Better Living (and Fighting) through Batteries

Private companies and governments are ushering in a fourth industrial revolution. Yet the advances—autonomous transportation, environmentally friendly cars and aircraft, and man-machine teaming—are being held back by an ever greater need for portable electrical power. Innovations ranging from smartphones to electric aircraft are tethered to fundamentals of the battery charge/discharge cycle. Although battery efficiency and portability have improved, growth has been incremental, hindering great technological leaps forward. Figuring out the next significant improvement in battery technology is the key to keeping the edge in emerging and disruptive technologies.

Until fairly recently, batteries were important but mostly minor pieces of military technology—useful but (except on submarines) disposable products that enabled communications and helped turn on other technology. As far back as World War I, though, they were vital to submarines, allowing what essentially were submersible surface ships to hide underwater when detected, making little noise. Only the advent of nuclear power diminished their importance in the undersea realm—and, even then, most navies continued to operate diesel submarines (and, now, air-independent propulsion systems) that still require high-capacity batteries for stealth. But today, all sorts of new technologies and capabilities are driving the charge for better batteries—higher capacity, longer lasting, and faster charging. A variety of new technologies have begun to emerge that could enable major leaps forward.

The Basics

Batteries convert stored energy—usually chemical energy—into electricity.

First, a few definitions: An ion is an atom or molecule that has lost or gained electrons. (An atom or a molecule usually has the same number of protons as electrons.) More electrons than protons make it a negative ion. Fewer electrons than protons make it a positive ion. Any system that has an electric current flowing through it has an anode (on a “coppertop” AA battery, it is the electrode that is the negative terminal) and a cathode (an electrode that is the positive terminal). This oversimplifies the chemistry a bit, but it works for this discussion.

A battery converts chemical energy to electricity by two means: the simultaneous flow of electrons through a circuit from the anode to the cathode and the oxidation reaction of ions in an electrolyte (the “battery acid” in our AA). Until fairly recently, battery electrolytes were always liquid, but solid ones have been discovered as well. While a variety of different elements and compounds have been used as anodes, cathodes, and electrolytes in different configurations, the best energy density available commercially today is in Lithium-ion (Li-ion) batteries, in which lithium acts as the base for the cathode. These batteries can be configured for myriad applications, shapes, and sizes, making batteries the workhorse for all sorts of consumer products: for example, power tools, wireless speakers, and wearables.

We need three more definitions: Energy density is a measure of how much energy can be stored in a given volume—joules per cubic meter are common units. Specific energy is the ratio of the energy stored in a battery to its mass, usually measured in joules per kilogram or per gram. Specific power is the ratio of power the battery can deliver to the mass of the battery, usually measured in watts per kilogram or per gram. It might help to know that joules are units of energy (like calories), while a watt is a measure of power (like horsepower), which is defined as one joule per second. Think of specific energy as how much air a tank can hold. Some tanks are strong, and can hold a lot of high-pressure air, while others are weaker and cannot hold much at all. Specific power tells you how fast you can get the air out of that tank. The higher the specific power, the faster the airflow will be.

Li-ion batteries have higher specific power than any other small batteries on the market, meaning they can push higher currents. When comparing a battery’s specific energy (this time using watt-hours per kilogram) to its specific power, you can see its energy density. Among common battery types, lead-acid batteries (such as those in your car) have the lowest energy density; nickel-cadmium (NiCd, the rechargeables you bought in the 1990s) are better, if toxic; nickel-metal hydride (NiMH, the rechargeable batteries you put in your mouse) are better still; but Li-ion batteries, such as those powering your smartphone, laptop, or hybrid-electric vehicle, have the best energy density.

Lithium-ion batteries hold an energy-by-mass rating of about 250 watt-hours per kilogram (wh/kg), and an energy-by-volume rating of 350 watt-hours per liter (wh/l). Continual pursuit of increased specific power and specific energy has led to higher performance over time. Yet, the energy density of even the best batteries is still overshadowed by most other sources of energy—everything from solid fuels, such as wood or coal, to petroleum distillates, to naturally occurring gases such as hydrogen. The next performance leap in energy storage will come through extensive research and development—and maybe rethinking the problem in new and scalable ways.

Graphene

One material in development is perhaps the biggest breakthrough in material science this century—graphene. Derived from graphite, graphene is a layer of carbon atoms a single atom thick, arranged in a honeycomb pattern. It has been celebrated as the wonder material of the future because it has multiple astounding properties. For example, it is 100 times stronger than steel, transparent, extremely flexible, and highly conductive, and it releases multiple electrons when hit with a single photon, which is key for converting solar energy into electricity. (Silicon, the material used in most current solar panels, releases only a single electron per photon.)

Korean tech-conglomerate Samsung has developed prototype “graphene ball” batteries, which promise a 45 percent increase in energy density and five times faster charging over Li-ion ones. And graphene has better stability at high temperatures than Li-ion, which is important in many applications, given Li-ion batteries’ tendency to explode at high temperatures. Graphene’s main downside is that it is not yet easily manufactured at scale. But its possibilities in batteries and many more applications have prompted substantial research into how to overcome this obstacle.

Solid-State

Solid-state batteries replace the liquid electrolyte found in most modern batteries (often in the form of a volatile and reactive gel) with a solid. This allows for lighter and more compact configurations and improved energy density, plus a lower risk of overheating. Solid-state batteries will therefore be cheaper on a dollar-per-kilowatt-hour basis. Although an ideal formula for such electrolytes has not been solidified (sorry!), they still show great promise. Toyota has announced plans to replace Li-ion batteries in its hybrid cars with solid-state batteries in the next few years.

Ultracapacitors

The fundamental flaw in chemical batteries is that they do not charge instantaneously, nor do they have the ability to release a quick burst of power. Capacitors store energy in an electric field, not chemically. Ultracapacitors typically have lower energy density than batteries, but their rapid charge/discharge ability gives them real advantages. Used in conjunction with batteries, ultracapacitors give vehicles (such as Tesla cars) the ability to run steadily on battery power while quickly accelerating with a discharge from the ultracapacitors.

Military Applications

Batteries are present in more or less every military vehicle—planes, tanks, aircraft, ships, etc. But except for in some submarines (and, recently, some small unmanned aerial vehicles), they do little more than provide power to start an engine or run some onboard electronics. But this is changing, and battery development needs to be prioritized to use its full potential.

In contested locations, power needs to be sourced locally, be reliable, and allow for greater usability. Autonomous vehicles will need the capability to charge independently and run indefinitely. Man-machine teaming will improve only if warfighters can rely on robotic companions with effective long-lasting batteries. It is unlikely any one technology will provide all the answers. Instead, the merging of multiple battery technologies and further development of existing types will permit technology that will no longer be limited by the potential of today’s batteries.