Safety accidents related to lithium-ion batteries occur frequently, causing the safety research of lithium-ion batteries to become a hot topic. This paper analyzes that most of the safety failure stem from the poor reliability of battery products. Reliability is the ability or possibility of a product to perform the predetermined function without failure within a specified period of time and under specified conditions, which is measured by probability. The safety accidents of lithium-ion batteries meet the definition characteristics of reliability. This paper answered why strict safety testing standards cannot eliminate battery safety failure, analyzed the causes of potential safety failure of lithium-ion batteries from the perspective of reliability, and explored their potential test methods. By this paper, measures to avoid or reduce the damage after safety failure are expected. It is emphasized that besides the research on the safety of lithium-ion battery, the reliability research should be paid more attention to.

The increasing demand for energy storage requires energy storage devices to have greater capacity, and there are high hopes for lithium-ion batteries (LIBs) in the energy storage field. Their structural stability as cathode materials and their voltage profiles for insertion/extraction directly determine the specific energy and power densities of the battery system. In recent years, research related to these characteristics has remained the core issue in the LIB research field, particularly the characterization of the material structure and electrochemical behavior. Important real-time and in-situ strategies were employed in designing and developing more types of materials with excellent performance. For the cathode materials, detailed insights, such as their microstructure, chemical composition, ion valence and states, character of morphology, ion transport, and electron transfer, are beneficial in the preparation, structure design and modification of electrode materials. In this review, the operating principles, usage scenarios, and corresponding information of characterization techniques are introduced, and some examples that use these techniques to characterize LIB anode materials are listed. Finally, the advantages and disadvantages of current characterization techniques are compared, and the major challenges in research studies are discussed. Hence, this article summarizes the typically used technologies that are applied in monitoring the structural changes and surface-interface behaviors of cathode materials, including the microscopic imaging, phase analysis, composition and chemical valence, and bonding and functional groups to provide a reference for the combined utilization of various characterization technologies and to promote the development of an ideal electrode material.

Lithium-ion batteries composed of single cathode materials, such as LiCoO₂, LiFePO₄, and LiMO₂ (M=Ni_xCo_yMn_z/Al_z, x+y+z=1), are widely used in aerospace aviation, electric vehicles, and electronic equipment due to their high specific capacity, large energy density, and long cycle life. However, these single cathode materials have an unstable structure, high irreversible capacity loss, poor cycle stability, low safety, and low conductivity, which restricts their application in large-scale power equipment. In this regard, we conducted a review of the literature on the application of single cathode materials in various fields and described their primary defects. It was found that a mono/binary composite cathode prepared by combining a single cathode material with other materials (cathode or non-cathode) can effectively solve the aforementioned problems, thereby improving the electrochemical performance and cycle stability of lithium-ion batteries. More specifically, this article presents a review on the research progress of 1+0 type and 1+0+0 type mono composite cathode materials, and 1+1 type and 1+1+0 type binary composite cathode materials based on lithium-ion battery cathode materials. In this series, 1 represents a lithium-ion cathode material and 0 represents a non-lithium-ion cathode material. Moreover, we analyzed the electrochemical performance of the four types of composite cathode materials with 14 different structural combinations. Finally, the main problems of composite cathode materials are explained and suggested development directions are proposed.

In between liquid electrolyte and solid electrolyte, gel polymer electrolyte (GPE) has attracted enormous interests because it can combine the advantages of liquid electrolyte and solid electrolyte. Polyvinylidene fluoride and its copolymer polyvinylidene fluoride-hexafluoropropylene have a high dielectric constant, which is conducive to the ionization of lithium ions, the improvement of ion conductivity, and thus can improve the electrochemical performance of lithium ion batteries. Is a kind of polymer material with application potential for preparing GPE. This article introduces the research progress of GPE based on polyvinylidene fluoride in the past five years, It focuses on the following four categories of GPE: (1) single polymer GPE based on polyvinylidene fluoride or polyvinylidene fluoride-hexafluoropropylene, (2) polymer composite GPE with other polymers (such as polymethyl acrylate, Polyisophthalamide, polyacrylonitrile, etc.), (3) composite GPE with inorganic nanomaterials (such as Aluminum oxide, silicon dioxide, Graphene oxide, etc), and (4) other new Other new types (polyvinylidene fluoride polymer and cellulose composite) GPEs. This article briefly describes the advantages and challenges of various types of composite GPE. Finally, the key problems for GPE industrialization were analyzed and some prospects for the future research directions of GPE were proposed.

In the process of LIB production, the formation processes aim at achieving the wetting of the electrodes and the activation of the electrode materials. Meanwhile, during the initial charge, with the insertion of lithium ions, the electrolyte components are reduced in the anode and form a stable solid electrolyte interface film on the anode for preventing the irreversible consumption of the electrolyte and lithium ions in the subsequent cycling. Consequently, these techniques are of great significance for the battery performance; the formation effect directly affects the subsequent performance of LIBs, including storage performance, cycle life, rate performance and safety. However, for each single battery in the battery pack of electric vehicles, it takes days or even weeks to carry out the formation and conditioning processing, resulting in low battery production efficiency. A large amount of charge/discharge equipment, temperature control equipment, and environmental space increases the cost of battery production. The traditional formation method cannot completely meet the requirements of high performance, such as capacity, life, and safety, in LIBs. To date, there have been many studies on improving the battery performance and reducing the formation time and the cost of battery production by optimizing the formation technique of LIBs. This review focuses on the optimization of the lithium-ion formation technique, introducing the significance of battery formation, cost analysis, and various technological parameters and forecasting the future research and improvement directions.

Graphite is the main anode material used in commercial LIBs, and graphite will remain the main anode material in the future. Failures often occur during the use or transportation of the LIB graphite electrode. These failures will affect the service life of an LIB. Therefore, how to extend the service life of the LIB graphite electrode becomes the most important issue. This article, as well as the recent relevant literature, summarizes the main LIB failure mechanism of the graphite electrode. Then, according to the failure mechanism of the graphite electrode material and the electrode design, two aspects are proposed to prolong the service life of the graphite electrode. Finally, the future development trend of the long-life graphite electrode is suggested.

