Aprotic Lithium–air battery

schematic of aprotic type li-air battery design

the non-aqueous li-air batteries demonstrated first (abraham , jiang 1996). use same mixed ethylene carbonate+ propylene carbonate solvents lipf6 or li bis-sulfonimide salts conventional lithium-ion batteries, however, gelled rather liquid electrolyte. (imanishi, matsui, et al. 2014) voltage difference upon constant current charge , discharge between 1.3 , 1.8 v (with ocp of ca. 4.2 v) @ such ridiculously low currents 0.01-0.5 ma/cm² , 50-500 ma/g of c on positive electrode (see fig. 2 example), (balaish, kraytsberg et al. 2014, mccloskey, burke, et al. 2015, liu, xu, et al. 2016). however, carbonate solvents evaporate , oxidized due high overvoltage upon charge, (lu , amine 2013) , other solvents, such end-capped glymes, dmso, dimethylacetamide, , ionic liquids, have been considered. (balaish, kraytsberg et al. 2014, imanishi, matsui et al. 2014) carbon cathode gets oxidized above +3.5 v v li during charge forming li2co3, leads irreversible capacity loss (imanishi, matsui et al. 2014).

most effort involved aprotic materials, consist of lithium metal anode, liquid organic electrolyte , porous carbon cathode. electrolyte can made of organic liquid able solvate lithium salts such lipf

6, liasf

6, lin(so

2cf

3)

2, , liso

3cf

3), typically consisted of carbonates, ethers , esters. carbon cathode made of high-surface-area carbon material nanostructured metal oxide catalyst (commonly mno

2 or mn

3o

4). major advantage spontaneous formation of barrier between anode , electrolyte (analogous barrier formed between electrolyte , carbon-lithium anodes in conventional li-ion batteries) protects lithium metal further reaction electrolyte. although rechargeable, li

2o

2 produced @ cathode insoluble in organic electrolyte, leading buildup along cathode/electrolyte interface. makes cathodes in aprotic batteries prone clogging , volume expansion progressively reduces conductivity , degrades battery performance. issue organic electrolytes flammable , can ignite if cell damaged.

in 2012, researchers announced dimethyl sulfoxide electrolyte , gold nanoparticle cathode achieved 100 charge cycles 5% capacity loss.

although studies agree li

2o

2 final discharge product of non-aqueous li-o2 batteries, there considerable body of evidence formation not proceed direct 2-electron electroreduction peroxide o2−

2 (which common pathway o2 reduction in water on carbon) rather via one–electron reduction superoxide o−

2, followed disproportionation:

2lio

2= li

2o

2+o

2 (1).

superoxide (o−

2) has been traditionally considered dangerous intermediate in aprotic oxygen batteries due high nucleophilicity, basicity , redox potential (balaish, kraytsberg et al. 2014, mccloskey, burke et al. 2015). however, recent reports argonne (zhai, lau et al. 2015, lu, lee et al. 2016) suggest superoxide (lio2) not intermediate during discharge peroxide (li

2o

2) , that lio2 can used final discharge product, potentially improved cycle life albeit lower specific energy (a little heavier battery weight). indeed, shown under conditions, superoxide can stable on scale of 20-70 h @ room temperature (zhai, lau et al. 2015). although irreversible capacity loss upon disproportionation of lio2 in charged battery not addressed in work.

pt/c seems best electrocatalyst o2 evolution , au/c o2 reduction when li

2o

2 product (lu, xu et al. 2010). nevertheless, “the performance of rechargeable lithium-air batteries non-aqueous electrolytes limited reactions on oxygen electrode, o2 evolution… conventional porous carbon air electrodes unable provide mah/g , mah/cm capacities , discharge rates @ magnitudes required high energy density batteries ev applications.” (lu, xu, et al. 2010) capacity (in mah/cm) , cycle life of non-aqueous li-o2 batteries limited deposition of insoluble , poorly electronically conducting liox phases upon discharge (balaish, kraytsberg et al. 2014). (li

3o

4 predicted have better li+ conductivity lio2 , li

2o

2 phases) (shi, xu et al. 2015). makes practical specific energy of li-o2 batteries smaller reagent-level calculation predicts. seems these parameters have reached limits now, , further improvement can expected alternative methods.

schematic of aqueous type li-air battery design