High-Rate Lithium Deintercalation from Lithiated Graphite Single

Apr 15, 2010 - by bringing a metal microfilament into contact with it in an electrolyte. In the case of a single-particle electrode, the current is su...
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High-Rate Lithium Deintercalation from Lithiated Graphite Single-Particle Electrode Kaoru Dokko,*,† Natsuko Nakata,‡ Yushi Suzuki,‡ and Kiyoshi Kanamura‡ Department of Chemistry and Biotechnology, Yokohama National UniVersity, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan, and Department of Applied Chemistry, Tokyo Metropolitan UniVersity, 1-1 minami-ohsawa, Hachioji, Tokyo 192-0397, Japan ReceiVed: February 06, 2010; ReVised Manuscript ReceiVed: April 03, 2010

The electrochemical behavior of a lithiated graphite single-particle electrode during high-rate Li deintercalation in an organic electrolyte was investigated using a microelectrode technique. A Ni-plated metal filament (diameter: 10 µm) was attached to a mesocarbon microbead (MCMB) in the electrolyte under optical microscope observation, and galvanostatic charge-discharge tests were carried out. The discharge capacity of a lithiated MCMB particle (diameter: 18 µm) was 2.02 nA h in the potential range of 0.005-2.5 V vs Li/Li+. The fully lithiated MCMB particle showed an extremely high rate capability and released more than 98% of the accommodated Li at a constant discharge current of 1000 nA within 10 s. At discharge currents lower than 200 nA, the charge transfer process at the interface controlled the reaction of the single-particle electrode, and the Li diffusion process in the MCMB particle did not significantly affect the Li deintercalation rate. The charge transfer resistance for Li intercalation/deintercalation was in the range of 20-50 Ω cm2, and the apparent chemical diffusion coefficient of Li in the MCMB particle was estimated to be 8.3 × 10-8 cm2 s-1. Introduction Graphite is the most popular anode material for Li ion batteries.1-13 Graphite reacts with Li to form graphite intercalation compounds (GICs). The electrochemical intercalation of Li into graphite takes place reversibly as shown in eq 1, and the theoretical capacity of graphite is 372 mA h g-1.1

6C + xLi+ + xe- T LixC6

(1)

Obtaining an insight into the kinetics of Li intercalation and deintercalation at graphite electrodes is important for determining the power density of Li batteries. The anode in commercial Li ion batteries is a porous composite electrode consisting of micrometer-sized graphite particles, an organic polymeric binder, and a conductive agent. The majority of the investigations reported in the literature have been carried out with porous composite electrodes.14-21 The binder and conductive agent have been reported to affect the charge/discharge properties of the composite electrode.14,15 Moreover, because there are distributions of potential, current density, and electrolyte salt concentration in porous electrodes during the charge-discharge process, the porous structure of the electrode has to be taken into account when analyzing the electrochemical response of a porous composite electrode.16-21 These distributions become particularly significant in the case of high-rate charge-discharge, in which electrochemical reactions do not take place uniformly within the porous electrode. Thus far, basic research on Li intercalation compounds has been conducted with use of a thin-film electrode as the model electrode. Thin-film electrodes, which can be fabricated by chemical vapor deposition, sputtering, pulsed-laser deposition, * To whom correspondence should be addressed. Phone: +81-453393942. Fax: +81-45-3393942. E-mail: [email protected]. † Yokohama National University. ‡ Tokyo Metropolitan University.

Figure 1. Schematic illustration of electrochemical cell with singleparticle electrode.

etc.,22-25 are ideal because the physicochemical properties of a flat electrode can be analyzed in detail. However, it is difficult to prepare thin-film graphite electrodes, and highly oriented pyrolytic graphite (HOPG) electrodes have been used as alternative model electrodes.26-33 Another useful method is the microelectrode technique.34-44 Uchida et al. developed a microelectrode technique to investigate the electrochemical properties of micrometer-sized single particles of battery active materials.34,35 As shown in Figure 1, the electrochemical properties of a micrometer-sized particle can be characterized by bringing a metal microfilament into contact with it in an electrolyte. In the case of a single-particle electrode, the current is sufficiently small (nanoampere level) such that the iR potential drop can be neglected. The distributions of potential and current density are then almost uniform at the particle surface. Using a single-particle electrode, therefore, enables the basic electrochemistry of the redox material to be investigated in detail. To assess Li intercalation into graphite particles, they performed cyclic voltammetry, potential step intermittent titration, and AC impedance measurements and investigated the Li diffusion process in the solid and the charge transfer process at the interface.41,42 However, the high rate capability of a single graphite particle to accommodate/release Li has not been clearly investigated through experiments thus far. In this work, the electrochemical behavior of a lithiated mesocarbon microbead

10.1021/jp101166d  2010 American Chemical Society Published on Web 04/15/2010

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Figure 2. Charge and discharge curves of a single MCMB particle measured at a constant current of 3 nA. From the discharge capacity, the diameter of the MCMB particle is estimated to be 18 µm.

