Saturday, 3 January 2015

Electric Motorcycle Battery development and manufacturing, plus problems and solutions

When fuel and air burn inside the cylinder of an internal-combustion engine, the energy being released comes from the electronic bonds that bind atoms together to form molecules. The bond energy of unburned hydrocarbon fuel and diatomic oxygen from the air is higher than that of the products of complete combustion, which are water and carbon dioxide. Upon combustion, this energy difference appears (mainly) as heat. This heat raises the pressure of the gases in the cylinder, driving the piston downward to turn the engine’s crankshaft.
The very same kind of electronic bond energy stores and delivers the power we take from batteries. A battery at its simplest consists of a positive and a negative electrode, exposed to an electrolyte. In the case of today’s powerful Lithium-ion batteries, the electrolyte consists of Lithium salts dissolved in an organic liquid. Just as table salt—sodium chloride or NaCl—separates into oppositely charged sodium and chlorine ions when dissolved in water, so the Lithium salts release Li+ ions into solution.

During charging, negative electrons are supplied by the charger to the battery’s negative electrode (anode). Because the electrolyte is an insulator to electrons, the only way charging current can move through it from one electrode to the other is by the movement of Li+ ions. They move to the anode (whose commonest material is carbon) where they wriggle between the layers of carbon, one Lithium ion nestling comfortably into each available six-carbon ring. This process has the wonderful name “intercalation.” Taking up an electron in the process of nestling into the carbon, the Li ion becomes neutral.
During discharge, Li atoms each give up an electron as they emerge from the carbon anode and migrate through the electrolyte to the cathode. In the first successful Li-ion batteries, the cathode was Cobalt Oxide. There, the ion enters the layered structure of the Cobalt Oxide. The electrons released in the discharge process move through the external circuit to power a cellphone, laptop computer, or other power-hungry application.
The voltage difference between positive and negative electrodes, which drives electrons through the external circuit and its load (the motor of an electric TT bike?) is the difference between the “electron affinities” of the two metals, their different electro-chemical potentials. In the experiment so many of us performed in school, two electrodes in the form of a strip of zinc and a strip of copper are inserted into a lemon. The water and mild acetic acid content of the lemon act as an electrolyte, allowing ions to move it to create an easily measured voltage difference between the two electrodes. The lemon has become a simple battery cell.
How We Feel
Why does battery power please some people and deeply offend others? Those who are pleased are those who see that battery electric vehicles could, if they became cheap enough to reach a mass market, clean up urban air. Many are also attracted to the high efficiency of electric motors, which, depending on price, varies between 90 and 97 percent. Electrics seem modern and progressive.
And those who are offended? Even though combustion power and battery power come from the same basic source—the electric charges that hold molecules together—the sound and fury of combustion give it romantic appeal. Understandably to these romantics, the hum of electric power is anticlimax, turning vehicles into appliances. Electrics seem like the leading edge of an era of standardized automatic vehicles, driving themselves identically in ranks and rows.

