There are two types of Coils; one without hormone (The copper IUD-Intra Uterine Device). The second, with hormone ( The Mirena IUS-Intra Uterine System)
The Copper IUD last for 10 years and works locally in the womb to stop pregnancy by two mechanisms. The presence of copper is toxic to sperm and the presence of copper in the endometrium makes it unsuitable for implantation.
Please see information leaflet below:
https://www.nhs.uk/conditions/contraception/iud-coil/