New Cost‐Effective Halide Solid Electrolytes for All‐Solid‐State Batteries: Mechanochemically Prepared Fe³⁺‐Substituted Li₂ZrCl₆

Abstract

, 0.7Li(CB 9 H 10)−0.3Li(CB 11 H 12), 6.7 mS cm −1), [11,12] and halides (e.g., Li 3 YX 6 [X = Cl, Br], 0.51-1.7 mS cm −1). [13,14] Thus far, oxide and sulfide SEs have been the most commonly investigated candidates. However, their pros and cons counteract each other. Oxide SEs possess high intrinsic electrochemical oxidation stabilities and relatively acceptable chemical stabilities; however, owing to their brittle nature, it is difficult to integrate them in devices. [3,10,15-17] On the other hand, the most important advantage of sulfide SEs, that is, mechanical deformability, which enables scalable cold-pressing-based fabrication protocols, is offset by their poor (electro)chemical stabilities. [3,16,18-20] On exposing sulfide SEs to humid air, the evolution of toxic H 2 S gases occurs. [21-26] Moreover, sulfide SEs exhibit oxidative decomposition at <3 V (vs Li/Li +) and are also incompatible with conventional layered LiMO 2 (M = Ni, Co, Mn, and Al) cathodes. [16,19,27] This issue can be alleviated by using protective coatings, such as LiNbO 3 and Li 3−x B 1−x C x O 3 ; [7,27] however, this constitutes additional processing costs. Furthermore, the oxidative decomposition of sulfide SEs at the surface of conductive carbon additives is unavoidable. [28-31] Recently, through reinvestigations on halide SEs, several compounds exhibiting Li + conductivities exceeding 10 −4 S cm −1 have been identified. [13,32−36] Asano and coworkers reported that trigonal Li 3 YCl 6 and monoclinic Li 3 YBr 6 showed high Li + Owing to the combined advantages of sulfide and oxide solid electrolytes (SEs), that is, mechanical sinterability and excellent (electro)chemical stability, recently emerging halide SEs such as Li 3 YCl 6 are considered to be a game changer for the development of all-solid-state batteries. However, the use of expensive central metals hinders their practical applicability. Herein, a new halide superionic conductors are reported that are free of rare-earth metals: hexagonal close-packed (hcp) Li 2 ZrCl 6 and Fe 3+-substituted Li 2 ZrCl 6 , derived via a mechanochemical method. Conventional heat treatment yields cubic close-packed monoclinic Li 2 ZrCl 6 with a low Li + conductivity of 5.7 × 10 −6 S cm −1 at 30 °C. In contrast, hcp Li 2 ZrCl 6 with a high Li + conductivity of 4.0 × 10 −4 S cm −1 is derived via ball-milling. More importantly, the aliovalent substitution of Li 2 ZrCl 6 with Fe 3+ , which is probed by complementary analyses using X-ray diffraction, pair distribution function, X-ray absorption spectroscopy, and Raman spectroscopy measurements, drastically enhances the Li + conductivity up to ≈1 mS cm −1 for Li 2.25 Zr 0.75 Fe 0.25 Cl 6. The superior interfacial stability when using Li 2+x Zr 1−x Fe x Cl 6 , as compared to that when using conventional Li 6 PS 5 Cl, is proved. Furthermore, an excellent electrochemical performance of the all-solid-state batteries is achieved via the combination of Li 2 ZrCl 6 and single-crystalline LiNi 0.88 Co 0.11 Al 0.01 O 2 .

Advanced High‐Voltage All‐Solid‐State Li‐Ion Batteries Enabled by a Dual‐Halogen Solid Electrolyte

“…Ionic Conductivity and Porosity Measurements: The porosity measurement as a function of applied pressure was similar to previous reports. [11,32] Ionic conductivity of as-prepared SEs was evaluated by using electrochemical impedance spectroscopy (EIS) with two stainless steel rods as blocking electrodes. The SE powders were cold-pressed into pellets under ≈400 Mpa.…”

“…have been reported not only possessing high ionic conductivity over 1 mS cm −1 , but also having excellent cathode compatibility to realize good battery performance without any additional protection coatings on the cathode materials. [9] The very recently developed Li 3-x Y 1-x Zr x Cl 6 [10] and Li 2+x Zr 1−x Fe x Cl 6 [11] using earth-abundant and low-cost elements make chloride SEs commercially important. Despite chloride SEs being able to support ASSLIBs cycling up to a cut-off voltage around 4.5 V, these compounds are still inadequate for LIBs operating at higher voltages.…”

Exploring Aliovalent Substitutions in the Lithium Halide Superionic Conductor Li3–xIn1–xZrxCl6 (0 ≤ x ≤ 0.5)

“…xZrxCl6, Li3-xY1-xZrxCl6, LixScCl3+x, Li2.25Zr0.75Fe0.25Cl6 and sodium analogue Na3-xEr1-xZrxCl6. 26,27,33,48 However, here the enhancement of the conductivity may not only arise from the decrease of lithium ion concentration but also from the changes to the lithium substructure that includes a cation-site disorder on M2/Li4 site, leading to more Li + on this site further simplifying a three-dimensional diffusion.…”

“…31,32 Just recently, Kwak et al showed that a mechanochemical synthesis stabilizes the trigonal structure for Li2ZrCl6, while annealing resulted in a monoclinic structure exhibiting conductivities of 0.4 mS•cm -1 and 5.7 • 10 -3 mS•cm -1 , respectively. 33 The conductivity of mechanochemically prepared Li2ZrCl6 was further enhanced up to ~ 1 mS•cm -1 by Fe 3+ substitution. The introduction of Zr 4+ in Li3ErCl6 and Li3YCl6 resulted in a conductivity increase by more than one order of magnitude by introducing vacancies into the structure.…”

Application of Ionic Liquids for Batteries and Supercapacitors

“…Electrolytes are commonly divided into three categories such as solid electrolytes, polymer electrolytes and liquid electrolytes. Solid electrolytes (SEs) exhibit high intrinsic oxidation stabilities and comparatively satisfactory electrochemical stabilities, which make them a promising candidate for EES devices, but their brittle nature often restricts them from integrating with EES devices [ 25 , 26 ]. The flexibility and versatility in the shape of polymer electrolytes (PEs) attract a massive interest in various EES devices.…”