  The addition of filler particles to polymer electrolytes is known to
increment their ionic conductivity (IC). A detailed understanding of how the
interactions between the constituent materials are responsible for the
enhancement, remains to be developed. A significant contribution is ascribed to
an increment of the polymer amorphous fraction, induced by the fillers,
resulting in the formation of higher ionic conductivity channels in the polymer
matrix. However, the dependence of IC on the particle weight load and its
composition on the polymer morphology is not fully understood. This work
investigates Li ion transport in composite polymer electrolytes (CPE)
comprising Bi-doped LLZO particles embedded in PEO: LiTFSI matrixes. We find
that the IC optimizes for very low particle weight loads (5 to 10%) and that
both its magnitude and the load required, strongly depend on the garnet
particle composition. Based on structural characterization results and
electrochemical impedance spectroscopy, a mechanism is proposed to explain
these findings. It is suggested that the Li-molar content in the garnet
particle controls its interactions with the polymer matrix, resulting at the
optimum loads reported, in the formation of high ionic conductivity channels.
We propose that filler particle chemical manipulation of the polymer morphology
is a promising avenue for the further development of composite polymer
electrolytes.
