Co-Sintering Reaction Analysis of Li1.3Al0.3Ti1.7(PO4)3/LiFePO4 and Li1.3Al0.3Ti1.7(PO4)3/LiNiPO4 Composites
Authors: Kobayashi Shuhei , Kubota Kei , Ichihara Fumihiko , Ode Machiko , Miyoshi Shogo , Masuda Takuya 
Published: December 2024

Tremendous effort has been dedicated to the research on all-solid-state lithium-ion batteries (ASSLIBs) using oxide-type solid electrolytes instead of flammable organic liquid electrolytes. ASSLIBs using oxide-based solid electrolytes are expected to be used for a wide range of applications because of their excellent safety, lifetime, and reliability. Despite those advantages, oxide-type ASSLIBs still have challenges before achieving widespread use. In conventional liquid-type lithium-ion batteries, electrolyte/electrode interfaces can be spontaneously formed by injecting electrolyte solution. However, co-sintering of solid electrolytes and electrode materials at high temperatures are required to form highly ion conducting interfaces with maintaining their intrinsic crystal structures and physicochemical properties [1, 2]. However, they often react with each other, resulting in the formation of highly resistive interlayers and/or decomposition of substances [1]. Therefore, it is very important to understand and control the reaction of solid electrolytes and electrode materials for developing a rational process to form a highly ion conducting interfaces from a variety of materials with suppressing the undesired side reactions.

In the present study, Li1.3Al0.3Ti1.7(PO4)3 (LATP), which is one of the oxide-based solid electrolytes with a relatively high ionic conductivity at room temperature, is co-sintered with positive electrode materials, LiFePO4 (LFP) and LiNiPO4 (LNP) at various temperatures and atmospheres, and the reaction products were quantitatively analyzed by X-ray diffraction (XRD), X-ray absorption fine structure (XAFS), energy-dispersive X-ray spectroscopy (EDS), and thermogravimetry-differential thermal analysis (TG-DTA).

When LFP/LATP composites were co-sintered in air, LFP started to decompose at a relatively low temperature; FePO4 at 300°C and Li3Fe2(PO4)3 in the temperature range of 500 – 700°C are the major products and Fe2O3 is the minor product existing in the temperature range of 300 – 800°C. Although LATP was remained unchanged up to 700°C, it reacted with Li3Fe2(PO4)3 to form a Fe-doped LATP, Lix(Al,Ti,Fe)2(PO4)3. In an Ar/H2 atmosphere, LFP and LATP retained their crystal structures up to 650°C, and they started to react with each other at 700°C to form Fe2P2O7 and Fe-doped Liy(Al,Ti,Fe)2(PO4)3. In contrast to LFP/LATP, both LNP and LATP retained their crystal structures throughout the temperature range when LNP/LATP composites were co-sintered in air. In an Ar/H2 atmosphere, however, various products such as Ni12P5 and Li4P2O7 at 500°C, Ni2P at 600°C were formed as well as Li-rich Liz(Al,Ti)2(PO4)3
at 800°C. Those reaction products were compared with the prediction based on thermodynamic calculations, and their consistence and contradiction were discussed based on the validity of thermodynamic parameters of reference materials in database, diffusion of metal ions and releasing oxygen and water during co-sintering.

Reference list

[1] F. Ichihara, S. Miyoshi, T. Masuda, Phys. Chem. Chem. Phys., 24, 25878 (2022)

[2] F. Ichihara, K. Niitsu, Y. Tanaka, Y. Niwa, K. Mitsuishi, S. Miyoshi, T. Ohno, T. Masuda, J. Phys. Chem. C, 127, 15043 (2023)