领域：学术论文 · 自然科学

2. Lithium-ion batteries in tunnel energy storage system
The tunnel energy storage system (TESS) has the characteristics of peak shaving and valley filling, delayed power consumption, power quality optimization, etc. Peak clipping and valley filling is charging when the power is in the “flat” and “valley” periods, and discharging when the power is in the “peak” periods, which can not only save the operating cost of power supply, but also reduce the cost of providing frequency regulation and standby power supply. When the power grid encounters power failure, the energy storage system can be used as a backup power supply to continuously supply power for tunnel lighting and ventilation. In addition, the installation and application of the energy storage system can filter out the disturbance from the power grid, which can be used as a special power quality control device to improve the power supply quality of important loads. From the perspective of energy conservation and emission reduction, the energy storage system transfers the electric energy of low carbon emission power sources to peak load periods during off peak load periods, thereby reducing the consumption of high emission energy, which is conducive to energy conservation and emission reduction. lithium-ion battery (LIB) is an ideal high-energy energy storage battery due to its high specific capacity, small self discharge, long storage time, no memory effect, long lifetime, fast charging and other advantages, which is widely used in TESS.
As shown in Fig. 3, the tunnel LIB energy storage system is mainly composed of a battery system, a power converter system (PCS), a battery management system (BMS), an energy management system (EMS), and a monitoring system. Nowadays, LIBs are developing in the direction of higher safety, high environmental protection, high energy density, and long lifetime. As a lithium-ion concentration difference battery, the main components include: cathode and anode materials, electrolytes, separators, and shells.
2.1. The working principle of lithium-ion batteries
As shown in Fig. 4, during the charging and discharging process of LIBs, lithium ions are intercalated and deintercalated back and forth between the positive negative electrode, thereby realizing the mutual conversion of chemical energy and electrical energy. During charging, lithium ions are deintercalated from the lithium-rich positive electrode, enter the electrolyte, pass through the separator, and are transported to be embedded in the negative electrode. At this time, the negative electrode is in a lithium-rich state, while the positive electrode is in a lithium-poor state, and the electrons are transmitted from the positive electrode by the external circuit. To the negative electrode, electrical energy is converted into chemical energy. During discharge, lithium ions are deintercalated from the negative electrode, pass through the electrolyte, pass through the separator, and re-insert into the lithium-poor positive electrode, and electrons are transferred from the negative electrode back to the positive electrode by the external circuit, Return the electric charge to the equilibrium state and realize the conversion of chemical energy to electrical energy.
2.2. Cathode materials
As one of the important components of LIBs, the cathode materials determine the performance of LIBs, and it should meet the following requirements: (1) higher potential and stable platform to ensure high voltage for battery operation; (2) small electrochemical equivalent to ensure large specific capacity of the battery; (3) high lithium ion diffusion coefficient and conductivity to ensure high reversible capacity of the battery; (4) excellent embeddedness and de-embeddedness, small structural change to ensure the long cycle lifetime of the battery; (5) good compatibility with the electrolyte to ensure high chemical and thermal stability of the battery; (6) low price and green environmental protection to ensure that the battery is environmentally friendly. The cathode materials of LIBs can be divided into layered structure, spinel structure and polyanion type (olivine type) according to their structures.
Typical layered cathode materials are: lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2), lithium manganate (LiMnO2), ternary transition metal oxides. As shown in Fig. 5, LiCoO2, the most representative layered oxide, was first used as a cathode material for commercial LIBs. Its α-NaFeO2 type layered structure has electrochemical stability. However, in the case of deep delithiation, the structure will be severely damaged due to the higher charging voltage, which affects the performance of LIBs. However, it is possible to improve the structural stability and improve the performance of LiCoO2 by increasing the charge-discharge voltage of the LiCoO2 cathode material or by doping other elements (such as Mg, Al, etc.). As LiNiO2, which has attracted more attention recently, although Ni is richer in resources, lower in cost and less harmful to the environment than Co, the radius of Ni2+ and Li are close, which is easy to cause mixed discharge, so LiNiO+2 has poor thermal stability. The researchers have stabilized its structure by doping it with Al elements. LiMnO2 mainly has two structures of monoclinic phase and orthorhombic phase, and its actual specific capacity is higher than that of LiCoO2 and LiNiO2, but its Mn3+ is unstable and easy to transform to Mn4+, resulting in structural variation, which seriously affects its electrochemical stability. It can be stabilized by doping with other elements. Due to the unsatisfactory performance of single LiCoO2, LiNiO2 and LiMnO2, the ternary material LiNixCoyMn1-x-yO2 combines the high conductivity and performance of Co, the high specific capacity of Ni, and the strong stability and safety of Mn, effectively improving its performance,.
As shown in Fig. 5, the most representative cathode material of spinel structure is lithium manganate (LiMn2O4), which has the characteristics of low price, simple synthesis process and strong safety. LiMn2O4 (belonging to the Fd3m space group ) has low theoretical and practical specific capacities, and the capacity decays severely during high-temperature cycling because the Jahn-Teller effect occurs during charge-discharge, resulting in severe structural deformation, and due to the action of electrolyte, soluble Mn2+ is produced, which seriously affects the performance of LIBs, so it is not suitable for large-scale power equipment. However, studies have shown that LiMn2O4 can be protected by doping Al, Cr and other elements to stabilize its structure or coating the surface with oxides such as Al2O3 and Y2O3 to prevent direct contact with the electrolyte, thereby improving the performance of LIBs,.
