Herein, we report a study on the structural and thermodynamic effects that cation size disparity may have in NASICONtype solid solutions. A sol−gel procedure was used to synthesize two new NASICON-type lithium-ion conductors with nominal compositions LiGe 2−y Sn y (PO 4 ) 3 and Li 1+x Al x Ge 2−y−(1/2)x Sn y−(1/2)x (PO 4 ) 3 . The effect of tin substitution on structure and lithium-ion conductivity was studied with powder X-ray diffraction, Raman spectroscopy, and dielectric spectroscopy. It is found that, although increased unit-cell dimensions derived from X-ray data suggest that tin incorporation should open the conduction bottleneck regions and improve conductivity, a decrease in conductivity is observed. Analysis of the electrical data shows that the conduction activation energy is comprised of contributions from carrier motion and generation, the latter accounting for up to 20% of the total activation energy. This result, currently unreported for NASICON-type materials, is correlated with local structural distortions observed in Raman spectra. It is deduced that the bottleneck regions suffer distortions due to the large ionic radius disparity among cationic constituents, which results in the "trapping" of charge carriers. Data estimated for the entropy of motion are also presented and discussed, considering the most probable thermodynamic equilibrium states.

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...

We report a solid-state Li-ion electrolyte predicted to exhibit simultaneously fast ionic conductivity, wide electrochemical stability, low cost, and low mass density. We report exceptional density functional theory (DFT)-based room-temperature single-crystal ionic conductivity values for two phases within the crystalline lithium–boron–sulfur (Li–B–S) system: 62 (+9, −2) mS cm–1 in Li5B7S13 and 80 (−56, −41) mS cm–1 in Li9B19S33. We report significant ionic conductivity values for two additional phases: between 0.0056 and 0.16 mS/cm –1 in Li2B2S5 and between 0.0031 and 9.7 mS cm–1 in Li3BS3 depending on the room-temperature extrapolation scheme used. To our knowledge, our prediction gives Li9B19S33 and Li5B7S13 the second and third highest reported DFT-computed single-crystal ionic conductivities of any crystalline material. We compute the thermodynamic electrochemical stability window widths of these materials to be 0.50 V for Li5B7S13, 0.16 V for Li2B2S5, 0.45 V for Li3BS3, and 0.60 V for Li9B19S33. Individually, these materials exhibit similar or better ionic conductivity and electrochemical stability than the best-known sulfide-based solid-state Li-ion electrolyte materials, including Li10GeP2S12 (LGPS). However, we predict that electrolyte materials synthesized from a range of compositions in the Li–B–S system may exhibit even wider thermodynamic electrochemical stability windows of 0.63 V and possibly as high as 3 V or greater. The Li–B–S system also has a low elemental cost of approximately 0.05 USD/m2 per 10 μm thickness, which is significantly lower than that of germanium-containing LGPS, and a comparable mass density below 2 g/cm3. These fast-conducting phases were initially brought to our attention by a machine learning-based approach to screen over 12,000 solid electrolyte candidates, and the evidence provided here represents an inspiring success for this model.

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In order to address the poor energy density of bulk all-solid-state lithium-ion batteries a new composite electrode preparation method is proposed. This study demonstrates the concurrent synthesis of the FeS 2 active material and a glass electrolyte which is similar to the (100-x)P 2 S 5 :xLi 2 S system of glass electrolytes. This is accomplished through the successive mechanochemical and thermal treatments of Fe 2 P, S, and Li 2 S. The composite electrode's ionic conductivity of 5.46 × 10 −5 S cm −1 at 60 • C and specific energy of 1.2 Wh g −1 versus a lithium metal anode suggest that this method may provide a more intimate solid-solid interfacial contact between active material and glass electrolyte. The results of this study provide a completely new perspective on the preparation of all-solid-state composite electrodes.

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