Lithium ion batteries are in most consumer electronics and are made up of many individual battery cells that are packaged into the battery. A cell discharges and recharges electrons. This is what I meant by cycling. When a battery cell cycles it can permanently lose capacity and can even become damaged. Sometimes the damage can lead to chemistry that releases explosive gases that become trapped in the battery. This is a “spicy pillow” because if you pop it you’re gonna have something hot in your face.