A Super-Ionic Solid-State Block Copolymer Electrolyte

Abstract

Polymer solid-state electrolytes offer great promise for battery materials with high energy density, mechanical stability, and improved safety. However, their low ion conductivities have so far limited their potential applications. Here, it is shown for poly(ethylene oxide) block copolymers that the super-stoichiometric addition of lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) as lithium salt leads to the formation of a crystalline PEO block copolymer phase with exceptionally high ion conductivities and low activation energies. The addition of LiTFSI further induces block copolymer phase transitions into bi-continuous Fddd and gyroid network morphologies, providing continuous 3D conduction pathways. Both effects lead to solid-state block copolymer electrolyte membranes with ion conductivities of up to 1·10^-1 S cm^-1 at 90 °C, decreasing only moderately to 4·10^-2 S cm^-1 at room temperature, and to >1·10^-3 S cm^-1 at -20 °C, corresponding to activation energies as low as 0.19 eV. The co-crystallization of PEO and LiTFSI with ether and carbonate solvents is observed to play a key role to realize a super-ionic conduction mechanism. The discovery of PEO super-ionic conductivity at high lithium concentrations opens a new pathway for fabrication of solid polymer electrolyte membranes with sufficiently high ion conductivities over a broad temperature range with widespread applications in electrical devices.

Keywords: block copolymer electrolytes; block copolymers; polyethylene oxide (PEO); solid‐state electrolytes; super‐ionic conductors.

Figure 1

LiTFSI and THF uptake by the PEO phase were measured by 19F‐NMR and TGA, and characteristic DSC scans. a) LiTFSI (blue) and THF (red) uptake in the PEO block copolymer phase showing a saturation limit above [Li⁺:O_EO]_n ≥3.1:1, indicated by the black vertical line. In the saturation limit uptakes of 26 wt.% for LiTFSI and 7 wt.% for THF for the SPEs from PI_14.6PS_34.8PEO_1.9, and of 18 wt.% for LiTFSI and 6 wt.% for THF for the SPE from PI_26.1PS_67.3PEO_1.9 are reached. LiTFSI uptake was determined by ¹⁹F‐NMR spectroscopy, and THF uptake by TGA. b) DSC scans of the neat components and their binary and ternary mixtures. We observe that the glass transition temperature ϑ_g,SPE of the SPE from PI_14.6PS_34.8PEO_1.9 with [Li⁺:O_EO]_n = 4.2:1 (orange) is identical to the ternary mixture LiTFSI/PEO/THF (green), and differs from the binary mixtures LiTFSI/PEO (blue) and LiTFSI/THF (red). The DSC‐scan of the neat block copolymer (black) differs as well, exhibiting a strong PEO melting peak. The absence of the ϑ_mp,THF at −105 °C indicates that no uncoordinated THF exists in the SPE electrolyte.

Figure 2

Crystal structure of the LiTFSI/PEO/THF phase. a) X‐ray diffraction (XRD) curves measured for the SPEs from PI_14.6PS_34.8PEO_1.9 at increasing LiTFSI concentrations of [Li⁺:O_EO]_n = 1.1–5.0:1. For comparison we also show the XRD curves (in blue) reported for the binary LiTFSI/THF and ternary LiTFSI/PEO/THF phase,^{[ 17 ]} for the neat LiTFSI (in pink), together with the peak positions expected for the trigonal unit cell. b) Structural model that is consistent with the unit cell dimensions and with structural information regarding the determined stoichiometry, from TGA and DSC, ¹⁹F‐NMR‐ and ¹⁹F‐¹H‐HOESY‐NMR spectroscopy. The color of the circles corresponds to the following atoms: yellow = sulfur, blue = nitrogen, gray = carbon, white = hydrogen, green = fluorine, and red = oxygen. Lithium is shown in dark pink, with dotted lines indicating oxygen coordinations. The PEO helix is viewed in axial direction. Pink rectangles indicate Li⁺‐mobility zones.

Figure 3

STEM images of SPEs from PI_xPS_yPEO_1.9 block copolymer morphologies with increasing LiTFSI concentrations. a) Hexagonally ordered cylindrical morphology (HEX) of the neat PI_14.6PS_34.8PEO_1.9 block copolymer. b) Ordered Fddd network structure observed for the SPEs from the same block copolymer at [Li⁺:O_EO]_n = 1.1:1, exhibiting alternating domains characteristic for the (110) projection. c) Gyroid structure observed for the SPE from the same block copolymer at [Li⁺:O_EO]_n = 4.2:1 with the undulating domain pattern characteristic for the (211) projection. d) Fddd network structure observed for the SPE from the PI_26.1PS_67.3PEO_1.9 block copolymer at a LiTFSI concentration of [Li⁺:O_EO]_n = 9.7:1 corresponding to the (001) projection. e–g) Schematically visualizations showing the HEX, Fddd, and Gyroid morphology.

Figure 4

Arrhenius plots of ionic conductivities measured by electrochemical impedance spectroscopy (EIS) of the block copolymer SPE membranes for a) LiTFSI/PI_14.6PS_34.8PEO_1.9/THF, for b) LiTFSI/PI_26.1PS_67.3PEO_1.9/THF, and for c) LiTFSI/PI_26.1PS_67.3PEO_1.9/DMC for a series of LiTFSI concentrations increasing from [Li⁺:O_EO]_n = 1.0:1 to 10.2:1. In each case the standard LiTFSI/PEO electrolyte with [Li⁺:O_EO] = 0.1:1 is also shown as a reference, in a temperature range from −20 to 90 °C. Remarkably high ionic conductivities of σ_90 °C = 1·10⁻¹ S cm⁻¹ are observed in block copolymer SPEs from LiTFSI/PI_14.6PS_34.8PEO_1.9/THF at [Li⁺:O_EO]_n = 4.2:1, concomitantly with a small activation energy E _a of ≈0.2 eV.

Figure 5

Temperature‐dependent ⁷Li relaxation times and relaxation rates. Spin relaxation NMR investigations in the temperature range from (0–90) °C for the SPE from LiTFSI/PI_14.6PS_34.8PEO_1.9/THF with [Li⁺:O_EO]_n = 4.2:1. In blue, correlation time (τ_c) as a function of the inverse temperature in an Arrhenius plot to determine the activation energy E _a = 0.19 eV, and in red the BPP fit with a single Lorentzian to determine the relaxation time.

Figure 6

Proposed model for the super‐ionic block copolymer SPE membrane. Left, Li⁺‐conduction planes in the crystalline LiTFSI/PEO/THF phase, middle, PEO continuous domain in the gyroid structure, and right, bi‐continuous gyroid structured conduction paths connecting two electrodes.