We find new halide solid electrolyte (HSE) materials that exhibit (a) high lithium ion conductivity, (b) high electrochemical stability against 4-V class cathode materials, and (c) high deformability for low inter-grain resistance realized with facile process. The lithium ionic conductivity of the new HSEs at room temperature reached 0.88 mS/cm. Bulk-type all-solid-state batteries (ASSBs) using these new HSE materials exhibit excellent charge/discharge performance; the initial reversible capacity and coulomb efficiency respectively reached as high as 119 mAh/g and 94 % with LiCoO₂ (LCO)-based active cathode material. These HSEs are the first non-sulfide inorganic materials, to our knowledge, that simultaneously satisfy all the above conditions (a) – (c) demanded for realization of all-solid-state batteries (ASSBs) for large capacity applications such as power sources for electric vehicles. In the presentation, we will reveal the details of the constituent elements, the material composition, the crystal structure, the simulated ion conducting path, and the battery characteristics.

Introduction:

The ASSBs are one of the most promising candidates for the post-lithium ion batteries. Towards realization of its commercialization, one of the key issues is development of solid-state electrolytes. The solid-state electrolytes for ASSBs are imposed to severe demands that need to simultaneously satisfy several requirements: not only high ionic conductivity, but also high deformability (for low inter-grain resistance and facile battery fabrication process), high electrochemical stability, high chemical stability, and viable mass production process, etc. Currently sulfides¹ and oxides² are the most widely studied/employed electrolytes for ASSB studies, and several other material systems^3,4 have also been proposed. It is, however, still challenging to satisfy all the above requirements at the same time, and discovery of alternative lithium ion conductors are expected.

We focus on halide materials as a solid electrolyte for ASSB application due to their advantageous natures; the halide ions are monovalent, fairly large ionic radii, high polarizability, high electrochemical oxidation stability, and high chemical stability of halide ionic materials against oxygen. Nevertheless, only several halide materials have been explored as lithium ion conductors so far, much less its use for ASSBs. We here report on our finding of the new halide lithium ion conductors and their use for bulk-type ASSBs.

Experiments, results and discussions:

The ionic conductivities of the HSE materials were measured on pressed pellets using electrochemical impedance spectroscopy (EIS). The conductivities at room temperature (at 25 °C) and its activation energies were, respectively, σ = 0.52 mS/cm and E_a = 0.40 eV for chloride HSE and σ = 0.88 mS/cm and E_a = 0.36 eV for bromide HSE. Their DC conductivities measured with Li – HSE – Li configuration were identical to the AC conductivities measured with blocking electrodes, SUS – HSE – SUS. The electromotive force generated by Li – HSE – LiIn cell was consistent with the value expected from Nernst equation (= 0.61 V). These electrochemical characterizations undoubtedly revealed that the dominant electric carrier of these HSEs is lithium ions. We note that all the above measurements were done on pellets formed by simply pressing powders of the HSEs, which indicate high deformability and low inter-grain resistance of these newly synthesized HSE materials.

The ASSBs using these HSE materials showed excellent charge/discharge performance using LiCoO₂ as a cathode material and Li-In alloy as an anode material. The bulk-type ASSBs were assembled based on the HSE powders, LiCoO₂ powders and Li-In alloy foil, simply by pressing them without any heat treatment. The characteristic plateau of the redox reaction of Li_1-xCoO₂ was clearly observed in the charge/discharge curves. The initial charged capacity reached 126 mAh/g (~ 93 % of the theoretical capacity) and the discharge capacity was 119 mAh/g, resulting in the coulomb efficiency for the first cycle as high as 94.4 %. The coulomb efficiency for the subsequent cycles were higher than 99 % till the measurement terminated at 12^th cycle. Such high coulomb efficiency is a manifestation of the high electrochemical stability of these HSE materials against 4-V class cathode materials.

All of the above characteristics of the HSEs clearly indicates that HSEs can be one of the candidate materials for bulk-type ASSBs. They show not only high lithium ion conductivity, but also showed excellent battery performance with bulk-type ASSBs with high oxidation stability.

¹ Y. Kato et al., Nat. Energy, 1, 16030 (2016).

² X. Han et al, Nature Materials 16, 572–579 (2017).

³ M. H. Braga et al, J. Mater. Chem. A, 2, 5470 (2014).

⁴ H. Maekawa et al, J. Am. Chem. Soc. 131, 894–895 (2009).