With the recent popularity of electric vehicles, the production demand for lithium-ion batteries has soared, and a corresponding number of spent lithium-ion batteries has also rapidly increased. Discarded lithium-ion batteries can contribute to environmental pollution, threaten human health, and are a waste of precious resources. From the point of view of environmental protection and resource utilization, recovering spent lithium-ion batteries directly addresses both the resource and pollution problems, so it is of the utmost importance to develop recycling technologies. Unfortunately, the recycling technologies of spent lithium-ion batteries are not widely used due to the complex process, high cost, and potential secondary pollution. At present, the hydrometallurgical technology is the most promising for recycling spent lithium-ion batteries. The process is simple, the recovery efficiency is high, and the cost is low. In this paper, the following topics are covered: ① the research progress using an inorganic acid as a leaching agent in hydrometallurgical recovery technologies is described in detail; ② the characteristics of several commonly used inorganic acids are introduced; ③ the reaction conditions and metal leaching efficiency of various inorganic acids are compared, and ④ the thermodynamic reaction mechanism during leaching is analyzed. In future research, it is necessary to vigorously promote the development of technologies for recycling spent lithium-ion batteries and to find a more environmentally friendly and universal process. Along with this development, the field should focus on understanding the reaction mechanism and explore more efficient recovery methods.

Solid-state lithium batteries have become the primary focus in the field of lithium batteries due to their high safety, high energy density, long cycle life, and wide operating temperature range. Using solid electrolyte as the core component of a solid battery is the primary difference from a traditional liquid battery. The solid electrolyte largely determines the performance parameters of the solid lithium battery, including the power density, cycle stability, safety performance, high and low temperature performance, and service life. Therefore, further study of solid electrolytes is ongoing in research institutions of various countries, large electronics companies, and automobile manufacturing companies. This article systematically introduces three types of solid electrolytes that are favored by the industry: Polymer, oxide, and sulfide, and analyzes the latest progress and reported results for each. Among these three types, polymer electrolytes demonstrate good viscoelasticity, excellent machining performance, are lightweight, and transmit lithium ions through the process of "complexation and decomplexation". Oxide solid electrolytes can be found in the crystal state or the glass state. NASICON, perovskite, garnet, and LISICON electrolytes are examples of crystal state electrolytes, while the LiPON electrolyte used in thin film batteries is a popular type of glass oxide electrolyte. Compared with oxide electrolytes, sulfide solid electrolytes exhibit higher ionic conductivity because sulfur ions have a large radius and strong polarization, and for this reason have attracted much attention in recent years. To analyze the development of these electrolytes, a search for solid-state electrolyte patent applications for all-solid-state lithium batteries was performed in the Derwent Innovations Index patent database (DII). After 2015, patent applications for solid electrolytes exhibited a stage of rapid growth. The number of patent applications from Japanese and Korean companies was exceptionally high, from companies such as Toyota, Fuji, and Samsung. These companies have mastered the advanced technology of solid-state lithium batteries. Tracking patent applications allows readers to understand the level and progress of solid electrolytes in different regions, and helps enterprises and universities to seek relevant cooperation and increase investment in related research fields. At present, mass production of solid electrolytes still faces many technical difficulties. It is our hope that in the future, research on solid electrolytes can overcome the technical bottlenecks, realize the industrialization of all-solid lithium batteries, and contribute to the clean and safe use of energy around the world.

Exploiting decoupled water is a novel strategy for hydrogen production that can detach the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) with the presence of electron coupled proton buffer medias (ECPBs). During this process, water electrolysis is split into two separate processes, HER and OER, that can proceed at different times and/or in different places. This decoupling strategy can effectively solve the potential hazard of the membrane being permeable to both hydrogen and oxygen, compared with traditional water electrolysis, as well as providing alternatives to storage and transportation of hydrogen. In this paper, recent developments of decoupled water electrolysis were reviewed. The principles of the electrolyser's structure and features of different ECPBs were introduced in detail. Based on the states of the ECPBs and the electrolytes concerned, the parameters of various ECPBs in an acidic electrolyte and an alkaline electrolyte, respectively, were listed. By comparing the advantages and disadvantages of present ECPBs' performance, the prospects and challenges faced by hydrogen production with decoupled water electrolysis were illustrated.

All-solid-state lithium-ion batteries have developed rapidly due to improved safety performance and energy storage capabilities. Chelated boron lithium salt is a new type of solid polymer electrolyte (SPE) with a larger anionic radius. It can disrupt the crystallinity of the matrix, such as polyethylene oxide (PEO) or thermoplastic polyurethane (TPU), so that more lithium ions can intercalate into the polymer segments of the matrix and therefore improve the dissociation of lithium-ion in the electrolyte. This study used chelated boron lithium salt and PEO/TPU/BaTiO₃ to form the SPE. Analysis of the morphology and structure, mechanical properties, thermodynamic properties, electrochemical properties, and battery performance showed that the lithium[(1,2-benzenediolate(2)-O,O′)(1,3-malonate-O,O′)] borate (LiBDMB) system gives the best SPE performance. At 60 ℃, the ion conductivity of the LiBDMB SPE system reached the order of 10^-4 S/cm. The discharge capacity of the assembled battery reached 142 mW·h/g at a rate of 0.2 C and performed well after 50 cycles.

Silicon/carbon (Si/C) composites were prepared from nano silicon and aniline by in-situ polymerization and carbonization. After further H₂O steam activation, the Si/AC composites were obtained. The microstructure and electrochemical performances of the as-prepared Si/AC composites were characterized by X-ray diffraction, N₂ isotherm adsorption, transmission electron microscopy, and galvanostatic chargedischarge measurement. The results show that when the activation temperature was 750°C and the activation time was 20 min, the obtained Si/AC composites showed a better high-rate and cycling performance than the Si/C composites. The specific capacities of the Si/AC 20 composites are 2860, 2482, 2212, and 1933 mA·h/g at the current densities of 100, 200, 300 and 500 mA/g. Moreover, the specific capacity is still as high as 1505 mA·h/g at the current density of 1000 mA/g. When the current density returns to 100 mA/g, the discharge specific capacity recovers to 2554 mA·h/g, with a capacity retention ratio of 89.3%.

A graphene-coated lithium rich manganese-based composite (G-LNMO) was synthesized using spray drying and was investigated as cathode material for lithium-ion batteries. The effect of graphene coating on the electrochemical performance was investigated systematically. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images revealed that in G-LNMO, graphene nanosheets are uniformly dispersed and particles of Li_1.22Mn_0.52Ni_0.26O₂ are coated with the graphene nanosheets. Charge-discharge curves and electrochemical impedance spectroscopy (EIS) indicate that the structure of particles coated by graphene nanosheets can enhance the electron migration and alleviate the polarization of a pristine sample, leading to improved cycling stability and a high-rate capability. At 0.1 C and 0.5 C, the pristine and graphene-coated samples delivered capacities of 199.8 and 220.2 mA·h/g, and after 100 cycles, retained a capacity of 71% and 88%, respectively. This simple and scalable approach can be applied to the industrial production of graphene coated and lithium rich manganese-based oxides.

