The development of novel high-performance electrolytes is an important method to solve the problems including safety hazard and insufficient energy density facing traditional lithium ion batteries. Liquid-crystalline electrolytes maintain fluidity of liquid and the anisotropy of crystal, so that it can be mixed with lithium salt to prepare liquid crystal electrolyte, which can form nano segregation structure of columnar, smectic or bicontinuous cubic phases through self-assembly to provide an efficient ion conduction path for Li⁺ transmission, exhibiting potential applications for lithium ion batteries. Despite some progress, however, there is no systematical review on the recent progress of liquid-crystalline electrolytes so as to gain the current development situation and guide the future development. Here, this review introduces the research progress of liquid-crystalline electrolytes through the discussion of related literatures, highlights on the description of the ion transport mechanism of lithium ions in non-ionic and ionic liquid-crystalline electrolytes and summarizes the electrochemical performance of liquid-crystalline electrolytes in lithium-ion batteries. Comprehensive analysis shows that liquid-crystalline electrolytes can improve the electrochemical performance by further optimizing the structure of liquid-crystalline molecules or adding liquid plasticizers, which is expected to be applied to high-performance lithium-ion batteries. Finally, this review presents the current challenges and the possible development trend of liquid-crystalline electrolytes in the future.

Lithium-sulfur (Li-S) batteries have attracted tremendous interest in the last decade because of the low cost of S and its high theoretical energy density. Extensive efforts have been devoted to designing advanced conductive cathode networks and functional electrolytes. The performances of Li-S batteries have been significantly improved. The working mechanisms have also been well understood. However, a huge gap still exists between the achievable energy density and their theoretical energy density and the limited cycle life for Li-S batteries under practical conditions, such as high S loading, low electrolyte usage, and low N/P ratio. Based on a brief review of the recent progress of Li-S batteries, we particularly discuss herein the obstacles of Li-S batteries under practical conditions, such as low S utilization, limited cycle life, lithium dendrite, and "dead Li." This review reveals the criticality of understanding and solving the reversible and sustainable reactions for S and Li metal under thick electrode, low electrolyte usage, and restricted N/P ratios for the future development of high-energy Li-S batteries. We conclude that a comprehensive solution that combines sulfur electrode architecture, electrolyte, and Li metal protection is important in enhancing the structural stability of Li and S electrodes and conducting networking to realize a high-energy Li-S battery technology and promote its commercialization.

Optimizing the initial coulombic efficiency and cycling stability of anode materials is important to improve the specific capacity of lithium-ion batteries. Acknowledgedly, silicon-carbon composite material has been regarded as the most promising candidate for the next generation of lithium- ion batteries. In this paper, to the point of higher specific capacity, the structure design of secondary particles of silicon-based carbon anode material is detailed. Meanwhile, the up-to-date achievements and bottlenecks in secondary particles design, pre-lithiation strategies and binder explorations of silicon-based anodes are systematically reviewed, given their importance in electrochemical performance promotion and specific capacity upliftment. Based on many basic scientific issues and critical technologies for specific capacity of silicon-based anodes, the suitable design of structure, process and selection of binder are recommended to the achieve in higher power and better stability of anodes on lithium-ion batteries.

As an anode material, the Li metal battery with metallic lithium has attracted tremendous attention for its extremely high-energy density. However, the development of Li metal batteries has been hindered by the parasitic reactions between metallic lithium and organic electrolytes and the uncontrolled growth of dendritic lithium, which brings serious safety hazards to Li metal batteries. The root causes of these challenges are the unstable solid electrolyte interphase (SEI) layer formed on the metallic lithium surface and the inhomogeneous Li deposition. Liquid electrolyte is a key component of a lithium metal battery. The compatibility of the liquid electrolyte with the lithium metal anode and the property of the liquid electrolyte determine the practicality of the lithium metal battery. This review first introduces the function mechanism of the liquid electrolyte in lithium metal batteries and presents recent progress of electrolyte regulation for Li metal batteries from three aspects of additives, conductive lithium salts, and organic solvents. The film-forming additives and those manipulating the lithium deposition behavior for liquid electrolyte additives are highlighted. Three strategies for conductive lithium salts are introduced: novel lithium salts; mixed lithium salts; and regulation of the lithium salt concentration. For organic solvents, the effects of carbonate, phosphate, and ether solvents on lithium metal batteries are introduced. Electrolyte optimization proves its effectiveness in regulating the Li deposition behavior and the SEI layer composition, which is one of the most facile and effective strategies for alleviating the abovementioned issues. Finally, we prospect the future research directions in this field.

In the recent years, the number of spent lithium-ion batteries has rapidly increased with the popularity of electric vehicles. Hence, the development of a recycling technology has become an urgent matter from the viewpoint of environmental protection and resource utilization. However, the recycling technology for spent lithium-ion batteries has not been widely used because of secondary pollution and high cost, which has consequently resulted to an urgent call for a green, economical, and efficient recycling technology. The hydrometallurgical technology is the most promising technology in recycling spent lithium-ion batteries because of its low energy demand, high purity, and low cost. This paper reviews the effects of organic acids on the hydrometallurgical recovery technology on metal leaching in spent lithium-ion batteries, introduces the characteristics of malic acid, citric acid, oxalic acid, and other organic acids in the acid leaching process, compares the reaction conditions of various organic acids in the leaching process and the efficiency of metal leaching, and analyzes the leaching kinetics between organic acids and active substances during the leaching process. The comprehensive analysis shows that the factors affecting metal leaching can be optimized; the metal leaching efficiency can be improved; and the overall recovery efficiency of the hydrometallurgical recovery technology can be elevated by exploring the leaching kinetics. In the future, the leaching kinetics of organic acids is expected to become an important research direction in the hydrometallurgical recovery process.

In this study, nanoscale silicon dioxide (SiO₂) is used to modify the polyvinyl alcohol potassium borate (PVAPB) hydrogel polymer electrolyte (HPE) prepared by electrodeposition. The structure, morphology, thermal properties, and conductivity of the PVAPB HPE with or without SiO₂ are analyzed. Consequently, the results demonstrate that the two kinds of electrolytes both have a stable chemical structure. Moreover, adding SiO₂ increases the salt content of the electrolyte and promotes ion dissociation and movement. The ionic conductivity of the SiO₂-modified electrolyte reaches 1.59 mS/cm. The supercapacitor (SC) assembled by the PVAPB HPE modified with SiO₂ has 75.6 F/g capacitance at 1.0 A/g current density and 33.3 s time constant. Compared with the unmodified SC, the capacitance of the SC with SiO₂ increases by 13.5%, and the time constant shortens by half. On the contrary, the SC modified with SiO₂ exhibits an excellent performance rate.

