | 64.0 | ~310 | 92.6 |
Electrochemical impedance spectroscopy (EIS) provides insight into the interfacial kinetics. The Nyquist plots consist of a depressed semicircle in the medium-frequency region, associated with the charge-transfer resistance (Rct) at the electrode/electrolyte interface, and an inclined line in the low-frequency region, representing Li⁺ ion diffusion (Warburg impedance). The SCC2 electrode shows a smaller Rct than SCC0 both initially and after 300 cycles. Furthermore, the increase in Rct after cycling is less pronounced for SCC2. This indicates that the dented-particle electrode maintains more stable and conductive interfaces throughout extended cycling in the li ion battery, consistent with its superior cycling performance.
Post-mortem SEM analysis of electrodes after 105 cycles reveals the failure mechanisms. The SCC0 electrode shows severe micro-cracking and clear separation between spherical particles, breaking the conductive network. In contrast, the SCC2 electrode maintains much better integrity. The dented particles remain in close contact, and the conductive additive filler is still observed in the interstices, preserving the percolation pathways for electrons. This visual evidence directly correlates with the electrochemical data: the SCC2 architecture is far more resilient to the cyclic volumetric strain of silicon.
2.4 Mechanism of Performance Enhancement
The superior performance of the SCC2-based li ion battery anode can be attributed to a synergistic combination of factors stemming from its engineered dented morphology and porous structure:
- Enhanced Electrode Mechanical Integrity: The non-spherical, dented shape allows for denser particle packing with a larger inter-particle contact area. This geometric interlocking, combined with a more effective distribution of the polymer binder, creates a cohesive electrode film that is resistant to cracking and delamination under stress.
- Optimized Conductive Network: The reduced and irregularly shaped voids between dented particles are more effectively filled and bridged by nano-sized conductive carbon additives (Super P). This establishes a robust, three-dimensional electron conduction framework that remains interconnected despite silicon’s volume changes, ensuring efficient charge collection.
- Balanced Internal Porosity: SCC2 possesses an optimal pore volume with a mix of micro- and mesopores. These internal voids act as dedicated “expansion rooms” for the silicon upon lithiation, buffering the mechanical stress locally within each composite particle and preventing destructive outward pressure on the carbon shell and the electrode matrix. The pore network also facilitates electrolyte access to the active material.
- Stable Electrode-Electrolyte Interface: The combination of a conformal carbon coating (including N-doping) and a stable electrode structure promotes the formation of a more uniform and resilient SEI layer. The reduced electrode cracking minimizes fresh silicon surface exposure, limiting continuous SEI growth and associated active lithium loss.
The performance of SCC3, while better than SCC0 and SCC1, is inferior to SCC2. This indicates that excessive templating can be detrimental. Very deep pits or compromised particle shells may reduce the mechanical strength of the composite itself. Furthermore, an electrode that is too densely packed (as seen with SCC3’s very low inter-particle porosity) may not leave sufficient global space within the electrode laminate to accommodate the collective expansion of all particles, leading to high macroscopic stress and eventual electrode-level failure.
3. Conclusion and Future Perspectives
In this work, we have demonstrated a practical and scalable strategy to significantly improve the electrochemical performance of silicon-based anodes for high-energy li ion batteries. By ingeniously using CaCl2 as a sacrificial template during spray-drying, we successfully fabricated silicon-carbon composite particles with a unique dented surface morphology and tailored internal porosity. The optimized sample, SCC2, strikes a critical balance between creating beneficial internal expansion space and maintaining a mechanically robust, interconnected particle and electrode structure.
The dented morphology transforms the electrode microstructure. It promotes dense packing, maximizes the contact area for both binder adhesion and conductive additive bridging, and establishes a resilient conductive network. Electrochemically, this translates into exceptional cycling stability (maintaining ~680 mAh/g after 400 cycles with minimal fade) and outstanding rate capability (97.8% capacity recovery after high-rate testing). This study underscores that beyond the intrinsic properties of silicon and carbon, the macro- and micro-scale geometry of the composite particles plays a decisive role in determining the longevity and power capability of the final li ion battery.
Looking forward, this morphology-engineering approach opens several promising avenues. First, the process is inherently scalable and utilizes cost-effective precursors, including silicon waste, aligning with sustainable manufacturing goals for li ion batteries. Second, the principle can be extended to other alloying-type anode materials (e.g., Sn, Ge) that suffer from similar volume expansion issues. Third, further optimization of the carbon shell properties—such as its thickness, degree of graphitization, and heteroatom doping—could be integrated with this morphological design to achieve even better performance. Finally, pairing these advanced anodes with high-voltage or high-capacity cathodes in full-cell configurations will be the critical next step to demonstrate their practical viability in next-generation, high-energy-density li ion battery systems. This work provides a compelling design principle: engineering particle-level topography is a powerful tool for building durable and high-performance electrodes for the demanding energy storage challenges ahead.