Characterization of the Particle-Polymer Interface in Dual-Phase Electrolytes by Kelvin Probe Force Microscopy

Abstract

In this study, the possibility to characterize the electrochemical characteristics of the particle-polymer interface in dual-phase electrolytes by measuring the contact potential difference with high local resolution is demonstrated. Two different polymer electrolytes, polyethylene oxide (PEO) and poly[bis-2-(2-methoxyethoxy)-ethoxyphosphazene] (MEEP), were investigated in combination with lithium ion conductive Li7La3Zr2O12 (LLZ) particles and two different mixed ionic-electronic conductive ceramic particles: uncoated and carbon coated LiFePO4 (LFP) as typical cathode material and uncoated Li4Ti5O12 as typical anode material. A distinct Volta potential gradient between the particles and the polymer was observable in all cases, except when no lithium salt was present within the polymer matrix. The measured potential gradients can be explained in terms of a contact potential between the polymer electrolyte and the ceramic electrolyte. A more negatively charged space charge layer around LFP particles in PEO matrix and around LLZ particles in MEEP can be explained by enrichment of salt anions in direct vicinity of the particle. Electrochemical characterization with impedance spectroscopy showed an increased conductivity for addition of LFP for PEO while the addition of various particles in different concentrations showed no effect on the conductivity of MEEP. The lithium transference number was unaffected by particle addition for all samples.