Graphene is a material with excellent properties, but its applications (e.g., energy storage) are limited because of its few surface-active sites. Nitrogen atom doping is an effective method for improving graphene properties. Nitrogen doping methods can generally be divided into two categories: 1) direct-doping, which uses small molecules or gases as nitrogen and carbon sources to perform an in-situ synthesis of nitrogen-doped graphene (e.g., chemical vapor deposition, solvothermal, and arc discharge methods); and 2) post-synthesis, which uses graphene or graphene oxide as raw materials to achieve nitrogen doping (e.g., thermal method, chemical treatment, and plasma method). Nitrogen-doped graphene shows different physical and chemical properties because of the various structural types of nitrogen atom entering the graphene lattice. As the electrode material of supercapacitors, which are an important application of nitrogen-doped graphene, no unified scientific conclusion has yet been provided as regards the mechanism of nitrogen doping to improve the capacitive performance. This study briefly introduces the characteristics of various graphene nitrogen doping methods and reviews the research progress of nitrogen atom doping control methods with different configurations. The effects of the reaction temperature, precursor structure, reaction energy, and nitrogen doping amount on the formation of the pyrrole-, pyridine-, and graphite-type doped nitrogen with different configurations are reviewed. Some main viewpoints on the influence mechanism of the pyrrole-, pyridine-, and graphite-type doped nitrogen on the capacitance characteristics of graphene are summarized. Finally, the future research direction of nitrogen-doped graphene is previewed.

Redox flow batteries (RFBs) are promising large-scale energy storage technologies. The commercialization of main RFBs is slow due to their high cost. Large-scale energy storage using RFBs consumes a large amount of electrolytes consisting of metals of different valences, ionic compounds, solvents, and additives. Elemental iron and iron compounds are the ideal active materials of positive and negative electrodes due to their abundant reserves and being environment friendly, and they have also been widely investigated. The research progress of iron-based RFBs in the recent years is briefly reviewed in this study. The iron-based RFBs are divided into hybrid iron-based RFBs and all-liquid iron-based RFBs based on the different active material states. The hybrid iron-based RFBs in the acid and alkaline condition are discussed. The factors influencing the RFB performance, such as hydrogen evolution, solubility, conductivity, and kinetics, are briefly described. The influence of complexing agents on solubility and kinetics is discussed. Different kinds of additives that inhibit the hydrogen evolution are introduced to improve the charging efficiency. The inhibition of the hydrogen evolution and the stability of the electrode capacity should be further improved, and new electrode structure and innovative aqueous RFB systems should be investigated.

Zinc-nickel single flow battery has become one of the hot technologies for electrochemical energy storage due to its advantages of safety, stability, low cost and high energy density. The working principle of zinc-nickel single flow battery is introduced. From the perspective of basic research, the main problems, influencing factors and solutions of the battery are summarized and analyzed: The morphology of zinc deposition is related to many factors such as electrolyte system, working current density and negative electrode substrate, so it can be controlled by optimizing the above conditions; The accumulation of zinc due to the imbalance of the charge consumed by the side reaction can reduce by suppressing the side reaction of the positive electrode, enhancing the side reaction of the negative electrode and preparing the composite positive electrode material; The polarization phenomenon is related to the current density, which can be reduced by optimizing the electrolyte flow field structure and using porous electrode materials; In addition, the development of new electrode materials can reduce battery costs and increase the area capacity of positive and negative electrodes. From the perspective of application research, this paper briefly analyzes the mathematical modeling of the battery and the current main engineering applications: By constructing different types of battery models, the influence of different factors on the battery can be explored; in practical applications, zinc-nickel single flow batteries have experienced three generations of large-scale products. Finally, some prospects for developing new battery structures, establishing accurate physical models and combining batteries with bionics are proposed.

The electrochemical reduction of carbon dioxide can deal with the mismatch between the demand and the supply of renewable energy, such as solar energy, converting excess electrical energy into chemical storage with a high added value while reducing carbon dioxide emissions, and alleviating environmental pressure. H-type electrolytic cells have mostly been adopted in early research, and this is far from the industrial application of the carbon dioxide electrochemical reduction technology. H-cells are susceptible to mass transfer limitations and often have low current density. This study no longer focuses on the research and development of cathode catalysts, but on studies using a continuous carbon dioxide electrochemical reduction reactor. We introduce herein several types of reactor structure commonly used in the current research and discuss the components, operating conditions, possible optimization methods (e.g., novel gas diffusion electrode structure), failure mechanism of the electrolyzer, and possible repair methods. A direct comparison of the performances of the cathode catalyst would be impossible due to the difference in structure, composition, and operating parameters of the electrolyzers used in each study; therefore, the reference electrode should be a necessary part of a CO₂ electrolyzer. The possible optimization methods of the carbon dioxide electrolyzer are as follows: ① preparation technology of the catalyst layer slurry and binder selection; ② preparation technology of the gas diffusion electrode and substrate selection; ③ development of a highly efficient and long-term stable polymer electrolyte membrane; and ④ optimization of the operating parameters.

The nickel-rich ternary material LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) is one of the cathode material candidates for new-generation high-energy density lithium-ion batteries due to its advantages of high specific capacity, low cost, and high safety. However, the inter-granular fracture cannot be avoided in polycrystalline NCM811 materials due to the contraction and expansion of the lattice volume during the charge and discharge processes, which causes an unsatisfied cycling life of the materials. Compared to polycrystalline materials, single-crystal materials have better mechanical property and thermal and cycle stabilities. In this work, the LiNO₃-LiOH mixed flux with a low-melting point is applied to prepare the single-crystal LiNi_0.8Co0.1Mn_0.1O₂ (NCM811) material. The influence of the synthesis conditions on the structure, morphology, and electrochemical performances of the final products (e.g., flux dosage) and the sintering temperature are systematically investigated through X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and electrochemical measurements. The results show that the molar ratio of flux in the mixture of the precursors and the flux is optimized to be 90 mol%, and the optimized sintering temperature is 800 °C. The as-prepared NCM811 material measuring 1-2 μm exhibits an excellent electrochemical performance. Furthermore, the Mg-doped single-crystal NCM 811 material achieves a large discharge specific capacity of 165.4 mA·h/g and a capacity retention of 97.7% after 100 cycles at 1 °C. For comparison, the discharge specific capacity of the polycrystalline NCM811 material synthesized from the commodity precursors is only 132.9 mA·h/g, and the capacity retention is 75.0% after 100 cycles at 1 °C. In conclusion, the electrochemical performance and the cycling capability of the single-crystal NCM811 material are superior to those of the polycrystalline NCM811 material.

