The degradation of LFP cells in BESS

What drives LFP battery degradation?

There are many reasons why LFP batteries degrade, and it is the combined effects of these degradation mechanisms that contribute to overall capacity loss over time. While LFP batteries are more resistant to different degradation factors compared to other cell chemistries, they are not immune. It’s expected that a grid-scale battery will lose between 20-30% capacity in its first decade. The main factors that govern degradation in LFP batteries in BESS setups include: 

• Temperature 
• C-rate 
• Depth of Discharge (DoD) 
• State of Charge (SoC) 

In terms of US$/cycle wear, temperature and C-rate have the biggest impact whereas SOC and the number of cycles per day have about half as much impact. The DoD has very little impact on the US$/cycle wear because deeper cycles age the cell quicker but deliver more energy, so these two factors cancel each other out. 

Temperature 

High and low temperatures can significantly increase degradation and is one of the biggest degradation factors. Capacity fade can significantly increase when the cells are cycled above 50°C, compared to 25°C, which reduces the lifetime of the cell. This is because high temperatures speed up the electrochemical side chemical reactions leading to a faster degradation. On the other hand, extreme cold reduces the efficiency of the cell and increases plating that degrades the cell quicker. 

C-Rate 

The C-rate is the speed at which a battery is charged/discharged relative to its capacity. The higher the C-rates, the faster the degradation. This is because faster charging increases structural decay, increases the temperature inside the cell (same effects as above), increases lithium plating on the anode (when the anode can’t absorb lithium ions quickly enough), and increases the solid electrolyte interphase (SEI) layer. The faster charging rates increase the overall internal strain of the battery which leads to an increase in both chemical and mechanical degradation that reduces cell life. Lower charging speeds reduces these factors, which is one reason why a lot of BESS are run at 0.5°C or less. 

Depth of Discharge (DoD) 

The DoD is the percentage of the battery’s capacity that is discharged during a cycle. Deeper discharges beyond 80% take the battery to below 20% SoC, which puts more stress on the battery than lower/shallower DoDs, e.g., 50% DoD. Prolonged deep discharge leads to a shorter life, but more energy is discharged during each cycle. So, it’s not always the best marker because a battery with regular deep discharges might have shorter lives but could deliver the same amount of energy in its lifetime as a shallow discharge battery that lasts longer.  

State of Charge (SoC) 

Linked to DoD, the SoC level the battery operates at for extended periods can affect the degree of degradation over time. Keeping the battery at either a very high (above 80%) or very low (below 20%) SoC for extended periods contributes to capacity loss. This is because at prolonged high SoC’s, the increased chemical stress inside the cell accelerates degradation. At frequent low SoCs, the current collectors can dissolve, electrode structures collapse quicker due to a lack of lithium ions, an imbalance of contractual and expansion stresses can occur on the electrodes, and an unstable SEI layer can form that regularly breaks and reforms.

All this speeds up degradation and capacity loss. The SoC is also closely linked to calendar aging because if the cells are sitting idle at high or low SoCs, they are not cycling, so it’s not cycling aging that is degrading them, it’s just the time spent in these more unstable states. 

Other factors 

Outside of the main degradation factors, there are a number of other factors that can speed up degradation, cause an increased capacity fade, and impact battery life. These include: 

• Frequent overcharging and over-discharging leading to higher electrical stresses 
• Loss of active lithium-ion sources 
• SEI film growth on the negative electrode as this film consumes lithium and increases internal resistance 
• Transition metal dissolution from the cathode that migrates to the anode and reduces lithium diffusivity and promotes mechanical fracture 

The flat curve problem makes it harder to determine degradation in LFP cells 

While batteries degrade at different rates, monitoring the degradation and capacity fade in LFP batteries is difficult because the open-circuit voltage (OCV) curve is almost flat across 20-80% SoC. The is an issue due to OCV hysteresis. Usually, measuring the OCV at a certain SoC is the same and reproducible regardless of if the battery was just charged or discharged. However, in LFP cells the OCV curve exhibits hysteresis with a measurable voltage gap (usually between 5–25mV) between the charge and discharge OCV curves instead of them being the same. This hysteresis is mostly seen in the flat curve regions of LFP cells. 

