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Article Cite This: ACS Appl. Energy Mater. 2019, 2, 873−881
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Lithium Acetylide: A Spectroscopic Marker for Lithium Deposition During Fast Charging of Li-Ion Cells Marco-Tulio Fonseca Rodrigues,† Victor A. Maroni,† David J. Gosztola,‡ Koffi P. C. Yao,† Kaushik Kalaga,† Ilya A. Shkrob,† and Daniel P. Abraham*,† †
Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States Nanoscience and Technology Division, Argonne National Laboratory, Argonne, Illinois 60439, United States
‡
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S Supporting Information *
ABSTRACT: Rapid charging of lithium-ion batteries being developed for electric vehicles is a formidable challenge. Electrochemical polarization of cells during fast charging favors deposition of metallic Li onto the surface of the graphite electrode, and this Li plating compromises safety and accelerates performance degradation. Observing the onset of Li nucleation is essential for elucidation of mechanisms and defining conditions favoring this Li plating, but presently available methods are not sufficiently sensitive and selective while also allowing satisfactory spatial resolution. Here we demonstrate the use of Raman spectroscopy as a sensitive means to identify Li nucleation and map Li deposition. Metallic Li is detected indirectly by probing the vibrations in an acetylide species (represented as LiCCX) that is formed on the exposed surface of Li nuclei in contact with the solid electrolyte interphase on graphite. Surface-enhanced Raman scattering (SERS) involving this species on Li nuclei appears to dramatically increase sensitivity and selectivity of this detection, making our method an excellent complement to the existing spectroscopy and microscopy approaches for determining Li deposition. KEYWORDS: Raman spectroscopy, SERS, fast charge, lithium plating, graphite bulk.3,4 The resulting Li deposits may assume a dendritic morphology and propagate through microscopic pores in typical polymer separators causing a short circuit.5,6 Such electrical shorts generate high currents and rapid temperature rise7 and have been responsible for numerous accidents. Less extreme levels of lithium plating can also contribute to performance degradation. Exposed Li has a highly reducing surface, and it rapidly reacts with the electrolyte after being deposited onto the Gr electrode.8 The reaction yields a solid electrolyte interphase (SEI) compositionally similar to the one found on lithiated graphite, which passivates the Li surface and prevents further decomposition of electrolyte.9 The formation of this protective layer contributes to capacity loss, as the Li+ containing ionic compounds in SEI deplete the inventory of cyclable Li+ in the cell. Another source of capacity loss is the plated lithium itself. Although some of the deposited Li can be stripped and reinserted into the cathode, a fraction of the plated Li becomes so encased by SEI that it is electronically disconnected from the electrode. These electrically isolated Li domains are referred to as “dead lithium”.7 This “dead lithium” is a safety concern, as it may enhance the energy released during battery failure10 and complicate the work of first
1. INTRODUCTION Li-ion batteries with graphite (negative) electrodes and layered oxide (positive) electrodes power most high-performing portable electronics and electric vehicles. While the steady increase in energy density1 has enabled mass adoption of this highly reversible Li-ion intercalation chemistry, there are still significant limits to the “user experience” that it can provide. These limits are especially relevant in the transportation sector, as the relatively long battery recharging times (hours) are a major impediment to a wider use of electric vehicles when compared to the minutes it takes to refuel a car at a gas station. Fully electric and hybrid vehicles have been projected to have a growing participation in the fleet, which may approach 25% of new-car sales by 2030.2 Reducing the gap between recharging and refueling time scales will lead to broader consumer acceptance of electric vehicles. The U.S. Department of Energy is dedicating resources to overcome this problem, having established a goal of developing Li-ion batteries that can achieve 80% recharge within 10 min.3 In an electrochemical cell, operation at high currents is limited by how quickly the charge carriers can be transported/ transferred within/across the different cell components (electrodes, electrolytes, and their interfaces). At high charging rates, direct deposition of metallic lithium onto the graphite (Gr) electrode surface becomes thermodynamically favorable, competing with the desired Li intercalation into the graphite © 2018 American Chemical Society
Received: November 13, 2018 Accepted: December 24, 2018 Published: December 24, 2018 873
DOI: 10.1021/acsaem.8b01975 ACS Appl. Energy Mater. 2019, 2, 873−881
Article
ACS Applied Energy Materials responders in accidents involving Li-ion batteries.3 Enabling fast charge thus necessarily involves identification of the exact conditions under which Li nucleation occurs on the Gr electrode. The direct detection of this nucleation, especially in situ, is challenging. Among the recent approaches, electron paramagnetic resonance (EPR) and 7Li nuclear magnetic resonance (NMR) spectroscopies allow the observation of microscopic domains of metallic Li directly with minimal interference from other phases.11,12 With the EPR spectroscopy, resonance signals from conduction band electrons in metallic Li domains are observed, whereas with the NMR spectroscopy, the resonance of 7Li spins interacting with these free electrons is distinguished from the 7Li nuclei present in all other phases. While these methods are both sensitive and selective, they do not probe spatial distribution of Li deposits. Conversely, electronic and optical microscopies can provide this spatial information under certain conditions,4 but these techniques are neither sensitive nor selective. Furthermore, such methods do not distinguish between the reversible and irreversible Li deposits, which need to be differentiated as they have different effects on capacity fade. This distinction is possible using electrochemical approaches.4,13,14 Nevertheless, while these latter methods are valuable, they offer no spatial resolution and have limited sensitivity and selectivity; furthermore, some of these approaches are only applicable at lower test temperatures.7 In this study, we sought a new technique to address this conundrum. Raman spectroscopy is used as a sensitive post mortem (and, potentially, operando) diagnostic for Li plating. Raman spectroscopy is an accessible tool that compares favorably with the magnetic resonance methods in terms of selectivity and sensitivity while providing excellent spatial resolution. The uniqueness of this method is that it relies on probing SEI on lithium, as opposed to detecting metallic Li directly. The emission originates selectively from lithium acetylide species (e.g., LiCCX and Li2C2) on small Li nuclei. These species yield an intense Raman band in a spectral region (1800 to 1900 cm−1) free of other spectral features and, hence, serve as a unique marker for the occurrence of Li deposition. Surface-enhanced Raman scattering appears to be amplifying this signal, thus making it a sensitive diagnostic of Li nucleation. Additional figures are provided as Supporting Information and are referenced herein using the designator “S”, as in Figure S1.
