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Analysis of intrinsic properties of Li4Ti5O12 using single-particle technique
Journal Pre-proof Analysis of intrinsic properties of Li4Ti5O12 using single-particle technique Nurzhan Umirov, Yuto Yamada, Hirokazu Munakata, Sung-Soo Kim, Kiyoshi Kanamura PII:
S1572-6657(19)30782-9
DOI:
https://doi.org/10.1016/j.jelechem.2019.113514
Reference:
JEAC 113514
To appear in:
Journal of Electroanalytical Chemistry
Received Date: 1 July 2019 Revised Date:
23 September 2019
Accepted Date: 23 September 2019
Please cite this article as: N. Umirov, Y. Yamada, H. Munakata, S.-S. Kim, K. Kanamura, Analysis of intrinsic properties of Li4Ti5O12 using single-particle technique, Journal of Electroanalytical Chemistry (2019), doi: https://doi.org/10.1016/j.jelechem.2019.113514. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Analysis of intrinsic properties of Li4Ti5O12 using single-particle technique Nurzhan Umirova, Yuto Yamadab, Hirokazu Munakatab, Sung-Soo Kima,* and Kiyoshi Kanamurab,* a
Graduate School of Energy Science and Technology, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 34134, Republic of Korea. E-mail: [email protected]
b
Department of Applied Chemistry, Tokyo Metropolitan University, 1-1 Minami-ohsawa, Hachioji, Tokyo 192-0397, Japan. E-mail: [email protected]
*Corresponding authors: Sung-Soo Kim, e-mail: [email protected], Tel: +82-42-821-8613, Fax: +82-42-821-8839; Kiyoshi Kanamura, e-mail: [email protected], Tel/Fax: +81-42-677-2828
Abstract Attractive electrochemical properties of Li4Ti5O12 (LTO) as an anode material for lithium-ion batteries originate primarily from the lithium-ion diffusion behavior in the crystal lattice. Therefore, it is extremely important to understand the inherent material properties that are favorable for superior kinetic performance. Here we report on the intrinsic electrochemical properties of LTO without the influence of inactive electrode components (e.g., binder, conductive agent) using single-particle measurement technique. Electrochemical analysis revealed an exceptionally high rate capability of a single LTO particle compared to the conventional LTO-based electrode. In particular, a single LTO particle demonstrates capacity retention of 88% even at 440 C-rate, while conventional LTO-based electrode shows a two-fold decrease in capacity at 30 C-rate, though it is temperature dependent. Particular attention is paid to determine the correlation of phase transition behavior in a single LTO particle with activation energies of exchange current (io), charge transfer resistance (Rct) at the electrode/electrolyte interface, and diffusivity (D) of lithium-ion in the lattice obtained by single-particle measurement technique.
Keywords: Lithium-ion battery; anode material; Li4Ti5O12; phase transition; single-particle measurement.
1. Introduction Spinel Li4Ti5O12 (LTO) with the theoretical capacity of 175 mAh g-1 possesses attractive electrochemical characteristics, such as stable cyclability, relatively high rate performance, constant electrical potential, and ‘zero-strain’ property during repeated (dis)charge owing to the close lattice parameters of spinel Li4Ti5O12 and lithiated end member Li7Ti5O12. Both materials are described with the cubic space group Fd-3m having very similar lattice parameters (8.3595 and 8.3538 Å). In the structure of spinel Li4Ti5O12, the O12 occupy 32e sites while the 5/6 fraction of octahedral 16d sites are randomly occupied by Ti5 and the rest occupied by Li1. In addition, Li3 occupies tetrahedral 8a positions. While, lithiated Li7Ti5O12 forms a rock-salt structure, where Li6 occupies octahedral 16c sites instead of tetrahedral 8a positions. [1,2]. Lithium-ion (Li+) diffusion is realized through the edge-sharing octahedral [Li1/6Ti5/6]O6 framework, which forms a three-dimensional network that connects 8a sites via the 16c sites [3]. The Li+ transport behavior in both crystal lattices provides exceptional properties. In general, the two-phase electrochemical reaction is accompanied by a flat voltage plateau, which can be derived from the Gibbs free energy change [4]. While the solid-solution based reaction, with sloping plateau, is favorable for high rate capability and long-term cycle life. Since the electrochemical reaction of LTO with Li+ coincides with the stated assumptions, the question of whether Li+ (de)intercalation into the lattice is solid-solution or two-phase reaction is still under dispute and needs to be carefully reconsidered once again. According to the literature, earlier studies have reported non-direct evidence of mutually inter-convertible two-phase formation upon Li+ insertion using high angle X-ray scans [1]. Moreover, the phase separation was visualized and confirmed by scanning transmission electron microscopy coupled with electron
energy loss spectroscopy [5,6]. In addition, the mechanism of the core-shell model was proposed to explain the phenomena [7,8]. Unlike other works, Wagemaker et al. [2] reported that the absence of strain and the observation of partial 16c occupation even at room temperature indicate the solid-solution behavior, and offered a model of reaction that satisfies the previous assumptions where the two-phase relaxes to a more stable solid-solution at room temperature. The authors described the solid-solution Li4+xTi5O12 as the nm-sized domains having either tetrahedral (8a) Li occupation or octahedral (16c) Li occupation. Furthermore, it has been suggested that the abundant domain boundaries and the associated disorder appear to be responsible for the facile Li+ diffusion through the lattice, and hence nm-sized domains are most likely the origin of the relatively high rate capability. The small nano-domain size makes the material electrochemically behave as a solid-solution [2,9]. Considering the controversial discussions above, in this study, the single-particle measurement technique is employed: i) to reveal the intrinsic electrochemical properties of the single LTO particle, i.e., without influence of conductive agent and binder; ii) to determine the correlation of phase transition behavior in a single LTO particle with activation energies of exchange current (io), charge transfer resistance (Rct) at the electrode/electrolyte interface, and diffusivity (D) of lithium-ion in the lattice. In particular, the single-particle measurement technique is advantageous for its accuracy and reliability [10,11], where a microelectrode tip (20 μm) in contact with a particle of active material itself, thus eliminating all impacts from the weak binding among particles and inactive electrode components.
