Technical Note pubs.acs.org/ac
CO2 and O2 Evolution at High Voltage Cathode Materials of Li-Ion Batteries: A Differential Electrochemical Mass Spectrometry Study Hongsen Wang,† Eric Rus,† Takahito Sakuraba,‡ Jun Kikuchi,‡ Yasuyuki Kiya,‡ and Héctor D. Abruña*,† †
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301, United States Subaru Technical Research Center, Fuji Heavy Industries, Ltd., 3-9-6 Oosawa, Mitaka-shi, Tokyo 181-8577, Japan
‡
S Supporting Information *
ABSTRACT: A three-electrode differential electrochemical mass spectrometry (DEMS) cell has been developed to study the oxidative decomposition of electrolytes at high voltage cathode materials of Li-ion batteries. In this DEMS cell, the working electrode used was the same as the cathode electrode in real Liion batteries, i.e., a lithium metal oxide deposited on a porous aluminum foil current collector. A charged LiCoO2 or LiMn2O4 was used as the reference electrode, because of their insensitivity to air, when compared to lithium. A lithium sheet was used as the counter electrode. This DEMS cell closely approaches real Li-ion battery conditions, and thus the results obtained can be readily correlated with reactions occurring in real Li-ion batteries. Using DEMS, the oxidative stability of three electrolytes (1 M LiPF6 in EC/DEC, EC/DMC, and PC) at three cathode materials including LiCoO2, LiMn2O4, and LiNi0.5Mn1.5O4 were studied. We found that 1 M LiPF6 + EC/DMC electrolyte is quite stable up to 5.0 V, when LiNi0.5Mn1.5O4 is used as the cathode material. The EC/DMC solvent mixture was found to be the most stable for the three cathode materials, while EC/DEC was the least stable. The oxidative decomposition of the EC/DEC mixture solvent could be readily observed under operating conditions in our cell even at potentials as low as 4.4 V in 1 M LiPF6 + EC/DEC electrolyte on a LiCoO2 cathode, as indicated by CO2 and O2 evolution. The features of this DEMS cell to unveil solvent and electrolyte decomposition pathways are also described.
L
the application of DEMS to Li-ion battery studies is still quite limited. Novák’s group has used DEMS to study the solvent decomposition at anode and cathode materials.10 In their electrochemical cells, Li is used as both the counter and reference electrodes. Cathode materials are directly deposited on porous Teflon membranes and used as working electrodes. As a result, the resistances in the solution and the working electrode are quite large, even though large amounts of carbon were used in order to lower the resistance of the working electrode. Therefore, the IR (ohmic) drop in their cell was quite large (several hundred millivolts). The voltage stability window of battery electrolytes strongly depends on the electrode meterials employed, surface area, and surface properties.10,11 Measurements on nonporous electrodes such as platinum and glassy carbon (which are also not used in real batteries) cannot represent well the behavior in real batteries. The best way is to use electrode materials that closely mimic the surface state and surface area of the electrodes used in real devices.11 In this article, we describe a new DEMS cell design with a three-electrode configuration for the study of solvent and electrolyte decomposition of Li-ion batteries, in which the IR
ithium-ion batteries have been widely used in many portable electronic devices and have the potential to be used in long-range electric vehicles. LiCoO2 is the most commonly used cathode material in today’s Li-ion battery market due to its excellent electrochemical cycling stability, even though Co is somewhat expensive and toxic.1−3 Alternatively, manganese-based oxides with a spinel structure are particularly attractive and promising as cathode materials for Li-ion batteries because of their low cost, nontoxicity, and high natural abundance.4,5 Recently, the high voltage cathode material LiNi0.5Mn1.5O4 has attracted a great deal of attention due to its high discharge voltage plateau at 4.7 V, high capacity, and stable intercalation/deintercalation processes.6−8 However, safety concerns have limited the full utilization of these batteries. In particular, gas evolution in Li-ion batteries