Temperature Dependent Local Structure of NaxCoO2 Cathode

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Temperature Dependent Local Structure of NaCoO Cathode Material for Rechargeable Sodium-ion Batteries Wojciech Olszewski, Marta Ávila Pérez, Carlo Marini, Eugenio Paris, Xianfen Wang, Tatsumi Iwao, Masashi Okubo, Atsuo Yamada, Takashi Mizokawa, Naurang L. Saini, and Laura Simonelli J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10885 • Publication Date (Web): 10 Feb 2016 Downloaded from http://pubs.acs.org on February 16, 2016

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The Journal of Physical Chemistry

Temperature Dependent Local Structure of NaxCoO2 Cathode Material for Rechargeable Sodium-ion Batteries Wojciech Olszewski, Marta Ávila Pérez, Carlo Marini, Eugenio Paris, Xianfen Wang, Tatsumi Iwao, Masashi Okubo, Atsuo Yamada, Takashi Mizokawa, Naurang Lal Saini, and Laura Simonelli †,

¶

†

¶

†

‡, ⊥

¶

§

¶

‡

∗,†

†ALBA

Synchrotron Light Facility, Carrer de la Llum 2-26, 08290 Cerdanyola del Vallés, Barcelona, Spain, EU ‡Dipartimento di Fisica, Universitá di Roma La Sapienza" - P. le Aldo Moro 2, 00185 Rome, Italy, EU ¶Department of Chemical System Engineering, School of Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan §Department of Applied Physics, Waseda University, Tokyo 169-8555, Japan Faculty of Physics, University of Bialystok, 1L K. Cioªkowskiego Str., 15-245 Biaªystok, Poland, EU ⊥Center for Life NanoScience@Sapienza, Istituto Italiano di Tecnologia, V.le Regina Elena 291, 00185 Rome, Italy, EU E-mail: [email protected]

Phone: +34 935924374. Fax: +34 935924301

)>IJH=?J

We have investigated the local structure of dierently charged Na CoO cathode x

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ACS Paragon Plus Environment

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material as a function of temperature by Co K-edge X-ray absorption ne structure (EXAFS) measurements. We have found that the charge/discharge process has direct eect on the bond characteristics of cathode in the Na-ion batteries. The results reveal that the local Co-O bonds get softer while the Co-Co bonds hardly show any change during discharging (sodiation). The present study underlines the key role of local atomic displacements in the diusion and the reversibility of ions in cathodes for batteries and points towards feasibility of NaxCoO2 to be used as cathode material. The results are discussed in comparison with the lithiation/delithiation of LixCoO2 battery materials. Introduction

The rising need of energy conversion and storage requires a constant development of battery cathode materials with higher energy and power densities as well as the satisfaction of several requirements such as cost-eectiveness, safety, sustainability, eco-friendliness, and large-scale manufacturing. Beside lithium ions derived cathode materials, whose outstanding electrochemical performances originate from their small ionic size, low atomic number, and the lowest redox potential, 1,2 several other functional materials have been recently proposed. Among these, sodium-ion batteries are attracting large attention to be of potential use because of the natural abundance and low toxicity of sodium. 35 In the commercialized Li-ion batteries, LixCoO2 (energy density 400-500 Wh·kg−1 ) 6 is the most commonly used cathode material, which stores Li + ions by inserting them into a layered structure. 7 NaxCoO2, being an analogue to LixCoO2, has been considered to be used as cathode material for promising Na-ion batteries, due to its relative high energy density ( ∼260 Wh·kg−1), 8 faster Na+ diusion 9 and good electrochemical reversibility. 1012 One of the key issues in cathode materials is their structural stability during the reversible cation diusion process. Indeed, during the charging/discharging processes the crystal structure of the used material could undergo bond breaking that have direct consequences on the reversibility of ion migration in the cathode. 1,13,14 Indeed, in the structure, small Li-ion 2


