Effect of Nanoparticle Shape on the Electrical and Thermal Properties

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Effect of Nanoparticle Shape on the Electrical and Thermal Properties of Solid Polymer Electrolytes Sandhya Vasudevan, and Susan K. Fullerton-Shirey J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08029 • Publication Date (Web): 11 Apr 2019 Downloaded from http://pubs.acs.org on April 11, 2019

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The Journal of Physical Chemistry

Effect of Nanoparticle Shape on the Electrical and Thermal Properties of Solid Polymer Electrolytes Sandhya Vasudevan† and Susan K. Fullerton-Shirey∗,†,‡ †Department of Electrical Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States ‡Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States E-mail: [email protected] Abstract The effect of nanoparticle shape on the ionic conductivity and thermal properties of solid polymer electrolytes is reported. Polyethylene oxide (PEO) and LiClO4 with an ether oxygen to Li+ ratio of 10:1 is filled with TiO2 nanorods and elliptical-shaped fillers in the concentration ranges of 1-2.5 and 1-10 wt.%, respectively. The filler concentration that provides the maximum improvement in the ionic conductivity scales inversely with aspect ratio (AR): 10 wt.% for spherical nanoparticles (AR = 1), 5 wt.% for ellipses (AR ≈ 4.4), and 1-2 wt.% for nanorods (AR ≈ 6.6). The same trend is true for Fe2 O3 nanofillers, suggesting that the observation is independent of the chemical identity of the metal oxide filler. While the filler concentration required to maximize the conductivity improvement is correlated to the nanofiller shape, the magnitude of the improvement is correlated to the surface-to-volume ratio of the filler. Specifically, the conductivity increases one order of magnitude by increasing the surface-to-volume

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ratio by 5x, illustrating the critical role of the filler surface and suggesting that ion transport is favored at the electrolyte-filler interface over the bulk. Thermal measurements indicate an unexpected relationship between crystallinity and conductivity: the room-temperature conductivity of the electrolyte with 2 wt.% TiO2 nanorods is one order of magnitude higher than with 5 wt.% elliptical TiO2 , even though the nanorod sample is semi-crystalline while the elliptical sample is amorphous. The results uncover shape-property relationships that can serve as design rules for solid polymer electrolytes filled with metal oxide nanofillers.

Introduction Solid polymer electrolytes (SPEs) have been studied as a potential replacement for liquid electrolytes in rechargeable lithium ion batteries. The solid polymer inhibits dendrite formation thereby improving chemical stability, 1,2 and the absence of flammable plasticizers improves safety, 2 and end-of-life disposal. 1 The solid-state nature of the SPE may also enable lighter and more flexible batteries because rigid casings would not be required. 1 Despite the predicted advantages, the room-temperature conductivity of SPEs is three to four orders smaller than the 10−3 S/cm required to power a portable device. 3,4 Polyethylene oxide (PEO) is often chosen for SPEs because it has a flexible backbone which permits polymer reorganization and hence ion mobility in the amorphous phase. 5 However, PEO will crystallize at room temperature 6–8 and pure crystalline PEO will inhibit ion transport. 9 Additional crystal phases can form depending on the salt concentration and temperature. According to the phase diagram for PEO:LiClO4 , three crystalline phases can form: pure PEO, (PEO)6 :LiClO4 , and (PEO)3 :LiClO4 . 6 In contrast to pure PEO, the (PEO)6 complex has been shown to enhance Li+ transport. 10 Adding plasticizers increases ionic conductivity by reducing crystallinity and increasing polymer mobility; 11–13 however, the improvement comes at the expense of mechanical properties. 14 In contrast, the addition of solid, metal oxide fillers both increases the ionic 2