In this study, the morphology characterization of carbon-coated Al foil and bare Al foil is performed to investigate their effect on the properties of lithium iron phosphate batteries. Scanning electron microscope images show that the surface of carbon black-coated Al foil is partially covered by carbon black particle aggregations, while that of graphene-coated Al foil is uniformly covered by graphene nanosheets. The Al foils are further assembled into lithium iron phosphate batteries to evaluate their effect on the electrochemical performances of lithium-ion batteries. Consequently, the electrochemical impedance spectroscopy curves of the batteries using the carbon-coated and bare Al foils are found to be totally different. The carbon-coated Al foil can reduce the internal resistance of the batteries, while the batteries made by the bare Al foil show an extra curve, which stands for the capacitive reactance attributed to the electrical double-layer capacitance between the Al foil and the active materials. The storage test results indicate that the carbon-coated Al foil can reduce the self-discharge capacity of batteries, leading to a better storage property. The rate charge and discharge tests show that the constant current charge ratio and the discharge median voltage of the batteries using the carbon-coated Al foil at a high-power condition are higher, indicating that the polarization of the batteries is smaller, and the power performance is batter. The batteries with the carbon-coated Al foil have advantages of low-temperature discharge capacity, initial voltage, and better cycling performance at the 1 and 3 C rate. Meanwhile, different kinds of carbon layer show different improvements. The graphene-coated Al foil show a better promotion on the battery performance under high-power conditions due to the improvement of the electron and heat conductivities by graphene nanosheets parallel to the Al foil surface.

As a new type of energy storage device, double-layer capacitors are gradually replacing traditional batteries because of their advantages of high-power density, long service life, cleanliness, and environmental protection. They also have a wide application prospect. However, their low energy density hinders application. The energy density can be increased by increasing their capacitance. Therefore, a molecular dynamics simulation method is used herein to study the influence of graphene surface spacing (slit pore width) and carbon nanotube diameter (cylinder pole diameter) on the area specific capacitance to indirectly reflect the influence of graphene surface spacing and carbon nanotube diameter on the energy density. An analysis of the K⁺ and H₂O distribution shows that when K⁺ is distributed in a single layer (surface spacing: less than 0.5 nm), the capacitance increases with the decrease of the surface spacing. When K⁺ is distributed in a double layer (surface spacing: between 0.5 nm and 0.803 nm), the result implies the opposite. In a circular hole, the capacitance oscillates with the diameter, and the area is much larger than that of the slit hole because of the curvature.

The thermochemical adsorption heat storage has a higher heat storage density, a lower heat loss, and a long-term heat storage, which exhibit significant advantages over both sensible heat and phase change heat storages. This paper summarizes the developments in inorganic salt-H₂O systems, called hydrated salts, for the thermochemical adsorption heat storage, including typical thermal chemisorption materials and chemisorption reactors. Hydrated salts for the thermochemical adsorption heat storage usually contain halide-H₂O and sulphate-H₂O. Although hydrated salts have high heat storage densities, their application in the thermochemical adsorption system still faces some problems, including the degradation of kinetic properties and reactor corrosion caused by the deliquescence and expansion agglomeration of hydrated salts. As a carrier for hydrated salts, a porous material is expected to solve the current technical problems. Moreover, the development of new porous carriers and the structural optimization of the thermochemical adsorption heat storage systems are expected as the keys to promoting the industrialization of the thermochemical adsorption heat storage technology.

Lignin is a kind of abundant, inexpensive, and renewable biological resource, which has been widely used in the industrial field. Lignin has made some progress, but its application in energy storage is very limited. The research progress of lignin and its derivatives in rechargeable batteries (i.e., lead acid and lithium-ion batteries), fuel and cells, and supercapacitors is reviewed herein to broaden its application scope in energy storage materials. Among them, the basic properties and characteristics of lignin from different sources are briefly introduced, and based on the design flexibility and diversity of lignin and its derivatives in molecular structure, additives, binders, catalysts, battery or supercapacitor electrode materials are prepared by directly selecting lignin or modifying or doping one or more heteroatoms. Moreover, the operation mechanism and electrochemical performance of different lignin-based energy storage devices are discussed and analyzed. The results show that lignin application in the energy storage device not only improves the cycle stability and extends the service life, but also reduces production costs and chemical pollution. The challenges and the possible development directions of lignin-based energy storage materials in the future are prospected herein to further improve the energy storage and output efficiency of lignin-based energy storage devices.

The factors affecting the process of phase change material (PCM) storage and heat release must be understood to improve the PCM storage and heat release efficiency. This study presents a plate heat exchanger heat unit model based on the solidification/melting model of FLUENT software, south of Switzerland group independent research and development of sodium acetate composite PCM on the exothermic process of solidification simulation, analyzed the different heat transfer structure, fluid velocity, and thickness of PCM for heat transfer process, the influence of PCM and heat unit on a flat plate heat exchanger structure optimization, the innovative design of microchannel plate heat exchanger heat units. The results show that the heat exchanger structure has an important effect on the heat transfer performance. The heat transfer velocity is improved with the fluid flow rate increase; however, the velocity and temperature fields are not uniform, and the heat transfer performance is limited. The total solidification time of the PCM increases with its thickness. The simulation results are used to prepare a phase change prototype, build a storage/heat release system, and verify the simulation results by experimental data comparison.

As a heat storage medium, nitrate molten salt has the advantage of high use temperature, good heat transfer performance, and large specific heat capacity. The development of a new low-melting point mixed molten salt heat storage material can solve the problems of low energy storage efficiency and pipeline freezing and blocking in binary solar salt. This study accurately predicts that the composition of the ternary eutectic point and the experimental results are basically consistent with the theoretical results based on the CALPHAD method and the results from PANDAT for the phase diagram of the NaNO₃-NaNO₂-KNO₂ ternary system. We add LiNO₃ on the basis of the ternary eutectic point composition ratio and prepare NaNO₃-KNO₂-NaNO₂-LiNO₃ quaternary molten salt. The quaternary low-melting point molten salt composition ratio is obtained through experimental optimization screening, resulting to 21.57% NaNO₃, 41.27% KNO₂, 17.16% NaNO₂, and 20% LiNO₃. The melting point of the low-melting molten salt is 84.2 °C, as obtained from the DSC test. The upper limit of temperature usage is 583.3 °C by the TG test. The step cooling curve test shows a solid-liquid phase transition temperature of 78.9 °C. The results show that the low-melting point molten salt can be widely used as a heat storage and heat transfer medium in CSP and industrial heat storage, among others.

Enhancing the specific heat capacity of solar salt can significantly improve its heat storage capacity, which is a central issue in the field of high-temperature energy storage in the recent years. This study synthesizes the NiO nanoparticles in situ in solar salt through the sol-gel combustion method to obtain modified solar salt. The effects of nano-NiO on the solar salt microstructures and properties are investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and differential scanning calorimetry. The results show that spherical NiO particles with 8—10 nm diameter are successfully prepared and can be agglomerated into small clusters measuring 0.1—0.5 μm. The NiO nanoparticles synthesized in situ by the sol-gel combustion method can markedly enhance the specific heat capacity of solar salt. The increase of the interfacial energy and the semi-solid layer formation are the main reasons for the enhanced specific heat capacity of the modified solar salt.

