See "Mods & Repairs" index list top of main forum page for links to lithium battery replacements; searches within this forum may give more results than Google.
French project http://www.kelrobot.fr/forum/faire-ses-pack-batteries-li-ion-t3293.html
At German Roboter-Forum "LIthium Ionen Upgrade".
XV pack cells are all 18650 (18x65mm).
With newer Rev64 Binky system boards (without the side charging jack) a USB command can configure the software for Vorwerk lithium charging procedure. On older Rev113 Cruz boards the heating of a balancer PCB must be used to heat the thermistors, simulating NiMh for charger control (NiMh heats up when full).
You need adequate capacity and drain rate cells.
The original balancers used in Germany are no longer available. A lower 12v version was made for Botvac
showing how to enhance some available balancers http://www.robotreviews.com/chat/viewtopic.php?f=20&t=19149