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Mechanisms of sodium insertion/extraction on the surface of defective graphenes James C. Pramudita, Aditya Rawal, Mohammad Choucair, Daniele Pontiroli, Giacomo Magnani, Mattia Gianandrea Gaboardi, Mauro Ricco, and Neeraj Sharma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13104 • Publication Date (Web): 12 Dec 2016 Downloaded from http://pubs.acs.org on December 19, 2016

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ACS Applied Materials & Interfaces

Mechanisms of sodium insertion/extraction on the surface of defective graphenes James C. Pramudita1,2, Aditya Rawal3, Mohammad Choucair4, Daniele Pontiroli5, Giacomo Magnani5, Mattia Gaboardi6, Mauro Riccò5, Neeraj Sharma1,* 1

School of Chemistry, UNSW Australia, Sydney NSW 2052, Australia Australia Nuclear Science and Technology Organisation, Kirrawee, DC NSW 2253, Australia 3 Mark Wainwright Analytical Centre, UNSW Australia, Sydney NSW 2052, Australia 4 School of Chemistry, University of Sydney, Sydney NSW 2006, Australia 5 Dipartimento di Fisica e Scienze della Terra, Università degli Studi di Parma, Parco Area delle Scienze 7/a 43124, Parma, Italy 6 ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK 2

Keywords: graphene, negative electrodes, sodium-ion batteries, solid state NMR, solid-electrolyte interface layer ABSTRACT: Two chemically synthesized defective graphene materials with distinctly contrasting extended structures and surface chemistry are used to prepare sodium-ion battery electrodes. The difference in electrode performance between the chemically prepared graphene materials is qualified based on correlations with intrinsic structural and chemical dissimilarities. The overall effects of the materials’ physical and chemical discrepancies are quantified by measuring the electrode capacities after repeated charge/discharge cycles. Solvothermal synthesized graphene (STSG) electrodes produce capacities of 92 mAh/g in sodium-ion batteries after 50 cycles at 10 mA/g while thermally exfoliated graphite oxide (TEGO) electrodes produce capacities of 248 mAh/g after 50 cycles at 100 mA/g. Solid state 23Na nuclear magnetic resonance spectroscopy is employed to locally probe distinct sodium environments on and between the surface of the graphene layers after charge/discharge cycles that are responsible for the variations in electrode capacities. Multiple distinct sodium environments of which at least 3 are mobile during the charge-discharge cycle are found in both cases, but the majority of Na is predominantly located in an immobile site, assigned to the solid electrolyte interface (SEI) layer. Mechanisms of sodium insertion and extraction on and between the defective graphene surfaces are proposed and discussed in relation to electrode performance. This work provides a direct account of the chemical and structural environments on the surface of graphene that govern the feasibility of graphene materials for use as sodium-ion battery electrodes.

1. Introduction The demand for lithium-ion batteries is rapidly increasing due to their recent use in electric vehicles,1 which has contributed to an increase in the price of lithium.2 Lithium-ion batteries do not yet meet all the requirements, such as energy density, cost and safety considerations, of emerging applications.3-4 Moreover, the development of sustainable and clean energy sources such as solar and wind requires scalable energy storage systems.5-6 The unstable supply of energy generated from these renewable energy sources requires a cheap, efficient and reliable energy storage system to ensure the availability of stored energy.5, 7 Scalable systems requiring lithium-ion batteries have proved limited in their feasibility due to the limited availability of lithium metal and increasing battery costs.6-7 Sodium-ion batteries could be a low-cost alternative to lithiumion batteries if feasible electrode materials are developed. Sodium-ion electrodes do not necessarily require overhauling the electrochemistry and basic set-up of lithium-ion batteries and thus could enable the parallel implementation of both solutions.4, 6, 8 The availability and accessibility of sodium-based raw materials makes it suitable for large scale production, ensuring that sodium can be acquired at a much cheaper price compared to lithium at

