Ternary layered oxide (NCM) cathode materials are widely used in today's energy storage systems (ESS) due to their advantages of high energy/power density, high specific capacity and high oxidation-reduction potential (ORP). Cathode material specific capacity increases with the improvement of Ni content while its stability, safety and capacity retention rate are decreasing. So how to deal with this contradiction effectively is the key to develop ternary material system. This paper starts from the failure phenomenon on account of bulk phase structure destruction and cathodeelectrolyte interface composition change during the cycle of NCM battery system. Combined with the new theory, new method and new application in the research of NCM failure mode at home and abroad in recent years, the possible decline mechanism and life decay reasons of mechanical damage, structural evolution, electrochemical polarization, chemical side reaction process and synergistic effect of cathode and anode electrodes are giving. The results guide users to rationally formulate charging and discharging protocols and alleviate electric vehicles (EV) range anxiety and the design of the material.

Lithium-ion batteries have grown rapidly and have changed our lives. The research on the cathode materials of lithium ion battery is the key to improve the performance of the battery. Therefore, understanding the relationship between the structure-performance relationship and explaining the electrochemical reaction mechanism (especially the performance degradation and failure mechanism) of the cathode materials can help to improve the energy density and power density of the materials. Magnetic Resonance techniques, including NMR (nuclear magnetic resonance) and EPR (electron paramagnetic resonance), has been continuously improved during the past three decades of material research, and has gradually become one of the key technologies for studying the structure-performance relationship of cathode materials. NMR could be used to study light elements commonly found in battery materials such as Li, Na, F, P, C, H and O, while EPR can be employed to study transition metals such as Co, Ni, Mn, Fe and V. This paper summarizes the progress of magnetic resonance research on several important commercial cathode materials (LiCoO₂, NCA, NMC and LiFePO₄), and demonstrates the important role of NMR and EPR in the study of structureperformance relationship of cathode materials. It is emphasized here that the development of in-situ technology has gradually shown its importance to investigate the electrochemical reaction mechanism. This article will help to understand the important value of magnetic resonance technology in battery materials research and further promote the development of magnetic resonance technology.

Solid state polymer lithium battery with the advantages of high energy density and high safety is promising to overcome the range anxiety and security issue of electric vehicles. However, there are several failure behaviors in solid state polymer lithium battery, such as capacity fading, overcharge, gas, internal short-circuit, and calendar failure. And the in-depth study and understanding on the failure mechanisms are limited by the suitable characterization techniques due to the low resistance to irradiation of polymer electrolyte and the difficulty to expose perfect electrode/electrolyte interface. So, developing suitable characterization techniques for solid state polymer lithium batteries is significant to better understand the failure mechanisms and help researchers find feasible solutions to overcome battery failure. In this review, the development of failure mechanisms and characterization techniques for solid state polymer lithium batteries are summarized from the aspects of lithium dendrite growth, cathode structure evolution and mechanical failure, interface microstructure evolution and interface reaction, as well as structure evolution of polymer electrolyte. Finally, new research strategies and methods of the failure mechanisms for solid state polymer lithium batteries are proposed.

Lithium-ion batteries (LIBs) present increasing applications in electrochemical energy storage component due to their comprehensive advantages in energy density, cycle life, energy efficiency and safety, there is still a need for further improvement. A large number of advanced characterization technologies has done great contributions to the development of the basic theory in LIBs. Ultrasound technique, as an advanced means for nondestructive testing (NDT), is widely used with the merits of ultrahigh sensitivity, low cost, easy operation and real-time measurement, which has great application potentiality in the field of LIBs characterization. The current research and application situation of the ultrasound technique are summarized and then the prospect is also discussed.

