Solid state batteries represent a transformative advancement in energy storage technology, offering enhanced safety, higher energy density, and improved cycle life compared to conventional lithium-ion batteries with liquid electrolytes. The integration of silicon anodes and sulfide-based solid-state electrolytes is particularly promising for developing high-performance solid state batteries. Silicon anodes provide a high theoretical capacity of approximately 4200 mAh/g, which is significantly higher than that of graphite anodes, making them ideal for next-generation solid state batteries. However, the practical implementation of silicon anodes in solid state batteries is hindered by substantial volume changes during lithiation and delithiation, leading to mechanical degradation and interfacial instability. Similarly, sulfide-based solid-state electrolytes, such as Li3PS4 (LPS) and Li10GeP2S12 (LGPS), exhibit high ionic conductivity but suffer from electrochemical instability at the anode interface, resulting in lithium dendrite growth and capacity fading. Understanding the atomic-scale mechanisms of lithiation-induced degradation is crucial for optimizing the performance and safety of solid state batteries.
In this study, we employ first-principles calculations based on density functional theory (DFT) to investigate the structural and energetic evolution of silicon anodes and sulfide-based solid-state electrolytes during cycling. We analyze the lithiation-delithiation processes, interface reactions, and cavity formation to provide insights into the degradation mechanisms in solid state batteries. Our computational approach is complemented by experimental validation using symmetric battery cells to ensure the reliability of our predictions. The focus is on how silicon anodes can mitigate the spontaneous lithiation of sulfide electrolytes and suppress interfacial cavity formation, thereby enhancing the stability and longevity of solid state batteries.
The first-principles calculations are performed using the Vienna Ab-initio Simulation Package (VASP) with the Perdew-Burke-Ernzerhof (PBE) functional under the generalized gradient approximation (GGA). We use a plane-wave cutoff energy of 450 eV and a Gamma-centered k-point grid for Brillouin zone sampling. The lithiation processes are modeled using ab initio molecular dynamics (AIMD) simulations with a melt-quench approach to generate realistic amorphous structures. For silicon anodes, we simulate the transformation from crystalline to amorphous states during lithiation, calculating key properties such as formation energy, voltage, and volume change. The reactions between silicon anodes and sulfide electrolytes are evaluated to determine the thermodynamic stability and interfacial evolution. The formation energy ($E_f$) for lithiated compounds is calculated using the equation:
$$E_f = E_{\text{Li}_x\text{Si}} – x E_{\text{Li}} – E_{\text{Si}}$$
where $E_{\text{Li}_x\text{Si}}$ is the total energy of lithiated silicon, $x$ is the lithiation amount, and $E_{\text{Li}}$ and $E_{\text{Si}}$ are the energies of bulk lithium and silicon, respectively. Similarly, for sulfide electrolytes, the formation energy during lithiation is computed to assess their degradation behavior. The voltage profile during lithiation is derived from the derivative of the formation energy with respect to the lithiation amount:
$$U_x = -\frac{dE_f}{dx}$$
Volume changes are analyzed to understand the mechanical stress and cavity formation at interfaces. The relative volume change ($\Delta V$) is given by:
$$\Delta V = V_{\text{Li}_x\text{Si}} – V_{\text{Si}} – x V_{\text{Li}}$$
where $V_{\text{Li}_x\text{Si}}$, $V_{\text{Si}}$, and $V_{\text{Li}}$ are the volumes of lithiated silicon, silicon, and lithium, respectively. These calculations provide a comprehensive view of the electrochemical and mechanical behavior in solid state batteries.
Our results for silicon anodes reveal a significant structural transformation during lithiation-delithiation cycles. Crystalline silicon initially has a diamond structure with a lattice constant of 5.47 Å and Si–Si bond lengths of 2.34 Å. Upon lithiation, the silicon network disrupts, forming an amorphous structure at a lithiation amount of $x = 1.0$ (LiSi). As lithiation progresses to $x = 4.4$ (Li4.4Si), the silicon atoms become isolated, leading to a volume expansion of approximately 426% compared to the original crystal. The delithiation process does not restore the crystalline structure; instead, it results in an amorphous silicon phase with increased volume and energy. The formation energy decreases during lithiation, reaching a minimum of -0.78 eV per formula unit at $x = 4.4$, indicating spontaneous reaction. However, upon delithiation, the formation energy increases to 0.48 eV, reflecting reduced stability. The voltage profile shows a decrease from 0.45 V to near 0 V during lithiation and an increase to 1.04 V during delithiation, consistent with experimental observations for solid state batteries. The volume changes highlight the mechanical challenges, with the amorphous silicon exhibiting a 207% volume increase compared to crystalline silicon after delithiation. These changes are critical for understanding the interfacial compatibility in solid state batteries.