Phase change materials (PCMs) can easily leak from building materials. A shape-stabilized phase change material (ssPCM), called LA-TD/SiO₂, is prepared herein through the sol–gel method to overcome the leakage defect. The optimum preparation process of LA-TD/SiO₂ is determined from the pH value and the stirring rate of the condensation reaction and from the amount of added LA-TD. Scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy, differential scanning calorimetry, and thermal gravimetric tests are performed to study the LA-TD/SiO₂ properties. The results show that LA-TD/SiO₂ is spherical in the SEM morphology. The maximum particle size of LA-TD/SiO₂ is less than 30 μm. LA-TD is effectively wrapped by SiO₂, and they are physically combined. The phase change temperature of LA-TD/SiO₂ is 24.81 °C, which is slightly lower than that of LA-TD (i.e., 25.0 °C). The latent heat of LA-TD/SiO₂ is 47.96 J/g, which is approximately 1/3 that of LA-TD (i.e., 136.28 J/g). Accordingly, 500 melting/solidification cycling experiments show that LA-TD/SiO₂ has good thermal stability and can be used for building application.

The performance of a molten salt-based nanofluid can be significantly enhanced, but its thermal stability caused by the nanoparticle cluster remains a main defect for practical application. This study prepared a solar salt-based SiO₂ nanofluid with 1% mass fraction via a two-step method. To study the stability of the molten salt-based nanofluid, a long-term high-temperature (LTHT) condition is set, and the reduction of the specific heat capacity (SHC) enhancement and micro-morphology is adopted to make a comprehensive stability assessment. The experimental results show an SHC enhancement; however, the SHC is significantly decreased, and the SiO₂ nanoparticle amount in the samples is reduced after the LTHT condition. In other words, the sample prepared by the two-step method is a poor-stability nanofluid. The investigation on the method improving the stability of the molten salt-based nanofluid is based on the preparation methods and the nanoparticle material selection. The high-temperature melting method is used to prepare the same nanofluid. The results show that the SHC enhancement is similar to that of the sample prepared by the two-step method; however, the SHC enhancement after the LTHT condition is slightly reduced with the nanoparticle amount increase. This result implies that the stability of the molten salt-based nanofluid can be improved to some extent using the high-temperature melting method. Hybrid nanofluids with total mass fraction of 1% (i.e., Al₂O₃-SiO₂, TiO₂-SiO₂, and CuO-SiO₂ nanofluids) are also investigated herein based on the high-temperature melting method. The results show that the attenuation rate of the SHC enhancement reduces to 6.1% after 100 h under the LTHT condition for the Al₂O₃-SiO₂ hybrid nanofluid, indicating that the stability is further improved. On the contrary, the stability of the TiO₂-SiO₂ and CuO-SiO₂ hybrid nanofluids is worse because no SHC enhancement occurs after the LTHT condition. The results are greatly significant for improving the stability of the molten salt-based nanofluid and enhancing its practicality.

This study investigates the energy storage performance of a plate-type phase change energy storage unit (PCESU) containing a paraffin-based phase change material. The heat transfer rate is analyzed considering the layout of the thermocouples, aspect ratio, and thickness of PCESUs. The results show that the thermocouples at measuring points accelerate the melting process. Natural convection accelerates the melting process of paraffin. The melting rate in the upper region of PCESUs is higher than that in the lower region. Affected by the buoyance force and the heat transfer area, the PCESU with an aspect ratio of 3:1 presents the fastest melting rate, whereas that with an aspect ratio of 2:3 shows the slowest melting rate. The total melting time of paraffin parabolically increases with the increasing PCESU thickness. An economic analysis shows that the PCESU with 3:1 aspect ratio and 30 mm thickness has an optimal structure.

This study investigates the thermoelectric cooling performance for a battery module. Cylindrical battery modules are arranged in a 3×5 array, and thermoelectric cooling systems are symmetrically arranged on both sides. A one-dimensional thermal resistance network of battery modules is established using a theoretical analysis to evaluate the thermal performance. The thermal resistance of the thermoelectric cooler (TEC) double-sided symmetric layout is obtained through an experimental measurement. The theoretical analysis method is used to study the maximum cooling power, COP, and optimal working current of the TEC by changing the TEC current, battery temperature, cold and hot-side thermal resistances, and TEC arrangement. The results show that the cold-side temperature of the TEC first decreases then increases with the TEC current increase, whereas the hot-side temperature gradually increases with the TEC input current increase. The cooling power first increases then decreases as the TEC current increases. The COP value of the TEC gradually decreases as the current increases. The cooling efficiency when the battery temperature is 30—50 ℃ is between 0.45 and 0.60. The optimal operating current corresponding to the maximum cooling power is between 5.50 A and 6.25 A. The cold- and hot-side thermal resistances affect the cooling power and the optimal cooling current of the TEC. The optimal TEC current corresponding to the maximum cooling power is greatly affected by the hot-side thermal resistance, but barely affected by the cold-side thermal resistance.

The Lattice Boltzmann method is used to simulate the natural convective melting in a triangular cavity. The effects of the Rayleigh number Ra, local heat source size L, and location S on its phase change heat transfer and energy storage characteristics are analyzed. The numerical results show that the full melting time in the triangular cavity with a full thermal boundary initially increases then decreases with the increase of the convective strength. The inflection point is at 31000. The melting process and the convective effect show a positive correlation when Ra > 31000. In contrast, a negative correlation is observed when Ra < 31000. Moreover, when the local heating size L is small, the melting time of the middle heat source is the shortest, and the energy storage efficiency is the highest. The melting time of the uppermost heat source is the longest. In addition, due to the different influence of the cold inclined the total melting time of the upper heat source increases with the convective effect enhancement wall. By contrast, the total melting time of the lower heat source shortens, while that of the intermediate heat source first increases then decreases with a critical value of Ra=19000. Meanwhile, when L≥0.5, the lowest heating becomes the best melting position, and the energy storage time is the shortest. As the convective intensity increases, the total melting time of the upper, middle, and lower local heating schemes first increases and then decreases, and the critical Ra values increase with the heating length increase. Furthermore, the change of the heating size in different positions has different effects on the energy storage process. This work will be helpful in providing theoretical basis and technical guidance for the optimal design and efficient energy storage of the actual phase change heat exchange equipment.

This study discusses the stress and deformation characteristics of a mandrel hub with a composite rim assembly to solve the structural design problems of a metal mandrel hub with a composite rim flywheel rotor in 25 MJ energy storage capacity. Using finite element analysis software, the stress and the deformation of the mandrel hub structure are solved, and the self-adaptive characteristics of the flange connection are analyzed. The shape, pin hole position, and mandrel structure are optimized. Moreover, a new structure of the elliptical pin hole is proposed. The finite element analysis results show that the overall maximum stress of the flywheel mandrel wheel hub structure is reduced from 734.23 to 487.28 MPa at a rated speed of 24,000 r/min.

