Effect of Nanosizing on Reversible Sodium Storage in NaCrO2 Electrode

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Effect of Nanosizing on Reversible Sodium Storage in NaCrO2 Electrode Yuka Tsuchiya, Alexey M. Glushenkov, and Naoaki Yabuuchi ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00207 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 28, 2017

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ACS Applied Nano Materials

Effect of Nanosizing on Reversible Sodium Storage in NaCrO2 Electrode

Yuka Tsuchiya1, Alexey M. Glushenkov, 2, 3 and Naoaki Yabuuchi1, 4*

1

Department of Applied Chemistry,Tokyo Denki University,

5 Senju Asahi-Cho, Adachi, Tokyo 120-8551, Japan,

2

Institute for Frontier Materials, Deakin University,

75 Pigdons Road, Waurn Ponds, Geelong, Victoria 3216, Australia 3

Department of Chemical and Biomolecular Engineering,

The University of Melbourne, Parkville, VIC 3010, Australia 4

Elements Strategy Initiative for Catalysts and Batteries, Kyoto University,

f1-30 Goryo-Ohara, Kyoto 615-8245, Japan

*Corresponding author, e-mail: [email protected]

Key words; sodium, insertion meterials, nanosize, mechanical milling, phase transtion

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Abstract

The effect of nanosizing on sodium storage performance in NaCrO2 is systematically examined. Cation-disordered rocksalt-type and nanosize NaCrO2 is prepared by mechanical milling, and layered O3-type and nanosize NaCrO2 is prepared by heat-treatment of the rocksalt phase.

The observation by high-resolution transmission electron microscopy reveals that

secondary particles consist of highly crystalline and nanosize NaCrO2 primary particles with enriched grain boundaries.

Such morphological features influence the voltage profiles in Na

cells, leading to an S-shaped profile with single phase reaction even for layered NaCrO2, in which a bi-phasic reaction dominates because of large repulsive interaction between Na ions. Moreover, O3-P3 phase transition is suppressed for the heat-treated sample with the presence of enriched grain boundaries.

The suppression of phase transition is proposed to be due to the

cancellation of CrO2 layers gliding for the incoherently aligned grain boundaries.

Thus, good

capacity retention as electrode materials is realized compared with as-prepared bulk O3 NaCrO2. Nanotechnology potentially changes materials design strategies for sodium insertion materials, leading to the development of innovative rechargeable sodium batteries in the future.

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Introduction Rechargeable Na batteries are promising to realize sustainable energy development in the future because of the material abundance for the constituents of batteries; consequently, many electrode materials have been actively researched in the world.1

Rechargeable batteries

consisting of abundant elements are required for large scale energy storage systems combined with renewable energy resources.2

Sodium-containing layered oxides are the most widely

studied as possible positive electrode materials for sodium batteries, and, among them, O3-type NaCrO2 is known to show a good cyclic performance. Additionally, O3 NaCrO2 shows the second highest operating voltage among O3-type layered oxides, inferior only to O3 NaFeO2.3-7 Thermal stability of electrode materials is of primary importance, especially for large scale energy storage applications, and a desodiated phase of NaCrO2, Na0.5CrO2, has been reported to be fundamentally safe, surprisingly even when it coexists with organic solvents on heating.5 Excellent chemical stability of Cr4+ is a significant advantage for battery applications.8 O3-type layered oxides translate into a P3-type layered structure after removing Na.9,10 Such phase transitions result in large volume changes associated with the increase in interlayer distances because of a large repulsive interaction between transition metal oxide layers. The deterioration of cyclability associated with O3-P3 transitions is, therefore, anticipated in these electrode materials. Moreover, Cr migration occurs when the Na extraction amount exceeds x

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= 0.5 in Na1–xCrO2, leading to the significant increase in polarization on electrochemical cycles.7,11

A similar problem is also evidenced for a P2-type Cr-based compound, P2

Na2/3(Cr2/3Ti1/3)O2, which has a different layered structure compared with the O3-type layered phase.12

Synthesis of layered polymorphs was not successful to extend the reversible limit of

Cr-based compounds for Na battery applications. In this study, nanosize NaCrO2 with enriched grain boundaries in individual particles is synthesized from nanosize cation-disordered rocksalt-type NaCrO2 prepared by mechanical milling. Electrochemistry and reaction mechanisms are examined in Na cells and the results are compared with conventional O3 NaCrO2.