The following table summarizes the key properties of silicon anodes during lithiation and delithiation:
| Lithiation Amount (x) | Structure | Formation Energy (eV/f.u.) | Voltage (V vs. Li/Li⁺) | Volume (ų/f.u.) | Density (g/cm³) |
|---|---|---|---|---|---|
| 0.0 | Crystalline | 0.00 | 0.45 | 20.4 | 2.28 |
| 1.0 | Amorphous | -0.25 | 0.30 | 35.2 | 1.95 |
| 2.0 | Amorphous | -0.50 | 0.15 | 50.1 | 1.65 |
| 3.0 | Amorphous | -0.68 | 0.05 | 65.0 | 1.40 |
| 4.4 | Amorphous | -0.78 | 0.00 | 87.0 | 1.12 |
| 0.0 (delithiated) | Amorphous | 0.48 | 1.04 | 42.3 | 1.10 |
For sulfide-based solid-state electrolytes, we analyze the lithiation of LPS and LGPS. The lithiation process involves the breaking of P–S and Ge–S bonds, leading to the formation of lithium phosphide, lithium sulfide, and lithium germanide as decomposition products. The formation energy for LPS lithiation becomes more negative with increasing lithiation amount, reaching -9.29 eV per formula unit at full lithiation ($x = 8$), indicating a spontaneous reaction. Similarly, LGPS shows a formation energy of -8.21 eV at full lithiation, suggesting that germanium doping enhances stability. The volume change during lithiation is negative for both electrolytes, with LPS contracting by 22.3% and LGPS by 19.2% at full lithiation. This contraction contributes to interfacial cavity formation when coupled with lithium metal anodes. However, when paired with silicon anodes, the volume change is mitigated due to the expansion of silicon during delithiation. The reaction between silicon anodes and sulfide electrolytes can be represented as:
$$\text{Li}_{4.4}\text{Si} + \text{Li}_3\text{PS}_4 \rightarrow \text{Li}_x\text{Si} + \text{Li}_y\text{PS}_z$$
where the formation of Li–Si and P–Si bonds alters the decomposition pathway, forming a solid electrolyte interphase (SEI) layer that includes Li2S and silicon phosphides. This SEI layer reduces further reaction but may increase interfacial resistance. The following table compares the properties of LPS and LGPS during lithiation:
| Electrolyte | Lithiation Amount (x) | Formation Energy (eV/f.u.) | Volume Change (ų/f.u.) | Decomposition Products |
|---|---|---|---|---|
| LPS | 0.83 | -2.5 | -2.1 | PS33-, S2- |
| 3.33 | -6.8 | -7.5 | Li3P, Li2S | |
| 8.0 | -9.29 | -11.2 | Li3P, Li2S | |
| LGPS | 1.50 | -3.2 | -3.0 | GeS44-, PS43- |
| 3.00 | -6.0 | -6.8 | Li3P, Li2S, Li4Ge | |
| 8.0 | -8.21 | -10.3 | Li3P, Li2S, Li4Ge |
To investigate the interfacial behavior, we construct models for Li/LPS and Li4.4Si/LPS interfaces. The Li/LPS interface shows rapid lithiation of LPS, leading to significant volume contraction and cavity formation. The initial cavities in LPS, with diameters around 3 Å, grow to 5 Å after lithiation and further expand during room-temperature annealing, forming cracks that facilitate lithium dendrite growth. In contrast, the Li4.4Si/LPS interface exhibits slower degradation, with cavity diameters remaining around 3.5 Å and minimal growth. The formation of P–Si bonds and a Li2S-rich SEI layer passivates the interface, reducing further reaction. The energy change during interface evolution is -46.5 eV for Li/LPS and -30.1 eV for Li4.4Si/LPS, indicating that silicon anodes suppress spontaneous lithiation. The cavity volume increases by 45.8% for Li/LPS but decreases by 0.4% for Li4.4Si/LPS, demonstrating the effectiveness of silicon anodes in maintaining interfacial integrity in solid state batteries.
Experimental validation is conducted using symmetric battery cells with Li-LPS and LiSi-LPS configurations. The cells are cycled at a current density of 0.1 mA/cm², and the voltage profiles are monitored. The Li-LPS cell shows a voltage increase from 0.0124 V to 0.0146 V after 67 hours, indicating rising internal resistance due to cavity formation. In contrast, the LiSi-LPS cell exhibits a gradual voltage increase to 0.0186 V after 72 hours, attributed to SEI formation. Scanning electron microscopy (SEM) images reveal large cracks (2–3 μm wide) in the LPS electrolyte of Li-LPS cells, while LiSi-LPS cells show smaller cracks (0.7–1.1 μm wide), confirming the computational predictions. These results underscore the importance of silicon anodes in enhancing the interfacial stability of solid state batteries.
In conclusion, our first-principles study provides atomic-scale insights into the lithiation-induced degradation of sulfide-based solid-state electrolytes on silicon anodes for solid state batteries. Silicon anodes mitigate the spontaneous lithiation of electrolytes through volume expansion during delithiation, reducing cavity formation and suppressing dendritic growth. The formation of a stable SEI layer, although increasing resistance, improves cycle life. These findings highlight the potential of silicon anodes in advancing solid state batteries, addressing key challenges such as interfacial instability and safety. Future work should focus on optimizing the SEI composition and exploring hybrid anode designs to further enhance the performance of solid state batteries.
The development of reliable solid state batteries hinges on understanding and controlling interface reactions. Our computational framework, combined with experimental validation, offers a robust approach for designing next-generation energy storage systems. By leveraging the unique properties of silicon anodes and sulfide electrolytes, we can overcome the limitations of current battery technologies and achieve high-energy, safe, and long-lasting solid state batteries for various applications, from electric vehicles to grid storage. The continuous improvement of solid state batteries will play a pivotal role in the transition to sustainable energy solutions.