The failure problems, associated with shortened cycle life, increased self-discharge rate, deteriorated power performance and ect., often occur in the application of lithium ion batteries. They may even lead to safety problems under harsh conditions. Lithium deposition is considered as one of the most important factors leading to these failures and even safety problems. Therefore, it is particularly important to analyze the formation and growth process of lithium dendrites. In this paper, the in-situ detection techniques of lithium deposition in lithium batteries are reviewed, including physical and electrochemical methods. Physical detection methods are systematically introduced, such as optical in-situ technology, in-situ X ray technology, in-situ nuclear magnetic resonance technology and in-situ neutron technology. The electrochemical methods include charge-discharge voltage curve method, Arrhenius method, internal resistance-capacity curve analysis method and capacity decay rate method. The principle, advantages, limitations, and corresponding special detection device of the above physical methods, and the principle and analysis method of electrochemical methods are introduced. Furthermoere, the problems of in-situ detection techniques are summarized and the future research direction is put forward.

This review summarizes recent important research progress in the field of in situ transmission electron microscopy (TEM) studies of batteries. Although enormous efforts have been devoted to understanding the fundamental electrochemistry of batteries, the structure, morphology and composition evolution of the electrode materials during charging and discharging are still unclear. Furthermore, how to correlate microstructure evolution with battery performance is still challenging. Thus, it is important to develop advanced in situ TEM techniques to monitor structural evolution of electrode materials during cycling in real time and to correlate the structural information with the batteries performance so as to advance our understanding of batteries. In this review, we firstly summarize the experimental techniques of assembling nano batteries under high vacuum condition in TEM to enable battery studies. Then discuss important research progress in the application of in situ techniques in the research of lithium/sodium ion batteries and metal air batteries. Finally, perspectives on future directions on the application of in situ TEM in battery studies are provided. The in situ TEM technology can directly and in real-time observe the microscopic behavior of battery materials in electrochemical reactions, deepen the understanding of the failure behavior of battery materials in charge and discharge, and provide a scientific basis for the design of high-stability batteries and broaden their application fields.

Lithium-ion batteries (LIBs) with high energy density are desired in many fields, however, the volume change during the electrochemical process of the high capacity materials (such as Si) would cause the mechanical failure of the LIBs. Thus, the in-situ experimental methods, which could obtain the inner information of the LIBs, are necessary for revealing the failure mechanisms of the LIBs. Here main characterization techniques for in-situ experiments have been introduced, including (1) in-situ mechanical-electrochemical experiments:atomic force microscopy (AFM), in-situ optical methods, in-situ scanning electron microscope (SEM) computed tomography (CT) and (2) inner temperature measurements:in-situ infrared method, embedded multi-field sensor.

An electric vehicles used commercial prismatic battery cell with lithium nickel manganese oxide cathode was selected to research the venting process of the products released during thermal runaway. The test was carried out in a sealed chamber with nitrogen as an inert protective environment to avoid combustion, and a high-speed photography was adopted to capture the rapid venting process. The sudden change of the detection pressure indicated that two intense injections occurred during the venting process under medium state of charge, and separately resulted in a sudden decrease and a rapid increase of the temperature in the jet region. With the aid of the auxiliary light source, high-speed images of the first venting evolution of the products caused by thermal runaway were captured. Further analysis revealed that the venting process experiences an initial strip and cone eruption first, followed by a longer period of amorphous eruption, and then a later inverted conical eruption. At the same time, the characteristics of gas-liquid-solid three-phase coexistence were also observed in the images.

In order to explorer the thermal runaway process of Li-ion batteries, based on the research and establishment of the Li-ion batteries thermal runaway method, the thermal runaway phenomenon of 18650 Li-ion batteries was analyzed by electrical heating trigger under different charging states, The gas leaked through thermal runaway process was both collected and analyzed. The results show that the electrical heating method can triggering the thermal runaway of 18650 Li-ion batteries, which will produce toxic gases, accompanied with smoke and high temperature. Improper protection will cause harm to human body and environment.

