New organogel electrolyte for Li-ion batteries offers ionic conductivity close to liquid electrolytes, with thermal stability
A research team at Ulsan National Institute of Science and Technology (UNIST), S. Korea, has found a new physical organogel polymer electrolyte for lithium-ion batteries (LIBs) with two novel characteristics: an irreversible thermal gelation and a high value of the Li+ transference number. In an open access paper published in Scientific Reports, they reported that LIBs using the organogel electrolyte delivered significantly enhanced cyclability and thermal stability.
Electrolytes are essential components of supercapacitors, batteries and fuel cells. The most widely used electrolyte is a liquid type, since its overall ionic conductivity and value of transference numbers are better than solid-type electrolytes. However, safety concerns caused by its leakage and explosive nature is encouraging research on the development of solid-type electrolytes. The solid electrolyte would enable batteries to be safer as well as the use of higher energy electrode materials.
However, the use of solid-state electrolytes has been limited due to low ionic conductivity caused by their immobile matrix regardless of their own merits such as no leak, non-volatility, mechanical strength and processing flexibility, the authors note.
Fluidity of matrix materials is not necessarily required (e.g. solid-state electrolytes) only if the movement of ions through the matrix is guaranteed. The most important parameter of electrolytes used in electrochemical cells is ionic conductivity (σi). A wide spectrum of electrolyte systems covers a wide range of σi from 10-5 ~ 100 mS cm-1 for solid electrolytes to 100 ~ 102 mS cm-1 for liquid electrolytes. The use of solid-state electrolytes has been limited due to low σi caused by their immobile matrix, even if they have their own merits such as no leak, non-volatility, mechanical strength and processing flexibility. Gel electrolytes were invented to combine high σi of liquid electrolytes with the advantages of solid electrolytes.
The second parameter we should consider is transference number [t+] of ions of interest...In this work, therefore, we materialized the two required properties simultaneously in a polymer gel electrolyte: a liquid-electrolyte-level σi (~101 mS cm-1) with high t+ (>0.8).—Kim et al.
The ionic conductivities of the electrolyte were estimated at the values close to that of liquid electrolytes at a temperature range from room temperature (σRT = 8.63 mS cm-1) to 60 °C (σ60 = 15 mS cm-1).
|Elastic behavior of a cylindrical monolith of the gel electrolyte based on 2 wt. % PVA-CN in 1 M LiPF6 in 1:2 (vol.) EC:EMC as the base liquid. Source: UNIST.|
Cyanoethly polyvinyle alcohol (PVA-CN) played a key role in the highly conductive gel electrolyte while another cyano resin, Cyanoethlyle pullulan (Pullulan-CN), was used as a control representing a liquid electrolyte containing cyano chains. The PVA-CN-containing liquid electrolyte was thermally gelated even without any chemical crosslinkers or polymerizations initiators.
Hyun-Kon Song and Noejung Park, both, professors of the Interdisciplinary School of Green Energy, UNIST, South Korea, led the effort. Fellow authors include: Young-Soo Kim, Yoon-Gyo Cho, and Dori Odkhuu from UNIST.
The organogel could be applied to other electrochemical cells including other type of rechargeable batteries, supercapacitors, dye-sensitized solar cells and electrochromic devices, accommodating various pairs of ionic species and solvents. We believe that this new type of electrolyte gel provides design flexibility of devices as well as enhanced safety and stability to electro-chemical devices.—Kim et al.
This research was funded by the World Class University (WCU) programs through the National Research Foundation of Korea (NRF).
Young-Soo Kim, Yoon-Gyo Cho, Dorj Odkhuu, Noejung Park & Hyun-Kon Song (2013) A physical organogel electrolyte: characterized by in situ thermo-irreversible gelation and single-ion-predominant conduction. Scientific Reports 3, Article number: 1917 doi: 10.1038/srep01917