There were huge potential safety hazards when commercialized lithium ion battery employing organic carbonate solvent and polyolefin-based separators. These potential risks (combustion and explosion) would retard the commercialization of electric vehicles or hybrid electric vehicles. Thus, the safety issue of lithium ion battery merits further study. Solid polymer electrolyte is attracting strong interest and regarded as a perfect way to handle the potential safety risk of lithium batteries.
2. Experimental
poly(propylene carbonate) (PPC) based solid polymer electrolytes were fabricated by a facile solution casting method. PPC (10 g) and lithium bis (trifluoromethanesulphonyl) imide (LiTFSI) (3 g) were added into anhydrous acetonitrile (100 mL) in sequence under intense stirring to form a homogeneous solution. Subsequently, flame retardant cellulose nonwoven membrane was prepared according to the method of our previous literature. Followingly, homogeneous solution poured into cellulose nonwovens followed by the evaporation of the solvent in a vacuum oven at 100 oC for 24 hrs. The thickness of the resultant cellulose-supported PPC solid polymer electrolyte (CPPC-SPE) was 75±5 μm. Pristine PPC solid polymer electrolyte (PPC-SPE) and pristine polyethylene oxide (PEO) solid polymer electrolyte (PEO-SPE) were fabricated via similar method without cellulose nonwoven supporting matrix.
3. Results and Discussion
In order to demonstrate the feasibility of the novel CPPC-SPE, all-solid-state polymer LiFePO4/Li battery was assembled and typical charge-discharge curves at various rates and long-term cycling stability were also examined (Fig. 1). It is obvious that rate capability and cycle performance of CPPC-SPE was much better than those of PEO-SPE at room temperature. This better rate performance is an indicator of a low resistance, which can be contributed to better ionic transport capability, demonstrating that CPPC-SPE has great potential in ambient temperature all-solid-state lithium batteries.
It was noted that the cell with CPPC-SPE displayed superior cycle durability after 1000 cycles at 20 oC, 95 % of the initial capacity. This outstanding cycle performance would be attributed to better electrochemically interfacial stability. It can be seen from Figure 4c that the value of charge-transfer resistance after first cycle was 73 Ω. After the 1000 cycles, the value became 83 Ω, indicative of a minor augment of charge-transfer resistance. More importantly, solid state soft-package lithium batteries using CPPC-SPE can present excellent safety characteristic even in extremely harsh condition. A corollary of this is that, this class of CPPC-SPE is very promising to develop next-generation all-solid-state lithium batteries that require high energy density, superior safety and excellent reliability. Most significantly, the main innovative of our work will highlight a reference for subsequent scientific research of room temperature solid polymer electrolyte and also boost the development of high performance all solid state polymer lithium batteries.
References
1) G. L. Cui, et. al Advanced Energy Materials DOI: 10.1002/aenm.201501082.
2) G. L. Cui, et. al Scientific Reports 2014, 4, 6272.