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Determination of the chemical diffusion coefficient of lithium ions in spherical Li[Ni0.5Mn0.3Co0.2]O2
Electrochimica Acta 66 (2012) 88–93
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Determination of the chemical diffusion coefficient of lithium ions in spherical Li[Ni0.5 Mn0.3 Co0.2 ]O2 Shunyi Yang, Xianyou Wang ∗ , Xiukang Yang, Yansong Bai, Ziling Liu, Hongbo Shu, Qiliang Wei Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, School of Chemistry, Xiangtan University, Hunan, Xiangtan 411105, China
a r t i c l e
i n f o
Article history: Received 8 September 2011 Received in revised form 20 December 2011 Accepted 13 January 2012 Available online 24 January 2012 Keywords: Lithium ion batteries Cathode material Li[Ni0.5 Mn0.3 Co0.2 ]O2 Diffusion coefficient of lithium ions
a b s t r a c t The chemical diffusion coefficients of lithium ions (DLi ) in spherical Li[Ni0.5 Mn0.3 Co0.2 ]O2 are systematically determined by galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS). The DLi obtained from GITT are greatly dependent on the lithium content and cell potential. In the charge process, the DLi values are in the range of 9.0 × 10−11 to 3.3 × 10−9 cm2 s−1 . In the discharge process, the DLi values are in the range of 1.9 × 10−11 to 3.9 × 10−9 cm2 s−1 . In addition, the minimum values in the DLi are observed at the stoichiometry x ≈ 0.25 for Li1−x [Ni0.5 Mn0.3 Co0.2 ]O2 , which coincide with the voltage-plateau region at about 3.75 V. The DLi obtained from EIS are in the range of 1.7 × 10−11 to 6.5 × 10−9 cm2 s−1 and the variation tendency with cell potential is consistent with the results obtained from GITT during the charge process. Crown Copyright © 2012 Published by Elsevier Ltd. All rights reserved.
1. Introduction During the past decade, lithium ion batteries have been extensively investigated and widely used; they are not only required to enable the moderately charge/discharge rates applications like mobile phone and portable computer, but also to meet an increasing need for new applications such as electric vehicles which need power sources with both high energy and high power density. To meet the need of this new trend, the rate capability of the cathode materials must improve proportionately [1]. It is well known that the factors which control the rate capability of lithium ion intercalation/deintercalation in the cathode materials mainly depend on the bulk properties including diffusion of lithium ion within the compound and its electronic conductivity as well as the kinetics of the electrochemical processes at the interface. Because the transport process of lithium ion in the cathode material is the key step for the energy storage and output, it is necessary to study the lithium diffusion coefficient and its variation with the lithium content or cell potential in cathode materials [2,3]. In the previous researches, much attention has been paid to focus on the kinetic behavior of LiCoO2 [4–6], LiMn2 O4 [7,8], Li3 V2 (PO4 )3 [9,10] and LiFePO4 [11,12], of which the chemical diffusion coefficient of Li ions, DLi , is one of the most important kinetic characteristics. Usually, the galvanostatic intermittent titration technique (GITT) [14–16] and electrochemical impedance
∗ Corresponding author. Tel.: +86 731 58292060; fax: +86 731 58292061. E-mail address: [email protected] (X. Wang).
spectroscopy (EIS) measurements [2,9,13,17] have been extensively used to measure the chemical diffusion coefficient of the electrode materials. The GITT is considered to be a reliable technique to determine the DLi with highly resolved data for intercalation compounds of varying potential or lithium content. The EIS is a powerful technique to determine the DLi due to the fact that the low frequency Warburg is directly related to the lithium-ion diffusion process in an electrode material. In our previous reports, EIS measurement had been successfully used to determine the chemical diffusion coefficients of lithium ions in Li3 V2 (PO4 )3 [18,19]. we successfully synthesized a spherical Recently, Li[Ni0.5 Mn0.3 Co0.2 ]O2 cathode material with narrow size distribution and high tap density (2.61 g cm−3 ) using a continuous hydroxide co-precipitation. This compound compromises between the increase of the discharge capacity due to the Co3+ and the increase of the thermal stability due to the Mn4+ ions [20–23]. XPS results [20] have testified that the valences of Co and Mn for Li[Ni0.5 Mn0.3 Co0.2 ]O2 are 2+ and 4+, respectively, while the valence number of Ni in this compound is a mixture of 2+ and 3+, which is different from the normal conditions in Li[Ni1/3 Co1/3 Mn1/3 ]O2 . The existence of mixed-valent cations (Ni2+ /Ni3+ ) significantly contributes to the inherent electronic conductivity of electrodes during charge and discharge [24]. Electrochemical tests indicated that Li[Ni0.5 Mn0.3 Co0.2 ]O2 delivers the high initial discharge capacity of 214 mAh g−1 (2.5–4.6 V, 0.2 C, 25 ◦ C), 175 mAh g−1 (3–4.3 V, 0.2 C, 25 ◦ C) and 166 mAh g−1 (3–4.3 V, 1 C, 25 ◦ C), as well as good cycling performance. Furthermore, it displays good high-temperature characteristics and excellent rate capability. The high performance of the cathode material in lithium ion
0013-4686/$ – see front matter. Crown Copyright © 2012 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2012.01.061
