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. 2020 Sep 9;12(36):40749-40758.
doi: 10.1021/acsami.0c09113. Epub 2020 Aug 27.

Investigating the Effects of Lithium Phosphorous Oxynitride Coating on Blended Solid Polymer Electrolytes

Jed LaCoste  1   2 , Zhifei Li  1 , Yun Xu  1 , Zizhou He  2 , Drew Matherne  2 , Andriy Zakutayev  1 , Ling Fei  1   2
Affiliations

Investigating the Effects of Lithium Phosphorous Oxynitride Coating on Blended Solid Polymer Electrolytes

Jed LaCoste et al. ACS Appl Mater Interfaces. .

Abstract

Solid-state electrolytes are very promising to enhance the safety of lithium-ion batteries. Two classes of solid electrolytes, polymer and ceramic, can be combined to yield a hybrid electrolyte that can synergistically combine the properties of both materials. Chemical stability, thermal stability, and high mechanical modulus of ceramic electrolytes against dendrite penetration can be combined with the flexibility and ease of processing of polymer electrolytes. By coating a polymer electrolyte with a ceramic electrolyte, the stability of the solid electrolyte is expected to improve against lithium metal, and the ionic conductivity could remain close to the value of the original polymer electrolyte, as long as an appropriate thickness of the ceramic electrolyte is applied. Here, we report a bilayered lithium-ion conducting hybrid solid electrolyte consisting of a blended polymer electrolyte (BPE) coated with a thin layer of the inorganic solid electrolyte lithium phosphorous oxynitride (LiPON). The hybrid system was thoroughly studied. First, we investigated the influence of the polymer chain length and lithium salt ratio on the ionic conductivity of the BPE based on poly(ethylene oxide) (PEO) and poly(propylene carbonate) (PPC) with the salt lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). The optimized BPE consisted of 100 k molecular weight PEO, 50 k molecular weight PPC, and 25(w/w)% LiTFSI, (denoted as PEO100PPC50LiTFSI25), which exhibited an ionic conductivity of 2.11 × 10-5 S/cm, and the ionic conductivity showed no thermal memory effects as the PEO crystallites were well disrupted by PPC and LiTFSI. Second, the effects of LiPON coating on the BPE were evaluated as a function of thickness down to 20 nm. The resulting bilayer structure showed an increase in the voltage window from 5.2 to 5.5 V (vs Li/Li+) and thermal activation energies that approached the activation energy of the BPE when thinner LiPON layers were used, resulting in similar ionic conductivities for 30 nm LiPON coatings on PEO100PPC50LiTFSI25. Coating BPEs with a thin layer of LiPON is shown to be an effective strategy to improve the long-term stability against lithium.

Keywords: LiPON; critical thickness; hybrid bilayer electrolyte; lithium-ion batteries; molecular weight; solid-state.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Cell arrangements for testing: (a) BPEs and (b) bilayer structured hybrid electrolytes. Arrangements shown for both blocking (B.E.) and nonblocking electrodes (N.B.E).
Figure 2
Figure 2
Typical XRD patterns for (a) PEO100, PEO100PPC50 blend, and the resulting blend electrolyte films with different salt ratios, and (b) optimal BPE (PEO100PPC50LiTFSI25), pure LiPON, and LiPON coated BPE.
Figure 3
Figure 3
Optical microscopy images of (a) PEO100, (b) PEO100PPC50 (1:1), (c) PEO100PPC50LiTFSI25, (d) PEO300, (e) PEO300PPC50 (1:1), (f) PEO300PPC50LiTFSI25, (g) PEO600, (h) PEO300PPC50 (1:1), and (i) PEO300PPC50LiTFSI25 [Magnification 20x/ Scale Bar: 50 μm].
Figure 4
Figure 4
DSC measurements for (a) PEO100 and (b) PEO100PPC50LiTFSI25-optimized BPE.
Figure 5
Figure 5
X-ray photoelectron spectroscopy (XPS) spectrum for LiPON showing peaks in descending binding energy for (a) O 1s, (b) N 1s, (c) P 2p, and (d) Li 1s.
Figure 6
Figure 6
Ionic conductivity of each polymer electrolyte system as a function of mass fraction of LiTFSI (a) PEO (100, 300, and 600) PPC50LiTFSIx and (b) temperature effect on PEO100PPC50LiTFSI25 (BPE) and LiPON-coated BPE’s ionic conductivity.
Figure 7
Figure 7
Loss tangent spectra for uncoated PEO100PPC50LiTFSI25 and different thickness LiPON-coated samples.
Figure 8
Figure 8
Linear sweep voltammograms of (a) PEOxPPC50LiTFSI-optimized systems and (b) PEO100PPC50LiTFSI25 (BPE) and LiPON.
Figure 9
Figure 9
Bulk resistivity of the PEO100PPC50LiTFSI25 (BPE), BPE with 20 nm LiPON, and BPE with 30 nm LiPON to assess the effects of storage times of the electrolytes against lithium metal.
Figure 10
Figure 10
Schematic illustration of the likely interface formed at the LiPON/Lithium contact (Li contact at the top side, BPE at the bottom side).
See this image and copyright information in PMC

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