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Garnet-structured Li7La3Zr2O12 is a promising solid Li-ion electrolyte for all-solid-state Li-metal batteries and Li-redox-flow batteries owing to its high Li-ion conductivity at room temperature and good electrochemical stability with Li metal. However, there are still three major challenges unsolved: (1) the controversial electrochemical window of garnet, (2) the impractically large resistance at a garnet/electrode interface and the fast lithium-dendrite growth along the grain boundaries of the garnet pellet, and (3) the fast degradation during storage. We have found that these challenges are closely related to a thick Li[sub 2]CO[sub 3] layer and the Li-Al-O glass phase on the surface of garnet materials. Here we introduce a simple method to remove Li2CO3 and the protons in the garnet framework by reacting garnet with carbon at 700 ºC; moreover, the amount of the Li-Al-O glass phase with a low Li-ion conductivity in the grain boundary on the garnet surface was also reduced. The surface of the carbon-treated garnet pellets is free of Li2CO3 and is wet by a metallic lithium anode, an organic electrolyte, and a solid composite cathode. The carbon post-treatment has reduced significantly the interfacial resistances to 28, 92 (at 65 ºC), and 45 Ω cm² at Li/garnet, garnet/LiFePO4, and garnet/organic-liquid interfaces, respectively. A symmetric Li/garnet/Li, an all-solid-state Li/garnet/LiFePO4, and a hybrid Li-S cell show small overpotentials, high Coulombic efficiencies, and stable cycling performance. [ABSTRACT FROM AUTHOR]

Potassium has as rich an abundance as sodium in the earth, but the development of a K-ion battery is lagging behind because of the higher mass and larger ionic size of K+ than that of Li+ and Na+, which makes it difficult to identify a high-voltage and high-capacity intercalation cathode host. Here we propose a cyanoperovskite KxMnFe(CN)6 (0 ≤ x ≤ 2) as a potassium cathode: high-spin MnIII/MnII and low-spin FeIII/FeII couples have similar energies and exhibit two close plateaus centered at 3.6 V; two active K+ per formula unit enable a theoretical specific capacity of 156 mAh g-1; Mn and Fe are the two most-desired transition metals for electrodes because they are cheap and environmental friendly. As a powder prepared by an inexpensive precipitation method, the cathode delivers a specific capacity of 142 mAh g-1. The observed voltage, capacity, and its low cost make it competitive in large-scale electricity storage applications. [ABSTRACT FROM AUTHOR]

Sodium extraction behavior in the rhombohedral Na3MnTi(PO4)3 with the NASICON structure was investigated and shown to have a 0.5 eV separation of the localized-electron Mn3+/Mn2+ and Mn4+/Mn3+ redox energies. Used as a cathode in a sodium-ion half-cell, the redox couples of Mn3+/Mn2+ and Mn4+/Mn3+ exhibit two voltage plateaus located at about 3.6 and 4.1 V vs Na+/Na, respectively. Na3MnTi(PO4)3 also exhibits a small volume change after the extraction of two sodium ions, and the NASICON structure is well-maintained after repeated extraction and insertion of sodium ions. [ABSTRACT FROM AUTHOR]

Garnet-structured solid electrolytes have been extensively studied for a solid-state lithium rechargeable battery. Previous works have been mostly focused on the materials' development and basic electrochemical properties but not the cathode/electrolyte interface. Understanding the cathode interface is critical to enhance chemical stability and electrochemical performance of a solid-state battery cell. In this work, we studied thoroughly the cathode/electrolyte interface between LiCoO2 and Li7La3Zr2O12 (LLZO). It was found that the high-temperature process to fuse LiCoO2 and LLZO induced cross-diffusion of elements and formation of the tetragonal LLZO phase at the interface. These degradations affected electrochemical performance, especially the initial Coulombic efficiency and cycle life. In a clean cathode interface without the thermal process, an irreversible electrochemical decomposition at > ∼ 3.0 V vs Li+/Li was identified. The decomposition was able to be avoided by a surface modification of LLZO (e.g., Co-diffused surface layer and/or presence of an interlayer, Li3BO3), and the surface modification was equally important to suppress a reaction during air storage. In a LiCoO2/LLZO interface, it is important to separate direct contacts between LiCoO2 and pure LLZO. [ABSTRACT FROM AUTHOR]