How Crystallite Size Controls the Reaction Path in Nonaqueous Metal

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How Crystallite Size Controls Reaction Path in Non-Aqueous Metal Ion Batteries: The Example of Sodium Bismuth Alloying. Jonas Sottmann, Matthias Herrmann, Ponniah Vajeeston, Yang Hu, Amund Ruud, Christina Drathen, Hermann Emerich, Helmer Fjellvåg, and David S. Wragg Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b00491 • Publication Date (Web): 01 Apr 2016 Downloaded from http://pubs.acs.org on April 4, 2016

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Chemistry of Materials

How Crystallite Size Controls Reaction Path in Non-Aqueous Metal Ion Batteries: The Example of Sodium Bismuth Alloying. Jonas Sottmann,*,† Matthias Herrmann,† Ponniah Vajeeston,† Yang Hu,† Amund Ruud,† Christina Drathen,‡ Hermann Emerich,§ Helmer Fjellvåg,*,† David S. Wragg† †

Department of Chemistry, University of Oslo, Blindern, P.O. Box 1033, 0315 Oslo, Norway.

‡

European Synchrotron, 71 Rue des Martyrs, 38043 Grenoble, France.

§

Swiss–Norwegian Beamlines, European Synchrotron, 71 Rue des Martyrs, 38043 Grenoble, France.

ABSTRACT: Crystallite size effects can influence the performance of battery materials by making the structural chemistry deviate from what is predicted by the equilibrium phase diagram. The implications of this are profound: the properties of many battery materials should be reassessed. Sodium ion battery anodes made from nanocrystalline bismuth form different phases during electrochemical cycling compared to anodes with larger crystallites. This is due to the formation of a metastable cubic polymorph of Na3Bi on the crystallite surfaces. The structural differences (weaker Na-Bi bonds, different coordination of Na to Bi) between the metastable cubic Na3Bi phase found in the nanocrystals and the hexagonal equilibrium polymorph which dominates the larger crystallites, offer an explanation for the improvements in cycling behavior observed for the nanostructured anode.

INTRODUCTION Energy storage is becoming a crucial issue as the number of intermittent renewable energy sources linked to our power grids increases. Although weight rules sodium ion batteries (SIBs) out of most of portable uses of solid state batteries, they emerge as an important technology for stationary energy storage. Their key advantage is that sodium is cheap and widely available, with a chemistry closely related to that of the well-studied lithium ion batteries (LIBs)1, 2. LIB cathode material analogues have been used successfully in SIBs, however, the graphite anodes commonly used in LiBs are not open to Na intercalation. The development of anode materials operating in the 0.2 to 0.6 V vs. Na/Na+ voltage range with high specific capacity, initial Coulombic efficiency, rate capability and long cycle life is therefore essential for the deployment of SIB technology3. Alloying anodes yield higher gravimetric and volumetric capacities compared to carbon-based or insertion systems, since one anode atom can combine with several Na atoms. At the same time they suffer from large volume changes and multiple phases changes (usually following the equilibrium phase diagram at ambient conditions) during sodiation and desodiation which reduce the cycling stability4, 5. Reversible Na alloying can be achieved with Ge, Sn, Pb, P, Sb, Bi3. In many cases this requires engineering of the crystallite size and careful selection of binder and carbon matrix to compensate for the large volume changes and the low electrical conductivity of all or some phases present during electrochemical cycling6-11.

The use of FEC (fluoroethylene carbonate) as electrolyte additive was found to lead to more stable (and thus beneficial) formation of the solid-electrolyte interface (SEI) layer11-14. Bi reacts with Na to form Na3Bi, giving a theoretical capacity of 385 mAh/g (1075 mAh/cm3). Bi has lower theoretical gravimetric capacity than other alloying anode materials, however, its volumetric capacity is readily comparable to those of Sn9, 10 and Sb11-13 and its associated volume expansion of 250 % is lower compared to 430 % and 290 %, for Sn and Sb, respectively. Furthermore, Bi has unusually low toxicity for a heavy metal. Bismuth particles embedded in graphene are reported to have 358 mAh/g specific capacity within a voltage range of 0.9 to 0.3 V vs. Na/Na+, with decent cycling stability and good rate performance15. The lithiation of Bi proceeds via sequential formation of LiBi and Li3Bi according to the Li-Bi equilibrium phase diagram16. Electrochemical Mg alloying with Bi shows two phase behavior as predicted by the equilibrium phase diagram17. Reports on the sodiation mechanism for Bi are inconsistent. Ellis et al. reported that the sodiation and desodiation mechanisms reversibly follow the Na-Bi equilibrium phase diagram18 with the formation of NaBi and Na3Bi19. Su et al. suggested Na intercalation in between Bi layers which are stacked along the c-axis15. It is important to clarify the sodiation mechanism for Bi since it is intimately linked with the cycling performance of the materials. Furthermore, it is crucial to identify factors that may influence the sodiation mechanism.

