Lithium dendrite and solid electrolyte interphase investigation using OsO4

Separators play an important role in lithium-ion battery operation; however, no comprehensive studies of their microstructure and its impact exist. To enable such studies, we present a simple method for separator microstructure visualization and quantification based on focused-ion-beam scanning electron microscopic tomography. Here, we use this approach to visualize a sample of commercial polyethylene separator, calculate its directional effective transport parameters, and explain the impact of these values on battery performance. We further extend this technique to visualize metallic deposition within the separator, which could facilitate the study of lithium plating and dendritic growth. Separators in Li-ion batteries (LIBs) prevent electronic contact between positive and negative electrodes while allowing ionic transport between them.1 In the event of cell failure, separators limit ion transport to avoid internal short-circuiting and thermal runaway.2 To achieve good transport and suitable mechanical, thermal, and chemical properties, LIB separators are commonly <25 μm-thick sheets consisting of a complex 3D structure featuring 40% porosity and submicron pore sizes.2 Though they are routinely characterized by thickness, pore size, porosity, as well as thermal and electrical properties, 3 there has been no quantitative measurement of their 3D structure. Microstructural data would be particularly helpful in light of recent studies that link separator structure to the likelihood of LIB failure (e.g., through Li dendrite growth 4 ) and changes in separator structure (e.g., through pore closure by mechanical stress and chemical degradation 5,6 ) to aging effects in LIBs. Here, we present a simple procedure for visualizing and quantifying separator microstructures. We use simulations to assess the implications of separator microstructure on battery performance and extend our technique to examine lithium metal deposition within the separator. ExperimentalWe employ focused ion beam scanning electron microscopic (FIB-SEM) tomography ( Figure S1 in the Supplementary Information 7 ), a technique that has been used for visualizing and quantifying of LIB electrode microstructures. 8,9 Figures 1a-1b show top view SEM images of polyethylene (PE) and polypropylene (PP) separators from Targray (PE16A and PP16). SEM images of cross-sectional cuts through the same separators (Figures 1c-1d) taken in the FIB-SEM configuration show pore edge contrast and depth information from the pores. Because these effects complicate image processing and quantification of the 3D microstructure, we in-fill the separator to enable imaging of a planar surface.Quantification of the 3D microstructure also requires sufficient contrast between the polymer skeleton and the in-filled pores. As a staining agent, we use OsO 4 , 10 which has previously been used with Celgard microporous membranes 11 as well as lithium dendrites and solid electrolyte interphases of graphite electrodes.12 PE and PP are inert toward OsO 4 staining, but OsO 4 reacts with unsa...

Section: Resultsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Communication—Technique for Visualization and Quantification of Lithium-Ion Battery Separator Microstructure

Lagadec

Ebner

Zahn

et al. 2016

“…The inset of Figure 4d shows dendritic structures which are typical for Li metal deposition on graphite anodes. 35,49,50 Figure 4e shows the GD-OES analysis of the cell cycled at 45…”

Section: Post-mortem Analysis Reveals Different Aging Mechanisms In 325mentioning

confidence: 99%

Electrochemical, Post-Mortem, and ARC Analysis of Li-Ion Cell Safety in Second-Life Applications

Waldmann

Quinn

Richter

et al. 2017

Li-ion cells are used in a variety of mobile and stationary applications. Their use must be safe under all conditions, even for aged cells in second-life applications. In the present study, different aging mechanisms are taken into account for accelerating rate calorimetry (ARC) tests. 18650-type cells are cycled at 0 • C (Li plating expected) and at 45 • C (SEI growth expected). After extensive evaluation of the electrochemical results (voltage curve analysis, capacity fade, energy fade, Coulombic efficiency), the cells are tested by PostMortem analysis (CT, GD-OES, SEM) to reveal the main aging mechanisms and by ARC to test the safety behavior. Besides typical ARC results such as onset-of-self-heating, onset-of-thermal runaway and maximum temperatures, as well as acoustic responses of thermal runaway are evaluated and a method is developed to compare fresh cells and cells aged until different SOHs. It turns out that the safety of aged cells is not simply a function of the SOH. However, safety is strongly affected by the main aging mechanism and to the history of operating parameters during the life-time of the cell. Driven by the demand for higher capacity, lower volume, and lower weight, Li-ion technology was developed in the 1980s.1,2 Nowadays, Li-ion technology is used for energy storage in a large variety of applications, e.g. smartphones, tablets, or drones. In particular, electric vehicles in combination with renewable energy sources are promising for reducing climate change and the corresponding glacier melting and sea level rise which is influenced by burning fossil fuels.3-6 For all applications, the safety behavior under all specified operating conditions is most important.7-9 The same is true for aged Li-ion cells used in second-life applications, for example cells which are utilized in stationary energy storage applications after their usage in electric cars.In the past, failure of Li-ion cells led to product recalls which are very expensive for the manufacturers and unsettle customers, even if such incidents happen only in very few cases (ppm range).10 At the moment, safety tests have to be performed with Li-ion prototypes cells before market release. In these tests, fresh cells are always used, however, aged cells are usually neglected. Even if fresh cells show an acceptable safety behavior, this can change for aged cells. 7,[11][12][13][14][15] While some authors observed decreases for certain safety properties, 7,11,12 others found improvements. 12,13,15 It is well known that aging mechanisms change the properties of the materials inside Li-ion cells. [16][17][18][19] Operating conditions and materials have a strong influence on these aging mechanisms. 12,[20][21][22][23] We recently reported on a change of the aging mechanism with temperature for cycling aging of commercial 18650-type cells with graphite anodes and NMC/LMO cathodes. 20 This mechanism change is expressed by a V-shape in Arrhenius plots of the capacity fade rates obtained from cycling at different ambient temperatures. 20 This c...

