Solid-state batteries (SSB) represent a promising future for energy storage, addressing technical issues and safety risks of conventional batteries, while offering higher energy densities and longer cycle life. Polymer solid electrolytes (SE) promise advantages such as higher mechanical stability, electrochemical stability, and easier processing, making them suitable for industrial applications. Solvent-free methods for producing polymer SSBs eliminate toxic solvents, reducing cost and environmental impact. Despite these benefits, challenges remain in achieving the best mixture with the lowest possible porosity to maximize ion conduction paths within the battery.
Our research focuses on cathode production using a solvent-free method involving a kneader device followed by direct calendering of the resulting granules. The kneader device operates at high temperatures to partially melt polymers within the cathode composite, forming granules that are later processed in a calender device to create a free-standing film, which is further laminated onto Al-foil. This process is crucial for producing cathode sheets with minimal porosity, essential for high-performance ASSBs.
In this study, NMC811 and C45 are used as the active material and electrical conductive additive, respectively. LiTFSI is employed as an ion-conducting salt due to its high thermal stability. PVDF-HFP serves as the basis for the polymer electrolyte, providing high mechanical stability and a wide electrochemical window. PEO is added to enhance ionic conductivity, as its oxygen atoms aid lithium-ion transference. The effects of carbon black, Li salt, and ionic liquid (PYR14TFSI) as additives, while maintaining a constant polymer ratio and varying the active material content, were investigated. Each recipe is examined for electronic conductivity, coating porosity, tortuosity, and pore size distribution. The produced cathode sheets are cycled at 80 °C and 0.1C for 20 cycles.
Data from pore size distribution tests are considered for pore diameters ranging from 10 µm (active material size) to 10 nm. The specific incremental pore volume and coating porosity remain below 0.05 cm³/g and 8%, respectively.
Increasing carbon black concentration improves electrical conductivity by providing additional pathways for electrons, benefiting cycling performance up to a certain level. However, excessive amounts can lead to aggregation, reducing pathway effectiveness. Low lithium salt content enhances lithium-ion mobility and reduces ion pairing risk, while high content increases system resistance, leading to competition between ions and hence, lower cycling capacities. Ionic liquids help maintain uniform ion distribution, enhancing the electrochemical performance of the cathode. However, excessive ionic liquid content increases resistance and obstructs electron flow, deteriorating battery performance. These experiments show that the composition of 10wt% IL, 10wt% LiTFSI and 12wt% CB has the best cycling performance with 1st cycle specific discharge capacity around 153 Ah/g and cycling of 20 cycles in 280 h.
In conclusion, optimizing solid-state batteries with polymeric solid electrolytes and solvent-free methods shows great potential. The balance of materials is crucial for performance, making these batteries a viable energy storage solution for EVs and industry. Continued research is essential to address remaining challenges and fully realize their potential.