The shortage of fossil energy and the deterioration of the climate environment have prompted the adjustment of the energy structure and the transformation of consumption worldwide. According to the Energy Production and Consumption Revolution Strategy (2016-2030) issued by the National Development and Reform Commission and the National Energy Administration in 2016, it is estimated that in 2050, the proportion of new energy sources replacing traditional fossil energy sources will exceed 50% [1]. As of the end of 2019, the proportion of new energy sources such as wind and solar energy connected to the grid in my country has been higher than 41%, which is a significant increase from the proportion in 2010, indicating that my countrys power-side structure is continuing to be optimized [2]. The new generation of power system is based on a variety of new energy sources, combining the main grid with distributed power sources or micro grids, in order to achieve a comprehensive sustainable development model of grid multi-energy, and promote the successful transformation of national energy strategy [3]. As one of the key technologies for future grid development [4], large-scale energy storage technology can not only assist in the safe grid connection of new energy sources, alleviate the pressure of grid peak shaving due to the uncertainty of new energy sources, but also can be used to improve power quality. , Improve grid operation efficiency or emergency backup. In order to promote the healthy and rapid development of energy storage technology in the future, many experts and scholars have carried out long-term in-depth research on the technical requirements of new energy grid connection, calculation of energy storage planning model methods, and optimization and improvement of energy storage materials [5-9]. The diversity of energy storage technology determines the difference between the applicable fields and the scale of energy storage. This article first introduces the overall situation of energy storage technology, and then based on the actual situation of the West Mengxi power grid and the basic characteristics of energy storage, select several electrochemical energy storage technologies that have great development potential in the field of power systems in recent years. The performance, application status and research and development of related energy storage materials are discussed, and finally, the future development trends and research trends of the above energy storage technologies are discussed in combination with relevant policy-oriented analysis.

1. General introduction of energy storage technology

Energy storage technology has a variety of classification methods, including mechanical energy storage, electromagnetic energy storage and chemical energy storage according to the energy form. Among them, mechanical energy storage mainly refers to pumped water storage, flywheel energy storage and compressed air energy storage; electromagnetic energy storage includes supercapacitors and superconducting energy storage; electrochemical energy storage includes mature lithium-ion batteries, sodium-sulfur batteries, and flow batteries , Lead-acid batteries and other derived new battery energy storage technologies such as sodium ion batteries, aluminum ion batteries, magnesium batteries, etc.[10-12]. The ideal energy storage technology should have both technical characteristics, economic benefits and environmental protection advantages. At present, no technology can meet the above conditions at the same time. Therefore, it is necessary to select a suitable energy storage technology according to the application field and conditions. Table 1 lists and compares the main characteristic parameters of several energy storage technologies that have been commercialized or are in the demonstration stage and are suitable for power systems [13-15].

Table 1 Comparison of main characteristic parameters of several common energy storage technologies

Tab.1 Comparison of main characteristic parameters of several common energy storage technologies

Pumped storage technology not only has a large energy storage capacity, but also has a long economic and safe life. It has been commercialized for many years in the fields of power system peaking and frequency modulation, peak shaving and valley filling, and backup power supply, and it has always occupied the global and national energy storage capacity. Absolute ratio (>90%). However, due to the shortage of water resources in the West Inner Mongolia region, and the pumped storage technology is not suitable for smoothing wind power output at wind farm ports, it is necessary to introduce other forms of energy storage to achieve multi-energy complementarity [16-17]. Electrochemical energy storage technology has fast response speed, small geographical conditions, flexible application and short construction period, which can play an important role in improving grid regulation and safety and stability [18]. According to the power supply characteristics shown in the California power market, electrochemical energy storage has a significant effect on frequency modulation, which is 1.7 times on average for hydropower generators and 2.5 times on average for gas generators [19]. Although the current high investment cost of electrochemical energy storage technology limits its extensive commercial development, the construction of many energy storage demonstration projects in many countries has proved its huge development potential. Some engineering project cases are listed in Table 2[14][17] [20-21].

The rapid development of energy storage technology requires innovation and progress in energy storage material systems. For key components such as batterys positive and negative electrodes, separators and electrolytes, relevant scholars have conducted in-depth research on energy storage mechanism, material physical and chemical properties, kinetics, thermodynamics, interface and surface, etc., through various combinations of existing materials or new materials The innovative research and development of the company is committed to achieving a substantial improvement in the performance of energy storage batteries, providing a strong technical guarantee for the development of energy storage.








2. Electrochemical energy storage technology

2.1 Lithium-ion battery

Lithium-ion battery is essentially a solid-state concentration difference secondary battery. Thanks to the rapid development of electric batteries, lithium-ion batteries used in the field of energy storage have good performance in terms of energy density, conversion efficiency, response time, cycle life, and safety, which are of great significance for enhancing the flexibility of the power grid. As of the end of 2018, among the electrochemical energy storage projects in my country established power systems, lithium-ion batteries accounted for about 72% of the total installed capacity[22], which were used in centralized new energy grid connection, alleviating the pressure of grid peaking, and grid Some application cases can be seen in Table 2 in the areas of auxiliary services, grid side and user side. For lithium-ion batteries, reducing battery costs while improving safety is the key to promoting their large-scale application in the power grid. Choosing and configuring appropriate electrode materials and electrolytes can not only effectively reduce the cost of lithium-ion batteries, but also improve safety performance. It is the research focus and focus of lithium-ion batteries as a key technology for chemical energy storage.

