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The a.c. impedance response of the LiNiO2 electrode in organic electrolyte solutions with different compositions
PERGAMON
Electrochimica Acta 44 (1999) 1607±1615
The a.c. impedance response of the LiNiO2 electrode in organic electrolyte solutions with dierent compositions Otoo Yamada, Masashi Ishikawa, Masayuki Morita * Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering, Yamaguchi University, Tokiwadai 2557, Ube 755-8611, Japan Received 20 April 1998; received in revised form 24 June 1998
Abstract The a.c. impedance of the LiNiO2 electrode was measured in organic electrolyte solutions based on mixed carbonate solvents containing inorganic lithium salts. The electrolyte composition in¯uenced the impedance response of the oxide electrode as well as the charge and discharge characteristics under constant-current polarization conditions. The impedance responses changed with the state of the charge of the oxide, i.e. the x value in Li1ÿxNiO2. The impedance diagrams were analyzed using three equivalent circuit models. The interfacial resistance and the apparent diusion coecient of lithium, evaluated from the impedance responses, varied with the sort of solvent and salt. The interfacial resistance showed a minimum value around x = 0.5 in Li1ÿxNiO2 for every electrolyte system, while the apparent diusion coecient decreased with decreasing x value. The relation between the electrolyte composition and the impedance response of the oxide is discussed. The electrolyte composition in¯uences the eective cross section of the diusion process in the solid phase through its eect on the surface chemistry of the oxide cathode. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Lithium batteries; LiNiO2 positive electrode; Organic electrolytes; A.c. impedance; Diusion coecient
1. Introduction Rechargeable lithium ion batteries on the market employ LiMO2 or LiM2O4 (M: transition metal) type oxides as the positive electrode (cathode) material where lithium species intercalate and deintercalate in layered or channel structures during discharge and recharge, respectively, in organic electrolyte solutions [1]. Much eort has been focused on improvements in the discharge capacity and cycleability of the oxide-based positive electrodes. Lithium nickelate, LiNiO2, is the most attractive replacement for the present LiCoO2-based positives because of its lower cost and higher theoretical capacity [2, 3]. The preparation conditions have in¯uences on the chem-
* Corresponding author. Fax: +81-836-35-9933; E-mail: [email protected]
istry and physics of the oxide, and then determine the electrode performances of the oxide in organic electrolyte solutions. Thus, dierent methods have been proposed to obtain higher cathode performances for the oxide [4±13]. For instance, the substitution of Ni by other transition metals, stabilizing the oxide structure, improves the charge and discharge performances [7, 10± 12, 14±16]. On the other hand, it is also recognized that the composition of the electrolyte system aects the charge and discharge performances of the batteries using this type of positive electrode [17]. We have demonstrated the in¯uences of electrolyte composition on the charge±discharge behavior of the LiNiO2 electrode in mixed alkyl carbonate media [18]. An electrolyte system having higher ionic conductivity tended to give a higher discharge capacity of the LiNiO2 cathode. However, the mechanism of the in¯uences of the electrolyte on the discharge performances have not been
0013-4686/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 9 8 ) 0 0 2 8 4 - 9
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clari®ed for this electrode/electrolyte system. It is quite important for us to understand the eects of the ionic structure and behavior on the electrode processes of the lithium-insertion materials. Practically, it is necessary to optimize the electrolyte composition for developing the batteries with advanced performances. In this paper, the a.c. impedance of the LiNiO2 electrode is measured in mixed carbonate electrolyte systems. Several research groups have so far reported the impedance response of LiNiO2 in organic electrolyte solutions [9, 19±21]. However, they presented dierent interpretations of the experimental results. Thus, in this work, the impedance pro®les are analyzed using possible equivalent circuit models, as functions of the depth of discharge and the electrolyte composition. The results assist the discussion of the electrochemistry at the interface between the oxide electrode and the organic electrolyte solution. The aim of this work is to give a rational explanation of the fact that the impedance of LiNiO2 depended not only on the oxide composition but also on the electrolyte solutions. 2. Experimental The electrolyte solutions consisted of mixed carbonate solvents dissolving lithium salts [18]. The high permittivity solvent, ethylene carbonate (EC) or propylene carbonate (PC), was mixed with a low viscosity linear alkyl carbonate, dimethyl carbonate (DMC) or diethyl carbonate (DEC) in the volumetric ratio of 50 + 50. These solvents were used as received (Mitsubish Chemical, Battery Grade) because of their high purity and low water contents (below 10 ppm). The electrolytic salts were well-dehydrated LiClO4 (Ishizu Pharmaceutical), LiPF6 (Tomiyama Chemical) and LiCF3SO3 (Morita Chemical Industries), which were dissolved in the mixed solvents to make 1 mol dm ÿ 3 (M) solutions. The composition of the electrolyte solution is given by the salt/solvent combination, as LiClO4/(EC + DMC), in this paper. The ionic conductivity of the electrolyte solution was measured at 10 kHz a.c. using an LCR meter (258C). The test electrode contained 20 mg of powdered LiNiO2 (Nihon Kagaku Sangyo) with 10 mg of acetylene black as a conducting support and 3 mg of poly(tetra¯uoroethylene) (PTFE) as a binder. These components were well mixed in a mortar and then pressed onto a current collector screen (13 mm diameter disc made of stainless steel). The resulting test electrode was dried at 1208C for 5 h or longer under a vacuum before use. The charge and discharge capacities were measured using a 2032-size coin cell, in which a lithium metal sheet with an excess amount of equivalence was used as the counter electrode (anode). The coin cell was
