Vacancy Disordered P2-Na0.67Co1âxTixO2: High-Energy and

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Na/vacancy disordered P2-Na Co TiO: high energy and high power cathode materials for sodium ion batteries Seok Mun Kang, Jae-Hyuk Park, Aihua Jin, Young Hwa Jung, Junyoung Mun, and Yung-Eun Sung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16077 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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ACS Applied Materials & Interfaces

Na+/vacancy Disordered P2-Na0.67Co1-xTixO2: High Energy and High Power Cathode Materials for Sodium Ion Batteries

Seok Mun Kang,†,‡ Jae-Hyuk Park,†,‡ Aihua Jin,†,‡ Young Hwa Jung,§ Junyoung Mun,*,¶ and Yung-Eun Sung*,†,‡

†

Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 08826, Republic

of Korea ‡

School of Chemical and Biological Engineering, Seoul National University (SNU), Seoul

08826, Republic of Korea §

Beamline Department, Pohang Accelerator Laboratory (PAL), Pohang 37673, Republic of

Korea ¶

Department of Energy and Chemical Engineering, Incheon National University (INU),

Incheon 22012, Republic of Korea

*CORRESPONDING AUTHORS Junyoung Mun e-mail: [email protected] Yung-Eun Sung e-mail: [email protected]

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ABSTRACT:

Although sodium ion batteries (NIBs) have gained wide interest, their

poor energy density poses a serious challenge for their practical applications. Therefore, high energy density cathode materials are required for NIBs to enable the utilization of a large amount of reversible Na ions. This study presents a P2-type Na0.67Co1-xTixO2 (x < 0.2) cathode with an extended potential range higher than 4.4 V to present high specific capacity of 166 mAh g-1. A group of P2-type cathodes containing various amounts of Ti is prepared using a facile synthetic method. These cathodes show different behaviors of the Na+/vacancy ordering. Na0.67CoO2 suffers severe capacity loss at high voltages due to irreversible structure changes causing serious polarization while the Ti-substituted cathodes have long credible cycleability as well as high energy. In particular, Na0.67Co0.90Ti0.10O2 exhibits excellent capacity retention (115 mAh g-1) even after 100 cycles whereas Na0.67CoO2 exhibits negligible capacity retention (105 mAh g-1), polarization higher than 0.2 V is observed in the curve of NCO (Figure 4b), and the high polarization is maintained until the completion of charging. On the contrary, Na0.67Co0.90Ti0.10O2 shows clearly different reaction pathways above 4 V in terms of polarization. With a relatively small increase in polarization, a sloping curve can be observed at charges from 4 V to 4.4 V. Na0.67Co0.90Ti0.10O2 exhibits the behavior of single-phase reaction by mitigated Na+/vacancy ordering during the extraction of Na+ above 4 V. Because its polarization is lower than that of NCO, it can utilize additional Na+ up to 4.4 V. Meanwhile, stabilized potential changes in NCO in a highly de-sodiated state. This observation is more evident in the voltage vs. time


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curve of GITT (Figure 4b). The part with increasing voltage is the sequence where the charging current is applied, and the part with decreasing voltage is the remaining section without charge. After charging to 4.36 V with large polarization (the 1st green arrow in the Figure 4b), a voltage drop appears as soon as the rest is applied. Open circuit voltage (OCV) is stabilized at 4.34 V after an instant voltage drop. Subsequently, the OCV falls deeper below 4.2 V. The highly de-sodiated NCO over 4 V is believed to be unstable and a phase change occurs when it is under the rest. This phase change is expected to be the irreversible reaction of NCO above 4 V. This reaction will be discussed further in the following section. Contrary to NCO, over the 4 V region, Na0.67Co0.90Ti0.10O2 undergoes relaxation immediately after charging (Figure 4c) without any voltage drop. In addition, its OCV remains higher than that of NCO. This result shows that Na0.67Co0.90Ti0.10O2 becomes more stable than NCO by substituting Co with Ti when highly de-sodiated and does not undergo any phase change. To examine the irreversible reaction and effect of Ti substitution, a more severe cut-off condition (2-4.5 V) is intentionally applied. During repeated cycling, NCO undergoes a sharp decrease in reversible capacity and its value is almost negligible after 100 cycles. On the other hand, even though Na0.67Co0.90Ti0.10O2 shows a steep decline similar to NCO at the beginning, a stable cycle retention appears after 10 cycles, as shown in Figure 5a. Each sample also shows a difference in coulombic efficiency. Na0.67Co0.90Ti0.

