Na Conductive Polymer/Inorganic Composite Electrolyte for Flexible All-Solid-State Battery
Authors: Hiraoka Koji , Kato Masaki , Hirose Ryo , Hakamada Yuta , Seki Shiro 
Published: February 2020

Recently, Na secondary batteries has been attracted attention by concerning depletion of elemental Li in the earth crust. However as with conventional Li-ion batteries, Na batteries use flammable electrolytes including organic solvents such as ethylene carbonate, propylene carbonate. Therefore, battery systems are demanded shift to all-solid-state battery because of the increasing safety. There are two type solid electrolytes, inorganic electrolyte such as oxides and sulfides, polymer electrolytes. Inorganic electrolytes are known to exhibit relatively high ionic conductivity (~10-4 Scm-1 at room temperature). But interfacial stability between electrode and electrolyte are relatively low due to it is hard and fragile. Moreover, it is exhibit high resistivity of grain boundary due to the most inorganic electrolytes are fabricated by sintered method. On the other hand, polyether-based polymer electrolytes has relatively high interfacial stability and flexibility, however, these electrolytes exhibit relatively low ionic conductivity (~10-5 Scm-1 at room temperature) and relatively slow ionic transport by segmental motion. In this study, we prepared polymer/inorganic composite electrolytes for utilize both advantages such as high ionic conductivity, high interfacial stability, and flexibility.

In the Ar-filled globe box ([H2O] <0.5ppm, [O2] <5ppm), polymer/inorganic composite electrolytes were prepared by mixing of polyether-based polymer, NaN(SO2CF3)2 as an alkaline salt, 2, 2-dimethoxy-2-phenylacetophenone as a photoinitiator, acetnitrile as a solvent and Na3Zr2Si2PO12 (NZSP) as an inorganic electrolyte. NZSP has structure of Na super ionic conductor (NASICON) and high ionic conductivity. In this study, composition percentage of NZSP were changed between 0 to 300wt%. After stirring, vacuum dried solution were casted to glass substrate and polymerized by UV irradiation to form hybrid electrolyte membrane. Thermal, transport and molecular properties of composite electrolytes were investigated by scanning electron microscopy (SEM), energy dispersed X-ray spectroscopy (EDX), fourier transform infrared (FT-IR), differential scanning calorimetry (DSC) and AC impedance method, respectively. Moreover, Na ion transference number were evaluated by AC impedance up to low frequency.

Fig. 1 show SEM image of cross-section of polymer/inorganic composite electrolyte containing 30wt% NZSP. The dark and bright point were detected as polymer phases and NZSP particles by EDX mapping, respectively. It is confirmed that the NZSP particles were uniformly dispersed to polymer phase in this composition. Furthermore, flexibility of composite electrolyte membranes improved compared to NZSP free sample. However, mechanical strength was decreased in the system containing over 100 wt% NZSP. In this study, glass transition temperature (T
g) were measured as the phase transition points of between glass and rubber states. Although T
g appeared approximately -25oC in composition 0wt%, T
gs were decreased to 3~10oC by compounding NZSP in the under 50 wt% NZSP range. These phenomena are expected improving ionic conductivity by increasing segmental motion and free volume. Similar value of T
g for NZSP free electrolyte were confirmed in the case of over 100wt% NZSP. The mainly cause of that is the decreasing composition of polymer phases in the composite electrolytes due to the many NZSP addition. The temperature dependence of ionic conductivity is shown Fig. 2. The 30wt% NZSP sample were exhibited the highest ionic conductivity in the all temperature range, especially, this composition was improved the value about 6 times compared to NZSP free electrolyte at the low temperature. On the other hand, composite electrolytes beyond approximately 100wt% NZSP were showed low ionic conductivity compared to NZSP free electrolyte in the all temperature range. In the Nyquist plots, observed semicircle shows approximately symmetric shape, but beyond 100wt% NZSP sample exhibited asymmetric semicircles. It is considered that these samples have two resistance components, bulk and grain boundary resistances of between NZSP particles by fitting impedance spectra. The resistance of grain boundary was occurred by Na ion conducting through the aggregation of NZSP particles which has higher ionic conductivity than polymer phase. Therefore, it was suggested that the Na ion path mainly conducts the polymer phase in the low composition of NZSP and mainly conducts NZSP particles in the high composition.

From the above, the prepared polymer/inorganic composite electrolyte showed high flexibility. Although the T
gs decreased in the system of under NZSP 50 wt%, the sample containing high composition of NZSP showed T
gs similar to NZSP free electrolyte. The 30wt% NZSP sample exhibited highest ionic conductivity, especially low temperature range, due to the low resistance of grain boundary of NZSP particles. Therefore, sufficient composition ratio of NZSP was suggested for low temperature operation of all-solid-state batteries.

Figure 1