Navigating Battery Safety Challenges in Thermal Runaway Scenarios

As we transition to net zero policies, battery safety is quite challenging. It's at the forefront of everyone's mind, especially in the UK, since policy at this moment requires that all new vehicles need to be either electric or hybrid by 2035. As a result, battery safety is of top concern right now, particularly in the UK, but also across the world.

In this newsletter, James Close provides a comprehensive overview of lithium-ion batteries, battery failure scenarios, and the characteristics of battery energy storage failure. He examines two case studies involving battery deflagrations and fires and the main factors that contributed to both incidents. He then presents a detailed look at testing methodologies aimed at improving the safety of Battery Energy Storage Systems (BESS).

Lithium-Ion Batteries

As chemical engineers, some might be unfamiliar with lithium-ion batteries. The first common example that everyone thinks of is known as the 18650 lithium-ion cell, and the name kind of says it all if you are a battery engineer. Eighteen means 18 millimeters in diameter, and then the 650 or 65 is the length of the battery or 65 millimeters. This is typically for cylindrical cells, but it's also true for different types of battery formats. There is a positive terminal, a positive connection, and an emergency event just in case. The importance of that will be discussed later.

A jelly roll ― a silly name, but that's what it is typically called in the industry ― is where the electrochemical reactions take place. This contains the electrodes, the electrolytes, and all the things that make lithium-ion batteries work. And then you have the actual housing, which is the canister. The higher energy density of lithium-ion batteries is the reason why they are so popular in the current sustainable industry, as you can see in Figure 2.

So how do they work? The jelly roll has two electrodes, which is how they operate, a positive electrode and a negative electrode. One is made of lithium metal while the other is made of graphite that contains lithium ions and functions as a lithium metal electrode. Then there is almost like a mediator or sort of like an electrolyte, and then a separator, which essentially prevents short circuits. That basically mediates the flow of lithium ions but then also that flow of lithium ions is a redox reaction. If you remember back from high school or university, redox was the gain and loss of electrons, and that's essentially how it works. These components work together to transfer lithium ions between them and therefore also eject and receive electrons generating currents. That is essentially how they work at the level of pure chemistry fundamentals.

Now you might be asking, well, okay, that's just one cell. One cell is limited to, let's say, 18650 cells which might have the capacity of 3 or 4 Ah or amp-hours. You can get larger formats with higher amounts of capacity, which is good. But in order to meet a certain capacity load, you do need multiple cells, either connected in a series or parallel, all depending on your goals, your required voltage capacity, your currents, and even your control systems.

Basically, you take all these cells and arrange them to meet your requirements into a module. Then you need to look again at your requirements, voltages, capacities, cycling behavior, and goals of the energy storage system. Then connect those modules into a rack or a pack of batteries, and then together the cells ― in terms of discharge and charging cycles, temperature control, and things like that ― are managed by a Battery Management System (BMS system). That is all within the rack stage. By combining multiple battery packs, you can form a Battery Energy Storage System (BESS) to deliver and store the required capacity of energy.

Battery Failure

However, because of their electrochemical nature, batteries also have great thermal instability. They have immense power and high energy capacity. With that amount of stored energy, they are quite inherently unstable. Now, this varies from chemistry to chemistry, but depending on the internal or external failures, you may have more thermal runaways and quite bad consequences.

For example, with a cell, you could overheat a cell, you could create an external short circuit in the connection, or you could cause mechanical deformation. If, for example, a batch of cells is dropped, this could damage the electrolytes and the electrodes within, causing internal short circuits and then some very bad consequences, such as starting fires or jet fires coming out of the batteries.

In another example, large format cells can cause large jet fires because they are prismatic in nature, they are bigger, they can store more energy, they can store more chemicals and more chemistry. They also have vents on top of them. If those vents open during a thermal runaway, what can happen? All these combustible gases, electrolytes, everything starts to catch on fire and cause massive jet fires. Other possible consequences include the release of toxic gases and the deflagration of any flammable materials.

Stages of Thermal Runaway

The first stage is when there is a solid interface breakdown and the very first bit of increase in temperature, that is when the mass transfer of the lithium ions breaks down. This can cause an energy release at the anode or the graphite electrode. This can then cause issues in terms of temperature rising and heat generation, which then could cause the anode to break apart, as shown in Figure 6. When the anode starts to decompose, this releases even more temperature. What happens further on is that the heat starts to cause decomposition in the electrolyte and could also cause a pop-off of the vent itself in the battery.