least for development purposes.6, 9 In terms of battery design, sodium-ion batteries offer the use of low cost and weight aluminum current collector foil, in fact sodium does not readily alloy with aluminum in contrast to lithium.2, 7 Beyond these intriguing aspects, sodium-ion batteries hide some drawbacks. Sodium typically has lower electrochemical activity compared to lithium due to the more positive reduction potential (-2.71 V for Na compared to -3.04 V for Li vs. H2/Pt2), larger size, and heavier mass which leads to lower operating voltage and typically lower energy densities.4, 7 Thus, while sodiumion batteries might not replace lithium-ion batteries in high power portable applications, it is a promising candidate for large scale stationary grid storage application.10 The development of sodium-ion batteries are limited by the lack of available anode materials that can reversibly insert/extract sodium at sufficient rates and energy density to match the available cathode materials.2 Graphite, the most commonly used anode material in lithium-ion batteries does not reversibly insert/extract sodium at sufficient rates to form Na-C compounds in contrast with graphite-lithium which forms compounds up to LiC6 during charge.6, 8 Hard carbon and pitch-cokes are carbon-based materials that show potential to be used as sodium-ion battery anodes

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giving capacities of 300 mAhg-1 after 120 cycles at C/10 (ref. 11), and 85 mAhg-1 (ref. 12) respectively. Modification has to be made for graphite to work, as recent work has shown using cointercalation electrolytes.13-14 Further modification of graphite into expanded graphite has been shown to produce capacities of 136 mAhg-1 at a current density of 100 mAg-1 after 1000 cycles.6 However, one aspect that is barely explored in the literature for electrodes in sodium-ion batteries is the solid electrolyte interface (SEI) layer, the layer formed during battery operation on the electrode surface by interaction of the electrolyte with the electrode and the charge transfer process. This has been explored rather extensively for lithium-ion battery electrodes and an approximate composition and structure of the SEI on some materials is known.15-17 The understanding of the SEI in carbon-based electrodes for sodium-ion batteries may allow for further improvements in performance and rational electrode design. This illustrates that carbon-based materials are still a possible candidate for sodium-ion battery anode materials especially considering their natural abundance and renewability. Single layer, defective graphene shows good electronic, mechanical, and chemical properties and has led to proposed use in various applications.18 For energy storage applications, the high theoretical specific surface area of up to 2630 m2g-1, conductivity, and broad electrochemical window of graphene makes it a potential electrode candidate.19-22 However, its use in this field requires large quantities of material, only achievable through chemical synthetic routes, that inevitably affect these properties. There are several known methods to synthesize graphene, and each approach appears to give modifications to the structure and chemistry of the graphene surface.18, 23-24 Extensive studies have been undertaken on using graphene as an anode material in lithium-ion batteries and in sodium-ion batteries.20, 25-34 However, studies detailing the insertion and extraction mechanisms of sodium ions on the surface of graphene materials and the role and composition of the SEI layer remain limited. In this study, we use solid state nuclear magnetic resonance (SS-NMR) to directly investigate the chemical environment of sodium-ions on the surface of defective and chemically modified graphenes. The materials employed include solvothermal synthesized graphene (STSG) and thermal exfoliation of graphite oxide (TEGO) which provide contrasting physical and chemical properties that exist in a range of chemically synthesized graphene materials and that correlate well with battery performance.

2.

Experimental

2.1 Materials Synthesis STSG was obtained using a bottom-up approach following procedures reported elsewhere35. A typical synthesis consists of heating a 1:1 molar ratio of sodium and ethanol in a sealed reactor vessel at 220oC for 72 h to yield the solid solvothermal productthe graphene precursor. This material is then rapidly pyrolyzed in air and the remaining product washed with deionized water, then acidified ethanol (1:4 vol/vol 2 M HCl/ethanol). The suspended solid is then vacuum filtered and dried in a vacuum oven at 200 o C for 24 h. The preparation of TEGO required the oxidation of graphite following the Brodie method36, where graphite powder (SGL Carbon, RW-A grade, average size 66 µm) was mixed with sodium chlorate powder and cooled with an ice bath in a fume-hood. The mixture was kept under continuous stirring and concentrated nitric acid was slowly added, with minimal change in temperature. The suspension was then heated at 60 °C for 8 h with a slow thermal ramp (20°C/h) and then cooled to room temperature (30 °C/h). After one day under continuous stirring, the powder appeared dark green. The suspension was then diluted in water and filtered

