Solid-state electrolyte


A solid-state electrolyte is a solid ionic conductor electrolyte and it is the characteristic component of the solid-state battery. It is useful for applications in electrical energy storage in substitution of the liquid electrolytes found in particular in lithium-ion battery. The main advantages are the increased safety, no issues of leakages of toxic organic liquids, low flammability, non-volatility, mechanical and thermal stability, easy processability, low self-discharge, higher achievable power density and cyclability.
This makes it possible, for example, the use of a lithium metal anode in a practical device, without the intrinsic limitations of a liquid electrolyte. The utilization of a high capacity anode and low reduction potential, like lithium with a specific capacity of 3860 mAh g−1 and a reduction potential of -3.04 V vs SHE, in substitution of the traditional low capacity graphite is the first step in the realization of a lighter, thinner and cheaper rechargeable battery. Moreover this allows the reach of gravimetric and volumetric energy densities, high enough to achieve 500 miles per single charge in an electric vehicle. Despite the promising advantages there are still some limitations that are hindering the transition of SSEs from academic research to large-scale production, however many car OEMs expect to integrate these systems into viable devices and to commercialize solid-state battery-based electric vehicles by 2025.

History

The first inorganic solid-state electrolytes were discovered by M. Faraday in the nineteenth century, the silver sulfide and lead fluoride. The first polymeric material able to conduct ions at the solid-state was PEO, discovered in the 1970s by V. Wrigh. The importance of the discovery was recognized in the early of 1980s.
However, unresolved fundamental issues remain in order to fully understand the behavior of all-solid batteries, especially in the area of electrochemical interfaces. In recent years the need of safety and performances improvements with respect to the state-of-the-art Li-ion chemistry are making solid-state batteries very appealable and are now considered an encouraging technology to satisfy the need for long range battery electric vehicles of the near future.
In March 2020, the Samsung Advanced Institute of Technology published the research on an all-solid-state battery using an argyrodite-based solid-state electrolyte with a demonstrated energy density of 900 Wh L−1 and a stable cyclability of more than 1000 cycles, reaching for the first time a value close to the 1000 Wh L−1.

Properties

In order to design a SSE with the optimal performances, several properties have to be met:
SSEs have the same role of a traditional liquid electrolyte and they are classified into all-solid-state electrolyte and quasi-solid-state electrolyte. All-solid-state electrolytes are furthermore divided into inorganic solid electrolyte, solid polymer electrolyte and composite polymer electrolyte. On the other hand, a QSSE, also called gel polymer electrolyte, is a freestanding membrane that contains a certain amount of liquid compontent immobilized inside the solid matrix. In general the nomenclatures SPE and GPE are used interchangeably but they have a substantially different ionic conduction mechanism: SPEs conducts ions through the interaction with the susbtitutional groups of the polymer chains, while GPEs conducts ions mainly in the solvent or plasticizer.

