Ionic Conduction in Composite Polymer Electrolytes: Case of PEO:Ga

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Ionic Conduction in Composite Polymer Electrolytes: Case of PEO:Ga-LLZO Composites Zhuo Li, HeMing Huang, Jiakun Zhu, Jianfang Wu, Hui Yang, Lu Wei, and Xin Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17279 • Publication Date (Web): 10 Dec 2018 Downloaded from http://pubs.acs.org on December 13, 2018

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ACS Applied Materials & Interfaces

Ionic Conduction in Composite Polymer Electrolytes: Case of PEO:Ga-LLZO Composites Zhuo Li#,1, He-Ming Huang#,1, Jia-Kun Zhu2, Jian-Fang Wu1, Hui Yang*,2, Lu Wei*,1, and Xin Guo*,1 1 School of Materials Science and Engineering, Huazhong University of science and Technology, Wuhan 430074, P.R. China. 2 Department of Mechanics, Huazhong University of science and Technology, Wuhan 430074, P.R. China # Z. Li and H.-M. Huang contributed equally to this work. KEYWORDS: composite polymer electrolyte, ionic conductivity, space charge region, phasefield simulation, percolation, Monte Carlo simulation

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ABSTRACT: By dispersing Li6.25Ga0.25La3Zr2O12 (Ga-LLZO) nanoparticles in polyethylene oxide (PEO) matrix, PEO:Ga-LLZO composite polymer electrolytes are synthesized. The PEO: Ga-LLZO composite with 16 vol.% Ga-LLZO nanoparticles shows a conductivity of 7.2×10-5 S cm-1 at 30 °C, about 4 orders of magnitude higher than the conductivity of PEO. The enhancement of the ionic conductivity is closely related to the space charge region (3 nm) formed at the interface between the PEO matrix and the Ga-LLZO nanoparticles. The space charge region is observed by transmission electron microscope (TEM) and corroborated by the phase-field simulation. Using the random resistor model, the lithium-ion transport in the composite polymer electrolyte is simulated by the Monte Carlo simulation, demonstrating that the enhanced ionic conductivity can be ascribed to the ionic conduction in the space charge regions and the percolation of the space charge regions.


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

INTRODUCTION Rechargeable lithium-ion batteries with increased energy density are widely used in electric

vehicles and portable electronic devices.1 However, commercial lithium-ion batteries suffer from a major safety problem due to the flammability of organic liquid electrolytes.2 All-solid-state batteries, in which solid electrolytes are used, are considered to be an effective solution to the safety problem. In addition to the better safety, all-solid-state batteries also possess a high energy density, long cycling life and good reliability.3-5 The commonly used solid electrolytes in all-solid-state batteries are ceramic and polymer electrolytes. Ceramic electrolytes, such as Li0.33La0.557TiO3 (LLTO),6 Li7La3Zr2O12 (LLZO)7-9 Li1.3Al0.3Ti1.7(PO4)3 (LATP),10 and Li10GeP2S12 (LGPS),11 usually have high lithium-ion conductivities (higher than 10-3 S cm-1 at room temperature),12 wide electrochemical window (> 5 V),8,13,14 and good thermal stability (ceramics can withstand temperatures > 1000 C). However, they are brittle, and the huge interface impedance between the electrolyte and electrodes is also a major disadvantage.15-17 In contrast, polymer electrolytes, such as polyethylene oxide (PEO),18,19 polyacrylonitrile (PAN),20 poly (propylene carbonate) (PPC),21 and poly (vinylene carbonate) (PVCA),22 are flexible and lightweight, and the interface resistance can be comparatively small.23,24 Unfortunately, the lithium-ion conductivity of polymer electrolytes is usually low (e.g. 10-6﹣10-8 S cm-1 at room temperature),25 the ionic transference number is small (< 0.5),23 and their thermal and electrochemical stabilities are also poor.26 To develop composites of ceramic and polymer electrolytes, i.e. composite polymer electrolytes, is an effective strategy to address the above issues; combining two kinds of electrolytes can mitigate the disadvantages, while without compromising the advantages of the 3 ACS Paragon Plus Environment


