High-Performance Na1.9K0.1VTi(PO4)3-GNP with Multielectron Redox for Printed Aqueous Symmetric Sodium-Ion Batteries

Aqueous symmetric sodium-ion batteries (ASSIBs) are gaining attention as attractive energy storage devices for flexible and wearable electronics due to their excellent structural stability, inherent safety, environmental friendliness, and the abundant sodium resources. NASICON-structured Na2VTi(PO4)3 (NVTP) features a three-dimensional framework structure; Ti4+/Ti3+ and V4+/V3+ redox couples can be used to fabricate symmetrical sodium-ion batteries as both the anode and the cathode. However, poor cycle stability and irreversible dissolution in aqueous solutions limit the application of NVTP in flexible ASSIBs. Herein, the Na1.9K0.1VTi(PO4)3-10% GNP (NKVTP-GNP) composite was synthesized by partially substituting Na+ in Na2VTi(PO4)3 with K+ and coating graphene nanoplatelets (GNP). The K+ doping and GNP coating successfully promoted the conversion of V4+ to V5+ and enhanced the cycling stability of the materials. The capacity retention rate of the NKVTP-GNP cathode is 99% after 1000 cycles, while the anode maintains 81% of its capacity after 500 cycles. Additionally, a flexible symmetric battery NKVTP-GNP//NKVTP-GNP was fabricated via a screen-printing technique, which delivered a wide voltage window of 1.8 V, an energy density of 63 Wh kg−1, and excellent cycling stability, retaining 90% of its capacity after 2000 cycles at 10 C. More importantly, the battery shows high flexibility, adjustable shapes, and high-throughput manufacturing. This work provides insights into the development of high-performance flexible ASSIBs.