How much does a phone battery degrade if kept in storage?

If you buy a brand new, unopened phone that has been in storage and released 2 years ago, approximately how much of the battery has already degraded? Does this vary with different types of smartphones like iphones or androids?

That is going to vary between phone, type of battery, and the particular construction run that battery was part of. This is why when there are battery recalls for iPhone or and battery maker, only certain serial number series' are part of the recall, since they are able to narrow the defect down to the batteries that were made during a particular batch, and ofter plant the bad batteries came from. At least those coming from more than one source/country.