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prefers to locates on octahedral or tetrahedral site in the cubic close-packed oxide ions, 15,16 in contrast with large Na-ion, that prefers to occupy octahedral or prismatic site between the CoO2 layers, 17,18 which strongly stabilizes the layered structure. It has been found that several layered polymorphs could occur, that aects the power capability depending on the crystal structures. 12 Among these polymorphs the P2-phase, 1921 where the Na-ions occupy the prismatic sites between the CoO 2 layers, generally delivers high capacity and good cyclability due to the small structural change associated with the charge/discharge process. 12 Experiments have shown that the structural and electronic properties of the layered Nax CoO2 are aected substantially by electrochemical intercalation or deintercalation of sodium (sodiation or desodiation), 19,22 having direct impact on Na + diusion velocity. 9 In particular, it has been found that the shapes of the CoO 6 octahedra, that make up the CoO 2 layers, are critically dependent on the electron count and on the distribution of the Na ions in the intervening layers. 22 These distortions occur at local scale and are not detectable by conventional diraction techniques. X-ray absorption spectroscopy in the soft or hard energy range is an atomic site-specic probe, 23 that does not require any long range crystal symmetry, and hence can be used to have a direct access to the local atomic structure of the matter, including crystalline, amorphous, and liquid states. Additionally, the large penetration depth of hard X-rays allows easily for in-situ measurements and in operando condition. Indeed, in-situ and ex-situ X-ray absorption measurements have been widely exploited to study battery materials, revealing important information on the atomic and valence electronic structure (redox chemistry) during charging and discharging. 10,11,2432 For example, Yoon et al. 32 studied the charge compensation mechanism of Li 1−x Co1/3 Ni1/3 Mn1/3 O2 cathode material, which shows higher energy density and better safety than LiCoO 2 . Similarly P. Shearing et al. 11 investigated the charge compensation mechanism on the Li-rich solid solution layered cathode xLi 2 MnO3 · (1x)LiMO2 (M = Ni, Mn, or Co) which exhibits a capacity of 250 mAh/g. I. Nakai et al. 30 instead investigated the structural changes accompanying the electrochemical Li deintercalation of Li 1−x NiO2 and Li1−x CoO2 , discovering that 3



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the Jahn−Teller distortion of the Ni −O octahedra found in LiNiO 2 decreased as the Li-ion are removed from the cathode. Incidentally, not so many eorts are dedicated to explore the local structure of Na x CoO2 cathode material. To our knowledge only Ding et al. !! by in situ XAS measurements on P2-phase of Na 0.74 CoO2 directly conrmed that deintercalation/intercalation of Na ions from/into the layered structure proceeds with the Co 3+ /Co4+ redox reaction. Here, we report the local structure of Na x CoO2 by temperature dependent ex-situ Co K-edge extended X-ray absorption ne structure (EXAFS) measurements. Generally, X-ray absorption studies on battery materials are performed at a single temperature as a function of charge, to access information on the electrochemical behavior and charge transfer mechanism during the intercalation or deintercalation process. !! However, the fact that the charging/discharging process is expected to have direct inuence on the bond lengths characteristics and disorder, it is important to perform temperature dependence measurements to nd a realistic correlation between the local structure and the battery characteristics. By a temperature dependent study it is possible to distinguish static and dynamic disorder and it is possible to have direct access to the local force constant between the atom pairs. For this reason we have investigated the local structure of Na x CoO2 cathode material as a function of temperature on dierently charged samples. We have extracted direct information on the bond strength and static disorder on dierently charged Na x CoO2 , parameter that are correlated to electrochemical performances like reversibility and capacity. The results suggest that Nax CoO2 remains highly feasible to be a promising cathode material for Na-ion batteries, however, it is likely that battery capacity associated with distinct local structure to remain one of the main concerns.