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through a coarse filter paper. The product was then suspended in a dilute solution of hydrochloric acid, filtered and carefully washed on the filter until the pH of the liquid phase increased to 7. Finally, the product was dried at 60 °C overnight producing graphite oxide. About 600 mg of this graphite oxide was placed in a quartz boat in a quartz tube under dynamic vacuum and was abruptly heated at 1150 °C and left for 30 minutes, while the pressure was continuously monitored.37 A sizeable expansion of the powder occurred, as the result of the sudden production of gases originating from the detachment and decomposition of functional groups38. This process also causes the removal of some carbon atoms leading to the formation of in-plane defects (vacancies or internal edges).39

2.2 Material Characteristics STSG contains a greater number of paramagnetic edge states and in-plane defects than the TEGO. The solid state N2 Brunauer– Emmett–Teller (BET) surface area of STSG has been found to range from 500 m2g-1 to values close to graphite of 11 m2g-1, decreasing with sample preparations that may disrupt the extended three-dimensional porous network of fused graphene sheets (ref. 40 ). The TEGO sample has been found to have accessible N2 BET surface areas exceeding 500 m2g-1 due to the extended porous structure of the layers arising from rapid sheet expansion.41 While the TEGO sample is never exposed to air or moisture, resulting in an absence (or close to an absence) of oxygen containing surface groups (2-3 wt%),4 the STSG sample is pyrolyzed in air to yield up to 16 weight percent of oxygen in the form of carboxyl and hydroxyl functional groups on the graphene materials’ surface.35 It has been shown that even after exposure to air, exfoliated graphene oxide does not readily undergo re-oxidation.42 These differences allowed for the employment of these two materials as suitable model systems to represent graphene surfaces that contain various degrees of defects and surface accessibility, and functional groups that alter the nucleophilic character of the graphene surface.

2.3 Electrode Construction The negative electrode materials were manufactured by mixing each of the two as-synthesized graphene materials, STSG and TEGO, with polyvinylidinefluoride powder (PVDF, MTI Corp.) in a 70:30 wt% ratio. N-methylpyrrolidone (NMP, MTI corporation) was then added dropwise to produce a viscous slurry which was left to stir overnight. This slurry was then pasted onto copper foil (Goodfellow) using a notch bar and the wet electrode film was dried at 100 °C under vacuum overnight. The electrode sheets were then pressed at a pressure of 100 kN using a flat-plate press (MTI Corporation) and dried overnight again under vacuum at 100 °C before transfer to an argon filled glovebox. The batteries were then constructed using research coin cells (CR 2032). The batteries used Na metal electrodes (∼1 mm thickness), and glassfiber separators which were soaked in electrolyte consisting of a solution of 1M NaPF6 in dimethyl carbonate and diethyl carbonate (1:1 wt%) solution. Further details regarding coin cell construction can be found in previous work.43 Typically, the masses of the STSG electrodes are around 0.60 ± 0.05 mg while the TEGO are around 0.15 ± 0.05 mg. The batteries were then typically discharged to 0.1 V and charged to 2.5 V at a variable current rate when conducting specific electrochemical tests. The graphene electrodes for NMR studies were obtained from coin cells cycled to either 0.1 V (discharged, Na-inserted) or 2.5 V (charged, Na-extracted). The electrodes were extracted from the coin cells in an Ar-filled glovebox, washed with dimethyl carbonate and dried overnight. They were subsequently packed in 2.5 mm zirconia rotors in an inert environment for NMR measure-


ments. The solid state NMR spectra were acquired on a Bruker Avance III 700 MHz spectrometer with a 16.4 T superconducting magnet operating at frequency of 185 MHz for the 23Na nucleus. The packed samples were spun to 30 kHz at the magic angle spinning (MAS) under a nitrogen atmosphere. Single-pulse excitation was used to detect the 23Na NMR spectra with a 2 µs hard pulse for excitation and 300 ms recycle delays. The spectra were referenced to 23Na spectrum of solid NaCl set to 0 ppm.44 The relative populations of the different sodium sites were determined by deconvolution of the central transition of the observable 23Na NMR signal using the DMFIT software.45

3.

