unable to prefix 'xsi'

Question

I am trying to add attributes to an XML root node. Using the Master-PowerShell book, I could successfully add attributes to the root node.

However, the xsi prefix disappears. I am sure it's something silly I have missed, grateful if I could please get some help. Below is my scenario.

XML file before modification:

<?xml version="1.0" encoding="UTF-8"?>
    <InvoiceBatch>
    ...
</InvoiceBatch>

Code I wrote in PowerShell to add new attributes (modify XML):

$xmldata = New-Object XML
$xmldata.Load("c:\users\SFDC_batch_0201.xml") 
$xmldata.InvoiceBatch.SetAttribute("batchSource","SFDC")
$xmldata.InvoiceBatch.SetAttribute("xmlns:xsi","http://www.w3.org/2001/XMLSchema-instance")
$xmldata.InvoiceBatch.SetAttribute("xsi:noNameSpaceSchemaLocation","g:\TrgFiles\xsd\invoice.xsd")
$xmldata.Save("c:\users\SFDC_batch_0201.xml")

I get below output:

<?xml version="1.0" encoding="UTF-8"?>
    <InvoiceBatch xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" noNamespaceSchemaLocation="g:\TrgFiles\xsd\invoice.xsd" batchSource="SFDC">

    ...
    </InvoiceBatch>

Output XML (expected result):

<?xml version="1.0" encoding="UTF-8"?>
    <InvoiceBatch batchSource="SFDC" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNameSpaceSchemaLocation="g:\TrgFiles\xsd\invoice.xsd" >
    ...
    </InvoiceBatch>

Did lot of googling, but was unable to find something related to my scenario. There were using C# etc., but not powershell. Pardon me if I missed any information as part of my search.