researchers at the University of Science and Technology of China developed all-solid-state batteries that match cutting-edge performance at just 4 per cent of the cost.

Shouldn't be surprising given that China has a huge and well educated population. China outnumber USA STEM Grads 8 to 1 and project to be 15 to 1 by 2030. China has also overtaken the US both in quantity and quality of research. And of course, China has a huge advantage with the state playing a major role in guiding the type of technology China focuses on instead of leaving it to the markets. So, it should be no surprise that China is already surpassing the west technologically now, and the gap will only keep growing going forward.

Literally the first paragraph of the article explains that it is 4% of the cost:

Chinese scientists say they have developed a new solid-state battery technology to match cutting-edge performance at just 4 per cent of the cost.

You seem to be confusing the cost of materials with the overall cost of production.

And 7/200 is 3.5%, maybe you should read that article again…

The one-word percent is standard in American English. Percent is not absent from other varieties of English, but most publications still prefer the two-word per cent. The older forms per-cent, per cent. (per cent followed by a period), and the original per centum have mostly disappeared from the language (although the latter sometimes appears in legal writing).