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Figure 1. (a/b) Voltage profile of Bi/C-20min / Bi/C-24h compared to (c/d) phase fractions of Na-Bi phases, (e/f) number of Na per Bi calculated from the phase fractions and (g/h) the relative shift in Bi L3 absorption edge position with respect to metallic Bi (13419 eV) for the first 1.5 galvanostatic cycles. The relative edge shift is initially positive due to presence of Bi2O3. A shift in the absorption edge position of an element to lower (higher) energies corresponds to a decrease (increase) of its average oxidation state.

It has been shown elsewhere3-5 that reducing the crystallite size in alloying anodes for SIBs/LIBs can improve their cycling stability. Tang et al. have reported that using nanocrystalline LiFePO4 as a LIB cathode can lead to deviation from the equilibrium phase diagram, exhibiting either two-phase (with an amorphous FePO4 phase formed at the expense of crystalline LiFePO4) or singlephase behavior (LiyFePO4, with y in the range 0 to 0.8)20. We show that reducing crystallite size changes the properties of the Na-Bi system and provide clear structural evidence from several techniques (including quasisimultaneous in operando synchrotron XRD/XAS and DFT calculations) explaining not only why nanosized crystallites lead to the change in battery cycling behavior; but also that, contrary to the previous findings4, 21, the alloying reaction in nanostructured anodes can deviate from the equilibrium phase diagram of the system at ambient temperature and pressure.

for further crystallographic information, Figure S8, Table S1). The Bi/C-24h crystallites are clearly in the size range (< 100 nm on one edge) of nanocrystals. The working electrodes are composed of Bi/C, additional conductive carbon black and poly(acrylic acid) (PAA) as binder, deposited on Al foil. The PAA binder was used to accommodate the large expected volume expansions6, 22, 23. All electrode preparations were performed under protective atmosphere to prevent oxidation of the Bi/C composites. SEM

RESULTS AND DISCUSSION Two bismuth/carbon (Bi/C) composite anodes were studied using quasi simultaneous in operando synchrotron XRD/XAS. The materials were produced by ball milling of Bi metal and carbon black. The first anode powder Bi/C-20min, was milled for 20 minutes; the second, Bi/C24h for 24h. The broadening of the Bragg reflections (Figure S7a) in the ex situ diffraction patterns of the two pristine materials gives average crystallite sizes of 129.6(4) and 34(1) nm for Bi/C-20min and Bi/C-24h, respectively, compared to 296(1) nm for the parent Bi metal (see ESI

Figure 2. Illustration of the different structural and electrochemical pathways in the Na-Bi system. Visualization by 25 VESTA .


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images show homogeneous coverage of the electrode film with Bi particles (mass loading of about 1 mg/cm2 in both cases) and finer Bi/C/binder aggregates for Bi/C-24h (Figures S7b and S7c). This may be indicative of the smaller Bi particle size. The best cycling stability for the Na-Bi/C composite system was obtained when FEC was used as electrolyte additive (Figure S1). The cycling profiles for the first 1.5 cycles of the two samples are shown in Figure 1 alongside the in operando XRD (phase fractions, number of Na per Bi) and XAS (Bi L3-edge position) results. Ellis et al.19 found that the alloying of Na and Bi in SIBs proceeds via a two stage reaction from Bi to NaBi (high voltage) and NaBi to Na3Bi (low voltage) in accordance with the equilibrium phase diagram18. The current XRD data show us immediately that, contrary to the findings of Ellis et al., Na-Bi alloying can lead to two different Na3Bi phases (Figure S9): for Bi/C20min we observed the previously reported hexagonal Na3Bi (h-Na3Bi), whereas for the nanosized Bi/C-24h a cubic Na3Bi phase (c-Na3Bi) occurs, previously only found at high pressures24 (see ESI for more crystallographic details). These different structural pathways (Figure 2) give rise to different electrochemical performances, especially with respect to cycling stabilities, for Bi/C-20min and Bi/C-24h. The charge/discharge voltage profiles show similar behavior in the two anode materials for the first 1.5 cycles (Figures 1a and 1b). The voltage profile of Bi/C-20min shows a pronounced step between flat high and low voltage plateaus, while it appears more sloped for Bi/C-24h. This may be due to a range of phases in the different nanocrystallites with slightly different unit cell parameters as evidenced by the microstrain contribution to the peak shape25 for Bi/C-24h (Table S1). During the first discharge the plateaus are observed at considerably lower voltage before a reversible regime is established. The high voltage plateaus during charge/discharge are found at 0.76 V / 0.70 V for Bi/C-20min and 0.79V / 0.62 V for Bi/C-24h while the low voltage plateaus occur at 0.58 V / 0.51 V for Bi/C-20min and 0.62 V / 0.38 V for Bi/C-24h (potential vs. Na/Na+) as determined from the differential capacity plots in Fig. S2. The voltage drop in the low voltage plateau of Bi/C-20min during the second discharge (Fig. 1a) is not produced by a change in potential of the working electrode, but by solvent interaction at the Na counter electrode which causes polarization26. Such a voltage drop is generally observed in flat discharge profiles at high rates of two-phase Na electrode materials. The voltage drop is not discernible for Bi/C-24h which might be due to the more sloped voltage profile. Voltages were also computed using known structures from the ICSD (cf. ESI section on DFT work) using the two-phase coexistence method27 of calculating average voltages with a Na body centered cubic (bcc) metal as the reference electrode:

