Multimodal Characterization of the Morphology and Functional

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Multimodal Characterization of the Morphology and Functional Interfaces in Composite Electrodes for Li-S Batteries by Li Ion and Electron Beams Vladimir P. Oleshko, Andrew A. Herzing, Kevin A Twedt, Jared J Griebel, Jabez J McClelland, Jeffrey Pyun, and Christopher L Soles Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b00978 • Publication Date (Web): 15 Jun 2017 Downloaded from http://pubs.acs.org on June 19, 2017

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Multimodal Characterization of the Morphology and Functional Interfaces in Composite Electrodes for Li-S Batteries by Li Ion and Electron Beams Vladimir. P. Oleshko,

1 *

Andrew A. Herzing,

1

Kevin A. Twedt,

2,3

Jared J. Griebel,

4

Jabez J.

McClelland, 3 Jeffrey Pyun, 4 and Christopher L. Soles 1 1

Material Measurement Laboratory, National Institute of Standards and Technology,

Gaithersburg, MD 20899 2

Maryland NanoCenter, University of Maryland, College Park, MD 20742

3

Center for Nanoscale Science and Technology, National Institute of Standards and Technology,

Gaithersburg, MD 20899 4

Department of Chemistry and Biochemistry, University of Arizona, Tucson, AZ 85721

KEYWORDS: interfaces, Li-S battery, sulfur copolymers, LiFIB, analytical electron microscopy ABSTRACT. We report the characterization of multiscale 3D structural architectures of novel poly[sulfur-random (1,3-diisopropenylbenzene)] copolymer-based cathodes for high-energy density Li-S batteries capable of realizing discharge capacities > 1000 mAh/g and long cycling life time > 500 cycles. Hierarchical morphologies and interfacial structures have been *

Corresponding author, [email protected]

ACS Paragon Plus Environment

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investigated by a combination of focused Li-ion beam (LiFIB) and analytical electron microscopy in relation to the electrochemical performance and physico-mechanical stability of the cathodes. Charge-free surface topography and composition sensitive imaging of the electrodes was performed using recently introduced low-energy scanning LiFIB with Li+ probe sizes of a few tens of nanometers at 5 keV energy and 1 pA probe current. Furthermore, we demonstrate that the LiFIB has the ability to inject a certain amount of Li cations in the material with nanoscale precision, potentially enabling control of the state of discharge in the selected area. We show that chemical modification of the cathodes by replacing the elemental sulfur with organosulfur copolymers significantly improves its structural integrity and compositional homogeneity down to the sub-5 nm length scale resulting in creating (a) robust functional interfaces and percolated conductive pathways involving graphitic-like outer shells of aggregated nanocarbons and (b) extended micro- and meso-scale porosities required for effective ion transport. Introduction Current Trends in Design of Next Generation High-Energy Density Li-S Batteries Li-S batteries are one of the most attractive electrochemical energy storage (EES) systems owing to their theoretical specific capacity (1675 mAh/g) and specific energy (2600 Wh/kg), which are the highest among the solid elemental redox couples.

Assuming the full two-electron

conversion, the relevant reaction is: 1, 2, 16Lio + S8o → 8Li2S

(1)

Specific discharge capacities for reaction (1) range from 800 mAh/g to 1400 mAh/g, which is 4 to 5 times that of incumbent Li-ion technology that is widely used today.

3

The low cost and

abundance of sulfur make light weight Li-S batteries attractive alternatives to Li-ion batteries for emerging applications (e.g., long distance electrical vehicles, unmanned aerial vehicles/high altitude pseudo satellites, portable electronics). 4, 5 Although the demonstrated energy density of 400 Wh/kg far exceeds that of current Li-ion systems, 6 Li-S batteries traditionally suffer from either poor electrode rechargeability or a poorly controlled Li/electrolyte interface, so called solid - electrolyte interphase (SEI), causing capacity fading and limiting their lifetime.

1, 2

The

poor long-term performance is mainly associated with the “shuttling” of linear polysulfides


2

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dissolved in the electrolyte from the cathode to the anode where they are reduced to lower polysulfides and subsequently transported back to the cathode. Unlike insertion cathodes, sulfur undergoes a series of bulk topochemical transformations during cycling which involve the linear soluble polysulfides, Li2Sn (n = 8, 6, 4, 3) and insoluble sulfides, Li2S2 and Li2S. 7 The utilization of the active material is limited by several factors: (a) the electrically insulating nature of sulfur (5 × 10-30 S·cm-1 at 25 oC), (b) losses due to the formation of insoluble lithium sulfides, (c) dendritic Li anode morphology, and (d) the expansion of sulfur (80% by volume) during lithiation, which may cause self-discharge, low Coulombic efficiency and capacity decay. 8, 9 To address these problems, several attempts have been made to tailor the morphology and the molecular structures of active sulfur species and confining polysulfides within the cathodes. Typical strategies have been used to “encapsulate” the orthorhombic α-S8 (and electrochemically generated lithium polysulfides) inside a cathode conductive matrix, which may comprise mesoporous carbons, graphene, or core-shell colloids. 10, very successful. Cui and co-workers

12

11

In general, these attempts have been

showed that yolk-shell S8-TiO2 colloids exhibit high

capacity retention and extended lifetimes achieving roughly 700 mAh/g at 1000 cycles at a C/2 rate (836 mAh/g) and demonstrated excellent capacity retention out to 1000 cycles with a capacity decay as low as 0.046 % per cycle 13. Alternatively, the use of polyacrilonytrile (PAN). 14

, sulfide polymers (e.g., 2,5-dimercapto-1,3,4-thiadiazole, DMAcT)

polyaniline nanotubes materials.

17

15, 16

and vulcanized

have been proven effective as a route for electroactive cathode

More recently, carbonization with Li2S salts has been explored to create bulk

electroactive materials for advanced Li-S cathodes.

18, 19

A cycle life of over 1000 charge-

discharge cycles at a capacity of 1470 mAhg-1 for cells comprised of pre-lithiated Si-carbon and all-carbon non-Li anodes coupled with sulfur-infused porous hollow sphere cathodes have been


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demonstrated. 20 All of these represent advances in the creation of electroactive cathode materials for Li-S batteries with enhanced performance. However, challenges persist in the development of chemistries that are inexpensive and amenable to large-scale production, which also exhibit high charge capacity and stability during prolonged cycle life.

