What’s the Difference Between LiFePO4 and LiIon?

Some peolpe have commented on LiIon batteries, but the question and this answer are about Lithium Ferro Phosphate batteries / LiFePO4 which I'll abbreviate in places as LFP4. .

These are related to LiIon and LiPo batteries but have major differences.

Notable, compared to LiIon and LiPo (which are chemically similar) LFP4 has an added internal inactive matrix which the Lithium resides within. This reduces mass and volume energy densities but also makes them free from the destructive melt down modes, gives them a much longer cycle life and a much longer calendar life, lower capacity, lower maximum voltage, better temperature operating and storage range, better overall energy efficiency - and a whole of life cost below to far below that of any lead acid battery.

LFP4 (LiFePO4) 4 cell batteries do make excellent replacements for normal 12V lead acid car batteries BUT proper attention to car and feeding is necessary to achieve the very long cycle and calendar lifetimes that they are capable of. They are far more whole of life cost effective" than any lead acid cells in (probably) any application when properly used. Wrongly used they will die an early and expensive death.

A few manufacturers of cells or batteries suited to this use, such as http://www.szaspower.com/2-subchannelproducts/12v-10Ah-lithium-battery.html    

Thundersky, Winston & SinoPoly

There are others which I can list but a serach on those names will get you into the right area. All those brands started out as Thundersky but there have been partings of the way and expensive litigation is happening.

Winston make 12V 4 cell batteries with no access to the inter-battery connections. Their 40 Ah and 60 Ah sizes are claimed to be well suited to automotive use. Simopoly an Winston both make single-cell batteries in sizes from around 10 Ah up to vastly-huge_Ah and you can strap these together as required. I am currently experimenting with 2 x SinoPoly 12v, 40Ah batteries each comprising 4 x 40 Ah single cells with strapping to produce a physical pack. A 12V, 40Ah 4 cell battery is about the size of a Ford-Prefect battery and less weight but is equal to about the largest LA battery you will see in a say 4 cylinder car.

HOWEVER

Claims from various battery makers overlap but are not identical, charging specs are suspect, claimed lifetimes vary depending on sales path and even reputable sellers disagree. The Winston battery charging specs use suspiciously high end point voltages - higher than I'd expect for LFP4 but below LiIon- almost like LiIon being run super conservatively to get good cycle life. Sellers warrant most brands of 10 Ah + cells for 5 years or 10 years use subjectto various conditions and probably subject to use of charge/discharge management electronics. Criteria for adequate mintoring varies - many claim that Vma and Vmin are enough as long as C rates of chage and discharge are within spec BUT a local supplier demands a gas-gauge type monitor and a low voltage cutoff. (I'd consider the gas-gauge excessive and the cutoff or similar protection essential)