Lithium Ion (LiFePO4)
Initially commercialized in power tool applications, Lithium-ion iron phosphate cells are being used today in numerous applications where high cycle life, quick charge, or rapid discharge specifications must be met. Lithium-ion iron phosphate (LiFePO4) cells utilize an iron phosphate cathode material. The chemistry has seen a substantial amount of market interest in the past 5 years. Many vendors now ship cells based on the technology.
The use of iron phosphate as a cathode material in advanced batteries was patented in 1996 by a group of scientists led by John Goodenough at the University of Texas. Through the years, the patent(s) have undergone challenges and litigation is still in process surrounding the use and commercialization of the technology by a number of companies.
The iron phosphate cathode chemical characteristics exhibit excellent thermal and chemical stability. The inherent stability of the chemistry yields a number of very positive attributes in cells constructed with the chemistry. Compared to other Lithium-ion chemistries, the phosphates are seen as being safer.
At the core of the iron phosphate cathode, the FeP-oxygen bond is, in electrochemical terms, much stronger than the bond created by a cobalt-oxygen material. The strength of this bond is interesting in situations where the bond is broken (extreme heat). Cobalt oxide cathodes release oxygen gas inside the cell when put under thermal stress (caused by external sources or potentially due to heating generated inside the cell during use). When this oxygen combines with the flammable electrolyte in the separator layer, the key elements for fire are available. As the heat increases, gases inside the cell can become dangerous and potentially cause the cell to “vent.” The iron phosphate bond holds the oxygen much more securely and thus is far less susceptible to the oxygen gas building up than oxide-based cathodes.
As the Lithium-ion industry has advanced, many variations of the original iron phosphate cathode have been explored. The use of nano-particles has also been utilized to dramatically increase the surface area of the cathode. In other iron phosphate variations, chemical compounds have been used to coat the surfaces of larger iron phosphate particles or chemically “doped” into the structure with good results. These advancements build on the underlying technology and produce a number of differences in the chemistry’s attributes.
- Low internal impedance – Provides high output currents with little internal heating
- Chemistry can support charge rates over 1C with some cells rated at up to 4C charge
- Low self-discharge
- Long cycle life
- High degree of safety
- Cost – Although many vendors and experts tout iron phosphate material as the “cheaper” of the lithium-ion chemistries (vs. cobalt oxide), current market pricing does not reflect this
- Availability – While many vendors are now building LiFePO4 cells, the number of vendors capable of producing high volumes of cells is still small, and the number of cell sizes is also very limited
- Energy density – A compounded effect of lower nominal voltage and chemical energy density give the chemistry a 40% to 60% lower energy density versus cobalt oxide cells which means that more cells are required which increases volume, weight and cost.
To explain the issue of low energy density it is useful to look at an example: 18650 cells are commercially available in CoO2 configurations that are rated at 3.7V and 3100 mAh. Most FePO4 18650 cells are rated at 3.2V and 1100 mAh. This results in an 18650 cell-level energy storage content difference of 11.5Wh vs. 3.5Wh. In applications where run-time is key, the energy density of CoO2 is hard to beat. Some FePO4 vendors are beginning to market a “realized” capacity at one year, two years, and three years to lessen this difference. As the CoO2 cell is cycled, it loses its effective capacity dramatically during that first year of use (in depletion mode applications like a laptop computer). The FePO4 cell retains nearly all of its capacity during that same time. Due to the lower delivered watt-hours on each cycle, the FePO4 cell would require 2x or 3x the “cycles.” Even with this in mind, the FePO4 cell should deliver 2x to 5x the calendar life than the cobalt oxide cell in a depletion mode application.
The energy density limitations of the iron phosphate chemistry has prevented its use in the high volume consumer electronics markets which continue to be dominated by the cobalt oxide products. Today, iron phosphate has had success and shows promise in many high-power and/or fast charge applications where power density is more important than energy density. Cordless power tools, hybrid electric vehicles, off grid power back-up and some military applications are all suited to this chemistry.
Accutronics can provide battery solutions usign Lithium ion - Iron Phosphate chemistry where fast charge, high discharge or long cycle life are a requirement. Please contact us to discuss your requirements.
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