Wearables: Energy Harvesting and the Evolution of Lithium-ion Batteries

Wearables 4.0

Wearables are electronic devices designed to be worn either on or in the user’s body. Wearables come in two main forms namely accessories and implants. The accessories include jewelry, clothing and portable fashion and entertainment devices while implants include smart tattoos, chips and medical devices. Wearables have been and are becoming ever more integral to our lives. Think of first generation wearables like wrist watches to fourth generation wearables like e-skin. The new generation wearables differ from the past ones in one major way: internet connectivity which enables the transfer and storage of vast volumes of data in the Cloud.

The key areas of application of wearables are health care, fitness and sports and entertainment. The use cases range from the monitoring of body vitals and performance to virtual reality.

Power source: Constraints

How do wearables power their operations? Like in any other electronic device, the power unit is one of the key components of a wearable device. Traditionally, the power unit of choice has been the lithium-ion battery cell due to its high energy density. High energy density is very significant given the size constraint of wearables. The wearables industry has promoted significant innovations around the lithium-ion battery cells including increased energy density, improved flexibility to allow safe deformation, customization of battery cell shape to match device shape and lower fire risk.

Trends: what’s new?

The power consumption of the wearable is based on the consumption of the integrated circuits, the sensors and the transmitters. Recently, several factors have led to the rethinking of the power unit of wearable devices. Key among them are the development of low power consumption electronic components. The other factors are related to the lithium-ion battery cells. Lithium-ion battery cells are costly, consume a lot of space, have a finite lifetime thus require maintenance and are a big source of environmental pollution. As such a need exists to remedy these challenges. Enter energy harvesting.

Energy harvesting: the key?

What is energy harvesting? Energy harvesting is the conversion of ambient energy into electrical energy. We are familiar with large scale energy harvesting techniques such as solar photovoltaics (PV) and wind turbines. Given their low power consumption, wearable devices can leverage microscale and nanoscale energy harvesters. An energy harvesting device is composed of a microgenerator, a storage unit and a power conversion and management circuit.

A microgenerator converts a certain type of ambient energy into electrical energy: micro solar PV (light), thermocouple (heat), piezoelectric generator (mechanical strain), triboelectric generator (electrostatic energy) and radio frequency harvester (electromagnetic energy).

Storage duel: lithium-ion batteries vs supercapacitors

As you are well aware, the intermittency of ambient energy sources means low reliability. As such, a form of storing the generated energy for later use is necessary. Energy storage can be provided by two main means: a next generation lithium-ion battery cell or a supercapacitor. Utilizing a capacitor instead of a battery enables a batteryless wearable device.

A supercapacitor is an electrostatic device that operates by separating positive and negative charges during the charge cycle and allowing the recombination of the charges during the discharge cycle.

The next generation lithium-ion battery cells, called thin-film solid state (TFSS) lithium-ion battery cells, employ two major technological advancements: vapor deposition (enables micrometer level layer thickness) and solid state (liquid electrolytes replaced by a solid electrolyte).

The electrochemical reactions in a TFSS lithium-ion battery cell are the same as those in a typical solid state lithium-ion battery cell.

TFSS LIB cells and supercapacitors have their own strengths and weaknesses. TFSS cells provide higher energy density and slower discharge rates but a shorter lifetime. Supercapacitors on the other hand offer a longer lifetime but lower energy density and faster discharge rates. A higher energy density, slower discharge rates and longer lifetime is the combination of performance characteristics of an ideal energy storage solution!