o2 battery has 
 been widely investigated because of its extremely high theoretical 
 energy density amounting to 3500 wh / kg for the overall battery reaction . in the past 
 10 years , significant progress has been realized in understanding 
 the complex chemistry that governs the functioning of this battery 
 system .
this has resulted in several strategies for improving the performance 
 of li  o2 batteries .
one deals with the reactivity 
 of the porous carbon - based gas diffusion electrode with the electrolyte 
 and the li2o2 discharge product .
this was mitigated 
 by the use of non - carbon porous electrodes made of gold , titanium carbide , metallic ti4o7 , to name a few , or by the alternative strategy of coating the porous 
 carbon substrate with a non - carbon - based material .
another important step forward has been establishing the 
 relationship between the electrolyte solvent donor number ( dn ) and 
 the morphology of the peroxide that forms .
the high solubility of the intermediate lio2 product 
 in high - dn electrolytes leads to a solution - mediated li2o2 growth mechanism , resulting in toroids of li2o2 that are responsible for large discharge capacities . 

in contrast , low - dn electrolytes lead to a surface growth mechanism 
 of li2o2 , resulting in thin li2o2 films that restrict the capacity and reversibility .
johnson et al . have pointed out that for 1,2-dimethoxyethane 
 ( dme ) , having an intermediate dn , both pathways for o2 reduction 
 occur simultaneously at a high voltage .
o2 battery 
 is mainly governed by the generation and decomposition mechanism of 
 li2o2 , which itself is a poor ionic and electronic 
 conductor .
in addition , the oxygen species 
 involved in the oxygen reduction reaction ( orr ) and in the inverse 
 oxygen evolution reaction ( oer ) are highly reactive with the organic 
 environment and traces of moisture , leading to the formation of parasitic 
 products like li2co3 , lioh , and co2 having a negative impact on li 
recent studies have shown that trace amounts of h2o 
 can enhance the formation of li2o2 and thereby 
 improve the performance of li  o2 batteries .
the water in the electrolytes catalyzes the reaction at the cathode , 
 typically through the presence of ruthenium and manganese nanoparticles 
 on carbon black , to form li2o2 and lioh during 
 discharge .
the formation of lioh occurs 
 via the reaction of li2o2 with h2o , and decomposition appears possible 
 below 3.2 v with ruthenium - based catalysts , making lioh an interesting cyclic discharge product for li 
highly cyclic crystalline lioh formation was achieved 
 by adding lithium iodide ( lii ) to a dme / li electrolyte 
 in combination with a graphene oxide electrode , in the absence and 
 presence of h2o .
the soluble 
 lii redox mediator was shown to reduce the overpotential of the charge 
 process , suggesting an iodine - mediated decomposition mechanism .
if this decomposition mechanism results in oxygen 
 evolution , it would make a new reversible mechanism for the li 
o2 battery , which is currently under debate as it has been proposed 
 to be thermodynamically unfavorable and 
 it has been suggested that i is oxidized rather 
 than lioh .
burke et al . have recently reported that the lioh crystallite was formed 
 by a four - electron reduction process with the addition of lii and 
 h2o in the dme electrolyte upon discharge ; however , the 
 decomposition of the lioh crystallite was primarily attributed to 
 iodo  oxygen electrochemistry rather than reversible oxygen 
 evolution . evidently , the performance of aprotic li 
o2 batteries 
 is directly determined by the reversible formation mechanism of the 
 discharge products , being either li2o2 or lioh . 

therefore , studying the nature and evolution of the discharge products , 
 preferably under realistic operando conditions , is 
 paramount for the future design of mechanisms and materials that aim 
 to improve performance . in a previous work , operando x - ray diffraction ( xrd )
was used 
 to study the decomposition of li2o2 in a tegdme 
 electrolyte , showing that the decomposition 
 mechanism via a substoichiometric li2xo2 proceeds during charging , in agreement with density 
 functional theory ( dft ) predictions , and 
 that thinner platelet crystallites decompose preferentially .
the decomposition 
 of individual li2o2 grains , by operando nanobeam synchrotron xrd , showed a slow concurrent li2o2 decomposition via the more reactive ( 001 ) facets .
