Wearable devices are transforming everything from remote patient monitoring to fitness tracking and hearing assistance. The next step is to shrink the size of these devices while offering more comfortable shapes and additional features and wireless communications capabilities. The biggest obstacle is the battery, and more specifically the Lithium-ion (Li-ion) batteries used in the majority of wearables. Today’s devices have been tethered to the form-factor and performance limitations imposed by these batteries’ liquid electrolyte chemistry.
An alternative has now emerged. Rechargeable 1 milliampere-hour (mAh) to 100 mAh solid-state lithium microbatteries use an inherently safer solid electrolyte than Li-ion. They increase energy density in a smaller, rectangular cuboid (rather than cylindrical) form factor whose length, width and height can be customized based on the needs of the wearable application. They also can be assembled onto a printed circuit board (PCB) using the same standard assembly processes as a wearable’s other components, dramatically simplifying how wearables can be designed and manufactured.
Wearable Device Requirements
Many wearable requirements are driven by the photoplethysmography (PPG) technology used to measure the volumetric variations of blood circulation in health and fitness monitoring applications. Because PPG sensors measure how light shined into the body is scattered from blood flow, they work better in some body locations than others, depending on the application. Heart rate can be monitored well using PPG sensors in nearly any body location, but blood pressure monitoring may not be as accurate as necessary from locations like the wrist or ankle because of their bone, muscle and tendon physiology and the effects of arm and leg movements.
Because of these dynamics, batteries must support many different wearable types, sizes and shapes. Smart-ring manufacturers have begun exploring alternative to smart rings based on Li-ion microbatteries because they that fit better into these wearables’ curved shape. Lithium-polymer (Li-poly) batteries also meet this need, but at the cost of energy density, which is half or less that of Li-ion. Manufacturers need battery alternatives, but they also need greater energy density, not less.
Users have their own needs. Smaller is better, but this reduces energy density. They also want new features that require batteries to do more in a smaller package so that wearables can support, besides PPG, electrocardiogram (ECG), accelerometer and gyroscope sensors and more, along with their associated communications requirements. Users also want a comfortable fit in many different situations at rest and during sleep, exercise, or daily activities.
These and other challenges can be solved with solid-state lithium microbattery technology that have been adapted to wearable needs.
Bringing Solid-State Lithium Technology to Wearables
One of the biggest obstacles to using solid-state lithium technology in wearables has been the lithium metal anode. The anode was deposited on top of the battery electrolyte during manufacturing, requiring a zero-humidity and argon (AR)-based environment.
When an anode-less solid-state microbattery is manufactured, no anode layer is deposited on the electrolyte. Instead, a layer of lithium is formed between the anode current collector and the solid-state electrolyte after manufacturing when it is first charged. This layer becomes the anode in an anode-less solid-state microbattery.
Benefits range from increased energy density and faster charging to greater form-factor freedom and new, simpler design and manufacturing options.
Energy density and faster charging are among the biggest benefits. The former is generally calculated as storage per unit volume, or volumetric energy density (VED). If two batteries have the same capacity and one is twice the size, then the smaller one has twice the VED. Tests show that the best-performing anode-less solid-state microbattery technology delivers up to twice the VED of Li-ion alternatives.
VED gains come from freeing up space inside the microbattery. Li-ion microbatteries require space-consuming safety mechanisms and interior packaging to protect them from the ambient environment and electrical conditions (overvoltage and overcurrent). Solid-state lithium microbatteries don’t need these structures, and further improve VED using an ultra-thin (10-μm) stainless steel substrate and ultra-compact stacking and packaging of the battery’s energy-producing cells. The higher VED enables designers to, for instance, create smaller hearing aids that integrate more features, and health tracking rings that monitor more sensor input over longer periods before recharging.
The same anode-less architecture offers faster charging speed than Li-ion and higher current discharge pulses for wireless communication. Since Li-ion batteries can only supply up to twice their rated current, wearable designers generally use higher-capacity barriers than needed just to support their high pulse current discharge requirement. Solid-state lithium microbatteries deliver up to 10 times their rated current. They also have lower leakage and, therefore, longer shelf life than Li-ion, taking up to four years or more to fall to half their fully charged state while a typical Li-ion microbattery might lose its entire charge in no more than three months.
Just as important as these performance improvements is form-factor freedom. Rather than forcing a cylindrical shape that has been the easiest for Li-ion battery manufacturers to build, solid-state lithium technology enable customizable rectangular form factors to suit the end-products ergonomically desired shape. Proven high-volume roll-to-roll manufacturing techniques are used to deposit the microbattery’s cathode and solid electrolyte, after which these unit cells are cut from the roll at customized rectangular lengths and widths and then stacked to the desired height based on capacity and other requirements.
Solid-state lithium technology also offers new ways to look at old wearable design challenges. For the first time, developers can optimize their wearables for either longer battery life or expanded feature sets, or a balance depending on the application. They will no longer need the protection and charging circuitry of Li-ion batteries, since solid-state lithium microbatteries require only a simple, low-cost constant voltage charging solution.
This new simplicity also extends beyond the design of wearables to their manufacturing. First, solid-state lithium microbatteries can be assembled onto PCBs using Surface Mount Technology (SMT) with a low temperature (up to +160 °C) reflow profile. This is because the absence of a traditional lithium anode, especially in the solid-date lithium microbattery’s as-manufactured or substantially discharged state, means there is no chance of lithium reacting to high SMT oven temperatures during assembly. Second, the microbattery’s stacked and encapsulated unit cells can be metallized at the side ends to create the electrical terminations needed for direct PCB connection. This eliminates the cost of coin cell socket and manual assembly or soldering required by Li-ion microbatteries.
Rechargeable solid-state lithium microbattery technology may one day also power wireless health monitoring devices such as continuous glucose monitors (CGMs), which currently use disposable primary batteries, that must be replaced and discarded about every ten (10) days. The technology may also benefit implantable devices like cochlear acoustic amplification solutions, pacemakers and neurostimulators. In the meantime, the technology is poised to help unlock innovation in the design of health, fitness and hearing-assistance wearables by enabling slimmer and more ergonomic form factors that are more comfortable for users and have slicker industrial designs while enabling more features, capabilities and longer operating times.