2. Experimental 2.1. Material characterization The Li4Ti5O12 powder was provided by Samsung Fine Chemicals and used as received. The morphology of the LTO powder was analyzed by a field emission scanning microscope (FESEM: JSM-6300). Powder X-ray diffraction (XRD, Rigaku Rint-2000) measurement was carried out using Cu Kα (λ = 1.5405 Å) radiation. The diffraction data collected in the 2θ range of 10150o with the step size 0.03o. 2.2. Electrode preparation and electrochemical measurements LTO-based composite electrode was prepared by mixing of active material powder with Ketjen black (KB) and polyvinylidene fluoride binder (PVDF) in N-methyl pyrrolidone (NMP) solution in 80:10:10 wt.% ratio. A thoroughly mixed slurry paste was coated onto Cu foil by a doctor blade method and dried in the convection oven for 12 hr at 100 oC. The loading level and density of the electrode was 2.7 mg/cm2 and 1.4 g/cc, respectively. The 2016 coin-type half-cells were assembled in the argon filled glovebox using Li metal as a counter/reference electrode. The electrolyte solution consists of 1M lithium hexafluorophosphate (LiPF6) salt dissolved in ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) (1:1:1 in vol.). For electrochemical evaluations the cells were (dis)charged at 30 oC in the potential window 1.0 – 3.0 V by applying current densities ranging from 0.1 C to 30 C (1 C corresponds to 175 mAh g-1). 2.3. Single-particle measurement The single particle experimental setup is schematically illustrated in Supplementary Information Figure S1. The tip of a fine Pt-Rh (20 µm in diameter) filament coated with a thin
film of Teflon is stand in contact with a single particle which is placed on a fritted glass disk soaked in the electrolyte solution. The electrolyte consists of 1M LiPF6 salt dissolved in ethylene carbonate and polyethylene carbonate (1:1 by vol.). The filament-to-particle contact was mechanically controlled by a micromanipulator (Eppendorf), and the operation is monitored using a microscope equipped with a CCD camera (Hirox). Electrochemical measurements were carried out with a galvano- / potentiostat IviumStat (IVIUM). The counter electrode was the Li foil wrapped on a Ni strip. The cut off voltage values were set to 1.0 V and 3.0 V vs. Li/Li+. First, the single LTO particle was (dis)charged at 3 nA three times. Further, the rate capability tests were performed at a constant current (CC) charge of 3 nA with the varying discharge currents (3 nA, 5 nA, 7 nA, 10 nA, 20 nA, 30 nA, 40 nA, 50 nA, 75 nA, 100 nA, 200 nA, 300 nA, 400 nA, 500 nA and 750 nA). Similarly, other particles were examined at different temperatures (-10 oC, 0 oC, 30 oC, 45 oC, 60 oC and 80 oC) to elucidate the temperature effect. It is noteworthy that single-particle electrochemical measurements were performed using different particles with control of diameter in the range of 17-20 µm. These measures were taken to obtain accurate and reliable experimental data by eliminating the side effects from the possible degradation of secondary particle caused by current or temperature changes. Moreover, these measurements were repeated at least 2 times to confirm the reproducibility of the results.
3. Results and discussions 3.1 Characterization Figure 1 shows the XRD diffraction patterns of commercial LTO powder. All of the peaks are in good accordance with the expected cubic spinel structure with Fd-3m space group. No
other impurities were observed. The powder is white in appearance and consists of 300 to 400 nm primary particles agglomerated into sphere shaped secondary particles with the sizes ranging from 7 to 20 μm (Figure 2).
Fig. 1. XRD patterns of LTO powder
Fig. 2. The SEM images of LTO powder at various magnifications (a) x1 000 (b) x5 000 (c) x10 000 (d) x50 000.