V(x) = [E(NaxBi) - E(Nax+∆xBi)] ∆x + Ebcc(Na) where x = Na/Bi ratio and ‘E’ is the first-principles total energy calculated at the optimized geometry. The average

Figure 3. Cycling performance of Bi/C-20min and Bi/C-24h at a current density of 50 mA/g. A 1M solution of NaPF6 in EC/DEC with addition of 5 wt. % FEC was used as electrolyte.

voltage should be close to the thermodynamic equilibrium voltages from experiments, but depends on the proper identification of the coexisting phases. Our calculated voltage for the Bi↔NaBi↔h-Na3Bi/c-Na3Bi reaction is 0.722 V, 0.555 V / 0.525 V respectively. Even if the theoretical voltage for NaBi↔c-Na3Bi is about 0.03 V lower than for NaBi↔h-Na3Bi overpotential is more pronounced in Bi/C-24h than in Bi/C-20min. The higher overpotential in Bi/C-24h could be due to more nucleation events required to form Na-Bi phases, i. e. greater Bi crystallite density (smaller crystallites) for the same mass loading of active material in the electrode. Each nucleation event requires an activation energy which is supposedly larger for a significant volume expansion (during sodiation) than for significant volume contraction (during desodiation). This would also explain the slight discrepancies during discharge and charge, respectively. The observation of a larger overpotential for smaller particles is counterintuitive to insertion materials where larger surface area and shorter diffusion paths would improve kinetics and thereby reduce overpotential. The long term cycling performances of Bi/C-20min and Bi/C-24h are compared in Figure 3. During the first cycles the specific capacity exceeds the theoretical value which might be due to reversible contributions from amorphous carbon28 and Bi2O3 [29] (cf. ESI section on electrochemistry). The capacity of Bi/C-20min degrades rapidly with cycle number, while superior capacity retention is observed for Bi/C24h. After 100 cycles as much as 80 % of the theoretical capacity is retained for Bi/C-24h compared to 20 % for Bi/C-20min. The Coloumbic efficiency drops over the first 5 cycles before it increases to 97 % and 95 % for Bi/C-20min and Bi/C-24h, respectively. The lower Coloumbic efficiency during the first 15 cycles is probably due to significant capacity loss and irreversible side reactions that occur mainly during discharge. The rate capabilities of Bi/C-20min and Bi/C-24h are presented in the ESI section on electrochemistry. The changes in phase fractions (Figures 1c and 1d) in the alloying/de-alloying reactions during discharge/charge can be quantified using the number of Na



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per Bi (Figures 1e and 1f). This number should, in the plateau region of the voltage profile, increase in a linear manner with the number of electrons exchanged (i. e. the specific discharge/charge capacity, indicated by the added trend line, Figures 1e and 1f). During the first discharge (sodiation) of Bi/C-20min the number of Na per Bi (Figure 1e) increases proportionally with specific discharge capacity. During subsequent charge (desodiation) the number of Na per Bi decreases first slowly and then abruptly in both lower and higher voltage plateaus. This is a result of the first minor and then abrupt changes in phase fractions. Similar irregularities are observed in the lower potential plateau during the second discharge. Bi/C20min shows thus clear deviations from the expected linear relationship between number of electrons exchanged and number of Na alloyed with Bi. These observations are mirrored by the average Bi oxidation state i. e. the shift in XAS absorption edge energy (Figure 1g) which was measured quasi simultaneously with the XRD. The changes in phase fractions of the corresponding alloying/de-alloying reactions during discharge/charge of Bi/C-24h are very similar and reversible (Figure 1d). Therefore, Bi/C-24h shows a linear relationship between the number of Na per Bi (Figure 1f) as well as the Bi oxidation state (Figure 1h), and the specific discharge/charge capacity during the first 1.5 cycles.

