A team from Tohoku University and Tokyo Tech have addressed one of the major disadvantages of all-solid-state batteries by developing batteries with a low resistance at their electrode/solid electrolyte interface. The fabricated batteries, reported in the journal ACS Applied Materials & Interfaces, showed excellent electrochemical properties that greatly surpass those of traditional and ubiquitous Li-ion batteries.

The team achieved an extremely low electrolyte/electrode interface resistance of 7.6 Ω cm² in solid-state Li batteries with Li(Ni_0.5Mn_1.5)O₄. Further, the researchers observed spontaneous migration of Li ions from the solid electrolyte to the positive electrode after the formation of the electrolyte/electrode interface. Finally, they were able to demonstrate stable fast charging and discharging of the solid-state Li batteries at a current density of 14 mA/cm₂.

Structure of the thin-film all-solid-state batteries. The batteries were made by stacking various layers via thin-film deposition methods. The LNMO/Li₃PO₄ interface showed spontaneous migration of Li ions and had an unprecedentedly low resistance.

Solid-state Li batteries (LBs) show great promise as energy-storage devices because of their high energy density and good safety. In particular, solid-state LBs with a 5 V-class positive electrode consisting of Li(Ni_0.5Mn_1.5)O₄ have beeen extensively investigated to further increase their energy density. However, in solid-state Lbs with LNMO, the high interface resistance (charge-transfer resistance, R_i) of ~200-2000Ω cm² at the LMNO/solid electrolyte interface hinder fast charging and discharging.

The interface resistance of solid-state LNMO LBs is reported to be an order of magnitude higher than that of liquid electrolyte-based batteries using LNMO. Thus, it remains high desirable to lower the interface resistance of solid-state LNMO LBs.

—Kawasoko

The scientists from Tokyo Tech and Tohoku University, led by Professor Taro Hitosugi, fabricated all-solid-state batteries with extremely low interface resistance using LNMO by stacking various layers via thin-film deposition methods. The team fabricated their batteries under ultrahigh vacuum conditions, ensuring that the electrolyte/electrode interfaces were free of impurities.

After fabrication, the electrochemical properties of these batteries were characterized to shed light on Li-ion distribution around the interface. X-ray diffraction and Raman spectroscopy were used for analyzing the crystal structure of the thin films comprising the batteries.

Spontaneous migration of Li ions was found to occur from the Li₃PO₄ layer to the LNMO layer, converting half the LNMO to L₂NMO at the Li₃PO₄/LNMO interface. The reverse migration occurs during the initial charging process to regenerate LNMO.

The resistance of this interface, verified using electrochemical impedance spectroscopy, was 7.6 Ωcm₂—two orders of magnitude smaller than that of previous LMNO-based all-solid-state batteries and even smaller than that of liquid-electrolyte-based Li-ion batteries using LNMO.

These batteries also displayed fast charging and discharging, managing to charge/discharge half the battery within just one second. Moreover, the cyclability of the battery was also excellent, showing no degradation in performance even after 100 charge/discharge cycles.

The research team hopes that these results will facilitate the development of high-performance all-solid-state batteries, which could revolutionize modern portable electronic devices and electric cars.

Resources

• Hideyuki Kawasoko, Susumu Shiraki, Toru Suzuki, Ryota Shimizu, and Taro Hitosugi (2018) “Extremely Low Resistance of Li3PO4 Electrolyte/Li(Ni0.5Mn1.5)O4 Electrode Interfaces” ACS Applied Materials & Interfaces doi: 10.1021/acsami.8b08506