Why LiFePO4?

I am going to get more scientific than ever before, but some context on why LiFePO4 batteries are a big deal today is required. One of the earliest mentions of the use of lithium as a battery electrode came in the early 1970s, with the image above taken from a journal article published in Science in 1976[1]. Titanium sulfide (TiS₂) as the anode and lithium metal as the cathode were used to create a rechargeable battery device. The energy density was measured to be 480 Wh/kg, which was not bad at the time, but there were serious issues related to the use of TiS₂, which did not require much energy to form H₂S gas when exposed. This is even before we get to the dangerous use of raw lithium, where moisture in the cell—be it as an electrolyte or otherwise—could be life-threatening. That having been said, the potential for lithium as an electron donor was quickly established, but it was recognized that it had to be used as a lithium compound.

The same decade also saw the study of graphite intercalation into alkali metal compounds, meaning carbon atoms as graphite layers would be interspersed with alkali metal compounds, including those using lithium as the alkali metal. It did not take long to suggest the use of graphite and lithium compounds as working electrodes for batteries[2]. There even have been lithium-graphite anodes[3], but the trend of safety, efficiency, and scalability led towards the adoption of graphite anodes paired with lithium compound anodes. So when you do see the use of lithium ion batteries, you are typically looking at an example of science that is three decades old!


There was a brief lull as different lithium compounds were tested, until polyanions involving iron were first suggested with sulfur[4] and eventually phosphorous[5]. A thin layer of carbon was even shown to benefit the electrochemical potential of LiFePO4 cathode[6], thus showing a symbiotic relation with a carbon (aka graphite) anode. The 2008 Energy Storage Research and Development Progress Report from the US DoE especially called out LiFePO4 for low toxicity, low cost due to higher abundance of source materials, high thermal stability and electrochemical potential, and high specific capacity (~170 mAh/g) to where it quickly became a source of research owing to government funding made available in different regions for the energy storage and battery technology sectors.


There is another good reason for the popularity of LiFePO4 cathodes today despite the intrinsically lower electrical conductivity. LiFePO4 has one Li+ ion per unit that can be extracted and transferred to the anode in the first charge process, which in turn compensates for the oxidation of iron. In all iron phosphates other than LiFePO4, the initial valence states of Fe are trivalent, which means they cannot be oxidized further within the electrolyte window typically available with current-gen electrolytes. Oxidation of LiFePO4 to FePO4 also reduces the size of the operating cell by ~7% in volume[7], which helps accommodate the expansion of the graphite anode during the cycle before things reverse again.


Remember where a thin coat of carbon helped increase the electrochemical potential of LiFePO4? Current research has helped tackle the intrinsically lower electrical conductivity, with an example the crystalline carbon coating the LiFePO4 powder packed into the final cathode. One such example comes in the form of multi-walled carbon nanotubes[8], which has a strong effect but also increases the cost. The electrodes used in this BLUETTI AC200P very likely are just LiFePO4, with BloombergNEF's survey results from last year showing prices of batteries with LiFePO4 electrodes reaching a low of just $80/kWh for much larger batteries than those used in the AC200P, a cost reduction of over 90% since 2010, when the lower thresholds were $1100/kWh.

So there is definitely a lot of profit in this sector for companies such as PowerOak, but the net cost to the consumer is also far lower today than even a few years ago, all thanks to LiFePO4.

References

[1] Whittingham, M. S., 1976, Science. 192 (4244),1126–1127
[2] Eichinger, G.; Besenhard, J. O.,1976, J. Electroanal. Chem. Inter. Electrochem. 72, 1–31
[3] Yazami, R.; Touzain, P., 1983, J. Power Sources. 9 (3), 365–371
[4] Manthiram, A.; Goodenough, J. B., 1989 J. Power Sources. 26 (3–4), 403–408
[5] Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B., 1997 J. Electrochem. Soc.144 (4), 1188-1194
[6] Masquelier, C.; Croguennec, L., 2013, J. Chem. Rev., 113, 6552-6591
[7] Yamada, A. et al., 2001, J. Electrochem. Soc., 148, A224-A229
[8] Susantyoko, R. A. et al., 2018, RSC Adv., 8, 16566–16573