Our batteries are not designed to handle the large amp draw that engines require when starting up.
There are specialty LiFePO4 batteries that can handle the high current demands of starters but they are not very mature in the industry and longevity is still a question.
In general, no you can NOT use LiFePO4 batteries to start an engine on a vehicle or boat.
However, small engines with smaller Amp requirements may work if you connect multiple LiFePO4 batteries together. For instance, the small RV generators could possibly be started if you have 3 or more of our 12V 100 Ah batteries connected together. This is an advanced use of our battery, and you should always contact an electrical expert for advice before attempting to do something like this.