cycled under constant-current conditions (current density: 0.5±2.0 mA cm ÿ 2, cut-o voltage: 4.2 V for charge, 2.5 V for discharge) at room temperature (18± 258C). A three electrode system was employed for the impedance measurements. A glass beaker cell equipped with a lithium tip reference electrode (Li/Li + ) and a lithium foil counter electrode (3.0 2.5 cm) was used. The geometric surface area of the test electrode exposed to the electrolyte solution was reduced to 0.071 cm2, which was about 1/100 that of the counter electrode, to minimize possible in¯uences arising from the lithium counter electrode. Further, the cell was equipped with no separator that might have in¯uenced the impedance response of this type of the electrode [22]. The volume of the electrolyte solution was 50 cm3. A frequency response analyzer (Solartron 1250) coupled with an electrochemical interface (potentiostat) was used for the impedance measurements. The a.c. frequency was scanned from 65 kHz to 10 mHz with an amplitude of 10 mV (p±p). The measurements were carried out under open circuit voltage (OCV) conditions before and after the constant current (1 mA cm ÿ 2) charging and discharging at room temperature (18±258C). To achieve the OCV conditions, sucient relaxation time was taken into account after each constant-current polarization. All experiments including the electrolyte preparation and the cell construction were performed under a dry argon atmosphere. 3. Results and discussion Figs. 1 and 2 show typical charge and discharge pro®les of the Li/LiNiO2 cells using dierent electrolyte solutions. As the cell capacity was controlled by the mass of the positive electrode, the pro®les are given as the voltage variations with the capacity (quantity of electricity passed) per mass of the positive electrode. These curves were obtained for each second cycle under CCV conditions (1 mA cm ÿ 2). The discharge capacity of the cell depended not only on the organic solvent but also on the electrolytic salt [18]. The Coulombic eciency for each charge and discharge cycle was practically 100%, except for the cell using LiPF6/(EC + DMC), where poor cycleability of the counter electrode (lithium metal) would be responsible for the low coulombic eciency of the full cell. Thus, the order of the discharge capacity would be a measure of the activity of the LiNiO2 electrode in the Li + -containing organic electrolyte. The capacity increases in the order of PC + DMC < EC + DEC < EC + DMC with respect to the solvent component, and in the order of LiCF3SO3 < LiPF63LiClO4 with respect to the electro-
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Fig. 1. Charge and discharge pro®les for Li/LiNiO2 cells with 1 mol dm ÿ 3 LiClO4 solutions. Current density: 1 mA cm ÿ 2, Solid line curve: EC + DMC, Broken line curve: EC + DEC, Dotted line curve: PC + DMC.
lytic salt. Since the ionic conductivity of the solution increases in the orders of EC + DEC < PC + DMC < EC + DMC (solvent) and of LiCF3SO3 < LiClO4 < LiPF6 (salt) [11], the activity of the LiNiO2 electrode is partly dependent on the ionic conduction. However, the process is not at all controlled by the mass transfer in the solution phase, as seen in the inconsistency between the discharge capacity of the electrode and the ionic conductivity of the solution. Fig. 3 shows the rate capability of the Li/LiNiO2 cells with dierent solvents of the electrolytes. When the cell was cycled under a lower rate (current), the eect of the electrolyte composition on the discharge capacity was not so signi®cant and tended to reach the theoretical maximum value (180±200 Ah kg ÿ 1) regardless of the electrolyte. However, the high rate cycling makes the dierences clearer in the discharge capacity among the solvent components. The electrode process of LiNiO2 in organic electrolytes, Eq. (1), consists of several steps. Li1ÿx NiO2 xLi xeÿ LiNiO2
0
1
That is, the direct current polarization includes Li + diusion in the liquid phase, charge transfer at the solid/liquid interphase, Li species diusion in the solid phase, electron insertion/extraction in the solid phase, and so on. Among these, the rate of electron transfer in the oxide itself would be very fast. The a.c. impedance technique is useful to analyze such an electrode process in detail. Figs. 4 and 5 show Cole±Cole plots (Nyquist plots: imaginary component, ÿZ0, vs. real component, Z 0 ) of the a.c. impedance obtained for the Li1 ÿ xNiO2 (0.3 < 1ÿ xR1.0) in dierent electrolyte systems. The impedance was measured under OCV conditions after the constant-current polarization with given quantity of electricity (charge and discharge) in each ®rst cycle. The depth of charge or discharge is presented as 1ÿ x in Li1ÿxNiO2, which is formally determined by the quantity of electricity passed, without any chemical analysis. In every electrolyte solution, the plot for the LiNiO2 electrode before charging (x = 0 in Li1ÿxNiO2) showed almost linear relation over a wide frequency range, which implies that the
Fig. 2. Charge and discharge pro®les for Li/LiNiO2 cells with EC + DMC solutions containing 1 mol dm ÿ 3 LiX salts. Current density: 1 mA cm ÿ 2. Solid line curve: LiClO4, Broken line curve: LiPF6, Dotted line curve: LiCF3SO3.
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Fig. 3. Rate capability of the Li/LiNiO2 cells with 1 mol dm ÿ 3 LiClO4 solutions. w: EC + DMC; t: EC + DEC; r: PC + DMC.
