Toward Understanding the Lithium Transport Mechanism in Garnet-type Solid Electrolytes: Li⁺ Ion Exchanges and Their Mobility at Octahedral/Tetrahedral Sites

The cubic garnet-type solid electrolyte Li₇La₃Zr₂O₁₂ with aliovalent doping exhibits a high ionic conductivity, reaching up to ∼10^–3 S/cm at room temperature. Fully understanding the Li⁺ transport mechanism including Li⁺ mobility at different sites is a key topic in this field, and Li_{7–2<i>x</i>–3<i>y</i>}Al_<i>y</i>La₃Zr_2–<i>x</i>W_<i>x</i>O₁₂ (0 ≤ <i>x</i> ≤ 1) are selected as target electrolytes. X-ray and neutron diffraction as well as ac impedance results show that a low amount of aliovalent substitution of Zr with W does not obviously affect the crystal structure and the activation energy of Li⁺ ion jumping, but it does noticeably vary the distribution of Li⁺ ions, electrostatic attraction/repulsion, and crystal defects, which increase the lithium jump rate and the creation energy of mobile Li⁺ ions. For the first time, high-resolution NMR results show evidence that the 24d, 96h, and 48g sites can be well-resolved. In addition, ionic exchange between the 24d and 96h sites is clearly observed, demonstrating a lithium transport route of 24d–96h–48g–96h–24d. The lithium mobility at the 24d sites is found to dominate the total ionic conductivity of the samples, with diffusion coefficients of 10^–9 m² s^–1 and 10^–12 m² s^–1 at the octahedral and tetrahedral sites, respectively

Copper Phosphate as a Cathode Material for Rechargeable Li Batteries and Its Electrochemical Reaction Mechanism

In the search for new cathode materials for rechargeable lithium batteries, conversion-type materials have great potential because of their ability to achieve high specific capacities via the full utilization of transition metal oxidation states. Here, we report for the first time that copper phosphate can be used as a novel high-capacity cathode for rechargeable Li batteries, capable of delivering a reversible capacity of 360 mAh/g with two discharge plateaus of 2.7 and 2.1 V at 400 mA/g. The underlying reaction involves the formation as well as the oxidation of metallic Cu. The solid-state NMR, <i>in situ</i> XAFS, HR-TEM, and XRD results clearly indicate that Cu can react with Li₃PO₄ to form copper phosphate and Li_<i>x</i>Cu_<i>y</i>PO₄ during the charging process, largely determining the reversibility of Cu₃(PO₄)₂. This new reaction scheme provides a new venue to explore polyanion-type compounds as high-capacity cathode materials with conversion reaction processes