represents a major problem, because it can cause ignition and explosion.9,10 When high voltage cathode materials such as LiNi0.5Mn1.5O4 and LiCoPO4 are used, the stability of solvents and electrolytes is even more challenging. In fact, most high voltage Li ion batteries operate beyond the thermodynamic limit of the solvent. Thus, an assessment of the stability of electrode/electrolyte combinations is necessary in order to establish safety parameters and metrics. Differential electrochemical mass spectrometry (DEMS) is a very useful technique for studying gas evolution during the charging/discharging processes of Li-ion batteries. However, © XXXX American Chemical Society
Received: October 14, 2013 Accepted: May 20, 2014
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have potentials of ∼3.9 V (LiCoO2) or 4.1 V (LiMn2O4), respectively, after charging. All potentials are quoted against a Li/Li+ electrode. The reference electrode is only 2 mm above the working electrode and was separated from the working electrode by a Teflon gasket with an inner diameter of 5 mm. The working electrode was placed on a porous Teflon membrane (Gore-Tex) with a mean thickness of 75 μm, a mean pore size of 0.02 μm, and a porosity of 50%. Beneath the Teflon membrane is a 25 μm thick FEP film (McMaster Carr), which was mechanically supported on a stainless steel frit and which can significantly reduce the evaporation of solvents into the vacuum due to lower porosity. A metallic lithium sheet (Aldrich, 99.9%) was used as the counter electrode. The DEMS cell was assembled in a dry glovebox with a high purity Argon atmosphere (residual water and oxygen below 0.1 ppm) and a cap was used to seal the DEMS cell. Afterward, the DEMS cell was transferred out of the glovebox. During DEMS measurements, the DEMS cell was also protected by placing it in a bag with a high purity argon atmosphere. Gases and volatile species generated at the working electrode can diffuse into the vacuum system of the mass spectrometer through the porous Teflon membrane and the FEP film. The time delay is estimated to be about 9 s from a calibration using formic acid oxidation. The electrolyte solvents, ethylene carbonate (EC, SigmaAldrich, 99%, anhydrous, less than 0.005% water), propylene carbonate (PC, Sigma-Aldrich, 99.7%, anhydrous, less than 0.002% water), dimethyl carbonate (DMC, Sigma-Aldrich, 99+ %, anhydrous, less than 0.002% water), diethyl carbonate (DEC, Aldrich, ≥99%, anhydrous, less than 0.002% water), and LiPF6 (≥99.99%, battery grade from Aldrich) were used as received. Three electrolyte solutions [1 M LiPF6 + PC, 1 M LiPF6 + EC/DEC (1:1 wt %), and 1 M LiPF6 + EC/DMC (1:1, wt %)] were prepared and stored in an argon-filled dry glovebox. Electrochemical experiments in the DEMS cell were carried out with an EG&G model 173 potentiostat/galvanostat, model 175 universal programmer, and a homemade data acquisition software using Labview, combined with a NI-DAQ card. The potential scan rate was 0.2 mV/s. All measurements were carried out at room temperature (21 ± 1 °C).
drop can be significantly mitigated, and the DEMS studies can be carried out under conditions quite similar to those in real Liion batteries since the working electrode employed was the same as in real Li-ion batteries. Moreover, the working electrode is separated by at least 7 mm from the counter electrode (Li sheet) thus avoiding the interference from the counter electrode. Hence, only gases generated at the working electrode are exclusively detected. Using this DEMS cell, we have studied the gas evolution (CO2 and O2) at LiCoO2, LiMn2O4, and LiNi0.5Mn1.5O4 cathode materials in three different carbonate-based electrolytes.
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EXPERIMENTAL SECTION The DEMS setup has been described in detail in previous publications.12 A homemade DEMS cell made of Kel-F was connected to the main chamber via an angle valve for DEMS experiments. This DEMS cell is quite similar to the traditional DEMS cell,13 as illustrated in Figure 1. The working electrodes
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Figure 1. Schematic diagram of the DEMS cell for Li-ion battery studies.