electrode reaction at x = 0 (1 ÿ x = 1) is simply controlled by a diusion process. When the electrode was anodically charged to 1 ÿ x = 0.4 or less, the plots formed arcs or ¯at semicircles. The size of the arc or semicircle once decreased with the discharging (increasing 1 ÿ x value), and then increased with the increase in the depth of discharge. Finally, for the electrodes with 1 ÿ xr0.8, the plots came back to the initial linear relations. Somewhat similar variations in the Cole± Cole plot with the depth of charge and discharge have also reported for LiNiO2 and related oxides [19±21]. The impedance diagrams shown in Figs. 4 and 5 are rather complicated, but we can analyze them using simpli®ed equivalent circuit models. Fig. 6 presents the equivalent circuits and the corresponding impedance responses. Model A consists of a series of the solution resistance (Rs), interface resistance (R1 and C1) and Warburg impedance (Zw). Model B is a simple series combination of two R±C (parallel) circuits, which is often observed for a charge transfer process with a protective ®lm formation or a combination of particleFig. 4. (a) Cole±Cole plots for a.c. impedance of Li1 ÿ xNiO2 in 1 mol dm ÿ 3 LiClO4/(EC + DMC). .: 1ÿ x = 1.00 (before charge); w: 1ÿ x = 0.31 (after full charge); r: 1ÿ x = 0.45; q: 1ÿ x = 0.60; t: 1ÿ x = 0.74; r: 1ÿ x = 0.82; +: 1ÿ x = 0.98 (after full discharge). (b) Cole±Cole plots for a.c. impedance of Li1 ÿ xNiO2 in 1 mol dm ÿ 3 LiClO4/ (EC + DEC). .: 1 ÿ x = 1.00 (before discharge); w: 1ÿ x = 0.40 (after full charge); r: 1ÿ x = 0.47; q: 1ÿ x = 0.62; t: 1ÿ x = 0.76; r: 1ÿ x = 0.98 (after full charge). (c) Cole±Cole plots for a.c. impedance of Li1 ÿ xNiO2 in 1 mol dm ÿ 3 LiClO4/(PC + DMC). .: 1 ÿ x = 1.00 (before charge); w: 1ÿ x = 0.34 (after full charge); r: 1ÿ x = 0.48; q: 1ÿ x = 0.56; t: 1ÿ x = 0.77; r: 1ÿ x = 0.92 (after full discharge).
O. Yamada et al. / Electrochimica Acta 44 (1999) 1607±1615
Fig. 5. (a) Cole±Cole plots for a.c. impedance of Li1 ÿ xNiO2 in 1 mol dm ÿ 3 LiPF6/(EC + DMC). .: 1ÿ x = 1.00 (before charge); w: 1ÿ x = 0.39 (after full charge); r: 1ÿ x = 0.61; q: 1ÿ x = 0.69; t: 1ÿ x = 0.76; r: 1ÿ x = 0.91 (after full discharge). (b) Cole±Cole plots for a.c. impedance of Li1 ÿ xNiO2 in 1 mol dm ÿ 3 LiCF3SO3/(EC + DMC). .: 1ÿ x = 1.00 (before charge); w: 1ÿ x = 0.47 (after full charge); r: 1ÿ x = 0.62; q: 1 ÿ x = 0.69; t: 1ÿ x = 0.76; r: 1ÿ x = 0.91 (after full discharge).
to-particle resistance of the oxide and absorption resistance at the oxide surface [20, 23]. Model C corresponds to the process involving a ®nite diusion step whose impedance is represented by Zw 0 [24]. The observed impedance showed no distinct response corresponding to a parallel R1±C1 circuit at a high frequency region, except for the case of LiCF3SO3/ (EC + DMC). This might be due to either very low R1
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(Rct) or small time constant (R1C1) that is out of the experimental frequency range ( fR65 kHz). That is, the intercept at Z 0 axis consisted mainly of the solution resistance, Rs. It depended on the electrolyte composition, but did not change with the depth of discharge in each electrolyte system. The Cole±Cole plots for the electrode with x1 0 ®tted the Model A, regardless of the electrolyte composition. If the impedance responses for the electrodes with 0.3R x approximate that of model B, we can evaluate the interface impedance of R2 and C2. Fig. 7 shows variations of R2 with the depth of discharge (1 ÿ x) of the electrode in dierent electrolyte compositions. In every electrolyte system, R2 has a minimum value around x = 0.5. A similar result has been reported by Yamada et al. [19] and Choi et al. [20]. The former authors had related the variation of the interface resistance with x in Li1ÿxNiO2 to the change in the interlayer distance of the crystal lattice [19]. The latter, on the other hand, interpreted the results as the changes in the concentrations of lithium and electron near the surface region of the oxide [20]. In this work, the resistance also depended on the sort of the electrolytic salt (LiClO4 < LiPF61LiCF3SO3) although dierences in the R2 values among the electrolyte solvent were rather small. This result means that the interface resistance include the contribution of the eects of the electrolyte solution. The physical meaning of the interface resistance is not clear in the present electrode/electrolyte system. However, it is generally accepted that the resistance of a ®lm formed on the electrode is a possible cause of the interface impedance [24, 25]. Aurbach et al. [21] reported the formation of surface ®lms at Li1ÿxMO2 type oxides in LiAsF6/ (EC + DMC), based on their FTIR observation. They suggests that the lithium intercalation process is partly controlled by the migration of lithium through the surface ®lms consisting of Li2CO3. On the other hand, Kanamura et al. [26] reported the in¯uences of the electrolytic salt on the anodic oxidation of PC-based electrolytes at an oxide-covered nickel substrate. The reactivity of the electrolyte was in the order of LiPF6 < LiCF3SO3 < LiClO4. Thus, the dierent electrolyte components should give dierent ®lm compositions, which would dierently in¯uence the interface resistance of the oxide electrode in the organic solutions. Another equivalent circuit model, Model C in Fig. 6, is also valid for the observed impedance. This equivalent circuit was ®rst adopted for the lithium metal electrode with polymer electrolyte interphase (PEI) in organic electrolyte solution [24, 27]. In this model, the arc observed in the low frequency range is considered as the result of the diusion process with a ®nite length. We can estimate the diusion rate from the Warburg component in the Cole±Cole plot. According
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Fig. 6. Electrochemical equivalent circuit models and corresponding impedance spectra. (A) Model controlled by an in®nite diusion process; (B) Model without a diusion control process; (C) Model with a ®nite diusion process.
Fig. 7. Interface resistance (R2) of Li1ÿxNiO2 electrode in dierent electrolyte solutions. (a) w: LiClO4/(EC + DMC); t: LiClO4/ (EC + DEC); r: LiClO4/(PC + DMC). (b) w: LiClO4/(EC + DMC); r: LiPF6/(EC + DMC); q: LiCF3SO3/(EC + DMC).