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respective materials.27 Compared with the polymer electrolyte, the composite polymer electrolyte has a significantly higher ionic conductivity, ionic transference number, and thermal and electrochemical stability, while the flexibility is superior to pure ceramic systems as well.28,29 In the composite polymer electrolyte, ceramic electrolyte nanoparticles are dispersed in the polymer matrix; the commonly used ceramic fillers are the nanoparticles of LLZO,19,29-31 LLTO,20,32,33 and LATP,34 while the polymer matrices mainly include PEO,29,30,35 PAN20,36 and PVDF.37 Among composite polymer electrolytes, the PEO-based composite has attracted the most interests due to their low cost, reasonable mechanical stability, good compatibility with electrodes, and excellent film-forming capability.38 However, the mechanistic understanding of the enhancement of the ionic conductivity is still lacking, impeding the optimization and targeted design of the composite polymer electrolyte. It is suggested that the percolation effect is an important factor in enhancing the ionic conductivity. Zhao et al. suggested that the lithium-ion conduction is due to the percolation effect in the PEOLLZO electrolyte (Li-salt free), when the size of the LLZO fillers decreases to nanoscale.39 Liu et al. also suggested that the percolation effect may make an important contribution to the lithiumion conductivity in the PAN-LLTO composite.20 However, how the percolation effect influences the lithium-ion transport in the composite polymer electrolyte remains unclear. In this work, we use the PEO:Ga-LLZO composite as a model system for the composite polymer electrolyte, and develop a conduction model based on the two-phase mixture theory. Using the phase-field simulation, the random resistor model and the Monte Carlo simulation, our study reveals that the space charge regions at the interfaces of the PEO matrix and the Ga-LLZO nanoparticles are the fast conduction pathway for lithium ions, and the percolation of the space


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charge regions can account for the enhanced lithium-ion conductivity in the composite polymer electrolyte. 

EXPERIMENTAL SECTION

1. Preparation of Materials Li6.25Ga0.25La3Zr2O12 (Ga-LLZO) nanoparticles were synthesized by the Pechini method. In a typical synthesis process, sol was prepared by firstly dissolving LiNO3, La(NO3)3·6H2O, ZrOCl2·8H2O, Ga(NO3)3·9H2O into a solution containing ethylene glycol and citric acid. After that, the solution was stirred intensively and evaporating-dried evenly to obtain gel, which was then calcined at 650 C in air. The Ga-LLZO nanoparticles thus obtained were transferred immediately into an Ar-filled glovebox (with H2O and O2 contents below 0.1 ppm). PEO (Mv = 600,000 g mol-1, Aladdin) was dried at 60 C for 24 h in vacuum. To avoid moisture and oxygen, all procedures were operated in Ar. PEO:Ga-LLZO composites were prepared by dissolving PEO into anhydrous acetonitrile, then Ga-LLZO nanoparticles were added into the solution, the mixture was intensively stirred for 24 h. The obtained homogenous slurry was then cast onto a Teflon substrate and dried in vacuum overnight at 60 C. 2. Sample characterizations The crystalline structures of the Ga-LLZO nanoparticles and the PEO:Ga-LLZO composite were characterized by X-ray diffraction (XRD-7000S, Shimadzu). The size distribution and microstructure of the Ga-LLZO nanoparticles were determined using transmission electron microscope (TEM, JEM-2100, JEOL). Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) (STA449F3, Netzsch) were used to determinate the thermal stability 5 ACS Paragon Plus Environment


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of the PEO:Ga-LLZO composite. The sample was heated from 40 to 500 C in air at a heating rate of 10 C min-1. 3. Electrochemical measurements The electrical conductivity of the PEO:Ga-LLZO composite was measured by an AC impedance spectrometer (1260 impedance/ gain-phase analyzer, Solartron) in the frequency range of 1 Hz to 5 MHz and the temperature range of 0 to 80 C. The sample dimension is 10 mm in diameter and 50 m in thickness. The ionic transference number ( TLi  ) was measured by the AC impedance spectroscopy and the DC polarization using Li/PEO:Ga-LLZO composite/Li symmetric cells. The applied polarization voltage was 10 mV. The polarization experiments were operated with an electrochemical workstation (Interface-1000E, Gamry). Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) were conducted with the electrochemical workstation (Interface-1000E, Gamry) using Li/composite/stainless steel cells at a scanning rate of 10 mV s-1. The scanning range was from 2.6 to 6.0 V for LSV and 0.5 to 6.0 V for CV. The PEO:16 vol.% Ga-LLZO composite was applied in coin cells (CR2016) of LiFePO4/electrolyte/Li, and the battery performance was evaluated. 

