Determining and Minimizing Resistance for Ion Transport at the

4 days ago - In this work, we report methods to quantify and minimize the interfacial resistance for Li ion transport, Rinterface, between a model pol...
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Determining and Minimizing Resistance for Ion Transport at the Polymer/Ceramic Electrolyte Interface ACS Energy Lett. Downloaded from pubs.acs.org by UNIV AUTONOMA DE COAHUILA on 04/18/19. For personal use only.

X. Chelsea Chen,*,† Xiaoming Liu,‡ Amaresh Samuthira Pandian,† Kun Lou,§ Frank M. Delnick,† and Nancy J. Dudney*,† †

Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, United States Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, United States § The Bredesen Center for Interdisciplinary Research and Graduate Education, University of Tennessee, Knoxville, Tennessee 37996, United States ‡

S Supporting Information *

ABSTRACT: In this work, we report methods to quantify and minimize the interfacial resistance for Li ion transport, Rinterface, between a model polymer electrolyte, poly(ethylene oxide) + LiCF3SO3 (PE), and a model Li+conducting ceramic electrolyte, LICGC from Ohara Corporation. By constructing a PE−ceramic−PE trilayer cell, we found Rinterface to be very large, 1.2 kΩ·cm2 at 30 °C, accounting for 66% of the total trilayer cell resistance. When dimethyl carbonate, a loose-binding solvent of Li+, was introduced into the trilayer, Rinterface decreased to essentially zero. As a result, a composite electrolyte with carbonate plasticizers wherein 40 vol % ceramic particles were dispersed in the polymer showed extraordinary room-temperature conductivity of approximately 10−4 S/cm, 3 orders of magnitude higher than that of the dry composite electrolyte. This discovery can be used as guidance in designing composite electrolytes to achieve synergistic effects.

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discovered that Rinterface was subject to processing methods. Recently, Langer et al.14 reported an Rinterface of 9 kΩ·cm2 at 70 °C between a polymer electrolyte P(EO)20-LiClO4 and a garnet ceramic electrolyte Li7La3Zr2O12 with a porous interface. The goal of this work is to quantify Rinterface between polymer and ceramic electrolytes and search for ways to minimize it. We used a model PE consisting of poly(ethylene oxide) (PEO) and LiCF3SO3 (lithium triflate, LiTf) salt with a molar ratio of Li+ to ether oxygen of [Li+]:[EO] = 1:16. A commercial Li+conducting ceramic, LICGC, with a general composition of Li2O−Al2O3−SiO2−P2O5−TiO2−GeO2 from Ohara corporation was used as the model ceramic electrolyte. We investigated the effects of two plasticizers, tetraethylene glycol dimethyl ether (TEGDME) and dimethyl carbonate (DMC) on Rinterface. In order to isolate the effect of the interface on the ion transport properties of the polymer−ceramic composite

he development of a safe electrolyte that can stabilize lithium anodes is the key to enable high-energydensity lithium metal batteries. Solid-state electrolytes are promising candidates owing to their intrinsic nonflammability. However, current solid electrolytes all have serious challenges when used alone: oxide ceramics are brittle, sulfide ceramics are air-sensitive, polymers are too resistive and soft, and many electrolytes react with lithium. Composites provide a clear route to address these issues. One design strategy of the composite electrolyte is to use a polymer electrolyte (PE) matrix with ceramic electrolyte fillers.1−9 Ideally, in such a composite, the ceramic fillers would increase the ionic conductivity and the mechanical modulus, and the polymer host would provide toughness, processability and adhesion. To achieve the required modulus and conductivity, more than 50 vol % ceramic is needed.10 However, our previous experiments indicate that ions transport only through the polymer phase of the composite electrolytes.11 As a result, composites with high loadings of ceramic electrolyte suffer low ionic conductivity.1,8,11 In the past, we investigated the interfacial resistance for ion transport, Rinterface, by using laminated or sputtered bilayer models.12,13 We © XXXX American Chemical Society

Received: March 4, 2019 Accepted: April 12, 2019

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DOI: 10.1021/acsenergylett.9b00495 ACS Energy Lett. 2019, 4, 1080−1085

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Cite This: ACS Energy Lett. 2019, 4, 1080−1085