O. Yamada et al. / Electrochimica Acta 44 (1999) 1607±1615
to Ho et al. [28], the Warburg impedance arising from the chemical diusion in a single solid phase is expressed by Eqs. (2) and (3), ÿz0 Aw o ÿ1=2 ,
2
Aw Vm dE=dx= zFD~ 1=2 S ,
3
where Aw is preexponential factor of the Warburg impedance, o the frequency (angle velocity) of a.c., Vm the molar volume of the electrode, dE/dx the dierential of the OCV curve, z the valence of the diusion species (Li + ), F the Faraday constant, D~ the chemical diusion coecient of the diusion species, and S the surface area of the electrode. We obtained Aw from the linear parts in the Cole±Cole plots and then D~ from Eq. (3) [29]. The apparent values of D~ are plotted in Fig. 8 as a function of the depth of discharge. As de®-
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nite dE/dx values were not determined in the lowest x region (1 ÿ xr0.8), the D~ vs. 1 ÿ x plots are limited in 0.3 < 1 ÿ x < 0.8. In every electrolyte solution, it tended that the chemical diusion coecient decreased with discharging, i.e. increasing 1 ÿ x in Li1ÿxNiO2. The values of D~ ranging 10 ÿ 8±10 ÿ 6 cm2 s ÿ 1 seem to be high for the lithium diusion in a solid oxide phase at room temperature. Bruce et al. [9] used the same a.c. impedance method and equations to evaluate the lithium diusion coecient in Li1ÿxNiO2, and obtained 2 10 ÿ 7 cm2 s ÿ 1 as a maximum value, which was higher than that in Li1ÿxCoO2 with a similar layered structure (10 ÿ 9±10 ÿ 8 cm2 s ÿ 1 [30±33]). They also observed that the chemical diusion coecient in Li1ÿxNiO2 depended on the depth of discharge, but it tended to decrease with decreasing 1 ÿ x in the composition range of 0.6R1ÿ xR0.95. As reported by Ohzuku et al. [5], the electrode process of reaction (1)
Fig. 8. Apparent diusion coecient of Li for Li1ÿxNiO2 electrode in dierent electrolyte solutions. (a) w: LiClO4/(EC + DMC); t: LiClO4/(EC + DEC); r: LiClO4/(PC + DMC). (b) w: LiClO4/(EC + DMC); r: LiPF6/(EC + DMC); q: LiCF3SO3/ (EC + DMC).
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may be accompanied by the structure change of the oxide. Thus, the observed changes in the diusivity with the electrode composition could include such a structure eect. If the equivalent circuit of Model C strictly re¯ects not only the chemistry but also the geometry of the electrode, the observed diusion process should include that in pore structures of the electrode [24, 27]. As the electrode consists of the composite of the active mass (LiNiO2) with considerable amount of the conductive support (acetylene black) and the binder (PTFE), it forms a heterogeneous structure having microscopic pores of the oxide itself as well as macroscopic channels that are resulted in mixing the conducting support and the binder. We have to take account of the dierent diusion processes in such pore structures. The observed diusion impedance which depended on the composition of the electrolyte solution might include the process in the pore structure of the electrode. On the other hand, we employed the apparent geometric area exposed to the electrolyte (0.071 cm2) as the surface area in the theoretical Eq. (3). As the surface of each oxide particle is considered to be partly wet with the electrolyte, the real surface area participating the electrode reaction will be higher than the geometric one. When we introduce ten times of the geometric area into the S value in Eq. (3), the calculation gives the diusion coecient of 10 ÿ 10±10 ÿ 8 cm2 s ÿ 1. Thus, it is not signi®cant for us to compare each absolute value of the diusion coecient in this type of the electrode structure. However, it is of special importance to discuss their variations with both the electrode and the electrolyte compositions. The eect of the electrolytic salt was relatively large, compared with that of the solvent. The order of LiCF3SO3 < LiPF6 < LiClO4 was mostly consistent with that of the discharge capacity at moderate rate (Fig. 2). With respect to the solvent composition, however, the order of EC + DEC < EC + DMC < PC + DMC was inconsistent with the rate capability of the discharge capacity, as shown in Fig. 3. These results also suggest that the process is not merely controlled by the diusion process in the bulk liquid phase. On the other hand, the dependence of the diusion coecient on the depth of discharge (the value of 1 ÿ x) suggest that at least the process is partly controlled by the structure of the oxide. It is quite reasonable that the lithium diusion becomes easier as the interlayer distance increases and the interaction of the diusing species decreases with decreasing the 1 ÿ x value [5, 19]. One of the possible reasons that explain the variations in the diusion coecient with the depth of discharge and with the electrolyte composition is that the process is controlled by the eective cross section of the diusion. According to the observation by Aurbach et al. [21], the surface of the oxide particle
would be covered with a thin ®lm whose chemical composition and transference properties must vary with the electrolyte composition. This would cause the dierence in the eective cross section, or in the activity of the diusion species in the solid phase. In other words, the variations in the diusion impedance that we can observe is that of the product D~ 1=2 S in Eq. (3). Thus, the apparent diusion coecient is aected by the electrolyte composition, while the intrinsic diusion coecient in the solid phase would not be changed by these factors. Consequently, if the intrinsic diusion coecient does not vary with the electrolyte composition, the cross section, or the eective surface area, for diusion increases in the order of EC + DEC < EC + DMC < PC + DMC for the solvent and LiCF3SO3 < LiPF6 < LiClO4 for the salt. Unfortunately, the intrinsic diusion coecient in the solid is not evaluated for the present electrode system by the impedance technique. However, it is noteworthy that the electrolyte composition would have signi®cant eect on the cathode performance of LiNiO2 through the dynamic parameter D~ 1=2 S at the oxide/electrolyte interphase.
