In general, batteries – regardless of size and capacity – consist of battery cells that are interconnected to form a battery module. By connecting multiple battery modules, the capacity of the battery is scaled. Different applications have different capacity and power requirements. To meet the requirements of an application, different battery chemistries are available, each with different characteristics. In general, lithium-ion batteries (LIB), familiar from cell phones, notebooks or even the electric car, are used for most applications.

However, within the supergroup of LIBs, there are large differences in battery characteristics due to the selection of the respective cathode material. LIB cathodes consist of a current conductor (usually aluminum foils) to which an active material is applied, in which the current and the lithium ions can be stored. The most widely used battery cell chemistries are lithium nickel cobalt manganese (NMC), lithium nickel cobalt aluminum oxide (NCA), and lithium iron phosphate (LFP).

ADVANTAGES OF LFP AS CATHODE MATERIAL

NCA and especially NMC batteries are the most widely used because they make a good compromise between performance, energy density and cost. Why we rely on the slightly more expensive LFP technology in our solutions is summarized in the following subsections.

I. Security – No Compromises

The safety of our solutions has the highest priority for us and for our customers, which is why we do not want to compromise here in our solutions. All our systems are protected with electrical and/or mechanical protection circuits against overtemperature, overcurrent, overvoltage and short circuit. In addition, LFP technology is out of competition in terms of safety compared to other LIB cell technologies.

Especially for cells with chemically and thermally unstable cathode material (e.g. NMC), strong heat generation during overcharge, an internal or external short circuit, mechanical damage, production-related impurities or strong external heat exposure can trigger a cell-internal exothermic chemical reaction. The thermal energy released increases the reaction rate of the cell chemistry and causes the internal cell temperature to rise further. When a specific temperature limit is exceeded, this self-accelerating process can no longer be stopped. Thermal runaway occurs, which can lead to fire or explosion of the cell. Because the oxygen bound in the cathode material is released in such a case, such a fire is very difficult to extinguish.
Unlike other LIBs, LFP batteries do not release oxygen during the chemical reaction, which reduces the tendency for thermal runaway. LFP batteries are not self-igniting (e.g. due to overcharging) and have no thermal effects up to 300°C.

The need for safe battery technology has been reaffirmed after a series of fires involving the less safe LIB, the most recent case being a major fire at a large storage facility in Australia that took three days to extinguish.

II. Cycle Stability & Longevity

Every battery cell is exposed to chemical, thermal and mechanical stresses such as expansion with every operating cycle (charging and discharging), which cause the cell to age and lose some of its original capacity. This phenomenon should be familiar to everyone from their smartphone – after 2 years, the battery charge only lasts half a day instead of one or even several as before.

Due to the somewhat lower cell voltage of 3.2 V, the energy density of LFP cells is not quite as high as that of NMC cells, but this supposed disadvantage is more than compensated for after only a short period of use by a cycle stability that is up to ten times higher. NMC cells age much more quickly in terms of cycling and have only 80 percent of their initial capacity after around 500-1000 cycles. This puts the somewhat higher initial costs of using lithium iron phosphate into perspective.

Due to higher stress on the battery in mobile applications, LFP batteries can contribute to a longer service life in the portable and mobile sector. Due to longer operating time of battery storage systems with LFP technology in stationary applications, the relative storage cost (Levelized Costs Of Storage / LCOS) can be reduced by 50% over the lifetime compared to NMC batteries.

III. High Charge & Output Performance

LFP technology ensures that our solutions can still deliver the specified power at the end of their life cycle. The memory effect of LFP cells is negligible compared to other LIB. The memory effect refers to a loss of capacity in a battery, as occurs when a battery is frequently partially discharged. The battery “remembers” the energy demand and, over time, instead of supplying the original energy, it supplies only the energy required during previous discharges.

LFP batteries have a higher power density compared directly to other LIBs, which allows for high charge and discharge currents and increased pulse load capacity. Via higher currents, LFP cells are capable of fast charging. Although continuous fast charging shortens cell life, this effect occurs only to a small extent with LFP cells. The output power of LFP cells is comparable to that of NMC cells.

Intelligent system design can be used to create cost-effective system designs in steady-state applications that provide a perfect trade-off between charge/discharge currents and cycle stability/lifetime and minimize lifetime LCOS.