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conductivity 15–18 and the mechanical properties of SPEs. 3 The conductivity improvement with fillers has been attributed to several mechanisms. One mechanism pertains to filler surface chemistry. Croce and co-workers showed that hydroxyl termination (i.e., acidic surface chemistry) of alumina (Al2 O3 ) nanofillers promotes ion transport while basic surface chemistry has no effect. 19 Another mechanism involves using nanofillers to stabilize highly conductive (PEO)6 :LiClO4 channels, 20 allowing them to persist longer to enhance ion transport. A third mechanisms involves the organization of nanofillers over large lengthscales. If the interface between the nanofiller and the electrolyte favors ion mobility, then longrange, conductive pathways (i.e., percolating pathways) could provide paths for enhanced ion transport compared to the bulk. 21–23 Because the percolation threshold varies inversely with filler aspect ratio, 24 we would expect high-aspect-ratio nanorods (NRs) to attain percolation at lower concentrations compared to, for example, elliptical or spherical nanoparticles (NPs). The impact of Fe2 O3 nanofillers with different aspect ratios was previously reported by our group, where we showed that the maximum conductivity improvement occurred at an nanorod concentration ten times lower than the spherical concentration. 21 However, it remains unclear whether or not the aspect ratio dependence is universal for metal oxides, or unique to Fe2 O3 fillers. Here, we seek to determine whether or not aspect ratio affects ion transport in a series of TiO2 nanofillers with various aspect ratios. Specifically, elliptical α−TiO2 nanofillers with an aspect ratio of ≈ 4.4, and NR α−TiO2 fillers with an aspect ratio of ≈ 6.6 fillers were added to PEO and LiClO4 at the eutectic concentration (ether oxygen:Li=10:1). The data are compared to our previous results for α−Fe2 O3 NPs with aspect ratio = 1, and α−Fe2 O3 NRs with aspect ratio ≈ 6.6. 21 We chose α−phase fillers because they provide acidic surface chemistry that is expected to enhance ion mobility. 19 We find that the TiO2 filler concentration required to improve conductivity decreases with increasing aspect ratio - the same as observed for Fe2 O3 -filled SPEs, showing that this dependence is not unique to one type of metal oxide but could be a universal shape

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effect. In addition, we show that while nanofiller shape controls the amount of filler required to boost the conductivity, the surface-to-volume ratio ( VS ) controls the magnitude of the conductivity improvement. That is, a smaller concentration of higher aspect-ratio fillers can be used to achieve a specific conductivity boost, while the magnitude of the boost can be controlled by

S . V

The results show how SPE performance relates to nanofiller shape and size

- information that could potentially be used for rational design of filler materials to enhance room-temperature conductivity.

Experimental Details Sample Preparation TiO2 NRs were synthesized at the Molecular Foundry at Lawrence Berkeley National Laboratory (LBNL) following a procedure reported by Cozzoli and co-workers. 25 The dimensions of the NRs has been reported previously as 20 x 3 nm, 26 and are confirmed for this study by SEM (Figure S1a) and transmission electron microscopy (TEM) (Figure S1b). TiO2 elliptical nanofillers were synthesized following a procedure described by Nian and Teng. 27 All materials used in this study were purchased from Sigma Aldrich. 5 g P25 TiO2 powder (Aeroxide) was mixed with 70 mL 10 N NaOH solution in a Teflon-lined autoclave and heated in an oven at 130 ◦ C for 20 hours. The filtered samples were washed with 0. 1 N HNO3 until the pH equaled 5.6. This mixture was autoclaved at 175 ◦ C for 48 hours, filtered and dried at 100 ◦ C for 3 hours. The elliptical nanofillers are sized as 17 ± 5 x 76 ± 27 nm by averaging the dimensions of 100 particles from scanning electron micrographs (SEM), shown in the Supplementary Information in Figure S1c and d. Both the elliptical and NR TiO2 fillers are bare; that is, they are not capped with organic molecules that could interfere with the interactions between the electrolyte and the filler surface. PEO (Molecular weight 600,000 g/mol) and LiClO4 were dissolved in anhydrous acetonitrile with an ether oxygen to lithium ratio of 10:1. The nanofillers were added to the 4


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PEO/LiClO4 /acetonitrile mixture in the desired concentrations. The solvent was evaporated at room temperature while undergoing sonication to avoid particle sedimentation. The samples were dried in a vacuum oven at 80 ◦ C for more than one week to remove solvent and water, then moved to an argon-filled glovebox for storage (water < 0.1 ppm). For each nanofiller concentration, two SPE samples were prepared and measured.

