What are the cathode materials for solid-state batteries? What are the problems? What is being researched specifically. An overview of the state of the art in positive electrode research.
For solid-state batteries, essentially the same cathode materials are being considered as for conventional Li-ion batteries. Overall, the development of new cathode materials is less dynamic than for anode materials, and there is relatively little research specifically addressing new cathode materials for solid-state electrolytes. Research is essentially benefiting from advances made on Li-ion batteries with liquid electrolytes. An overview of which cathodes are used for solid-state batteries is described in this article.
Cathode types at a glance
Cathode materials are divided into the different types of their crystal structure. There are oxides with layered structure (LiMO2 M=Co, Ni, Mn, Al), olivins (LiMPO4 M= Fe, Mn) and spinels (LiM2O4 M=Mn, Ni). Figure 1,2 and 3 show the crystal structures of each type. The crystal structure of oxides with layered structure includes NMC, NCA and LCO. These have a 2021 market share of over 70% of all Li-ion batteries, making them the most widely used structure[1]. The lithium ions are sandwiched between a composite layer of oxygen and a metal (or metal mixture) (Figure 1). LCO was the first oxide material to reach production maturity, but due to the high cost of cobalt, it is quite expensive overall. In further development, cobalt was substituted by nickel and the addition of inexpensive aluminum (equivalent to NCA) or manganese (equivalent to NMC) stabilized the cells and reduced production costs [2].
Figure 1: Crystal structure of cathodes with layered structure (here: LCO), own illustration.
Figure 2 shows the crystal structure of phosphates. This group includes the LFP cells, which have seen increasing market shares in recent years[1] because they are increasingly used in entry-level electric cars with shorter ranges. The phosphate cathodes crystallize in an olivine structure. The oxygen atoms group together as an extremely stable octahedron (six oxygen atoms) or tetrahedron (four oxygen atoms). An iron atom is placed in the center of the octahedron, and the metal is placed in the center of the tetrahedron (which is iron in LFP). The lithium ions are deposited between the oxygen sphere packings [3]. The volume of this type of cathode varies only by a few percent during charging and discharging, which means that mechanical stresses are very low and very high cycling stability and lifetime can be achieved. The strong covalent bonds between oxygen and phosphorus also result in excellent temperature stability, so no exothermic reactions are expected up to 300°C. A major disadvantage of the technology, however, is the low operating voltage of 3.4 V (compared to 3.7 V for NMC), which means that the cells have a significantly lower energy density [2].
Figure 2: Crystal structure of olivine cathodes (here: LFP), own representation.
The third group of cathodes are the spinel crystallized variants to
which Li1–x Mn2 O4 – better known as lithium
manganese oxide (LMO) – belongs. The oxygen atoms are grouped here as a cubic
dense spherical packing. The manganese and lithium ions are deposited in the
interstices (see Figure 3). Some interstices remain free, through which the
Li-ion transport can then take place. LMO, as the only relevant representative
of spinels, shows extremely poor cycling behavior, which is mainly due to the
fact that LMO reacts with parts of the electrolyte and leads to its
decomposition [2]. For this reason, LMO is hardly used in today’s systems.
Figure 3: Crystal structure of spinel cathodes. Here, oxygen forms a cubic dense sphere packing (ccp). Manganese is located in the interstices and the Li-ions are deposited there, own illustration.
Research goals for solid-state cathodes
NMC, NCA and LFP represent the cathode types that are considered to have the greatest potential and whose use has already been tested in Li-ion batteries with liquid electrolytes. There are also prospects for LMO to improve solid-state batteries. In this context, the development of the materials has not yet been exhausted and improvements can be expected in the coming years:
- NMC/NCA: Due to the high raw material costs of cobalt and the critical extraction conditions of the raw material (in particular unsecured mines and child labor in the Congo (cf. Deutschlandfunk[4]), research efforts are going towards reducing the proportion of cobalt or doing without it completely. However, a lower cobalt content leads to poorer stability and a reduced ability to intercalate lithium between the crystal layers, which reduces capacity. Special protective layers or dopants are used to try to counteract this. It is assumed that the cobalt content in NMC/NCA cells will gradually decrease to below 10 % in the next few years[5].
- LFP: LFP cathodes today represent a low cost, high performance and durable cathode. However, the energy density is significantly inferior to NMC or NCA, mainly due to the low potential of only 3.3 V compared to Li+. Research is underway to try to improve the voltage. By (partially) replacing the iron with manganese, the potential can be increased up to 4.1 V[6], which would represent an improvement of over 20% compared to normal LFP. TESLA and CATL have announced that they will deliver the first vehicles with LMFP cells in early 2023, so it can already be assumed that they are ready for series production[7].