10O2

shows a high

coulombic efficiency of 99% along cycles, but NCO has an average coulombic efficiency of 91%, implying that a severe irreversible reaction occurs. Figure 5b and 5c show charge/discharge curves of NCO and Na0.67Co0.90Ti0.10O2, respectively. During the first charge process, a plateau appears at 4.375 V for NCO and its initial charge and discharge capacity is 130.8 mAh g-1 and 149.0 mAh g-1, respectively. After the first cycle, the charge plateau at 4.375 V disappears and the discharge capacity of NCO dramatically decreases to 130.5 mAh g-1 with large polarization. After 100 cycles, the reversible capacity of NCO becomes almost zero owing to high polarization. For Na0.67Co0.90Ti0.10O2, no charge plateau exists at 2.0-4.4 V but it appears at 4.43 V during the first charge process. Its initial charge and discharge capacities are 150.9 mAh g-1 and 167.2 mAh g-1, which are higher than those of NCO under the same condition. Similar to NCO, Na0.67Co0.90Ti0.10O2 shows a polarization increase and a capacity fading after the first cycle. However, after 10 cycles, it shows much more stable cycle retention. In particular, it has 114 mAh g-1 after 100 cycles whereas NCO has a



negligible specific capacity at the same cycle number (Figure 5a). To identify structural changes after the capacity decay of NCO when cycled up to 4.5 V, XRD analysis was performed for the NCO electrode after 300 cycles of charge/discharge with various cut-off values (Figure S3). XRD patterns of the long-cycled sample and a sample discharged at 2.0 V immediately after cell assembly were analyzed. However, no peak of the impurity phase was found except for the peak shift by the state of charge (SOC). Therefore, the major reason for the poor cyclability is not bulk structure changes but kinetic hindrances of surface resistance. This can be confirmed by electrochemistry (Figure S4). After the capacity decay, NCO recovered 83 % of initial discharge capacity when applied a low current density (2 mA g-1). Because XRD analysis is not sensitive to surface analysis, TEM SAED analysis was additionally performed to investigate the surface of Na0.67CoO2 after capacity decay caused by charging to 4.5 V, as shown in Figure 6. From the TEM SAED analysis, not only hexagonal NCO but also cubic Co3O4 were clearly detected at the particle surface as shown in Figure 6a and 6b, respectively. Figure 6c and 6d show calculated SAED patterns of NCO with [0 0 1] zone axis and Co3O4 with [1 0 0] zone axis obtained by CrysTBox software.40 The real patterns are almost identical to the calculated patterns. Unlike NCO, Na0.67Co0.90Ti0.10O2 exhibits only hexagonal [0 0 1] SAED pattern at particle surfaces (Figure S5). The Co3O4 phase is expected as the side product of the irreversible reaction at a high voltage, which causes capacity decay. Similar to previous reports on the capacity decay of the Li analogue, LiCoO2, impedance growth by Co3O4 is expected to occur at the surface.41 The formation of Co3O4 through a reaction between P2-type Na0.67CoO2 and the electrolyte at a high temperature (ARC test) was reported by Dahn et al.42 The reaction is expressed as below: NaxCoO2→ 1/2Na2xCoO2 + 1/6 Co3O4 + 1/6O2 The above reaction is also expected to be accelerated by an electrochemical side reaction with the electrolyte or the electrode when a high voltage (≥4.4 V) is applied.43 The difference between the two reactions is that, in electrochemical reactions, passivation by Co3O4 limits the reaction to the surface, but under heating conditions, the reaction occurs not only at the surface but also in the bulk such that it becomes detectable by XRD. As shown above, this Co3O4 formation involves the discharge of electrode and oxygen evolution.