LIBs have been widely used in consumer electronics, electric vehicles, and large-scale energy storage grids. The next generation of LIBs, with silicon/carbon as the anode, will exhibit higher energy density. Nevertheless, the large volume expansion, the particle pulverization, the separation of the active material and the current collector, and the unstable SEI of the silicon anode hinder its large-scale application. Here, carbon nanotubes (CNTs) with high conductivity and an easily formed conductive network are used as the conductive agent, significantly improving the cycle life of the battery (the capacity retention after 500 cycles is 92%). Through the differential electrochemical curves, EIS, DCIR, SEM and XPS, it is found that CNTs can limit the capacity degradation of an SiO anode in the initial several cycles by improving the conductivity of the battery and the stability of the interface. It can be predicted that CNTs will also play a positive role in improving the performance of other electrode materials in LIBs.

Silicon/hard carbon composites have been fabricated with the four carbon precursors of sucrose, soluble starch, chitosan, and wheat starch in combination with recycled industrial silicon waste through a water-phase coating plus low temperature carbonization two-step approach. The content of the carbon within the silicon/hard carbon composites of the four precursors and the further influence on cycling performance of the anode have been studied. The results show that chitosan is an outstanding representative for fabrication of silicon/hard carbon composites with a higher carbon content, especially as (c2@Si), after secondary recombination, is more favorable for the cycling stability of the anode. Furthermore, by adding graphite during the fabrication process, the as-prepared silicon/hard carbon/graphite anode (SCG0.2) is able to achieve the specific capacity of 617 mA·h/g after 100 cycles, with the coulombic efficiency being 99.24%. Even after 300 cycles, 637 mA·h/g of specific capacity can be maintained under the high retention of 103.2%; this indicates that the volume expansion issue of Si has been optimized with better cycling stability and the utilization value of silicon waste in energy storage has been realized.

With the rapid development of electric vehicles, the requirements for power batteries now includes fast-charging and high energy densities. The traditional graphite anode material cannot meet these needs, especially for fast-charging applications. A negative electrode made of tin has a high specific capacity and fast rate performance, but its battery cycle life is limited due to volume expansion/shrinkage and interface stability problems during cycling. In this article, the synthesis of a graphite/nano-Sn composite material is presented, using chemical deposition and high-temperature sintering using SnCl₂ and natural graphite as precursors. With surface deposition of nano-sized Sn particles, the graphite prevents aggregation and pulverization of the tin particles during charging and discharging. The graphite/nano-Sn composite anode material significantly improves the rate performance and maintains excellent cycle stability. The anode material demonstrates a stable capacity of 360 mA·h/g after 150 cycles at a current density of 90 mA/g, and a stable capacity of 180 mA·h/g at a current density of 900 mA/g. In contrast, the traditional graphite anode material is only capable of capacities less than 20 mA·h/g at the same current densities.

As an important way to improve the energy density of lithium-ion batteries is to utilize concentrated electrolytes. Concentrated electrolytes are able to regulate the structure, chemical composition, and stability of the interface between electrolytes and electrodes in lithium-ion batteries because of their differences from low-concentration electrolytes. The features of concentrated electrolytes have a potential broad application in improving the comprehensive electrochemical performance of batteries including the safety, cycle stability, and discharge rate. The special advantages of concentrated electrolytes compared with low-concentration electrolytes in recent years were reviewed in this article. It was found that concentrated electrolytes had the ability to widen the electrochemical stability window of electrolytes, inhibit the corrosion of aluminum current collectors, prevent graphite stripping, and other markers of improved compatibility between electrolytes and electrodes. We focused on the mechanism analysis of concentrated electrolytes on the electrode/electrolyte interface with increased lithium salt concentration, including the occupation of lithium salt anions in the electric double layer and solvated structures formed from lithium salt anions, solvents, and lithium ions. The structural and chemical composition of the interface film was also analyzed, along with its ability to form a thin and dense interface film. The application of new characterization techniques to the special solvation structure generated in the film formation process of concentrated electrolytes at the electrode/electrolyte interface were introduced, and the future development and research direction of concentrated electrolytes were also discussed.

In this work, gas chromatography (GC) and rechargeable symmetrical lithium-ion batteries were used to explore the causes of H₂ generation in NCM lithium-ion batteries (LIBs). In addition to determining whether H₂ is produced by the reduction of trace water in the battery, this paper explores whether the H₂ generation mechanisms for NCM LIBs are best described as arising from the proton electrolyte oxide (R-H⁺) or carbonate dissociation into H^?. Considering that R-H⁺ deposited on the negative electrode is the related product between the positive and negative electrodes, the graphite/graphite negative symmetrical cell (NSC), the NCM/NCM (LiNi_0.6Co_0.2Mn_0.2O₂ is defined as NCM) positive symmetrical cell (PSC), and graphite/NCM pouch cell (PC) with charge and discharge capacity were prepared. After a room temperature cycling test and an overcharge test, the GC results indicated that H₂ was produced in the soft package full cell and the negative symmetrical battery, but not in the positive symmetrical battery. This result supports the R-H⁺ mechanism, where H2 is produced by the reduction of the R-H⁺ deposited on the negative electrode, so in the positive symmetrical cell, no H₂ is produced. In order to eliminate any interfering signal of H₂ produced by the reduction of trace water in the battery, the positive symmetrical battery without H₂ generation after cycles was selected. After adding trace water to the system, H₂ was indeed detected in the GC results. Therefore, H₂ production from trace moisture in the original cells can be ignored. Finally, the mechanism of H₂ production by the dissociation of carbonate to H^? was tested using the positive symmetric cell. The influence of R-H⁺ and water on the final H₂ production can be ignored according to the previous experimental results. After a high temperature storage and a high temperature overcharge test, no H₂ was detected in the cathode symmetry battery after cycling, so the mechanism of H₂ production by dissociation of carbonate to H^? is not the active mechanism.

In the manufacturing of lithium-ion batteries (LIBs), a common phenomenon occurs where wrinkles containing residual air bubbles are formed between the separator and electrodes, usually observed on local areas of the separator when the liquid electrolyte is added. Wrinkles on the separator and defects at the interface between the separator and electrodes will result in a non-uniform internal resistance distribution in the batteries. In the low internal resistance area, over-discharge or overcharge may occur during cycling, which might affect the cycle performance of the batteries. In this manuscript, various kinds of separators were investigated to address this issue. When dimethyl carbonate (DMC) is used to wet the separator surface, wrinkles form on all types of separators, and the wrinkle spacing increases with the thickness of the separator. The wetting frontier of the separator by DMC was investigated, and the resulting separator wrinkle can be attributed to two factors. When DMC is flowing on and wetting the separator, capillary action leads to the uplift of the separator, and a small gap can form between the separator and electrode. The air driven out of micropores in the separator and electrode after wetting with DMC aggregates at the interface to form air bubbles, resulting in local deformation and wrinkles. To address the wrinkle problem in LIB manufacturing, adhering polyvinylidene difluoride (PVDF) coated composite separators to electrodes under a hot press can counteract the capillary action and mitigate the formation of wrinkles. It was found that when the peeling strength is smaller than 10 mN/cm, the adhesion force cannot counteract the capillary action. Although local wrinkles may still be present, the number of wrinkles was significantly decreased. When the peeling strength is larger than 15 mN/cm, separator wrinkles are completely eliminated, suggesting that an increase in the adhesion force between the separator and electrodes reduces the defects at the interface. Reducing defects such as separator wrinkles is a valuable strategy to promote the consistency and cycle stability of LIBs.