This bimonthly review paper highlights 100 recent published papers on lithium batteries. We searched the Web of Science and found 3062 papers online from Aug. 1, 2020 to Sept. 30, 2020. 100 of them were selected to be highlighted. High-nickel ternary layered, spinel, high-voltage LCO layered cathode materials are still under extensive investigations for studying Li⁺ intercalation-deintercalation mechanism and evolution of structure, and the influences of doping and interface modifications on their electrochemical performances. The researches of silicon-based composite anode materials mainly focus on the design of electrode structure, pre-lithiation and stabilization of the interface while the researches of lithium metal anode mainly focus on the design of electrode structure and further to regulate the growth of SEI and inhibit the formation of lithium dendrites. Large efforts have been devoted to solid state electrolytes including oxide, sulfide, polymer and composite solid state electrolytes. The research works on liquid electrolytes involves mainly the optimal design of solvents, lithium salts and additives, particularly electrolyte for high-voltage spinel cathode materials. For solid-state batteries, there are papers studying the design of alloyed anodes to improve the uniformity of lithium deposition; there are also papers studying the wet coating technology of sulfide solid electrolyte-based all-solid electrodes. The research focus of lithium-sulfur batteries is to improve the activity of sulfur cathodes. The focuses of characterization techniques are on in-situ methods to observe and analyze the material structure, electrode/electrolyte interface, thermal failure mechanism, interface of solid-state batteries etc. Furthermore, there are a few papers related to the theoretical works for the electric structure of materials, Li⁺ transport and Li metal deposition.

The battery energy storage system with an excellent control performance has become a new generation of support means for dealing with the frequency problem after faults or high-power disturbances. However, the market mechanism for energy storage applications has not yet been clarified, and the value measurement has not yet become systematic, restricting its development. In response to this problem, based on the analysis of the research results, combines the cost analysis of equipment, infrastructure, operation, and maintenance, etc., from the dimensions of the flow of value to the revenue, environmental benefits and social benefits, the operation benefit evaluation index and mathematical mode of energy storage frequency modulation system are constructed, it includes quantifiable evaluation index models such as financial net present value, rate of return, investment payback period, fossil energy conservation, and gas and smoke emission reduction. Using the least square method to determine the weighted decision ideas of subjective and objective indicators, this study puts forward a comprehensive evaluation model that can reflect the economics of different criterion layers and judge the operating benefits of each criterion layer. A 9 MW/4.5 MWh energy storage combined with a 300 MW thermal power unit is taken as an example, by which the effectiveness of the operational benefit evaluation method is verified.

Renewable energy sources, such as solar energy, have the characteristics of intermittence and instability that lead to their temporal, spatial, and intensity mismatch and seasonal characteristics. The seasonal thermal storage technology is one of the most effective solutions for these problems, but the traditional seasonal thermal storage system encounters problems of large heat loss and low system efficiency. To solve these problems, this study proposes a new type of composite thermal storage system coupled with an underground borehole storage and a water tank thermal storage. This system uses Fluent simulation software to perform research on the thermal storage and release characteristics of the composite thermal storage system and the change law of the temperature field of the thermal storage body. The results show that the new composite thermal storage system is technically feasible with a 67.29% system efficiency. The two thermal storage methods are complementary and coordinate with each other. The temperature fields of the coupled thermal storage bodies are superimposed on each other, thereby showing a significant temperature delay, and are more conducive to the thermal storage and release. In addition, after one operating cycle of the composite thermal storage system, the temperature level of the coupled thermal storage body increases, which is conducive to the system operation for many years. This research will be helpful in expanding and improving the energy transmission and heat transfer control theory of the underground seasonal thermal storage system and provide theoretical guidance for further expansion and application.

Distributed energy systems have the advantage of high-energy utilization and have been rapidly developed. However, problems, such as large design capacity, reduced operating efficiency, and poor security of coupled renewable energy systems, are still being encountered. Cold and heat storage technologies are applied to distributed energy systems, which are widely used, to solve the abovementioned problems. However, the related work mostly focuses on a single case, which lacks a systematic arrangement and discussion. Therefore, this study first analyzes the current status of cold and heat storage materials. The characteristics of different cold and heat storage materials are then discussed. The coupled application of distributed energy systems and cold and heat storage technologies is summarized. Moreover, the application effects are analyzed to determine the development trend of cold and heat storage technologies based on distributed energy systems. The results show that water, molten salt, refractory brick, ice, paraffin, and hydrated salt are more suitable for cold and heat storage in commercial applications. The cold storage and heat storage technologies coupled with distributed energy systems are mainly water, ice, molten salt, phase change thermal, and thermochemical thermal storage technologies. Among them, the application of water, ice, and molten salt storage technologies is more mature. The coupled application with the phase change thermal storage technology is in the demonstration application stage, while that with the thermochemical thermal storage technology is in the laboratory research stage. The application of the cold and heat storage technology with renewable energy distributed systems is an important development direction in the future. This study could provide reference and basis for the efficient application of China's distributed energy system.

The container energy storage system is an effective means of solving the energy waste problem caused by the mismatch between the generation and consumption peaks. The development of the container energy storage system is limited by the reason that the life of the lithium battery (hereinafter referred to as the battery) is affected by the batch battery consistency and the heating characteristics. The thermal performance of the battery module of a container energy storage system is analyzed based on the computational fluid dynamics simulation technology. The air distribution characteristics and the temperature distribution of the battery surface are then obtained. Moreover, the influence of the size and the arrangement angle of the guide plate on the gas flow and heat transfer characteristics is studied. The results show that the maximum and average temperatures of the cooling surface of the battery have a downward trend with the increases of the guide plate width and layout angle of the guide plate. The reasonable arrangement of the guide plate can reduce the temperature of the cooling surface of the battery to below 60 ℃, which is in line with the reasonable working environment of the general battery. The research results provide a technical reference for the wide application of the container energy storage system.

This study takes a certain type of container energy storage system as the research object. A personalized uniform air supply scheme in the form of "main duct + riser" is proposed for the energy storage battery packs on the left and right sides of the container. Based on the computational fluid dynamics technology, the flow field characteristics of the whole duct are analyzed, and the air characteristics and uniformity data of each outlet are obtained. Measures, such as adding deflectors and reducing the area of the air outlets, are used to optimize the structure of the cooling air ducts. Consequently, a good air uniformity is obtained. The results show that the flow field uniformity is determined by the deflectors added at the corner of the main air duct inlet, at the inlet of each riser, and under the six air outlets at the upper part of each riser, thereby making the dispersion coefficient of the area-weighted average velocity in the vertical direction of each air outlet of the left cooling air duct decrease from 0.837 to 0.074 and that of the right cooling air duct decrease from 0.867 to 0.059.

