Nanocomposites and nanomaterials

Synthesis and investigation of components of composite solid electrolyte based on 
inorganic nano-particles of NASICON and LiClO4-doped polyethylene oxyurea 

V'yunov O.I. 1, Kovalenko L.L.1, Belous A.G.1, Klimenko N.S.2, Gumenna M.A.2, Stryutsky A.V.2

1 V.I. Vernadsky Institute of General & Inorganic Chemistry, NAS of Ukraine. Palladina ave. 32/34, Kyiv 03142, Ukraine. E‑mail: vyunov@ionc.kiev.ua
2 Institute of Macromolecular Chemistry, NAS of Ukraine. Kharkiv highway 48, Kyiv 02160, Ukraine. E‑mail: striutskyi@gmail.com

The stand-alone small-size lithium-ionic chemical current sources with metallic lithium anode and a ceramic electrolyte are required for practical application in modern electronics. At present, the compounds with perovskite structure Li3xLa2/3‑xTiO3 are the most promising electrolytes as oxides with highest conductivity (20оС  3.10-3 S/cm). However, the metallic lithium interacts with lanthanum lithium titanate in contact area to form Ti3+, resulting in unwanted electron conductivity of the ceramic electrolyte. Phosphate-containing compounds NASICON (Li1+xAlxTi2-x(PO4)3) and LiPON (Li3+yPO4-xN(y+2x)/3) are chemically stable in contact with a metallic lithium anode. However, NASICON-based ceramics are very fragile. The deposition methods of the LiPON compound (RF magnetron sputtering, laser spraying) are complex. The ionic conductivity of LiPON is low (20оС  10-6 S/cm), but in the form of thin film (<1 μm) can be used as electrolyte. Therefore, the development of solid electrolyte of lithium-ion chemical current sources as a film composite is promising.
The purpose of the work is to check the possibility of preparation a composite solid electrolyte as a film based on inorganic nanoparticles and an organic polymer with an ion-conductive component that will be characterized by high ionic conductivity and chemical stability in contact with a metallic lithium anode. At the first stage of work, the synthesis and investigation of individual components of a composite solid electrolyte is important.
As inorganic component, the nanoparticles of compound with NASICON structure (Li1.3Al0.3Ti1.7(PO4)3) were used. Particles were synthesized by solid phase reactions technique at 850 °C for 4 hours. As the initial reagents, lithium carbonate, aluminum nitrate, titanium oxide and phosphoric acid were used. Fine particles down to the nanometre range were produced by a grinding in a planetary mill. The conductivity of synthesized inorganic compounds was evaluated using ceramic sintered based on them. The complex impedance measurements in a wide frequency (1 Hz-32 MHz) and temperature (20-150 °C) ranges were performed. The X-ray diffraction analysis revealed that the inorganic particles of the NASCON compound were single-phase. An electron spectroscopy shown a particle size of <1 μm. The total conductivity and conductivity of grain inner of the NASICON ceramic at room temperature was 1.6·10-6 and 3.6·10-5 S/cm respectively. The temperature dependence of the total conductivity of the ceramic material NASICON was found to have an activation character with an activation energy of 0.41 eV.
The synthesis method of LiClO4-doped polyethylene oxyurea as a model lithium-conductive polymer system has been developed. This system was expected to be effective as ion-conductive polymer component of film composite electrolytes. As initial reagents, oligoethylene oxide with a molecular weight of 1000 and 4,4'-diphenylmethane di-isocyanate were used. Polymer was synthesized at 80 °C in dimethyl formamide under a nitrogen atmosphere followed by the introduction of LiClO4 and removal of the solvent under low pressure. The oligo-oxyethylene flexible segments of prepared polymer contribute to lithium-ionic conductivity due to the cation solvation and high self-segmental mobility. The rigid urethane segments form and maintain the domain structure of the system that is one of the basic conditions for ensuring a high level of ionic conductivity. At the ratio (CH2CH2O)/Li+ = 8, the maximum conductivity of prepared systems was observed. For impedance spectroscopy, discs of Li-conductive polymer with a diameter of 11 mm and thickness of 80 μm were used. The total conductivity of lithium conductive polymer at room temperature was found to be 3.8·10-7 S/cm. The temperature dependence of the total conductivity of the Li-conductive polymer was found to have an activation character with an activation energy of 0.3 eV.
Thus the experimental results indicate that NASCON inorganic nanoparticles (Li1.3Al0.3Ti1.7(PO4)3) and LiClO4-doped polyethylene oxyurea has significant prospects for the development of a composite solid electrolyte with high ionic conductivity and chemical stability in contact with a metallic lithium anode. Lithium-ion current sources containing a composite solid electrolyte based on inorganic nanoparticles and a polymer will be characterized by a low cost of the initial reagents, a simple preparation technology, and give the possibility to effectively solve the issues of miniaturization of modern electronic equipment.

The authors would like to thank the Ukrainian National Academy of Sciences for providing the research grant (85/19‑Н) to support this work.