Electrolyte Lithium–air battery

1 electrolyte

1.1 aprotic
1.2 aqueous
1.3 mixed aqueous–aprotic
1.4 solid state

electrolyte

efforts in li-air batteries have focused on 4 different chemical designs. designs have distinct advantages , significant technical challenges.

aprotic

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


aqueous

an aqueous li-air battery consists of lithium metal anode, aqueous electrolyte , porous carbon cathode. aqueous electrolyte combines lithium salts dissolved in water. avoids issue of cathode clogging because reaction products water-soluble. aqueous design has higher practical discharge potential aprotic counterpart. however, lithium metal reacts violently water , aqueous design requires solid electrolyte interface between lithium , electrolyte. commonly, lithium-conducting ceramic or glass used, conductivity low (on order of 10 s/cm @ ambient temperatures).

schematic of mixed aqueous-aprotic type li-air battery design


mixed aqueous–aprotic

the aqueous–aprotic or mixed li-air battery design attempts unite advantages of aprotic , aqueous battery designs. common feature of hybrid designs two-part (one part aqueous , 1 part aprotic) electrolyte connected lithium-conducting membrane. anode abuts aprotic side while cathode in contact aqueous side. lithium-conducting ceramic typically employed membrane joining 2 electrolytes.

the use of solid electrolyte (see fig. 3) 1 such alternative approaches allows combination of lithium metal anode aqueous cathode (visco 2004). ceramic solid electrolytes (cses) of nasicon type (e.g., li1−xaxm2−x(po4)3 ∈ [al, sc, y] , m ∈ [ti, ge]) 1 family of li+ conducting materials has been studied. albeit compatible water @ alkaline ph , having large electrochemical window (see figs. 3,4), low li+ ion conductivity near room temperature (< 0.005 s/cm, >85 Ω cm) (imanishi, matsui et al. 2014) makes them unsuitable automotive , stationary energy storage applications demand low cost of power (i.e., operating current densities on 100 ma/cm). further, both ti , ge reduced metallic li, , intermediate layer between ceramic electrode , negative electrode required. in contrast, solid polymer electrolytes (spes) can provide higher conductivity @ expense of faster crossover of water , of other small molecules reactive toward metallic li. among more exotic membranes considered li-o2 batteries single-crystal silicon (lu , amine 2013).

in 2015 researchers announced design used highly porous form of graphene anode, electrolyte of lithium bis(trifluoromethyl) sulfonylimide/dimethoxyethane added water , lithium iodide use mediator . electrolyte produces lithium hydroxide (lioh) @ cathode instead of lithium peroxide (li

2o

2). result offered energy efficiency of 93 percent (voltage gap of .2) , cycled more 2,000 times little impact on output. however, design required pure oxygen function, rather ambient air.

schematic of solid-state type li-air battery design


solid state

a solid-state battery design attractive safety, eliminating chance of ignition rupture. current solid-state li-air batteries use lithium anode, ceramic, glass, or glass-ceramic electrolyte, , porous carbon cathode. anode , cathode typically separated electrolyte polymer-ceramic composites enhance charge transfer @ anode, , electrochemically couple cathode electrolyte. polymer-ceramic composites reduce overall impedance. main drawback of solid-state battery design low conductivity of glass-ceramic electrolytes. ionic conductivity of current lithium fast ion conductors still lower liquid electrolyte alternatives.