•The charge/discharge electrochemical window of Li-S batteries is limited to a narrow range (1.95-2.45 V).•In this narrow electrochemical window, the electrochemical reactions only occur in the liquid phase.•This narrow electrochemical window provides improved capacity performance and cycling performance.In this research, a narrow charge/discharge electrochemical window is enforced for lithium sulfur batteries. In this way, the active material of the electrode (which is sulfur in the initial state) is limited to Li2S8 and Li2S3, leading the electrochemical reactions to take place in the liquid phase and thereby improving the capacity performance and cycling performance. After 50 cycles at a current density of 0.1 mA cm−2, the specific capacity obtained using a narrow electrochemical window (1.95-2.45 V) (490 mAh g−1) was greater than that obtained using a wide electrochemical window (1.7-2.8 V) (435 mAh g−1). This finding demonstrates the feasibility of improving the electrochemical performance by employing a suitable electrochemical window to restrain the phase transformation to the middle liquid phase.

A ternary polyaniline/sulfur/acetylene black (PANi@S-C) composite cathode with variable PANi content has been synthesized by a continuous two-step liquid phase route. Different ratios of the PANi@S-C composite are characterized by SEM, TEM, FTIR, XRD and electrochemical methods. The conductive polyaniline plays multiple roles in the composite, acting as a conducting additive and a porous adsorbing agent. It is uniformly coated onto the surface of a S-C composite powder to form a core/shell structure, which significantly enhances the electrochemical performance and cycle life of the sulfur cells. The PANi@S-C composite with 12.5 wt% PANi presents optimum electrochemical performance, where the initial discharge specific capacity is 1257 mAh g−1 and remains ∼600 mAh g−1 after 100 cycles at 0.16 mA cm−2. After a rate test from 0.1 to 1 mA cm−2, the cell remained at ∼600 mAh g−1 sulfur after 50 cycles when the current density returned to 0.1 mA cm−2.

A novel sulfur–nitrogen co-doped carbon material (SNC), which is obtained by taking polyaniline as the nitrogen-containing carbon precursor and then incorporating sulfur atoms in situ as the matrix material for lithium sulfur batteries, is investigated. XPS reveals the formation of strong chemical bonding (–C–S–C– and C–SOx–C (x = 2–4)) in the structure of SNC materials. Moreover, the results demonstrate that SNC–S composites showed higher specific capacity and better cycle performance than the pristine nitrogen co-doped carbon material (NC)/sulfur (NC–S) composites. Except for providing more lithium storage sites due to faradaic reactions, the incorporation of sulfur not only immobilizes the sulfur and polysulfide species, thus improving the interfacial characterization between the electrode and the electrolyte, but also influences the reversible dissolving balance of long-chain polysulfides and suppresses the “shuttle effect”, resulting in better electrochemical behavior. This facile approach of the SNC matrix could provide a practical research direction for lithium sulfur batteries.

Co3O4 microtubes with balsam pear-shaped outer surfaces and void spaces were synthesized by the Kirkendall effect. Intermediate phase Li1.47Co3O4 was detected by X-ray diffraction (XRD) to illustrate the severe volume change during electrode reactions and detail the conversion of Co3O4 to Li2O and Co. Changes in and influences on Li2O and Co during the first cycle were explored through calculations based on electrochemical impedance spectroscopy (EIS) tests to obtain a better understanding of the electrode reaction processes. The void spaces in the tube walls accommodated the volume change during electrode reactions, and the balsam pear-shaped outer surfaces expanded the available active surface for electrode reactions. As a result, the prepared Co3O4 microtubes exhibited strong electrochemical performance. The first discharge capacity reached ∼907 mAh g−1 at 5.00 mA cm−2, and discharge capacity remained above 400 mAh g−1 until the 40th cycle at 0.05 mA cm−2.

Lithium vanadium phosphate (Li3V2(PO4)3) has been extensively studied because of its application as a cathode material in rechargeable lithium ion batteries due to its attractive electrochemical properties, including high specific energy, high working voltage, good cycle stability, and low price. In this review, the preparation of technology, structure, Li+ insertion/extraction mechanism, and electrochemical properties of Li3V2(PO4)3 are introduced, and with particular focus on the relationship of these topics each other. The synthetic techniques of Li3V2(PO4)3, such as high-temperature solid-state method, sol–gel method, hydrothermal method, etc. And progress of techniques in modification, such as coating and elemental doping, is reviewed. Finally, the directions for further development and prospective applications for the material are proposed.

