Thermodynamic and Kinetic Properties of the Lithium–Silver System

A carbon–silver anode has recently been shown
to suppress
dendrite formation in all-solid-state lithium-ion batteries. The role
that silver plays in enabling the reversible deposition and stripping
of lithium remains unknown. Furthermore, very little is known about
the thermodynamic and kinetic properties of LixAg1–x alloys. Here, we report
on an in-depth first-principles study of phase stability and diffusion
mechanisms in the Li–Ag alloy system. We identify two new intermetallic
phases that are predicted to be stable in Li-rich LixAg1–x alloys with stoichiometries
of Li3Ag and Li11Ag2. Our calculations
show that the peculiar and highly anharmonic energy surface of pure
Li along the Bain and Burgers paths persists upon the addition of
Ag to BCC Li. This has important implications for room-temperature
phase stability and mechanical properties. We have also performed
a systematic study of diffusion mechanisms in the LixAg1–x alloy system as a
function of alloy concentration x. Diffusion in alloys
and intermetallics is mediated by vacancies. High vacancy formation
energies are predicted in the LixAg1–x alloy, especially in Ag-rich FCC
solid solutions. Complex diffusion mechanisms are identified in the
B2 and γ-brass intermetallic phases that include two-atom hops
and second-nearest neighbor hops. The migration barriers are found
to decrease with increasing Li concentration, with predictions of
exceptionally low migration barriers of 0.1 eV in the D03 Li3Ag phase.

近期研究证实，碳银负极可抑制全固态锂离子电池中的锂枝晶生长。但银在实现锂可逆沉积与剥离过程中所发挥的具体作用仍不明晰。此外，学界对LixAg1–x合金的热力学与动力学性质所知甚少。为此，本研究针对Li-Ag合金体系的相稳定性与扩散机制开展了深入的第一性原理研究。我们识别出两种全新的金属间化合物相，经预测，它们在化学计量比为Li3Ag与Li11Ag2的富锂LixAg1–x合金中呈稳定状态。计算结果显示，纯锂沿贝恩（Bain）路径与伯格斯（Burgers）路径的特殊且高度非简谐的势能面，在向体心立方锂（BCC Li）中掺入银后依然存在。这一发现对室温相稳定性与力学性能具有重要意义。我们还针对LixAg1–x合金体系的扩散机制，系统探究了其随合金浓度x的变化规律。合金与金属间化合物中的扩散过程由空位介导，研究预测LixAg1–x合金（尤其是富银的面心立方固溶体（FCC solid solution））中存在较高的空位形成能。在B2相与γ黄铜型金属间化合物相中，我们发现了包含双原子跃迁与次近邻跃迁在内的复杂扩散机制。研究发现，迁移势垒随锂浓度升高而降低，其中D03结构的Li3Ag相的迁移势垒极低，预测值仅为0.1 eV。