What are the advantages of lithium-ion batteries in energy storage ?

Energy is an important material basis for human survival and social development, and an important cornerstone for the national economy, national security and sustainable development. With the development of human society, human beings' demand for energy is increasing, but the ecological environment is deteriorating, especially the greenhouse gas emissions leading to increasingly severe global climate change. In recent years, this contradiction has become more severe. At present, China has become a major energy producer and consumer in the world. China's demand for energy continues to grow. Therefore, it is urgent to adjust the energy structure: on the one hand, we must develop new energy to meet demand, on the other hand, we must use it reasonably and effectively. renewable energy.

Renewable energy sources include: wind energy, solar energy, biomass energy, ocean energy and small hydropower. It is a primary energy source and is usually converted into electricity. In the process of developing and utilizing renewable energy, electric energy storage technology plays an important role. As we all know, there is discontinuity and instability in the use of wind energy and solar energy. It is necessary to go through the energy storage system and then re-enter the network. At the same time, the wind energy generator system with off-grid power generation mode is also essential. In the process of using energy, there is an imbalance in use. The energy storage system can be used for “peak clipping” of the power grid to improve energy utilization. In order to promote the large-scale utilization of renewable energy power generation, improve the efficiency of alternative energy power stations and maintain national energy security, it is of great economic and social significance to study energy storage technology. Industrialized countries attach great importance to the research and development of large-scale energy storage systems. For example, the Japanese government's “New Sunshine Plan”, the US “DOE Project Plan” and the EU “Framework Plan” all focus on energy storage technology. China also attaches great importance to the development of energy storage technology. High-efficiency energy conversion and energy storage technology has been listed as the priority development technology field of the National Torch Program in the future. The China Energy Storage Battery Industry, Education and Research Technology Innovation Alliance was established in November 2009. In addition, according to the “Key Points for the Development of New Energy and Renewable Energy Industry 2000-2015” issued by the State Economic and Trade Commission, by 2015, the annual production capacity of small wind turbines will reach 50,000 units, and the total output will reach 340,000 units. The total installed capacity is 105,000 kW, and the total output value is about 900 million yuan. The sales of energy storage systems should exceed 100 million yuan, which will definitely drive the development of the energy storage industry.

Driven by strong social development needs and huge potential markets, new energy storage systems based on new concepts, new materials and new technologies are emerging. Energy storage technology is developing in the direction of large-scale, high-efficiency, long-life, low-cost, and non-polluting.

I. Classification and development trend of energy storage technology

So far, various energy storage technologies have been proposed and developed to meet the needs of different fields and different needs. Global energy storage technologies mainly include physical energy storage, chemical energy storage (such as sodium-sulfur batteries, all-vanadium flow batteries, lead-acid batteries, lithium-ion batteries, supercapacitors, etc.), electromagnetic energy storage and phase change energy storage.

Physical energy storage

Physical energy storage technologies mainly include pumped storage, compressed air energy storage, and flywheel energy storage. Compared with chemical energy storage, physical energy storage is more environmentally friendly and green, using natural resources to achieve. Pumped Storage Power Station (PSH, Pumped Storage Hydroelectricity) is equipped with two reservoirs upstream and downstream. When the load is low, the equipment works in the motor state, and the water in the downstream reservoir is pumped to the upstream reservoir for storage. When the load is high, the equipment works in the state of the generator. Use the water stored in the upstream reservoir to generate electricity, as shown in Figure 1. Due to the maturity of technology, pumped storage power stations have become the most widely used energy storage technology in power systems. At present, the installed capacity of pumped storage power stations in China is about 11,400 MW. It is estimated that the total installed capacity of pumped storage power stations will reach 17500 MW by the end of 2010.

Compressed Air Energy Storage (CAES) is a gas turbine power plant for peak shaving. It mainly uses the remaining power compressed air when the grid load is low, and stores it in a high-pressure sealing facility with a typical storage pressure of 7.5 MPa. The gas peak is released to drive the gas turbine to generate electricity. The world's first commercial CAES power station was a Huntdorf power station built in Germany in 1978 with an installed capacity of 290 MW and a conversion efficiency of 77%. Up to now, it has been launched more than 7,000 times, mainly for hot standby and smooth load. Compared with pumped storage power stations, CAES power stations are flexible in location. They do not need to build surface reservoirs, and the terrain conditions are easy to meet. At present, compressed air storage power stations have been widely used in some developed countries.

Flywheel energy storage (FW, FlyWheels) is achieved by the mutual conversion of mechanical energy and electrical energy to achieve charge and discharge. It uses a high-speed rotating flywheel core as a medium for mechanical energy storage, using electric/generators and energy conversion control systems to control energy input and output. Flywheel energy storage has high requirements on the raw materials and technology for making flywheels. It was not developed until the 1990s for uninterruptible power supply (UPS)/emergency power supply (EPS), power grid peaking and frequency control. Our country’s research in this area has just begun.