It is found that O3-P3 phase transition is

suppressed for nanosize NaCrO2, which probably originates from the enrichment of grain boundaries and randomly oriented crystalline structures in individual particles.

Indeed, the

suppression of the phase transition results in a better cyclic performance of electrode materials. From these results, we discuss the factors affecting reaction mechanisms of materials with the O3-type layered structure used as positive electrode materials for the application in rechargeable Na batteries. At first, bulk O3NaCrO2 was prepared by calcination of a mixture between Na2CO3 and Cr2O3.

By using bulk NaCrO2 as a precursor, nanosize cation-disordered rocksalt-type

NaCrO2 was prepared by mechanical milling, as shown in Figure 1. O3 NaCrO2 was ball

4


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milled using a planetary ball mill with a zirconia pot and zirconia balls at 600 rpm for 36 h. After mechanical milling, the intensity of the 003 diffraction line at 16.5o in the 2θ range is drastically reduced, and the majority of the material is changed into a cation-disordered rocksalt phase with low crystallinity.

O3 NaCrO2 with plate-shaped particles changes into

round-shaped particles, as observed by using a scanning electron microscope (Figure S1). Large monocrystalline particles of O3 NaCrO2 with smooth edges for O3 are no longer observed after mechanical milling (Figure 2), as evidenced by transmission electron microscopy (TEM) examination.

Instead, agglomerated secondary particles consisting of

nanosize primary particles are observed (Figure 2b). These findings are further supported by electron diffraction (Figure S2). While the electron diffraction patterns of the particles in the original O3 NaCrO2 sample are periodic arrays of spots typical for monocrystals (Figure S2a), a ring pattern containing diffraction rings from the rocksalt phase is observed in the milled sample (Figure S2c). It is also found that a small fraction of the layered phase remains in the sample, as evidenced by the same electron diffraction pattern. In accordance with these results, a fraction of the particles with the cation-disordered rocksalt phase is directly observed in the high-resolution TEM image in Figure 2b.

An enlarged image is shown in Figure S3.

A

phase transition from the cation-ordered layered phase to cation-disordered rocksalt phase by mechanical milling is also known for the Li system,13 and recently the formation of Na-excess

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cation-disordered rocksalt phase by mechanical milling has been also reported.14 BET analysis reveals that the surface area of the sample is increased from 1.0 m2/g before milling to 6.8 m2/g after milling. The surface area of the milled sample is relatively small for the nanosize sample. In general, the surface area reaches 100 m2/g for non-agglomerated nanosize particles with an average particle size of 10 nm (the sample possessing similar density with NaCrO2).15

This

fact clearly indicates that the nanosize primary particles are tightly and densely agglomerated with each other as shown in Figure 2b and Figure S1, leading to a small surface area, suggesting that the particles contains enriched grain boundary consisting of nanosize particles. Heat treatment at 700 oC for 1 h was further conducted for the mechanically milled sample; the resulting sample is denoted as “HT-700” in the following discussion. The heat treatment of the sample results in the structural rearrangement and crystal growth of nanosize O3 NaCrO2 particles from the predominantly cation-disordered rocksalt-type material. The intensity of the 003 diffraction line at 16.5o clearly increases after heat treatment, but a broad peak profile is also noted in Figure 1c. Although the peak profile is broad, formation of a well-defined and layered structure with nanosize domains (approximately 5 – 15 nm) is evidenced from the TEM images (Figure 2c and Figure S3). The electron diffraction pattern also indicates that HT-700 is polycrystalline. The individual particles of HT-700 are nanosize and contain a layered NaCrO2 structure with high crystallinity.

