I've been reading this thread all weekend on Mobile and it's been killing me... On desktop now and can reply.
Working at the system level, I'm responsible for designing and implementing (often lithium based) battery systems for motion control applications.
Lithium ion as a generic name, and Lithium Iron Phosphate (LiFePo4) as a specific chemistry are two very different things. Lithium ion is to dog as LiFePO4 is to german shepherd.
Lithium ion is commonly used as the name for most 18650s and cell phone cells, which I believe are commonly Lithium Cobalt Oxide, Lithium Manganese Oxide or some combo of the two. These cells are optimized for energy density (not power density) and not safety. There is a relatively low temperature thermal runaway condition that can be caused by physical damage (as this allows the anode and cathode to directly short) or temperature increase.
Lithium Polymer batteries (very popular in RC industry, and make a great show on battlebots) are different in that they use a polymer electolyte instead of a lithium salt. They are capable of higher power density than the cobalt and Mn lithium ion, but are often less energy dense. These also are not optimized for safety, and have the same thermal runaway condition.
Both of the above are used far more commonly than LiFePO4 and are made safe by the system design. They are spec'd where overcurrent is not a concern, they are balanced during charging (and sometimes during use), often have external current monitoring to prevent over current, temperature monitoring, etc... All of this with the goal of preventing thermal runaway.
Now let's get to the good stuff!
LiFePO4 is a very safe chemistry as compared to above, and comes in cell and pouch style. Some 18650s are LiFePO4, and a 26650 size was popularized by A123 Systems... but they do come in all shapes and sizes. LiFePO4 are far safer (which mostly comes from about 1/3 the energy density of the above). They are optimized for power density and safety.
LiFePO4 are commonly penetration tested (nail, etc... inserted entirely through the cell and out the other side) without any flame event, and often with the cell remaining perfectly functional. There will be some capacity loss, but it will keep trucking. They can be overcharged and discharged to 0V and recovered without safety issue (do not try this with other chemistires...), just some capacity loss.
LiFePO4 is a great option for FRC batteries as they are commonly used in SLA replacement applications where you you do 2/3 as many SLA cells with LiFePO4 cells, and use the same equipment. This isn't what I would recommend for FRC, as I believe we should still have a balance unit but it would work.
Considering the SLA batteries we use are essentially 4-5Ah batteries (see spec and explanation of law), a LiFePO4 pack could be made that would safely run an FRC robot for 4-5 matches and have a retail price of $200-250. The battery would last for a great deal more cycles than the SLA batteries as they aren't affected by the deep discharg nearly as much. Our team goes through 12 SLA batteries per year, and would we switch to probably 3-4 LiFePO4 batteries and one new one every other year. Most teams could just run 2 and buy a new one every 3 years.
Lithium Titanate would also be another safe option if we stay in the stone ages long enough and price comes down...
I don't think we really fit into those rigidly defined categories, my team builds our robot. - AdamHeard [more] 
06-07-2016, 02:51
Hybrid Mode
