Nanoscale Mapping of Extrinsic Interfaces in Hybrid Solid Electrolytes

Abstract

•Mechanical properties of extrinsic interfaces mapped for model system using AFM•Sub-surface ceramic distribution correlated to variation in surface mechanics•Electrochemical performance linked to improved adhesion at extrinsic interface•Tailoring interfacial properties will enable high-performance ASSBs ASSBs are promising high-energy density energy storage devices. Hybrid electrolytes are material systems containing an organic and inorganic ion conductor that can achieve manufacturing, transport, and mechanical requirements of ASSBs. Hybrid electrolytes contain material junctions between the organic and inorganic phases (intrinsic) and between the electrode and electrolyte (extrinsic). Both extrinsic and intrinsic interfaces in these systems witness dynamic physical and chemical changes that can adversely impact the battery performance. Limited experimental knowledge is available for these interfaces due to the relative difficulty of accessing these interfaces. Rational design of interfaces is crucial to achieve stable, high-performance ASSBs. This work evaluates extrinsic interfaces for a model PEO-LLZO hybrid electrolyte system and shows the impact of interfacial properties on electrochemical performance. Inorganic-organic hybrid solid electrolytes are promising material systems for all solid-state batteries (ASSBs). These electrolytes contain numerous solid|solid interfaces that govern transport pathways, electrode|electrolyte compatibility, and durability. This paper evaluates the role that electrode|electrolyte interfaces and electrolyte structure have on electrochemical performance. Atomic force microscopy techniques reveal how mechanical, adhesion, and morphological properties transform in a series of model hybrid solid electrolytes. These measurements are mapped to sub-surface microstructural features using synchrotron nano-tomography. Hybrid solid electrolytes with shorter polymer chains demonstrate a higher adhesion (>100 nN), Young’s Modulus (6.4 GPa), capacity (114.6 mAh/g), and capacity retention (92.9%) than hybrid electrolytes with longer polymer chains (i.e., higher molecular weight). Extrinsic interfacial properties largely dictate electrochemical performance in ASSBs. Microstructural control over inorganic constituents may provide a means for tailoring interfacial properties in hybrid solid electrolytes. Inorganic-organic hybrid solid electrolytes are promising material systems for all solid-state batteries (ASSBs). These electrolytes contain numerous solid|solid interfaces that govern transport pathways, electrode|electrolyte compatibility, and durability. This paper evaluates the role that electrode|electrolyte interfaces and electrolyte structure have on electrochemical performance. Atomic force microscopy techniques reveal how mechanical, adhesion, and morphological properties transform in a series of model hybrid solid electrolytes. These measurements are mapped to sub-surface microstructural features using synchrotron nano-tomography. Hybrid solid electrolytes with shorter polymer chains demonstrate a higher adhesion (>100 nN), Young’s Modulus (6.4 GPa), capacity (114.6 mAh/g), and capacity retention (92.9%) than hybrid electrolytes with longer polymer chains (i.e., higher molecular weight). Extrinsic interfacial properties largely dictate electrochemical performance in ASSBs. Microstructural control over inorganic constituents may provide a means for tailoring interfacial properties in hybrid solid electrolytes. Hybrid solid electrolytes are an emerging family of solid electrolytes that are promising alternatives to liquid electrolytes for energy and power dense solid-state batteries.1Dirican M. Yan C. Zhu P. Zhang X. 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Variation in physical properties at the extrinsic interfaces can be correlated to the distribution of inorganic material in the hybrid electrolytes. Synchrotron X-ray nano-tomography is used to evaluate the heterogeneous distribution of inorganic particles in order to quantify the role that microstructure plays on interfacial mechanical properties. Three-dimensional (3D) structural reconstructions of hybrid solid electrolytes obtained from synchrotron experiments are combined with physics-based modeling to elucidate the origin of heterogenous interfacial properties. The findings reveal that mechanical properties at the extrinsic interface (rather than transport properties) dictate solid-state battery performance. Microstructural control over the sub-surface inorganic particles within a hybrid electrolyte may enable