Battery Basics

BATTERY BASICS

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Or why do lithium-ion batteries cost so much?

Kevin Cameron

The term "lithium-ion battery" includes a wide variety of possible electrode chemistries and electrolytes, and as these types of batteries proliferate, we decided it was time to provide a basic primer on them.

One of the most important facts is that lithium reacts vigorously with water or water vapor. Therefore, lithiumion batteries must be sealed to exclude the atmosphere, and the electrolyte used cannot contain water.

While most Li-ion batteries employ graphite anodes, cathode types and applications are numerous, as follows: Lithium cobalt oxide: achieves high energy density but current is somewhat limited by electrode resistance and the heat generation that it produces.

Lithium manganese oxide: good for electric tools requiring high current. Less energy density than cobalt oxide.

Lithium iron phosphate: lower energy density but long life, inherent thermal safety.

Lithium nickel manganese cobalt oxide: good for low-drain medical equipment.

Lithium nickel cobalt aluminum oxide: able to tolerate many chargedischarge cycles; might be useful for electrical grid storage (storing solar power by day for discharge at night).

In all cases, the charging process stores lithium ions in the negative electrode, or anode. Discharge moves lithium ions from anode to cathode.

Think of electrode structure as analogous to the familiar problem of airliner seating: To shorten loading/ unloading time at airports, more aisles are essential, but providing such aisles means the space they occupy cannot be filled by more paying passengers.

Cathodes are made with structures that provide large surface area (Li-cobalt oxide is a layered structure, but lithium manganese oxide is a triangulated “spinel”). So, in general, having maximum energy storage capacity makes it more difficult to achieve rapid charge/discharge. Electrode resistance— chiefly the anode—generates heat.

It was natural for users seeking maximum performance (laptop and mobile-phone makers, Boeing, and others for aircraft use) to be attracted to lithium cobalt oxide, but a number of well-publicized laptop, handheld device, and other fires resulted, including one in a Cessna CJ4 business jet, which caused the FAAto stipulate that this model’s Li-ion main battery be replaced by either lead-acid or nickelmetal hydride batteries. Boeing was allowed to put Li-cobalt oxide aboard

its new 787 Dreamliner because four levels of security were provided. As we now know, even that did not prevent “overheating.”

Fire results when a battery enters “thermal runaway,” develops internal current, and becomes hot enough to vaporize its electrolyte, generating internal pressure that bursts the battery’s containment. The combination of the electrolyte—an organic solvent such as ethylene carbonate—high temperature, and atmospheric oxygen generates an intense fire. Lithium plus atmospheric water vapor reacts to

lithium hydroxide plus hydrogen gas. Big bangs!

Industry’s response has taken several forms: to shift to inherently safer electrode chemistries such as Li-iron phosphate; to protect high-performance batteries with charge/discharge controls and temperature sensing circuitry; to add fire-retardant substances to battery electrolyte.

In the case of the Shorai motorcycle battery, it employs the safe lithium iron phosphate cathode chemistry. Even though this cathode choice reduces energy storage in comparison with

a Li-cobalt oxide chemistry, it still displays much more energy storage than traditional lead-acid.

Every week one can read of “breakthrough” developments in Liion battery technology, most of them taking the form of ways to create electrodes with extremely large surface area and an open structure allowing rapid ion movement. No large company can afford to bet the farm on new developments that have not been thoroughly explored, so it can be years before such refinements make their way to market.