An inflexion point during battery aging, turning the capacity retention curve from a gradually decreasing region into an abrupt drop, has been reported for a diversity of lithium-ion battery materials. Understandings such a multi-stage aging phenomenon are vital to lifespan design and cascade use of batteries, while they remain hitherto elusive. We unravel that the multi-stage aging behavior can result from the multi-slope nature of open-circuit-voltage (OCV) curve of batteries. Increasing kinetic polarization during aging renders the discharging process terminated in the OCV region with different slopes, leading to distinct aging profiles. In this work, we firstly demonstrate this basic idea using a two-stage example. Then, a general theory of inflexion point in battery aging considering both the positive and negative electrode is extended which is rather facile to incorporate major aging mechanisms and battery chemistries. This work provides new insights in understanding the multi-stage aging behavior, which can further contribute to the lifespan design and reuse of lithium-ion batteries.

The lead redox flow battery (LRFB), as a novel type of lead battery, which has bright prospects in future research and application, is becoming a research focus in electrochemistry. Since it was proposed by Pletcher in 2004, a number of relative researches have been done. This paper analyzes the theoretical performances, reviews the development history, and introduces the research status of LRFB. Research outcomes exhibited that the performances of SLFB on the 100 cm² electrode scale reached 90% charge efficiencies and 80% voltage efficiencies across 100 cycles, and the tests on the 1000 cm² electrodes of SLFB were already done. According to theoretical calculation and analysis, the energy storage cost of LRFBs is only 0.265 ￥·(kW·h)^-1, which is lower compared with conventional lead acid batteries. The exigent problems of LRFB to be solved include:① scaleup experiments are necessary; ② cost reduction by developing current collector materials; ③ failure mechanisms study to improve cycle life;④ restore or repair methods for spent LRFB.

Phase change energy storage is a technology to realize energy storage through the absorption/release of latent heat during phase change processes. It can balance the mismatch of heat supply and demand in time, space and intensity. It has become the focus of attention in the field of energy storage due to its high energy storage density. However, its application is limited because of the low thermal conductivity of phase change materials. Aiming at the problem of slow heat transfer rate caused by low thermal conductivity of phase change materials in the melting/solidification process, this paper reviews the latest progress of phase change materials heat transfer enhancement research at home and abroad from three aspects:optimizing the structure of energy storage devices, improving the thermal conductivity of phase change materials with additives, and their combination. By comparing the advantages and disadvantages of various heat transfer enhancement methods, both experiments and simulations show that the combined heat transfer enhancement can solve the problems of low thermal conductivity and increase heat transfer area, thus improves the heat transfer performance of phase change materials; Porous metal materials as thermal conductive additives enable better thermal conductivity enhancement. The related technical problems to be solved in future for phase change energy storage technology to enhance heat transfer are also put forward.

In order to improve the energy density of LiFePO₄ power lithium ion batteries, a small amount of S was introduced to the LiFePO₄ cathode materials to obtain the LiFePO₄/S composite materials by a ball-milling method. The crystalline structure, morphology and electrochemical performance were characterized by X-ray diffraction, scanning electron microscopy and electrochemical tests by coin cells/pouch cells. The results showed that the sulfur particles were covered with nanosized LiFePO₄ nanoparticles on the surface, which is analogous to the core-shell structure. The results of electrochemical tests of coin cells showed that the LiFePO₄/S composite with 15% S content had the best electrochemical performance, which exhibited a reversible capacity up to 251.5 mA·h·g^-1 at 0.1 C and an excellent long-term cycling stability with 94.9% capacity retention after 100 cycles. Moreover, 0.5A·h pouch cell with LiFePO₄/S-85/15 composite as cathode LiFePO₄ showed a capacity retention of 80.1% after 100 cycles. As a polar carrier, LiFePO₄ has a certain adsorption for polysulfides. The addition of a small amount of sulfur can greatly improve the discharge capacity of LiFePO₄ materials, while maintaining excellent cycling stability. The LiFePO₄/S composites can provide a new strategy for the development of LiFePO₄ power lithium ion batteries.

