Quasi-Solid Polymer Electrolyte with Multiple Lithium-Ion Transport Pathways by In Situ Thermal-Initiating Polymerization

Tailoring Li+ Local Coordination Environment of Solid Polymer Electrolytes to Boom Ionic Conductivity

The low ionic conductivity is the key issue of solid polymer electrolytes (SPEs), hampering their practical application. Great efforts have been devoted to reducing their crystallinity to increase ionic conductivity but ignored their Li+ local coordination environment influence. Here, Li+ local coordination environment tunable poly(vinylidene fluoride-co-hexafluoropropylene)-based solid-state electrolytes are experimentally realized via d-cellobiose octaacetate. d-cellobiose octaacetate competes with anions and polymer chains for coordination of Li+ thorough C═O groups to weaken their coordination of Li+, increase the number of carriers, and strengthen the transport kinetics of Li+, booming the ionic conductivity of SPEs at room temperature. When used in lithium metal symmetric batteries and full batteries, SPEs greatly improve their electrochemical performance at 25 °C. This work clarifies the important influence of Li+ local coordination environment on Li+ transport and provides a promising strategy to improve the ionic conductivity of SPEs.

Opening and Constructing Stable Lithium-ion Channels within Polymer Electrolytes

Lithium-ion batteries play an integral role in various aspects of daily life, yet there is a pressing need to enhance their safety and cycling stability. In this study, we have successfully developed a highly secure and flexible solid-state polymer electrolyte (SPE) through the in situ polymerization of allyl acetoacetate (AAA) monomers. This SPE constructed an efficient Li+ transport channel inside and effectively improved the solid-solid interface contact of solid-state batteries to reduce interfacial impedance. Furthermore, it exhibited excellent thermal stability, an ionic conductivity of 3.82×10-4 S cm-1 at room temperature (RT), and a Li+ transport number (tLi+) of 0.66. The numerous oxygen vacancies on layered inorganic SiO2 created an excellent environment for TFSI- immobilization. Free Li+ migrated rapidly at the C=O equivalence site with the poly(allyl acetoacetate) (PAAA) matrix. Consequently, when cycled at 0.5C and RT, it displayed an initial discharge specific capacity of 140.6 mAh g-1 with a discharge specific capacity retention rate of 70 % even after 500 cycles. Similarly, when cycled at a higher rate of 5C, it demonstrated an initial discharge specific capacity of 132.3 mAh g-1 while maintaining excellent cycling stability.

Multiple Na+ transport pathways and interfacial compatibility enable high-capacity, room-temperature quasi-solid sodium batteries

Sodium-metal batteries (SMBs) are ideal for large-scale energy storage due to their stable operation and high capacity. However, they have safety issues caused by severe dendrite growth and side reactions, particularly when using liquid electrolytes. Therefore, it is critically important to develop electrolytes with high ionic conductivity and improved safety that are non-flammable and resistant to dendrites. Here, we developed polymerized polyethylene glycol diacrylate (PEGDA)-modified poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) electrolytes (PPEs) with highly conductive sodium bis(trifluoromethanesulfonyl)imide and corrosion-inhibitive sodium bis(oxalato)borate salts for SMBs. Well-complexed PEGDA not only increases the amorphicity of the PVDF matrix, but also offers numerous Lewis basic sites through the polar groups of carbonyl and ether groups (i.e., electron donors). The presence of the Lewis basic sites facilitates the dissociation of sodium salt and transportation of Na+ within the PVDF matrix. This results in the generation of additional Na+ transport pathways, which can enhance the performance of the battery. Among PPEs, the optimized PPE-50 exhibits a high ionic conductivity of 3.42 × 10-4 S cm-1 and a mechanical strength of 14.0 MPa. A Na||Na symmetric cell with PPE-50 displays high stability at 0.2 mA cm-2 for 800 h. PPE-50 further displays high capacity, e.g., a Na3V2(PO4)3|PPE-50|Na battery delivers a decent discharge capacity of 101.5 mAh g-1 at 1.0C after 650 cycles. Our work demonstrates the development of high-performance quasi-solid polymer electrolytes with multiple transport pathways suitable for room-temperature SMBs.