Electroactive Sulfur Copolymers as Emerging Cathode Materials for Li-S Batteries A facile, low-cost synthetic route, to prepare electroactive organosulfur polymers poly(sulfurrandom-1,3-diisopropenylbenzene (DIB)), or poly(S-r-DIB), was introduced Pyun and coworkers

21, 22, 23

for the use as cathode materials in Li-S batteries. This approach, termed inverse

vulcanization, achieved through direct copolymerization of a cross-linking agent (DIB) in liquid sulfur, represents an alternative strategy to the encapsulation schemes. The poly(S-r-DIB)-based cathodes exhibit an initial specific discharge capacity of 1225 mAh/g, but also realize high reversible specific discharge capacity and cycle stability (1005 mAh/g at 100 cycles, 0.1 C rate). 21, 22

The sulfur copolymers are members of an emerging class of electroactive polymers such as

conjugated conductive polymers, sulfide polymers and nitrosyl radical polymers. 24, 25 Recently, several new members of the class, e.g., sulfur-rich copolymers with 1,3-diethylbenzene (DEB) 26, soluble inverse-vulcanized hyperbranched polymers (SIVHPs) terminated

poly(3-hexyl-thiophene-2,5-diyl)

penylbenzene) copolymers [poly(S-r-TIB)]

29

27

[poly(S-P3HT)]

sulfur copolymers with allyl28

,

poly(S-r-1,3,5-triisopro-

, poly(S-r-styrene) copolymers [poly(S-r-Sty)]

sustainable poly(S-r-C-a) copolymers of sulfur and cardanol (agro waste), DIB)/MoS2 composites (MolyS10-DIB10)

32

31

30

,

and poly (S-r-

have been synthesized. Porous trithiocyanuric acid

(TTCA) crystals have been utilized as a soft template, where the ring opening polymerization of elemental sulfur could occur along the thiol surfaces to create three-dimensionally (3D)


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interconnected sulfur-rich polymers.

33

Poly(S-TTCA)-based cathodes in Li-S cells demonstrate

excellent capacity retention over 83 % and notable rate capability at various current rates from 0.1 C (1210 mAh/g) to 5 C (730 mAh/g). The electrochemical properties of the cathodes utilizing various sulfur copolymers in Li-S cells are summarized in Table 1. The performance and cycle stability of sulfur copolymers in cathodes depends on the nature and content of crosslinking agents. 21, 22, 26 Table 1. Electroactive sulfur copolymers and their electrochemical performance Sulfur copolymer

Non-sulfur comonomer (copolymer)

Number of cycles

Cycling performance

References

Discharge capacity, Capacity mAhg-1 retention, % Poly(S-r-1,3-diisopropenylbenzene), Poly(S-r-DIB)

DIB

S-polyacrylonitrile (SPAN)

21, 22

100 300 500

1005 (0.1 C) 817 (0.1 C) 635 (0.1 C)

52

PAN

40

500 (0.1 C)

65

25

C-S copolymer-1,3- DEB diethynylbenzene (DEB)

50

700 (0.1 C, C-S copolymer-20% DEB-2h) 504 (1 C, C-S carbon blackDEB) 1217 (0.1 C, SIVHPs–graphene ultralight aerogels (GUAs)) 796 (0.3 C, SIVHPs–GUAs) 685 (0.5 C, SIVHPs–GUAs) 583 (1 C, SIVHPs– GUAs) 482 (2 C, SIVHPs–

66

26

500

Soluble inversevulcanized hyperbranched polymers (SIVHPs) via thiolene addition of S8 to DIB

DIB followed by functionalization with thiolene (sulfhydryl) and Menschutkin reaction (propargyl bromide)

10

10 10 10 10


70 27

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10 400 Poly(S-trithiocyanuric acid), Poly (S-TTCA)

TTCA

Poly(S-allylterminated poly(3hexyl-thiophene2,5-diyl)), Poly(SP3HT) Poly(S-r-styrene), Poly(S-r-Sty) Poly(S-r-C-a), S90 with 90% of sulfur by weight

P3HT

100 450

100 100

33

83

66

28

70

Styrene (Sty)

1000

485 (0.2 C)

46

30

Cardanol benzoxazine, (C-a)

100

975 (0.12C, S90- 84 2.5% reduced graphene oxide 86 (rGO)) 700 (0.6C, S902.5% rGO) 350 (1.2 C, S902.5% rGO) 680 (0.2 C) 68

31

300 500

Poly (S-r-DIB)/ MoS2 composite, MolyS10-DIB10

GUAs) 255 (5 C, SIVHPs– GUAs) 450 (0.1 C, SIVHPs–GUAs) 945 (0.2 C) 850 (0.2 C) 872 (1 C) 803 (3 C) 730 (5 C) 799 (0.5 C, S:P3HT=9.5: 0.5) 838 (0.5 C, S/P3HT=8:2)

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DIB

595

32

The new electroactive polymeric materials show potential for next technology breakthrough with high societal impact. The underlying structure-processing-property relations and detailed mechanisms responsible for outstanding electrochemical properties and physico-mechanical stability of new electroactive organosulfur polymers, however, remains to be fully understood and appreciated.

Impact of Spatially-Resolved Instrumental Techniques on Mechanistic Understanding of the Li-S System


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Remarkable progress has been made in mechanistic understanding of the Li-S system during last decade using advanced characterization techniques, including in situ and in operando studies of cycling batteries.

34, 35

Spatially-resolved analytical electron microscopy, i.e., scanning and

transmission electron microscopies (S/(T)EM) along with associated techniques (electron diffraction, energy-filtering TEM, electron energy-loss (EEL) and energy-dispersive X-ray (EDX) spectroscopies) are often utilized to characterize the morphology, crystallography and chemical distributions across diverse length scales that span down to the atomic level. 3, 8-13, 18-20, 22, 32, 36, 37

In this approach, several imaging, diffraction and analytical modes are integrated in a

design to provide analytical synergism having obvious advantages over any single technique. As a result, the great deal of structural and compositional information can be acquired from the same area or feature by different techniques simultaneously or sequentially. The S(T)EM characterization of cathode structures and interfaces is critical for establishing the structure property relationships that underpin the electrochemical performance of Li-S batteries.

32-34

Several obstacles, however, should be overcome since lithium compounds and the SEI are all vulnerable to knock-on displacement damage by ionizing radiation. 38 Elemental sulfur will also sublimate in a TEM vacuum (9.3×10-5 Pa) at a rate of ≈ 1015 atom cm-2 s-1, approximately one monolayer per second

39

, because of its relatively high vapor pressure (8.0×10-5 Pa) at room

temperature. 40 Although SEM and scanning ion microscopy (SIM), with Ga+, Xe+, or He+ focused ion beams (FIB) are often employed for the characterization of battery materials, the use of SIM/FIBs is mostly limited to specimen sectioning due to high sputtering rates and intensive radiation damage. This is unfortunate because SIM/FIB offer often benefits in terms of resolution, high surface sensitivity, charge control and distinct material contrast, providing


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unique image information that complements or exceeds that from SEM with its potential only recently being explored.

Like helium ions, low mass lithium ions are ideal for imaging

applications where sample sputtering should be minimized. However, cold atom Li-ion sources are well suited for operation at low beam energies below 6 keV, a regime where He-ion microscopes suffer. 41 These light ion sources offer the distinct advantage since they can be used to probe the topmost surface compositions and surface structures by ion scattering spectroscopy. Recently we have introduced low-energy scanning LiFIB as a new tool for lithiating battery materials comparing FIB lithiation with conventional electrochemical lithiation of isolated β-Sn microspheres.

42

We have yet to illustrate the use of the LiFIB as a tool to characterize battery

electrode materials and specifically the cathodes in Li-S batteries. In this paper, we explore 3D structural architectures and functional interfaces of poly(S-rDIB)-carbon nanocomposite cathodes for Li-S batteries seeking correlations with their electrochemical performance and physico-mechanical stability. Combining advanced low-energy LiFIB, SEM and analytical electron microscopy in multiple imaging, diffraction and analytical modes, we analyze the surface topography, morphology, chemical compositions, and molecular bonding in poly(S-r-DIB)-based cathodes with different DIB content. Through in-depth analyses of the structure-property relations at multiple length scales, this work provides insights into the underlying mechanisms of the enhanced capacity retention in emerging organosulfur polymerbased cathodes for the next generation of Li-S batteries.