more operando xrd studies have 
 explored the time - dependent formation and decomposition of li2o2 crystallites by comparing the changes in the 
 li2o2 peak area and the full width at half - maximum 
 ( fwhm ) , revealing the character of the li2o2 crystallites during ( dis)charge . however , no 
 detailed insight exists for the formation of the li2o2 crystallites during discharge .
moreover , little is known 
 about the operando formation and decomposition of 
 lioh presently under debate and how the presence of the lii 
 redox mediator and water affects the structure of the formed species . in this work ,
operando xrd is used to reveal the 
 detailed structural evolution during a complete ( dis)charge cycle 
 of li2o2 and lioh in dme and dme / lii electrolytes , 
 respectively .
detailed rietveld refinement of crystalline li2o2 and lioh formation and decomposition yields a model 
 for the growth and decomposition process , providing novel insights 
 into these relevant battery systems .
the cathodes were prepared by coating 
 a slurry of activated carbon ( kuraray chemicals ) and a lithiated nafion 
 binder on carbon paper ( spectracarb ) .
the activated carbon was mixed 
 with a nafion binder [ 5% in a mixture of lower aliphatic alcohols 
 and water ( aldrich ) ] with a mass ratio of 60:40 in a milling 
 bowl , and subsequently , an amount of n - methyl-2-pyrrolidone 
 ( nmp ) ( sigma - aldrich , anhydrous , 99.5% ) was added to adjust the viscosity 
 of the slurry .
the slurry 
 was coated on a sheet of carbon paper , and the coated activated carbon 
 sheets were dried at 100 c for 24 h in a vacuum oven to remove 
 surface - adsorbed water , after which 12.7 mm disks were punched 
 out .
the final carbon loading on the carbon paper was determined to 
 be 3.04.0 mg .
two different electrolyte 
 solutions were 
 used , each consisting of a dme ( sigma - aldrich , > 99.5% ) solvent 
 that 
 was further dried for several days over freshly activated molecular 
 sieves ( type 4  ) ( sigma - aldrich ) and lithium bis(trifluoromethanesulfonyl)imide 
 salt ( litfsi , 99.95% , aldrich ) , dried in a vacuum oven at 80 c 
 for 24 h. one electrolyte consisted of a solution of 0.5 m litfsi 
 dissolved in dme , while the other consisted of a solution of 0.05 
 m lii and 0.5 m litfsi dissolved in dme .
these electrolytes will be 
 termed the dme and dme / lii electrolytes , respectively .
all the electrolyte 
 preparations were performed in an argon - filled glovebox ( h2o and o2 content of < 1 ppm ) . on the basis of the liquid 
 chromatogram test ,
there is still a large amount of water ( 4000 
 ppm ) in the electrolyte during the battery test . a li 
o2 battery designed and 
 constructed in house was used for the operando x - ray 
 diffraction measurements as described in 
 detail elsewhere . the li 
o2 battery , 
 comprising the cathode , a glass microfiber separator ( whatman ) soaked 
 with the electrolyte , and a lithium metal anode , was assembled in 
 the operando xrd battery in an argon - filled glovebox . 

the battery was subsequently connected to o2 ( linde , 99.9999% ) 
 under a pressure of 0.5 bar where it was allowed to equilibrate for 
 3 h before it was tested .
xrd measurements 
 were taken using a panalytical xpert pro pw3040/60 diffractometer 
 with cu k radiation operating at 45 kv and 40 ma in a 2 
 range of 3165. scans ( each 30 min in duration ) 
 were recorded for the batteries during a complete ( dis)charge cycle 
 with a current density of 0.3 ma / cm .
refinement of the 
 diffraction data was performed using the rietveld method as implemented 
 in the fullprof program .
to more accurately fit the zero position 
 ( effectively positioned at a different height in the cathode ) of the 
 li2o2 diffraction pattern , peaks arising from 
 the current collector as well as carbon paper were excluded from the 
 fits .
electrodes were rinsed 
 with dry tetrahydrofuran ( thf ) prior to analysis , and samples were 
 prepared in an argon - filled glovebox , using a stainless steel holder 
 as the substrate and double - sided carbon tape as the contact point 
 between the electrode and holder .
samples were transferred into the 
 scanning electron microscope ( jeol jsm-6010la ) under anaerobic conditions , 
 and images were taken using an accelerating voltage of 5 kv .
