High Specific Energy Li-ion/air Battery Research

Portable electronic equipment and devices have been developing at a rapid pace, and this progress demands ever-increasing energy and power density in power sources. Due to their energy density, which is higher than that of previous power sources such as Ni-MH batteries, lithium-ion batteries are being considered in the hope of being able to meet these demands. However, the maximum energy density of current lithium-ion batteries is limited owing to electrode materials having intercalation chemistry and slow lithium ion diffusion coefficient, thus such batteries are not satisfactory for the practical application of electric vehicles. Metal-air batteries have attracted much attention in recent years, especially for lithium air battery, which has rather high specific energy and a potential for a variety of applications. The theoretical capacity of lithium air battery can reach11140Wh kg-1if excluding O2, which is ten times greater than lithium-ion batteries and is comparable with gasoline.The mechanism of lithium air batteries are base on the reaction of oxygen with lithium in non-aqueous electrolyte, and meanwhile, lithium oxides are deposited on air cathode. In general, the research on lithium air batteries is still in its initial stage and there are many obstacles must be solved in order to achieve long operating life and large discharge capacity. In this paper, our research focuses on the oxygen reduction catalysts, air cathode and the decomposition of organic electrolyte.1. Perovskite oxide La0.8Sr0.2MnO3as ORR (oxygen reduction reaction) catalyst is synthesized by soft chemistry method, compared with traditional solid state reaction method, the catalyst synthesized by soft chemistry method with uniform particle size about100nm, and there is no agglomeration. Besides it has a large specific surface area of32m2g-1, as to the catalyst which synthesized by traditional solid state reaction method, it’s only1m2g-1. Electrochemistry test shows that it exhibits higher electrocatalysis activity, which can increase discharge plateau, decrease polarization and increase specific capacity. In all, the prepared La0.8Sr0.2MnO3catalyst can enhance the cell performance.Large specific surface area catalyst shows high electrocatalysis activity. Herein, a series of manganese oxide nanorods are prepared as ORR catalyst for lithium air batteries. The dependence of discharge capacity and cycle performance on various crystal structures and specific surface areas are evaluated and believed to be responsible for the electrochemical properties. From the experimental results, we hypothesize the catalytic model that the tunnel structure is the catalytic activity site and if this structure be choked by lithium oxides will lead to electrochemical activity decrease.Among manganese oxides, α-MnO2catalyzed batteries exhibit superior cell performance with higher discharge capacity of2300mAh g-1and reversible capacity when cycling, while β-MnO2, γ-MnO2and y-MnOOH catalyzed batteries show a lower capacity and fading rapidly when cycling. Although their specific surface area is similar, that is the nature of the catalyst play the key role. It has proved that high surface area catalyst could facilitate oxygen reduction reaction. The clew-like γ-MnO2catalyzed lithium air batteries are observed with highest reversible capacity about2350mAh g-1over5cycles, which is5times of γ-MnO2nanorod. Besides, the high surface area catalyst also helps to decrease the voltage for charging.Base on experimental results, we hypothesize that the tunnel structure is the catalytic activity site, the electrocatalytic activity depends on the mount of tunnel structure on the MnOx surface, this tunnel structure can accommodate both Li+and O2-, when this structure chocked by discharge product-Li2O2may lead to catalytic activity decreasing, and the (1×1) tunnel structure is more easily be choked than (2x2), while most of activity sites are covered, the catalyst fails and results in cell performance drop sharply.2. Lithium air batteries must operate in an ambient environment because oxygen is obtained from the atmosphere. However, lithium metal is easily corroded by moisture, it may leads to low capacity and eventually cell failure. In order to analysis hydrophobic properties of air cathode, electrochemical impedance spectra (EIS) and postmortem analyses of lithium anodes are incorporated. When expose in ambient environment, we find that lithium air batteries with single layer air cathode exhibit inferior performance, such as low discharge capacity, largely increased electrochemical resistance, and lithium anode also be corrode, even though, there are precipitations deposited onto the lithium surface. Optimized air cathode using polyaniline (PAN) as water proof can effectively protect lithium from moisture invasion, the optimized air cathode exhibits more stable interface impedance and much better cell performance than the unprotected air cathode and also shows a higher capacity and rate capability, after full discharge anode lithium still in good condition. Highly conductive PANI membranes is synthesized by a proton doping method, it promotes lithium ion transport into the electrode and blocks the moisture entrance, which can protect the lithium anode from erosion. In ambient environment, the relative humidity (RH>20%), the optimized air cathode provides stable interface, and deliver a much higher specific capacity3241mAh g-1, there is little scathe on the lithium surface, the batteries also have an excellent rate capability.