Lithium thionyl chloride battery, a new choice for wireless sensor network power supply
For the power supply of wireless sensor network systems for monitoring the deterioration of buildings and farmland environment, there is a battery that can be used for 10 years with minimal power consumption, and that is lithium thionyl chloride battery. This means that the lithium thionyl chloride battery has less natural discharge electrodes, and it can still be used normally even after 10 years of storage. In general, most batteries will naturally discharge when there is no load, and will be unusable after a few years. Moreover, the price of this battery is cheaper than energy harvesting equipment that uses heat, vibration, and light energy to generate electricity.
"Currently, lithium thionyl chloride batteries are mainly used as the power source of the wireless sensor network." The wireless sensor network lays out sensors in a wide range and sends environmental data such as temperature wirelessly. As its power source, energy harvesting equipment that does not require battery replacement is a good choice.
Nonetheless, the reason is: "Energy harvesting equipment uses heat and vibration to generate electricity, so sometimes there is uneven power generation, which is inconvenient to use. Especially for industrial purposes, reliable power must be used when needed. Many uses All require the use of batteries that can provide quantitative performance indicators at the design stage." Of course, there are also applications suitable for the use of energy harvesting equipment. However, industry experts say that if the power consumption of the module can be reduced, long-life batteries can meet the demand for many purposes. For system designers who hate the instability of the natural environment and bring uncertainty, cheap and used batteries are a realistic choice.
Use the coating on the electrode surface to suppress natural discharge
The reason why lithium thionyl chloride batteries are favored among several types of batteries is the small natural discharge capacity of this type of battery. According to reports, if stored for 10 years when not in use at all, this battery can retain most of the initial capacity at room temperature. "The annual natural discharge is only about 0.5% of the initial value at room temperature. Although the discharge will vary with conditions, the natural discharge is less than that of a manganese dioxide lithium battery whose annual power is reduced by 2%." The reason for the low natural discharge of lithium thionyl chloride batteries is that the natural reaction between the positive and negative electrodes inside the battery is difficult to proceed smoothly.
Generally speaking, a battery generates electric current by connecting a load and moving electrons from negative to positive. In lithium thionyl chloride batteries, electrons generated after ionization of lithium (Li) in the negative electrode flow to the positive electrode via a load. In the positive electrode, thionyl chloride (SOCl2) that has accepted electrons will chemically react with lithium ions to become lithium chloride and other substances (LiCl, S, SO2). Therefore, lithium ions move between the two electrodes through the separator inserted between the positive electrode and the negative electrode inside the battery. That is to say, the energy of the lithium thionyl chloride battery is based on the chemical reaction of lithium and thionyl chloride into lithium chloride. Chemical energy.
Therefore, if the chemical reaction does not proceed smoothly, it will become a "battery dead" state. Natural discharge is a phenomenon in which electrons generated by the negative electrode move to the positive electrode through the battery and undergo a chemical reaction. If there are few chemical reactions inside the battery, the natural discharge rate will be very slow. In most dry batteries, although the separator inserted between the positive electrode and the negative electrode hinders the movement of electrons, in fact the electrons still move slowly, causing natural discharge. In lithium thionyl chloride batteries, when there is no load, a lithium chloride coating is formed on the lithium surface of the negative electrode, which inhibits the chemical reaction (ionization) between lithium and electrons. Therefore, this battery has less natural discharge than other batteries.
The output voltage is 3.6V, which can be kept at a certain level until the end
Currently, companies selling lithium thionyl chloride batteries have published graphs on storage characteristics on the Internet, and advertised that the product can be stored for 100,000 hours (about 11 years) at 20°C. The most used applications are backup power supplies such as semiconductor memory, and are also used in MCU meters such as gas and tap water.
The characteristic of the lithium thionyl chloride battery is that the output voltage (nominal voltage) reaches 3.6V, the highest among the disposable batteries that have been commercialized. Even if it continues to discharge, it can maintain a basic voltage before the power is exhausted. However, it is difficult for this kind of battery to obtain a large current, and the voltage will drop in a short period of time when the current is first passed, so this situation needs to be considered when designing the power supply.