Many alternative cathode and anode chemistries have been discovered since that first commercial Li-ion battery hit the market in 1991. The original Cobalt Oxide cathode’s vulnerability to overheating, producing oxygen, and possibly catching on fire led to the 2006 “era of flaming laptops”. Meanwhile, other cathode types such as Lithium Manganese Oxide (LMO) and Lithium Iron Phosphate (LPO) were developed, offering greater resistance to overheating but having less energy density (measured in kilowatt-hours per kilogram, or kWh/kg). These types have become the principal players in the electric vehicle field.
How They’re Made
What are these batteries, physically? First of all, they are completely sealed and contain no water (lithium and water react violently). Each of the two electrodes is usually implemented as a thin metal foil carrying a layer of the electrode material in powder form, held together and onto the foil by a polymer binder. The positive electrode begins with a thin aluminum foil to function as a current collector, with electrode material and binder on its surface. The negative electrode material is commonly carbon, again held in place by polymer binder but on a thin copper foil current collector. The active surfaces of the two face each other, separated by a thin (0.001 inch) polymer membrane separator whose cost can be half of total cell cost. Electrolyte wets both electrodes.
The two obvious packaging schemes are cylindrical and flat. In a cylindrical cell, such as the “billions served” 18650, the electrode material is made in the form of long strips, which are sandwiched over the separator. This is then rolled up to fit in the cylindrical container. The 18650 is so called because it is nominally 18mm in diameter (a little under 3/4 inch) and 65mm long (a bit over 2 1/2 inches). A potential advantage of a flat format is that the cell container can be a flexible flat bag or a pouch that packages densely.
Although much is made of the possible scarcity and high cost of materials such as Cobalt or Lithium, material cost is said to be only a small element in finished cell price.
Problems and Solutions
Many problems have beset Lithium-ion batteries, and many problems remain to be solved. Back in the 1980s, researchers found that if the cell was charged too rapidly, Lithium ions did not obediently wriggle between the layers of the carbon anode but instead plated out on its surface. Then the plated surface developed bumps, which developed into whisker-like dendrites. Such dendrites could either grow right through the separator membrane, shorting out the cell, or they could become loose particles during discharge, causing loss of lithium that had to be made good by providing more than just necessary for normal operation.
Lithium’s burrowing act also had consequences. Each time the cell was charged, the carbon anode swelled up as Lithium ions took up their positions between its many layers, then shrank again as Lithium departed during discharge. This, over time, led to cracking and the shedding of particles. In response, the industry has developed other anode chemistries such as LTO, or Lithium Titanate Oxide, which eliminates dendrite formation and speeds charging but reduces cell voltage and energy density.
If aggressive charging went on too long, it drove reactions between the Lithium and electrolyte. Such reactions gradually consumed Lithium, causing a drop in cell properties, and ultimately releasing oxygen. Since the liquid part of the electrolyte is an inflammable organic, the combination of fuel, oxygen, and heat is a recipe for fire. To prevent this, the high-power-density Lithium Cobalt Oxide cells are provided with electronic battery-management systems to supervise and control charge/discharge rates and monitor temperature. Such systems add considerable expense.
Also, fire retardants may be added to electrolytes. In the celebrated case of Boeing’s 787 “Dreamliner,” the engineers’ inability to understand and overcome battery overheating led to placing each $16,000 cell assembly inside a fire-resistant steel box, vented outside the aircraft. Sadly, the weight saved by adoption of energy-dense Li-ion cells was neutralized by the weight added as containment.
Intensive development work on every aspect of the Lithium-ion cell is ongoing around the world. Many kinds of high surface area electrode materials—extremely fine powders, spinel-structured minerals, and nano-wires—seek to provide so much area onto which Li-ions can attach that they need not burrow into layered solids such as carbon or silicon, causing swelling, cracking, and flaking. You will find announcements of such work almost every day on sites such as greencarcongress or gizmag. As one battery expert put it in 2009, Li-ion batteries are “boxes of technical trade-offs and compromises.”
For some, this intensive research fuels a certainty that any day now, a complete solution will be found—compatible anode and cathode chemistries offering near-zero heating, record energy density, long cycle life, high cell voltage, fast charge, and low cost. For others, the modest gains achieved by all this work and investment suggest the work must continue for a long time yet.
Stanley Wittingham, an original pioneer of Lithium-ion, has said electric vehicles will be used only for trips of less than 100 miles. He expects energy density to eventually double, but not much more. J.B. Straubel, an engineer at electric automaker Tesla, says battery technology improves by “of the order” of double in 10 years, which implies a rate of improvement of about seven percent per year.
Brammo motorcycle parts
Battery Cooling
The greater the energy density of a cell system, and the more tightly it is packaged, the greater is its need of active cooling. Because the charge/discharge cycle cannot be 100 percent efficient, heat is generated. Standard sources list charge/discharge efficiency as 66 percent for Ni-metal hydride batteries such used on Toyota’s hybrid Prius, and 80 to 90 percent for Lithium-ion.
Cooling can be as simple as spacing cells apart enough to allow air circulation or placing strips of aluminum sheet between flat “pouch” cells, leaving part of the sheet projecting into circulating air that carries away heat. In the most intensive systems, liquid coolant is circulated to an external radiator by pump. Reminds me of what former Rolls-Royce CEO Lord Hives said when told of the simplicity of the gas turbine, “We’ll soon design the simplicity out of it!”
Change of Auto Industry Emphasis
When I reviewed my back issues of Automotive Engineering, the magazine of the Society of Automotive Engineers (SAE), I saw that articles on battery technology, electric motors, and motor controllers peaked in 2008/09 and declined thereafter. In conversations with auto engineers, I have learned that attitudes have changed. It is now clear that there is little market demand for electric vehicles at present price levels; the “electric-car buzz” has arisen mainly from government.
Because the industry now faces the mandated 54-mile-per-gallon fleet average fuel economy, it has for the present chosen a dual-path strategy. One path is to continue to improve the internal combustion engine and the other is to develop hybrids, which are of two basic kinds, parallel and serial:
1. In a parallel hybrid, either the IC engine or the electric motor can propel the vehicle. The electric motor is used at low speeds and loads at which the IC engine is least efficient, and the IC engine propels it the rest of the time.
2. In a serial hybrid, the wheels are driven by an electric motor drawing power from a battery, but the battery is charged by what marketeers are now calling a “range extender.” That is an IC engine, so in fact this system’s prime mover is an IC engine, driving through a “transmission” consisting of battery and electric motor.
Either type of hybrid may become a “plug-in hybrid” by carrying a charging system that can pull power from a 120V household outlet (power limited to 1500 W, meaning long charging time), a 220V stove/drier outlet, or a dedicated charging point.
Hybrids cost about 30 percent more than equivalent all-combustion-powered vehicles yet can reach much more of the market than can expensive present-day pure electrics. Hybrids have what electrics presently most lack: range and quick refueling from the hundreds of thousands of existing gasoline stations.
Lithium-ion batteries have been expensive, around $500 to $650 per kilowatt-hour (kWh) of capacity. That would price an electric motorcycle’s 14-kWh battery at $8,000. Tesla’s recently announced new battery plant aims to bring the price down to $300 per kWh or $4,200 for a notional electric motorcycle’s battery. And $150 to $200 per kWh is regarded as a possible turning point in the market competitiveness of pure electric cars.
Can we believe recent announcements that Li-ion prices are about to drop by 50 percent? Or do we extrapolate Bloomberg’s price-versus-year graph, which shows Li-ion battery price dropping at just five percent a year, a rate that requires 14 years to achieve that 50 percent price cut?
Other Battery Futures
One way to compare battery chemistries is by their theoretical properties, unmodified by the compromises of usable, affordable commercial products. Comparing in this way, a Lithium-air battery is tantalizing, as its numbers suggest an energy storage device that could be close in energy density to that of hydrocarbon fuels.
Lithium is among the very lightest of the elements and the air doesn’t have to be carried; it comes from the atmosphere. In 2009, there was intense interest in Lithium-air, peaking in 2012, but more recently, major labs such as those of IBM and Argonne have all but given up Lithium-air as unworkable. Li-air work continues at St. Andrews University in Scotland.
Now, some believe more actual performance can be realized from a Sodium-air battery, despite its having only half as much theoretical energy density.
There is much work to be done.


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