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Surface area measured by BET

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analysis is 7.1 m2/g, which is comparable to that of the cation-disordered rocksalt phase, indicating that the sample is dense with a small fraction of pores in the particle. The phase transition process on mechanical milling and the subsequent heating process is summarized in Figure 3. The nanosize cation-disordered rocksalt domains are formed by mechanical milling, and homogeneous nucleation of layered NaCrO2 starts within the particles on heating. Crystal growth from these nucleated sites results in the formation and enrichment of grain boundaries consisting of highly crystalline and nanosize NaCrO2. Growth of nanosize NaCrO2 is inhibited when grain boundaries are created, leading to the formation of highly crystalline nanosize samples. These nanosize domains are, therefore, randomly aligned with each other within the secondary particles as evidenced in Figure 2c and Figure S3. The electrode performance of these particles was also examined in Na cells. As-prepared O3 NaCrO2 delivers 115 mAh g-1 of the reversible capacity in a Na cell when the cut-off voltage is set to 3.5 V, and the result is consistent with literature.4

After mechanical

milling, the sample with the cation-disordered rocksalt structure delivers only 50 mAh g-1 of the reversible capacity within the same voltage range. An anomalously large irreversible capacity is observed for the initial charge process. Nevertheless, the nanosize sample delivers large discharge capacity of 180 mAh g-1 when the cut-off voltage is lowered to 1.2 V vs. Na metal, as shown in Figure S4a. The voltage plateaus associated with phase transitions8 disappear, and

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charge/discharge curves change to sloping profiles.

The S-shaped voltage profile is

theoretically predicted for cation disordered materials.16

Different statistical distributions of

metal sites around Li sites in disordered structures, compared with conventional layered materials, result in the S-shaped sloping voltage profiles of electrode materials.

Charge

compensation mechanisms were examined by X-ray absorption spectroscopy, as shown in Figure S4b. For the half-charged state (x = 0.5 in Na1–xCrO2) in the layered phase, Cr ions are oxidized from trivalent into tetravalent states.17

Further oxidation beyond x = 0.5 results in the

formation of hexavalent Cr ions, leading to the deterioration of the electrode performance (Figure S4c). In contrast, for nanosize cation-disordered NaCrO2, hexavalent Cr ions, which are evidenced from the appearance of a pre-edge peak at 5993 eV in the spectrum, are formed in the half-charged state.

This pre-edge peak is further intensified on further oxidation.7

However, good reversibility is noted on both electrochemistry and X-ray absorption spectra, as shown in Figure S4a and 4c.

Nevertheless, cyclability is not acceptable for battery

applications, and reversible capacities rapidly decline on subsequent cycles.

Such an

insufficient cyclability is expected to originate from surface defects and residual strains introduced into the samples by mechanical milling, coupled with the formation of Cr6+ with high oxidative properties. heating.18

In general, residual strains are known to be effectively relieved by

Therefore, electrode performance of heat-treated and nanosize layered NaCrO2 with

8


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enriched grain boundaries (HT-700) was also examined in Na cells. The sample delivers approximately 100 mAh g-1 of a reversible capacity with good capacity retention. Its voltage profile completely changes into an S-shaped profile, and no voltage plateau is observed, as compared in Figure 4. Changes in voltage profiles for nanosize oxides containing alkali metal ions are consistent with the literature.19,20

Grain boundaries for Na-rich and Na-deficient

domains formed on Na extraction are expected to be energetically destabilized for HT-700, leading to the S-shaped voltage profile even for the layered material containing Na ions, for which large repulsive interaction is expected.1

Moreover, good capacity retention for HT-700

is evidenced from an accelerated cycle test at 100 mA g-1, as shown in Figure 4c. HT-700 delivers more than 60 % of the original reversible capacity after the 500 cycle test, which is much better than the behavior of micrometer size O3 NaCrO2. A marked

difference

is

further

observed

in

the

current

responses

to

chronoamperometry. Figure 5 compares potential-step chronoamperograms for O3 NaCrO2 before milling and nanosize NaCrO2 (HT-700). Clearly different responses are noted in both samples, and the highlighted current responses for the voltage step from 2.96 V to 3.00 V are also compared.

In the case of O3 NaCrO2, a voltage step from 2.96 to 3.00 V results in

gradually increase of current for initial 30 s and a nearly constant value of current for 500 s. This trend for Faradaic current decay cannot be explained by the Cottrell equation and is often

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observed for a biphasic process, associated with nucleation and growth of a phase transition process.21,22

This fact is also consistent with the de-sodiation mechanism for NaxCrO2.

