The Lithium iron phosphate accumulator is a version of a lithium-ion accumulator with a cell voltage of 3,2 V or 3,3 V. Lithium iron phosphate (LiFePO₄) instead of conventional lithium cobalt (III) oxide (LiCoO₂) is used. The negative electrode consists of graphite or hard carbon with incorporated lithium. Accumulators with electrode materials made of lithium iron phosphate have a lower energy density than the widespread lithium-ion accumulator with lithium cobalt electrode, but do not tend to thermal runaway even in the event of mechanical damage.

Development and function

lifepo₄ was first proposed in 1997 as a material for lithium-ion batteries. It replaces the lithium cobalt used in conventional lithium batteries.

In 2010, Süd-Chemie (now Johnson Matthey) built the world's largest production facility (2500 t per year) in Canada for the production of lithium iron phosphate using a wet chemical process. Production started in April 2012.

Early LiFePO₄Cathodes suffered from low electrical conductivity for ions and electrons, which inhibited power density. The conductivity could be increased through the use of LiFePO₄- Nanoparticles and the coating with carbon are improved. The doping of the LiFePO₄ for example with yttrium (LiFeYPO₄) or sulfur atoms also improves the technical properties.^[

In contrast to conventional lithium-ion cells with lithium cobalt (III) oxide (LiCoO₂) no oxygen is released during the chemical reaction. In lithium-ion accumulators, lithium-cobalt, this can lead to thermal runaway, which under unfavorable conditions leads to the cell igniting automatically.

Compared to the conventional cathode materials (LiCoO₂) the entire lithium content is used in the lithium iron phosphate battery. For batteries with LiCoO₂-Cathode only 50-60% of the lithium is used, otherwise the layer structure would be unstable. When using Li₂Mn₂O₄-Cathodes, only 50% of the existing lithium can be used, the rest is built into the crystal.

The mass fraction of lithium in LiFePO₄ is approx. 4,5% by weight. For an accumulator with an energy content of 1000 Wh, only approx. 11,3 mol (≈ 80 g) lithium are required for a lithium iron phosphate battery, compared to approx. 20 mol or 140 g for the lithium-cobalt or lithium-manganese battery. The energy density of lithium iron phosphate batteries is 90 Wh / kg to 110 Wh / kg. For lithium batteries with LiCoO₂Cathode, an energy density almost twice as high can be achieved.

lifepo₄-Accumulators have no memory effect like the nickel-cadmium accumulator. A so-called anomaly during discharge is very small and insignificant in normal operation. LiFePO₄-Cells can be temporarily stored, discharged and charged at any time. Long storage times are detrimental to life expectancy only when fully charged and almost discharged.

Features

Cell voltage range

The exact voltages differ slightly between the cell types and manufacturers, in the application they can be found in the respective data sheet. The end-of-charge voltage is usually 3,6-3,65 V. The protective circuits against overcharging usually respond at 3,8 V.^[

The end-of-discharge voltages differ depending on the type and are mostly 2,0 V, with some types also just above it in the range of 2,5 V. In the charge range of 10% to 90%, the cells show both charging and discharging only a slight change in the cell voltage, as in the diagram opposite with the cell voltage as a function of the charge for a LiFePO₄Cell shown with a nominal capacity of 2,3 Ah. In the area of ​​the end of discharge, in the diagram the end in the course of the red line in the right area of ​​the image, and in the area of ​​the charge end, in the diagram the end in the course of the green line in the right area of ​​the image, there is a strong voltage reduction (during discharge) or a strong voltage increase (when charging) available. Slightly reduced end-of-charge voltages (3,4-3,5 V) and reduced depths of discharge have a positive effect on the usable number of cycles and thus the service life.

designs

There are only a few standardized designs. A basic distinction can be made between round cells and flat cells.