Results and Discussion

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Figure 1 shows the electrochemical characteristics of the STSG based electrode in sodium-ion batteries. STSG electrodes produce a 1st discharge capacity of 588 mAhg-1 before decreasing to 240 mAhg-1 at the 2nd discharge. The 1st charge capacity was 200 mAhg-1, illustrating that only one-third of the sodium ions inserted in the 1st discharge can be cycled. This capacity drop from the 1st discharge is relatively common in carbon-based materials, and is often attributed to the formation of the solid electrolyte interface (SEI) layer at the anode surface.4, 24 While the first few cycles of the battery show promising capacity, further cycling shows that the capacity continues to decrease without stabilizing (Figure 1b) and by the 50th cycle the capacity is 90 mAhg-1 at a rate of 10 mAg-1. Cycling at different current rates shows the STSG undergoes significant capacity loss with increased current rate (Figure 1c). Increasing the current rate from 10 mAg-1 to 20 mAg-1 results in 25 % loss in capacity, while increasing from 20 mAg-1 to 50 mAg-1 a large reduction of 42% is observed with a capacity at 50 mAg-1 of 62 mAhg-1. In this case, the gradual decrease with continuous cycling also influences the rate dependent studies. Overall the capacities observed at high current rates suggest there is insufficient time for the sodium-ions to populate the surface or insert into the STSG electrode.

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Figure 1. Electrochemical data of STSG in sodium ion batteries. (a) Electrochemical profile (b) after extended cycling, and (c) with a variable current cycling rate. Note the capacity is not stable over extended cycling and produces lower capacities at high current rate cycling. The electrochemical characterization of TEGO based electrodes versus sodium is presented in Figure 2. Further electrochemical details about the TEGO electrode can be found in previous work4. The capacity of TEGO stabilizes at 248 mAhg-1 after the 50th cycle at 100 mAg-1 (Figure 2b). A 10-fold higher current with sodium-ion cells still produces 2.7 times more capacity using TEGO compared to STSG. It should be noted that the TEGO electrodes tested in this work generally have lower electrode loadings than the STSG. An 80 % irreversible capacity loss is observed after the 1st discharge in TEGO-containing batteries, compared to 62 % loss in the STSG. The TEGO electrode appears to stabilize in terms of capacity after about the 20th cycle while the STSG shows a gradual decrease in capacity with cycling and no evidence of stabilization. The TEGO electrodes produce a capacity of 175 mAh g-1 at a current rate cycling of 5 Ag-1. This difference in performance between STSG and TEGO correlates well with differences in the overall surface area of the graphene materials resulting in an unequal amount of sodium-ions that can be inserted to the structure. As shown in Figure 1a and 2a, the lack of a plateau region in the electrochemical profile after



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the first discharge suggests that the insertion mechanism of sodium-ions into the graphene electrodes are dominated by surface reactions, thus enforcing the importance of the surface area in potential applications. Recent research on electrodes with pseudocapacitive behavior has been linked to excellent rate capability.46 This property would also hold in the graphene systems, especially in high surface area TEGO.4 Furthermore, results reported using nitrogen-doped graphene produce capacities of 154 mAhg-1 at 5 Ag-1 after 10000 cycles.47

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Electrochemical characterization of STSG and TEGO electrodes in lithium-ion batteries have also been demonstrated in parallel to show that the similarities in cycling performance translate across to lithium-ion batteries. Detailed results of the electrochemical characterization of STSG and TEGO electrodes in lithium-ion batteries can be found in the Supporting Information (Figure S1 and Figure S2).

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To understand the differences in the performance, 23Na solid state NMR data of the TEGO and STSG based electrodes were collected at the sodium-rich discharged state (0.1 V) and the sodium-deficient charged state (2.5 V), Figure 3a and Figure 3c respectively. Furthermore 23Na NMR data was also collected at the discharged and charged states on the 20th cycle, Figure 3b and Figure 3d for TEGO and STSG respectively, to identify the effects of prolonged cycling on the Na environment. A detailed lineshape fitting routine was carried out to elucidate the changes in the 23Na environments and the full details of the fitting parameters are presented in the Supporting Information (Table S1 and Table S2, and Figure S3). 13C NMR was not undertaken in this instance due to low sample quantities and the possibility of the probe arching effect from the conductive graphene. In all the 23Na spectra, there is no sodium metal peak, which implies that sodium metal does not plate or form significant quantities of dendrites during cycling. Considering first the Na environment inside/on TEGO after the 1st discharge (Figure 3a), there are at least 4 distinct sites that can be identified with NMR shifts of -2 ppm, -10 ppm -14 ppm and -16 ppm. Akin to our previous study on Na-insertion in carbon nanotubes (CNTs),48 a peak with an approximate second order quadrupolar lineshape (ηQ = 0.98) at -2 ppm is related to an ordered, but asymmetric environment. The specific site associated with the -2 ppm signal is assigned to Na in-between graphene sheets and its presence in the difference spectra of Figure 3 indicates that the site is effectively cycled during the charging and discharging processes. The NMR peaks at 14 ppm and -16 ppm which have been previously attributed to the Na inside carbon nanopores49-50 are also cycled out, with the peak