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batteries depends on its ability to intercalate lithium reversibly into the host lattice. Lithium diffusion in the oxide is a key factor that determines the rate of the intercalation/deintercalation process. However, until recently, there is some lack of knowledge about the investigation of chemical diffusion of lithium ions in Li[Ni0.5 Mn0.3 Co0.2 ]O2 composite material. In this paper, GITT and EIS techniques are employed to investigate the chemical diffusion of lithium ions in spherical Li[Ni0.5 Mn0.3 Co0.2 ]O2 . 2. Experimental Spherical Li[Ni0.5 Mn0.3 Co0.2 ]O2 cathode material was prepared by a continuous hydroxide co-precipitation method. Details of the preparation procedures were described in a previous work. The [Ni0.5 Mn0.3 Co0.2 ](OH)2 precursor was first synthesized by a continuous hydroxide co-precipitation using two continuous stirred-tank reactors (CSTR). The obtained [Ni0.5 Mn0.3 Co0.2 ](OH)2 precursor was then mixed thoroughly with appropriate amount of Li2 CO3 . Finally, the mixture was pre-heated at 500 ◦ C for 5 h and calcined at 820 ◦ C for 12 h in air to obtain the final product Li[Ni0.5 Mn0.3 Co0.2 ]O2 . The phase identification of the sample was performed with a diffractometer (D/Max-3 C, Rigaku, Japan) using ˚ and a graphite monochromator at Cu K␣ radiation ( = 1.54178 A) 36 kV, 20 mA. The scanning rate was 8◦ min−1 and the scanning range of diffraction angle (2) was 10◦ ≤ 2 ≤ 80◦ . The morphology of the sample was observed using scanning electron microscopy (JSM-5600LV, JEOL, Japan). For the electrochemical measurements, a two electrode Swagelok cell was assembled in an Ar filled glove box. The cathode was consisted of a mixture of active material (80 wt.%), acetylene black (10 wt.%), graphite (5 wt.%) and polyvinylidene fluoride (5 wt.%) as binder agent, Lithium was served as counter and reference electrodes, a Celgard 2400 was used as separator, and the electrolyte was a 1 mol L−1 LiPF6 solution in ethylene carbonate (EC)–dimethyl carbonate (DMC) (1:1 in volume). The GITT and EIS of the cells were measured on a CHI 660A electrochemical workstation (Chenhua, China). Before each measurement, the cells were galvanostatically charged and discharged in two cycles at a current density of 32 mA g−1 in Neware battery test system (BTSXWJ-6.44S-00052, Newell, China). The GITT was employed at a pulse of 20 A for 1 h and with 4 h interruption between each pulse. EIS measurements were performed in the frequency range of 10 kHz–1 mHz with an amplitude voltage of 5 mV.
Fig. 1. X-ray diffraction pattern and SEM image for Li[Ni0.5 Mn0.3 Co0.2 ]O2 powders.
3. Results and discussion Fig. 1 illustrates the XRD pattern of spherical Li[Ni0.5 Mn0.3 Co0.2 ]O2 . Apparently, the as-prepared powder has a typical hexagonal structure of ␣-NaFeO2 with a space group ¯ (No. 166). The diffraction peaks are quite narrow, indicating of R3m high crystallinity, and no impurity diffraction peak is observed. The splits in the (0 0 6)/(1 0 2) and (1 0 8)/(1 1 0) doublets indicate the formation of a highly ordered ␣-NaFeO2 -type layered structure. The calculated lattice parameters of the powder are a = 0.2870 nm, c = 1.426 nm, and V = 0.1017 nm3 , respectively. The inset in Fig. 1 shows the SEM image of the Li[Ni0.5 Mn0.3 Co0.2 ]O2 . It can be seen that the secondary particles of Li[Ni0.5 Mn0.3 Co0.2 ]O2 show uniform spherical morphology, each of the spherical particle is made up of numerous primary grains. The first and third charge/discharge cycle profiles of Li[Ni0.5 Mn0.3 Co0.2 ]O2 and the corresponding differential capacity profiles are presented in Fig. 2. As shown in Fig. 2, the Li[Ni0.5 Mn0.3 Co0.2 ]O2 shows three anodic peaks and two cathodic peaks during the first cycle. Three anodic peaks positioned at about 3.77 V, 3.80 V and 4.55 V are identified as Ni2+ /Ni3+ , Ni3+ /Ni4+ ,
Fig. 2. (a) The first and third charge/discharge profiles of Li[Ni0.5 Mn0.3 Co0.2 ]O2 at a current density of 32 mA g−1 in the voltage range of 2.5–4.6 V and (b) the corresponding differential capacity (dQ/dE) vs. cell potential profiles.
and Co3+ /Co4+ redox couple, respectively; while two cathodic peaks at about 3.74 V and 4.53 V match along with Ni4+ /Ni2+ and Co4+ /Co3+ redox [25,26], respectively. In the third cycle, two shoulder anodic peaks at about 3.7–3.8 V have emerged into one broaden peak at 3.75 V, while the cathodic at 3.74 V has little change. Previous reports [9] have testified that the valences of
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Co and Mn for Li[Ni0.5 Mn0.3 Co0.2 ]O2 are 2+ and 4+, respectively, and the valence number of Ni in this compound is a mixture of 2+ and 3+. Since Mn4+ is inactive over the voltage range of 2.5–4.6 V, the charge/discharge reaction should correspond to Ni2+ /Ni4+ , Ni3+ /Ni4+ , and Co3+ /Co4+ redox couples. These results were well consistent with the previous reports by Delmas et al. [27] and Liao et al. [28]. The chemical diffusion coefficient of the insertion electrode materials is an important kinetic parameter to determine lithium ion charge/discharge rate. It has been reported that the electrochemical galvanostatic intermittent titration technique and electrochemical impedance spectroscopy have been used to determine the chemical diffusion coefficient of lithium ions in lithium transitional metal oxides [16,17]. GITT technique was firstly developed by Weppner and Huggins [29] and widely recognized as a useful method to determine the chemical diffusion coefficient in electron and ion mixed conductors. Assuming that lithium transport in the electrode obeys Fick’s second law, the chemical diffusion coefficients can be obtained by the following equation:
4 Vm I0 DLi = FS
2 dE/dx 2 dE/dt 1/2
(1)
