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A sodium bis(fluorosulfonyl)imide (NaFSI)‐based multifunctional electrolyte is developed by partially replacing NaPF6 salt in the electrolyte to improve the wide temperature range working capability of NaNi1/3Fe1/3Mn1/3O2/hard carbon (NNFM111/HC) sodium‐ion batteries (SIBs). The capacity retention of the SIBs with NaFSI‐NaPF6 dual salt electrolyte increases from 47.2 % to 75.5 % after 250 cycles at 25 °C, and from 51.0 % to 82.3 % after 80 cycles at 45 °C, and the 1 C discharge capacity retention at the low temperature of −20 °C also increases 26.8 %. In the single salt system, NaPF6 effectively passivate the aluminum foil and NaFSI passivate the electrode/electrolyte interface. The synergistic effect of NaPF6 and NaFSI greatly improves the battery performance in a wide temperature range. This NaFSI‐based dual salt electrolyte also effectively overcomes the flaws when the SIBs using NaFSI or NaPF6 independently, and makes it more suitable for SIBs, indicating promising prospects in the commercial application of NNFM111/HC SIBs. [ABSTRACT FROM AUTHOR]

Solid electrolyte interphases (SEIs) play a crucial role in keeping sodium metal anodes (SMAs) intact and improving battery life. However, the SEIs arising from irreversible reactions between metallic Na and electrolytes fail to suppress Na dendrite growth and have sluggish Na+ kinetics. Herein, a functionalized separator modified by a sp2 carbon conjugated‐covalent organic framework (sp2c‐COF) is proposed to induce a robust SEI. X‐ray photoelectron spectroscopy (XPS) analyses and theoretical calculations demonstrate that the SEI is rich in NaF because the structure of NaPF6 is unstable due to influences from the COF separator. In situ observations show that the Na dendrite is effectively suppressed even at a high current density of 20 mA cm−2. Satisfactorily, the COF separator exhibits a high transference number of 0.78, achieving a fast Na plating/stripping process. Based on these superiorities, a symmetric cell Na|COF|Na shows stable cycling for over 1500 h at 20 mA cm−2. In addition, full cells Na|COF|NaTi2(PO4)3 (NTPO) present good rate performance (30 and 50 C) and excellent cycling stability over 5000 cycles at 5 and 10 C. The application of COFs to improve SMAs in this work demonstrates a new strategy for improving sodium metal batteries. [ABSTRACT FROM AUTHOR]

The separator, as an important inner part of the sodium‐ion battery (SIB), has a significant impact on the electrochemical performance and security of the battery. However, conventional polyolefin separators are inapplicable for SIBs due to their poor wettability to liquid electrolytes and unsatisfactory heat resistance. To address these problems, a novel polyethylene (PE)‐ hydroxyethyl cellulose (HEC)‐TiO2 composite separator modified on the PE matrix is proposed and successfully prepared by a multistep synthesis procedure of HEC coating and TiO2 in situ self‐growth, while almost maintaining the initial separator thickness. Compared with conventional PE separators, this composite separator possesses remarkable wettability which benefits from the introduction of a polar HEC‐TiO2‐incorporated coating. Besides, thanks to a significant improvement in wettability, the separator presents high electrolyte uptake of up to 186.5% and an extraordinary ionic conductivity of 0.342 mS cm−1. As expected, a Na|Na3V2(PO4)3 battery with the PE‐HEC‐TiO2 separator exhibits a reversible capacity of 99.0 mAh g−1 and a capacity retention of 94.8% after 1000 cycles at 5 C with a steady Coulombic efficiency of nearly 100%. These brilliant performances convincingly make it a promising separator for advanced SIBs with high reversibility, high capacity, and long life. [ABSTRACT FROM AUTHOR]