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Experimental methods P2-Nax CoO2 compounds (polymorph with CoO 6 -Na-CoO6 stacking) were synthesized by solid-state reaction of high purity raw materials. Details on the preparation and characterization along with the electrochemical studies can be found elsewhere. & For the present study we have selected samples that have suered full charging (x = 0.35), full discharging (x = 0.80) and intermediate charging (x = 0.60). The x values are calculated under the assumption that the charge/discharge current was completely consumed for Na-ion insertion/extraction in the layered structure with the starting composition being NaCoO 2 . Finely powdered samples were mixed uniformly with cellulose and pressed in pellets to ensure the Co Kedge X-ray absorption step close to 1. X-ray absorption measurements were performed at CLAESS beamline at ALBA synchrotron facility. A wiggler source was monochromatized using a Si(111) double crystal monochromator, while Rh-coated mirrors were used to reject higher harmonics. The pellets were mounted into a liquid nitrogen cryostat and the spectra were recorded in transmission mode at dierent temperatures from 77 K to 400 K (within accuracy of 1 K) by means of two ionization chambers. Several scans were acquired at each temperature to ensure spectral reproducibility and good signal to noise ratio. Standard procedure based on the cubic spline t to the pre-edge subtracted absorption spectra was used to extract the EXAFS signal ! to determine local structural parameters.

Results and discussion Fig. 1 displays the EXAFS oscillations extracted from the Co K-edge X-ray absorption spectra measured at several temperatures (77 K, 250 K and 400 K) on polycrystalline Na x CoO2 samples (x = 0.8, 0.6 and 0.35). The measured oscillations are visible up to k ∼13 Å−1 out of the noise level. There are apparent changes in the EXAFS oscillations associated with temperature and charging, indicating evolving local environment around the photo-absorbing Co atoms. 5



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Figure 1: k2 χ(k) EXAFS signals at 77 K (black squares), 250 K (red circles) and 400 K (blue triangles) obtained for samples fully charged (x = 0.35), x = 0.6 and fully discharged (x = 0.8), respectively. The lowest panel shows the comparison on the EXAFS signals at 77 K for the dierent charged samples: green diamonds, blue dots, and orange stars are representing the spectra relative to x = 0.35, 0.6, 0.8, respectively. 6


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The Fourier transform (FT) magnitudes of the EXAFS oscillations provide information on the partial atomic distribution around the photo-absorbing Co atom. The FTs were performed using the k-range of 3-13 Å −1 and are shown in Fig. 2. The main peaks at ∼1.5 Å and ∼2.4 Å correspond to single scattering contributions from the nearest neighbor O atoms (at a distance ∼1.9 Å), and the next nearest neighbors Co atoms (at a distance ∼2.8 Å), respectively. The peaks appearing at longer distances are due to multiple scattering contributions. For quantication of the local structure, we have analyzed the EXAFS by modeling the signal following general equation assuming the single-scattering approximation: 23 χ(k) =

Ni S 2 i

0 kRi2

fi (k, Ri )e−2Ri /λ e−2k

2 σ2 i

sin[2kRi + δi (k)]

(1)

where Ni is the number of neighboring atoms at a distance Ri from the absorbing atom, S02 is the passive electrons reduction factor, fi(k, Ri) is the backscattering amplitude, λ is the photo-electron mean free path, δi(k) is the phase shift, and σi2 is the Debye-Waller factor (DWF) measuring the mean square relative displacements (MSRDs) of the photo-absorber back-scatter pairs. For NaxCoO2 system the EXAFS contributions from Co-O and CoCo bond lengths are well separated from any multiple scattering contributions and can be analyzed separately. We have used Athena package 34 and Fe6L code 35 to model the EXAFS oscillations with the single scattering contributions due to Co-O and Co-Co bonds, where the phase shifts are calculated in harmonic approximation. The Ni was xed to the average values known from diraction studies. 16,19,22 E0 and S02 have been chosen and then xed after a number of t trials on dierent scans. The best values were nd 1.076 eV for E0, while for S02 0.6 and 0.9 for the Co-O and Co-Co shells, respectively. Since Ni is well known from the diraction studies, only the distances Ri and the corresponding σi2 were the t parameters in the model, keeping the correlations between the tting parameters to minimum in the least-squares ts. The real space model ts (the real space window considered in the ts is ΔR = 2 Å) are shown in Fig. 2 along with the ts in the k space (insets of Fig. 2). The number of independent parameters for the least-squares ts, that depend on the Δk 7



2

k χ(k)

77 K 250 K 400 K

0

4

k [Å

-1

8

12

]

