Electron and Ion Modulation at the Nasicon Solid-Electrolyte/Electrode Interfaces at Static and Operating Conditions in All-Solid-State Batteries: A DFT Study
Authors: Tian Hong-Kang , Tateyama Yoshitaka , Jalem Randy , Gao Bo 
Published: December 2020

High interfacial resistance has been one of the main obstacles that limit the application of all-solid-state Li-ion batteries (ASSLB), which relates to Li+ transport and distribution around the interfaces. Sulfide solid-electrolytes (SEs), many of which have higher ionic conductivity in bulk, usually showed significant deterioration of the resistance or interface states. In this respect, oxide SEs, even with lower ionic conductivity in bulk, is a promising route for acquiring better interface performance. Recently, several studies have shown the stability of phosphate moiety (PO4
3-) [1]. In this work, we examined the characteristics of interfaces of Li1+xAlxTi2-x(PO4)3 (LATP) SE with LiCoO2 (LCO) cathode and Li metal anode as a representative model system, for better understanding of the interfacial electron and ion modulations, and Li+ transport.

We used Density Functional Theory (DFT)-based first-principles calculation to evaluate the electronic states and the Li chemical potential around the Li/LATP/LCO interfaces. We especially examined the interfacial band alignments and redox processes as well as the possible Li+ movement. To explore the probable interface structures, we built the pristine atomic interface structures at various displacements via the CALYPSO technique [2], and then explored the exchange of cations for the LATP/LCO interface.

Single and double exchanges of cation pairs, including Ti-Co, P-Co, and Al-Co, at LATP/LCO interface, and all the 288 possible configurations are considered. The calculation results show that the interface with the Ti-Co exchange is more thermodynamically favorable than the pristine interface and the other exchange combinations, which agrees with the previous experiments [3, 4]. This indicates that the mutual diffusion of Ti-Co may occur around the LATP/LCO interface upon contact of LATP and LCO. Though the P-Co exchange was expected according to the previous studies, it does not seem to be the case at the LATP/LCO interface, indicating the strong and stable PO4
3- moiety in LATP.

We also calculated the partial density of states (PDOS) to evaluate the band alignment at the LATP/LCO and LATP/Li interfaces and electron modulation. The results reveal that there is a possible electron transfer from LCO to LATP upon contact through thermal excitation and/or interfacial states induced by oxygen vacancies, as the valence band maximum of LCO is higher than LATP and close to conduction band minimum of LATP. It would cause the reduction of Ti in LATP that has been observed by the experiments [3]. On the other hand, the LATP/Li interface model shows the Fermi level that lies in the conduction bands of LATP, indicating the electron transfer from Li to LATP, and it would likely cause the reduction of LATP at the interface.

By calculating the formation energy of Li vacancy (EV) in bulk and at interfaces [2], which relates to taking electron and Li+ altogether, we can correlate with Li chemical potential. Upon contact of LATP and LCO, Li+ is likely to move from LCO to LATP due to the lower Li chemical (-4.7 eV) in bulk LATP compared with bulk LCO (-4.2 eV), which can be compensated by negative charges that are from LCO to keep the neutrality. The EV of Li at the probable LATP/LCO interface (3.7 eV) is smaller than that of bulk LCO and bulk LATP. Thus, while charging, the Li+ will be removed primary from the LATP/LCO interface toward the bulk LATP. This removing tendency will cause the Li-depletion around the interface while charging, which was also observed by the experiments [5].

This work provides a modeling method and scenario that captures the electrons and ions modulation at the SE/electrode interfaces, which relates to the interfacial resistance, and agrees with experimental observations. It can be further applied to investigate other all-solid-state battery systems in future works.

This work was partly supported by MEXT “Fugaku Battery & Fuel Cell Project” and JSPS KAKENHI “Interface IONICS” Grant Number JP19H05815.

[1] F. Walther, J. Janek et al., Chem. Mater., 31, 3745–3755 (2019).

[2] B. Gao, R. Jalem, Y. Tateyama et al., Chem. Mater., 32, 85–96 (2020).

[3] Y. Yamamoto, Y. Iriyama, S. Muto, J. Am. Ceram. Soc., 103, 1454–1462 (2020).

[4] Y. Liu, X. Sun et al., ACS Appl. Mater. Interfaces, 12, 2293–2298, (2020).

[5] B. Tsuchiya, Y. Iriyama et al., Adv. Mater. Interfaces, 6, 1900100 (2019).