Optimization Of Non-Aqueous Electrolyte And Its Performance Research In Lithium Air Battery

Since the energy shortage and environment pollution becoming two of the biggest issues, lithium air battery has attracted much attention by researchers because of its high specific energy density which is close to gasoline. For the highly researched non-aqueous electrolyte based lithium air battery now, the reaction mechanism is that Li anode reacts with the oxygen in air to generate/decompose Li₂O or Li₂O₂ during discharge/charge process. The development of lithium air battery has been hindered by the problems of the cathode pore blockage, cathode passivation and decomposition, electrolyte decomposition and lithium anode corrosion. The problems of electrolyte and cathode materials are the significant issues in research nowadays. One of the questions which bothered the researchers is how to improve the cycle performance of lithium air battery and the electrolyte’s stability at the same time.The main experiments for this paper focus on the research of electrolyte solvents, lithium salts and additives aiming to find an excellent non-aqueous electrolyte with efficient and stable cycle performance by researching the influence of solvents, lithium salts and additives on the electrolyte physic properties and battery cycle performance. A series of non-aqueous electrolytes are investigated by exploring the species of lithium salts, organic solvents and additives. We systematically investigate the impacts of organic solvents, lithium salts and additives on electrolytes and lithium air battery by analyzing the properties of compatibility to lithium anode, viscosity, conductivity, dissolved oxygen, electrochemical stability, battery impedance and cycle performance. At the same time, the reaction products after charge-discharge processes and the reaction mechanism are examined. At last, conclusions as follows:（1） While using TMS, TEGDME, DMSO, DMA as organic solvents with LiTFSI as lithium salt and LiTFSI, LiN0₃, LiOTf, LiClO₄ as lithium salts with TMS as organic solvent to prepare electrolytes, the lithium air battery with LiTFSI-TMS electrolyte shows the best cycle performance which can highly and stably cycled 120 circles at the current density of 0.1mA/cm2. Meanwhile, this electrolyte has the widest electrochemical window of 5.5V and the highest dissolved oxygen of 7.44mg/L. These excellent properties make the lithium air battery with LiTFSI-TMS electrolyte had a high research values. However, the LiTFSI-TMS electrolyte has the high viscosity and low conductivity which influence the transportation of Li+ ion and oxygen in electrolyte. What’s more, the compatibility between lithium anode and LiTFSI-TMS electrolyte is not ideal due to their mutual reaction which undoubtedly influences the cycle performance of lithium air battery. So, It is necessary to optimize the LiTFSI-TMS electrolyte in order to decrease the viscosity and increase the conductivity also improve the compatibility of lithium to the electrolyte to obtain highly cycle performance.（2） LiNO₃ was proved to be a useful electrolyte additive which can generate a stable SEI film to protect the lithium anode and effectively decrease the by-reactions between the lithium anode and electrolyte. Here we also discover that the addition of LiNO₃ has no influence to the charge-discharge mechanism of lithium air battery while being added into LiTFSI-TMS electrolyte as a film-forming additive. Although the addition of LiNO₃ into LiTFSI-TMS electrolyte has disadvantages such as a higher electrolyte viscosity, a higher electrolyte impedance and a lower electrolyte oxidation potential, the lithium air battery with LiTFSI-TMS-LiNO₃ electrolyte shows a low charge-discharge over potential during the first cycle and a highly stable cycle performance. Meanwhile, LiNO₃ can react with lithium anode and generate a more stable SEI film at the surface of lithium anode thus effectively improving the compatibility between lithium anode and electrolyte.（3） FTBA had been proved to be an effective oxygen carrier additive which can improve the capacity and cycle performance of lithium air battery when it was added into the PC/EC electrolyte to enhance the dissolved oxygen in electrolyte. Here, the FTBA is added into LiTFSI-TMS electrolyte with a ratio of 0.5wt%. It is discovered that the addition of FTBA could greatly improve the dissolved oxygen and conductivity of electrolyte, while it will not change the charge-discharge reaction mechanism of lithium air battery. At the same time, the viscosity of electrolyte, the over-potential of first cycle and the charge transfer resistance are all decreased with the addition of FTBA. However, the FTBA containing electrolyte based lithium air battery shows a lower cycle performance which can be attributed to a poor compatibility between the lithium anode and electrolyte, although it can promote the reaction kinetics of lithium air battery.（4） We investigate the physic and electrochemistry properties of the LiTFSI-TMS electrolyte by the co-addition of 0.5wt% FTBA as an oxygen carrier additive and 0.4M LiNO₃ as a film forming additive. It is discovered that the compatibility between electrolyte and lithium anode, electrochemistry stability, conductivity and dissolved oxygen are well improved based on the FTBA’s dissolved oxygen mechanism and LiNO₃’s film-forming mechanism. At the same time, the lithium air battery with LiTFSI-TMS-FTBA-LiNO₃ electrolyte has been successfully cycled for 150 stable circles without any change of the charge-discharge reaction mechanism and suppressed the generation of by-products during charge process. In summary, the cooperation of FTBA and LiNO₃ can obviously improve both the electrolyte properties and lithium air battery’s cycle performance.