2

k χ(k)

77 K 250 K 400 K

0

4

k [Å

-1

8

12

k [Å

-1

8

12

]

77 K

2

k χ(k)

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

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250 K 400 K

0

4

]

Figure 2: Fourier transform magnitudes weighted by k2 of the Co K-edge EXAFS (dotted curves) measured on Nax CoO2 fully charged (upper), x = 0.6 (middle) and fully discharged (lower) at three dierent temperatures (77 K, 250 K and 400 K) and model ts to the FT (solid curves). Model ts to the experimental ltered EXAFS in the k-space are shown in the insets. 8


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and ΔR windows used for the ttings are Nind ∼ 2ΔkΔR/π .

!

In the present study the Nind

results about 13 (Δk = 10 Å−1 , ΔR = 2 Å), well above the 4 t parameters. (i.e., Co-O and Co-Co distances and the corresponding DWF). The uncertainties were determined by tting independently the dierent scans that were merged to achieve the reported statistics, to take into account the errors coming from normalization and background subtraction. Any possible contribution of Co-Na ( ∼2.97 Å) was assumed negligible due to the expected smaller scattering factor of Na in compare to that of Co.

Figure 3: Distances of Co-O and Co-Co in the Na x CoO2 fully charged (black squares), x = 0.6 (red empty circles) and fully discharged (blue triangles) as a function of temperature. The distances Ri corresponding to the Co-O and Co-Co bonds for dierent Na contents as a function of temperature are reported in Fig. 3. The thermal contraction of the bondlengths seems within the experimental uncertainties. Nevertheless, the fully discharged compound 9



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(x = 0.8) shows smaller thermal contraction of the Co-O bond if compared with the charged samples. The Co-O distance exhibits an increase with increasing Na content, in agreement with previous studies. 19,22,33 Similarly, Co-Co distance also tends to increase, however, the increase is smaller than the one for the Co-O distance. This is consistent with what has been reported previously by Ding and coworkers 33 by a constant temperature variable charge study of the P2-phase of Na 0.74 CoO2 compounds, which shows the Co-Co distance being almost constant, that seems also the case of Li x CoO2 . 30,36

Figure 4: Temperature dependence of Co-O (lower) and Co-Co (upper) MSRDs for the Nax CoO2 fully discharged (left), x = 0.6 (middle) and fully charged (right). The solid lines represent the correlated Einstein model ts.

The correlated DWF ( σi2 ), i.e. the MSRDs of the Co-O and the Co-Co pairs as a function of temperature for dierent Na contents are shown in Fig. 4. The DWF ( σi2 ) comprises the temperature independent structural disorder eects ( σ02 ) as well as the thermal disorder (σ 2 (T )): i.e.,

σ 2 = σ02 + σ 2 (T ).

The static disorder ( σ02 ) can be estimated extrapolating

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the temperature behavior to 0 K (see Table 1). The static disorder

σ02

of Co-O bond tends

to increase with increasing Na content, however any such variation seems negligible for the Co-Co pair. Table 1: The Einstein temperatures ( θE ), force constants ( k ) and temperature independent DWF (σ02 ) for the fully charged (FC), x = 0.6 and fully discharged (FD) cathodes. FD, x = 0.8

θE [K] k [eV·Å−2 ] σ02 [Å2 ]

x = 0.6

FC, x = 0.35

Co-O

Co-Co

Co-O

Co-Co

Co-O

Co-Co

654±38

435±29

677±35

437±27

715±42

431±24

∼9.56±0.58

∼9.91±0.68

∼10.25±0.55

∼10.00±0.64

∼11.43±0.70

∼9.72±0.56

0.00068

0.00173

0.00081

0.00155

0.00108

0.00150

In the harmonic and single scattering approximation the simplest model to describe the temperature dependence of the Debye-Waller factor ( σ 2 (T )) is the Einstein model. !5,37 Considering the investigated temperature range we have used the Einstein equation to describe the temperature dependent

σ 2 (T ):