MXene with large specific surface area and good conductivity properties as a new 2D material has been explored as electrode materials for supercapacitor. Silver nanowire (Ag Nw) has been attracting more and more attention because of their intrinsic optical, thermal and electric properties. In this research, MXene/Ag Nw composites have been fabricated by a simple and "green" filtration selfassembly method. Their structure and morphology were analyzed with scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectrometer (FT-IR), energy dispersive spectrometer (EDS) as well as electrochemical properties tested on electrochemical workstation. The synergetic effect of MXene and silver nanowire constructed a three dimensional (3D) conductive network and enlarged the distance of MXene layers. The results revealed that MXene/Ag Nw composites exhibited a high specific capacitance (379.06 F·g^-1 at 10 mV·s^-1) when the concentration of Ag Nw is 5% which is higher than pure MXene (176.95 F·g^-1) and long-term cycling stability (98.20% capacitance retention over 1000 cycles). In a word, the MXene/Ag Nw composite films have exhibited an improved electrochemical performance when used as electrode materials for supercapacitor.

MnFe₂O₄ with one dimensional porous rod structure was synthesized based on microemulsion method and calcination with ferrous sulfate and manganese acetate as raw materials, bis (2-ethylhexyl) sulfosuccinate sodium salt as surfactant. The prepared MnFe₂O₄ rods show a diameter of about 200 nm and length of 2~3 μm with abundant mesopores of 13~35 nm. When tested as anode material of lithium ion batteries, the prepared MnFe₂O₄ exhibited a capacity over 630 mA·h·g^-1 after 200 cycles at a current density 100 mA·g^-1. It also shows good rate capability. The research shows that the MnFe₂O₄ with porous rod structure could be a good candidate for anode materials of lithium ion batteries.

Graphene is a two-dimensional carbon material with high conductivity, good chemical stability and excellent electrochemical performance, which has promising prospects in energy storage field. Nitrogen doping can not only create structural defects, but also change the electronic distribution for graphene, which is beneficial for the electrochemical energy storage properties. In our work, the nitrogendoped graphene (N-rGO) was prepared using low cost nitrogen source (urea) and graphene oxide solution by freeze-drying and the subsequent high-temperature thermal reduction. The effects of thermal reduction temperature on the chemical composition, structure and electrochemical properties of the N-rGO were also studied. The results show that with the increase of thermal reduction temperature, the nitrogen content decreases, while the graphitization degree and the conductivity increases (72.3 S·cm^-1), indicating the improved lithium storage performance. Tested in half-cell as anode material, the N-rGO-800 exhibits a high reversible capacity of 876 mA·h·g^-1 at 0.05 A·g^-1, superior to the values reported previously. Meanwhile, the specific capacity can retain 584 mA·h·g^-1 at 1 A·g^-1 and remains stable after 850 cycles, indicating excellent cyclic stability and good rate performance.

In order to deal with the inefficient utilization of regenerative braking energy generated by high-speed trains during braking process, a regenerative braking energy storage scheme for highspeed railway based on super capacitor (SC) was proposed. In the scheme, railway power conditioner (RPC) was used as the interface circuit to connect the energy storage device with the traction power supply system. The super capacitor, used as the energy storage medium, was connected with the DC link of the railway power conditioner through the bi-directional DC/DC converter, so as to achieve the function of energy storage and compensation of negative sequence current. On the basis of studying the topology of energy storage scheme, the compensation principle of negative sequence current was analyzed, and the control strategy of energy storage scheme was studied according to the compensation principle. In the strategy, hysteresis control method was used for RPC two converters and current closed loop control method was used for bi-directional DC/DC converters in energy storage device. The simulation results show that the proposed storage scheme can effectively recycle the regenerative braking energy generated by high-speed trains and compensate the negative sequence current to improve the power quality of the grid side.

