Researches On Key Materials For Rechargeable Lithium Air Batteries

Recently, lithium-air batteries have attracted great interest due to theirhuge theoretical specific energy of5200Wh kg-1including O2. The firstlithium-air battery with a structure of Li|organic electrolyte|air was reportedby Abraham and Jiang, and further developed by many scientists over theworld. However, the investigation and development on lithium-air batteriesis still in its initial stage. Much fundamental research is required before itcan be considered further for technological applications: first, if there is notan effective bifunctional electrocatalyst (oxygen reduction reaction (ORR)and oxygen evolution reaction (OER)), discharge products (Li2O and Li2O2)will precipitate in the pores of the carbon based cathodes, which blocksfurther intake of oxygen and thus abruptly ends cell life. On the other hand,organic carbonates are not suitable as electrolytes for Li/O2batteries. In thisdissertation, high performance non-noble metal catalysts and compositepolymer electrolytes were developed. Electrochemical properties of theprepared materials were studied. The major research contents are presentedas follows: 1. Phenanthroline (phen) was used as a ligand to prepare Co(phen)2complexes, which were coated on BP2000and then heat-treated to obtaincarbon-supported Co-N catalysts (Co-N/C). The influence of heat treatingtemperature on the structure and catalytic performance of the Co-N/Ccatalysts were investigated. The results indicated that the Co-N/C catalystprepared at800℃showed the best performance. Charge/discharge tests ofthe lithium-oxygen cells using the prepared Co-N/C catalyst showeddischarge capacities of3221mAh g-1(The specific capacities werereferenced with respect to the total mass of carbon and cobalt catalyst in thecathode). The catalytic activity of Co-N/C is similar to other non-noblecatalysts, such as Co-P/C electrocatalyst.2. We synthesized a PVDF-HFP composite polymer electrolyte. Theinfluence of silica on the structure and electrochemical performance of thepolymer electrolyte were investigated. The results revealed that polymerelectrolyte membranes showed smooth morphology properties. Theaddition of3%silica in the polymer electrolyte displayed the highest ionicconductivity (1.3×10-5S cm-1). Li ion transference number is as high as0.36. The lithium-oxygen cells using this polymer electrolyte showed thefirst discharge capacity of about3163mAh g-1, at0.2mA cm-1currentdensity.3. Due to low conductivity of PVDF-HFP polymer electrolyte, Ionicliquids PP13TFSI was used to improve the performance of polymer electrolyte. The electrolyte with ionic liquid showed a better ionicconductivity (4.9×10-5S cm-1), and had a smaller polarization on charge(3.6V) and discharge (2.75V). The cycling performance of the lithium-airbattery was also improved.4. Because of poor stability of carbonate based organic solvents inlithium air batteries, we explored dimethyl methyl phosphonate (DMMP)as an organic solvent of electrolyte. The results showed that1.0MLiTFSI-DMMP electrolyte presented high ion conductivity (5.1×10-3Scm-1) and wide electrochemical window (5.5V vs. Li/Li+) at roomtemperature. CV measurements indicate that oxygen reduction inDMMP-based electrolyte is a multistep process. The onset of the oxygenreduction reaction on GC electrode occurred at the potential~2.5VLi/Li+forLiTFSI-DMMP electrolyte and oxidation current peak appeared at~3.2VLi/Li+, lower than the carbonate electrolyte (~3.28VLi/Li+) and TEGDMEbased electrolyte (~3.56VLi/Li+). The CV curves show good reproducibilitywith cycle number increase, only slightly decrease of the peak area isobserved after50cycles, which indicates less accumulation of insolubleORR products on the electrode surface.5. Based on perfluorinated sulfonic, we designed and synthesized anew composite polymer electrolyte LiTFSI-DMMP/PFSA-Li.Electrochemical tests show that the ionic conductivity of the compositeelectrolyte was1.4×10-4S cm-1. The Li ion transference number of the electrolyte membrane is as high as0.48. The lithium-oxygen cells using theLiTFSI-DMMP/PFSA-Li electrolyte showed high rate discharge capacitieswith the first discharge capacity of2471mAh g-1, at1mA cm-1. Noobvious evidence of electrolyte decomposition was observed from theresults of1H,13C and31P NMR experiments.