What electrolytes are there? What can they do? What are the advantages and disadvantages and which electrolyte has the greatest potential to dominate the market? This article provides the answers.
The main innovation of solid-state batteries compared with conventional Li-ion batteries is that the liquid electrolyte is replaced by a solid electrolyte, which then simultaneously takes over the task of the separator. There are various types of solid-state electrolytes, which differ in their application and stage of development. This article provides an overview of the properties and differences between the individual electrolytes.
Electrolyte requirements
For the development of electrolytes, some requirements must be fulfilled so that they can be used commercially. Wei, Chen et al [1] define nine requirements, all of which must be fulfilled simultaneously. Figure 1 shows the requirements graphically. The requirements are as follows:
Figure 1 : Electrolyte requirements for solid-state batteries, after Wei, Chen et al. [1], Own illustration
- The electrolyte must have good ionic conductivity (>10^-3 S/cm at room temperature) to allow Li+ ions to migrate from the anode to the cathode and back again
- The electrolyte must have a very low electrical conductivity (<1^-9 S/cm) so that there are no short circuits and the self-discharge is low
- The resistance of the anode/electrolyte and cathode/electrolyte interface must be minimal
- The system must be chemically stable and no parasitic reactions must occur to prevent self-decomposition of the system.
- The system must be electrochemical stable and have a wide voltage range vs. Li+
- The system must be mechanically stable and fit well with the other cell components (e.g., the coefficient of expansion must be similar)
- The electrolyte must be easy to produce
- The electrolyte must be cheap
Overview on electrolyte types
Three main groups of solid-state electrolytes can be considered for solid-state battery applications in the automotive sector: oxide-based, sulfide-based and polymer-based electrolytes. The main properties of these three types and their advantages and disadvantages are described below. Figure 2 shows a comparison of the strengths and weaknesses of the different electrolytes.
Figure 2 : Comparison of the properties of polymer, oxide and sulfide electrolyte, own illustration.
Polymers
Polymer-based electrolytes are the closest to commercialization today and are already being used in initial projects (e.g. in the elite Mercedes Benz bus [2]). Polymer electrolytes consist of a polymer matrix in which lithium salt is dissolved and other additives. Their properties are most similar to liquid electrolytes [3]. Typical polymer electrolytes are PEO (polyethylene oxide), PAN (polyacrylonitrile), PMMA (polymethyl methacrylate) and PVdF (polyvinylidene fluoride) [4].
Ionic conductivity: Ionic conductivity is the biggest problem of polymer-based electrolytes and is at best about 1 mS/cm at room temperature, although it has not yet been possible to achieve these values in commercial use. The ionic conductivity is strongly temperature-dependent and increases with higher temperatures. Heating of the cells is therefore necessary for use in automobiles [3].
Electrical conductivity: In the literature, conductivity values of 4.4 x 10^-8 S/cm are given for PEO/PVdF combinations, for example [5]. Even if there is still potential for improvement here, it can be assumed that the conductivity is sufficient.
Interface compatibility anode/electrolyte: The basic interface stability is described as good [6]. The material copes well with the volume changes caused by the charging and discharging processes, which is otherwise often a problem, especially with Li metal as the anode. Another problem is that there are tendencies for polymers to form dendrites [3].
Interface compatibility cathode/electrolyte: There is good interface compatibility with conventional cathode materials [7].
Chemical stability: Chemical stability is sufficient and for some electrolyte subtypes (e.g. PEO) the use of Li metal as anode is possible [3].
Electrochemical stability: The electrochemical stability can be described as low. The use of high-voltage cathodes is not readily possible [3].
Mechanical stability: The mechanical stability is considered to be good [6], especially due to the high flexibility of the material. Oxides, however, still perform somewhat better [7].
Manufacturability: Manufacture is simple and processes already exist for industrial production [7], especially thin-film manufacturing [6].
Cost: The materials used are cheap and readily available. Little expensive lithium is needed because the structural stability of polymer compounds is achieved. Overall, polymer electrolytes are the cheapest [3].