figure 1a depicts 
 the ( dis)charge curves obtained for the dme and dme / lii electrolytes 
 at a current density of 0.3 ma / cm . for both electrolytes , 

a typical discharge plateau is observed around 2.7 v. the gradual 
 decrease , resulting from an increasing overpotential , most likely 
 reflects the increasing thickness of the insulating discharge products 
 increasing the electrode s resistance . for the dme and dme / lii 
 electrolytes , capacities of 2000 and 4000
mah / g , respectively , 
 can be obtained when discharging to the cutoff voltage of 2.2 v versus 
 li / li . during charging ( oer ) ,
the voltage profile steadily 
 increases toward a voltage plateau at 4.4 v for the dme electrolyte , 
 whereas a continuously increasing voltage is observed for the dme / lii 
 electrolyte between 3.5 and 4.5 v , which has been attributed to the 
 electrochemical oxidation of lii .
( a ) galvanostatic 
 ( dis)charge profiles for the operando li  o2 batteries with dme and dme / lii electrolytes 
 in a 2.24.5 v voltage window vs
( b ) xrd patterns of a pristine cathode 
 and the cathodes at the end of discharge , with a 2.2 v cutoff voltage , 
 in the dme and dme / lii electrolytes , respectively .
( c and d ) sem images 
 for the cathodes measured after discharge in dme and dme / lii electrolytes , 
 respectively , at a current density of 0.3 ma / cm .
ex situ xrd measurements were 
 taken to identify the discharge products 
 for the two different electrolytes .
figure 1b presents the xrd patterns of the cathodes 
 discharged to a 2.2 v cutoff voltage at a current density of 0.3 ma / cm in the dme and dme / lii electrolytes .
compared to the xrd 
 pattern of the pristine electrode , new diffraction peaks appear at 
 around 33 , 35 , 41 , 49 , and 59 for the 
 cathode discharged in the dme electrolyte , which can be attributed 
 to the crystalline li2o2 phase . in the cathode 
 discharged in the dme / lii electrolyte , the diffraction peaks at 32.5 , 
 35.6 , 49 , 51.5 , 56 , and 62 can be indexed 
 to crystalline lioh , the formation of which is attributed to the presence 
 of lii and h2o in the electrolyte .
the morphology 
 of the discharge products in the dme and dme / lii 
 electrolytes is shown in the sem images in panels c and d of figure 1 .
figure 1c shows the two types of primary 
 aggregates observed for li2o2 in the dme electrolyte 
 discharged down to 2.2 v. a fraction of the li2o2 particles aggregate with several parallel plates , resulting in a 
 loosely stacked layer - by - layer toroid - like structure , and others stack 
 more tightly to assemble into toroids . in addition , many individual 
 li2o2 platelets are observed , distributed throughout 
 the remaining space on the carbon cathode surface .
the diameter of 
 the platelet layers , stacked in toroids , is 2.0 m , 
 and the thickness is between 0.5 and 0.8 m .
prior reports have 
 described li2o2 toroids as consisting of stacked 
 platelets with a range of shapes and sizes in ether electrolytes .
experimental studies performed at different current densities in 
 a dme electrolyte by aetukuri et al . suggested 
 that this was due to varying levels of water contamination in the 
 cells and different discharge currents .
described a particle growth mechanism in which 
 nucleation progresses via a ring - shaped primary structure , rather 
 than via a linear or hemispherical primary structure . in our work , 

the trace water left in the dme electrolyte and in the o2 supplemental system may be responsible for the formation of toroidal 
 li2o2 .
recently , a number of studies have explored 
 the formation of toroidal li2o2 ; however , there appears to be no clear consensus about the crystallite 
 growth and aggregation mechanism .
figure 1d shows 
 the discharge product morphology of the cathode discharged in the 
 dme / lii electrolyte representing thick lioh plates .
the length of 
 the lioh plates reaches up to tens of micrometers , and the thickness 
 is in the range of 100200 nm .
these lioh plates , forming on 
 the activated carbon surface , are distinctly different from the glassy 
 film lioh morphology on the carbon electrode 
 during the orr process and from the  flowerlike  agglomerated 
 lioh particles formed on the reduced 
 graphene oxide cathode in li  o2 batteries .
