Could BESS move towards LMFP battery chemistry?

The main difference between LFP and LMFP cathodes is that some of the iron atoms are replaced by manganese atoms. While this may seem like a small difference, small chemical changes can have big macroscale impacts, especially on the energy density. The fundamental chemical structure, however, remains the same. Both LFP and LMFP posses an olivine crystal structure.

LMFP has an atomic lattice composed of tetrahedrons occupied by phosphorus and octahedrons with lithium, iron and manganese, all inside a hexagonal close-packed oxygen framework. This rigid crystal structure allows lithium ions to intercalate and de-intercalate with a high efficiency, while the olivine structure resists mechanical distortion a lot better than other cathode crystal structures—which is why both LFP and LMFP are stable as this resistance reduces battery deformation during charging and discharging and leads to longer cycle lifetimes than other Li-ion chemistries.

But what does this mean in practical terms?

While all the ions in the cathode play a role in increasing the energy density of the cathode, the manganese is more effective than the iron it replaces in LMFP cathodes compared to LFP cathodes. Lithium ions are mobile and adopt a +1 oxidation state (Li⁺), and as they move from the cathode, the iron acts a redox centre inside LFP cathodes, changing its oxidation state from Fe²⁺ to Fe³⁺ to maintain charge balance in the cell.

In LMFP cells, some of these iron redox centres are replaced with manganese oxidation centres. When lithium ions leave the cathode, the manganese ions also change from Mn²⁺ to Mn³⁺. So, they play the same role in as iron, but manganese has a higher oxidation potential than iron. This means that manganese can lose and gain electrons a lot easier than iron can and causes LMFP cells to have a higher potential difference between the cathode and the anode than LFP cells. This results in a higher energy density—roughly an extra 15% at the cell level— and a higher nominal voltage of 3.7V for LMFP compared to 3.2V for LFP. So, it’s a small chemical change, but it brings the energy density performance closer to NMC batteries while retaining the safety of LFP. LMFP cells are also a similar cost to LFP and possess a better low-temperature performance than LFP.

However, while adding manganese to LFP cathodes is beneficial, it’s not without its challenges, and is a big reason why LFP is still the dominant battery chemistry for BESS. Battery cathodes with a high amount of manganese destabilises the cathode, leading to a poor cycling stability. At higher temperatures, these effects get worse leading to manganese dissolution that breaks down the cathode structure, reduces ionic conductivity, worsens electrode kinetics, and ultimately can lead to cell failure.

Even before cell failure occurs, the dissolution of manganese reduces the energy density of the LMFP cells. Over time, the cells lose the benefit of the manganese, and they revert to regular LFP performance. Overcoming this dissolution is key to their commercial success and competing with LFP.

There are many companies who are trying to make the electrolyte-electrode interface more stable. One company has so far been commercially successful, but it’s not for BESS. That company is BYD, which has developed LMFP cells for its second-generation Blade batteries. If there is progress happening in the EV industry, the BESS application space could be next.

Why the voltage increase matters

As noted above, the higher potential difference between the cathode and the anode from manganese’s higher oxidation potential leads to an operating voltage of 3.7–3.8V in LMFP compared to 3.2V for LFP. The higher voltage allows a higher energy density because the energy density is directly calculated by multiplying the capacity of the cell by the voltage. So, the higher the voltage, the higher the energy density. Theoretically, it would be better for batteries to operate at higher voltages, but it can reduce the cycle life, and some chemistries are not as stable at higher voltages.

LMFP does have similar voltage characteristics to LFP in that it still has two flat curved regions where the voltage doesn’t vary. There is a region between 50-60% where it does vary, but the rest of the voltage curve is similar to LFP. While the LMFP nominal operational voltage is around 3.7V, they can reach peak charging voltages up to 4.2V, which is similar to NMC chemistries. By comparison, the peak charging voltage of LFP is around 3.65V.

Even though LFMP is similar in many aspects to LFP, because it does have slightly different voltage curves, the systems require their own control system and BMS algorithms, meaning that they all have to be designed from scratch instead of using LFP control technology. This extends to state-of-charge estimation which requires new logic controls, and the BMS needs to be designed to understand and manage the voltage change between 50-60% for the cells to be effective.

The potential benefits of LMFP for BESS

LMFP has a lot of the intrinsic benefits that has made LFP a staple battery chemistry for BESS installations, including a potentially long cycle and calendar life (if the dissolution issues are sorted), high thermal stability, and a long-term stability.

The obvious benefit of LMFP is the higher energy density. For large installations, a 15% increase in energy density for a similar cost is a big benefit when you get to large scales because it enables site developers to do one of two things. The first is achieve a greater site energy density across the same footprint. This means that more energy can be stored in the same installation footprint, so for developers involved with energy markets or grid services, it could mean a higher ROI given that the CAPEX and OPEX is going to be similar.

The second option for developers is that there’ll be the option to lower the footprint. If buying land, it could lower the initial CAPEX from a land procurement perspective while still delivering the same amount of energy. So, even though less energy is stored, if the cost of land being scoped is expensive, then it could lead to a higher ROI for developers. Likewise, it could be more beneficial to use LMFP in space constrained sites, such as in urban settings where space is at a premium or where there is limited space to expand the current installation capacity, to deliver more energy in a smaller footprint.

While there are lot of benefits for developers, the extra energy density per given area also benefits any connected infrastructure and could provide more backup energy to better stabilise local grids. LFP BESS have already become preferable over NMC because the extra performance benefits of NMC don’t outweigh the improved safety of LFP for longer duration and large-scale installations. However, with LMFP, there is the chance to have the best of both worlds and deliver BESS that have a higher energy density almost at NMC level but with the inherent safety and stability features that have attracted developers to LFP.

It is not a completely new chemistry, but is rather the next potential step for BESS manufacturers. Whether the dissolution challenges can be overcome and managed in a timely manner before other potential battery chemistries take the commercial reins off of LFP remains to be seen.