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Lithium thionyl chloride: “A primary battery is a matter of trust”
Lithium thionyl chloride: “A primary battery is a matter of trust”
If they were to produce a podcast, it would probably be called “Batterietechnologisches Herrengedeck”: Kurt Korn and Robert Pilz are business partners. Operating at the interface between “distributor” and “manufacturer”, they have worked closely together for many decades. Collectively, they boast almost a century of experience and expertise in the battery sector. Kurt Korn is a technical sales representative for battery solutions at Omnitron Griese GmbH. Robert Pilz is enjoying an active retirement, but is still regarded in the industry as a proven expert in primary cells. In the late 1970s, he introduced new lithium technologies – including primary cells based on lithium thionyl chloride – to the German market. Together, the two battery specialists answer our questions about lithium thionyl chloride cell chemistry in a joint interview:
Is lithium thionyl chloride the best cell chemistry for a primary battery?
Kurt Korn: “The best cell chemistry does exist – but only ever for a specific application. Cells based on lithium thionyl chloride are ideal when the application does not require high currents and recharging is not possible. They have the highest energy density of any primary cell and are extremely long-lasting at very low currents. Their self-discharge rate is less than one per cent. They do not leak and are extremely temperature-resistant. Consequently, lithium thionyl chloride cells can also be stored safely for extremely long periods. However, there is one particular point to bear in mind in the event of prolonged inactivity.”
What needs to be considered when storing lithium-thionyl chloride cells?
Robert Pilz: “The challenge with lithium-thionyl chloride cell chemistry lies in its passivation layer. This forms on the lithium surface during storage and prevents the cells from self-discharging. The initial energy released when the battery is put into operation breaks down this passivation layer – which, however, leads to a significant drop in voltage. It must therefore be ensured that the voltage never falls below the minimum voltage required to start the application. A real-world example illustrates this requirement: some time ago, product developers working on basal thermometers extensively tested a lithium thionyl chloride battery in their laboratory and found it to be adequate. The company subsequently ordered thirty thousand of these Li-SOCl₂ batteries and installed them in the clinical thermometers used to measure fertile days. They fitted them with an ‘end-of-life’ indicator set at 2.5 volts and sealed them permanently. After several months in storage, the devices displayed ‘end of life’ when switched on and shut down irreversibly. Yet the cells still had a residual capacity of more than 99 per cent. However, due to the design-in fault, this capacity could no longer be utilised. The entire batch was lost. The reason: During laboratory testing, the battery was never stored. Consequently, it had not developed a passivation film and its voltage always remained above the required 2.5 volts during the tests.”
Kurt Korn: “During the design-in phase, we can easily ensure that the required minimum voltage of a lithium-thionyl chloride battery is maintained throughout its entire service life. This is achieved via the electronics, which occasionally draw small amounts of current using indication pulses to maintain the passivation at the desired level. This is important for all applications that are either stored for a long period or remain dormant for months, such as an emergency torch. As battery developers, it is therefore important to have a detailed understanding of the complete current and temperature profile over the entire service life of the application, as well as the requirements placed on the devices. Only in this way can we calculate the relevant values, decide for or against a particular cell chemistry and, where necessary, incorporate control mechanisms. The clean design of the battery is crucial to the function and quality of the product.”
Why is lithium thionyl chloride considered an absolute niche application?
Kurt Korn: “Their unique properties often make lithium thionyl chloride cells a product for specialised applications and specific tasks. The high energy density generates a lot of power even in small battery sizes. They are also extremely mechanically stable and can withstand a very wide temperature range.”
Robert Pilz: “I’ll never forget a visit to a Formula 1 factory. One of the team’s developers was looking for an autonomous power supply for a sensor designed to record data during test drives of a newly developed Formula 1 car. The exact application was – like almost everything in the offices, some of which were fitted with privacy screens – classified. Suffice to say: the battery was required to deliver as much energy as possible in the smallest possible space and withstand strong mechanical forces caused by vibration and shock, as well as very high temperatures. For this application, a lithium-thionyl chloride battery was the method of choice.”
So does this cell chemistry serve small production runs for exciting and exclusive highlights?
Robert Pilz: “Formula One is, of course, a thrilling highlight for all racing fans and car enthusiasts. Animal tracking can also be highly emotional, for example. We recently saw this in the media in Germany when a humpback whale became stranded in Lübeck Bay and, fitted with a GPS transmitter, was towed back into supposed freedom. But apart from this single, well-known whale, the practice of animal tracking is quite widespread. It serves to research and document the lifestyles, migration routes and behavioural patterns of wild animals. On the one hand, this forms an essential basis for nature conservation. On the other hand, it plays an important role in conflict and risk management involving and for humans and their habitats.
Kurt Korn: “All tracking functions require high-energy-density cells that last a long time, are light and small, and cannot be recharged whilst on the move. The lithium-thionyl chloride cell chemistry provides the optimal solution for these requirements. And this is where we then move into mass production.”
In which trends and markets does the future of lithium thionyl chloride in serial production lie?
Robert Pilz: “Tracking is indeed a growing market. As mobility and global economic interconnection increase, so too does the need for the traceability of goods and assets. Secure systems are a prerequisite for this. That is why accurate calculations and expert advice in the early stages are so important. Primary cells are a matter of absolute trust. They determine the service life of the device.”
Kurt Korn: “The future of lithium thionyl chloride therefore lies in a wide range of applications – both in extremely small quantities for niche markets and in mass-production applications. The market for transponder chips is booming. Monitoring takes place across multiple climate zones. Container tracking, in particular, is a growing trend. But even in the fields of smoke and fire detectors, alarm systems and metering systems for gas, water or electricity, this cell chemistry is sure to remain a permanent part of everyday life for each and every one of us.”