operando xrd patterns were collected for the dme electrolyte 
 during a complete ( dis)charge cycle at a current density of 0.3 ma / cm . the two - dimensional contour xrd plot in the 2 regions 
 of 32.433.4 , 34.535.5 , and 58.259.2 
 in figure 2 shows the 
 evolution of the li2o2 { 100 } , { 101 } , and { 110 } 
 reflections demonstrating the gradual formation and decomposition 
 of li2o2 .
the { 101 } reflection is significantly 
 broader than the { 100 } reflection , indicating anisotropic broadening 
 of the li2o2 reflections in the { 00l } crystal plane direction .
although it is not possible to distinguish 
 between size and strain broadening , given the limited range of the 
 2 data , we assume the broadening to be a consequence of size 
 broadening because electrochemically formed li2o2 is known to form toroidal aggregates consisting of stacked li2o2 crystalline platelets , 
 with the plate normal in the { 00l } direction .
two - dimensional 
 contour plots ( left ) of the operando xrd patterns 
 showing the 2 region between 32.4 and 
 33.4 , 34.5 and 35.5 , and 58.2 and 59.2 
 during a complete ( dis)charge cycle demonstrating li2o2 formation ( by the { 100 } , { 101 } , and { 110 } li2o2 indexed reflections ) and decomposition in the li 
o2 battery was tested by using the dme - based electrolyte at 
 a current density of 0.3 ma / cm .
the integrated and normalized areas as a function of discharge 
 time for the { 100 } and { 101 } reflections of li2o2 ( figure s2 ) show a linear increase in 
 the peak area during discharge . however , during charging , it decreases 
 via two different stages , as reported previously . during the initial stage of charging ,
the integrated area 
 decays very slowly , indicating preferential oxidation of the surface 
 species ( lio2 * and o2 * ) and/or 
 an amorphous lithium component at the lower voltages ( 2.83.4 
 v ) , the latter possibly comprising an 
 amorphous li2o2 species , or both amorphous li2o2 and side products arising from electrolyte degradation 
 such as formate , that can be oxidized at relatively low potentials 
 without a catalyst . in the second stage , 

the integrated li2o2 peak areas decrease linearly , 
 indicating the oxidation of the crystalline fraction of li2o2 . assuming that the observed anisotropic broadening 
 of the xrd reflections 
 is solely due to the crystallite size , the xrd patterns were refined 
 with the fullprof program using a thompson  cox  hastings 
 pseudo - voigt profile function based on spherical harmonics ( sph ) to 
 fit the anisotropic size broadening .
the 
 broadening of each reflection translates in an apparent crystallite 
 size in the direction perpendicular to the planes specified by the 
 miller indices , shown for a number of reflections in figure s3 .
the large apparent size in the { 100 } direction , 
 coincident with the a lattice parameter , compared 
 to the { 004 } reflection , coincident with the c lattice 
 parameter , is in agreement with the reported platelet - shaped li2o2 crystallites .
when 
 the crystallites are assumed to be platelets , cylinders with a large d / t ( diameter / thickness ) aspect ratio , 
 the average apparent size of the li2o2 plates 
 in the { 00l } direction represents the thickness of 
 the plates , and the average apparent size in the { hk0 } direction , multiplied by 3/8 , represents the average diameter 
 of the cylinders .
the evolution of the 
 aspect ratio during the complete ( dis)charge cycle is shown in figure 3b .
finally , by taking 
 into account the size broadening of all observed reflections , we can 
 determine a detailed crystallite shape , images of which are shown 
 in figure 3a at different 
 stages during the complete ( dis)charge cycle . (
( b ) aspect ratio of the li2o2 crystallite shape 
 ( diameter in the a 
( c ) li2o2 lattice parameters , resulting from rietveld refinement during the 
 full galvanostatic ( dis)charge cycle in the dme electrolyte . in each 
 panel
, the blue line represents the voltage curve during the ( dis)charge 
 cycle at a current density of 0.3 ma / cm . during the discharge process ,
the d / t aspect ratio , shown in figure 3b , decreases drastically , indicating that 
 on average 
 the platelets become more isotropic in shape . during charging ,
the 
 aspect ratio increases slightly , whereas at the end of charging , it 
 decreases again .
