All-solid-state lithium batteries have become an important direction for next-generation energy storage technologies due to their combined high safety and high energy density, and finding highly compatible inorganic solid electrolytes is key to realizing this technology. At present, the few conventional electrolytes with lithium compatibility are generally limited by thermal runaway risks, low ionic conductivity, or excessively high raw material costs, making them unable to meet multiple practical requirements. In contrast, zirconium-based oxychloride solid electrolytes, with their excellent ionic conductivity, low density, good cathode compatibility, and outstanding cost advantages, are regarded as highly promising candidate materials with commercial potential.

However, these electrolytes face a severe incompatibility challenge with lithium metal in practical applications. Because they contain non-lithium metal cations, they are easily reduced to electronic conductors upon contact with highly reducing lithium metal, thereby forming electronic pathways at the interface and triggering continuous side reactions. Existing coating or doping modification strategies either rely heavily on fragile protective thin films that are vulnerable under high pressure, or are limited by expensive raw materials such as rare earth elements, making it difficult to balance the structural stability of all-solid-state batteries with low-cost mass production requirements. Therefore, how to effectively block interfacial electron percolation and construct an electronically insulating stable passivation layer in such cost-effective systems is the core bottleneck that urgently needs to be overcome in this field.

To address the above issues, a team composed of the University of Science and Technology of China, Yanshan University, and others conducted systematic research using ZEPTOOLS' FIB cryo-transfer system, and innovatively proposed a non-metal cation introduction strategy. By introducing hexavalent sulfur cations into the electrolyte, the reduction produces predominantly electronically insulating products, successfully blocking the interfacial electron percolation pathway and significantly improving the lithium metal compatibility of zirconium-based oxychloride solid electrolytes.

Title: Endowing Metal Oxychloride Solid Electrolytes with Improved Li Compatibility
Journal: Journal of the American Chemical Society
URL:http://dx.doi.org/10.1021/jacs.5c13676

The research team first focused on the mechanochemical synthesis and amorphous structural characterization of the novel xLi₂SO₄–ZrCl₄ solid electrolyte, systematically investigating the effect of compositional variation on the material's microscopic network structure. Through high-resolution transmission electron microscopy, neutron total scattering pair distribution function analysis, and X-ray absorption spectroscopy measurements, the study confirmed that introducing Li₂SO₄ to replace LiCl can significantly promote amorphization of the electrolyte during mechanical ball milling. As the lithium sulfate content x increases, the material structure exhibits extremely weak long-range order, forming a disordered bonding network dominated by an amorphous state. This highly amorphous character endows the material with excellent ionic conductivity exceeding 10⁻⁴ S/cm and good mechanical deformability.

Subsequently, the study focused on evaluating the electrochemical compatibility of different compositional amorphous solid electrolytes with lithium metal, revealing the intrinsic law governing the regulation of symmetric cell cycling stability by non-metal cation content. Tests showed that when the lithium sulfate molar ratio x ≤ 0.6, the cell voltage rises rapidly within a short time, indicating continuous reaction between the electrolyte and lithium. However, when x is increased to 0.8 or above, the lithium symmetric cells exhibit extremely stable charge-discharge voltage profiles. Achieving stable cycling for up to 5000 hours at 25 °C and a critical current density as high as 1.0 mA/cm² strongly confirmed that increasing the non-metal cation component can fundamentally suppress the reductive decomposition of conventional zirconium-based electrolytes.

To thoroughly elucidate the microscopic mechanism by which the interface blocks electron percolation, the research team performed high-precision direct microscopic morphological and compositional analysis of the lithium/electrolyte interface after cycling. During focused ion beam preparation of ultrathin transmission electron microscopy specimens, the team employed ZEPTOOLS' FIB cryo-transfer system to perform sectioning, thinning, and polishing at an ultra-low temperature of –170 °C, successfully avoiding thermal damage and structural disruption of the sensitive interface caused by the high-energy ion beam. With the aid of this non-destructive cryo-transfer technique, the team clearly observed a highly dense reaction interfacial layer approximately 15 μm thick under transmission electron microscopy, and through electron energy loss spectroscopy analysis confirmed that the interface is mainly composed of Li₂S, LiCl, Li₆Zr₂O₇, and a very small amount of the electronically conductive phase Zr₃O. The successful application of the FIB cryo-transfer system precisely corroborated that because the volume fraction of Zr₃O is only 0.93%, it cannot form a continuous electron percolation pathway, rendering the entire interface excellently electronically insulating.

Finally, the study further verified the practical application value and commercial mass production potential of this solid electrolyte in all-solid-state full cells. The research not only achieved high initial discharge capacity and excellent rate performance in cathode composite systems paired with lithium metal anodes and commercial high-voltage cathodes, but also completely eliminated the high-pressure creep limitation of lithium metal by introducing Li₁₃Si₄ anodes. Under a high stack pressure of 190 MPa, the cell achieved highly stable cycling for up to 1312 cycles with a capacity retention above 80%, demonstrating that this zirconium-based oxychloride solid electrolyte can directly interface with highly reactive anodes without the need for expensive sulfide buffer layers, significantly reducing the overall manufacturing cost of all-solid-state lithium batteries and enhancing the overall energy density.

The following figure shows the ZEPTOOLS FIB cryo-transfer system used in this research: