Although sodium-ion batteries (SIBs) are considered as alternatives to lithium-ion batteries (LIBs), the electrochemical performances, in particular the energy density, are much lower than LIBs. A metal-organic compound, cuprous 7,7,8,8-tetracyanoquinodimethane (CuTCNQ), is presented as a new kind of cathode material for SIBs. It consists of both cationic (Cu ↔Cu ) and anionic (TCNQ ↔TCNQ ↔ TCNQ ) reversible redox reactions, delivering a discharge capacity as high as 255 mAh g at a current density of 20 mA g . The synergistic effect of both redox-active metal cations and organic anions brings an electrochemical transfer of multiple electrons. The transformation of cupric ions to cuprous ions occurs at near 3.80 V vs. Na /Na, while the full reduction of TCNQ to TCNQ happens at 3.00-3.30 V. The remarkably high voltage is attributed to the strong inductive effect of the four cyano groups.
Pyrite FeS has attracted extensive interest as anode material for sodium-ion batteries due to its high capacity, low cost, and abundant resource. However, the micron-sized FeS usually suffers from poor cyclability, which stems from structure collapse, exfoliation of active materials, and sulfur dissolution. Here, we use a synergistic approach to enhance the sodium storage performance of the micron-sized FeS through voltage control (0.5-3 V), binder choice, and graphene coating. The FeS electrode with the synergistic approach exhibits high specific capacity (524 mA h g), long cycle life (87.8% capacity retention after 800 cycles), and excellent rate capability (323 mA h g at 5 A g). The results prove that a synergistic approach can be applied in the micron-sized sulfides to achieve high electrochemical performance.
Nanostructured δ-MnO2incorporated with alkali cations (A-δ-MnO2, A = K+and Na+) has been synthesized and evaluated as a cathode material for aqueous sodium-ion batteries.
Carbon coated K0.8Ti1.73Li0.27O4 (KTLO) has been synthesized by a facile flux method followed by ball-milling and gaseous carbon coating. The carbon coated KTLO delivers a reversible specific capacity of 119.6 mA h g(-1) at 20 mA g(-1) with no capacity loss after 250 cycles as an anode material in sodium ion batteries, exhibiting an improved rate capability of 66 mA h g(-1) at 200 mA g(-1). It was found that carbon coating of KTLO not only enhances its electronic conductivity, but also improves the structure stability, proving that the carbon coated KTLO is a promising anode material for sodium ion batteries.
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