Improving the Cycling Life of Aluminum and Germanium Thin Films for use as Anodic Materials in Li-Ion Batteries.

The cycling of high-capacity electrode materials for lithium-ion batteries results in significant volumetric expansion and contraction, and this leads to mechanical failure of the electrodes. To increase battery performance and reliability, there is a drive towards the use of nanostructured electrode materials and nanoscale surface coatings. As a part of the Visiting Faculty Program (VFP) last summer, we examined the ability of aluminum oxide and gold film surface coatings to improve the mechanical and cycling properties of vapor-deposited aluminum films in lithium-ion batteries. Nanoscale gold coatings resulted in significantly improved cycling behavior for the thinnest aluminum films whereas aluminum oxide coatings did not improve the cycling behavior of the aluminum films. This summer we performed a similar investigation on vapor-deposited germanium, which has an even higher theoretical capacity per unit mass than aluminum. Because the mechanism of lithium-alloying is different for each electrode material, we expected the effects of coating the germanium surface with aluminum oxide or gold to differ significantly from previous observations. Indeed, we found that gold coatings gave only small or negligible improvements in cycling behavior of germanium films, but aluminum oxide (Al2O3) coatings gave significant improvements in cycling over the range of film thicknesses tested. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. SAND2014-17662R Improving the Cycling Life of Aluminum and Germanium Thin Films for use as Anodic Materials in Li-Ion Batteries 2 Gerald Gulley Introduction: Nanostructured anode and cathode materials offer some of the best promise for dramatically increasing the performance (energy and power density) of Li-ion batteries.[1] One of the many advantages of nanostructuring is that nanostructures are able to accommodate larger volumetric changes during Li insertion and de-insertion than bulk materials.[2] This enables the use of high-capacity electrode materials that require large volume increases for full lithiation. Further research is needed to evaluate and improve the mechanical performance and reliability of nanostructured battery materials before they can be utilized in commercial battery systems. One approach to improve cycle life is to form nanocomposite electrodes. An intriguing example of this is the use of atomic-layer deposited (ALD) aluminum oxide layers, Al2O3, applied to anode and cathode phases, as described by the Dillon and George groups.[3] When these Al2O3 layers are extremely thin, they apparently act as a solid electrolyte, permitting ion transport while also providing mechanical support to the electrodes. Aluminum oxide is not thermodynamically stable at low potentials, so the aluminum oxide phases should convert into a stable Li-Al-O phase when polarized at potentials close to Li 0 . At the Center for Integrated Nanotechnologies (CINT), we have seen evidence for this transformation through the use of in situ transmission electron microscopy (TEM) of aluminum oxide layers on Al nanowires that were polarized at potentials close to Li 0 .[4] During lithiation, the aluminum oxide layer converted to a rigid Li-AlO layer, which was at least partially responsible for the poor cycling behavior observed. Aluminum and germanium have high theoretical capacities for electrochemically alloying with lithium: 993 mAh/g and1680 mAh/g, respectively. However, both suffer from poor cycling. Because of this, we chose as our objective of this proposal to examine the ability of aluminum oxide and gold coatings to improve the mechanical and cycling properties of these high-capacity anode materials. As a part of the Visiting Faculty Program (VFP) last summer, we observed that a 5-nm layer of gold significantly improved the cycling behavior of aluminum films of submicron thickness, but the addition of aluminum oxide coatings did not improve the cycling properties of aluminum. This improvement was attributed to the absence of a native aluminum oxide layer, and thus the absence of the rigid Li-Al-O layer, on gold-coated aluminum. This summer, we found the reverse for our germanium samples. The addition of a 5nm layer of gold did little to enhance the cycling behavior of germanium films, but the aluminum oxide coatings significantly improved their cycling behavior. Hypothesis and research objectives: The cycling of Li into high capacity Li-ion battery electrode materials results in volumetric expansion and contraction that leads to mechanical failure of the electrodes and capacity loss in the battery. Our results from last summer showed promise for improving the mechanical behavior of high-capacity anode materials for Li-ion batteries. In the case of the aluminum films studied last summer, the gold coating acts as an oxygen