Li3V2(PO4)3/C (LVP/C) composite materials have been successfully synthesized via a sol–gel method with oxalic acid as chelating agent and polyethylene glycol (PEG) as the supplementary carbon source, in which oxalic acid and PEG serve as double carbon sources. The X-ray diffraction patterns indicate that all of the samples are well crystallized. Transmission electron microscopy images reveal that the LVP/C sample prepared with 10 wt% PEG is uniformly coated by carbon layer with an appropriate thickness of 10–16 nm, resulting in a high electrical conductivity and a fast kinetics. The Li+ diffusion coefficient in the LVP/C sample prepared with PEG is 3.482 × 10−13 cm2 s−1, which is larger than that of the LVP/C sample prepared without PEG. In the range of 3.0–4.3 V, the LVP/C-10 electrodes exhibit good rate capability and excellent cyclic performance, which discharge capacities are 131.9 mAh g−1 at 0.1 C and 105.3 mAh g−1 at 5 C. The present work provides a valuable route for preparing lithium metal phosphates with double carbon sources to improve the conductivity and hence the electrochemical performance.

Currently, studies with sulphur electrode materials are focused primarily on carbon/sulphur and polymer/sulphur composites. Despite carbon/sulphur composites being a more popular research interest than polymer/sulphur composites, improving the cycle performance of sulphur by using polymers is also a major research focus. Therefore, we review the latest developments for polymer/sulphur composites in Li–S batteries. The various polymer/sulphur composites and their impacts on the electrochemical performance are discussed. Meanwhile, the synthesis approaches toward the various polymer/sulphur composites are also summarised. Finally, the future research directions involving polymer/sulphur composites are addressed.

The feasibility of a semi-solid flow battery with polysulfide as catholyte is demonstrated, which gives a power density of 1.823 mW cm−2 at 4 mA cm−2. Compared to Li–S batteries with sulfur as cathode, the feasibility and flexibility using polysulfide as catholyte in flow-through mode create new potential for the practical application of conventional Li–S batteries.

A variety of MnO2 nanorods containing one or two transition metals (M) (with M = Al and/or Ni) have been successfully synthesised via a facile hydrothermal synthesis route. The physical–chemical properties and electrochemical performance of manganese oxide were analysed by X-ray diffraction (XRD), inductively coupled plasma atomic emission spectrometry (ICP-OES), Fourier transform infrared spectrometer (FT-IR), scanning electron microscopy (SEM), Brunauer–Emmett–Teller method (BET), galvanostatic discharge and cyclic voltammetry (CV). The result indicated that α-type MnO2 was obtained, and a small quantity of Al and/or Ni were embedded into the crystal lattice of manganese oxide instead of the partial Mn ion, which resulted in anisotropic expansion of the MnO2 unit cell. The doping of Al can strengthen Mn–O bonds in the [MnO6] octahedral and increases the specific surface area of the modified material (i.e., Al–MnO2 is 119 m2 g−1). Interestingly, MnO2 electrode co-doped with equimolar Al and Ni exhibited the highest specific capacity of 169 mAh g−1 at 0.05 mA cm−2. The substantial enhancement of the electrochemical lithium storage capacity was due to the ameliorating of integrative factors, such as high specific surface area, excellent lattice parameters and lower electrical resistance, as well as short Li+ and electron transport length. In addition, a more stable host skeleton also guaranteed an endurable Li+ intercalation behaviour during the discharge process.

Highly ordered mesoporous β-MnO2 (HMM) was prepared using KIT-6 templates, and mesoporous β-MnO2/S (HMM/S) composites with four sulfur content were synthesized by thermally treating a mixture of sublimed sulfur and HMM for use as cathode in Li–S batteries. The results of thermal gravimetric analysis, Brunauer–Emmett–Teller measurements, X-ray diffraction, environmental scanning electron microscopy and transmission electron microscopy for samples of HMM and HMM/S material indicate that HMM has a highly ordered pore structure and highly crystalline walls, facilitating electrolyte flooding of these pores within the cathode. The mesopores of HMM contain the majority of the sulfur mass, and effectively suppress the diffusion of polysulfide species into the electrolyte. The thin (nm scale) walls of HMM ensure short diffusion distances for Li+ on intercalation in charge/discharge cycles. Electrochemical experiments show that HMM/S composites with low sulfur content exhibit high discharge capacities and good cycling stability.

Currently, studies with sulphur electrode materials are focused primarily on carbon/sulphur and polymer/sulphur composites. Despite carbon/sulphur composites being a more popular research interest than polymer/sulphur composites, improving the cycle performance of sulphur by using polymers is also a major research focus. Therefore, we review the latest developments for polymer/sulphur composites in Li–S batteries. The various polymer/sulphur composites and their impacts on the electrochemical performance are discussed. Meanwhile, the synthesis approaches toward the various polymer/sulphur composites are also summarised. Finally, the future research directions involving polymer/sulphur composites are addressed.