IV. Wider Thermal Operating Window

Overall, LFP cells are much less sensitive to heat and even operation at sub-zero temperatures is possible. The temperature range of commercially available LFP cells here extends from -30 to 65 °C. The operating temperature range of our LFP batteries is deliberately specified to be between -10 and 55 °C: On the one hand, it is no longer possible to charge the cells practicably at extreme minus temperatures. On the other hand, the cells within a battery pack already reach a cell temperature of 65 °C in normal operation at an ambient temperature of 55 °C due to self-heating and would therefore be overloaded at higher ambient temperatures. This is an important detail that should be taken into account when comparing the temperature specifications of different cell and battery packs.
The operating temperature range can be extended by active heating or cooling of the battery cells, depending on the solution and application.

V. Environmental Friendliness

LFP is the only cathode material that also occurs as a natural mineral in its chemical composition and does not require any additional raw materials for the chemical reaction. Accordingly, there is no cobalt or nickel in our solutions, both of which are considered toxic heavy metals. Cobalt is also a potential conflict raw material. Although cobalt used in Europe often comes from Canada or Australia, a large part of the mining, especially for Chinese production, is located in the Congo, where the raw material is mined under questionable circumstances.

In addition, all the lithium used in an LFP cell is used for the chemical reaction. In other LIBs, on the other hand, only about 50 – 60 % of the lithium used is utilized, since otherwise instabilities would occur in the layer structure, or the rest would be integrated into the crystalline structure of the cathode. This reduces the required mass per kWh from approx. 140 g (NMC / NCA) to approx. 80 g (LFP).

LFP – CURRENTLY THE SAFEST AND BEST CELL TECHNOLOGY FOR DEMANDING APPLICATIONS

In addition to conventional lithium-ion cells based on lithium cobalt oxide (LCO) or MNC, LFP in particular has established itself as a particularly robust, safe and durable cell chemistry. With up to 10 times the number of cycles compared to LCO/NMC and a low total cost of ownership (TCO), LFP battery storage offers optimal long-term properties with low maintenance requirements and a high level of investment protection and functional safety. Due to the wide operating temperature range, excellent cycling stability, low internal resistance and high efficiency, LFP batteries are the best choice for demanding use in stationary and mobile applications.

Since the cathode material is the largest cost item in a battery cell, it has a great impact on the active cost of the battery cells, modules and battery storage system. Due to a future increasing scaling of LFP production capacities worldwide, the currently high costs for the LFP cathode material will further decrease in the future and thus also realize a cost advantage over NMC.

AXSOL GmbH was already awarded the Federal Prize for Innovative Achievement in 2016 for the combination of LFP battery chemistry of power electronics in portable format and thus takes a pioneering role in the field of European battery integrators.

LIB and especially LFP batteries are ideally suited for mobile and portable applications due to their high energy density. The market share is also continuously increasing in power tools and electromobility. Due to the increasing spread of stationary applications, battery chemistry is also being used more and more in the home or large-scale storage sector. Due to the independence of AXSOL GmbH from battery cell manufacturers, we can (also in the future) select the best available battery chemistry and offer our customers the best solutions on a permanent basis. Through the possibility of technology exchange, we guarantee our customers future security. Thus, at the end of the battery life cycle, it is possible to switch to a new, market-available battery technology without having to replace the power electronics or control system.

References:

https://www.theguardian.com/australia-news/2021/aug/02/tesla-big-battery-fire-in-victoria-burns-into-day-three

https://sonnen.de/wissen/4-gruende-fuer-lithium-eisenphosphat-einem-batteriespeicher/

https://sonnen.de/wissen/4-gruende-fuer-lithium-eisenphosphat-einem-batteriespeicher/

https://www.elektroniknet.de/power/energiespeicher/die-qual-der-wahl.170603.html

https://www.dke.de/resource/blob/933404/3d80f2d93602ef58c6e28ade9be093cf/kompendium-li-ionen-batterien-data.pdf

https://www.energie-experten.org/erneuerbare-energien/photovoltaik/stromspeicher/lithium-eisen-phosphat

https://nickelinstitute.org/blog/2020/june/battle-of-the-batteries-cost-versus-performance/