Effect of Polymeric Interlayers in Sulfide-based Solid-state Batteries

Abstract

Sulfide-based solid-state batteries (SSBs) are emerging as a top contender for next-generation rechargeable batteries with improved safety and outstanding energy densities. SSBs incorporate non-flammable solid-electrolytes (SEs), eliminating safety hazards for batteries in electric vehicles and electronic devices. In addition, the development of SSBs with SEs allows the conventional graphite anode to be replaced by a lithium metal anode, theoretically surpassing the energy densities of lithium-ion batteries (LIBs). However, SSBs with high nickel cathode materials such as LiNi0.8Mn0.1Co0.1O2 (NMC), exhibit several cathode interface-related issues preventing the large-scale adoption of this technology. Specifically, the problems include challenges such as mechanical and chemical instability, which results in particle cracking, contact loss, and decomposition of the solid-electrolyte.
To overcome these challenges, this study used polymeric interlayers at the surface of the NMC to buffer volume changes and passivate chemical side reactions. Herein, the Ni-rich layered oxides polycrystalline NMC811 (PC-NMC) and single-crystal NMC Ni82 (SC-NMC) were coated with PDDA-TFSI, poly(diallyldimethy-lammonium) bis(trifluoro-methanesulfonyl)imide, using a microencapsulation method. Additionally, this study aimed to maximize the utilization of cathode active material with the use of the conductive additive carbon nanofibers (CNFs) and the addition of lithium salt (LiTFSI) in the polymeric coating. Full lab-scale SSBs consisted of a coated NMC-based cathode, a Li-In alloy anode, and Li6PS5Cl solid-electrolyte.
The conductive carbon additive severely increased SSB degradation, which was associated with the formation of decomposition elements sulfates/sulfites (SOx ), polysulfides (P2Sx ), phosphates (POx ), and lithium phosphate phases (P/LixP). The polymeric coating on PC-NMC slightly improved cycle stability. Here, improvements were associated with the significant reduction of contact loss between NMC and Li6PS5Cl particles. The cathode with SC-NMC with a PDDA-(Li)TFSI polymeric coating demonstrated exceptional performance improvements in SSBs, mitigating chemical and mechanical degradation. It showcased improved cycle stability with a capacity retention of 95% observed after 100 cycles at 0.2C, compared to a capacity retention of 84% for SSBs without a cathode interface coating. Furthermore, a significantly improved initial capacity of 155 mAh g−1 at 0.2C was established, compared to an initial
capacity of 144 mAh g−1 for uncoated SSBs. Overall, the results highlight the performance-enhancing effect of a polymeric coating with added lithium salts in Li6PS5Cl-based SSBs.