The Short Version
The claim is directionally accurate: peer-reviewed research confirms that higher state-of-charge accelerates lithium-ion battery degradation through well-understood mechanisms like SEI growth and lithium plating. Real-world smartphone tests also show measurably better capacity retention when charging is capped below 100%. However, the claim lacks important context: modern phones use battery management systems that reduce stress near full charge, and the practical effect over a typical phone's lifespan is often modest — not dramatic. The biggest factor is time spent at high charge levels, not simply reaching 100%.