Experimental Section Materials


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The composite cathodes were fabricated by blade coating a ball-milled slurry, consisting of either elemental α-S8 or the poly(S-r-DIB) copolymers (10% and 50% DIB mass fraction), mixed with the Timcal Super C65 conductive carbon particles and a polyethylene binder onto an aluminum current collector. The mass ratio of the components (sulfur containing material: carbon : binder) in the slurry was held constant at 75 : 20 : 5, respectively. The composite cathodes with a sulfur loading of 0.75 mg/cm2 for a standard 2032 coin cell were prepared with a Li metal foil anode and cycled according to procedures described elsewhere. 22

Methods FESEM and Low-Energy LiFIB The topography and elemental compositions of the pristine and cycled poly(S-r-DIB) – carbon cathodes were characterized using a cold field-emission Hitachi S4700 II FESEM* (Hitachi, Japan) equipped with an Oxford high-speed 80 mm2 Tmax silicon-drift EDX detector. The FESEM was operated at accelerating voltages of 2, 5 and 20 kV. The LiFIB used in this study (Supporting Information (SI), Figure 1Sa) consists of a magneto-optical trap (MOT) ion source incorporated into a commercial FIB optical column. Figure 1Sb, SI shows the LiFIB build at the NIST. In the MOT source, trapped Li atoms laser-cooled down to about 600 µK are photoionized by an additional laser to produce high brightness 7Li+ ion beams. A resistive tube accelerator is used to accelerate the ions to the final incident beam energy in the range of (0.5 to 6) keV, probe current from (1 to 50) pA, and focal spot size as small as 27 nm, while keeping a low electric

*

Certain equipment, instruments or materials are identified in this paper in order to adequately specify the experimental details. Such identification does not imply recommendation by the National Institute of Standards and Technology nor does it imply the materials are necessarily the best available for the purpose.


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field over the region of ionization, reducing the energy spread of the beam to less than 100 meV. For FESEM/EDX and LiFIB analyses, the cathode films were mounted on aluminum alloy stubs.

Monte Carlo Simulations Full cascade stopping power and range of ions in matter (SRIM) simulations of Li ion interactions with cathode components (1 µm sulfur and 1 µm carbon layers) for chosen operating conditions (5 keV accelerating voltage, 103 Li+ trajectories, etc.) were performed using the Ziegler-Biersack-Littmark (ZBL) universal screening potential with a SRIM 2013 package.

43

Similarly, Monte Carlo (MC) simulations of electron beam interactions with the same materials were performed using a Casino software, version 2.48. 44

High-Resolution Analytical S/TEM S/TEM imaging, selected-area electron diffraction (SAED), EEL and EDX spectroscopic analyses of air-dried cathode powders deposited onto lacey carbon Cu grids were performed in a Schottky field-emission Titan 80-300 S/TEM (FEI, USA) equipped with S-TWIN objective lenses and operating at 300 kV accelerating voltage with a point-to-point resolution of 0.19 nm. For nanoanalyses with (0.14 to 0.2) nm diameter probes, the instrument was equipped with a high-angle annular dark-field (HAADF) detector, BF- and ADF-STEM detectors, an Enfina electron spectrometer, and a 30 mm2 EDAX Si/Li EDX detector with a 0.13 sr acceptance angle. To ensure optimal counting rates, the specimens were tilted 15o towards the EDX detector. To reduce beam-induced damage of the specimens, the electron beam dose rates were minimized to 4.5 e·nm-2s-1 and beam blanking was employed between acquisitions. Multivariate statistical analysis (MSA) of STEM images was performed using a scatter diagram construction using a


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Gatan Digital Micrograph script based on earlier described analysis routines 45 and an automatic AXSIA software package. 46

Electron Tomography STEM-based tomography (ET) was performed on powders of the poly(S-r-DIB10%)-based cathode using a Cs probe-corrected FEI Titan 80-300 TEM/STEM equipped with an EDAX Octane SDD X-ray detector with a solid angle of ≈ 0.4 sr. Due to the large size of the copolymercarbon agglomerates ranging from 5 µm to 20 µm, it was necessary to operate the instrument in such a way so as to produce a large depth of field. This was done by using a 10 µm probe forming aperture resulting in a probe semi-convergence angle of 0.6 mrads. Since the use of such an aperture severely limits the final probe current, a low spot number was used along with a small inner collection angle for the annular detector (18 mrads) in order to increase the resulting signal. STEM images were collected over a range of specimen tilts from -70o to + 70o using a 1.5o tilt increment. We aligned data using an intensity-based subpixel registration method 47 and carried out reconstruction using 50 iterations of the simultaneous iterative reconstruction technique (SIRT)

48

as implemented in the ASTRA Toolbox.

49

Segmentation of the

reconstructed volume was performed using the Avizo software package, version 9 (FEI, USA).

Results and Discussion The Effect of DIB on the Electrochemical Performance and Microstructure of Composite Cathodes Analyzing the effect of the poly(S-r-DIB) copolymer composition on the electrochemical properties as the active material in cathodes for Li-S batteries 22, we have found that the poly(S-


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r-DIB) copolymers with DIB mass fraction of 5 %, 10 %, and 15% all exhibited high initial discharge capacities (above 1,100 mAh/g), low initial capacity loss, and consistently reduced capacity loss per cycle, whereas copolymers with DIB mass fraction of 1 % by demonstrated cycling performance comparable to elemental α-S8, and copolymers with DIB mass fraction of 20 % or greater showed little to no improvement over elemental sulfur. The copolymers with DIB mass fraction of 10 % were found to perform optimally (SI, Figures S2a and S2b). The enhanced battery performance and excellent capacity retention with the poly(S-r-DIB10 %) active material, i.e., 1005 mAh/g at 100 cycles and 817 mAh/g at 300 cycles, arises from in situ generation of organosulfur moieties and linear polysulfides via electrochemical fragmentation of the initial copolymer.

22

The analyses of normalized charge and discharge capacity curves for

copolymer cathodes with different DIB content suggest that the transformations involving S-S and organosulfur moieties occur in both high and low voltage plateau regimes, regardless of the DIB content, as schematically described below:22 High Voltage Plateau

+

(2)

Li2S8 (2)

Discharge

Charge

+

(3)

Li2S4 (4)

Charge

Low Voltage Plateau

Discharge Discharge Charge

Discharge

Charge


12

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+

Li2Sx (6) x~2-3

(4)

We propose that organosulfur moieties from the copolymer suppress irreversible deposition of insoluble discharge products Li2Sx (x ~1-3).