Figure 3. Discharge curves of a single MCMB particle (diameter: 18 µm) measured at various currents. Prior to each discharge, the MCMB particle was fully lithiated by charging at a low current of 3 nA.

(MCMB) during high-rate Li deintercalation was investigated with use of a microelectrode technique.

ment with previous reports.1,2 Although a small irreversible capacity was observed in the first cycle, the Coulombic efficiency after the second cycle was more than 97%. The MCMB single particle exhibited excellent reversibility, and the iR potential drop was very small. The contact resistance between the microfilament and the MCMB particle is considered to be negligibly small because the electrical conductivity of graphite is sufficiently high. Umeda et al. performed AC impedance measurements on MCMB single particles and reported that the contact resistance did not significantly affect their electrochemical behavior.41 The measured discharge capacity of the MCMB single particle was 2.02 nA h. The specific discharge capacity of the MCMB, which was evaluated by using a porous composite electrode, was 304 mA h g-1 (see the Supporting Information, Figure S1), and the density of the MCMB particle was 2.16 g cm-3.47 The mean diameter of the MCMB particle, calculated from the specific capacity and the density of MCMB, was 18 µm, which was slightly smaller than that estimated by optical microscopy. The MCMB particle is not a perfect sphere, and observations with an optical microscope may lead to overestimation of the particle size. In this paper, the MCMB particle was treated as a sphere with 18 µm diameter. To evaluate the rate capability of a single MCMB particle, discharge measurements were carried out at various currents. Prior to each discharge measurement, the MCMB particle was charged galvanostatically at a constant current of 3 nA. As shown in Figure 3, the single particle showed excellent discharge rate capability. The rate of discharge of the electrode, n C-rate, is defined as n ) (applied current)/(full capacity of the electrode). In the case of MCMB, 1 C-rate corresponds to a gravimetric current density of 304 mA g-1. A current of 3 nA corresponds to a rate of ca. 1.5 C-rate for the MCMB particle. The discharge capacity measured at 2000 nA, which corresponded to 1000 C-rate, was 1.75 nA h, more than 85% of the full capacity of the single particle. Several processes are involved in Li intercalation and deintercalation at the MCMB single-particle electrode: (i) diffusion of solvated Li+ ions from the bulk electrolyte to the particle surface; (ii) interfacial charge transfer (Li+ ion transfer) at the interface between the particle and the electrolyte; (iii) solid-state Li diffusion within the particle; and (iv) crystallographic structural changes in the solid. In general, the diffusion coefficient of Li+ ions in a liquid electrolyte is higher than that in a solid.1 Therefore, the reaction rate at a single MCMB particle is apparently controlled by charge transfer, solid-state diffusion, and/or crystallographic structural changes. In the following section, we discuss the factors that determine the reaction rate of a single-particle electrode of MCMB.

