Solid-state lithium microbattery technology is poised to leapfrog Li-ion and Li-poly alternatives for hearable and wearable devices.
By Arvind Kamath, Ensurge Micropower
The more popular digital health and fitness wearables and hearables become, the more frustrating the shortcomings of their rechargeable microbatteries.
Decades-old Lithium-Ion (Li-ion) technology has reached its limits while handcuffing product developers to inflexible form factors. Newer technologies like Lithium-Polymer (Li-poly) provide some form-factor relief with their bendable pouch shapes and more stable solid polymer electrolyte as compared to Li-ion, but likewise don’t deliver the energy density, charging speed or other key capabilities that device manufacturers need in the 1 milliampere-hour (mAh) to 100 mAh rechargeable microbattery class.
This has changed with recent advancements in solid-state lithium microbatteries that significantly improve volumetric energy density (VED) and enable much faster charging from a smaller, lighter microbattery with better reliability and a longer lifespan. Solid-state lithium battery technology also introduces the option to customize microbattery form factors and adopt new, simpler design and manufacturing processes. Plus, solid-state lithium microbatteries are expected to be the first to meet the most demanding peak current requirements of today’s Bluetooth and other low-power radio-frequency (RF) system-on-chip (SoC) devices that give wearables and hearables their most compelling capabilities.
Today’s products stress battery limits
Users have embraced sensor-based health fitness products, from smart rings worn on a finger to electronic skin patches, armbands, belts, gloves, shoes, shirts, vests and leggings. These devices monitor organ function, heart rate, blood chemistry, circulation and other metrics in real time. Fitness versions offer many of the same capabilities while also monitoring environmental conditions such as location, temperature, wind velocity and terrain incline.
“Hearable” products include medical hearing aids, headphones, and true wireless stereo (TWS) earbuds. These products assist people with impaired hearing, perform noise-cancellation, and allow users to listen to music, sports broadcasts and even private, real-time speech translations. Many also issue notifications, display information and accept phone calls.
Wireless connectivity consumes a significant amount of battery power. This will get worse as digital health and fitness monitoring devices are incorporated into wireless body area networks (WBANs) that allow medical professionals to access patient data online. Powering these medical wearable devices has been a major question mark because of the limitations of the prevailing technologies. There must be enough battery power and capacity for WBAN nodes to support longer spans of wireless connectivity, higher current pulses for wireless transmission, and increased functionality, while encrypting data to ensure it is transmitted securely that can add to the computational load and subsequent battery drain.
Solid-state lithium battery technology can meet these needs, either alone or as a companion to energy-harvesting solutions. It delivers superior VED in a microbattery and enables microbatteries to be produced using more simplified and scalable manufacturing and assembly processes than Li-ion.
Maximizing energy density
A commercial solid-state lithium microbattery must meet VED requirements while also delivering the necessary charging speed and charge cycles for today’s connected wearable and hearable devices. This needs careful interfacial engineering of the microbattery chemistry’s materials and critical interfaces.
Tests show that solid-state lithium microbattery can now achieve a full charge to more than 80 percent of capacity in 20 minutes or less, while targeting an industry-high VED of 700 watt-hours per liter (Wh/L) (see Figure 1 below). This is twice that of the equivalent Li-ion microbatteries used in 1 milliampere-hour (mAh) to 100 mAh applications, using constant voltage charging at 4.2 volts (V). Li-ion batteries must typically charge more slowly and use a costly and complex charging solution to avoid dendrite formation and other reliability issues.
To achieve this industry-high VED, solid-state lithium microbatteries minimize the packaging required to protect a Li-ion microbattery’s liquid electrolyte.
Further VED improvements come from reducing the thickness of the substrate onto which the solid-state lithium microbattery’s cathode and electrolyte are deposited. Today’s substrates are as little as 10 microns thick. VED can also be increased by stacking the battery’s energy-producing core cells as compactly as possible while still maintaining low impedance. This is now possible using proven high-volume roll-to-roll manufacturing techniques to fabricate the core battery cells. The unit cells are then cut from the roll and stacked to the desired height based on the end-product’s specific capacity requirements. Figure 2 below compares the VED of this type of solid-state lithium microbattery and Li-ion alternatives.
The ability to cut and stack cells provides two additional benefits: form-factor customization, and simplified product assembly. The microbattery’s core cells can be cut from a sheet at specified lengths and widths as needed for the wearable or hearable device’s size, fit, comfort, and other requirements. These stacked core cells can also be encapsulated and packaged with cathode and anode metal connectors (see Figure 3 below). This enables the microbattery to be directly connected to the Printed Circuit Board (PCB) using the same standard surface mount technology (SMT) as a product’s other components while using a low temperature (up to +160°C) reflow profile.
These advances have not come at the expense of charging speed and cycles. Today’s solid-state lithium battery technology doubles VED as compared to Li-ion while also enabling up to double the charging speed of the fastest-charging Li-ion battery, and longer product lifetimes. Plus, while Li-ion batteries can only supply pulse currents up to twice their rated current (also known as their “C” or “pulse” rate, or the rate at which the battery discharges in a short period of time to support communication), solid-state lithium microbatteries increase this number 10 times or more. This eliminates the need to specify batteries with more capacity than required, just to support high enough peak current rates for delivering at least 5C pulse current discharge for supporting Bluetooth and Wi-Fi communication protocols (see Figure 4 below). This 5C rating means the battery can provide five times its mA current for several seconds.
Solid-state lithium battery technology is poised to leapfrog today’s Li-ion and Li-poly alternatives across all performance metrics required in today’s hearable and wearable devices.
Understanding how to leverage these benefits will enable developers to pursue concepts for new products that offer more features and new capabilities in smaller, more comfortable, and better-fitting shapes and sizes.
About the author
Arvind Kamath, Ensurge VP of technology and engineering, has built and led several technology development, engineering and operations teams. Most recently, he was responsible for the flexible substrate roll- to-roll printed dopant polysilicon (PDPS) manufacturing scale-up and led the teams that built a global ecosystem for enabling it.