Ionic Conduction in NH4(Mg0.8Li0.2)F2.8 Treated By Mechanical Milling
Authors: Motohashi Kota , Kimura Yuta , Nakamura Takashi , Uchimoto Yoshiharu , Amezawa Koji 
Published: December 2020

All-solid-state fluoride-ion batteries (ASSFIBs) attract attentions because of their potential of higher energy density and safety operation [1]. The theoretical energy density of ASSFIBs is expected to reach 5000 Wh L-1, which is more than twice of the practical energy density of conventional lithium ion batteries. However, ASSFIBs are still not into practical use. One of problems with ASSFIBs is low ionic conductivity of the electrolyte.
Recently, CH3NH3PbI3, one of typical hybrid organic inorganic perovskite compounds, was reported to show anion conduction [2]. This suggested that compounds containing large-size molecular cations can be candidates of anion conductors. Based on this idea, our group has been investigating fluoride-ion conduction in compounds containing a molecular cation, and found that NH4(Mg0.8Li0.2)F2.8 exhibited 1.7×10-4 S cm-1 of conductivity at 373 K [3]. However, its conductivity is still not high enough for the practical application, and thus necessary to be further improved. In this work, we aimed to improve the ionic conductivity of NH4(Mg0.8Li0.2)F2.8 by mechanical milling treatments as sometimes done for sulfide based lithium-ion conductors [4]. In particular, the influence of the phase change by the mechanical milling on the ionic conductivity was investigated.
Crystalline NH4(Mg0.8Li0.2)F2.8 powder was prepared by the solid-state reaction and thereafter were mechanically milled by using a planetary ball mill apparatus. It was found that electrical conductivity of a pressed compact of NH4(Mg0.8Li0.2)F2.8 was significantly enhanced by the mechanical milling treatment. The conductivity reached to 4.1×10-5 S cm-1 at 293 K after the milling at 300 rpm for 168 hours. The temperature dependence of the conductivity was not monotonical, and the activation energy changed from 1.0 to 0.45 eV around 273 K as temperature increased. Dominant charge carrier species in the milled sample was identified as fluoride-ion from conductivity measurements using an electron-blocking cell consisting of Pb/PbSnF4/sample/PbSnF4/Pb. The conductivities measured by applying AC and DC voltages were comparable, indicating dominant fluoride ion conduction.
The phase state and morphology of prepared NH4(Mg0.8Li0.2)F2.8 samples were examined by XRD, TEM/ED, and DSC. Broadening of the XRD peaks was seen for the milled samples, suggesting the partial amorphization and/or pulverization. Similar tendencies were confirmed by TEM/ED analysis. It was found from DSC that the glass transition and crystallization occurred around 273 and 313 K, respectively. The glass transition temperature agreed with the temperature where the change in the activation energy of the electrical conductivity was observed. These indicated that, at least, one reason for the conductivity enhancement by mechanical milling in NH4(Mg0.8Li0.2)F2.8 was the amorphization of the sample.

References:

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Acknowledgement: This work was partly supported by JST-Mirai Program JPMJMI18E2, Japan.