A lithium battery container energy storage system consumes electrical energy during energy storage; hence, reducing the energy consumption of the container energy storage system can effectively improve the power efficiency. The energy consumption of the container energy storage system is mainly divided into air conditioning system energy consumption, PCS energy consumption, BMS energy consumption, and other energy consumption, of which the total energy consumptions of the air conditioning system and the PCS account for 92%. This study analyzes the energy consumption reduction plan of the air conditioning system and the PCS equipment. Through testing and theoretical calculations, we find that the actual energy consumption of the air conditioning system is reduced by approximately 41.8%, while that of the container system is reduced by approximately 33.0%. Some SI-based IGBT modules have been replaced by SIC IGBT modules. The theoretical calculation can reduce the energy consumptions of the PCS equipment and the container systems by 32.6% and approximately 7.1%, respectively. The abovementioned solution reduces the total energy consumption of the container energy storage system by approximately 40.1%.

To solve the problem that the doubly fed adjustable speed pumped storage unit with speed as control target cannot participate in the primary frequency modulation of power grid, this paper proposed a kind of coordinated control of doubly fed adjustable speed pumped storage unit and pump turbine for primary frequency modulation. The mathematical models of pump turbine and doubly fed induction machine are established, and the process of speed optimization of doubly fed induction machine is analyzed according to the efficiency characteristic curve of pump turbine. By adding frequency controller to the speed control of doubly fed induction machine and guide vane opening control of pump turbine separately, the grid frequency deviation is tuned as additional command for speed control and guide vane opening control. Moreover, the influence of frequency controller parameters on the frequency modulation performance of doubly fed adjustable speed pumped storage unit are analyzed. The simulation results show that the doubly fed adjustable speed pumped storage unit can absorb or release energy in short term by adjusting the speed to reduce the amplitude of frequency fluctuation at the beginning of frequency fluctuation. And the mechanical torque of pump turbine is changed by controling the opening of guide vane, and then the output power is adjusted to reduce the steady-state deviation of gird frequency in later period.

System efficiency of air-cooled proton exchange membrane fuel cell (air-cooled PEMFC) is experimentally studied on full power range. Results show air-cooled PEMFC system efficiency is a parabola with the system power. With the increasing of the system power, system efficiency increases dramatically; system efficiency decreases slowly after it reaches the peak efficiency, and then almost approaches to a flat. The system efficiency is about 47% in the rated power, and the peak efficiency is about 56% nearby the 50% of rated power. A "dual-stack" PEMFC system is designed, that is to say the PEMFC system consists of two stacks. Hydrogen enters into the PEMFC stacks in series mode. Hydrogen from the previous stack anode end enters into the subsequent stack through a separator. Experimental results show performance of the dual-stack system is only better in higher power range compared to the single stack system with the same cells, and there has litter change in lower power range. The delightful result is the system efficiency significantly improves, and a 60.23% of peak efficiency is obtained. The efficiency of dual-stack system increases by 2%—4% in lower power range, and approximately increases by 5% in higher power range. This dual-stack PEMFC system has great practical valve.

In this study, the mode of conserving income for the electricity and subsystem investment costs of the battery energy storage system (BESS) is analyzed based on a two-part tariff. An economic mathematical model of the user-side BESS is established for a large industry enterprise, whose transformer capacity is above 315 kVA. Considering the seasonal feature of the time-of-use electricity price in a specific region and the hourly load characteristics, different typical days are extracted as the calculation object of the model. A lithium iron phosphate battery with high safety, high charge-discharge efficiency, and long cycle life is selected as the BESS charging medium. The model parameter constraints are set. The regional subsidy policy is also considered. Taking the optimal economy of the energy storage device as the goal, the BESS configuration, including the rated capacity and the rated charge–discharge power, and the charge-discharge strategy are calculated using genetic algorithms. In addition, a large industrial enterprise in Suzhou is taken as an example for calculating the BESS using the established model. The optimal system configuration, optimal system charge-discharge strategy, and system recovery period are obtained under two modes: ① peak load shifting and ② peak load shifting combined with load demand regulation. Accordingly, the scientific method for calculating the configuration, charge-discharge strategy, and payback period of the user-side BESS are provided. By comparing the configuration and the payback period of the BESS under the two modes, we conclude that considering the load demand regulation income would effectively reduce the system payback period and provide favorable conditions for the large-scale commercialization of the energy storage technology.

In this study, the transaction mode of the energy storage independently participating in the grid peak shaving is sorted out and a simulation is used to analyze the dispatching operation and market income of the energy storage independently participating in the peak shaving market scenario to promote electricity storage energy to actively participate in the grid peak shaving under the background of the peak shaving auxiliary service market being liberalized to electricity storage. Considering that the energy storage lacks bidding experience in the early market stages, we use herein the principle of more energy being absorbed from the grid and higher energy storage quotations to formulate a segmented quotation strategy for it. Combined with the prediction of the energy storage charging and discharging behavior, we construct a declaration model for the energy storage to participate in the peak shaving market. Based on the peak shaving demand of the power grid on the next day and considering the peak shaving quotation information of the energy storage and thermal power, the auction clearing model of the peak shaving market is constructed such that the power dispatching agency can call the peak shaving resources with the minimum system shaving cost as the goal. The relationship between the market marginal clearing price and the winning bid power of each market member is also analyzed. In an actual dispatch operation, the energy storage can participate in the electrical energy market through discharge to obtain electrical energy gains. To optimize the discharge behavior to maximize market profitability, a real-time energy storage optimization scheduling model is constructed based on the real-time charge-discharge status of the energy storage after the peak shaving market clearance and electricity price fluctuation and considering the battery consumption costs. The usage of a real-coded genetic algorithm is proposed to solve the real-time optimal scheduling model of energy storage. Taking the energy storage built in the metering outlet of wind farms and photovoltaic power plants as an example, we use the abovementioned model to simulate and analyze the energy storage independently participating in peak shaving of the power grid based on the actual operating data of a power grid in Northwest China. The results show that participating in the peak regulation market for energy storage can reduce the peak regulation cost of the power grid and increase its own profit through market revenue compensation to achieve expected economic benefits. This study provides a certain theoretical basis for promoting energy storage to independently participate in the peak shaving auxiliary service market.

The flywheel energy storage system (FESS) is a form of energy storage with a high dynamic response performance. This study proposes a fault ride-through control strategy suitable for the FESS based on the analysis of the operating characteristics of the system under low- and high-voltage faults in the power grid. The proposed control strategy is applicable to both symmetric and asymmetrical faults in the power grid. Through a multi-mode coordinated control of the grid- and machine-side converters during the low- and high-voltage faults of the power grid, the proposed method enables a continuous non-off-grid operation of the flywheel energy storage grid-connected system. At the same time, it can meet the fault ride-through operation requirement of the FESS being able to provide a certain reactive power support to the grid during fault. The effectiveness of the proposed control strategy is verified by a simulation. Moreover, the fault ride-through capability of the flywheel energy storage grid-connected system is evaluated.