When that happens, as everyone can imagine, it opens to the atmosphere. This can get a mixture of oxygens coming in, causing further combustion reactions of any gases being generated during the runaway process. What’s worse is that the metal oxides start to break down as temperatures continue to rise. The key point is that the final electrode that breaks down is a metal oxide, which releases oxygen, which then can react further with the electrolytes, which can cause combustion. Flammable material can also be ejected. And then what's worse is if the separator between the battery melts, you can also get short circuits[LR1]?, releasing a huge amount of electrical energy as well. These are the main stages of how thermal runaway can occur.

The issue becomes if one cell experiences thermal runaway, depending on how I packed these cells in the module, thermal runaway can spread, temperatures can accumulate, and can spread to other cells. Then what else could happen? I have a thermal runaway in my module. Depending on what's in place thermal barrier-wise, that can then spread to another module within the pack. Then in the Battery Energy Storage System (BESS), it could go from pack to pack. And then what's worse is, depending on the format, cell type, or chemistry, flammable gases can be ejected. Not only are the chemicals toxic, they can also accumulate. And if they are still an ignition source, they can ignite and cause a deflagration.

BESS Failures and Case Studies

Unfortunately, this has been happening all across the world. From 2018 to about 2020 in South Korea alone, there have been 25 or 26 BESS fires and a few explosions as well. There was a dreadful BESS fire and explosion on Carnage Road in Liverpool, England.

Surprise, Arizona, US Case Study

The battery incident that kicked a lot of safety protocols into gear occurred on April 19, 2019, at an enterprise BESS in Surprise, Arizona. We can see there was definitely a meltdown in rack 15 shown on the left-hand side in Figure 9. There was also a lot of debris. A lot of the BESS structure was crumpled and ejected outwards. This occurred because there was a secondary explosion that happened when firefighters arrived on the scene.

If we look at the timeline, this essentially is how the incident played out. In rack 15, there was identified an internal short circuit within the pouch cell. This thermal runaway caused bursting within that cell, releasing toxic flammable gases. Then the cell caught fire and the fire spread throughout the module and then also rack 15. The fire was detected and the Battery Energy Storage System (BESS) did have a Novak 1230 dispenser, a fire suppression system, to suppress fires.

The problem was that even though the Novak 1230 gas was released and the fire was contained, unfortunately, the thermal runaway was still occurring, spreading through to rack 15. While the fires were well contained, the flammable gases being produced rose and accumulated on top of the Battery Energy Storage System (BESS). Shown in Figure 11 in blue are the Novak 1230 gases pushing the flammable gases up to the top layer.

Unfortunately, when the firefighters arrived on the scene and opened the door to the BESS, the Novak 1230 gases came out of the system, and then air rushed in. Then the flammable gases came, back down, and mixed with the air. And then, as we mentioned, the thermal runaway in rack 15 was still continuing. The thermal runaway caused an ignition point and then a deflagration occurred. So this was a very insightful case study, but it also just shows that, although we're moving in a more sustainable direction, there are still a lot of chemical hazards we need to look out for.

Liverpool, England Case Study

So that incident in Arizona was a failure from a design perspective, but what about regulatory? You might say, well, 2019 was a long time ago. Were there any sort of regulatory protections back then? If we take a look at the September 15, 2020, battery incident in Liverpool, England, you can see in Figure 13 the dreadful fire that occurred at the Battery Energy Storage System (BESS) facility on the left-hand side, and then the aftermath on the right-hand side. This was a very similar incident to the explosion and fire that happened in Arizona. Fortunately, there were no injuries, and only property within the facility was damaged.

The good thing about this case study is that the incident investigation made it very clear what regulations and standards were followed, including a lot of the current regulations. You can see in Figure 14 that the standards and good engineering practices for the time were followed. The issue is, that although these regulations are helpful in terms of lithium-ion battery safety, a lot of the regulations are, to put it bluntly, a complete mess.

For example, Figure 15 shows all of the connections to lots of different types of international standards and regulations for mechanical, electrical, environmental, and chemical industries. You have to follow these regulations if you want to make a module, a battery pack for an electric vehicle, etc., and then perform all the different types of tests that fall under the specific regulation you're trying to follow.

