Wearable Electronics
What to expect from a new generation of rechargeable microbatteries for wearables and hearables
Solid-state lithium battery technology will remove Li-ion performance limitations and enable both form-factor flexibility and more scalable production and assembly techniques. By Arvind Kamath, vice president of technology and engineering, Ensurge Micropower
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ince the emergence of rechargeable hearable and wearable products in the mid-2010s, there has been a continuous push for smaller devices with more functionality.
Ongoing improvements in size and functionality have challenged the traditional Lithium Ion (Li-ion) and Lithium Polymer (Li-polymer) battery technologies to meet the requirements of improved battery life, smaller size, custom shapes, high performance, and manufacturability.
These and other Li-ion deficiencies are focusing attention on rechargeable solid- state lithium battery technology, which has moved beyond the proof-of-concept stage and on through its first wave of validation testing by hearable and wearable device manufacturers. Commercialized solid-state lithium microbatteries will enable wearable and hearable device original equipment manufacturers (OEMs) to circumvent Li-ion microbattery’s limitations, transforming how microbatteries are designed, manufactured, assembled into end products, and used to power exciting new capabilities.
Solid-state lithium vs. Li-ion Several technology advances including interface chemistry, substrate engineering and packaging encapsulation have enabled solid- state lithium battery technology to deliver many advantages over Li-ion microbatteries. One example is fast charging, with the latest solid-state lithium microbattery tests demonstrating significantly better performance in this area than Li-ion or Li polymer (Li-poly) microbatteries.
The typical Li-ion microbattery – as well as Li-poly alternatives – goes through repeated cycles of charging and discharging across its lifetime. Each charging and discharging cycle must be managed extremely carefully to maximize the battery’s potential lifespan. This requires a purpose-built battery-charging integrated circuit (IC) that combines Constant
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Current (CC) and Constant Voltage (CV) charging methods. The former method maintains a constant current over time during a charging event, while the latter maintains a constant voltage over time during a charging event. CC mode is used first to steadily increase the battery’s voltage until it reaches a certain threshold (typically the battery’s voltage rating). At this point, the charging system switches over to CV mode, which holds the charging voltage steady while the amperage (amount of potential current) is reduced. This Li-ion charging cycle ends when the battery’s voltage and current ratings are both reached.
Another key purpose of this combined “CC- CV” process is to help ensure that Li-ion and Li-poly batteries are charged safely, including minimizing the risks of fire, overcharging, and dendrite formation. The special battery charging IC that maximizes the battery’s potential lifespan also prevents overheating, and includes a protection circuit required for safety. The downsides are increased costs, board area, and design complexity. The approach also results in a relatively slow charging process – total charging time can often be greater than two hours, starting from a depleted state to a full charge, even in microbatteries having less than 100 milliampere-hour (mAh) capacity. Solid-state lithium battery technology does not rely on this two-phase CC-CV charging protocol. These batteries charge in CV mode, only, and without the need for complicated battery charge control and protection circuitry. The charge voltage is held constant at 4.2 V while the current varies until the solid-state lithium microbattery is fully charged safely and without any performance or reliability issues. Tests show they achieve an 80 per cent charge capacity in less than 20 minutes (see Figure 1). Discharging is another big problem. Li-ion and Li-poly batteries use their battery charge control ICs to prevent overheating while being
Components in Electronics
discharged, and their discharge rates are also usually capped at 1C to 2C. This hinders the ability to support wireless transmissions or other high current load events. This problem, too, can be eliminated with solid-state lithium battery technology, along with other challenges that are solved using an “anode-less” chemistry, high-density core-cell stacking, and packaging techniques that enable these batteries to join other modern components in supporting today’s standard product assembly processes.
New chemistry and construction Solid-state lithium microbatteries eliminate the flammable liquid electrolyte and can also use an anode-less architecture. With this architecture, the microbattery, as manufactured, contains a reservoir of lithium but does not have a pre-assembled lithium anode. The lithium anode is formed during the first charge and is generated from the cathode. This action forms a thin, uniform film of lithium opposite the cathode that, functionally, serves as the anode. Again, this only occurs after the first charge is complete, which provides several important advantages beyond the fast charging and discharging speeds described earlier.
First, the anode-less architecture enables battery production in a high-throughput, low-cost, conventional manufacturing environment. Second, this anode-less chemistry is expected to achieve an industry- leading volumetric energy density (VED) of 600-75 0 Watt-Hours per Liter (Wh/L) in the 1 mAh to 100 mAh capacity class. A big reason for this VED breakthrough is the elimination of the space-wasting packaging
Figure 1: Ensurge used constant voltage charging at 4.2V to demonstrate that a solid-state lithium microbattery requires no more than 20 minutes to go from zero to 80 per cent of capacity. [Image courtesy of Ensurge]
www.cieonline.co.uk.uk
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