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ACS Energy Letters

that of the Ohara plate, ROhara, was 27 Ω·cm2. If there were no interfacial resistance between PE and the Ohara ceramic, one would expect the area-specific resistance of the trilayer, RTrilayer‑dry, to be equal to ROhara + RPE‑dry = 1171 Ω. However, our experimental data show that RTrilayer‑dry is 3503 Ω·cm2, a factor of 3 larger than ROhara + RPE‑dry. This suggests that interfacial resistance makes a major contribution to RTrilayer‑dry. The Nyquist plots of PE, the Ohara plate, and the trilayer cell at 70 °C, above the melting temperature of PEO, are shown in Figure 2c. At this temperature the samples exhibit only a capacitive tail. The impedance response can be fitted with the serial RC circuit model, as shown in the inset of Figure 2c.15 At 70 °C, RPE‑dry = 17 Ω·cm2, ROhara‑dry = 8 Ω·cm2, and RTrilayer‑dry = 109 Ω·cm2, which is 4 times larger than ROhara + RPE‑dry = 25 Ω. This indicates that above the melting temperature of PE, when ion transport is much faster, interfacial resistance remains a major contributor to RTrilayer‑dry. We obtained RPE‑dry, ROhara, and RTrilayer‑dry as a function of inverse temperature, 1/T, shown in Figure 2d,e. There is a change in the activation energy in RPE‑dry below 60 °C due to the crystallization of PEO (Figure 2d). Figure 2e clearly shows that in the temperature range of 20−80 °C, RTrilayer‑dry > ROhara + RPE‑dry. The interfacial resistance Rinterface can be quantified by the following equation: Rinterface = (1/2)[RTrilayer‑dry − (ROhara + RPE‑dry)]. An alternative method of extracting Rinterface by fitting the impedance spectra of the trilayer cell with an unknown interface element and the known properties of PE and Ohara single layers produced very similar results.12 The temperature dependence of Rinterface is shown in Figure 2f. At 30 °C, Rinterface = 1166 Ω·cm2. The interfacial resistance accounts for 66% of the trilayer cell resistance. At 70 °C, Rinterface = 42 Ω·cm2, accounting for 77% of the trilayer cell resistance. Rinterface follows similar temperature dependence as RPE as a transition in slope was observed below 60 °C. A common strategy to enhance the conductivity of PEs is by adding plasticizers.16−21 Both glyme and carbonate solvents are common plasticizers used to enhance the conductivity of PEs. We investigated the effects of two plasticizers on Rinterface: a glyme solvent, TEGDME, and a carbonate solvent, DMC. The results pertaining to TEGDME are shown in Figures S1 and S2. The impedance responses of TEGDME-plasticized PE and trilayer samples were very similar to those without TEGDME (Figure S1). Compared to RPE‑dry, the area-specific resistance of TEGDME-plasticized PE, RPE‑TEGDME, decreased by 50%. However, the area-specific resistance of TEGDME-plasticized trilayer, RTrilayer‑TEGDME, is still much larger than ROhara + RPE‑TEGDME, resulting in a large Rinterface (Figure S2). These results revealed that TEGDME as a plasticizer did not effectively decrease the PE−ceramic interfacial resistance. Contrary to TEGDME, DMC strongly affected Rinterface (Figure 3). We note here that DMC has a vapor pressure of 364 mmHg at 25 °C and a boiling point of 90 °C. Adding DMC into a solution of PEO and LiTf followed by spray coating and drying will lead to complete evaporation of DMC. Instead, DMC was introduced into PE via vapor absorption, after PE was thoroughly dried. Vapor absorption, unlike dispensing liquid DMC directly onto PE, prevents solvent from seeping around the edge of the trilayer cell and provides consistent solvent uptake. Details of the sample preparation are described in the Supporting Information. TGA results showed that the amount of DMC infused into the samples was approximately 5.4 wt % (Figure S3). At 30 °C (Figure 3a), we obtained RPE‑DMC = 329 Ω·cm2, a factor of 3 smaller than

electrolyte, we designed a trilayer cell in which a piece of tapecast and polished Ohara ceramic plate was sandwiched by two thin layers of PE (Figure 1a). The thickness of the Ohara

Figure 1. (a) Schematic of the trilayer cell. Polymer electrolyte PEO + LiTf with 10 μm thickness was spray-coated on each side of the Ohara ceramic plate (155 μm in thickness). The area of the PE was defined by a PTFE washer. Cu was used as the blocking electrodes. The schematic is not drawn to scale. (b) SEM image showing that the PE was homogeneously coated on the Ohara ceramic surface.

