Flexible & Solid-State Batteries: Li-ion batteries, challenges for other batteries
Li-ion batteries using polycarbonate-based electrolytes
Energy storage devices represented by Li-ion secondary batteries are indispensable to modern society. In recent years, the need for higher energy density and long-term operation has increased, and there is a trend toward full-scale commercialization of the automobile industry in the future, and expectations for establishment of basic research and breakthrough are extremely increased. Researches on "complete solidification" of electrolytes have attracted much attention in recent social backgrounds. Research and development of inorganic (glass) solid electrolyte showing high ionic conductivity is a typical example, recently research on next generation batteries such as lithium air batteries is also full-scale. Solidification of this electrolyte is an important research leading to improvement of device safety (the risk of liquid leakage, volatilization, ignition and explosion) and weight reduction which can not be achieved with conventional electrolytic solution system. On the other hand, the solid polymer electrolyte (SPE) is a new electrolyte material that is rich in processability and flexibility, which is difficult for inorganic materials, as well as safety as a solid material, further enabling further weight reduction and thinning of devices. In the future, expectations will be gathered as a new material contributing to the next generation electronics industry, such as the realization of flexible storage batteries.
Tominaga Group conducts coin cell fabrication using a polycarbonate type electrolyte showing specific ionic conduction behavior (example on the left figure, positive electrode: LiFePO4, negative electrode: Li) and battery characteristics evaluation (bottom left) [1,2]. The charge/discharge curves at the third cycle at room temperature of P (EO/PO)-LiFSI electrolyte (5 mol%) and PEC-LiFSI electrolyte (188 mol%) are shown at the bottom left. Since the ether type P (EO/PO) electrolyte (blue lines) has a partial melting peak based on the crystallization of the EO part around 40 °C, the conductivity at around room temperature is on the order of 10-5 S/cm. Therefore, the discharge capacity of P (EO/PO) electrolyte is about 60 mAh/g. On the other hand, the charge/discharge curve (red lines) of the PEC type electrolyte showed a clear plateau based on precipitation/dissolution of Li around 3.4 V, indicating that the discharge capacity was high at around 120 mAh/g . It is thought that the Li-ion transference numbers (t+) of each electrolyte greatly affect the difference in discharge capacities. Polyether generally has a low t+ value as low as 0.1 to 0.4, and it is strongly dependent on anion conduction due to strong solvation of cations by ether chains. Therefore, it is conceivable that the battery characteristics deteriorate due to accumulation in the vicinity of the anion's electrode. One of the polycarbonate types is known to exhibit a high t+ of 0.5 or more, which is considered to have an influence on excellent charge/discharge characteristics.
- Y. Tominaga, Polymer Journal (Focus Review), 49 (3), 291-299 (2017).
- K. Kimura, M. Yajima, Y. Tominaga*, Electrochemistry Communications, 66, 46-48 (2016).
Mg-ion batteries using polycarbonate-based electrolytes
Magnesium (Mg)-ion batteries have a great potential to be a viable alternative to Li-ion batteries due to the highly abundance of Mg source and a better safety characteristics. Interest in Mg-ion batteries has been increasing because of their substantial prospective benefits, higher volumetric capacity (3833 mAh/cm3 vs. 2205 mAh/cm3 for Li) and reduction in potential of -2.4 V (vs. standard hydrogen electrode). Magnesium is also naturally abundance, safe, non-toxic and environmentally friendly. As an anode material, Mg has a low standard electrode potential and fast deposition/stripping kinetics with nearly 100% reversibility without any formation of dendritic structures. Additionally, Mg is also chemically more stable compared to Li and Na; thus, with an established design and architecture, it could also lead to gravimetric energy densities ranging from 150-200 Wh/Kg with an operating voltage in the range of 2-3 V. Ionic radius of 0.86 Å Mg is considered similar to Li with 0.9 Å; hence, the intercalation of Mg2+ with double charge of Li+ would result in twofold capacity of the cathode. Furthermore, since Mg-ion battery has similar characteristics to Li-ion, it is known to operate on the same principles and performances with Li-ion system. However, to deliver a good performance, this Mg battery needs appropriate electrolyte and cathode materials.
Many attempts have been made to develop Mg batteries with high efficiency and high specific capacity, but practical application is still limited by the cathodes and electrolytes materials. Polymer electrolytes (PEs) are promising materials for Mg batteries with the advantages of low flammability, high flexibility, good ionic conductivity and no leakage problems. Mg ion-conductive PEs based on poly(ethylene oxide), PEO, polyvinyl alcohol, PVA, poly(methyl methacrylate), PMMA have already been reported and detailed discussion has been given on their electrochemical properties especially on the Mg plating/stripping behaviour of the PEs. However, only a few studies have attained high cycling stability and high columbic efficiency for Mg redox reaction. Besides, only some reports discussed the details on electrochemical performance of Mg-ion conductive in PEs. PEO as a well-known polymer in PEs has been displayed insufficient for the development of Mg ion-conductive, which is due to the ionic conduction in PEO based on the coupling mechanism that strongly depends on the segmental motion of polymer chains. The interaction of ions with the ethers chains causes the physical cross-linking that affects the decrement of segmental motion. PEO-based electrolytes suffer low conductivity due to the migration of ions in PEO arisen from the local motion of oxyethylene (OE) chains in the amorphous region, which highly depends the cation-dipole interactions. The interaction inhibits a fast migration of ions as the OE chains trap and coordinate cations with dipoles and form stable complexes that increase the Tg. This new study use poly(ethylene carbonate) (PEC) was used as a polymer host in Mg-based electrolytes. The increasing attention on the aliphatic poly(alkylene carbonate)s derived from CO2/epoxide coplymerisation has been increased due to their various functionalities and concept of CO2 utilisation.
- A. Aziz, Y. Tominaga*, Polymer Journal, in press (2018).
- A. Aziz, N. Yoshimoto, K. Yamabuki, Y. Tominaga*, Chemistry Letters (OpenAccess), 47 (10), 1258-1261 (2018).
- A. Aziz, Y. Tominaga*, Ionics, 24 (11) 3475-3481 (2018).