The rapidly growing markets for electric vehicles and various forms of energy storage share a common need for lithium-ion batteries which can address performance requirements and be at a cost compatible with the end user desired economics. 60 to 70 per cent of the cost of a lithium-ion battery is attributed to the raw material inputs and of these by far the major contributor is the cathode. Three dominant categories of cathodes have emerged over the last several decades – layered, spinel and polyanion (figure 1).
Figure 1: Three major classes of cathodes
Source: DOI: 10.1038/s41467-020-15355-0, used under CC BY 4.0 license
The electric vehicle and energy storage markets originally favoured the same layered cathode materials used by portable electronics – Lithium Cobalt Oxide (LCO), Lithium Nickel Manganese Cobalt Oxide (NMC) and Lithium Nickel Cobalt Aluminium Oxide (NCA). However, two major realizations in recent years have driven both markets increasingly towards the olivine 1 Lithium Iron Phosphate (LFP).
The first of these was that the electric vehicle market is segmented between those willing to pay a significant premium for higher performance, such as longer range, and those who are less concerned with this and for whom cost is more important. The second was that lithium-ion batteries containing an LFP cathode are inherently safer and more thermally stable than those containing one of the layered materials. The early growth of the energy storage market was plagued by fires, precipitating a directive from China’s National Energy Administration for medium-to-large energy storage installations, which banned the use of lithium-ion batteries containing NCM cathodes and essentially directed the industry towards LFP (figure 2).
Figure 2: LFP as a percentage of global Cathode Active Material (CAM) supply
Source: Benchmark Mineral Intelligence
Compared to nickel-based cathodes, LFP suffers from comparatively low voltage, capacity, and energy density, with adverse effects on electric vehicle range and acceleration. However, in addition to its cost and safety advantages, LFP also offers long cycle life, fast charging and a wide operating temperature range, and its raw materials are abundant, with no need to source them from areas of geopolitical concern. Tesla’s move in cell cathode chemistry from NCA to LFP in certain of its models is thought to have been driven by a combination of its market analysis on electric vehicle driver requirements and its concerns related to nickel and cobalt – origin, availability and cost.
LFP’s crystal structure only offers one-dimensional diffusion pathways 2 for lithium ions, which makes their movement relatively slow and limits cell charge and discharge efficiency at high rates. LFP also has a low electronic conductivity (several orders of magnitude lower than NMC’s). These two issues were effectively addressed over a decade ago by a combination of a small particle size and carbon coating. A third issue relates to its flat voltage characteristics in the middle state of charge (SOC) range, making it difficult to determine SOC in application. Various mathematical techniques can help with the latter, although it has not yet been fully addressed. Since LFP’s inception, various incremental improvements have been made. The cathode powder has been enhanced by incorporation of various dopants 3, helping to extend cell cycle life. The cell safety has been leveraged to improve (almost doubling in just over a decade) the effective energy density by battery pack level engineering, for example in cell-to-pack technology.
Despite the huge growth in usage, there is an overriding concern that the percentage of LFP manufactured outside China is too low, forcing a Western dependence on a China-based supply chain. This is driving investment and development particularly in Europe and the United States, often with substantial government support and funding. The major patents governing LFP and its use as a cathode material in lithium-ion batteries expired at or before the end of 2022, making widescale global production possible without the need to negotiate a license. Furthermore, there is considerable relevant academic expertise outside China so Western manufacture is realistic, albeit with the expected competitive cost challenges.
Now in the early stages of commercialization, Lithium Manganese Iron Phosphate (LMFP, with chemical formula LiMnxFe(1-x)PO4) is another olivine material which offers some of the benefits of LFP (safety and thermal stability) and some of the benefits of NCM (higher voltage and energy density) with an added advantage of (potentially) being 20% cheaper than LFP on an energy (kWh) basis. Other benefits include the relatively straightforward transition from LFP to LMFP for cathode manufacturers and cell manufacturers since the same processes and equipment can be employed, although for the cathode material production the hydrothermal (liquid phase) production route appears a better match than the solid phase reaction method whereas both are employed in LFP production. Other cell components, such as anode and electrolyte, do not require substitution so established supply chain relationships can be maintained. Figure 3 shows the olivine structure for LFP compared with that for LMFP.
Figure 3: The olivine crystal structure of LFP compared with that of LMFP
Source: Zagorac, D., Müller, H., Ruehl, S., Zagorac, J. & Rehme, S., J. Appl. Cryst. 52 (2019), 918-925; DOI: 10.1107/S160057671900997X ICSD-47937 and ICSD-15448 (ICSD release 2023.2)
The transition to LMFP is far from trivial and research has been ongoing for more than ten years inside and outside China. There remain some major challenges associated with the material structure and properties, which caused early development efforts to be abandoned in the mid-2010’s. The comparatively recent commercial shift to LFP has reopened the interest in LMFP and research is once more progressing rapidly. Several companies, including Chinese majors CATL, Gotion and SVOLT, appear to have resolved the issues sufficiently to begin commercialization, with CATL’s M3P leading the way, although solutions may include cathode chemistries modified from standard LMFP with dopants such as vanadium and magnesium.
The LMFP properties of greatest concern are the low lithium ion diffusion coefficient (an order of magnitude lower than LFP’s), the low electronic conductivity (several orders of magnitude lower than LFP’s) and the low tap density (typically 15% lower than LFP’s without process modification). The former two are intrinsically linked to the crystal structure where the difference in ionic radius between manganese and iron and the complex crystal structure convolute the lithium ion diffusion pathway. The ratio between manganese and iron is critical in establishing the viability of the material. If the amount of manganese is too low, the energy density increase will not be sufficient to justify the commercial transition from LFP. If it’s too high, crystal distortion associated with the Jahn-Teller effect 4 reduces the cell cycle life and stability. Figure 4 shows microscopic images comparing LFP and LMFP made by conventional methods.
Figure 4: Example secondary electron microscopy (SEM) images of commercial LFP (0% Mn) and LMFP (79% Mn) samples
Source: Modified from DOI: 10.1149/1945-7111/ac76e5, used under CC BY 4.0 license
One frequent misconception is that LMFP is only a next-generation substitute for LFP, whereas the opportunity for its inclusion in cells is far wider. Actually the direct replacement has the most challenges in manufacturing, whereas its combination with NMC is less challenging and can help increase the safety and reduce the cost of NMC-based cells, forging an effective way to a competitively-priced, mid-range electric vehicle. This combination can be as simple as a blend or as complex as a coated composite. In conjunction with other cathode chemistries such as LCO, LMFP can also help improve cell characteristics such as cycle life and safety in portable electronics and other lithium-ion battery applications.
The strong re-emergence of LFP in electric vehicle and energy storage applications coupled with the expiration of the key LFP patents is causing huge global growth in LFP cathode material and cell production and is unsurprisingly associated with a drive to lowest cost. The potential benefits of LMFP over LFP and the intrinsic challenges of the material are creating an unprecedented opportunity for innovation in research. Effective solutions arising from material and process modifications can be quickly adopted for commercial applications, with the related IP potentially of substantially higher value than the IP for all earlier generations of cathode material combined.
Notes:
1. Olivine refers to a crystal material structure from within the orthorhombic system
2. Lithium-ion batteries’ functionality is based on movement of lithium ions and the pathway for these ions has a fundamental impact on performance characteristics such as charging and discharging rates
3. Dopants are small quantities of additional elements added to modify material properties
4. The Jahn-Teller effect refers to a geometric distortion of a molecule which occurs to reduce its symmetry and energy so it can achieve a more stable state