Acknowledgements This work was ®nancially supported by Grant-inAid for Scienti®c Research (Nos. 08650980, 09237250, 09650907) from the Ministry of Education, Science, Sports and Culture.
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Electrochimica Acta 44 (1999) 1607±1615
The a.c. impedance response of the LiNiO2 electrode in organic electrolyte solutions with dierent compositions Otoo Yamada, Masashi Ishikawa, Masayuki Morita * Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering, Yamaguchi University, Tokiwadai 2557, Ube 755-8611, Japan Received 20 April 1998; received in revised form 24 June 1998
Abstract The a.c. impedance of the LiNiO2 electrode was measured in organic electrolyte solutions based on mixed carbonate solvents containing inorganic lithium salts. The electrolyte composition in¯uenced the impedance response of the oxide electrode as well as the charge and discharge characteristics under constant-current polarization conditions. The impedance responses changed with the state of the charge of the oxide, i.e. the x value in Li1ÿxNiO2. The impedance diagrams were analyzed using three equivalent circuit models. The interfacial resistance and the apparent diusion coecient of lithium, evaluated from the impedance responses, varied with the sort of solvent and salt. The interfacial resistance showed a minimum value around x = 0.5 in Li1ÿxNiO2 for every electrolyte system, while the apparent diusion coecient decreased with decreasing x value. The relation between the electrolyte composition and the impedance response of the oxide is discussed. The electrolyte composition in¯uences the eective cross section of the diusion process in the solid phase through its eect on the surface chemistry of the oxide cathode. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Lithium batteries; LiNiO2 positive electrode; Organic electrolytes; A.c. impedance; Diusion coecient
1. Introduction Rechargeable lithium ion batteries on the market employ LiMO2 or LiM2O4 (M: transition metal) type oxides as the positive electrode (cathode) material where lithium species intercalate and deintercalate in layered or channel structures during discharge and recharge, respectively, in organic electrolyte solutions [1]. Much eort has been focused on improvements in the discharge capacity and cycleability of the oxide-based positive electrodes. Lithium nickelate, LiNiO2, is the most attractive replacement for the present LiCoO2-based positives because of its lower cost and higher theoretical capacity [2, 3]. The preparation conditions have in¯uences on the chem-
* Corresponding author. Fax: +81-836-35-9933; E-mail: [email protected]
istry and physics of the oxide, and then determine the electrode performances of the oxide in organic electrolyte solutions. Thus, dierent methods have been proposed to obtain higher cathode performances for the oxide [4±13]. For instance, the substitution of Ni by other transition metals, stabilizing the oxide structure, improves the charge and discharge performances [7, 10± 12, 14±16]. On the other hand, it is also recognized that the composition of the electrolyte system aects the charge and discharge performances of the batteries using this type of positive electrode [17]. We have demonstrated the in¯uences of electrolyte composition on the charge±discharge behavior of the LiNiO2 electrode in mixed alkyl carbonate media [18]. An electrolyte system having higher ionic conductivity tended to give a higher discharge capacity of the LiNiO2 cathode. However, the mechanism of the in¯uences of the electrolyte on the discharge performances have not been
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clari®ed for this electrode/electrolyte system. It is quite important for us to understand the eects of the ionic structure and behavior on the electrode processes of the lithium-insertion materials. Practically, it is necessary to optimize the electrolyte composition for developing the batteries with advanced performances. In this paper, the a.c. impedance of the LiNiO2 electrode is measured in mixed carbonate electrolyte systems. Several research groups have so far reported the impedance response of LiNiO2 in organic electrolyte solutions [9, 19±21]. However, they presented dierent interpretations of the experimental results. Thus, in this work, the impedance pro®les are analyzed using possible equivalent circuit models, as functions of the depth of discharge and the electrolyte composition. The results assist the discussion of the electrochemistry at the interface between the oxide electrode and the organic electrolyte solution. The aim of this work is to give a rational explanation of the fact that the impedance of LiNiO2 depended not only on the oxide composition but also on the electrolyte solutions. 2. Experimental The electrolyte solutions consisted of mixed carbonate solvents dissolving lithium salts [18]. The high permittivity solvent, ethylene carbonate (EC) or propylene carbonate (PC), was mixed with a low viscosity linear alkyl carbonate, dimethyl carbonate (DMC) or diethyl carbonate (DEC) in the volumetric ratio of 50 + 50. These solvents were used as received (Mitsubish Chemical, Battery Grade) because of their high purity and low water contents (below 10 ppm). The electrolytic salts were well-dehydrated LiClO4 (Ishizu Pharmaceutical), LiPF6 (Tomiyama Chemical) and LiCF3SO3 (Morita Chemical Industries), which were dissolved in the mixed solvents to make 1 mol dm ÿ 3 (M) solutions. The composition of the electrolyte solution is given by the salt/solvent combination, as LiClO4/(EC + DMC), in this paper. The ionic conductivity of the electrolyte solution was measured at 10 kHz a.c. using an LCR meter (258C). The test electrode contained 20 mg of powdered LiNiO2 (Nihon Kagaku Sangyo) with 10 mg of acetylene black as a conducting support and 3 mg of poly(tetra¯uoroethylene) (PTFE) as a binder. These components were well mixed in a mortar and then pressed onto a current collector screen (13 mm diameter disc made of stainless steel). The resulting test electrode was dried at 1208C for 5 h or longer under a vacuum before use. The charge and discharge capacities were measured using a 2032-size coin cell, in which a lithium metal sheet with an excess amount of equivalence was used as the counter electrode (anode). The coin cell was
