creating a button

I am using the log out of password-protected post plug-in. I am trying to make my link to a button. The only thing I can’t seem to do is get quotes around the URL. This is the code that I have now.

echo '<input type="button" value="Log out of password protected posts" onclick="window.location.href=' . posts_logout_url() . '">';

if I add ” after href= it doesn’t wrap it URL quotes. How do I add quotes around the URL.