Aiming at the problem that the single urban rail train impacts the traction network voltage during the starting and braking process and causes the instability of the network voltage, this paper proposes an MMC-based vehicle supercapacitor energy storage system (vehicle MMC-SCESS). The system uses MMC topology as the main circuit and the control flexibility and fault tolerance of the system are improved by dispersing the supercapacitor energy storage unit into the MMC sub-module. Then taking the overall structure of the vehicle MMC-SCESS as the research object, the working principle of the a-phase bridge arm circuit is analyzed in detail, and the general working mode of each energy storage sub-module is summarized. Aiming at the main circuit structure of the energy storage system, an integrated control strategy is proposed. By controlling the charge and discharge state of the supercapacitor energy storage unit, the energy is realized to flow between the urban rail train, the DC traction network, and the super capacitor energy storage system. Finally, a three-phase fivelevel vehicle MMC-SCESS simulation model is built in Matlab/Simulink. The simulation waveform verifies the feasibility of the energy storage system control strategy.

Based on the self-developed lithium-ion capacitor, a 48 V start-stop power system vehicle model was built based on AVL-Cruise. Combined with the Ah metrology, open circuit voltage method and extended Kalman filtering algorithm, the device's state-of-charge (SOC) estimation module is designed to realize online SOC estimation. The energy management model based on fuzzy control is established in MATLAB/Simulink to realize the functions of engine start and stop, pure electric drive start, braking energy recovery and active coasting. Finally, the SOC of the power system and the fuel consumption of the vehicle are evaluated according to the New European Driving Cycle (NEDC) operation condition. The research results show that the system can achieve SOC estimation within 10% of the error, and the 48 V start-stop power system based on lithium ion capacitor has good fuel economy.

In order to study the influence factors of floating charge performance of lithium ion battery in high-voltage system, the gas-producing composition, structure change of cathode and anode materials, metal dissolution, separator morphology and Gurley value change of floating charge failure battery were deeply analyzed. The results show that:In the process of floating charge at high temperature for a long time, the cathode material will undergo phase transformation, and the metal elements will be dissolved. Meanwhile, O₂ will be released to cause the oxidation and decomposition of the electrolyte. Under the condition of high temperature and high voltage, the SEI film on anode surface will also be destroyed, and continuous reforming and repair reactions will occur. These reaction products are deposited on the surface of anode and in the separator pores, resulting in the blockage and even penetration of separator pores near the anode electrode side. In other words, the cathode and anode micro-short circuit will be caused, and a large amount of gas will be released. By improving structure stability of cathode materials, optimizing electrolyte and increasing the puncture strength of separator, the high-temperature floating charge performance of the battery can be significantly improved.

AC impedance of Li(Ni_1/3Co_1/3Mn_1/3)O₂/graphite power battery was tested under simulated UPS. The effect of different storage time and SOC on ohmic resistance R_s, charge transfer impedance R_ct and diffusion impedance CPE_W was analyze by the equivalent circuit. The variation trend of capacity and impedance of Li(Ni_1/3Co_1/3Mn_1/3)O₂/graphite power battery under UPS was studied. The results show that the capacity of the cell decrease about 1.7%, the ohmic resistance R_s at different SOC has the same tendency to change and the charge transfer impedance increase significantly with increasing storage days. The charge transfer impedance generated by the electric double layer is increases significantly as SOC decreases. The diffusion resistance (SOC=0%) increases and the diffusion impedances at SOC=100% and 50% slightly increase as storage day increases. Capacity degradation and impedance results show that the Li(Ni_1/3Co_1/3Mn_1/3)O₂/graphite power battery can be used well in UPS.

