Rechargeable lithium ion batteries typically use graphite for the negative electrode and a lithium cobalt oxide for the positive electrode. The electrolyte is a lithium salt dissolved in an organic solvent which is flammable. When a lithium ion battery is first charged a protective layer (called the Solid Electrolyte Interface or SEI) is formed on the surface of the highly reactive negative electrode.

Under normal operating temperatures, the SEI layer maintains a safety barrier between the reactive negative electrode and the electrolyte. However, if the temperature of the battery rises above about 120° C, the SEI breaks down. In this situation the negative electrode has a high tendency to chemically react vigorously with the electrolyte in a heat-generating reaction that accelerates exponentially as the breakdown of the SEI occurs. This uncontrollable reaction is called a thermal runaway and ultimately leads to the destruction of the battery, and a resulting fire which could ignite the device to which the battery is connected such as an electric vehicle, laptop or cellphone.

The initial increase in temperature could be caused by a number of problems including external shorting of the battery, internal shorting of the electrodes resulting from mechanical damage to the battery or a manufacturing defect, overcharging of the battery, electronic control unit failure or external heat. Impurities in the battery could be introduced during the manufacturing process ultimately leading to an internal shorting of the battery.

(The recall of Sony li-ion batteries provides one recent example of the potential risks. In a less publicized event in 2004, a prototype li-ion battery pack for a vehicle caught fire as it was being loaded into a FedEx cargo jet.)

Altair Nano replaces the graphite with a patented nano-titanate material—Lithium Titanate Spinel (Li₄Ti₅O₁₂)—in its NanoSafe lithium-ion batteries. Spinel is one of a group of minerals which crystallize with an octahedral habit. (Earlier post.)

By removing the reactive graphite from the battery design, and instead using nano-titanate materials as the negative electrode material, no interaction takes place with the electrolyte in the Altairnano batteries—resulting in a high-power, thermally stable battery, according to the company.

During charging, lithium ions deposit inside the negative electrode. In a typical battery, the rate at which lithium ions can deposit is limited by the electro-chemical properties of the graphite. If the ions can not enter the graphite particles they, instead, may collect (plate) on the negative electrode’s surface as lithium metal.

This plating can occur if ions are deposited too rapidly on the graphite electrode—a possible outcome if the battery is charged too quickly, for example. If plating occurs, the battery will degrade in performance and in extreme cases, will short, causing overheating and thermal runaway.

The charge rate is thus restricted by the ion incorporation rate capability. Charge rate can also be affected by external factors such as low temperature.

Altair Nano says that the electro-chemical properties of its nano-titanate material support the deposition of lithium ions in the electrode structure at high rates, even at low temperatures. In recent laboratory testing, Altairnano demonstrated that a NanoSafe cell can be charged to more than 80% charge capacity in about one minute. Actual charge rates achieved in specific applications will vary due to the application environment.

The same technology also increases battery discharge rates which could be important to applications that require bursts of power, for example, a freeway electric vehicle accelerating rapidly. Altair Nano claims that the NanoSafe cell has demonstrated that surges of power can be delivered without risking thermal runaway or performance damage to the battery.

Altair Nanotechnologies is providing battery packs to Phoenix Motorcars for its development of its all-electric vehicles. (Earlier post.)

In August, Alcoa AFL Automotive announced a development agreement with Maxwell Technologies for the development of an ultracapacitor-based cold start system for the commercial transportation market. (Earlier post.)