ceramic plate was 155 μm. The thickness of each PE layer was approximately 10 μm, deposited by spray coating. The details of the spray coating process are described in the Supporting Information. Briefly, a small quantity of PE solution in acetonitrile was delivered through the spray nozzle of an airbrush onto the ceramic surface, forming a very thin layer. This was followed by drying with a laboratory heat gun at a low temperature setting for 1 min. The spraying/drying process was repeated until 10 μm of thickness was reached. The area of the spray-coated layer was defined by a custom-made PTFE washer. The spray coating method created a PE layer with very good thickness control, excellent uniformity, and good contact with the substrate. A cross-sectional SEM image of the PE− ceramic interface is shown in Figure 1b. PE formed a very homogeneous coating on the Ohara ceramic surface. Copper was used as blocking electrodes for the trilayer cell. A thickness of 10 μm is ideal for PE. If too thin of a PE layer (100 μm), it would account for more than 70% of the total trilayer cell resistance, making the deconvolution of Rinterface difficult. The trilayer cell as well as its individual layers of PE and the Ohara plate were investigated by AC impedance spectroscopy, shown by Figure 2a−c. Note that the thickness of PE is equal to the combined thickness of the two PE layers in the trilayer cell. At 30 °C, PE, Ohara, and the trilayer cell showed a semicircle in the high-frequency region and a linear tail in the low-frequency region (Figure 2a,b). An equivalent circuit model was used for fitting the impedance data to extract the resistance of each sample (inset Figure 2b).15 The area-specific resistance of PE in the dry state, RPE‑dry, was 1144 Ω·cm2 and 1081

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Figure 2. (a) Nyquist plots of the Ohara ceramic plate (black squares), PE (blue circles), and trilayer cell (red triangles) at 30 °C. The highfrequency region of the Nyquist plot of the Ohara ceramic plate is shown in (b). (b, inset) Equivalent circuit model to extrapolate the resistance at 30 °C. (c) Nyquist plots of the Ohara ceramic plate, PE, and trilayer at 70 °C. (c, inset) Equivalent circuit model to extrapolate the resistance at 70 °C. The numbers on the Nyquist plots indicate the log of frequency. For example, 3 means a frequency of 1 × 103 Hz. (d) Arrhenius plots of ROhara (black diamonds) and RPE‑dry (blue circles). (e) Comparison of RTrilayer‑dry (red triangles) with ROhara + RPE‑dry (black squares). (f) Arrhenius plot of Rinterface between dry PE and the Ohara ceramic.

Figure 3. (a,b) Nyquist plots of the Ohara ceramic plate (black squares), DMC-plasticized PE (blue circles), and DMC-plasticized trilayer (red triangles): (a) at 30 °C and (b) at 70 °C. The numbers on the Nyquist plots indicate the log of frequency. (c) Arrhenius plots of ROhara (black diamonds) and RPE‑DMC (blue circles). (d) Comparison of RTrilayer‑DMC (red triangles) and ROhara + RPE‑DMC (black squares). (e) Temperature dependence of Rinterface between PE-DMC and the Ohara ceramic. 1082

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ACS Energy Letters RPE‑dry. Amazingly, RTrilayer‑DMC = 265 Ω·cm2, a more than 1 order of magnitude decrease from RTrilayer‑dry of 3503 Ω·cm2. At 70 °C (Figure 3b), RPE‑DMC = 17 Ω·cm2, similar to RPE‑dry. RTrilayer‑DMC = 30 Ω·cm2, a factor of 3 smaller than RTrilayer‑dry = 109 Ω·cm2. The Arrhenius plot of RPE‑DMC is shown in Figure 3c. A transition in slope below 60 °C is observed, again due to crystallization of PEO. At all temperatures measured, RTrilayer‑DMC ≈ ROhara + RPE‑DMC. This leads to Rinterface(DMC) ≈ 0 (Figure 3d,e). With the presence of DMC, the interfacial resistance was brought down to below the resolution of our method. The area-specific resistance of trilayers, R Trilayer‑dry , RTrilayer‑TEGDME, and RTrilayer‑DMC, are summarized in Figure 4a. TEGDME did not significantly decrease R interface .