A Ti_1.06Pr_0.04Fe_0.6Ni_0.3Zr_0.1Mn_0.2 alloy was prepared by vacuum induction and high energy ball milling, and the effect of ball milling time on the phase composition, microstructure, and electrochemical properties was studied by X-ray diffraction (XRD), scanning electron microscope (SEM), and in a LAND battery test system. The studies indicated that the main phase in the as-cast alloy was TiFe and the secondary phase was ZrMn₂. Ball milling causes the formation of an amorphous structure, as the lattice parameter and cell volume decreased with increased ball milling time. SEM analysis showed that the particle size becomes smaller with increased milling time. Electrochemical performance tests indicated that the activation of alloys and the electrochemical discharge capacity were significantly improved by ball milling. The discharge capacity of as-milled alloys was 52.8 mA·h/g, higher than the as-cast alloy. The maximum discharge capacity was 170.7 mA·h/g with a milling time of five hours. The P-C-T curves indicated that the plateau of hydrogenation was elevated with the milling time, and a hysteretic pressure of hydrogenation/dehydrogenation was also observed with the milling time.

Five mixed nitrate molten salts were prepared via the addition of either Additive A or Additive B into the binary formula, i.e. 47% Ca(NO₃)₂-53% KNO₃, and the thermophysical properties were measured. The results indicated that the working temperature range of the mixed molten salts was extended compared to solar salt, with the lowest freezing point at 135.2 ℃ and the highest decomposition temperature at 639.1 ℃. The optimum formula containing 34% Additive B demonstrates a thermal energy density and a thermal conductivity as high as 734 kJ/kg and 0.74 W/(m·K), respectively. A comprehensive analysis of the various thermophysical properties of mixed molten salts indicate that compared to traditional solar salt and Hitec salt, the developed salts demonstrate superior performance and are good candidates for application as a heat transfer and storage medium in concentrating solar power technologies.

The organic phase change materials show good potential for development in the future; they are widely used in building energy saving, low-temperature storage of solar energy, waste heat recovery, intelligent textiles, constant temperature protection of electronic devices, and battery thermal management. Myristic acids (MAs) were used as the main phase change materials, and activated carbons (ACs) of different sizes were used for the framework. All the AC/MA composites as form-stable phase change materials were prepared by the melt-blending method. The physical properties and performance of the composites as form-stable phase change materials were characterized by a tablet pressing machine, infrared thermal imagery, a thermal conductivity instrument, and a resistivity meter. The optimum mass fractions of AC with 200, 300, 325 and 400-mesh in the MA were 47%, 42%, 38% and 35%, respectively. The mass fractions decreased with the decrease in particle size of the AC. The density of the composites as form-stable phase change materials increased with the increasing CA mass fraction and the molding pressure, while the leakage rate decreased with the increase of the pressure and the CA mass. The temperature field distributions of the composites, as form-stable phase change materials, were more evenly distributed, and the heat storage and release time were shorter than that of the pure MA. The thermal conductivity of the composite materials was increased by 1.91～4.11, 2.05～3.93, 1.71～3.93, and 1.97～4.11 times that of the pure MA. The resistivity of the composite material would decrease with the increase of pressure and the graphite adding mass fraction. After adding 10% graphite, the resistivity would decrease by 1～2 orders of magnitude. The resistivity of the mixed material fluctuates less in a liquid state. The fitting analysis shows that there is an exponential attenuation trend between the resistivity and the pressure.

To study the effect of expanded graphite on the melting and solidification properties of paraffin wax, the solidification and melting process of expanded graphite paraffin wax composite phase change heat storage material was numerically simulated and compared with that of pure paraffin wax phase change heat storage material. The effects of a different content of expanded graphite and a different wall temperature on the melting and solidification process of paraffin wax were analyzed. The results show that the addition of expanded graphite to paraffin wax can obviously shorten the solidification and melting time of the paraffin wax, and the melting and solidification time decreases with the increase of the expanded graphite content. Under the same working conditions, compared with pure paraffin, the melting times of composite paraffin with 1%, 2%, and 5% expanded graphite were reduced by 2.14, 2.81 and 9.74 times, respectively, and the solidification time was reduced by 0.77, 1.05, and 3.76 times, respectively. The wall temperature has a significant effect on the melting process of composite paraffin but not on the solidification process. When the initial temperature is the same, compared with the melting time of 5% EG composite paraffin wax under the wall temperature of 327° K, when the wall temperature is 332° K and 337° K, the melting time of 5% EG composite paraffin wax is shortened by 0.83 and 1.58 times, respectively.

The charge storage capacity of supercapacitors is affected by many factors. A study of the voltage holding ability of commercial large-capacitance power supercapacitors is systematically studied from five aspects: charging current, charging voltage, constant voltage time, storage temperature, and the electrolyte system. The results indicate that lower charging current, lower charging voltage, lower ambient temperature, and longer constant voltage time are conducive to charge storage and improved monomer voltage retention capability. When the electrolyte salt is fixed, specifically tetraethylammonium tetrafluoroborate (TEA-BF₄), the solvent with the best voltage retention ability was propylene carbonate (PC). With a fixed solvent (PC) and concentration of electrolyte, the TEA-BF₄ salt showed better voltage retention than spiro-(1,1′)-bipyrrolidinium tetrafluoroborate (SBP-BF₄).

