Battery passivation effects in lithium thionyl chloride batteries after 4.5 years of storage

Lithium-based primary batteries have become a go-to energy source for many embedded and IoT applications. In particular, lithium thionyl chloride (Li-SOCl₂) has proven to be highly desirable due to its very high energy density, extremely low self-discharge rate, and wide operating temperature range. It continues to demonstrate exceptional reliability in remote environments and has seen significant growth in global utility metering—especially in gas, water, and electricity meters.

However, one challenge with this battery type is its tendency to undergo passivation—a chemical effect that, if not adequately accounted for, can lead to unpredictable behavior and significantly shorten the product’s operational life.

The degree of battery passivation depends on the duration of rest, temperature, and state of charge of the battery. In this article, we investigate the effects of passivation based on how long the battery has been resting, specifically examining a previously unused Saft LiSOCl₂ battery that has been in storage for 4.5 years.

Battery passivation

LiSOCl₂ batteries undergo a chemical process that increases their internal resistance when the battery or device is in sleep mode—if the current is low enough. A thin film, primarily composed of lithium chloride (LiCl), forms on the lithium anode as a byproduct of this process. This phenomenon is known as passivation.

Passivation isn’t entirely negative; in fact, it enables long-term storage of the battery due to its extremely low self-discharge rate.

For a device, the impact is seen when transitioning to active mode—for example, when the radio is turned on for transmission. At that point, the initial elevated internal resistance causes a deeper voltage drop. However, during the rest of the active period, the passivation layer on the lithium anode is broken down, and the internal resistance returns to normal.

Lithium thionyl chloride batteries in the study

In this case study, we investigated the passivation effects of two batteries:

• Saft LS14500, 3.6V
• Saft LSP14500, 3.6 V. LSP refers to a hybrid lithium primary battery that combines a lithium thionyl chloride cell with a lithium-ion capacitor (LIC) in parallel.

We initially started with two packs of LSP14500. To make a comparison, we opened one of the LSP14500 packs and removed the super capacitor, allowing us to measure only the LS14500.

During battery handling, great care is taken to ensure the cells are not connected in any way, preserving their unused state since manufacturing. Extra caution is exercised when opening the battery pack to avoid any risk of short circuits. This process has been conducted in a professional laboratory environment, and it’s a critical practice that should be carefully considered when working with any battery.

Both batteries were manufactured in December 2020 and remained unused.

The measurement set-up for battery passivation evaluation

The batteries are evaluated in a compact and scalable setup with Otii Ace Pro (see picture below), utilizing the Otii Battery Toolbox. The complete step-by-step setup is explained and can be followed here.

Both batteries are tested in the same way, with a battery connector connected to the banana connectors of the Otii Ace Pro. The same cable length is also used.

Battery discharge profiles

Most utility meters with wireless communication globally use one of the following technologies: Wireless M-Bus, NB-IoT, LTE-M, or LoRaWAN. For this exercise, we chose to investigate the passivation effect, assuming a LoRaWAN device and a typical LoRaWAN discharge profile.

The discharge profile is selected for LoRaWAN spreading factor 9 (SF9), with a sleep current of 10 μA for 20 seconds.

The discharge profile is simplified to:

• 10uA for 20s (sleep period)
• 7.5mA for 6ms (wakeup)
• 115mA for 168ms (TX)
• 8.5mA for 20ms (post TX)
• 10uA for 4.96s (sleep, waiting for RX)
• 6.2mA for 290ms (RX)

The profile is iterated for 20 minutes to investigate the battery behaviour over time.

Battery passivation results

Passivation effects in Saft LS14500

It is observed that the first pulse is affected by an initially elevated internal resistance, caused by the buildup of the passivation layer during the 4.5 years of storage, as shown in Fig. 1. At the start of the first pulse, the internal resistance exceeds 100 ohms. For the second pulse, it drops to approximately 15 ohms. From the third pulse onward, the internal resistance stabilizes in the range of 3.5 to 5 ohms.

Voltage drops due to passivation have been observed as high as 1.8V. For devices powered by 3.6 volts, such drops can lead to device resets or shutdowns, directly affecting the product’s operability and longevity.

Passivation effects in Saft LSP14500

Even after 4.5 years since the battery was manufactured, no significant passivation effect has been observed, as seen in Fig. 1. The voltage drop during the first and second pulses is roughly the same, and the internal resistance during the first pulse is approximately 200 mΩ, based on the initial voltage drop.

The passivation effect has been mitigated by the super capacitor connected in parallel, as shown in Fig. 3. The super capacitor absorbs the high pulse when the device enters active mode, maintaining a stable voltage while the passivation layer disappears.

Summary

Battery passivation in lithium thionyl chloride cells is a well-known phenomenon and a risk. Despite their high energy density, passivation can compromise the performance of smart meters or IoT applications. Using a super capacitor in parallel is one effective way to compensate for these effects and mitigate the risk.

To understand the level of risk you’re facing—and how effectively you are mitigating it—measuring the passivation effect is essential. In this article, we investigated the impact of passivation in an unused SAFT LiSOCl₂ battery that had been in storage for 4.5 years. We demonstrated how to assess passivation using a compact and scalable setup with the Otii Ace Pro and Otii Battery Toolbox.

Stay tuned for more studies on battery passivation! If you’d like to learn more about the Otii Product Suite and how it can help you evaluate battery passivation, don’t hesitate to contact us.