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An electrochemical investigation into the lithium insertion properties of LixCoO2
Electrochimica
Pergamon PII:SOO13-4686(96)00036-9
AN ELECTROCHEMICAL LITHIUM INSERTION
Vol. 41, No. 15. pp. 248-2488. 1996 Copyright 0 1996 Elsevier Science Ltd. Printedin Great Britain. All rights reserved 0013-4686/96 515.00 + 0.00 Acra.
INVESTIGATION INTO THE PROPERTIES OF Li,CoO,
J. BARKER,* R. PYNENBURG, R. KOKSBANC and M. Y.
SAIDI
Valence Technology, Inc., 301 Conestoga Way, Henderson, NV 89015 USA (Received 10 September 1995, acceptedfor publication 13 December 1995)
Abstract-The thermodynamic, kinetic and interfacial properties of lithium insertion in Li,CoO, have been probed by ac and dc electrochemical techniques. The electrochemical voltage spectroscopy method, which was used to cycle the cells, indicated that a significant coulombic inefficiency was present in the first chargdischarge cycle. The second cycle data, however, were close to 100% coulombic efficiency and demonstrated that around x = 0.64 in Li,CoO, , lithium could be inserted reversibly. This value is significantly higher than has been previously reported. In agreement with the work carried out by Reimers and Dahn[14] we also find two additional reversible differential capacity features at cell potentials of approximately 4.08 V and 4.20 V vs. Li on charge. The kinetic investigation revealed the average diffusion coefficient for the composite Li,CoO, cathode to be in the range 10-9cm2s-’ indicating the relatively facile reaction kinetics for the lithium insertion reaction. The dc impedance data collected by the current interrupt method were broadly consistent with the ac impedance spectra and showed the presence of two distinct impedance regions dependent on the cell state-of-charge. One region exists below approximately x = 0.6 where the system generates relatively low impedance characteristics. The second region occurs above about x = 0.7 where significantly higher impedance properties are found. We suspect morphological changes at the lithium/electrolyte interface to be responsible for these impedance variations. Copyright 0 1996 Elsevier Science Ltd Key words: lithium cobalt oxide, insertion, batteries, lithium ion, impedance.
INTRODUCTION
Due to its high voltage and its ability to insert lithium reversibly, the LiCoO, compound has attracted considerable interest as a cathode material for rechargeable lithium batteries[ l-1 11. It has already found commercial application in rocking chair/lithium ion type batteriesC4, 7,9, 12, 131. The LiCoO, compound has the rock salt structure consisting of COO, layers of edge shared octahedra[14-151. Within the phase, the lithium ions are normally located in the octahedral sites between the layers, while the cobalt atoms are coordinated in alternate (111) octahedral sites in the planes. It has previously been reported that the structure has accessible sites for reversible electrochemical lithium insertion such that the material can liberate lithium range 0.1 < x < 1 in over the composition Li,CoO,[l] at potentials exceeding 4.5 V. In practice the reversible lithium extraction process has hitherto been limited to x-values around 0.4-0.5[4, 6, 143. For example, a detailed study comparing the lithium insertion properties of L&Coo, to Li,NiO* and Li,Ni,,,Co,,,O, indicated that although the lithiated cobalt oxide was comparatively easy to prepare, the rechargeable specific capacity was limited to x = 0.46, especially over extended cycling periods[ 151. The relationship between the electrochemical and structural properties of Li,CoO, as a function of * Author to whom correspondence should be addressed.
lithium concentration have been studied in detail by Reimers and Dahn[l4]. These measurements indicated a sequence of three distinct phase changes occurring as x values were electrochemically varied between 1 and 0.4. Lithium ordering was evidenced at the lithium concentration, x = 0.5 and was concluded to be coupled to a lattice distortion in which there was a transition from a hexagonal to a monoclinic symmetry for the host lattice. To elucidate the mechanism of lithium insertion and extraction in Li,CoO, we have undertaken detailed electrochemical measurements to generate thermodynamic, kinetic and interfacial information. These measurements have been conducted in metallic lithium anode half cells incorporating a composite Li,CoO,-containing cathode together with a plasticized polymer electrolyte. For detailed studies of the electrochemical properties of Li,CoO, we have employed the voltage-step electrochemical voltage spectroscopy (EVS) technique which has been previously demonstrated by this group to be an extremely powerful method for characterizing the properties of alkali metal insertion reactions[l6-191. AC and dc measurements have also been employed to investigate the impedance changes accompanying the lithium insertion and extraction reactions. EXPERIMENTAL
The LiCoO, compound was obtained from a commercial source, and used as received. Atomic 2481
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absorption (AA) measurements, used to determine the Li and Co contents of the electrode films, were conducted using an Instrumentation Laboratory AA/AE Spectrophotometer Model 357. These measurements were undertaken on both as-made and cycled electrodes to allow determination of the exact composition of the active material. In each case the cathode was removed from the current collector and rinsed several times in THF until the washings showed lithium content below the sensitivity of the spectrophotometer. This procedure ensured the sample material was free from residual lithium originating from the electrolyte salt. About OSg of the material was then acid digested prior to the AA analysis for Co and Li. Duplicate samples were used in all cases to ensure reproducibility. Corrections for broad band absorption and zero point drift were applied. The phase purity was determined using a Siemens D5000 powder X-ray diffractometer and CuK, radiation. Fig. 1 compares the powder XRD trace of the LiCoO, with that of the calculated powder X-ray diagram. The compound was shown to be pure LiCoO, rock salt structure with no traces of impurity phases. The hybrid polymeric electrolyte, which has been described in detail elsewhereC16, 17, 201, possesses a room temperature conductivity exceeding 1 mS cm- ’ and the salt diffusion coefficient was determined to be > 1O-6 cm’s_‘[16]. The transport properties of the electrolyte are comparable to those of similar liquid electrolytes, and as a consequence, we regarded the transport properties of the test cells to be
1100 1000
dominated entirely by the electrodes under the experimental conditions used in this work. The composite Li,CoO, electrode was formulated using the same polymer composition as that used in the electrolyte phase as the electrode binder. The composite cathode contains 56 w/o active material and around 10% by weight of Shawinigan Black to aid electronic conductivity. The electrode thickness was about 1OOpm. Polymerization of both the cathode and electrolyte films was accomplished by exposure of the films to an electron beam source, using an ES1 175/15/lOL Electrocurtain Lab Unit. The maximum accelerating voltage was 180 keV and a maximum current of 10mA was used. Generally, the maximum curing was achieved at a dose of 6 MRad. The test cell configuration is sketched in Fig. 2, and consists essentially of the composite electrode, an electrolyte layer and a lithium foil electrode in a parallel stack configuration. The electrode stack was placed in a flexible, moisture and oxygen impermeable encapsulation, which was subsequently heat sealed under vacuum. The active electrode area of each cell was 24cm*. All preparation and assembly operations involving air- and moisture-sensitive components were conducted inside high integrity Ar or N, containing glove boxes (Vacuum Atmospheres), in which the moisture and oxygen levels were lower than 25 ppm and lOppm, respectively. The bulk of the electrochemical measurements were carried out under thermostatic conditions at 23 + 0.2”C using the EVS technique. EVS was orig-
1
i”-
t 900
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Trace = Experimental Bars = Calculated
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t-
t 800
Unit Cell Parameters: S.G.: l73m a = 2.808(l) A c = 14.020(7) A
t
Scattering
Angle, 2 Theta (“)
Fig. 1. Comparison between the calculated and measured powder XRD patterns.
