Internal Morphologies of Cycled Li-Metal Electrodes Investigated by

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Internal Morphologies of Cycled Li-Metal Electrodes Investigated by Nano-Scale Resolution X-Ray Computed Tomography Sarah Frisco, Danny Liu, Arjun Kumar, Jay F. Whitacre, Corey T. Love, Karen Swider-Lyons, and Shawn Litster ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 09 May 2017 Downloaded from http://pubs.acs.org on May 10, 2017

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ACS Applied Materials & Interfaces

Internal Morphologies of Cycled Li-Metal Electrodes Investigated by Nano-Scale Resolution X-Ray Computed Tomography Sarah Friscoa‡, Danny X. Liub,d‡, Arjun Kumarc, Jay F. Whitacrea, Corey T. Loveb, Karen E. Swider-Lyonsb, Shawn Litsterc,* ‡These two authors contributed equally to the manuscript. a

Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA

15213, USA b

Chemistry Division, U.S. Naval Research Laboratory, Washington, D.C. 20375, USA

c

Department Mechanical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA

d

NRL/NRC Post-doctoral Research Associate

KEYWORDS: X-ray computed tomography, lithium battery, lithium metal anode, morphology, cycling

ABSTRACT

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While some commercially available primary batteries have lithium metal anodes, there has yet to be a commercially viable secondary battery with this type of electrode. Research prototypes of these cells typically exhibit a limited cycle life before dendrites form and cause internal cell shorting, an occurrence that is more pronounced during high rate cycling. To better understand the effects of high rate cycling that can lead to cell failure, we use ex-situ nano-scale resolution X-ray computed tomography (nano-CT) with the aid of Zernike phase contrast to image the internal morphologies of lithium metal electrodes on copper wire current collectors that have been cycled at low and high current densities. The Li that is deposited on a Cu wire and then stripped and deposited at low current density appears uniform in morphology. Those cycled at high current density undergo short voltage transients to >3 V during Li-stripping from the electrode, during which electrolyte oxidation and Cu dissolution from the current collector may occur. The effect of temperature is also explored with separate cycling experiments performed at 5oC and 33oC. The resulting morphologies are non-uniform films filled with voids that are semispherical in shape with diameters ranging from hundreds of nanometers to tens of micrometers, where the void size distributions are temperature dependent. Low temperature cycling elicits a high proportion of sub-micrometer voids, while the higher temperature sample morphology is dominated by voids larger than 2 µm. In evaluating these morphologies, we consider the importance of non-idealities during extreme charging, such as electrolyte decomposition. We conclude that nano-CT is an effective tool for resolving features and aggressive cycling – induced anomalies in Li films in the range of 100 nm to 100 µm.


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INTRODUCTION Lithium metal is an energy-dense anode used in commercial, primary lithium batteries. However, careful stripping and deposition of lithium and the use of protective solid state layers1 is commonly required to achieve rechargeable (secondary) systems (ie Li-S and Li-O2 batteries). Eqs. 1 and 2 show the reactions for lithium-metal oxidation during discharge and the lithium-ion reduction during charging, respectively. During battery discharge, the oxidation of lithium metal to Li+, Eq. [1], is kinetically limited due to impeded charge transfer. Inversely, the electroplating charging reaction in Eq. [2] can lead to the formation of uncontrolled dendritic Li metal structures due to bulk and interfacial mass and charge transfer processes2-3. To minimize the impact of these kinetic and mass resistances, secondary lithium-metal batteries in traditional electrolytes are typically only stable during low rate charging, limiting their usefulness. Li0

Li+ + e-

[1]

Li+ + e-

Li0

[2]

Most of the safety concerns with lithium and lithium-ion batteries have been related to the charging process in Eq. [2], during which metallic lithium dendrites can form. Lithium dendrite growth during charging can puncture the separator, shorting the anode and cathode, which leads to rapid heating that can result in battery fires and explosions4. A second issue with dendrite growth is the generation of significant new surface area that is passivated by additional solid electrolyte interphase (SEI) formed by electrolyte breakdown, which consumes the electroactive Li+ and reduces cell capacity 5 6. Cycling at low temperatures or high current densities appears to exacerbate non-uniform electrodeposition and dendrite growth of Li7 8 9 10. These morphologies have been often described as initially mossy before transitioning to dendritic11,


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consisting of aggregate deposits and tangled wires 12 or protuberances transitioning to metallic filaments 13. The progression of morphology is often current density dependent but is also rooted in nucleation and growth processes consisting of thermodynamic and kinetic driving forces that should be heavily influenced by temperature14. Using optical microscopy with resolution on the order of tens of micrometers and an in-situ Li0|Li0 electrochemical cell, Love et al.7 attempted to quantify the models of Alkokar15, which predicted the temperature-dependent growth of dendrites based on the competition of kinetic vs mass transport factors in the growth mechanisms. They found that the initiation rate and morphology of the Li dendrites depended on temperature, and that charging the Li-electrodes at temperatures around 5°C was the most likely condition to form hazardous dendrites.