RESULTS AND DISCUSSIONS In view of the high lithium-ion conductivity and chemical/electrochemical stability, cubic

LLZO is one of the most promising ceramic electrolytes for all-solid-state batteries.40 According to our previous work, the ionic conductivity of LLZO can be remarkably enhanced by Ga doping, namely, the ionic conductivity of Ga-LLZO reaches 1.4610-3 S cm-1 at room temperature,8 we synthesized Ga-LLZO

nanoparticles in this work for our PEO:Ga-LLZO composites. The 6 ACS Paragon Plus Environment

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crystalline structure and size distribution of the Ga-LLZO nanoparticles were investigated. As shown in Figure S1, the nanoparticles have a cubic garnet structure, and the particle size is about 4050 nm. By dispersing different amounts of the Ga-LLZO nanoparticles into the PEO matrix, the PEO:Ga-LLZO composites were fabricated, and the corresponding phase structure was investigated by XRD. According to the XRD pattern shown in Figure S2, the crystalline phases of the Ga-LLZO nanoparticles and the PEO matrix do not change in the composite. The thermal stability of the composite was investigated by TGA and DSC; up to about 180 C, the electrolyte is stable (Figure S3). The microstructure of the PEO:16 vol.% Ga-LLZO composite was investigated by SEM. According to the SEM images shown in Figure S4, the Ga-LLZO nanoparticles are homogeneously distributed in the PEO matrix with no aggregation. The electrical properties of the PEO:Ga-LLZO composite were measured by the AC impedance spectroscopy; typical spectra are presented in Figure 1a. The semicircle at high and intermediate frequencies can be ascribed to the bulk, while the spike at low frequencies is due to the double-layer capacitance of the interface between the electrodes and electrolyte. Total conductivity can be calculated according to:  total  l / RS , where R is the sample resistance, l is the sample thickness, and S is the cross-section area. The composite with 16 vol.% Ga-LLZO exhibits a conductivity of 7.210-5 S cm-1 at 30 C and 4.110-4 S cm-1 at 60 C; compared with pure PEO, the conductivity of the PEO:Ga-LLZO composite is significantly enhanced by almost 4 orders of magnitude. Figure 1b displays the Arrhenius plot of the composite with 16 vol.% GaLLZO. The activation energy of the PEO:Ga-LLZO composite is calculated according to:

   exp(Ea / kBT ) , where A is the pre-exponential factor, kB is the Boltzmann constant, Ea is 7 ACS Paragon Plus Environment


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the activation energy, and T is the absolute temperature. The activation energy Ea in the temperature region of 50 to 80 C is 0.27 eV, while it is 0.62 eV in the temperature range of 0 to 50 C. Such a phenomenon is due to the crystallization of amorphous PEO at around the melting point of 50 C,41 which is in excellent agreement with the DSC result (Figure S3). The melting point of the PEO:Ga-LLZO composite is 55 C, according to the DSC result. The Ga-LLZO content has a significant influence on the conductivity of the composite. The total conductivity as a function of the volume fraction of Ga-LLZO nanoparticles is depicted in Figure 1c. When the Ga-LLZO content is lower than 4 vol.%, the total conductivity of the PEO:GaLLZO composite keeps at a low level. When the Ga-LLZO content exceeds 4 vol.%, the conductivity continuously increases until the Ga-LLZO content reaches 16 vol.%, at which the PEO:Ga-LLZO composite shows a maximal conductivity of 7.210-5 S cm-1 at 30 C. With further increase of the Ga-LLZO volume fraction, the conductivity begins to decrease, which may be due to the aggregation of the nanoparticles at high volume ratio loading, leading to the disruption of lithium-ion conducting paths, which will be discussed later.


