Sodium batteries[1] could be the answer to grid storage batteries. Sodium can be extracted from seawater (e.g. desalination plants, there's a huge supply of salt readily available), and while it's heavier than lithium, it's chemically similar. It's heavier weight probably makes it less suitable for transport, at least initially but ideal for grid storage if/when they figure out how to produce the batteries commercially.
[Side note: some interesting quotes from the above link...
In 2012, the French researcher decided to take the bull by the horns and make the most of France's know-how in sodium batteries. "For lithium, all fundamental research had been conducted in Europe, especially in France," Tarascon points out. "Yet it was in Japan that the technology was transferred and brought to market, allowing Sony to launch its first lithium-ion battery in 1991. As a result, 95% of Li-ion production today takes place in Asia." It is out of the question to repeat history. The CNRS (responsible for fundamental research) and the LITEN-CEA (in charge of technology transfer) have thus joined forces with around 15 industrial players such as Renault, Saft, and Alstom to create the RS2E network dedicated to new-generation batteries. Their stated objective is to ensure research AND development, in order to bring sodium-ion batteries to market on European soil as soon as they are ready.]
[1] https://news.cnrs.fr/articles/a-battery-revolution-in-motion
[Side note: some interesting quotes from the above link... In 2012, the French researcher decided to take the bull by the horns and make the most of France's know-how in sodium batteries. "For lithium, all fundamental research had been conducted in Europe, especially in France," Tarascon points out. "Yet it was in Japan that the technology was transferred and brought to market, allowing Sony to launch its first lithium-ion battery in 1991. As a result, 95% of Li-ion production today takes place in Asia." It is out of the question to repeat history. The CNRS (responsible for fundamental research) and the LITEN-CEA (in charge of technology transfer) have thus joined forces with around 15 industrial players such as Renault, Saft, and Alstom to create the RS2E network dedicated to new-generation batteries. Their stated objective is to ensure research AND development, in order to bring sodium-ion batteries to market on European soil as soon as they are ready.]