(Invited) All-Solid-State Sodium-Ion Battery with Nasicon

Abstract

All solid-state battery is an ideal in the viewpoint of safety and reliability. However, to achieve room temperature drivable all-solid-state battery, it is necessary to improve the battery reaction rate in the solid phase. So far, many efforts of researchers have been devoted to realize the high bulk ionic conductivity of solid electrolyte at room temperature as same as that of the liquid organic electrolytes. Although the target values were achieved by the sulfide solid electrolyte, Li9.54Si1.74P1.44S11.7Cl0.3with 25 mS/cm [1], the realization of the all solid state battery is still far away. Because the interface resistance between the solid electrolyte and the electrode active material is actually more than 100 times larger than that of the bulk resistance of the solid electrolyte, and charge transfer at the interface is a rate limiting step on the charge/discharge reaction in the all-solid-state batteries. Previously, in order to reduce the interface resistance, we adopted the solid electrolyte and electrode active material with the same crystal structure. While perovskite and NASICON are well known as the matrix of solid electrolyte, layered rocksalt, spinel, olivine and NASICON are popular framework to electrode active materials. So, if NASICON-type electrode active materials were applied on the both side of NASICON-type solid electrolyte, all-solid-state NASICON battery can be constructed. Actually, we chose Na3Zr2(SiO4)2PO4 as NASICON-type solid electrolyte and Na3V2(PO4)3 as NASICON-type electrodes. Here, we chose Na-ion battery instead of Li-ion battery, becase Na is cheaper than Li and the Na ionic conductivity of NASICON-type Na3Zr2(SiO4)2PO4 (s = 1.06 x 10-3 S/cm) [2] is higher than Li ionic conductivity of NASICON-type Li1.3Al0.3Ti1.7(PO4)3 (s = 7 x 10-4 S/cm) [3]. On the other hand, Na3V2(PO4)3 electrode has 2 voltage plateaus, 3.2 and 1.5 V against Na, corresponding to V3+/V4+ and V3+/V2+ redox. Fortunately, it can be used as cathode and anode, respectively. However, when the sintering temperature is increased to increase the degree of sintering, the foreign substances such as SiP and Na4SiO4 are produced between NASICON-type solid electrolyte and electrode active materials as reaction products, which results in increased interfacial resistance. In order to prevent such side reaction at the contact interface between NASICON-type Na3Zr2(SiO4)2PO4 and NASICON-type Na3V2(PO4)3 the sintering temperature must be reduced less than 700 °C. This restriction of the sintering temperature disturbs to increase the sufficient sintered density of the NASICON components. The obtained all-solid-state battery annealed at 700 °C can operate reversibly even at room temperature and it shows flat 1.7 V discharge voltage corresponding to the potential discrepancy between V3+/V4+ and V3+/V2+ redox. However, the rate capability was insufficient and the output current density is less than 10 mA/cm2 [4]. Therefore, as the alternative approach, we used NASICON-type Na3V2(PO4)3 as both electrode and solid electrolyte and attempted to set up NASICON single phase solid-state battery (Na3V2(PO4)3 cathode/Na3V2(PO4)3 electrolyte/Na3V2(PO4)3 anode) for the first time. Essentially, it will be suffered by the self discharge, because the electrode active material has some electron conductivity. But, fortunately, Na3V2(PO4)3 has a poor electron conductivity as an electrode active material and this drawback may allow the diversion to the solid electrolyte. Another problem of Na3V2(PO4)3 is the low ionic conductivity as a solid electrolyte. In order to solve the latter problem, Zr doped Na3V2-xZrx(PO4)3 was considered and the optimization of the doping amount, x, was performed. Although it has the disadvantage of the self-discharge caused by the electron conductivity of Na3V2-xZrx(PO4)3, the advantage is zero interfacial resistance between the solid electrolyte and electrode active materials. In addition, the restriction of the sintering temperature is not needed, because there is no worry of the side reaction between the solid electrolyte and electrode active materials. So, Na3V2-xZrx(PO4)3 pellets were annealed at 1000 °C and thin Pt layer as current collector was suputtered on the both side of Na3V2-xZrx(PO4)3 pellets, to build the Na3V2-xZrx(PO4)3NASICON single phase solid-state Na-ion battery. The typical charge/discharge profiles at room temperature are shown in Fig. 1. The details will be shown at the presentation.