Cem Özbilgin1,2,Kiyoshi Kobayashi2,Shinji Tamura3,Nobuhito Imanaka3,Tohru Suzuki2,1
Waseda University1,National Institute for Materials Science2,Osaka University3
Cem Özbilgin1,2,Kiyoshi Kobayashi2,Shinji Tamura3,Nobuhito Imanaka3,Tohru Suzuki2,1
Waseda University1,National Institute for Materials Science2,Osaka University3
Multivalent ion-conducting ceramics are attracting attention as key materials for the production of high-safety and large-capacity rechargeable batteries. However, due to the low ionic conductivity of solid electrolytes and compatibility issues between battery components, their extensive use has not yet been realized. One aspect of the compatibility is that the thermal expansion mismatch between the electrode and electrolyte causes cracks at the interface due to the thermal history of the material.<sup>1</sup> Moreover, in polycrystalline ceramics, the anisotropic thermal expansion of grains causes inter/intra-grain microcracks during the sintering due to surpassing critical grain size.<sup>2</sup> Some physical properties of ceramic materials with anisotropic crystal structures can be controlled by the crystallographic orientation of their grains. It is highly desirable to have simultaneous control over material properties such as ionic conductivity and thermal expansion in one step.<br/><br/>(Mg<sub>x</sub>Hf<sub>1-x</sub>)<sub>4/4-2x</sub>Nb(PO<sub>4</sub>)<sub>3</sub> is a NASICON (Na<sup>+</sup> Super Ionic CONductor)-type Mg<sup>2+</sup>-conducting solid electrolyte having a 3-dimensional ion-conduction network within a rhombohedral structure (Space group: R-3c). Based on the symmetry in the crystal, several second-rank tensorial properties such as electrical conductivity (ionic conductivity (σ)), thermal expansion (α), magnetic susceptibility (χ) show directional dependencies in the lattice.<br/><br/>In this study, Mg<sup>2+</sup>-conducting (Mg<sub>0.1</sub>Hf<sub>0.9</sub>)<sub>4/3.8</sub>Nb(PO<sub>4</sub>)<sub>3</sub> solid electrolyte was synthesized by a co-precipitation method. The crystallographic orientation of the grains in the polycrystalline end product was controlled during the slip-casting process using strong static and rotating magnetic fields. The c-axis of the (Mg<sub>0.1</sub>Hf<sub>0.9</sub>)<sub>4/3.8</sub>Nb(PO<sub>4</sub>)<sub>3</sub> crystals was determined to be the easy magnetization axis and could be oriented along the magnetic field. The c-axis-oriented sample, having 90% of its grains oriented within the 20° tilt angle, was produced under a static magnetic field. The total conductivity of the c-axis-oriented sample at 150°C was determined to be higher (9.44 × 10<sup>−8</sup> S.cm<sup>−1</sup>) than those of the a-axis and the randomly oriented samples by 27% and 18%, respectively. (Mg<sub>0.1</sub>Hf<sub>0.9</sub>)<sub>4/3.8</sub>Nb(PO<sub>4</sub>)<sub>3</sub> was established to be a negative thermal expansion material despite having a positive linear thermal expansion along its c-axis. The findings of this study confirm that any (Mg<sub>0.1</sub>Hf<sub>0.9</sub>)<sub>4/3.8</sub>Nb(PO<sub>4</sub>)<sub>3</sub> solid electrolyte surface with thermal expansion values ranging from −1.73 to 2.13 ppm/°C can be produced by controlling the tilt angle. To achieve a zero thermal expansion surface, the tilt angle should be 30°. The control of the orientation of the anisotropic grains is beneficial for enhancing the apparent performance of materials; additionally, the grain orientation can be effectively tailored to reduce the thermal expansion mismatches microscopically between the grains and macroscopically between the other adjacent components used in devices.<br/><br/>1. Bertrand M, Rousselot S, Aymé-Perrot D, Dollé M. Compatibility assessment of solid ceramic electrolytes and active materials based on thermal dilatation for the development of solid-state batteries. <i>Mater Adv</i> 2021;2:2989–2999.<br/>2. Jackman SD, Cutler RA. Effect of microcracking on ionic conductivity in LATP. <i>J Power Sources</i> 2012;218:65–72.