As technology literally embeds itself in sports venues and on the athletes themselves, concerns about lithium-ion battery safety and protection naturally follow. Battery structures can be compromised by physical forces, exposing their users to serious injury if Li-ion cells fail catastrophically, resulting in heat, fire, and even explosion. To mitigate these effects, battery manufacturers continue to build a wide range of protection mechanisms into these devices.
Thanks to their low weight and high energy capacity, Li-ion batteries have earned a central role in applications ranging from electric vehicles to sports and fitness activity trackers. In fact, Li-ion cells can be designed to deliver high specific energy and power that outstrips the performance of traditional battery technologies.
Concerns about safety of Li-ion technologies have been highlighted in high-profile Li-ion battery failures in Tesla electric vehicles and Boeing’s latest flagship airliner, the 787 Dreamliner. Battery fires during Dreamliner flight operations led to the worldwide grounding of all Dreamliner aircraft. For Boeing, the event initiated a detailed examination of the 787’s battery design. For the public at large, it led to broader questions about the safety of Li-ion batteries in any consumer application. In fact, Li-ion batteries are ubiquitous in consumer devices, performing safely and reliability when operated within recommended margins. For Boeing, a redesigned Li-ion battery system allowed the 787 to return to flight—its Li-ion batteries enclosed in an airtight stainless-steel cabinet vented through a pressure-relief valve to a tube passing through the pressurized fuselage of the aircraft itself.
Batteries for sports and fitness need not (and of course cannot) take such heroic measures as the 787’s robust battery system design. Thanks to a combination of internal protection mechanisms and packaging methods, however, these components can reliably serve to power devices for even the most active sports and fitness activities.
Li-ion Cell Structure
A lithium-ion cell is a simple structure where lithium ions move through a non-aqueous electrolyte from the negative and positive electrodes during discharge and in the reverse direction when charging. A typical 18650 battery, for example, is built as a three-layered structure comprising the positive plate, a separator layer, and the negative electrode plate. This structure is wound to form the familiar battery “jelly roll.”
Figure 2: A typical 18650 cylindrical battery comprises a three-layered Li-ion cell structure wound into the characteristic “jelly roll” and packaged in a protection case.
(Source: Panasonic)
Li-ion cells operate within a narrow temperature region (Figure 3) whether used as single-cell devices or combined together in multi-cell battery packs. In normal operation, even a battery in close contact with the human body remains within this safe operating region and so presents no inherent danger to the user. Elevated a few degrees above their optimum temperature range, Li-ion cells remain safe but begin to experience accelerated aging and exhibit an associated loss in energy capacity.
Figure 3: Li-ion cells deliver optimum performance when operated within a narrow window of ambient temperature and with minimal temperature differences between separate cells in a multi-cell stack. (Source: University of Oxford, Energy and Power Group)
Serious problems arise, however, if Li-ion cells are allowed to reach higher temperatures through a combination of continued body heat, high ambient temperature, solar heating, poor ventilation, or other factors. Beyond the optimum temperature range, chemical processes speed up dramatically—and increased internal temperature and pressure can ramp quickly to reach thermal runaway.
Thermal Runaway Mitigation
The process of thermal runaway starts gradually with an increase in temperature within a cell due to an electrical fault, mechanical abuse, or the presence of an external heat source such as a neighboring cell that is further along in the thermal runaway cycle. As temperature in the cell rises, chemical interactions result in exothermic breakdown of cell components, further elevating cell temperatures. Eventually, electrodes break down, releasing flammable gases. As temperature and pressure build within the cell, the separator disintegrates and the cell short circuits, releasing yet more heat. Different battery chemistries and cell designs shift the threshold leading to irreversible breakdown and thermal runaway but eventually, cell temperature can spike to hundreds of degrees Centigrade (Figure 4). At this point, the battery catastrophically fails, ruptures its enclosure, and is typically consumed by fire as the released gases react with oxygen in the surroundings.
