batteries Review A Performance and Cost Overview of Selected Solid-State Electrolytes: Race between Polymer Electrolytes and Inorganic Sulfide Electrolytes Duygu Karabelli 1,*, Kai Peter Birke 1,2 and Max Weeber 1 1 Fraunhofer Institute for Manufacturing Engineering and Automation IPA, Nobelstr. 12, 70569 Stuttgart, Germany; [email protected] (K.P.B.); [email protected] (M.W.) 2 Chair for Electrical Energy Storage Systems, Institute for Photovoltaics, University of Stuttgart, Pfaffenwaldring 47, 70569 Stuttgart, Germany * Correspondence: [email protected] Abstract: Electrolytes are key components in electrochemical storage systems, which provide an ion-transport mechanism between the cathode and anode of a cell. As battery technologies are in continuous development, there has been growing demand for more efficient, reliable and environmen- tally friendly materials. Solid-state lithium ion batteries (SSLIBs) are considered as next-generation energy storage systems and solid electrolytes (SEs) are the key components for these systems. Com- pared to liquid electrolytes, SEs are thermally stable (safer), less toxic and provide a more compact (lighter) battery design. However, the main issue is the ionic conductivity, especially at low tem- peratures. So far, there are two popular types of SEs: (1) inorganic solid electrolytes (InSEs) and (2) polymer electrolytes (PEs). Among InSEs, sulfide-based SEs are providing very high ionic conduc- −2 Citation: Karabelli, D.; Birke, K.P.; tivities (up to 10 S/cm) and they can easily compete with liquid electrolytes (LEs). On the other Weeber, M. A Performance and Cost hand, they are much more expensive than LEs. PEs can be produced at less cost than InSEs but their Overview of Selected Solid-State conductivities are still not sufficient for higher performances. This paper reviews the most efficient Electrolytes: Race between Polymer SEs and compares them in terms of their performances and costs. The challenges associated with the Electrolytes and Inorganic Sulfide current state-of-the-art electrolytes and their cost-reduction potentials are described. Electrolytes. Batteries 2021, 7, 18. https://doi.org/10.3390/ Keywords: solid-state batteries; solid electrolytes; polymer electrolytes; inorganic sulfide electrolytes; batteries7010018 lithium ion batteries; lithium metal batteries Academic Editor: Carolina Rosero-Navarro 1. Introduction Received: 30 January 2021 The first lithium batteries were already based on “Li metal” technology where metal- Accepted: 26 February 2021 Published: 5 March 2021 lic lithium was used as the negative electrode, achieving the highest theoretical energy densities [1]. However, the use of lithium in the metallic form coupled with an organic Publisher’s Note: MDPI stays neutral liquid electrolyte resulted in dendrite formation, which eventually leads to an internal with regard to jurisdictional claims in short circuit and thus, a thermal runaway. The serious safety problems associated with this published maps and institutional affil- system stunted their growth during their years on the market. In 1991, Sony presented and iations. marketed the first Li-ion battery (LIB) technology in which Lithium was no longer present in metallic form but only in ionic form (Li+) in a “host” material at a higher potential than lithium metal, thus limiting the formation of dendrites [2]. Since then, LIBs have been widely developed and are now present in all portable devices requiring a rechargeable battery (mobile phone, laptop, etc.). Today, the low manufacturing cost of LIBs makes Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. them the leading technology on the market for applications in electromobility (e-mobility). This article is an open access article However, as e-mobility (especially Electric Vehicle, EV) is an increasing market and becom- distributed under the terms and ing more and more attractive for millions of customers, there is a need for higher energy conditions of the Creative Commons density cells with increased charge–discharge and thermal performances. This could be Attribution (CC BY) license (https:// achieved through the optimization of existing LIB chemistries. creativecommons.org/licenses/by/ Conventional Li-ion technology is reaching its performance limits, as there can be no 4.0/). compromise on lifetime or safety. The latest “advanced” Li-ion systems with a silicon anode Batteries 2021, 7, 18. https://doi.org/10.3390/batteries7010018 https://www.mdpi.com/journal/batteries Batteries 2021, 7, x FOR PEER REVIEW 2 of 14 Batteries 2021, 7, 18 2 of 13 discharge and thermal performances. This could be achieved through the optimization of existing LIB chemistries. −1 −1 will not exceed energy densitiesConventional of 800 Li-ion