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at -16 ppm disappearing completely after the first charge cycle. This suggests that the Na located at these environments are electrochemically mobile, and along with the site at -2 ppm, are responsible for the reversible cycling and hence capacity of TEGO. In these electrode materials, it is not unexpected to find few layers of graphene sheets stacked together and a Na environment associated with intercalation into the stacked layers. Previous work by Zhou et al on carbon microspheres showed similar sodium environments to that observed here.49 In contrast to the narrow 23Na signal, the Na environments associated with the -10 ppm signal show no distinct change upon

charge and discharge, indicating the Na in these environments are immobile after the 1st discharge and they contribute to the irreversible capacity observed in the 1st cycle. As such we are able to assign this signal predominantly to the SEI layer, which has also been observed previously by independent works.49-50 Integration of the 23Na signal (Table S1) shows that the immobile Na environment makes up approximately 80 % of the total Na environment, implying only 20 % of the Na environments are able to be cycled and this correlates to the 80 % capacity drop from the 1st discharge (Figure 2a).

Figure 3. 23Na MAS NMR of Na/TEGO and Na/STSG. (a) Na/TEGO 1st cycle and (b) Na/TEGO 20th cycle, and (c) Na/STSG at 1st cycle and (d) Na/STSG 20th cycle. In the spectra, blue is discharged or Na-inserted, red is charged or Na-extracted and green is the difference. Note that the intensity of the charged spectrum is scaled relative to the discharged spectrum such that the resulting difference spectrum subtracts out the broad immobile sodium site (-10 ppm). As such, the signals in the green (difference) spectrum represent the Na sites that are cycled out during the charging.

One key aspect of battery performance is its stability under long term cycling. To probe this, the 23Na environments in TEGO after its 20th cycle are examined (Figure 3b). At the discharged state of the 20th cycle, three environments identical to the 1st discharge cycle at -2 ppm, -10 ppm and -14 ppm are observed, although the signals at -2 ppm and -14 ppm show a lower intensity as compared to the 1st cycle. The peak at -16 ppm is no longer clearly present and this suggests that the carbon pores became less accessible after prolonged cycling, contributing to the decrease in the observed capacity. In comparison to TEGO in the 1st discharged state, the electrochemically mobile sites make up only 12

% of the total 23Na NMR signal (Table S1) which is comparable to the drop in the capacity observed for these electrodes in the sodium-ion batteries. A new Na environment was observed at -27 ppm (Figure 3b) with a narrow lineshape indicating a symmetric environment and is electrochemically mobile, since it can be removed completely with charging. This environment is associated to Na in a NaPF6 salt environment (see Supporting Information Figure S4), and may indicate that after prolonged cycling there is an aggregation of NaPF6 within the TEGO electrode material or on its surface. A comparison of the Na environment in the native NaPF6 salt and in



