Basic Research Of The Lithium-air Battery

Crucial environmental issues together with energy resource crisis have combined to make the development of improved lithium batteries a worldwide imperative. Whereas the performance of conventional lithium battery systems are limited because they must include cathode materials that constrain the energy storage capacity of these cells on a per unit volume and mass basis. Although lithium has the highest columbic capacity (3860 mAh g-1),most available cathode materials have specific capacities that are less than 200 mAh g-1.In addition, since lithium diffusion coefficients are in the range of about 10-8 to 10"11 cm2 s-1,the use of conventional cathode materials limits the discharge rates and thereby power output capability of these cells. Thus there is a great incentive to find a way to improve the specific capacity of cathode materials in lithium battery systems.On the other hand, metal/oxygen batteries offer excellent performance because cathode active material is not stored in the battery. Theoretically oxygen as an active material is essentially unlimited from the environment. Lithium oxygen (air) battery has the highest theoretical specific energy of 11140 Wh kg-1 excluding O2 among all the metal/oxygen battery systems. Abraham was the first to report a non-aqueous electrolyte lithium oxygen battery. The cell had an open-circuit potential about 3 V and specific energy density of 250-350 Wh kg-1.Further studies have been concentrated on discharge mechanism and the effect of electrolyte composition, cathode formulation, oxygen reduction catalysts and moisture barrier on the electrochemical properties of the lithium oxygen batteries. In general, the research on lithium-air battery is still in the initial stage and many obstacles must be overcome in order to achieve long operating life and large discharge capacity. In this paper, we focused on the oxygen reduction catalysts, air electrode and ionic liquid electrolyte.1.A series of manganese oxide nanorods were prepared as catalysts for lithium oxygen batteries. The dependence of discharge capacity and cycle performance on various crystal structures and specific surface areas was evaluated and believed to be responsible for the electrochemical properties. We found that the addition of oxygen reduction catalyst can reduce the polarization and improve the electrochemical performance of the cell.Among these manganese oxides,α-MnO2 andγ-MnOOH catalyzed cells exhibit superior initial discharge performance with the discharge capacity of 2300 mAh g-1 and 2600 mAh g-1 of carbon, respectively. Whileβ-MnO2,γ-MnO2 and Mn2O3 catalyzed cells show lower initial discharge capacity. Although excellent first discharge performance was obtained from a y-MnOOH catalyzed cell, no distinct following charge capacity was observed and capacity retention was rather low during the following cycles. Significantly, as toα-MnO2 andβ-MnO2,superior initial discharge capacity and reversible capacity were obtained. y-MnO2 and Mn2O3 catalyzed lithium oxygen cells showed lower discharge capacity than others, and fading occurs significantly after 3 cycles.It is also shown that high surface area catalyst could facilitate oxygen reduction and Li2O2 decomposition. The clew-likeγ-MnO2 catalyzed lithium oxygen cell was found with the highest reversible lithium storage capacity exceeding 2350 mAh g-1 over 5 cycles, which is 4 times greater than y-MnO2 nanorod. The specific capacity associated with the total mass of the cathode can be calculated as 1175 mAh g-1 which promises to be about ten times higher than that of the conventional lithium batteries. Besides, the high surface area catalyst also helps to lowering the activation energy, and thus reducing the voltage required for charging.2.Although most of the researchers mentioned the existence of polarization in air electrode, intensive investigations were hardly reported. In this section, the polarization of an oxygen electrode in rechargeable lithium oxygen batteries was investigated and we found that it is closely related to the fade of discharge capacity and the cease of cycling. Electrochemical impedance spectra (EIS) responses and scanning electron microscopy (SEM) observations indicate that the discharged product could block air pathway, increase electrochemical resistance and lose electrical contact. The results also show that the increment of overpotential makes the cell failure. The main reasons including are listed as follows:the generation of large amount of low electronic conductive Li2O and Li2O2 precipitates; the fully blocked and expanded air electrode causes partial loss of electrical contact between active materials and current collector; the large interface resistance etc. Besides, it need to be defined that although P. G Bruce's group demonstrated that the electrochemical reaction of 2Li++ 2e-+O2 (?) Li2O2 is reversible, the specific cycleable mechanism is far from clear, and the potential reasons still unknown. By limiting the depth of discharge, the operation time of rechargeable lithium oxygen battery could be prolonged and the cell exhibits good capacity retention during 10 cycles. 3.Hydrophobic ionic liquid is a promising waterproof electrolyte for lithium air battery due to its hydrophobicity and negligible vapor pressure. In this section, hydrophobic ionic liquid 1,2-dimethyl-3-propylimidazolium bis(trifluoromethane sulfonyl)imide was successfully prepared and employed in lithium-air batteries to repel moisture. The cell gave specific capacity of about 3600 mAh g-1 in oxygen atmosphere, which is much higher than the conventional organic electrolyte system. However, unfortunately, less than half of the capacity could be obtained under the ambient condition. So, we attempted to synthesize hydrophobic ionic liquid-silica-PVdF-HFP polymer composite electrolyte film as the barrier. After optimizing the electrolyte, the discharge capacity of the cell with composite electrolyte extended to 4080 mAh g-1,which can be calculated as 2040 mAh g-1 associated with the total mass of the cathode. The flat discharge plateau and large discharge capacity indicate that the hydrophobic ionic liquid-silica-PVdF-HFP polymer composite electrolyte membrane can effectively protect lithium from moisture invasion.