Electrolyte-Free Graphite Electrodes Employing Lithium-Substitution-Modulated Binders for Practical Fabrication of All-Solid-State Batteries

The challenges associated with electrically inert binders have been mitigated in conventional lithium-ion batteries (LIBs) by utilizing permeable liquid electrolytes (LEs) for ionic connectivity and carbonaceous additives for electronic connectivity in the electrodes. Unlike electro-conductive binders to exclude the inactive conductive agent, the development of ion-conductive binders has been limited due to the presence of LEs in the electrodes [1].<br/>Meanwhile, all-solid-state batteries (ASSBs) are promising for high-energy storage and safety by using solid electrolytes (SEs) instead of LEs, though poor SE wettability hampers Li⁺ transport and sulfide-based SEs may decompose with electrode components [2,3]. To address these issues, our group has proposed a simplified electrolyte-free ASSBs electrode that relies on interfacial conduction between electrode particles, potentially facilitated by ion-conducting binders [2].<br/>In this study, we propose a strategic approach to enhance interfacial Li⁺ conduction in ASS, electrolyte-free electrodes, where electrolyte components are entirely excluded, by employing lithium-substitution-modulated (LSM) binders [2]. By precisely adjusting the lithium substitution ratio, we synthesized conductive LSM-carboxymethyl cellulose (CMC) through a controlled direct Na⁺/Li⁺ exchange reaction, avoiding the use of hazardous acids.<br/>Our findings demonstrate that the electrolyte-free graphite electrode using LSM as the binder, with a maximum degree of lithium substitution (DS_Li) of approximately 68%, exhibits significantly higher rate capability and capacity retention compared to those using sodium-CMC (Na-CMC) and LSM with ~35% lithium substitution.<br/>Moreover, we systematically investigated the correlation between phase transition near the bottom region of the graphite electrode and the state of charge (SOC), revealing that the enhancement of interfacial conduction is directly proportional to the DS_Li of the CMC binders. We speculate that the creation of continuous interfaces with abundant pathways for mobile ions using the Li⁺-conductive binder is the primary mechanism for improved interfacial conduction in the electrolyte-free graphite electrode, thereby reducing significant charge transfer resistance.<br/><br/>References<br/>[1] J. Qian, C. G. Wiener, Y. Zhu, B. D. Vogt, Polymer, 143 (<b>2018)</b> 237-244.<br/>[2] Y. Yamagishi, H. Morita, Y. Nomura, E. Igaki, J Phys Chem Lett., 12 (<b>2021)</b> 4623-4627.<br/>[3] Y.-T. C. D. H. S. Tan, H. Yang, W. Bao, B. Sreenarayanan, J.-M. Doux, W. Li, B. Lu, S.-Y. Ham, B. Sayahpour, J. Scharf, E. A. Wu, G. Deysher, H. E. Han, H. J. Hah, H. Jeong, J. B. Lee, Z. Chen, Y. S. Meng, Science, 373 (<b>2021)</b> 1494-1499.<br/>[4] D. O. Shin, H. Kim, S. Jung, S. Byun, J. Choi, M. P. Kim, J. Y. Kim, S. H. Kang, Y.-S. Park, S. Y. Hong, M. Cho, Y.-G. Lee, K. Cho, Y. M. Lee, Energy Storage Materials, 49 (<b>2022)</b> 481-492.