22, 33

For further insight into the enhanced

performance of the copolymers in Li-S batteries, we examine in detail the morphologies, compositional variations and interfaces between the copolymers and conductive carbons in the cathodes with DIB mass fraction of 10 % and 50 % in comparison with the analogous conventional α-S8 - carbon cathodes. The set of large-area plane view secondary electron (SE) and bright-field (BF-) TEM images of the cathodes together with corresponding X- ray maps and BF-light microscopy (LM) images (Figure 1 and SI, Figure 3S) demonstrate macroscale and microscale surface topographies of the pristine cathodes, which reveal drastic morphological and compositional transformations in the cathodes as the DIB loading increases. BF-LM images, close in magnification to the low-magnification FESEM images are, however, dark because they are dominated by the contrast from black carbon particles. The conventional cathode contains multiple cracks that are 10 µm to 20 µm wide (SI, Figures 3Sa1 and 3Sb1) that must have developed during the coating of the cathode slurry, from the volume reduction that occur as the coating dries. Constrained by the rigid substrate, this volume reduction induces mechanical stresses in the cathode resulting in cracks when the mechanical strength of the composite is exceeded. It is striking that the incorporation of the organosulfur copolymer remarkably improves the mechanical integrity of the as-cast cathode composites; the propensity for cracking


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is strongly reduced in the cathodes made from the poly(S-r-DIB50%) copolymer which appear to be practically crack-free. There is also large scale compositional heterogeneity in the conventional cathode in terms of phase separated domains, rich in either sulfur or carbon, on length scales that approach tens of micrometers (see Figures 1a1, 1b1, 1c1; SI, Figures 3Sa1 and 3Sb1). EDX mapping (SI, Figure 3Sd1) confirms the strong segregation between the orthorhombic α-S8 microparticles, sometimes displaying facets and aggregated spherical carbon nanoparticles (Figure 1a1). The encapsulation of α-S8 particles inside cavities and micropores formed by surrounding aggregated conductive carbons promotes its stabilization leading to reduced sublimation rates of sulfur in a vacuum of ≈ 1.3 ×10-6 Pa in the FESEM specimen chamber. No evidence of sulfur sublimation from the cathode composites were found during several hours of observations. The magnitude of the large-scale compositional heterogeneity is significantly reduced as the DIB mass fraction increased from 10% to 50 %. The poly(S-r-DIB50%)-based composite cathode did not show any signs of macroscale phase segregation of either of the carbons or sulfur-rich particles. Simultaneously, the inverse vulcanization evidently improves the stability of the copolymer particles against sublimation in the microscope vacuum. The X-ray map (SI, Figure 3Sd3) confirms the enhanced compositional uniformity of the poly(S-r-DIB50%)-based composite cathode. Replacing the α-S8 with the poly(S-r-DIB) copolymers can be therefore used to achieve molecular level homogeneity producing 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 cycling. 50


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Microscale

FESEM

Mesoscale BF-TEM

(a1)

(a2)

(a3)

(a4)

(b2)

(b3)

(b4)

(c2)

(c3)

(c4)

α-S8

α-S8

1 µm

(a1)

Poly(S-r-DIB10%)

(b1)

Poly(S-r-DIB

1 µm

)

10%

(b1)

(c1) Poly(S-r-DIB50%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

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1 µm

(c1)

Figure 1. (a1, b1, c1) FESEM, pristine cathodes and (a2-a4, b2-b4, c2-c4) BF-TEM, powders of the cathodes with different DIB contents: (a1-a4) conventional α-S8-carbon cathode, (b1-b4) poly(S-r-DIB10%)-based cathode, (c1-c4) poly(S-r-DIB50%)-based cathode.


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Low-Energy Scanning LiFIB as a New Tool for Surface Characterization of Electrode Materials In this section, we introduce our low-energy scanning LiFIB as a new tool for characterization of cathode materials in electrochemical batteries. With focused ion beams, the interaction volume that governs the attainable resolution by FIBs, is usually smaller as compared to electron beams under the same conditions. As the interaction volume reduces with lowering beam energy, composition and surface topography sensitive imaging with backscattered Li+ ions (BSIs) and ion-induced SEs (iSEs) could be advantageous. This is illustrated by simulations of interactions of Li+ ions and electrons with carbon and sulfur films. SRIM simulations of the interaction volumes and ion ranges for a 5 keV Li+ ion beam (SI, Figure 4S) have been compared with similar MC simulations for a 5 keV electron beam (SI, Figure 5S). The size of the calculated interaction volumes for Li+ ions appear to be 15 times smaller (for carbon) and 8 times smaller (for sulfur) than those for electron beams indicating potentially superior capabilities of focused Li+ probes for high sensitive surface imaging and analyses. Large-area LiFIB images of a poly(S-r-DIB10%)-based composite cathode are shown together with reference FESEM images and an overlaid X-ray map of the same cathode are shown in Figure 2. Using signals generated by iSEs (Figure 2a) and BSIs (Figure 2b), Li ion imaging enables visualization of both the cathode surface topography and composition, including the electrically insulating copolymer domains (bright areas in BSI image). These observations are perfectly consistent with the reference FESEM images obtained using a combination of the pure topographic SEI signal (Figure 2c, U = 5 kV), mixed SEI+II signal (Figures 3d and 3f, U = 20 kV), (e) X-ray map of the area (Figure 2e), and compositional backscattered electron (BSE) signal. The contrast variations in the images in panels (c) through (f) result from the different nature of signals generated by electron beam - material interactions within the interaction


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volume. In Figure 2c, the dark region elongated in the direction of a horizontal scan is due to the attraction of emitted SEIs to the surface of predominantly insulating copolymer-rich domains and iSE

Charging

SEI

(c) SEI+II

(d) (e)

Poly(S-r-DIB

)

10%

(a) BSI S

Al

C

SEI+II

(f) BSE

Poly(S-r-DIB

)

10%

Poly(S-r-DIB

)

10% (g) (b) Figure 2. LiFIB vs. FESEM, macroscale surface topography of the poly (S-r-DIB10 %)-based cathode, plane view acquired at working magnification of ×1000. (a, b) LiFIB, U = 5 k V, (a) iSE, (b) BSI. (c-g) FESEM, the same scale as for the LiFIB images. (c) SEI, U = 5 kV, (d-g) U = 20 kV, (d, f) SEI+II, (e) overlaid X-ray S Kα (orange), C Kα (green), and Al Kα (red) map of the area (d), (g) BSE with a Robinson type detector.


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beam displacement caused by positive charging during scanning. Contrary to the SEI image in Figure 2c, LiFIB images do not show any charging artifacts or evident radiation damage. LiFIB and FESEM-EDS with their greater depth of field and phase and chemical sensitivity provide in this case more useful information particularly concerning the surface topography and phase distributions than the light microscopy. A similar set of LiFIB and FESEM images recorded at iSE

SEI

(c) SEI+II

(a)

(d)

BSI

BSE Poly(S-r-DIB

)

10%

(e) (f) Poly(S-r-DIB

)

10%

(b)

S

Al

C

Figure 3. LiFIB vs. FESEM, microscale surface topography and composition of the poly(S-rDIB10 %)-based cathode, plane view, ×5000. (a, b) LiFIB, U = 5 k V, (a) iSE, (b) backscattered ions (BSI). (c-f) FESEM, the same scale, (c) SEI, U = 5 kV, (d-f) U = 20 kV, (d) SEI+II, (e) BSE, (f) overlaid S Kα (orange), C Kα (green), and Al Kα (red) X-ray maps of the area (d).


