理解固体电解质中的离子输运现象对于提高全固态电池、固体氧化物燃料电池、传感器等各种固体器件的性能至关重要。特别是锂离子全固态电池，由于其安全性问题，作为下一代电池已经引起了人们的广泛关注。
Fig. 1 Schematic representation of ionic conductivity calculations in conventional MD and CCD-NEMD.
a Conventional MD: equilibrium MD (left) and color-diffusion NEMD (right) and b CCD-NEMD: the color charge is defined on the basis of the charge valency of the chemical units. For example, the color charges ci of Li+ and the Xx–-unit are denoted as +1 and –X, respectively. The conductivity considering the ion–ion distinct correlation effect can be calculated faster using CCD-NEMD than EMD.
寻找具有高离子电导率的固体电解质对于其性能的进一步提升尤为关键，而原子模拟能够准确、快速地估计离子电导率，有助于材料设计。目前，从头算分子动力学（AIMD）由于不需要拟合参数，是计算电导率最有效的工具。
Fig. 2 Crystal structure and transport property of Li10GeP2S12
然而，计算相关离子电导率的高成本迫使几乎所有的从头算分子动力学都依赖于能斯特-爱因斯坦（N-E）稀解近似，这忽略了互相关效应。基于颜色扩散算法的非平衡分子动力学（CD-NEMD）可以加速N-E近似电导率的计算，但在避免依赖于N-E近似方面还有改进的空间。
Fig. 3 Validation and efficiency of CCD-NEMD.
来自日本东京工业大学材料与化学工程学院的Ryoma Sasaki等人，将原有的CD-NEMD加以扩展，开发了一种化学颜色扩散非平衡分子动力学（CCD-NEMD）方法，能够以比传统MD更少的采样步骤计算相关电导率。
Fig. 4 Temperature dependence and anisotropy of ionic conductivity.
CCD-NEMD可以很好地模拟典型固体电解质Li10GeP2S12和Li7La3Zr2O12的电导率。作者发现，CCD-NEMD的计算成本低、统计精度高并且比使用能斯特-爱因斯坦稀解近似的平衡分子动力学计算效率更高。作者也将CCD-NEMD应用于Li7La3Zr2O12的晶界，证明了该方法适用于界面局域电导率的计算。
Fig. 5 Haven ratio HR = σdilute/σNEMD of LGPS along c-axis and in ab-plane as a function of temperature, respectively.
他们进一步发现，CCD-NEMD方法可以通过局部通量来估计界面离子的导电性，这对于增强复合材料的电导率和晶界电阻至关重要。本工作提出的CCD-NEMD有助于进一步准确理解离子相关效应，并促进固体器件的发展。该文近期发布于npj Computational Materials 9: 48 (2023).
Fig. 6 Structures and grain-boundary ionic conductivity of the Σ3(112) grain boundary of LLZO.
Editorial Summary
Correlated conductivity solid-state batteries: Nonequilibrium molecular dynamicsUnderstanding ionic transport phenomena in solid electrolytes is fundamentally important for improving the performance of various solid devices, for example, all-solid-state batteries, solid oxide fuel cells, and sensors.
In particular, Li-ion all-solid-state batteries have attracted considerable attention as next-generation batteries owing to their safety concerns. Finding solid electrolytes with a high ionic conductivity is crucial for further improvement, and atomistic simulations are required to enable accurate and fast estimation of ionic conductivities for the material design. Currently, ab initio molecular dynamics (AIMD) is the most effective tool for calculating the conductivity as they are free from fitting parameters.
However, the high cost of computing correlated ionic conductivities has forced almost all ab initio molecular dynamics to rely on the Nernst–Einstein (N–E) dilute-solution approximation, which ignores the cross-correlation effect. The color-diffusion algorithm-based nonequilibrium molecular dynamics (CD-NEMD) has been applied to accelerate the calculations of N–E approximated conductivity. However, there is room for improvement to avoid the reliance on N–E approximation. In this work, Ryoma Sasaki et al from the School of Materials and Chemical Technology, Tokyo Institute of Technology, extended the CD-NEMD and developed a chemical color-diffusion nonequilibrium molecular dynamics (CCD-NEMD) method, which enables to calculate the correlated conductivities with fewer sampling steps than the conventional MD. CCD-NEMD was demonstrated to well evaluate the conductivities in the representative solid electrolyte bulk Li10GeP2S12 and Li7La3Zr2O12.
The results showed that CCD-NEMD leads to low computational cost with high statistical accuracy and is more efficient than the equilibrium molecular dynamics using N–E approximation. The authors also applied CCD-NEMD to the grain boundary of Li7La3Zr2O12and demonstrated its applicability for calculating interfacial local conductivities.
This indicates that it can also be employed to estimate interfacial ionic conduction using the local flux, which is essential for enhanced conductivity in composites and grain-boundary resistance. CCD-NEMD can provide further accurate understanding of ionics with ionic correlations and promote developing solid devices. This article was recently published in npj Computational Materials 9: 48 (2023).
原文Abstract及其翻译
Nonequilibrium molecular dynamics for accelerated computation of ion–ion correlated conductivity beyond Nernst–Einstein limitation (超越能斯特-爱因斯坦限制的非平衡分子动力学加速计算离子-离子关联电导率)Ryoma Sasaki, Bo Gao,Taro Hitosugi & Yoshitaka Tateyama
Abstract
Condensed matters with high ionic conductivities are crucial in various solid devices such as solid-state batteries. The conduction is characterized by the cooperative ionic motion associated with the high carrier density. However, the high cost of computing correlated ionic conductivities has forced almost all ab initio molecular dynamics (MD) to rely on the Nernst–Einstein dilute-solution approximation, which ignores the cross-correlation effect. Here we develop a chemical color-diffusion nonequilibrium MD (CCD-NEMD) method, which enables to calculate the correlated conductivities with fewer sampling steps than the conventional MD.
This CCD-NEMD is demonstrated to well evaluate the conductivities in the representative solid electrolyte bulk Li10GeP2S12 and Li7La3Zr2O12. We also applied CCD-NEMD to the grain boundary of Li7La3Zr2O12and demonstrated its applicability for calculating interfacial local conductivities, which is essential for investigating grain boundaries and composite electrolytes. CCD-NEMD can provide further accurate understanding of ionics with ionic correlations and promote developing solid devices.
摘要
具有高离子电导率的凝聚态物质在固态电池等各种固体器件中至关重要。导电性可通过与高载流子密度相关的协同离子运动表征。然而，计算相关离子电导率的高成本迫使几乎所有的从头算分子动力学（MD）都依赖于能斯特-爱因斯坦稀解近似，这忽略了互相关联效应。
这里，我们开发了一套化学颜色扩散非平衡MD（CCD-NEMD）方法，该方法能够以比传统MD更少的采样步骤计算关联电导率。CCD-NEMD可以很好地评估典型固体电解质Li10GeP2S12和Li7La3Zr2O12的电导率。我们也将CCD-NEMD应用于Li7La3Zr2O12的晶界，证明了该方法适用于界面局域电导率的计算，这对研究晶界和复合电解质至关重要。CCD-NEMD有助于进一步准确理解离子相关效应，并促进固体器件的发展。