This topic really helped me, so I'd like to share my improvements. All credits go to the nixda, this is based on his answer.
For those who need to convert multiple csv's in a folder, just modify the directory. Outputfilenames will be identical to input, just with another extension.
Take care of the cleanup in the end, if you like to keep the original csv's you might not want to remove these.
Can be easily modifed to save the xlsx in another directory.
$workingdir = "C:\data\*.csv"
$csv = dir -path $workingdir
foreach($inputCSV in $csv){
$outputXLSX = $inputCSV.DirectoryName + "\" + $inputCSV.Basename + ".xlsx"
### Create a new Excel Workbook with one empty sheet
$excel = New-Object -ComObject excel.application
$excel.DisplayAlerts = $False
$workbook = $excel.Workbooks.Add(1)
$worksheet = $workbook.worksheets.Item(1)
### Build the QueryTables.Add command
### QueryTables does the same as when clicking "Data ยป From Text" in Excel
$TxtConnector = ("TEXT;" + $inputCSV)
$Connector = $worksheet.QueryTables.add($TxtConnector,$worksheet.Range("A1"))
$query = $worksheet.QueryTables.item($Connector.name)
### Set the delimiter (, or ;) according to your regional settings
### $Excel.Application.International(3) = ,
### $Excel.Application.International(5) = ;
$query.TextFileOtherDelimiter = $Excel.Application.International(5)
### Set the format to delimited and text for every column
### A trick to create an array of 2s is used with the preceding comma
$query.TextFileParseType = 1
$query.TextFileColumnDataTypes = ,2 * $worksheet.Cells.Columns.Count
$query.AdjustColumnWidth = 1
### Execute & delete the import query
$query.Refresh()
$query.Delete()
### Save & close the Workbook as XLSX. Change the output extension for Excel 2003
$Workbook.SaveAs($outputXLSX,51)
$excel.Quit()
}
## To exclude an item, use the '-exclude' parameter (wildcards if needed)
remove-item -path $workingdir -exclude *Crab4dq.csv