Figure 4: Using three different types of commercial Li-ion batteries (blue, green, and red), researchers found the batteries exhibited thermal breakdown spikes with a predictable profile but with thresholds shifted depending on battery type, external heat (1 vs. 2), or simply due to variations across similar devices. (Source: Royal Society of Chemistry)
Lithium-ion power sources are designed with dedicated circuits able to manage individual cells and balance charge in multi-cell packs while providing protection from over- and under-voltage conditions, excessive charge current, and high temperatures. Besides integrated electronic protection, lithium batteries are manufactured with a number of built-in mechanical safeguards designed to mitigate the factors leading to thermal runaway (Figure 5). Battery components are typically sealed with gaskets or welds that are designed to fail under high internal pressure. For extra protection, battery caps are often scored or otherwise built with specific burst disks intended to open to relieve pressure. Charge interrupt devices (CIDs) physically break the connection between the cell and the external circuit in response to overcharge, over-discharge, overheating or internal shorting.
Figure 5: A typical cylindrical battery cap includes a number of mechanical structures designed to prevent conditions that can lead to catastrophic failure and thermal runaway.
(Source: U.S. Department of Energy National Renewable Energy Laboratory)
One of the more effective mitigation methods in commercial Li-ion batteries uses a current-limiting positive temperature coefficient (PTC) structure. Ringing the battery cap internally (again, see Figure 5), the PTC device limits external currents if the battery is shorted. Battery PTCs are typically built out of a matrix of a crystalline polyethylene impregnated with conductive particles, exhibiting a sharp increase in resistance with increasing temperature. If the battery is shorted, the rapid increase in current causes self-heating in the PTC device. As a result, PTC device resistance increases quickly and reduces current flow. While the battery remains shorted, PTC device resistance remains high, limiting current until the shorting condition is removed. When the battery returns to a safe operating condition, the PTC device resistance returns to normal—providing the battery with a protective, resettable fuse.
Physical Protection
In long-term operation, cell electrodes can gradually break down and even separate, leading to loss of overall capacity. In some cases, fractures in these electrodes or even simply impurities in electrodes can lead to dendrite formation, where metallic lithium essentially crystallizes on the battery’s anode. Over time, as metallic lithium continues to accrete on the dendrite, the dendrite can grow into a spike. Eventually the dendrite can reach sufficient length that it pierces the separator between the electrodes, creating a short circuit in the cell.
Figure 6: Micrographs of dendrite formation show the rapid increase in size of these metallic lithium spikes and their random growth, which can eventually compromise internal structures and result in a short circuit in the Li-ion cell. (Source: U.S. Department of Energy National Renewable Energy Laboratory)
Although dendrite formation can be a longer-term effect, external forces such as vibration or sudden acceleration can damage electrodes, increasing the chance of dendrite formation; degrade the structural integrity of the battery’s physical structure, compromising protection; or even break electrical connections, resulting in battery failure.
Sudden external mechanical trauma to a lithium-ion battery can literally expose a battery to the danger of thermal runaway. In sports and fitness, punctures from equipment such as a racquet, hockey stick or cleats can break through the external package and disrupt the separator, creating a conductive path between the electrodes. Worse, a puncture can cause the non-aqueous electrolyte to leak and even release flammable gases if the damaged battery has already begun to degrade chemically.
Similarly, if a lithium battery is crushed, damage to electrodes can erode battery performance at best. At worst, if the battery structure is crushed so much that the separator is penetrated, a short circuit will likely occur, threatening thermal runaway.
Manufacturers anticipate these concerns with battery enclosures appropriate to the application. In familiar consumer devices, battery packs might be sealed in stiff metal or plastic enclosures—or even bound in shrink wrap when weight requirements demand and safety considerations permit minimal packaging.
Custom battery packaging manufacturers such as iTECH and Nuvation match specific Li-ion cell designs with a combination of materials and manufacturing methods to create suitable enclosures for these devices. The familiar cylindrical cell typically relies on a stainless steel package that can be too bulky for wearable sports and fitness applications. For these applications, manufacturers often turn to prismatic cell designs, which use a compact layered structure that results in thinner cell geometries, and are generally packaged in metal containers. For ultra-compact designs, pouch cell designs are simple structures where the electrolytes and electrodes are contained within a thin sealed pouch with connections brought out as conductive foil tabs welded to the electrode.
Li-ion technologies offer energy densities substantially higher than earlier battery technologies. Packaged in suitable enclosures and used within recommended operating conditions, Li-ion cells offer an efficient energy source able to meet the physical and energy demands of sports and fitness applications.