cycled under constant-current conditions (current density: 0.5±2.0 mA cm ÿ 2, cut-o voltage: 4.2 V for charge, 2.5 V for discharge) at room temperature (18± 258C). A three electrode system was employed for the impedance measurements. A glass beaker cell equipped with a lithium tip reference electrode (Li/Li + ) and a lithium foil counter electrode (3.0 2.5 cm) was used. The geometric surface area of the test electrode exposed to the electrolyte solution was reduced to 0.071 cm2, which was about 1/100 that of the counter electrode, to minimize possible in¯uences arising from the lithium counter electrode. Further, the cell was equipped with no separator that might have in¯uenced the impedance response of this type of the electrode [22]. The volume of the electrolyte solution was 50 cm3. A frequency response analyzer (Solartron 1250) coupled with an electrochemical interface (potentiostat) was used for the impedance measurements. The a.c. frequency was scanned from 65 kHz to 10 mHz with an amplitude of 10 mV (p±p). The measurements were carried out under open circuit voltage (OCV) conditions before and after the constant current (1 mA cm ÿ 2) charging and discharging at room temperature (18±258C). To achieve the OCV conditions, sucient relaxation time was taken into account after each constant-current polarization. All experiments including the electrolyte preparation and the cell construction were performed under a dry argon atmosphere. 3. Results and discussion Figs. 1 and 2 show typical charge and discharge pro®les of the Li/LiNiO2 cells using dierent electrolyte solutions. As the cell capacity was controlled by the mass of the positive electrode, the pro®les are given as the voltage variations with the capacity (quantity of electricity passed) per mass of the positive electrode. These curves were obtained for each second cycle under CCV conditions (1 mA cm ÿ 2). The discharge capacity of the cell depended not only on the organic solvent but also on the electrolytic salt [18]. The Coulombic eciency for each charge and discharge cycle was practically 100%, except for the cell using LiPF6/(EC + DMC), where poor cycleability of the counter electrode (lithium metal) would be responsible for the low coulombic eciency of the full cell. Thus, the order of the discharge capacity would be a measure of the activity of the LiNiO2 electrode in the Li + -containing organic electrolyte. The capacity increases in the order of PC + DMC < EC + DEC < EC + DMC with respect to the solvent component, and in the order of LiCF3SO3 < LiPF63LiClO4 with respect to the electro-
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Fig. 1. Charge and discharge pro®les for Li/LiNiO2 cells with 1 mol dm ÿ 3 LiClO4 solutions. Current density: 1 mA cm ÿ 2, Solid line curve: EC + DMC, Broken line curve: EC + DEC, Dotted line curve: PC + DMC.
lytic salt. Since the ionic conductivity of the solution increases in the orders of EC + DEC < PC + DMC < EC + DMC (solvent) and of LiCF3SO3 < LiClO4 < LiPF6 (salt) [11], the activity of the LiNiO2 electrode is partly dependent on the ionic conduction. However, the process is not at all controlled by the mass transfer in the solution phase, as seen in the inconsistency between the discharge capacity of the electrode and the ionic conductivity of the solution. Fig. 3 shows the rate capability of the Li/LiNiO2 cells with dierent solvents of the electrolytes. When the cell was cycled under a lower rate (current), the eect of the electrolyte composition on the discharge capacity was not so signi®cant and tended to reach the theoretical maximum value (180±200 Ah kg ÿ 1) regardless of the electrolyte. However, the high rate cycling makes the dierences clearer in the discharge capacity among the solvent components. The electrode process of LiNiO2 in organic electrolytes, Eq. (1), consists of several steps. Li1ÿx NiO2 xLi xeÿ LiNiO2
0
1
That is, the direct current polarization includes Li + diusion in the liquid phase, charge transfer at the solid/liquid interphase, Li species diusion in the solid phase, electron insertion/extraction in the solid phase, and so on. Among these, the rate of electron transfer in the oxide itself would be very fast. The a.c. impedance technique is useful to analyze such an electrode process in detail. Figs. 4 and 5 show Cole±Cole plots (Nyquist plots: imaginary component, ÿZ0, vs. real component, Z 0 ) of the a.c. impedance obtained for the Li1 ÿ xNiO2 (0.3 < 1ÿ xR1.0) in dierent electrolyte systems. The impedance was measured under OCV conditions after the constant-current polarization with given quantity of electricity (charge and discharge) in each ®rst cycle. The depth of charge or discharge is presented as 1ÿ x in Li1ÿxNiO2, which is formally determined by the quantity of electricity passed, without any chemical analysis. In every electrolyte solution, the plot for the LiNiO2 electrode before charging (x = 0 in Li1ÿxNiO2) showed almost linear relation over a wide frequency range, which implies that the
Fig. 2. Charge and discharge pro®les for Li/LiNiO2 cells with EC + DMC solutions containing 1 mol dm ÿ 3 LiX salts. Current density: 1 mA cm ÿ 2. Solid line curve: LiClO4, Broken line curve: LiPF6, Dotted line curve: LiCF3SO3.
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Fig. 3. Rate capability of the Li/LiNiO2 cells with 1 mol dm ÿ 3 LiClO4 solutions. w: EC + DMC; t: EC + DEC; r: PC + DMC.
