How to remove braces from Get-CsCertificate cmdlet output and ConvertTo-Html

I'm trying to take the output from get-cscertificate | select-object "alternativenames" which is: 

AlternativeNames

----------------

{sip.domain.nl, lyncdiscoverInternal.domain.nl, lyncdiscover.domain.nl, sr-xxxxx.vitens.lan...}

When piping this to ConvertTo-Html the output in the Html file becomes:

System.Collections.Generic.List`1[System.String]

The command i use is: Get-CsCertificate | fl 'AlternativeNames', Issuer, NotAfter, SerialNumber, Subject, Thumbprint, Use |

ConvertTo-HTML -head $a -building "<H2>$Servername Certificate Information $Date</H2>" | 

Out-File C:\Install\LyncCertificateCheck.htm

I know it's because of the {} braces but i can't figure out how to remove or edit or change the formatting.

Anyone?