The polymer-composite binder used in lithium-ion battery electrodes must both hold the electrodes together and augment their electrical conductivity while subjected to mechanical stresses caused by active material volume changes due to lithiation and delithiation. We have discovered that cyclic mechanical stresses cause significant degradation in the binder electrical conductivity. After just 160 mechanical cycles, the conductivity of polyvinylidene fluoride (PVDF):carbon black binder dropped between 45-75%. This degradation in binder conductivity has been shown to be quite general, occurring over a range of carbon black concentrations, with and without absorbed electrolyte solvent and for different polymer manufacturers. Mechanical cycling of lithium cobalt oxide (LiCoO 2 ) cathodes caused a similar degradation, reducing the effective electrical conductivity by 30-40%. Mesoscale simulations on a reconstructed experimental cathode geometry predicted the binder conductivity degradation will have a proportional impact on cathode electrical conductivity, in qualitative agreement with the experimental measurements. Finally, ohmic resistance measurements were made on complete batteries. Direct comparisons between electrochemical cycling and mechanical cycling show consistent trends in the conductivity decline. This evidence supports a new mechanism for performance decline of rechargeable lithium-ion batteries during operation -electrochemically-induced mechanical stresses that degrade binder conductivity, increasing the internal resistance of the battery with cycling. Lithium-ion batteries (LIB) are an enabling energy storage technology for portable consumer electronics, electric vehicles and renewable power generation in part due to their high energy densities. The energy density is driven by not only the relatively large potential of lithium-ion chemistries, but also the ability of active materials to store large amounts of lithium.1 The most common graphitic carbon anode can absorb up to one lithium for every carbon atom. Recent research on higher capacity anodes such as silicon has highlighted an increased need for understanding the mechanics of lithium-ion batteries. As the lithium is shuttled between the anode and cathode, the active materials expand and contract to accommodate the lithium. The resulting volume changes are accentuated for high capacity materials such as silicon which can increase in volume by up to 400% during lithiation. 2Because most LIB electrodes are porous multicomponent composites, understanding the generation and impact of mechanical stresses on batteries can be difficult. The electrode is generally 50-75 vol% solid fraction with active material consisting of micron-sized particles held together by an active binder, which is itself a composite of conductive carbon particles and polymer. The performance of the battery is highly dependent on this complex structure which must allow efficient ion and electron transport through the electrode. The void space in the porous structure allows lith...
The contribution from loss of Li + inventory to capacity fade is described for slow rates (C/10) and long-term cycling (up to 80 cycles). It was found through electrochemical testing and ex-situ Raman analysis that at these slow rates, the entirety of capacity loss up to 80 cycles can be explained by loss of Li + inventory in the cell. The Raman spectrum of LiCoO 2 is sensitive to the state of lithiation and can therefore be leveraged to quantify the state of lithiation for individual particles. With these Raman derived estimates, the lithiation state of the cathode in the discharged state is compared to electrochemical data as a function of cycle number. High correlation is found between Raman quantifications of cycleable lithium and the capacity fade. Additionally, the linear relationship between discharge capacity and cell overpotential suggests that the loss of capacity stems from an impedance rise of the electrodes, which based on Li inventory losses, is caused by SEI formation and repair. Lithium ion batteries (LIB) provide the highest energy densities compared to other rechargeable battery types.1 For this reason, they are being pursued for usage in advanced applications such as electric vehicles and storage of intermittent renewable energies (solar, wind, etc.). Despite their promise, degradation caused by both chemical and physical mechanisms diminishes LIB performance. Chemically, formation of a solid electrolyte interphase (SEI) on the surface of both the graphite anode and the LiCoO 2 cathode accompanied by other side reactions between the electrolyte and the electrode surface 2 results in the loss of cycleable Li + and thus capacity. These surface films also increase electrode impedance, which can shorten the time for the cell to reach cutoff voltages and affect rate capability of the cell. 3,4 Capacity loss can also originate due to physical mechanisms. For example, stress may accumulate in electrode materials during lithiation and delithiation owing to the large volumetric alterations induced by these processes. 4,5 Volumetric expansion can be up to 10% in graphite and ∼1.6% in LiCoO 2 .6,7 Such expansion and the induced stress can, in turn, lead to fracturing of active particles, cracking and reforming of the SEI, and loss of contact among electrode components (i.e., active material, polymeric binder, and current collectors). 4 Taken together, these chemical and physical degradation mechanisms induce capacity fade with repeated cycling that curtail the lifetime of the battery.Beyond identification, it is necessary to assess the relative influence of individual mechanisms to the totality of capacity fade. 3,8,9 Specifically, comparisons between the relative impact of several mechanisms-primary active material loss (Li + inventory), secondary active material loss (LiCoO 2 and/or graphite), and increased internal cell resistance (caused by surface films)-remain the topic of continued investigation.3 Such effort is complicated by the dependence of these mechanisms to several parameters as it has been...
The study of various degradation mechanisms in lithium ion batteries (LIB) that lead to capacity fade during cycling is of primary interest to the future progress of consumer electronic products, including the desire for advancements in hybrid electric vehicles. Considering that significant improvements to performance properties such as cycle life and energy density are crucial; it is necessary to advance the understanding of electrode and electrolyte materials, and the physical and chemical processes that ultimately lead to capacity fade in commercial LIB. By quantifying the degradation processes that occur within LIB, the predictive capability in modeling battery performance will improve. This work introduces a novel approach to studying, in-situ, the degradation mechanisms of the cell as a whole with focus on the LiCoO2 electrode. The research involves utilizing fluorescence confocal microscopy to observe direct changes on the mesoscopic structural level as well as changes in the electrochemical cell performance in order to quantify the mechanical stresses that develop due to various processes (SEI formation, electrode expansion/ contraction, external loading, etc.). It is expected that this work will realize, for the first time, the effect of stress on particle configuration over time, loss of electronic contact to particles, and the role of binder in controlling and affecting stress evolution. In this report, we report validation testing of the use of fluorophores in LIB for studying the interactions among particles and binder during cycling. Cyclic voltammetry (CV) is used to determine the electrochemical stability of fluorescents in electrolyte of 1.2M LiPF6 EC:EMC (3:7 by wt). CV testing was performed within the potential window of an operating LiCoO2 cell; 0 to 4.2 V. Fluorescents that were tested include acridine, acriflavine, boron-dipyrromethene (BODIPY FL), fluorescein, naphthofluorescein, quinine, rhodamine 6G, and rhodamine B. Figure 1 shows the CV results for all fluorescents. Many of these dyes are not suitable for use in LIB due to strong reduction and/or oxidation peaks within the potential window. The most stable dyes that are best suited for use in LIB are fluorescein and naphthofluorescein. Meanwhile, coin cell testing is performed to ensure that the addition of fluorescents to the electrode and electrolyte materials does not adversely affect the long-term performance of the cell. Coin cells were made with and without naphthofluorescein and fluorescein added to the electrolyte and binder, respectively. Cells are cycled for a minimum of 100 charge-discharge cycles at a C/10 charge and discharge rate. In addition to performing validation testing and ensuring that dyes do not leach from their tagged component, preliminary imaging of the dyed cell using the confocal microscope is done in preparations for in-situ imaging and testing. Sandia National Laboratories is a multi-program 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. Figure 1
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