The commonly used cathode materials for lithium-ion batteries are lithium cobalt oxide, lithium manganate, lithium iron phosphate or ternary materials. Lithium iron phosphate batteries have become the first choice for large-scale energy storage due to their long cycle life, safety and environmental protection, and low prices [13]. The negative electrode materials of commercial lithium-ion batteries are often modified graphite, but the dendrite problem of graphite negative electrode materials during rapid charging will cause short circuit of the positive and negative electrodes, which will reduce the cycle life and safety of the battery [23]. Regarding the improvement and innovation of lithium-ion battery cathode materials, in addition to the zero-strain material lithium titanate or silicon-based cathode materials, the addition of carbon nanotubes or graphene as supplementary materials also shows obvious advantages in improving battery performance. For example, by compounding with electrode materials, used as a conductive agent or used for separator modification, not only the battery rate characteristics and cycle performance are effectively improved, but also the low-temperature performance defects of lithium-ion batteries can be improved [24]. It can be seen that new materials such as graphene are one of the new directions and trends in future energy storage materials research. Wei et al. [25] added an appropriate amount of graphene to the lithium iron phosphate electrode as a conductive agent, and the performance test results showed that the conductivity is better than that of the traditional lithium iron phosphate battery, which provides a new way for the future preparation of high-performance lithium iron phosphate batteries. Ideas. The energy density characteristics of lithium-ion batteries are in contradiction with the stability of the organic electrolyte used. All-solid-state batteries prepared with polymers or inorganic electrolytes can not only overcome the problem of dendrites, increase battery life, but also reduce costs. It is one of the main research and development directions for future batteries [26]. Sodium-ion batteries are similar to lithium-ion batteries in battery structure and working principle, and have outstanding sodium resource advantages, so they have also received hot attention in recent years. The Institute of Physics, Chinese Academy of Sciences has been committed to the development and research of high-performance room temperature sodium ion battery electrodes, electrolytes, diaphragms and other materials and related devices for many years. The O3-Na0.9[Cu0.22Fe0.30Mn0.48]O2 cathode has been developed The material and the soft carbon anode material prepared from anthracite make the battery cycle stability excellent, and both have the practical ability [27]. In addition, the arrival of the first batch of power battery retirement peaks heralds the retirement of a large number of lithium-ion batteries. The use of valuable retired battery ladders in energy storage power stations can effectively reduce the performance requirements. The investment cost of energy storage batteries can also reduce environmental threats caused by large-scale battery disposal. In the future, it is possible to further strengthen the technical research on detection methods, energy management, and safety of decommissioned batteries and the regulatory requirements of various links and systems to promote the realization of a complete and efficient commercial industrial chain of lithium-ion batteries [28].

2.2 Sodium-sulfur battery

Sodium-sulfur batteries were developed by Ford in the United States in 1967, and entered the commercial application stage of large-scale energy storage technology in 2002 [29-30]. Contrary to conventional batteries, it is a tubular battery composed of molten electrodes and solid electrolytes. The beta-Al2O3 ceramic with excellent sodium ion conductivity is used as the electrolyte and separator of the sodium-sulfur battery to safely separate the metal sodium negative electrode and the external sulfur positive electrode loaded in the ceramic tube. The energy density of sodium-sulfur batteries is much higher than that of lithium-ion batteries and all-vanadium flow batteries, and it has the advantages of large capacity, high charge and discharge efficiency, low raw material costs, and low environmental pollution [31]. However, in order to meet the high sodium ion conductivity and high reaction efficiency, sodium-sulfur batteries need to operate in a high temperature environment of 300℃-350℃, which causes huge safety hazards and heat preservation and energy consumption problems [32]. Table 2 demonstrates the positive effects of sodium-sulfur batteries in power system peak shaving, load balancing and power quality improvement. The development of electrolyte materials for sodium-sulfur batteries is the key to the improvement of electrochemical performance and one of the effective ways to solve battery economic and safety issues. The selection of electrolyte raw materials and impurity control, preparation process methods and technical maturity will directly determine the quality and performance of the ceramic electrolyte. In addition, room temperature sodium-sulfur batteries, born by replacing ceramic electrolytes with polymers or organic solvents, have a significant contribution to the battery first discharge capacity. However, serious self-discharge and rapid capacity degradation have caused them to be far from commercial applications. Long distance [33-35].
2.3 All vanadium flow battery

The flow battery is a redox battery that realizes the mutual conversion and storage of electrical energy and chemical energy through changes in the valence of positive and negative active materials. Among many flow battery systems such as zinc-bromide system, iron-chromium system, vanadium-bromide system, and all-vanadium system, all-vanadium battery is the most promising energy storage technology and the most built demonstration project [36]. The combination of positive and negative active materials of the same element in different valence states of the all-vanadium redox flow battery and the unique operating structure independent of each other give it advantages in response time, energy conversion efficiency, cycle life, power capacity adjustment, safety and environmental protection, and design placement. prominent. However, the thermal instability of the solubility of the active material of the all-vanadium redox flow battery and the complexity of the system device connection make the all-vanadium redox flow battery have a low energy density, high initial investment cost, and only suitable for large-scale stationary with low site requirements. Energy storage field [37-38]. Table 2 shows the role of all-vanadium redox flow batteries in the grid connection of renewable energy, peak and frequency modulation, emergency backup, and improvement of grid power quality in the power system.