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higher magnification (Figure 3) display further details of the microscale surface topography and compositional variations over the cathode film, including clearly seen 5 µm to 10 µm in size copolymer domains surrounded by aggregated carbons and extended porous structures. The results show the potential of low-energy LiFIB as an advanced tool for high quality, charge-free surface topography and composition-sensitive imaging of the heterogeneous battery electrode materials. Furthermore, injecting a certain amount of Li+ ions in the cathode with nanoscale precision, the LiFIB initiates the discharge reaction where lithium ions react with sulfur copolymer producing sulfides: +

o

-

12Li + Poly(S 8-r-DIB) + 12e = (LiS)4-r-DIB + 4Li2S,

(5)

making it potentially possible to monitor the discharge process in the electrode. For the iSE image in Figure 4a acquired with a 50 nm diameter Li+ probe at an accelerating voltage U = 5 kV, ion current I = 1 pA, area size S = 28.3×108 nm2, and acquisition time t = 173.8 s, this corresponds to the total injected charge: QLi+ = It = 0.17×10-9 C

(6)

= 1.08 ×109 Li+ ions or 1.2×10-14 g Although the amount of injected Li+ ions is much less than that for the active material (2.1×10-8 g) to induce noticeable changes, it can be essentially increased by optimizing parameters I, S, and t. By this way, it may be possible to control quantitatively the state of discharge (SOD) in the desired electrode area in situ without presence of a liquid electrolyte and avoid side reactions associated with the SEI formation. This work is currently in progress, and is beyond the scope of this paper.


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Micro-, Meso- and Nanoscale Structural Organization of Poly(S-r-DIB)-Based Composite Cathodes Complementing topography and phase composition sensitive FESEM and LiFIB imaging, BFTEM of air-dried powders extracted from the cathodes with different DIB content (Figure 1) shows remarkable transformations of the internal structures involving microscale copolymercarbon agglomerates and mesoscale aggregates of conductive carbons. Multimodal FESEM, LiFIB and TEM imaging indicate that the composite homogeneity appears to result from (a) an intimate mixing of the copolymers and conductive carbons and (b) densification of agglomerates formed by the two components. The complex hierarchical morphologies of the cathodes exhibit random chain- and/or branch-like 3D aggregates of the carbon nanoparticles, which form percolated conductive networks embedding electrochemically active but electrically insulating microparticles of α-S8 or copolymers. The mean size of segregations composed of α-S8 and copolymer particles decreased with the DIB content from (75.6 ± 6.7) µm for the conventional cathode and (56.4 ± 9.2) µm for the poly(S-r-DIB10%)-based cathode down to (7.3 ± 1.7) µm for the poly(S-r-DIB50%)-based cathode. The average size of the aggregated carbon clusters was estimated as (610 ± 130 nm) for the poly(S-r-DIB10%)-based cathode (Figures 1b1-1b2) and (520 ± 110 nm) for the poly(S-r-DIB50%)-based cathode (Figure 1c1-1c2), respectively. This is much smaller than the large nodular aggregates of carbon particles (132 ± 30.4) µm found in the conventional cathode, reflecting the tendency to reduce the cathode heterogeneity as the DIB content increases. The mean diameter of individual carbon particles determined by counting of 300 particles was d = (39.8 ± 1.2) nm (SI, Figure 6Sa). The particle size distributions fit satisfactorily to a normal distribution curve (SI, Figure 6Sb).


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SAED patterns from the cathode powders (Figure 4) exhibit a series of broad diffuse rings due to scattering from aggregated carbons, which indicate a short range ordering (SRO) defined as ordering of the first- or second-nearest neighbors of a carbon atom. The first ring indicates a spacing of 0.39 nm, which corresponds to the partially disordered onion-like fullerene shells. This value is close to the natural spacing of the basal planes in graphite and suggests that the carbon nanoparticles have highly conductive shells. The other two rings at 0.21 nm and 0.12 nm are typical interatomic dimensions for amorphous carbon materials.

51

Phase-contrast HRTEM

(Figure 5) shows that the carbons are partially coalesced onion-like particles with 3 nm to 6 nm

(a)

0.12 nm

(b) 0.11 nm

0.21 nm

0.21 nm

0.39 nm

0.40 nm

Figure 4. SAED patterns from cathode powders display a series of broad diffuse rings due to scattering from aggregated carbons, which indicate a SRO (see text for details): (a) a pristine composite cathode; (b) a cycled cathode after 30 cycles.

ill-defined cores and 10 nm to 20 nm outer shells, exhibiting the typical 0.38 nm to 0.39 nm (002) graphite-like interlayer spacings. HRTEM micrographs in Figures 5b and 5c show carbon particles partially embedded into the copolymer matrix further supporting the notion of intimate


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mixing of the copolymers with carbons in the composites. Aggregated carbon particles with electron conductive graphitic-like outer shells having hyperconjugated π- bonding form percolated and presumably conductive random networks, which propagate through the cathode (a)

C65

(b) Poly(S-r-DIB10%)

0.39 nm

C65

0.39 nm

(c)

Poly(S-r-DIB50%)

0.38 nm C65

Figure 5. HRTEM, pristine composite cathodes with different DIB contents: (a) conventional αS8-carbon cathode, (b) poly(S-r-DIB10%)-based cathode, (c) poly(S-r-DIB50%)-based cathode. Images in (b) and (c) show carbon particles embedded into the copolymer matrix). White circles mark ill-defined cores of partially coalesced carbon particles.


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matrix. The observations point to the origin of electron conductivity in the system involving random percolated π-bonding networks formed by aggregated carbons. At the interfaces with the copolymer or sulfur (or sulfide ions from the electrolyte), the conductive carbons could create a multitude of electroactive cites where the reaction steps (2) – (4), including charge generation, occur during cycling.

Analytical Multimode STEM-MSA Phase Analysis of Poly(S-r-DIB)-Based Cathodes Due to inherent inhomogeneity and complex 3D morphologies of copolymer - carbon agglomerates, it is quite challenging to analyze the internal structures of the composites by conventional 2D TEM imaging. As one of the possible ways to overcome these challenges, we propose to collect simultaneously multiple image (and analytical) STEM signals and to inspect the relationships between them in order to produce pseudo-phase discrimination at high spatial resolution using correlative MSA. A set of multimodal STEM images (Figure 6) acquired simultaneously in BF-, low-angle ADF- (LAADF), medium-angle ADF- (MAADF), and highangle ADF- (HAADF) STEM modes shows characteristic features of the internal microstructure of copolymer – carbon agglomerates extracted from the cathode film. Strong contrast variations resulting in complete contrast reversal (e.g., between BF and MAADF images) allow to visualize structural features of copolymer domains, aggregated carbon nanoparticles, interfaces, and pores even in areas of several micrometers in thickness. In HAADF STEM, the image intensity varies monotonically with a mean atomic number, making it possible to visualize lateral distributions of the copolymer within the cathode. STEM offers evident benefits over the broad-beam illumination TEM, which normally requires samples less than 100 nm in thickness (e.g., see Figure 1). STEM is also compatible with spectroscopic techniques such as EDX and EELS. The


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(a)

(b)

(c)

(d)

