From electronic devices to high-efficiency vehicles, consumer demand continues to grow for more compact, lightweight, quick-charging battery technologies with higher energy densities. At the same time, batteries should also be safe, even in catastrophic events. Lithium-ion (Li-ion) has become a favorite battery technology among engineers and designers because it satisfies many of these demands, and is cost-efficient. As battery designers continue to push the limits of Li-ion battery capabilities, however, many of these requirements can conflict with one another.
The act of charging and discharging a Li-ion battery produces changes in the temperature, electrochemistry, and mechanics of its internal components. These dynamics also cause changes in interface pressure within the battery housing. Many familiar with the design of a Li-ion battery will say that these changes in pressure give the effect of a battery “breathing.” Over time, this can affect battery performance, and, in extreme cases, can lead to potentially dangerous reactions.
Recently, battery designers have begun using piezoresistive force and pressure sensor technology to analyze the effects of charging and discharging Li-ion batteries in long-term lifecycle tests. These same types of sensors can also be embedded within the actual device to help alert end users to potential battery failures.
A Single Core Technology Wears Two Hats: R&D Testing and Embedded Component
Even between relatively flat surfaces, one finds that interface pressure distribution is often not uniform in localized regions. Whether as part of a turnkey pressure mapping system, or as an embedded component in a final product, thin, flexible, piezoresistive sensors offer engineers and designers the ability to capture relative changes in force and pressure.
Piezoresistive sensors consist of a semi-conductive material contained between two pieces of thin, flexible polyester. They are passive elements that act as force-sensing resistors in an electrical circuit. When unloaded, the sensor has a high resistance (about 2MΩ) that drops when loaded. If you consider the inverse of resistance (conductance), the conductance response of touch sensors is linear as a function of force within the sensor’s designated force range.
As shown in Figure 1, piezoresistive sensors are produced as both single-point force sensors and as multiple-point matrix sensors.
Matrix Sensors for Pressure Mapping
Matrix sensors are commonly used in R&D applications to dynamically measure pressure distribution across two mating surfaces — a process known as pressure mapping. Pressure mapping systems consist of sensors, scanning electronics, and software.
As two surfaces come in contact with the sensor, the scanning electronics collects the analog signal from the sensor and converts it into a digital signal. The software displays real-time activity across the sensing area. This allows the user to track the center of force, locate regions of peak pressure, and identify exact moments of pressure changes in a frame-by-frame recording.
Additional features of matrix sensors include:
Sensors typically have about 2000 sensing points, but some sensors can be designed with over 16,000 .
Sensing element spacing (pitch) can as narrow as 0.64 mm (0.025 in.).
Sensors can measure pressure ranges up to 25,000 psi (172 MPa).
High-temperature sensor options up to 200 °C (400 °F).
Scanning speeds available up to 20,000 Hz.
Single-Point Sensors for Embedded Sensing Applications
Single-point piezoresistive force sensors measure force feedback on a single sensing area. This sensor type lends itself well to integration within a product or device not only because it is thin and flexible, but because it can function as a component of an op-amp circuit or voltage divider. Depending on the setup, the force range of the sensor can be adjusted by changing drive voltage and resistance of the feedback resistor. This allows the user to have control over parameters such as maximum force range, and measurement resolution within that range.
Since piezoresistive sensors are passive components with linear conductance response and have a wide dynamic range of resistance, engineers integrating them can use simple electronics that do not require a lot of filtering.
An important benefit of this linearity is that piezoresistive sensors only require simple calibration. Force-sensing applications that use load cells or strain gauges, may need to be factory calibrated after repeated use, while devices with piezoresistive sensors can have their calibration routine embedded into the device firmware for on-the-fly recalibration.
Because of the flexibility of this technology, along with its ability to function with low-power electronics, piezoresistive force sensors have been successful in enhancing many different types of devices, without adding complexity to the design or difficulty for the user.