“…123 Zier et al showed that it is possible to enhance the material contrast of Li deposition on graphite electrodes by reaction with OsO 4 . 119 The study of other anode materials such as Li 4 Ti 5 O 12 by SEM has so far not given precise information on the degradation mechanisms. 124 On the cathode side, often no changes are visible by SEM imaging between pristine and aged cathodes 16,40,125 (see lower part of Figure 7).…”

Section: 121mentioning

confidence: 99%

“…119,136,137 Therefore, TEM unveils sample characteristics in terms of particle morphology, crystallinity, stress or even magnetic domains. However, due to the higher energy, beam damage has to be taken into account for battery materials.…”

Section: 121mentioning

confidence: 99%

Review—Post-Mortem Analysis of Aged Lithium-Ion Batteries: Disassembly Methodology and Physico-Chemical Analysis Techniques

Waldmann

Iturrondobeitia

Kasper

et al. 2016

214

149

Improvement of life-time is an important issue in the development of Li-ion batteries. Aging mechanisms limiting the life-time can efficiently be characterized by physico-chemical analysis of aged cells with a variety of complementary methods. This study reviews the state-of-the-art literature on Post-Mortem analysis of Li-ion cells, including disassembly methodology as well as physicochemical characterization methods for battery materials. A detailed scheme for Post-Mortem analysis is deduced from literature, including pre-inspection, conditions and safe environment for disassembly of cells, as well as separation and post-processing of components. Special attention is paid to the characterization of aged materials including anodes, cathodes, separators, and electrolyte. More specifically, microscopy, chemical methods sensitive to electrode surfaces or to electrode bulk, and electrolyte analysis are reviewed in detail. The techniques are complemented by electrochemical measurements using reconstruction methods for electrodes built into half and full cells with reference electrode. The changes happening to the materials during aging as well as abilities of the reviewed analysis methods to observe them are critically discussed. Li-ion batteries are currently used in everyday objects such as smart-phones, power tools and tablet computers as well as in the growing fields of light electric vehicles (LEVs), unmanned aerial vehicles (UAVs), battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). [1][2][3][4] Furthermore, the rise of renewable energy sources like wind and solar power, which are only intermittently available, demands reliable and highly flexible stationary energy storage solutions, which provide high capacities and predictable life-times. 2,5Aging of Li-ion batteries is a general problem for manufacturers as they have to guarantee long-term reliability of their products. For state-of-the-art cells, degradation effects on the material level lead to capacity fade and resistance increase on the cell level. The aging state of a battery is often characterized by the state-of-health (SOH) in % according to 3,16,22,[29][30][31] SO H(t) = discharge capacity (t) discharge capacity (t = 0)[1]where t represents the aging time. In general, one has to differentiate between cycling 7,16,18,21,[23][24][25]32 and calendar aging. 7,19,[21][22][23][24]27 Since commercial Li-ion cells can be subject to calendar aging in the time between manufacturing and delivery, it is good practice to measure the discharge capacity at t = 0 for each cell that undergoes an aging test. Since the discharge capacity depends mainly on temperature, depth-of-discharge (DOD), and discharge current, the SOH is usually monitored by regular check-ups with defined parameter sets, 7,16,21,23,24 which can vary depending on the application. Typically, a temperature of 25• C, 16,22,24 DOD of 100%, 16,21 and discharge rates of 1C 7,16,21,22,24 or lower 23 are used in check-ups. The performance dec...