Morphology of electrodeposited tin alloys.

Lance Hoffman

University of Rochester
Materials Science Program
OPT 407: SEM Practicum
Spring 2015 Project


Contents

Introduction
Sample Preparation
Characterization
Conclusion
References

Introduction

The current generation of Li-ion batteries (LIBs) use carbonaceous anodes, a mature technology capable of achieving capacites close to its theoretical limit of 372 mAh/g. There has been much interest in increasing the capacity of LIBs by using anode materials with greater native capacities. Materials such as silicon or tin have higher theoretical capacities than carbon (4000 and 991 mAh/g, respectively). However, the lithiation cycle causes these materials to expand and contract, by as much as 400%, leading to cracking and degrading performance. One approach to mitigate the effects of volumetric expansion is to include a non-Li metal in an intermetallic compound. In our lab, we investigate the use of Ni (Li inactive) with Sn to improve the cycleability of LIB electrodes.1

It has been shown2 that the composition of electrodeposited alloys can be controlled via current density. The plating bath should have a significantly smaller concentration of the metal with the more positive reduction potential. While this one preferentially plates at small current densities, its ionic current is limited at large current densities, leading to higher a higher deposition rate of the counterpart metal.

In this project, I have used the scanning electron microscope (SEM) to examine the affect of current density and bath composition the morphology of several electrodeposited samples of Ni-Sn and Cu-Sn (copper is also inactive toward Li). The elemental compositions of the samples were evaluated using energy dispersive spectroscopy (EDS), and a cross section of one sample was created using the focused ion beam (FIB) to examine the grain structure of the deposit through its thickness.


Sample Preparation

Ni samples

A Watts bath (240 g/L NiSO4 · 6 H2O, 72 g/L NiCl2 · 6 H2O, and 25 g/L H3BO3) was used to electrodeposit Ni on a copper substrate in order to compare to the Ni-Sn codepositions. Galvanostatic depositions were done for five minutes at three current densities (2, 4, and 20 mA/cm2).

Ni-Sn samples

To attempt the codeposition of Sn with Ni, the Watts bath was modified with tin salts (19.5 g/L SnSO4, 6.8 g/L SnCl2 · 2 H2O). These concentrations give a molar Ni(II) to Sn(II) ratio of 10:1, while maintaining the 1.5:1 ratio of SO42− to Cl. Samples were plated using the same current densities for ten minutes. Additional Ni-Sn samples were prepared using a bath to which a small amount surfactant, Triton X-100, was added. Due to competitive adsorption on the electrode, surfactants can reduce hydrogen evolution during deposition,
3 but can also affect the morphology of the deposit. It was desired to see what affect a low concentration of this surfactant had on the shape of the Ni-Sn deposits.

Cu-Sn samples

A bath containing SnSO4 and CuSO4· 5 H2O, with a Sn(II) to Cu(II) ratio of 10:1, was used to create samples on Cu substrates. Low (0.08 mA/cm2) and high (4 mA/cm2) current galvanostatic depositions were performed for 1 hour and 10 minutes, respectively.

Characterization

Ni deposition: SEM

Figures 1, 2, and 3 show the results of Ni deposition from the Watts bath. The secondary electron micrographs were taken with the in-lens detector. Despite an order of magnitude difference in current density between the samples, their grain size was relatively similar.

Ni film, 2 mA/cm2
Figure 1: Ni film, 2 mA/cm2
Ni film, 4 mA/cm2
Figure 2: Ni film, 4 mA/cm2
Ni film, 20 mA/cm2
Figure 3: Ni film, 20 mA/cm2

Ni-Sn deposition: SEM, EDS

SEM images of deposits

Figures 4 through 8 show Ni-Sn electrodes prepared using the modified Watts bath, at different current densities. Even though these electrodes were prepared for the same time as the Ni samples, these films appeared much thinner. The micrographs show that rather than achieving uniform deposition, plating occured from isolated nucleation spots. The area density of nucleation sites increased with increasing current density. High magnification details (Figures 7 and 8) show that high current densities promote dendritic growth.