the evolution of the average crystallite shape , shown 
 in figure 3a , demonstrates 
 that the decrease in the d / t aspect 
 ratio during discharge is caused by both an increase in average crystallite 
 thickness and a decrease in average diameter .
it should be realized 
 that the crystallite dimensions , derived from the broadening of the 
 xrd reflections , represent average values .
therefore , the observed 
 decrease in platelet width during discharge most likely does not indicate 
 that the crystallite diameter decreases during li2o2 formation .
it suggests that during discharge the li2o2 platelets that form progressively are more isotropically 
 shaped , having a smaller diameter , thereby decreasing the average 
 diameter .
similarly , the increasing platelet thickness and diameter 
 during charging , during decomposition of the li2o2 crystallites , imply that thin and small platelets are decomposed 
 first during charging , consistent with previous work .
the evolution of the li2o2 lattice 
 parameters 
 during the full ( dis)charge cycle is shown in figure 3c .
the a lattice parameter 
 is practically constant during li2o2 formation 
 and decomposition , whereas the c lattice parameter 
 is 7.659  0.020  at the beginning of discharge and then 
 decreases to 7.634  0.002  near the end of the discharge 
 process . because li2o2 platelets with a thickness 
 on the order of nanometers show an expansion in the average c lattice parameter due to surface relaxation , the decreasing c lattice parameter during discharge 
 is consistent with the progressive formation of thicker , more isotropic 
 platelets , consistent with the evolution of the crystallite shape 
 in panels a and b of figure 3 . during charging ,
the c lattice parameter 
 is initially constant at 7.643  0.002  before it increases 
 slightly to 7.651  0.063  toward the end of charging . 

as suggested previously , the increase in c lattice 
 parameters during charging may be caused by the decomposition of li2o2 taking place via substoichiometric li2xo2 intermediates .
previously , detailed rietveld refinement of operando xrd data performed during charging revealed the decomposition mechanism . at present , the formation of li2o2 during discharge is additionally studied by operando xrd . moreover ,
the better signal - to - noise ratio and better time 
 resolution obtained allow a more detailed refinement of the anisotropic 
 broadening of the xrd reflections during a full discharge 
charge 
 cycle , giving insight into the evolution of the crystallite shape . 
 on the basis of the evolution of the apparent crystallite shape of 
 li2o2 and the aspect ratio d / t
, the li2o2 crystallites 
 follow a distinct formation and decomposition mechanism . as is well - known , 
 the toroidal li2o2 particles consist of arrays 
 of plateletlike li2o2 crystallites . during discharge ,
the thinner li2o2 crystallites , having a more anisotropic shape , form 
 at the initial stages , whereas progressive discharging results in 
 more isotropically shaped li2o2 crystallites . 
 concurrently , several li2o2 crystallites aggregate 
 to form toroidal li2o2 particles . in the meantime ,
small li2o2 crystallites with a less anisotropic shape are also formed . during 
 charging ,
the relatively thin li2o2 anisotropic 
 crystallites are oxidized first , followed by the thicker and more 
 isotropic li2o2 crystallites .
figure 4 presents the two - dimensional contour plots 
 of the operando xrd patterns in the 2 regions 
 of 31.533.5 , 34.836.8 , 50.552.5 , 
 54.956.9 , and 61.263.2 , collected for 
 the dme / lii electrolyte during a complete ( dis)charge cycle at a current 
 density of 0.3 ma / cm .
the evolution of the { 101 } , { 110 } , 
 { 200 } , { 112 } , and { 211 } reflections demonstrates the formation and 
 decomposition of lioh crystallites during discharge and charge , respectively , 
 also supported by the linear increase ( discharge ) and decrease ( charge ) 
 in the integrated area of the lioh { 101 } , { 110 } , and { 200 } reflections 
 shown in figure s5 .
two - dimensional contour 
 plots ( left ) of the operando xrd patterns showing 
 the 2 regions between 31.5 and 
 33.5 , 34.8 and 36.8 , 50.5 and 52.5 , 
 54.9 and 56.9 , and 61.2 and 63.2 , during a 
 complete ( dis)charge cycle demonstrating lioh formation ( { 101 } , { 110 } , 
 { 200 } , { 112 } , and { 211 } reflections ) and decomposition .
o2 battery was tested by 
 using the dme / lii electrolyte at a current density of 0.3 ma / cm .
