Bi₂O₃ promotes the densification of the LATP electrolyte, thereby improving its structural integrity, and at the same time, facilitating Li⁺ conduction, leading to reduced grain boundary resistance, the team writes in a paper accepted for publication in the journal ChemSusChem.

All-solid-state Li batteries are being considered as a future energy storage technology because of their advantages over conventional liquid electrolyte-based batteries in terms of energy density, stability, and safety. For practical applications of solid electrolytes to all-solid-state Li batteries, however, a number of technical issues should be addressed: (i) high Li⁺ conductivity but negligible electronic conductivity; (ii) a wide electrochemical window; (iii) chemical compatibility with electrode materials during battery fabrication and operation; (iv) mechanical stability for avoiding short-circuit phenomena; (v) chemical stability against air and moisture; and (vi) easy and cost-effective fabrication.

To date, considerable efforts have been devoted to the development of highly reliable solid-state inorganic Li⁺ conductors with various crystal structures, including garnet, perovskite, NASICON (Na superionic conductor), and LISICON (Li superionic conductor). Among these candidates, a NASICON-type Li⁺ conductor, LiTi₂(PO₄)₃ (LTP), has been highlighted as a promising solid electrolyte, owing to fast Li⁺ conduction in the bulk (grain interior) as well as its excellent mechanical strength and chemical stability against moisture.

Technical approaches aimed at maximizing Li⁺-conducting properties of LTP electrolytes may be divided into two categories: (i) chemical substitution for tuning crystal structures and (ii) incorporation of secondary phases (additives) for modifying the microstructures. … The approaches mentioned above have been shown to be effective; however, LTP-based electrolytes still suffer from low ionic conductivities due to large grain boundary resistances.

With regard to battery fabrication, an important concern is lowering the processing temperature of electrolytes. In general, LTP requires a high-temperature sintering process (~950 ˚C) to ensure acceptable ionic conductivity (to reduce grain boundary resistances); however, such a high-temperature process may cause deleterious reactions between the electrolytes and electrodes as well as significant damage to other components. It is thus of practical significance to fabricate high-density LTP electrolytes at reduced temperatures without sacrificing their high ionic conductivity.

—Lee et al.

The team selected as a microstructural modifier to reduce the fabrication temperature of the solid electrolyte and to enhance its ionic conductivity.

To investigate the effect of bismuth tetroxide (Bi₂O₃) on the structure of the electrolyte composite, the team synthesized LATP–Bi₂O₃ composite electrolytes with different Bi₂O₃ content, with the sintering temperature was fixed at 850 ˚C—i.e., lower by 100 ˚C as compared to that of LATP.

To examine the feasibility of the composite electrolyte, the researchers designed an constructed an all-solid-state Li battery with the following multi-layered structure: Li metal anode – PEO LiClO₄ interlayer – LATP-Bi₂O₃ electrolyte – interlayer – Li(Ni_0.6Co_0.2Mn_0.2)O₂ (NCM)–PEO–LiClO₄ cathode.

Charge and discharge capacities were estimated to be 124 and 122 mAh g^–1, respectively. Even at high current densities of 60 and 120 mA g^–1, the discharge capacities for LATP–Bi₂O₃ were determined to be as high as 70 and 29 mAh g^–1, respectively, which are higher than those of LATP.

… an all-solid-state Li battery assembled with the LATP–Bi₂O₃ composite electrolyte shows reversible charge–discharge operation without significant capacity fading. It is expected that further interfacial engineering of the battery configuration would be helpful to improve the electrochemical performance.

—Lee et al.

Resources

• Lee, S.-D., Jung, K.-N., Kim, H., Shin, H.-S., Song, S.-W., Park, M.-S. and Lee, J.-W. (2017) “NASICON-Bi₂O₃ composite electrolyte for all-solid-state lithium batteries: low-temperature fabrication and conductivity enhancement.” ChemSusChem doi: 10.1002/cssc.201700104