• Round cells are mainly offered in the single-digit to low double-digit Ah range. There are also designs that correspond to unofficial industry standards for round cell dimensions and are similar to device batteries. For example, batteries of the types 18650 and 26650 are used more often, the type designation of which reflects the approximate size, 18650 means approx. 18 mm in diameter and 65 mm in length, 26650 is approx. There are also cells of type 26, these are 65 mm in diameter and approx. 38140 mm long. The weight is around 38 grams per cell and has an M140 screw connection on the poles. These cells are mainly used in industry.
• Flat cells are available for almost all capacity sizes. They are sold in the form of film cells and cuboid cell blocks.
  • The former are produced in the form of flat cells covered with foil and also as Foil cells designated. However, this design is only an intermediate product that must be treated with care for the assembly of battery packs or for direct installation in a housing. Sizes range from the mAh range to the two-digit Ah range.
  • The large cuboid shapes with plastic housing and screw connections (range approx. 20–1000 Ah), often referred to as single cells, consist of several film cells combined in parallel in a common housing. They are much easier to handle than pure film cells, but here too there are no standardized dimensions or grid dimensions.

Anomaly in the discharge voltage curve

In the course of the discharge voltage curve of LiFePO₄-Accumulators can detect small humps. This anomaly was called the “memory effect” by the discoverers. The effect is due to the phase transition of individual particles of the active material and, according to previous knowledge, occurs exclusively with LiFePO₄Cathodes and similar olivine cathodes. This so-called "memory effect" is not related to the well-known memory effect Nothing: D- and NiMH batteries comparable. It occurs from the first discharge, is limited in time and can be reversed by charging the battery. The performance and service life of the battery are not directly affected by the anomaly, but the effect can lead to the charge status display being falsified. By researching the LiFePO₄- The previously difficult determination of the state of charge with LiFePO can have a "memory effect"₄-Accumulators will be improved in the future.

Advantages and disadvantages

lifepo₄-Accumulators have clear advantages in terms of cycle stability, size, capacity and weight compared to common lead-acid batteries, the disadvantage is the higher price of LiFePO₄-Accumulators compared to electrically equivalent solutions with lead-acid batteries. Then there are those at LiFePO₄-Accumulators require balancers, which are not necessary with lead-acid batteries.

Benefits

• High security: Due to the solid electrolyte and cell chemistry, LiFePO apply₄-Cells as intrinsically safe, d. H. Thermal runaway and membrane melting, as with lithium-ion batteries, is excluded.
• Power density up to 3000 W / kg, higher than the conventional Li-Ion battery on LiCoO₂-Base, therefore high load capacity (continuous power consumption)
• Very high pulse load capacity up to 40 C.
• High charging currents possible (0,5 C - 3 C), pulse charging currents up to 6 C (10 s)
• High cycle stability: I) Sony Fortelion: 74% remaining capacity after 8 cycles with 000% degree of discharge (DoD) II) still 100% original capacity (nominal capacity, NC) after 80 cycles and 1000% capacity after 60 cycles. Also in cylindrical (2000) cells,> 18650% residual capacity was achieved after 85 cycles with 10% DoD. Other manufacturers state more than 000 cycles with a respective discharge to 100% (Depth of Discharge, DoD) and 5000 cycles with minimal discharge to just 70%, resulting in a long service life and low operating costs.
• high electrical efficiency for a total cycle of charge and discharge of> 91%
• flat voltage profile for charging and discharging
• Wide temperature range for storage (e.g .: −45 to +85 ° C, −15 to +60 ° C). Practical experience shows that it can be used up to approx. +10 ° C without any problems; below this, high current consumption above 0,5 C (traction application) leads to greater voltage drops, but without a noticeable loss of battery capacity.
• The self-discharge was improved by doping and is low at approx. 3–5% per month
• improved environmental compatibility by eliminating cobalt

Disadvantages

• There are only a few standardized designs. This makes the application and the exchange more difficult.
• Lower energy density by 90 Wh / kg. As a result, higher weight and space requirements than lithium-cobalt dioxide batteries for the same capacity. (→ energy density and efficiency)
• As with all lithium-ion batteries, protective circuits (overcharge and deep discharge protection) are necessary for safe operation, since lithium cells are generally not overcharge-proof or deep discharge-proof. When several cells are connected in series, balancing circuits and battery management systems ensure that individual cells in the network are not overcharged or deeply discharged.
• The flat voltage curve makes it difficult to determine the state of charge.

