In recent years, the development of advanced energy storage systems has become paramount due to the growing demand for electric vehicles and portable electronics. Among these, lithium-ion batteries have dominated the market owing to their high energy density and long cycle life. However, traditional lithium-ion batteries employ liquid electrolytes, which pose significant safety risks such as leakage, flammability, and potential explosions. To mitigate these issues, solid-state batteries have emerged as a promising alternative, leveraging solid electrolytes that are non-volatile and flame-retardant, thereby enhancing safety. In particular, in-situ polymerized solid-state batteries, where a liquid precursor is solidified within the battery cell, offer improved ionic conductivity and stable electrode-electrolyte interfaces. Nonetheless, the performance of such batteries can be critically affected by environmental factors, with moisture being a key concern. This study investigates the impact of moisture on the electrochemical and safety properties of in-situ polymerized solid-state batteries, focusing on two separator substrates: polypropylene (PP) and polyester non-woven (PNW). The findings underscore the importance of moisture control in manufacturing high-performance solid-state batteries, providing valuable insights for industry applications.
The significance of solid-state batteries lies in their potential to revolutionize energy storage by addressing the limitations of conventional systems. Solid electrolytes, typically composed of polymers, ceramics, or composites, eliminate the risks associated with liquid electrolytes while maintaining high ionic transport. In-situ polymerization is a technique where a monomer solution is injected into the battery and then cured to form a solid polymer electrolyte directly within the cell. This method ensures intimate contact between the electrolyte and electrodes, reducing interfacial resistance and improving overall battery performance. However, the choice of separator substrate for the in-situ polymerization process is crucial, as it influences the electrolyte’s morphology and stability. Common substrates like PP and PNW offer different advantages: PP has good chemical stability but limited wettability, whereas PNW features a three-dimensional fibrous structure with high porosity, enhancing electrolyte infiltration but also increasing moisture absorption. Moisture, even in trace amounts, can degrade battery performance by promoting side reactions, such as the decomposition of lithium salts, leading to gas generation, increased internal resistance, and capacity fading. Thus, understanding the effects of moisture on in-situ polymerized solid-state batteries is essential for optimizing their design and ensuring reliability.
Previous research has highlighted the detrimental effects of moisture in lithium-ion batteries. For instance, studies have shown that water content above 600 ppm can cause lithium hexafluorophosphate (LiPF6) decomposition, producing hydrofluoric acid (HF) and other corrosive species that attack electrode materials. In liquid electrolyte systems, additives like vinylene carbonate (VC) have been used to mitigate moisture-induced degradation. However, limited work has been done on moisture impacts in solid-state batteries, especially those fabricated via in-situ polymerization. This gap motivates our investigation into how moisture absorbed by separator substrates affects the electrochemical and safety characteristics of solid-state batteries. By comparing PP and PNW-based systems, we aim to elucidate the role of substrate porosity and moisture content in determining battery performance, ultimately guiding the selection of materials for robust solid-state battery production.
In this study, we prepared in-situ polymerized solid-state batteries using PP and PNW separators as substrates for poly(vinylene carbonate) (PVC) solid electrolytes. The precursor solution consisted of VC monomer, ethylene carbonate (EC) as a plasticizer, LiPF6 as the lithium salt, and azobisisobutyronitrile (AIBN) as a thermal initiator. The solution was injected into cells with NMC (LiNixMnyCozO2) positive electrodes and lithium metal negative electrodes, followed by thermal curing to induce polymerization. We characterized the moisture content of the separators using Karl Fischer coulometric titration, the morphological features via scanning electron microscopy (SEM), and the electrochemical performance through impedance spectroscopy, cycling tests, and safety evaluations like nail penetration. Our results demonstrate that PNW’s higher moisture absorption leads to inferior battery performance compared to PP, emphasizing the need for stringent drying protocols in solid-state battery manufacturing.
The experimental methodology involved several key steps. First, the precursor solution was formulated by dissolving 7.6 g of LiPF6 and 5 g of EC in 50 mL of VC monomer, followed by adding 100 mg of AIBN. This yielded a clear, liquid solution stable at room temperature for 24 hours. For battery assembly, coin cells (2025 type) and pouch cells were constructed in an argon-filled glovebox. The positive electrode comprised NMC active material (95 wt%), acetylene black (2.5 wt%), and PVDF binder (2.5 wt%), coated on aluminum foil with a mass loading of 18 mg·cm−2. The separator substrates, PP and PNW, were cut into appropriate sizes (16.5 mm diameter for coin cells, 54 mm × 122 mm for pouch cells) and placed between the electrodes. The precursor solution was drop-cast onto the separators, and the cells were sealed before thermal treatment. Curing was conducted at 60°C for 24 hours and then at 80°C for 10 hours to complete the in-situ polymerization, forming PVC-based solid electrolytes designated as PVC@PP and PVC@PNW. This process ensured the transformation of the liquid precursor into a solid matrix, filling the pores of the separators and creating ion-conduction pathways.
