Tohoku team develops rechargeable quasi-solid state lithium battery with organic crystalline cathode
A pair of researchers at Tohoku University in Japan have developed a novel rechargeable solid-state lithium battery with an organic crystalline cathode. The researchers achieved a total cell energy density of 120 Wh/kg-cell for a cell that tolerated more than 100 cycles; 200 Wh/kg-cell was achieved when cyclability was sacrificed, according to a paper published by the team in Scientific Reports—an online and open access publication from the Nature Publishing Group.
The battery features a novel cell design which prevents dissolution of the organic cathode compounds. The cell features a completely encapsulated cathode; polyethylene oxide (PEO) layer; quasi-solid electrolyte and controlled electrolyte-anode interface. The team primarily investigated tetracyanoquinodimethane (TCNQ) for the cathode for its relatively high redox potentials and abundance of literature.
Lithium ion batteries (LIB) have traditionally been used for portable electronic devices due to low weight, high energy density but high price. However, recent emerging demands for secondary batteries extend to large- scale applications such as electric vehicles and peak load leveling installations due to environmental concerns and energy security. However, large-scale installations using current methods would leave large ecological footprint and face resource restrictions, since current cathodes use rare metals such as cobalt. Redox-active organic materials do not require such material and also possess large energy density, primarily owing to their two-electron reactions. Many of such compounds are low-cost, and some are even biomass in origin. Furthermore, if organic cathode can be integrated in a solid-state lithium batteries that accommodate energy-dense metallic lithium anode and do not require flammable organic electrolytes, it would offer possible solution for a much needed energy-dense, durable, low-cost and safe large-scale lithium battery.
While properties of organic cathodes are desirable, irreversible reaction by singly reduced radical anions, low conductivity, and dissolution issues currently pose critical safety and cyclability problems...Moreover, if potentials of organic cathodes—low cost, large capacity and molecular design versatility—are to be fully exploited, a generally applicable methodology that accommodates any monomeric, polymeric or composite cathode must be devised.—Hanyu and Honma 2012
The cell designed by Yuki Hanyu and Itaru Honma features a cathode containing a large surface-area carbon current collector, room-temperature ionic liquid (RTIL) and organic crystals in a sealed environment. The RTIL secures lithium ion conduction paths to the organic crystals, which are the active cathode material. A polyethylene oxide (PEO) membrane and silica-RTIL composite solid electrolyte separates the cathode from the metallic lithium anode.
The solid electrolyte is a critical component of the solid cell. However, due to the low temperature tolerance of organic cathodes, many of the known ceramic solid electrolytes are unsuitable. A suitable solid electrolyte for such a cell needs to display sufficiently high ionic conductivity below 340 K (67 °C), suppress dendritic formation on the lithium anode side, and prevent cathode dissolution, the authors noted.
Their solution was a 3-layer composite electrolyte: ~20 µm thick PEO layer on the cathode side; ~400 µm thick RTIL-Silica “saggy sand” layer in the middle; and a controlled SEI on the anode side.
PEO film is soluble in most RTILs; upon cell assembly, the PEO layer transforms into a highly viscous thin matrix of PEO-RTIL-silica mixture that glues solid electrolyte with cathode and prevents dissolution.
The quasi-solid electrolyte, PEO membrane, TCNQ cathode paste and current collector were stacked in this order and compressed in a die at 100 MPa pressure to form a combined pellet that encapsulated the cathode paste. The combined pellet and metallic lithium were integrated in a coin-type cell for characterization and measurement. Among the results:
A lithium cell using 1-ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide ([EMIm][Tf2N]) RTIL composite quasi-solid electrolyte and TCNQ cathode reached the theoretical capacity of 262 mAh/g-TCNQ at 323K (50 °C), 0.2 C discharge rate between 2.1 V–4.0 V.
At room temperature, the initial capacity was 215.8 mAh/g-TCNQ and after 100 cycles, 170 mAh/g-TCNQ of capacity was retained.
Another cell using 1-butyl-2-methyl pyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP][Tf2N]) RTIL composite quasi-solid electrolyte recorded 73% capacity retention over 170 cycles at 0.2 C rate, 323 K. Comparable cyclability was observed at 2 C rate.
In general, when the temperature was elevated, test cells recorded higher initial capacities but faster capacity loss. When the active material content was high, the capacity fading was more rapid despite comparable initial specific capacities.
Further investigations confirmed that the approach is effective for numerous other redox-active organic compounds. This implies hundreds of compounds dismissed before due to low cyclability would worth a re-visit under such a solid state design, they suggested.
In conclusion, we have demonstrated the effectiveness of an all-solid approach for using organic cathodes, many of which have been previously dismissed for being excessively soluble. Given these results, we consider re-visiting such compounds a worthwhile effort. TCNQ-based all-solid cells have reached the theoretical capacity and hence reversible cathodic reactions were demonstrated. Besides molecular designing of cathode compounds, we believe further room for improvements exist in the electrolyte and cathode preparation. The basic specifications of test cells, “all-solid, ~300 µm thick cathode, 50~80 wt.% active material” offers a strong case for adopting organic cathodes for large-scale rechargeable lithium batteries for mass energy storage applications. Further improvements would be found in integration of polymeric or sulphur cathode, or the use of improved ceramic solid electrolytes.—Hanyu and Honma 2012
Honma suggested to the Nikkei that a next step will be to look further for organic materials that more efficiently store power and boost the battery’s capacity, with a goal of developing a secondary battery for electric vehicles.
Yuki Hanyu & Itaru Honma (2012) Rechargeable quasi-solid state lithium battery with organic crystalline cathode. Scientific Reports 2, Article number: 453 doi: 10.1038/srep00453