Electrochemical energy storage, which stores energy dissolved in liquid electrolytes, has become a focus of attention in recent years and has found many practical applications. Currently, electrochemical energy storage mainly includes various secondary batteries, such as lead-acid batteries, lithium-ion batteries, sodium-sulfur batteries, and flow batteries, most of which are technologically mature.
1. Lead-acid batteries
Lead-acid batteries are among the most widely used batteries in the world. In a lead-acid battery, the anode (PbO2) and cathode (Pb) are immersed in an electrolyte (dilute sulfuric acid), generating a potential of 2V between the two electrodes. This is the principle behind lead-acid batteries.
Lead-acid batteries are commonly used as emergency or backup power sources in power systems, and were previously the primary source of power in most stand-alone photovoltaic (PV) systems. However, they are gradually being replaced by other battery types, such as lithium-ion batteries.
2. Lithium-ion batteries
A lithium-ion battery is actually a lithium-ion concentration cell, with the positive and negative electrodes composed of two different lithium-ion intercalation compounds. During charging, Li+ ions are deintercalated from the positive electrode, pass through the electrolyte, and intercalate into the negative electrode. At this time, the negative electrode is in a lithium-rich state, and the positive electrode is in a lithium-poor state. During discharging, the opposite occurs: Li+ ions are deintercalated from the negative electrode, pass through the electrolyte, and intercalate into the positive electrode. The positive electrode is in a lithium-rich state, and the negative electrode is in a lithium-poor state.
Due to its use in electric vehicles, computers, mobile phones, and other portable and mobile devices, lithium-ion batteries have become the most widely used batteries in the world.
3. Sodium-sulfur batteries
The anode of a sodium-sulfur battery consists of liquid sulfur, and the cathode consists of liquid sodium, separated by a beta-aluminum tube made of ceramic material. The battery needs to be operated at a temperature above 300°C to keep the electrodes in a molten state.
4. Vanadium redox flow battery
In a flow battery, energy is stored in electroactive species dissolved in a liquid electrolyte, which is stored in a tank outside the battery. The electrolyte stored in the tank is pumped into the battery stack and, through electrodes and a thin film, electrical energy is converted into chemical energy, or vice versa.
The power and energy of a battery are independent of each other; the energy stored depends on the size of the storage tank, thus it can store energy for hours to days, with capacities reaching the MW level, making it suitable for use in power systems.
