Impact of sulfur-containing additives on lithium-ion battery performance

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Impact of sulfur-containing additives on lithium-ion battery performance - from computational predictions to full cell assessments Piotr Jankowski, Niklas Lindahl, Jonathan Weidow, Wladyslaw Grzegorz Wieczorek, and Patrik Johansson ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00295 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 31, 2018

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ACS Applied Energy Materials

Impact of sulfur-containing additives on lithiumion battery performance - from computational predictions to full cell assessments Piotr Jankowski*,a,b,c Niklas Lindahl,b Jonathan Weidow,b Władysław Wieczoreka,c and Patrik Johanssonb,c a

Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland b

Department of Physics, Chalmers University of Technology, SE-412 96, Gothenburg, Sweden

c

ALISTORE-ERI European Research Institute, CNRS FR 3104, Hub de l’Energie, Rue Baudelocque, 80039, Amiens, France

* [email protected]

Abstract

Electrolyte additives are pivotal for stabilization of lithium-ion batteries, by suppressing capacity loss through creation of an engineered solid electrolyte interphase (SEI) layer at the negative electrode and thereby increasing life-time. Here we compare four different sulfurcontaining 5-membered ring molecules as SEI-formers: 1,3,2-dioxathiolane-2,2-dioxide (DTD), propane-1,3-sultone (PS), sulfopropionic acid anhydride (SPA) and prop-1-ene-1,3-

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sultone (PES). Density functional theory calculations and electrochemical measurements both confirm appropriate reduction potentials. In order to connect the protective properties of the SEIs formed to the chemical structure of the additives, the decomposition paths are computed and compared with spectroscopic data for the negative electrode surface. The performance of full cells cycled using a commercial electrolyte and the different additives reveals the formation of organic dianions to play a crucial beneficial role; more so for DTD and SPA than for PS and PES.

Keywords Li-ion battery; Solid Electrolyte Interphase; Additives; Sulfur-containing compounds; DFT; Battery cycling


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1. Introduction An increasing demand for high-capacity lithium-ion batteries (LIBs), forecast to be worth up to ca. $220 billion in 2024, is driving battery research towards new solutions to improve both energy density and long-term stability.1,2 Application in electric cars and renewable energy storage require a cycle life with more or less maintained capacity, often benchmarked at 75-80%, for several years. Decrease in LIB capacity is usually caused by the intrinsic thermodynamic instability of the cells, having very wide electrochemical operation windows,3 and thus relying on kinetic stabilization. In LIBs an initial reduction of electrolyte solvent molecules occurs during the very first cycles and the decomposition products ideally cover the surface of the negative electrode, creating a solid electrolyte interphase (SEI).4 The SEI hinders further reduction and decomposition of the electrolyte, while still being conductive to lithium cations – hence keeping the LIB operational. The properties of the SEI thus have a huge impact on performance, cyclability and long-term stability. Apart from salt and solvent, any functional LIB electrolyte will need one or more additives to enhance the overall performance – most often targetting safety properties, but also salt solubility, wetting of electrodes, etc.5 Additives targetting the SEI, SEI-formers, are thus designed to better control the SEI creation and build-up process – foremost by being reduced at a higher potential than other electrolyte components and thereby passivate the negative electrode prior to any reductive decomposition of the electrolyte.6 Several families of SEI-formers have been reported and employed, including compounds with double bonds,7–13 fluorinated molecules,14,15 isocyanates,16–18 and oxalates.19,20 The most used and known electrolyte additive is vinylene carbonate (VC)7, which undergoes polymerization at the negative electrode surface creating a protective SEI-layer.21 However, VC has a negative impact on performance due to oxidation at the positive electrode,



especially at elevated temperatures.22–24 Recently, sulfur-containing additives have been the subject of many studies, due to higher oxidation stabilities. Replacing the carbonyl carbon atom by a sulfur atom in organic carbonates lowers the LUMO level energy and thus facilitates reduction. The simplest such SEI-former, ethylene sulphite (ES), a direct analogue of ethylene carbonate (EC), renders an SEI with protective properties, but just like VC suffers from poor oxidation stability, ca. 3 V vs. Li+/Li° (due to the presence of lone pair electrons at the sulfur atom).25 Addition of another oxygen atom results in 1,3,2-dioxathiolane-2,2-dioxide (DTD), shown to improve the oxidative stability up to ca. 6 V vs. Li+/Li°.26 Sano and Maruyama proposed a radical polymerization mechanism at the negative electrode surface for this additive.27 Also sultone compounds have been studied; propane-1,3-sultone (PS) 28,29 and prop-1-ene-1,3-sultone (PES)

30,31

were both proven to stabilize the SEI-layer. Some of these

sulfur-containing SEI-former additives have been studied by density functional theory (DFT) calculations to elucidate the mechanism of reduction.32–35 The aim of this work is to connect differences in the chemical structure of the SEI-forming additives to the observed reduction behaviour and finally the SEI-layer composition and overall battery performance. We choose four additives systematically differing in the presence of oxygen atoms and double bonds (Figure 1). By initial DFT computations and analysis of the additives, via electrochemical and spectroscopic studies of the SEI-layers formed, to the final battery cycling studies, we gain knowledge enabling a more rational design of future SEI-forming additives.