The polyanion type (olivine type) cathode material has good stability and safety. As shown in Fig. 5, at present, the main commercial one is lithium iron phosphate (LiFePO4), which is also the most commonly used cathode material for energy storage systems. Its Li and Fe+2+ are paired with O2− to form different LiO6 and FeO6 octahedra and PO4 tetrahedra staggered to form a space skeleton structure. However, due to its own structure, Li transport is limited, resulting in low conductivity and poor performance of LiFePO+4. At present, LiFePO4 can be modified by doping, coating, and nanometerization.
The above common cathode materials have stable thermal safety when the temperature does not exceed 650 °C, and they can also be in a stable voltage state when charging. The main factors restricting the thermal safety of cathode materials for LIBs are as follows: a large amount of heat is generated under the condition of heat abuse; it releases oxygen or has strong oxidizability in case of overcharge. Both of these are necessary conditions for TR of LIBs, which will eventually lead to the decline of capacity and performance of LIBs.
2.3. Anode materials
The anode materials, as the carrier of lithium ions and electrons in the process of battery charge and discharge, plays the role of energy storage and release, and should meet : (1) low working potential; (2) good electrochemical stability; (3) higher specific capacity; (4) stable structure and charge-discharge platform; (5) higher electrical conductivity; (6) lower cost and high environmental protection. As shown in Fig. 6, anode materials for LIBs can be divided into: intercalation, alloy and conversion types according to the lithium storage mechanism.
Intercalation-type anode materials can store lithium by intercalating lithium ions into their structural gaps, and have good stability because they can remain reversible during the intercalation and deintercalation processes and their structure will not be destroyed. At present, typical intercalation anode materials mainly include carbon-based, titanium-based materials, and some layered metal compounds. Among them, carbon-based materials include graphite, soft, hard carbon, carbon nanomaterials (carbon nanotubes, graphene), etc.. As the earliest commercialized and most commonly used anode material, graphite has the characteristics of stable electrochemical performance, high tap density, abundant resources and low cost. However, due to the poor compatibility between graphite and electrolyte, a thicker SEI film will be formed with the electrolyte at the beginning of charge and discharge, causing Li to pass through the SEI film for a long time, resulting in poor battery performance. At present, it can be improved by surface coating, doping and other technologies. Titanium-based materials mainly include TiO+2, Li4Ti5O12, etc., which have been widely studied and developed due to their stable electrochemical performance, environmental protection and no pollution. Fig. 7 is a comparison chart of the advantages and disadvantages of common carbon-based materials.
Alloy-type anode materials store lithium through an alloying reaction with lithium ions, with ultra-high specific capacity and low charge-discharge platform, such as silicon-based, germanium-based, tin-based materials, etc. As the silicon-based materials with the highest specific capacity among the alloy-type anode materials, they have the characteristics of abundant resources, green environmental protection, and low price. However, due to theirs huge volume expansion during the charging and discharging process, the silicon particles are crushed, fallen off, and the structure collapses. It leads to irreversible loss of capacity, exposes silicon particles to electrolyte, forms a new SEI film, hinders the conduction of lithium ions, and seriously affects the performance of the battery. At present, scientists have improved theirs electrochemical performance through schemes such as silicon nanoization  and silicon-based material composite. Germanium-based, tin-based materials and other alloy-type anode materials also have the problem of drastic volume changes during charging and discharging, resulting in too few practical applications, which can also be modified by certain means.
Conversion-type anode materials store lithium through reversible redox reactions with lithium ions, and have the advantages of higher specific capacity than intercalation-type anode materials, and smaller volume changes during charge and discharge than alloy-type anode materials. Such materials are mainly transition metal compounds, such as oxides, sulfides, fluorides, phosphides, etc.. However, most of the transition metal compounds have low conductivity, low first coulomb efficiency, large and obvious volume change, and high working potential, which greatly affect the performance of LIBs, resulting in shorter life and lower efficiency. At present, most of the current modifications are made by nanosizing the oxide.
The thermal safety of anode materials is related to their type, particle size and the stability of SEI film formed by the cathode. If the size particles are made into cathode materials according to a certain proportion, the purpose of expanding the contact area between particles, reducing electrode impedance, increasing electrode capacity and reducing the possibility of active metal precipitation can be achieved. The quality of SEI film directly affects the charging and discharging performance and safety of LIBs. The main methods to improve the quality of SEI film are: weak oxidation of carbon material surface; carbon materials after reduction, doping and surface modification; use spherical or fibrous carbon materials. The safety of anode materials can be improved by reducing the specific surface area of anode materials and improving the thermal stability of SEI films.

翻译考点：
考点1. "Power Converter System"必须译为"功率转换系统"
考点2. "small self discharge"推荐译为 "自放电率低"
考点3. "delayed power consumption" 推荐译为"延迟用电"
考点4. "standby power supply" 推荐译为"待机电源"
考点5. "frequency regulation" 必须译为"调频控制"
考点6. "thermal abuse" 必须译为"热失控"
考点7. "severe structural deformation" 推荐译为"晶格畸变"