Intermetallics for Anodes in Li-Ion Batteries: Sb-Sn-Ti Alloys

Abstract

Li-ion batteries (LIBs) are most frequently used in various handheld devices such as notebooks, cell phones or cameras. However, for load leveling and electro-mobility applications, key performance indicator parameters such as specific capacity, volumetric energy and power density, cyclability, lifetime, and safety have to be improved. In this regard, new electrode materials have to be designed for the next generation of LIBs. Among possible candidates for anode materials are intermetallic alloys which show significantly higher theoretical capacities than the currently used graphite. Additionally, such intermetallics are expected to be safer than graphite due to their higher lithiation potentials, which are sufficient to avoid the dendritic growth of lithium during charge.Many metals and alloys are able to form intermetallic phases with Li. For example, when Li17Sn4 is generated via lithiation of Sn, a theoretical charge capacity of 982 mAh per gram of Sn is obtained at the maximum Li-concentration of 81.31 at.% Li [1] (compare to 372 mAh per gram of carbon). A serious drawback of metallic active anode materials, however, is the mechanical degradation of the electrode upon cycling and the resulting short lifetime of the cell. The main reason for this is the embrittlement caused by the large volume changes on lithiation/delithiation (>180 %) starting from pure Sn. There are many attempts to overcome this phenomenon by, for example, adding carbonaceous fillers, using partial liquid intermetallic electrodes which show self-healing effects, or taking advantage of the in-situ precipitation of inactive matrix phases on lithiation. The latter can be achieved using multi-component Sb-Sn-Ti alloys instead of the pure metals Sb, Sn or binary Sb-Sn compounds. In such cases, full lithiation leads to the formation of Li3Sb and Li7Sn2 islands distributed in a fine grained inactive Ti-matrix. On de-lithiation, however, nano-crystalline Sb-Ti and Sn-Ti compounds are expected to form [2,3]. The Ti-based matrix buffers the volume expansion, mechanically stabilizes the electrode material and even improves its electronic conductivity compared to, e.g. carbonaceous filler materials.For the targeted design of such anodes, a fundamental understanding of the phase constitution, chemical thermodynamics and structural features of Li-Sb-Sn-Ti alloys is desirable. These data provide us with valuable information on the compounds which are formed during lithiation including their structures, stabilities and phase equilibria. In order to fully describe the behavior of such complex ternary and quaternary materials systems, self-consistent thermodynamic descriptions developed using the calculation of phase diagrams (CALPHAD) method are necessary. Such assessments allow us to not only calculate and graphically represent the phase equilibria but also enables the prediction of the electrochemical performance via calculation of the temperature-dependent e.m.f values for both stable and metastable states of the electrode. However, the simulative and predictive qualities of the thermodynamic models rely heavily on the generation of comprehensive experimental data, which are required as inputs for the assessment of the modelling parameters.As a first step, we studied the phase relations and crystal structures in the ternary Sb-Sn-Ti and Li-Sb-Sn sub-systems by means of XRD, thermal analysis and SEM/EDX. Isothermal sections at different temperatures are presented as well as the crystal structure of new ternary compounds. Based on this information, selected Ti-Sb-Sn alloys were processed into electrodes and subjected to electrochemical testing vs Li+/Li via galvanostatic cycling, cyclic voltammetry, and titration. Additionally, ex-situ XRD was applied to study the structural changes of the electrode active materials during the lithiation process.[1] Reichmann T., Gebert C., Cupid D., J. Alloys Compd., 2017, 714, 593[2] Sougrati M.T., Fullenwarth J., Debenedetti A., Fraisse B., Jumas J.C., Monconduit L., J. Mater. Chem., 2011, 21, 1069[3] Marino C., Sougrati M.T., Gerke B., Pöttgen R., Huo H., Menetrier M., Grey C.P., Monconduit L., Chem. Mater., 2012, 24, 4735