An electrochemical investigation into the lithium insertion properties of Li,CoO,
Key I Packa$ngmataial 2 Poutive Electrode Current Coktor 3. Li.CoOg Positive Elktrode 4 Polylncr i3ectrolyte 5. L,,hium Foil Negwive Electrode 6. Negauvc Electrode Cunmt
Collector
Fig. 2. Schematic representation of the cell design. Key: (1) packaging material; (2) positive electrode current collector; (3) Li,CoO, positive electrode; (4) polymer electrolyte; (5) lithium foil negative electrode; (6) negative electrode current collector.
inally devised by Thompson[Zl, 221 to investigate lithium intercalation reactions in the layered chalcogenide TiS,. EVS is a voltage step technique which provides a high resolution approximation to the open circuit voltage discharge-charge curves for the electrochemical system under investigation. The technique initially involves the measurement of the cell current resulting from the application of a small potential step, dV to the cell. The current density is continuously monitored until it decays to a pre-set minimum value, Slim. The cell potential is then stepped by dV again. The value of Is,,, is chosen such that the cell is close to thermodynamic equilibrium at the end of the current transient to ensure that voltage corrections due to the cell ir drop and diffusion overvoltages are insignificant. By using small potential steps the voltage-charge relation generates an accurate approximation to the thermodynamic properties of the cell between the chosen voltage limits. During each step the current is integrated to allow calculation of the differential capacity, dQ/dV, where Q is the charge, or dx/dV, where x is the lithium ion concentration in, for example, Li,CoO, In the experiments conducted here the values of dV and In,,, were + 10 mV and 50 ,uA cm-’ respectively. Kinetic properties such as the solid state lithium ion diffusion within the cathode host may also be estimated by analyzing the decay of the cell current following each voltage step. This, then, is proportional to t -112 for linear diffusion in a semiinfinite system[23]. This treatment is essentially a solution of the current-time response from the well known Cottrell equation[24]. Separate experiments were performed to verify the diffusional data using the so-called galvanostatic intermittent titration Technique, GITT[25]. In this method a small constant current pulse is applied across a cell, while the transient voltage is measured as a function of time. The change of the steady state voltage following the pulse then determines the dependence of the cell voltage on the concentration of the electroactive species. The EVS instrumentation used was an Advantest R6142 Precision Voltage source/sink coupled to a
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Hewlett Packard Model HP344OlA Digital Multimeter (DMM) for current monitoring. The experimental instrument control was performed using custom software written in Hewlett Packard HP Instrument Basic in conjunction with a standard HP-IB (IEEE488) parallel interface bus. AC measurements were conducted using a Solartron FRA 1250 or 1255, connected to a Solartron 1286 Electrochemical Interface. The set-up was controlled by a personal computer using the commercially available Zplot software. The complex impedance parameters were extracted by data fitting using Zplot. An amplitude of f 1OmV was used to ensure only a small perturbation of the interface layers during the measurements. Since only interfacial characteristics were of interest in this particular study, the frequency range was limited to 65 kHz-1 Hz. The complementary dc impedance (current interrupt) experiments were conducted using the Solartron 1286 EI as the constant current source, coupled to an HP3144A for voltage transient measurements. The experiments were again carried out using custom software written in HP Instrument Basic in conjunction with a standard HP-IB (IEEE488) parallel interface bus. The test conditions consisted of application of a current density (I = 0.25 mA cm-*) for approximately 1 min, followed by a rest period (I = 0). The time delay following current interruption was carefully controlled so as to allow an estimation of the ohmic and nonohmic (polarization) contributions to the overall cell impedance to be made. For a detailed discussion of the current interruption method and its application to these kinds of measurement the reader is referred to the work reported by[26].
.
RESULTS
AND DISCUSSION
The open circuit cell voltage of the as-made Li/Li,CoO, cells were typically around 3.1 V vs. Li. The EVS technique was used to charge (lithium extraction from the Li,CoO,) the cells to a stable ecu of about 4.25 V vs. Li, and subsequently to discharge (lithium insertion) the cells to 3.2V vs. Li. The precise experimental parameters described in the experimental section, ensured that the cells were close to thermodynamic equilibrium during the cycling process. The technique approximates to about a 1OOh chargedischarge rate. By essentially removing any unnecessary overvoltage, this method also minimizes the risk of electrochemical oxidation of the electrolyte. Figure 3 shows the relationship between the pseudo-open cell circuit voltage and lithium concentration, x, in Li,CoO, derived from the first EVS charge-discharge cycle of a typical Li/Li,CoO, The x values shown in the figure have been calculated assuming all charge passed is associated with either lithium extraction or insertion reactions. The voltage hysteresis (ie voltage difference between extraction and insertion waves) is relatively small, consistent with the expected thermodynamic reversibility of the system. The oxidation step results in a charge equivalent to x = 0.79 being extracted from the Li,CoO, .
J. BARKERet al.
34 I
I 0.2
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08
0.9
10
x in LixCoO,
07.
03
04
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09
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Fig. 3. First cycle EVS cell voltage-composition profile for the Li/Li,CoO, cell cycled between voltage limits of 3.2V and 4.25 V vs. Li.
Fig. 5. Second cycle EVS cell voltage-composition profile for the Li/Li,CoO, cell cycled between voltage limits of 3.2 V and 4.25 V vs. Li.
It can be seen clearly from inspection that the overall lithium extraction-insertion reactions are coulombitally inefficient to such an extent that charge equivalent to only x = 0.61 can be subsequently re-inserted. The reason for this inefficiency is presently unclear but may be associated with some electrolyte decomposition occurring during the initial charge stage. It has been proposed by Thomas and co-workers[2] that the electrolyte degradation process may result in formation of a surface passivation layer on the cathode. A similar irreversible charge consumption was also noted during the work of Reimers and Dahn[14]. It was speculated that this way be due to a combination of both cathode degradation and electrolyte decomposition at the high oxidation voltage encountered during cell charging stage. As lithium is extracted from Li,CoO, , Co3+ is oxidized to Co4+ which is expected to be a relatively unstable oxidation state for this element. Thus it was proposed that the presence of Co4+ may induce cathode degradation[14]. Certainly, if this were the case then it should be expected that the precise control of co3+/co4+ ratio for the Li,CoO, would be advantageous in maintaining cathode integrity during cell cycling. The associated differential capacity-cell voltage relationship for the first EVS cycle is shown in Fig. 4.
There appear to be no irreversible oxidation peaks which we can reliably assign to electrolyte or cathode degradation processes. In fact the main extraction-insertion peaks (at 3.94V and 3.86V vs. Li respectively) appear to be well-defined and with small voltage peak separation Al/’ = 0.06 V as indicated by the low voltage hysteresis described in Fig. 3. In fact the definition of the insertion peak is superior to that previously reported for the Li/Li,CoO, system in liquid electrolyte cells[14]. Also shown on this figure (see arrows) are the two (small) additional reversible features first reported by Reimers and Dahn[14]. The origin of these features will be discussed in more detail later in this section. From AA analysis the cathode composition for one representative electrode following the first cycle indicating that all Li was found to be Li ,,.&oO extracted had been re-inserted within the host lattice. For this reason we begin the second cycle data at a nominal composition of Li,CoO, The second cycle EVS voltage-composition and differential capacity profiles are shown in Figs 5 and 6 respectively. Inspection of the voltage profile data indicates that in contrast to cycle 1, the coulombic efficiency for the second cycle is now close to lOO%, suggesting that the irreversible reactions encountered during the first cycle are absent during this cycle. This observ-
Fig. 4. First cycle EVS differential capacity plot for the Li/Li,CoO, cell cycled between voltage limits of 3.2V and 4.25V vs. Li. The arrows denote the two high voltage features described in the text.
Fig. 6. Second cycle EVS differential capacity plot for the Li/Li,CoO, cell cycled between voltage limits of 3.2 V and 4.25V vs. Li. The arrows denote the two high voltage features described in the text.
