$\beta -$NMR Measurements of Lithium Ion Transport in Thin Films of Pure and Lithium-Salt-Doped Poly(ethylene oxide)

$\beta -$Detected nuclear spin relaxation of $^{8}$Li$^{+}$ has been used to study the microscopic diffusion of lithium ions in thin films of poly(ethylene oxide) (PEO), PEO with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), PEO with lithium triflate (LiTf) and PEO with lithium trifluoroacetic acid (LiTFA) with monomer-to-salt ratios of 8.3:1. Hopping of Li$+$ above $\sim$ 250 K follows an Arrhenius law in all of the films. Diffusion of Li$^{+}$ is fastest in pure PEO and decreases in order LiTFSI \textgreater LiTf \textgreater LiTFA. We observed the activation energy for hopping ($E_{\mathrm{A}})$ and the intrinsic hop rate ($\tau _{0}^{-1})$ both increasing in order LiTFA \textless LiTf \textless LiTFSI \textless PEO but the larger $\tau _{0}^{-1}$ outweighs the larger $E_{\mathrm{A}}$ and results in Li$^{+}$ motion being fastest in the pure polymer. The results will be discussed in terms of the ionicity of the salt.