For the application of the solid-state battery in the electric car, high storage capacities and high charging speeds are necessary. Li-ion batteries have already been able to achieve both goals in automobiles for years. However, solid-state batteries have yet to prove that they also meet these requirements.
In addition to the goal of using solid electrolytes to reduce the risk of fires, the desire is to increase the energy and power density of batteries so that people can drive further on a battery charge and spend less time at the charging station. This article analyzes how realistic it is that this desire will be achieved.
Energy density:
By definition, a solid-state battery differs from a lithium-ion battery in that it has a solid electrolyte, whereas lithium-ion batteries have a liquid or gel-like (polymer) electrolyte. If you only replace the liquid electrolyte with a solid electrolyte in a lithium-ion battery, this does not initially result in any major energy density benefits. So where does the claim come from that high energy densities can be achieved with solid-state batteries?
Higher energy density with Li metal anode
Normally, higher energy densities are achieved by replacing the anode. Li-ion batteries have so far generally used graphite. Although this can be easily integrated into the battery chemistry, the energy density of 375 mAh/g is not outstanding. Already in the early stages of Li-ion development, there were concepts to use pure lithium metal as anode material and thus increase the energy density. However, Li metal is very reactive and chemical reactions occur with the liquid electrolyte, causing the cells to break down very quickly. With solid electrolytes such as those found in solid-state batteries, the interface with the lithium metal is much less susceptible electrochemically, making it possible to develop cells with comparatively long lifetimes for the first time[1]. In theory, the capacity of the anode can thus be increased more than tenfold (375 mAh/g for graphite anodes and 3860 mAh/g for lithium anodes). It should be noted, however, that multiplying the anode does not automatically result in multiplied capacity at the cell level, since the anode is only one building block of many components in the cell. Therefore, in practice, the actual increase in energy density is not quite as high. Therefore, increases in energy density of only 5-25% are predicted for initial commercial solid-state batteries [2].
Silicon anode energy density
In addition to the use of lithium metal as anode, the use of pure silicon as anode is also being discussed. With this, very high theoretical energy densities of 3590 mAh/g can also be achieved [3]. However, pure silicon anodes are still very far from being ready for series production, which is why small amounts of silicon are often mixed with graphite instead. The increase in energy density at cell level is then correspondingly very small. When higher energy densities are advertised for solid-state batteries, this is therefore usually in combination with Li-metal anodes.
Further optimization of the energy density
In addition to replacing the anodes, there is also discussion of using other cathode materials that can increase the voltage of the cell, allowing for higher energy densities. Due to the robustness of the electrolyte, it is also expected that the use of lithium-sulfur or even lithium-air cathodes could be possible[4]. However, the development process for these is not yet far advanced, so they are not expected in the next few years.
Announcements from the manufacturers
If we look at the manufacturers’ announcements, it also becomes apparent here that the increases in energy density are essentially rooted in the use of Li-metal anodes. It is striking that some of the planned energy increases are significantly higher than the 5-25% stated in the literature. QuantumScape, SolidPower or even Toyota are planning with energy densities between 350-450 Wh/kg [5], compared to today’s 250 Wh/kg for Li-ions. However, it is critical to illuminate under which conditions the improvements in energy density were achieved. Measurements in the laboratory generally cannot be transferred to a later series product.
In principle, however, science and industry agree that an increase in energy density can be achieved with solid-state batteries. It only remains to be seen whether this will be as high as the industry promises.
Power density:
The power density of a battery indicates the charging current with which a battery can be charged. A high power density is necessary to quickly charge an e-car battery in a few minutes and to enable a rapid continuation of the journey.
While fast charging has been normal for lithium-ion batteries for years, the picture is different for solid-state batteries. In fact, there are reasonable doubts about whether solid-state batteries are suitable for fast charging. Since the cells are still at an early stage of commercialization, it is not yet possible to draw a hundred percent conclusion, but there are weighty reasons that speak against high power densities:
Dendrite formation
Solid-state batteries have a much higher tendency to form dendrites. Dendrites are spear-like deposits of Li-ions that form during operation of the cell. With service life, these deposits increase and there is a risk that they will penetrate the separator and cause a short circuit. Figure 1 shows the formation process of these dendrites in solid-state batteries. With the solid electrolyte, the dendrites can form particularly easily along the grain boundaries, so the probability of occurrence is greatly increased compared to liquid electrolytes [6].
Figure 1: Course of dendrite formation in solid-state batteries, Own representation based on investigation of an LLZO electrolyte by [7].
Li metal cavity formation
Solid-state batteries with Li-metal anode also suffer from high charging currents. If this is too high, it leads to an uneven load on the cell, so that voids and cavities can form [6].
Poor ionic conductivity of the electrolyte
In solid-state cells, the solid electrolyte can essentially be differentiated into polymer, oxide and sulfur-based systems. While very good ionic conductivities are achieved for sulfur-based systems, this is not the case for oxide- and polymer-based systems. Polymer-based systems in particular suffer from high resistivity. This then leads to high temperatures during fast charging and also significantly reduces the charging efficiency of the cell. The poor contact resistances at the electrode interfaces and the instability due to the high volume changes during charging and discharging exacerbate this problem [7].
This article provides only an introduction to the topic of fast charging. A deeper insight with further challenges for high power densities and an overview of start-up announcements on fast charging can be found here.
Conclusion:
Because development work is still ongoing, it is not possible to say with certainty which promises will really be fulfilled in terms of energy and power density. However, there is a lot to suggest that there will be a leap in energy density with solid-state batteries. Power density is a different story: The strong tendency to form dendrites and the poor conductivity of some electrolyte variants could still prove to be show-stoppers for broad commercialization, unless these problems can be solved, or at least work-arounds (such as hybrid cells or semi-solid-state cells) are developed. The next years of industrialization will be decisive here in how far the cells can establish themselves on the market.
Sources:
[1] Nithyadharseni Palaniyandy, K.P. Abhilash, B. Nalini (2022): Solid State Batteries: Design, Challenges and Market Demands, South Africa, Czech republic, India
[2] Fraunhofer Institute for systems and Innovation Research ISI: Solid-State Battery Roadmap 2035, 2022,Karlsruhe
[3] Ashuri, M., He, Q.: Silicon as a potential anode material for Li-ion batteries: where size, geometry and structure matter, 2015, Royal Society of Chemistry
[4] Xin, G. u.a.: All-Solid-State Lithium–Sulfur Batteries Enhanced by Redox Mediators, 2021, https://doi.org/10.1021/jacs.1c07754
[5]Reid Max: Will semi-solid battery technology render solid-state batteries redundant? https://www.woodmac.com/news/opinion/will-semi-solid-battery-technology-render-solid-state-batteries-redundant/, 2022
[6] Vishnugopi B., Kazyak E. et al. : Challenges and Opportunities for Fast Charging of Solid-State Lithium Metal Batteries. 2021, ACS Energy Letters
[7] Zhang, C., Hu, Q. et al. :Fast-Charging Solid-State Lithium-Metal Batteries: A review, 2022, Advanced Energy Storages