Abstract: Charge‐discharge reaction mechanisms of sodium‐ion batteries under various condition are studied by using a three‐electrode setup of a pouch‐type sodium‐ion battery. The sodium‐ion battery is constructed by using cost‐effective ternary layered Na0.76Ni0.3Fe0.4Mn0.3O2 and commercial hard carbon as the cathode and anode materials, respectively, and 1.0 M NaPF6 in mixed carbonate solvent as the electrolyte. The electrochemical impedance spectroscopy (EIS) results and transmission electron microscopy (TEM) images show that an apparent solid‐electrolyte interphase (SEI) film is formed on anode material surface during the formation (pre‐charging) process, and the potentials for the SEI film formation of the solvents (ethylene carbonate and ethylene carbonate) and the additive (fluorinated ethylene carbonate) in the electrolyte are 2.04 and 2.79 V, respectively. The X‐ray diffraction (XRD) results demonstrate that, during charge, the crystal structure of cathode material changes significantly with the deintercalation of Na+. When the battery is charged to 5.0 V, the diffraction peak corresponding to the (002) plane disappears, as Na+ is further deintercalated, and the structure changes from the hexagonal phase to the monoclinic phase, causing the rapid degradation of the cycle performance. When the battery is overdischarged to 0 V, the EIS results and TEM images show that the SEI film is destroyed completely, and the cycle life performance is significantly deteriorated. [ABSTRACT FROM AUTHOR]

Organic electrode materials bring about new possibilities for the next generation green and sustainable lithium/sodium ion batteries (LIBs/SIBs) owing to their low cost, environmental benignity, renewability, flexibility, redox stability and structural diversity. However, electroactive organic compounds face many challenges in practical applications for LIBs/SIBs, such as high solubility in organic electrolytes, poor electronic conductivity, and low discharge potential as postive materials. Quinone organic materials are the most promising candidates as electrodes in LIBs/SIBs because of their high theoretical capacity, good reaction reversibility and high resource availability. While quinone electrode materials (QEMs) have so far received less attention in comparison with other organic electrode materials in secondary batteries. In this paper, an overview of the recent developments in the field of QEMs for LIBs/SIBs is provided, emphasizing on the modifications of the quinone compounds in solubility, electronic conductivity, and discharge plateaus. Finally, multifaceted modification approaches are analyzed, which can stimulate the practical applications of QEMs for LIBs/SIBs. [ABSTRACT FROM AUTHOR]

[Display omitted] • Hard carbon in the PC/EC-based electrolyte can achieve excellent cycle performance and rate performance for sodium-ion batteries. • For the first time, the differences of the ion solvation/desolvation behavior in the electrolytes are proposed to uncover the underlying mechanisms of the distinct electrochemical performance of HC in the PC/EC-based electrolyte and the PC-based electrolyte. • A SEI film with a higher percentage of organic composition is formed in the PC/EC-based electrolyte. • The PC/EC-based electrolyte exhibits a faster ion desolvation process compared to the PC-based electrolyte. Although hard carbon in propylene carbonate / ethylene carbonate (PC/EC)–based electrolytes possesses favorable electrochemical characteristics in rechargeable sodium–ion batteries, the underlying mechanism is still vague. Numerous hypotheses have been proposed to solve the puzzle, but none of them have satisfactorily unraveled the reason at the molecular–level. In this study, we firstly attempted to address this mystery through a profound insight into the disparity of the ion solvation/desolvation behavior in electrolyte. Combining the results of density functional theory (DFT) calculations and experiments, the work explains that compared to the sole PC–based electrolyte, Na+–EC 4 molecules in the PC/EC–based electrolyte preferentially undergo reduction and contribute to the emergence of a more stable protective film on the surface of hard carbon, leading to the preferable durability and rate capability of the cell. Nevertheless, applying the ion solvation/desolvation model, it also reveals that Na+–(solvent) n molecules in the PC/EC–based electrolyte can achieve faster Na+ desolvation processes than in the PC–based electrolyte alone, contributing to the enhancement of charge transfer kinetics. This research holds great importance in uncovering the possible mechanism of the remarkable electrochemical– properties of hard carbon in PC/EC–based electrolytes, and advancing its practical utilization in future sodium–ion batteries. [ABSTRACT FROM AUTHOR]

