Three main reasons to use lithium iron phosphate batteries for storage
Which battery cells are best suited for battery storage systems? At RCT Power, we have utilised lithium iron phosphate, also known as LiFePO4 or LFP, for our battery units from the beginning. There are three main reasons for using lithium iron phosphate:
1. Durability and performance of LiFePO4 battery storage
The durability of a battery can best be described by this simple question: How often can the battery be recharged before the capacity drops significantly? The parameter to look at for comparison of batteries is charge cycles.
A charge cycle is a connected charging and discharging process. We distinguish between full and partial charge cycles. For a full charge cycle, the battery is discharged to 0 per cent residual capacity followed by a subsequent charge to 100 per cent capacity. Partial charge cycles are characterised by an incomplete discharge of the battery. Partial charge cycles can be summed up to full-cycle equivalents when calculating the overall charge cycle numbers for a battery.
Important parameters that determine the cycle life of the battery storage are the Depth of Discharge (DoD), the State of Charge (SoC) and the ambient temperature at the installation site. SoC and DoD values indicate the states of charge and discharge of a battery in per cent to its total capacity.
For self-consumption optimised storage systems, the battery will be charged and discharged around 250 times a year. RCT Power's lithium iron phosphate battery storage systems can be charged and discharged around 5000 times. Beyond that, battery storage systems can still be charged efficiently, but a slight loss of capacity is to be expected. A lead-acid battery, such as a car battery, in comparison, has between 300 and 500 full charging cycles.
The performance of the battery also plays a significant role during the charging and discharging process. The LiFePO4 battery can be charged and discharged at high power. A feature that is especially beneficial during changing weather conditions or when used with backup power systems.
The efficiency η is a dimensionless value. For batteries, η is determined as the ratio of energy supplied to energy extracted. No energy storage system has the ideal efficiency of 100 per cent, which means, that in practice, not all of the charging energy of the battery can be utilised. The efficiency of a LiFePO4 battery is very high and can range between 93 and 98 per cent. Compared to other battery technologies lithium iron phosphate-based battery not only have higher charge cycle numbers but also a higher energy conversion efficiency.
Batteries have a so-called "design lifetime" which is independent of the charging cycles. Here, LFP-based batteries are superior to other technologies. Lithium iron phosphate batteries used by RCT Power, last around 15-20 years. This durability does make economic sense and is also beneficial for the environment as batteries are built with rare raw materials. This connects us to our second argument in favour of lithium iron phosphate batteries: environmental compatibility.
2. Environmental compatibility of lithium iron phosphate battery storage
Let's take a look inside a battery: A battery consists of two electrodes and the electrolyte. The negative anode is usually made of graphite, the positive cathode often consists of metal oxides of various combinations of cobalt and other metals (Lithium Nickel Cobalt Aluminum Oxide (LiNiCoAlO2) — NCA, Lithium Nickel Manganese Cobalt Oxide (LiNiMnCoO2) — NMC, Lithium Cobalt Oxide (LiCoO2) — LCO, etc.) or Lithium Iron Phosphate (LiFePO4) — LFP.
Lithium iron phosphate batteries are one of the few cobalt-free lithium-ion batteries. They also do not contain other toxic heavy metals like lead and nickel which are commonly used in other batteries. Lithium iron phosphate, in contrast, is a substance that is in its chemical composition a naturally occurring mineral. Lithium iron phosphate-based batteries are considered to be safe, not toxic and of low environmental impact.
An equally important aspect: the mining of cobalt is often undertaken without any or only very limited health and safety precautions for the miners and proper consideration of the environmental impact.
Giving up on controversial and toxic raw materials such as cobalt for the storage batteries makes lithium iron phosphate batteries the prime choice for energy storage. They combine low ecological impact with economic benefits.
3. Safe battery technology
LFP battery storage systems have superior thermal and chemical stability than those based on cell technologies made with other cathode materials. They provide better safety characteristics, especially concerning potential fire hazards caused by improper overcharging or short circuit conditions.
A lithium iron phosphate cell has a lower energy density which provides additional fire protection. The Fe-P-O bond of the cathode material in LFP cells is stronger than the bond of the oxides used in other cells. Temperatures ( >400°C) required to release the oxygen from this bond and burn the cathode or cause a thermal runaway will not be reached, making them in effect incombustible. The cells will also withstand high temperatures without decomposing. LFP batteries are, therefore, considered to be intrinsically safe.
A high-quality battery management system, as provided by RCT Power, will reduce thermal loads on the battery storage system to a minimum.
From a today perspective, lithium iron phosphate-based batteries are one of the safest battery technologies.
Quick Digested Summary: Lithium iron phosphate battery storage
Currently, lithium iron phosphate batteries are the "state of the art" for the storage of solar energy. They feature a long service life and high efficiency. Lithium iron phosphate battery storage systems are particularly safe and environmentally friendly.