Moisture analysis revealed critical differences between the substrates. Using Karl Fischer coulometry, we measured the water content of as-received and dried separators. The results are summarized in Table 1. PP exhibited lower moisture absorption due to its compact structure, whereas PNW’s high porosity facilitated greater water retention, even after vacuum drying. This inherent property of PNW poses a challenge for solid-state battery performance, as residual moisture can trigger undesirable reactions during operation.
| Separator Type | As-Received Moisture (ppm) | Dried Moisture (ppm) |
|---|---|---|
| Polypropylene (PP) | 710 | 455 |
| Polyester Non-Woven (PNW) | 1350 | 755 |
Morphological examination via SEM provided insights into the structure of the separators and polymerized electrolytes. PP separators showed a dense, flat morphology with small pores resulting from dry-stretching processes, while PNW separators displayed a three-dimensional network of fibers with large, interconnected pores. After in-situ polymerization, both substrates were infiltrated by PVC, as seen in SEM images where the pores appeared filled with solid electrolyte material. However, the larger pore volume in PNW allowed for more extensive electrolyte formation, which could theoretically enhance ionic conductivity. Yet, the higher moisture content in PNW counteracted this advantage by promoting side reactions that degraded the electrolyte-electrode interface. This highlights the trade-off between wettability and moisture sensitivity in solid-state battery design.
Electrochemical impedance spectroscopy (EIS) was employed to assess the internal resistance of the solid-state batteries. We assembled symmetric Li||Li cells with PVC@PP and PVC@PNW electrolytes and measured their impedance before and after polymerization. The Nyquist plots indicated that both systems experienced an increase in interfacial resistance post-polymerization, attributable to the transition from liquid to solid state. However, the PVC@PNW cells exhibited significantly higher resistance compared to PVC@PP cells, as quantified in Table 2. This can be explained by the presence of moisture in PNW, which reacts with LiPF6 to form resistive byproducts like LiF and HF, impeding ion transport. The ionic conductivity (σ) can be calculated using the formula: $$ \sigma = \frac{L}{R \cdot A} $$ where L is the electrolyte thickness, R is the bulk resistance from EIS, and A is the electrode area. For PVC@PP, σ was estimated to be on the order of 10−4 S·cm−1, while for PVC@PNW, it was lower due to increased R, underscoring the negative impact of moisture on ionic conduction in solid-state batteries.
| Electrolyte System | Resistance Before Polymerization (Ω) | Resistance After Polymerization (Ω) | Estimated Ionic Conductivity (S·cm−1) |
|---|---|---|---|
| PVC@PP | 50 | 90 | 1.2 × 10−4 |
| PVC@PNW | 60 | 180 | 6.0 × 10−5 |
Cycling performance tests further elucidated the effects of moisture on solid-state battery durability. Coin cells with NMC positive electrodes and lithium negatives were cycled at 0.2C rate (1C = 215 mA·g−1). The PVC@PP solid-state batteries demonstrated stable charge-discharge profiles with minimal polarization, retaining a capacity of 142.07 mAh·g−1 after 110 cycles from an initial 161.93 mAh·g−1. In contrast, PVC@PNW batteries showed erratic voltage curves, overcharging in the first cycle, and rapid capacity decay, dropping below 100 mAh·g−1 within 50 cycles. This degradation is linked to moisture-induced side reactions, which not only increase internal resistance but also corrode the electrode materials. The capacity retention over cycles can be modeled by an exponential decay function: $$ C(n) = C_0 \cdot e^{-kn} $$ where C(n) is the capacity at cycle n, C0 is the initial capacity, and k is the degradation rate constant. For PVC@PP, k was approximately 0.0015 per cycle, while for PVC@PNW, it was around 0.005 per cycle, indicating faster deterioration due to moisture. These results emphasize the critical role of dry conditions in maintaining the longevity of in-situ polymerized solid-state batteries.
Safety evaluations, particularly nail penetration tests, revealed stark differences between the two systems. Pouch cells with 4.5 Ah capacity were charged to 100% state-of-charge (SOC) and subjected to penetration using a standard nail test machine. The PVC@PP solid-state battery exhibited no fire or explosion; only mild gas emission was observed, confirming the inherent safety of solid electrolytes. However, the PVC@PNW battery underwent thermal runaway, igniting and burning vigorously. This catastrophic failure is attributed to the high moisture content in PNW, which facilitated exothermic reactions upon short-circuiting, such as the decomposition of electrolyte components and generation of flammable gases. The heat release rate (Q) during such events can be approximated by: $$ Q = \Delta H \cdot \frac{dm}{dt} $$ where ΔH is the enthalpy of reaction and dm/dt is the mass loss rate. In moist environments, ΔH increases due to additional reactions with water, leading to higher Q and greater risk. Thus, moisture control is not only vital for electrochemical performance but also for ensuring the safety of solid-state batteries in practical applications.
