Battery Storage Technology

Introduction

        Driven by the vigorous development of global renewable energy generation, electric vehicles, and emerging energy storage industries, various types of energy storage technologies have made significant progress in recent years. In addition to the already commercialized pumped hydro storage and cavern compressed air energy storage technologies, battery energy storage technologies, led by lithium-ion batteries, have shown initial commercial application potential on the power generation, grid, and load sides.

        Battery energy storage technology realizes the storage and output of electric energy through the conversion between electric and chemical energy. It not only boasts technical characteristics such as rapid response and bidirectional regulation but also exhibits technical advantages of strong environmental adaptability, small-scale and decentralized configuration, and short construction periods. It has overturned the traditional concept of source-grid-load and broken the inherent attribute of simultaneously completing all links of power generation, transmission, distribution, and utilization. It can play different roles and functions on the power side, grid side, and user side of the power system. By the end of 2018, the installed capacity of global battery energy storage technologies reached 6058.9 MW, with China's installed capacity at 1033.7 MW, ranking third after the United States and South Korea.

        This article, combining with the actual industrial development, analyzes the technical level, market applications, problems and challenges, as well as future development trends of major battery energy storage technologies, providing a multi-dimensional perspective and basic data for the development of battery energy storage technologies.

1 Typical Battery Energy Storage Technologies

Battery energy storage technologies primarily include lead-acid batteries, lithium-ion batteries, flow batteries, sodium-based batteries, and other types of battery energy storage technologies, with subcategories illustrated in Figure 1.

Figure 1 Classification of Commercialized or Demonstrated Battery Energy Storage Technologies

1.1 Lead-Acid Batteries

Lead-acid batteries used in energy storage projects include lead-acid batteries and lead-carbon batteries. Lead-carbon batteries, which have undergone capacitive improvements to the negative electrode material based on traditional lead-acid batteries, combine the advantages of both lead-acid batteries and supercapacitors. The addition of carbon materials prevents sulfation of the negative electrode, significantly enhancing the battery's cycle life. The cycle life has increased from 500-1000 times (60%-70% DOD, Depth of Discharge) for lead-acid batteries to 3700-4200 times (60%-70% DOD) for lead-carbon batteries. The investment cost of the energy storage system is 1000-1300 yuan/kWh, and the levelized cost of energy (LCOE) is 0.5-0.7 yuan/kWh. In recent years, the application of lead-acid batteries in the energy storage field has mostly focused on lead-carbon batteries with lower LCOE, especially in regions such as Jiangsu, Guangdong, and Beijing, where the peak-to-valley price difference for industrial and commercial electricity is high, and where conditions for commercial application have been preliminarily established.

1.2 Lithium-Ion Batteries

A wide variety of lithium-ion batteries are used in energy storage projects, including polymer lithium batteries, lithium manganate batteries, and lithium titanate batteries, which were predominantly put into operation from 2011 to 2015, as well as lithium iron phosphate batteries, ternary lithium batteries, and repurposed lithium batteries, which have developed rapidly in recent years. From the perspectives of one-time investment cost, cycle life, and safety, lithium iron phosphate batteries are undoubtedly the most superior lithium-ion battery energy storage system in the energy storage field, widely used in all links of power generation, transmission, distribution, and utilization in the power system. Lithium iron phosphate batteries have the advantages of high stability and long cycle life, making them a popular and widely used lithium-ion battery technology in domestic power energy storage systems. The energy density of lithium iron phosphate batteries for energy storage is 120-150 Wh/kg, with a system energy conversion efficiency of 85%-88%. The cycle life for low-rate charging and discharging is 3500-5000 times. The investment cost of the energy storage system is 1600-2000 yuan/kWh, and the LCOE is 0.7-1.0 yuan/kWh. In recent years, due to the decrease in the cost and improvement in comprehensive performance of lithium iron phosphate batteries, this technology has been widely applied in all links of power generation, transmission, distribution, and utilization in the power system.

