Energetics of Ion Transport in NASICON-Type Electrolytes

Herein we report a study on the energetics of ion transport in NASICON-type solid electrolytes. A sol–gel procedure was used to synthesize NASICON-type lithium-ion conductors with nominal compositions Li1+X Al X Ge2–X (PO4)3 where 0 ≤ X ≤ 0.6. Trends in the conductivity and activation energy, including both enthalpic and entropic contributions, were examined with electrochemical impedance spectroscopy. Physical interpretations of these results are drawn from structural characterizations performed by synchrotron powder X-ray diffraction and Raman spectroscopy. Considering X = 0 → 0.6, we conclude that initial drops in activation energy are driven by a growing Li+ population on M2 sites, while later increases in activation energy are driven by changes in average bottleneck size caused by the Al-for-Ge substitution. Values of the entropy of motion are rationalized physically by considering the changing configurational potential of the mobile Li+ population with changes in X. We conclude that entropic contributions to the free energy of activation amount to ≤22% of the enthalpic contributions at room temperature. These insights suggest that while entropic contributions are not insignificant, more attention should be paid to lowering the activation energy when designing a new NASICON-type conductor.