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This behavior can be explained by the differences in the reaction paths via h-Na3Bi or c-Na3Bi (Figure 2, Table S2). In h-Na3Bi (Na3As-type), Na1 and Bi1 form simple, 2D BNlike honeycomb layers that are stacked along the c-axis and separated by Na2 which are connected to Bi (Figure 4b). The Na1-Bi1 distances within the 2D BN-like layers are 3.146 Å and the vertical Na2-Bi1 bonds between the adjacent layers are 3.209 Å. c-Na3Bi crystallizes with the Li3Bi-type structure. Here Na2 and Bi1 form a 3D network with interatomic distances of 3.319 Å (Figure 4c). The Na2-Bi1/Na1 distances are 3.833 Å. For the transformation from h-Na3Bi to NaBi (Figure 4a) major reconstructions are required; the interlayer Na2 coordinated to a single Bi1 has to be removed, and the Na1 to Bi1 coordination has to change from three to four, and several strong Na-Bi bonds (average 3.171 Å) are replaced by weaker bonds in NaBi with Na-Bi distances of 3.428 Å. At low concentrations of Na2 the h-Na3Bi structure appears to collapse forming NaBi. In the c-Na3Bi structure interlayer Na2 is shared between the Na1-Bi1 layers with two Na2-Bi1 bonds per layer and Na1 additionally fourfold coordinated to Bi1. This means fewer and weaker bonds need to be broken and reformed as the structure changes from c-Na3Bi and NaBi, resulting in the smoother conversion between structures observed for Bi/C-24h. The deviation from linearity in number of Na per Bi (Bi oxidation state) vs. charge state in the higher voltage plateau (transformation from NaBi to Bi) during the first desodiation of Bi/C20min might be a result of the preceding h-Na3Bi to NaBi transformation described above since a linear trend is observed in the subsequent sodiation as well as for the nanostructured Bi/C-24h electrode where the same conversion between NaBi and Bi is observed. In the transformation of Bi into NaBi the volume expands by 62 % (per Na). When NaBi subsequently trans-

Figure 4. Side and top view of the Na-Bi layers in (a) NaBi, (b) h-Na3Bi and (c) c-Na3Bi. Na and Bi atoms are shown in yellow and purple, respectively. Purple planes highlight the Bi layers. Numbers on the atoms represent the symmetry 25 inequivalent Na and Bi sites. Visualization by VESTA .

Figure 5. Phase fractions of the various Na-Bi phases in the th charged (2 V) and discharged state (0 V) at the 100 cycle for Bi/C-20min and Bi/C-24h. The number of Na per Bi (x) was calculated from the phase fractions obtained from Rietveld refinements (R-values are given in Table S6).


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Figure 6. Curves of calculated total energy as a function of volume for the different polymorphs of Na3Bi. Structure types are labelled on the illustration. A magnified version of the low-energy part of the curve is shown in the inset.

forms to Na3Bi, the volume changes by 117 % (59 % per Na) for h-Na3Bi, but only by 97 % (48 % per Na) for cNa3Bi. Relative to the initial Bi volume, a total expansion of 250 % and 219 % is calculated for full sodiation to hNa3Bi and c-Na3Bi, respectively. For Bi/C-24h both the smoother transformation from c-Na3Bi to NaBi and the significantly reduced volume expansion act to reduce the macrostrain in the electrode particles, which is beneficial for their structural integrity and results in enhanced cycling stability. Strong indications of particle pulverization (cracking of the crystallites) were found in Bi/C-20min by comparing ex situ XRD average crystallite size of the Na-Bi alloys in the 100th cycle to the initial size of the pristine Bi/C composites. In Bi/C-20min the crystallite size is reduced by a factor of three, from 129.6(4) nm to 41.8(2) nm whereas the much smaller crystallites in Bi/C-24h do not change significantly with prolonged cycling. The particle pulverization reduces the particle-to-particle contacts which are essential for maintaining electronic conductivity in the electrode and is accompanied by the formation of inactive residuals (i. e. originally active particles trapped inside an inactive matrix) which leads to loss of capacity with cycle number. During the first charge of Bi/C-20min, about 10 wt. % NaBi does not transform back to Bi and remains as