Oxides
Oxide-solid electrolytes are a group of electrolytes consisting of compounds of lithium and oxygen. The whole is supplemented by other elements such as titanium, lanthanum or germanium. There are various subtypes of oxide electrolytes, some of which differ significantly in chemical composition. The most important oxide types are LiPon, NASICON, GARNET and perovskite, whereby LiPon and perovskite are actually excluded from being suitable for use as large-format solid-state batteries [3].
Ionic conductivity: The ionic conductivity of oxide electrolytes is in the middle range with max. 1mS/cm. This is better than for polymer electrolytes, but worse than for sulfide electrolytes [3].
Electrical conductivity: The electrical conductivity has so far only been marginally investigated in the literature. For the oxide electrolyte LLZO, a value of 10^-8 – 10^-7 S/cm is given in the literature, but measures are mentioned on how to improve this value [8]. Thus, the electrical conductivity does not seem to be a major obstacle.
Interface compatibility anode/electrolyte: The interface between electrolyte and electrodes is one of the main problems of the oxide electrolyte. The electrolyte is so stiff and brittle that it does not interface well with the electrodes, resulting in contact losses [3]. Although there are approaches such as adding additional protective layers or applying an artificial SEI layer. However, these measures are still in the research stage, which is why the interfaces are still one of the weak points of oxide electrolytes today [1].
Interface compatibility cathode/electrolyte: The ionic conductivity is too poor to be used as a thick electrolyte layer. As a solution, for example, a gel coating is proposed to be applied, which ensures a good transition of ions from cathode to electrolyte [3]. Small amounts of liquid electrolyte can also be applied instead of gel [9]. If gel or liquid is added, however, this is no longer referred to as an all-solid-state battery (ASSB), but as a semi-solid-state battery (SSSB).
Chemical stability: The system is so stable that Li metal anodes are possible in principle [3]. It is also characterized by the fact that it can be operated at ambient conditions [7]. Oxide electrolytes also work at very high temperatures [3]. For LLZTO, it has also been shown that operation at ambient conditions is possible [10].
Electrochemical stability: oxides are electrochemically stable and it is possible to use high voltage cathodes [3].
Mechanical stability: Oxide-based electrolytes are considered to be particularly mechanically stable by comparison [3], but they still suffer from a high susceptibility to dendrites growing along the grain boundary [1].
Manufacturability: The material is particularly hard and brittle during production. In addition, the electrolytes must be sintered at very high temperatures in order to achieve dense layers with low grain boundary resistances. Currently, the manufacturing process is therefore very complex and only wet chemical processing is possible [3]. How to scale up to large quantities is currently still the subject of research [7].
Cost: Due to the difficulty of fabrication, because the material is hard and brittle, and because an energy-intensive sintering process is required, the electrolyte is expensive [3].
Sulfides
Sulfide-based electrolytes are all electrolytes consisting of compounds containing at least lithium and sulfur. Phosphorus, silicon, germanium or halides (elements of the seventh main group: fluorine, chlorine, etc.) are often used in addition [3]. Typical sulfides include glassy Li-P-S (LPS), glass ceramics, agryodite (Li6PS5X), LISICON (lithium superionic conductor), and Li10GeP2S12 (LGPS) [11].
Ionic conductivity: Ionic conductivity is one of the advantages of sulfide electrolytes. With 10^-2 S/m, values are already achieved here which are in a similar order of magnitude to liquid electrolytes [11].
Electrical conductivity: Electrical conductivity is a weak point in some variants of Sulifid electrolytes (e.g. Thio-LISICON electrolytes). It has also been shown that high electrical conductivity increases the tendency to form dendrites [12].
Interface compatibility anode/electrolyte: Due to poor electrochemical stability (see below), batteries with this chemistry tend to form parasitic layers on the surface of the interfaces [11]. This results in high resistances. Interface compatibility is considered the biggest problem with sulfide electrolytes.
Interface compatibility cathode/electrolyte: The interface cathode/electrolyte is also one of the problems hindering the use of the electrolyte. The cause of the problem here is also poor electrochemical stability. However, the problem can be reduced with the help of protective coatings. In general, the cathode/electrolyte interface is seen as less critical than the anode/electrolyte interface [3].
