Li-alloying anodes⁠ are well-known to suffer large volumetric expansion during lithiation^⁠1,⁠^2⁠⁠.This causes compressive and shear stresses within the anode that leads to loss of electrical contact with the current collector, and hence poor cycle life. Several schemes have been employed to mitigate this effect. Approaches such as thin films, nanoparticles and other nanostructures^3,⁠⁴⁠ prevent the stress to reach the critical limit of material failure. Incorporation of other materials that act as a structural matrix has also been shown to prevent the Li-alloying (or “Li-active”) material from losing electrical contact⁶⁠. Most designs are optimized through a combination of such strategies.

Here, we report on a simple approach for improving cycle life of Li-alloying anodes by preparing a thin-film anode material with controlled stress distribution. This will be achieved by preparing thin-film anodes with graded composition: i.e., the concentration of the expanding active material increases with the distance from the current collector. Such graded materials are commonly applied for resisting deformation and damage at the contact between two different materials⁷⁠, and indeed, Tamura et al. have reported on improved adherence of active materials by gradient composition achieved through annealing^8⁠. However, to the best of our knowledge, this is the first report on using a single-batch approach that enables control over the gradient at will. In particular, focus will be placed on the effect of the magnitude of the gradient on the anode performance.

We apply an electrodeposition-based approach similar to that reported by Dahn et al.⁹⁠ where composition control is achieved by tuning the current density during deposition. Ni-Sn alloy film was chosen as a model system, where the Ni acts as Li-inactive structural matrix, and Sn acts as Li-alloying active material. In this approach, a plating bath with a high concentration of the metal with the more negative reduction potential (here, Ni) and a low concentration of the metal with more positive reduction potential (here, Sn) is prepared. Due to its more positive reduction potential, Sn is preferentially plated at lower current densities. However, as the current density is increased, Sn will become diffusion limited (due to its low concentration), and the composition of Ni will increase. In contrast to Dahn’s approach where the current was pulsed to achieve distinctively layered structures, our study will also report on results where current is gradually changed to achieve gradient structures.

Electrodeposited Ni-Sn films were characterized by scanning electron microscope (SEM), and the dependence of Sn concentration of the film on current density was established using energy dispersive x-ray spectroscopy (EDS). Within our current limits, we achieved a range of 13 to 47 at.% Sn in the deposits. Using these results, samples with different distributions of Ni-Sn composition were prepared and their depth profiles were characterized by x-ray photoelectron spectroscopy (XPS).

To further extend our understanding of the deposition phenomena, numerical analysis of the binary-alloy deposition model using Butler-Volmer kinetics was studied with COMSOL Multiphysics^® software. The results on differences in the internal stress induced by lithiation was also modeled and compared for the different sample configurations achieved experimentally.

Finally, we will report on the charge capacities and cyclability of the gradient-based Ni-Sn electrodes as lithium-ion battery anodes. We compare the performance of gradient electrodes to that of single-composition Ni-Sn electrodes of similar thickness and Sn content. We will discuss how the results of the numerical simulations of induced stress in the various electrode configurations relate to the different cycling characteristics of the electrodes seen experimentally.

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