magnitude decrease in the trilayer resistance. This reduction is a result of reduced resistance in the PE layers but is more importantly due to significantly reduced interfacial resistance. We can use this discovery to design composite electrolytes with minimized interfacial resistance. We made a composite electrolyte consisting of PEO, LiTf, Ohara ceramic powder (58 wt %, nominal 40 vol %), and a mixture of 3:7 w/w EC:DMC plasticizers ([EO]:[Li+]:[DMC] = 16:1:4). The composite showed extraordinary ionic conductivity (Figure 4b). The room-temperature conductivity was close to 10−4 S/cm, 3 orders of magnitude higher than that of the dry composite electrolyte and comparable to that of the pure Ohara ceramic. In the last part of the discussion, we want to gain some insight into DMC’s efficacy in decreasing the interfacial resistance. We first examined the activation energy, Ea, of the trilayer cell, the individual layers, and the interface, summarized in Table 1. We divided Ea values into two groups, below and above the melting point, Tm, of PEO. In the semicrystalline state (below Tm), the Arrhenius equation R = Ro exp(Ea/kT) was used to calculate Ea,23 where Ro is the preexponential factor, k is the Boltzmann constant (8.617 × 10−5 eV K−1), and T is the temperature of measurement. In the melt state, the Vogel−Tammann−Fulcher (VTF) equation is typically used to calculate Ea. However, in the case of the PEO-LiTf system, it has been reported that above 60 °C it has an apparent Arrhenius behavior rather than VTF behavior due to the presence of an additional crystalline phase (PEO)3LiTf.23,24 Therefore, we used the Arrhenius equation to fit the data above Tm. The activation energy of Ohara, Ea‑Ohara, is 0.39 eV in the entire measured temperature range as it does not go through a melting transition. In the dry condition, the activation energies of PE, the trilayer, and the interface, Ea‑PE, Ea‑Trilayer, and Ea‑interface, were 0.83, 0.65, and 0.81 eV, respectively. The value of Ea‑interface is very close Ea‑PE and much larger than Ea‑Ohara. This suggests that Li+ transport at the interface is controlled by Li+ dissolution from the polymer, not the ceramic. This is consistent with studies on interfacial ion transport between the liquid electrolyte and ceramic electrolyte.25,26 DMC significantly decreased Ea‑PE and Ea‑Trilayer below Tm. Due to a nearzero Rinterface, we could not obtain Ea‑interface with the presence of DMC through the Arrhenius relationship. Above Tm and below 80 °C, Ea‑PE and Ea‑Trilayer were similar to Ea‑Ohara. In this temperature range, it is more difficult to determine the ratelimiting step for interfacial ion transport. We then examined the structure of PE and PE + DMC with infrared (IR) spectroscopy (Figure 4c,d). In the dry polymer electrolyte PEO + LiTf (blue line in Figure 4c), a single band at 760 cm−1 is observed in the spectral region of 700−800 cm−1. This band arises from the symmetrical bending mode of CF3 (δs(CF3)) in the triflate anion. The peak position of 760 cm−1 suggests ionically associated CF3 ions.27 In some reports, this peak was assigned to δs(CF3) in the crystalline compound (PEO)3LiCF3SO3.28,29 With the addition of DMC, we did not

Figure 4. (a) Comparison of RTrilayer‑dry (black squares) with RTrilayer‑TEGDME (blue triangles) and RTrilayer‑DMC (red circles). (b) Arrhenius plots of the ionic conductivity of the Ohara ceramic plate (black stars), dry composite electrolytes (black squares), TEGDME-plasticized composite electrolytes (blue triangles), and EC:DMC-plasticized composite electrolytes (red circles). All of the composite electrolytes contained 58 wt % (40 vol %) Ohara ceramic powder. (c,d) IR spectra of DMC, PEO, PEO + LiTf, PEO + LiTf + DMC, and PEO + DMC: (c) between 800 and 700 cm−1 and (d) between 1800 and 1700 cm−1.