3.2 Electrochemical performance of the LTO-based composite electrode and a single LTO particle at 30 oC. The typical charge-discharge profiles of the composite and a single particle LTO at the near room temperature (30 oC) demonstrated in Figure 3. The LTO-based composite electrode shows approx. 185 mAh g-1 in charge capacity and 169 mAh g-1 in discharge capacity, corresponding to 91.4% of initial Coulombic efficiency (ICE) at a current rate of 0.1 C. Whereas a single LTO particle, with the size of 17.4 µm, demonstrates charge and discharge capacities of ~2 nAh and 1.75 nAh, respectively (corresponding to the ICE of 87.5%) at a current rate of 1.8 C. Both samples show an irreversible capacity loss at the first (dis)charge cycle. However, the coulombic efficiency (CE) reached almost 100% on the second (dis)charge cycle for the LTO-based composite electrode whereas a single LTO particle reached that after three cycles. A similar irreversible capacity loss during the first (dis)charge cycle was also observed by other researchers [12]. The polarization observed in the (dis)charge profile of the LTO-based composite electrode (Fig.3a) is not found in single-particle measurement experiment (Fig4b), meaning small IR drop in single LTO particle.
Fig. 3. The first (dis)charge profiles of (a) LTO-based composite electrode and (b) single LTO particle at 30 oC. Capacity retentions of both composite LTO electrode and a single LTO particle at different current rates are shown in Figure 4 as capacity retention vs. log C-rate. The rate capability evaluation was carried out after three cycles with the constant current charge rate of 0.1 C and increasing the discharge current rate from 0.1 C up to 30 C for composite LTO electrode. While for the single particle experiment, the particle was (dis)charged three cycles at 3 nA (∼1.8 C) with the consequent increasing discharge currents from 3 nA to 750 nA, which corresponds to 1.8 C and 440 C, respectively. A single LTO particle shows exceptional capacity retention of 88% at high discharge rate as 440 C in comparison to the composite LTO electrode which capacity abruptly decreases and retains 52% at 30 C. The exceptionally high rate capability, which results from the rapid and reversible intercalation of Li+ means that the reaction is single-phase rather than two-phase. At room temperature, Li+ have enough energy to migrate into 16c sites resulting in a disorder that makes possible for Li+ diffuse faster in the lattice, which in turn results in a high rate capability.
Fig. 4. Rate capabilities of LTO-based composite electrode and a single LTO particle represented as capacity retention vs. log C-rate at 30 oC. 3.3 The effect of temperature on a single LTO particle The single-particle measurements were carried out at the different temperature ranges from 10 oC to 80 oC to understand the dependence of the electrochemical performance on temperature. Figure 5 demonstrates the first three (dis)charge profiles of single LTO particles at the current rate of 1.8 C. All profiles exhibit irreversible capacity loss in the initial (dis)charge process, and no other differences were observed, except for discrepancy in the capacity value, which is particle size dependent. An example of expected capacity calculation is given in Supplementary Materials.
Fig. 5. The first three cycles of single LTO particles at different temperatures (as specified in the graph) As proposed by C. Pecharroman et al. [13], Li-ions originally placed at 8a tetrahedral sites in the spinel structure, partially migrate to the near empty 16c octahedral sites and the migration process increases with temperature. Such behavior suggests a fast and versatile Li+ intercalation at high current rates and temperatures. Figure 6 shows the (dis)charge profiles of the single LTO particles at different temperatures (as specified in the graph) with subsequent discharge C-rates increment every single cycle. Notably, with the consequent increasing the discharge currents from 3 nA to 750 nA, which corresponds to 1.8 C-rate and 440 C-rate, respectively, the single
LTO particles demonstrate clear deterioration in the electrochemical performance with a decrease in temperature. In particular, at the high discharge rate as 440 C, the capacity retentions are 65%, 80%, 88%, 88%, 97%, and 98% at the experimental temperatures -10 oC, 0 oC, 30 oC, 45 oC, 60 oC and 80 oC, respectively. The deterioration of the rate capabilities at low temperatures and high current densities is most likely due to the large contribution of charge and mass transfer resistances. This is also has been suggested as the advent of kinetic two-phase reaction, especially at low temperature by Wagemaker et al. [2]. Thereby, it might be implied that the solid-solution reaction mechanism occurs at the abundant disorder of 8a/16c occupation, in specific chemical composition and temperatures.
Fig. 6. The rate capabilities of single LTO particles at different temperatures (as specified in the graph).
