Is there a type of keyboard that doesn't require batteries or any electricity of any kind? also one that is extremely portable like a guitar? It should also sound like a normal piano without any synths or pads and all those other "features" they usually come with. If no such device exists, than why haven't companies created one by now?
The sound of a normal piano comes from its heavy, large construction. That's just physics. If you want the same sound from something smaller, you need a sampler (of some kind), and that will require electricity. Maybe you can rig something together with a keytar and a wearable amplifier powered by a solar panel on your back and generators in your shoes, or just use a piano app on an iPad, but I think what you really want is a...
The melodica is a great instrument! It's portable; you hold it with one hand and play with the other, it's powered by your own breath, and sounds like happiness feels. You may have recently seen one in the hands of Jon Batiste in Stephen Colbert's Tonight Show band, or playing the tastiest of jazz solos in one of YouTube phenom Jacob Collier's multitrack videos. Check it out!
Well, you could go for a small accordion. They are fairly portable, the energy supply are your arms, but of course it sounds nowhere like a piano and the left hand uses a different system than a piano keyboard. But it's a polyphonic instrument, and if you really want to play piano scores, you can go for an instrument with a free bass system in the left hand where each button is just a single note. But for many well-known piano pieces there are accordion arrangements that are playable with the standard bass system reasonably well. So that's not a must.
Schoenhut makes ones that sound kind of like a piano, sort of.