where I0 is the applied current (2 × 10−5 A), Vm is the molar volume (20.42 cm3 mol−1 ), F is the Faraday constant (96485 C mol−1 ) and S is the surface area of the electrode (0.785 cm2 ). Fig. 3a shows the GITT curves of Li[Ni0.5 Mn0.3 Co0.2 ]O2 during the third cycle as a function of time in the voltage range of 2.5–4.6 V. The cell was first charged at a constant current flux (I0 = 20 A) for an interval of 1 h followed by an open-circuit stand for 4 h to allow the cell potential to relax to its steady-state value (Es ). The procedure was repeated for the full voltage window of operation. It can be seen from Fig. 3b that the cell potential stabilizes to a constant value after the 4 h open-circuit stand after each current flux. The evolution of the quasi-equilibrium potential (Es ) vs. x in Li1−x [Ni0.5 Mn0.3 Co0.2 ]O2 is obtained from the GITT results as shown in Fig. 4a. These are equivalent to open-circuit voltage (OCV) at varying x. From this figure, a differential factor (dE/dx) as a function of the stoichiometry x was obtained as given in Fig. 4b. Fig. 5a shows an example of an E vs. t1/2 plot recorded for Li1−x [Ni0.5 Mn0.3 Co0.2 ]O2 after application of 20 A current pulse. It can be seen that in the time domain from 10 to 400 s, the plot is nearly linear. In the literatures using GITT techniques [12,29,30], the time range of 10–100 s is identical. The slope is taken from this linear range to calculate DLi . Fig. 5b shows the plot of dE/dt1/2 vs. the x value. It is found that the dE/dt1/2 values exhibit an analogous W-type variation varied from 4.5 × 10−4 to 8.3 × 10−3 V s−1/2 . Since the applied current is constant and the same for each step, the variation of dE/dt1/2 also reveals the change of the electrode resistance. The DLi of the Li[Ni0.5 Mn0.3 Co0.2 ]O2 can be calculated according to the values of dE/dt1/2 and dE/dx in Eq. (1), the results as a function of the cell potential and as a function of the stoichiometry x are presented in Fig. 6a and b. It can be seen from Fig. 6a that the variation tendency of the DLi during the charge/discharge process is very similar, and its values are greatly dependent on the cell potential. In the charge process, the DLi values are in the range of 9.0 × 10−11 to 3.3 × 10−9 cm2 s−1 . With the increase of the cell potential, the DLi increases gradually, but the DLi will decrease after the cell potential is more than 4.4 V. In the discharge process, the DLi values are in the range of 1.9 × 10−11 to 3.9 × 10−9 cm2 s−1 . However, no matter whether it is in the charge process or discharge process, the DLi is almost unchanged with a value of 3.0 (±0.5) × 10−9 cm2 s−1 when the cell potential is in the range of 3.9–4.3 V. In addition, the minimum values at about 3.75 V can be
Fig. 3. (a) The GITT curves of Li[Ni0.5 Mn0.3 Co0.2 ]O2 during the third cycle as a function of time in the voltage range of 2.5–4.6 V and (b) schematic illustration of a single step of the GITT.
observed. It is worth noting that the observed minimum values in DLi coincide with the plateau observed in the voltage profile during charge–discharge cycling in Fig. 2. As shown in Fig. 6b, it can be found that the lithium diffusion coefficients are also dependent on the lithium content in Li1−x [Ni0.5 Mn0.3 Co0.2 ]O2 , the minimum values in the DLi are observed at about the stoichiometry x ≈ 0.25 for Li1−x [Ni0.5 Mn0.3 Co0.2 ]O2 . A minimum in DLi vs. cell potential or lithium content coinciding with the plateau in the cell potential vs. capacity curves is commonly observed in oxide cathodes [6–16]. In the previous reports [7,15,16], most researchers attribute this phenomenon to a possible reversible structural phase transition or order–disorder transition in the compound during cycling, but it has not yet definitely settled. As mentioned by Arachi et al. [31], in situ synchrotron XRD on Li[Ni1/2 Mn1/2 ]O2 revealed that the material underwent a phase transition from hexagonal (space group, x ≈ 0.25 for Li[Ni1/2 Mn1/2 ]O2 ) to monoclinic (C2/m for Li0.5 [Ni1/2 Mn1/2 ]O2 ) structure during Li-extraction at ∼4.2 V. However, the results reported by Shaju et al. [15] showed that the minimum in DLi vs. cell potential appeared at the beginning of Li-extraction, in the range of x = 0.95–0.80 for Lix [Ni1/2 Mn1/2 ]O2 at around 3.8 V. Thus, the reported structural change does not agree with the voltage profile and the trend of variation of DLi . Even though the DLi vs. cell potential profiles show a minimum, the values are not sufficiently accurate to clearly indicate the phase transition occurring in the Li[Ni1/2 Mn1/2 ]O2 compound in the voltage range of 3.7–3.85 V.
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Fig. 4. (a) The equilibrium potentials (Es ) as a function of the stoichiometry x and (b) dE/dx as a function of the stoichiometry x.
The DLi values derived from Eq. (1) is based on the assumption that the molar volume (Vm ) remains unchanged with the change in Li content in the compound. The possible phase change during charge–discharge cycling makes the Li-ion diffusion to occur through the phase boundary and through each phase and the measured DLi would be the resultant value. Hence, the above DLi are apparent rather than the true values. The geometric surface area of the composite cathode (0.785 cm2 ) was used for the above DLi calculations. However, in the composite electrode, the electrolyte can permeate through the additives (carbon and binder) and the determination of the actual electrochemically active surface area is very difficult. Therefore, from the available literature, it can be seen that the geometric surface area of the electrode and the actual surface area of the active material (mostly obtained from BET method) have often been used for the DLi calculation. Pyun and Bae [32] analyzed the Warburg region of the impedance response of the composite V2 O5 electrode and found that only ∼50% of the actual surface area is available for electrochemical activity. Thus, taking into account the measured surface area of the active material in the composite electrode A = 34.3 cm2 (obtained from BET method), the calculated DLi values at 4.2 V (charge process) from the GITT data are 3.3 × 10−9 cm2 s−1 (geometric area, A = 0.785 cm2 ), 1.7 × 10−12 cm2 s−1 (actual surface area, A = 34.3 cm2 ), and 6.8 × 10−12 cm2 s−1 (electrochemical surface area, A /2). Obviously, the DLi values calculated using electrochemical surface area are three orders of magnitude lower than
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Fig. 5. (a) The typical transient voltage changes as a function of the square root of the time (t1/2 ) during a single titration process and (b) dE/dt1/2 as a function of the stoichiometry x.