3.4 Derivation of the values of kinetic parameters by quasi Tafel plots. Li+ (de)insertion at single LTO particle involve certain processes: i) diffusion of solvated Li+ from the bulk electrolyte to the particle surface; ii) interfacial charge transfer (Li+ transfer) at the interface between the particle and the electrolyte; solid-state Li+ diffusion within the particle; and crystallographic structural changes in the solid. In general, the diffusion coefficient of Li+ in a liquid electrolyte is higher than that in a solid. Therefore, the reaction rate at a single particle is apparently controlled by charge transfer, solid-state diffusion, and/or crystallographic structural changes [14]. When the Li+ reaction is controlled by the charge transfer process (a mass-transfer process is negligible, small current), the relationship between delithiation current and overpotential can be expressed by the Butler-Volmer equation. When the applied current is small, the departure from the equilibrium potential Eeq during electrochemical lithiation and delithiation are also small. Thus, midpoint potential can be treated as shown in equation (1). Eeq = (Elithiation + Edelithiation) /2
(1)
At large overpotentials, the kinetic parameters can be obtained using the Tafel equation (equation 2) ia = log i0 +αFɳ/2.303RT
(2)
Here, ia is the applied current density, i0 is the exchange current density, α is the transfer coefficient for the anodic reaction, F is the Faraday constant, R is the gas constant, T is the temperature, and η = (E - Eeq) is the overpotential. Charge transfer coefficient, α is the parameter that signifies the fraction of overpotential that affects the current density and can be approximated to be 0.5. Exchange current, io is defined as the current flowing in both directions
per unit area when an electrode reaction is at equilibrium (and, hence, at its equilibrium potential). If io is small, then little current flows and the reactions at dynamic equilibrium are generally slow. Likewise, a high io gives a fast reaction. The exchange current density can be obtained from the intercept of the linear segment αF/2.303RT. Here, Tafel slopes were derived based on galvanostatic curves at various current rates. Using the following correlation for a oneelectron process, we can obtain the charge-transfer resistance Rct value, at the interface as Rct = RT/Fio [15]. The exchange current densities, charge transfer resistances and diffusivity of Li+ were obtained from the Tafel plots at different temperatures, and the values of kinetic parameters are summarized in Table 1. The charge transfer resistance Rct decreases with temperature increase, while both the exchange current density i0 and diffusivity D increases with increasing temperature from -10 oC to 80 oC, which means the reaction is faster at higher temperatures. The increase of diffusivity at high temperatures can be explained by the fact that Li+ have enough energy to self-diffuse [16] and generate chaos, making it easy to move for Li+ while providing fast migration through hopping mechanism. The temperature dependence of exchange current density, charge transfer resistance, and diffusion shows Arrhenius-type behavior as presented in Fig. 7. In the case of diffusivity, two distinct slopes are observed at low and high temperatures. The activation energies were obtained from the slopes of the Arrhenius plots according to the Arrhenius equation. The activation energies of exchange current density and charge transfer of Li+ were calculated to be 23.1 kJ mol-1 and 20.5 kJ mol-1 respectively, whereas diffusivity activation energy showed 4.3 kJ mol-1 at low temperatures and 24.8 kJ mol-1 at higher temperatures. As suggested previously, at low temperatures (large domain size also included) the Li+ deintercalation induces the two-phase reaction kinetically, whereas, at high temperatures, the
solid-solution behavior is dominant. The activation energy variation with temperature could be the indirect evidence of multiple reaction mechanisms. The small activation energy of diffusion (4.3 kJ mol-1) at a lower temperature can be attributed to the energetically favorable 8a and 16c sites occupation without Coulombic repulsion [9].
Table 1. The values of kinetic parameters (exchange current density io, charge transfer resistance Rct, diffusivity D) obtained at different temperatures (DOD 50%) Particle diameter, µm 19.6 20.2 17.4 18.4 17.4 17.9
Temperature, o C -10 0 30 45 60 80
io, mA cm-2 0.5 2.2 3.2 6.5 8.8 9.6
Rct, Ωcm2 45.4 10.0 8.2 4.2 3.2 3.1
D, cm s-1 1.7× 10-8 1.8× 10-8 2.2× 10-8 3.5× 10-8 4.5× 10-8 9.3× 10-8
Fig. 7. Arrhenius plots as lni0 against 1/T, lnD against 1/T and ln1/Rct against 1/T. The straight lines represent a fit to the data yielding the activation energy as indicated (DOD 50%)
4. Conclusions In this study, the electrochemical performances of LTO-based composite electrode and a single LTO particle were comparatively analyzed. It was found that a single LTO particle, indeed, shows outstanding performance in comparison to the LTO-based composite electrode, where intrinsic properties of active material were affected by the inactive components of the electrode. As a result, a single LTO particle demonstrates superior capacity retention of 88% at high discharge rate as 440 C, while conventional LTO-based electrode shows a two-fold decrease in capacity (52% retention) at 30 C-rate. The exceptionally high rate capability that results from the rapid and reversible intercalation of Li+ proves that the reaction is single-phase rather than twophase at room temperature. Also, the performance of a single LTO particle was examined in the temperature range from -10 oC to 80 oC. Through Arrhenius plots, the activation energies of exchange current density (io) and charge transfer resistance (Rct) at DOD 50% evaluated as 23.1 kJ mol-1 and 20.5 kJ mol-1, respectively. The activation energy of solid-state diffusivity (D) varies with temperature and has a smaller value at the low temperatures. At elevated temperatures, the activation energy of D is 24.8 kJ mol-1, whereas that of low temperature is 4.3 kJ mol-1, implying the two-phase segregation.
Acknowledgment The authors are very grateful for the financial support of the KETEP (Grant 20164010201070) and Chungnam National University.