- LMO: LMO reacts with the liquid electrolyte in today’s Li-ion batteries, leading to decomposition of the cathode. With solid-state batteries, it can be assumed that the problem does not occur or only to a reduced extent. The most interesting aspect is that by replacing the manganese with nickel (Li(Mn,Ni)2 O4 ), the cell voltage can be increased to 4.6 V, which would enable the production of high-voltage cells with a correspondingly higher energy density. However, development is not yet far advanced, which is why entry into the mass market cannot be expected here until the 2030s[5].
- Conversion type cathodes: This type differs significantly from the concepts described above. In conversion type cathodes, the Li-ions do not intercalate into the lattice structure of the cathode, but they form solid chemical bonds with the carrier structure. The most studied carrier structure is iron fluorine (FeFx with x=2.3), which reacts with lithium to form LiF and metallic iron when discharged. There is still a relatively long way to go before it is ready for mass production, and problems such as decomposition of active material or melting of particles leading to passivation of the materials still need to be solved. Thus, the theoretical energy density of the cell can be increased by over 30% compared to commercial NMC811 [8].
Commercialisation approaches
Most commercialization approaches do not focus on the development of the cathode. Mostly, the focus is on the solid-state electrolyte and the anode (lithium metal or silicon graphite). Accordingly, there is no information from the manufacturers about the exact cathode composition so far. However, the published information supports the thesis that in most cases proven technologies are used. Most manufacturers focus on optimizing energy density, which is why NMC-based cathodes are favored (e.g. Quantumscape [9]). ProLogium claims to use NMC811 together with a catholyte for stabilization [10]. Blue Solutions takes a different approach, using LFP as the cathode and thus focusing more on safety, longevity, and ease of recycling [11]. SolidPower states on their website that their system is compatible with conversion-type cathodes in addition to NMC cathodes [12], but does not provide any further information on what stage of development this concept is at and exactly which materials are used.
Conclusion
Cathode research for solid-state batteries essentially benefits from the developments also achieved for Li-ion batteries. The challenges are similar. The integration approaches of the manufacturers show that the implementation of standard materials such as NMC and LFP for solid-state batteries is not an insurmountable obstacle. The use of improved NMC cathodes (NMC811) is planned, but there is no known use for other cathode materials (now ready for series production) such as LMFP. The announcement by Solid Power that it intends to use conversion type cathodes is striking. However, since no further data are known here and Solid Power also works with NMC, it can be assumed that this is more of a long-term prospect. However, a more precise picture of which cathode material will dominate the market will only emerge in the next few years when more precise data on the materials used becomes known.
Sources
[1] IDTechEx: The state of the Li-Ion industry, 2022, Webinar
[2] Korthauer, Reiner (2013): Handbuch Lithium-Ionen-Batterien, Frankfurt
[3] Kasavayee K.: Synthesis of Li-ion battery cathode materials via freeze granulation, 2015, swerea IVF
[4] Deutschlandfunk: Der hohe Preis für Elektroautos und Smartphone, 2019, Kobaltabbau im Kongo – Der hohe Preis für Elektroautos und Smartphones | deutschlandfunk.de
[5] Fraunhofer Institute for Systems and Innovation Research ISI: Solid-State Battery Roadmap 2035+, Karlsruhe, 2022
[6] Deng, Y., Yang, C.: Recent Advances of Mn-Rich LiFe1-yMnyPO4 (0.5 ≤ y < 1.0) Cathode Materials for High Energy Density Lithium Ion Batteries, 2017, AdvancedEnergyMaterials
[7] TESMANIAN: CATL to Supply LMFP Batteries to Tesla in Q4 for Model Y Production, 2022, https://www.tesmanian.com/blogs/tesmanian-blog/catl-to-supply-lmfp-batteries-to-tesla-in-q4-for-model-y-production
[8] Olbrich, L., Xiao, A., Pasta, M.: Conversion-type fluoride cathodes: Current state of the art, 2021, Elsevier Electrochemistry
[9] QuantumScape: Delivering on the promise of solid-state technology, https://www.quantumscape.com/technology/, 2023
[10] Prologium: Core Technology, https://prologium.com/tech/core-technology/, 2023
[11] Blue-Solutions: Battery Technology, 2023, Blue Solutions – Battery technology (blue-solutions.com)
[12] Solid Power: All-Solid-State Battery Cell Technology, https://www.solidpowerbattery.com/batteries/, 2023