those shown in Fig. 6, but the general trend of variation of DLi vs. V (or x) will be the same. EIS is considered to be another useful method to measure the chemical diffusion coefficient. The impedance spectra at open circuit condition under different charge states for Li[Ni0.5 Mn0.3 Co0.2 ]O2 were measured and the Nyquist plots (Z vs. Z ) are shown in Fig. 7. The plots consist of three regions [15,16], a semicircle in the high-frequency region as well as another depressed one in the intermediate-frequency region and the Warburg-type element (the sloping line) in the low-frequency region. The semi-circle in the high frequency range can be attributed to the surface film resistance and the bulk resistance, whereas, the semicircle in the middle frequency range represents the charge transfer resistance. The Warburg impedance in the low frequency is mainly corresponding to the diffusion of lithium ion in the bulk of the electrode, which has been used to determine the Li-ion diffusion coefficient in the compound. By using the model proposed by Ho et al. [33], the diffusion coefficient of Li ions for Li[Ni0.5 Mn0.3 Co0.2 ]O2 can be calculated as the following equation: DLi =
1 2
V dE 2 m FS
dx
(2)
where Vm is the molar volume (20.42 cm3 mol−1 ), S is the surface area of the electrode (0.785 cm2 ), F is the Faraday constant (96485 C mol−1 ), dE/dx is the slope of the electrode potential (as shown in Fig. 4b). The is the Warburg coefficient, the values
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Fig. 6. The lithium diffusion coefficients calculated from the GITT curves as a function of the cell potential (a) and as a function of the stoichiometry x (b).
under different charge states for Li[Ni0.5 Mn0.3 Co0.2 ]O2 (as shown in Fig. 8a) can be obtained from the slope of Z vs. ω−1/2 plots (ω is the angular frequency) in the Warburg region. Typical plots of the Z vs. ω−1/2 for the Li[Ni0.5 Mn0.3 Co0.2 ]O2 during the third charge process along with the linear fitting are shown in Fig. 8b. Both
Fig. 7. Impedance spectra for Li[Ni0.5 Mn0.3 Co0.2 ]O2 under different charge states.
Fig. 8. (a) as a function of the cell potential and (b) real parts of the complex impedance Z vs. ω−1/2 for Li[Ni0.5 Mn0.3 Co0.2 ]O2 under different charge states.
the plots show parallel straight lines characteristic of the Warburg impedance. The diffusion coefficients of lithium for Li[Ni0.5 Mn0.3 Co0.2 ]O2 under different charge states are obtained by substitution of the curve slopes in Eq. (2), as shown in Fig. 9. It can be seen that the DLi values obtained from EIS are in the range of 1.7 × 10−11 to 6.5 × 10−9 cm2 s−1 . Further, it can be found that the DLi (EIS) values are larger by nearly one time compared to the corresponding DLi (GITT) in the voltage range of 4.0–4.5 V. Similar differences in DLi measured for the Li1-x [Ni1/3 Mn2/3 ]O2 [34] compound by the above two techniques were also observed earlier. The DLi for Li1-x [Ni1/3 Mn2/3 ]O2 in the composition range 0.1 < x < 0.3 obtained by GITT was about 5 × 10−9 cm2 s−1 , whereas by the EIS method, the DLi values are higher by one order of magnitude. It must be mentioned at this juncture that Levi and Aurbach [35] clarified that even though there is no principle difference in the resolution of diffusion time with respect to voltage, the GITT could provide a some what more precise data on (here, the diffusion coefficient DLi = l2 /, where l is the characteristic diffusion length) as compared to EIS, provided the current pulse height for the GITT measurements and the amplitude of the AC current for EIS are small. This is due to the fact that the ohmic potential drop during GITT titration is independent of the potential and can be readily eliminated. This result was consistent with the reports by Bruce
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Acknowledgments This work was financially supported by the National Natural Science Foundation of China under project no. 20871101, Joint Fund of Natural Science of Hunan Province and Xiangtan City under project no. 09BG005, Industrial Project of Colleges and Universities of Hunan Province under project no. 10CY005, Project of Condition Research of Hunan Province under project no. 2010TC2004 and Colleges and Universities in Hunan Province plans to graduate research and innovation under project no. CX2009B133. References
Fig. 9. Diffusion coefficient of Li ions for the Li[Ni0.5 Mn0.3 Co0.2 ]O2 calculated from the EIS.
[36] and Boukamp and co-workers [37]. Besides, Shaju et al. [16] had proved that GITT provided more accurate diffusion coefficients compared to EIS when calculated the DLi for Li[Ni1/3 Co1/3 Mn1/3 ]O2 . In this paper, the diffusion coefficients of Li[Ni0.5 Mn0.3 Co0.2 ]O2 obtained by the two techniques are close to those of similar layered cathode material Li[Ni1/3 Mn1/3 Co1/3 ]O2 . Therefore, it could be considered that the GITT is also a more reliable method for the determination of the diffusion coefficient of lithium ions in spherical Li[Ni0.5 Mn0.3 Co0.2 ]O2 . Besides, it is worth noting that the values of the DLi (GITT) in our work for Li[Ni0.5 Mn0.3 Co0.2 ]O2 (with a value of 3.0 (±0.5) × 10−9 cm2 s−1 in the voltage range of 3.9–4.3 V) is higher by nearly one order of magnitude than that for Li[Ni1/3 Co1/3 Mn1/3 ]O2 (with a value of ∼3 × 10−10 cm2 s−1 in the same voltage range obtained from GITT) measured by Shaju et al. [16]. This is probably attributed to different inherent properties of two cathode materials such as structure, surface area, grain size, morphology and so on, and different parameters setting when the values of DLi were calculated. 4. Conclusions Two electrochemical measurement techniques, GITT and EIS, have been successfully employed to determine the chemical diffusion coefficient of Li ions in spherical Li[Ni0.5 Mn0.3 Co0.2 ]O2 . The variation of DLi derived from GITT and EIS is correlated with the performance of lithium ion intercalation/deintercalation in the Li[Ni0.5 Mn0.3 Co0.2 ]O2 and greatly dependent on the cell potential. The DLi obtained from GITT and EIS are in the range of 1.9 × 10−11 to 3.9 × 10−9 cm2 s−1 and 1.7 × 10−11 to 6.5 × 10−9 cm2 s−1 respectively. The results obtained by the two techniques are well consistent.