An electrochemical investigation into the lithium insertion properties of Li,CoO,
ation may be consistent with the formation of a stable, ionically conducting interface layer on the LiCoO, compound during the initial cycle as described[2]. This layer is then expected to minimize further decomposition of the electrolyte. The total charge passed suggested that Li+ was extracted during the oxidation step to generate the approximate composition Li ,,34Co02. This degree of lithium extraction is higher than previously reported[4, 6, 143. This observation may be due to the extremely low rates of extraction and insertion used in this study. The EVS method adopted here approximates to a 100 h rate for both charge and discharge reactions. Thus, as we described in the experimental section, the data we present here are expected to be close to the thermodynamic equilibrium case and as such allow us to approach the highest possible extraction and insertion levels attainable for the system under the prevailing test conditions. The associated differential capacity plot for the second cycle shows the main extraction and insertion peaks positioned at 3.94 V and 3.87 V vs. Li respectively. These peak positions, as well as peak separation, are consistent with the data presented for the first cycle. The extraction peak, although still welldefined is less sharp, perhaps indicating a slight structural degradation of the cathode material. Also shown on Fig. 6, are the two additional high voltage features noted during cycle 1. To improve the clarity of these features we have enlarged the differential capacity data from cycles 1 and 2 and presented these in Figs 7(a) and 7(b) respectively. The voltage range in these figures is restricted to the one of interest. The peak voltages have been clearly marked on the two figures. On cycle 1 we have maxima during charge at 4.08 V and 4.2OV vs. Li and at 4.13 V and 4.02V vs. Li on the subsequent discharge. The relatively wide differential capacity minima lie at approximately 4.15 V and 4.07 V on the charge and discharge steps respectively. The data from cycle 2 are consistent with those collected for cycle 1. The equivalent features were previously reported[14] at voltages of 4.08 V and 4.18 V on charge with the minimum in the dx/dV data situated at approximately 4.15V. It appears that our observations, strictly from an electrode potential standpoint, are consistent with those previously reported. The work by Reimers and Dahn[14] also attempts to assign the minima to an ordered lithium superlattice within the host lattice. For this purpose the dx/dV data were presented as a function of lithium composition. Certainly minima in differential capacity data have previously been ascribed to ordered compositions of the host structure[27]. By taking an average value for x from their charge and discharge data they assign the minima in dx/dV to a composition of Li 0.533Co02. As the authors correctly assert, however, it is difficult to reconcile an ordered superlattice existing at such an unusual composition. For this reason Reimers and Dahn assumed that the lithium superlattice occurred precisely at x = 0.5. This determination was to a large extent based on an estimation of overall cathode utilization. It was assumed for this purpose that not all the cobalt oxide grains were electrically connected within the
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J
J Fig. 7. Expanded view of (a) the first cycle differential capacity data and (b) the second cycle differential capacity data, showing high voltage features in each case.
composite electrode structure. Thus the dx/dVcomposition data could be shifted to allow the minimum to correspond precisely to x = 0.5 and at an electrode potential of around 4.15 V vs. Li. We find that the dx/dV minimum in our data is consistent with this electrode potential bu not with the figure based on lithium composition. As we have described, after taking into account the first cycle coulombic inefficiency, we find that for cycle 2 the lithium extraction-insertion reactions are essentially 100% reversible. Using cycle 2 data we determine that the dx/dV minimum occurs at approximately x = 0.38 (based on the average lithium extraction and insertion data). There is a large discrepancy between the two sets of data. In our case we could only position the minima close to a cathode composition, x = 0.5 if there were a gross underestimation of the active material loading within the composite Li,CoO, electrode. We believe this to be an unlikely scenario since the precise composition of the electrodes were cross-checked by detailed AA analysis. At present we consider that the suggestion that the minima correspond to an ordered lithium superlattice at x = 0.5 requires further clarification and confirmation. Chemical diffusion coefficient measurements for the Li,CoO, were determined by the two electrochemical pulse methods described in the experimental section, and indicated relatively fast reaction kinetics for the lithium insertion process. The values
J. BARKERet
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from the two techniques are essentially consistent and only the data collected from the EVS experiments will be presented here. Careful collection of the data was performed to ensure that the Cottrell fit of the current-time transient for the system was satisfied by the semi-infinite condition (ie t 4 12/D, where t is time, I = electrode thickness and D is the chemical diffusion coefhcient)[24]. This procedure entailed collecting the current transient data at relatively short time periods following the voltage step. It also involved discarding data where a non-Cottrell behavior (within either the semi-infinite or infinite regions) was detected. Effective diffusion coefficients for the electrodes composites were then calculated from these measurements based on the geometric surface area of the electrodes. We recognize that these values will be dependent on the intrinsic diffusion coefficients for the electrode materials in question. It should be noted, however, that other electrode parameters will also affect the magnitudes. As these factors do not depend on the state-ofcharge of the electrode, the variation in the measured effective diffusion coefficients are expected to directly reflect the compositional variation of the intrinsic diffusion coefficient for Li in the electrode material. In Fig. 8, we show the variation of the Li’ ion diffusion coefficients for the Li,CoO, composite cathodes as a function of the lithium concentration during the lithium insertion reaction. The magnitude of the diffusions coefficients are generally around 10-9cm2s-1. The variation of the diffusion coefhcient in the insertion range 0.3 < x < 0.85 is consistent with filling/removal of lithium ions from a single site within the host lattice over this particular composition range. Outside this range there was insufticient current transient data for us to reliably calculate kinetic data for the system. At a composition around x = 0.65, the diffusion coefficient reaches a local minimum. Within the compositional range described there exists a two-phase system with the phase transition being first order involving a significant increase in the c-lattice parameter of the hexagonal unit cell[ 143. Relatively substantial structural rearrangement is therefore taking place as lithium is inserted or extracted from this particular lattice site. We believe that this may explain the observed
al.
minimum in the diffusion coefficient-composition data since the kinetic properties will be driven by guest-host and guest-guest coulombic interactions within the Li,CoO, . The results from the UC impedance study of an uncycled (open circuit voltage, 3.1 V vs. Li) and for a charged (open cell voltage, 4.2V vs. Li) cell are shown in Fig. 9(a) and 9(b) respectively. The impedance properties for a two electrode cell incorporating a carefully formulated composite insertion cathode, are normally dominated, at all states of charge, by the characteristics of the lithium metal anode[28]. Only under circumstances where the cathode has been improperly formulated (eg poor interparticle contact, insuficient conductive diluent or poor active material dispersion) or where the electrode has been compromised during cycling, does the cathode impedance become dominant and therefore begin to affect significantly the overall cell performance. The spectrum in Fig. 9(a) is consistent with the cell impedance being dominated by the pristine Li/ electrolyte interface. In the uncycled state the lithium electrode is still covered by the native oxide layer which generates a relatively high interfacial impedance. It is well known that the Li/electrolyte interface film is modified significantly upon passage of charge through the film. It has been clearly demonstrated in both liquid[29] and polymer[28] electrolyte systems that the Li interfacial impedance decreases rapidly following charge transfer. Thus, the spectrum for the charged cell shown in Fig. 9(b) is consistent with the low cell impedance expected for the system following charge transfer. Since the cathode impedance is expected to be less that 1 R (for the 24cm2 electrode area) over the complete insertion range investigated, no adverse effects of the cathode impedance on the rate capability of the cell are expected. ._._ -10 0
(a)
uncyded Cdl
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5
10
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06
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10
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Fig. 8. The variation in chemical diffusion coefficient with lithium concentration, x in Li,CoO,. The data were collected from the current transient data of the EVS method during lithium insertion.
Fig. 9. AC impedance spectra for (a) uncycled cell (open circuit voltage 3.1 V vs. Li); and Li/Li,CoO, cell (open circuit voltage 4.2 V vs. were collected between frequency limits of with an ac oscillation of C 1OmV.
Li/Li,CoO, (b) charged Li). The data 65 kHz-1 Hz
An electrochemical investigation into the lithium insertion properties of Li,CoO,
Complementary cell impedance data were also collected by the dc current interrupt technique described in the experimental section. This technique has the advantage of generating impedance properties for the complete cell under dynamic conditions. This procedure allows one to realistically probe the cell impedance variations as a function of the cell state-of-charge in a time effective fashion. The data for cycles 1 and 2 of a representative cell are summarized in Figs 10 and 11 respectively. The measurements were conducted over the voltage range 3.0-4.25V vs. Li and at a current density for charge and discharge of 0.25 mA cm-‘. From inspection we can denote two distinct regions, ie one below about x = 0.6 in Li,CoO, where the system demonstrates relatively low impedance behavior, and one above about x = 0.7 where significantly higher impedance characteristics are found: Thus the impedance difference during cycling is close to an order of magnitude, being lowest when the cell approaches the fully charged state. The cell impedance does not appear to increase to any appreciable amount towards the end of charge step. This observation is somewhat at variance with the findings from Ohzuku and coworkers[lS] who reported a significant increase in cell overvoltage at low insertion levels during constant current cycling of Li/Li,CoO, cells. It was
Fig. 10. First cycle dc impedance data collected by the current interrupt method described in the text. The cell was cycled between voltage limits of 3.2V to 4.25V using densities for charge and discharge of current +0.25mAcn1-~.
Fig. 11. Second cycle dc impedance data collected by the current interrupt method. Experimental conditions the same as denoted in Fig. 10.
