Na+/K+-codoped amorphous manganese oxide with enhanced performance for aqueous sodium-ion battery

Abstract

The cycling performance of aqueous rechargeable metal-ion batteries is highly dependent on the cathodes. Metal-ion doping is an effective means to improve the ion mobility, surface reactivity, and structural stability of metal oxide cathodes. In this paper, we report on the synthesis and electrochemical performance of Na+-, K+-, and Na+/K+-codoped amorphous manganese oxide (NAMO, KAMO, and NKAMO) cathodes for aqueous sodium-ion battery (ASIB). Na+ ion is more easily doped in the AMO matrix than K+ ion, whether single-ion doping or Na+/K+-codoping, presenting NAMO, KAMO, and NKAMO with different doping levels. Also, the specific surface area (SBET), Mn3+ concentration, and surface defect are linearly correlated with the doping level. NKAMO with high doping level manifests improved electrode performance as compared with NAMO, KAMO, and commercially available MnO2, due to the large SBET, high Mn3+ concentration, and more surface defects. The charge/discharge mechanism is mainly attributed to the reversible extraction/insertion of Na+ ions from/into the AMO matrix. NKAMO can deliver a discharge specific capacity (Cdis) of 41.3 mA h g1 after 1000 cycles at 1 A g1. Moreover, a typical ASIB with NKAMO cathode and NaTi2(PO4)3 (NTP) anode can retain a Cdis of 33.2 mA h g1 after 5000 cycles with continuously changing the current density from 1 to 5 C, showing capacity retention of ca. 72% relative to the initial cycle.