Potassium ion batteries can meet the requirements of energy storage system owing to their advantages of low cost, long life and high energy density, since potassium is the element with abundant resource, low price and low electrochemical potential. However, challenges coming from disadvantages of larger and heavier of potassium ion still confront of the development of electrode materials and electrolyte for potassium ion batteries. In recent years, the high capacity electrode materials and electrolyte of potassium ion batteries have been studied extensively under the traction of electric vehicles and energy storage applications. The cathode materials such as Prussian blue and its analogues, transition metal oxides and polyanionic materials show the application prospect. Available anode materials consist of carbon, titanium and alloy materials. Effective electrolytes involve ester-based electrolytes and ether-based electrolytes. These results are significant for the basic and applied research of potassium ion batteries.

Safety concern imposes an uncertainty for the large-scale applications of high energy density and high capacity lithium-ion batteries (LIBs) in electrical vehicles and electric storage devices. Thermal runaway is recognized to be the main cause for the unsafe behaviors, such as firing and explosion. From the electrochemical point of view, an effective way for resolving this problem is to build a thermal shutdown mechanism in the internal LIBs to switch off the ions or electrons transport at risky temperatures, thus terminating the battery reactions and preventing the thermal runaway from happening. Based on this consideration, various thermal runaway-preventing technologies have been proposed for improving the safety of LIBs in recent years, such as positive-temperature-coefficient electrode (i.e. PTC electrode), thermoresponsive microsphere-coated separator or electrode, thermally polymerizable additives, and so on. This paper intends to review the recent progress of these technologies after a brief introduction to their implementation approaches and working mechanisms. Furthermore, the problems and future development orientations in this area have also been discussed from a viewpoint of practical application demand.

The development of lithium ion battery with high energy density is one of the key means to achieve long mileage for new energy vehicles, especially for pure electric vehicles. However, with the continuous improvement of battery energy density, the cycle life and safety performance of batteries will be decreased. Taking the cell with energy density of 300W·h/Kg as the target, this paper discusses the material chemistry, the design of cell structure and the safety protection measures of the system, and explores the technical route of lithium ion battery system with high safety and high energy density.

Developing the high energy density cathode materials for lithium ion batteries is the key to satisfy the demands of rapid development of electric vehicles and hybrid electric vehicles. Compared with traditional cathodes including LiCoO₂ and LiFePO₄, layered Li-rich Mn-based cathode materials are expected to achieve the index requirement of 300 W·h·kg^-1 because their specific capacity could reach up to 250 mA·h·g^-1 and the average discharge voltage exceeds 3.5 V. However, Li-rich materials have a poor cycle performance and gradual voltage decay during cycling, which is due to the irreversible phase transition from layered to spinel. In recent years, researchers have carries out tremendous works to solve these issues. In this paper, according to modification technology, the current studies including lattice doping, surface coating, designs of microstructure and control of active crystal plane are reviewed.

Power batteries have significant influence on the development of electric vehicles.Lithium ion batteries are regarded as the most promising candidate among different electrochemical energy storage devices due to their outstanding characteristics. Cycling performances and safety issues of lithium ion batteries are highly concerned and tightly related to the properties of electrolytes. State-of-the-art electrolytes are composed of organic solvents, lithium salts,and additives, and among which the additive plays an important role and shows great influence on the electrochemical performances and cycle life of the batteries. This paper reviews the recent progress on electrolyte additives, especially the ones related to high-voltage cathode and silicon-carbon anode materials. The functionalities and mechanisms of typical additives are presented in details. We believe that high performance power batteries could be realized by developing novel multi-functional electrolyte additives.

The lithium ion battery solid-electrolyte interface (SEI) is a thin-layer film formed on the surface of electrodes due to redox decomposition of electrolyte in the initial charging process. SEI film with high ionic conduction and electrical resistance is quite necessary for the long-term usage of lithium ion batteries and has a crucial impact on their capacity, rate, cycling and safety performances. However, because of its complex formation processes and great difficulties in making accurate characterization, only a superficial knowledge of SEI derive from some experimental observation or model hypothesis, thus quantitative analysis and controllable structural optimization are still needed to be further investigated. This paper reviews the formation process, the influence factors,some research ideas and current research status of SEI film. In addition, some potential research directions of SEI have been presented, including investigating the formation mechanism and role of SEI on the surface of cathode materials, optimizing the electrolyte formulas through solvents, lithium salts and additives to facilitate the formation of more stable SEI films, adopting advanced in-situ analysis methods and theoretical calculation methods to analyze chemical composition, morphology and microstructure of SEI, exploring effective ways to construct artificial SEI film and realize controllable structural modification.

