Characterization of Advanced High-Energy Density Li-S Batteries by FE-AEM, SEM/EDS X-ray Spectral Imaging and Feature Sizing/Chemical Typing Techniques

Abstract

Environmentally clean technologies for sustainable energy generation and use are continuously demanding a next generation of long-cycle life high-energy density battery systems, which have higher capacity than existing electrochemical Li-ion and Li-ion/polymer batteries. A light-weight rechargeable cell based on the Li/elemental S redox couple has a highest theoretical specific capacity of 1670 mAh/g of active material and a specific energy of 2600 Wh/kg, assuming complete reaction 2Li. This Sion Li-S battery type utilizing liquid electrolytes with a specific energy 350 Wh/kg is superior to commercial type Li-ion batteries delivering specific energies 64-108 Wh/kg that demonstrated in 2007-2008 by setting two unofficial world endurance records for a flight by an unmanned Zephyr-6 aircraft (QinetiQ, UK) [2]. To allow a broader range of emerging applications such as electrical vehicles, performance of Li-S batteries at continuous high current discharge will be further improved via novel design of cells and electrode materials. Rechargeability and utilization of active material limited by the insulating nature of sulfur and Li 2 S and losses of active material in the form of insoluble polysulfides (PS) should be enhanced. Here we report results on combined structural and analytical characterization of commercially available unprotected Li foil anodes and sulfur-carbon cathodes down to the atomic scale by FE-AEM, FESEM, X-ray spectral imaging (SI) and feature sizing/chemical typing techniques performed to address several key problems for Li-S cells.3D-morphology, crystallography and elemental distributions over porous sulfur-carbon cathodes (FIG. 1). Sulfur-carbon cathodes exhibited extended aggregated and porous structures. Lowvoltage high-resolution FESEM enabled to characterize in detail surface morphology of faceted sulfur microcrystals, aggregated carbons and individual carbon particles down to 5-10 nm. EDS X-ray SI using drift compensation followed by data multivariate statistical analysis indicated an appearance of sulfur-enriched segregations on the scale of 10-15 µm. FESEM and HRTEM combined with SAED and EDS revealed crystalline structures and local compositions of sulfur and PS particles and Li 2 S nanocrystallites formed under cycling.Morphological and chemical classification of a cycled Li anode surface by automated feature sizing/chemical typing analysis (FIG. 2). The anode was fabricated using a 50.8 µm-thick commercially available unprotected Li foil. Thermo NORAN System SIX feature sizing and chemical typing analysis module provides flexible means to measure the surface feature size/shapes and count their number in selected anode areas from SE or BSE images using various morphological parameters (area, circularity, aspect ratio, etc.), filters and sieves to tailor desired analysis. Feature sizing analysis data evidently pointed to significant morphological changes of the anode surface under chosen cycling conditions. In addition, chemical typing allowed classifying of analyzed surface areas vs. cont...

Analytical Multimode Scanning and Transmission Electron Imaging and Tomography of Multiscale Structural Architectures of Sulfur Copolymer-Based Composite Cathodes for Next-Generation High-Energy Density Li–S Batteries

“…It could result in increased utilization of the electrochemically active poly(S- r -DIB) copolymer in the cathode and retention of this active material upon cycling. These findings are consistent with through-plane direct current (DC) conductivity measurements that show a slight increase in electrical conductivity of the cathode as the fraction of DIB content increases, despite the fact that DIB itself is electrically insulating (Oleshko et al, 2015 a , 2015 b ). This indicates that the incorporation of DIB probably could simultaneously improve the electrical percolation of the carbon nanoparticles in the composite electrodes.…”

“…The morphological and compositional variations in the distributions of the poly(S- r -DIB) copolymers and conductive carbon nanoparticles in both the poly(S- r -DIB 10% )-based and poly(S- r -DIB 50% )-based composites will be compared, when possible, to the analogous elemental sulfur-based composite cathodes. Recently, we demonstrated that replacing the elemental α -sulfur (S 8 ) with the poly( S-r -DIB) copolymers may result in molecular level homogeneity and high-capacity/long-life cathode structures that more efficiently generate and transfer charge and are robust against the mechanical stresses that arise from the repeated expansions and contractions during charge/discharge cycling (Simmonds et al, 2014; Oleshko et al, 2015 b ).…”

“…The cathode films were fabricated by coating a ball-milled slurry, consisting of either elemental sulfur or the poly(S- r -DIB) copolymers, mixed with Timcal Super C65 conductive carbons (TIMCAL Ltd., Bodio, Switzerland) and a polyethylene binder onto an aluminum current collector as described elsewhere (Oleshko et al, 2015 a , 2015 b ). The mass ratio of the components (sulfur-containing material:carbon:binder) in the slurry was held constant at ~75:20:5, respectively.…”

“…Interest in prospective candidate Li–S and Li–O 2 EES technologies is increasing because they offer the interconversion of chemical and electrical energy on a relatively short time scale, theoretically with higher efficiency than that of a heat engine and without releasing polluting gas emissions to the environment (Bruce et al, 2012; Li et al, 2015). However, high-energy density electrochemical rechargeable batteries should also be safe, inexpensive, nontoxic, and reliable through thousands of charge–discharge cycles to deliver long-term discharge capacities above 1,000 mAh/g and specific energy densities above 600 W h/kg required for emerging practical applications, e.g., in high-endurance unmanned aerial vehicle systems (UAVs) and electric vehicles capable of traveling over 500 km on a single charge (Ji et al, 2009; Oleshko et al, 2009 a , 2015 a , 2015 b ). Specifically, light-weight rechargeable Li–S batteries based on the Li/S redox couple generating ~2.2 V with respect to Li + /Li° have the highest theoretical specific capacity of 1,672 mAh/g of active material among all solid elemental redox couples and a theoretical specific energy of 2,567 W h/kg, assuming a complete two-electron reaction (Peled et al, 1989; Marmorstein et al, 2000): …”

Miniature all-solid-state heterostructure nanowire Li-ion batteries as a tool for engineering and structural diagnostics of nanoscale electrochemical processes

“…Electron microscopy techniques are widely applied for the determination of structure-property relationships in EES materials by obtaining morphological, crystallographic and local chemical information down to the atomic level. [10][11][12][13][14][15][16][17][18][19][20] S/TEM methods permit real-time examination of various types of batteries during galvanostatic (GS) testing of charge-discharge electrochemical reactions. These methods can additionally probe compression effects and the formation and spatial distribution of the solid-electrolyte-interphase (SEI) under varying processing conditions (temperature, pressure) in situ and ex situ.…”