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Determination of the chemical diffusion coefficient of lithium ions in spherical Li[Ni0.5 Mn0.3 Co0.2 ]O2 Shunyi Yang, Xianyou Wang ∗ , Xiukang Yang, Yansong Bai, Ziling Liu, Hongbo Shu, Qiliang Wei Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, School of Chemistry, Xiangtan University, Hunan, Xiangtan 411105, China
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Article history: Received 8 September 2011 Received in revised form 20 December 2011 Accepted 13 January 2012 Available online 24 January 2012 Keywords: Lithium ion batteries Cathode material Li[Ni0.5 Mn0.3 Co0.2 ]O2 Diffusion coefficient of lithium ions
a b s t r a c t The chemical diffusion coefficients of lithium ions (DLi ) in spherical Li[Ni0.5 Mn0.3 Co0.2 ]O2 are systematically determined by galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS). The DLi obtained from GITT are greatly dependent on the lithium content and cell potential. In the charge process, the DLi values are in the range of 9.0 × 10−11 to 3.3 × 10−9 cm2 s−1 . In the discharge process, the DLi values are in the range of 1.9 × 10−11 to 3.9 × 10−9 cm2 s−1 . In addition, the minimum values in the DLi are observed at the stoichiometry x ≈ 0.25 for Li1−x [Ni0.5 Mn0.3 Co0.2 ]O2 , which coincide with the voltage-plateau region at about 3.75 V. The DLi obtained from EIS are in the range of 1.7 × 10−11 to 6.5 × 10−9 cm2 s−1 and the variation tendency with cell potential is consistent with the results obtained from GITT during the charge process. Crown Copyright © 2012 Published by Elsevier Ltd. All rights reserved.
1. Introduction During the past decade, lithium ion batteries have been extensively investigated and widely used; they are not only required to enable the moderately charge/discharge rates applications like mobile phone and portable computer, but also to meet an increasing need for new applications such as electric vehicles which need power sources with both high energy and high power density. To meet the need of this new trend, the rate capability of the cathode materials must improve proportionately [1]. It is well known that the factors which control the rate capability of lithium ion intercalation/deintercalation in the cathode materials mainly depend on the bulk properties including diffusion of lithium ion within the compound and its electronic conductivity as well as the kinetics of the electrochemical processes at the interface. Because the transport process of lithium ion in the cathode material is the key step for the energy storage and output, it is necessary to study the lithium diffusion coefficient and its variation with the lithium content or cell potential in cathode materials [2,3]. In the previous researches, much attention has been paid to focus on the kinetic behavior of LiCoO2 [4–6], LiMn2 O4 [7,8], Li3 V2 (PO4 )3 [9,10] and LiFePO4 [11,12], of which the chemical diffusion coefficient of Li ions, DLi , is one of the most important kinetic characteristics. Usually, the galvanostatic intermittent titration technique (GITT) [14–16] and electrochemical impedance
∗ Corresponding author. Tel.: +86 731 58292060; fax: +86 731 58292061. E-mail address: [email protected] (X. Wang).
spectroscopy (EIS) measurements [2,9,13,17] have been extensively used to measure the chemical diffusion coefficient of the electrode materials. The GITT is considered to be a reliable technique to determine the DLi with highly resolved data for intercalation compounds of varying potential or lithium content. The EIS is a powerful technique to determine the DLi due to the fact that the low frequency Warburg is directly related to the lithium-ion diffusion process in an electrode material. In our previous reports, EIS measurement had been successfully used to determine the chemical diffusion coefficients of lithium ions in Li3 V2 (PO4 )3 [18,19]. we successfully synthesized a spherical Recently, Li[Ni0.5 Mn0.3 Co0.2 ]O2 cathode material with narrow size distribution and high tap density (2.61 g cm−3 ) using a continuous hydroxide co-precipitation. This compound compromises between the increase of the discharge capacity due to the Co3+ and the increase of the thermal stability due to the Mn4+ ions [20–23]. XPS results [20] have testified that the valences of Co and Mn for Li[Ni0.5 Mn0.3 Co0.2 ]O2 are 2+ and 4+, respectively, while the valence number of Ni in this compound is a mixture of 2+ and 3+, which is different from the normal conditions in Li[Ni1/3 Co1/3 Mn1/3 ]O2 . The existence of mixed-valent cations (Ni2+ /Ni3+ ) significantly contributes to the inherent electronic conductivity of electrodes during charge and discharge [24]. Electrochemical tests indicated that Li[Ni0.5 Mn0.3 Co0.2 ]O2 delivers the high initial discharge capacity of 214 mAh g−1 (2.5–4.6 V, 0.2 C, 25 ◦ C), 175 mAh g−1 (3–4.3 V, 0.2 C, 25 ◦ C) and 166 mAh g−1 (3–4.3 V, 1 C, 25 ◦ C), as well as good cycling performance. Furthermore, it displays good high-temperature characteristics and excellent rate capability. The high performance of the cathode material in lithium ion
0013-4686/$ – see front matter. Crown Copyright © 2012 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2012.01.061
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batteries depends on its ability to intercalate lithium reversibly into the host lattice. Lithium diffusion in the oxide is a key factor that determines the rate of the intercalation/deintercalation process. However, until recently, there is some lack of knowledge about the investigation of chemical diffusion of lithium ions in Li[Ni0.5 Mn0.3 Co0.2 ]O2 composite material. In this paper, GITT and EIS techniques are employed to investigate the chemical diffusion of lithium ions in spherical Li[Ni0.5 Mn0.3 Co0.2 ]O2 . 2. Experimental Spherical Li[Ni0.5 Mn0.3 Co0.2 ]O2 cathode material was prepared by a continuous hydroxide co-precipitation method. Details of the preparation procedures were described in a previous work. The [Ni0.5 Mn0.3 Co0.2 ](OH)2 precursor was first synthesized by a continuous hydroxide co-precipitation using two continuous stirred-tank reactors (CSTR). The obtained [Ni0.5 Mn0.3 Co0.2 ](OH)2 precursor was then mixed thoroughly with appropriate amount of Li2 CO3 . Finally, the mixture was pre-heated at 500 ◦ C for 5 h and calcined at 820 ◦ C for 12 h in air to obtain the final product Li[Ni0.5 Mn0.3 Co0.2 ]O2 . The phase identification of the sample was performed with a diffractometer (D/Max-3 C, Rigaku, Japan) using ˚ and a graphite monochromator at Cu K␣ radiation ( = 1.54178 A) 36 kV, 20 mA. The scanning rate was 8◦ min−1 and the scanning range of diffraction angle (2) was 10◦ ≤ 2 ≤ 80◦ . The morphology of the sample was observed using scanning electron microscopy (JSM-5600LV, JEOL, Japan). For the electrochemical measurements, a two electrode Swagelok cell was assembled in an Ar filled glove box. The cathode was consisted of a mixture of active material (80 wt.%), acetylene black (10 wt.%), graphite (5 wt.%) and polyvinylidene fluoride (5 wt.%) as binder agent, Lithium was served as counter and reference electrodes, a Celgard 2400 was used as separator, and the electrolyte was a 1 mol L−1 LiPF6 solution in ethylene carbonate (EC)–dimethyl carbonate (DMC) (1:1 in volume). The GITT and EIS of the cells were measured on a CHI 660A electrochemical workstation (Chenhua, China). Before each measurement, the cells were galvanostatically charged and discharged in two cycles at a current density of 32 mA g−1 in Neware battery test system (BTSXWJ-6.44S-00052, Newell, China). The GITT was employed at a pulse of 20 A for 1 h and with 4 h interruption between each pulse. EIS measurements were performed in the frequency range of 10 kHz–1 mHz with an amplitude voltage of 5 mV.