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TEGO shows that the TEGO 23Na NMR signal is broader with a Gaussian lineshape as compared to Na in the NaPF6 salt which has a narrower 23Na NMR signal with a Lorentzian lineshape. Furthermore the Na in the -27 ppm environment is effectively cycled out on charging. These results indicate that the NaPF6 environment formed upon prolonged cycles at the discharged state is not the same as found in the crystalline salt. Importantly this appears to show that the NaPF6 species from the electrolyte can aggregate into an amorphous-like domain with nano-scale sizes and these NaPF6 domains are likely to be formed within the nanopores of the electrode material where the size confinement would prevent crystallization. In the case of the STSG (Figure 3c), three Na environments similar to those in TEGO with signals at -2 ppm, -10 ppm and -14 ppm are observed, however the signal at -16 ppm was absent. The lack of significant intensity in the -16 ppm peak suggests that there is a difference in the pore structure of the stacked graphene layers (few layer) in STSG electrode as compared to the TEGO electrode. Comparison with the charged spectrum in Figure 3c shows, similar to TEGO, during the first cycle the Na environments at -2 ppm and -14 ppm are cycled out. However the difference spectrum which in principle is characteristic of the cycled Na sites shows intensity for the -10 ppm signal along with the -2 and -14 ppm signals, Figure 3c. Recent operando 23Na solid state NMR study on hard carbon electrodes suggests on discharge at higher voltage Na is initially intercalated to the defects between graphene sheets subsequently filling the nanopores of hard carbon at lower voltage.51 The work emphasizes the importance of controlling pore structure during synthesis which has the potential ability to tune the relative capacities of the high and low-voltage process.51 These results are comparable in part with those reported here. For example, the Na environments associated with the nanopores and graphene sheets are observed on both TEGO and STSG and are electrochemically active. The difference in pore structure observed in our results between TEGO and STSG illustrates the importance of design for these nanopores for efficient cycling. The presence of the -10 ppm signal which was associated with the Na in the SEI layer in TEGO implies an unusual case for the STSG electrodes. A careful comparison of the 23Na NMR signal of the discharged and charged STSG electrodes shows that there is a change in the lineshape of the broad -10 ppm peak indicating that there is a rearrangement of the SEI structure upon charging as indicated by the intense signal for the -10 ppm peak in the difference spectrum (green, Figure 3d). Correlating this to electrochemistry, a smaller percentage capacity drop was noted for STSG compared to TEGO on the first cycle and the changing environment of the SEI may in part be responsible for this. However, solid state NMR studies on a larger library of compounds are required to understand the precise role and re-arrangements of the SEI. For the 20th charge-discharge cycle (Figure 3d), the Na environment associated with the -2 ppm and -14 ppm environments are only minimally cycled out, whereas the -27 ppm signal (NaPF6-type) is completely cycled out. The small intensity of the cycled Na in the difference spectrum is consistent with the continued drop in capacity in the STSG. In the 20th cycle, particularly large signal intensity in the difference spectrum of the SEI component of the STSG electrode is also noted, indicating the persistence of a large change in the structure of the SEI layer during cycling. Correlation of the 23Na NMR with the electrochemistry of the two distinct graphene samples demonstrates that it is likely to be the difference in the SEI structures that have the primary effect on the overall performance of these materials. This difference is particularly evident when fitting the experimental 23Na NMR lineshapes for the different materials (see results of the fitting in the

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SI). The fit of the -2 ppm signal yield a well-defined second-order quadrupolar lineshape indicating that the Na is in an ordered but asymmetric co-ordination environment (ηQ = 0.98). The signals at -14 ppm, -16 ppm and -27 ppm are best fit by narrow Gaussian lineshapes indicating a mobile or highly symmetric environment. The signal of the SEI in the TEGO is best fit by a broad Gaussian lineshape, whereas the SEI signal in the STSG requires a Czjzek distribution with a Gaussian isotropic model (GIM).52 The Gaussian model represents a distribution of 23Na chemical shifts due to a predominantly disordered structure of the SEI layer. Importantly it also means that there is a negligible distribution of the quadrupolar coupling for the 23Na in the TEGO SEI, which is consistent with an environment with a high degree of local molecular mobility. On the other hand, the Czjzek distribution with GIM implies that there is a distribution of chemical shifts and quadrupolar couplings for the 23Na environments in the STSG SEI, which is consistent with a rigid environment with a low degree of local molecular mobility.52 Correlating the electrochemistry to the structure of the SEI, the Czjzek model explains the much lower capacity observed for the STSG (Figure 1b and 1c) particularly during high rate cycling. Further details about NMR data acquisition and analysis can be found in the SI, Figures S6 and S7. Essentially the more rigid -10 ppm site would inhibit the fast transfer of ions during high rate cycling. Furthermore, the 23Na NMR signal of the TEGO SEI is stable at the charged/discharged states and with continued cycling, while there is a continual change (or evolution) in the 23Na NMR signal of the STSG SEI with cycling. This continuously changing (and most likely electrochemically active) SEI structure may explain the observed degradation of the electrochemical performance of the STSG electrode with cycling (Figure 1b) while the TEGO electrode shows a stable performance (Figure 2b) as a relative “inactive” SEI layer is formed. This can be intuitively rationalized by a greater mobility for Na in the TEGO SEI, the SEI is adaptive to an extent, while the rigid STSG SEI shows less Na mobility and therefore this SEI has to change to accommodate Na insertion/extraction. The constituents of SEI are complex and are likely to be made up of multiple inorganic or organic components. Formation of nanophases and spatial distribution around active materials and on the bulk surfaces are likely to play an important role in the ionic transfer processes between the electrode and electrolyte both at a particle and bulk scale. Intuitively a SEI structure with a series of continuous ion pathways that allow rapid ion exchange are expected to promote better charge and discharge. Thus the NMR-based fitting of the -10 ppm signal could act as a fingerprint for the “active” or “inactive” SEI which can be used to screen materials for sodium-ion electrode performance.

