Research On Organic Electrolyte Of Rechargeable Lithium-air Cells

Lithium-air battery has received widespread concern in the academic due to its ultra-high theoretical energy density, excellent power characteristics, temperature characteristics. It is considered a potential viable large-capacity electrical storage system. However, its defects in catalyst, discharge mechanism, porous air electrode, electrolyte and other aspects, especially electrolyte, seriously affect its performance and application. Thus, the development of reliable organic electrolyte is of great significance.In this dissertation, lithium nitrate（Li NO₃） in the solvent N, N-dimethylacetamide（DMA） is regarded as a base, by adding thereto solvent dimethyl sulfoxide（DMSO）, tetra（ethylene glycol） dimethyl ether（TEGDME） and salt, lithium hexafluorophosphate（LiPF₆）, lithium perchlorate（LiClO4）, bistrifluoromethylsulfonyl imide（LiTFSI）, we concluded as follow from the results of chemical and physical properties:1. The electrochemical performances of two LiNO₃-DMA based electrolytes LiNO₃-DMA/DMSO and LiNO₃-DMA/TEGDME were investigated. Electrolyte with DMSO added in LiNO₃-DMA is capable of forming a protective solid-electrolyte interphase（SEI） on Li. Effectively enhance the chemical compatibility and reduce the polarization resistance of LiNO₃-DMA so that the charging profile has improved, stably cycles 40 times with capacity limited to 1000 mAhg-carbon-1 at current density 0.2mA/cm². However, electrolyte with TEGDME added lowering cell polarization resistance, but increases the impedance of electrolyte and oxide reaction with lithium. Consequently, it appeared only 12 cycles and discharge capacity fade exceed 20% till 18 cycles. At a current density of 0.1 mA/cm², Since the addition of TEGDME notably reduce the vapor pressure, it improves the cycle performance to some extent, but LiNO₃-DMA/DMSO has higher capacity retention than LiNO₃-DMA/TEGDME. Furthermore, the affect of solvent on the electrolyte viscosity is more significantly than the impact of mixing lithium salt. Analytical techniques such as SEM and XRD identified the discharge products of both LiNO₃-DMA and LiNO₃-DMA/DMSO are Li2O2, but hydrophobic methacrylic in DMSO inhibited formation of by-product LiOH. In addition, during cycling LiNO₃-DMA has generated S-containing compound which is an important cause of discharge termination.2. The electrochemical performances of organic electrolyte of mixing lithium salt, LiPF₆/LiNO₃-DMA、 LiClO4/LiNO₃-DMA、 Li TFSI/Li NO₃-DMA were investigated. With LiPF₆ as an additive to LiNO₃-DMA, the chemical stability of the electrolyte with lithium has significantly improved while LiClO4 and LiTFSI are exacerbated oxide electrolyte. Since the total amount of the lithium salt is 1mol/L, viscosity of the electrolyte compared to LiNO₃-DMA has no significant change. The conductivity of mixed electrolyte is improved so as to polarization resistance and LiPF₆/LiNO₃-DMA has the most significant upgrade. The full cycle performance of LiPF₆/Li NO₃-DMA is superior to LiNO₃-DMA; It is shown to cycle 42 times with the specific capacity of 1000 mAh g-carbon-1, at a current density of 0.2mA/cm². However, both LiNO₃-DMA and LiPF₆/LiNO₃-DMA occurred failure when decrease current density to 0.1mA/cm², while the addition of LiClO4 and LiTFSI improved interfacial stability with lower discharge capacity. XRD analysis of LiPF₆/LiNO₃-DMA showed that the discharge product is Li2O2, but it formed more LiOH due to LiPF₆’s easily hydrolyzation.3. Based on the optimization effect of DMSO and LiPF₆, thus LiNO₃/LiPF₆-DMA/ DMSO has been investigated compared with singal lithium salt in solvent DMA or DMSO. Results suggest that LiNO₃/LiPF₆-DMA/DMSO has a higher viscosity and electrical conductivity than LiNO₃-DMA and other single lithium salt in solvent. Its impedance of electrolyte is between LiNO₃-DMSO and LiPF₆-DMA. LiPF₆-DMSO has the best cycle performance but with high polarization resistance and a low solubility of LiPF₆ in DMSO. Li NO₃/LiPF₆-DMA/DMSO has a good optimization effect on LiNO₃-DMA when capacity limited to 1000 mAhg-carbon-1 at current density 0.2mA/cm². It is proved that not only does it has the characteristic of high discharge capacity of LiNO₃-DMA and high capacity retention ratio of LiPF₆-DMSO, but also effectively alleviated the problem of degradation. XRD analysis of LiNO₃/LiPF₆-DMA/DMSO showed that the discharge product is Li2O2, but it has no significant inhibition of the generation of LiOH.