1.3 Sodium-based Batteries

Sodium-based batteries applied in energy storage projects include high-temperature sodium-sulfur batteries, sodium-nickel batteries, and aqueous sodium-ion batteries operating at room temperature. Sodium-sulfur batteries, a typical representative of sodium-based batteries, are the most mature energy storage technology in high-temperature operation systems (350-400°C). Led by Japanese companies like NGK, over 430 MW of energy storage projects have been implemented in countries such as Japan, the United States, the United Arab Emirates, Germany, Italy, and France before 2015. However, in September 2011, a fire caused by a sodium-sulfur battery (manufactured by NGK) at Mitsubishi Materials Tsukuba Plant in Ibaraki, Japan, lasted for two weeks. Additionally, the high technical barriers of solid ceramic electrolytes and the concentration of core intellectual property rights in a few enterprises like NGK have led to severe intellectual property blockades, slow industrial progress, and stagnation in market applications in recent years. Sodium-nickel batteries, a relatively mild high-temperature battery system, use nickel chloride instead of sulfur in the cathode. Companies like GE Energy Storage and FIAMM Energy Storage Solutions implemented approximately 19 MW of energy storage projects in the United States and Italy from 2011 to 2014. Aqueous sodium-ion batteries, operating at room temperature with aqueous solutions as electrolytes, were gradually introduced into the small-capacity distributed and microgrid energy storage markets by Aquion Energy in 2013. In 2017, China's Titan Energy Tech Group acquired Aquion Energy, shifting its business to China. Recently, driven by the emerging global energy storage market, high-safety, potentially low-cost, and environmentally friendly water-based energy storage systems have attracted significant attention. Titan Energy Tech Group and Enly Energy Technology Co., Ltd. have introduced their aqueous ion battery energy storage products into the market. Additionally, the ternary sodium-ion batteries of the Institute of Physics, Chinese Academy of Sciences, and Zhongju Battery Co., Ltd. have entered the battery module development stage, while companies like Contemporary Amperex Technology Co., Limited (CATL) and Ruihaipo (Qingdao) Energy Technology Co., Ltd. are actively deploying aqueous batteries (including aqueous sodium-ion batteries and aqueous zinc-lithium batteries).

1.4 Second-life Lithium-ion Batteries

Second-life lithium-ion batteries primarily refer to large-scale electric vehicle traction lithium-ion batteries retired after reaching 80% of their initial capacity. After retirement, these batteries can be reused in some energy storage applications through sorting, recombination, and integration.

Currently, China's second-life lithium-ion batteries mainly consist of lithium iron phosphate batteries. As high-energy-density ternary lithium-ion batteries are widely adopted, they will gradually enter the second-life market. Considering the significant dispersion and unpredictability of the state parameters of lithium-ion batteries retired after 80% capacity, the integration design of second-life lithium-ion batteries is challenging, and their applications are mainly focused on small and decentralized scenarios, such as backup power for communication base stations, peak shaving and valley filling at terminals, and small-scale photovoltaic energy storage configurations.

1.5 Other Types of Batteries

Apart from the aforementioned battery energy storage technologies, there are also supercapacitors, nickel-based batteries, and zinc-air batteries. Supercapacitors belong to power-type energy storage technologies. In application scenarios dominated by frequency regulation and capacity-type energy storage demands, their low energy density and short charge-discharge time limit their application in energy storage. Zinc-air batteries are currently the only type of air battery energy storage technology applied in energy storage projects, with notable industry players like EOS Energy Storage and Fluidic Energy. Zinc-air batteries are primarily energy-type products with a discharge duration of more than 2 hours. According to reports, the cost of a 1 MWh energy storage system is approximately 200 $/kWh (equivalent to approximately 1371 yuan/kWh), which is comparable to lead-carbon battery energy storage systems. However, zinc-air batteries suffer from overly complex system designs, low production automation, and relatively low system efficiency (below 75%).