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Figure 6. Multimode STEM of a powder of a poly(S-r-DIB10%)-based composite cathode: (a) BF; (b) Low-angle-ADF (LAADF); (c) Medium-angle ADF (MAADF); (d) HAADF. All scale bars correspond to 500 nm. HAADF signal is generated by the incoherent Rutherford-like electrons scattered out to high angles (>50 mrad), in which the registered images have different levels of contrast related to the atomic number (chemical composition), the density and thickness (mass-thickness) of the material. 52 Moreover, if field emission sources are used in STEM, partial temporal coherence


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(a)

(b)

(c)

S-K

C-K

500 nm (d)

(e)

(f)

Figure 7. (a) HAADF STEM and (b-d) drift-corrected STEM//EDX SI of the poly(S-r-DIB10 %) copolymer carbon agglomerate shown in Figure 6. Red rectangular box in (a) marks the area where EDX analyses were performed; (b) sulfur S Kα map; (c) carbon C Kα map acquired over the same area; (d) X-ray spectrum acquired in the spot marked by red cross; (e) low-loss EEL spectrum acquired in the spot marked by red cross, a single scattering distribution (blue curve) was derived using a Fourier-log technique 52; (f) core-loss EEL spectrum acquired at the same point shows sulfur S L2,3 edge at 165 eV and carbon C K -edge at 284 eV, respectively. The net S L2,3- and C K- ionization edges obtained by subtracting a power-law fitted background (red curves) are shown in green.


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may arise only because of the relatively low spread in energies of the illuminating beam.

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53

Contrary to stationary-beam TEM, STEM imaging is not limited by the spherical aberrations of the objective lens, which may cause contrast delocalization. 54 In addition, STEM enables higher contrast and less blurring in thick specimens up to several µm, as in this case. With 1 mrad to 2 mrad small convergence angles of the incident beam, it is possible to increase depth of field so that the entire thick sample will be in focus along its depth, i.e., z-direction. Utilizing an axial BF-detector instead of a HAADF-detector reduces blurring of specimen features situated towards the bottom of thick samples. Even if diffraction effects are not completely avoidable, Z-contrast in the incoherent HAADF-STEM mode relates the image intensity monotonically to the atomic number, material density and thickness making a much better approximation to a mass thickness image than a coherent BF image. On the other hand, the ability of HAADF-STEM to suppress diffraction and phase contrast makes it insensitive to differentiating between crystalline and amorphous phases. Conversely, phase contrast BF-STEM provides both crystallographic and orientation relationship information. Figure 7 presents results of spatially-correlated driftcorrected EDX spectroscopic imaging (SI) and EELS of the poly(S-r-DIB10 %) copolymer carbon agglomerate shown in Figure 6. The red rectangular box in HAADF STEM image (Figure 7a) marks the region where sulfur S K map (Figure 7b) and carbon C K map (Figure 7c) were acquired. The low background X-ray spectrum extracted from the spot marked by red cross (Figure 7d) shows the carbon C Kα1 peak at 0.28 keV and sulfur S Kα1 peak at 2.31 keV. Due to large thickness variations, EELS measurements were performed at an interfacial region near the edge of the agglomerate. The core-loss EEL spectrum (Figure 7f) acquired at the same point as X-ray spectrum in Figure 7d shows sulfur S L2,3 edge at 165 eV and carbon C K -edge at 284 eV, respectively. The graph also displays the net S L2,3- and C K-edges (green curves) obtained by


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subtracting a power-law fitted background (red curves). Dark blue curves denote Hartree-Slater ionization cross-sections calculated for both edges.

55

Spatially-correlated EDX SI and EELS

confirm both elemental compositions of the components and the compositional correlation between analyzed STEM images (Figure 6). Ideally, by collecting multiple STEM signals (i.e., images, spectra and diffraction patterns), and analyzing correlations between the intensity (structural fingerprints), morphology, crystallinity and composition in a multidimensional space, one can identify and ultimately quantify all phase components. MSA, used to analyze large datasets of multimode STEM images, provides a powerful means for systematic multiparameter data evaluation and an unbiased classification of the phases. 56 An image can be defined in MSA by its components along the axis chosen in a hyperdimensional space with as many dimensions as there are pixels in the image. MSA seeks correlations between the number of components and the “fingerprint” image or concentration of each component. Such components should be incorporated in a set of imaging and/or spectral data recorded from the samples in which the compositions of the constituent phases may vary with spatial positions. described earlier

44

57

A multivariate scatter diagram construction based on analysis routines

was adopted here to quantify spatial correlations between the intensities of

three different input STEM signals and phase distributions in the poly(S-r-DIB10%) copolymer– based cathode (Figures 8a and 8b). When considering the correlations between two or more images, the points could be distributed over an ideal curve corresponding to the ultimate correlation; the distribution width reflects the contribution of the statistical noise. A trivariate scatter histogram in Figure 8a revealed distinct clusters of the major phases. The histogram is generated by comparison of the intensities contained within the source images and a sampling width w determined by the histogram resolution r = 128 pixels as


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wsignal = [maxsurvey(I)-minsurvey(I)]/r,

(7),

where I is the intensities of MADF, HAADF or BF signals, respectively. The intensities are

(b)

(a) Poly(S-r-DIB10%)

(a)

C65 carbons

(b)

background

(c)

Figure 8. MSA of multimode STEM of a powder of a poly(S-r-DIB)10%-based composite cathode: (a) Trivariate scatter diagram of normalized BF, MAADF and HAADF image intensities revealing distinct clusters corresponding to the major phase components. (b) Phase image produced by selecting the outlined areas with clusters from the scatter diagram. Copolymer domains are in green, carbon particles are in yellow, and background (vacuum and supporting lacey film) are in black. (c) Phase image produced from four STEM signals using an automated AXSIA package. Copolymer domains are in blue, carbon particles are in green, and background is in red.


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plotted as a fraction of the total sum of intensities for each pixel from the three source images after applying the intensity range limits to the source images and rescaling with the minimum intensity set at zero and the maximum at the specified histogram resolution. Such rescaling allows the contribution of each source image to be independent of its relative intensity. Figure 8b shows the composite image obtained from the inverse transformation of the selected intensity clusters from the outlined areas back into the real space. In Figure 8b, voids are black, the copolymer domains are green, and carbon particles are yellow. For comparison, Figure 8c presents a similar composite image obtained by an automatic MSA analysis of all four STEM signals in Figure 8 using the AXSIA package developed by Kotula et al. 45 Here, voids are in red, the copolymer domains are in blue and aggregated carbon particles are in green. Despite of large thickness variations, the composite images in Figures 8b and 8c display lateral distributions of the extracted phase components quite similarly, although the unbiased automatic MSA appears to be more sensitive to the separation of heavily overlapped carbon particles and copolymer domains as compared to the interactive histogram construction. For the analyzed cathode fragment, MSA indicates the structural correlation between STEM signals and phase components. Providing the morphological evidence of the enhanced chemical compatibility of the components down to molecular length scales, these data point to stronger cohesion between copolymers and conductive carbons in the composites. The results show that the cathode uniformity arises from the compatibility of the copolymer and carbon fillers at the sub-5 nm length scales, particularly when compared to the gross morphology of conventional α-S8-carbon cathodes. The copolymers form nearly conformal coatings with intimate contacts around the carbons for both compositions and appear to be in wetting contact with the carbon nanoparticles at potentially active interfaces.


