The world is changing and with it our reliance on fossil fuels. As we become more aware of the environmental impact of polluting the air with carbon dioxide (CO2) emissions, society has been moving towards renewable energy sources. One way to achieve this goal is through renewable batteries that can store solar or wind power for use at a later time. These new technologies are essential in achieving sustainable development and reducing CO2 emissions by 2050.
Extensive research has been done on batteries in the past decade to improve their performance in capacity, flexibility, efficiency, cycling stability, as well as decreasing their cost and impact on the environment. As a result, the current market of rechargeable batteries is mainly dominated by high-energy-density Li-ion batteries. However, a drastically rising demand, largely driven by the electrification of mobility, has brought to light multiple issues that still need to be overcome. This post will attempt to explain these issues and how they can be managed.
The limitations and challenges of current lithium-ion battery designs are well-documented, which is why researchers have been seeking myriad ways to improve these widely used energy-storage devices. As lithium-ion batteries are used increasingly over a multitude of applications, we shall aim our focus on them. In this post, we will look at what thermal runaway is and how it can be avoided.
Thermal runaway begins when the heat generated within a battery exceeds the amount of heat that is dissipated to its surroundings. If the cause of excessive heat creation is not remedied, the condition will worsen. Internal battery temperature will continue to rise – causing battery current to rise – creating a domino effect. The rise in temperature in a single battery will begin to affect other batteries in close proximity, and the pattern will continue, thus the term “runaway.”
Lithium-ion (Li-ion) battery thermal runaway occurs when a cell, or area within the cell, achieves elevated temperatures due to thermal failure, mechanical failure, internal/external short-circuiting, and electrochemical abuse. At elevated temperatures, exothermic decomposition of the cell materials begins. Eventually, the self-heating rate of the cell is greater than the rate at which heat can be dissipated to the surroundings, the cell temperature rises exponentially, and stability is ultimately lost. Losing stability results in all remaining thermal and electrochemical energy being released to the surroundings.
A few things can trigger thermal runaway events. Thermal runaway can be initiated from mechanical or thermal failures. Electro-chemical abuse from overcharging or over-discharging the cell can also initiate thermal runaway. Also, there’s the possibility of an internal short circuit within the cell leading to thermal runaway. Any of these events can lead to elevated temperatures that are high enough to induce rapid exothermic decomposition of the cell materials.
The causes of thermal runaway are connected to:
- Ambient temperature
Battery life is compromised when continually subjected to ambient temperatures above 77° F (25° C) because the battery can’t shed heat. When this happens, it leads to an increase in the internal chemical reaction and increased float charging current that further increases temperature.
- Age of the battery
Batteries that are going through an end-of-life phase (commonly defined as a reduction to 80% capacity) can become vulnerable in the long term. They will have trouble holding a charge and may require a longer time for charging; they might also experience higher current, heat generation, and internal wear over time from stress.
- Float charging voltage
Float charging occurs when a battery is on standby and not in use, such as at night or during work. If the voltage remains too high for an extended period, it can cause excessive current to flow into the batteries, which will lead to increased internal temperatures.
Overcharging your battery will wear down the internal components, leading to a shorter lifespan.
- Internal short circuit
Due to an accident or similar mechanical impact, e.g. if a tool falls from a great height, the battery is deformed, material penetrates the battery cell, and triggers an internal short circuit.
- External short circuit
Deformation of the battery cell causes an external short circuit.
A chain of exothermic reactions that are driven by thermal runaway leads to severe redox exothermic reactions at relatively high temperatures. These types of events generate a tremendous amount of heat and ultimately result in an uncontrollable rise in temperature. Thus, the removal or reduction of the main exothermic reactions during the evolution of thermal runaway is essential to guarantee the safety of lithium-ion batteries.
How can the risk be reduced?
To prevent thermal runaway, the mechanical and thermal stability of a battery must be maintained. This can be done by ensuring that there are appropriate monitoring mechanisms in place for cells and batteries as a whole unit. If left unchecked by built-in system protections or the Battery Management System (BMS), this process can continue to drive up temperature and pressure until the battery cell ruptures, which can cause fires in affected and adjacent cells.
Fortunately, early warning systems have been developed in order to identify a unique pre-cursor event that occurs up to 30 minutes prior to thermal runaway, known as off-gassing. By definition, an off-gas is the by-product of a chemical process. This enables problems or the system to shut down before thermal runaway can even happen.
Although most quality BMS equipment monitors temperature and other variables to prevent thermal runaway by triggering protections before temperatures change unplanned, the early detection of off-gassing provides a critical additional layer of protection for the entire system, as well as both facility personnel and infrastructure.
“Once a Li-ion BESS goes into a thermal runaway, you cannot stop it [in that cell] – your goal is to try to stop the propagation of heat and thermal runaway to adjoining cells,” explains retired NYC firefighter Paul Rogers, a co-founder of Energy Storage Response Group (ESRG)
- Battery cell
Monitoring of the battery cell is crucial, as a thermal runaway is created here and can spread to other battery cells, the entire battery as well as batteries/applications that might be surrounded or connected to it.
- Battery pack
The number and arrangement of the battery cells play a key role in the battery pack:
- Twenty-four lithium-ion cells in a row: Relatively uniform temperature distribution
- 3 x 8 lithium-ion cells: Hotspot inside
- 5 x 5 lithium-ion cells: Hotspot with a higher temperature inside
Liquid cooling systems, fans, thermal conducting plates, and thermal conducting film are available to cool down the battery or to dissipate heat from a hotspot.
Besides the arrangement of the battery cells, the arrangement of the fan is also crucial when using fans. Tests have shown that when discharging the battery with a 1-fold, 2-fold, and 3-fold rated current, surprising temperature distributions occur and hazardous hotspots can arise. Therefore, it is not sufficient to rely on assumptions when constructing a battery with a fan; rather, exact measurements need to be carried out or existing investigations used.
To dissipate heat effectively, a thermal conducting film is recommendable. To also reduce the heat at the hotspots, NASBIS insulating film is available in addition to the PGS. Positioned between the battery cells, the NASBIS film prevents thermal runaway from one battery cell reaching neighbouring cells and thus the entire battery. Its thermal conductivity of 0.02W/mK is lower than air and it, therefore, serves as thermal insulation. The NASBIS films are also extremely thin and flexible and can, as a result, also be used in confined spaces.