Increasing the proportion of inorganic electrolytes in organic–inorganic composite electrolytes can significantly improve the ionic conductivity of composite electrolytes. However, this method is confronted with some problems, such as the uneven dispersion of inorganic electrolytes and the aggregation of nanoparticles, which results in a worse performance of the electrolytes’ interface. In this study, we had the dopamine hydrochloride molecules self-polymerized on the surface of inorganic ionic conductor lithium titanium aluminum phosphate (LATP) particles to obtain PDA-coated LATP nanoparticles. Then, the coated LATP particles were composited with PEO at a ratio of 80 : 20, with the LATP's evenly dispersed in the PEO, and no aggregation happened. On adding a small amount of electrolyte (5 μL cm−2, 1 M Li TFSI), the ionic conductivity of the electrospun PI-loaded PEO0.2–PDA@LATP0.8 flexible composite solid electrolyte reached 2.07 × 10−4 at 30 °C and 2.05 × 10−3 S cm−1 at 80 °C. NCM811‖Li batteries assembled with this electrolyte had an initial discharge capacity of 178.0 mA h g−1 at 30 °C, and still remained at 172.1 mA h g−1 (96.7%) after 200 cycles. The research shows that there are three main conduction pathways of Li+ in this electrolyte: through the segments of the polymer chains, through the interface between the polymer and inorganic ionic conductor, and through the passage composed of a continuous inorganic ionic conductor. Further results show that the coating of LATP by PDA does not block the transmission of Li+ both inside the crystal and at the crystal interface of LATP.
Solid electrolytes are generated through in situ polymerization within batteries, which is one of the most promising methods for achieving solid-state lithium metal batteries with good interfacial contact, high safety, and high performance. Poly-1,3-dioxolane (PDOL), although a good in situ solidified electrolyte for lithium stability, has poor applicability in high-voltage battery systems. In order to improve the interfacial compatibility between PDOL and both the cathode and anode of high-voltage batteries. we herein design a sacrificial additive, diethyl (2,2,2-trifluoroethyl) phosphite (DETFPi), with a lower lowest unoccupied molecular orbital (LUMO) and higher highest occupied molecular orbital (HOMO) energy, through methods of calculation. After being loaded into the battery, DETFPi-DOL is in situ polymerized to form DETFPi-PDOL electrolyte. The Li||DETFPi-PDOL||Li symmetric battery operates stably for 2000 h at 0.5 mA cm–2, with an overpotential of only 16 mV. XPS analysis shows that an SEI layer with high LiF content is formed on the surface of the lithium anode after cycling, which promotes the uniform deposition of lithium ions and inhibits the growth of lithium dendrites. After 300 cycles, the NCM811||DETFPi-PDOL||Li battery exhibits a remaining capacity of 154.8 mAh g–1 (81%) within the 3.0–4.35 V range, meanwhile demonstrating excellent rate performance. Moreover, a uniform CEI layer containing pentavalent phosphorus and low LiF content is formed on the surface of the cathode after battery cycling. Finally, due to the improvement of the cathode interface, the increase of interfacial impedance of the battery after 300 cycles is reduced to half that of the PDOL battery.
Fabricating flexible composite polymer electrolytes (CPEs) with vertically aligned ion transport pathways is necessary for the application of solid-state lithium metal batteries (LMBs), yet the orientation of polymer matrix usually is disorder, thus the improvement in ionic conductivity is unsatisfied. Herein, we develop a simple electric field induced molecular orientation strategy to fabricate the flexible CPEs with vertically aligned ion transport pathways. Driven by direct current (DC) electric field (e.g. 1.5 MV m −1 ), the polar 15C5-Li + dipoles and monomer molecules are oriented regularly and cured to form flexible three dimension (3D) cross-linked structure. As a result, numerous vertically-aligned ion transport pathways have been generated in the vertically oriented arrangement CPEs, thus presenting highly enhanced ionic conductivities, mechanical strength and superb inhibition ability to lithium dendrite growth. When used as CPEs for Li||LiFePO 4 (LFP) batteries, the vertically oriented CPEs exhibit excellent rate and long-term cycle stability with a reversible discharge capability of 124.9 mAh g −1 after 350 cycles at 1C and 60 °C. However, the randomly oriented CPEs deliver much poor cycle and rate performance at the same conduction. It can also be compatible with NCM811|Li batteries at 30 °C. Therefore, our simple DC electric field induced molecular orientation strategy holds a great application promise to construct the fast ion transport channels for the all-solid-state LMBs.
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