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Probing of Valence Electronic States and Bonding by EELS In addition to the compositional analyses, EELS was used to probe the valence electronic states and bonding in different cathode components. Figure 7e presents a low-loss EEL spectrum collected at the same spot as X-ray spectrum in Figure 7d. The spectrum reflects excitations of outer-shell electrons within the probed material that arise from the inelastic scattering of the beam. This leads to a net displacement at a given wavelength of the full momentum-dependent electronic structure of 2s1/2, 2p3/2 and 2p1/2 occupied states (for carbon atoms) and 3s1/2, 3p3/2 and 3p1/2 occupied states (for sulfur atoms), respectively. For the insulating poly(S-r-DIB10%) copolymer agglomerated with conductive carbons, the low energy losses from 0 to 30 eV are dominated by the collective excitations of plasmons and excitons. The spectrum exhibits typical

π plasmon at 5.9 eV and σ + π bulk plasmon at 22.0 eV as bonding fingerprints. Depending on the contributions from each component, positions of the plasmon peaks vary in the range from 5.6 eV to 6.3 eV for the π plasmon and from 21.5 eV to 25.1 eV for the bulk plasmon, respectively.

47

The highest values for π and σ + π plasmon losses at 6.3 eV and 25.1 eV were

found for pure onion-like carbon nanoparticles. Plural scattering of incident electrons in the thick agglomerate causes a broad peak above 40 eV energy loss, which could alter spectral features. The plural scattering effect is removed in a single scattering distribution (blue curve). The single scattering distribution displays only π and σ + π plasmons with an enhanced spectral resolution. The strong π plasmon excitations observed along the percolated aggregates of carbon nanoparticles indicate the appearance of electrically conductive pathways formed by hyperconjugated π-bonding systems. However, no π plasmons are found in any copolymers, despite the existence of the benzene moiety in the DIB molecule.


47

This is consistent with 30

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strongly localized preferentially σ bonding and the insulating nature of the poly(S-r-DIB) macromolecules. For the poly(S-r-DIB50%) copolymer, the bulk plasmon occurs at 22.6 eV, while for poly(S-r-DIB10%) the bulk plasmon shifts to 21.5 eV indicating that the nature of the molecular bonding in the copolymer changes with composition. 47 In the core-loss region (Figure 7f), the net sulfur S L2,3-edge at 165 eV (green curve) has a featureless and rounded profile that is dominated by atomic effects rather than contributions from solid state band structure and coordination. This leads to a broad maximum delayed by 20 eV beyond the ionization threshold of 165 eV. The electron loss near-edge structure (ELNES) of the net carbon C K-edge at 284 eV (Figure 7f, green curve) shows characteristic excitations from 1s to the π* band, corresponding to sp2 bonding with a sharp peak just above the edge onset typical for graphitic-like outer shells as well as excitations from 1s to the σ band with a broad peak above 291 eV corresponding to sp3 bonding. Contrary to low losses, core π excitations are evident for both carbon nanoparticles and copolymers, thus confirming the presence of the benzene moieties of the DIB fragments in the copolymer rich regions. 47

3D Structural Architectures STEM-ET was employed to visualize the 3D hierarchical architectures formed within the poly(Sr-DIB)-based composite cathodes. The extreme topographical variation in these composites means that the projection images formed in a single STEM image can be difficult to interpret since the image intensity at any given point represents the integrated scattered signal resulting from the beam passing through multiple phases and particles. This is especially true in the case of the carbon nanoparticles, which are present in sufficient number density that they often cannot


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be individually distinguished, as can be seen in the HAADF STEM image (Figure 9a). In addition, the distribution of the carbon particles along the surface of the copolymer agglomerates

(b)

(a)

S C

2 1

2

1

(d)

(c)

2 1

2

Figure 9. (a) STEM-HAADF image of a agglomerated fragment of the poly(S-r-DIB10%)-based cathode showing two irregular copolymer particles (brighter regions) surrounded by an interconnected carbon network. The right upper inset shows overlaid S Kα (orange) and C Kα (green) X-ray maps of the copolymer particles. 47 (b) A volume rendering of the ET reconstruction shows the 3D extent of the copolymer microparticles ((1) violet and (2) green) and the carbon network. Individual slices extracted from orthogonal directions in the


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reconstructed volume (c and d) demonstrate that the surfaces of the copolymer are decorated by the carbons, providing electrical contact with the rest of the conductive network.

cannot be readily determined for the same reason. By contrast, the 3D reconstruction of the STEM-ET data (SI, Movie 8S) allows us to visualize the structure either as a whole through volumetric rendering (Figure 9b) or plane by plane (Figure 9c and 9d). The volumetric rendering shows the extent of the carbon network parallel to the incident beam direction, which was missing from the single projection image. The entire network in this case was found to fit into a cubic volume of just over 900 mm3, where the span from top to bottom was nearly 6 µm. From the estimated mass of the active material in the fragment, its maximum expected discharge capacity is ≈ 1.7×10-8 C or 4.7 pAh.

47

The carbon nanoparticles can be seen to surround the

copolymer and form an interconnected 3D percolation network composed of conductive outer shells of partially coalesced aggregated carbons (SI, Figure 7S). For closer inspection, the slices extracted from the X-Y orientation (i.e. parallel to the electron beam, Figure 9c) as well as the orthogonal X-Z orientation (Figure 9d), show that the carbon nanoparticles intimately decorate the surface of the copolymers, thus facilitating charge transfer during reaction. When S/TEM and ET are combined with spatially-resolved low-loss EELS, the lateral distributions of mixed sp2/sp3 carbon-carbon bonding explicitly corroborate the picture that the onion-like outer shells of partially coalesced carbon nanoparticles form electrically conductive percolated networks of conductive pathways. This approach can essentially complement traditional electrochemical techniques such as electro-chemical impedance spectroscopy (EIS, e.g., 58 by enabling real-space visualization of conductivity pathways and interfaces formed within the porous cathodes. Furthermore, spatially-resolved 3D visualization of cathode structural architectures could promote developing of realistic physical models for equivalent circuits used in EIS for


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explaining mechanisms of charge generation and interfacial impedance phenomena in Li-S batteries and related EES systems. The incorporation of the cross-linking fragments into the poly(S-r-DIB) domains through enhanced compatibility with carbons strongly affect an access of the conductive networks to the active interfaces at the surfaces and in pores. It could lead to an increased utilization of the active material in the cathode upon cycling. These findings are consistent with through-plane DC conductivity measurements that show a slight increase in electrical conductivity of the cathode as the fraction of DIB content increases, despite that the DIB molecule itself is insulating.

33

The incorporation of DIB fragments improves the electrical

percolation of the carbons in the cathodes. Simultaneously, increasing the DIB content reduces the propensity for shrinkage-induced cracking while the cathodes being dried, suggesting an important role of the DIB for improving the physico-mechanical stability.

Structural Transformations Under Cycling To investigate the effects of cycling on the cathode integrity, the discharged cathodes were examined by FESEM and TEM (see SI, Figure 9S and Figure 10). In conventional Li-S batteries, capacity fading is often attributed to mechanical detachment of the cathode fragments from the current collector due to irreversible build-up of dense lower Li2Sx discharge products, where x ≈ 2 to 3 that block the active material during the prolonged cycling.

