Solid Electrolytes

Despite significant improvements since their first commercialization in 1991, modern rechargeable lithium ion batteries do not fully meet the growing demands of industry in terms of capacity, charge/discharge performance and safety. Higher capacity can be achieved with a lithium metal anode, but lithium dendrites are likely to grow through liquid or gel electrolytes, eventually leading to short circuits and battery failure. Electrochemical cells utilizing solid ceramic or polymer electrolytes can circumvent these issues. Many ceramic electrolytes provide a rigid pathway for lithium-ion migration and show comparatively high conductivity, but suffer from inflexible-interface problems that are inherent with solid ceramic electrolytes.

Hybrid electrolytes with 3D bicontinuous ordered ceramic and polymer microchannels for all-solid-state batteries

There are many challenges on the road to an all-solid-state battery with a lithium metal anode. One such challenge is identifying a solid electrolyte that has sufficient ionic conductivity without being too soft or too brittle. Polymer electrolytes suffer from the former and ceramics from the latter problem. Composites formed of ceramic and polymer electrolytes introduce multiple interfaces between the two phases that often impede ions crossing the interfaces. We present a hybrid solid electrolyte, composed of 3D ordered bicontinuous ceramic and polymer microchannels, generated by 3D printing. The ceramic endows the hybrid with high ionic conductivity at room temperature due to continuous pathways for ions through the ceramic phase, whereas the polymer mitigates the brittleness of the ceramic, rendering the hybrid electrolyte more resilient to fracture. The conductivity of the hybrid is reduced by only the volume fraction of space that the non-conducting polymer occupies, demonstrating a well-sintered ceramic phase. The microarchitecture with gyroidal channels exhibits the best properties, sustaining longer cycling in contact with lithium metal electrodes than a dense ceramic disk. The hybrid solid electrolyte decouples conductivity from mechanical properties, which may offer a way forward in the quest for an all-solid-state battery.

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Hybrid Electrolytes with 3D Bicontinuous Ordered Ceramic and Polymer Microchannels for All-Solid-State Batteries, S. Zekoll, C. Marriner-Edwards, A. K. Hekselman, J. Kasemchainan, C. Kuss, D. Armstrong, D. Cai, R. Wallace, F. H. Richter, J. Thijssen, P. G. Bruce, Energy & Environmental Science, 11, 185-201 (2018).

 

Compared to ceramic electrolytes, polymer electrolytes are more able to accommodate for changing interfacial structures occurring during cycling of the battery. By combining salts and polyethers such as polyethylene oxide (-CH2-CH2-O-)n, it is possible to synthesise thousands of metal-polyether complexes, alternatively known as polymer electrolytes. Such materials are in effect co-ordination compounds in the solid state.

The 6:1 complexes (6 ether oxygen's per lithium), poly(ethylene oxide)6:LiXF6, where X=P,As,Sb, have a structure composed of polymer tunnels within which the Li+ ions reside. We predicted that this structure should support ionic conductivity and went on to show that this is so. This work represented the discovery of ionic conductivity in crystalline polymer electrolytes when all such materials had previously been considered to be insulators. It represents a new direction in the study of ion transport in the solid state that is quite different from the conventional picture of ion transport in amorphous polymers that has dominated the field since the late 1970's.

We have gone on to show that it is possible to dope the 6:1 complexes thus raising the conductivity of these materials substantially to levels equalling and exceeding that of the best amorphous polymers and paving the way for the application of polymer electrolytes in devices such as all-solid-state rechargeable lithium batteries.

In addition, we have developed a new class of solid ionic conductors that are different from both ceramic and polymer electrolytes: small molecule electrolytes, in which cations are coordinated by discrete low molecular weight ligands such as glymes. Unlike ceramic electrolytes, they are soft solids, yet, unlike amorphous polymer electrolytes, they are highly crystalline, of low molecular weight, and have no polydispersity or chain entanglement. Due to their monodispersity and different crystal structures, the relation between structure and conductivity can be investigated.