The development of commercial solid-state batteries has to date been hindered by the individual limitations of inorganic and organic solid electrolytes, motivating hybrid concepts. However, the room-temperature conductivity of hybrid solid electrolytes is still insufficient to support the required battery performance. A key challenge is to assess the Li-ion transport over the inorganic and organic interfaces and relate this to surface chemistry. Here we study the interphase structure and the Li-ion transport across the interface of hybrid solid electrolytes using solid-state nuclear magnetic resonance spectroscopy. In a hybrid solid polyethylene oxide polymer–inorganic electrolyte, we introduce two representative types of ionic liquid that have different miscibilities with the polymer. The poorly miscible ionic liquid wets the polymer–inorganic interface and increases the local polarizability. This lowers the diffusional barrier, resulting in an overall room-temperature conductivity of 2.47 × 10−4 S cm−1. A critical current density of 0.25 mA cm−2 versus a Li-metal anode shows improved stability, allowing cycling of a LiFePO4–Li-metal solid-state cell at room temperature with a Coulombic efficiency of 99.9%. Tailoring the local interface environment between the inorganic and organic solid electrolyte components in hybrid solid electrolytes seems to be a viable route towards designing highly conducting hybrid solid electrolytes.
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The data that support the findings of this study are available at the online depository Zenodo (https://doi.org/10.5281/zenodo.6334099).
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We thank F. Ooms, F. Zhang and C. Ma for their assistance with experiments. M.L. and M.W. acknowledge the financial support from the Netherlands Organization for Scientific Research (NWO) under the VICI grant nr. 16122. E.R.H.v.E. further acknowledges NWO for their support of the solid-state NMR facility for advanced materials science, which is part of the uNMR-NL grid (NWO grant 184.035.002). M.W. gratefully acknowledges the financial support from the Advanced Dutch Energy Materials (ADEM) programme of the Dutch Ministry of Economic Affairs, Agriculture and Innovation.
The authors declare no competing interests.
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(a) SEM image of the pristine micron-sized Li6PS5Cl. (b) SEM image showing the morphology of the HSE where the Li6PS5Cl particles are marked with circles.
1H-6Li CPMAS spectra of the LiTFSI-PEO-Li6PS5Cl HSE measured at contact times ranging from 0.2 ms (lightest grey) to 6 ms (black).
Extended Data Fig. 3 1D 6Li magic angle spinning (MAS) spectrum corresponding to the Li6PS5Cl -LiTFSI-PEO HSE.
1D 6Li magic angle spinning (MAS) spectrum corresponding to the Li6PS5Cl -LiTFSI-PEO HSE.
DSC measurements showing the heat flow of the HSE, HSE-EMIM, HSE-PP13 under heating up from 0 to 65 °C.
Extended Data Fig. 5 1H-1H cross peak intensity buildup of protons in EMIM-TFSI and PP13-TFSI correlated to PEO from the 2D 1H-1H NOESY spectra of HSE-PP13 and HSE-EMIM.
1H-1H cross peak intensity buildup of protons in EMIM-TFSI (a) and PP13-TFSI (b) correlated to PEO from the 2D 1H-1H NOESY spectra of HSE-PP13 and HSE-EMIM given in Fig. 4. All the cross peaks between EMIM-TFSI and LiTFSI-PEO appear at nearly at the same mixing time which means that there is no preferred orientation of the EMIM-TFSI species with respect to PEO. While a sequence of cross peak evolution with mixing time is observed in HSE-PP13. At the shortest mixing times, 1H-1H correlations are first observed between 1H resonances at positions 1.0 ppm and 1.6 ppm on the piperidine ring of PP13-TFSI and the –OCH2- protons from PEO. These ring protons are the furthest away from the bulky propyl and methyl groups attached to the N atom on the piperidine ring.
Extended Data Fig. 6 1D 7Li CPMAS spectra and intensity plots measured of the HSE-EMIM and HSE-PP13, and the full build-up of the peak intensity of the broad component.
1D 7Li CPMAS spectra and intensity plots measured of the (a,c) HSE-EMIM and (b,d) HSE-PP13 at contact times between 200 µs and 12 ms (e) Full build-up of peak intensity at 0.2–0.7 ppm as function of contact time of the spectra given in (a) and (b).
Extended Data Fig. 7 Li+ transport characterization in HSE-EMIM and HSE-PP13 using 6,7Li-6,7Li 2D EXSY NMR.
Li+ transport characterization in HSE with PP13-TFSI and EMIM-TFSI IL additives. 7Li–7Li, 6Li-6Li 2D-EXSY corresponding to the HSE-EMIM (a, b) and HSE-PP13 (c, d) ILs measured under MAS at a spinning speed of 5 kHz mixing time of 1.5 s and 2 s at 328 K.
Extended Data Fig. 8 Plating and stripping curves of a Li metal symmetrical cell with HSE-EMIM and HSE-PP13.
Plating and stripping curves of a Li metal symmetrical cell with LiTFSI-PEO-Li6PS5Cl HSEs with PP13 TFSI and EMIM TFSI ionic liquids. The cell with HSE-EMIM shows quick polarization after 300 h of cycling at a current density of 0.05 mA/cm2. In comparison the cell with HSE-PP13 shows a very stable over-potential (lower than 200 mV) during 800 hours of cycling, indicating a higher ionic conductivity and better interfacial stability against Li-metal.
Extended Data Fig. 9 Electrochemical impedance spectroscopy measurements (EIS) of the cell with LiTFSI-PEO-Li6PS5Cl HSE with both PP13-TFSI and EMIM-TFSI ionic liquids.
Electrochemical impedance spectroscopy measurements (EIS) of cell with LiTFSI-PEO-Li6PS5Cl HSE with both PP13-TFSI and EMIM-TFSI (PP13-TFSI and EMIM-TFSI, 0.25:1 molar ratio IL:Li-ion, HSE—EMIM-PP13) ionic liquids.
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Liu, M., Zhang, S., van Eck, E.R.H. et al. Improving Li-ion interfacial transport in hybrid solid electrolytes. Nat. Nanotechnol. 17, 959–967 (2022). https://doi.org/10.1038/s41565-022-01162-9