To solve the problem of intermittent and fluctuating energy output in renewable energy power generation systems, a small-scale combined cooling, heating and power (CCHP) system with compressed air energy storage and supercharged internal combustion engine technology is proposed based on the laws of thermodynamic and the principle of cascade energy utilization. The thermodynamic model of the system is established and the thermodynamic properties of the system are analyzed. The effects of isentropic efficiency and outlet pressure of compressor, isentropic efficiency and inlet pressure of expander, efficiency of heat exchanger and inlet pressure of diesel engine on system performance are emphatically discussed. The simulation indicates that under the design condition, the energy efficiency, exergy efficiency and energy saving rate (ESR) are respectively 86.08%, 59.45% and 43.50%. The inlet pressure of the expander has little influence on the energy efficiency of the system, which effectively improves the problem of variable operating conditions in the process of power generation. Energy efficiency, exergy efficiency and energy saving rate of the system increase with the rising the efficiency of heat exchanger and the inlet pressure of diesel engine in the system. The results provide a theoretical basis for the engineering application of the system.

Since its invention, the lithium-ion battery has been widely used in various aspects of human life due to its advantages of high-energy density and long cycle life, among others. However, the safety problem of the lithium-ion battery cannot be ignored. Accidents, such as fire hazards and explosions caused by the thermal runaway of a lithium-ion battery cause inevitable property loss and casualties. Hence, many researchers have been paying attention to the safety problems of the lithium-ion battery. This study analyzes existing early warming methods of the lithium-ion battery thermal runaway from characteristic parameters like temperature, resistance, voltage, and inside pressure and the produced gas of the batteries combined with the knowledge of thermal runaway, concludes the normal early warning methods, and performs analyzation and prospect of the development of future early battery warming methods.

Beset by energy shortage and environmental pollution with the sharp rise in car ownership, electric vehicles (EVs) are currently receiving high praise from people. However, the energy used in EVs must be supplied by hundreds of cells when they are driven for a limited energy density of the lithium ion; as a result, the battery management system (BMS) can be viewed as the core technology for EVs. Unluckily, some parameters cannot be measured directly and are obtained only through model estimation. The appropriate model of a lithium-ion battery is the key to the efficiency, precision, and stability of the BMS. Several factors, including material, ambient temperature, work mode, and aging degree, are well known to be closely related to the lithium-ion battery model; nevertheless, having such elements during modeling is difficult. This study introduces the function and structure of the BMS, enabling the listing of concerned references regarding the battery model in the recent years. Three sorts of model (i.e., electrical characteristic, thermal, and electric-thermal coupling models) are then discussed separately. Although the first and second models can clearly reveal the work mechanism of the lithium-ion battery, the large amount of calculation makes it hard to be accepted in engineering considering the current situation. Adversely, the third model combines the advantages of the two models and is widely used for its relative simplicity. Herein, the applications of such models on internal states, such as state of charge (SOC), state of health (SOH), and inner temperature of battery, are described based on a correlative discussion. The SOC variable of EV is as significant as the oil gauge of the internal combustion engine vehicle; thus, it is relatively mature. The SOH is influenced by the current, temperature, and SOC and is very relevant to mechanical vibration and overpotential. Meanwhile, the internal temperature is vital to the capacity, discharging efficiency, span life, and safety of the lithium-ion battery; therefor, how to maintain the proper temperature is very crucial. Considering the coupling of estimation variables (e.g., SOC and inner temperature variable), the estimation accuracy should be greatly enhanced to ensure the BMS reliability. In the future, the lithium-ion battery model is expected to be simplified continually, thereby enabling the invariable satisfaction of a real-time BMS.

In order to improve the low temperature charge performance of lithium-ion battery, reduce the aging rate and charging time in low temperature, thereby promoting the promotion of new energy vehicles in the low temperature region, made a series of lithium-ion battery charge and discharge cycle aging test in low temperature, based on a large number of low temperature test, charge and discharge test data under low temperature environment is analyzed under different charge conditions on the influence of lithium-ion battery aging rate. A BP neural network model for low temperature charge aging rate estimation of lithium-ion batteries was established. On this basis, particle swarm optimization algorithm is introduced to optimize the traditional CC-CV charging strategy, and the whole charging process is divided into two stages. In the first stage, particle swarm optimization algorithm is used to find the approximate optimal charging curve before reaching the charging cut-off voltage, and in the second stage, conventional constant voltage charging is used. Based on the capacity decline rate estimation model at low temperature, low temperature aging rate charging and charging time weighted summation of multi-objective optimization equation as the fitness function of particle swarm optimization algorithm, introduction of the weights coefficient "g" in the fitness function to weigh the two optimization goal, iterative optimization using particle swarm optimization algorithm. The results show that the model has high estimation accuracy for the decline rate of low temperature charging capacity, and the optimized charging strategy can effectively reduce the low temperature charging aging rate and charging time of lithium batteries.

The prediction of the state of charge (SOC) of lithium-ion batteries by the least squares support vector machine (LSSVM) shows a faster convergence speed and gives an extraordinary method for the global optimal solution. The prediction ability is enhanced more than ever before. However, the parameter selection of the LSSVM will greatly affect the prediction result. A prediction method for the SOC of lithium-ion batteries by estimating the distribution algorithm (EDA) with an LSSVM is proposed herein. The operating voltage, current, and temperature of the lithium-ion batteries are used as the input quantities. The SOC of the batteries is used as the output quantity. Moreover, a non-linear system model is built using the LSSVM. The EDA is designed to optimize the regularization parameter and the radial basis kernel width of the model. We then obtain the optimal model. The simulation results show that compared with the conventional prediction model for the SOC of lithium-ion batteries, the proposed EDA-LSSVM method has a higher prediction accuracy for the SOC.

With the increasing extensive use of lithium-ion batteries in electric vehicles and microgrid, much research has been performed to ensure a safe and reliable operation and reduce the maintenance costs of the lithium-ion battery management system (BMS). As one of the key functions of the BMS, the state of health (SOH) estimation is very important. A dynamic cuckoo search algorithm is proposed based on the analysis of the traditional cuckoo search optimization algorithm to improve the estimation accuracy. The relationship between search speed and accuracy is balanced by improving the step size and the discovery probability and by introducing the change trend of the function value into the step update equation. A dynamic cuckoo search algorithm is proposed to solve the particle degradation problem existing in the traditional particle filter itself. The particles are represented by cuckoo bird's nests. Moreover, the cuckoo group search simulation is used to guide the distribution of updated particles, while the improved dynamic cuckoo search is used to optimize the particle filter algorithm. The health index (HI) is extracted from the measurable parameters of the lithium-ion battery, and the mapping model between the HI index and the SOH is established and applied to the state space model observation. A battery SOH estimation method based on the improved particle filter algorithm is proposed. The experimental results show that this method is superior to the traditional particle filter algorithm and has good adaptability and accuracy in predicting the degradation process of lithium-ion batteries.

