Eliminating Batteries as the Main Innovation Barrier for Smart, Connected Devices

As solid-state lithium microbattery technology enters commercialization, the industry is getting a much better picture of how it will transform the manufacturing and use of rechargeable hearables, wearables, and implantable devices. The latest validation tests confirm just how much the technology will leapfrog traditional Lithium Ion (Li-ion) and Lithium Polymer (Li-polymer) alternatives in terms of battery life, performance, and manufacturability.

But even more intriguing are the opportunities for device innovation when products can be powered by microbatteries that enable smaller and slimmer wearables, hearables, and implantables. There is also the opportunity for new curved, bendable, and round product shapes, including sleeker smart rings and hearables that fit more comfortably in the ear canal. Additionally, ultrathin flexible formats will also be possible, subject to further testing and use case requirements. Ensurge is engaging with a number of partners as part of industry efforts to drive the commercialization of these types of new form factors.

Key Differences Between Solid-state Lithium and Alternatives

Solid-state lithium microbatteries use a considerably different chemistry and architecture approach than Li-ion and Li-poly alternatives. Most notably, solid-state lithium batteries eliminate the flammable liquid electrolyte/gel and separator that these batteries need, which must be contained with metal or other packaging to prevent swelling, breaches, and safety events. These packaging measures typically reduce the amount of volume available for energy-containing compounds, limiting the energy density. For example, rather than having a pre-assembled anode as Li-ion and Li-poly batteries do, Ensurge’s solid-state lithium batteries instead form a functional Lithium anode after the manufacturing process, during the battery’s first charge.

This foundational difference between solid-state lithium microbatteries and other approaches delivers a substantial improvement in volumetric energy density (VED). These batteries are designed to achieve an industry-leading VED of 600-750 Watt-Hours per Liter (Wh/L) in the 1-100 mAh capacity class by minimizing the packaging and substrate materials contained in the battery. The additional packaging and materials take up space that could otherwise be used to contain energy

There are other important differences. First, solid-state lithium microbatteries are produced in a low-cost manufacturing environment (no dry room required), using a stacked-layer architecture and proven high-throughput roll-to-roll manufacturing processes that enable capacity and form factor customization. During core battery cell fabrication, the cathode and electrolyte are deposited on an ultra-thin 10um stainless steel substrate as These core cells are then cut from their roll at specified lengths and widths as needed for the desired product size, fit, comfort and other requirements. This is a major departure from the restrictive cylindrical shapes and limited standard sizes typical of Li-ion alternatives.

Layers are stacked to meet the end-product’s specific capacity needs. At this point, the stacked core is packaged and then encapsulated and metallized with cathode and anode metal connectors to facilitate direct connection to the Printed Circuit Board (PCB). This enables batteries to be integrated into products in a “pick-and-place” manner similar to the product’s other electronic components using Surface Mount Technology (SMT) with a low-temperature (up to +160°C) reflow profile.

Two other microbattery attributes are especially important: fast charging and pulse discharge.

A Closer Look at Charging and Discharging

Simplification of the battery management system (BMS) is particularly important. Every secondary (i.e., rechargeable) battery, including solid-state lithium and the traditional Li-ion and Li-poly alternatives, repeatedly charges and discharges across its lifetime. The difference is that Li-ion and Li-poly batteries’ charging and discharging cycles must each be carefully managed to maximize the battery’s lifespan and protect them from the safety risks of their liquid electrolytes.

To do this, a purpose-built battery-charging integrated circuit (IC) is used that combines two charging methods over time during a charging event: Constant Current (CC) and Constant Voltage (CV). CC mode is used initially to steadily increase the battery’s voltage, typically to the battery’s voltage rating. Then the charging system switches to CV mode, holding the charging voltage at a steady level while the amperage (amount of potential current) is reduced. This charging cycle ends when the battery reaches its voltage and current ratings.

Using this type of battery-charging circuit and charging method maximizes the Li-ion or Li-poly battery’s potential lifespan while preventing overheating and reducing the risks of fire, overcharging and dendrite formation. But it increases costs, board area, and design complexity while creating a relatively slow charging process. It often takes more than two hours of total charging time to go from a depleted state to a full charge, even in microbatteries having less than 100 milliampere-hour (mAh) capacity.