As a new type of clean energy, lithium-ion battery has been used in various fields for its high energy density and extended cycle life, but in some conditions such as overheating, overcharging and short circuit, the lithium-ion battery would get into thermal runaway or even thermal explosion. To study the thermal explosion behaviors of the lithium-ion battery, a series of thermal explosion experiments of 18650 lithium-ion battery under different stages of charge (SOCs) in hermetic space was carried out using extend volume accelerating rate calorimeter (EV-ARC) and a stainless steel pressure canister(the volume of this vessel is 292mL). In these experiments, the lithium-ion battery was overheated until it got into thermal explosion and the stage SOC of sample cells was 0, 25%, 35%, 50%, 65%, 75%, 85% and 100%, respectively. By the experiment trails, we could get some vital thermal hazard characters of batteries, such as the thermal explosion initial temperature, the maximum temperature, the maximum pressure, temperature rise rate, pressure rise rate, et al. The experimental results showed that there was no thermal explosion at 0% SOC, while it occurred under other conditions. When the battery got into thermal explosion, the maximum surface temperature of the battery and the maximum canister internal pressure increase with the increase of SOC. The thermal energy released from the battery under different SOCs during thermal explosion was calculated using the initial temperature and the maximum temperature of the battery. When the battery was fully charged, the explosion equivalent value was 5.45 g TNT, which is about 1.5 times higher than that of 25% SOC, and the maximum canister internal pressure was 40.69 bar at 100% SOC. In hermetic space, the thermal explosion hazards of 18650 lithium-ion batteries increase with the increases SOC.

The relationship of ion transport and pressure in poly(ethylene oxide)/lithium salts(PEO/LITFSI) solid polymer electrolyte with different lithium salts concentration is firstly simulated by molecular dynamics (MD) method. Our simulation results demonstrate that the PEO polymer chains fold with the increased pressure, which will increase the interaction between lithium ions and TFSI anions and thus hinders the lithium ions transport in PEO/LITFSI solid electrolyte. Moreover, the molecular simulation results show a reduced lithium-ion diffusion coefficient and decreased ion transport ability in PEO/LITFSI under increased pressure. The present study will offer a theoretical basis for the applications of these solid electrolytes in high pressure environment.

Magnesium (Mg) ion batteries and aluminum (Al) ion batteries have become new hot spots in beyond lithium ion batteries due to their high energy densities, abundant reserves as well as safety. In this work, the theoretical gravimetric energy densities (GED), volumetric energy densities (VED) and output voltage (OV) of nearly 300 electrochemical couples of Mg/Al ion batteries are calculated by thermodynamic method. As our evaluation, considering of gravimetric energy density, volumetric energy density, output voltage, toxicity, corrosion, flammability and environmental friendliness and so on, a series of materials are screened out as promising candidates including Mg ion cathodes (O₂, S, MnO₂, MoO₃, Fe₂O₃, Fe₃O₄, NiO, MoO₂, CuO and Cu₂O) and Al ion cathodes (O₂, S, MnO₂, MoO₃, NiO, CuO and Cu₂O).

Two kinds of flexible anodes (Si/PDCNT and Si@PDCNT) were prepared by the combination of silicon and multi-walled carbon nanotube (MWCNT), using coating and mixing methods according to the advantages of MWCNT and nanostructured silicon. The morphology and electrochemical properties of the flexible anodes were comparatively analyzed by means of scanning electron microscopy (SEM), energy spectrum analysis (EDS) and electrochemical analysis. Results show that in the Si/PDCNT anode, nanostructured silicon is evenly distributed on the surface of anode, and the Si and PDCNT are closely combined. Si/PDCNT anode delivered a specific capacity of 170 mA·h·g^-1 after 200 cycles,which is much better than traditional Si/Cu batteries. The Si@PDCNT anode prepared by the mixing method is evenly dispersed in the three-dimensional conductive network structure of PDCNTs. Si@PDCNT anode retained a specific capacity of 200 mA·h·g^-1 after 500 cycles, which is better than Si/PDCNT anode. This study is helpful to promote the application of flexible anode of silicon based carbon nanotubes, and provides experimental basis for the research and development of high specific energy flexible battery technology.