The heat production of the lithium-ion battery has a very important impact on its safety and life. Based on COMSOL Multiphysics, this study proposes a three-dimensional electrochemical thermal coupled finite element analysis model for a 51 A·h laminated lithium-ion pouch battery. The effects of six design parameters on the temperature field (i.e., positive electrode thickness, plate width, positive electrode tab thickness, positive electrode tab width, negative electrode tab thickness, and negative electrode tab width) are studied using the response surface method. The linear weighted sum and random gradient descent methods are used to obtain the optimal scheme to reduce the average temperature rise and the maximum temperature difference of the battery. The average temperature rise and the maximum temperature difference of the battery can be reduced by using the random gradient descent method. The results show that the positive electrode thickness greatly influences the temperature field positively related to the temperature rise; however, the influence is weakened when the thickness is reduced to a certain extent. The increase of the plate width and the tab size can reduce the temperature rise of the battery at the end of the discharge. In addition, the maximum temperature difference of the battery reaches the minimum value within a certain range. The error of the scheme is less than 2.68%. The temperature rise is reduced by 2.93 ℃. The temperature difference is reduced by 0.596 ℃, which is helpful in improving the safety and life of the battery, and provides a reference for the multi-objective thermal optimization of other batteries.

In this study, a long–short-term memory (LSTM) recurrent neural network is used to establish an estimation model of the state of charge (SOC) to estimate the SOC of power batteries. The model is trained and tested with a laboratory's constant current discharge data. The maximum absolute error is 2.7%. A further verification by the FSEC racing battery measured data shows a maximum test error of 3.9%. However, considering the complexity of the environment during the actual operation and the inconsistency caused by different driving habits to the power battery in engineering applications, training and testing must be performed according to the actual driving conditions of the vehicle. The SOC in driving conditions are directly translated from the battery management system (BMS) message; hence, we cannot affirm whether the SOC algorithm in the BMS is accurate. Consequently, the SOC in the driving conditions cannot be used as a label while training the model. At this time, the correct training label must be calculated or the existing model trained by the labeled data must be used. Its model parameters must then be dynamically adjusted based on the actual operating unlabeled data. This study takes the second method to solve the training problem of unlabeled data, proposing for the first time the combination of the domain adaptive neural network (DaNN) in transfer learning with LSTM to form the SOC estimation algorithm of LSTM-DaNN using the labeled data to train the LSTM model in advance, transfer its model parameters to LSTM-DaNN, and finally train the LSTM-DaNN model by combining the labeled and unlabeled data. The result shows that LSTM-DaNN can complete the training without the label of the actual driving condition (i.e., SOC) and ensure an absolute error of 4.8%. Compared with the model before the adaptive adjustment, the error decreases by 14.1%, and the absolute error is guaranteed to be <5%, meeting actual needs.

A full-battery capacity estimation method using the maximum chemical capacity of the lithium battery is proposed herein to solve the problems of slow update and low accuracy in the full-battery capacity estimation method based on the Coulomb integral in the existing lithium battery management system. During the charging process, the battery charging time constant is estimated according to the equivalent resistance-capacitance model of the lithium battery and the current sampling sequence. According to the charging time constant and the charging cut-off current, the virtual capacity in the constant voltage stage is then calculated by assuming that charging is started from the cut-off current until an infinite time. Meanwhile, the maximum chemical capacity of the lithium battery is calculated based on the discharge depth of the two selected relaxation states of the lithium battery and the charge capacity between these two states. Finally, the full charge capacity of the battery is calculated according to the approximate relationship among the full charge capacity, maximum chemical capacity, and virtual capacity. The experimental results show that the full charge capacity errors are less than 3% under different temperatures and health states. In other words, the proposed full charge capacity estimation method has high accuracy and strong adaptability and can meet the needs of the actual lithium battery management system. This method has the advantages of simple calculation and easy implementation and wide application prospects in various lithium battery management systems. This study explores and utilizes the internal relationship between the full charge and maximum chemical capacities, which provides a new method of estimating the state parameters of lithium batteries.

The carrier phase shift pulse width modulation (CPS-PWM) is a modulation method widely used in modular multilevel converter-based battery energy storage systems (MMC-BESS). At present, the mathematical modeling of MMC-BESS based on CPS-PWM is mostly concentrated on the AC side of the system, and its analysis of the output voltage waveforms is mostly focused on the frequency domain. The mathematical modeling for the DC side of the system and the time-domain analysis of the output waveforms has not yet been studied. Moreover, the instantaneous equivalent circuit for the DC side of MMC-BESS is still unclear. This study performs a theoretical research on CPS-PWM and the DC side of MMC-BESS. Accordingly, time-domain triangle wave mathematical expressions for the carrier waveforms are established, including the two most commonly used CPS-PWM (i.e., N + 1 and 2N+1 modulations). The intersection point sequence is proposed for a constant modulation wave and the carrier waveforms in a carrier period. The expressions of the intersection time interval and the number of sub-modules opened in a single phase, which are only related to the modulation ratio and the number of sub-modules in an arm, are then obtained. Based on this, an equivalent model of the DC side of MMC-BESS is established. We find that the DC side of the system can be equivalent to two voltage sources and one switch, and its parameters are only related to the modulation ratio and the number of bridge modules. Subsequently, the ripple expression of the DC current in the equivalent switching cycle is calculated based on this model. A method for determining the bridge inductance, which can meet the design requirements for the DC current ripple in the steady state, is also proposed. Finally, a three-phase 48-module MMC-BESS simulation model is built in MATLAB/Simulink. The simulation results of important system parameters, such as the number of open sub-modules, equivalent switching frequency and duty cycle, and DC current ripple, are consistent with the theory, thereby verifying the mathematical model accuracy. The simulation results of the current ripple can meet the design requirements when the method for choosing the bridge inductance is used. This also confirms the method effectiveness.

The mechanism model of a lithium-ion battery has high precision and can reveal the internal information that cannot be described in an experiment. Moreover, this model is greatly significant to aging research, fault diagnosis, and design of a battery management system. This study proposes a simplified method coupled with temperature to tackle the high computational complexity and temperature-coupled influence of the battery model. The proposed model is based on the single particle (SP) model. First, a two-parameter parabolic profile method is used to simplify the solid diffusion equations. Parabolic profile approximation and the finite difference method are then used to solve the liquid diffusion. Second, a whole model, called the simplified pseudo two-dimensional (SP2D) model, is built considering Ohm's law and solid electrolyte interface film polarization. The average temperature of the battery is calculated through the lumped model simplification method or the central difference method. The thermal model is related to the heat production calculated by the mechanism model. The electrochemical parameters in the mechanism model are influenced by the temperature from the thermal model. The mechanism and thermal models are finally coupled. The results show that when current is constant, the simplified coupled model is more accurate than the SP and SP2D models in terms of the battery temperature and battery voltage, respectively. The model also shows a good performance in complex conditions. The root mean square error of the battery terminal voltage and the temperature are less than 0.041 and 0.66, respectively, for the constant current test. In addition, the absolute percentage error is less than 1% and 0.15% for the urban dynamometer driving schedule test.