Solid-state electrolyte


A solid-state electrolyte is a solid ionic conductor and electron-insulating material 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 the lithium-ion battery. Their main advantages are their absolute safety, no issues of leakages of toxic organic solvents, low flammability, non-volatility, mechanical and thermal stability, easy processability, low self-discharge, higher achievable power density and cyclability.
This makes possible, for example, the use of a lithium metal anode in a practical device, without the intrinsic limitations of a liquid electrolyte thanks to the property of lithium dendrite suppression in the presence of a solid-state electrolyte membrane. The use of a high-capacity and low reduction potential anode, like lithium with a specific capacity of 3860 mAh/g and a reduction potential of -3.04 V vs standard hydrogen electrode, in substitution of the traditional low capacity graphite, which exhibits a theoretical capacity of 372 mAh/g in its fully lithiated state of LiC6, is the first step in the realization of a lighter, thinner and cheaper rechargeable battery. This allows for gravimetric and volumetric energy densities high enough to achieve 500 miles per single charge in an electric vehicle. Despite these promising advantages, there are still many limitations that are hindering the transition of SSEs from academic research to large-scale production, mainly the poor ionic conductivity compared to that of liquid counterparts. 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 Michael Faraday in the nineteenth century, these being 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. Wright. The importance of the discovery was recognized in the early 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 needs of safety and performance improvements with respect to the state-of-the-art Li-ion chemistry are making solid-state batteries very appealing 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 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. This was achieved by using warm isostatic pressing to improve the contact between the electrode and the electrolyte.

Properties

For Solid State Batteries / Solid Electrolytes to become a major market challenger it must meet some key performance measurements. The major criteria that an SSB/SE should have are:
  • Ionic conductivity: Historically, SSBs have suffered from low ionic conductivities due to poor interfacial kinetics and mobility of ions in general. Hence an SE with a high ionic conductivity is of primary importance. High ionic conductivity can be measured through electrochemical impedance spectroscopy analysis.
  • Volumetric Energy Density: Along with high ionic conductivity the candidate must have the ability to be stacked within a single package, so it supplies high energy density to the Electric Vehicles. A high volumetric energy density is required so that the driving range of EVs can be increased between charges.
  • Power density: Sufficient power density is needed to make energy available when needed which is also a measure of how quickly charging and discharging can take place.
  • Cycle life: Long cycle and shelf life are needed as conventional Li-ion batteries degrade after a few years.
  • Ionic transference number: High ionic transference number can be measured through a combination of chronoamperometry and EIS analysis.
  • Thermal, mechanical and electrochemical Stability: During device or car operation the SSBs may undergo large volume variations and face mechanical stress. Also, electrochemical stability at high operating electrode potentials which are of advantage when it comes to high energy density. Hence, it is important that their mechanical, thermal, and electrochemical stability are considered. High mechanical strength can be measured through a traditional tensile test. Wide electrochemical stability windows can be measured through linear sweep voltammetry or cyclic voltammetry.
  • Compatibility: The SE must be compatible with the electrode materials used in batteries as there is already a high chance of increased resistance in SSBs due to limited contact area between electrolyte and electrode materials. It should also be stable in contact with Lithium metal. It should be lighter so that it can be used in portable electronic devices. High compatibility with the electrode material can be measured through EIS analysis repeated over more consecutive days.
  • Economic fabrication technologies: If SEs contain expensive materials like Ge it will make the production cost go up significantly. The production of an exemplar SSB will require the convergence of uncomplicated fabrication technologies like particle dispersion, mechanical mixing, film formation etc.
It is hard for one material to fulfill all the above criteria, hence a number of other approaches can be used for example a hybrid electrolyte system which combines the advantages of inorganic and polymer electrolytes.

Categories

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 component 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 substitutional groups of the polymer chains, while GPEs conducts ions mainly in the solvent or plasticizer.

All-solid-state electrolyte

All-solid-state electrolytes are divided into inorganic solid electrolyte, solid polymer electrolyte 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 Electrolytes (ISEs)

Inorganic solid electrolytes are a class of all-solid-state electrolytes composed of inorganic materials in either crystalline or glassy form, which conduct ions via diffusion through the solid lattice. Nature+1 These electrolytes present significant advantages including relatively high ionic conductivity and a high mechanical modulus, as well as a high cation-transfer number.
Traditional ISEs are typically single-phase materials. Oxide-based conductors are widely studied for their excellent chemical stability and mechanical robustness. Within oxide ISEs there are many crystalline structure types, include LISICON , argyrodite-like, garnets, NASICON , lithium nitrides, lithium hydrides, lithium phosphidotrielates and phoshidotetrelates, perovskites. Sulfide-based or halide-based ISEs offer particularly high ionic conductivities and comparatively soft mechanical behavior, which benefits interfacial contact. These sulfide and halide conductors often have crystalline frameworks that facilitate fast Li+ transport, and some glass-ceramic variants that exploit amorphous structure for improved interface conformity.
However, despite these strengths, single-phase ISEs face significant challenges. Oxide ceramics, though mechanically robust, tend to be brittle, develop high interfacial resistance at the electrode/electrolyte interface, and often require high sintering or stack pressure to achieve good contact. On the other hand, sulfide and halide materials, while offering high ionic conductivity, may suffer from chemical instability, a limited electrochemical window, and mechanical softness.

Solid polymer electrolyte (SPE)

Solid polymer electrolytes 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 compatible 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 crystallinity.
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. Thus 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 amorphous-to-crystalline ratio.