In order to promote the large-scale commercial development of vanadium redox flow batteries, key energy storage materials such as electrodes, electrolytes and diaphragms of vanadium redox flow batteries need to be improved and optimized to improve battery performance. The electrochemical activity, resistance, chemical stability and porosity of the electrode materials will all have an impact on the performance of the vanadium redox flow battery. Oxidation treatment of carbon electrode materials such as graphite or carbon felt, surface-supported electrocatalyst treatment or carbon nanomaterial modification can effectively improve the physical and chemical properties of the material surface, reduce polarization, and improve battery reversibility and power density [35] [39]. Flox et al. [40] developed a new type of graphene-supported cathode material with Pt and PtCu3 nanocubic catalysts. The specific surface area was significantly increased, which greatly improved the electronic conductivity and battery energy efficiency. He et al. [41] found that the use of chemical vapor deposition to support carbon nanotubes on the surface of graphene can significantly optimize battery performance and make the energy efficiency as high as 85%. The commonly used sulfuric acid electrolyte has good solubility and stability for polyvalent vanadium ions only in the temperature range of 10-40℃. The applicable temperature range can be expanded by optimizing the concentration of sulfuric acid, changing the electrolyte type or quantitatively introducing electrolyte additives. , Thereby improving the energy density and operational stability of the battery [42]. Hwang et al. [43] used taurine and its derivatives as the positive electrode electrolyte additive, and found that it can effectively reduce the thermal instability of the active material, inhibit the irreversible high-temperature precipitation phenomenon, and make the battery energy efficiency as high as 87.9% and the capacity retention rate is low. After 100 cycles, it still reaches 87.6%. At present, the most widely used diaphragm in flow batteries is the Nafion series perfluorinated proton conducting membrane of DuPont, but the diaphragm is expensive and accounts for 10-15% of the total cost of investment. Therefore, how to improve the ion selectivity of the diaphragm and ensure its long-term stability At the same time, reducing the cost of diaphragm manufacturing has become a key issue that urgently needs to be broken through [44]. Regarding the modification of perfluorinated membrane materials and the exploration of non-perfluorinated ion membranes, Mai et al. [45] first combined high-hydrophobic polyvinylidene fluoride with Nafion membranes, which effectively limited the swelling of Nafion membranes and reduced Permeability of vanadium ions. In addition, non-perfluorinated ionic membranes are cheap, easy to prepare, and have excellent performance. They have also received a lot of attention, but its long-term stability has yet to be verified.

2.4 Lead-acid batteries

The advantages of traditional lead-acid batteries such as mature technology, low cost, safety and reliability, and high recycling rate have made them widely used in power system peak and frequency modulation, peak shaving and valley filling, power quality improvement and backup power supply. However, the corrosion of the positive plate of lead-acid batteries and the irreversible sulfation of the negative plate in the high-rate partial charge state (HRPSoC) will seriously affect the battery performance and lead to premature battery life failure [46]. Under the situation of increasing battery performance requirements, lead-carbon batteries have attracted attention due to their excellent performance in terms of safety, economy and cycle life. By selecting a suitable carbon material,  internally or internally mixed in the negative plate of the lead-acid battery at an appropriate ratio to form a combination of lead-acid battery and supercapacitor, the high specific surface area and high conductivity of the carbon material make the battery Performance has been greatly improved [47]. Compared with traditional lead-acid batteries, lead-carbon batteries have significantly improved specific power, rapid charge and discharge, and cycle life [48]. It has shown broad prospects in the fields of domestic and foreign wind power grid access, peak shaving and valley filling, grid frequency modulation, and micro grid systems.

The FCP lead-carbon battery jointly developed by Shengyang Power and Japans Furukawa Battery Co., Ltd. has increased the battery cycle life to 4200 times through a series of optimization measures, which proves that the commercial application of lead-carbon battery energy storage is inseparable from positive and negative battery materials. Intensive breakthroughs in extremely active materials, electrolytes and additives [49]. However, the complex diversity of carbon materials has caused the unclear mechanism of carbon materials. The selection of carbon materials and the amount of carbon materials still needs further qualitative and quantitative research. In addition, how to effectively suppress the hydrogen evolution reaction that is aggravated by the introduction of carbon materials and The battery s manufacturing process, environmental pollution and other aspects are awaiting further exploration and improvement.

Information source: Energy Storage Technology Engineering Center

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