36

The unevenly distributed

sulfide deposits exacerbate the cracks incurred during fabrication due to mechanical stresses induced by repeated cycling. Cathode cracks foreshadow irreversible sulfide deposition and correlate with the lowered capacity retention. Large-area plane view FESEM images and X-ray maps of discharged, solvent-rinsed cathodes made from either the copolymers with DIB mass fraction 50 % and 10 % or elemental sulfur are compared in Figure 9S, SI. The conventional α-


34

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S8-carbon cathode after 30 cycles revealed the deep cracks spreading throughout the cathode and extend down to the current collector exemplifying the limitations of the conventional fabrication


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Microscale

Mesoscale

FESEM

BF-TEM (a2)

(a3)

(a4)

(b2)

(b2)

(b3)

(c2)

(c2)

(c3)

α-S8

(a1)

1 µm Poly(S-r-DIB10%)

(b1)

1 µm (c1)

Poly(S-r-DIB50%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

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1 µm Figure 10. FESEM (a1, b1, c1) and BF-TEM (a2-a4, b2-b4, c2-c4), powders of cycled discharged cathodes: (a1-a4) the conventional α-S8-carbon cathode, (b1-b4) the poly(S-r-DIB10.%)-based cathode; and (c1-c4) the poly(S-r-DIB50.%)-based cathode.


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(a)

(b) C65 0.40 nm

C65

0.41 nm (c) 0.41 nm

0.32 nm (111)

C65

Figure 11. HRTEM, the cycled discharged composite cathodes with different DIB contents: (a) the conventional α-S8-carbon cathode; (b) the poly(S-r-DIB10 %)-based cathode; (c) the poly(S-rDIB50 %)-based cathode. White arrows in image (a) indicate multiple local disruptions and/or insertions in graphitic-like outer shells of carbons. The green circle in image (c) marks a 4 nm Li2S cluster. scheme. These reflect an integral effect of mechanical stresses incurred initially during drying of the cathode film followed by the damage incurred through cycling, which involve multiple contraction/expansion steps. On the contrary, the copolymer-based composite cathodes exhibit


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only a few small cracks or almost crack-free morphologies over large areas. FESEM at higher magnifications (Figure 10) show that extended meso- and nanoscale porosities are preserved despite the prolonged cycling, while the deposition of insoluble sulfides is suppressed. Remarkably reduced degradation after 120 cycles was found with the best preforming poly(S-rDIB10%)-based cathode. FESEM and BF-TEM of the cycled composite cathodes with different DIB content (Figure 10) show that typical chain-like and branch-like morphologies of aggregated carbons as well as their core-shell structures, including the SRO (Figure 4b), did not change drastically upon cycling. Diffraction rings, however, appear more diffuse after cycling. HRTEM observations (Figure 11) indicate that although the carbon core-shell structures are generally preserved during cycling, gradual disruption and expansion of (002) interlayer spacings for graphitic-like outer shells from (0.38 to 0.39) nm to (0.40 to 0.41) nm occurred. Multiple local defects and/or insertions into the shells of the carbons were observed (Figure 11a). In addition, nano-sized lithium sulfide precipitates generated during cycling and deposited onto carbons were found occasionally. The green circle in Figure 11c marks a 4 nm diameter Li2S cluster exhibiting 0.32 nm (111) lattice fringes, which are consistent with a fcc CaF2 structure type, Fm-3m (225) space group.

59

The observations correlate well with the comparative electrochemical

performance of the cathodes illustrating the important role of the interfaces and agglomerated copolymer – carbon composites for enhancing capacity retention, suppressing the deposition of insoluble sulfides and improving physical integrity of the cathodes.

Conclusions Probing 3D structural architectures and functional interfaces in poly(S-r-DIB)-based composite cathodes by focused Li+ ion and electron beams provide critical insights into the origins of the


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enhanced capacity retention in emerging organosulfur polymeric electrodes for high-energy density Li-S batteries. Using multiple operating modes of FESEM, LiFIB, high-resolution analytical (S)/TEM coupled with MSA and tilt-angle ET, we have identified the topographic, morphological, and compositional effects of replacing α-S8 with electrochemically active high molecular mass poly(S-r-DIB) copolymers for DIB mass fraction of 10 % and 50 %. We show that organosulfur moieties from copolymers strongly influence electrochemical conversion reactions via improving the compatibility between the conductive onion-like carbon nanoparticles and the electroactive copolymer domains, which lead to an intimate mixing of the components and creation of the functional interfaces capable to enhance capacity retention and suppress the deposition of insoluble polysulfides. The enhanced homogeneity promotes the creation of a unique hierarchical cathode morphology that is electrochemically and mechanically robust and exhibits reduced levels of cracking but preserves extended porosities required for effective ion transport. We introduce the low-energy scanning LiFIB as an advanced new tool capable of high quality charge-free surface topography and composition sensitive imaging of the heterogeneous battery electrode materials. LiFIB and FESEM-EDS with their chemical sensitivity and greater depth of field provide much more detailed information particularly concerning the surface topography and phase distributions than the light microscopy. Furthermore, injecting a certain amount of Li+ ions in the material with nanoscale precision, the LiFIB initiates the discharge process, making it potentially possible to control the SOD in the selected electrode area in situ without presence of a liquid electrolyte and side reactions associated with the SEI formation. Through direct HRTEM observations and spatially-resolved SAED and EELS, we identify the origin of electron conductivity in the system, i.e., stochastic percolated carbon networks


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composed of 30 nm to 60 nm diameter onion-like particles with graphitic outer shells having hyperconjugated π-bonding. The conductive networks randomly propagate through the cathode matrix forming its rigid skeleton. Although the core-shell carbon structures were preserved during cycling, gradual structural disruption and expansion of (002) interlayer spacings for graphitic-like shells from (0.38 to 0.39) nm to (0.40 to 0.41) nm occurred. Local defects and insertions into the carbon shells as well as nano-sized Li2S precipitates generated during cycling particles were observed. Tilt-angle STEM ET with a Cs-corrected electron probe using advanced iterative reconstruction algorithms was successfully applied to visualize 3D hierarchical structural architectures created in poly(S-r-DIB)-carbon agglomerates of several micrometers in thickness. Combining comprehensive multiparameter characterization at multiple scales ranging from the macroscale up to the atomic level with probing of lithiation mechanisms in situ, this can constitute a powerful integrated focused ion and electron beam platform for advanced battery research and diagnostics. Furthermore, such innovative multimodal inter-instrumentation platform utilizing LiFIB, FESEM, (HR)TEM, ED, STEM, MSA, ET, EDXS, and EELS can essentially complement conventional electrochemical techniques enabling detailed real-space visualization of conductivity pathways and functional interfaces formed with highly porous inhomogeneous electrode structures. This approach can promote the development of realistic physical models for equivalent circuits used in EIS for explaining mechanisms of charge transfer and interfacial impedance phenomena in Li-S batteries and related EES systems.

Acknowledgments


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The authors gratefully acknowledge Paul Kotula (Sandia National Labs) for providing the AXSIA software package for MSA, Joshua Taillon, Gery Stafford and John Henry Scott (NIST) for useful discussions and the NSF (CHE-1305773) for support of this work.


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Multimodal Characterization of the Morphology and Functional

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