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1. Thermal and electronic transport properties of A Cr X ₂ superionic conductors (A=Cu, Ag and X=S, Se)

   https://doi.org/10.1088/2515-7655/addf7e  

   Rahman, Md Towhidur; Ciesielski, Kamil; Pelkey, Joseph; Shawon, A_K_M Ashiquzzaman; Toberer, Eric; Zevalkink, Alexandra (June 2025, Journal of Physics: Energy)

   Abstract Superionic conductors, includingACrX₂(A=Ag, Cu; X = S, Se) compounds, have attracted attention due to their low lattice thermal conductivity and high ionic conductivity. These properties are driven by structural characteristics such as anharmonicity, soft bonding, and disorder, which enhance both fast ion transport and thermal resistance. In the present study, we investigate the impact of various factors (e.g.A-site disorder, microstructure, speed of sound and chemical composition) on the thermal conductivity of the compounds CuCrS₂, CuCrSe₂, AgCrS₂and AgCrSe₂. The samples were synthesized using solid state reaction, ball milling and subsequent spark plasma sintering, and thermal diffusivity, electrical resistivity, Hall coefficients and Seebeck coefficients were measured as a function of temperature. The selenides were found to behave as degenerate semiconductors, with reasonable thermoelectric figure of merit (up to 0.79 in CuCrSe₂), while the sulfides behaved as non-degenerate semiconductors with high electrical resistivity. At room temperature, all samples are in the ordered phase and show low lattice thermal conductivity ranging from 0.60 W m⁻¹-K in AgCrSe₂to 1.1 W m⁻¹-K in CuCrSe₂. Little reduction in lattice thermal conductivity was observed in the high-temperature phase, despite the increased disorder on the cation site and the onset of superionic conductivity. This suggests that the low lattice thermal conductivity inACrX₂compounds is an inherent property of the crystal structure, caused by anharmonic bonding and diffuson dominated transport. 

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2. Tailoring Ion Transport in Li _3‐3y Ho _1+y Cl _6‐x Br _x via Transition‐Metal Free Structural Planes and Charge Carrier Distribution

   https://doi.org/10.1002/advs.202409668  

   Ogbolu, Bright O; Poudel, Tej P; Dikella, Thilina_N_D D; Truong, Erica; Chen, Yudan; Hou, Dewen; Li, Tianyi; Liu, Yuzi; Gabriel, Eric; Xiong, Hui; et al (February 2025, Advanced Science)

   Abstract Localized atomistic disorder in halide‐based solid electrolytes (SEs) can be leveraged to boost Li⁺mobility. In this study, Li⁺transport in structurally modified Li₃HoCl₆, via Br⁻introduction and Li⁺deficiency, is explored. The optimized Li_3‐3yHo_1+yCl_6‐xBr_xachieves an ionic conductivity of 3.8 mS cm⁻¹at 25 °C, the highest reported for holmium halide materials.^6,7Li nuclear magnetic resonance and relaxometry investigations unveil enhanced ion dynamics with bromination, attaining a Li⁺motional rate neighboring 116 MHz. X‐ray diffraction analyses reveal mixed‐anion‐induced phase transitions with disproportionate octahedral expansions and distortions, creating Ho‐free planes with favorable energetics for Li⁺migration. Bond valence site energy analysis highlights preferred Li⁺transport pathways, particularly in structural planes devoid of Ho³⁺blocking effects. Molecular dynamics simulations corroborate enhanced Li⁺diffusion with Br⁻introduction into Li₃HoCl₆. Li‐Ho electrostatic repulsions in the (001) plane presumably drive Li⁺diffusion into the Ho‐free (002) layer, enabling rapid intraplanar Li⁺motion and exchange between the 2d and 4h sites. Li_3‐3yHo_1+yCl_6‐xBr_xalso demonstrates good battery cycling stability. These findings offer valuable insights into the intricate correlations between structure and ion transport and will help guide the design of high‐performance fast ion conductors for all‐solid‐state batteries. 

