Search for: All records

Designing the solid–electrolyte interphase (SEI) is critical for stable, fast-charging, low-temperature Li-ion batteries. Fostering a “fluorinated interphase,” SEI enriched with LiF, has become a popular design strategy. Although LiF possesses low Li-ion conductivity, many studies have reported favorable battery performance with fluorinated SEIs. Such a contradiction suggests that optimizing SEI must extend beyond chemical composition design to consider spatial distributions of different chemical species. In this work, we demonstrate that the impact of a fluorinated SEI on battery performance should be evaluated on a case-by-case basis. Sufficiently passivating the anode surface without impeding Li-ion transport is key. We reveal that a fluorinated SEI containing excessive and dense LiF severely impedes Li-ion transport. In contrast, a fluorinated SEI with well-dispersed LiF (i.e., small LiF aggregates well mixed with other SEI components) is advantageous, presumably due to the enhanced Li-ion transport across heterointerfaces between LiF and other SEI components. An electrolyte, 1 M LiPF₆in 2-methyl tetrahydrofuran (2MeTHF), yields a fluorinated SEI with dispersed LiF. This electrolyte allows anodes of graphite, μSi/graphite composite, and pure Si to all deliver a stable Coulombic efficiency of 99.9% and excellent rate capability at low temperatures. Pouch cells containing layered cathodes also demonstrate impressive cycling stability over 1,000 cycles and exceptional rate capability down to −20 °C. Through experiments and theoretical modeling, we have identified a balanced SEI-based approach that achieves stable, fast-charging, low-temperature Li-ion batteries. 

Maximizing ion conduction in single-ion-conducting ionomers is essential for their application in energy-related technologies such as Li-ion batteries. Understanding the anion chemical composition impacts on ion conduction offers new perspectives to maximize ion transport, since the current approach of lowering T g has apparently reached a limit (lowest T g ∼ 190 K, highest conductivity ∼10 −5 –10 −4 S cm −1 ). Here, a series of random ionomers are synthesized by copolymerizing poly(ethylene glycol)methacrylate with either sulfonylimide lithium methacrylate (MTLi) or sulfonate lithium methacrylate (MSLi) using reversible addition–fragmentation chain transfer (RAFT) polymerization. Li-Ion conduction and self-diffusion coefficients ( D Li + ) of the ionomers are characterized with dielectric relaxation spectroscopy (DRS) and pulsed-field-gradient (PFG) NMR diffusometry, respectively. Increasing ion content decreases the Li-ion conductivity and D Li + , as expected from the increased T g . Moreover, a considerably lower ionic conductivity and D Li + are observed for MSLi compared to MTLi at constant ion content and T g / T . As revealed from X-ray scattering, strong ion aggregation in MSLi results in much lower conductivity and D Li + compared with less aggregated MTLi based on the more delocalized sulfonylimide anion. These results emphasize the detrimental and molecularly specific role of ion aggregation in Li-ion conductivity, and highlight the necessity for minimizing ion aggregation via the rational choice of anion chemical composition. 

Molecular ionic composites (MICs) are a new class of solid electrolytes that combine ionic liquids (ILs) and a rigid-rod double helical polyelectrolyte, poly(2,2′-disulfonyl-4,4′benzidine terephthalamide) (PBDT). In this study, we focus on the mechanical, dielectric, and ion diffusive dynamics of MICs with a fixed PBDT weight percent (10 wt%) and varying IL chemistry and molecular volume ( V m ). All six MICs produce tensile moduli in the range of 50–500 MPa at 30 °C, up to 60× higher than the shear moduli of the same MICs. The high range of moduli and tensile to shear modulus ratio emphasizes that the distribution of PBDT chains and the strong ionic interactions between IL ions and PBDT chains dictate the modulus and the mechanical strength in MICs. Additionally, these MICs exhibit high ionic conductivities ranging from 1–6 mS cm −1 at 30 °C, consistent with the measured diffusion coefficients of the IL ions. The tunability of the extraordinary mechanical properties and high ionic conductivities of MIC electrolytes greatly inspire their use in advanced electrochemical devices.