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Figure 1. Electrical properties of the composite polymer electrolyte: (a) AC impedance spectra of the composite with 16 vol.% Ga-LLZO nanoparticles at different temperatures; (b) Arrhenius plot; (c) total conductivity and lithium-ion conductivity as a function of the volume fraction of GaLLZO nanoparticles.



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Figure 2. Current evolution under a polarization voltage of 10 mV. The inset shows the impedance spectra of the PEO:16 vol.% Ga-LLZO composite with Li electrodes before and after 10 mV polarization.

To simplify the modelling, no Li-salt was added in the PEO:Ga-LLZO composite. Owing to the segmental motion in the amorphous region of the polymer, some other species in PEO might also be mobile,39 leading to an ionic transference number TLi  < 1 in the PEO:Ga-LLZO composite. Following the classic electrochemical measurement of the transference number in polymer electrolytes,42 we determined the ionic transference number of the composite. The variation of current with time and the impedance spectra before and after polarization for the PEO:16 vol.% Ga-LLZO composite are shown in Figure 2. The polarization curves of the composites with 4 vol.%, 8 vol.%, 12 vol.%, 20 vol.% Ga-LLZO are also provided in the supporting information, Figure S5. Then the ionic transference number of lithium ions, TLi  , for the composite can be calculated according to Equation (1)


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TLi  

I s (V  I 0 R0 ) I 0 (V  I S RS )

(1)

where ΔV is the polarization voltage, I0 and R0 are the initial current and resistance, and Is and Rs are the steady state current and resistance, respectively. The values TLi  for the composites with different Ga-LLZO contents are listed in Table 1; the ionic transference number for the PEO:16 vol.% Ga-LLZO composite is 0.39. Then the lithium-ion conductivity,  Li , can be calculate according to:

 Li   total  TLi 

(2)

Table 1. Ionic transference numbers ( TLi  ) for composites with different volume fractions of GaLLZO nanoparticles. Sample

I0/μA

Is/μA

R0/Ω

Rs/Ω

TLi 

PEO:4%Ga-LLZO

0.27

0.08

823

935

0.29

PEO:6%Ga-LLZO

1.29

0.42

322

643

0.31

PEO:8%Ga-LLZO

2.47

0.82

289

424

0.33

PEO:10%Ga-LLZO

6.33

2.37

123

176

0.36

PEO:12%Ga-LLZO

5.50

1.98

121

162

0.35

PEO:14%Ga-LLZO

15.45

6.07

43

48

0.37

PEO:16%Ga-LLZO

18.68

8.22

35

38

0.39

PEO:18%Ga-LLZO

15.71

6.22

56

62

0.36

PEO:20%Ga-LLZO

7.90

2.69

101

122

0.32

The lithium-ion conductivities are also plotted in Figure 1c, which show a changing tendency very similar to the total conductivity. In fact, the enhancement in the lithium-ion conductivity is the synergistic effects of the decreasing crystallinity of polymer,35 the anion-immobilized effect,29 11 ACS Paragon Plus Environment


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and the percolation effect in composite electrolytes.20,39 The crystallinity of the composite electrolytes was characterized by DSC, and the results are given in Figure S6 and Table S1. The crystallinity of the PEO composites with different Ga-LLZO contents is roughly identical; therefore, the crystallinity of the polymer has almost no effect on the lithium-ion conductivity. Additionally, the anion-immobilized effect no longer play leading roles due to the absence of Lisalt in the PEO:Ga-LLZO composites. The conductivity profiles shown in Figure 1c suggest a percolation-type behavior typically observed for two-phase mixture systems with high interfacial conduction.43,44

In the presence of surfaces/interfaces, some of the lithium ions at regular Ga-LLZO lattice sites ( LiLi ) can move to surface sites ( Vs , here s denotes the surface site), leaving negatively charged vacancies ( VLi ) behind in the lattice and positively charged ions ( Lis ) on the surfaces. The defect reaction is:  V LiLi s

Lis  VLi

(3)

