Mitigating Gas Evolution in Electron Beam-Induced Gel Polymer Electrolytes through Bi-Functional Cross-Linkable Additives
Authors: Park Soojin , Nam Seoha , Son Hye Bin , Kim Yeongseok , Jeong Jin-Hyeok 
Published: December 2024

Currently, high-capacity LIBs commonly employ LiPF6-based liquid electrolytes (LEs) and high-nickel (Ni) layered oxide cathodes, such as Li[NixMnyCo1-x-y]O2 (NCM, x > 0.7). Although the LEs provide superior ionic conductivity, their intrinsic flammability and potential for leakage pose significant safety challenges. In addition, the generation of acidic species (e.g., POF3, PF5, and HF) from LiPF6 salt decomposition exacerbates the degradation of solid-electrolyte-interphase (SEI) and cathode-electrolyte-interface (CEI) layers. The high-Ni NCMs are particularly vulnerable to structural degradation and accelerate side reactions with the LEs under elevated voltages and temperatures, generating considerable gaseous byproducts, which, in turn, significantly increase the risk of battery explosion. Instead of LEs, solid-state electrolytes are considered safer alternatives owing to their enhanced safety. Yet, they have encountered obstacles such as unstable interface, low ionic conductivity, and the need for high processing temperatures exceeding 1000 °C. Therefore, designing a suitable electrolyte that simultaneously provides both safety and excellent interfacial compatibility is paramount.

One of the emerging solutions is in-situ gel polymer electrolytes (GPEs), developed by incorporating a liquid precursor into the cell followed by polymerization, thus ensuring commendable electrode compatibility. Commonly, the production of in-situ GPEs employs a thermal curing process. However, its demand for an initiator, elevated temperatures exceeding 50 °C, and prolonged polymerization time can negatively impact battery components. Conversely, high-energy electron beam (E-beam) irradiation can initiate polymerization uniformly and rapidly within a minute without an initiator. Further, it can penetrate deeply into thick materials, even aluminum (Al)-based packaging of pouch-type batteries, making it an advantageous technique for widespread battery applications and conducive to mass production.

In our research, we have developed a safe and commercially viable E-beam-induced GPE (E-Gel) with enhanced interfacial compatibility and minimal gas-related hazards using bi-functional cross-linkable additives (CIA). Dipentaerythritol hexaacrylate (DPH) is selected as a CIA for its proficiency in forming E-Gel with sufficient mechanical strength and Li-ion conductivity, achieving up to 80% of LEs with minimal addition of 3%.

Key aspects of our work are as following.

1. Innovative GPE fabrication process

Our work diverges fundamentally from existing methodologies by leveraging the bi-functional capability of DPH in a two-stage process involving initial electrochemical activation followed by E-beam-induced polymerization. Unlike conventional methods where a liquid precursor solution containing a monomer is injected and cured immediately after cell assembly, our process involves assembling a cell with a precursor solution and initiating the polymerization through electron beam irradiation after the first charge and discharge cycle. This sequence allows
DPH to function as an additive during the initial cycle, establishing stable and robust polymeric SEI and CEI and as a crosslinker during the E-beam irradiation step, forming a polymer framework. Consequently,the DPH-derived SEI and CEI layers obtained by utilizing DPH as an additive had lower Young’s modulus values of 110.0 and 42 MPa, respectively, than the LE-derived SEI (289.2 MPa) and CEI (111.5 MPa). These findings suggest that the polymeric DPH-modified SEI and CEI layers possess soft and flexible attributes, enabling them to endure mechanical deformation during the Li-ion insertion-extraction processes occurring within the layered Gr and NCM811 materials.

Enhanced interfacial compatibility at the electrode/E-Gel interfaces

DPH first forms a stable, polymeric interface layer which acts as a protective shield for the electrodes during the first cycle. Then, during the E-beam irradiation step, the C=C bonds in the decomposition products of DPH on the electrode surface can be co-crosslinked with the acrylates within the residual DPH monomer. In other words, the polymer matrix within the GPE is integrated with cathode and anode surfaces, thereby augmenting the E-Gel's structural integrity and interfacial compatibility. Such an integrated system establishes continuous ion transport pathways at the GPE/electrode interphases, promoting ion conductivity and facilitating Li-ion migration.

Improved safety and performance in practical applications

Our research demonstrates that the E-Gel significantly reduces the release of hazardous gases by 2.5 times compared to LE, particularly during the initial formation stage. This is because the preferential decomposition of DPH to form stable SEI/CEI effectively alleviates the breakdown of EC. Outstandingly, the 1.2 Ah pouch cell incorporating E-Gel maintains a high capacity of 1.0 Ah after 200 cycles at 55 °C while preserving good contact between the electrodes with low gas generation.

In summary, our research introduces a novel approach to the fabrication and application of gel polymer electrolytes for LIBs, leveraging the unique properties of DPH and a strategic E-beam irradiation process.

Figure 1