This hysteresis is due to both thermodynamic and mechanical factors at the material level. LFP cathodes undergo lithiation (ions entering the cathode) during charging and de-lithiation (ions leaving the cathode) during discharge. However, the movement of lithium ions is not identical in each direction, leading to a voltage gap, i.e. the hysteresis, between the charge and discharge OCV curves. However, because the electronic and ionic conductivity of LFP is lower than other Li-ion cells, the cells can’t internally equalise these inhomogeneities as well as other cell chemistries. 

This can have a big impact on determining how much an LFP cell has degraded over time. Taking an average LFP cell at a nominal voltage of 3.3V, the cell will normally operate within ±30mV of the nominal voltage at 20-80% SoC. A ±10mV measurement error due to the hysteresis will cause a ±15% SoC error in LFP cells. For reference, NMC cells with the same mV error will only have a ±3% SoC error, so it is an issue that affects LFP cells the most. 

The main issue with OCV hysteresis in LFP batteries is the complication of SoC estimation. BMS rely on the OCV-SoC relationship to estimate the level of remaining charge in a cell. So, because the same OCV value can correspond to more than one SoC value, depending on the recent charge/discharge history, it can lead to these huge errors which makes it difficult to estimate the true SoC of a cell―which makes it harder to determine the true degradation and capacity loss over time. 

In large BESS installations, this can be issue when so many cells potentially have much lower State of Health (SoH) than the BMS states them to be, as this would unknowingly lead to a significantly reduced capacity and output of the BESS―which for grid backup power situations can be an issue if the anticipated amount of backup power is not supplied due to inaccurate BMS measurements. This is particularly an issue below 80% SoH because the capacity fade roughly doubles below this SoH, so the true SoH could be much lower, and the capacity loss much greater, once it goes below these thresholds. 

However, not all capacity loss is gone forever. Some capacity loss in BESS is at the module level as well as the cell level. In short, each container is only as good as its worst cell, i.e. the cell with the highest degradation. This means that some capacity loss is reversible. In BESS containers, cells are connected in series to build capacity, but the weakest cell dictates the performance in a string. So, if a single cell is underperforming compared to the others, it will reduce the overall capacity of the BESS installation. 

This is because there is often an imbalance in SoCs across cells, and it is a common issue in LFP systems. Different internal resistances, self-discharging rates, and balancing inefficiencies all contribute to SoC imbalance. In these cases, the cell with the lowest SoC determines when discharge finishes which limits energy extraction while the cell with the highest SoC determines when the charging has completed, which can limit charging if there is an imbalance. However, while these issues cause capacity loss in LFP BESS, a lot of this degradation is at a system level and is reversible through internal battery management systems (BMS) balancing, forced balancing, and external balancing using a balancing device. 

How LFP degrades across different use cases 

Many BESS applications have different DoDs and cycling behaviour. Across different applications, it can be difficult to determine the number of full cycles that a BESS has undergone because many use cases implement multiple microcycles, or incomplete charge/discharge cycles, with differing DoDs―and the DoD can also vary within the same application use case depending on the situation. All these different cycle situations need to be accounted for and observed over a time period to get the true full-cycle equivalent that a BESS has undergone, as this enables the degradation to be better quantified across use cases. Here a few LFP BESS application used case examples and how their operations (and the way they are used) can affect the level of degradation. 

Frequency regulation 

While there are different frequency regulation operations, the majority tend to maintain a 50% SOC and the cycles are not too heavy that they cause prolonged degradation. However, even though each cycle is not deep and doesn’t have a high DoD, BESS used for frequency regulation can have shorter lifetimes that other applications because they are cycled much more frequently (often through microcycles where less than 1% capacity is discharged), increasing the cycling aging much quicker than other use cases. Frequency regulation operations respond to the grid multiple times a day to maintain grid stability, which means that they can be cycled multiple times a day depending on the local grid volatility. 

