Li-metal anodes make it possible to significantly increase the energy density of solid-state batteries. However, the use of this anode material is not straightforward. This article explains the opportunities offered by the Li anode and the challenges that still need to be solved before it is ready for series production.
To increase the energy density of a battery, attempts can be made to increase either the energy density of the cathode or the anode. On the cathode side, for example, attempts are made to increase the proportion of nickel for NMC cathodes or to introduce an additional proportion of manganese for LFP cathodes. In the past, graphite was almost always used for the anode because it is durable and stable. The lithium in lithium metal anodes is so chemically reactive that it would react with the liquid electrolyte and damage it. Solid electrolytes are more stable, which is why a new development momentum has emerged in recent years to develop Li-metal anodes for solid-state batteries. In addition to the silicon-based anode, the Li-metal anode has established itself as the favored approach for solid-state cells.
Expectations and challenges
Due to the electrochemical potential of 0 V compared to Li+, high cell voltages are possible and with a theoretical energy density of 3860 mAh/g (or 2061 mAh/cm3), Li anodes perform many times better than graphite anodes [1]. However, the Li metal anode is not yet ready for mass production, as some problems hinder its industrialization.
The strong volume change of the anode during operation is the biggest challenge that has to be solved. This arises because Li-ions are deposited on the anode surface during charging and thus expand. During discharge, the process reverses and the anode shrinks to nearly its original volume. If no countermeasures are taken, the volume change can lead to a more than tenfold increase after a few dozen cycles [2], which represents an enormous mechanical load on the cell. This also promotes the growth of dendrites. Dendrites are Li-ions that pile up on the anode like spears and are potentially capable of piercing through the solid electrolyte and triggering a short circuit [1].
Another challenge is the poor interface quality between Li anode and electrolyte, which causes Li to lose contact with the electrolyte and no longer participate in the charging and discharging reactions as “dead lithium” [2].
There are also some subtypes of solid electrolytes (especially sulfide-based systems) that undergo chemical reactions with the Li metal, leading to severe corrosion and necessitating additional protective layers, for example, by an artificial SEI (solid-electrolyte interface) layer [3].
Li metal anode types
The structure of the Li-metal anode is the subject of current research and several approaches are being pursued. These are usually a compromise between high energy density with simultaneously high cycle stability and good producibility, and usually not all requirements can be met equally.
2D lithium anode
The 2D lithium anode is the simplest approach to realize a lithium metal anode and is widely used in laboratory studies. There are several approaches for fabrication. It is possible to fabricate thin Li films from pure lithium. Figure 1 (a) shows the structure of a solid-state battery with Li foil. Foil densities of 20um[4] are required for good energy density, but this is difficult to produce in terms of process technology. Alternatively, the lithium can be melted or vaporized and deposited directly onto a substrate (e.g., the current collector)[5]. However, a 2D anode results in the volume of the anode changing very significantly with each charge and discharge process, and dendrites form particularly easily. Therefore, further approaches are being researched to get a grip on these problems.

Figure 1: (a) Li anode as 20 um foil on copper collector (b) Intercalation of lithium ions in C6 layers of a graphite anode.
3D host/grid structures
One of the approaches for volume-stable cells is based on the concept of the graphite anode commonly used today. Graphite consists of several layers of C6 rings. During charging, the lithium is deposited in the middle of the C6 rings between two layers [6] and ensures a stable structure with only a slight change in volume (see Figure 1b). For Li metal anodes, it is investigated whether three-dimensional structures are suitable to keep the volume stable and reduce the formation of dendrites. Studies show that lattice-like 3D structures can reduce the local current density and thus actually reduce the mechanical and electrical stress. However, the 3D lattices are usually passive structures that reduce the energy density, sometimes significantly, due to the extra volume. The lattice material can be carbon, zinc-oxygen, or nickel or copper foam [4]. It is possible to deposit pure lithium in a 3D structure, which according to Chi, Liu et al. prevents dendrites from being formed[7]. However, the development of 3D structures is still in the early stages and it is not yet clear which materials will become popular.

