Co-continuous hard Carbon/NASICON electrodes for all-solid-state batteries: Insights into structural stability and interfacial Na-ion transfer

Abstract

Sodium-ion batteries (SIBs) have recently been launched as alternatives to lithium-ion batteries (LIBs). Since the superiority of SIBs over LIBs lies in the abundance and low cost of raw materials, SIBs are expected to be deployed in large-scale storage systems, where long-term stable operation with a high level of safety is demanded rather than high energy density. On this account, all-solid-state sodium-ion batteries (ASS-SIBs) comprising an air-stable oxide solid electrolyte are of great promise but still in their infancy. In this study, we focus on hard carbon, which is first in line for SIB anodes, in conjunction with a NASICON-type solid electrolyte, Na₃Zr₂Si₂PO₁₂ (NZSP), and elucidate the underlying electrode capability of hard carbon for ASS-SIBs. Monolithic NZSP/carbon electrode layers with an elaborately designed co-continuous microstructure have been fabricated via the sol–gel technique to exploit the potential of hard carbon. Consequently, the favorable performance of hard carbon in the NZSP-based ASS cell is validated: a reversible capacity of >300 mAh g^–1, an initial coulombic efficiency of 94 %, and a stable cycling over 500 cycles. The striking longevity underpins a minuscule volume change of carbon during sodiation/desodiation. The kinetic study on the interfacial Na⁺-transfer highlights the lower activation energy (∼0.4 eV) at NZSP/carbon interface than that in a liquid electrolyte. All the findings in this study clearly corroborate the significant potential in the combination of hard carbon anodes and NASICON-type solid electrolytes for ASS-SIBs.