Experimental Section The experimental setup for the electrochemical measurements, which is shown in Figure 1, is similar to that reported elsewhere.35 A Pt microfilament (diameter: 10 µm) was sealed in glass, using a previously described method.45 The tip of the glass-sealed Pt microfilament was cut to yield a microdisk, and then polished to a mirror finish. The microdisk was electroplated with Ni; this is necessary to avoid electrochemical reactions of Pt when electrochemical measurements on the MCMB particles are being performed. The Ni plating was carried out at a current of 15 nA for 20 min in a Watt bath consisting of 0.35 mol dm-3 NiCl2, 0.9 mol dm-3 NiSO4, and 0.5 mol dm-3 boric acid. MCMB powder (MCMB-28-25, heat-treated at 2800 °C) was supplied by the Osaka Gas Chemicals Co. and used as received. MCMB particles are spherical in shape,46,47 and it is convenient to analyze their electrochemical responses as a model electrode. The MCMB powder was spread on a glass-fiber filter, and the filter was placed in the electrochemical cell (Figure 1). The Niplated microfilament was attached to a single MCMB particle in the electrolyte with use of a micromanipulator (InjectMan NI2, Eppendorf) under an optical microscope (BX51W1, Olympus). Subsequently, the electrochemical measurements were performed. The electrolyte was a mixed solvent of propylene carbonate and ethylene carbonate (1:1 by volume) containing 1 mol dm-3 LiClO4. The counter electrode was a Li foil (area: 1 cm2), and the electrochemical measurements were performed with a two-electrode system. The charge-discharge tests were carried out with a galvanostat (ALS Model 660A) in the potential range of 0.005-2.5 V vs Li/Li+. All electrochemical measurements were carried out in an Ar-filled glovebox at room temperature (25 °C). Results and Discussion Figure 2 shows charge and discharge curves measured at 3 nA, during the initial three cycles of a single MCMB particle. The MCMB particle was observed with an optical microscope, and its diameter was estimated to be ca. 20 µm. The single particle showed several potential plateaus at potentials lower than 0.25 V in both the charge and the discharge curves. It is well-known that Li is intercalated into the layered structure of graphite to form GICs.1,2 Four different stage structures of LiGICs are known depending on x in LixC6, and a phase transition takes place at each potential plateau. This electrochemical behavior is characteristic of graphite particles and is in agree-

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As shown in Figure 3, at currents less than 1000 nA, the MCMB particle exhibited a constant discharge capacity, although the overpotential increased with increasing discharge current. If there had been a large Li concentration gradient in the particle during the discharge reaction, the electrode potential would have risen rapidly to the cutoff potential when x in LixC6 reached zero at the particle surface,48 while x at the center of the particle would not reach zero, and the full-discharge capacity would not be attained. It was therefore concluded that solidstate diffusion of Li did not control the reaction rate of the MCMB particle at currents less than 1000 nA. It is possible that crystallographic structural changes may affect the reaction rate of the MCMB single-particle electrode. The crystallographic structural changes and stage transformations depend on the value of x in LixC6.1,2 Funabiki et al. suggested that phase-boundary movements take place in the course of a stage transformation.29,30 They performed potential-step chronoamperometry and AC impedance measurements on HOPG electrodes and analyzed the electrochemical response of the electrode using a simple geometric model. They concluded that the phase-boundary movement was determined initially by the rate of the interfacial electrochemical reaction and thereafter controlled by the diffusion of Li in the solid phases.30 In other words, the phaseboundary movement does not control the rate of Li intercalation and deintercalation at graphite electrodes. These studies confirm that the reaction rate of the MCMB particle (diameter: 18 µm) is controlled by charge-transfer processes at currents less than 1000 nA. It is noteworthy that 1000 nA corresponds to a 500 C-rate for a single MCMB particle (diameter: 18 µm). As demonstrated in Figure 3, the discharge of a fully lithiated MCMB particle is completed within 7.2 s at 500 C-rate, and almost all the Li in the MCMB particle is released. As shown in Figure 3, the operating potential (E) of the MCMB electrode shifted in the anodic direction because of the increase in the overpotential with increasing discharge current. When the Li deintercalation rate is controlled by the charge transfer process, the relationship between discharge current and overpotential can be expressed by the Butler-Volmer equation.48 In the case of a Li intercalation compound, the opencircuit potential, which is very close to the equilibrium potential, changes depending on x in LixC6. In this study, the midpoint potential of the charge curve and the discharge curve at a constant x, measured at a low current of 3 nA, was regarded as the equilibrium potential (Ee) for Li intercalation/deintercalation in LixC6. The midpoint potential ([E(charge) + E(discharge)]/ 2) curve as a function of the depth of discharge (DOD) is shown in Figure S2 in the Supporting Information. The fully charged state (lithiated state) and the fully discharged state (delithiated state) correspond to 0% and 100% DOD, respectively. The electrode potentials of the discharge curves at a constant DOD value of 10% were plotted as a function of applied current density ia, as shown in Figure 4. The current was normalized by the geometric surface area of the particle (1.0 × 10-5 cm2). The Tafel equation is expressed as follows:48

log ia ) log i0 +

RF η 2.303RT

(2)

where ia is the applied current density, i0 is the exchange current density, R is the transference coefficient for the anodic reaction, F is the Faraday constant, R is the gas constant, T is the temperature, and η ()E - Ee) is the overpotential for the anodic reaction. In the Tafel plot for the MCMB particle electrode at DOD ) 10% (Figure 4), a linear region can be seen in the

Figure 4. Tafel plot for a single MCMB single-particle electrode (diameter: 18 µm) at 10% DOD.