RESULTS AND DISCUSSION
LiCoO2. LiCoO2 is the most commonly used cathode material in commercial Li-ion batteries. In order to examine the performance of our newly designed DEMS cell, and the sensitivity of our DEMS system, the standard material LiCoO2 was first used as the working electrode to study solvent decomposition. Figure 2 presents cyclic voltammograms (CVs) (a) and corresponding mass spectrometric cyclic voltammograms (MSCVs) of CO2 at m/z = 44 (b) and O2 at m/z = 32 (c) for LiCoO2 in PC, EC/DMC, and EC/DEC solutions with 1 M LiPF6, respectively. Panel b presents the potentialdependent CO2 evolution, while panel c panels presents the potential-dependent O2 evolution, respectively. The CVs of
were prepared by doctor blade-coating the oxides, LiCoO2, LiMn2O4, and LiNi0.5Mn1.5O4 (TODA KOGYO Corp.), respectively on a 30 μm thick porous aluminum foil current collector (KDK, Japan). The slurry used for the coating was composed of 86 wt % active mass, 4 wt % PVDF binder, and 10 wt % carbon black. The working electrode was vacuum-dried at 120 °C overnight and then protected in gastight bags with dry argon. The aluminum foil can significantly increase the conductivity of the working electrode, even when only 5− 10% carbon is present in the cathode. The specifications of all three working electrodes are listed in Table 1. LiCoO2 or LiMn2O4 was used as the reference electrode. These materials
Table 1. Specifications for LiCoO2, LiMn2O4, and LiNi0.5Mn1.5O4 Electrodesa
a
samples
electrode thickness/mm
Al foil thickness/mm
metal oxide thickness/mm
loading/g/cm3
loading/g/cm2
particle size/μm
LiCoO2 LiMn2O4 LiNi0.5Mn1.5O4
0.081 0.074 0.075
0.030 0.030 0.030
0.051 0.044 0.045
2.23 1.76 1.65
0.0112 0.0077 0.0074
6.3 9.3 9.5
The thickness of the electrode and metal oxide was determined by SEM. B
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Under overcharging conditions, CO2 and oxygen have been detected at LiCoO2 electrodes by gas chromatography.9 The generation of O2 has been attributed to the decomposition of overdelithiated LiCoO2.9 However, there are very limited systematic studies of potential-dependent gas evolution in different electrolytes at this (and virtually all) cathode materials. To our knowledge, this work represents the first report that O2 generation is strongly correlated to the type of electrolyte employed. The possible mechanism of CO2 and O2 generation can be described as follows: LiCoO2 → CoO2 + Li+ + e−
(1)
3CoO2 → Co3O4 + O2
(2)
CH3CH 2OCO2 CH 2CH3 (DEC) → CO2 + CH3CH 2OCH 2CH3+ + e− Figure 2. DEMS data for LiCoO2 electrode in 1 M LiPF6 + PC (left panel), 1 M LiPF6 + EC/DMC (1:1 wt %) (middle panel), and 1 M LiPF6 + EC/DEC (1:1 wt %) (right panel), respectively, at a scan rate of 0.2 mV/s. (a) Electrochemical data, (b) intensity of mass spectra at m/z = 44 (CO2+), (c) intensity of mass spectra at m/z = 32 (O2+). The first scan is shown.
(3a)
CH3OCO2 CH3 (DMC) → CO2 + CH3OCH3+ + e− (3b) +
C4 H6O3 (PC) → CO2 + C3H6O + e
−
(3c)
CH3CH 2OCO2 CH 2CH3 (DEC) + O2
LiCoO2 in the DEMS cell, shown in Figure 2a, are quite similar to those in the coin cell test (see Figure S1 in the Supporting Information). This suggests that our DEMS cell is a good approximation to coin cells, has low resistance and thus small IR drops. In the positive-going scan up to 5.0 V, two distinct oxidation peaks occur at ∼4.1 and 4.7 V in the EC/DMC and EC/DEC electrolytes and 4.2 and 4.8 V in the PC electrolyte, corresponding to partial and complete delithiation processes, respectively. In the negative-going scan, two Li-intercalation peaks are observed at ∼4.2 and 3.7 V. In the EC/DEC solution, CO2 and O2 start to form at potentials higher than 4.4 V (vs Li/ Li+). The O2 peak occurs at ∼4.8 V, while CO2 evolution reaches its limiting current at around 5.0 V. However, only CO2 formation can be detected in the PC and EC/DMC solutions at potentials between 4.7 and 5.0 V, and no potential-dependent O2 evolution was observed. In the EC/DMC electrolyte, CO2 evolution