Newswise — As the globe shifts towards a more eco-friendly and sustainable energy system, the reliance on lithium (Li)-ion batteries is predicted to increase. Scientists worldwide are striving to develop smaller yet efficient batteries that can meet the growing demand for energy storage. Recently, the focus has turned towards all-solid-state lithium batteries (ASSLBs) due to their use of solid electrolytes instead of liquid ones, which has sparked significant research interest. These solid electrolytes not only enhance battery safety by preventing leakage and fire hazards but also offer superior energy and power characteristics. However, their inflexibility results in inadequate wetting of the cathode surface and an uneven supply of Li ions to the cathode. Consequently, the solid-state battery experiences a loss in capacity. This problem becomes more evident when using thicker battery cathode electrodes, such as millimeter-thick ones, which offer a more cost-effective and high-energy-density battery package compared to conventional electrodes with a typical thickness of less than 0.1 mm.
Luckily, a recently published study in Science has discovered a solution to this issue. The research, conducted by Prof. Ryoji Kanno and his team from the Tokyo Institute of Technology (Tokyo Tech), unveils a novel method to enhance the Li-ion conductivity of solid electrolytes. Their findings introduce a design principle for creating high-entropy crystals of lithium superionic conductors using a multi-substitution approach.
"Numerous studies have demonstrated that inorganic ionic conductors exhibit improved ion conductivity through multi-element substitution, likely due to the reduction of the potential barrier for Li-ion migration, which is crucial for enhancing ion conductivity," emphasized Prof. Kanno. This formed the starting point for their research. To design their new material, the team drew inspiration from the chemical compositions of two well-known Li-based solid electrolytes: the argyrodite-type (Li6PS5Cl) and LGPS-type (Li10GeP2S12) superionic crystals. They introduced modifications to the LGPS-type Li9.54Si1.74P1.44S11.7Cl0.3 through multi-substitution and synthesized a series of crystals with the composition Li9.54[Si1−δMδ]1.74P1.44S11.1Br0.3O0.6 (M = Ge, Sn; 0 ≤ δ ≤ 1).
In their experiments, the researchers utilized a crystal with M = Ge and δ = 0.4 as a catholyte in an ASSLB, with either a 1- or 0.8-millimeter-thick cathode. The ASSLB with the 1 mm cathode exhibited a discharge capacity of 26.4 mAh cm−2 at 25 °C, while the ASSLB with the 0.8 mm cathode showed a discharge capacity of 17.3 mAh cm−2 at -10 °C. These values corresponded to area-specific capacities that were 1.8 and 5.3 times higher, respectively, than those previously reported for state-of-the-art ASSLBs. Theoretical calculations indicated that the enhanced conductivity of the solid electrolyte could be attributed to the reduction of the energy barrier for ion migration, resulting from the slight chemical substitution in the aforementioned crystal.
This study introduces a novel approach to fabricate high-entropy solid electrolytes suitable for millimeter-thick electrodes while maintaining their superionic conduction pathways. Prof. Kanno emphasizes that this design principle establishes a solid foundation for exploring new superionic conductors with exceptional charge-discharge performance, even under room temperature conditions. In essence, this research offers a promising avenue for further advancements in the field of solid electrolytes and their application in high-performance energy storage systems.
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