While the effects of higher rate charging are being thoroughly studied, work is also needed to study the limits on battery discharging when the Li is stripped from the anode (Eq [1]). Even if a battery is deemed safe for use at low rates, anomalies in the charging and discharging can occur due to cell imbalances and even problems with the battery management system, which can allow voltage transients to occur.

Several techniques have been reported for imaging the morphology of Li electrodes, mainly around the purpose of imaging dendrites formed during charging. Some of these methods include: scanning transmission electron microscopy (STEM) for 2D images with resolution in the nanometer regime16 17 , scanning electron microscopy-focused ion beam (SEM-FIB) where the sample cross-sections are obtained through destructive ion milling18 19 , 7Li magnetic


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resonance imaging (MRI) and nuclear magnetic resonance (NMR) where the Li chemical shifts are correlated and used to determine the local microstructures20 21 , and micro-scale resolution xray computed tomography (micro-CT)22 23 24 25 26. Micro-CT allows for the 3D morphologies of internal structures to be non-destructively reconstructed with a resolution of 1 µm and can be performed on samples sealed in gas or liquid environments. 22-23 24 25 26. Previous micro-CT studies have revealed the existence of subsurface structural changes during electrodeposition of Li, but the origin and details of the growth were difficult to determine at this resolution22. It is important to note that the prior micro-CT Li imaging studies have all been done on synchrotron beamlines with phase contrast imaging for low atomic number (Z) materials. Because of its low Z of 3, Li metal is almost completely transparent in conventional X-ray absorption contrast imaging. Phase contrast provides a high contrast representation of the metal surface, but requires complex model-based corrections for volumetric contrast for image segmentation25, 27.

In this paper, we use a laboratory X-ray nano-CT to study the 3D structure of Li-metal electrodes that have been cycled at different temperatures and current densities ex situ. The laboratory system has a maximum optical resolution of 50 nm (16 nm voxels) with a 16 µm field of view. However, here we use optics for 150 nm resolution with 130 nm voxels that provide a larger 65µm field of view for imaging the post-mortem Li samples grown on 80 µm diameter copper wires. In its higher resolution mode, the nano-CT could be used to visualize dendrite nucleation sites (which Stark et al28 determined by SEM observation to have length scales on the order of 100 nm), but here we use the larger field of view to investigate the effects of cycling on the electrode morphology. The first objective of this study is to demonstrate that nano-CT with the aid of Zernike phase contrast optics for low Z materials can resolve Li at this scale. The second


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objective of this study is to gain insight into the effect of temperature (5 vs 33 °C) and current density (2.4 vs 40 mA cm-2) on the morphology of cycled Li electrodes. The two current densities are chosen to reflect a low current density (LCD) where Li-stripping and deposition should be uniform and high current density (HCD) where there can be mass transport and kinetic limitations. Rather than studying a single constant current electrodeposition typical of the prior imaging work, the lithium morphologies are the result of 30 galvanostatic charging and discharging cycles in order to mimic Li-metal battery operation. In our HCD experiments, we encountered significantly increased impedance with stripping that have been previously attributed the formation of ‘dead lithium’. In these cases, the cell voltage was allowed to rapidly rise from the stable potential (~1 V vs. Li/Li+) to high potentials (3-6 V vs. Li/Li+) over 1-2 s before the Li stripping portion of the cycle was terminated. This is consistent with abusive charging, particularly in battery packs, which can induce electrolyte decomposition and copper current collector dissolution. The resulting samples were then analyzed with the nano-CT, and the data was processed to correlate the effects of temperature and current density on the external and internal morphology of the deposited Li films.

EXPERIMENTAL Sample preparation As shown in Figure 1, the cycles of electrodeposition and stripping of Li were carried out on 80 µm diameter copper wire working electrodes (99.9%, McMaster-Carr) submerged 0.5 cm into in a beaker cell (20 mL) filled with 1M LiPF6 in battery grade EC:DEC 1:1 v/v (Aldrich). The counter/reference (CE/RE) electrode was a strip of 0.75-mm thick lithium metal (99.9%, Aldrich). An Ar-filled glovebox with continuous H2O and O2 monitoring was


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used to store all materials and contain the electrochemical experiments. H2O and O2 levels were measured to be

Internal Morphologies of Cycled Li-Metal Electrodes Investigated by

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