When a second phase, e.g. PEO, is present, the second phase can exert a stabilizing effect, whereby lithium ions are increasingly drawn from the Ga-LLZO lattice. The driving force for the defect reaction is the free energy; according to Dudney’s work, 45 the Ga-LLZO surface free energy is decreased with the formation of the interfacial region. Therefore, a space charge region, in which lithium vacancies and ions are redistributed, is formed at the Ga-LLZO/PEO interface. The presence of the space charge region is evidenced by the TEM observation; in Figure 3a, a surface layer of 3 nm is observed. In ionic systems, the width of a space charge region is typically a few nanometers, depending on the material system. 46,47 As an 12 ACS Paragon Plus Environment

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approximation, the width of the space charge region can be estimated by the Debye length,

D   kBT  e2C )1/2 , which is 5 nm for the Ga-LLZO case. In the equation,  is the dielectric constant and C is the dopant concentration.

Figure 3. Space charge region at the Ga-LLZO/PEO interface. (a) TEM images of the GaLLZO/PEO interface. (b) Schematic illustration of Ga-LLZO nanoparticle in the PEO:Ga-LLZO composite. The domain of the Ga-LLZO nanoparticle  is surrounded by the Ga-LLZO/PEO interface . The significance of space charge regions for the ionic conduction was firstly brought about by Liang,43 who made a systematic study of the electrical properties of the two-phase system LiIAl2O3 and found that the ionic conduction was anomalously higher in comparison with that of pure phases. Since then a whole range of similar effects were observed in the systems of AgCl-A12O3,48 AgCl-BaTiO3,49 AgCl-AgI50 and CaF2-BaF2.51 Wagner studied the defect reactions in the space charge region.47,48 Using the Monte Carlo simulation, Bunde et al. established a percolation model for the space chare regions in AgX (X = Cl, Br, I)-Al2O3.52,53 Based on the defect chemistry, Maier proposed targeted design of Ag-ion composite electrolytes.50,54-56 In this work, we devote to model the Li-ion conduction in the composite polymer electrolytes. 13 ACS Paragon Plus Environment


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To simulate the formation of the space charge region at the Ga-LLZO/PEO interface, the phase-field method based on the Poisson-Cahn equations is adopted.57-61 Considering the concentrations of vacancies and lithium ions (Li+) in the Ga-LLZO nanoparticle domain () surrounded by the Ga-LLZO/PEO interface () (Figure 3b), the defect concentration in the space charge region can be quantized. The simulation details and corresponding parameters are provided in the Supporting Information (Figure S7 to S10, and Table S2). The driving force for the formation of the space charge region is the free energy. Our phase-field simulation results, given in Figure S7, S8 and Movie S1, reveal that the free energy density decreases in the surface layer of the nanoparticle with the formation of the space charge region, and it reaches the lowest value on the outer surface of the space charge region when the system gets to a new equilibrium state. Figure 4a shows the evolution of the space charge region in a single Ga-LLZO nanoparticle. Both lithium ion and vacancy concentrations are normalized by their corresponding initial bulk values. The distributions of the normalized lithium ion and vacancy concentrations from the surface to the center of the Ga-LLZO nanoparticle along the radial direction are plotted in Figure 4b and 4c, respectively. Initially, as lithium ions and vacancies are assumed to be uniformly distributed in the nanoparticle, the normalized lithium ion and vacancy concentrations are constant throughout the nanoparticle at t = 0 ns. When the Ga-LLZO nanoparticle contacts with PEO matrix, the driving force that stems from the interfacial chemical potential drives lithium ions at regular Ga-LLZO lattice sites to migrate to surface sites, leading to the aggregation of positively charged ions on the surfaces and negatively charged vacancies behind in the lattice (t = 150 ns). With the migration process proceeding further, the system finally reaches at a new equilibrium state (t = 300 ns), leaving a high concentration of lithium ions and a low concentration of vacancies in the surface layer of the Ga-LLZO nanoparticle. As shown in Figure 4b and 4c, at the equilibrium state, 14 ACS Paragon Plus Environment

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the lithium ion and vacancy concentrations reach a minimum and a maximum at 2 nm away from the Ga-LLZO/PEO interface, respectively. The corresponding dynamic processes are exhibited in Movie S2.