In this paper, four kinds of aging states, capacity retention ratio (CRR) of 100%, 85%, 75% and 65% lithium ion phosphate power battery were selected as the research objects. The combustibility and smoke generation of key components of the battery (positive, negative and separator, all containing electrolyte) were studied by cone calorimeter. The fire risk assessment index system of batteries with different aging states was established by using AHP (analytic hierarchy process). The results show that the effective combustion heat value of the negative electrode decreases as the battery CRR declines, And the yield of CO₂ and total smoke production of battery components gradually decrease. The normalized risk index of battery components with a capacity retention rate of 100%~85% is significantly higher than that of battery components with a capacity retention rate of 75%~65%.

Charging strategies of traction battery play a crucial role in improving the performance and life of electric vehicles. In this paper, the pulse charging strategy for a ternary material system 18650 cylindrical battery is studied, and it is compared with the standard constant current and constant voltage charging strategy. The impact of the key parameters of the pulse charging strategy on the electrical performance is comprehensively and objectively analyzed. The impact of the pulse charging strategy on battery life was systematically evaluated. In the aspect of cycle life research, electrochemical Impedance Spectroscopy (EIS) was used to analyze the AC impedance under different cycles and SOC, and the cell structure was characterized by X-ray tomography (CT) nondestructive analysis. The relationship between the pulse charging strategy and the cycle performance and structure of the battery is revealed.

In order to solve the problem that pure electric vehicles are susceptible to current fluctuations and non-linear conditions during SOC prediction, a method for dynamic prediction of lithium battery SOC is proposed. Firstly, the parameter combination of the particle swarm clustering algorithm is optimized and combined with the preferred results to improve the radial basis function (RBF) neural network. Then, by analyzing the characteristics of the battery under different working conditions, the battery is divided into charging and static. Set and discharge three states. Different strategies are used to predict the SOC for the working state of the battery. In the battery discharge phase, the improved PSO-RBF algorithm is used to dynamically predict the SOC. in the battery standing and charging state, the two-point look-up table method is used to make the open circuit voltage curve considering the temperature drift and the current node abrupt curve during charging into two dimensions. Array table, the value of SOC is corrected by using the created two-dimensional array table. Thereby reducing system response time while improving accuracy. The experimental results show that the maximum error of the prediction correction model is about 1.9%, which verifies the effectiveness of the method.

Inconsistent internal resistance of the battery pack will cause overcharge and over discharge of the short board cells, which will induce gradual fault and aggravate battery pack failure. In this paper, after analyzing the charging curves of LiFePO₄ with different aging degrees, an online detection method based on incremental capacity was proposed. The incremental capacity peak can be obtained by analyzing the charging data, and thus characterize internal resistance differences between cells, last use the box plots for the abnormal detection. This Online detection carried out by the designed battery detection system was compared with HPPC detection of the battery pack. It was found that the normalized internal resistance distribution of the two has high consistency. Meanwhile, the online detection method has low cost and simple operation. It is suitable for large-scale commercial battery for internal resistance consistency detection and does not affect engineering efficiency and battery life. The online detection method provides guidance for the life-cycle preventive safety detection of lithium-ion batteries.

This paper reviews the research progress of three main data-driven methods, artificial neural network, support vector regression and Gaussian process regression, in the estimation of state of health (SOH). Artificial neural network is suitable for simulating power batteries and can achieve high precision. Support vector regression has a small amount of calculation and perfect theoretical foundation. It is widely used in the research of power battery SOH estimation. The Gaussian process has high regression accuracy and can give a confidence interval for the prediction results. In recent years, the number of related literatures shows an increasing trend. In view of the shortcomings of the current SOH definition that fail to reflect the rated voltage decay of lithium-ion batteries, it is proposed to define SOH by using battery full charge energy. In this paper, BP neural network, support vector regression and Gaussian process regression model are established respectively. The new energy vehicle big data is used to predict the battery charging energy. Quantitative comparison results verify the characteristics of the three methods in terms of calculation volume and accuracy. Finally, the application prospects of data-driven methods and new energy vehicle big data in power battery SOH estimation research are prospected.