Physical energy storage, such as pumped storage and compressed air storage, has the advantages of large scale, long cycle life and low operating cost, but requires special geographical conditions and sites. The construction has limited limitations and the one-time investment cost is high. Not suitable for off-grid power generation systems with lower power. From the perspective of development level and practicality, chemical energy storage has broader application prospects than physical energy storage.

2. Chemical energy storage - Lithium-ion battery energy storage is currently the most feasible technical route

Lead-acid batteries are the oldest and most mature chemical energy storage methods. They have been used for more than 100 years and are widely used in automotive start-up power supplies, electric bicycle or motorcycle power supplies, backup power supplies and lighting power supplies. Lead-acid battery electrodes are mainly made of lead and its oxide, and the electrolyte is a sulfuric acid solution. When charging, the main component of the positive electrode is lead dioxide, and the main component of the negative electrode is lead. When discharging, the main components of the positive and negative electrodes are lead sulfate. The lead-acid battery has good reliability, easy availability of raw materials and low price, but its optimal charging current is about 0.1C, the charging current cannot be greater than 0.3C, and the discharge current is generally required to be between 0.05 and 3C. It is difficult to meet both power and capacity. A large-scale power storage requirement. At the same time, the lead-acid battery can not be deeply charged and discharged, and the life of the battery is very affected under the condition of 100% discharge (the cycle life of the battery is less than 300 times under full charge and discharge conditions), and the water at the end of the charge will be decomposed into hydrogen and oxygen gas. It is necessary to add acid and water regularly, and the maintenance work is heavy, so it is not suitable for application in the smart grid field.

The chemical power sources currently available in the field of smart grids mainly include sodium-sulfur batteries, flow batteries and lithium-ion batteries.

The sodium-sulfur battery (NaS) was first invented by Ford in 1967. It uses sodium metal as the negative electrode, sulfur as the positive electrode, and ceramic tube as the electrolyte separator. At a certain working temperature, sodium ions pass through the electrolyte membrane and reversibly react with sulfur to form energy release and storage, as shown in Figure 2. Sodium-sulfur battery has higher specific energy (theoretical specific energy up to 760Wh/kg), high current charge and discharge, and long service life (10-15 years). It is one of the most economical and practical energy storage methods. The main application target is power station load. Leveling, UPS emergency power supply and instantaneous compensation power supply. The current leading country for sodium-sulfur battery technology is Japan. As of 2007, Japan's annual production of sodium-sulfur batteries has exceeded 100MW. In 2008, Japan's second wind power station introduced NGK's 17 sodium-sulfur battery systems with a storage capacity of 34 MW, successfully suppressing the power variation of wind power plants with a maximum power of 51 MW, and achieving planned power output. It provides the basis for grid-connected power generation of wind power. In 2009, China's Shanghai Silicate Research Institute successfully developed the key technology of 100kW class, becoming the second country in the world to master the core technology of large-capacity sodium-sulfur monomer battery after Japan. The sodium-sulfur battery developed is shown in Figure 3. Show. However, sodium-sulfur batteries require a high temperature of 350 ° C to melt sulfur and sodium, and additional heating equipment is required to maintain the temperature, and it is dangerous when overcharged, so there are disadvantages in terms of safety and maintenance-free.

The study of all vanadium redox flow batteries began in 1984 at the Skyllas-kazacos research team at the University of New South Wales, Australia. It is a redox renewable fuel cell energy storage system based on metal vanadium. The schematic diagram of its working principle is shown in Figure 4. The flow battery uses a proton exchange membrane as a separator of the battery pack, and the electrolyte solution flows in parallel through the surface of the electrode and electrochemically reacts, and the chemical energy stored in the solution is converted into electric energy by collecting and conducting current through the double electrode plate. The rated power and rated capacity of the liquid energy storage battery system are independent of each other. The power depends on the battery stack. The capacity depends on the electrolyte. The battery capacity can be increased by increasing the amount of electrolyte or increasing the concentration of the electrolyte. The liquid achieves "instant recharge". The theoretical storage period of the flow battery is unlimited, the storage life is long, there is no self-discharge, and it can be 100% deep discharged without damaging the battery. These characteristics make the flow battery one of the preferred technologies for energy storage technology. At present, liquid energy storage technology has been applied in developed countries such as the United States, Germany, Japan and the United Kingdom, and China is still in the research and development stage. The difficulty of the all-vanadium redox flow battery is that the total vanadium ion concentration generally used is less than 2 mol/L, resulting in a specific energy of only 25-35 Wh/kg. The electrolyte storage tank is large and difficult to manage, and the pentavalent vanadium in the positive electrode liquid is When standing or the temperature is higher than 45 ° C, the precipitation of vanadium pentoxide is easily precipitated, which affects the service life of the battery.

In comparison, lithium-ion battery energy storage is currently the most feasible technical route in the development of energy storage products. Lithium-ion batteries are called green batteries because of their high energy density, small self-discharge, no memory effect, wide operating temperature range, fast charge and discharge, long service life and no environmental pollution. Table 1 is a comparison of lead-acid batteries, sodium-sulfur batteries, flow batteries and lithium-ion batteries with lithium titanate as the negative electrode. It can be seen that the service life of lead-acid batteries is short, and the disadvantage of sodium-sulfur batteries is that the operating temperature is lower. High, flow batteries have lower energy density, while lithium ion batteries with lithium titanate as the negative electrode show a comprehensive performance advantage. Fig. 5 is a schematic view showing the working principle of a lithium ion battery using lithium titanate as a negative electrode.