Fig. 1. X-ray diffraction pattern and SEM image for Li[Ni0.5 Mn0.3 Co0.2 ]O2 powders.
3. Results and discussion Fig. 1 illustrates the XRD pattern of spherical Li[Ni0.5 Mn0.3 Co0.2 ]O2 . Apparently, the as-prepared powder has a typical hexagonal structure of ␣-NaFeO2 with a space group ¯ (No. 166). The diffraction peaks are quite narrow, indicating of R3m high crystallinity, and no impurity diffraction peak is observed. The splits in the (0 0 6)/(1 0 2) and (1 0 8)/(1 1 0) doublets indicate the formation of a highly ordered ␣-NaFeO2 -type layered structure. The calculated lattice parameters of the powder are a = 0.2870 nm, c = 1.426 nm, and V = 0.1017 nm3 , respectively. The inset in Fig. 1 shows the SEM image of the Li[Ni0.5 Mn0.3 Co0.2 ]O2 . It can be seen that the secondary particles of Li[Ni0.5 Mn0.3 Co0.2 ]O2 show uniform spherical morphology, each of the spherical particle is made up of numerous primary grains. The first and third charge/discharge cycle profiles of Li[Ni0.5 Mn0.3 Co0.2 ]O2 and the corresponding differential capacity profiles are presented in Fig. 2. As shown in Fig. 2, the Li[Ni0.5 Mn0.3 Co0.2 ]O2 shows three anodic peaks and two cathodic peaks during the first cycle. Three anodic peaks positioned at about 3.77 V, 3.80 V and 4.55 V are identified as Ni2+ /Ni3+ , Ni3+ /Ni4+ ,
Fig. 2. (a) The first and third charge/discharge profiles of Li[Ni0.5 Mn0.3 Co0.2 ]O2 at a current density of 32 mA g−1 in the voltage range of 2.5–4.6 V and (b) the corresponding differential capacity (dQ/dE) vs. cell potential profiles.
and Co3+ /Co4+ redox couple, respectively; while two cathodic peaks at about 3.74 V and 4.53 V match along with Ni4+ /Ni2+ and Co4+ /Co3+ redox [25,26], respectively. In the third cycle, two shoulder anodic peaks at about 3.7–3.8 V have emerged into one broaden peak at 3.75 V, while the cathodic at 3.74 V has little change. Previous reports [9] have testified that the valences of
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Co and Mn for Li[Ni0.5 Mn0.3 Co0.2 ]O2 are 2+ and 4+, respectively, and the valence number of Ni in this compound is a mixture of 2+ and 3+. Since Mn4+ is inactive over the voltage range of 2.5–4.6 V, the charge/discharge reaction should correspond to Ni2+ /Ni4+ , Ni3+ /Ni4+ , and Co3+ /Co4+ redox couples. These results were well consistent with the previous reports by Delmas et al. [27] and Liao et al. [28]. The chemical diffusion coefficient of the insertion electrode materials is an important kinetic parameter to determine lithium ion charge/discharge rate. It has been reported that the electrochemical galvanostatic intermittent titration technique and electrochemical impedance spectroscopy have been used to determine the chemical diffusion coefficient of lithium ions in lithium transitional metal oxides [16,17]. GITT technique was firstly developed by Weppner and Huggins [29] and widely recognized as a useful method to determine the chemical diffusion coefficient in electron and ion mixed conductors. Assuming that lithium transport in the electrode obeys Fick’s second law, the chemical diffusion coefficients can be obtained by the following equation:
4 Vm I0 DLi = FS
2 dE/dx 2 dE/dt 1/2
(1)
where I0 is the applied current (2 × 10−5 A), Vm is the molar volume (20.42 cm3 mol−1 ), F is the Faraday constant (96485 C mol−1 ) and S is the surface area of the electrode (0.785 cm2 ). Fig. 3a shows the GITT curves of Li[Ni0.5 Mn0.3 Co0.2 ]O2 during the third cycle as a function of time in the voltage range of 2.5–4.6 V. The cell was first charged at a constant current flux (I0 = 20 A) for an interval of 1 h followed by an open-circuit stand for 4 h to allow the cell potential to relax to its steady-state value (Es ). The procedure was repeated for the full voltage window of operation. It can be seen from Fig. 3b that the cell potential stabilizes to a constant value after the 4 h open-circuit stand after each current flux. The evolution of the quasi-equilibrium potential (Es ) vs. x in Li1−x [Ni0.5 Mn0.3 Co0.2 ]O2 is obtained from the GITT results as shown in Fig. 4a. These are equivalent to open-circuit voltage (OCV) at varying x. From this figure, a differential factor (dE/dx) as a function of the stoichiometry x was obtained as given in Fig. 4b. Fig. 5a shows an example of an E vs. t1/2 plot recorded for Li1−x [Ni0.5 Mn0.3 Co0.2 ]O2 after application of 20 A current pulse. It can be seen that in the time domain from 10 to 400 s, the plot is nearly linear. In the literatures using GITT techniques [12,29,30], the time range of 10–100 s is identical. The slope is taken from this linear range to calculate DLi . Fig. 5b shows the plot of dE/dt1/2 vs. the x value. It is found that the dE/dt1/2 values exhibit an analogous W-type variation varied from 4.5 × 10−4 to 8.3 × 10−3 V s−1/2 . Since the applied current is constant and the same for each step, the variation of dE/dt1/2 also reveals the change of the electrode resistance. The DLi