The electric heating and solid sensible heat thermal storage system is of great significance for the consumption of renewable energy and the clean utilization of energy. The key parameters design and economic analysis of the electric heating and solid sensible heat thermal storage device are important means to improve economic benefits. Therefore, the calculation method of investment and operation cost of electric heating and solid sensible heat thermal storage device is presented in this paper. By comparing the cost of different heating methods, the economics of electric heating and solid sensible heat thermal storage device were analyzed. Meanwhile, the influence of the utilization coefficient of the off-peak power on the economy of the electric heating and solid sensible heat thermal storage device was studied. Finally, a case study is used to verify the rationality and correctness of the economic evaluation method proposed in this paper. The research content of the paper can provide reference for users to choose the electric heating and solid sensible heat thermal storage device.

Latent heat thermal energy storage (LHTES) has been a hot research topic because of the advantage of coordinating the mismatch between energy supply and demand. This paper presents numerical investigation to study the thermal behavior of shell-and-tube LHTES system. The twodimensional physical model was considered to simplify. The three-dimensional physical model was also used to monitor outlet temperature and time needed to accomplish phase change. The enthalpy method was used to solve the energy equation in liquid and solid regions of phase change material (PCM). The natural convection (NC) in liquid PCM was considered by adopted Boussinesq approximation. The numerical model is validated by literature data. The result indicates that the inlet temperature of heat transfer fluid (HTF) has a significant influence during the phase change process. It also confirms that the melting process is significantly influenced by natural convection, vorticity distribution directly affects the heat exchange efficiency; and the thermal conduction plays a dominant role during the solidification process. The three-dimensional simulation shows that outlet temperature of HTF depends on the average Nusselt number at the tube, in the meantime, the horizontal arrangement of heat exchange tube has slightly lower heat transfer efficiency than vertical arrangement.

Molten salt as heat transfer and storage medium has been widely used in solar power generation. Nitrate has been successfully applied in commercial power plants due to its low melting point, low cost and low corrosiveness. In this paper, different characterization techniques including differential scanning calorimetry, thermogravimetric method, DIN method, laser flash method, Archimedes principle and rotation method will be used to measure and compare the melting point, decomposition temperature, specific heat, thermal conductivity, density and viscosity of solar salt (60% NaNO₃+40% KNO₃), Hitec (7% NaNO₃+53% KNO₃+40% NaNO₂), Hitec XL[7% NaNO₃+45% KNO₃+48% Ca(NO₃)₂] and quaternary mixed nitrates(16.67% Ca(NO₃)₂·4H₂O+44.17% KNO₃+5.83% NaNO₃+33.33% NaNO₂) independently developed by our group. The advantages and disadvantages of thermal properties of four kinds of mixed nitrates were analyzed, which provided basic data for engineering application. The results show that among the four kinds of mixed nitrates, the quaternary salt has the lowest melting point, the highest decomposition temperature, the highest average specific heat and thermal conductivity, while the Hitec XL has the highest density and Solar salt has the the lowest viscosity.

In this paper, a new type of cage drawer phase change heat storage tank is introduced. The phase change heat storage tank and ordinary heat storage tank are compared and analyzed through experimental tests on their solar energy guarantee rate and system energy efficiency ratio. The experimental results show that the hourly heat collection with the phase change heat storage tank is 3.7 times of that with the ordinary heat storage tank at the same tank size. The phase change heat storage water tank is beneficial to improve the solar energy guarantee rate and the energy efficiency ratio of the system. When the solar radiation intensity is similar, the phase change heat storage tank will increase the solar energy guarantee rate by 72% on average and increase the energy efficiency of the system by 26% compared with the average EER. At the same time, the phase change heat storage tank can reduce the heat loss in the upper part of the water tank at night, and the water temperature drop in the upper part of the water tank can be reduced by 50%.

