Growth and dissolution of NaO 2 in an ether-based electrolyte , as the discharge product in the NaO 2 cell

Deposition and dissolution of sodium superoxide (NaO2) was investigated by atomic force microscopy. Rectangular prisms consisting of 8 smaller sub-structures grew from NaO2 platelets, when discharged in 0.5 M NaClO4, diethylene glycol dimethyl ether on highly ordered pyrolytic graphite. During oxidation the 8 substructures is conserved. 200 nm diameter ring-like structures of Na2CO3 remain at the end of oxidation.

The sodium-oxygen battery (Na-O2) is under investigation due its advantageous theoretical specific energy of 1106 Wh kg - 1 for the reaction (equation 1). 1 Na + + e -+ O2  NaO2 E o = 2.27 V vs. Na + /Na (1)   The reduction of dissolved O2 in the present of Na + results in the precipitation of large micrometre sized cubes (1 -50 µm 2,3 ) of NaO2 on the surface of the electrode.Understanding the growth and dissolution mechanism of NaO2 cubes formed from the one electron reduction of O2 is critical towards the realisation of Na-O2 as a practical technology.NaO2 cubes are often of uniform size; however their morphology changes based on the rate of formation.At high rates (>600 µA cm -2 ) 2 icosahedral features are formed on the electrode surface. 2 At lower rates (<400 µA cm -2 ) 2 they precipitate out as cubes. 4The explanation of these observations is that at higher rates the amount of soluble NaO2 formed rapidly saturates the electrolyte near the electrode interface and precipitates out as smaller cubes at many nucleation points. 2 Whereas the slower formation and saturation of the electrolyte at lower rates forms larger cubes at a decreased amount of nucleation points.Subsequently the reaction is halted by the formation of compact, insulating NaO2 films. 5,6cent reports on the growth mechanism of NaO2 suggest that the soluble formation of NaO2, aided by HO2 7 catalysed intermediates, agglomerate within the solution to form cubic structures. 7The sudden death of the cell chemistry is attributed to the formation of a passivation film between the cube structures at the end of discharge process, which the thickness is discharge rate dependent. 8though many factors upon NaO2 growth and morphology have been reported including: rate 2,8,9 , impurity concentration 7,10 , and solvent dependence [11][12][13][14][15][16][17][18] , a unified model of the fundamental control parameters of the discharge process remains unclear, with different conclusions operating under similar conditions being made.However, there has been an extensive review carried out by Bender et al 1 that ascertains the differences and similarities, Bi et al 19 analysed the cell setup concluding that glass cell setups obtain different discharge products (mainly Na2O2.2H2O) to stainless steel cell setups (NaO2 only).
Atomic force microscopy (AFM) is a powerful method for the investigation of NaO2 growth and dissolution reaction, as size and morphology of the deposits can be directly measured with nm precision.This experimental approach has been already successfully undertaken in the lithium-oxygen (Li-O2) battery system by Wen et al 20 on highly ordered pyrolytic graphite (HOPG) and Liu et al 21 on Au.On HOPG agglomerates of LiO2 nanoparticles collect and grow at step edges forming Li2O2 nanoplates. 20Upon Au, due to the lack of step edges and more uniform surface reactivity, Liu et al 21 concluded that a soluble LiO2 species initially form that subsequently precipitates out on the surface as Li2O2. 22S. E. Herrera et al 23 studied the formation of Li2O2 in DMSO on HOPG using ex situ AFM and herein an analogous AFM study in the Na-O2 system of the growth and dissolution of NaO2 precipitates upon HOPG is presented.
The CV of O2 saturated 0.5 M NaClO4, diethylene glycol dimethyl ether (DEGDME) on a HOPG surface is shown in Figure 1a.All stated potentials are vs.Na + /Na.The reductive current rise begins at the thermodynamic potential of NaO2 formation (2.33 V) where it peaks at 1.5 V.
During the reverse scan a broad peak beginning at 2. 35   reaching a maximum at 3.06 V, signals the electrooxidation of NaO2 back to O2 and Na + .The current increases at 4.0 V suggesting further oxidation of remaining reaction products. 16,24,12e sequential description of the AFM images collected around the CV curve begins at OCP (2.8 V) where the small striations of the freshly cleaved HOPG surface are observed that follow the step edges of the stacked graphene sheets, the height profiles showing that the step edges are 0.8 nm in height (Figure 1b/Figure S1).At 2.2 V (Figure 1c) an inhomogeneous 5 nm film of NaO2 was observed on the surface with 20x20 nm sized 5 nm thick deposits.Calculating the weight of the discharge product, via the charge passed, estimates an idealised film of 4.7 nm formed on the surface (Table S1).Height profiles are shown in the ESI for all measurements (Figures S2-S7).
