3D spiny AlF3/Mullite heterostructure nanofiber as solid-state polymer electrolyte fillers with enhanced ionic conductivity and improved interfacial compatibility

Abstract

Lithium metal batteries assembled with solid-state electrolyte can offer high safety and volumetric energy density compared to liquid electrolyte. The polymer solid-state electrolytes of poly(ethylene oxide) (PEO) are widely used in lithium metal solid-state batteries due to their unique properties. However, there are still some defects such as low ionic conductivity at room temperature and weak inhibition of lithium dendrite growth. Herein, the spiny inorganic nanofibers heterostructure with mullite whiskers grown on the surface of aluminum fluoride (AlF3) nanofibers are introduced into the PEO-LiTFSI electrolytes for the first time to prepare composite solid-state electrolytes. The AlF3 as a strong Lewis acid can adsorb anions and promote the dissociation of Li salts. Besides, the specially three-dimensional (3D) structure enlarges the effective contacting interface with the PEO polymer, which allows the lithium ions to be transported not only along the large aspect ratio of AlF3 nanofibers, but also along the mullite phase in the transmembrane direction rapidly. Thereby, the transport channel of lithium ions at the spiny inorganic nanofibers-polymer interface is further improved. Benefiting from these advantages, the obtained composite solid-state electrolyte has a high ionic conductivity of 1.58 × 10−4 S cm−1 at 30 °C and the lithium ions transfer number of 0.53. In addition, the AlF3 has strong binding energy with anions, low electronic conductivity and wide electrochemical stability window, and reduced nucleation overpotential of lithium during cycling, which is positive for lithium dendrite suppression in solid-state electrolytes. Thus, the assembled symmetric Li/Li symmetric batteries exhibit stable cycling performance at different area capacities of 0.15, 0.2, 0.3 and 0.4 mA h cm−2. More importantly, the LiFePO4 (LFP)/Li battery still has 113.5 mA h g−1 remaining after 400 cycles at 50 °C and the Coulomb efficiency is nearly 100% during the long cycle. Overall, the interconnected structure of 3D spiny inorganic heterostructure nanofiber constitutes fast and uninterrupted lithium ions transport channels, maximizing the synergistic effect of interfacial transport of inorganic fillers and reducing PEO crystallinity, thus providing a novel approach to high performance solid-state electrolytes.