In Situ Polymerized Quasi-Solid Electrolytes Compounded with Ionic Liquid Empowering Long-Life Cycling of 4.45 V Lithium-Metal Battery

High-voltage resistant quasi-solid-state polymer electrolytes (QSPEs) are promising for enhancing the energy density of lithium-metal batteries in practice. However, side reactions occurring at the interfaces between the anodes or cathodes and QSPEs considerably reduce the lifespan of high-voltage LMBs. In this study, a copolymer of vinyl ethylene carbonate (VEC) and poly(ethylene glycol) diacrylate (PEGDA) was used as the framework, with a cellulose membrane (CE) as the supporting layer. Based on density functional theory calculations, 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr14TFSI), an ionic liquid, was screened because of its lowest unoccupied molecular orbital energy level as a modifying agent for the in situ P(VECx-EGy)/Pyrz/LiTFSI@CE QSPEs synthesis. Pyr14+, with a lithiophobic alkyl chain, forms a dense positive ion shielding layer on the protruding tips of deposited lithium, facilitating uniform and smooth lithium deposition. Pyr14TFSI assists in constructing a stable solid electrolyte interphase (SEI) layer on the Li surface enriched with LiF, Li3N, and RCOOLi. The modulation of lithium deposition behavior on the anode by Pyr14TFSI ensures stable Li plating/stripping for >1500 h. A Li-Cu cell exhibits stable cycling for >200 cycles at a current density of 0.05 mA cm-2, with an average Coulombic efficiency of 92.7%. In situ polymerization ensures that P(VECx-EGy)/Pyrz/LiTFSI@CE QSPEs exhibit excellent interface compatibility with the anode and the cathode. The CR2032 button cell Li|P(VEC1-EG0.06)/Pyr0.4/LiTFSI@CE|LiCoO2 demonstrates stable cycling with a negligible capacity decay of 0.083% per cycle for >390 cycles at 25 °C and 0.2 C when using a high-voltage LiCoO2 (4.45 V) cathode. Furthermore, a 7.1 mAh pouch cell achieves stable charge-discharge cycles, confirming the pronounced stability of the as-fabricated QSPE at the interfaces of the high-voltage LiCoO2 cathode and Li anode.

LLZTO Nanoparticle- and Cellulose Mesh-Coreinforced Flexible Composite Electrolyte for Stable Li Metal Batteries

Composite electrolytes have been regarded as the most prospective electrolytes for commercial application because they acquire the advantages of both polymer and inorganic electrolytes, commonly exhibiting appreciated flexibility and suitable ionic conductivity. Nevertheless, the conventional solution-casting method with toxic solvent and poor interfacial contact still hamper their commercialization process. Moreover, electrolytes with higher ionic conductivity and transference number are urgently needed for satisfying fast-charging batteries. Herein, a novel composite electrolyte (LZEC) reinforced by mechanically robust LLZTO nanoparticles and flexible cellulose mesh was fabricated by a simple and advanced in situ thermal polymerization method, with adding of highly ion-conductive liquid plasticizer. Consequently, the rationally designed LZEC composite electrolyte exhibits superior flexibility and remarkable electrochemical properties in the form of high ionic conductivity, wide electrochemical stability window, and high Li+ transference number. Importantly, the in situ synthesis method is expected to help construct an enhanced electrolyte/electrode interface inside the battery, and the LZEC composite electrolyte is capable of suppressing Li dendrite growth effectively, as evidenced by the prolonged stable cycling of the Li/Li symmetric cell. Therefore, the LFP/LZEC/Li full cell exhibits superior rate performance and long cyclic life. These attractive properties make LZEC a potential composite electrolyte for boosting the practical application of safe and long-life Li metal batteries.