Unveiling Cation–Polymer Coordination: A Reverse Approach from Neutron Scattering in Dilute Acetonitrile Solutions

Quantifying cation coordination is essential for understanding ion transport in solid polymer electrolytes, yet reliable experimental determination of coordination numbers remains challenging. Here, we establish a calibrated framework that integrates Fourier transform infrared spectroscopy with neutron total scattering data to quantify cation coordination environments relevant to polymer electrolytes. Building on FTIR measurements using acetonitrile as a vibrational probe, apparent cation–acetonitrile coordination numbers can be extracted but are systematically overestimated due to coordination-dependent absorbance effects. Here, neutron total scattering provides an independent structural reference that enables correction of these biases. The corrected coordination numbers are subsequently converted to cation–polymer coordination numbers through a competitive coordination framework based on a site-balance assumption. Using this approach, quantitative coordination numbers for Li+, Na+, and Mg2+ are determined for poly(ethylene oxide) in acetonitrile solution across a range of compositions. This approach yields quantitatively reliable coordination numbers across multiple cation chemistries and polymer concentrations, providing a robust and experimentally accessible basis for interpreting spectroscopic coordination data in polymer electrolyte-relevant environments.