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With advantages of high theoretical capacity and low cost, manganese sulfide (MnS) has become a potential electrode material for sodium-ion batteries (SIBs). However, complicated preparations and limited cycle life still hinder its application. Inspired by cream rolls in our daily life, a MnS/N,S-co-doped carbon tube (MnS/NSCT) composite with a 3D cross-linked tubular structure is prepared via an ultra-simple and low-cost method in this work. As the anode for SIBs, the cream roll-like MnS/NSCT composite has delivered the best electrochemical performance to date (the highest capacity of 550.6 mA h g−1 at 100 mA g−1, the highest capacity of 447.0 mA h g−1 after 1400 cycles at 1000 mA g−1, and the best rate performance of 319.8 mA h g−1 at 10 000 mA g−1). Besides, according to several in situ and ex situ techniques, the sodium storage mechanism of MnS/NSCTs is mainly from a conversion reaction, and the superior electrochemical performance of MnS/NSCTs is mainly attributed to the unique cream roll-like structure. More importantly, this simple method may be feasible for other anode materials, which will greatly promote the development of SIBs.

Heterostructures show excellent performance in energy and environment fields due to the unique interface structure and synergistic effect of multi-components. Here, we introduce a stepwise method to prepare multi-element Co3O4@C@MnO2 heterostructures on nickel foam as binder-free supercapacitor electrode material, where crystalline Co3O4 acts as intimal “core” and wrinkled ultrathin MnO2 nanosheets as the outer “shell”. In three-electrode mode, Co3O4@C@MnO2 shows excellent electrochemical performance with a high cyclic specific capacitance of 1335.3 F g−1 at a current density of 20 A g−1. Furthermore, the asymmetry supercapacitor device assembled with Co3O4@C@MnO2 heterostructures and active carbon exhibits high specific capacity (126.6 F g−1 at a current density of 0.5 A g−1) and long life (about 61 F g−1 at a relatively high current of 20 A g−1 after 40,000 cycles). Compared with Co3O4@MnO2 or C@MnO2, the improved performance of Co3O4@C@MnO2 heterostructures is proposed to be attributed to the synergistic effect in multi-components. Remarkably, the introduction of C between Co3O4 and MnO2 could promote the electron transfer, reduce the impedance and thus improve the electrochemical performance of the heterostructures.

We demonstrate the availability of Fe 7 Se 8 as anode for sodium-ion batteries (SIBs) for the first time. A Fe 7 Se 8 nanocomposite encapsulated by nitrogen-doped carbon layers (Fe 7 Se 8 @NC) has been fabricated via a facile thermal method using Prussian Blue as precursor, and its sodium storage mechanisms and extraordinary activation process during cycling are revealed for comprehensive understanding of its reaction process. It shows that the sodium storage mechanisms of Fe 7 Se 8 can be affected by the activation process, especially in the charge process. As SIB anode material, the Fe 7 Se 8 @NC exhibits an outstanding electrochemical performance with a reversible discharge capacity of 367 mAh g −1 at 500 mA g −1 . Even at 2000 mA g −1 , the capacity can retain 251 mAh g −1 . Moreover, the Fe 7 Se 8 @NC shows excellent cyclability, which maintains the capacity up to 339 mAh g −1 at 1000 mA g −1 after 1200 cycles. The results demonstrate that Fe 7 Se 8 @NC is a promising anode material for SIBs.

Pyrite FeS2 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 FeS2 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 FeS2 through voltage control (0.5–3 V), binder choice, and graphene coating. The FeS2 electrode with the synergistic approach exhibits high specific capacity (524 mA h g–1), long cycle life (87.8% capacity retention after 800 cycles), and excellent rate capability (323 mA h g–1 at 5 A g–1). The results prove that a synergistic approach can be applied in the micron-sized sulfides to achieve high electrochemical performance.

Gamma titanium phosphate (G-TiP) possesses a lamellar framework with larger interlayer spacing than α-type TiP, in which the protons within the P–OH groups can be exchanged by alien ions. Herein, the electrochemical performance of the bulk G-TiP in both lithium and sodium storage has been systematically investigated. The results reveal that the layered G-TiP can store both Li+ and Na+ ions via a local ion exchange process and redox reaction in a wide voltage range. Despite the large radius of Na+, G-TiP exhibits a higher structural tolerance upon (de)sodiation and better capacity retention when cycled at low voltage. For a Na-ion cell, the G-TiP electrode delivers a specific capacity of 161.3 mA h g−1 within 0.1–2.8 V, showing an obvious plateau in the range of 1.6–2.2 V with a solid solution reaction. For a Li-ion cell, a reversible capacity of 109 mA h g−1 is attained within 0.8–2.8 V, and a voltage plateau occurs at 2.2–2.6 V.

Exploring simple synthesis method of high-performance sodium-ion batteries (SIBs) electrode materials has always been highly concerned in large-scale energy storage technology filed. Here, ultrathin two-dimensional (2D) FexCo1-xSe2 nanosheets as anode material for SIBs are successfully synthesized by one-step solvothermal method. The average thickness of Fe0·08Co0·92Se2 nanosheets is approximately 3.08 nm, which is about equivalent to the thickness of six b-axis crystal cells. When employed as anode for SIBs, Fe0·08Co0·92Se2 nanosheets display a high initial discharge specific capacity of 679.8 mAh/g, a high initial coulombic efficiency of 85.5% and superior reversible specific capacity of 510.4 mAh/g at 1 A/g. Simultaneously, Fe0·08Co0·92Se2 also exhibits an enhanced rate capability (480.0, 460.0, 435.1 mAh/g at 0.2, 2, 4 A/g respectively) and outstanding cycling stability at relatively large current density (397.0 mAh/g at 4 A/g after 1000 cycles). The excellent Na-ion storage performance of FexCo1-xSe2 nanosheets could be ascribed to the abundant electrochemical active sites and shorten ion transfer tunnels provided by this ultrathin 2D structure and bimetallic ion synergistic effect by heteroatom doping. Noticeably, the capacitive contribution may play the key role in the admirable rate capability through electrochemical kinetics analysis of Fe0·08Co0·92Se2 nanosheets. Therefore, this work presents a simple preparation of ultrathin 2D selenide nanosheets and an effective low-cost regulation strategy for improving Na-ion storage performance.

Nanostructured δ-MnO2 incorporated with alkali cations (A-δ-MnO2, A = K+, Na+) has been synthesized and evaluated as a cathode material for aqueous sodium-ion batteries. It is observed that Na+ ions are easier than K+ ions to intercalate into the layered δ-MnO2 due to their smaller ion radius. The incorporation of K+ and Na+ into δ-MnO2 shows a great influence on the electrochemical performance of the layered δ-MnO2. The full cell with (K, Na)-co-incorporated δ-MnO2 (K : Na : Mn = 0.15 : 0.26 : 1) hierarchical nanospheres as the cathode and NaTi2(PO4)3 as the anode exhibits a specific capacity of 74.6 mA h g−1 at 150 mA g−1, and the capacity remains at ∼62% at a high current density of 600 mA g−1. The electrochemical cycling does not induce observable structural degradation even after 200 cycles. K-incorporated and (K, Na)-co-incorporated δ-MnO2 electrodes have superior capacity and rate capability, which can be ascribed to their hierarchical structure and adequate crystallinity.

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 (CuII↔CuI) and anionic (TCNQ0↔TCNQ−↔ TCNQ2−) reversible redox reactions, delivering a discharge capacity as high as 255 mAh g−1 at a current density of 20 mA g−1. 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 TCNQ0 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.