To enable high performance of all solid-state batteries, a catholyte should demonstrate high ionic conductivity, good compressibility and oxidative stability. Here, a LaCl₃-based Na⁺ superionic conductor (Na_1−xZr_xLa_1−xCl₄) with high ionic conductivity of 2.9 × 10⁻⁴S cm⁻¹ (30 °C), good compressibility and high oxidative potential (3.80 V vs. Na₂Sn) is prepared via solid state reaction combining mechanochemical method. X-ray diffraction reveals a hexagonal structure (P6₃/m) of Na_1−xZr_xLa_1−xCl₄, with Na⁺ ions forming a one-dimensional diffusion channel along the c-axis. First-principle calculations combining with X-ray absorption fine structure characterization etc. reveal that the ionic conductivity of Na_1−xZr_xLa_1−xCl₄ is mainly determined by the size of Na⁺-channels and the Na⁺/La³⁺ mixing in the one-dimensional diffusion channels. When applied as a catholyte, the NaCrO₂||Na_0.7Zr_0.3La_0.7Cl₄||Na₃PS₄||Na₂Sn all-solid-state batteries demonstrate an initial capacity of 114 mA h g⁻¹ and 88% retention after 70 cycles at 0.3 C. In addition, a high capacity of 94 mA h g⁻¹ can be maintained at 1 C current density.

Vibration of Br⁻ during the Na⁺ ion migration can significantly reduce ion migration activation energy. Therefore, compared to chlorides, bromide should be able to achieve lower migration activation energy and higher Na⁺ ion conduction.

An in situ fluorination strategy is designed to pre-construct a stable LiF layer on the surface for achieving an ideal SEI.

Solid state reaction is widely used in the synthesis of electrode materials, due to its low cost and good scalability. However, the traditional solid-state reaction is not suitable for the synthesis of materials with multiple elements, such as high entropy or medium entropy materials, due to the poor homogeneity of raw material mixing. Here, we prepared multi-element doped LiNi_0.5Mn_1.5O₄ (medium entropy) cathode material by two step solid state reaction. X-ray diffraction and Raman image show that the homogeneity of multi-element doped LiNi_0.5Mn_1.5O₄ cathode has been greatly improved with this two-step method. As a result, the electrochemical performance is greatly improved, comparing to traditional solid-state reaction. First, the specific capacity at 0.1 C is increased from 126 mAh/g to 137 mAh/g. With a high current density of 10 C, the specific capacity is even increased from 64 mAh/g to 89 mAh/g with this two-step method. Second, the cycle stability is enhanced, with capacity retention of 86% after cycling at 1 C for 500 times (vs. 71% for the one-step method).

High ionic conductivity is the key point in the development of new solid‐state electrolytes. Herein, a combining strategy of anion (O²⁻) doping and structure distortion is applied to enhance the Li⁺ ion conductivity in Li₁₅P₄S₁₆Cl₃, thus converting the nonionic conductor into fast ionic conductor. Solid‐state ⁶Li nuclear magnetic resonance analysis shows redistribution of Li⁺ ions in Li₁₅P₄S₁₆Cl₃ with O²⁻ doping or local structure distortion via ball milling, indicating energy changes at different lithium sites. As a result, the activation energy is reduced from 0.50 to 0.35 eV for the ball‐milled Li₁₅P₄S_15.6O_0.4Cl₃, and the ionic conductivity is enhanced from 10⁻⁹ to 10⁻⁴ S cm⁻¹. The electrochemical stability of Li₁₅P₄S_15.6O_0.4Cl₃ is broadened at the anode side as well. The symmetric cell Li|Li₁₅P₄S_15.6O_0.4Cl₃|Li can cycle more than 1000 h with negligible voltage increase. The LiCoO₂|Li₁₅P₄S_15.6O_0.4Cl₃|Li‐Si all‐solid‐state battery demonstrates an initial capacity of 106 mA h g⁻¹ and retains 92% capacity after 200 cycles at 0.5 C, highlighting excellent rate performance and electrochemical stability.