Is there an alternative to the Lithium-ion Battery?
Lithium-ion batteries (LIBs) have certainly been in the news recently, and not always for positive reasons. The characteristics of LIBs - excellent energy density, small memory effect and low self-discharge - have made them to first choice for a wide range of applications from cellphones to the latest generation of electric vehicles. However, LIBs may be state-of-the-art, but they are still an electromechanical batteries and have a few disadvantages. They can be affected by temperature, especially during the charging cycle, and they do not have an unlimited life. And, as Samsung have found out to their great cost, they can pose a safety risk.
Capacitors are sometimes presented as an alternative to rechargable batteries. They certainly offer a number of advantages that, in theory, suggest they are the future. They can be charged much more quickly, and they have an effectively unlimited life. However, their discharge characteristics are not identical to that of batteries (see fig 1.), and they exhibit much higher self-discharge, meaning that once disconnected from their load, they will lose their charge relatively quickly. In addition, traditional types have low capacitance, offering a much lower energy density when compared to batteries.
Fig 1. Capacitors do not provide stable discharge voltage when compared to batteries.
Filling the void between capacitors and batteries are the supercapacitors. These monsters of the capacitor world offer capacitance values of up to 100 times higher than traditional types, which make them more attractive as replacements for batteries. They still suffer from some of the same disadvantages as other capacitors - in particular high self discharge, and they still cannot compete with batteries with respect to total energy. However, their virtually unlimited life and quick charging means that Supercapacitors have found a number of key applications for themselves.
They can be used as a backup battery, covering momentary losses of power for applications ranging from household appliances to railroad systems. Another up-to-date application is in the field of energy harvesting, one of the highest profile examples of which is the KERS (Kinetic Energy Recovery System) utilised in Formula 1. This system harvests the energy generated during braking, stores it and then provides the driver with an energy boost during acceleration. The repeated cycle of this system makes good use of the rapid charge/discharge characteristics of supercapacitors, and highlights their advantages over lithium-ion batteries.
Despite these specific applications, it would appear that the end of LIBs may not be upon us yet. However, a lot of work is being conducted to close the gap between capacitors and batteries, and one manufacturer has released a product that brings the two technologies closer together.
Murata has developed a tiny, high-capacity, cylindrical energy device that is lithium-ion based, but behaves more like a supercapacitor. Designed to fill the gap in the available range of energy storage solutions, the UMAC is intended for use in wearables and wireless sensor applications, applications that demand reliability and safety.
The miniature device offers a high energy storage capacity with low internal resistance, fast-charging/discharging and the capability of withstanding fluctuations in load. The UMAC can be used in the same way as a capacitor to provide a secondary battery, although it offers improved charge/discharge attributes and an extended lifecycle over conventional batteries.
The use of chemically stable materials combined with UMAC’s small capacity prevents thermal runaway, which means that even in the unlikely event of a short circuit occurring, there won’t be any smoke or fire – a critical necessity for the wearables market.
The UMAC has an impressively high rate of charge/discharge cycles and is capable of 10 cycles every hour (maximum discharge rate of 30 mA). Its extended lifecycle is even more impressive with a minimum capacity recovery rate of 90% after 1,000 cycles. UMAC’s high-rate discharge characteristics make it particularly suited as an energy device for sensor nodes within wireless networks as peak-assist capacitors are not required. Stored energy is maximised and lost energy minimised due to the UMAC’s low self-discharge levels.
In addition to use in sensor networks and wearables, the UMAC’s long lifestyle characteristics that allow repeated use after rapid charging are also well suited for use as a back-up power supply for devices like mobile phones, as well as home appliances.
So, while it may be too early to predict the end of lithium-ion batteries, technologies like UMAC are opening up new possibilities to the engineer when safety, reliability and long-life are key.