current range of 10-200 nA. At currents higher than 500 nA, the Tafel plot deviates from linearity because of the limitations of mass transport in the MCMB particle. According to the Tafel equation, the exchange current density for the reaction can be determined from the intercept of the linear line at η ) 0, and the transference coefficient R can be obtained from the slope of the linear line. The exchange current densities and transference coefficients evaluated at various DOD values are shown in Figure 5. The charge transfer resistance at the interface and the exchange current density can be correlated as follows:48

Rct )

RT Fi0

(3)

where Rct is the charge transfer resistance. The charge transfer resistance is in the range of 20-50 Ω cm2, which shows good agreement with values obtained from AC impedance measurements in a previous study.41 It has been reported that the film formed on the particle surface causes resistance during charge and discharge of graphite electrodes.1,33,41,49 This surface film resistance may be involved in the charge transfer resistance estimated from the Tafel plots. The transference coefficient R is in the range 0.25-0.4 and decreases gradually with increasing DOD. At present, it is not clear how the Li concentration in the MCMB particle affects the transference coefficient. Further investigations are needed to understand the kinetics of Li intercalation/deintercalation at the interface between graphite and the electrolyte. As shown in Figure 3, at currents higher than 1000 nA, the discharge capacity of the MCMB particle decreased gradually with increasing discharge current. As mentioned before, the electrode potential reached the cutoff potential because x dropped to zero at the particle surface before the full capacity of the particle had been achieved. Apparently, solid-state diffusion of Li becomes a significant factor for the reaction rate at discharge currents higher than 1000 nA. The Li concentration gradient (in the solid) at the particle surface becomes steeper with increasing discharge current. It is likely that solid-state diffusion of Li completely controls the reaction rate at currents higher than 2000 nA. The apparent chemical diffusion coefficient of Li in the MCMB particle can be estimated if the diffusion of Li in the particle takes place (i) spherically and (ii) in a single phase. For simplification, we do not take the phase transition into account in this paper. The estimated chemical diffusion coefficient is therefore an apparent value. If the particle is initially at a uniform Li concentration CO, and there is a constant flux JO of Li at the particle surface, then

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Figure 5. (a) Exchange current density (io) and charge-transfer resistance (Rct) and (b) transference coefficient R for Li deintercalation evaluated at various DOD values.

Figure 6. Simulated Li concentration profiles of an MCMB particle (radius: 9 µm) during galvanostatic discharge at discharge currents of (a) 200 and (b) 2000 nA. The initial concentration of Li in the MCMB particle was assumed to be 25 × 10-3 mol cm-3 and was considered to be uniform within the particle. The chemical diffusion coefficient (D) of Li in the MCMB particle was 8.3 × 10-8 cm2 s-1.

JO ) -D

∂C(r, t) ∂r

at r ) a

(4)

where r is the radial distance from the center of the particle, C(r,t) is the concentration of Li at time t, D is the diffusion coefficient of Li in the particle, and a is the particle radius. Solving the diffusion equations for spherical diffusion in the particle gives the following equation:50

CO - C(r, t) )

{

JOa 3Dt 1 r2 3 + 2 2 D a 2a 10 2

}

∞ sin(βnr) a exp(-Dβn2t) 2 r n)1 β a2 sin(β a) n n

∑

(5)

where the aβn values are the positive roots of aβn cot(aβn) ) 1. If the applied current for the MCMB particle is I, then JO ) I/FA (where A is the geometric surface area of the particle). At transition time τ, C(a,t) drops to zero, and the electrode potential of the single particle rises rapidly.48 If the charge-transfer process controls the rate of Li deintercalation, the transition time is independent of the Li diffusion coefficient, and τ ) Q/I, where Q is the full-discharge capacity of the MCMB particle. When the rate of Li deintercalation is controlled by Li diffusion in the particle, the diffusion coefficient can be determined from eq 5 and τ. From the discharge curve shown in Figure 3, τ was 3.2 s when the applied current was 2000 nA. The particle radius, a, was 9 µm. The initial concentration of Li in the particle, CO,