The formation of 50-150 nm thick plate-like structures of NaO2 were observed at 1.9 V (Figure 1d / Figure S3), and at 1.5 V the formation of cube-like NaO2 crystals at the interface is observed (Figure 1e / Figure S4).The NaO2 crystals have the dimensions of 2x2x1 m, with axial lines visible on the surface along with a centralised recess that is half the deposits height.These deposits are smaller than those observed in the literature which are generally 2-10 m in size. 25Raman spectra (Figure S8) of these deposits show a peak at 1165cm -1 , consistent with a superoxide species.The axial lines reveal a structure whereby the discrete crystal consists of stacked particles that have agglomerated on the surface and grown together.Some crystals appear prismatic or tetragonal in shape coming out of the surface, but still contain the same axial lines defining different cube-like features together, but emerging out of plane.The depth of the hole within the centre of the crystal is ~0.5 µm in height as shown from the line profile (Figure S4).
Taking an extended view of the surface the NaO2 crystals formed at the HOPG surface are all fairly uniform in size and are scattered both across the basal plane and agglomerated along the step edge (Figure S9).) Visible along x and y axis of these features are faint lines that reveal cubic building blocks of the overall structure (Figure 1e).The scale of the hole here suggests that the depth of this hole is in fact the height of the cube (1.0 µm).Upon closer inspection of the height profile it is actually 0.5 µm (Figure S4).For other deposits the electrode surface is visible in the middle of the cube (Figure S10).The secondary image taken on top of NaO2 shows the imperfect facets of the cube structure.This is interesting as the majority of studies show uniform NaO2 cubes that appear perfect plate like structures. 26versing the potential back to 2.6 V the NaO2 are diminished in size with the axial outlines of smaller cubes making up the larger crystals still apparent (Figure 1f/Figure S7).This observation concurs with the previous state that these NaO2 crystals formed are built essentially from 8 smaller Please do not adjust margins Please do not adjust margins building blocks.By moving the over potential to 3.25 V (Figure 1g/Figure S8), beyond the peak current density of the redox process, very little NaO2 is left upon the surface of the HOPG.Only tiny agglomerates of nanoparticles (ca.20 nm in height) are observed.At 4.0 V little remains on the surface of the HOPG electrode (Figure 1h/Figure S9).Nonetheless, in comparison with HOPG at OCP (Figure 1b) the image shows some residual material on the electrode surface, some of which forms rings of ca.200 nm diameter, 1.5-7 nm height, suggests incomplete oxidation of the discharge products.This is possibly imprints of NaO2 deposits on the surface of the electrode (Line profile shown in Figure S9).The plates could be a coating of side products from other reactions as indicated by Ledano et al 26 and Black et al. 27 Only the carbon G band (1583 cm -1 ) of HOPG is present in the Raman spectrum (Figure S11a).However in the FTIR a band for Na2CO3 at 1429 cm -1 (Figure S11b) is detected.
Observations from the AFM study are summarised within Schematic S1 where initially a thin film (2-3 nm) is formed on the surface with deposits of 5 nm nanoparticles.Platelets appear on the surface an order of magnitude (100 nm) bigger than the film.On the surface as you increase the over potential you see the agglomeration of cubic structures that grow into rectangular prisms of NaO2 that consist of 8 discrete building blocks.The start of the oxidation process the edges of each crystal loses definition, leading to the presence of nanoagglomerates left at the interface.The resulting "ghost" shells of the crystal can be seen on the interface identified as Na2CO3.
Further AFM measurements (Figure S12) showed that NaO2 crystallisation chemistry was found to be salt dependent and reduction of O2 in 0.5M NaTFSI, DEGDME in the electrolyte resulted in the formation of a holed plate like deposit with larger dimensions of 16x16x0.5 m.This identifies that further work need to be undertaken to fully understand the role of the anion, as well as ion coordination and solvent types in electrolytes on NaO2 precipitation. 28e observation of imperfections on the NaO2 rectangular prism including both holes and axial lines indicate the high mobility of NaO2 particles at the interface to agglomerate and grow in discrete, potential dependent morphologies.This has implications for practical cells where controlling type of NaO2 precipitate will be necessary for predictable cell behaviour.The high mobility of NaO2 means that agglomerates will migrate along the surface until an energy favourable site, such as a defect, is available to deposit.At the initial stage of reduction the NaO2 is dissolved into the electrolyte.Once the double layer is saturated with NaO2, the deposition and formation of NaO2 crystals can occur.
