Rechargeable lithium-ion batteries (LIBs) have developed rapidly over the last three decades and have become an integral part of the world economy. However, commercial LIBs fall short of the surging demand for high-energy-density devices due to the limited anode capacity. Recently, Li-metal batteries (LMBs) have attracted substantial interest owning to the ultrahigh specific capacity and lowest redox potential of metallic Li. Nevertheless, Li metal is highly reactive with conventional liquid electrolytes, resulting in inhomogeneous solid electrolyte interphase, fast capacity fade, and severe safety concerns. Different from the liquid electrolyte, polymer-based electrolytes are less leakage, low toxic, and appear to be promising electrolytes for LMBs. However, the implementation of polymer-based electrolytes is hindered by their intrinsically low ionic conductivity and poor interfacial contact with the electrodes. In addition, their mechanical strength, and (electro)chemical stability against Li metal need to be improved. Therefore, the objective of the dissertation is to develop new polymer-based electrolytes (e.g., solid-like or quasi-solid electrolytes), which offer good cyclic stability and high safety for LMBs.
This dissertation comprises four projects. In the first project, we developed free-standing composite polymer electrolytes (CPEs) which exhibited unique characteristics including non-flammability, high flexibility, and good thermal stability. A fluoroethylene carbonate (FEC) additive was used on the surface of Li metal to facilitate the formation of a LiF-rich solid electrolyte interphase layer. The FEC-coated Li PE iFePO
4 battery exhibited excellent cycling and rate performance. To improve the energy density of the LMB, the LiFePO
4 cathode was replaced with a high-voltage material LiNi
1/3Mn
1/3Co
1/3O
2. The obtained Li PE iNi
1/3Mn
1/3Co
1/3O
2 cell exhibited a discharge capacity of 109 mAh g
−1 after 100 cycles at 0.2 C.
In the second project, highly conductive CPE membranes were prepared by integrating a poly(vinylidene fluoride) matrix (PVDF) with a Li-conductive perovskite (i.e., Li
0.38Sr
0.44Ta
0.70Hf
0.30O
2.95F
0.05, LSTHF), a flame-retarding solvent (i.e., trimethyl phosphate (TMP)), and a Li salt (i.e., LiClO
4). The CPE membrane with 10 wt% LSTHF (CPE-10) exhibited conductivity as high as 0.53 mS cm
-1 at room temperature and 0.36 mS cm
-1 at 0 °C. Furthermore, prototype batteries including the CPE-10 electrolyte, showed high initial discharge capacities, good rate capabilities, and stable cycling performance at either RT or 5 and 60 °C.
In the third project, we prepared a solid-like dual-salt polymer electrolyte (DSPE), consisting of coordinated solvents and thermally stable Li-salts within a temperature-resistant polymer matrix. The developed DSPE demonstrated high ionic conductivity of 0.16, 0.73, and 1.93 mS cm
−1 at -20, 20, and 100 °C. Li SPE iFePO
4 batteries delivered excellent rate performance and exceptional long-term stability. Further, the battery showed stable cycling between −10 and 80 °C. The impressive electrochemical performance is ascribed to the high Li conductivity of the membrane, the stabilization of the Al current collector, and the formation of a robust, LiF-rich solid-electrolyte interphase containing conductive Li-B-O-based species.
Last, we developed a novel quasi-solid polymer electrolyte (QSPE-40) via in-situ polymerization of 1,3-dioxolane (DOL). QSPE-40 showed high ionic conductivity from -30 to 30 °C, satisfactory interphase compatibility, and a wide electrochemical window of up to 5.0 V vs. Li/Li
+. Theoretical analysis has clarified the Li
+ transport mechanism of QSPE-40, in which the TFSI
-@MP rich inner shell tends to maintain the fast Li
+ conduction. In contrast, the DOL-dominated frigostable outer shell forms a protective layer between the electrode and electrolyte. Consequently, the Li SPE-40 iNi
0.8Co
0.1Mn
0.1O
2 batteries delivered excellent discharge capacities and cycling stability between -30 °C and room temperature.
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