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reported that this property limited the useful specific capacity for their electrodes[15]. We suspect that the superior impedance characteristics for the electrodes tested in our work may result from improved cathode formulation effects. Generally, we find that the current interrupt measurements are consistent with the UC measurements described above. Presently we do not fully understand the large step in the magnitude of the cell impedance but suspect variations associated with morphological changes at the lithium/electrolyte interface[30]. We are currently investigating the impedance properties of three-electrode cells (incorporating a lithium reference electrode) in an attempt to deconvolute the cathode and anode contributions to the overall cell impedance.
CONCLUSIONS It is well known that Li,CoO, has been used successfully as the cathode active material in commercially available lithium ion battery systemsC12, 131. We have undertaken a detailed electrochemical study to elucidate the mechanisms of lithium insertion and extraction in this material. The work was conducted using two electrode cells incorporating a metallic lithium anode together with a hybrid polymer electrolyte. We find that during the initial charge-discharge cycle there is a relatively large coulombic inefficiency which may be related to the formation of a passivation layer on the surface of the active material. This is supported by the observation that the second cycle is close to 100% reversible suggesting that any side reactions were completed during the first cycle. The first cycle differential capacity profile, which has been demonstrated previously to be an extremely powerful method for probing reversible and non-reversible reactions[l6193, did not, however, indicate specific charge consumption related to any reactions not associated with the lithium extraction process. In fact the differential capacity profile indicated the main extractioninsertion peaks to be well-defined and with only minor peak separation consistent with a reversible system demonstrating low overvoltage. The charge passed during the EVS second cycle indicated that around x = 0.66 Li could be reversibly inserted and extracted from the Li,CoO, material. This degree of reversibility is far higher than previously reported in the open literatureC4, 6, 143. We suspect that the combination of using well-formulated composite cathodes together with the slow charge/discharge EVS regimen employed during the study contributed to this observation. The differential capacity profiles indicated two additional high voltage peaks positioned at 4.08V and 4.20 V vs. Li on lithium extraction and at 4.13 V and 4.02 V vs. Li on lithium insertion. These features have previously been reported in the work by Reimers and Dahn[14], who ascribed the minima between the peaks as evidence for creation of a stable lithium superlattice within the Li,CoO, host. This was supported by in situ XRD studies. Following an estimation of the cathode utilization Reimers and Dahn then compositionally positioned the
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ordered phase at precisely x = 0.5 in Li,CoO,. In our data we could only find the minima close to x = 0.5 if there was a gross underestimation of the active material loading within the composite Li,CoO, electrode. This we consider unlikely since the compositional figures were confirmed independent analytical analysis. At present we contend that the composition for the ordered superlattice requires further clarification. Solid state lithium ion diffusion coefficients were estimated using electrochemical measurements. We recognize that collecting reliable kinetic data for powdered materials is notoriously difficult to achieve due to the fact that several other electrode factors will affect their magnitude. However, since these factors are not expected to vary with state-of-charge we expect that our measured values will reflect the compositional variation of the intrinsic diffusion coefficient in the Li,CoO,. The average diffusion coefficient (at 23°C) for our electrodes was found to be around 1O-9 cm2 s- ‘. This indicated the relatively facile reaction kinetics for the lithium insertion reaction. We also find that the diffusion coefficient reaches a local minimum at the lithium composition, x = 0.65. We believe this observation is consistent with coulombic interactions within the cathode host. The ac impedance measurements demonstrated that the impedance characteristics for a Li/Li,CoO, cell, at all states-of-charge, are expected to be dominated by the properties of the lithium/electrolyte interface. The cell impedance for the 24cm’ cell in the charged state (ie ecu = 4.2V) was found to be around 1 R. The complementary dc impedance data, collected by the current interrupt method, were consistent with the UC spectra. We found two distinct impedance regions as a function of the cell state-ofcharge. One region exists below approximately x = 0.6 where the system generates relatively low impedance characteristics. The second region occurs above about x = 0.7 where significantly higher impedance properties are found. We suspect that these observations are consistent with morphological changes at the lithium/electrolyte interface occurring during the lithium stripping/plating reactions.
E. Plichta, S. Slane, M. Uchiyama and M. Salomon, 136, 1865 (1989). M. Yoshio, H. Tanaka, K. Tominaga and H. Noguchi, J. Power Sources 40,347 (1992). R. S. McMillan and E. E. Andrukaitis, in Proceedings of the Symposium on Primary and Secondary Lithium Batteries (Edited by K. M. Abraham and M. Salomon),
8. 9.
10. 11.
Vol. 91-3, pp. 416. The Electrochemical Society, Pennington, NJ, (1991). M. Broussely, F. Perton, J. Labat, R. J. Staniewitz and A. Romero, J. Power Sources 43-44,209 (1993). K. Sekai, H. Azuma, A. Omaru, S. Fujita, H. Imoto, T. Endo, K. Yamaura and Y. Nishi, J. Power Sources 4344, 241 (1993). G. T. K. Fey, M. C. Hsieh, H. K. Jaw and T. J. Lee, J. Power Sources 43-44,673 (1993). G. Nagasubramanian and A. I. Attia, in Proceedings of the Symposium on Primary and Secondary Lithium Batteries (Edited by K. M. Abraham and M. Salomon),
Vol. 91-3, pp. 207. The Electrochemical Society, Pennington, NJ, (1991). 12. K. Tanaka, M. Itabashi, M. Aoki, S. Hiraka, M. Kataoka, S. Fujita, K. Sekai and K. Ozawa in The Electrochemical Society Fall Meeting, New Orleans, Extended Abstract 21, p. 36. lo-15 October (1993). 13. T. Nagaura and T. Tozawa, Prog. Batteries and Solar Cells 9,209 (1990).
14. J. N. Reimers and J. R. Dahn, J. electrochem. Sot. 139, 2091 (1992). 15. T. Ohzuku, A. Ueda, M. Nagayama, Y. Iwakoshi and H. Komori. Electrochim. Acta 38. 1159 (1993). 16. J. Barker, Synth. Met. 32,43 (1989). 17. J. Barker, D. Baldwin, D. C. Bott and S. J. Porter, Synth. Met. u(, D127 (1989). 18. J. Barker, R. Pynenburg and R. Koksbang, J. Power Sources 52, 185 (1994). 19. J. Barker, Electrochim. Actn 40, 1603 (1995). 20. J. S. Lundsgaard, S. Yde-Andersen, R. Koksbang, D. R. Shackle, R. A. Austin and D. Fauteux, Proceedings of the Second International Symposium on Polymer Electrolytes, Sienna, Italy, 14-16 June 1989, pp. 395. Else-
vier Applied Science Publishers, Ltd, UK (1990). 21. A. H. Thompson, Phys. Rev. Lett. 23, 1511 (1978). 22. A. H. Thompson, J. electrochem. Sot. 126,608 (1979). 23. T. Ohzuku and A. Ueda, Solid State Ionics 69, 201 (1994). 24. A. J. Bard and L. R. Faulkner, Electrochemical Methods: Fundamentals and Applications, pp. 143. Wiley, New York (1980). 25. W. Weppner and R. A. Huggins, J. electrochem. Sot. 124, 1569 (1977).
REFERENCES 1. K. Mizushima, P. C. Jones, P. J. Wiseman and J. B. Goodenough, Mater. Res. Bull. 15, 783 (1980). 2. M. G. S. R. Thomas, P. G. Bruce and J. B. Goodenough, J. electrochem. Sot. 132, 1521 (1985). 3. M. G. S. R. Thomas, P. G. Bruce and J. B. Goodenough, Solid State lonics 18-19, 794 (1986). 4. J. N. Reimers, J. R. Dahn and U. von Sacken J. electrothem. Sot. 140,2752 (1993).
26. F. H. van Heuveln, J. electrochem. Sot. 141, 3423 (1994). 27. 0. Tillement and M. Quarton, J. electrochem. Sot. 140, 1870 (1993). 28. E. Peled in Lithium Batteries (Edited by J. P. Gabano), pp. 43. Academic Press, London (1983). 29. J. Barker and R. Koksbang, Electrochim. Acta 40, 1706 (1995). 30. I. I. Olsen, J. Barker and R. Koksbang, lonics 83 125 (1996).
