Apple is certainly investigating all kinds of advanced battery technology, but I'd put money on the next devices iterating on its already-leading-edge power optimisation, rather than solar or kinetic charging.
The Times guesses that Apple may use solar in a watch, somehow getting enough energy out of a small surface worn on the wrist under a sleeve. They go on to postulate that Apple could use the motion of your arm or wireless charging tech or some other charging method to get power to the watch.
Me? I don’t buy any of these pie in the sky ideas. These don’t feel like informed sources and it feels like a lot of guessing going on. My money is on Apple optimizing everything from the battery to the chips as much as science will allow and then putting a super efficient OS on top of it and eeking out a full day’s power– which is the gold standard.