Anion-Mixed Chloride Solid Electrolytes for All-Solid-State Lithium Batteries

Abstract

All-solid-state batteries utilizing inorganic solid electrolytes are widely regarded as one of the most promising technological pathways to address the safety concerns associated with traditional liquid lithium-ion batteries. Compared to flammable organic electrolytes, solid electrolytes significantly reduce the risk of thermal runaway and maintain stability under higher voltage conditions. Furthermore, all-solid-state batteries with a broader electrochemical stability window, enable the use of high-capacity cathode materials and lithium metal anodes, opening new avenues for further enhancing battery energy density. Among various solid electrolyte systems, chloride solid electrolytes stand out as highly promising candidates due to their high oxidation potential and excellent ductility. However, the low ionic conductivities of chloride electrolytes limits rapid ion transport at room temperature, thereby affecting the rate performance. In this thesis, we mainly focus on several strategies to tune the local structure and improve the ionic conductivities of chlorides solid electrolytes (SEs).

Firstly, we proposed a strategy to design an oxychloride electrolyte Li_2.176ZrCl_6.176Sm_0.176O_0.265 (LZCSO) via one-step mechanochemical route using Sm₂O₃ as the oxygen source. The LZCSO exhibits improved ionic conductivity at 30 °C, from 0.40 to 1.1 mS cm⁻¹ compared to Li₂ZrCl₆ (LZC). The mechanochemical process with Sm₂O₃ enables the formation of amorphous phase, facilitating the Li ions conduction. Moreover, a surface-derived Sm₂O₃ protective layer enables LZCSO to retain 0.8 mS cm⁻¹ after 24 h exposure at 5% relative humidity, which is over 100-fold improved humidity tolerance compared to pristine LZC. In addition, the mechanochemical reaction pathway during co-milling was further elucidated by in-situ synchrotron X-ray diffraction, revealing a real-time structural evolution of the chloride framework. When deployed in full cells with layered oxide cathodes, LZCSO SEs deliver stable cycling with 78% capacity retention over 400 cycles.

Secondly, we designed new family of amorphous nitrichloride SEs Li_3xMN_xCl_y (M=Al or Hf, 1≤3x≤1.4, y=5 or 3), synthesized by mechanochemical method with low-cost precursors. Notably, the introduction of N atoms with high charge density formed highly amorphous phase in Li_3xMN_xCl_y SEs, which was confirmed by Cryo-TEM. The amorphous phase can eliminate the grain boundary for Li conduction. LMCN SEs feature with bridge N atoms between Metal-Metal atoms, enabling the formation of disordered polyhedra that support fast ion conduction, which was verified by a combination of XPS, XAS, PDF and Raman spectra. LHNC achieves ionic conductivities as high as 2.16 mS cm^‒1 and the ionic conductivity of LANC was improved to 0.24 mS cm^‒1. When applied in SSBs, LHNC enables stable cycling with 80% capacity retention over 500 cycles at 1 C.

Thirdly, we designed a novel class of selenichloride-based amorphous solid-state electrolytes, which exhibit impressive ionic conductivities of up to 6.6×10⁻³ S cm⁻¹ at 30 °C, ranking among the highest reported for amorphous SEs and approaching the performance of widely studied crystalline electrolytes. The local structures of the Li_2xMSe_xCl_y amorphous electrolytes are comprehensively investigated and correlated with their ionic conductivities. Our analysis reveals that the Se²⁻ with high polarizability network achieves a highly disordered structure in long and medium range, playing a crucial role in enabling fast Li-ion transport within these materials. Systematic investigations into the mechanochemical process reveal that high-energy ball-milling induces rapid amorphization and significant grain refinement, effectively optimizing the particle morphology and contact area for ion transport.

In conclusion, we systematically explored the synthesis and structural regulation strategies for anion-mixed chloride SEs to enhance their ionic conductivities. This thesis has guiding significance for the structural design and the development of anion-mixed chemistry in designing amorphous SEs and provides new avenues for the development of cost-effective, high-performance solid-state batteries.

Date of Award	9 Jul 2026
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Yang REN (Supervisor)