Solid State
Pergamon PII:SOO13-4686(96)00036-9
AN ELECTROCHEMICAL LITHIUM INSERTION
Vol. 41, No. 15. pp. 248-2488. 1996 Copyright 0 1996 Elsevier Science Ltd. Printedin Great Britain. All rights reserved 0013-4686/96 515.00 + 0.00 Acra.
INVESTIGATION INTO THE PROPERTIES OF Li,CoO,
J. BARKER,* R. PYNENBURG, R. KOKSBANC and M. Y.
SAIDI
Valence Technology, Inc., 301 Conestoga Way, Henderson, NV 89015 USA (Received 10 September 1995, acceptedfor publication 13 December 1995)
Abstract-The thermodynamic, kinetic and interfacial properties of lithium insertion in Li,CoO, have been probed by ac and dc electrochemical techniques. The electrochemical voltage spectroscopy method, which was used to cycle the cells, indicated that a significant coulombic inefficiency was present in the first chargdischarge cycle. The second cycle data, however, were close to 100% coulombic efficiency and demonstrated that around x = 0.64 in Li,CoO, , lithium could be inserted reversibly. This value is significantly higher than has been previously reported. In agreement with the work carried out by Reimers and Dahn[14] we also find two additional reversible differential capacity features at cell potentials of approximately 4.08 V and 4.20 V vs. Li on charge. The kinetic investigation revealed the average diffusion coefficient for the composite Li,CoO, cathode to be in the range 10-9cm2s-’ indicating the relatively facile reaction kinetics for the lithium insertion reaction. The dc impedance data collected by the current interrupt method were broadly consistent with the ac impedance spectra and showed the presence of two distinct impedance regions dependent on the cell state-of-charge. One region exists below approximately x = 0.6 where the system generates relatively low impedance characteristics. The second region occurs above about x = 0.7 where significantly higher impedance properties are found. We suspect morphological changes at the lithium/electrolyte interface to be responsible for these impedance variations. Copyright 0 1996 Elsevier Science Ltd Key words: lithium cobalt oxide, insertion, batteries, lithium ion, impedance.
INTRODUCTION
Due to its high voltage and its ability to insert lithium reversibly, the LiCoO, compound has attracted considerable interest as a cathode material for rechargeable lithium batteries[ l-1 11. It has already found commercial application in rocking chair/lithium ion type batteriesC4, 7,9, 12, 131. The LiCoO, compound has the rock salt structure consisting of COO, layers of edge shared octahedra[14-151. Within the phase, the lithium ions are normally located in the octahedral sites between the layers, while the cobalt atoms are coordinated in alternate (111) octahedral sites in the planes. It has previously been reported that the structure has accessible sites for reversible electrochemical lithium insertion such that the material can liberate lithium range 0.1 < x < 1 in over the composition Li,CoO,[l] at potentials exceeding 4.5 V. In practice the reversible lithium extraction process has hitherto been limited to x-values around 0.4-0.5[4, 6, 143. For example, a detailed study comparing the lithium insertion properties of L&Coo, to Li,NiO* and Li,Ni,,,Co,,,O, indicated that although the lithiated cobalt oxide was comparatively easy to prepare, the rechargeable specific capacity was limited to x = 0.46, especially over extended cycling periods[ 151. The relationship between the electrochemical and structural properties of Li,CoO, as a function of * Author to whom correspondence should be addressed.
lithium concentration have been studied in detail by Reimers and Dahn[l4]. These measurements indicated a sequence of three distinct phase changes occurring as x values were electrochemically varied between 1 and 0.4. Lithium ordering was evidenced at the lithium concentration, x = 0.5 and was concluded to be coupled to a lattice distortion in which there was a transition from a hexagonal to a monoclinic symmetry for the host lattice. To elucidate the mechanism of lithium insertion and extraction in Li,CoO, we have undertaken detailed electrochemical measurements to generate thermodynamic, kinetic and interfacial information. These measurements have been conducted in metallic lithium anode half cells incorporating a composite Li,CoO,-containing cathode together with a plasticized polymer electrolyte. For detailed studies of the electrochemical properties of Li,CoO, we have employed the voltage-step electrochemical voltage spectroscopy (EVS) technique which has been previously demonstrated by this group to be an extremely powerful method for characterizing the properties of alkali metal insertion reactions[l6-191. AC and dc measurements have also been employed to investigate the impedance changes accompanying the lithium insertion and extraction reactions. EXPERIMENTAL
The LiCoO, compound was obtained from a commercial source, and used as received. Atomic 2481
EA41-15-F
J.
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BARKERet al.
absorption (AA) measurements, used to determine the Li and Co contents of the electrode films, were conducted using an Instrumentation Laboratory AA/AE Spectrophotometer Model 357. These measurements were undertaken on both as-made and cycled electrodes to allow determination of the exact composition of the active material. In each case the cathode was removed from the current collector and rinsed several times in THF until the washings showed lithium content below the sensitivity of the spectrophotometer. This procedure ensured the sample material was free from residual lithium originating from the electrolyte salt. About OSg of the material was then acid digested prior to the AA analysis for Co and Li. Duplicate samples were used in all cases to ensure reproducibility. Corrections for broad band absorption and zero point drift were applied. The phase purity was determined using a Siemens D5000 powder X-ray diffractometer and CuK, radiation. Fig. 1 compares the powder XRD trace of the LiCoO, with that of the calculated powder X-ray diagram. The compound was shown to be pure LiCoO, rock salt structure with no traces of impurity phases. The hybrid polymeric electrolyte, which has been described in detail elsewhereC16, 17, 201, possesses a room temperature conductivity exceeding 1 mS cm- ’ and the salt diffusion coefficient was determined to be > 1O-6 cm’s_‘[16]. The transport properties of the electrolyte are comparable to those of similar liquid electrolytes, and as a consequence, we regarded the transport properties of the test cells to be
1100 1000
dominated entirely by the electrodes under the experimental conditions used in this work. The composite Li,CoO, electrode was formulated using the same polymer composition as that used in the electrolyte phase as the electrode binder. The composite cathode contains 56 w/o active material and around 10% by weight of Shawinigan Black to aid electronic conductivity. The electrode thickness was about 1OOpm. Polymerization of both the cathode and electrolyte films was accomplished by exposure of the films to an electron beam source, using an ES1 175/15/lOL Electrocurtain Lab Unit. The maximum accelerating voltage was 180 keV and a maximum current of 10mA was used. Generally, the maximum curing was achieved at a dose of 6 MRad. The test cell configuration is sketched in Fig. 2, and consists essentially of the composite electrode, an electrolyte layer and a lithium foil electrode in a parallel stack configuration. The electrode stack was placed in a flexible, moisture and oxygen impermeable encapsulation, which was subsequently heat sealed under vacuum. The active electrode area of each cell was 24cm*. All preparation and assembly operations involving air- and moisture-sensitive components were conducted inside high integrity Ar or N, containing glove boxes (Vacuum Atmospheres), in which the moisture and oxygen levels were lower than 25 ppm and lOppm, respectively. The bulk of the electrochemical measurements were carried out under thermostatic conditions at 23 + 0.2”C using the EVS technique. EVS was orig-
1
i”-
t 900
-
Trace = Experimental Bars = Calculated
-
t-
t 800
Unit Cell Parameters: S.G.: l73m a = 2.808(l) A c = 14.020(7) A
t
Scattering
Angle, 2 Theta (“)
Fig. 1. Comparison between the calculated and measured powder XRD patterns.