NMC811/SiO-C cell is one of the most potential battery systems with high specific capacity, however, it is difficult to improve its poor cycling performance in practical application. To analysis the cycling failure mechanism of NMC811/SiO-C cell, electrode characterizations have been carried out by electrochemical impedance spectroscopy (EIS), X-ray diffraction spectroscopy (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM),transmission electron microscope (TEM). The result showed that NMC811 cathode material can keep stable crystal structure and yield dissolving of metal ions during cycling, while SiO-C anode material suffered from structure cracking and displayed increasing consumption of electrolyte as well as growing SEI layer thickness. The changes in electrodes cause the serious specific capacity degradation and the poor cycling performance of NMC811/SiO-C pouch cells.

With the emergence and development of flexible mobile electronic, flexible lithium-sulfur (Li-S) batteries with higher energy density have received more and more attention. In this perspective, we summarize the key materials and current status of flexible Li-S batteries, and presents its future development. The key factors in the flexible Li-S battery lies in the conformal design of its materials. By integrating the positive sulfur electrode to a flexible substrate, such as carbon nanotube membrane, graphene film and polymer, an integrated flexible Li-S composite sulfur electrode can be obtained. In the design, these substrates act as flexible support. Compared with conformal Li-S cathode, the fabrication of flexible lithium metal anode possess greater challenge. By developing new host material for lithium metal or using lithium-free anode, flexible anode materials for Li-S batteries is expected. Although there are many problems to be solved, flexible Li-S batteries with improved electrochemical properties and mechanical properties are anticipated in the field of mobile electronics.

This bimonthly review paper highlights 100 recent published papers on lithium batteries. We searched the Web of Science and found 2731 papers online from Feb. 1, 2018 to Mar. 31, 2018. 100 of them were selected to be highlighted. Layered oxide、spinel cathode materials and organic cathode materials are studied. Li⁺ intercalation-deintercalation mechanism and evolution of surface structure, and the influences of doping, coating and interface modifications on their cycling performances are studied for oxide materials and the oxygen redox in lithiun-rich oxides are extensively studied. Large efforts were devoted to Si based composite anode materials for analyzing the mechanism for Li storage and SEI formation. The cycling properties of metallic lithium electrode are improved by using different kinds of surface cover layer. In-situ technologies are used to analyze the kinetic process and SEI and theoretical work covers the machnism for Li storage, kinetics, SEI and solid state electrolytes. There are a few papers related to electrolyte additives, new solid state electrolytes, solid state lithium batteries, Li/S batteries, and modeling

Compressed air energy storage system, as one of the most promising energy storage technologies, is developing rapidly. Gas storage room is one of the main components of compressed air energy storage system. Firstly, the development, application and research status of compressed air energy storage system are reviewed, and the characteristics of commercial energy storage power station and demonstration power station are put forward. Secondly, the development and application of the gas storage device are analyzed in detail, including the classification of gas storage devices, the technical characteristics and application status of different types of gas storage devices, and the energy storage characteristics of the gas storage devices are analyzed in detail. Finally, the technical problems and related parameters and evaluation indexes of the gas storage device are analyzed and arranged. The development direction and research emphasis of compressed air energy storage and its storage device were put forward.

Supercapacitor has been recently evaluated as a new type of energy storage device because of its high energy density and the rapid charge/discharge.The niccolite-type MOF compound was synthesized by a facile solvothermal method. By thermal treatment at 800℃, the precursor FeMn-MOF was calcinated to the complex structure C-MOF. FTIR Spectroscopy, XRD and SEM were used to characterize the structure of samples. The electrochemical properties of the supercapacitor applying C-MOF as the electrode material were measured. Results show that the specific capacitance of C-MOF electrode reaches 388.9 F·g^-1 at the current density of 0.5 A·g^-1. Furthermore, the capacitance retention is up to 201.5% after 5000 cycles at current density of 2 A·g^-1 compared with the initial specific capacitance. It indicates not only a good electrochemical stability but also an activation process of C-MOF electrode. This research not only broadens the application field of MOF material but also promotes the application value of MOF based composites as the electrode material of supercapacitor.