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Figure 1 Chemical structures of the studied SEI-forming additives. 2. Computational and experimental methods 2.1. Computational All calculations were carried out using the Gaussian0936 package and a methodology developed previously.37 All geometries were optimized using the M06-2X DFT functional and the 6-311++G(d,p) basis set. The effect of a liquid surrounding was modelled using the conductor-like polarizable continuum model (C-PCM) using parameters for water – as previously shown adequate.37 Local minima and transition states geometries were both confirmed by frequency calculations. Reduction potentials were calculated based on reduction reactions with a lithium cation coordinating to the additive.37 The energies for each of the different path steps were calculated and referenced to the neutral additive, divided by the number of sulfur atoms. The analysis of the charge distribution was performed using the Natural Population Analysis (NPA).38 2.2. Materials 1 M LiPF6 in EC:DMC (99.9%, Solvionic) was used as base electrolyte to which different SEI-forming additives: DTD (98%, TCI Chemicals), PS (99%, Sigma Aldrich), PES (99%, TCI Chemicals) and sulfopropionic acid anhydride, SPA (95%, Azepine), were added (1 wt% and 5 wt%) to create functional electrolytes. Commercially available graphite (MTI) and LFP



(MTI) electrodes were cut out as 12 mm diameter discs with capacities of 2.7 and 2.3 mAh, respectively, and subsequently dried in vacuum for 24 h at 100 °C. Half- and full cells were assembled using glass fibre separators (150 µm, Chemland, dried in vacuum for 24 h at 100 °C) in Swagelok cells using 70 µl of each electrolyte. All preparations were made inside an argon-filled glovebox (both O2 and H2O < 10 ppm). 2.3. Electrochemistry and battery cycling The oxidation potential of the additives was assessed by cyclic voltammetry (CV) using a Pt disk (99.9%, Mennica Polska) as working electrode (WE) and Li foil (99.9%, Gallium Source) as reference (RE) and counter electrode (CE). The reduction potential and behaviour was examined both by CV and by the differential capacity curve for the first charging process of half-cells, both using a graphite/electrolyte/Li set-up. The half-cells were further cycled using a C/20 current between 0.01 - 1.5 V vs. Li+/Li° and after 10 cycles disassembled in the discharged (delithiated) state inside the glovebox. Electrodes were rinsed with DMC (99%, Sigma Aldrich) and after a few hours put in the antechamber to remove residual solvent molecules as described elsewhere,39 and subsequently transferred to the spectroscopic analyses. Battery performance tests employed graphite/electrolyte/LFP full cells of 2.3 mAh theoretical capacity cycled using a C/10 current between 2.6 - 3.8 V vs. Li+/Li°. Electrochemical impedance spectroscopy (EIS) (500 kHz – 100 mHz, 10 points per decade, 2 measurements per frequency, 10 mV AC amplitude) of full cells was performedbefore cycling and during cycling after full discharge, i.e. at 3.8 V vs. Li+/Li°. All measurements were performed at room temperature using a VMP3 potentiostat (Bio-logic). 2.4. XPS and FT-IR spectroscopy The graphite electrodes were cut into three pieces for spectroscopic and microscopic analysis. The first parts were transferred without any exposure to the ambient atmosphere from the glovebox to the X-ray photoelectron spectroscopy (XPS) system (Perkin Elmer PHI


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5000 C ESCA) to study the surface composition of the electrodes including the SEI-layers formed. The X-ray source was monochromatic Al Kα (1486.7 eV) and the concentric hemispherical analyzer was positioned at 45° angle from the sample normal. Energies were shifted to set the position of the C-C (sp3) peak to 284.8 eV. Deconvolution of the peaks was performed using XPSPEAK41 software. C 1s spectra were fitted using 5 peaks, whereas for O 1s and S 2p spectra only 2 peaks were considered. In case of S 2p, both p-peaks consisted of double-peaks with set interdistance of 1.16 eV and area ratio 1:2. The second parts of the graphite electrodes were subject to FT-IR spectroscopy using an Avatar System 370 FT-IR spectrometer with a resolution of 2 cm−1 and a Golden Gate ATR with a diamond crystal in the range 550-4000 cm-1. Electrodes were transferred inside argonfilled plastic bags, exposed to the ambient atmosphere only briefly during the time of measurement. While the SEI-layer is very thin and thus the signal weak, FT-IR spectra were collected for 64 scans. 2.5. SEM microscopy The third part of the samples were used to obtain SEM images, using a Ultra 55 scanning electron microscope equipped with a field emission gun. Imaging was carried out with 5 kV acceleration voltage with secondary electrons. A 60 µm objective aperture was used and the working distance was set to 5 mm. 3. Results and Discussion Our comparative study starts with DFT calculations of molecular parameters, to predict basic abilities of the additives to act as SEI-formers, followed by experimental studies of electrochemical stabilities. The analysis of DFT computed reaction paths after reduction connects the chemical structure to the composition of the SEI-layer, as probed by