An electrochemical investigation into the lithium insertion properties of Li,CoO,
Key I Packa$ngmataial 2 Poutive Electrode Current Coktor 3. Li.CoOg Positive Elktrode 4 Polylncr i3ectrolyte 5. L,,hium Foil Negwive Electrode 6. Negauvc Electrode Cunmt
Collector
Fig. 2. Schematic representation of the cell design. Key: (1) packaging material; (2) positive electrode current collector; (3) Li,CoO, positive electrode; (4) polymer electrolyte; (5) lithium foil negative electrode; (6) negative electrode current collector.
inally devised by Thompson[Zl, 221 to investigate lithium intercalation reactions in the layered chalcogenide TiS,. EVS is a voltage step technique which provides a high resolution approximation to the open circuit voltage discharge-charge curves for the electrochemical system under investigation. The technique initially involves the measurement of the cell current resulting from the application of a small potential step, dV to the cell. The current density is continuously monitored until it decays to a pre-set minimum value, Slim. The cell potential is then stepped by dV again. The value of Is,,, is chosen such that the cell is close to thermodynamic equilibrium at the end of the current transient to ensure that voltage corrections due to the cell ir drop and diffusion overvoltages are insignificant. By using small potential steps the voltage-charge relation generates an accurate approximation to the thermodynamic properties of the cell between the chosen voltage limits. During each step the current is integrated to allow calculation of the differential capacity, dQ/dV, where Q is the charge, or dx/dV, where x is the lithium ion concentration in, for example, Li,CoO, In the experiments conducted here the values of dV and In,,, were + 10 mV and 50 ,uA cm-’ respectively. Kinetic properties such as the solid state lithium ion diffusion within the cathode host may also be estimated by analyzing the decay of the cell current following each voltage step. This, then, is proportional to t -112 for linear diffusion in a semiinfinite system[23]. This treatment is essentially a solution of the current-time response from the well known Cottrell equation[24]. Separate experiments were performed to verify the diffusional data using the so-called galvanostatic intermittent titration Technique, GITT[25]. In this method a small constant current pulse is applied across a cell, while the transient voltage is measured as a function of time. The change of the steady state voltage following the pulse then determines the dependence of the cell voltage on the concentration of the electroactive species. The EVS instrumentation used was an Advantest R6142 Precision Voltage source/sink coupled to a
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Hewlett Packard Model HP344OlA Digital Multimeter (DMM) for current monitoring. The experimental instrument control was performed using custom software written in Hewlett Packard HP Instrument Basic in conjunction with a standard HP-IB (IEEE488) parallel interface bus. AC measurements were conducted using a Solartron FRA 1250 or 1255, connected to a Solartron 1286 Electrochemical Interface. The set-up was controlled by a personal computer using the commercially available Zplot software. The complex impedance parameters were extracted by data fitting using Zplot. An amplitude of f 1OmV was used to ensure only a small perturbation of the interface layers during the measurements. Since only interfacial characteristics were of interest in this particular study, the frequency range was limited to 65 kHz-1 Hz. The complementary dc impedance (current interrupt) experiments were conducted using the Solartron 1286 EI as the constant current source, coupled to an HP3144A for voltage transient measurements. The experiments were again carried out using custom software written in HP Instrument Basic in conjunction with a standard HP-IB (IEEE488) parallel interface bus. The test conditions consisted of application of a current density (I = 0.25 mA cm-*) for approximately 1 min, followed by a rest period (I = 0). The time delay following current interruption was carefully controlled so as to allow an estimation of the ohmic and nonohmic (polarization) contributions to the overall cell impedance to be made. For a detailed discussion of the current interruption method and its application to these kinds of measurement the reader is referred to the work reported by[26].
.
RESULTS
AND DISCUSSION
The open circuit cell voltage of the as-made Li/Li,CoO, cells were typically around 3.1 V vs. Li. The EVS technique was used to charge (lithium extraction from the Li,CoO,) the cells to a stable ecu of about 4.25 V vs. Li, and subsequently to discharge (lithium insertion) the cells to 3.2V vs. Li. The precise experimental parameters described in the experimental section, ensured that the cells were close to thermodynamic equilibrium during the cycling process. The technique approximates to about a 1OOh chargedischarge rate. By essentially removing any unnecessary overvoltage, this method also minimizes the risk of electrochemical oxidation of the electrolyte. Figure 3 shows the relationship between the pseudo-open cell circuit voltage and lithium concentration, x, in Li,CoO, derived from the first EVS charge-discharge cycle of a typical Li/Li,CoO, The x values shown in the figure have been calculated assuming all charge passed is associated with either lithium extraction or insertion reactions. The voltage hysteresis (ie voltage difference between extraction and insertion waves) is relatively small, consistent with the expected thermodynamic reversibility of the system. The oxidation step results in a charge equivalent to x = 0.79 being extracted from the Li,CoO, .
J. BARKERet al.
34 I
I 0.2
0.3
04
05
0.6
07
08
0.9
10
x in LixCoO,
07.
03
04
05
0.0
0.7
08
09
10
x in LI,COO,
Fig. 3. First cycle EVS cell voltage-composition profile for the Li/Li,CoO, cell cycled between voltage limits of 3.2V and 4.25 V vs. Li.
Fig. 5. Second cycle EVS cell voltage-composition profile for the Li/Li,CoO, cell cycled between voltage limits of 3.2 V and 4.25 V vs. Li.
It can be seen clearly from inspection that the overall lithium extraction-insertion reactions are coulombitally inefficient to such an extent that charge equivalent to only x = 0.61 can be subsequently re-inserted. The reason for this inefficiency is presently unclear but may be associated with some electrolyte decomposition occurring during the initial charge stage. It has been proposed by Thomas and co-workers[2] that the electrolyte degradation process may result in formation of a surface passivation layer on the cathode. A similar irreversible charge consumption was also noted during the work of Reimers and Dahn[14]. It was speculated that this way be due to a combination of both cathode degradation and electrolyte decomposition at the high oxidation voltage encountered during cell charging stage. As lithium is extracted from Li,CoO, , Co3+ is oxidized to Co4+ which is expected to be a relatively unstable oxidation state for this element. Thus it was proposed that the presence of Co4+ may induce cathode degradation[14]. Certainly, if this were the case then it should be expected that the precise control of co3+/co4+ ratio for the Li,CoO, would be advantageous in maintaining cathode integrity during cell cycling. The associated differential capacity-cell voltage relationship for the first EVS cycle is shown in Fig. 4.
There appear to be no irreversible oxidation peaks which we can reliably assign to electrolyte or cathode degradation processes. In fact the main extraction-insertion peaks (at 3.94V and 3.86V vs. Li respectively) appear to be well-defined and with small voltage peak separation Al/’ = 0.06 V as indicated by the low voltage hysteresis described in Fig. 3. In fact the definition of the insertion peak is superior to that previously reported for the Li/Li,CoO, system in liquid electrolyte cells[14]. Also shown on this figure (see arrows) are the two (small) additional reversible features first reported by Reimers and Dahn[14]. The origin of these features will be discussed in more detail later in this section. From AA analysis the cathode composition for one representative electrode following the first cycle indicating that all Li was found to be Li ,,.&oO extracted had been re-inserted within the host lattice. For this reason we begin the second cycle data at a nominal composition of Li,CoO, The second cycle EVS voltage-composition and differential capacity profiles are shown in Figs 5 and 6 respectively. Inspection of the voltage profile data indicates that in contrast to cycle 1, the coulombic efficiency for the second cycle is now close to lOO%, suggesting that the irreversible reactions encountered during the first cycle are absent during this cycle. This observ-
Fig. 4. First cycle EVS differential capacity plot for the Li/Li,CoO, cell cycled between voltage limits of 3.2V and 4.25V vs. Li. The arrows denote the two high voltage features described in the text.
Fig. 6. Second cycle EVS differential capacity plot for the Li/Li,CoO, cell cycled between voltage limits of 3.2 V and 4.25V vs. Li. The arrows denote the two high voltage features described in the text.
