Intercalation in Li-ion batteries: thermodynamics and its relation to non-ideal solid-state diffusion

Marco Lagnoni, Gaia Armiento, C. Nicolella, A. Bertei
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Abstract

Intercalation is the key phenomenon taking place in lithium-ion batteries: while its thermodynamics sets the equilibrium voltage of active materials, solid-state diffusion of intercalated lithium determines the rate at which the battery can operate. This study revisits the thermodynamics of intercalation by treating the active material as a binary mixture of filled and empty sites, thus relating the equilibrium potential to the chemical potential difference of intercalated lithium. By setting a reference to unitary activity at half state-of-lithiation, the non-ideal behaviour of the active material is quantified via a revisited form of the thermodynamic enhancement factor, revealing that common solid-solution cathode materials as LiNixMnyCo1-x-yO2, LiNi0.8Co0.15Al0.05O2, and LiCoO2 show strong super-ideal behaviour. The latter is related to the thermodynamic enhancement of the diffusion coefficient of intercalated lithium. A comprehensive overview of the functional forms of Li diffusion flux according to linear irreversible thermodynamics is provided and related to the chemical diffusion coefficient obtained by conventional characterisation techniques. A literature analysis made on solid-solution cathode active materials reveals that while the chemical diffusion coefficient varies significantly with state-of-lithiation, there exists a convenient functional form of diffusion flux according to linear irreversible thermodynamics that enables a fairly stable diffusion coefficient with state-of-lithiation. This has clear benefits from both modelling and experimental viewpoints and potentially sheds light on the mechanistic fundamentals of solid-state diffusion.
锂离子电池中的互锁:热力学及其与非理想固态扩散的关系
插层是锂离子电池中发生的关键现象:插层热力学设定了活性材料的平衡电压,而插层锂的固态扩散则决定了电池的运行速度。本研究将活性材料视为填充位点和空位点的二元混合物,重新审视了插层热力学,从而将平衡电位与插层锂的化学位差联系起来。通过设定半锂化状态下的单位活性作为参考,活性材料的非理想行为通过热力学增强因子的重访形式得到量化,揭示了常见的固溶正极材料,如 LiNixMnyCo1-x-yO2、LiNi0.8Co0.15Al0.05O2 和 LiCoO2,表现出强烈的超理想行为。后者与插层锂的热力学扩散系数增强有关。根据线性不可逆热力学全面概述了锂扩散通量的函数形式,并将其与通过传统表征技术获得的化学扩散系数联系起来。对固溶阴极活性材料的文献分析表明,虽然化学扩散系数随硫化状态的不同而变化很大,但根据线性不可逆热力学,存在一种方便的扩散通量函数形式,可使扩散系数随硫化状态的不同而相当稳定。这对建模和实验都有明显的好处,并有可能揭示固态扩散的机理基础。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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CiteScore
18.20
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