Recent study of SSEs is focused on RT ionic conductivites, higher charge-transport efficiency. Why is that? There are three major market demands for EES which are transportation, grid storage, and residential power-backup applications. This means, in the future, we need to concern about practical issues like safety problems.

Now, all solid-state processing uses bipolar electrodes, which make grid storage be larger and increase energy density. Also, Multiple electrolyte and electrolyte-electrode interface designs enable the long-term, dendrite-free cycling of energy-dense metals, so primary requirements for SSEs will be decreasing the cost of weight, increasing efficiency of interfacial ion transport, environmental stability and long-term chemical, structural and mechanical stabilities

In historical views, SSEs started about 200 years ago by Micheal Faraday, however most of the design concepts emerged in 1960s with classifing SSEs by SIEs(solid inorganic electrolytes) and SPEs(solid polymer electrolytes)

Properties of SIEs and SPEs

Pros and cons for SIEs and SPEs are shown by table in below.

  SIEs SPEs
Pros - high ionic conductivites
- high moduli
- wide and high electrochemical stability windows
- excellent thermal stability		 - ease of synthesis
- chemical stability
- low cost
- compatibility with large-scale manufacturing process
- mechanical toughness
Cons - manufacturing difficulties (such as fragility on large areas)
- poor interfacial charge transport (compared with liquid electrolytes
- risk of metal dendrite growth along grain boundaries at low current densities
- high cost
- poor environmental stability		 - low dielectric constant -> hard to facilitate ion-pair dissocation
- crystaline lattice hinder the motion of cation

To overcome these challenges, SIE materials have been designed to enable elevated ionic conductivity (but ionic conductivity of SIEs is pros… No idea..) or more fundamentally, decreasing electrostatic forces1 which drop the transportation and ion conductivity within the material by increasing distance between the mobile cation and neighbouring anions.

The process responsible for ion-pair dissociation for SPE materials , even if we use PEO, PAN, PMMA, etc in which the electron-withdrawing groups enable ion-pair dissocation, lead to dynamic association between mobile cations and the long-chain molecules. That is why crystaline lattice disturb the motion of cation. Therefore, to conquest these issues, there are physical and chemical strategies to design SPEs that frustrate crystallization to enable acceptable ionic conductivities.

As above, there are distinct advantage and disadvantage of each material. If we make the formation of SIE-SPE composites which has both good features, It will be attractive step. For example, addition of inert inorganic particles (SIEs) into ion-conductive engineering polymers such as PEO (SPEs) increases the mechanical strength and thermal stability etc. Over past 5 years, lots of remedies of various SSEs has emerged. those areas of study aslo include the mechanical heterogenity. Still, there are two major materials that we classify, the interest of composites will cross these boundaries.

  1. Coulomb Force ($=\frac{q_1q_2}{\epsilon r^2}$). Therefore, increasing distance will drop it. ↩