Figure 2 : “Anodeless” design of a solid-state battery. Design before and after the first charging process
“Anodeless” anodes
It is possible to build batteries “anodeless”. This means that no anode is installed during manufacture. Only during initial charging do Li-ions migrate from the cathode to the other side and then form the anode in-situ by depositing the Li-ions on the current collector (see Figure 2). The advantage here is that production can be simplified, as anode production is completely eliminated and reactive Li films can be dispensed with. For “anodeless” anodes, it is necessary to introduce an excess of Li on the cathode side so that they can build up the anode during charging [5]. Since the “anodeless” anode essentially builds up a 2D Li layer, this equally leads to large volume changes during charging and discharging and consequently a high dendrite tendency. Efforts are therefore also being made to form the anode in 3D structures of copper, for example. A compound of silver and carbon has proved to be particularly successful here, with which over 1000 cycles have already been achieved [3].
Known commercialization approaches of Li anode
There are several companies working on lithium anodes. These include ProLogium, Toyota, SolidPower, Gangfeng Lithium Group or Solid Energy [5]. Exact information about the anode concept is usually not published. However, at least basic information is known for QuantumScape and Blue-Solutions:
QuantumScape is one of the suppliers working on an “anodeless” anode. The concept does not use 3D structures to host the Li-ions. There is no information on how large the volume change is during charging and discharging. However, in tests carried out, the cells had to be compressed at a pressure of 3.4 bar. The anode proved to be relatively stable in tests and after 800 full cycles at 25° C, a residual capacity of approx. 90 % was achieved [8].
Blue-Solutions uses a lithium foil as the anode. The company is the first to have solid-state batteries on the market, equipping the Mercedes Bus eCitaro[9]. In 2021, this has already produced cells with a capacity of 500 MWh/year, with the goal of soon increasing this to three times that amount. Blue-Solutions is also working with the company Li-Metall to develop anodes that are not based on Li foils in the future [10].
Conclusion
Li metal anodes are one of the most promising ways to increase the capacity of Li batteries. While no major changes are expected for the pure graphite anode, the Li anode still offers much development potential. However, the strong volume change and the reactivity of lithium are problems that have to be tackled until series production is ready. Meanwhile, virtually all major OEMs are working with development companies researching solid-state batteries with Li-ion anodes. In parallel, publications on lithium metal anodes have increased steadily in recent years. It can therefore be assumed that solutions to the current problems will be worked out with new approaches such as 3D host materials or extra protective layers, and that the series maturity of the Li-metal anode will progress.
Sources
[1] Wang, R., Cui, W. et al: Lithium metal anodes: Present and future, 2019, Journal of Energy chemistry.
[2] Wang, Q., Liu, B. et al: Confronting the Challenges in Lithium Anodes for Lithium Metal Batteries, 2021, Advanced Science.
[3] Varzi, A., Thanner, K., et al.: Current status and future perspectives of Lithium Metal Batteries, 2020, Journal of Power Sources.
[4] M. Gao, H. Li, L. Xu, Q. Xue, X. Wang, Y. Bai, C. Wu, Lithium metal batteries for high energy density: fundamental electrochemistry and challenges, Journal of Energy Chemistry (2020), doi:https://doi.org/10.1016/j.jechem.2020.11.034
[5] Fraunhofer Institute for Systems and Innovation Research ISI: Solid-State Battery Roadmap 2035+, Karlsruhe, 2022
[6] Korthauer, Reiner: Handbuch Lithium-Ionen-Batterien, 2013, Frankfurt
[7] Chi,S., Liu, X. et al: Solid polymer electrolyte soft interface layer with 3D lithium anode for all-solid-state lithium batteries, 2019, Energy Storage Materials.
[8] QuantumSacpe: InvestorPresentation October 12,2022, https://s29.q4cdn.com/884415011/files/doc_presentation/2022/QS-IR-Presentation-Oct-’22.pdf.
[9] electrive.net, “We are the real pioneer of the solid state battery”, 2021, “We are the real pioneer of the solid state battery” – electrive.net
[10] Li-Metal: Technology, 2023, Metal (li-metal.com)