Why are lithium-ion batteries such a headache for mobile device designers?
What can be done about it?
The applications, features and connectivity that mobile technologies offer today’s consumers would have been impossible to imagine a decade ago. Brighter and bigger screens deliver HD videos and 3D graphics. High speed 4G and WiFi wireless chipsets and high-capacity flash memory enable a high quality mobile experience with streaming, caching, download and storage of video, music and games.
Unfortunately, while mobile technology is racing into the future, lithium-ion (Li-ion) battery technology is stuck somewhere in the ’90s. Neither consumers nor mobile product manufacturers are happy with its performance and battery life issues. The problem is thermal instability. Fundamental limitations in current Li-ion chemistry make it vulnerable to elevated temperatures, which not only degrade battery life, performance, and safety, but make it harder to deliver the sleek, ever-slimmer devices consumers demand.
Heat and the Death of a Li-ion Battery
The cells in today’s Li-ion batteries, just as in the early ’90s, are made of an anode (+) and a cathode (-) between which is a liquid electrolyte made up of a lithium salt and a combination of organic solvents and additives. Unfortunately, the particular salt used — lithium hexafluorophosphate (LiPF6) — tends to react with residual moisture in the cell to produce hydrofluoric acid (HF), one of the most chemically reactive substances imaginable. As with all chemical reactions, the process speeds up as the temperature rises.
Thus, Li-ion batteries operating at the high temperatures found in consumer devices deliver shorter run-times and fewer charge-discharge cycles. The battery can even balloon or rupture, destroying the device or worse. These possibilities mean unhappy consumers, overburdened help desks, and lost customers.
Li-ion cell datasheets generally show test results for cells and batteries at a “room temperature” of 20°C (68°F ).But mobile device reality is nowhere near this temperate. Designers want to be able to pack in more features, resulting in more heat-generating components surrounding the battery. Consumers demand longer run-times while carrying their phone in a pocket or leaving it in their car on a sunny day — in other words, at double or triple the 20°C design spec.
The heat maps illustrate why the thermal instability of Li-ion batteries makes their placement a major obstacle to designing super-svelte and reliable smartphones, tablets and ultrabooks. The bright orange spots represent temperatures well beyond the advertised specs of the Li-ion batteries, and that’s before the consumer parks it on a surface that blocks the vents.
The Real Measures of Mobile Battery Life
To consumers, battery life is a very simple metric: when device run-time drops below about 80 percent of its initial value they become dissatisfied. It’s time to replace the battery. For mobile device designers, it’s more complicated. They need to ask pointed questions of their battery vendors and carefully probe three factors:
* Cycle life: how many charge/discharge cycles can the battery sustain?
* Calendar life: over what period of its life can it sustain an acceptable device run-time?
* Dimensional stability: how subject is it to swelling or ballooning?
Most Li-ion battery manufacturers today advertise between 350 and 500 charge/discharge cycles. This is based on accelerated, room-temperature testing that is not representative of longer-term consumer use, especially if depth of discharge (DOD), state of charge (SOC), and temperature are considered.
Not stressing the cell over its full range, especially not fully charging the cell (100 percent SOC, where cell degradation is accelerated), will generate misleading cycle life figures. For instance, a DOD of five percent or less is how low earth orbit satellites achieve the necessary hundreds of thousands of cycles.
In a mobile consumer device, reducing DOD or maximum charge means either a shorter run time, or more cells and a bulkier device (less volumetric efficiency). Designers should ask the battery supplier what the cycle life specification looks like at 100 percent DOD at 40°C or even 60°C .
Calendar life measures how well the battery maintains device runtime over the life of the battery regardless of the number of charge/discharge cycles. The interplay between state of charge and temperature has a major effect on calendar life.
Consumers often plug an almost fully-charged device into the power outlet and leave it connected all day long: a worst case scenario of high temperature with 100 percent SOC. In addition, although batteries are shipped from the factory at about 50 percent charge both to maximize battery life and to comply with transportation safety regulations, uncontrolled temperatures in shipment and distribution can significantly shorten battery life.
Any cell may experience slight expansion during usage; in fact, very slight swelling is part of normal operation, and is taken into account in the total cell thickness provided in data sheets. But HF generation in a Li-ion cell can sometimes trigger a runaway state in which gas generation grossly deforms or even bursts the battery. This is one of the leading causes of battery recalls in consumer electronics with embedded batteries.
What Can Be Done About Thermal Instability?
Mobile device manufacturers are trying to educate consumers about how to care for their mobile devices. Apple has done an especially good job here.1 But education is not enough when the underlying battery chemistry is flawed. Mobile device designers should carefully consider alternative lithium electrolyte chemistries such as the one patented by Leyden Energy (www.leydenenergy.com). Leyden Energy’s lithium-imide (Li-imide) electrolyte chemistry provides superior thermal stability because it is not sensitive tothe humidity present inside the cell.
As the chart below demonstrates, the greater thermal stability of Leyden Energy batteries delivers more than 750 cycles before its capacity drops below the 80 percent runtime at which consumers become discontented. That represents a consumer fully discharging and recharging their device every day for over 2.5 years, even at temperatures of 40°C.Battery placement in device design becomes much easier, at a cost on par with that of traditional Li-ion technology and which can be available from multiple manufacturing sources.
About the Author
Marc Juskow is an industry leader with a unique skill set from lithium ion battery, ultracapacitor research and development, application engineering, sales, marketing and executive management. Marc has designed and built a variety of world class products. In the field of lithium batteries and ultracapacitors, Marc has worked for EaglePicher, PolyStor, Cooper Bussmann (PowerStor), Qynergy and OMG (cobalt supplier), and founded three companies: Volt Source (consulting), UNCAP (ultra-capacitors) and Mobile Power Solutions (independent battery testing). Marc began his battery career at Moli Energy in Vancouver, Canada in rechargeable lithium battery research. In the early 1990's, he managed the Product Development and Evaluation teams during the development of the first lithium-ion cell in North America and later assembled their Sales and Marketing Group to commercialize Moli's lithium-ion technology. Marc has over 35 publications and formal presentations, including keynote, plenary and many invited presentations internationally. He holds an Executive MBA and an MS in Chemistry from Simon Fraser University in Vancouver, BC.