Indeed, the biphasic reaction progresses for NaxCrO2 (0.95 < x < 0.82). In contrast, in the case of HT-700, the current for the same potential step (and for the all steps) decays exponentially as a function of time as expected from the Cottrell equation, indicating a diffusion-controlled process. This trend is generally observed for a solid-solution phase (a monophasic process) during de-sodiation. The phase transition process of HT-700 in Na cells is examined in the next section. Phase transition behavior in Na cells was examined by ex-situ X-ray diffraction (XRD), and results are compared for O3 NaCrO2 and HT-700. For O3 NaCrO2, as shown in Figure 6a and Figure S5, the O3 layered phase immediately translates into O’3 phase with an in-plane distortion on Na extraction; this phase coexists with the parent O3 phase in a two-phase reaction. O’3 phase finally changes into P’3 phase, which is accompanied by gliding of CrO2 layers. Large and discontinuous volume changes on Na extraction/re-insertion inevitably result in deterioration of cyclability. In contrast, surprisingly, ex-situ XRD data clearly indicates that the O3-P3 phase transition is effectively suppressed during charge/discharge processes. Continuous changes in the in-plane Cr-Cr distance and interslab distance are observed, and a single phase reaction is consistent with the electrochemical behavior.

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Oxygen-oxygen

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repulsive interaction between the CrO2 layers is large for P3 phase, and thus interlayer distance of P3 phase is longer than that of O3 phase. Moreover, the suppression of the gliding of layers for HT-700 is advantageous for the electrode materials, as shown in Figure 4c.

According to

Figure 2c, nanosize and highly crystalline NaCrO2 domains are randomly aligned in the individual secondary particles, and the enrichment of grain boundaries is a unique nanostructure prepared by heating of metastable phases. It is proposed that randomly aligned particles with many grain boundaries cancel and suppress gliding of transition metal layers (see Figure 7). To test this hypothesis, nanosize NaCrO2 with less grain boundary was prepared by solid-state calcination method with acetylene black (AB) used as an inhibitor for crystal growth. The mixture of Cr2O3, Na2CO3, and AB (10 wt%) was heated at 800 oC for 6 h. An XRD pattern of thus obtained sample, particle morphology, and electrochemical properties are shown in Figure S6.

Although crystallinity of the sample is slightly higher than HT-700, peak width of

diffraction line is much wider than NaCrO2 synthesized without AB. Primary particle size is reduced to nanoscale by the addition of AB as observed by SEM, and voltage profile is also modified with less pronounced voltage plateaus on electrochemical cycle. O3-P3 phase transition is unavoidable as shown in Figure S7.

Nevertheless, the

Simple size reduction of

primary particles cannot alter the phase transition process, and the enrichment of grain boundary is believed to be essential to suppress the O3-P3 phase transition.

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Lastly, impact of the enrichment of grain boundaries on electrode kinetics was examined. Rate capability of as-prepared O3 NaCrO2 and HT-700 is compared in Figure S8. As-prepared O3 NaCrO2 shows excellent rate capability, and delivers over 100 mAh g-1 even at a rate of 2,560 mA g-1. In contrast, electrode kinetics is deteriorated by the enrichment of grain boundaries. Rate capability is much better for O3 NaCrO2 when compared with HT-700. Better electrode kinetics is expected for the P3 phase because of a much wider interlayer distance. Sodium migration for HT-700 is impeded by the presence of grain boundaries, and higher diffusion barriers are anticipated because of a narrower migration path of sodium for the O3 phase and larger electrostatic repulsion from transition metal layers.1 The phase evolution process of sodium layered materials would be effectively modulated through the optimization of nanostructure and enrichment of grain boundaries, and this finding potentially results in the development of cost-effective rechargeable battery applications based on Na chemistry with long cycle life.

Conclusions Two factors affecting the voltage profile and phase transition behavior are found for NaCrO2, i.e., nanosizing and the enriched grain boundaries. Single phase reaction is more energetically preferable for the nanosized samples, and enriched grain boundaries effectively

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suppresses the O3-P3 phase transition. Moreover, the highly crystalline nanosize sample with enriched grain boundaries has been successfully prepared by heating of the metastable phase prepared by mechanical milling.