An electrochemical investigation into the lithium insertion properties of Li,CoO,
ation may be consistent with the formation of a stable, ionically conducting interface layer on the LiCoO, compound during the initial cycle as described[2]. This layer is then expected to minimize further decomposition of the electrolyte. The total charge passed suggested that Li+ was extracted during the oxidation step to generate the approximate composition Li ,,34Co02. This degree of lithium extraction is higher than previously reported[4, 6, 143. This observation may be due to the extremely low rates of extraction and insertion used in this study. The EVS method adopted here approximates to a 100 h rate for both charge and discharge reactions. Thus, as we described in the experimental section, the data we present here are expected to be close to the thermodynamic equilibrium case and as such allow us to approach the highest possible extraction and insertion levels attainable for the system under the prevailing test conditions. The associated differential capacity plot for the second cycle shows the main extraction and insertion peaks positioned at 3.94 V and 3.87 V vs. Li respectively. These peak positions, as well as peak separation, are consistent with the data presented for the first cycle. The extraction peak, although still welldefined is less sharp, perhaps indicating a slight structural degradation of the cathode material. Also shown on Fig. 6, are the two additional high voltage features noted during cycle 1. To improve the clarity of these features we have enlarged the differential capacity data from cycles 1 and 2 and presented these in Figs 7(a) and 7(b) respectively. The voltage range in these figures is restricted to the one of interest. The peak voltages have been clearly marked on the two figures. On cycle 1 we have maxima during charge at 4.08 V and 4.2OV vs. Li and at 4.13 V and 4.02V vs. Li on the subsequent discharge. The relatively wide differential capacity minima lie at approximately 4.15 V and 4.07 V on the charge and discharge steps respectively. The data from cycle 2 are consistent with those collected for cycle 1. The equivalent features were previously reported[14] at voltages of 4.08 V and 4.18 V on charge with the minimum in the dx/dV data situated at approximately 4.15V. It appears that our observations, strictly from an electrode potential standpoint, are consistent with those previously reported. The work by Reimers and Dahn[14] also attempts to assign the minima to an ordered lithium superlattice within the host lattice. For this purpose the dx/dV data were presented as a function of lithium composition. Certainly minima in differential capacity data have previously been ascribed to ordered compositions of the host structure[27]. By taking an average value for x from their charge and discharge data they assign the minima in dx/dV to a composition of Li 0.533Co02. As the authors correctly assert, however, it is difficult to reconcile an ordered superlattice existing at such an unusual composition. For this reason Reimers and Dahn assumed that the lithium superlattice occurred precisely at x = 0.5. This determination was to a large extent based on an estimation of overall cathode utilization. It was assumed for this purpose that not all the cobalt oxide grains were electrically connected within the
2485
J
J Fig. 7. Expanded view of (a) the first cycle differential capacity data and (b) the second cycle differential capacity data, showing high voltage features in each case.
composite electrode structure. Thus the dx/dVcomposition data could be shifted to allow the minimum to correspond precisely to x = 0.5 and at an electrode potential of around 4.15 V vs. Li. We find that the dx/dV minimum in our data is consistent with this electrode potential bu not with the figure based on lithium composition. As we have described, after taking into account the first cycle coulombic inefficiency, we find that for cycle 2 the lithium extraction-insertion reactions are essentially 100% reversible. Using cycle 2 data we determine that the dx/dV minimum occurs at approximately x = 0.38 (based on the average lithium extraction and insertion data). There is a large discrepancy between the two sets of data. In our case we could only position the minima close to a cathode composition, x = 0.5 if there were a gross underestimation of the active material loading within the composite Li,CoO, electrode. We believe this to be an unlikely scenario since the precise composition of the electrodes were cross-checked by detailed AA analysis. At present we consider that the suggestion that the minima correspond to an ordered lithium superlattice at x = 0.5 requires further clarification and confirmation. Chemical diffusion coefficient measurements for the Li,CoO, were determined by the two electrochemical pulse methods described in the experimental section, and indicated relatively fast reaction kinetics for the lithium insertion process. The values
J. BARKERet
2486
from the two techniques are essentially consistent and only the data collected from the EVS experiments will be presented here. Careful collection of the data was performed to ensure that the Cottrell fit of the current-time transient for the system was satisfied by the semi-infinite condition (ie t 4 12/D, where t is time, I = electrode thickness and D is the chemical diffusion coefhcient)[24]. This procedure entailed collecting the current transient data at relatively short time periods following the voltage step. It also involved discarding data where a non-Cottrell behavior (within either the semi-infinite or infinite regions) was detected. Effective diffusion coefficients for the electrodes composites were then calculated from these measurements based on the geometric surface area of the electrodes. We recognize that these values will be dependent on the intrinsic diffusion coefficients for the electrode materials in question. It should be noted, however, that other electrode parameters will also affect the magnitudes. As these factors do not depend on the state-ofcharge of the electrode, the variation in the measured effective diffusion coefficients are expected to directly reflect the compositional variation of the intrinsic diffusion coefficient for Li in the electrode material. In Fig. 8, we show the variation of the Li’ ion diffusion coefficients for the Li,CoO, composite cathodes as a function of the lithium concentration during the lithium insertion reaction. The magnitude of the diffusions coefficients are generally around 10-9cm2s-1. The variation of the diffusion coefhcient in the insertion range 0.3 < x < 0.85 is consistent with filling/removal of lithium ions from a single site within the host lattice over this particular composition range. Outside this range there was insufticient current transient data for us to reliably calculate kinetic data for the system. At a composition around x = 0.65, the diffusion coefficient reaches a local minimum. Within the compositional range described there exists a two-phase system with the phase transition being first order involving a significant increase in the c-lattice parameter of the hexagonal unit cell[ 143. Relatively substantial structural rearrangement is therefore taking place as lithium is inserted or extracted from this particular lattice site. We believe that this may explain the observed
al.
minimum in the diffusion coefficient-composition data since the kinetic properties will be driven by guest-host and guest-guest coulombic interactions within the Li,CoO, . The results from the UC impedance study of an uncycled (open circuit voltage, 3.1 V vs. Li) and for a charged (open cell voltage, 4.2V vs. Li) cell are shown in Fig. 9(a) and 9(b) respectively. The impedance properties for a two electrode cell incorporating a carefully formulated composite insertion cathode, are normally dominated, at all states of charge, by the characteristics of the lithium metal anode[28]. Only under circumstances where the cathode has been improperly formulated (eg poor interparticle contact, insuficient conductive diluent or poor active material dispersion) or where the electrode has been compromised during cycling, does the cathode impedance become dominant and therefore begin to affect significantly the overall cell performance. The spectrum in Fig. 9(a) is consistent with the cell impedance being dominated by the pristine Li/ electrolyte interface. In the uncycled state the lithium electrode is still covered by the native oxide layer which generates a relatively high interfacial impedance. It is well known that the Li/electrolyte interface film is modified significantly upon passage of charge through the film. It has been clearly demonstrated in both liquid[29] and polymer[28] electrolyte systems that the Li interfacial impedance decreases rapidly following charge transfer. Thus, the spectrum for the charged cell shown in Fig. 9(b) is consistent with the low cell impedance expected for the system following charge transfer. Since the cathode impedance is expected to be less that 1 R (for the 24cm2 electrode area) over the complete insertion range investigated, no adverse effects of the cathode impedance on the rate capability of the cell are expected. ._._ -10 0
(a)
uncyded Cdl
-7 5
0’
o~“-*-o-o\
O\
0’
% \
.’ -5 0
0’
8 4
/
4 9 “* b
% %
L
*
ox’o-**
0’ 0
5
10
15
20
25
i 01
02
03
04
05
06
0.7
08
09
10
x in Li,CoO,
Fig. 8. The variation in chemical diffusion coefficient with lithium concentration, x in Li,CoO,. The data were collected from the current transient data of the EVS method during lithium insertion.
Fig. 9. AC impedance spectra for (a) uncycled cell (open circuit voltage 3.1 V vs. Li); and Li/Li,CoO, cell (open circuit voltage 4.2 V vs. were collected between frequency limits of with an ac oscillation of C 1OmV.
Li/Li,CoO, (b) charged Li). The data 65 kHz-1 Hz
An electrochemical investigation into the lithium insertion properties of Li,CoO,
Complementary cell impedance data were also collected by the dc current interrupt technique described in the experimental section. This technique has the advantage of generating impedance properties for the complete cell under dynamic conditions. This procedure allows one to realistically probe the cell impedance variations as a function of the cell state-of-charge in a time effective fashion. The data for cycles 1 and 2 of a representative cell are summarized in Figs 10 and 11 respectively. The measurements were conducted over the voltage range 3.0-4.25V vs. Li and at a current density for charge and discharge of 0.25 mA cm-‘. From inspection we can denote two distinct regions, ie one below about x = 0.6 in Li,CoO, where the system demonstrates relatively low impedance behavior, and one above about x = 0.7 where significantly higher impedance characteristics are found: Thus the impedance difference during cycling is close to an order of magnitude, being lowest when the cell approaches the fully charged state. The cell impedance does not appear to increase to any appreciable amount towards the end of charge step. This observation is somewhat at variance with the findings from Ohzuku and coworkers[lS] who reported a significant increase in cell overvoltage at low insertion levels during constant current cycling of Li/Li,CoO, cells. It was
Fig. 10. First cycle dc impedance data collected by the current interrupt method described in the text. The cell was cycled between voltage limits of 3.2V to 4.25V using densities for charge and discharge of current +0.25mAcn1-~.
Fig. 11. Second cycle dc impedance data collected by the current interrupt method. Experimental conditions the same as denoted in Fig. 10.