All-solid-state electrolyte

All-solid-state electrolyte are divided into inorganic solid electrolyte, solid polymer electrolite and composite polymer electrolyte. They are solid at room temperature and the ionic movement occurs at the solid-state. Their main advantage is the complete removal of any liquid component aimed to a greatly enhanced safety of the overall device. The main limitation is the ionic conductivity that tends to be much lower compared to a liquid counterpart.
Inorganic solid electrolyte are a particular type of all-solid-state electrolyte that is constituted by an inorganic material in the crystalline or glassy state, that conducts ion by diffusion through the lattice. The main advantages of this class of solid-state electrolyte are the high ionic conductivity, high modulus and high transfer number compared to other classes of SSEs. They are generally brittle and with this comes a low compatibility and stability towards the electrode, with a rapidly increasing interfacial resistance and a complicated scale-up from academic to industry. They can be oxides, sulfides or phosphates-based and the crystalline structures include LISICON , argyrodite-like, garnets, NASICON , lithium nitrides, lithium hydrides, perovskites, lithium halides. Some ISEs can be glass ceramics assuming an amorfous state instead of a regular crystalline structure, popular examples are lithium phosporus oxynitride and the lithium thiophosphates.
Solid polymer electrolyte are defined as a solvent-free salt solution in a polymer host material that conducts ions through the polymer chains. Compared to ISEs, SPEs are much easier to process, generally by solution casting, making them greatly compatibile with large-scale manufacturing processes. Moreover, they possess higher elasticity and plasticity giving stability at the interface, flexibility and improved resistance to volume changes during operation. A good dissolution of Li salts, low glass transition temperature, electrochemical compatibility with most common electrode materials, a low degree of crystallinity, mechanical stability, low temperature sensitivity are all characteristics for the ideal SPE candidate. In general though the ionic conductivity is lower than the ISEs and their rate capability is restricted, limiting fast charging. PEO-based SPE is the first solid-state polymer in which ionic conductivity was demonstrated both through inter and intra molecular through ion hopping, thanks to the segmental motion of the polymeric chains because of the great ion complexation capability of the ether groups, but they suffer from the low room-temperature ionic conductivity due to the high degree of cristallinity. The main alternatives to polyether-based SPEs are polycarbonates, polyesters, polynitriles, polyalcohols, polyamines, polysiloxane and fluoropolymers. Bio-polymers like lignin, chitosan and cellulose are also gaining a lot of interest as standalone SPEs or blended with other polymers, on one side for their environmentally friendliness and on the other for their high complexation capability on the salts. Furthermore different strategies are considered to increase the ionic conductivity of SPEs and the amorfous-to-crystalline ratio.
With the introduction of particles as fillers inside the polymer solution, a composite polymer electrolyte is obtained, the particles can be inert to the Li+ conduction, with the sole purpose of reducing the cristallinity, or active if ISE's particles are dispersed and depending on the polymer/inorganic ratio the nomenclature ceramic-in-polymer and polymer-in-ceramic is often used. Copolymerization, crosslinking, interpenetration, and blending may also be used as polymer/polymer coordination to tune the properties of the SPEs and achieve better performances, introducing in the polymeric chains polar groups like ethers, carbonyls or nitriles drastically improve the dissolution of the lithium salts.

Quasi-solid-state electrolyte

Often confused with SPEs, quasi-solid-state electrolytes are also called gel polymer electrolytes, but they have a substantially different ionic conduction mechanism: SPEs conducts ions through the interaction with the susbtitutional groups of the polymer chains, while GPEs conducts ions mainly in the solvent or plasticizer. They consist of a polymer network swollen in a solvent that contains the active ions, so it possesses both the mechanical properties of solids and the high transport properties of liquids. Several GPEs with a number of polymer hosts have been studied, by using the same polymers as SPEs, but synthetized with increased porosity in order to easily allocate a low-evaporation solvents like ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, used as plasticizers. Low molecular weight poly or other ethers or aprotic organic solvents with high dielectric constant like dimethylsulfoxide can also be mixed the SPE matrix. UV and thermal crosslinking are useful ways to polymerize in-situ the GPE directly in contact with the electrodes for a perfectly adherent interface. Values of ionic conductivity of few mS cm−1 can be easily achieved with GPEs, as demonstrate the numerous research articles published on the argument.

Opportunities

The versatility and properties of the solid-state electrolyte widen the possible applications towards high energy density and cheaper battery chemistries that are otherwise prevented by the current state-of-the-art of Li-ion batteries. Indeed, by introducing a SSE in the battery architecture there's the possibility to use metallic lithium as anode material, with the possibility to achieve a high energy density battery thanks to its high specific capacity of 3860 mAh g−1. The utilization of a lithium metal anode is prevented in a liquid electrolyte above all because of the dendritic growth of a pure Li electrode that easily cause short circuits after few cycles; other related issues are volume expansions, solid-electrolyte-interface reactivity and 'dead' lithium. The usage of a SSE guarantees a homogeneous contact with the metallic lithium electrode and possess the mechanical properties to impede the uncontrolled deposition of Li+ ions during the charging phase. At the same time, a SSE finds very promising application in lithium-sulfur batteries solving the key issue of the polysulfide "shuttle" effect by blocking the dissolution of polysulfide species in the electrolyte that rapidly causes a reduction of capacity.