the xrd reflections from the crystalline 
 lioh show considerable 
 anisotropic broadening as clearly observed in figure 4 , where the { 101 } reflection appears to be 
 much broader than the { 110 } and { 200 } reflections .
analysis of the 
 anisotropic broadening using fullprof as described in the previous 
 section results in platelet - shaped lioh crystallites with the platelet 
 normal aligned with the c lattice parameter , very 
 similar to the li2o2 crystallites .
sem images 
 of lioh ( figure 1d ) 
 show large grains , suggesting layered stacks of thin crystalline lioh 
 plates .
the evolution of the crystallite shape during the complete 
 ( dis)charge 
 cycle is shown figure 5a , and the evolution of the resulting d / t ( diameter / thickness ) aspect ratio is shown in figure 5b .
consistent with 
 the constant width of the xrd reflections during discharge , the aspect 
 ratio and crystallite shape remain constant during discharge .
this 
 indicates that the average crystallite shape and size do not evolve 
 during discharge , indicating continuous nucleation and growth of lioh 
 crystallites with a constant crystallite size distribution .
this suggests 
 a particle - by - particle nucleation and growth process , rather than 
 concurrent growth of lioh crystallites , where the latter would lead 
 to an increase in average crystallite size .
in contrast , figure 5a shows that during 
 charging the average crystallite dimensions increase significantly 
 ( see also figure s6 ) .
therefore , crystallite 
 size evolution is highly asymmetric with respect to charge and discharge , 
 indicating at a very different growth and decomposition mechanism . 

because lioh decomposition should reduce the crystallite size , and 
 with the realization that the xrd results represent average crystallite 
 sizes , this may be explained by preferential decomposition of the 
 smallest lioh crystallites that will increase the average crystallite 
 size .
( b ) aspect ratio of the 
 lioh crystallite shape ( diameter in the a 
( c ) lioh lattice parameters , resulting from rietveld refinement during 
 the full galvanostatic ( dis)charge cycle in the dme / lii electrolyte . 
 in each figure
, the blue line represents the voltage curve during 
 the full galvanostatic ( dis)charge cycle in the dme / lii electrolyte 
 at a current density of 0.3 ma / cm .
the evolution of the lioh lattice parameters during discharge 
 and 
 charge is shown in figure 5c .
in contrast , 
 the c parameter increases near the end of discharge 
 and decreases at the early stages of charging , after which it remains 
 relatively stable until the end of charging .
the crystallite size 
 in the c direction , parallel to the platelet normal , 
 is on average approximately 20 nm during charge and discharge . for 
 nanostructured metal oxides , crystallite dimensions in the nanorange 
 typically result in expansion of the lattice parameters , attributed 
 to an increase in the surface energy due to larger exposed surfaces . 
 as a consequence ,
because the average crystallite size does not change significantly 
 during discharge , the increase in the c lattice parameter 
 must have an origin different from that of the crystallite size .
one 
 possibility is the presence of li or oh vacancies , where the latter appears to be more relevant as the effect 
 of oh vacancies has been studied in mg(oh)2 and lioh . in a lioh crystal ,
the li ions are located around the apexes of a square that slightly 
 fold downward along a diagonal under the o atom of the oh ion .
it is therefore likely that the 
 presence of vacancies will affect the lattice parameters anisotropically . 
 to investigate the dependence of the lattice parameters on the presence 
 of vacancies , dft calculations were performed on a 2  2  
 2 supercell of lioh by removing one hydrogen atom , lithium atom , and 
 oh ion from the supercell , resulting in h , li , and oh vacancies , respectively 
 ( figure s7 ) .
relaxation of the supercell 
 with a single hydrogen vacancy in the oh layer results in an increase 
 of the a lattice parameter and a decrease in the c lattice parameter ( table s1 ) . upon relaxation of the supercell with either a li or a oh vacancy , 

both structures show an increase in the c lattice 
 parameter [ li , 1.12% ; oh , 2.60% ( table s1 ) ] .
however , the supercell shows a decrease in the a lattice parameter ( 0.79% ) for a li vacancy and 
 an increase ( 1.05% ) for a oh vacancy .
