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.

Thermoelectric superlattices are expected to decouple the strong correlation between various thermoelectric parameters, and are an important strategy for excellent thermoelectric performances. The superlattices of (Bi₂)_m(Bi₂Te₃)_n homologous series are well-known for low lattice thermal conductivity and intriguing topological surface states. However, the impacts of electronic structure on the thermoelectric performance were still not well-understood in (Bi₂)_m(Bi₂Te₃)_n. To cope with this issue, Bi₂–Bi₂Te₃ superlattice-like films with adjustable Bi₂/(Bi₂+Bi₂Te₃) molar ratio (R) were successfully fabricated by the molecular beam epitaxy technique. Angle-resolved photoemission spectroscopy measurements combined with theoretical calculations revealed the conduction band evolution from single-valley to multi-valley as R ≥ 0.30, leading to intrinsically high carrier effective mass and improved thermoelectric power factor. Also, the superlattice film (R = 0.46) with the structure close to Bi₄Te₃ possesses the topological surface state feature around the high symmetry point. As a result of the high effective mass of 3.9 m₀ and very high electron density of 2.31 × 10²¹ cm⁻³, the film with R = 0.46 acquired the highest power factor of 1.49 mW·m⁻¹·K⁻² at 420 K, outperforming that of other (Bi₂)_m(Bi₂Te₃)_n superlattices. This work lays an essential foundation on understanding the electronic structure and further improving thermoelectric performances of (Bi₂)_m(Bi₂Te₃)_n homologous series.