a pathway toward controlling extrinsic interfacial properties with spatial control. This control is important for achieving facile and long-lasting Li-metal solid-state batteries. Three model hybrid solid electrolytes were processed in a ratio of 50:50 wt % between (PEO) and Li7La3Zr2O12 (LLZO) (see Supplemental Experimental Procedures). Lithium perchlorate (LiClO4) is used as the salt in each polymer electrolyte. The polymer molecular weight is a control variable in this study that enables a systematic approach toward altering the extrinsic interfacial properties. The molecular weight of the PEO was altered between 300,000 g mol–1 (300 K), 1,000,000 g mol−1 (1 M), and 5,000,000 g mol−1 (5 M). The viscosity increases with increasing molecular weight, which can impact the processing step (Figure S1B). A shear thinning behavior for 1 M and 5 M PEO is observed and suggests entangled polymer chains at a ≈4 wt % concentration in an acetonitrile solvent. The ionic conductivity for all solid electrolytes are similar and increase from 10−6 S/cm at room temperature to 10−3 S/cm at 80°C. The electrolytes’ transport properties are all similar and independent of the molecular weight of the polymer (Figure 1C). Hybrid electrolytes show improvement over the polymer electrolytes, especially at lower temperatures.40Langer F. Bardenhagen I. Glenneberg J. Kun R. Microstructure and temperature dependent lithium ion transport of ceramic-polymer composite electrolyte for solid-state lithium ion batteries based on garnet-type Li7La3Zr2O12.Solid State Ion. 2016; 291: 8-13Crossref Scopus (80) Google Scholar Hybrid electrolytes may have higher transport properties than polymer electrolytes because of a decrease in the crystallinity in the polymer matrix as well as a lower cation-polymer interaction.11Zheng J. Tang M. Hu Y.-Y. Lithium ion pathway within Li7 La3 Zr2 O12 polyethylene oxide composite electrolytes.Angew. Chem. Int. Ed. 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Enhancement of ionic conductivity of composite membranes for all-solid-state lithium rechargeable batteries incorporating tetragonal Li7La3Zr2O12 into a polyethylene oxide matrix.J. Power Sources. 2015; 274: 458-463Crossref Scopus (215) Google Scholar Above >60°C (melting temperature of PEO [Tm]) polymer segmental motion is enhanced. At higher ceramic loading, LLZO particles can increase the tortuosity of ion transport pathways, leading to a reduction in ionic conductivity enhancement in hybrid electrolytes over the polymer electrolytes. This effect is stronger for hybrid electrolytes processed with PEO of 300 K molecular weight owing to the shorter chain lengths of these polymers. Hybrid electrolytes show lower activation energies compared to the polymer electrolytes (Table 1). Changes in the activation energies suggest the presence of an additional ion transport pathway in the system. The intrinsic interface between the organic and inorganic phase is a potential pathway for ion conduction within these hybrid electrolytes.6Chen L. Li Y. Li S.P. Fan L.Z. Nan C. Goodenough J.B. PEO/garnet composite electrolytes for solid-state lithium batteries: from “ceramic-in-polymer” to “polymer-in-ceramic”.Nano Energy. 2018; 46: 176-184Crossref Scopus (810) Google Scholar,9Zheng J. Hu Y.-Y. New insights into the compositional dependence of Li-Ion transport in polymer-ceramic composite electrolytes.ACS Appl. Mater. Interfaces. 2018; 10: 4113-4120Crossref PubMed Scopus (240) Google Scholar,11Zheng J. Tang M. Hu Y.-Y. Lithium ion pathway within Li7 La3 Zr2 O12 polyethylene oxide composite electrolytes.Angew. Chem. Int. Ed. Engl. 2016; 55: 12538-12542Crossref PubMed Scopus (370) Google Scholar,18Zheng J. Dang H. Feng Xuyong Chien P.-H. Hu Y.-Y. Li-ion transport in a representative ceramic–polymer–plasticizer composite electrolyte: Li7La3Zr2O12–polyethylene oxide–tetraethylene glycol dimethyl ether.J. Mater. Chem. A. 2017; 5: 18457-18463Crossref Google Scholar,43Yang F. Xin L. Uzunoglu A. Qiu Y. Stanciu L. Ilavsky J. Li W. Xie J. Investigation of the interaction between nafion ionomer and surface functionalized carbon black using both ultrasmall angle X-ray scattering and Cryo-TEM.ACS Appl. Mater. Interfaces. 