moreover , according 
 to the dft calculations , the formation energies 
 of li , h , and oh vacancies are 1.399 , 0.317 , 
 and 1.578 ev , respectively , suggesting that the oh vacancies are relatively stable in lioh ( theoretical calculation and table s1 ) . hence , the observed increase in 
 the lioh c lattice parameter and the slight increase 
 in the a lattice parameter at the end of discharge 
 seen in figure 5c may 
 be a consequence of the formation of oh vacancies in the lioh lattice . 
 this may be rationalized as follows ; during the initial stages of 
 discharge , h2o will be relatively abundant , resulting in 
 the formation of lioh crystallites with a small number of oh vacancies . 
 at the later stage of discharge ,
most of the h2o can be 
 anticipated to be consumed , most likely the origin of the formation 
 of oh substoichiometric li(oh)1x crystallites , explaining the observed increase in the c lattice parameter . during the initial stages of charging , the c lattice parameter decreases , suggesting that the oh vacancy 
 rich li(oh)1x material is preferentially 
 decomposed . to investigate the cyclic formation of lioh over 
 multiple cycles , operando xrd was performed for the 
 battery based on the 
 dme / lii electrolyte during three ( dis)charge cycles , restricted to 
 a capacity of 1.5 mah at a current density of 0.3 ma / cm .
the electrochemical curves for three cycles are shown in figure 6a , and the two - dimensional 
 contour plots of the operando xrd patterns in figure 6b demonstrate the 
 cyclic formation and decomposition of lioh crystallites during three 
 discharge  charge cycles .
this gives direct evidence that lioh 
 is a cyclic discharge product in the presence of a lii mediator over 
 multiple cycles .
additionally , the linear increase ( discharge ) and 
 decrease ( charge ) in the integrated area of the lioh { 101 } and { 110 } 
 reflections over three cycles as shown in figure s8 also support the formation and decomposition of crystalline 
 lioh during ( dis)charge .
( b ) 
 two - dimensional contour 
 plots of the operando xrd patterns showing the 2 
 region between 31.5 and 33.5 and between 34.8 and 
 36.8 , during three complete discharge  charge cycles demonstrating 
 lioh formation ( { 101 } and { 110 } reflections ) and decomposition using 
 the dme / lii electrolyte employing a capacity - restricted cycling capacity 
 of 1.5 mah at a current density of 0.3 ma / cm . to explore the influence of h2o on the 
 formation of 
 lioh , the li 
o2 battery , combining an activated 
 carbon electrode and the dme / lii electrolyte , was ( dis)charged while 
 the cathode was being exposed to h2o - saturated o2 .
the ( dis)charge curve obtained is shown in figure s9 , where it is compared to that obtained for the same 
 system exposed to dry o2 ( results shown in figure 5 ) . because of the presence 
 of h2o in the provided o2 gas , the discharge 
 overpotential for the h2o - saturated o2 battery 
 is lower than that of its dry o2 counterpart .
conversely , 
 the charge overpotential for the h2o - saturated o2 battery is larger , possibly because of the higher discharge capacity 
 and associated increased resistance of the insulating lioh crystallites 
 and/or blocking of the gas diffusion electrode .
the resulting operando xrd contour plots and the results from the rietveld 
 refinement of the lioh structure are shown in figures s10 and s11 , respectively .
the evolution of the lioh 
 reflections in the presence of h2o - saturated o2 is similar to that observed for the dry o2 shown in figure 4 , indicating the 
 formation and decomposition of crystalline lioh .
the main difference 
 caused by the presence of h2o - saturated o2 is 
 that the intensities of the lioh reflections are larger and that the 
 { 101 } reflection becomes sharper .
these results indicate that larger 
 lioh crystallites are generated during the discharge process because 
 of the presence of h2o - saturated o2 .
both the 
 apparent crystallite size and the lattice parameters evolve in a manner 
 similar to that observed for dry o2 , also indicating that 
 small lioh crystallites are decomposed first during the charging process . in summary , from the detailed refinement of the cyclic appearance 
 of the xrd lioh reflections in the dme / lii electrolyte , a formation 
 and decomposition mechanism is shown in figure 7a . on the basis of the evolution of the apparent 
 crystallite shape and the d / t aspect 
 ratio of lioh , the lioh crystallites follow a continuous nucleation 
 and growth mechanism with a constant crystallite size distribution 
 during discharge .
this mechanism represents a particle - by - particle 
 rather than concurrent nucleation and growth process , forming the 
 platelike lioh particles ( figure 7b , images a 
, we expect that the low concentration of water in the electrolyte 
 induces oh substoichiometric lioh , explaining the observed increase 
 in the c lattice parameter . during charging , the 
 average lioh crystallite dimensions increase significantly , whereas 
 the aspect ratio and crystallite shape remain constant , suggesting 
 that the smallest oh deficient lioh crystallites oxidize first ( figure 7b , images d  f ) . 

