Recharging Lead Acid Batteries  

The Economist has an article on a humble staple of the energy storage industry that may still have a future ahead of it - lead acid batteries - Lead-acid batteries: Recharged.

LEAD-ACID batteries seem to have been around for ever. They were invented in 1859 by Gaston Planté, a French physicist, and have done sterling work over the decades starting car engines and powering slow-moving vehicles such as fork-lift trucks and milk floats. Compared with the newer energy technologies that are now sweeping the world, however, it has to be admitted that they look old-fashioned and a bit frumpy. These days the catwalk is crowded with nickel-metal hydride and lithium-ion batteries, showing off their ability to pack a lot of energy into a small space and deliver a steady current over a long period. The fact that these modern batteries are also lighter (lead is, after all, one of the densest elements in the periodic table) has made them the first choices for powering truly serious electric vehicles, as opposed to the ones that potter about warehouses and suburban streets.

It is, nevertheless, a mistake to dismiss something just because it is old. Another way of looking at things is that lead-acid batteries are tried and trusted. They may just need a bit of pepping up. And that is what is now happening. Axion Power, a firm based near Pittsburgh, Pennsylvania, has found that the ideal tonic is carbon.

A conventional lead-acid battery is a simple affair, made up of a series of cells each containing a positive electrode made of lead dioxide and a negative electrode of metallic lead. These are immersed in an electrolyte of dilute sulphuric acid. Car batteries tend to have thin electrode plates, which allows a lot of energy to be discharged quickly, but only for a short period of time. That is fine for turning a starter motor, but it is not so good for turning an electric motor intended to move a car any distance. Moreover, a lead-acid battery can be ruined if it is discharged completely, as many motorists discover to their cost when trying to start their car on an icy morning. Lead-acid batteries with thicker electrodes can tolerate such “deep” discharges better than those with thin ones, but only at the expense of making a heavy battery even heavier.

In Axion’s battery the negative electrode is replaced with one made from activated carbon, a material used in supercapacitors. Normal capacitors—those that power the flashguns in cameras for instance—can be charged and discharged rapidly, but cannot store much energy. Supercapacitors are meatier versions that are able to hold a reasonable amount of energy as well as taking it in and releasing it quickly. Some, indeed, are already used in tandem with the lithium-ion batteries in electric cars to boost acceleration and recapture energy during so-called “regenerative” braking. Axion’s plan, therefore, is to have the best of both worlds by building a lead-acid/carbon hybrid, or PbC.

The carbon in the hybrid, which is protected within a sandwich of other materials, is more effective than metallic lead at releasing and absorbing protons to and from the acid during charging and discharging. In tests, Axion says, its PbCs have withstood more than 1,600 charges and deep discharges before they failed, which is three times better than standard lead-acid batteries specifically designed for such deep cycles.

True, the hybrids are still heavy compared with lithium-ion batteries. “But not everyone needs or can afford an electric car that accelerates like a Tesla,” says Ed Buiel, Axion’s chief technical officer, referring to the fastest electric car yet to be put into production, which uses a huge pack of lithium-ion cells. And for those who do not require Tesla-like performance, this makes sense. The hybrids are durable and also cheap to make because, according to Dr Buiel, they can be produced on existing lead-acid production lines. A Tesla costs $109,000. Axion, by contrast, has converted a pickup truck to run on a pack of its hybrids for around $8,000. (It has a range of 70km or about 45 miles.) The company is also working with a number of other small engineering firms to convert other sorts of vehicles.

Lead-carbon batteries are different from other types of batteries because they combine the high energy density of a battery and the high specific power of a supercapacitor in a single low-cost device. The primary goals of lead-carbon research have been to extend the cycle lives of lead-acid batteries and increase their power. Basically, developers start with conventional lead-acid chemistry and add carbon components to the negative electrodes. While the carbon components do not change the basic electrochemistry, they increase specific power and reduce a chemical reaction called “sulfation” that occurs during charging cycles and is the principal reason ordinary lead-acid batteries fail. Over the last several years, lead-carbon researchers have followed three different development paths:

* Blending carbon additives into the lead sulfate paste that is used for negative electrodes;
* Developing split-electrodes where half of the negative electrode is lead and the other half is carbon; and
* Completely replacing the lead-based negative electrode with a carbon electrode assembly.

The DOE’s 2008 Peer Review for its Energy Storage Systems Research Program included a slide presentation from Sandia that summarized the results of its cycle-life tests on five different batteries including a deep-cycle lead-acid battery, two lead-acid batteries with carbon enhanced pastes, a split-electrode lead-carbon battery (the Ultrabattery) and an advanced lithium-ion (Li-FePO4) battery. While the tests performed by Sandia focused on smoothing power output from wind turbines and used a 10% depth of discharge from a 50% initial state of charge, which means more testing will be required before comprehensive comparisons are possible, the following graph highlights the magnitude of the cycle-life improvements that lead-carbon technologies offer today. ...

A 10-fold improvement in the performance of any technology is by definition highly disruptive. The fact that lead-carbon achieved these disruptive performance gains using cheap and plentiful raw materials that are readily available from domestic sources and easily recyclable for use in new batteries using existing infrastructure is an absolute game changer; particularly when the closest comparable technology is based on expensive imported raw materials that are not easily recyclable for use in new batteries using existing infrastructure.