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Highlights •
The intrinsic properties of LTO were investigated by single particle measurement
•
Single LTO particle shows outstanding rate capability vs. composite LTO electrode
•
Activation energies of io, Rct, and D evaluated in the range of -10 oC to 80 oC
•
LTO reaction is single-phase rather than two-phase at certain temperatures.
S1572-6657(19)30782-9
DOI:
https://doi.org/10.1016/j.jelechem.2019.113514
Reference:
JEAC 113514
To appear in:
Journal of Electroanalytical Chemistry
Received Date: 1 July 2019 Revised Date:
23 September 2019
Accepted Date: 23 September 2019
Please cite this article as: N. Umirov, Y. Yamada, H. Munakata, S.-S. Kim, K. Kanamura, Analysis of intrinsic properties of Li4Ti5O12 using single-particle technique, Journal of Electroanalytical Chemistry (2019), doi: https://doi.org/10.1016/j.jelechem.2019.113514. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Analysis of intrinsic properties of Li4Ti5O12 using single-particle technique Nurzhan Umirova, Yuto Yamadab, Hirokazu Munakatab, Sung-Soo Kima,* and Kiyoshi Kanamurab,* a
Graduate School of Energy Science and Technology, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 34134, Republic of Korea. E-mail: [email protected]
b
Department of Applied Chemistry, Tokyo Metropolitan University, 1-1 Minami-ohsawa, Hachioji, Tokyo 192-0397, Japan. E-mail: [email protected]
*Corresponding authors: Sung-Soo Kim, e-mail: [email protected], Tel: +82-42-821-8613, Fax: +82-42-821-8839; Kiyoshi Kanamura, e-mail: [email protected], Tel/Fax: +81-42-677-2828
Abstract Attractive electrochemical properties of Li4Ti5O12 (LTO) as an anode material for lithium-ion batteries originate primarily from the lithium-ion diffusion behavior in the crystal lattice. Therefore, it is extremely important to understand the inherent material properties that are favorable for superior kinetic performance. Here we report on the intrinsic electrochemical properties of LTO without the influence of inactive electrode components (e.g., binder, conductive agent) using single-particle measurement technique. Electrochemical analysis revealed an exceptionally high rate capability of a single LTO particle compared to the conventional LTO-based electrode. In particular, a single LTO particle demonstrates capacity retention of 88% even at 440 C-rate, while conventional LTO-based electrode shows a two-fold decrease in capacity at 30 C-rate, though it is temperature dependent. Particular attention is paid to determine the correlation of phase transition behavior in a single LTO particle with activation energies of exchange current (io), charge transfer resistance (Rct) at the electrode/electrolyte interface, and diffusivity (D) of lithium-ion in the lattice obtained by single-particle measurement technique.
Keywords: Lithium-ion battery; anode material; Li4Ti5O12; phase transition; single-particle measurement.
1. Introduction Spinel Li4Ti5O12 (LTO) with the theoretical capacity of 175 mAh g-1 possesses attractive electrochemical characteristics, such as stable cyclability, relatively high rate performance, constant electrical potential, and ‘zero-strain’ property during repeated (dis)charge owing to the close lattice parameters of spinel Li4Ti5O12 and lithiated end member Li7Ti5O12. Both materials are described with the cubic space group Fd-3m having very similar lattice parameters (8.3595 and 8.3538 Å). In the structure of spinel Li4Ti5O12, the O12 occupy 32e sites while the 5/6 fraction of octahedral 16d sites are randomly occupied by Ti5 and the rest occupied by Li1. In addition, Li3 occupies tetrahedral 8a positions. While, lithiated Li7Ti5O12 forms a rock-salt structure, where Li6 occupies octahedral 16c sites instead of tetrahedral 8a positions. [1,2]. Lithium-ion (Li+) diffusion is realized through the edge-sharing octahedral [Li1/6Ti5/6]O6 framework, which forms a three-dimensional network that connects 8a sites via the 16c sites [3]. The Li+ transport behavior in both crystal lattices provides exceptional properties. In general, the two-phase electrochemical reaction is accompanied by a flat voltage plateau, which can be derived from the Gibbs free energy change [4]. While the solid-solution based reaction, with sloping plateau, is favorable for high rate capability and long-term cycle life. Since the electrochemical reaction of LTO with Li+ coincides with the stated assumptions, the question of whether Li+ (de)intercalation into the lattice is solid-solution or two-phase reaction is still under dispute and needs to be carefully reconsidered once again. According to the literature, earlier studies have reported non-direct evidence of mutually inter-convertible two-phase formation upon Li+ insertion using high angle X-ray scans [1]. Moreover, the phase separation was visualized and confirmed by scanning transmission electron microscopy coupled with electron
energy loss spectroscopy [5,6]. In addition, the mechanism of the core-shell model was proposed to explain the phenomena [7,8]. Unlike other works, Wagemaker et al. [2] reported that the absence of strain and the observation of partial 16c occupation even at room temperature indicate the solid-solution behavior, and offered a model of reaction that satisfies the previous assumptions where the two-phase relaxes to a more stable solid-solution at room temperature. The authors described the solid-solution Li4+xTi5O12 as the nm-sized domains having either tetrahedral (8a) Li occupation or octahedral (16c) Li occupation. Furthermore, it has been suggested that the abundant domain boundaries and the associated disorder appear to be responsible for the facile Li+ diffusion through the lattice, and hence nm-sized domains are most likely the origin of the relatively high rate capability. The small nano-domain size makes the material electrochemically behave as a solid-solution [2,9]. Considering the controversial discussions above, in this study, the single-particle measurement technique is employed: i) to reveal the intrinsic electrochemical properties of the single LTO particle, i.e., without influence of conductive agent and binder; ii) to determine the correlation of phase transition behavior in a single LTO particle with activation energies of exchange current (io), charge transfer resistance (Rct) at the electrode/electrolyte interface, and diffusivity (D) of lithium-ion in the lattice. In particular, the single-particle measurement technique is advantageous for its accuracy and reliability [10,11], where a microelectrode tip (20 μm) in contact with a particle of active material itself, thus eliminating all impacts from the weak binding among particles and inactive electrode components.