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To further investigate the irreversible reactions associated with the Co3O4 formation, transmission in situ XRD analysis was performed. Figure 7a and b show the obtained voltage curves of NCO and Na0.67Co0.90Ti0.10O2 during in situ XRD measurements. In the right side of Figure 7a and b, (002) peak shift of Na0.67CoO2 (Figure 7a) and Na0.67Co0.90Ti0.10O2 (Figure 7b) are depicted. The lattice parameter of c corresponding to each peak is marked in Figure 7c. This is directly related to the sodium content in the electrode. The lattice parameter c values of Na0.67Co0.90Ti0.10O2 appear to be larger than those of Na0.67CoO2 throughout the charge and the rest process. As sodium ions are extracted from both electrodes, the value of lattice parameter c gradually increases up to ~4 V because a repulsive interaction between oxygen planes grows. After the increase, differences in the lattice parameter behavior occur between Na0.67CoO2 and Na0.67Co0.90Ti0.10O2. For Na0.67Co0.90Ti0.10O2, a nearly constant c value is maintained until the end of charge, continuing even to the following rest stage. However, for NCO, the c value is similarly maintained as the charge continues, but abruptly decreases as the voltage curve reaches higher than 4.0 V (0.2 > x mole of Na in the form of NaxCoO2). This shows that unfavorable changes in the crystal structure occur beyond 4.0 V for NCO. Nevertheless, this problematic behavior is significantly mitigated by Ti-substitution as shown in Figure 7c. When Ti ions substitute a part of Co ions in NCO, the tendency of the irreversible reaction occurring at high voltage is significantly reduced. This is due to the following two factors. For the irreversible reaction to occur, disproportionation of Co ions (2Co3+→ Co2+ + Co4+) and oxygen evolution (O2-→ 1/2O2 + 2e-) must occur. In the abovementioned cycleability in Figure 5a, Ti substitution effectively reduces the irreversible reaction in the above two aspects. First, disproportionation of Co3+ ions occurs via charge transfer between two Co3+ ions. Substitution of Ti4+ reduces the possibility of Co3+ pair formation. Because Ti ions are less electronegative than Co ions, charge transfer from Co to Ti is inhibited. When Ti is substituted, the formation probability of Co3+ pair is reduced. Second, Ti ions are more ionic in the bonding with oxygen. Therefore, it is more difficult for oxygen to break away from the bonding with the metal ion when Ti is substituted. Irreversible reactions in the Ti substituted sample occur at a higher voltage (4.425 V) than in the case of early stages (4.375 V) at the same current density. Furthermore, it maintains a more stable phase under high voltage. Consequently, Ti substituted NaxCo0.90Ti0.10O2 resists phase transformation that occurs at high voltages and shows superior cycle stability than pristine NCO.



4. CONCLUSIONS In this study, we demonstrated the effect of Ti substitution on P2-type NaxCoO2 through electrochemical analyses, TEM SAED, and transmission in situ XRD analysis. Various types of P2-Na0.67Co1-xTixO2 (x = 0, 0.05, 0.1, 0.15, and 0.2) were synthesized via solid-state synthesis. Among these, P2-Na0.67Co1-xTixO2, with an amount of Ti ions higher than x = 0.1, develops an unfavorable second crystal structure. Ti substitution effectively mitigated Na+/vacancy ordering in the P2-type NaxCoO2 as the amount of Ti increases showing smooth voltage profiles. In particular, NaxCo0.90Ti0.10O2 shows small increases in polarization even at over 4 V, which is contrary to the sharp increase in the polarization of NaxCoO2. These results show that Ti substituted Na0.67Co0.90Ti0.10O2 can be used in developing cells with long cyclability even under high cut-off potential. Naturally, Na0.67Co0.90Ti0.10O2 has higher specific energy than pristine Na0.67CoO2. The rate capabilities of the P2-type Na0.67Co1-xTixO2 (x = 0, 0.05 and 0.1) are effectively enhanced as the amount of Ti increases. In the case of pristine Na0.67CoO2, Co3O4 formation occurs on the surface at high voltage, leading to severe capacity degradation. However, Ti substituted Na0.67Co0.90Ti0.10O2 is much more stable and can withstand over 100 mAh g-1 of discharge capacity after 150 cycles and maintain a cycle life over 300 cycles with 4.5 V of high cut off because of its stronger bonding. Our research highlights that the energy density of sodium layered oxide materials can be increased with enhanced electrochemical stability by controlling Na+/vacancy ordering through chemical substitution.


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■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: (Figure S1) FE-SEM images of as-synthesized P2-type Na0.67Co1-xTixO2 (x = 0, 0.05, 0.1, and 0.15); (Figure S2, Table S1, Table S2) Rietveld plots and detailed information for the samples; (Figure S3) XRD patterns of Na0.67CoO2 and Na0.67Co0.90Ti0.10O2 after 300cycles; (Figure S4) Discharge profile of Na0.67CoO2 at very low current density (2 mA g-1) after 100 charge/discharge cycling in a voltage window 2-4.5 V at 100 mA g-1 with capacity decay; (Figure S5) TEM SAED patterns of Na0.67Co0.90Ti0.10O2 after 100cycles between 2.0-4.5 V; (Figure S6) Transmission in situ XRD patterns of Na0.67CoO2 and Na0.67Co0.90Ti0.10O2 from 10 to 50°

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGEMENTS This research was supported by the Institute of Basic Science (IBS) in Korea (IBSR006-D1) and Basic Science Research Program through the National Research



Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2014R1A1A1038248).