a residual phase. During the second discharge this NaBi residual (10 wt. %) remains electrochemically inactive. On the other hand, during the first 1.5 cycles of Bi/C-24h complete alloying and de-alloying reactions take place reversibly. These observations are in good agreement with the shifts in Bi absorption edge energy. After the first discharge the absorption edge energy shifts by about -3.3 eV from that of metallic Bi for both samples. After the first charge the Bi/C-20min absorption edge position lies about -0.3 eV lower than metallic Bi, while Bi/C-24h returns to the metallic Bi value. After the second discharge the edge position for Bi/C-20min shifts only to about -3.1 eV, while for Bi/C-24h the -3.3 eV absorption energy shift is reversibly obtained. Residual phase formation is more pronounced after 100 cycles, as shown for both electrodes at charged (2 V) and discharged states (about 0 V) in Figure 5. The formation of residual phases in Bi/C-20min continues with prolonged cycling. In the charged state in the 100th cycle, as much as 44 wt. % NaBi and also 4 wt. % h-Na3Bi are found in Bi/C-20min. Only 52 wt. % Bi is available for the alloying reaction with sodium, however, as much as 29 wt. % of Bi remains as residual and only 20 wt. % Na3Bi (mainly h-Na3Bi) is found after discharge. The main phase in the discharged state is NaBi with 54 wt. %. In total only 0.56 Na (72 mAh/g) can be alloyed with Bi in



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the Bi/C-20min electrode in the 100th cycle. For Bi/C24h, only Bi is found at charged state before the 100th cycle (no residual phases). However, some capacity is lost because 22 wt. % Bi and 6 wt. % NaBi are not fully sodiated. Still 72 wt. % Na3Bi (mainly c-Na3Bi) is formed during discharge. This results in alloying of 2.21 Na (283 mAh/g) per Bi in the Bi/C-24h electrode in the 100th cycle. For both electrodes the electrochemically measured specific capacity is in good agreement with the theoretical value obtained from the phase fractions. So far we have focused on the conversion between Na3Bi and NaBi (low voltage plateau) as a possible cause for capacity degradation. However, some loss of capacity might also be due to inefficiency of the NaBi to Bi reaction (high voltage plateau) which might give rise to the residual phases observed in Bi/C-24h in the 100th cycle. Both electrode systems should be similarly affected by the inefficiency of the conversion between NaBi and Bi. In summary, the abrupt changes in phase fractions during conversion between h-Na3Bi and NaBi in the lower voltage plateau combined with a large volume change are the most plausible reasons for electrode particle pulverization and formation of inactive residuals resulting in pronounced capacity degradation in Bi/C-20min. The ease of the structural conversion between c-Na3Bi and NaBi and the smaller associated volume change appear to reduce the formation of residual phases, giving rise to improved cycling stability for Bi/C-24h. In order to understand the relative stability of the Na3Bi chemical composition the following four structures were considered in a theoretical simulation: Na3As-type (P63/mmc; 194; Z=2), anti-LaF3-type (P-3c1; 165; Z = 6), Cu3P-type (P63cm; 185; Z = 6) and Li3Bi-type (Fm-3m; 225; Z =1). The calculated total energies as a function of unit cell volume for the four phases are displayed in Figure 6. The anti-LaF3-type derived structure is lower in energy than the Na3As-derived structure. The energy difference between the two structures is 4 meV/Z (see inset Figure 6). This finding is consistent with earlier theoretical studies30, 31. It is interesting to note that in the volume range from 125 Å3/Z to 130 Å3/Z both anti-LaF3-type derived and Cu3P-type derived structures have almost identical total energies (the difference is only 0.3 meV/Z). The energy difference between these three closely related structures (anti-LaF3-type, Na3As- and Cu3P-type) is very small. This closeness in energy suggests that external factors like temperature, minor impurities and residual lattice strain may easily affect these modifications and their relative appearance. It is not practically possible to differentiate between the three possible hexagonal polymorphs by PXRD, so the highest symmetry model (space group 194) was used to fit the diffraction data. The calculated results are valid only at zero Kelvin and for defect free ideal materials. The simulations show that on application of pressure, the anti-LaF3 modification transforms into the cubic Li3Bi-type structure at 0.82 GPa, in good agreement with earlier experimental18 and theoretical work30, 31. The energy difference between the antiLaF3- and the Li3Bi-type is 60 meV/Z. The estimated vol-