electrode reaction at x = 0 (1 ÿ x = 1) is simply controlled by a diusion process. When the electrode was anodically charged to 1 ÿ x = 0.4 or less, the plots formed arcs or ¯at semicircles. The size of the arc or semicircle once decreased with the discharging (increasing 1 ÿ x value), and then increased with the increase in the depth of discharge. Finally, for the electrodes with 1 ÿ xr0.8, the plots came back to the initial linear relations. Somewhat similar variations in the Cole± Cole plot with the depth of charge and discharge have also reported for LiNiO2 and related oxides [19±21]. The impedance diagrams shown in Figs. 4 and 5 are rather complicated, but we can analyze them using simpli®ed equivalent circuit models. Fig. 6 presents the equivalent circuits and the corresponding impedance responses. Model A consists of a series of the solution resistance (Rs), interface resistance (R1 and C1) and Warburg impedance (Zw). Model B is a simple series combination of two R±C (parallel) circuits, which is often observed for a charge transfer process with a protective ®lm formation or a combination of particleFig. 4. (a) Cole±Cole plots for a.c. impedance of Li1 ÿ xNiO2 in 1 mol dm ÿ 3 LiClO4/(EC + DMC). .: 1ÿ x = 1.00 (before charge); w: 1ÿ x = 0.31 (after full charge); r: 1ÿ x = 0.45; q: 1ÿ x = 0.60; t: 1ÿ x = 0.74; r: 1ÿ x = 0.82; +: 1ÿ x = 0.98 (after full discharge). (b) Cole±Cole plots for a.c. impedance of Li1 ÿ xNiO2 in 1 mol dm ÿ 3 LiClO4/ (EC + DEC). .: 1 ÿ x = 1.00 (before discharge); w: 1ÿ x = 0.40 (after full charge); r: 1ÿ x = 0.47; q: 1ÿ x = 0.62; t: 1ÿ x = 0.76; r: 1ÿ x = 0.98 (after full charge). (c) Cole±Cole plots for a.c. impedance of Li1 ÿ xNiO2 in 1 mol dm ÿ 3 LiClO4/(PC + DMC). .: 1 ÿ x = 1.00 (before charge); w: 1ÿ x = 0.34 (after full charge); r: 1ÿ x = 0.48; q: 1ÿ x = 0.56; t: 1ÿ x = 0.77; r: 1ÿ x = 0.92 (after full discharge).
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Fig. 5. (a) Cole±Cole plots for a.c. impedance of Li1 ÿ xNiO2 in 1 mol dm ÿ 3 LiPF6/(EC + DMC). .: 1ÿ x = 1.00 (before charge); w: 1ÿ x = 0.39 (after full charge); r: 1ÿ x = 0.61; q: 1ÿ x = 0.69; t: 1ÿ x = 0.76; r: 1ÿ x = 0.91 (after full discharge). (b) Cole±Cole plots for a.c. impedance of Li1 ÿ xNiO2 in 1 mol dm ÿ 3 LiCF3SO3/(EC + DMC). .: 1ÿ x = 1.00 (before charge); w: 1ÿ x = 0.47 (after full charge); r: 1ÿ x = 0.62; q: 1 ÿ x = 0.69; t: 1ÿ x = 0.76; r: 1ÿ x = 0.91 (after full discharge).
to-particle resistance of the oxide and absorption resistance at the oxide surface [20, 23]. Model C corresponds to the process involving a ®nite diusion step whose impedance is represented by Zw 0 [24]. The observed impedance showed no distinct response corresponding to a parallel R1±C1 circuit at a high frequency region, except for the case of LiCF3SO3/ (EC + DMC). This might be due to either very low R1
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(Rct) or small time constant (R1C1) that is out of the experimental frequency range ( fR65 kHz). That is, the intercept at Z 0 axis consisted mainly of the solution resistance, Rs. It depended on the electrolyte composition, but did not change with the depth of discharge in each electrolyte system. The Cole±Cole plots for the electrode with x1 0 ®tted the Model A, regardless of the electrolyte composition. If the impedance responses for the electrodes with 0.3R x approximate that of model B, we can evaluate the interface impedance of R2 and C2. Fig. 7 shows variations of R2 with the depth of discharge (1 ÿ x) of the electrode in dierent electrolyte compositions. In every electrolyte system, R2 has a minimum value around x = 0.5. A similar result has been reported by Yamada et al. [19] and Choi et al. [20]. The former authors had related the variation of the interface resistance with x in Li1ÿxNiO2 to the change in the interlayer distance of the crystal lattice [19]. The latter, on the other hand, interpreted the results as the changes in the concentrations of lithium and electron near the surface region of the oxide [20]. In this work, the resistance also depended on the sort of the electrolytic salt (LiClO4 < LiPF61LiCF3SO3) although dierences in the R2 values among the electrolyte solvent were rather small. This result means that the interface resistance include the contribution of the eects of the electrolyte solution. The physical meaning of the interface resistance is not clear in the present electrode/electrolyte system. However, it is generally accepted that the resistance of a ®lm formed on the electrode is a possible cause of the interface impedance [24, 25]. Aurbach et al. [21] reported the formation of surface ®lms at Li1ÿxMO2 type oxides in LiAsF6/ (EC + DMC), based on their FTIR observation. They suggests that the lithium intercalation process is partly controlled by the migration of lithium through the surface ®lms consisting of Li2CO3. On the other hand, Kanamura et al. [26] reported the in¯uences of the electrolytic salt on the anodic oxidation of PC-based electrolytes at an oxide-covered nickel substrate. The reactivity of the electrolyte was in the order of LiPF6 < LiCF3SO3 < LiClO4. Thus, the dierent electrolyte components should give dierent ®lm compositions, which would dierently in¯uence the interface resistance of the oxide electrode in the organic solutions. Another equivalent circuit model, Model C in Fig. 6, is also valid for the observed impedance. This equivalent circuit was ®rst adopted for the lithium metal electrode with polymer electrolyte interphase (PEI) in organic electrolyte solution [24, 27]. In this model, the arc observed in the low frequency range is considered as the result of the diusion process with a ®nite length. We can estimate the diusion rate from the Warburg component in the Cole±Cole plot. According
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Fig. 6. Electrochemical equivalent circuit models and corresponding impedance spectra. (A) Model controlled by an in®nite diusion process; (B) Model without a diusion control process; (C) Model with a ®nite diusion process.
Fig. 7. Interface resistance (R2) of Li1ÿxNiO2 electrode in dierent electrolyte solutions. (a) w: LiClO4/(EC + DMC); t: LiClO4/ (EC + DEC); r: LiClO4/(PC + DMC). (b) w: LiClO4/(EC + DMC); r: LiPF6/(EC + DMC); q: LiCF3SO3/(EC + DMC).
