In general, batteries – regardless of size and capacity – consist of battery cells that are connected together to form a battery module. By connecting several 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), which are familiar from mobile phones, notebooks or even the electric car, are used for most applications.
However, within the supergroup of LIBs, there are major differences in battery characteristics, which are determined by the choice of the respective cathode material. LIB cathodes consist of a current conductor (usually aluminium foils) to which an active material is applied in which the current and the lithium ions can be stored. The most common battery cell chemistries are lithium-nickel-cobalt-manganese (NMC), lithium-nickel-cobalt-aluminium-oxide (NCA) and lithium-iron-phosphate (LFP).
ADVANTAGES OF LFP AS CATHODE MATERIAL
NCA and especially NMC batteries are the most widely used, as they make a good compromise between performance, energy density and cost. Why we rely on the somewhat more expensive LFP technology in our solutions is summarised in the following subsections.
1. SAFETY – NO COMPROMISES
The security of our solutions has the highest priority for us and for our customers, which is why we do not want to make any compromises in our solutions. All of our systems are protected against excess temperature, excess current, excess voltage and short circuits with electrical and/or mechanical protective circuits. In addition, the LFP technology is unrivaled in terms of safety compared to other LIB cell technologies.
Particularly in the case of cells with chemically and thermally unstable cathode material (e.g. NMC), excessive heat generation in the event of overcharging, an internal or external short circuit, mechanical damage, production-related contamination or strong external heat can trigger an exothermic chemical reaction within the cell. The heat energy released increases the reaction speed of the cell chemistry and causes the cell-internal temperature to rise further. If a specific temperature limit is exceeded, this self-accelerating process can no longer be stopped. Thermal runaway occurs, which can result in cell fire or explosion. 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, reducing the risk of 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 was particularly confirmed after a series of fires involving the less safe LIB, most recently a major fire at a large storage facility in Australia, which could only be extinguished three days later.
2. CYCLE RESISTANCE & DURABILITY
Every battery cell is exposed to chemical, thermal and mechanical stresses such as expansion with every operating cycle (charging and discharging), which causes the cell to age and lose part of its original capacity. Everyone should be familiar with this phenomenon from their smartphones – after 2 years, the battery charge lasts only half a day instead of one or even several as before.
Due to the slightly 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 a short period of use by a cycle stability that is up to ten times higher. NMC cells age cyclically much faster and after about 500-1000 cycles they only have 80 percent of their initial capacity. To this end, the slightly higher initial costs when using lithium iron phosphate are put into perspective.
Due to higher battery demands in mobile applications, LFP batteries can contribute to a longer service life in the portable and mobile area. Due to the longer service life of battery storage systems with LFP technology in the stationary sector, the relative storage costs (Levelized Costs Of Storage / LCOS) can be reduced by 50% over the lifetime compared to NMC batteries.
3. HIGH LOAD AND OUTPUT REGULATION
The LFP technology ensures that our solutions can still call up the specified performance even at the end of their life cycle. The memory effect of the LFP cells is negligible compared to other LIBs. The memory effect means a loss of capacity in a battery, which occurs when a battery is partially discharged frequently. The battery “remembers” the energy requirement and over time only supplies the energy required for the previous discharging processes instead of the original energy.
In direct comparison to other LIBs, LFP batteries have a higher power density, which enables high charging and discharging currents and an increased pulse load capacity. LFP cells can be charged quickly using higher currents. Although the lifetime of the cells is shortened by constant rapid charging, this effect only occurs to a small extent in LFP cells. The output power of LFP cells is comparable to that of NMC cells.
Through intelligent system design, cost-efficient system designs can be created in the stationary area, which enable a perfect trade-off between charging and discharging currents and cycle stability/lifetime and minimize the LCOS over lifetime.
Overall, LFP cells are much less sensitive to heat and can even be used at sub-zero temperatures. The temperature range of commercially available LFP cells extends from -30 to 65 °C. The working temperature range of our LFP batteries is deliberately specified at -10 to 55 °C: On the one hand, it is no longer possible to charge the cells in a practical way at extremely low temperatures. On the other hand, the cells within a battery pack in normal operation at an ambient temperature of 55 °C already reach a cell temperature of 65 °C due to self-heating and would therefore be overloaded at higher ambient temperatures. An important detail that should be considered when comparing different cell and battery packs in terms of temperature information.
Depending on the solution and application, the working temperature range can be extended by actively heating or cooling the battery cells.
In addition, all of the lithium built into an LFP cell is used for the chemical reaction. In other LIBs, on the other hand, only about 50-60% of the built-in lithium is used, as otherwise instabilities would result 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 – THE CURRENT 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 a cycle count that is up to 10 times higher than that of LCO/NMC and a low total cost of ownership (TCO), LFP battery storage systems offer optimal long-term properties with low maintenance requirements and a high degree of investment protection and functional safety. Due to the wide operating temperature range, excellent cycle 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 major impact on the active costs 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 continue to decrease in the future and thus also realize a cost advantage compared to NMC.
As early as 2016, AXSOL GmbH was awarded the federal prize for innovative achievement for the combination of the LFP battery chemistry with power electronics in a portable format and thus plays a pioneering role in the field of European battery integrators.
LIB and in particular LFP batteries are ideal for mobile and portable applications due to their high energy density. The market share is also increasing continuously in power tools and electromobility. With 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 over the long term. Through the possibility of technology exchange, we guarantee our customers future security. At the end of the battery life cycle, it is possible to switch to a new, commercially available battery technology without having to replace the power electronics or control.
Sources:
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.energie-experten.org/erneuerbare-energien/photovoltaik/stromspeicher/lithium-eisen-phosphat https://nickelinstitute.org/blog/2020/june/battle-of-the-batteries-cost-versus-performance/