Interchangeability with other battery types

The nominal voltage of two-cell LiFePO₄-A battery is in the same order of size as that of 6 V lead-acid batteries. The same applies to multiples z. B. 12 V, 24 V, 48 V, etc. LiFePO₄-Batteries are well suited for replacing conventional lead-acid batteries. Often, despite the higher capacity and resilience, space and weight can be saved, which is offset by the higher costs of LiFePO₄Batteries in relation to lead batteries. Protective and control electronics are seldom available in lead-acid batteries, as these are overcharge-proof in a large area.

However, the replacement of other lithium-ion battery technologies or the conversion of these to LiFePO₄Batteries are more difficult due to the different typical cell voltage of 3,2–3,3 V (3,6 V for lithium-ion batteries based on cobalt, 3,7 V for lithium-polymer batteries). Existing battery management systems, balancers and chargers for use with cobalt-based lithium-ion batteries can only rarely be run on LiFePO₄- Reconfigure the battery.

Manufacturers

A2012Systems, which went bankrupt in 123, offered lithium iron phosphate batteries as round cells under the name of lithium nanophosphate batteries. A123Systems participated in the development of the serial plug-in hybrid sports car Fisker Karma from Fisker Automotive.

GAIA Akkumulatorenwerke GmbH in Nordhausen, Thuringia, manufactures cylindrical cells with 18 Ah and 38 Ah in LFP (iron phosphate) technology, which are assembled into starter batteries or customer-specific traction batteries.^[

Winston Battery Ltd (formerly Thunder Sky Ltd) from China manufactures a wide range of prismatic battery cells based on LiFePO₄, especially with yttrium doping (LiFeYPO₄) to increase durability and performance.

The Chinese company BYD is the world's largest manufacturer of lithium batteries with a production capacity of over 10 GWh per year. The subsidiary BYD Auto installs the cells in its own electric vehicles as well as in stationary electricity storage systems. The BYD ebus is the world's first battery bus with lithium iron phosphate batteries.

According to EuPD Research, the southern German company sonnen is the leading company on the German and European home storage market, which exclusively uses lithium iron phosphate cells, with a market share of approx. 24%.

Varta AG with Varta Storage GmbH is a leading European company that offers energy storage systems for private households and for industry.

Applications

The largest cell blocks up to 30 Ah are used in submarines, in uninterruptible power supplies and in the storage of regenerative energy. Due to its high reliability, the lithium iron phosphate accumulator has an outstanding position in new stationary storage systems for grid stabilization: measured by the output in MW of all Li-ion storage power plants planned in 000, 2014% of them are based on lithium iron phosphate. 39 MWh are used in a battery storage power plant in Hong Kong.

Other fields of application are power tools and the $ 100 laptop. In RC model making, LiFePO₄- Batteries are used because they can be fully recharged within 15–20 minutes and have a higher cycle stability. Ordinary lithium-polymer batteries often require more than an hour to charge if you do not want to accept any loss in service life.

Further applications are starter batteries in internal combustion engines, where prismatic lithium iron phosphate blocks or assembled round cells are used. Porsche is the first automobile manufacturer to offer a LiFePO ex works for a surcharge₄-Starter battery on.

