How is a solid-state battery constructed? How does this battery differ from today’s systems? What is a semi-solid-state battery? This article gives an overview.
Solid state batteries differ from conventional Li-ion batteries in a number of ways. The internal structure depends on which variant is involved.The general structure of a (lithium) cell is usually very similar. Basically, a battery consists of two electrodes (anode and cathode) and a separator. The two electrodes are located in an electrolyte, which allows ions to move through the cell. The separator separates the anode and cathode. It is permeable to ions, but acts as a barrier for electrons. The anode and cathode are each connected to a current collector (copper or aluminum). The current arresters are then the plus and minus pole of the respective battery. [1, p.14ff].
Normally, the electrolyte is in a liquid or gel form (lithium polymer batteries). Due to its viscosity, the electrolyte can contact the two electrodes very well. The idea of solid state batteries is now to replace the liquid or gel electrolyte with a solid electrolyte. The solid electrolyte also acts as a separator, so that the separator is no longer an independent component [2 p. 2].
Within solid state batteries, there is a distinction between all solid state batteries (ASSB) and semi solid state batteries (SSSB). All solid state batteries use only a solid electrolyte. Semi solid state batteries represent a hybrid variant between all solid state batteries and conventional batteries with liquid electrolyte: in addition to the solid electrolyte, there is also a liquid electrolyte that takes up a few percent by volume, which improves the contact between the electrolyte and the electrodes. The improved contacting results in significantly lower internal resistance and thus higher performance [3]. Figure 1 shows the structure of the different technologies.
Picture 1: Structure of conventional Li-Ion Battery (a) in comparison with Semi-solid-state-battery (b) and All-solid-state-battery (c), own illustration
Overview of the materials used
Cathode: In principle, the same material as for Li-ion batteries can be used as the cathode material. Accordingly, NMC (nickel-manganese-cobalt-oxide) or NCA (nickel-cobalt-aluminum) is often used, as these enable high energy density and are also the most widely used materials for Li-ion batteries [4].
Anode: The same material for Li-ion batteries is also possible as anode material. For today’s liquid electrolyte cells, graphite is usually used and there is a lot of practical experience with this material. One of the goals in solid-state battery development is to increase the energy density of the cell. However, with normal graphite as the anode material, no significant increase in energy density can be achieved. Instead, Li metal as an anode material is much studied in research. This has a theoretical energy density of 3860 mAh/g [5]. In comparison, a classical graphite anode achieves only 372 mAh/g [6]and thus only one tenth of Li-metal. In practice, however, only a doubling of capacity is expected compared to current systems [7]. To date, however, the use of Li metal has been associated with some considerable difficulties, due in particular to the high reactivity of lithium and, on the other hand, to the tendency of the metal to form dendrites. Dendrites are spear-like deposits that can form on the anode and are capable of puncturing the electrolyte and causing a short circuit. It is the subject of research to suppress the formation of the dendrites (see e.g. [8]). An alternative to Li metal is silicon. It is predicted to increase volumetric energy density by over 40%. However, the material tends to undergo extreme volume changes during charging and discharging, so more research is needed here [9].
Electrolyte: There are various materials that can be used as electrolytes. Basically, electrolytes can be divided into three groups: Polymers, oxides and sulfides. Development has already progressed furthest with polymers, and the first small-scale productions are available. For oxides and sulfides, there are still some challenges that need to be solved in order to reach series production readiness. More information on the differences and challenges of the electrolytes can be found here.
Sources
[1] Korthauer, Reiner (2013): Handbuch Lithium-Ionen-Batterien, Frankfurt
[2]Nithyadharseni Palaniyandy, K.P. Abhilash, B. Nalini (2022): Solid State Batteries: Design, Challenges and Market Demands, South Africa, Czech republic, India
[3]Wood Mackenzie: 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/
[4] Fraunhofer Institute for Systems and Innovation Research ISI: Solid-State Battery Roadmap 2035+, Karlsruhe, 2022
[5] Li, D., Liu, H. et al.: Challenges and Developments of High Energy Density Anode Materials in Sulfide-Based Solid-State Batteries, 2022, Chemistry Europe
[6] Andersen, H., Foss, C. et al. : Silicon-Carbon composite anodes from industrial battery grade silicon, 2019, scientific reports
[7] Mun Sek Kim : Lithium Metal Anode for Batteries, 2020, Stanford, Lithium Metal Anode for Batteries (stanford.edu)
[8] Sastre J. , Futscher, M. et al.: Blocking lithium dendrite growth in solid-state batteries with an ultrathin amorphous Li-La-Zr-O solid electrolyte, 2021, Communication Materials
[9] Piwko M.: Siliziumanoden zur Steigerung der Energiedichte von Lithium-Batterien, 2016, Fraunhofer, Siliziumanoden zur Steigerung der Energiedichte von Lithium-Batterien (fraunhofer.de)