2. Experimental 2.1. Material characterization The Li4Ti5O12 powder was provided by Samsung Fine Chemicals and used as received. The morphology of the LTO powder was analyzed by a field emission scanning microscope (FESEM: JSM-6300). Powder X-ray diffraction (XRD, Rigaku Rint-2000) measurement was carried out using Cu Kα (λ = 1.5405 Å) radiation. The diffraction data collected in the 2θ range of 10150o with the step size 0.03o. 2.2. Electrode preparation and electrochemical measurements LTO-based composite electrode was prepared by mixing of active material powder with Ketjen black (KB) and polyvinylidene fluoride binder (PVDF) in N-methyl pyrrolidone (NMP) solution in 80:10:10 wt.% ratio. A thoroughly mixed slurry paste was coated onto Cu foil by a doctor blade method and dried in the convection oven for 12 hr at 100 oC. The loading level and density of the electrode was 2.7 mg/cm2 and 1.4 g/cc, respectively. The 2016 coin-type half-cells were assembled in the argon filled glovebox using Li metal as a counter/reference electrode. The electrolyte solution consists of 1M lithium hexafluorophosphate (LiPF6) salt dissolved in ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) (1:1:1 in vol.). For electrochemical evaluations the cells were (dis)charged at 30 oC in the potential window 1.0 – 3.0 V by applying current densities ranging from 0.1 C to 30 C (1 C corresponds to 175 mAh g-1). 2.3. Single-particle measurement The single particle experimental setup is schematically illustrated in Supplementary Information Figure S1. The tip of a fine Pt-Rh (20 µm in diameter) filament coated with a thin
film of Teflon is stand in contact with a single particle which is placed on a fritted glass disk soaked in the electrolyte solution. The electrolyte consists of 1M LiPF6 salt dissolved in ethylene carbonate and polyethylene carbonate (1:1 by vol.). The filament-to-particle contact was mechanically controlled by a micromanipulator (Eppendorf), and the operation is monitored using a microscope equipped with a CCD camera (Hirox). Electrochemical measurements were carried out with a galvano- / potentiostat IviumStat (IVIUM). The counter electrode was the Li foil wrapped on a Ni strip. The cut off voltage values were set to 1.0 V and 3.0 V vs. Li/Li+. First, the single LTO particle was (dis)charged at 3 nA three times. Further, the rate capability tests were performed at a constant current (CC) charge of 3 nA with the varying discharge currents (3 nA, 5 nA, 7 nA, 10 nA, 20 nA, 30 nA, 40 nA, 50 nA, 75 nA, 100 nA, 200 nA, 300 nA, 400 nA, 500 nA and 750 nA). Similarly, other particles were examined at different temperatures (-10 oC, 0 oC, 30 oC, 45 oC, 60 oC and 80 oC) to elucidate the temperature effect. It is noteworthy that single-particle electrochemical measurements were performed using different particles with control of diameter in the range of 17-20 µm. These measures were taken to obtain accurate and reliable experimental data by eliminating the side effects from the possible degradation of secondary particle caused by current or temperature changes. Moreover, these measurements were repeated at least 2 times to confirm the reproducibility of the results.
3. Results and discussions 3.1 Characterization Figure 1 shows the XRD diffraction patterns of commercial LTO powder. All of the peaks are in good accordance with the expected cubic spinel structure with Fd-3m space group. No
other impurities were observed. The powder is white in appearance and consists of 300 to 400 nm primary particles agglomerated into sphere shaped secondary particles with the sizes ranging from 7 to 20 μm (Figure 2).
Fig. 1. XRD patterns of LTO powder
Fig. 2. The SEM images of LTO powder at various magnifications (a) x1 000 (b) x5 000 (c) x10 000 (d) x50 000.
