An All-Solid State Nasicon Sodium Battery Operating at 200°C

Na-ion batteries have attracted recent interest and start now to be counted as viable alternatives vs. Li‑ion technologies for specific applications. Indeed, recent works on phosphate-based Na-containing positive electrodes such as Na₃V₂(PO₄)₃¹ and Na₃V₂(PO₄)₂F₃² have demonstrated excellent performances and can be considered as a new step on the way of sodium-ion technology development. However, like for the Li-ion technology, safety issues related to the use of flammable liquid electrolytes remain, especially due to the high reactivity of sodium with moisture and oxygen. All-solid state batteries, which use non-flammable solid electrolytes instead of organic liquid ones, have been proposed as strong candidates for alternative energy storage devices ^3-5.

Following a recent approach developed for Li-ion all-solid state batteries^6,7, we were able to assemble a monolithic all-solid state Na-ion battery using NASICON-type electrodes and electrolyte in a single 10’ step using the spark plasma sintering technique at 900°C (see Fig. 1). Na₃V₂(PO₄)₃ was used as both positive (V⁴⁺/V³⁺) and negative (V³⁺/V²⁺) electrodes while Na₃Zr₂Si₂PO₁₂ was used as the solid electrolyte. Both compositions present order-disorder phase transitions and present decent ionic conductivities of 1.5 x10^‑3 S cm^-1 and 1.9 x10^-4 S cm^-1 at 200°C for Na₃Zr₂Si₂PO₁₂ and Na₃V₂(PO₄)₃, respectively.

Fig. 1: (top) Photo of the battery and (down) SEM backscattered electrons of the cross-section of the battery.

Thanks to a new experimental set-up, we report for the first time the electrochemical characteristics of an all-solid state Na-ion battery at temperatures as high as 200°C⁸. The potential limitations were selected so that the optimal capacity of Na₃V₂(PO₄)₃ could be attained at both negative and positive electrodes, given that it can be either reduced (Na₃V₂(PO₄)₃ → Na₄V₂(PO₄)₃ at 1.6 V vs. Na⁺/Na) or oxidized (Na₃V₂(PO₄)₃ → NaV₂(PO₄)₃ at 3.4 V vs. Na⁺/Na), hence providing a theoretical voltage of 1.8 V for the symmetrical charged cell. The full battery supplies 85% of the theoretical capacity attained at C/10 with satisfactory capacity retention and low polarization, for an overall energy density of 1.87 x10^-3 W h cm^-2 and a capacity of 1.04 mA h cm^-2(see Fig. 2).

Fig. 2: (top) Galvanostatic cycling at current rate of C/2 or C/10 with potential limitation of 2.2 V for the 3^rd, 6^th and 26^thcycles and (down) capacity retention over cycling at 200°C.

References

 

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