By controlling nucleation and growth of the

thermodynamically stable phase by optimized heating conditions, size of particles and concentration of grain boundaries would be adjustable.

These findings would open a new path

to designing high performance sodium insertion compounds as positive electrode materials through nanoenginnering, leading to the development of rechargeable sodium batteries for energy storage applications in the future.

Experimental Section O3 NaCrO2 was prepared from Na2CO3 (99.5%, Wako Pure Chemical Industries, Ltd.) and Cr2O3 (98%, Wako Pure Chemical Industries, Ltd.) by heat treatment at 900 oC in Ar. Thus prepared sample was stored in an Ar-filled glovebox. The composite electrodes consisted of 80

wt % active materials, 10 wt % acetylene black (AB, HS-100, Denka Corp.), and 10 wt%

polyacrylonitrile (PAN) pasted on aluminum foil as a current collector. Metallic sodium was used as

a negative electrode. The electrodes were dried at 80 °C for 2 h in vacuum and then heated at

120 °C for 2 h. Cation-disordered rocksalt NaCrO2 was prepared from O3 NaCrO2 by mechanical milling with a planetary ball mill (PULVERISETTE 7; FRITSCH) at 600 rpm. 1.5 g of O3

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NaCrO2 was mixed with 15 g of ZrO2 balls in a ZrO2 container (45 mL) for 36 h.

To enhance the

electrode performance, the sample was mixed with AB (sample:AB = 90:10 wt.%) by using the same

planetary ball mill at 300 rpm for 6 h with a zirconia pot and balls. For the preparation of

electrodes, 85 wt.% cabon composite sample, 5 wt.% additional AB, and 10 wt.% PAN were mixed

and then pasted on Al foil.

The final composition of the electrode was sample:AB:PAN =

76.5:13.5:10 by weight. Highly crystalline and nanosize NaCrO2 was prepared by heat treatment

of the cation-disordered rocksalt NaCrO2 at 700 oC for 1 h in inert atmosphere and thus obtained sample is denoted as “HT-700” in this article. 1 M of NaPF6 dissolved in propylene carbonate (Kishida Chemical) was used as an electrolyte.

A glass filter (GB-100R, Advantec) was used as a

separator. Two-electrode cells (TJ-AC, Tomcell Japan) were assembled in the Ar-filled glovebox. The cells were cycled at a rate of 10 mA g−1 at room temperature. X-ray diffraction (XRD) patterns

of samples were collected using an X-ray diffractometer (D2 PHASER, Bruker) equipped with a

one-dimensional X-ray detector using Cu Kα radiation generated at 300 W (30 kV and 10 mA) with a Ni filter. Structural analysis was carried out using RIETAN-FP.23

Schematic illustrations of

crystal structures in Figure 3 and 6 were drawn using VESTA software.24 The particle morphology

of the samples was observed using a scanning electron microscope (JCM-6000, JEOL) with an

acceleration voltage of 15 keV.

Transmission electron microscopy (TEM) examination was

conducted on a JEOL JEM 2100F instrument operated under 200 kV. Hard X-ray absorption

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spectroscopy (XAS) at the Cr K-edge was performed at beamline BL-12C of the Photon Factory

Synchrotron Source in Japan.

Hard X-ray absorption spectra were collected with a silicon

monochromator in a transmission mode. The intensity of incident and transmitted X-ray was

measured using an ionization chamber at room temperature. Composite electrode samples for

analysis were prepared using two-electrode cells with sodium counter-electrodes by cycling at a rate of 10 mA g−1. The composite electrodes were rinsed with dimethyl carbonate and sealed in a

water-resistant polymer film in the Ar-filled glovebox. Normalization of the XAS spectra was carried out using the program code IFEFFIT.25 The post-edge background was determined using a

cubic spline procedure.

Acknowledgements This study was in part supported by MEXT program “Elements Strategy Initiative to Form Core Research Center”, MEXT; Ministry of Education Culture, Sports, Science and Technology, Japan and by the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST). The synchrotron X-ray absorption work was done under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2017G005). TEM work was carried out with the support from Deakin Advanced Characterization Facility.

