Research Update: Ca doping effect on the Li-ion conductivity in NASICON-type solid electrolyte LiZr2(PO4)3: A first-principles molecular dynamics study

Yusuke Noda,Masashi Kotobuki,Li Lu,Ryo Kobayashi,Masanobu Nakayama,Koki Nakano,Masanari Otake

doi:10.1063/1.5033460

Abstract

In this work, we used a density functional theory-based molecular dynamics simulation to investigate the Ca content-dependent Li-ion conductivity of NASICON-type Li1+2xCaxZr2-x(PO4)3 (LCZP) solid electrolytes (0.063 ≤ x ≤ 0.375) which exhibit a Li-excess chemical composition. The LCZP systems show a higher room temperature Li-ion conductivity and a lower activation energy than pristine LiZr2(PO4)3 (LZP), and the tendencies of those properties agree with the experimental results. In addition, the Li-ion conduction mechanisms in LCZP were clarified by analyzing the radial distribution functions and site displacement functions obtained from our molecular dynamics simulations. For minimal Ca substitution for LZP, the Li-ion conductivity is enhanced because of the creation of interstitial Li ions by Ca doping in the LCZP systems; the frequency of collisions with Li ions dramatically increases. For substantial Ca substitution for LZP, the Li-ion conductivity gradually worsened because some Li ions were trapped at the M1 (most stable) and M2 (metastable) sites near Ca atoms.

Highlights

Much scientific attention has been devoted to developing all-solid-state Li-ion batteries (LIBs) as alternatives to batteries that contain liquid electrolytes
We investigate the dependence of the Li-ion conductivity in LCZP on Ca substitution using first-principles molecular dynamics (FPMD) simulations and elucidate the Li-ion conduction mechanism in the system by analyzing the Li-ion trajectories
We show the methodology of firstprinciples calculations based on density functional theory (DFT)

Summary

INTRODUCTION

Much scientific attention has been devoted to developing all-solid-state Li-ion batteries (LIBs) as alternatives to batteries that contain liquid electrolytes. A promising oxide-based solid electrolyte is Na super ionic conductor (NASICON)-type materials that are similar to perovskite-type and garnet-type ionic conductors. Li1+2xCaxZr2−x(PO4)[3] (LCZP) has exhibited a better Li-ion conductivity than pristine LZP.[15,16] The highest bulk and total ionic conductivities were 1.2 × 10−4 and 4.9 × 10−5 S/cm, respectively, at room temperature for Li1.2Ca0.1Zr1.9(PO4)[3]. It is necessary to gain fundamental knowledge of the structural properties and ionic conductivity of LCZP at an atomic level to develop solid-state electrolyte materials. We describe the discussions of the resulting Li-ion conduction properties and analyze the Li-ion migration behavior in LCZP systems. The target materials in this paper exhibit Li-excess compositions by replacing Zr4+ with 2 Li+ + Ca2+, and there is a possibility of Li-ion occupation at M1 and M2 sites

COMPUTATIONAL DETAILS

RESULTS AND DISCUSSIONS

CONCLUSIONS