In contrast, solid-state lithium battery technology can charge in CV mode and does not require complicated battery charge control and protection circuitry. The charge voltage is held constant at 4.2 V while the current varies, with no performance or reliability issues as the battery completes its full charge. Tests show solid-state lithium microbatteries can achieve an 80% charge capacity in less than 8 minutes (Figure 1).

Low impedance contributes to a well-formed stack discharge curve with a relatively “flat profile.” It can be challenging to achieve low impedance, but it is critical when integrating anode-less chemistry materials. These challenges exist between the substrate and the cathode, the cathode and solid electrolyte, and the solid electrolyte and the battery package. Figure 2 shows the results of Ensurge electrochemical impedance spectroscopy (EIS) tests and how low impedance contributes to the solid-state lithium microbattery’s discharge performance, among other key metrics.

Solid-state lithium technology’s benefits are equally valuable during discharging. Connected wearable and hearable devices must be able to support wireless transmission via Bluetooth and other high-current load events during health measurements. Even though this type of demand may occur in a small fraction of the time of the duty cycle, and therefore uses very little of a battery’s energy capacity, it is critical that the battery support this much higher current draw, when needed, for transmitting wireless signals to another electronic device. For this reason, product developers generally must specify more battery capacity than they actually need.

Consider the example of a product with a battery capacity requirement of only 4mAh. If the product’s application needs it to deliver a 20mA pulse during Bluetooth Low Energy communication, the designer must specify a Li-ion or Li-polymer battery with about 10mAh of capacity to meet the pulse requirement during wireless transmission. Replacing the Li-ion battery with a solid-state lithium battery will allow for pulse discharge rates greater than 5C. Ensurge testing of its solid-state lithium battery technology has demonstrated it can produce pulse discharge cycles of 12.5C (see Figure 3), with further improvements planned for the future.

Other benefits of this new microbattery technology include low self-discharge, which is ideal for low-duty-cycle applications where recharging is not always available, and a more rugged and reliable connection than batteries that require wires, sockets, and connectors, which are susceptible to corrosion.

Opportunities for Form Factor Customization

As product manufacturers prepare for the availability of solid-state lithium microbatteries with these capabilities, they are exploring a wide and growing range of new form factors. For the first time, it will be possible to specify customized microbattery sizes and shapes. Figure 4 illustrates the roll-to-roll deposition, ultra-thin packaging, and encapsulation and metallization process that, together, make this form factor’s flexibility and customization possible.

The types of products that will be possible using solid-state lithium microbatteries include smaller, thinner, and more capable and comfortable wearables and hearables for fitness tracking and remote patient monitoring. These products will be able to do more in the same or smaller form factors, during the same or longer periods of time between charges. The technology will also spur innovation in the implantable space for products ranging from cochlear acoustic amplification solutions to pacemakers and neurostimulators. Other implantable examples include ophthalmic devices for sensing, diagnostic, and therapeutic applications, such as treating presbyopia, photophobia, and glaucoma, and for delivering augmented and virtual reality capabilities, and even smart contact lenses that leverage curved form factors that are conformal to the shape of the product application and available in ultrathin form factors.

Changing the Shape of Innovation

A roadmap is critical to creating more options for product designers using safe, high-performance solid-state batteries. On the horizon are patent-pending technologies that enable form factors to support round and curved batteries that maintain the high performance and high volumetric energy density of the core stacked solid-state architecture. These will be especially attractive to designs that already use such architectures with Li-ion or Li-polymer batteries (Figure 5).

Conclusion

In summary, the coming generation of solid-state lithium microbatteries will bring much higher VED to wearable and hearable devices, along with faster charging and discharging performance. Perhaps more importantly, it will also remove the innovation obstacles that conventional Li-ion microbatteries have created. Solid-state lithium microbatteries will open the door for thinner, smaller, more comfortable, and trend-setting new products that can be worn in more locations both on and inside the body while delivering more features that can be enjoyed for longer periods between charges.