These regulatory bodies are all separated. They all have their own opinion on required testing, what's applicable, what's not applicable, what's valid, and what's not valid. And they all have different pass/fail criteria. Some require you to have footage of the actual incident when you do a test, for example. To do a nail penetration test, you must poke a cell with a nail. Some standards and some regulations require filming that test, while some of them don't. It's very confusing and it's a huge mismatch of different parts and parcels to try to achieve a safer battery pack and battery system. That's also the case for these Battery Energy Storage Systems (BESS) incidents.

Lessons Learned

These were the most important lessons learned and the main factors that contributed to both incidents. Internal failures in cells initiated thermal runaways. The fire suppression systems were not capable of stopping thermal runaway. There was also a lack of thermal barriers not only between cells but also between modules within the rack. Also, the cooling system for maintaining the temperatures of the cells during operation, in my opinion, is what made things worse. The cells were air-cooled, so this spread the flammable gases throughout the energy storage system quicker, therefore it became a lot easier to collect the flammable gases into an accumulation at the top of the energy storage system. So again, all these factors as shown in Figure 16 contributed unfortunately to these explosions and fires on both of these Battery Energy Storage Systems (BESS).

Exploring Mitigation Methodologies

Now you might be saying, it’s very helpful to know these case studies, but what can we do about it? What can we do to improve the safety of these Battery Energy Storage Systems (BESS)? My method is to examine first what the issues were, explore different methodologies, and then take an optimistic approach to improve safety. Here my focus will be primarily on testing.

So the first step is determining what happened. There was some dendrite formation and some deformation internally within the cell. That caused internal short circuits and then a huge thermal runaway reaction. There was also some ejection of material in the cell, as mentioned earlier, some debris and flammable gases that accumulated and caused the deflagration. The thermal runaway spread was not being controlled. There was also the potential of secondary thermal runaways that occurred within the rack. Then the fire suppression systems just didn't do their job.

The current approaches that we have available to us include thermal runaway modeling, similar to performing thermal runaway dynamic modeling of a storage tank. You can do a detailed kinetic model or an ISO conversion model, which is just as helpful.

The next method is propagation modeling and design which essentially looks at the thermal runaway spread from cell to cell. But this method also has to be backed up by data, some validation. At ioKinetic , we have the capability to do single cell module tests using an EVx Calorimeter instrument. But one of the big issues around 2020-2021 was that there was no discussion about any gas ejection or any material ejection, analyzing the flammable limits of these gases being produced during that thermal runaway. That is becoming crucial now.

In terms of approved methodologies, at ioKinetic and ioMosaic Corporation , we can perform computational fluid dynamics (CFD) modeling. In a previous presentation, I spoke about 1D explosion dynamics to model BESS deflagration and how we can size blast panels using Process Safety Office? SuperChems? software which looks at the Layers of Protection (LOPA) aspect. This is very helpful for these sorts of installations.

If we look at the module, pack, or cell level, we really need to understand the energy distribution because this will help us design better modules, and also different thermal resistant materials and thermal barriers to help prevent the propagation to the rack itself.

We have lots of tools at our disposal at the ioKinetic lab. We do more component testing to look at the individual chemistry of electrodes and electrolyte mixtures, things like that. We also do a lot of heat capacity and conductivity testing. This is very important if you wish to understand the heat of reaction of your battery and what your heat capacity is. For heat transfer calculations, you need to understand the thermal diffusivity conductivity. It is absolutely vital to have those accurate measurements to simulate and better improve thermal barriers and double resistances at a module level.

At ioKinetic , we have the EVx calorimeter and with it, we do a lot of battery testing to test different chemistries, different gas collection methodologies to characterize things like temperature rises and pressure rises. We can store the gas and run actual testing in terms of composition and then also use Process Safety Office? SuperChems? software to see what type of combustion reactions we would get, flammability limits, etc.

We can take all these experimental data, all these pressure measurements, voltage measurements, and then the collected gas to put it through GC mass spectrometry and really identify what kind of compositions we have and then really nail down the flammability limits.

This really helps from a standards point of view and a regulatory point of view. For example, after these case study incidents occurred, regulation UL9540A changed its testing structure. If you wish to follow UL9540A testing structure now, you are required to do a single cell level test, module test, rack test, and then an installation level test.

Throughout all of those levels, you need to understand the gas ejection, what the gas composition is, what flammability limits it has, and also the diesel migration potential as well. It is really good for meeting compliance and also design purposes as well, to make safer products, safer batteries.