σ 2 (T ) = where

is the Planck constant;

is the temperature and

θE

μ

2 θE coth( ) 2μkB θE 2T

is the reduced mass;

k

is the Boltzmann constant,

T

is the Einstein temperature. From the Einstein temperature we

have extracted the Einstein frequency ( ωE constant

kB

(2)

for the Co-O and Co-Co pairs ( k

= kB θE /) = μωE2 ).

to quantify the eective local force

The Einstein temperatures and force

constants are also included in Table 1. It appears that the local Co-O bonds are getting more exible by discharging (sodiation), while the Co-Co bonds hardly show any changes. Let us compare the local structure properties of Na x CoO2 and Lix CoO2 systems. The Einstein temperatures have been reported for single crystal samples and polycrystalline powder samples of LiCoO 2 , 36,38 showing dierent values. While the Einstein temperature of Co-O pair in fully discharged Na 0.8 CoO2 sample (θE = 654 K) is found to be slightly higher than that for Li0.99 CoO2 single crystal samples ( θE = 586 K), 36 it turns out to be clearly smaller than powder sample of LiCoO 2 (θE = 720 K). 38 However, it should be recalled that the

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single crystal data provided anisotropic displacements while this information is averaged out in the powder samples. Therefore, for the consistency, a quantitative comparison should be made only between the Einstein temperatures on the powder samples. Comparing with powder LiCoO 2 compounds (θE = 720 K), it is clear that the Co-O bond is softer in the case of Nax CoO2 . The softer bond could be the reason for a faster diusion in Na x CoO2 bulk, however, the capacity is expected to be substantially deteriorated, by reducing the amount of charge that can be stored in the CoO 2 layers, in agreement with the measured capacity for Nax CoO2 cathodes that appears smaller (around 110 mAh/g) 8,33 than that in Lix CoO2 systems (around 140 mAh/g). 39,40 In addition, while the behavior with charging/discharging appears similar to Li x CoO2 , showing hardening with the desodiation, it has been found that reduction of particle size (a process similar to the delithiation) makes the Co-O bond softer in the case of Lix CoO2 . 38 On the other hand the Co-Co bonds hardly suer any change. In the present case the desodiation in Na x CoO2 appears to make the Co-O bond harder similar to the Lix CoO2 , however the changes are more prominent. On the other hand, Co-Co bond characteristics for Na x CoO2 seem comparable to what has been obtained for Li x CoO2 . 38 Therefore, similar to Li x CoO2 the charging/discharging of Na-batteries aects the congurational disorder in the Co-O sub-lattice, aecting the battery characteristics as the reversible diusion, however, the bond strength changes more prominently with the desodiation, which may be the key factor for the limited battery capacity for Na x CoO2 cathodes. In summary, by means of Co K-edges EXAFS we have studied the temperature dependent local structure of dierently charged Na x CoO2 . The EXAFS data provide important information on the static disorder and bond characteristics as a function of charging/discharging. The local Co-O and Co-Co distances tend to increase with the increasing Na (i.e., during the discharging). Instead, the temperature dependent MSRD reveal that the local Co-O bond are softer for fully discharged cathode (i.e. in the sodiated cathode), while the Co-Co bond strength appears to remain unaltered. Recalling the faster diusion rate for Na + with respect to the Li + ions, it is likely that local Co-O atomic disorder and bond strength being

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limiting factors for the diusion capacity in the Na-ion batteries.

Acknowledgement The work is partially supported under the executive protocol of the general agreement for cooperation between the Sapienza University of Rome and the University of Tokyo.

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18


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Na+

CoO6

Na+

Figure 5: Graphical TOC.

19



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2 5 0 K 4 0 0 K

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)

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-1

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1 2

[ Å -1 ] k

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7 7 K

2

k χ( k

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4

[Å ] k

)

7 7 K

k χ( k

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


2

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2 5 0 K 4 0 0 K

0


4

k

[Å ]


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


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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



he Journal of Physical Page CoO 24Chemist of 24 Na+ 6

Na+

1 2 3 4 5 ACS 6 Paragon Plus Environmen 7 8

Temperature Dependent Local Structure of NaxCoO2 Cathode

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