reaches its limiting value at ∼4.8 V; while in the PC electrolyte, CO2 evolution is still increasing at potentials beyond 5.0 V. It appears that the decomposition of PC is more significant than that of EC/DMC at potentials beyond 5.0 V. Our DEMS data strongly suggest that CO2 and O2 evolution are solvent dependent. Among the three solvents, the EC/DEC mixture was the most unstable for LiCoO2 at high potentials. Novák et al. reported that CO2 is detected in PC electrolytes using a similar DEMS cell,10 which is in agreement with our DEMS data. However, in the EC/DMC electrolyte, we detected CO2 formation on LiCoO2, but they could not detect any CO2 evolution using a similar DEMS cell.10 However, they did detect CO2 formation using another electrochemical cell.14 On the other hand, we directly detected O2 formed during overcharging of LiCoO2 using DEMS. These preliminary results indicate that our DEMS system performs very well and is quite sensitive. Overdelithiating LiCoO2 often results in a rapid loss of capacity, which is thought to be caused by the phase transition of LixCoO2 with x < 0.5,15,16 oxygen loss,9,17 Co dissolution,18 the formation of electrochemically resistive surface films,19,20 and side reactions with the electrolyte at high potentials.9
→ CO2 + 2CH3CHO + H 2O
(4)
The aldehydes can be further oxidized to carboxylic acids and CO2. In PC and EC/DMC electrolytes, O2 evolution was not observed, and thus CO2 is considered to be generated through eq 3. In contrast, in the EC/DEC electrolyte, CoO 2 decomposition to generate O2 was significant, and thus CO2 could be generated through both eqs 3 and 4. Carbon black is also likely to be oxidized to CO2. However, in the EC/DMC electrolyte, CO2 was not detected when using LiNi0.5Mn1.5O4 as a cathode (see Figure 4), and thus carbon black oxidation can be ruled out as a source of CO2, at least under these experimental conditions. LiMn2O4. LiMn2O4 is an alternative cathode material to LiCoO2 due to its low cost and enhanced safety. Figure 3 presents CVs (a) and corresponding MSCVs of CO2 at m/z = 44 (b) for LiMn2O4 in PC, EC/DMC, and EC/DEC with 1 M LiPF6, respectively. Similar to the CV results from the coin cell (see Figure S2 in the Supporting Information), the CVs of LiMn2O4 in the DEMS cell also exhibited a pair of reversible oxidation and reduction peaks, corresponding to lithium extraction and insertation. In the positive-going scan up to 5.0 V, in addition to the main oxidation peak at ∼4.3 V in the PC and EC/DMC electrolytes, or 4.4 V in the EC/DEC electrolyte, a small peak was also observed at around 4.7 V, which could be related to defects in the spinel structure.21 In the EC/DEC electrolyte, the delithiation and lithiation peaks are broader than those in the PC and EC/DMC electrolytes, suggesting slower kinetics of lithium extraction and insertation. In the PC and EC/DEC electrolytes, CO2 starts to form at potentials higher than 4.7 V, while in the EC + DMC electrolyte, CO2 is observed at potentials higher than 4.9 V. More CO2 is formed in the EC/DEC electrolyte than in PC and EC/DMC electrolytes, which is similar to the case of LiCoO2. Moreover, the lithiation and delithiation kinetics were the fastest in the EC/DMC electrolyte, as suggested by the sharp peaks in Figure 3a. Hence, among the three solvents, the C
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Figure 4. DEMS data for LiNi0.5Mn1.5O4 electrode in 1 M LiPF6 + PC (left panel), 1 M LiPF6 + EC/DMC (1:1 wt %) (middle panel), and 1 M LiPF6 + EC/DEC (1:1 wt %) (right panel), respectively, at a scan rate of 0.2 mV/s. (a) Electrochemical data and (b) intensity of mass spectra at m/z = 44 (CO2+). The first scan is shown.
Figure 3. DEMS data for LiMn2O4 electrode in 1 M LiPF6 + PC (left panel), 1 M LiPF6 + EC/DMC (1:1 wt %) (middle panel), and 1 M LiPF6 + EC/DEC (1:1 wt %) (right panel), respectively, at a scan rate of 0.2 mV/s. (a) Electrochemical data and (b) intensity of mass spectra at m/z = 44 (CO2+). The first scan is shown.
