Solid-Solution or Intermetallic Compounds: Phase Dependence of the Li-Alloying Reactions for Li-Metal Batteries

Electrochemical Li-alloying reactions with Li-rich alloy phases render a much higher theoretical capacity that is critical for high-energy batteries, and the accompanying phase transition determines the alloying/dealloying reversibility and cycling stability. However, the influence of phase-transition characteristics upon the thermodynamic properties and diffusion kinetic mechanisms among the two categories of alloys, solid-solutions and intermetallic compounds, remains incomplete. Here we investigated three representative Li-alloys: Li-Ag alloy of extended solid-solution regions; Li-Zn alloy of an intermetallic compound with a solid-solution phase of a very narrow window in Li atom concentration; and Li-Al alloy of an intermetallic compound. Solid-solution phases undertake a much lower phase-transition energy barrier than the intermetallic compounds, leading to a considerably higher Li-alloying/dealloying reversibility and cycling stability, which is due to the subtle structural change and chemical potential gradient built up inside of the solid-solution phases. These two effects enable the Li atoms to enter the bulk of the Li-Ag alloy to form a homogeneous alloy phase. The pouch cell of the Li-rich Li20Ag alloy pairs with a LiNi0.8Co0.1Mn0.1O2 cathode under an areal capacity of 3.5 mAh cm(-2) can retain 87% of its initial capacity after 250 cycles with an enhanced Coulombic efficiency of 99.8 +/- 0.1%. While Li-alloying reactions and the alloy phase transitions have always been tightly linked in past studies, our findings provide important guidelines for the intelligent design of components for secondary metal batteries.

Automated summary

In this study, three representative Li-alloys were investigated, and it was found that solid-solution phases have lower phase-transition energy barriers compared to intermetallic compounds, resulting in higher Li-alloying/dealloying reversibility and cycling stability. These effects enable the formation of a homogeneous alloy phase and provide important guidelines for the intelligent design of components for secondary metal batteries.