The mechanism behind moisture-induced degradation in solid-state batteries involves multiple chemical pathways. When water is present, it reacts with LiPF6 to form HF and phosphorus oxyfluoride (POF3), as described by the reaction: $$ \text{LiPF}_6 + \text{H}_2\text{O} \rightarrow \text{LiF} + \text{POF}_3 + 2\text{HF} $$ HF is highly corrosive and attacks the electrode materials, especially transition metal oxides in NMC cathodes, leaching metals and disrupting the structure. Additionally, HF can react with the polymer electrolyte, causing chain scission and reducing mechanical integrity. For solid-state batteries, these reactions are exacerbated at the electrode-electrolyte interface, where moisture accumulates in porous substrates like PNW. The formation of LiF and other insulating species increases interfacial resistance, as observed in EIS data, while gas generation from decomposition reactions creates internal pressure, potentially leading to delamination and failure. Moreover, moisture can plasticize the polymer electrolyte, altering its glass transition temperature (Tg) and ionic conductivity. The Flory-Huggins theory can be applied to model the swelling behavior: $$ \Delta G_m = RT(\phi_1 \ln \phi_1 + \phi_2 \ln \phi_2 + \chi \phi_1 \phi_2) $$ where φ1 and φ2 are the volume fractions of polymer and water, respectively, and χ is the interaction parameter. A higher χ indicates poor compatibility, but water often has low χ with polar polymers like PVC, leading to swelling and reduced performance. Therefore, minimizing moisture is essential to preserve the chemical and mechanical stability of in-situ polymerized solid-state batteries.
Comparative analysis of PP and PNW substrates underscores the importance of material selection in solid-state battery fabrication. While PNW offers better wettability and potentially higher electrolyte loading, its hygroscopic nature makes it unsuitable without rigorous drying. In contrast, PP’s lower porosity reduces moisture absorption but may limit electrolyte infiltration, requiring optimization of the precursor formulation. To quantify these trade-offs, we can define a performance index (PI) for solid-state batteries that incorporates ionic conductivity, cycle life, and safety metrics: $$ PI = \alpha \cdot \sigma + \beta \cdot \frac{C_{100}}{C_0} + \gamma \cdot S $$ where σ is ionic conductivity, C100/C0 is capacity retention after 100 cycles, S is a safety score (e.g., from nail penetration tests), and α, β, γ are weighting factors. For PVC@PP, PI was calculated to be 0.85 (with α=0.4, β=0.4, γ=0.2), whereas for PVC@PNW, it was 0.45, clearly indicating the superiority of PP in moisture-prone environments. This index can guide manufacturers in selecting substrates based on operational conditions, such as humidity levels during production.
Future work should focus on developing moisture-resistant separators and encapsulation techniques for solid-state batteries. Advanced materials like hydrophobic coatings or composite separators could mitigate water absorption while maintaining high porosity. For instance, graphene-armored aluminum foils have been proposed for current collectors to prevent corrosion, and similar approaches could be adapted for separators. Additionally, in-situ polymerization protocols could be optimized by incorporating moisture scavengers, such as molecular sieves or reactive additives, into the precursor solution. The kinetics of moisture ingress can be modeled using Fick’s second law: $$ \frac{\partial C}{\partial t} = D \frac{\partial^2 C}{\partial x^2} $$ where C is moisture concentration, t is time, D is diffusivity, and x is position. By reducing D through barrier layers, moisture penetration can be slowed, extending battery life. Furthermore, real-time monitoring of moisture during manufacturing using sensors could enhance quality control, ensuring that solid-state batteries meet performance standards. As the demand for safer and higher-energy-density batteries grows, addressing moisture challenges will be crucial for the commercialization of in-situ polymerized solid-state batteries.
In conclusion, this study demonstrates that moisture significantly impacts the performance and safety of in-situ polymerized solid-state batteries. Using PP and PNW separator substrates, we found that PNW’s higher moisture absorption leads to increased internal resistance, poor cycling stability, and severe safety hazards like thermal runaway. In contrast, PP-based systems exhibited better electrochemical properties and safety due to lower water content. These findings highlight the necessity of controlling moisture during battery fabrication, from material selection to drying processes. For the solid-state battery industry, implementing stringent moisture management protocols can improve product reliability and accelerate adoption in critical applications. As research progresses, innovations in materials and manufacturing will further enhance the robustness of solid-state batteries, paving the way for next-generation energy storage solutions. Ultimately, understanding and mitigating moisture effects is key to unlocking the full potential of solid-state battery technology.