SS-501 Soryu - Lithium-ion Batteries - Technology
Japan's submarine powered by lithium-ion (Li-ion) batteries, dispensing with lead-acid batteries and Stirling air-independent propulsion (AIP) system. Air Independent Propulsion (AIP) technology had revolutionized the traditional diesel-electric submarine. Performance capabilities approaching the the domain of highly expensive and large nuclear submarines became available for a fraction of the cost.
Now the diesel-electric submarine is on the verge of yet another revolution. Lithium-ion batteries (LIB) power the lives of millions of people each day. From laptops and cell phones to hybrids and electric cars, this technology is growing in popularity due to its light weight, high energy density, and ability to recharge. These batteries arewell on their way to meeting the challenging technical goals that have been set for vehicle batteries. However, they are still far from achieving the cost goals.
The lithium-ion battery is one of the most promising new battery types, in part because of its high energy and power densities, and also because it has the potential to last the lifetime of the submarine, a major economic advantage over most other batteries.
Among the major producers are Sony, Sanyo, Varta, and SAFT. Most Li-ion production is in Japan, but Polystor has begun producing small cells in California. The technology used to produce these small consumer cells is essentially transferable to production of the larger cell sizes that would be put together into larger battery packs.
Lead-acid batteries suffer from certain disadvantages as well. They offer relatively low cycle life, particularly in deep-discharge applications. Due to the weight of the lead components and other structural components needed to reinforce the plates, lead-acid batteries typically have limited energy density. If lead-acid batteries are stored for prolonged periods in a discharged condition, sulfation of the electrodes can occur, damaging the battery and impairing its performance. In addition, hydrogen can be evolved in some designs.
A battery is made up of an anode, cathode, separator, electrolyte, and two current collectors (positive and negative). The anode and cathode store the lithium. No lithium metal is present in the cell, therebyalleviating some serious safety concerns.
The electrolyte carries positively charged lithium ions from the anode to the cathode and vice versa through the separator. The movement of the lithium ions creates free electrons in the anode which creates a charge at the positive current collector. The electrical current then flows from the current collector through a device being powered (cell phone, computer, etc.) to the negative current collector. The separator blocks the flow of electrons inside the battery.
While the battery is discharging and providing an electric current, the anode releases lithium ions to the cathode, generating a flow of electrons from one side to the other. When plugging in the device, the opposite happens: Lithium ions are released by the cathode and received by the anode.
The two most common concepts associated with batteries are energy density and power density. Energy density is measured in watt-hours per kilogram (Wh/kg) and is the amount of energy the battery can store with respect to its mass. The energy stored in the battery serves the same function as the fuel of a conventional vehicle. An important objective in development of such batteries is to maximize energy density, the energy stored per unit volume, or specific energy, the energy stored per unit mass. Li-ion cells can be manufactured with energy densities as high as 175 W·h/L (specific energy, 144 W·h/kg), with a targeted value of 310 W·h/L. Lead-acid batteries typically achieve only 73 W·h/L. A lithium-ion battery has an energy storage density of 10 kg/kWh, and a lead battery has an energy storage density of 26 kg/kWh.
Power density is measured in watts per kilogram (W/kg) and is the amount of power that can be generated by the battery with respect to its mass. To draw a clearer picture, think of draining a pool. Energy density is similar to the size of the pool, while power density is comparable to draining the pool as quickly as possible. High-power Li-ion cells currently achieve a specificpower greater than 1,300 W/kg and a power density greater than 2,700 W/L.
Li-ion batteries may suffer thermal runaway, if overheated, overcharged, or over-discharged. This can lead to cell rupture, exposing the active material to the atmosphere. In extreme cases, this can cause the battery to catch fire. The possibility of such an event occurring on commercial airliners, where many passengers carry laptop computers and cell phones with such batteries, is especially disconcerting. These events have occurred on much larger scale, and have caused industry-wide concern in the continued use of this important technology.
Deep discharge may short-circuit the Li-ion cell, causing recharging to be unsafe. To manage these risks, Li-ion batteries are typically manufactured with expensive and complex power and thermal management systems. In a typical Li-ion application for a hybrid automobile, two-thirds of the volume of the battery module may be given over to collateral equipment for thermal management and power electronics and battery management, dramatically increasing the overall size and weight of the battery system, as well as its cost.
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