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ume difference at the equilibrium points between these phases is 12.4 Å3/Z (see Figure 6). Usually by the loss of covalency with pressure, one expects increased coordination number as found in c-Na3Bi. It should be noted that the equilibrium volume (i. e. the minimum of the energy volume curve) of c-Na3Bi is positioned below the phase transition point – explaining why it is possible to stabilize c-Na3Bi by either high pressure synthesis or particle size engineering (as in Bi/C-24h). We note that c-Na3Bi is metastable to the polymorphs of h-Na3Bi and might transform into one of them during cycling, at increased temperature or upon long term storage at ambient. Bi/C-20min and Bi/C-24h form different amounts of cNa3Bi during alloying. After the first discharge of Bi/C20min h-Na3Bi is the majority phase, however, some 4 wt. % of c-Na3Bi is present. Small amounts of the high pressure polymorph were similarly found for Na3Sb during electrochemical cycling of microcrystalline Sb vs. Na12, 13. After the first discharge of Bi/C-24h we observe mainly cNa3Bi, but with 8 wt. % of h-Na3Bi. c-Na3Bi is clearly favored in the sample with larger surface area (nanocrystalline Bi/C-24h) which indicates that nucleation and growth of c-Na3Bi occurs on the crystallite surfaces. This is further supported by the increasing c-Na3Bi to h-Na3Bi ratio in Bi/C-20min (cf. Fig. 5, discharged) due to electrochemical grinding during prolonged cycling. By controlling the crystallite size it is thus possible to direct the electrochemical reaction pathway out of the equilibrium phase situation at ambient conditions and stabilize phases that provide improved cycling performance.

CONCLUSIONS The alloying of sodium and bismuth in SIB anodes proceeds via two distinct structural mechanisms depending on the crystallite size in the Bi/C anode. This leads to significantly different cycling behavior for the nanocrystallite and microcrystallite anodes, with the former displaying significantly superior capacity stability over 100 cycles. The transformation of NaBi into c-Na3Bi requires less disturbance of the crystal structure than the NaBi to h-Na3Bi conversion, explaining this improvement. The cracking of the crystallites in the microcrystalline anode after 100 charge/discharge cycles supports this conclusion. c-Na3Bi is favored in the nanocrystalline anode as it forms on the crystallite surfaces. This insight suggests the need to further investigate other alloying systems for both Na and Li anodes in which a reduction in particle size may lead to improved cycling performance. Research should be addressed to systems for which it might be possible to deviate from the equilibrium phase diagram to create better batteries.

ASSOCIATED CONTENT Supporting Information. Experimental details; electrochemical, cycling performance with different electrolytes, differential capacity plots, discussion of surplus capacity and rate performance; Na-Bi DFT work showing calculated total energy as a function of volume for the different polymorphs of Bi and NaBi; SEM images of


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the electrode films; crystallographic information, XRD profiles and Rietveld refinement results. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected]. * E-mail: [email protected].

Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.

Funding Sources We acknowledge the University of Oslo and the Research Council of Norway (project number 143732, Nanomilib) for financial support.

ACKNOWLEDGMENT We thank Sabrina Sartori (University of Oslo, Oslo, Norway) for assistance with ball-milling and Serena Margadonna (Swansea University, Swansea, UK) for discussions. We thank the ESRF for granting beamtime at SNBL and ID22. We acknowledge use of the Norwegian national infrastructure for X-ray diffraction and Scattering (RECX) and the Research Council of Norway for providing the computer time at the Norwegian supercomputer facilities.

ABBREVIATIONS SIB, sodium ion battery; LIB, lithium ion battery; FEC, fluoroethylene carbonate; SEI, solid-electrolyte interface; PAA, poly(acrylic acid); h-Na3Bi, hexagonal Na3Bi; c-Na3Bi, cubic Na3Bi; bcc, body centered cubic.

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Size Matters: Crystallite size can influence the performance of battery materials. Sodium ion battery anodes made from nanocrystalline bismuth form different phases during electrochemical cycling compared to anodes with larger crystallites. This is due to the formation of a metastable cubic polymorph of Na3Bi on the crystallite surfaces. The structural differences offer an explanation for the improvements in cycling behavior observed for the nanostructured anode.


How Crystallite Size Controls the Reaction Path in Nonaqueous Metal

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