But we are not just stopping at the GC mass spectrometer, we're taking it a step further. I mentioned that the spread and the understanding of the distribution of energy is really important when dealing with single cells, single cells to modules, and things like that. The EVx Calorimeter instrument has its benefits, but what we really need to start thinking about is the distribution of energy during a thermal runaway when cells go off.

There's a huge amount of ejected material coming from either the top or bottom of the cell. It can go in either direction. Then also the currents can get very, very hot and melt away. We need to understand what the distribution is between the vented material and then also the EVx Calorimeter itself, because currently this equipment is great for a sort of lumped look at the thermal runaway, but do need to take it a step further and understand geometrically where the energy distributions are.

This brings me to the equipment at ioKinetic called Fractional Thermal Runaway Calorimetry (FTRC). This calorimeter can quantify the distribution of energy based on the location of the cell. When we enter a cell into this chamber in the middle, it will trigger a thermal runaway either by overheating, or nail penetration, or another form of overcharge or mechanical deflagration. The FTRC captures and quantifies the energy at the actual cell itself, and then also the vented gas and the potential solids coming out.

The FTRC allows us to quantify the energy distribution down to every component. The results are presented as a pie chart that will be incredibly handy for any future design improvements and modeling purposes. We are able to quantify what portion of the energy is through the actual casing and then what portion of the energy is distributed through the ejector. You can see in Figure 25 that a good majority of that energy goes through the ejected material, 74.7%, and then the remainder of the proportion, 21.7%, typically goes through the main cell body itself.

The FTRC data is very important because when we understand the distribution of the cell, we can put this into our battery modeling and design and start to answer two questions. We can see the fire suppression system's effectiveness and look at the energy carryover from fireball materials. We can also start to look at how effective the barriers would be in terms of stopping thermal runaway. For example, one of the current big challenges is, that a lot of people are working on thermal barriers, but they place cells with no barriers in between. Sometimes they are shocked to find out that, depending on the thermal barriers they use, a thermal runaway still occurs further down the cell module.

What I suspect, and what I'm also pursuing in my doctoral research, is that a significant amount of energy is being carried and distributed further across the module by the temperatures, combustion processes, and energy that are happening. We need to understand this distribution of energy a lot better to solve this problem and make these systems a lot more safe.

Figure 27 shows another example of thermal propagation modeling. We need to understand how these energy distributions work. You can see below that if I didn't change the efficiency, it would cause these problems.

The Fractional Thermal Runaway Calorimetry (FTRC) data shows the energy released via the gas and the ejected flames. This new equipment will help battery designers, module designers, and battery safety system designers optimize their thermal runaway mitigation processes and improve overall safety.

In Conclusion

Lithium-ion batteries are great in terms of sustainability for capturing energy and discharging it, but they are highly thermally unstable. Thermal runaways can result in fire and spread from module to module and vertically in a pack. During thermal runaway, lithium-ion batteries release flammable and toxic gases. These gases can accumulate in a BESS if ignited and also cause deflagrations.

Two case studies involving battery deflagrations and fires were examined. The main factors that contributed to both incidents were a lack of thermal barriers and venting of the flammable gases, and the ineffectiveness of the suppression system to reduce the heat output of the thermal runaway. These factors all aggregated into the right conditions for an explosion.

The confusion around applicable international standards and regulations and their impact on testing requirements was discussed. Next, a detailed look at thermal runaway and explosion mitigation strategies was presented, along with recommended testing.

Thermal runaway models, safeguard and mitigation designs, the usage of firefighting equipment, thermal barrier designs, and other applications can all benefit from battery testing. ioMosaic Corporation and ioKinetic can help.

#BatterySafety #BESS #ThermalRunaway

Overpressure Protection of Battery Energy Storage Systems (BESS) White Paper

Explore the mechanics of deflagrations that can occur during a thermal runaway event in a Battery Energy Storage System (BESS) and methods for sizing deflagration vents to protect against such explosions in elongated geometries in this white paper by James Close and Charles Lea . They present methods for adequately sizing blast panels to reduce the impact of deflagration on the storage system structure by incorporating venting dynamics, burning rate models, defining hot spots, and representing blockages in the geometry.?

To download the white paper Overpressure Protection of Battery Energy Storage Systems (BESS), visit our website > https://bit.ly/3BdQhxI

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