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Figure 1. (a) Schematic illustration of P2-type Na0.67Co1-xTixO2 crystal structure. Two different sites exist for Na; Na1 and Na2. (b) Synchrotron X-ray powder diffraction (SXRPD) patterns of Na0.67Co1-xTixO2(x = 0, 0.05, 0.1, 0.15 and 0.2). The wavelength of the radiation is 1.49150 Å. Inset shows a peak from an unknown impurity phase near the (002) peak. The asterisks indicate impurity phases. (c) Variations of lattice parameters a and unit cell volume with the amount of substituted Ti4+ in Na0.67Co1-xTixO2 (x = 0, 0.05 and 0.1)


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Figure 2. Electrochemical cycling performance of the Na/Na0.67Co1-xTixO2 (x = 0, 0.05 and 0.1) cells. (a)-(c) Galvanostatic charge/discharge curve of Na0.67Co1-xTixO2 (x = 0, 0.05 and 0.1) during 100 cycles. The test was conducted at 100 mA g-1 between 2 and 4.4 V. (d) dQ/dV plots of the 2nd charge/discharge curve from Fig. 2 (a)–(c). (e) Specific capacity and coulombic efficiency of each cell as a function of cycle number. (f) Specific energy of each cell as a function of cycle number.



Figure 3. (a) Rate performance of the Na/Na0.67Co1-xTixO2 (x = 0, 0.05 and 0.1) cells. (b), (c) Discharge curves with various current densities for Na0.67CoO2 and Na0.67Co0.90Ti0.10O2, respectively.


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Figure 4. (a) Galvanostatic Intermittent Titration Technique (GITT) curves of Na0.67CoO2 and Na0.67Co0.90Ti0.10O2. These GITT curves involve repeated charge at 5 mA g-1 for 1 h followed by resting for 2 h. Voltage plots as a function of time for (b) Na0.67CoO2 and (c) Na0.67Co0.90Ti0.10O2.



Figure 5. Electrochemical cycling performance of the Na/Na0.67Co1-xTixO2 (x = 0 and 0.1) cells, where 100 mA g-1 of current density is applied between 2.0 and 4.5 V. (a) Discharge capacity and coulombic efficiency of each cell as a function of cycle number up to 300 cycles. (b), (c) Charge/discharge curves of Na0.67CoO2 and Na0.67Co0.90Ti0.10O2, respectively.


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Figure 6. (a), (b) Two different TEM SAED patterns observed from the surface of capacity decayed Na0.67CoO2 (after 100 cycles between 2.0-4.5 V). (c) Simulated SAED pattern of a zone axis [0 0 1] of NaxCoO2. (d) Simulated SAED pattern of a zone axis [1 0 0] of Co3O4.



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Figure 7. Transmission in situ XRD patterns (right side) with voltage profiles (left side) of (a) Na0.67CoO2 and (b) Na0.67Co0.90Ti0.10O2 during galvanostatic charge at a rate of 10 mA g-1. The peak is hexagonal (0 0 2) peak. (c) Evolution of lattice parameter c for Na0.67CoO2 and Na0.67Co0.90Ti0.10O2 according to Na content.

Table 1. Atomic compositions of Na0.67Co1-xTixO2 (x = 0, 0.05 and 0.1) samples confirmed by ICP-AES analysis.

Sample

Theoretical formula

Measured atomic ratio

x=0

Na0.67CoO2

Na0.66CoO2

x = 0.05

Na0.67Co0.95Ti0.05O2

Na0.66Co0.95Ti0.05O2

x = 0.1

Na0.67Co0.90Ti0.10O2

Na0.66Co0.90Ti0.10O2


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Table of Contents

Na+/vacancy disordered P2-Na0.67Co1-xTixO2: high energy and high power cathode materials for sodium ion batteries Seok Mun Kang,†,‡ Jae-Hyuk Park,†,‡ Aihua Jin,†,‡ Young Hwa Jung,§ Junyoung Mun,*,¶ and Yung-Eun Sung*,†,‡


Vacancy Disordered P2-Na0.67Co1âxTixO2: High-Energy and

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