All-solid-state lithium battery, which has higher energy density and safety than conventional lithium ion batteries are desired to stationary storage battery and electric vehicle battery. Solid polymer electrolyte (SPE) are one of candidate materials for all-solid-state lithium battery in term of high flexibility, compact, and the ability to prevent lithium dendrite formation.^[1-2]

Li₇La₃Zr₂O₁₂(LLZO) with garnet structure is most representive oxide-based inorganic electrolyte. and suitable stability for lithium metal anodes. Representive crystal structure of LLZO is reported cubic structure and tetragonal structure. Cubic LLZO shows relatively high ionic conductivity (10^-4~10^-3[S cm^-1] ) at room temperature in oxide-based inorganic elctrolyte. Ionic conductivity of tetragonal phase is about 2 orders of magnitude lower than that for the cubic phase.^[3-4]

In this study, we prepared polyether-based polymer-LLZO composite electrolyte and measured temperature dependence of bulk ionic conductivity, interfacial resistance of Li/electrolyte and glass transition temperature (T_g) used by AC impedance method and differential scaning calorimetry (DSC). We investigated that the influence of LLZO additions for ionic conduction and thermal property by hybridization of LLZO and polyether-based solid polymer electrolyte.

Experimental

Preparation and characterization of hybrid solid electrolyte film

Preparation of sample and cell preparation were examined in Ar-filled grovebox. LiTFSA was dissolved in polyether-based macromonomer solution ([Li] / [O] =0.1, amount of O was based on oxide unit from polyether). Tetragonal Li₇La₃Zr₂O₁₂ 1.0wt% and 5.0wt%, DMPA (photoinitiator) and acetonitrile were added to the solution and stirred by stirrer. The solution was dried over 10h and obtained complete homogenerous solution. The solution was casted on glass plate and covered by two glass plate and 0.5mm teflon spacer. Hybrid solid electrolyte films were fabricated by radical photo polymerization under UV irradiation at 5 min.

Characterization of hybrid solid electrolyte

Ionic conductivity of hybrid solid electrolyte were measured by AC impedance method. Measurement samples were 12mm diameter disk. Frequence range is from 100kHz to 10mHz with a 500mV amplitude. Temperature range is -30 to 80 and all samples were thermally equilibrated at each temperature at least 1.5 h prior to the measurement.

T_g of hybrid solid electrolytes were measured by DSC measurement. The hybrid solid electrolyte films were cut into circle of 3mm diameter and enclosed into Al pans in the Ar-filled glovebox. The DSC measurements of hybrid solid electrolyte were conducted with the samples cooling to -100, followed by heating to 150 at cooling and heating rate of 10/min.

The hybrid solid electrolyte films were cut into circle of 19mm diameter and then were sandwiched between Li metal nonblocking electrodes. These samples were enclosed into 2032 coin cell completely. Coin cells were settled in a constant temperature chamber at 60 over 100h to stabilize interface of Li/hybrid solid electrolyte. After that, temperature dependence of interfacial resistance of Li/hybrid solid electrolyte were mesured by AC impedance method under the same condition of ionic conductivity measurement.

Results and discussion

Fig.1 shows the temperature dependences of ionic conductivity for hybrid electrolyte. Ionic conductivity of the hybrid solid electrolyte showed larger values than that of LLZO free electrolyte. This tendency was remarkable in the case of low temperature.

Fig. 2 shows DSC thermograms of hybrid solid electrolyte. T_g of hybrid solid electrolyte were lower than LLZO free system. It is assumed that local segment motion of polymer were improved by interaction of between LLZO and matrix in polymer.

Fig.3 shows temperature dependence of interfacial resistance of Li/hybrid solid electrolyte. Interfacial resistance of hybrid solid electrolyte composited 5.0wt% LLZO was lower than LLZO free system. It is assumed that solvation and desolvation reaction was promoted at Li/electrolyte interface by improving of local segment motion in matrix.

References

[1] F.Croce, G.B. Appetecchi, L. Peris, B. Scrosati

Nature, 394 (1998) 456-458

[2] J. W. Fergus, J.Power Sources, 195 (2010) 4554–

4569.

[3] Y. Matsuda, M. Matsui, N. Imanishi, Netsu Sokutei,

42 (2015) 62-68.

[4]T.Thomason.et.al. J. Mater. Chem. A, 2 (2014) 13431