Conductivity Measurement Impedance measurements were made on a Cascade Microtech Summit 11861 probe station using an Agilent 4294A Precision Impedance Analyzer in the frequency range of 40 Hz - 110 MHz with a 500 mV AC bias. The sample was prepared in a parallel-plate geometry where approximately 30 mg of the SPE was hot pressed at 100 ◦ C between two polished stainless steel blocking electrodes (diameter = 1.26 cm) inside the Argon-filled glovebox. The SPE was pressed until the thickness was reduced to ≈ 100 µm and SPE diameter equaled that of the electrode. All samples were heated to 100 ◦ C for 2 minutes on a hot plate inside the glovebox to ensure that every sample shared the same thermal history. Data were collected at five temperatures between 22 - 100 ◦ C, and the temperature was controlled to within ± 0.1 ◦ C using a Temptronic TP03000A−X300 Thermo Chuck system. A 10 minute hold time at each temperature was used before the measurement to ensure thermal equilibrium. Measurements were performed under N2 environment. The conductivity was calculated using equation (1)

σ(f ) =

dcosφ(f ) AZ0 (f )

(1)

where f is the frequency, d is the distance between the electrodes separated by the SPE, A is the area of the electrodes, Z is the magnitude of the measured impedance, and φ is the phase angle of the impedance.

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Differential Scanning Calorimetry The melting (Tm ) and glass transition temperatures (Tg ) were measured using differential scanning calorimetry (DSC). 9−10 mg of each sample was hermetically sealed in Aluminum DSC pans inside an Argon-filled glovebox and measured under Nitrogen in a Mettler Toledo DSC. The instrument has a resolution of ± 0.02 K and a sensitivity of 0.04 mW and is calibrated by an indium standard. A heat/cool/heat cycle was performed on each sample, where the first heat cycle is used to remove thermal history. The 5 steps of the heat/cool/heat cycle were: 1) heat from 22 to 100 ◦ C at 5 ◦ C/min, 2) cool from 100 to 10 ◦ C at 5 ◦ C/min, 3) cool from 10 to -70 ◦ C at 2 ◦ C/min, 4) head from -70 to 10 ◦ C at 2 ◦ C/min and 5) heat from 10 to 100 ◦ C at 5 ◦ C/min. A slower sweep rate of 2 ◦ C/min is used at low temperature to improve the resolution of the Tg . Because thermal history affects the structure and therefore the conductivity of the sample, a second type of DSC measurement was made where the samples were treated in an identical manner to the thermal treatment described for the conductivity measurements. That is, each sample was preheated to 100 ◦ C for 2 mins, and equilibrated at 23 ◦ C for 10 mins. A heat cycle from 23 ◦ C to 100 ◦ C was then performed at a heating rate of 5 ◦ C/min. Note that because the perfect heat of fusion is unknown at this ion concentration, the crystal fraction cannot be quantified using DSC. Instead, another technique, such as small-angle neutron scattering (SANS), would be required. 9

Results and discussion Impedance, phase angle and conductivity data for the unfilled SPE (PEO10 :LiClO4 ) are provided as a function of frequency and temperature in Figure 1. The AC conductivity corresponds to the region of the data where the conductivity is frequency independent.

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Figure 1: (a) Magnitude of the impedance, |Z|, (b) phase angle, θ, and (c) ionic conductivity, σ, versus frequency for PEO10 :LiClO4 at five temperatures in the range of 21 to 100 ◦ C. The AC conductivity is calculated using equation 1. The thickness of the sample is 100 µm and the diameter is 1.26 cm.

The impedance data for the samples filled with nanofillers is similar to Figure 1, and a summary of the temperature-dependent conductivity is shown in Figure 2 for SPEs filled with (a) TiO2 elliptical particles and (b) TiO2 NRs in the range of 22 to 100◦ C. Regardless of filler shape, TiO2 increases the conductivity at all filler concentrations and temperatures. However, TiO2 NRs give the largest conductivity improvement at all measured temperatures. Because the surface chemistries are the same, this result highlights the role of filler shape on the the enhancement of the conductivity.

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Figure 2: Temperature-dependent ionic conductivity for PEO10 LiClO4 SPEs filled with (a) TiO2 ellipse and (b) TiO2 NRs. The symbols represent the average of two measurements on two samples, and the error bars represent the largest and smallest values.

The conductivity data in Figure 2 exhibits two regimes, one where the conductivity depends more strongly on the temperature (T < 50 ◦ C) than the other (T > 50 ◦ C). The change in temperature dependence occurs at the melting point of the SPE, 20 and can be quantified by calculating the activation energies (Ea ) in each regime (Table S1). The activation energies range from 0.4 - 1.1 eV, in agreement with previous reports for PEO-based electrolytes. 28,29 The energies reflect the energy barrier encountered by the cations and anions when moving through the PEO. Because SPEs are semi-crystalline at T

Effect of Nanoparticle Shape on the Electrical and Thermal Properties

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