Chemical stability: Sulfide electrolytes are not well suited for Li metal because they form chemical bonds with the material [3]. The chemical stability is significantly worse compared to oxide and polymer electrolytes [7].
Electrochemical stability: The stability window of sulfidic electrolytes is very small and is only in the order of 1. 7V – 2.3 V versus Li+ [3].
Mechanical stability: Due to the softness and malleability of the material, the electrolyte can conform very well to the anode. By pressing the components together, the grain boundary resistance can be significantly reduced and the tendency to form dendrites is reduced [3]. He [7] nevertheless judges the stability to be inferior to that of oxides and sulfides.
Manufacturability: The production of sulfide electrolytes is very complex because it must take place in an inert gas environment, as the material is susceptible to humid environments. With inert gas it is also possible to prevent the formation of the toxic gas H2S. While it is possible to fabricate the materials at ambient temperatures, it is difficult to reduce the film thickness [11]. Overall, however, it is believed that production is possible even in larger quantities [7].
Cost: Manufacturing costs are expected to be between those of oxides and polymers [7].
Outlook
From today’s perspective, it is not yet clear which electrolyte variant will dominate. Although polymers are the most widely used systems today, the advantages of the solid-state battery cannot be fully exploited due to the poor ionic conductivity at room temperature and the heating that is therefore required.
Oxide and sulfide electrolytes are both still further away from commercialization, and considerable research efforts still need to be made to bring these systems into series production. However, their potential properties could lead to polymer electrolytes being displaced in the medium term.
Oxide electrolytes are particularly promising due to their operation at ambient temperatures. Before commercialization, however, interface problems in particular still need to be resolved and inexpensive production processes developed.
Today, sulfide cells struggle in particular with poor interface compatibility and insufficient (electro-)chemical stability. If these problems can be eliminated, they could occupy a significant share of the market due to their excellent ionic conductivity.
Sources
[1] Wei, R., Chen, S. et. al.: Challenges, fabrications and horizons of oxide solid electrolytes for solid-state lithium batteries, 2021, Shanghai.
[2] Mercedes Benz: eCitaro, Battery Technology, 2023, https://www.mercedes-benz-bus.com/de_DE/models/ecitaro/technology/battery-technology.html
[3] Fraunhofer Institute for Systems and Innovation Research ISI: Solid-State Battery Roadmap 2035+, Karlsruhe, 2022
[4] Stephan, A., Nahm, K.: Review on composite electrolytes for lithium batteries, 2006, Polyner, Volume 47.
[5] Rathika, R., Padmaraj, O.: Electrical conductivity and dielectric relaxation behavior of PEO/PVdF-based solid polymer blend electrolytes for zinc battery applications, 2017, Ionics 24, 243-25.
[6] Aziz, S., Woo, T.: A conceptual review on polymer electrolytes and ion transport models, 2018, Journal of Science: Advanced Materials and Devices.
[7] IDTEchEx, Xiaoxi He: Solid State Batteries for EV Applications, June 2022, Conference Battery Show Stuttgart
[8] Kim, A., Woo, S. et al: Research Progresses of Garnet-Type Solid Electrolytes for Developing All-Solid-State Li Batteries, 2020, Pohang University of Science and Technology.
[9] Zhaoa, Zhaoa et al: Liquid phase therapy to solid electrolyte-electrode interface in solid-state Li metal batteries: A review, 2020 Energy Storage Materials, Pages 75-84.
[10] Mauger, A., Julien, C., Paolella, A. et.al. : Building Better Batteries in the Solid State: A review, 2019, Sorbonne Université.
[11] Zhang, Q., Cao, D. et al: Sulfide-Based Solid-State Electrolytes: Synthesis, Stability, and Potential for All-Solid-State Batteries, 2019, Advanced Materials, Northeastern University.
[12] Wang, S., Fang, R. et al : Interfacial challenges for all-solid-state batteries based on sulfide solid electrolytes, 2020, Journal of Materionmics.