While aging can occur due to frequent cycling, frequency regulation operations don’t require deep cycling, so they tend to utilise lower C-rates (0.5 or 0.25°C) because frequency deviations are small compared to what can be handled by LFP systems. Again, because only a small amount of power is provided during each discharge, the temperature doesn’t elevate which minimises degradation through thermal factors. 

Peak shaving 

Peak shaving involves discharging the battery when consumption is high and charging it when consumption is low. DoD doesn’t heavily affect LFP BESS in peak shaving applications because many maintain a 70-80% SoC to balance capacity with longevity, but some use cases (residential, commercial & industrial) use the BESS at lower depths between 40-60%. In both cases, the DoD is not kept at either extreme to accelerated degradation. Peak shaving C-rates tend to be around 1-1.5°C which is higher than frequency regulation and can contribute to a higher degradation (as higher C-rate is one of the more detrimental degradation factors).  

The temperature can fluctuate in peak shaving applications, but keeping the LFP cells at 25°C is a key to extending their life because once they regularly start operating at 40°C, the degradation rate increases. A study on 220Ah LFP modules being used at a storage power station showed that peak shaving degrades LFP cells 1.8 times faster than using them in frequency regulation applications, taking into account the same SoC range and operating temperature. At temperatures of 40°C and above, the degradation rate against frequency regulation increases to 1.92 times faster. The study showed that even though the number of microcycles in frequency regulation increases degradation, the main factor that causes peak shaving to have much higher degradation rate against frequency regulation is the higher C-rate. 

Energy trading 

BESS can be used as flexible energy storage assets that can generate revenue for their owners by trading electricity in volatile power markets. Energy trading applications have deep cycles with higher DoDs that can accelerate degradation and shorten the calendar life of LFP BESS used for energy trading. The C-rate of an energy trading BESS is not usually that high, typically 0.5°C, so this doesn’t have a major bearing on the degradation of the installed BESS. However, because of the deep discharge cycles, higher operating temperatures are common which can also accelerate degradation.  

For energy trading, the SoC can vary depending on the markets. The aging and degradation in energy trading hinges on whether high SoCs can be avoided while idling. If they can, then the degradation is less but the need for higher DoDs means that the BESS will spend long periods of time at higher SoCs that could accelerate the degradation over time. The degradation of LFP BESS in energy trading applications is therefore very dependant on how they are used, because there is a lot more variation on how they can be used due to being tied to energy markets. 

Strategies being employed to minimise degradation 

While battery degradation is an issue across all cells, LFP batteries are more resistant to some forms of degradation. In addition to this, a number of different strategies are being employed to identify, slow down, and manage the degradation of LFP BESS. These strategies include: 

• Passive balancing (BMS) 
• Active balancing (BMS) 
• Thermal management systems keeping temperatures at optimum levels 
• Overbuilding capacity from the outset 
• Employing field diagnostics to monitor cells 
• Augmentation when cells start to degrade 
• Advanced data monitoring analytics 
• Incremental capacity analysis 
• Optimal SoC management 
• Optimising and controlling charge rates 
• Predictive health assessments 
• Advanced cooling systems to prevent overheating 
• Predictive and preventative maintenance  

Concluding remarks 

Understanding the degradation modes, how they affect BESS installation use cases, and the challenges with monitoring an installation’s SoH is critical from a warranty perspective. BESS come with a guaranteed warranty that ensures that certain level of degradation will not be exceeded (and a certain level of capacity will be retained) so long as the system is used within certain limits―such as number of cycles per year/cycles per day and operating within certain DoD levels. Ensuring that the systems installed are working to these operational requirements is key to maintaining the warranty if anything goes wrong, but with the potential for the wrong SoCs to be recorded, understanding how to mitigate these errors and monitor the correct SoCs, SoH, and degradation become vital.