Energetics of Ion Transport in NASICON-Type Electrolytes

Abstract

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 Li 1+X Al X Ge 2−X (PO 4 ) 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. ■ INTRODUCTIONWith the demand for high-performing rechargeable lithium-ion batteries continually on the rise, much research effort over the past decade has been spent on the development of materials with enhanced electrochemical properties. As all-solid-state batteries show great promise to meet many performance needs, a targeted research effort to develop high-conducting solid-state separator materials has developed. Materials of the sodium superionic conductor (NASICON) family are promising candidates, as many have demonstrated high conductivity, electrochemical stability, and mechanical integrity. 1−4 With these and other classes of ion conductor showing promise for use in next-generation energy storage technologies, it is helpful to have a clear picture of the various factors that contribute to the measured conductivities.In the case of ion-conducting solids such as the NASICONtype materials, the conductivity (σ) is governed by the relation σ = cμq, where c is the density of charge carriers, μ is the mobility of the charge carriers, and q is the charge carried by each carrier. Here we can see two obvious approaches to enhance the conductivity of such a material: increasing the concentration of charge carriers or increasing carrier mobility. Indeed, a common method of enhancing the conductivity of the established NASICON-type conductor lithium germanium phosphate is with a heterovalent doping scheme: 5,6 LiGe 2 (PO 4 ) 3 → Li 1+X Al X Ge 2−X (PO 4 ) 3 . Since the radii of Ge 4+ (0.53 Å) and Al 3+ (0.535 Å) are nearly the same and there are many unfilled sites available for the additional lithium ions in the structure, the substitution is easily accomplished. This doping has been reported to increase the conductiv...

Thermophysical Properties of Lithium Aluminum Germanium Phosphate with Different Compositions

“…The thermal transport behavior of superionic conductors has been studied by a number of numerical models [22,28,29,30] using different approaches. The main results of the modeling can be summarized in the following predictions: The thermal conductivity as a function of the temperature should show the same exponential behavior like the ionic conductivity.…”

Structural elucidation of NASICON (Na3Al2P3O12) based glass electrolyte materials: effective influence of boron and gallium

“…However, so far, only a few reports were found highlighting the sodium ionic conductors with the NASICON-type structure containing Al cations. 19,25,26 Therefore, the present research reported here, aims at developing the Al-containing NASICON (Na 3 Al 2 P 3 O 12 ) glass for use as a solid electrolyte material by understanding the glass structure and the modications that occur in the structure with the changes in its chemical composition.…”

The role of metal substitutions in the development of Li batteries, part II: solid electrolytes

“…147 Higher substituent content increases activation energy due to the smaller bottleneck size. 148 Further co-substituting with Y tweaks the microstructure and grain morphology producing a denser product with lower intergrain resistance. 149 Other co-substitution explored on Al-LGP is Sr where due to its large ionic radius has limited solubility of 0.17 per unit formula above which secondary phase (ScPO 4 ) forms at the grain boundary lowering intergrain lithium diffusion.…”