If we speak about what's available in laboratories, Lithium–sulfur battery gives around 550 Wh/kg with up to 1500 charge-discharge cycles ([1]). Not available commercially, but there are a few companies that try to commercialize them ([2], [3], [4]).

I would not bet that these batteries arrive fast enough to save Lillium.

1. https://en.wikipedia.org/wiki/Lithium%E2%80%93sulfur_battery

2. https://lyten.com/

3. https://oxisenergy.com/

4. https://www.thebatt.com/news/lithium-sulfur-battery-startup-...