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ASSOCIATED CONTENT

Supporting Information: SEM images of O3 NaCrO2 before and after mechanical milling, electron diffraction patterns and TEM images of as-prepared NaCrO2, mechanically milled sample, and heat-treated sample (HT-700), electrochemical properties of mechanically milled sample, and

changes in XAS spectra for mechanically milled NaCrO2, structural evolution of as-prepared O3 NaxCrO2 on desodiation, a proposed mechanism of the suppression of O3/P3 phase transition, and synthesis and characterization of nanosize NaCrO2 with less grain boundaries.

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Figure Captions Figure 1. Changes in the XRD patterns of NaCrO2 before (a) and after (b) mechanical milling. SEM images of the samples are shown in Supporting Figure 1.

An XRD pattern of the

heat-treated sample (HT-700) is displayed as well in (c).

Figure 2. TEM images of the samples shown in Figure 1; (a) as-prepared NaCrO2, (b) after mechanical milling, and (c) after heating at 700 oC (HT-700). Electron diffraction patterns of the 18


Page 19 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


samples are also compared in Supporting Figure S2. For as-prepared NaCrO2, periodic patterns of approximately 5 nm, corresponding to the interlayer distance, are observed, indicating that sample

is parallel to the electron beam. HT-700 consists of highly crystalline and nanosize layered NaCrO2. Other TEM images of the samples are also found in Supporting Figure S3. The TEM image

shown in a blue dotted circle in (b) is enlarged in Supporting Figure S3b.

Figure 3. Schematic illustrations of phase transition processes by mechanical milling and the

subsequent heating process.

Figure 4. (a) Charge/discharge curves of NaCrO2 before and after mechanical milling at a rate of 10 mA g-1 at 25 oC.

Electrochemical data of HT-700 is also shown in (a).

Second cycle

charge/discharge curves and capacity retention of as-prepared O3 NaCrO2 and HT-700 are also compared in (b) and (c), respectively.

Figure 5. Potential-step chronoamperograms of as-prepared O3 NaCrO2 and HT-700 samples in the voltage range of 2.5 – 3.5 V with a voltage step of 40 mV.

from 2.96 to 3.00 V for both samples are also compared.

19


Chronoamperograms for the step


Figure 6. (a) Selected XRD patterns of as-prepared O3 NaCrO2 on charge. Whole XRD patterns are shown in Supporting Figure S4. (b) XRD patterns of HT-700 on charge/discharge. (c)

Calculated crystallographic parameters for both phases. (d) A schematic illustration of the phase

transition for HT-700 in Na cells.

Figure 7. A proposed mechanism of the suppression of O3/P3 phase transition for HT-700 with

enriched grain boundaries.

20


Page 20 of 28

003

(a)

ahex. = 2.973 Å

108 110 113

200

20

40

As-prepared O3 NaCrO2

a = 4.285 Å After mechanical milling

110 113

108

ahex. = 2.961 Å 107

006 101 012

(c)

104

202

111

(b)

107

006 101 012

chex. = 15.953 Å

003

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56


104

Page 21 of 28

60 2deg. (Cu K)

chex. = 15.914 Å

HT-700 80

Y.ACSTsuchiya et al., Figure 1 Paragon Plus Environment

100

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56

(a)

200 nm

10 nm

(b)

20 nm

5 nm

(c)

100 nm

10 nm


Page 22 of 28

Page 23 of 28


Crystal Structure 1 2 A A (Na, Cr)O6 3 B Naoct. B Na 4 oct. C C 5 CrO2 CrO2 A A 6 layer layer B B 7 Heat C Mechanical C 8 A A Treatment Milling 9 B B 10 Layered Structure Layered Structure Cation-Disordered 11 Rocksalt 12 Morphology 13 14 15 16 17 18 Agglomerated Nanoparticles with 19 Micrometer Size Cation-disordered Cation-disordered Cation-disordered -ordered Cation-ordered Cation-ordered Nanosize layered NaCrO2laye Nanoparticles enriched grain boundaries 20 Nanosize layered NaC Nanosize rocksalt-type NaCrO rocksalt-type NaCrO 21 2 rocksalt-type NaCrO pe NaCrO 2 with 2 yered-type 2NaCrO2NaCrO2 layered-type random orientation with random orientat with random 22 (nanoparticles) (nanoparticles) (nanoparticles) (HT-700) 23 (HT-700) (HT-7 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 ACS Paragon Plus Environment 54 55 56