TEGDME, similar to PEO, dissolves Li+ through coordination between EO groups and Li+. Both TEGDME and PEO are tight-binding solvents of Li+.22 On the other hand, the presence of the carbonate solvent DMC led to a 1 order of

Table 1. Activation Energy of Ohara, PE, Trilayer Cells and the Interface Tm

condition

Ea‑PE (eV)

Ea‑Trilayer (eV)

Ea‑interface (eV)

Ea‑PE (eV)

Ea‑Trilayer (eV)

Ea‑interface (eV)

Ea‑Ohara (eV)

dry TEGDME DMC

0.83 0.67 0.46

0.65 0.59 0.46

0.81 0.57 na

0.37 0.21 0.37

0.32 0.37 0.46

0.36 0.42 na

0.39 0.39 0.39

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below the experimental resolution. This discovery can be used as guidance in designing composite electrolytes to achieve synergistic effects.

observe a change in this band (purple line in Figure 4c). This is not surprising as DMC is not capable of solvating large anions like the triflate.30 From IR, we can also obtain information on the molecular structure of DMC by examining the stretching vibration band of CO. In neat DMC, this band was observed at 1750 cm−1 (Figure 4d). In samples of PEO + DMC and PEO + LiTf + DMC, this band remained the same. Similarly, the OCO2 rocking mode δ(OCO2) observed at 793 cm−1 in neat DMC (Figure 4c) remained the same in PEO + DMC and PEO + LiTf + DMC samples. These results suggest that DMC did not coordinate with LiTf salt in an ocean of PEO chains. This is consistent with literature reports.22,28,29,31,32 Generally, a carbonate group forms looser bonding to Li+ compared to an EO group.33−38 Recently, we performed quasi-elastic neutron scattering measurements on PE/Ohara ceramic composites and discovered that the segmental mobility of PE confined by the Ohara ceramic surface was significantly slowed.39 This finding suggests that the ion transport rate in PE near the Ohara ceramic surface is much slower compared to that in bulk PE. Without the presence of plasticizers, the slow layer near the interface contributes to the large Rinterface. The addition of DMC molecules may positively affect this slow layer near the interface. We will investigate the effect of plasticizers on the structure of thin film PE confined by a ceramic surface in a follow-up study. To this end, our study suggests that adding a loose-binding Li+ solvent such as DMC into the tight-binding solvent (PEO) greatly decreases the interfacial resistance for Li+ transport across the polymer−ceramic interface. We believe that DMC may have several effects: first, DMC greatly reduces Ea in PE below Tm. This means that the rate-limiting step for interfacial ion transport is faster in the presence of DMC. Second, DMC may increase the segmental mobility of the slow PE layer near the ceramic interface, which facilitates Li+ transport. Third, having a loose-binding Li+ solvent like DMC near the ceramic interface may facilitate Li+ dissolution from PEO. It is worth mentioning that the dielectric constant of DMC at 298 K is 3.2,36 smaller than that of TEGDME of 7.8.37 Therefore, a high dielectric constant is not a requirement for decreasing interfacial resistance. Our past work indicated that interfacial resistance is subject to processing methods. In this work, samples containing TEGDME and DMC were prepared with different methods due to their drastically different physical properties. The microscopic spatial distribution of DMC molecules in the PE layer may be different from that of TEGDME due to different processing methods, which could in turn affect interfacial properties. We also note that our preliminary data showed that DMC also reduced the interfacial resistance between PE and a garnet ceramic electrolyte, Li7La3Zr2O12(LLZO). In summary, we quantified the interfacial resistance for Li ion transport between a model polymer electrolyte PEO + LiTf and a model ceramic electrolyte LICGC from Ohara corporation. By constructing a PE−ceramic−PE trilayer cell and comparing the AC impedance spectra of the trilayer cell with its individual components, a large interfacial resistance of 1.2 kΩ·cm2 at 30 °C was identified. The addition of a tightbinding solvent of Li+, TEGDME, into the trilayer did not effectively decrease the interfacial resistance. When a loosebinding solvent of Li+, DMC, was introduced into the trilayer, the interfacial resistance was decreased to essentially zero,

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.9b00495. Experimental section; Nyquist plots of the TEGDMEplasticized PE and trilayer; and interfacial resistance of the TEGDME-plasticized trilayer (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected] ORCID

X. Chelsea Chen: 0000-0003-1188-7658 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE), under Contract DE-AC05-00OR22725, was sponsored by the Office of Energy Efficiency and Renewable Energy for the Vehicle Technologies Office’s Advanced Battery Materials Research program. SEM experiment was conducted using resources of the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. IR experiments were conducted by K. Lou. We thank Dr. Robert L. Sacci for discussion on the activation energy.

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DOI: 10.1021/acsenergylett.9b00495 ACS Energy Lett. 2019, 4, 1080−1085