Influences on service life and economy

Depending on the application, the lithium iron phosphate batteries are optimized for high energy density for storing large amounts of energy, e.g. as a traction battery for purely electric vehicles or for delivering high currents, e.g. for buffer batteries in hybrid electric vehicles or as starter batteries. Lithium iron phosphate accumulators, with the appropriate design and mode of operation, have the prerequisite to function for the entire life of the vehicle without replacement. Various factors can be specifically influenced in order to increase economic efficiency and service life:

• Although a higher temperature generally has a positive effect on the mobility of the electrons and the course of the chemical processes (current resistance), it increases with LiFePO₄-Accumulators also the formation of surface layers on the electrodes and thus the aging or the gradual loss of capacity and the reduction of the current carrying capacity. Since this is especially true above about 40 ° C, the temperature influence is practically usually less than that of other factors and mainly affects cells that continue to warm themselves due to cyclical and permanent high loads. Investigations have shown that aging disproportionately deteriorates performance and usability, especially at low temperatures. A study in which cells were aged at 50 ° C and then measured at different temperatures summarizes the results as follows:

“Capacity fade after 600 cycles is 14.3% at 45 ° C and 25.8% at −10 ° C. The discharge pulse power capability (PPC discharge) at 45 ° C does not decrease with cycling (namely, there is little power fade) from 0 to 600 cycles, whereas the power fade after 600 cycles is 61.6% and 77.2%, respectively, at 0 and −10 ° C. The capacity and power fade evidently becomes more severe at lower temperature due to greatly increasing cell resistance. In particular, the power fade at low temperatures (eg, 0 and −10 ° C) rather than capacity loss is a major limitation of the LiFePO₄ cell. "

“The decrease in capacity after 600 cycles is 14,3% at 45 ° C and 25,8% at −10 ° C. There is only a slight decrease in the current carrying capacity at 45 ° C after 600 cycles, while the power decrease after 600 cycles is 61,6% and 77,2% at 0 and −10 ° C. Capacity and current carrying capacity decrease more at low temperatures. In particular, the decrease in current-carrying capacity at low temperatures (e.g. 0 and −10 ° C) is a major limitation of LiFePO₄ Cell."

• The regularly used depth of discharge has a major influence on the cyclical service life. When the voltage level is low, irreversible processes start in the cells. Storage in a discharged state is therefore also harmful. Shallow discharge depths multiply the achievable number of cycles, the possible energy consumption and thus increase the service life compared to operation with full cycles. The lower voltage limit is usually monitored by the battery management system with a limitation of the power that can be drawn and shutdown, but often at a very low voltage level in order to enable high amounts of energy to be drawn. The manufacturer Winston recommends designing the capacity of a traction battery in such a way that a regular discharge of less than 70% is required.
• Also in the range specified by the manufacturer upper voltage limit of the cells use irreversible chemical processes, which in the long run lead to a decrease in capacity and thus cell wear. Overcharging beyond this voltage limit damages the cell irreversibly. In current applications, the upper charging voltage is often set high when balancing, as this allows the charging states of the individual cells to be better determined and the entire capacity to be used, at the expense of service life. Even with balancers with balancing currents that are too low, voltages are often reached in the uppermost operating range of the cells. Maintenance or continuous charging with constant voltage with the upper voltage limit is not necessary due to the low self-discharge and is detrimental to the service life. It is therefore recommended that the charging current be switched off after the full charge criterion has been reached.
• The current load should be as even as possible, extreme current load peaks (especially with smaller-sized batteries, e.g. in hybrid vehicles) increase wear. The manufacturer's limit specifications are the technical maximum values ​​that the battery can provide, but whose regular use shortens the service life. It is less about the currents of the on-board chargers, which are usually limited in their performance, but rather about extreme current peaks, for example during acceleration, but also through recuperation or fast charging processes with currents> 1 C, for example with CHAdeMO, whose high-current charging therefore to protect cells at around 80% of the nominal capacity is terminated.

A study from 2012 on high-current A123 cells in hybrid application states:

“The longest lifetime is observed for cells cycled with low peak currents and a narrow SOC range. In addition, high charge current is found to affect the cycle life profoundly. On the contrary, a moderate temperature increase did not result in a shorter cycle life. "

“The longest service life is achieved for cells that are used with low current peaks and in a narrow area of ​​the SOC. In addition, high charging currents have a very severe impact on the service life. In contrast, a moderate increase in temperature did not result in a shorter service life. "