Mechanical Mechanical Mechanical Milling Milling Milling

Heat Heat Heat HT-700 Treatment Treatment Treatment

Y. Tsuchiya et al., Figure 3


3.5 3.0 2.5

Voltage / V

4.0

Voltage / V

(a)

(b) 3th 1st 2nd

O3 NaCrO2

2.0 3th 2nd

3.5

1st

3.0

O3-NaCrO2 HT-700

3.5 3.0 2.5 2.0

2.5

After mechanical milling

2.0

3th 2nd

3.5

1st

3.0 2.5

HT-700

2.0 0

50

100

Capacity / mAh g

150 -1

Normalized Capacity / a. u.

(c)

150

Capacity / mAh g-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56

Page 24 of 28

O3-NaCrO2

100

50

0 0

HT-700 100


200 300 Cycle Number

400

500

Page 25 of 28

3.6

HT700

3.2

0.06 3.0 0.04

3.4

0.08

I / mA

0.08

3.2

0.06 3.0 0.04

2.8 0.02

2.96 to 3.00 V

0.06

Current Voltage

0.10

3.4

I / mA

Voltage

0.10

0.12

O3 NaCrO2

Current

Voltage / V

I / mA

3.6

0.12

Voltage / V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56


HT700

0.04

O3 NaCrO2 0.02

2.8 0.02

2.6 0.00

2.6 0.00

0

2

4

6

t / h

8

10

12

0

2

4

6

8

10

0.00

12

t / h


0

200

400

600

t / s

800

1000


(a) 1

*Page 26 of 28 *

(b)

* Al foil

*

*

* Al-foil full-discharge

P’3 2full-charge 3 45 mAh g-1 discharge 4 580 mAh g-1 O’3 6 charge full-charge 7 8 O3 + 940 mAh g-1 80 mAh g-1 charge 10 charge O’3 11 12 40 mAh g-1 charge 13 as-prepared O3 14 as-prepared 15 HT-700 16 30 35 40 45 50 17 18 20 40 60 2 deg. (Cu K) 19 20 2deg. (Cu K) 21 22 3.00 A 23 2.95 B Napri. 24 2.90 As-prepared 25 B HT-700 26 2.85 A C 27 5.8 Na oct. B P'3 28 C C 5.6 O'3 29 A O3 A 30 5.4 O3 A B No phase 31 5.2 B 32 C transition 33 125 P’3 Na CrO A 0.5 2 34 120 B 35 115 Nanosize 36 O3 Na0.5CrO2 37 110 O3 NaCrO2 0.0 0.1 0.2 0.3 0.4 0.5 38 39 x in Na1-xCrO2 40 41 42 43 44 45 46 47 48 49 50 51 52 53 ACS Paragon Plus Environment 54 55 56 110

107

108

104

012

101

006

O3

003

104

*



Cell Volume / Å3

Cr-Cr

Interlayer

(d)

Distance / Å

distance / Å

(c)

Y. Tsuchiya et al., Figure 6

Page 27 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56


As-prepared micro-size particles

10 nm

Nano-size particles with enriched grain boundaries

Grain boundaries

5 nm Suppression of CrO2 layer gliding

CrO2 layer gliding and formation of P3 phase

Ordered Orientation

Random Orientation



4.0

O3-NaCrO2 HT-700

3.5 3.0 2.5 2.0 1.5

0.06

I / mA

Voltage / V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56

0.04

HT700

2.96 to 3.00 V

O3 NaCrO2

0.02

10 nm

0.00 0.00 0.05 0.10 0.15 0.20 0.25 0.30

t / h

1.0

Normalized Capacity / a. u.

Nanosize NaCrO2 with enriched grain boundaries

TOC


Page 28 of 28

Effect of Nanosizing on Reversible Sodium Storage in NaCrO2 Electrode

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