The Japanese Self Defense Forces (JSDF) is looking to update their Soryu-class submarines, and the next batch will switch from an air-independent propulsion (AIP) system to using lithium ion batteries to replace the current lead acid technology:

Japan has decided to power its new batch of Soryu-class submarines with Lithium-ion batteries instead of air-independent propulsion (AIP) technology — a move that could raise eyebrows after similar types batteries were faulted for fires aboard the Boeing 787 Dreamliner.

However, experts brush aside those concerns and instead say this type of technological leap increases power and performance, while reducing maintenance. It also could make Japanese subs more marketable overseas.

Yasushi Kojima, a spokesman for the Maritime Self-Defense Force (MSDF), said the change would affect the next four remaining Soryu-class submarines in Japan’s 10-boat class.

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The existing Soryu-class diesel-electric submarines (16SS) use AIP technology based on Kockums Stirling engines license-built by Kawasaki Heavy Industries, allowing them to stay submerged for long periods. The engines power Sweden’s smaller Gotland-class submarines for up to two weeks at 5 knots.

The current Soryu-class submarines are propelled by a large electric motor that has three power sources: diesel engines, the AIP engines and main storage batteries. Diesel engines, which require oxygen for combustion, power the boats on the surface or while snorkeling. The boats can snorkel for extended periods to limit their detectability while transiting submerged (only the snorkel mast is above the water) or for short periods to quickly recharge their batteries after operating underwater. The AIP engines — which burn small quantities of diesel fuel and liquid oxygen — are used for long-range underwater cruising at low speed, and to keep the batteries topped off. The batteries are used for ultra-quiet operation as well as high-speed underwater operations, which quickly depletes them.

By shifting to Lithium-ion batteries, the new Soryus would retain their main propulsion diesels, but be equipped with more powerful and far lower maintenance batteries than lead-acid types widely in use.

LiIon batteries have at least 4x the energy density of lead-acid batteries, and I would assume that the spaces for the LOX tanks and Sterling engines would be changed to battery storage.

If you assume twice the volume of batteries, and 4x the energy density, it means that you could have about a 2 week undersea patrol before it would have to surface or use the snorkel.

I think that this might give an underwater patrol performance approaching that of the AIP system, with a much longer underwater dash range, and logistical issues would be simpler, as a tanking up the submarine with liquid oxygen is not something that can be done at sea or at a foreign port.

My (barely educated) guess is that this change is being considered because they expect their submarines to range further from their home ports in the future, and the ability to refuel at sea from Japanese or allied tenders would be an important part of this, and that they feel that losing some underwater endurance is worth this.

If this is indeed the case, it reflects a major change in the Japanese military posture.

Since the end of WWII, this sort of power projection has been an anathema to both the JSDF and the Japanese government.