https://www.scmp.com/news/china/science/article/3197670/chinas-submarine-fleet-may-soon-be-powered-lithium-batteries

https://www.thyssenkrupp-marinesystems.com/en/teaser/the-submarine-revolution--lithium-ion-battery-system-for-a-better-performance

Japanese also have been working on it

https://www.navalnews.com/naval-news/2021/03/japan-commissions-its-2nd-li-ion-battery-submarine-js-toryu%E3%80%8C%E3%81%A8%E3%81%86%E3%82%8A%E3%82%85%E3%81%86%E3%80%8D/