Dual Substitution Strategy to Enhance Li⁺ Ionic Conductivity in Li₇La₃Zr₂O₁₂ Solid Electrolyte

Solid state electrolytes could address the current safety concerns of lithium-ion batteries as well as provide higher electrochemical stability and energy density. Among solid electrolyte contenders, garnet-structured Li₇La₃Zr₂O₁₂ appears as a particularly promising material owing to its wide electrochemical stability window; however, its ionic conductivity remains an order of magnitude below that of ubiquitous liquid electrolytes. Here, we present an innovative dual substitution strategy developed to enhance Li-ion mobility in garnet-structured solid electrolytes. A first dopant cation, Ga³⁺, is introduced on the Li sites to stabilize the fast-conducting cubic phase. Simultaneously, a second cation, Sc³⁺, is used to partially populate the Zr sites, which consequently increases the concentration of Li ions by charge compensation. This aliovalent dual substitution strategy allows fine-tuning of the number of charge carriers in the cubic Li₇La₃Zr₂O₁₂ according to the resulting stoichiometry, Li_{7–3<i>x</i>+y}Ga_<i>x</i>La₃Zr_2–<i>y</i>Sc_<i>y</i>O₁₂. The coexistence of Ga and Sc cations in the garnet structure is confirmed by a set of simulation and experimental techniques: DFT calculations, XRD, ICP, SEM, STEM, EDS, solid state NMR, and EIS. This thorough characterization highlights a particular cationic distribution in Li_6.65Ga_0.15La₃Zr_1.90Sc_0.10O₁₂, with preferential Ga³⁺ occupation of tetrahedral Li_24<i>d</i> sites over the distorted octahedral Li_96<i>h</i> sites. ⁷Li NMR reveals a heterogeneous distribution of Li charge carriers with distinct mobilities. This unique Li local structure has a beneficial effect on the transport properties of the garnet, enhancing the ionic conductivity and lowering the activation energy, with values of 1.8 × 10^–3 S cm^–1 at 300 K and 0.29 eV in the temperature range of 180 to 340 K, respectively

Crystallographic Control at the Nanoscale To Enhance Functionality: Polytypic Cu₂GeSe₃ Nanoparticles as Thermoelectric Materials

The potential to control the composition and crystal phase at the nanometer scale enable the production of nanocrystalline materials with enhanced functionalities and new applications. In the present work, we detail a novel colloidal synthesis route to prepare nanoparticles of the ternary semiconductor Cu₂GeSe₃ (CGSe) with nanometer-scale control over their crystal phases. We also demonstrate the structural effect on the thermoelectric properties of bottom-up-prepared CGSe nanomaterials. By careful adjustment of the nucleation and growth temperatures, pure orthorhombic CGSe nanoparticles with cationic order or polytypic CGSe nanoparticles with disordered cation positions can be produced. In this second type of nanoparticle, a high density of twins can be created to periodically change the atomic plane stacking, forming a hexagonal wurtzite CGSe phase. The high yield of the synthetic routes reported here allows the production of single-phase and multiphase CGSe nanoparticles in the gram scale, which permits characterization of the thermoelectric properties of these materials. Reduced thermal conductivities and a related 2.5-fold increase of the thermoelectric figure of merit for multiphase nanomaterials compared to pure-phase CGSe are systematically obtained. These results are discussed in terms of the density and efficiency of phonon scattering centers in both types of materials