3.2 Electrochemical performance of the LTO-based composite electrode and a single LTO particle at 30 oC. The typical charge-discharge profiles of the composite and a single particle LTO at the near room temperature (30 oC) demonstrated in Figure 3. The LTO-based composite electrode shows approx. 185 mAh g-1 in charge capacity and 169 mAh g-1 in discharge capacity, corresponding to 91.4% of initial Coulombic efficiency (ICE) at a current rate of 0.1 C. Whereas a single LTO particle, with the size of 17.4 µm, demonstrates charge and discharge capacities of ~2 nAh and 1.75 nAh, respectively (corresponding to the ICE of 87.5%) at a current rate of 1.8 C. Both samples show an irreversible capacity loss at the first (dis)charge cycle. However, the coulombic efficiency (CE) reached almost 100% on the second (dis)charge cycle for the LTO-based composite electrode whereas a single LTO particle reached that after three cycles. A similar irreversible capacity loss during the first (dis)charge cycle was also observed by other researchers [12]. The polarization observed in the (dis)charge profile of the LTO-based composite electrode (Fig.3a) is not found in single-particle measurement experiment (Fig4b), meaning small IR drop in single LTO particle.
Fig. 3. The first (dis)charge profiles of (a) LTO-based composite electrode and (b) single LTO particle at 30 oC. Capacity retentions of both composite LTO electrode and a single LTO particle at different current rates are shown in Figure 4 as capacity retention vs. log C-rate. The rate capability evaluation was carried out after three cycles with the constant current charge rate of 0.1 C and increasing the discharge current rate from 0.1 C up to 30 C for composite LTO electrode. While for the single particle experiment, the particle was (dis)charged three cycles at 3 nA (∼1.8 C) with the consequent increasing discharge currents from 3 nA to 750 nA, which corresponds to 1.8 C and 440 C, respectively. A single LTO particle shows exceptional capacity retention of 88% at high discharge rate as 440 C in comparison to the composite LTO electrode which capacity abruptly decreases and retains 52% at 30 C. The exceptionally high rate capability, which results from the rapid and reversible intercalation of Li+ means that the reaction is single-phase rather than two-phase. At room temperature, Li+ have enough energy to migrate into 16c sites resulting in a disorder that makes possible for Li+ diffuse faster in the lattice, which in turn results in a high rate capability.
Fig. 4. Rate capabilities of LTO-based composite electrode and a single LTO particle represented as capacity retention vs. log C-rate at 30 oC. 3.3 The effect of temperature on a single LTO particle The single-particle measurements were carried out at the different temperature ranges from 10 oC to 80 oC to understand the dependence of the electrochemical performance on temperature. Figure 5 demonstrates the first three (dis)charge profiles of single LTO particles at the current rate of 1.8 C. All profiles exhibit irreversible capacity loss in the initial (dis)charge process, and no other differences were observed, except for discrepancy in the capacity value, which is particle size dependent. An example of expected capacity calculation is given in Supplementary Materials.
Fig. 5. The first three cycles of single LTO particles at different temperatures (as specified in the graph) As proposed by C. Pecharroman et al. [13], Li-ions originally placed at 8a tetrahedral sites in the spinel structure, partially migrate to the near empty 16c octahedral sites and the migration process increases with temperature. Such behavior suggests a fast and versatile Li+ intercalation at high current rates and temperatures. Figure 6 shows the (dis)charge profiles of the single LTO particles at different temperatures (as specified in the graph) with subsequent discharge C-rates increment every single cycle. Notably, with the consequent increasing the discharge currents from 3 nA to 750 nA, which corresponds to 1.8 C-rate and 440 C-rate, respectively, the single
LTO particles demonstrate clear deterioration in the electrochemical performance with a decrease in temperature. In particular, at the high discharge rate as 440 C, the capacity retentions are 65%, 80%, 88%, 88%, 97%, and 98% at the experimental temperatures -10 oC, 0 oC, 30 oC, 45 oC, 60 oC and 80 oC, respectively. The deterioration of the rate capabilities at low temperatures and high current densities is most likely due to the large contribution of charge and mass transfer resistances. This is also has been suggested as the advent of kinetic two-phase reaction, especially at low temperature by Wagemaker et al. [2]. Thereby, it might be implied that the solid-solution reaction mechanism occurs at the abundant disorder of 8a/16c occupation, in specific chemical composition and temperatures.
Fig. 6. The rate capabilities of single LTO particles at different temperatures (as specified in the graph).