barrier preventing the formation of an oxide layer. The dramatically improved cycling behavior can be attributed to decreased rigidity in the surface layer. Because surface characteristics are closely related to the functionality of all nanomaterials, the application of surface coatings to germanium materials are likewise expected to significantly alter their behavior in lithium-ion cells. Improving the Cycling Life of Aluminum and Germanium Thin Films for use as Anodic Materials in Li-Ion Batteries 3 Gerald Gulley The objectives of this proposal were to: 1. examine the ability of thin gold and Al2O3 coatings to change the mechanical and cycling properties of germanium thin-film electrodes in lithium-ion batteries; 2. Reach a better understanding of the role of surface composition in stabilizing or destabilizing the cycling of lithium-ion anodes, 3. (if time and resources allow) examine the cycling behavior of other germanium nanostructures (e.g. nanoparticles or nanowires) with or without coatings. R&D approach: The same experimental techniques from last year’s VFP project were applied to this project. We deposited germanium thin films of various thicknesses (50 nm, 100nm, 0.25 μm, and 1.25 μm) onto copper substrate disks. The copper disks were made from copper foil obtained from Alfa Aesar (0.25 mm thick, 99.99985% purity) and were de-oxidized prior to use. The germanium films were prepared by electron-beam (E-beam) deposition onto the copper disks. Some of these germanium films were kept bare while other samples were coated further either with 5nm of gold or 5nm of Al2O3 . The gold layer was deposited using E-beam deposition, immediately after germanium deposition, with the sample maintained under high vacuum during and between both depositions. The Al2O3 layer was applied to bare germanium films using Atomic Layer Deposition (ALD). Two-electrode electrochemical cells were made using Swagelok fixtures assembled in a glove box[5]. The germanium films on copper disks formed the working electrodes while lithium foil pressed onto steel disks formed the counter and reference electrodes. A glass microfiber separator (Whatman GF/D) soaked with lithium electrolyte is placed between the two electrodes. The cycling behavior of germanium films was characterized by galvanostatic (constant-current) cycling techniques on PAR 263A potentiostats. In this type of cycling, the voltage is monitored as a function of time. The voltage ranges over which lithium electrocehmically alloys with germanium were observed, and the capacity of the cell during subsequent charges and discharges was measured. The measured capacity is the amount of electrical charge that, in this case, the anode material can deliver. In the cycle life of a battery cell with a lithium-alloy anode, there is capacity loss over time as it is charged and discharged (cycled) due to pulverization of the anode material from volumetric expansion. Extending the capacity of electrode materials over many periods of charge and discharge is desirable and is the ultimate goal of this research. A cycling rate of 1C was used, which corresponds to one theoretical charge or discharge of the cell in 1 hour. Cycling behavior of the bare, gold-coated, and ALD-modified germanium films was compared to observe any differences in capacity and cycling stability. Results: The alloying and de-alloying reaction between germanium and lithium can be observed through the voltage profile during galvanostatic cycling. Potential profiles for a 1.25-μm bare germanium sample and a 1.25-μm germanium sample with Al2O3 coating during galvanostatic cycling are shown in Figure 1. As shown, the alloying and de-alloying reactions occurred largely between 0.1 V and 0.6 V vs. Li/Li + . The general shape of the voltage curve remained the same with repeated cycling, which shows that the lithiation/de-lithiation mechanism remained the same. However, the length of time for each charge or discharge decreases with cycling, as Improving the Cycling Life of Aluminum and Germanium Thin Films for use as Anodic Materials in Li-Ion Batteries 4 Gerald Gulley the active material (germanium) pulverizes and loses electrochemical activity. Comparison between the voltage profiles in Figure 1a and Figure 1b shows that the length of time for a charge or discharge is longer over more cycles for the Al2O3-coated germanium. From these figures, we can also see that all 26 cycles for the 1.25 μm bare germanium sample have been completed in 10.4 hours whereas only 11 cycles have been completed for the Al2O3-coated germanium in this amount of time. This indicates that the Al2O3-coated sample is retaining more of its capacity over a longer period of time. Figure 1. Voltage vs. Time for bare and Al2O3 -coated 1.25 μm germanium samples. The width of the charge/discharge curves are wider over a longer period of time for the Al2O3coated 1.25 μm germanium sample indicating better rete