Garnet-type solid-state electrolytes (SSEs) are a remarkable Li-ion electrolyte for the realization of next-generation all-solid-state lithium batteries due to their excellent stability against Li metal as well as high ionic conductivities at room temperature. However, garnet electrolytes always contain undesired and hardly removable Li₂CO₃ contaminations that have persistently large resistance and unstable interface contact with Li metal. This is a critical bottleneck for the practical application of garnet electrolytes. Here, we design a novel strategy to completely root out Li₂CO₃ both inside and on the surface of garnet. This is achieved by a so-called double replacement reaction between Li₂CO₃ and SiO₂ during one-step hot press process for garnet electrolyte densification. It leads to in-situ transformation of Li_xSiO_y (LSO) mostly locating around the grain boundaries of garnet. Due to the higher ion conductivity and better electrochemistry stability of LSO than Li₂CO₃, the modified garnet electrolyte shows much improved electrochemical performance. Moreover, the wettability between modified garnet electrolyte and lithium metals was significantly enhanced in the absence of surface Li₂CO₃. As a proof of concept, an assembled Li symmetric cell with modified garnet electrolyte displays a high critical current density (CCD) of 0.7 mA cm⁻² and a low interfacial impedance (5 Ω cm²) at 25 ℃. These results indicate that the upcycling of Li₂CO₃ is a promising strategy to well-address the degradation and interfacial issue associated with garnet electrolytes.

The rhombohedral α-GeTe can be approximated as a slightly distorted rock-salt structure along its [1 1 1] direction and possesses superb thermoelectric performance. However, the role of such a ferroelectric-like structural distortion on its transport properties remains unclear. Herein, we performed a systematic study on the crystal structure and electronic band structure evolutions of Ge_1-xSn_xTe alloys where the degree of ferroelectric distortion is continuously tuned. It is revealed that the band gap is maximized while multiple valence bands are converged at x = 0.6, where the ferroelectric distortion is the least but still works. Once undistorted, the band gap is considerably reduced, and the valence bands are largely separated again. Moreover, near the ferro-to-paraelectric phase transition Curie temperature, the lattice thermal conductivity reaches its minima because of significant lattice softening enabled by ferroelectric instability. We predict a peak ZT value of 2.6 at 673 K in α-GeTe by use of proper dopants which are powerful in suppressing the excess hole concentrations but meanwhile exert little influence on the ferroelectric distortion.