3.4 Derivation of the values of kinetic parameters by quasi Tafel plots. Li+ (de)insertion at single LTO particle involve certain processes: i) diffusion of solvated Li+ from the bulk electrolyte to the particle surface; ii) interfacial charge transfer (Li+ transfer) at the interface between the particle and the electrolyte; solid-state Li+ diffusion within the particle; and crystallographic structural changes in the solid. In general, the diffusion coefficient of Li+ in a liquid electrolyte is higher than that in a solid. Therefore, the reaction rate at a single particle is apparently controlled by charge transfer, solid-state diffusion, and/or crystallographic structural changes [14]. When the Li+ reaction is controlled by the charge transfer process (a mass-transfer process is negligible, small current), the relationship between delithiation current and overpotential can be expressed by the Butler-Volmer equation. When the applied current is small, the departure from the equilibrium potential Eeq during electrochemical lithiation and delithiation are also small. Thus, midpoint potential can be treated as shown in equation (1). Eeq = (Elithiation + Edelithiation) /2
(1)
At large overpotentials, the kinetic parameters can be obtained using the Tafel equation (equation 2) ia = log i0 +αFɳ/2.303RT
(2)
Here, ia is the applied current density, i0 is the exchange current density, α is the transfer coefficient for the anodic reaction, F is the Faraday constant, R is the gas constant, T is the temperature, and η = (E - Eeq) is the overpotential. Charge transfer coefficient, α is the parameter that signifies the fraction of overpotential that affects the current density and can be approximated to be 0.5. Exchange current, io is defined as the current flowing in both directions
per unit area when an electrode reaction is at equilibrium (and, hence, at its equilibrium potential). If io is small, then little current flows and the reactions at dynamic equilibrium are generally slow. Likewise, a high io gives a fast reaction. The exchange current density can be obtained from the intercept of the linear segment αF/2.303RT. Here, Tafel slopes were derived based on galvanostatic curves at various current rates. Using the following correlation for a oneelectron process, we can obtain the charge-transfer resistance Rct value, at the interface as Rct = RT/Fio [15]. The exchange current densities, charge transfer resistances and diffusivity of Li+ were obtained from the Tafel plots at different temperatures, and the values of kinetic parameters are summarized in Table 1. The charge transfer resistance Rct decreases with temperature increase, while both the exchange current density i0 and diffusivity D increases with increasing temperature from -10 oC to 80 oC, which means the reaction is faster at higher temperatures. The increase of diffusivity at high temperatures can be explained by the fact that Li+ have enough energy to self-diffuse [16] and generate chaos, making it easy to move for Li+ while providing fast migration through hopping mechanism. The temperature dependence of exchange current density, charge transfer resistance, and diffusion shows Arrhenius-type behavior as presented in Fig. 7. In the case of diffusivity, two distinct slopes are observed at low and high temperatures. The activation energies were obtained from the slopes of the Arrhenius plots according to the Arrhenius equation. The activation energies of exchange current density and charge transfer of Li+ were calculated to be 23.1 kJ mol-1 and 20.5 kJ mol-1 respectively, whereas diffusivity activation energy showed 4.3 kJ mol-1 at low temperatures and 24.8 kJ mol-1 at higher temperatures. As suggested previously, at low temperatures (large domain size also included) the Li+ deintercalation induces the two-phase reaction kinetically, whereas, at high temperatures, the
solid-solution behavior is dominant. The activation energy variation with temperature could be the indirect evidence of multiple reaction mechanisms. The small activation energy of diffusion (4.3 kJ mol-1) at a lower temperature can be attributed to the energetically favorable 8a and 16c sites occupation without Coulombic repulsion [9].
Table 1. The values of kinetic parameters (exchange current density io, charge transfer resistance Rct, diffusivity D) obtained at different temperatures (DOD 50%) Particle diameter, µm 19.6 20.2 17.4 18.4 17.4 17.9
Temperature, o C -10 0 30 45 60 80
io, mA cm-2 0.5 2.2 3.2 6.5 8.8 9.6
Rct, Ωcm2 45.4 10.0 8.2 4.2 3.2 3.1
D, cm s-1 1.7× 10-8 1.8× 10-8 2.2× 10-8 3.5× 10-8 4.5× 10-8 9.3× 10-8
Fig. 7. Arrhenius plots as lni0 against 1/T, lnD against 1/T and ln1/Rct against 1/T. The straight lines represent a fit to the data yielding the activation energy as indicated (DOD 50%)
4. Conclusions In this study, the electrochemical performances of LTO-based composite electrode and a single LTO particle were comparatively analyzed. It was found that a single LTO particle, indeed, shows outstanding performance in comparison to the LTO-based composite electrode, where intrinsic properties of active material were affected by the inactive components of the electrode. As a result, a single LTO particle demonstrates superior capacity retention of 88% at high discharge rate as 440 C, while conventional LTO-based electrode shows a two-fold decrease in capacity (52% retention) at 30 C-rate. The exceptionally high rate capability that results from the rapid and reversible intercalation of Li+ proves that the reaction is single-phase rather than twophase at room temperature. Also, the performance of a single LTO particle was examined in the temperature range from -10 oC to 80 oC. Through Arrhenius plots, the activation energies of exchange current density (io) and charge transfer resistance (Rct) at DOD 50% evaluated as 23.1 kJ mol-1 and 20.5 kJ mol-1, respectively. The activation energy of solid-state diffusivity (D) varies with temperature and has a smaller value at the low temperatures. At elevated temperatures, the activation energy of D is 24.8 kJ mol-1, whereas that of low temperature is 4.3 kJ mol-1, implying the two-phase segregation.
Acknowledgment The authors are very grateful for the financial support of the KETEP (Grant 20164010201070) and Chungnam National University.
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Highlights •
The intrinsic properties of LTO were investigated by single particle measurement
•
Single LTO particle shows outstanding rate capability vs. composite LTO electrode
•
Activation energies of io, Rct, and D evaluated in the range of -10 oC to 80 oC
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LTO reaction is single-phase rather than two-phase at certain temperatures.