Note in my previous post , I referred to the freezing of the electrolyte, this is related to ultimate low temperature performance, Here I try and deal with the issues related to sub zero ( c) performance and why it drops off dramatically in commercial
For those of you with an academic bent I would cite
"Research on cathode material of Li-ion battery by yttrium doping,JOURNAL OF RARE EARTHS
, Vol. 26, No. 2, Apr. 2008, p. 279 by TIAN Yanwen (田彦文)1, KANG Xiaoxue (康晓雪)1, LIU Liying(刘丽英)2, XU Chaqing (徐茶青)1, QU Tao (曲 涛)1"
Certainly its clear from that article that Y doping in LiFePo4
in certain amounts improves both initial discharge ability and also cyclic performance was improved. This is because Yttrium doping improves the conductivity of the cell over that of non doped Lifepo4
Low temperature performance of LI cells, is still a matter of considerable scientific debate, but can be summarised as
(a) reduced conductivity of the electrolyte and solid electrolyte interface on the elec- trod surface
(b) limited diffusivity of lithium ions within graphite anode
(c) high polarization of the graphite anode, ( as per (b)
(d) substantially increased charge-transfer resistance on the electrolyte–electrode interfaces
( S.S. Zhang∗, K. Xu, T.R. Jow, Electrochemical impedance study on the low temperature of Li-ion batteries,US Army Research Laboratory, Adelphi, MD 20783-1197, US
, Electrochimica Acta 49 (2004) 1057–1061.)
Of this the primary factor has been to kinetics of Ion transfer at low temperature, and it has been shown that while discharge can occur at increasingly lower temperatures, the equivalent resistance of the cell reaction kinetics, increases with discharge at low termoperatures , making re-charging such cells difficult or even impossible at low temperature.
The key thing here is that Yttrium doping was designed to counter the lower conductivity and improve cyclic performance, in an attempt to recover some of the energy capacity lost
in deploying LiFepO4 as a cathodic material. It has little or no effect on temperature.
As is pointed out in the above article, commercial
grade LI-ion suffers greatly from low temperatures, a typical 18650 cell, will have less then 5% of original room temperature capacity at -40 degree C ( electrolyte freezing point)
Its furthermore interesting that most of this research
work, seems to be in the hands of Asians scientists , working in American research institutions from about 2000-2005, theres nothing new in Yttrium doping , nor is it exclusive to Winston Chung.
What this means, is that for commercial LIfePo4 cells ( with or without doping) both Lithiation and delithiation become very poor at temperatures below zero, primarily due to the kinetics around the Graphite anode. Note that often manufacturers temperature limits , merely mean the battery can be technically
used at stated low temperature, ( i.e. the electrolyte will no freeze) , it does NOT mean that in particular the standard charging
regime can be applied at sub zero conditions,
IN particular at low temperatures , The battery is easy to "voltage stress " , to simply it looks like a battery of significantly reduced capacity and applying a standard charging regime will over stress the battery and cause irreversible plating of the electrodes. Subsequent attempts can then cause massive oxidisation of the electrolyte and the usual Li issues.
Li technology can be improved with exotic anodes etc and much research is ongoing , but today what we get in commercial cells is not in any way optimised for low temperatures.