Ion Conduction and Electrochemical Performance of Poly(oxetane)-Based Electrolytes with Tri(cyanoethoxymethyl) Moiety As a Side Chain
Authors: Nakano Yohei , Tsutsumi Hiromori 
Published: February 2020

Solid polymer electrolytes (SPEs) have been investigated by many scientists and engineers who would like to enhance safety of batteries and/or reduce their weight and volume. The large number of studies about SPEs has focused on enhancement of their ionic conductivity. Many SPEs’ matrixes have polyether structure, especially, polyethylene oxide (PEO) structure in their main chain and/or side chains. The highest ionic conductivity of PEO-based solid polymer electrolyte films is approximately 0.1 mS/cm.

In this study, we prepared a poly(oxetane) derivative with tri(cyanoethoxymethyl) moiety, PCOA (Fig. 1). The repeating units of PCOA are trimethylene oxide which is flexible as similar as ethylene oxide is. PCOA also has a nitrile groups on each branched side chain for dissociation of the added lithium (Li) salt. Characterization of PCOA-based solid polymer electrolytes was carried out with FT-IR, DSC, ionic conductivity measurements. We also demonstrated electrochemical deposition and dissolution of lithium on a nickel (Ni) plate in the PCOA-based solid polymer electrolyte films with potential cycling.

Oxetane derivative (COA) was prepared by coupling between oxetane with carboxylic acid moiety and amine with tri(cyanoethoxymethyl) group. COA was polymerized through ring opening reaction of oxetane by using a cationic initiator, boron trifluoride - ethyl ether (1:1) complex. The structure of PCOA was confirmed by FT-IR and NMR measurements. Typical preparation procedure of PCOA-based solid polymer electrolyte films was as following. PCOA, poly(vinylidene fluoride-co-hexafluoropropylene), (PVDF-HFP), and Li salt (for example, lithium bis(trifluoromethanesulfonyl)amide, LiTFSA) were dissolved into acetone. The acetone solution was cast on a Teflon plate. The acetone was removed under dynamic vacuum condition. Other PCOA-based solid polymer electrolyte films were also prepared through similar procedure. The film (PCOA, PVDF-HFP, and LiTFSA (0.279 mol per molar of nitrile group)) is abbreviated as PCOA(LiTFSA)(Li/ CN=0.279).

Temperature dependence of ionic conductivity for PCOA-based solid polymer electrolyte films is shown in Fig. 2. The PCOA(LiTFSA)(Li/ CN=0.557) film indicated the highest ionic conductivity in this work. The value at 293 K was 0.0978 mS/cm and at 343 K was 0.738 mS/cm. The temperature dependance curves of ionic conductivity for the PCOA-based solid polymer electrolyte films are slightly convex. The curves are fitted to Vogel-Tamman-Fulcher (VTF) equation. The estimated activation energy of PCOA-based solid polymer electrolyte films on ionic conduction is in the range from 7.44 kJ/mol to 9.81 kJ/mol. The values of PCOA-based electrolyte films are similar to that in the PEO-Li trifluoromethanesulfonate systems (8.69-12.1 kJ/mol) [1] and the PEO-based electrolytes with propylene carbonate or ethylene carbonate as a plasticizer (5.57-7.87 kJ/mol) [2].

IR spectra of the PCOA-based electrolyte films with various Li salt concentration and the Li salt-free PCOA film were also recorded. We checked the peak position of CN stretching mode in PCOA matrix. The peak position of the PCOA-based electrolyte films shifted to higher wavenumbers by addition of Li salts. This indicates that the Li ions in the PCOA matrix interact with the nitrile groups of the PCOA matrix. The nitrile groups in the PCOA act as a dissolution enhancer of Li salt.

The cyclic voltammogram of a Ni plate in PCOA(LiTFSA)(Li/CN=0.557) is shown in Fig. 3. In cathodic scan increase in current suggests that Li ions deposit on the Ni plate as Li metal. Anodic peak based on dissolution process of Li from the Ni electrode is also observed.

References

[1] N. K. Karan et.al., Solid State Ionics, 179 (2008) 689-696.

[2] Y.-J. Wang et.al., Polymer International, 56 (2007) 381-388.