Basically, electric waste does not belong in household waste and this also includes smartphone batteries. The batteries can be recycled very well and this also benefits the environment.
There are three different ways to dispose of your old battery. All mobile phone providers are obliged to accept the batteries purchased from the dealer and to dispose of them professionally. In many larger supermarkets there are containers for used batteries and you can also dispose of your smartphone battery. You can also go to the next scrap yard and put the battery in the hazardous waste.