A finite element model of energy storage flywheel was established in order to obtain the stress characteristics and to optimize its structure. The stress characteristics analyses of the aluminum alloy (7075) flywheel at a given speed, at different rotational speed, and with different materials were carried out based on Workbench. The results showed that the maximum radial stress appeared on the inner wall of aluminum alloy (7075) flywheel at a rotational speed of 5000 r/min and gradually decreased outwardly, with a maximum value of 27.047 MPa. The maximum circumferential stress appeared at the intersection of the flywheel flange and the wheel disc, with a maximum value of 13.623 MPa. With the increase of rotational speed, the stress value also increases, but the position of the maximum stress does not change. At the same speed, aluminum alloy (7075) is superior to the other three alloys as flywheel material. The research results can provide important reference for the design and optimization of energy storage flywheel structure.

This study was expected to provide the theory references for the design of a scroll expander used in micro-compressed air energy storage (micro-CAES) systems, by conducting the computational fluid dynamics (CFD) method to obtain the pressure, velocity and temperature field distributions in the working chambers. The effects of different external expansion ratio on the transient performance of the scroll expander and the flow fields of the working chambers were investigated under a constant back-pressure. The results showed that the variable external expansion ratio has little influence on the fluctuation of the inlet mass flow rate, and the outlet mass flow rate fluctuation increases with the increase of the external expansion ratio. The heat utilization degree of the compressed air in the expander and the unsteady driving moment of the expander increase with the increase of the external expansion ratio. The secondary flow loses in the back pressure chamber also increase with the increase of the external expansion ratio. There exist obvious local high temperature areas in the back pressure chamber.

The performance of air compressor has a great influence on the performance of fuel cell. In order to establish the mathematical model of air compressor accurately, the nonlinear function of rotational speed, flow and pressure is established by using the equivalent circuit structure method. The mathematical model parameters of air compressor equivalent circuit and the data of air compressor performance parameters are fitted, according to the fitting effect, the method based on maximum flow deviation and outlet pressure weighting is used to improve the fitting method. The results show that the two methods can realize high precision fitting of static model.

In order to improve the frequency modulation effect of the energy storage battery with different performance on the power grid and solve the problem of overcharge and over discharge of the battery during the frequency modulation process, an adaptive control strategy based on doublelayer fuzzy device is proposed. The performance of the energy storage battery with inconsistent performance can better maintain the state of charge (SOC) of the battery while meeting the frequency modulation requirements of the power grid, avoiding overcharge and over discharge, thereby prolonging the cycle life and improving the economy of the energy storage battery. Comparing the results of the example with the traditional method, the superiority of the proposed control strategy is verified, especially for the performance of the poor performance of the energy storage battery SOC state is better than the traditional method.

Understanding the structure-function relationship is the eternal topic of functional materials, which is also true for lithium ion battery materials. Thus, various kinds of characterizations, such as XRD, neutron diffraction, NMR, XPS, et al., were used to study lithium ion battery materials. However, these characterization methods are not sensitive to micro-structure, which can only provide global structure information. Micro-structure has huge impact on the properties of materials, so it is important to acquire micro-structure of lithium ion battery materials. Transmission electron microscope can resolve micro-structure at atomic scale, which helped a lot for the study of lithium ion battery materials. This paper is based on the relationship between transmission electron microscopy and lithium ion battery materials in terms of basic principles and experiment methods. We hope this paper can facilitate the research and cooperation for both TEM and lithium ion battery related field.

This bimonthly review paper highlights 100 recent published papers on lithium batteries. We searched the Web of Science and found 3077 papers online from Aug. 1, 2019 to Sep. 30, 2019.100 of them were selected to be highlighted. Layered oxide and lithium-rich materials are still under extensive investigations for studying Li⁺ intercalation-deintercalation mechanism and evolution of surface structure, and the influences of doping, coating and interface modifications on their cycling performances. 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 mechanism for Li storage, kinetics, SEI. There are a few papers related to electrolyte additives, solid state lithium batteries, Li/S batteries and modeling.