Figure 4. Formation of the space charge region at the Ga-LLZO/PEO interface: (a) Evolution of the normalized Li+ ( cLi  ) and vacancy concentration ( cV ) from 0 to 300 ns; (b) distributions of the lithium ion concentrations, and (c) distributions of the vacancy concentrations from the surface to the center of the Ga-LLZO nanoparticle along the radial direction at t = 0, 150 and 300 ns (both the lithium ion and vacancy concentrations are normalized by their initial bulk values); (d) schematic illustration of the fast ionic conduction pathway along the space charge regions. In addition, the formation of the space charge region in the system with two Ga-LLZO nanoparticles is provided in Figure S9, Figure S10, Movie S3 and Movie S4. These results uncover 15 ACS Paragon Plus Environment


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that a continuous pathway of the space charge regions can be formed, when the generated space charge regions in individual nanoparticles are connected to each other, as illustrated in Figure 4d. The space charge region has two effects: first, the ionic conductivity is influenced by changing the defect concentration in the region;44 second, it provides a new kinetic pathway for the ionic conduction. Notably, a highly conductive region surrounding an isolated particle barely affects the ionic conductivity; however, if continuous paths are formed (i.e. the situation above the percolation threshold), the contribution of the space charge region to the ionic conductivity is significant. Therefore, in the PEO:Ga-LLZO composite, when the Ga-LLZO content exceeds the percolation threshold value, the formed continuous pathway of the space charge regions can be treated as the fast channel for Li+ ion transportation, as illustrated in Figure 4d. To describe the influence of the Ga-LLZO content on the ion transport in the PEO:Ga-LLZO composite, the random resistor model for the two-phase mixture is used to elaborate the particular role of the space charge conduction, and the Monte Carlo simulation is used to determine the ionic conductivity, ∑, as a function of the concentration, p, of the conductive phase.53,62 In the random resistor model, a cubic space is used to study the two-phase mixture. The unit cubes, representing the dispersed conductive Ga-LLZO particles (α), randomly occupy the space with probability p, while the remaining is the PEO phase (β). The bonds connecting two neighboring sites are identified with electrical resistors, of which are distinguished for three types, as defined in Figure 5. Bonds at the interface between α and β phases correspond to the highly conducting pathway (A), and bonds through the interiors of the α and β phases are the bulk conducting pathway (B) and the matrix conducting pathway (C), respectively. The bond A is highly conductive, as explained in Figure 4. Although the conductivity of Ga-LLZO is high, the conductivity of the bond B (σB) 16 ACS Paragon Plus Environment

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should be low; without sintering, the Ga-LLZO nanoparticles form mechanical contacts among them, the Ga-LLZO/Ga-LLZO contacts are highly resistive to any lithium-ion transport across them, which is supported by two facts: 1. without sintering a compacted body of the Ga-LLZO nanoparticles is almost insulating (the conductivity of the compacted but not sintered LLZO particles is 10-9 S cm-1 at 100 C, as demonstrated in Figure S11); 2. the conductivity decreases when the Ga-LLZO content is > 16 vol.% (Figure 1c). Since the conductivity of PEO is on the order of 10-8 S cm-1 (Figure 1c), the conductivity of the bond C (σC) should be much lower than that of the bond A (σA).

Figure 5. Two-phase mixture with different concentrations, p, of Ga-LLZO nanoparticles (denoted by the blue cubes): (a) p < pC , (b) pC  p < pC , onset of the interface percolation, and (c) p  pC , threshold for the disruption of the percolation paths. The highly conductive space charge regions at the Ga-LLZO/PEO interfaces are marked with green lines (bond A), the Ga-LLZO nanoparticles are denoted as the bulk conduction (bond B), marked with red line, while the PEO matrix is bond C, marked with yellow lines. Using a random resistor network, the ionic conduction in the composite with the special feature of a modified conductivity in the space charge regions is simulated. For a very small p, ∑ 17 ACS Paragon Plus Environment


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is governed by the conductivity of PEO (σC), as shown in Figure 5a. At a concentration where a percolation path of the bond A (σA) is formed for the first time (Figure 5b), the high space charge conductivity becomes dominant, and ∑ increases strongly. By further increasing p, the conductivity ∑ firstly passes through a maximum and then decreases to a low value at a concentration pC , where all the conducting paths are about to be disrupted (Figure 5c). The computational details of the Monte Carlo simulation are given in the supporting information (Figure S12).