Evidence of coarsening or Ostwald ripening is exhibited in the initial film and final stages of the reduction of O2 (Figure 1c) and oxidation of NaO2 (Figure 1g).This is provided by the spherical nature of the particles mapped within the AFM images.These are thermodynamically more stable due to surface energy minimisation of the particles through particle dissolution and ion precipitation. 29At this stage of formation the precipitation of primary nanoparticles is similar to classical nanoparticle growth.Therefore at these stages of the discharge, a high concentration of monomers i.e.O2 -and Na + should prevent particle coarsening and changes in morphology. 30,31hese effects would be heightened at an electrode interface due to the formation of the double layer structure.The anion plays a considerable role in the structure of the double layer in each system, hence, the electrolyte anion affects these stages of NaO2 growth, as demonstrated by the variation in deposit size between NaClO4 (Figure 1) and NaTFSI (Figure S12).However, classical crystallisation does not characterise the complete mechanism here due to the observation of Rubik cube like structures.
Precursor layered plates and then Rubik cube like structures is unsurprising given observations of oxide formation in this solvent in the lithium analogue of this system. 20From the evidence here the steps between spherical nanoparticles of NaO2 and rectangular prismatic structures suggest a reoriented aggregation (RO) or mesocrystal mechanism is taking place.RO mechanisms simply encompass the aggregation of building block nanoparticles that then reorient to form single faced structures again minimising surface energy.It is observed within the film the initially formed NaO2 nanoparticles (Figure 1c).These favourable aggregation sites will then form the precipitation sites at the interface.Here collisions of aligned nanocrystals in suspension or the rotation of collided misaligned nanoparticles form the basis of growth at the interface. 31,32Once an agglomerate of a certain size is reached in some instances coarse ripening has been shown to smooth the edges of the final crystal structures.However, this model is not the best fit.Mesocrystal formation is a better model in this case because of the initial stages of growth proceeding into subsequent smoothing of the final precipitate.In mesocrystal formation the primary nanoparticles are iso-oriented into a crystal via oriented aggregation (OA). 33Organic media has been shown to temporarily stabilise these particles and form a mesocrystal via mesoscale assembly. 33Fusion can then occur to form an oriented crystal and then finally to a single crystal.These previous observations help to explain the observations of the Rubik cube like structures within this data series.Within this the final stage of fusion to form a single crystal is not observed (in contrast to many literature reports 33,34 ), most likely due to the much shorter timescale of the voltammetry experiment and the high current rate generation of superoxide.
The process of meso-crystals fusion, which is thermodynamically favoured under the displacement of the solvent molecules can often include part of the organic molecule as a coating. 33and this is proposed as a pathway for the formation of a Na2CO3 coating upon the crystals forming, and a crust of Na2CO3 remaining on the surface of the electrode after oxidation of NaO2. 26e dissolution mechanism should firstly oxidise the surface layer in contact with the surface and the edges of the rectangular prism agglomerates.The stages characterised in this study would suggest that similar stages of dissolution are present upon dissolution as on formation of these mesocrystals.This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins

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The scan rate used (100 mVs -1 ) resulted in a high rate of O2 reduction leading to initial film formation from supersaturation of the double layer providing many smaller nucleation points. 35n the medium ionic strength electrolyte (NaClO4/DEGDME) the amount of time a particle of NaO2 can reorient itself before being fixed, is lowered. 8This observation is different to that reported by Ledrano et al 6 through in situ electrochemical quartz mass balance measurements upon Au electrodes.They concluded that there is only solution based NaO2 in this instance and that no film is formed until the end of the discharge.Recently Lutz et al 36 have highlighted the different behaviour of this process due to substrate, especially the difference in observed morphology between carbon (cubic NaO2) and Au (NaO2 flakes) surfaces.Undoubtable the role of electrode surface, solvent, salt and H2O level are play a major role on the growth and morphology of NaO2 crystals and further investigations are required to understand these influences.
In conclusion, ex situ AFM images on HOPG demonstrate the rich and intricate dissolution and recrystallisation chemistry during O2 reduction in an aprotic ether-based solvent in the presence of Na + .An initial formation of a 5 nm NaO2 film, followed by subsequent deposition of stacked NaO2 platelets, is observed.These plates then grow into crystalline rectangular prisms (2x2x1 m) consisting of 8 smaller sub-structures.These were found to preferentially agglomerate at the step edges of HOPG, but also upon the basal plane itself.The observation of building block rectangular prisms that have stacked on top of each other, with striations on the deposit resembling those found on the Rubik's Cube puzzle, was unexpected and this method of NaO2 precipitation results in the observed 0.5 µm deep holes present in the central face of the NaO2.Upon oxidation the dissolution of rectangular prismatic structure, conserves an unevenly shaped collection of 8 sub-structures that degrades further to groups of 40 nm sized deposits.Rings of 200 nm diameter remain on the surface after oxidation, these rings are thought to be the un-oxidised side reaction product (Na2CO3) that forms upon the surface of NaO2.Support from the Engineering and Physical Sciences Research Council grant EP/J020265/1 and the Energy SuperStore, Early Career Researcher Award is acknowledged.