(Invited) Development of a Superionic Conductor for High Power All-Solid-State Battery
Authors: Kato Yuki 
Published: February 2020

Batteries has been widely used as energy source of mobile electronic devices and considered to be promising for large scale energy storage for electric vehicle and smart power grid due to their high energy density. Although high power density is required to regulate a large amount of electric power, the highest energy density battery, such as lithium ion batteries, generally offer lower power density than supercapacitors, because of slow ionic transport under the charge-discharge reactions. As the high rate capabilities of solid-state electrodes were confirmed, the rate determining steps that prevent their high power densities might be the ion transport in liquid electrolyte and/or at the liquid-solid interface. At high current condition, a concentration gradient is produced due to slow diffusion both of anions and cations of the electrolyte salts in solution, and lithium near the electrode region is consumed by the electrode reaction, this is well known as a solution phase diffusion limitation. In addition, reaction kinetics at the interface of liquid electrolyte / solid electrode is slow because of the (de-)solvation reactions of lithium ion that derives from a strong interaction between lithium ion and solvents.

Since issues preventing the lithium ion batteries to be high power derive from the fundamental properties of liquid electrolyte, our strategy for high energy and power is to fabricate an energy storage device with solid electrolyte instead of liquids. Although the solidification of electrolyte has been investigated over the past few decades, none of the high power secondary systems have been actually realized. Low ionic conductivity of solid electrolytes (10-5 ~ 10-3 S/cm; three to one order of magnitude of lower than those of liquid electrolytes) causes high ohmic resistivity, resulting in poor electrochemical performance; none of the all-solid-state battery systems could operate at the high current density region where solution phase limitation occurs in lithium ion battery systems, therefore, the advantage of solid configurations have not been clarified experimentally.

In this study, we reported a superionic conductor of Li9.54Si1.74P1.44S11.7Cl0.3 the materials group, Li10GeP2S12. It exhibits the ionic conductivity of 25 mS cm-1 at room temperature. The wide three dimensional distributed lithium in the framework of this crystal has been clarified using careful structural analysis using neutron diffraction technique. Such a high ionically conducting solid may improve the power characteristics of the all-solid-state battery. Therefore, here we clarify the rate characteristics of an all-solid-state battery using the developed solid electrolyte and compare the performance with that of a lithium-ion battery with a standard liquid organic electrolyte of 1M LiPF6 in EC:DEC.

The rate capability of all-solid-state batteries was superior to that of lithium ion battery using liquid electrolyte although the configuration of batteries were same for each systems except for electrolytes. The results of high rate capability of all-solid-state battery could be summarized as below:

(Low temperature) A significant difference in the discharge profiles of the solid-state and liquid electrolyte systems was evident; while the all-solid-state battery exhibited large charge-discharge capacity at current densities of 0.03 mA cm-2, large voltage drops were observed for the liquid electrolyte battery.

(Room temperature) Differences in rate characteristics between the solid-state and lithium ion batteries are clearly indicated by the rate dependence of the discharge capacities. Discharge capacity of the liquid electrolyte battery decreased rapidly at a current density of 30 mA cm-2 (45 C), although no differences in discharge capacities of the solid-state and liquid electrolyte batteries were found in the low current density region of 0.06 mA cm-2 (0.09 C) to 20 mA cm-2 (30 C).

(High temperature) The performance of lithium ion battery and all-solid-state battery were evaluated at high temperature 100 °C. The lithium ion battery could not function due to thermal instability of the LiPF6-based liquid electrolyte19. In contrast, the present all-solid-state battery exhibits the excellent performance at high temperature owing to good thermal stability of solid electrolyte.

The advantages of the all-solid-state battery are discussed using electrochemical evaluation techniques: (1) simple lithium transfer at the electrode / electrolyte interface with low activation energy that reduces the charge-transfer resistance at low temperatures. (2) High lithium concentration and high lithium-ion transport number prevent a concentration gradient under high rate current conditions, promoting a high rate performance. (3) Thermal stability of solid electrolyte allows to exhibit ultrahigh rate discharging performance at high temperature where lithium ion battery could not be operated. These results showed the experimental evidence of high power in an all-solid-state battery.