2487
reported that this property limited the useful specific capacity for their electrodes[15]. We suspect that the superior impedance characteristics for the electrodes tested in our work may result from improved cathode formulation effects. Generally, we find that the current interrupt measurements are consistent with the UC measurements described above. Presently we do not fully understand the large step in the magnitude of the cell impedance but suspect variations associated with morphological changes at the lithium/electrolyte interface[30]. We are currently investigating the impedance properties of three-electrode cells (incorporating a lithium reference electrode) in an attempt to deconvolute the cathode and anode contributions to the overall cell impedance.
CONCLUSIONS It is well known that Li,CoO, has been used successfully as the cathode active material in commercially available lithium ion battery systemsC12, 131. We have undertaken a detailed electrochemical study to elucidate the mechanisms of lithium insertion and extraction in this material. The work was conducted using two electrode cells incorporating a metallic lithium anode together with a hybrid polymer electrolyte. We find that during the initial charge-discharge cycle there is a relatively large coulombic inefficiency which may be related to the formation of a passivation layer on the surface of the active material. This is supported by the observation that the second cycle is close to 100% reversible suggesting that any side reactions were completed during the first cycle. The first cycle differential capacity profile, which has been demonstrated previously to be an extremely powerful method for probing reversible and non-reversible reactions[l6193, did not, however, indicate specific charge consumption related to any reactions not associated with the lithium extraction process. In fact the differential capacity profile indicated the main extractioninsertion peaks to be well-defined and with only minor peak separation consistent with a reversible system demonstrating low overvoltage. The charge passed during the EVS second cycle indicated that around x = 0.66 Li could be reversibly inserted and extracted from the Li,CoO, material. This degree of reversibility is far higher than previously reported in the open literatureC4, 6, 143. We suspect that the combination of using well-formulated composite cathodes together with the slow charge/discharge EVS regimen employed during the study contributed to this observation. The differential capacity profiles indicated two additional high voltage peaks positioned at 4.08V and 4.20 V vs. Li on lithium extraction and at 4.13 V and 4.02 V vs. Li on lithium insertion. These features have previously been reported in the work by Reimers and Dahn[14], who ascribed the minima between the peaks as evidence for creation of a stable lithium superlattice within the Li,CoO, host. This was supported by in situ XRD studies. Following an estimation of the cathode utilization Reimers and Dahn then compositionally positioned the
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ordered phase at precisely x = 0.5 in Li,CoO,. In our data we could only find the minima close to x = 0.5 if there was a gross underestimation of the active material loading within the composite Li,CoO, electrode. This we consider unlikely since the compositional figures were confirmed independent analytical analysis. At present we contend that the composition for the ordered superlattice requires further clarification. Solid state lithium ion diffusion coefficients were estimated using electrochemical measurements. We recognize that collecting reliable kinetic data for powdered materials is notoriously difficult to achieve due to the fact that several other electrode factors will affect their magnitude. However, since these factors are not expected to vary with state-of-charge we expect that our measured values will reflect the compositional variation of the intrinsic diffusion coefficient in the Li,CoO,. The average diffusion coefficient (at 23°C) for our electrodes was found to be around 1O-9 cm2 s- ‘. This indicated the relatively facile reaction kinetics for the lithium insertion reaction. We also find that the diffusion coefficient reaches a local minimum at the lithium composition, x = 0.65. We believe this observation is consistent with coulombic interactions within the cathode host. The ac impedance measurements demonstrated that the impedance characteristics for a Li/Li,CoO, cell, at all states-of-charge, are expected to be dominated by the properties of the lithium/electrolyte interface. The cell impedance for the 24cm’ cell in the charged state (ie ecu = 4.2V) was found to be around 1 R. The complementary dc impedance data, collected by the current interrupt method, were consistent with the UC spectra. We found two distinct impedance regions as a function of the cell state-ofcharge. One region exists below approximately x = 0.6 where the system generates relatively low impedance characteristics. The second region occurs above about x = 0.7 where significantly higher impedance properties are found. We suspect that these observations are consistent with morphological changes at the lithium/electrolyte interface occurring during the lithium stripping/plating reactions.
E. Plichta, S. Slane, M. Uchiyama and M. Salomon, 136, 1865 (1989). M. Yoshio, H. Tanaka, K. Tominaga and H. Noguchi, J. Power Sources 40,347 (1992). R. S. McMillan and E. E. Andrukaitis, in Proceedings of the Symposium on Primary and Secondary Lithium Batteries (Edited by K. M. Abraham and M. Salomon),
8. 9.
10. 11.
Vol. 91-3, pp. 416. The Electrochemical Society, Pennington, NJ, (1991). M. Broussely, F. Perton, J. Labat, R. J. Staniewitz and A. Romero, J. Power Sources 43-44,209 (1993). K. Sekai, H. Azuma, A. Omaru, S. Fujita, H. Imoto, T. Endo, K. Yamaura and Y. Nishi, J. Power Sources 4344, 241 (1993). G. T. K. Fey, M. C. Hsieh, H. K. Jaw and T. J. Lee, J. Power Sources 43-44,673 (1993). G. Nagasubramanian and A. I. Attia, in Proceedings of the Symposium on Primary and Secondary Lithium Batteries (Edited by K. M. Abraham and M. Salomon),
Vol. 91-3, pp. 207. The Electrochemical Society, Pennington, NJ, (1991). 12. K. Tanaka, M. Itabashi, M. Aoki, S. Hiraka, M. Kataoka, S. Fujita, K. Sekai and K. Ozawa in The Electrochemical Society Fall Meeting, New Orleans, Extended Abstract 21, p. 36. lo-15 October (1993). 13. T. Nagaura and T. Tozawa, Prog. Batteries and Solar Cells 9,209 (1990).
14. J. N. Reimers and J. R. Dahn, J. electrochem. Sot. 139, 2091 (1992). 15. T. Ohzuku, A. Ueda, M. Nagayama, Y. Iwakoshi and H. Komori. Electrochim. Acta 38. 1159 (1993). 16. J. Barker, Synth. Met. 32,43 (1989). 17. J. Barker, D. Baldwin, D. C. Bott and S. J. Porter, Synth. Met. u(, D127 (1989). 18. J. Barker, R. Pynenburg and R. Koksbang, J. Power Sources 52, 185 (1994). 19. J. Barker, Electrochim. Actn 40, 1603 (1995). 20. J. S. Lundsgaard, S. Yde-Andersen, R. Koksbang, D. R. Shackle, R. A. Austin and D. Fauteux, Proceedings of the Second International Symposium on Polymer Electrolytes, Sienna, Italy, 14-16 June 1989, pp. 395. Else-
vier Applied Science Publishers, Ltd, UK (1990). 21. A. H. Thompson, Phys. Rev. Lett. 23, 1511 (1978). 22. A. H. Thompson, J. electrochem. Sot. 126,608 (1979). 23. T. Ohzuku and A. Ueda, Solid State Ionics 69, 201 (1994). 24. A. J. Bard and L. R. Faulkner, Electrochemical Methods: Fundamentals and Applications, pp. 143. Wiley, New York (1980). 25. W. Weppner and R. A. Huggins, J. electrochem. Sot. 124, 1569 (1977).
REFERENCES 1. K. Mizushima, P. C. Jones, P. J. Wiseman and J. B. Goodenough, Mater. Res. Bull. 15, 783 (1980). 2. M. G. S. R. Thomas, P. G. Bruce and J. B. Goodenough, J. electrochem. Sot. 132, 1521 (1985). 3. M. G. S. R. Thomas, P. G. Bruce and J. B. Goodenough, Solid State lonics 18-19, 794 (1986). 4. J. N. Reimers, J. R. Dahn and U. von Sacken J. electrothem. Sot. 140,2752 (1993).
26. F. H. van Heuveln, J. electrochem. Sot. 141, 3423 (1994). 27. 0. Tillement and M. Quarton, J. electrochem. Sot. 140, 1870 (1993). 28. E. Peled in Lithium Batteries (Edited by J. P. Gabano), pp. 43. Academic Press, London (1983). 29. J. Barker and R. Koksbang, Electrochim. Acta 40, 1706 (1995). 30. I. I. Olsen, J. Barker and R. Koksbang, lonics 83 125 (1996).
Solid State