S/PPy composite materials with different mass ratios (the weight of sulfur to that of polypyrrole) of 6:4, 7:3, 8:2 were prepared by coating polypyrrole (PPy) on the surface of sulfur with a solution drop addition method to investigate the optimum synthesis condition. The structure and electrochemical properties of S/PPy composites were studied by means of X-ray diffraction (XRD), Fourier transform infrared spectrum analysis (FTIR), field emission scanning electron microscopy (FESEM), transmission electron microscope (TEM) and electrochemical measurements. And the morphology observation results show that PPy granules disperse uniformly and exert outstanding covering effect when mass ratio of sulfur and PPy is 7:3 (S/PPy-30). Furthermore, S/PPy-30 composite manifests best electrochemical performance, which displays high initial discharge capacity of 1151 mA·h·g^-1 and maintains 623 mA·h·g^-1 after 120 cycles at a current density of 200 mA·g^-1. And S/PPy-30 presents distinctive advantage in rate capacity compared to the other two S/PPy materials.

Cellulose has attracted much attention due to their eco-friendly property and low cost. Recently, it could be widely used in fields of electrochemical research as an efficient carbon material. Fe₃O₄/C-CNFA nanocomposite was prepared by a solvothermal method while ferric trichloride hexahydrate as the iron source and carbonized cellulose nanofiber aerogel as the carrier. X-ray diffraction and scanning electron microscopy were employed to characterize the structure and micro-morphology of the as-prepared nanomaterial. Meanwhile, electrochemical properties were also measured to compared with pure Fe₃O₄ nanoparticles. The results show that carbonized cellulose nanofiber aerogel maintains the 3D porous network and Fe₃O₄ nanoparticles with uniform size are uniformly distributed in the carbon matrix. Fe₃O₄/C-CNFA nanocomposite demonstrates an excellent cycling stability with a reversible capacity of 847 mA·h·g^-1 over 100 cycles at a current density of 100 mA·g^-1, as well as an initial specific discharge capacity of 1064 mA·h·g^-1. Compared to pure Fe₃O₄ nanoparticles, the electrochemical properties of the synthesized nanomaterials have been greatly improved. This study is beneficial for promoting the application of cellulose based carbon material in electrochemical field. Furthermore, it also provides experimental basis for the development of composite electrode material.

Based on the abundant database of nationalthird-party vehicle detection institutioncovering entire vehicle, system and core components, the paper comparatively studied the performances and give a comprehensive evaluation of typical 30 kW PEMFC commercial systems from international mainstream manufacturer, joint venture with internationaltechnical background and domestic mainstream manufacturer. A series of comparative studies were carried out, which includes U-I curve, FCS power and efficiency, FCE power and efficiency, BOP power and power factor, hydrogen consumption rate and flux. The results provide the necessary reference for related technology development and industrialization.

In terms of the high operation density and regenerative braking power of the urban mass transit, the flywheel energy storage system(FESS) can effectively reduce the DC traction network voltage fluctuation and the traction energy consumption. Because the charge or discharge operation for the FESS is just based on the high or low DC traction network voltage, so the regenerative braking power may not be identified accurately in addition, the state of charge (SOC) of the FESS cannot be automatically adjusted which leads to the incorrect response to the traction network voltage in the actual operating conditions. In this paper, the unload transit network voltage identification and the SOC optimized control strategy is adopted to solve these problem, the effectiveness of the proposed control strategy is verified by rail transit platform test.

The operating parameters of VRFB are important factors affecting the performance of the battery. The existing equivalent circuit model often fail to study the effect of flow rate on the performance and efficiency of VRFB due to the lack of coupling with hydrodynamic model, so they are not perfect in practical projects; In addition, the pure hydrodynamic model only studies the relationship between flow rate and pump loss, but the electrical characteristics of the battery itself are too simplified, which cannot fully reflect the characteristics of VRFB system. Therefore, the equivalent circuit model of the basic principle and the equivalent loss of vanadium battery was established based on basic principle and the equivalent loss of VRFB while the hydrodynamic model was established based on mechanical loss and structure parameters in this paper. Based on the equivalent model of the VRFB system, the influence of the operating parameters of the battery on the battery performance was analyzed. The simulation analysis shows that the optimal flow rate during charging-discharging is a function of the state of charge. According to this phenomenon, the flow is controlled piecewise with the change of SOC. Through the analysis of simulation results, the efficiency of VRFB can be effectively improved by the optimal operation strategy of subsection control flow rate according to the change of SOC.

Cyclic voltammetry (CV) is a very important electrochemical measurement method, which has been widely used in electrochemistry research especially for the study of lithium batteries. CV is commonly used to study the reversibility, mechanism and kinetic properties of electrode reactions in lithium batteries. Here, we overviewed the fundamental principles, experimental methods and the commonly used equipments for the CV measurement. Besides, its applications on the study of lithium batteries were introduced in detail, combing with practical experimental cases.