was estimated to be 25 × 10-3 mol cm-3 from the discharge capacity of 304 mA h g-1 (Figure S1, Supporting Information) and the density of 2.16 g cm-3.47 The concentration of Li at the particle surface C(a,t) is 0 when t ) τ. Substituting these values in eq 5 gives D ) 8.3 × 10-8 cm2 s-1, which is in agreement with values reported by other research groups.51-53 Although the estimated chemical diffusion coefficient is an apparent value, it is useful in predicting the charge and discharge rate capability of the single-particle electrode. Figure 6 shows the Li concentration profiles in the MCMB particle simulated with use of eq 5. The concentration gradient within the particle can scarcely be observed at currents lower than 200 nA, which corresponds to a 100 C-rate. At high currents, such as 2000 nA, the concentration gradient becomes steep, and the Li remains inside the particle when the surface concentration reaches zero. It should be noted that the normal operating conditions of practical Li ion batteries are lower than 50 C-rate. Under battery operating conditions, the Li diffusion process in the micrometersized graphite particle never limits the rate capability of the battery. Electrochemical measurements of MCMB single particles suggest that the charge transfer process controls the electrode reaction rate. According to the reports by Abe et al.,33,54 the rate-determining step of the charge transfer process is the solvation/desolvation of Li+ ions at the interface. The surface condition of the graphite and the electrolyte composition are therefore essential factors in the kinetics of Li intercalation/ deintercalation into/from a graphite particle. As shown in this study, single-particle electrodes of lithiated graphite can be discharged at extremely high rates by applying

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large overpotentials. However, high-rate charging (Li intercalation) is difficult to perform because the operating potential should be kept above 0 V vs Li/Li+ to avoid the dendritic deposition of metallic Li. The equilibrium potential of Li intercalation at graphite electrodes is usually lower than 0.25 V, and smaller overpotentials can be applied to the electrode during charging. Conclusions The high-rate Li deintercalation capability of a single-particle electrode of lithiated graphite (diameter: 18 µm) was investigated with use of a microelectrode technique. The fully lithiated MCMB particle showed an extremely high rate capability (500 C-rate) and released more than 98% of the accommodated Li within 10 s. At discharge rates lower than 100 C-rate, the charge transfer process at the interface controlled the reactions of the single-particle electrode, and the Li diffusion process in the MCMB particle did not significantly affect the Li deintercalation rate. The charge transfer resistance for Li intercalation/deintercalation was in the range of 20-50 Ω cm2, and the apparent chemical diffusion coefficient of Li in the MCMB particle was estimated to be 8.3 × 10-8 cm2 s-1. Acknowledgment. This work was partially supported by the Industrial Technology Research Grant Program (07A22007a) under the New Energy and Industrial Technology Development Organization (NEDO), Japan. Supporting Information Available: Supporting figures showing the charge and discharge curves of composite electrode consisting of MCMB powder and PVDF binder and the midpoint potential of charge and discharge curves of a single MCMB particle measured at a current of 3 nA plotted as a function of DOD. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Ogumi, Z.; Inaba, M. Bull. Chem. Soc. Jpn. 1998, 71, 521. (2) Dahn, J. R. Phys. ReV. B 1991, 44, 9170. (3) Yazami, R. Electrochim. Acta 1999, 45, 87. (4) Zaghib, K.; Tatsumi, K.; Sawada, Y.; Higuchi, S.; Abe, H.; Ohsaki, T. J. Electrochem. Soc. 1999, 146, 2784. (5) Guyomard, D.; Tarascon, J. M. J. Electrochem. Soc. 1992, 139, 937. (6) Zaghib, K.; Song, X.; Guerfi, A.; Kostecki, R.; Kinoshita, K. J. Power Sources 2003, 124, 505. (7) Takami, N.; Satoh, A.; Hara, M.; Ohsaki, T. J. Electrochem. Soc. 1995, 142, 371. (8) Tarascon, J. M.; Armand, M. Nature 2001, 414, 359. (9) Bruce, P. G. Chem. Commun. 1997, 1817. (10) Winter, M.; Besenhard, J. O.; Spahr, M. E.; Nova´k, P. AdV. Mater. 1998, 10, 725. (11) Aurbach, D. J. Power Sources 2000, 89, 206. (12) Scrosati, B.; Garche, J. J. Power Sources 2010, 195, 2419. (13) Ohzuku, T.; Iwakoshi, Y.; Sawai, K. J. Electrochem. Soc. 1993, 140, 2490. (14) Dominko, R.; Gaberek, M.; Drofenik, J.; Bele, M.; Pejovnik, S. Electrochem. Solid-State Lett. 2001, 4, A187. (15) Dominko, R.; Gaberscek, M.; Drofenik, J.; Bele, M.; Pejovnik, S.; Jamnik, J. J. Power Sources 2003, 119-121, 770.

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