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3. On the structure of heptalithium distannide: Li₇Sn₂ or Li_7−xSn₂ (0.3 < x < 0.5)

   https://doi.org/10.1039/D5DT01022E  

   Rahman, Salina; Ghosh, Kowsik; Ovchinnikov, Alexander; Nandakumar, Anirudh; Chan, Candace K; Bobev, Svilen (July 2025, Dalton Transactions)

   New structural insights into Li_7−xSn₂reveal partial/vacant Li occupancy. 

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4. Oxygen‐Induced Structural Disruption for Improved Li ⁺ Transport and Electrochemical Stability of Li ₃ PS ₄

   https://doi.org/10.1002/aenm.202302785  

   Deck, Michael J.; Chien, Po‐Hsiu; Poudel, Tej P.; Jin, Yongkang; Liu, Haoyu; Hu, Yan‐Yan (January 2024, Advanced Energy Materials)

   Abstract The performance of all‐solid‐state batteries (ASSBs) relies on the Li⁺transport and stability characteristics of solid electrolytes (SEs). Li₃PS₄is notable for its stability against lithium metal, yet its ionic conductivity remains a limiting factor. This study leverages local structural disorder via O substitution to achieve an ionic conductivity of 1.38 mS cm⁻¹with an activation energy of 0.34 eV for Li₃PS_4−xO_x(x = 0.31). Optimal O substitution transforms Li⁺transport from 2D to 3D pathways with increased ion mobility. Li₃PS_3.69O_0.31exhibits improvements in the critical current density and stability against Li metal and retains its electrochemical stability window compared with Li₃PS₄. The practical implementation of Li₃PS_3.69O_0.31in ASSBs half‐cells, particularly when coupled with TiS₂as the cathode active material, demonstrates substantially enhanced capacity and rate performance. This work elucidates the utility of introducing local structural disorder to ameliorate SE properties and highlights the benefits of strategically combining the inherent strengths of sulfides and oxides via creating oxysulfide SEs. 

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5. Electrochemical lithiation of the layered superionic conductors AgCrSe₂ and CuCrSe₂

   https://doi.org/10.26434/chemrxiv-2025-wg75n  

   Moon, Seongbak; Rahman, Md Towhidur; Holzapfel, Noah P; Zevalkink, Alexandra; Augustyn, Veronica (October 2025, ChemRxiv)

   The layered transition metal chalcogenides MCrX₂ (M = Ag, Cu; X = S, Se, Te) are of interest for energy storage because chemically Li-substituted analogs were reported as superionic Li⁺ conductors. The coexistence of fast ion transport and reducible transition metal and chalcogen elements suggests that this family may offer multifunctional capability for electrochemical storage. Here, we investigate the electrochemical reduction of AgCrSe₂ and CuCrSe₂ in non-aqueous Li- and Na-ion electrolytes using electrochemical measurements coupled with ex situ characterization (scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy). Both compounds delivered high initial specific capacities (~ 560 mAh/g), corresponding to 6.64 and 5.73 Li⁺/e^- per formula unit for AgCrSe₂ and CuCrSe₂, respectively. We attribute this difference to distinct reduction pathways: 1) Li⁺ intercalation to form LiCrSe₂ and extruded Ag or Cu, 2) conversion of LiCrSe₂ to Li₂Se, and 3) formation of an Ag-Li alloy at the lowest potential, operative only in AgCrSe₂. Consistent with this proposed mechanism, step 3 was absent during reduction of AgCrSe₂ in a Na-ion electrolyte since Ag does not alloy with Na. These results demonstrate the complex reduction chemistry of MCrX₂ in the presence of alkali ions, providing insights into the use of MCrX₂ materials as alkali ion superionic conductors or high capacity electrodes for lithium or sodium-ion type batteries. 

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