Since lithium titanate is a zero strain material, structural damage can be avoided due to the back and forth expansion of the electrode material, thereby greatly improving the service life of the lithium ion power battery; and since the lithium titanate has a high operating potential, even overcharge It is also difficult to form lithium dendrites on the negative electrode, thereby greatly improving the safety of the lithium ion power battery. These improvements make the application of lithium-ion power batteries in the field of energy storage possible. At present, lithium-ion battery energy storage technology with lithium titanate as the negative electrode is becoming a hot spot for competition at home and abroad. In 2008, Altairnano Company of the United States developed a 1MW lithium titanate energy storage battery system. After trial operation, it can output 250kWh of energy, and the energy conversion efficiency is greater than 90%. In 2010, Toshiba announced that it will develop a super lithium battery (SCiB) for energy storage using lithium titanate anode materials at its annual business policy meeting. With the successful commercialization of high-power SCiB lithium titanate battery, Toshiba's SCiB is expected. Energy storage batteries will soon be available to the market. After five years of technology development, China CITIC Guoan Mengli Power Technology Co., Ltd. developed a 35Ah battery for energy storage applications in 2010.

The cycle life of the battery is close to 8000 times, it can be charged and discharged at 5C rate, and the safety performance is excellent. At present, the company is working with the cooperation unit to develop the megawatt energy storage system. It is expected that the product can be sold to the market in 2011.

In addition to the lithium ion battery with lithium titanate as the negative electrode, it can be used in the field of energy storage. With the application of lithium iron phosphate cathode material, the life and safety of the traditional carbon negative lithium ion battery have also been greatly improved. Can be applied to the field of energy storage. In 2010, Sony introduced a 1.2kWh lithium iron phosphate energy storage battery module with a maximum output power of 2.5kW. However, there is still a serious problem of consistency in the lithium iron phosphate battery. Even if the life of the single battery can reach 2000 times or more, the life of the battery after the group is greatly reduced, and the core patent of the lithium iron phosphate material is controlled by some international companies. In the hands, the production of lithium iron phosphate batteries will face patent disputes. Therefore, the current use of lithium titanate lithium ion batteries for energy storage in lithium ion energy storage battery products should be the most feasible technical route.

3. Other energy storage technologies

The superconducting magnetic energy storage is to convert electrical energy into magnetic energy and store it in the magnetic field of the superconducting coil, and realize the charging and discharging of the energy storage device through electromagnetic mutual conversion. Since the coil has no resistance in the superconducting state, the energy loss of superconducting energy storage is very small. However, since the superconducting state requires the coil to be at a very low temperature, the low temperature requires a large amount of energy and is not easy to be miniaturized, so the technology is in the research and development stage.

Phase change energy storage is the use of certain substances to absorb or release energy through phase change at a specific temperature, such as ice storage, water storage and energy storage. It can be applied to central air conditioning and other fields, and is an emerging energy storage technology.

Second, the market prospects of energy storage technology - lithium-ion batteries will become an ideal choice

According to the data of the Wind Energy Professional Committee of the China Renewable Energy Society, in 2009 China (excluding Taiwan Province) accumulated wind power installed capacity of 25805.3MW. Then, according to the research and calculation of Guodian, China's energy storage industry contains about 5161~7742MW market. By 2020, China's wind power and solar installed capacity will reach 10 million kilowatts, the market for energy storage batteries will reach 70 billion yuan, and energy storage products will become the most valuable market area for investment and capital in the future.

Lithium-ion batteries are one of the most important achievements in high-tech research in the past 10 years and represent the most advanced level of chemical power development. Due to its remarkable advantages such as high specific energy, long cycle life and environmental friendliness, this new system has become the main supporting power source for all kinds of advanced portable electronic products. It has absolute advantages in mobile applications. Currently, the global year of lithium ion batteries With a demand of 1.3 billion, with annual sales of $27 billion, it is undoubtedly one of the leaders in the rechargeable battery market. With the development of new materials for lithium-ion batteries, the innovation of battery-making technology, and the participation of many scientific research institutions and enterprises, the performance of lithium-ion batteries is increasing, the cost of batteries is decreasing, and the safety performance of batteries is greatly improved. Batteries are gradually showing application advantages in the field of electric vehicles. Japan's Fuji economy believes that lithium-ion batteries will gradually replace nickel-metal hydride batteries in 2011, and lithium-ion batteries as the mainstream technology route in the future is beyond doubt. With the development and application of new materials such as nano-titanate lithium and lithium iron phosphate, lithium-ion batteries will become an ideal choice for a series of major high-tech applications such as clean transportation and photovoltaic energy storage. At present, China State Grid Corporation is actively developing 10MW. Test project for a lithium-ion battery energy storage system, which will trigger related manufacturing equipment