Ni-Sn deposition, 2 mA/cm2
Figure 4: Ni-Sn deposition, 2 mA/cm2
Ni-Sn deposition, 4 mA/cm2
Figure 5: Ni-Sn deposition, 4 mA/cm2
Ni-Sn deposition, 20 mA/cm2
Figure 6: Ni-Sn deposition, 20 mA/cm2

Ni-Sn deposition, 2 mA/cm2
Figure 7: Ni-Sn deposition, 2 mA/cm2
Ni-Sn deposition, 20 mA/cm2
Figure 8: Ni-Sn deposition, 20 mA/cm2

EDS spectra

EDS spectra were collected of the modified Watts bath samples (Figure 9). Apart from the signal from the Cu substrate, in the lower current density samples (2 and 4 mA/cm2) the only element with a measurable quantity was Sn. In the high current density sample (20 mA/cm2), we begin to see a small signal for Ni at about 7.5 keV. High current density favors the deposition of Ni in our system, though the amount of Ni deposited relative to Sn was expected to be much higher.

Ni-Sn EDS
Figure 9: Ni-Sn EDS

Ni-Sn with surfactant

The samples prepared from the bath with the added surfactant are shown in Figures 10 and 11. It is clear that a small concentration of surfactant has a large affect of the morphology of the deposit, givng both large flakes and thin, fibrous deposits.

Ni-Sn deposition with surfactant, 4 mA/cm2
Figure 10: Ni-Sn deposition with surfactant, 4 mA/cm2
Ni-Sn deposition with surfactant, 4 mA/cm2
Figure 11: Surfactant deposition detail.

Cu-Sn deposition: SEM, EDS, FIB

SEM images of deposits

Figures 12 through 14 depict the Cu-Sn samples plated at low current density (0.08 mA/cm2); Figures 15 through 17 show the high current density samples (4 mA/cm2). It is shown that low current densities favor the growth of larger crystallites that higher current densities.

Here we can also demonstrate the advantages of using the different detectors in the SEM. Figures 12 and 15 were taken with the in-lens detector; Figures 13 and 16 were taken with the SE2 detector, and the backscatter detector was used in Figures 14 and 17. We can see that the topography of larger features such as in the low current density samples look more natural using the SE2 detector. However, the higher contrast achievable by the in-lens detector is particularly useful for finely detailed grain structures as in the high current density sample. It is interesting to note that any difference in local atomic concentration was not shown by the backscatter detector.

Cu-Sn film, Inlens image
Figure 12: Cu-Sn, 0.08 mA/cm2, in-lens detector
Cu-Sn film, SE2 image
Figure 13: Cu-Sn, 0.08 mA/cm2, SE2 detector
Cu-Sn film, BSD image
Figure 14: Cu-Sn, 0.08 mA/cm2, BSD detector

Cu-Sn film, Inlens image
Figure 15: Cu-Sn, 4 mA/cm2, in-lens detector
Cu-Sn film, SE2 image
Figure 16: Cu-Sn, 4 mA/cm2, SE2 detector
Cu-Sn film, BSD image
Figure 17: Cu-Sn, 4 mA/cm2, BSD detector

EDS maps

An EDS map of the low current density sample was collected and shown in Figures 18 through 20. It can be seen that the deposit was not entirely uniform, with a Sn-rich region appearing in the bottom portion of Figure 20.

Cu-Sn image
Figure 18: Cu-Sn deposition, 0.08 mA/cm2.
Cu-Sn EDS Cu map
Figure 19: EDS Cu map.
Cu-Sn EDS Sn map
Figure 20: EDS Sn map.

FIB cross section

The FIB was used to mill a cross section in low current density Cu-Sn sample (Figures 21 and 22). The Cu substrate is the dark gray portion in the bottom of Figure 22. It is possible that the lighter region above this is a region of diffusion of Cu into the deposit. ImageJ was used to determine a deposit thickness of 1,300 nm, with an average grain diameter of 340 nm.

FIB cross section
Figure 21: FIB cross section.
FIB cross section detail
Figure 22: FIB cross section detail.


Conclusion

Electrodeposited samples of Ni-Sn and Cu-Sn were examined using several detectors in the SEM, and elemental analysis was performed with EDS. The FIB was used to create a cross section in a Cu-Sn sample, allowing for an examination of the grain structure through the thickness of the deposit. The techniques used have shown how our results differ from what was expected; specifically, how the nickel content of the codeposited samples was much lower than predicted. The insight we gain using SEM tools is invaluable as we refine our process.

I would like to thank Hanyuan Zhang and Brian McIntyre for their time and their help in learning the techniques presented here.


References

1. H. Mukaibo, Electrochem. Solid-State Lett. 10, A70 (2007)
2. S.D. Beattie, J.R. Dahn, J. Electrochem. Soc. 150, A894 (2003)
3. Xiao, F. Int. J. Miner. Metall. Mat., 20, 472 (2013)

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