A Novel Method to Validate Li-ion Battery Stack Stress
Although, battery manufacturers prioritize maintaining constant battery stack construction during manufacturing, many do not validate stack pressure as part of their quality procedure, nor do they quantify internal stresses on the battery as it is charged or discharged.
However, research has found that high stack pressures can have a strong effect on long-term cell performance, with higher levels of stress leading to higher rates of capacity fade. Over time, significant internal pressures can lead to plastic deformation and delamination. The mechanical expansion and contraction from temperature changes causes the anode/cathode layers to separate over time. When those layers separate, the impedance of the battery goes up, derating its capacity. On the other hand, while lower internal pressures may provide better long-term performance, there may be too much movement from thermal expansion within the battery housing. Therefore, battery designers must find a “sweet-spot” for housing pressure that can maintain over the charge and discharge cycles.
One method to characterize Li-ion battery swelling in charge/discharge cycle testing is with a fixture that incorporates a load cell to detect swelling. However, because load cells can only collect average force feedback across a battery assembly, and not insights on the position of excessive pressure occurring in the battery itself, battery designers would be missing out on important data.
More recently, battery designers have turned to pressure mapping systems to collect comprehensive data during Li-ion battery testing. The thin and flexible array of sensing elements can wrap around the battery to provide a 360° view within a testing fixture. These systems can be used to evaluate pressure profiles over thousands of charge and discharge cycles, which can last several months.
Figure 3 shows an example of charge/discharge characterization data from a fixture incorporating a piezoresistive pressure mapping system.
Insights from R&D Initiate an Embedded Battery Safety Feature
Recently, a team of engineers designing a new laptop with a high-efficiency Li-ion battery found a unique application inspired by their R&D efforts. After characterizing the battery performance using a fixture similar to the one shown in Figure 3, the team had an idea to embed the same piezoresistive sensing technology into their laptop to serve as a method to monitor abnormal battery swelling while in use.
To begin, the design team purchased sample piezoresistive force sensors, specifically selecting a pressure-sensitive ink variety formulated for high-temperature and high-humidity operating environments. Because piezoresistive sensors are so thin — only 0.203 mm (0.008 in.) — and can function with low-power electronics, the team did not need to make any significant adjustments to their design.
For their prototype, the team positioned two sensors on either end of the battery compartment to detect localized changes in force, as shown in Figure 4. Based on the characterization data the team acquired during the design of the battery itself, they were able to determine a relative force threshold signifying the battery was approaching failure. They then developed a feature that would trigger an alert to the user’s screen before the battery pressure reaches a critical threshold.
Accounting for Sensor Drift in the Relative Measurement Application
Because sensor drift will cause the piezoresistive sensor’s output voltage to change gradually over time, using absolute voltage outputs to determine battery swelling becomes a challenge. However, this battery safety application only required measuring relative changes in force, which is not affected by drift since the slope of the Voltage vs Force curve stays relatively constant regardless of how much the output has changed.
When the sensor is powered with the circuit shown in Figure 2, our piezoresistive force sensors typically have an output drift rate <5% / logarithmic time. Therefore, for relative measurement applications, the design engineer should look for the differential voltage output as a function of force (the slope of the V vs F curve) as shown in Figure 5.
On the other hand, for an application requiring measuring an absolute measure of force to produce some sort of an action (e.g., an actuator pressing on the sensor at exactly 5 lbs would cause “X” response, while 10 lbs would produce “Y” response), then the engineer would need to follow a different calibration procedure.
Measurement Tools and Embedded Components Help You Understand the Full Scope of your Battery Design Decisions
Satisfying consumer demands while maintaining a safe, repeatable design, is a difficult balance for battery designers. Temperature increase during charging causes lithiation, which leads to gassing. Internal pressures from the additional gasses in the battery can cause housing or separator failures. This can lead to thermal runaway reactions in extreme cases.
Ultra-thin piezoresistive sensing technology, whether as a test and measurement tool in the design process, or an embedded component in the final product, helps measure and identify regions of excessive pressure that can signal complications or potential battery failures. This, in turn, helps battery designers develop advanced energy technologies to power our lives safely.