Lithium Ion and Lithium Iron Phosphate batteries

Lithium Ion and Lithium Iron Phosphate
batteries
Introduction
The modern drive towards electric cars has been made possible by recent advances in batty
technology. Lithium based rechargeable batteries are the closest yet to achieving energy
densities of gasoline, which would give electric cars the range that would allow them to
completely replace internal combustion engines. Lithium batteries have other limitations on
their charge and discharge rates, and on their operational lifetime, which must also be
considered. This paper will examine lithium batteries, their limitations, and how to work around
them.
Lithium Ion Battery Technology
Lithium ion batteries are secondary cells constructed from layers of lithium sandwiched with an
electrolyte and stacked into rectangular packs, although they can also be wrapped into a
cylindrical shape. The distinction between lithium, lithium ion and the other varieties of lithium
packs is the kind of electrolyte used. Their primary advantages are their energy density and
faster charge/discharge times compared to the nickel based batteries historically used [1].
Limitations of Lithium Ion Cells
The primary problem with lithium cells is their degradation. Over time a lithium ion cell will lose
capacity, with a total lifetime of 2-3 years [2]. The exact lifetime is a function of the amount of
use, the amount discharged between recharging, and other factors such as the temperature of
the cells. This reduction in lifespan is a problem for electric cars as the battery system is an
expensive system to have to completely replace every three years. An additional problem is
their low charge and discharge rates compared with traditional liquid fuels in automotive
applications, which require a car to charge for hours each night instead of only taking a few
minutes at a filling station [3].
Improvements
The high internal resistance of the lithium ion cells can be changed by changing the electrolyte
material to improve its conduction of lithium ions, for example with cobalt oxide, but this
introduced environmental concerns with the disposal of cobalt. Better performance has
recently been obtained using olivine type materials such as iron phosphate (FePO4) which has
an improved energy density compared to even the cobalt cells [4]. Recent developments with
the manufacture of the LiFePO4 cells has determined that the conductivity can be drastically
improved by having channels of more conductive electrolyte permeating the FePO4 material,
allowing cells to be fully charged in as little as 10 minutes if adequate charging power levels are
available [5].
Other Considerations
While some of the innovations discussed above haven’t yet hit the market, LiFePO4 cells are
currently available and have excellent specifications making them fit for the automotive
market. A particular cell (HW 38120) available since 2007 claims max charge and discharge
currents which could result in a full charge in 20 minutes and a full discharge in 6, providing and
making use of all the power available for quick accelerations and decelerations experienced by
electric cars using regenerative braking [6]. These particular cells were subjected to a number
of tests to prove that they are safe enough for automotive use, and can withstand a 1m trop
onto concrete, operate at temperatures as high as 130 degrees centigrade, and be discharged
through a wire with 50 mΩ resistance without exploding or catching fire as one might expect a
short circuited battery to [6]. The energy density of these cells comes out to .38 MJ/kg, which
compares to 47 MJ/kg of gasoline and allows a car with 21 kg of these cells to go as much as
100 miles on a charge [3]. Furthermore these cells were shown to still have 70% capacity after
2000 cycles, which for automotive use would mean that a car with 100 mile range could drive
200000 miles without replacing the battery pack [6]. This is however unrelated to the issue of
cell degradation over stored time, which for normal lithium ion cells would reduce the capacity
to 70% within 3 years [2]. LiFePO4 cells are rumored to have a longer shelf life, but they
haven’t been around long enough for this to become apparent, and certainly hadn’t when the
datasheet was written in 2007.
References
[1] J. Kim and H. Lee, “Lithium Ion Battery,” U.S. Patent 0 154 795, A1, Jul. 5, 2007.
[2] Battery University. (2011). How to prolong lithium-based batteries. Battery University.
[Online]. Available:
http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries
[3] A. Hadhazy, (2009). A Better Battery? The Lithium Ion Cell Gets Supercharged. Scientific
American. [online] Available: http://www.scientificamerican.com/article.cfm?id=betterbattery-lithium-ion-cell-gets-supercharged
[4] M.S. Islam, D.A. Driscoll & C.A. Fisher, "Atomic-scale investigation of defects, dopants, and
lithium transport in the LiFePO4 olivine-type battery material," Chemistry of Materials, vol. 17,
no. 20, pp. 5085–5092, 2005.
[5] S. K. More. (2009). A Rapid-Recharge Lithium Battery. IEEE Spectrum. [online] Available:
http://spectrum.ieee.org/energy/renewables/a-rapidrecharge-lithium-battery
[6] Headway . (2007) Technical Specifications for li-ion battery. Ecocarforum.com [online]
Available:
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