of the Li[Ni0.5 Mn0.3 Co0.2 ]O2 can be calculated according to the values of dE/dt1/2 and dE/dx in Eq. (1), the results as a function of the cell potential and as a function of the stoichiometry x are presented in Fig. 6a and b. It can be seen from Fig. 6a that the variation tendency of the DLi during the charge/discharge process is very similar, and its values are greatly dependent on the cell potential. In the charge process, the DLi values are in the range of 9.0 × 10−11 to 3.3 × 10−9 cm2 s−1 . With the increase of the cell potential, the DLi increases gradually, but the DLi will decrease after the cell potential is more than 4.4 V. In the discharge process, the DLi values are in the range of 1.9 × 10−11 to 3.9 × 10−9 cm2 s−1 . However, no matter whether it is in the charge process or discharge process, the DLi is almost unchanged with a value of 3.0 (±0.5) × 10−9 cm2 s−1 when the cell potential is in the range of 3.9–4.3 V. In addition, the minimum values at about 3.75 V can be
Fig. 3. (a) The GITT curves of Li[Ni0.5 Mn0.3 Co0.2 ]O2 during the third cycle as a function of time in the voltage range of 2.5–4.6 V and (b) schematic illustration of a single step of the GITT.
observed. It is worth noting that the observed minimum values in DLi coincide with the plateau observed in the voltage profile during charge–discharge cycling in Fig. 2. As shown in Fig. 6b, it can be found that the lithium diffusion coefficients are also dependent on the lithium content in Li1−x [Ni0.5 Mn0.3 Co0.2 ]O2 , the minimum values in the DLi are observed at about the stoichiometry x ≈ 0.25 for Li1−x [Ni0.5 Mn0.3 Co0.2 ]O2 . A minimum in DLi vs. cell potential or lithium content coinciding with the plateau in the cell potential vs. capacity curves is commonly observed in oxide cathodes [6–16]. In the previous reports [7,15,16], most researchers attribute this phenomenon to a possible reversible structural phase transition or order–disorder transition in the compound during cycling, but it has not yet definitely settled. As mentioned by Arachi et al. [31], in situ synchrotron XRD on Li[Ni1/2 Mn1/2 ]O2 revealed that the material underwent a phase transition from hexagonal (space group, x ≈ 0.25 for Li[Ni1/2 Mn1/2 ]O2 ) to monoclinic (C2/m for Li0.5 [Ni1/2 Mn1/2 ]O2 ) structure during Li-extraction at ∼4.2 V. However, the results reported by Shaju et al. [15] showed that the minimum in DLi vs. cell potential appeared at the beginning of Li-extraction, in the range of x = 0.95–0.80 for Lix [Ni1/2 Mn1/2 ]O2 at around 3.8 V. Thus, the reported structural change does not agree with the voltage profile and the trend of variation of DLi . Even though the DLi vs. cell potential profiles show a minimum, the values are not sufficiently accurate to clearly indicate the phase transition occurring in the Li[Ni1/2 Mn1/2 ]O2 compound in the voltage range of 3.7–3.85 V.
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Fig. 4. (a) The equilibrium potentials (Es ) as a function of the stoichiometry x and (b) dE/dx as a function of the stoichiometry x.
The DLi values derived from Eq. (1) is based on the assumption that the molar volume (Vm ) remains unchanged with the change in Li content in the compound. The possible phase change during charge–discharge cycling makes the Li-ion diffusion to occur through the phase boundary and through each phase and the measured DLi would be the resultant value. Hence, the above DLi are apparent rather than the true values. The geometric surface area of the composite cathode (0.785 cm2 ) was used for the above DLi calculations. However, in the composite electrode, the electrolyte can permeate through the additives (carbon and binder) and the determination of the actual electrochemically active surface area is very difficult. Therefore, from the available literature, it can be seen that the geometric surface area of the electrode and the actual surface area of the active material (mostly obtained from BET method) have often been used for the DLi calculation. Pyun and Bae [32] analyzed the Warburg region of the impedance response of the composite V2 O5 electrode and found that only ∼50% of the actual surface area is available for electrochemical activity. Thus, taking into account the measured surface area of the active material in the composite electrode A = 34.3 cm2 (obtained from BET method), the calculated DLi values at 4.2 V (charge process) from the GITT data are 3.3 × 10−9 cm2 s−1 (geometric area, A = 0.785 cm2 ), 1.7 × 10−12 cm2 s−1 (actual surface area, A = 34.3 cm2 ), and 6.8 × 10−12 cm2 s−1 (electrochemical surface area, A /2). Obviously, the DLi values calculated using electrochemical surface area are three orders of magnitude lower than
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Fig. 5. (a) The typical transient voltage changes as a function of the square root of the time (t1/2 ) during a single titration process and (b) dE/dt1/2 as a function of the stoichiometry x.
