Brief Introduction The lithium-airbattery (Li-air) is a metal-air electrochemical cell or battery that oxidizesat the anode and reduction of oxygen occurring at the cathode therefore aprocess that may induce a current flow. 1 For a short period of time Li-airbatteries possessed a hazard which was greater than its benefits deeming theproduct not worth pursuing at the time. This is as a result of the both thenegative (lithium metal) and positive (air or oxygen electrodes). Thenon-aqueous lithium-air (or Li-O2) battery is a relatively new battery conceptproposed by Abraham et al. in 1996 2. As he demonstrated the workingprinciple of a non-aqueous Li-air cell When compared to the lithium-ionbattery, the lithium air battery could be described as an open system thatallows oxygen from the atmosphere (or from an oxygen storage tank) anddissolves in the electrolyte which is then reduced at the positive cathode toform lithium peroxide (Li2O2).
Henceforth heavy transition metals are avoidedin the cathode and lighter elements are utilized to reduce battery weightsignificantly e.g. Carbon. According to _____ the energy density of the Li-airbattery ranges from 3.8-11.7 kWh/kg depending on whether the oxygen is includedin the calculation or not .
This is impressive when compared to fossil fuelsincluding gasoline, if realizable, exceeding the improvements seen in theprevious 150 years since Gaston Plante discovered the Lead- Acid battery in1859. The figure above portrays a side-by-side comparison of Li-O2chemistry with other battery technologies. The cell performance of the firstpractical rechargeable batteries has shown a major development within the lastcentury.
As mentioned earlier the product wasn’t worth pursuing, it wasn’tuntil a man named Peter Bruce performed experiments with his groupdemonstrating rechargeability. By performing this action, the technology becameacknowledged and launched a competitive race in the scientific community topublish results as fast as possible. Major companies including IBM made effortsto contribute to the development of the Li-air battery. IBM initiated theBattery 500 project to develop a EV Li-air battery consisting of a drivingrange of 500 miles.
The expansion did not only receive the contribution of increasingattention companies only but global research groups as well. The Agency ofIndustrial Science and Technology (AIST), under the jurisdiction of theministry of International Trade and Industry (MITI) of Japan, initiated abattery research program for electric vehicles in 1971. Lithium air batteriesare now attracting increasing attention and R&D effort as possible powersources for electric vehicles (EVs). Although significant strides have beentaken in the development of Li-air technology, the energy density hasn’tcompletely been able to suffice for portable electric devices and EV’s. OperationIn generallithium ions move between the anode and the cathode sides of the electrolyte. The above figure portrays the function of a lithium battery. This operatesby moving Li+ ions through the electrolyte from the negative electrode to thepositive electrode at the time of discharge and vice versa during charge.
Thetwo electrode materials absorb, store and release Li+ ions at two differentchemical potentials. The difference between these chemical potentials producesan open circuit cell voltage (OCV). Subsequently, ___ is the change in Gibbsfree energy of the reaction z, is the charge number (1 in Li-batteries), and Fis the Faraday constant. The lithium ion battery may be discharged byelectrically connecting the two materials through an external circuit. As aresult of this electrons flow from the negative electrode to the positiveelectrode with an energy corresponding to the voltage difference multiplied bythe elementary charge.
During charge (when an external potential becomesgreater than the standard potential for the discharge reaction) the lithiumplates are onto the anode allowing the O2 at the cathode. Lithium metals aretypically used at the anode. Electrochemical potential forces the lithium metalto send off electrons through oxidation The half reaction isLi ? Li+ + e?At the cathodeduring charge, oxygen will donate electrons to the lithium through reduction.In modeling of the first pore-scale lithium batteries, the micro structure ofthe cathode effects the battery capacity in both non-pore-blocking andpore-blocking regimes. Energy produced can be used to power a laptop oranother electrical device.On the otherhand, Li-O2 batteries possess a different electrode.
As the electrodes grow(shrink) during reduction (oxidation) Voltage dropcould play apart in the performance of the efficiency at the cathode. Theactual mechanism of the Li-O2 battery depends on the choice of the electrolyte Therefore, battery chemistries have beenevaluated and distinguished by electrolytes. More specifically, aprotic andaqueous electrolytes. The aprotic system has received most attention inliterature which is due to its issues with ohmic losses, low energy density,rechargeability observed in other systems.
Therefore, making the aprotic systemthe responsible for the work. As shown above the battery is open to allowoxygen into the battery. The oxygen is then diffused to the positive electrodeand reduced to form Li2O2. In the charging process, the Li2O2 is oxidizedhence, oxygen is released. The Li+ ions are transferred between the two electrodesto maintain a steady balance on the charge and on the negative electrode,lithium is thus plated to keep constant number of Li+ ions in the electrolyte.Challenges Associated with Li-air BatteriesAs the demandfor independence on fossil fuels and clean and secure energy for our futuregrows, it pushes for the development of low or zero emission electric andhybrid electric cars which would be powered by a new generation of electricenergy storage (e.g. batteries and supercapacitors) .
Due to the inherent lowdensity of the lithium ion batteries we look towards a replacement. Thus whenthe heavy cathode material in a lithium-ion battery is replaced by alight-weight porous (usually carbon based) O2-breathing electrode, a lithium-O2or lithium air battery is formed which as mentioned earlier possesses a largerenergy density. For the Li-air battery, difficulties include a much lower practicalenergy density (than the theoretical one). Li-air batteries are categorized byelectrolyte, namely aqueous and non-aqueous. It is estimated that the energydensity for alkaline aqueous electrolyte is 1,300 Wh/kg and 1,400 Wh/kg foracidic aqueous electrolyte. On the other hand, energy density of non-aqueousLi-air batteries is predicted to be 2,790 Wh/kg and battery cells areterminated by air cathode being clogged by lithium oxides, known to beinsoluble in electrolyte. Solutions are being sought through research whichsuggest that a combination of both types of electrolyte could provide a solutionto both limitations which includes adding additives to dissolve lithium oxides.Current energy densities of the Li-air are significantly lower than the valuesmentioned above.
A reason behind theinequality is mainly from the substantial weight percentage of electrolyte percell (~70%) while both lithium and carbon content is only ~11%. . Anotherchallenge facing the product is a much larger polarization resistance duringboth discharge and re-charge and lower cycling rates.
These issues are due tothe inefficiency of the O2-breathing electrode which includes the transport of oxygenthrough the pores and the deposition of insulating products on active sites foroxygen reduction and evolution. To facilitateO2 transport successfully to (or from) the active sites the O2- breathingelectrode must be porous with adequate porosity and minimal tortuosity. Thismust be done in order for the electrode reactions on the interior surface ofthe porous electrode with minimum energy loss. A sufficient amount of electrical conductivity must be present to injectelectrons to the active sites for oxygen reduction.