Portable Batteries: Part 1
There is an ever-growing need for portable energy storage solutions ranging from cell-phone batteries to electric vehicle batteries, and from microbatteries for tiny disposable sensors to lightweight powerpacks for flying cars.
This is a four-part overview of the current state of the lithium battery industry. We explore the future of portable battery technology, and how to evaluate new battery technologies for applications ranging from electric aviation, autonomous vehicles, medical implants, to portable electronics.
Introduction
Battery technology has been one of the major enablers of mobility innovation over the last 50 years. From medical implants to powered vehicles, every advance made in stored energy density has been accompanied by a sizeable step forward in miniaturization and capability in the broader device market.
Limitations in portable energy storage are one of the biggest reasons that we aren’t yet moving around in powered exoskeletons or commuting in flying cars. In general, the physical size and capability of any device is fundamentally limited by its power source. For portable electronics, the batteries are very often the bulkiest/heaviest component.
Performance Criteria
Improving energy density has been the primary factor driving ongoing battery research. Energy density is typically expressed in units of watt hours, (abbreviated Wh), which is the amount of power (in watts) the battery can deliver for a one-hour period. Energy density is described in two different ways. There is volumetric energy density (expressed in Wh/Liter), where the goal is to have more energy stored in a smaller package volume, such as medical implants or hearing aids. And then there is gravimetric energy density, where the weight of the battery is of primary concern such as electric vehicles and drones, and expressed in watt-hours per kilogram (Wh/kg). A closely related measure is Capacity (expressed in Amp hours, Ah), representing the discharge current a battery can deliver over time.
Energy density is only one of many factors that might drive selection for a particular application. Also of importance is power density, describing how much electrical current can be delivered by a battery without undesirable effects such as sagging voltage, build-up of heat (which can lead to bursting or fire), or unwanted chemical side-reactions inside the cell. This raises another important factor, safety; both in terms of potential for fire/explosion if the stored energy gets suddenly released, and also in the form of toxicity of the chemical compounds themselves; both during operation, and at end of life when it ends up in a landfill. Other important considerations are the overall cycle-life of a battery, describing how many times it can be discharged and charged before the performance drops below some threshold (usually 80% of the amount of energy it originally stored). Finally, of course, the cost of production and purchase price is a key factor driving batter selection.
Chemistries
In the search for better battery technology, many different chemistries have been tried. The majority of practical battery technologies involve oxidation/reduction reactions of metals in an acidic or basic electrolyte. The ubiquitous “alkaline” AA and AAA battery derives its power from the reaction between zinc metal and manganese dioxide electrodes in a caustic potassium hydroxide electrolyte. The equally common lead-acid battery reacts lead(Pb) metal and water in a sulfuric-acid electrolyte. It is typical that a chemical battery will contain a significant quantity of reactive metal, and a liquid electrolyte, both of which are quite heavy materials.
It is no surprise that lithium arose as a leading candidate for advanced battery chemistries.
Lithium has the lowest weight of all metals, while having the highest electrochemical potential (voltage) on the periodic table. Lithium is not only the latest and greatest material in battery science, it’s the theoretical best there is.
There are many different Lithium-based chemical reactions that are suitable for encapsulation into battery cells, but all of them share a common mechanical anatomy:
Battery Components
- Anode: The negative electrode during discharge. Typically a porous carbon graphite sheet that serves like a conductive sponge to loosely bind lithium ions in a process known as intercalation.
- Electrolyte: A conductive liquid or gel through which ions are transported as charge carriers. This is typically a strong organic solvent, which is quite flammable if exposed to air, and the source of most battery fires.
- Separator: A porous membrane which serves as a spacer, preventing the anode and cathode electrodes from coming into contact, and also acting as a reservoir for the liquid electrolyte. Can be made from plastics, polymers, paper, or more exotic materials. Must allow lithium ions to pass through.
- Cathode: A highly-porous metal oxide layer that participates in the chemical reactions of the cell. Lithium ions migrate into this lattice during discharge. This is usually the largest contributor to energy capacity and power density.
- Current Collectors: Metal foil applied as a backing to each electrode to conduct electrons into and out of the cell. Copper(Cu) is used on the anode; aluminum(Al) on the cathode. The materials are chosen to be passivated and stable at the potential of each electrode, and do not normally take part in the chemical reactions. Many different metals/materials would work, but Al and Cu are cheap and abundant.
- Housing/packaging: The outer enclosure of the battery. Does not participate in reactions, but must be able to withstand elevated temperatures and mechanical effects (expansion, pressure, etc). Contributes to the overall weight and volume of the battery cell.
Advances in any individual cell component can lead to substantial improvements in energy density, safety, power output, or cycle life of the assembled battery cell. Entire materials companies are built around developing and supplying a single one of these components to the major battery makers. Battery manufacturers can choose the materials for each component (cathode, electrolyte, separator, etc.) to enhance specific qualities desirable for their use-case; however, an enhancement in one property often compromises something else. Thus true advances in any one of these components can lead to new optimal combinations, resulting in substantial gains in overall energy density, safety, power output, and cycle life of the assembled battery cell. However, there are countless material companies pursuing this approach; and getting mass adoption of a particular material can be as much a matter of luck as of actual performance.
Conclusion
In part one of this piece, we have introduced an overview of battery technology and introduced lithium ion as the leading chemistry in high-performance applications. In parts two, three and four we will cover the battery manufacturing process, the current state of the art technology and what we see coming in the future.
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