Electrochemical Performance of All-Solid-State Sodium-Ion Model Cells with Crystalline NaxCoO2 Thin-Film Cathodes

Abstract

A common way to improve the electrochemical performance of the NaxCoO2 thin-film cathodes is to increase their crystallinity. Here we present our study of the electrochemical performance of all-solid-state sodium ion batteries with NaxCoO2 thin-film cathodes having two different degrees of crystallinity tuned by their post-deposition annealing at 700°C. The NaxCoO2 cathode thin-films were grown by pulsed laser deposition onto a bulk Na3.4Sc0.4Zr1.6(SiO4)2(PO4) (Nasicon) solid electrolyte substrates and assembled with sodium metal into a Swagelok battery cells. Cells with the low-crystalline NaxCoO2 cathodes show discharge capacities of up to 124 mAh g−1 over 800 charge/discharged cycles. However, cells with highly crystalline NaxCoO2 cathodes revealed a significant capacity loss down to 9 mAh g−1 and a pronounced increase of the overpotential from 100 to 890 mV during the 200 cycles. The observed loss in capacity can be attributed to a strong increase of the interface resistance between the highly crystalline annealed NaxCoO2 films and Nasicon during cycling.