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spectroscopic techniques. Finally, the influence of each of the additives is examined by studying full cell performance. 3.1. DFT computed molecular parameters Comparisons of DFT computed parameters of the additives to those of EC show all the former to have lower LUMO energy levels and higher EAs, indicating them to be favoured for reduction (Table 1). In more detail the introduction of a double bond and/or carbonyl group significantly enhance the reduction ability; the LUMO energies are 0.04, -0.27 and 0.10 eV for DTD, PES and SPA, respectively. Additional information on kinetics of reduction can be obtained from the chemical hardness – harder means larger resistance to accept an electron, but in turn most often immediate decomposition upon acceptance.37 In contrast, softer molecules are more stable in their reduced form, which can either hinder any SEI-layer formation altogether or, preferably, create the SEI-layer in a slower, more controlled way. The introduction of an additional double bond usually makes a molecule softer, obvious here by comparing PES (η=3.67 eV) to DTD (η=4.48 eV). As all additives are to be used in LIB electrolytes, i.e. with Li+ cations present, the Gibbs free energies of interaction with a Li+ cation was computed for both the neutral species, resulting in no complex formation, and the reduced, anionic species, resulting in spontaneous ion-pair formation. Hence ion-pair formation should impact the reduction of each additive. Table 1. Dipole moments, HOMO and LUMO level energies, ionization potentials (IP), electron affinities (EA), chemical hardnesses (η), and Gibbs free energies of interaction with lithium cation (∆G). DTD

PS

SPA

PES

EC

Dipole /Debye

7.99

8.45

7.40

8.64

7.33

HOMO /eV

-10.77

-10.33

-10.67

-9.89

-10.62

LUMO /eV

0.04

0.03

-0.10

-0.27

0.06


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IP /eV

9.60

9.02

9.21

8.78

9.55

EA /eV

0.66

0.69

1.08

1.45

0.65

η /eV

4.48

4.16

4.06

3.67

4.36

Neutral 3.76

0.85

3.70

1.20

0.45

Anion

-15.72

-24.73

-18.42

-41.54

∆G(Li+) /kJ·mol1

-19.56

3.2. Experimental and computational electrochemical stability Searching for SEI-formers essentially focuses on the electrochemical stability/instability, that should match the electrode and solvent decomposition potentials. All of the tested sulfurbased molecules have a high oxidation stability – higher than 5 V vs. Li+/Li°. Their addition to a commercial electrolyte only slightly reduces the oxidation stability, making these functional electrolytes compatible with all popular cathodes, including high-voltage electrodes (Figure 2a).40



Figure 2 Electrochemical study of the 1 M LiPF6 EC:DMC electrolyte without and with 5 wt% of additive; a) oxidation stability on Pt as WE; sweep rate of 5 mV·s-1 b) differential capacity curves of graphite electrode charging; rate of C/20 c) cyclic voltammograms of graphite electrode; sweep rate of 1 mV·s-1.


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At the other end of the potential range, we observed signatures arising from reduction reactions of the additives. The reduction peaks, all above 0.6 V vs. Li+/Li° for the electrolytes with 5 wt% of additive, can be attributed to their reduction and SEI formation (Figure 3 and 4). The lowest observed reduction potential, a weak signal at ca. 0.74 V vs. Li+/Li°, thus just above the solvent decomposition limit, was obtained for PS – in accordance with other studies.15,28,30 Much more pronounced signals were obtained for DTD, PES and SPA at 1.05, 1.12 and 1.23 V vs. Li+/Li°, respectively. Also at higher potentials reduction peaks are observed; for SPA a feature can be observed already at 2.29 V in the differential capacity curve (Figure 2b), while the signals from DTD and PS are much weaker and only clearly observed by CV at 2.20 and 1.75 V (Figure 2c), respectively – with the onsets corresponding to the weak differential capacity features. In contrast, the electrolyte with PES has no additional peak at these potentials. The origins of the experimental observations can be explained by DFT calculations of reduction potentials using thermodynamic cycles (Table 2). The low potential features correspond to reductions without any ring-opening reaction, with the computationally obtained potentials of DTD, SPA and PES having a standard deviation of only ca. 0.14 V. For PS the error is larger, with a very low reduction potential of 0.44 V, which is connected with the lower charge at the sulfur atom than for DTD, +2.28 and +2.51, respectively, as a result of a carbon atom close rather than an electronegative oxygen atom. This implicates that the LUMO of PS is “pushed out of” the sulfur atom to other atoms of the ring (Figure 3). The same sulfur charges were obtained for SPA and PES, +2.28 for both. For these additives, however, the preference of electron acceptance is changed towards the carbonyl group or the double bond, accompanied by a decreased energy needed. The observed high-potential peaks for DTD, PS and SPA are all related to simultaneous reduction and ring-opening reactions, while the lack of such a feature for PES can be explained by its softness; the double bond ACS Paragon Plus Environment


