I purchased some ISO 15693 icode slix tags (not from seritag, sorry, my fault, just found you). I understood that they had a much longer reading range over a metre or something like that. the tags i have can only read a few centimeters like a normal ntag213. are my tags not correct ?
ISO 15693 tags read range
The scan distance of ISO 15693 are very misleading. NXP's information on the ICODE SLIX tag, for example, quotes a scan distance of up to 1.5m. In reality, with mobile phones, most users will get scan distances in the region of standard ISO 14443A NTAG type of tags.
In essence, the scan distances quoted are possible but only with very high powered readers and large antennas. A typical mobile phone antenna will have a power in the region of milliwatts (a fraction of a single watt). To achieve these kind of scan distances, the antenna power would need to be many watts - a factor of 100 or thereabouts greater than a mobile phone. Even standard USB readers typically operate in hundreds of milliwatts.
With a standard mobile phone, you should expect to achieve scan distances similar to any other NFC tag.