Wh technology L or 300 is Whreaching kg itson performance a cell scale limits,[3,4]. Inas there order can be no to achieve higher energycompromise densities, on lifetime it is possible or safety. to The use latest Li metal “advanced” instead Li-ion of graphite systems as with the a silicon −1 negative electrode.anode Li metal will not has exceed about energy ten-times densities higher of 800 specific Wh L capacity−1 or 300 Wh (3.860 kg−1 mAhon a cell g scale) [3,4]. than graphite [5]. However,In order to achieve as stated higher previously, energy densities, Li metal it is is possible not compatible to use Li metal with instead a liquid of graphite electrolyte system becauseas the negative of the electrode. formation Li metal of dendrites. has about Porous ten-times polymer-based higher specific separatorscapacity (3.860 mAh do not provide ag sufficient−1) than graphite physical [5]. barrierHowever, to as stop stated the previously, breakthrough Li metal of is dendrites. not compatible In with a addition, the existingliquid liquid electrolyte electrolytes system because are toxic of andthe fo flammablermation of duedendrites. to the Porous fluorinated polymer-based separators do not provide a sufficient physical barrier to stop the breakthrough of salt LiPF6 carbonate solvents. A battery system with a liquid electrolyte can cause many dendrites. In addition, the existing liquid electrolytes are toxic and flammable due to the safety problems in the event of accidents. Its replacement with a solid electrolyte, which fluorinated salt LiPF6 carbonate solvents. A battery system with a liquid electrolyte can is also acting as acause separator, many safety would problems create anin inert,the event solid of systemaccidents. that Its couldreplacement solve with the a solid problems mentionedelectrolyte, above. which Solid-state is also acting batteries as a doseparato not haver, would a liquid create junction,an inert, solid which system that facilitates the formationcould solve of series-connected the problems mentioned cells in above. a pack. So Thelid-state absence batteries of this do junctionnot have a liquid eliminates unnecessaryjunction, volume, which facilitates resulting the in formation higher volumetric of series-connected energy densities.cells in a pack. Hence, The absence these new all-solidof state this batteriesjunction eliminates (ASSB) are unnecessary currently consideredvolume, resulting as the nextin higher generation volumetric of energy lithium batteries. densities. Hence, these new all-solid state batteries (ASSB) are currently considered as the For a successfulnext ASSB, generation the of solid lithium electrolyte batteries. must meet several key criteria such as (i) high ionic conductivity,For a (ii) successful wide electrochemical ASSB, the solid electrolyte stable window must meet and several chemical key stability,criteria such as (i) high ionic conductivity, (ii) wide electrochemical stable window and chemical stability, (iii) simple management of the interfaces between the components of the cell, (iv) good (iii) simple management of the interfaces between the components of the cell, (iv) good mechanical properties,mechanical flexibility properties, and (v) flexibility affordable and cost (v) [af6fordable]. There havecost [6]. been There many have stud- been many ies to find the moststudies suitable to find solid the most electrolyte suitable tosolid make electrolyte ASSBs to competitivemake ASSBs competitive with today’s with today’s Li-ion technology. Li-ion technology. SEs are generally classifiedSEs are generally into two classified main groups:into two main inorganic groups: electrolytes inorganic electrolytes and polymer and polymer electrolytes (PE). Theelectrolytes most commonly (PE). The most studied commonly SEs are studied given SEs in Figureare given1. in Figure 1. Figure 1. The most common solid electrolytes (SEs) and their examples [7,8]. Figure 1. The most common solid electrolytes (SEs) and their examples [7,8]. Under inorganic electrolytes,Under inorganic Lithium electrolytes, SuperIonic Lithium CONductor SuperIonic (LiSICON) CONductor andderiva- (LiSICON) andderivatives are widely used as oxide-type electrolytes due to their lower reactivity tives are widely used as oxide-type electrolytes due to their lower reactivity with water and with water and air. However, they show lower ionic conductivity at room temperature air. However, they show lower ionic conductivity at room temperature (RT) (~10−7 S cm−1) (RT) (~10−7 S cm−1) compared to sulfide electrolytes [9]. In 1989, Aono et al. showed that compared to sulfideSodium electrolytes (Na) SuperIonic [9]. In 1989,CONductor Aono et(NaS al.ICON)-type showed that electrolytes Sodium such (Na) as Su-Li1+xAlxTi2−x