Flexible & Solid-State Batteries: Li-ion batteries, challenges for other batteries

     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 [2].  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.