Reducing of Grain Boundary Influences By Hybridization of Inorganic / Polymer Materials for Li-Conductive Solid Electrolyte
Authors: Suzuki Naamo , Fujii Kenta , Ohara Koji , Seki Shiro 
Published: December 2020

Introduction

Recently, rechargeable batteries are strongly desired as energy storage systems. In particular, Li-ion batteries have been realized for practical use from consumer use (e.g., smartphone, laptop computer) to large-scaled energy storage devices such as for in/output control of renewable energy and electric vehicle owing to their high-energy-density and cycle performances. However, safety will be the most important demands with the scale of batteries, and many ignition accidents have been reported. ‘Solvent-free’ all-solid-state batteries exhibit quite high safety and have high-energy-density due to realization of thin-film electrolyte layer without separators. Generally, solid electrolytes are mainly categorized to polymer and inorganic electrolytes. Polymer electrolyte have a sufficient self-standing property, formability of stable interface with electrode and mechanical properties. However, it exhibits relatively low ionic conductivity and Li cation transport number. On the other hand, inorganic electrolyte shows relatively high ionic conductivity, even though it have grain boundary (GB) and be easily broken by external force (e.g. dropping and expansion / contraction with charge-discharge processes). Therefore, in this study, we propose polymer / inorganic hybrid electrolyte for usage of both advantages. Hybrid electrolytes of P(EO/PO) network polymer and two-types of Li1.5Al0.5Ge1.5(PO4)3 (LAGP) presence/absence GB resistance were investigated for developing high-performance all-solid-state Li batteries.

Experimental

Sample preparation

All samples were prepared in a glove box under Ar atmosphere. LiN(SO2CF3)2, rhombohedral (r, having GB resistances) and amorphous (a, not having GB resistances) LAGP (Toshima mfg.), DMPA (photo initiator) and acetonitrile (solvent) were dissolved into polyether-based macromonomer (P(EO/PO) (TA-210, Dai-ichi Kogyo Seiyaku)). Amount of LiTFSA was [Li]/[O]=0.1 per mole of oxygen units of P(EO/PO). Amount of hybridized LAGP((a) or (r)) were 0, 1, 3, 5, 10, 20, 30, 40, 50, 75, 100, 150, 200 wt% with polymer electrolytes, respectively. Mixed samples were dried in vacuum for 12h to evaporate acetonitrile. Then the slurry was casted between two glass plates separated by spacers (Teflon sheet with a thickness of 0.5 mm) and radical polymerized by UV irradiation at 5 min.

Measurements

Ionic conductivities of hybrid solid electrolyte were measured by AC impedance method. In this study, we used two types of electrode, one is blocking electrode and the other is non-blocking electrode. First, blocking electrode cells (SUS / electrolyte (0.5 mm thickness) / SUS) were prepared Measurement samples were 12mm diameter disk. Frequency range is from 200kHz to 10mHz with 100 mV amplitude. Temperature range is 80oC to -30oC and all samples were thermally equilibrated at each temperature at least 1.5 h prior to the measurement. Secondly, non-blocking electrode cells (Li metal / electrolyte (0.5 mm thickness) / Li metal) were prepared. Measurement samples were 18mm diameter disk (diameter of Li metal were 16mm). Samples were measured under similar measurement conditions as above described.
Additionally, molecular-level structures of prepared samples were analyzed by high-energy X-ray diffraction at SPring-8 (BL04B2). LAGP-free polyether electrolyte, Polyether/ LAGP(a)150wt% hybrid electrolyte and LAGP(a) powder were enclosed in a sample tube and measured.

Results and discussion

From the result of impedance measurement by blocking electrode systems, possibility of influence of GB may not appear when using an amorphous material was suggested. To compare the effects of GB, non-blocking electrode systems, led clear difference between amorphous and rhombohedral by the difference of frequency properties [1]. Fig.1 shows impedance spectrum of several electrolytes by non-blocking electrode systems. LAGP-free (0%) electrolyte showed two semicircular arcs and all amorphous LAGP systems also showed the two semicircular arcs. In both systems, high and low frequency components were assigned as bulk resistance and interfacial resistance with Li metal. However, rhombohedral LAGP systems showed clearly different spectrum. GB resistance appears between the two resistance components. This result shows that amorphous LAGP obtain no GB resistance component into hybrid electrolyte. By using amorphous inorganic electrolyte into polyether electrolyte, the influence of GB could be eliminated. Therefore, one of reducing technique of GB influences was established.
Fig.2 shows radial distribution function of three samples. A periodic structure appeared in all electrolytes. The radial distribution function of hybrid electrolyte was confirmed between polyether electrolyte and LAGP powder. This result exhibits the spectral proximity, suggestion of good interfacial matching (effective interactions) between the polymer and the inorganic electrolyte. Moreover, this measurement method was also effective for the local structure analysis because of their ability to analyze hybrid material of amorphous polymer and inorganic systems.

Conclusion

From the standpoint of electrochemical and spectroscopic measurements, we clarified the influence of grain boundary by using amorphous inorganic electrolyte into polyether electrolyte.
[1] M. Kato, K. Hiraoka, S. Seki, J. Electrochem. Soc., 167, 070559 (2020).

Figure 1