those shown in Fig. 6, but the general trend of variation of DLi vs. V (or x) will be the same. EIS is considered to be another useful method to measure the chemical diffusion coefficient. The impedance spectra at open circuit condition under different charge states for Li[Ni0.5 Mn0.3 Co0.2 ]O2 were measured and the Nyquist plots (Z vs. Z ) are shown in Fig. 7. The plots consist of three regions [15,16], a semicircle in the high-frequency region as well as another depressed one in the intermediate-frequency region and the Warburg-type element (the sloping line) in the low-frequency region. The semi-circle in the high frequency range can be attributed to the surface film resistance and the bulk resistance, whereas, the semicircle in the middle frequency range represents the charge transfer resistance. The Warburg impedance in the low frequency is mainly corresponding to the diffusion of lithium ion in the bulk of the electrode, which has been used to determine the Li-ion diffusion coefficient in the compound. By using the model proposed by Ho et al. [33], the diffusion coefficient of Li ions for Li[Ni0.5 Mn0.3 Co0.2 ]O2 can be calculated as the following equation: DLi =
1 2
V dE 2 m FS
dx
(2)
where Vm is the molar volume (20.42 cm3 mol−1 ), S is the surface area of the electrode (0.785 cm2 ), F is the Faraday constant (96485 C mol−1 ), dE/dx is the slope of the electrode potential (as shown in Fig. 4b). The is the Warburg coefficient, the values
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Fig. 6. The lithium diffusion coefficients calculated from the GITT curves as a function of the cell potential (a) and as a function of the stoichiometry x (b).
under different charge states for Li[Ni0.5 Mn0.3 Co0.2 ]O2 (as shown in Fig. 8a) can be obtained from the slope of Z vs. ω−1/2 plots (ω is the angular frequency) in the Warburg region. Typical plots of the Z vs. ω−1/2 for the Li[Ni0.5 Mn0.3 Co0.2 ]O2 during the third charge process along with the linear fitting are shown in Fig. 8b. Both
Fig. 7. Impedance spectra for Li[Ni0.5 Mn0.3 Co0.2 ]O2 under different charge states.
Fig. 8. (a) as a function of the cell potential and (b) real parts of the complex impedance Z vs. ω−1/2 for Li[Ni0.5 Mn0.3 Co0.2 ]O2 under different charge states.
the plots show parallel straight lines characteristic of the Warburg impedance. The diffusion coefficients of lithium for Li[Ni0.5 Mn0.3 Co0.2 ]O2 under different charge states are obtained by substitution of the curve slopes in Eq. (2), as shown in Fig. 9. It can be seen that the DLi values obtained from EIS are in the range of 1.7 × 10−11 to 6.5 × 10−9 cm2 s−1 . Further, it can be found that the DLi (EIS) values are larger by nearly one time compared to the corresponding DLi (GITT) in the voltage range of 4.0–4.5 V. Similar differences in DLi measured for the Li1-x [Ni1/3 Mn2/3 ]O2 [34] compound by the above two techniques were also observed earlier. The DLi for Li1-x [Ni1/3 Mn2/3 ]O2 in the composition range 0.1 < x < 0.3 obtained by GITT was about 5 × 10−9 cm2 s−1 , whereas by the EIS method, the DLi values are higher by one order of magnitude. It must be mentioned at this juncture that Levi and Aurbach [35] clarified that even though there is no principle difference in the resolution of diffusion time with respect to voltage, the GITT could provide a some what more precise data on (here, the diffusion coefficient DLi = l2 /, where l is the characteristic diffusion length) as compared to EIS, provided the current pulse height for the GITT measurements and the amplitude of the AC current for EIS are small. This is due to the fact that the ohmic potential drop during GITT titration is independent of the potential and can be readily eliminated. This result was consistent with the reports by Bruce
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Acknowledgments This work was financially supported by the National Natural Science Foundation of China under project no. 20871101, Joint Fund of Natural Science of Hunan Province and Xiangtan City under project no. 09BG005, Industrial Project of Colleges and Universities of Hunan Province under project no. 10CY005, Project of Condition Research of Hunan Province under project no. 2010TC2004 and Colleges and Universities in Hunan Province plans to graduate research and innovation under project no. CX2009B133. References
Fig. 9. Diffusion coefficient of Li ions for the Li[Ni0.5 Mn0.3 Co0.2 ]O2 calculated from the EIS.
[36] and Boukamp and co-workers [37]. Besides, Shaju et al. [16] had proved that GITT provided more accurate diffusion coefficients compared to EIS when calculated the DLi for Li[Ni1/3 Co1/3 Mn1/3 ]O2 . In this paper, the diffusion coefficients of Li[Ni0.5 Mn0.3 Co0.2 ]O2 obtained by the two techniques are close to those of similar layered cathode material Li[Ni1/3 Mn1/3 Co1/3 ]O2 . Therefore, it could be considered that the GITT is also a more reliable method for the determination of the diffusion coefficient of lithium ions in spherical Li[Ni0.5 Mn0.3 Co0.2 ]O2 . Besides, it is worth noting that the values of the DLi (GITT) in our work for Li[Ni0.5 Mn0.3 Co0.2 ]O2 (with a value of 3.0 (±0.5) × 10−9 cm2 s−1 in the voltage range of 3.9–4.3 V) is higher by nearly one order of magnitude than that for Li[Ni1/3 Co1/3 Mn1/3 ]O2 (with a value of ∼3 × 10−10 cm2 s−1 in the same voltage range obtained from GITT) measured by Shaju et al. [16]. This is probably attributed to different inherent properties of two cathode materials such as structure, surface area, grain size, morphology and so on, and different parameters setting when the values of DLi were calculated. 4. Conclusions Two electrochemical measurement techniques, GITT and EIS, have been successfully employed to determine the chemical diffusion coefficient of Li ions in spherical Li[Ni0.5 Mn0.3 Co0.2 ]O2 . The variation of DLi derived from GITT and EIS is correlated with the performance of lithium ion intercalation/deintercalation in the Li[Ni0.5 Mn0.3 Co0.2 ]O2 and greatly dependent on the cell potential. The DLi obtained from GITT and EIS are in the range of 1.9 × 10−11 to 3.9 × 10−9 cm2 s−1 and 1.7 × 10−11 to 6.5 × 10−9 cm2 s−1 respectively. The results obtained by the two techniques are well consistent.
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