Elevated temperature is the most direct trigger of thermal runaway in lithium-ion batteries, so it is crucial to study the thermal runaway characteristics and mechanism of lithium-ion batteries at elevated temperatures. This paper presents the study of 109 A·h large-scale lithium iron phosphate power batteries, and an oven thermal runaway model at six different temperatures (140 ℃, 145 ℃, 150 ℃, 155 ℃, 160 ℃, 165 ℃) is presented via COMSOL Multiphysics software to simulate the thermal runaway characteristics and temperature distribution of the battery under high temperatures. The studies showed that the battery does not trigger thermal runaway at temperatures of 140 ℃ and 145 ℃, but does trigger thermal runaway at higher temperatures. The higher the oven temperature, the earlier the thermal runaway occurs, and the rate of temperature rise is also accelerated. Through the analysis of the decomposition concentration of each side reaction in the thermal runaway, it is observed that only the decomposition of the solid electrolyte interphase layer and anode occurred in the non-thermal runaway cases. The reaction between the cathode and the electrolyte is the main cause of thermal runaway. Finally, by comparing the thermal runaway cases with the non-thermal runaway cases, it was found that the temperature distribution of the battery is uniform in the case of a non-thermal runaway, while the temperature uniformity is poor in the case of a thermal runaway. At higher temperatures, the thermal runaway of the battery is more severe, where the temperature distribution is extremely uneven and changes rapidly before and after thermal runaway. In such thermal runaways, it is predicted that the electrode materials have undergone irreversible decomposition leading to battery damage.

A hexagonal cell module was designed based on the 21700 capacity NCM811 lithium-ion cell, and a cylindrical structure of graphite paraffin composite phase change material was used to cover the battery. The thermal characteristics of the module was investigated using numerical simulations by varying the discharge rate and distance between two adjacent batteries. The results show that the distance between adjacent batteries has a greater influence on the battery temperature for a high rate discharge process compared to a low rate discharge process. At a fixed discharge rate, the temperature rise of the small distance battery modules is higher than that of the middle distance and large distance battery modules. The change of battery temperature lags behind the thermal flow in time. By monitoring the thermal flow, the failure of battery thermal management can be predicted in advance to improve the safety of batteries.

It is of great significance to study the thermal runaway of lithium-ion power batteries to create an early warning system and to optimize the design of batteries to prevent thermal runaway. In this work, we investigated the thermal runaway characteristics of different anode materials at different states of charge (SOC) of the batteries using the positive active material NCM811. The heat generated by thermal runaway was calculated using the equivalent temperature of a steel nail, and was then converted into the heat released per unit capacity with varied SOC. Heat transfer in the form of flame and ejected high-temperature solids were also analyzed. The studies indicated that at the same SOC, cells with a SiO_x/graphite anode exhibited more severe thermal runaway than cells with graphite only anodes. When the SOC was 25%, the cells with SiO_x/graphite anode still experienced severe thermal runaway during the penetration test, while the cells with a graphite only anode showed relatively mild thermal changes. When the batteries with a SiO_x/graphite anode were at 100% SOC, 50% SOC, and 25% SOC, the region surrounding the battery was heated to a dangerously high temperature that could endanger the thermal propagation of nearby batteries. The weight loss of the battery in the nail penetration test increased with a higher SOC, and the weight loss ratio of the 100% SOC SiO_x graphite anode battery was the highest, reaching 75.2%.

Magnetron sputtering technology was used to deposit SiO₂ and AlF₃ ceramic particle layers with a thickness of 200 nm on both sides of a polypropylene (PP) separator. The preparation of a SiO₂/PP/AlF₃ functional separator prevents the pitfalls of increased thickness and decreased porosity caused by traditional coating methods. In general, SiO₂ and AlF₃ ceramic particles have excellent mechanical properties and chemical stability, properties which can improve the heat resistance of a polyolefin (PP) separator. In addition, the nonpolar SiO₂ and AlF₃ can synergistically improve the affinity of the separator to the electrolyte, thereby promoting the conductivity of lithium ions and reducing the internal resistance of the battery. In addition, the strong Lewis acidity and low surface energy of AlF₃ effectively suppresses the decomposition of the electrolyte and the growth of lithium dendrites, improving the electrochemical performance and safety of lithium-ion batteries. The modified separator was characterized using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), differential scanning calorimetry (DSC), and X-ray photoelectron spectroscopy (XPS). The porosity, electrolyte affinity, and electrochemical performance were also measured. The results show that magnetron sputtering has little impact on the thickness and porosity of the modified separator. The lithium-ion battery with a SiO₂/PP/AlF₃ separator has an initial discharge specific capacity of 164.98 mA·h/g at 0.2 C, a specific capacity of 154.87 mA·h/g after 50 cycles, and a cycle decay rate of 0.12%. The battery prepared with a SiO₂/PP/AlF₃ separator has a discharge specific capacity of 102.07 mA·h/g at a high current density of 5.0 C. Overall, the cycle and rate performance were significantly better than batteries with a traditional PP separator. The application of a SiO₂/PP/AlF₃ functional separator has the potential to inform the development of high-performance lithium-ion batteries.

The estimation of the battery state of charge (SOC) is a core feature of the on-board battery management system (BMS). Its accurate estimation can prolong the service life of a battery and ensure the normal driving of a vehicle. Using lithium-ion batteries as the model, this paper proposes a battery SOC estimation method based on the combination of the adaptive unscented Kalman filter (AUKF) and the BP neural network. This method improves the estimation accuracy of UKF through adaptive sampling and uses the SOC output value of the trained BP neural network for the observation of UKF. Based on the battery test data under mixed working conditions and the FUDS working conditions collected by the Arbin battery test platform at varied temperatures (0 ℃, 25 ℃, and 40 ℃), the accuracy of the AUKF-BP algorithm versus the BP algorithm were evaluated. The results indicate that the average mean error of the AUKF-BP algorithm at different temperatures was 0.82%, and the average mean error of the BP algorithm was 1.63%. Overall, an SOC estimation method based on the AUKF-BP algorithm is the most accurate.

Based on the Thevenin equivalent circuit model, this paper proposes a joint estimation SOC algorithm combining the forgetting factor least squares (FFRLS) and adaptive extended Kalman filter (AEKF) methods. FFRLS identifies and provides the model parameters for the SOC estimation. AEKF estimates the SOC online and provides an accurate open circuit voltage for the model parameter identification. The Beijing bus dynamic stress test (BBDST) was used to simulate and compare with the FFRLS online identification and the SOC estimation based on ampere-hour integration. The algorithm realizes the fast tracking of terminal voltage, and the accuracy is improved by 85% compared with FFRLS. The SOC estimation results can be rapidly converged with an accuracy up to 1.5%～2%. The results show that the algorithm in this paper can modify the model system in a closed-loop manner, thus achieving higher accuracy and better adaptability.