• NaDFP preferentially deposits on HC anode can induce a low-impedance interphase. • Interphase on NFM cathode inhibits iondissolution and maintains structure integrity. • NaDFP improves battery's cycle and storage performance over a wide temperature range. An electrolyte containing a sodium difluorophosphate (NaDFP) dual-functional additive is developed to enhance the temperature performance of NaNi 0.33 Fe 0.33 Mn 0.33 O 2 (NFM)/hard carbon (HC) sodium-ion batteries (SIBs). The addition of NaDFP in the electrolyte not only induces solid electrolyte interphases (SEIs) and cathode electrolyte interphases with excellent properties on the anode and cathode surfaces but also optimizes of the solvation structure of sodium ions, resulting in a better cycle stability of NFM/HC SIBs over a wide temperature range from −30 °C to 60 °C. In addition, high capacity retentions of 85 % after 700 cycles at 25 °C, 90 % after 150 cycles at −10 °C and 90 % after 150 cycles at 45 °C are achieved, respectively. And those common problems such as gas generation and sodium evolution of pouch NFM/HC SIBs are also effectively relieved, indicating the promising application prospect of NaDFP additive in the functional electrolyte of NFM/HC SIBs. [ABSTRACT FROM AUTHOR]

A commercially feasible sodium-ion pouch cell with a capacity of 650 mAh is fabricated using the ternary layered Na 0.76 Ni 0.3 Fe 0.4 Mn 0.3 O 2 and the hard carbon as the cathode and anode materials, respectively. 1.0 M NaPF 6 in the mixed carbonate solvents is served as the electrolyte. According to the calculation of the frontier molecular orbital energy, adiponitrile (ADN) has a stronger ability of electron donating than the carbonate solvents and thus is easier to be reduced on the surface of the cathode material to form the solid-electrolyte interphase (SEI) film, so ADN is selected as the electrolyte additive to improve the performance of sodium-ion battery for the first time. The transmission electron microscopy (TEM) images show that the addition of ADN enables the formation of more uniform and compact SEI film on the cathode material surface. The X-ray photoelectron spectroscopy (XPS) results indicate that ADN promotes the formation of NaF and result in the formation of NaCN as the effective components of the SEI film. The electrochemical test results demonstrate that ADN effectively improves the electrochemical performance at high -low-temperature and cycling stability of Na 0.76 Ni 0.3 Fe 0.4 Mn 0.3 O 2 as the cathode material of sodium-ion batteries, that is because the adding of ADN in electrolyte result in the formation of more effective SEI film. In particular, the battery using the electrolyte with 3% ADN exhibits the most obvious improvement, which owe to the formation of most compact, stable and effective SEI film after adding 3% ADN into the electrolyte. At the operating temperature of 45 °C, −10 °C, and −20 °C, the discharge capacity increases by 10.5%, 8%, and 13%, respectively. For the cycle life, the capacity retention of the battery without addition of ADN drops rapidly to 75% after 40 cycles, while the capacity retention of the battery with 3% ADN still remains 78% after 220 cycles. [ABSTRACT FROM AUTHOR]

Na x Mn 0.54 Ni 0.13 Fe 0.13 O 2 ( x = 0.46, 0.67) with P2-type hexagonal phase are synthesized via a sol-gel method as novel cathode materials for sodium-ion batteries. The followed characterization indicates the existence of coaxial hexagonal phase structure in two samples, among which Na 0.46 Mn 0.54 Ni 0.13 Fe 0.13 O 2 (NMNF-0.46) presents neat hexagonal nanosheets with a sharp edge, whereas Na 0.67 Mn 0.54 Ni 0.13 Fe 0.13 O 2 (NMNF-0.67) shows granular structure. Tested in sodium-ion batteries at the potential window of 2.0–4.2 V, the specific capacity of NMNF-0.46 is 132.5 mAh g −1 at a current density of 0.1C, and the capacity retention rate was 78% after 50 cycles. In contrast, the specific capacity of NMNF-0.67 is 99.5 mAh g −1 , but a high capacity retention rate of 86.8% is delivered. The promising electrochemical performance of Na x Mn 0.54 Ni 0.13 Fe 0.13 O 2 provides a meaningful reference for developing advanced cost-effective electrode materials for sodium-ion batteries. [ABSTRACT FROM AUTHOR]