2017; 9: 6530-6538Crossref PubMed Scopus (60) Google Scholar Conductivity of these intrinsic interfaces are estimated using an effective mean field theory model. This model considers a three-component system: (1) backbone matrix (PEO), (2) filler material (LLZO), and (3) interfacial region and predicts the effective conductivity of the resultant effective medium. The following implicit equation is solved to estimate the conductivity of the intrinsic interface44Li Yuanji Zhao Y. Cui Y. Zou Zheyi Wang D. Shi S. Screening polyethylene oxide-based composite polymer electrolytes via combining effective medium theory and Halpin-Tsai model.Comput. Mater. Sci. 2018; 144: 338-344Crossref Scopus (15) Google Scholar:(1−fc)σpol−σcompσcomp+Li*(σpol−σcomp)+fcσint−σcompσcomp+Li*(σint−σcomp)=0(Equation 1) where σpol, σcomp, and σint is the conductivity of the polymer matrix, composite, and the interface, respectively; Li* is the effective depolarization factor; and fc is the volume fraction of the inserted grains (details in Supplemental Information).Table 1Activation Energies (Ea) of Hybrid Solid Electrolyte and Polymer Solid ElectrolytesEa (eV)300K1M5M< Tm> Tm< Tm> Tm< Tm> TmPolymer0.660.340.640.360.640.36Hybrid0.480.220.480.180.460.21 Open table in a new tab The model estimates higher ionic conductivity for the intrinsic interfaces compared to the polymer and hybrid solid electrolytes at temperatures below the melting temperature of PEO (Figures 1D and S3). It is hypothesized that the interface could be a preferred ion transport pathway in this temperature range. Above the melting temperature, the interfacial conductivity shows a decrease for the 300 K and 1 M system. The differences in the behavior of the intrinsic interface could arise from the varying chain lengths of the polymer in these systems. Above the melting temperature, the ion transport should be favored within the polymer matrix as it shows higher ion conductivity. The transference numbers for the three hybrid solid electrolytes were calculated to be 0.18 ± 0.02, 0.17 ± 0.02, and 0.18 ± 0.02 (Figure S2). While the transport properties in each electrolyte are similar and independent of the molecular weight of the polymer, the electrochemical properties (stability and capacity) vary significantly. Hybrid solid electrolytes processed with 5 M molecular weight PEO failed significantly faster (≈100 h) than those processed with 300 K (>180 h) and 1 M molecular weight PEO (≈130 h). The charge passed to failure (Qfailure) is >2× times longer for hybrid solid electrolytes processed with 300 K than those processed with 5 M (Figure 2A; Table 2). Furthermore, full-cell experiments using a lithium iron phosphate cathode (LiFePO4 or LFP) || demonstrate that the capacity and capacity retention is also dependent on the molecular weight. The 5 M hybrid solid electrolyte has the lowest capacity (92.0 mAh/g, 30th cycle) and poor capacity retention over cycle life (Figures 2B and S15). The 300 K hybrid solid electrolyte shows greater capacity retention and a significantly higher capacity (114.6 mAh/g, 30th cycle). The trends in capacity and retention for full cells and charge passed to failure (Qfailure) for symmetric cells were verified with multiple tests. Since transport properties in each solid electrolyte are similar, the variability in full-cell performance and symmetric stability testing must arise from either (1) electrode microstructure or (2) extrinsic interfacial properties.Table 2Electrochemical Performance Metrics of Hybrid ElectrolytesPEO MW (g/Mol)Qfailure (C)Retention (%)300 K44.9 ± 1.992.91 M25.8 ± 1.792.95 M17.6 ± 0.788.8Qfailure is the charge passed to failure in a symmetric cell experiment. Retention is capacity retained in a full cell experiment after 30 cycles. Open table in a new tab Qfailure is the charge passed to failure in a symmetric cell experiment. Retention is capacity retained in a full cell experiment after 30 cycles. The electrodes for all tests were processed using identical cathode inks and processing conditions. The inherent microstructure thus should be identical across the three systems. Differences, if any, should arise from the cell assembly procedure (pressure driven). Galvanostatic intermittent titration technique (GITT) and electrode imaging (scanning electron microscopy) were carried out on the three systems to quantify the effective diffusion coefficients in the electrodes. Cross-sectional scanning electron microscopy images demonstrate similar LFP microstructures that are independent of the electrolyte molecular weight (Figures 2D–2F and S5). Experimentally, Li diffusion coefficients can provide an indirect measure of the electrode microstructure. Tortuous ion transport pathways will lead to lower diffusion coefficients and vice-versa. The galvanostatic intermittent titration technique is used to estimate the Li diffusion coefficient for each respective cathode:DGITT=4π[I0VMAFdE/dxdE/dt1/2]2,t≪τ(Equation 2) where I0 (A) is the applied current, A (m2) is the electrode area, F is the Faraday constant (96,485 C mol−1), and VM is the electrode molar v