Thermal parameter estimation, of fundamental importance for thermal design and management of lithium-ion batteries (LIBs), is challenging as LIBs possess complicated structure with remarkable heterogeneities. The experimental setup to estimate these thermal parameters have many conditions at play, the influence of which on the precision of the estimation methods is yet largely unclear. Employing a non-dimensional thermal model of LIB, we analyze how these experimental conditions, including the radius and applied power of the circular planar heater, the placement of thermocouples, the heating duration, and the thickness of the LIB, dictate the precision of a thermal parameter estimation method we developed recently. Generally, a higher power of the heater leads to higher precision, but there is an upper bound imposed by the safety concerns. we identify the lateral limit for the placement of thermocouples, which is extended when the heater's power is higher. The allowable range of heating duration is reduced at greater ratio of the parallel to perpendicular thermal conductivity. In addition, the thermal parameter estimation method following the non-dimensional approach enjoys a much wider applicability for thermal conductivities of the LIBs with lower thickness-width ratio.

To solve the problem of the difficulty in balancing the accuracy and efficiency in the process of capacity estimation for an LIB, this paper proposes a remaining capacity estimation method for an LIB based on feature engineering and a radial basis neural network. First, the features associated with the remaining available capacity from the data during battery charging is extracted; then, the local anomaly factor algorithm is used to clean the abnormal points accurately in the features that increase the amount of effective information contained in the feature quantity; next, the dimensionality reduction process of the feature vector group is performed by the local linear embedding dimensionality reduction algorithm to reduce the computation complexity; and finally, a radial basis function neural network is introduced to establish an estimation model for the remaining capacity. The model is verified on different batteries; the results show that the model has strong robustness, the maximum average absolute error does not exceed 0.06, the maximum root mean square error is 0.05, and, when compared with the Elman neural network and the BP neural network algorithm, it has faster estimation efficiency while ensuring high accuracy.

Various equalization models can be used to balance a battery pack, but their equalization performance is different. In previous reports, a qualitative analysis was the primary method used to evaluate balancing technologies. To clarify the advantages and disadvantages of various models and to find a better equalization model, this paper presents a quantitative evaluation system for a battery pack equalization model. The equalization structure cost, equalization time, available state of charge (SOC), and average thermal power are used as evaluation indexes. The equalization model is composed of an equalization structure and an equalization strategy, and the battery pack model consists of 96 lithium-ion batteries in series, with an initial SOC of the battery pack set to conform to a normal distribution. The value of the evaluation index is obtained through simulation and is normalized. The advantages and disadvantages of different models are compared using a radar chart and the comprehensive performance value is calculated to compare the performance of different models. Four typical flying equalization models are used as examples, and analysis with the quantitative evaluation system indicate that the four equalization models are able to effectively equalize the battery pack. The balance time of the flying inductance model was the shortest, the balancing structure cost of the flying resistance model was the lowest, the comprehensive performance of the flying winding model was the worst, and the comprehensive performance of the flying capacitor model was the best. In summary, the quantitative evaluation system can quickly and effectively evaluate the multi-dimensional and comprehensive performance of multiple battery pack equalization models.

A phase change latent heat storage system (LHTES) based on phase change materials (PCMs) can reversibly absorb and release a large amount of latent heat over a small temperature range. These systems have a potential application in solving the mismatch of low-grade thermal energy in time and space. The thermophysical properties of PCMs can determine the heat storage density, heat storage/release efficiency, operating cost, and lifetime of the heat storage system. It is therefore important to quickly, accurately, and easily measure the thermophysical parameters of PCMs that may meet the needs of engineering applications. This article reviews the principles, experimental devices, and mathematical models of the T-history method. In light of the over-simplification of the T-history method and the difficulty in defining the solid-liquid interface in the phase transition process, a way to improve the test accuracy of the T-history method is emphasized. The application of the T-history curve (T-t curve) in the determination of a phase change point, subcooling, and the phase change temperature range of PCMs is reviewed, and the application of the T-t curve to determine the specific heat capacity and phase change latent heat of PCMs is introduced. Mathematical models and applications for the specific heat capacity-temperature curve and phase change enthalpy-temperature curve are presented. A comprehensive analysis shows that the T-history method can be used to easily obtain thermophysical parameters including the subcooling degree, phase transition interval, specific heat capacity, and phase transition enthalpy of PCMs.

Adding phase change materials (PCMs) to building envelopes is an effective method to reduce the heat transfer between the outdoor and indoor environments. PCMs are materials that can change their phase by absorbing or releasing large amounts of energy, which prevents a large temperature variation. The utilization of PCMs in building envelopes realizes this heat transfer regulation between the indoor and outdoor environments. This paper reports the use of PCMs with a phase change temperature of 25 ℃ encapsulated in hollow HDPE balls with inner diameter of 24 mm and outer diameter of 25 mm. The PCM balls were evenly distributed in a concrete panel with dimensions of 880 mm×500 mm to form a PCM-concrete layer. Outdoor solar radiation was simulated using an electrical heating film. The thermal performance of walls with a PCM-concrete layer at various positions in a building envelope were measured and simulated using Ansys software. The effective thermal conductivity of the PCM-concrete layer was calculated to evaluate the thermal performance of the walls. It was found that when the solar radiation is less than 80 W/m² or greater than 200 W/m², the effective thermal conductivity is lowest when the PCM-concrete layer is adjacent to the interior surface. When the solar radiation is between these values, it is more effective to place the PCM-concrete layer adjacent to the exterior surface to allow the PCM to absorb heat and achieve better thermal performance.

This bimonthly review paper highlights 100 recent published papers on lithium batteries. We searched the Web of Science and found 2731 papers online from Oct. 1, 2020 to Nov. 30, 2020. 100 of them were selected to be highlighted. Ni-rich layered oxides and Li-rich oxide cathode materials with oxygen redox reaction drawed large attentions. Doping and coating are used to improve the performances of mateials. The swelling of cell using Si-based anode material is investigated. The methods for improving the cycling peroperties of Si-based anode include using new binders, to form artificial SEI, etc. Carbon coated Ti₂Nb₁₀O₂₉, grapheneand its commposites and 3D porous carbon anode materials are also reported. Electrolyte with additives are developped for high voltage cathode matreials including Ni-rich oxides, Li-rich oxides, and spinel Ni-Mn oxides, also for Li-S and thickelectrode cells. Sullfide based solid state elelctrolyte and its composite with polymer are studied. Enhanced conductivity is obtianed for Si doped Li₆PS₅I and B doped Li₇La₃Zr₂O₁₂.There are few papers related to solid state battery, LI-S battery and Lithium oxygen battery.Analysis work focuses on interface SEI, Lithium deposition/striping in solid state cell, Li distributuon in electode, gasing of the cell, etc. Theoretical simulations cover the interface SEI formation mechanism, kinetics of thick electrodes, etc.