this indicates that the crystallite size and presence of vacancies 
 are important parameters to consider when aiming to increase the charge 
 rate and to decrease the charge overpotential .
( a ) voltage profile , 
 including a schematic illustration of the 
 formation and decomposition mechanism of the lioh platelet - shaped 
 crystallites during the discharge  charge process , as determined 
 from refinement of the operando xrd patterns .
( b ) 
 sem images recorded at different capacity stages of lioh formation 
 and decomposition .
operando x - ray diffraction is performed to investigate 
 the structure evolution of li2o2 and lioh during 
 discharging and charging in the li  o2 battery employing 
 a dme or dme / lii electrolyte in combination with an activated carbon 
 cathode . in the dme electrolyte , li2o2 forms 
 and decomposes reversibly as platelet crystallites with the normal 
 of the platelet in the c lattice parameter direction . 
 during discharge ,
the average li2o2 crystallite 
 shape becomes more isotropic because of the formation of more isotropic 
 crystallites .
first , amorphous 
 species and small li2o2 crystallites are oxidized , 
 and second , the larger crystallites are decomposed . in the dme / lii 
 electrolyte , lii appears to act as a redox mediator resulting in the 
 cyclic formation of crystalline lioh over multiple cycles . during 
 discharge , platelet lioh crystallites form , with the normal of the 
 platelet in the c lattice parameter direction .
as 
 the average crystallite size and shape do not evolve during discharge , 
 this indicates continuous , particle - by - particle nucleation and growth . 

the increase in the average c lattice parameter is , 
 on the basis of dft calculations , proposed to be a result of the formation 
 of oh vacancies in lioh , suggested to be a consequence of the depletion 
 of water in the dme electrolyte during lioh formation . during decomposition 
 during charging ,
the smallest oh deficient lioh crystallites decompose 
 first , indicating that the decomposition of these particles is most 
 facile .
providing h2o - saturated o2 does not 
 change the crystallite growth mechanism significantly ; however , the 
 amount of crystalline lioh increases , consistent with the larger electrochemical 
 discharge capacity .
 the 
 lithium air , or li  o2 , battery system is 
 a promising electrochemical energy storage system because of its very 
 high theoretical specific energy , as required by automotive applications . 
 
 fundamental research has resulted in much progress in mitigating detrimental 
 ( electro)chemical processes ; however , the detailed structural evolution 
 of the crystalline li2o2 and lioh discharge 
 products , held at least partially responsible for the limited reversibility 
 and poor rate performance , is hard to measure operando under realistic electrochemical conditions . 
 this study uses rietveld 
 refinement of operando x - ray diffraction data during 
 a complete discharge  
 charge cycle to reveal the detailed structural 
 evolution of li2o2 and lioh crystallites in 
 1,2-dimethoxyethane ( dme ) and dme / lii electrolytes , respectively . 
 
 the anisotropic broadened reflections confirm and quantify the platelet 
 crystallite shape of li2o2 and lioh and show 
 how the average crystallite shape evolves during discharge and charge . 
 
 li2o2 is shown to form via a nucleation and 
 growth mechanism , whereas the decomposition appears to start at the 
 smallest li2o2 crystallite sizes because of 
 their larger exposed surface . in the presence of lii 
 , platelet lioh 
 crystallites are formed by a particle - by - particle nucleation and growth 
 process , and at the end of discharge , h2o depletion is 
 suggested to result in substoichiometric li(oh)1x , which appears to be preferentially decomposed during 
 charging . 
 operando x - ray diffraction proves the cyclic 
 formation and decomposition of the lioh crystallites in the presence 
 of lii over multiple cycles , and the structural evolution provides 
 key information for understanding and improving these highly relevant 
 electrochemical systems . 
Introduction
Materials
and Methods
Results
and Discussion
Conclusions