The big problem is that modern day technology works via incremental improvements and sharing knowledge.
You can get out ahead of other people, but it will be comparatively marginal, and it will of necessity be based on known technology.
Such a research endevour is going to be economically limited to a modest number of researchers with modest budgets. They'll either come up with something novel and useful, or they'll have their research department slashed. Research is blindly groping through the sphere of all knowledge; you don't know if you are getting anywhere.
Long-term research is mostly done by public institutions, and those scientists are rewarded by sharing as much of their results as they can as soon as they can (publish or perish).
When you want to reverse engineer it, you'll have a valuable target (amazing batteries) that you know is reachable. You'll have budgets that blow out of the water the original research team, a fair amount of lee way in not producing results right away, and a concrete object that provides piles of hints asto where to research next.
Now, there are long and deep research traditions that are not public. They are mostly military research, into nuclear or stealth or similar technologies.
Such military research tends to be smaller scale than the public commercial research, but secrets are kept for years, commercial applications are not the goal, and even the products are kept secret. For the most part, people interested in reverse engineering are also keeping their results secret, preventing the attackers from forming a huge collaborative endevour.
This does, however, give us a way for technology to be sufficiently secret that reverse engineering is difficult. There is some long-standing secret military research going on that uses up a non-trivial amount of the world's GDP. It is focused on something else, besides battery technology. Maybe nanoscale construction of materials to make them invisible to radar, or nanoscale construction of fissile cores to permit tiny nuclear devices that can handle decay, or whatever. The point is some serious fundamental research into materials science that "falls off" some military research.
That technology can easily develop in secret for 10, 20, 30 years.
Then someone discovers that it can be used to construct a better battery. You develop even that technology in secret. As an arm of the military, you build a huge battery complex somewhere, use it originally for military purposes.
If you successfully kept the first endevour secret, now your competitors have to reverse engineer 30 years of technology to work out how your construction method works.
Unfortunately, nanoscale technology probably is a bad example; it is currently a hot research field. As is biological manufacture, or almost any other semi-practical high-tech materials science I can think of.
For the science to be 30+ years of practical application iteration beyond the current bleeding edge research, it would have to sound ridiculously science fictiony.
- Post-trans uranic stable isotopes (PTUs).
- Nuclear decay suppression field.
- Atomic-scale macroscopic materials design.
- Low energy H2<->He reversible nuclear reactions.
- Non-terrestrial biological energy storage.
The thing is, it would be ridiculous that only one ridiculous technology came out of a completely new branch of materials science.
But, if you get over all of this -- a 30 year long research effort by a nation-state level lab resulting in an unexpected battery technology, which is then nation-state backed into mass production, while successfully holding off other nation-state espionage -- you could have a tech that might take 5-10 years to reverse engineer.
Note, however, that at this time the only one doing this scale of secret research is the USA and possibly China. And China hasn't really been doing it long enough.