Lithium-ion batteries have gradually replaced lead-acid batteries to become the mainstream batteries on the market due to their high energy density, long cycle life and good safety performance. At the same time, lithium batteries have also been widely used in various fields such as energy storage power stations, communication base stations, substations and other backup power systems and commonly used notebook computers. Replenishing the energy of lithium-ion batteries by floating charging is a common way to charge backup batteries, and long-term floating charging will cause changes in the internal structure of the battery, resulting in reduced battery cycle life and even safety issues. This article summarizes the impact of different factors on the floating charge performance and the impact of the floating charge on the lithium-ion battery from three aspects: the influence of external temperature, the difference of float voltage, and the inconsistency of battery cells. It is convenient to optimize the floating charging conditions of energy storage lithium-ion batteries, to ensure that the battery life is increased under stable operation, and to provide guidance for the research progress of energy storage lithium-ion batteries.

A 18650 NCM LIB was tested at 0.5 C, 1 C, and 2 C discharge rates at 40 ℃ and 25 ℃ in the incubator, and 0.5 C at 0 ℃ and -25 ℃, respectively. The voltage and temperature curves under different conditions were obtained. The reliability of the electrochemical thermal coupling model was verified; the accuracy of the model was the highest at 25 ℃, the voltage error was 0.07 V, and the temperature error was 0.8 ℃; the accuracy was the lowest at -25 ℃, the voltage error was 0.6 V, and the temperature error was 1.5 ℃. The model was used to analyze the electrode heat generation at 25 ℃ and simulate the temperature field during 2 C discharge at 25 ℃. At the end of discharge, the temperature at the positive and negative electrode ears of the battery was the highest, with the specific value of 34.8 ℃. The temperature of the battery surface directly opposite to the air flow was the lowest, with the value of 34 ℃. At the model boundary, 50mm away from the battery center, behind the air flow, the temperature rose by 4 ℃.

In the process of estimating the state of charge (SOC) for lithium-ion batteries, the estimation results can be significantly influenced by a reduction of available capacity and changes in the internal parameters of aging batteries. To address this problem, this paper proposes a novel method for estimating the SOC of aging lithium-ion batteries based on the migration model, by regarding the aging of batteries as an uncertain factor that affects the estimation of the SOC. First, with the second-order resistance-capacitance (RC) equivalent battery model at the original state as an initial battery model, recursive least squares (RLS) and polynomial fitting are employed to extract the relationship between parameters of the initial model and the SOC, which is then linearly migrated to obtain a state equation of the migration model. Second, migration factors of the model are updated in the actual running of batteries with the risk sensitive particle filter algorithm (RSPF). Finally, a precise estimation of the SOC is attained in combination with low-pass filters. Based on four sets of Urban Dynamometer Driving Schedule (UDDS) data at different degrees of aging, the migration model algorithm was verified and compared with the extended Kalman filter (EKF) and the adaptive extended Kalman filter (AEKF) algorithms. The experimental studies indicated that the model proposed in this paper is precise over a range battery ages and is effective, with a root-mean-square error (RMSE) of the SOC calculation less than 1.04%. The study can help to promote the application of the migration model in the estimation of the SOC of aging lithium-ion batteries, and can provide guidance and a reference for the estimation of the SOC of electric vehicle batteries throughout their lifecycles.

The state-of-charge (SOC) estimation of lithium batteries is an important part of battery management, and the accuracy of SOC estimation results can directly affect the performance of battery management systems. To address the difficult problem of improving the accuracy of the SOC estimations, a time-varying equivalent circuit model and an improved fractional extended Kalman algorithm are proposed. Varying the time in model parameters was used to accurately describe the SOC of lithium batteries over the entire cycle, and the memory characteristics of fractional derivatives were used to improve the state prediction equation. Given the estimation error usually found with the traditional fractional extended Kalman reference to historical data, the adaptive noise factor was added to improve the accuracy of the algorithm. To overcome the data redundancy caused by a fractional order algorithm and the short-term memory characteristics of lithium batteries, a fixed window with a size of 20 (M=20) was designed using the sliding window idea, and the data in the window was updated in real time with the charging and discharging of the battery. Fractional order operation was carried out with 20 data in the window, which reduces the error caused by data redundancy and improves the estimation accuracy. The feasibility and accuracy of the proposed algorithm were verified using two different working conditions. The experimental results showed that the maximum error of the fractional extended Kalman estimation was 0.02, while the maximum error of the traditional extended Kalman algorithm was 0.05. The error fluctuation of the proposed algorithm was also smaller than the traditional algorithm. Overall, the proposed estimation method has a high accuracy and anti-interference ability, which will be helpful to promote the development of lithium battery management systems and the application of more accurate state-of-charge estimation methods.

The current challenges of modeling aging lithium-ion batteries include oversaturated model parameters and time-varying parameters, which cannot be evaluated with an online parameter identification of the model using the traditional least squares method with a fixed forgetting factor. This paper proposes a least squares method with a variable forgetting factor, which continuously updates the forgetting factor to better track the run-time utilization of battery aging characteristics. Using the first-order RC equivalent circuit model of the lithium battery as a model, a test platform was established for charge and discharge experiments, and the results were compared with the traditional least squares method with a fixed forgetting factor. The experimental results indicated that the proposed method can quickly converge and dynamically track battery aging. The average absolute error of the voltage parameters at the model terminal was found to be less than 25 mV. When the proposed method was run under a dynamic stress test with the typical working conditions of an energy storage system, the corresponding parameter identification accuracy was improved by 38.33%, indicating that the proposed method is highly accurate.

To fulfill integration and application of retired Li-ion batteries in a PV and energy storage micro grid system, from the perspective of whole package using, checking appearance, nameplate, open circuit voltage (OCV), BMS communication of the 80 V-60 A·h batteries for initial filtration, then finishing second filtration through charging and discharging batteries, sorting batteries in accordance with valid capacity, grouping batteries by multichannel power conversion system (PCS), then applying in a PV and energy storage micro grid system. The results show the following: There are eight batteries that have a problem with appearance, OCV, or communication of the BMS, and 22 batteries also cannot be reused due to the capacity and the CD-OCV curve being unqualified. In the 50 remaining batteries, the maximum capacity is 49.46 A·h, and the minimum capacity is 45.58 A·h; so, the difference in value is 3.88 A·h, which is only 6.5% of the rated capacity, meaning these batteries present good uniformity. By charging and discharging under different rates, such as 0.1, 0.2, 0.3 and 0.5 C, the dispersion of the CD-OCV curves is small after grouping; when charging at a rate of 0.05 C and discharging at a rate of 0.1 C for peak cutting and valley filling, the charging energy is 111 kW·h and the discharging energy is 103 kW·h; so, the efficiency is 92.79%. This shows that the characteristics of the batteries are normal after filtrating and grouping, and the batteries have cascade utilization value.