EC/DMC seems to be the best solvent for the LiMn2O4 cathode material. Compared to LiCoO2, the oxidative decomposition of all three solvents on LiMn2O4 is slower. In all three electrolytes, no potential-dependent oxygen evolution was observed for LiMn2O4. Therefore, we believe that CO2 generation on LiMn2O4 at potentials up to 5.0 V is likely through eq 3. Tarascon et al. screened a series of electrolyte compositions for LiMn2O4 cathode materials and identified an electrolyte with an optimum composition of DMC/EC (3:7 to 8:2) + (1 M to 2M) LiPF6, which is highly stable against oxidation and can be used for voltages up to 4.9 V at room temperature.22 Novák et al. studied CO2 evolution at LiMn2O4 in PC and EC/ DMC solvents with 1 M LiN(SO2CF3)2 with DEMS and found that CO2 formation started around 4.8 V in the PC electrolyte, while no CO2 was observed for the EC + DMC electrolyte for potentials below 5.5 V. Our DEMS data show that the 1 M LiPF6 + EC/DMC electrolyte is most stable for LiMn2O4 among the three studied electrolytes, which is in good agreement with Tarascon’s findings.22 LiNi0.5Mn1.5O4. For high voltage cathode materials, solvent oxidative decomposition is a significant problem. So far, there are very few studies of solvent oxidative decomposition on LiNi0.5Mn1.5O4-based cathodes. Figure 4 presents CVs (a) and corresponding MSCVs of CO2 at m/z = 44 (b) for LiNi0.5Mn1.5O4 in PC, EC/DMC, and EC/DEC with 1 M LiPF6, respectively. The CVs of LiNi0.5Mn1.5O4 in the DEMS cell exhibit two pairs of redox peaks, similar to the CV from the coin cell (see Figure S3 in the Supporting Information). They are attributed to the Mn3+/Mn4+ and Ni2+/Ni4+ redox couples, respectively.7,8 In the EC/DEC electrolyte, CO2 starts to form at potentials higher than 4.7 V on the LiNi0.5Mn1.5O4 electrode. Moreover, more CO2 is observed on LiNi0.5Mn1.5O4 in the EC/ DEC electrolyte, compared to LiMn2O4, suggesting that LiNi0.5Mn1.5O4 can catalyze the oxidation of EC/DEC. However, if we use PC or EC/DMC as solvent, the opposite behavior is observed. We can see less evolution of CO2 on LiNi0.5Mn1.5O4 than on LiMn2O4 in the PC electrolyte. In the EC/DMC electrolyte, we did not observe any CO2 formation on LiNi0.5Mn1.5O4 at potentials below 5.0 V. It appears that for
LiNi0.5Mn1.5O4, among the three solvents studied, the EC/ DMC mixture is the most stable solvent at potentials below 5.0 V (vs Li/Li+). In addition, in the EC/DMC electrolyte, the lithiation and delithiation kinetics of LiNi0.5Mn1.5O4 were also the fastest. Similar to the LiMn2O4 electrode, no potentialdependent oxygen evolution was detected for the LiNi0.5Mn1.5O4 electrode, suggesting that CO2 generation can again be described by eq 3. It is known that LiNiO2 has a stronger oxidation ability than LiCoO 2 and LiMn 2 O 4 .10 However, Ni doped LiMn 2 O 4 (LiNi0.5Mn1.5O4) did not exhibit enhenced oxidation activity. In particular, we did not observe CO2 evolution in the EC/ DMC electrolyte on LiNi0.5Mn1.5O4 at potentials up to 5.0 V. This could be due to different structures and thus different electronic properties of LiNi0.5Mn1.5O4 from LiNiO2.
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CONCLUSIONS
In this work, a three-electrode DEMS cell was designed for Liion battery studies. The combination of three cathode materials and three electrolytes were studied with respect to oxidative decomposition to form CO2 and O2, which were monitored by DEMS. The following conclusions can be drawn on the basis of our DEMS data. (1) The electrolyte 1 M LiPF6 + EC/DMC is the most stable with respect to solvent oxidative decomposition for the three cathode materials: LiCoO2, LiMn2O4, and LiNi0.5Mn1.5O4. In particular, for the high voltage cathode material LiNi0.5Mn1.5O4, this electrolyte appears to be stable for potentials up to 5.0 V. (2) LiCoO2 was the most oxidatively active cathode material studied; while EC/DEC was the least stable solvent among the three studied. In 1 M LiPF6 + EC/ DEC electrolyte, large amounts of CO2 and O2 were detected on LiCoO2 at potentials higher than 4.4 V. (3) To unravel the stability of electrolytes, one should use the same working electrode materials as in real Li-ion batteries, since cathodes with different surface properties can exhibit different oxidative activity. D
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ASSOCIATED CONTENT
* Supporting Information S
Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: 1-607-255-4720. Fax: 1-607-255-9864. E-mail: hda1@ cornell.edu. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported as part of the Energy Materials Center at Cornell, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001086. This work was also funded by Subaru Technical Research Center, Fuji Heavy Industries, Ltd.
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