4. Conclusions Two chemically synthesized graphene materials with markedly different physical and chemical modifications were investigated with respect to their electrochemical performance in sodium-ion batteries. An understanding of the interaction between sodium ions and defective and chemically modified graphene surfaces was formulated using 23Na SS-NMR which showed at least three distinct Na chemical environments in both graphene materials. These chemical environments were attributed to ‘mobile sites’ and a fourth site identified with a chemical shift of -10 ppm associated with an ‘immobile site(s)’. The immobile site integrated intensity correlates to the capacity loss in the first current cycle and the mobile sites decrease in intensity with cycling while the immobile site(s) increase with cycling correlating with the change in performance of these electrodes between the 1st and 20th cycle, e.g. if prolonged cycling showed a growth of the immobile sites and the loss of mobile sites a decrease in capacity is observed.


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The immobile -10 ppm site associated with the SEI required different lineshapes during fits for the graphene materials employed. The more disordered (organic material derived) graphene material, STSG, showed a rigid SEI, that was active during charge/discharge when compared to the more ordered (graphite derived) graphene material, TEGO, SEI. The more ordered graphene material TEGO SEI demonstrated greater mobility for Na during insertion and charge/discharge. The rigidity of the SEI during charge/discharge could be the root cause determining the performance of sodium ion transport on the surface of graphene electrodes. Interestingly, an additional Na site(s) forms on the 20th discharge cycle in both graphene materials (23Na NMR peak shift at -27 ppm) which is associated to NaPF6 aggregation from the electrolyte. It appears that an “active” SEI is adverse for performance in the graphene samples explored here, however, a more expansive study is required to correlate the link between SEI and performance and the results may be material-dependent rather than a generalizable trend. Interestingly, since the mobile sites in the examined graphene samples appear to be associated with the stacked layers and carbon nanopores, the design of carbon based materials which feature few stacked layers and more nanopores may improve the insertion/extraction of sodium ions and hence improve the performance of the electrode. Therefore, this work illustrates that NMR can be employed to detect fingerprints for ‘active’ or ‘inactive’ SEI formations on electrodes. This can then be used to tailor electrode materials to form the desired SEI’s and hence performance.

5. ASSOCIATED CONTENT Supporting Information Electrochemical studies of STSG and TEGO in lithium-ion batteries, site populations of Na based on NMR signals, fits and fitting parameters of NMR spectra, NMR spectra of NaPF6, plot showing sidebands, initial MQMAS spectra, comparison of NMR spectra collected at different T1. Supporting information for this work is available free of charge via the Internet at http://pubs.acs.org.

6. AUTHOR INFORMATION Corresponding Author *Neeraj Sharma, +61293854714, [email protected].

Notes The authors declare no competing financial interests.

7. ACKNOWLEDGMENT James C. Pramudita acknowledges UNSW/ANSTO and AINSE for the PhD scholarships. Neeraj Sharma would like to thank the ARC for providing support through the DECRA (DE160100237) scheme. IRSES-EU Project MagNonMag nr. 295180.

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