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to Ho et al. [28], the Warburg impedance arising from the chemical diusion in a single solid phase is expressed by Eqs. (2) and (3), ÿz0 Aw o ÿ1=2 ,
2
Aw Vm dE=dx= zFD~ 1=2 S ,
3
where Aw is preexponential factor of the Warburg impedance, o the frequency (angle velocity) of a.c., Vm the molar volume of the electrode, dE/dx the dierential of the OCV curve, z the valence of the diusion species (Li + ), F the Faraday constant, D~ the chemical diusion coecient of the diusion species, and S the surface area of the electrode. We obtained Aw from the linear parts in the Cole±Cole plots and then D~ from Eq. (3) [29]. The apparent values of D~ are plotted in Fig. 8 as a function of the depth of discharge. As de®-
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nite dE/dx values were not determined in the lowest x region (1 ÿ xr0.8), the D~ vs. 1 ÿ x plots are limited in 0.3 < 1 ÿ x < 0.8. In every electrolyte solution, it tended that the chemical diusion coecient decreased with discharging, i.e. increasing 1 ÿ x in Li1ÿxNiO2. The values of D~ ranging 10 ÿ 8±10 ÿ 6 cm2 s ÿ 1 seem to be high for the lithium diusion in a solid oxide phase at room temperature. Bruce et al. [9] used the same a.c. impedance method and equations to evaluate the lithium diusion coecient in Li1ÿxNiO2, and obtained 2 10 ÿ 7 cm2 s ÿ 1 as a maximum value, which was higher than that in Li1ÿxCoO2 with a similar layered structure (10 ÿ 9±10 ÿ 8 cm2 s ÿ 1 [30±33]). They also observed that the chemical diusion coecient in Li1ÿxNiO2 depended on the depth of discharge, but it tended to decrease with decreasing 1 ÿ x in the composition range of 0.6R1ÿ xR0.95. As reported by Ohzuku et al. [5], the electrode process of reaction (1)
Fig. 8. Apparent diusion coecient of Li for Li1ÿxNiO2 electrode in dierent electrolyte solutions. (a) w: LiClO4/(EC + DMC); t: LiClO4/(EC + DEC); r: LiClO4/(PC + DMC). (b) w: LiClO4/(EC + DMC); r: LiPF6/(EC + DMC); q: LiCF3SO3/ (EC + DMC).
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may be accompanied by the structure change of the oxide. Thus, the observed changes in the diusivity with the electrode composition could include such a structure eect. If the equivalent circuit of Model C strictly re¯ects not only the chemistry but also the geometry of the electrode, the observed diusion process should include that in pore structures of the electrode [24, 27]. As the electrode consists of the composite of the active mass (LiNiO2) with considerable amount of the conductive support (acetylene black) and the binder (PTFE), it forms a heterogeneous structure having microscopic pores of the oxide itself as well as macroscopic channels that are resulted in mixing the conducting support and the binder. We have to take account of the dierent diusion processes in such pore structures. The observed diusion impedance which depended on the composition of the electrolyte solution might include the process in the pore structure of the electrode. On the other hand, we employed the apparent geometric area exposed to the electrolyte (0.071 cm2) as the surface area in the theoretical Eq. (3). As the surface of each oxide particle is considered to be partly wet with the electrolyte, the real surface area participating the electrode reaction will be higher than the geometric one. When we introduce ten times of the geometric area into the S value in Eq. (3), the calculation gives the diusion coecient of 10 ÿ 10±10 ÿ 8 cm2 s ÿ 1. Thus, it is not signi®cant for us to compare each absolute value of the diusion coecient in this type of the electrode structure. However, it is of special importance to discuss their variations with both the electrode and the electrolyte compositions. The eect of the electrolytic salt was relatively large, compared with that of the solvent. The order of LiCF3SO3 < LiPF6 < LiClO4 was mostly consistent with that of the discharge capacity at moderate rate (Fig. 2). With respect to the solvent composition, however, the order of EC + DEC < EC + DMC < PC + DMC was inconsistent with the rate capability of the discharge capacity, as shown in Fig. 3. These results also suggest that the process is not merely controlled by the diusion process in the bulk liquid phase. On the other hand, the dependence of the diusion coecient on the depth of discharge (the value of 1 ÿ x) suggest that at least the process is partly controlled by the structure of the oxide. It is quite reasonable that the lithium diusion becomes easier as the interlayer distance increases and the interaction of the diusing species decreases with decreasing the 1 ÿ x value [5, 19]. One of the possible reasons that explain the variations in the diusion coecient with the depth of discharge and with the electrolyte composition is that the process is controlled by the eective cross section of the diusion. According to the observation by Aurbach et al. [21], the surface of the oxide particle
would be covered with a thin ®lm whose chemical composition and transference properties must vary with the electrolyte composition. This would cause the dierence in the eective cross section, or in the activity of the diusion species in the solid phase. In other words, the variations in the diusion impedance that we can observe is that of the product D~ 1=2 S in Eq. (3). Thus, the apparent diusion coecient is aected by the electrolyte composition, while the intrinsic diusion coecient in the solid phase would not be changed by these factors. Consequently, if the intrinsic diusion coecient does not vary with the electrolyte composition, the cross section, or the eective surface area, for diusion increases in the order of EC + DEC < EC + DMC < PC + DMC for the solvent and LiCF3SO3 < LiPF6 < LiClO4 for the salt. Unfortunately, the intrinsic diusion coecient in the solid is not evaluated for the present electrode system by the impedance technique. However, it is noteworthy that the electrolyte composition would have signi®cant eect on the cathode performance of LiNiO2 through the dynamic parameter D~ 1=2 S at the oxide/electrolyte interphase.
Acknowledgements This work was ®nancially supported by Grant-inAid for Scienti®c Research (Nos. 08650980, 09237250, 09650907) from the Ministry of Education, Science, Sports and Culture.
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