A small off-grid wind-photovoltaic pumped hydroelectric energy storage (PV-PHES) hybrid system for remote rural areas is proposed, based on the complementary characteristics of wind and solar energy and an open well common in rural areas as a lower reservoir for a PHES system. In this paper, a novel power generation technique is used, where the output power and voltage of the system are unaffected by changes in wind speed or solar irradiation and does not require the use of batteries, inverters, controllers, or controlling circuits. A mathematical model of the proposed system is also established in this paper. The simulation and comparative analysis of the power output characteristics of the constant voltage power supply system is carried out using real data of a remote rural area in the Hunan Province over a period of 24 hours. The results indicate that the proposed system can achieve continuous and stable operation day and night, with an output power almost equal to the design value of 0.3 kW, and a nearly constant output voltage of 220 V. During the peak hours of 822 h, the average output power of the whole system was 294.5 W, the maximum value reached 300 W, and the average output voltage was 217.8 V, with a power deviation of 1.8% and a voltage deviation of 1% from the design value. Compared to the results of the combined operation of wind-PHES and PV-PHES hybrid systems, the power deviation is reduced by 28.8% and 34.8%, respectively, and the voltage deviation is reduced by 27.7% and 34.1%, respectively. The proposed system has the desirable features of simplicity, reliability, low failure rate, no pollution, and the ability to generate constant power at a constant voltage, which helps to provide low-cost and high-quality electricity to remote rural residents in areas with poor wind and solar resources.

Combining compressed air energy storage (CAES) and renewable energy is recognized as an effective way to solve the dependence of the conventional compressed air energy storage system on fossil fuels and to increase the renewable energy penetration. In this paper, by integrating the CAES with an electrical heater (CH-CAES), the amount of heat storage in the thermal storage device is greatly improved, and, simultaneously, the capacity of the expander to output mechanical power in the compressed air energy storage system is improved. To recover the high temperature residual heat of the thermal storage device in the CAES, a novel combined heat and power system (CH-CAES-AHP) coupled with the CH-CAES and absorption heat pump cycle (AHP) is proposed in this paper for achieving the cascade utilization of energy. Parametric analyses were carried out in this paper to investigate the effect of four key parameters on the integration system performance using the control variable method. Meanwhile, the exergy analysis method based on the second law of thermodynamics was applied as a beneficial supplement to the energy analysis method to improve the evaluation of the system performance. The results show that the round-trip efficiency of the CH-CAES is greatly improved, also improving the energy utilization rate by integrating with the AHP. Under the fundamental working conditions, the round-trip efficiency and exergy efficiency of the CH-CAES-AHP increased by 29.96% and 1.87%, respectively, due to the integration system output being an additional 5790.53 kW heating power compared with the CH-CAES. The parametric analysis shows that the discharging pressure and the electrical heating temperature have a great influence on the performance of the integration system, while the rectification column pressure and the rectifier reflux ratio have little influence on the performance of the integration system. The parametric analysis also shows that the round-trip efficiency of the integration system is decreased with the discharging pressure and increased with the electrical heating temperature. Moreover, reducing the rectification column pressure and the rectifier reflux ratio can lead to more net heat being released by the ammonia absorption heat pump cycle and increasing the performance coefficient of the AHP; this is conducive to the improvement of the round-trip efficiency of the integration system. Simultaneously, the exergy efficiency of the integration system is increased with the discharging pressure and decreased with the electrical heating temperature, while the rectification column pressure and the rectifier reflux ratio have little influence on the exergy efficiency of the integration system.

To address the problem of mutually restricted thermodynamic and economic performance of CAES systems, both factors were considered in this paper to optimize the system. Thermodynamic and economic models of the system were established and the influence of key node parameters on the system were revealed. A multi-objective optimization analysis with a genetic algorithm was carried out. The studies indicated that the thermodynamics and economics of the system increase with an increase in the expansion ratio and inlet temperature of the turbine. The optimized results indicated that a solution with an exergy efficiency of 55.12% and a unit energy cost of 396.60 $/kW is attainable for the system.

To determine the benefits of heat storage technologies applied to distributed energy systems with renewable energy, a distributed system in Dalian utilizing solar energy, wind energy, and gas was studied. An evaluation model was established, and the effects of heat storage on the electrical balance, thermal balance, fuel consumption, greenhouse effect, acidification, and pollution of the distributed energy system were analyzed. The economic feasibility of sensible heat storage applications in water, heat transfer oils, refractory bricks, and phase change heat storage technologies such as hydrated salts and paraffin, was determined using static and dynamic economic evaluation methods. The calculations indicate that heat storage technologies have no effect on the electricity balance of the distributed energy system, but could do the following: supply heating of 14261.14 kW·h on a typical day, reduce the heat generated with gas by 63.95%, reduce 1822.74 m³ of gas consumption, save 13.16% of primary energy, reduce the greenhouse effect caused by 372165.90 g of CO₂, decrease acidification caused by 278.30 g of SO₂, and lower pollution caused by 150.74 g of PM2.5 particulates. Heat storage technologies utilizing water, refractory brick, hydrated salts, and paraffins have high economic feasibility. Water is especially practical, with a static investment recovery period of 4.91 years and a dynamic investment recovery period of 6.57 years. The investment recovery periods of heat storage technologies using oil for heat transfer are relatively long, resulting in poor economic feasibility. In summary, this study provides a reference and a basis for the efficient application of heat storage technologies in distributed energy systems.

The thermodynamic properties of compressed air energy storage (CAES) have been reported in recent years, including the mean gas pressure and temperature in a gas reservoir and the influence on the security of the surrounding rock, but the temperature and pressure distribution in the cavity have not been reported. To study the thermodynamic properties of a CAES gas storage cavern, the effects of different shape parameters K (ratio of length and radius), and the inlet position on the internal thermodynamic properties of the cylindrical cavity, a non-isothermal conjugate heat transfer model was used to build a 3D model of a CAES gas storage cavern. The thermodynamic process of the charging stage was calculated under different values of K and with varied air inlet layouts. The results of the study indicated that: ① the temperature field in the CAES hole was heterogeneous, but the pressure field was uniformly distributed; ② a cylindrical cavity shape had a significant influence on the temperature field of the cavity, but little influence on the pressure distribution and exergy storage. In other words, the bigger the K value, the bigger the mean temperature and the maximum; ③ placing the air inlet in the center of the cavity significantly reduced the mean and maximum temperatures by 16 K and 159.61 K, respectively, but did not affect the exergy storage; ④ there was inhomogeneity in the temperature field of the CAES air storage cavern, where extremely high temperatures could form locally, which seriously threatens the safety of the lining and surrounding rock. The inhomogeneity of the temperature field can be improved by placing the air inlet in the center and designing reasonable shape parameters to avoid the thermal stress damage of surrounding rock caused by the temperature inhomogeneity.