carbonate containing 1.2 mol/L lithium hexafluorophosphate (“Gen2,” supplied by Tomiyama). A microporous separator (Celgard 2325) was sandwiched between the electrodes. Each cell contained 40 μL of electrolyte, which is ∼6× greater than the total pore volume of materials in the cell.15 2.2. Electrochemical Testing. All electrochemical tests were conducted with a Maccor 2300 cycler. The cells underwent a single formation cycle at a rate of ∼ C/20 (where 1C designates the current corresponding to full charge in 1 h), and 4.4 and 3.0 V were set as the upper and the lower voltage limits for charge and discharge of the cell, respectively. The 1C rate was defined as the current required by a NCM523//Gr cell to be fully discharged in 1 h (1.52 or 2.02 mA/cm2 when thin or thick cathodes were used, respectively). One set of fullcells (containing the thinner NCM523 cathodes) was then exposed to cycling regimes that included higher rates in the charge step. In these experiments, cells were charged at a given rate until the capacity reached 2.2 mAh (capacity-limited cycling), rested at open circuit for 1 h, and discharged to 3.0 V at a C/5 rate. The cycles of fast charge and slow discharge were repeated to varying extents (up to seven times) and with charging rates increasing from 2C to 10C. Note that the 2.2 mAh capacity is only about 80% of the capacity available in the Gr electrode. Thus, the test protocol ensured extended exposure to high charge currents with the assurance that the Gr electrode would not be overfilled with lithium; note that the electrode potentials were allowed to polarize freely to achieve the capacity cutoff. After the final discharge, the cells were held at 3.0 V for 10−100 h to maximize lithium extraction from the graphite. In another set of experiments using the thicker NCM523 cathodes, the formed cells were deliberately exposed to conditions facilitating Li plating. These cells were cycled at a rate of C/10 with a capacitylimited charge step (corresponding to Gr lithiation) defined as 105% or 125% of the practical capacity of our Gr electrodes. The use of a thicker cathode was necessary to achieve these overlithiated states without damaging the NCM523 oxide. After all sites in the Gr were filled (100% capacity), the excess Li was expected to deposit in its metallic form. These cells were also held for several hours at 3.0 V after the final discharge. Reference electrode cells were also assembled to monitor electrode potentials during cycling at high rates. These cells contained Gr electrodes, thin NCM523 cathodes (34 μm) and a Li/Cu reference electrode sandwiched between the separators with the probing tip placed at the center of the electrode stack; detailed cell assembly is described in ref 16. Electrochemical tests were carried out between 3.0 and 4.39 V. Cells initially underwent three formation cycles (two C/10 cycles followed by a single C/25 cycle), and were subsequently exposed to different charging rates increasing from C/5 to 6C. After reaching the upper cutoff at each rate, the voltage was held constant until the current dropped below a C/25 rate, to ensure full electrode utilization. Discharge was always carried out at a slow rate of C/5. 2.3. Raman Spectroscopy. The Gr electrodes were extracted from the fully discharged cells. The cells were disassembled inside an argon-filled glovebox, and the electrodes dried for ∼1 h before being placed in a custom-assembled optical cell shown in Figure S1. Rinsing the harvested Gr electrodes with dimethyl carbonate to remove loose deposits and residual electrolyte had little effect on the Raman spectra, so most of the results reported here were obtained on samples that were not rinsed. The optical cell comprised a two-piece Teflon assembly (International Crystal Laboratories, Micro Quik Cell 00006-2093) housing a circular BaF2 window (10 mm diameter, 2 mm thick). An O-ring was used to minimize air leaks, and the harvested electrode was positioned between the window and a stainless steel backing disc (see Figure S1). The Raman spectra were obtained through the BaF2 window using a Renishaw inVia Raman microscope. Unless stated otherwise, 785 nm laser excitation was used in all experiments; some measurements using 532 nm laser radiation were also performed. The power density of the excitation used in all measurements was