Figure 6. Comparison of the ionic conductivity data obtained from the Monte Carlo simulation with those acquired via the experimental measurement for the PEO:Ga-LLZO composite.

The results of the Monte Carlo simulation for the relation ∑ p are plotted in Figure 6; the stimulation results are in excellent agreement with our experimentally data. Therefore, the enhanced conductivity of the PEO:Ga-LLZO composite can be ascribed to the fast ionic conduction in the space charge regions, in conjunction with the percolation effect. The percolation of the space charge regions can be successfully applied not only to understand the ionic conduction 18 ACS Paragon Plus Environment

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in the composite, but also to strategically optimize the conductivity. One key parameter to enhance the ionic conductivity is the specific contact area between Ga-LLZO and PEO; decreasing the size of the Ga-LLZO nanoparticles is an effective way to get a larger contact area. Another key parameter is the distribution of the Ga-LLZO nanoparticles (characterized by the percolation efficiency); structure inhomogeneity, such as particle agglomeration and porosity, must be avoided in the composite. Combined the flexibility of PEO with the high ionic conductivity and excellent stability of Ga-LLZO, the PEO:Ga-LLZO composite shows an enormous potential in all-solid-state batteries. The electrochemical window greater than 4.6 V (Figure S13) and the excellent stability against metallic lithium (Figure S14) suggest that the PEO:Ga-LLZO composite is suitable for high voltage all-solid-state batteries using metallic Li anode. Additionally, operating at 60 C, the solidstate battery LiFePO4/PEO:Ga-LLZO/Li shows a discharge capacity of 145 mAh g-1 at 0.1 C (Figure S15). 

CONCLUSIONS The enhanced ionic conductivity of the PEO:Ga-LLZO composite polymer electrolyte can be

ascribed to the fast ionic conduction in the space charge regions at the interfaces of the PEO matrix and the Ga-LLZO nanoparticles. When the space charge regions and the phase distribution meet the demand of forming the percolation threshold, the percolation effect takes effect, and continuous fast conduction pathways are formed; as a result, the ionic conductivity increases significantly. The mechanistic understanding of the ionic conduction paves the way to the further optimization and targeted design of the composite polymer electrolyte. Moreover, owing to the high ionic conductivity, the electrochemical window greater than 4.6 V and the excellent stability against 19 ACS Paragon Plus Environment


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metallic lithium, the PEO:Ga-LLZO composite polymer electrolyte opens up the opportunity for flexible all-solid-state batteries of high safety and energy density. Although the conduction model was developed using the PEO:Ga-LLZO composite as a model system, this model is applicable to other composite polymer electrolytes as well, because the experimental and theoretical techniques used in this work can obviously also be applied to other superionic conductor/insulator systems.

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ASSOCIATED CONTENT

Supporting Information The supporting information is available free of charge on the ACS Publications website. XRD patterns, SEM and TEM images, TGA and DSC curves, ionic transference number characterization, Ga-LLZO/PEO interface characterization, phase-field simulation, Monte Carlo simulation and electrochemical measurements.

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AUTHOR INFORMATION

Corresponding Author * E-mail: [email protected] (X.G.) * E-mail: [email protected] (H.Y.) * E-mail: [email protected] (L.W.)


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Author contributions X.G. acquired the funding and supervised the project, X.G. and Z.L. conceived the idea and designed the experiments, Z.L., J.-F.W. and L.W. conducted the experiments, Z.L. and H.-M.H. did the Monte Carlo simulation, J.-K.Z. and H.Y. did the phase-field simulation, X.G., Z.L. and H.Y. wrote the manuscript. Notes The authors declare no competing financial interest. 

ACKNOWLEDGMENTS

This work is supported by the National Natural Science Foundation of China (Grant No. 51672096) and the fund of the State Key Laboratory of Solidification Processing in NWPU (Grant No. SKLSP201710). H.Y. acknowledges the National 1000 Talents Program of China tenable at the Huazhong University of Science and Technology (HUST), China. 

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Ionic Conduction in Composite Polymer Electrolytes: Case of PEO:Ga

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