Advantages and disadvantages of lithium thionyl chloride batteries

Advantages and disadvantages of lithium thionyl chloride batteries

Li/SOCl2 batteries are manufactured in a wide variety of sizes and configurations. The capacity from 400mAh cylindrical charcoal and wound electrode structure batteries to 10000Ah square batteries and many others to meet special requirements. The Li/SOCl2 system originally has safety and voltage delay problems. The safety problem was particularly likely to occur at high rate discharge and over discharge. And the voltage delay occurs when the battery continues to be discharged at low temperature after storage at high temperature.

Low discharge rate batteries are mainly used for backup voltage of memory and other applications requiring long working life. Large square batteries have been used for military applications as an emergency application power source. And medium and high discharge rate batteries have also been developed for powering various electrical and electronic devices. In some of the batteries, thionyl chloride and other oxyhalide electrolytes are usually added with additives to improve the battery specific performance.

Chemical Principle

The Li/SOCl2 battery consists of lithium negative electrode, carbon positive electrode and a non-aqueous SOCl2:LiAlCl4 electrolyte. Thionyl chloride is both electrolyte and positive electrode active material. Other electrolyte salts like LiAlCl4 have been used in specially designed batteries.
The general reaction mechanism is:

Sulfur and sulfur dioxide are dissolved in an excess of thionyl chloride electrolyte, and during discharging, a certain amount of pressure is generated due to the production of sulfur dioxide. During storage, the lithium negative electrode reacts with the thionyl chloride electrolyte to form LiCl upon contacting with the electrolyte. The lithium negative electrode is protected by the LiCl film formed thereon. This passivation film is beneficial for prolonging the storage life of the battery but causes a voltage delay at the beginning of discharge.After long-term storage at a high temperature and then discharged at a low temperature, the battery voltage hysteresis is particularly remarkable.

Li/SOCl2 batteries working principles

Li/SOCl2 batteries use lithium as the negative electrode and carbon as the positive electrode. SOLi2 used as the electrolyte and the positive electrode active material. Polypropylene felt or fiberglass paper is used as the separator. The open circuit voltage is 3.65V. The battery system can be expressed by the following formula:

Negative electrode: 4Li=4Li+ +4e
Positive electrode: 2SOCL2 +4e=2SO2 +4Cl- 2SO→←(SO)2 (SO)2→←S+SO2
Total battery response:
4Li + 2SOCl2→4LiCl + S + SO2 SO2
Dissolved in SOCl2, S is precipitated in a large amount and deposited in the positive carbon black, and LiCl is insoluble.
When Li contacted with SOCl2, the following reaction occurs:
8Li + 4SOCl2 →6LiCl + Li2S2O4 + S2Cl2 or 8Li + 3SOCl2 →6LiCl + Li2SO3 +2S

Due to this reaction, although the positive active material SOCl2 of the Li/SOCl2 battery closely surrounds the negative electrode, the short circuit phenomenon does not actually occur because a very thin dense LiCl protective film is formed on the surface of the negative electrode ( Primary film). This film is electronically insulating and can penetrate ions, thus preventing the external reaction of SOCl2 and lithium, making lithium become very stable in SOCl2 electrolyte. With the increase of ambient temperature and storage time of the battery is prolonged, the film will gradually enlarge and become thicker to form a so-called secondary film, so the battery has a good storage life.

As a result, the Li/SOCl2 battery has a relatively severe voltage hysteresis, which causes the voltage to recover to 95% of the peak voltage in a matter of minutes. The Li/SOCl2 battery stored at 25 °C for two years, due to the LiCl passivation layer formed on the surface of the lithium, the initial voltage is low, such as battery short circuit or multiple high-current brake discharge, the LiCl film can be broken, so that the working voltage restore. The products LiCl (white) and S (yellow) are deposited in the positive carbon black, partially blocking the micropores in the positive electrode. On the one hand, the positive electrode is somewhat expanded, on the other hand, the diffusion of the electrolyte is hindered, the concentration polarization is increased, and the battery is gradually failed.

Advantages of Li/SOCl2 batteries:

Higher energy density and lower weight.
High and stable load voltage
Superior drain capacity.
Low self-dischage rate (less than 1% after 1 year of storage at 20 °C)
Stainless steel container.
Hermetic glass-to-metal sealing.
Non-flammable electrolyte.
Laser weilding.

Disadvantages of Li/SOCl2 batteries:

1, Voltage delay: when discharged with a larger current after long-term normal temperature storage, the operating voltage drops sharply, and then slowly returns to normal.
2, Safety issues: despite some measures, an uncontrollable heat eruption occurs at high temperature discharge and explode.
3, High price
4, Environmental pollution: SOCl2 is decomposed into hydrochloric acid and sulfur dioxide after water absorption, which is extremely corrosive, so the production site must be well ventilated.

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