stabilizes the reduced form and hinders ring-opening. Hence the values for the ring-opening reduction potentials can be used as a very good indicator for the first step of the additive decomposition. 3.3. Reaction paths – from SEI-former to SEI-layer By calculating the different reaction paths it becomes obvious how very sensitive the process of forming an SEI-layer is to changes in the molecular structure of the SEI-formers (Figs. 4 and 5). Furthermore, many paths have optional branches and therefore it becomes even more crucial to validate the energetic/thermodynamic picture, even if the reaction barriers do affect the kinetics, by in detail study the composition of the SEI-layers formed (see section 3.4). All the here below computed possible SEI-layer compounds are summarized in Table 3. Starting with the chemically rather hard DTD it first undergoes reduction to an S–O bondbroken structure (1: -338 kJ·mol-1) via a meta-stable closed ring-structure (-248). Further reduction of molecule 1, leading to inorganic compounds like Li2SO3, requires much less energy from the polarized electrode (4.92 V vs. Li+/Li°). Another possibility is a reorganization of the structure towards ROSO3Li compounds (2: -373) with an energy barrier of 148 kJ·mol-1. The termination reactions of 2 result in formation of preferred di-anionic structures (12: -529; 13: -487). The mechanism proposed by Sano and Maruyama27: polymeric structure creation (14: -330) by radical polymerization initiated by 1, was found not to be energetically preferred due to the stabilization of 1 by a lithium cation to create a 7membered ring structure. The replacement of one of the oxygen atoms by the CH2 group drastically decreases the reduction ability of this additive. For PS the corresponding ring-opening reaction with S–O bond breaking (3: -312) is limited by an energy barrier of 90 kJ·mol-1, explaining the weak features in Figs. 2b and c. Subsequent possible reorganization towards 4 (-346) and 5 (-322) comes with energy barriers


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of 167 and 101 kJ·mol-1, respectively, and lead to the creation of the di-anions R(SO3Li)2 or R(OSO2Li)2 (15: -499; 16: -464; 17: -471; 18: -442). Yet another option is a second reduction of the radicals, made possible by a second electron from the electrode, and creation of inorganic Li2SO3 and Li2S end products. Moving to SPA, it in analogy with DTD has a barrier-less reaction via a meta-stable closed ring-structure, but here leads to a C–O bond cleaved radical (6: -335). Addition of the second electron requires a potential of 1.83 V vs. Li+/Li° and results in Li2SO3 and gaseous products. Here the alternative path involves a reorganization of 6 to 7 (-406) with an energy barrier of 82 kJ·mol-1, or less likely further to 8 (-361) with a rather large barrier of 203 kJ·mol-1. The highly preferred reduction of 7 (4.17 V vs. Li+/Li°) results in a very stable di-anionic structure (19: -952), but it does require contact between an already reduced molecule (7) and the electrode – hence solubility of the former will matter. Yet another possibility is a reaction between two radicals (7, 8) creating di-anionic structures (20: -520; 21: -428; 22: -519). Finally, for PES a slightly different behaviour was found; being the by far softest additive, and hence its reduced form much more stable, three initial pathways are possible – as also outlined in the literature.35 These paths involve initial breaking of different bonds: the C–O bond (11: -397) (preferred), the S–O bond (9: -314) and the S–C bond (10: -286). However, they all have similar, and low, energetic barriers: 18, 15 and 16 kJ·mol-1. The C–O and S–C bond-breaking paths is more likely related to the formation of dimeric structures, for the latter it might involve a re-arrangement of 10 via a proton transfer that stabilizes the structure by delocalization of the radical electron (23: -407). Dimerization of 23 and 11 gives ROSO2Li and RSO3Li end products, respectively (24: -500; 25: -429; 26: -495; 27: -430). In contrast, the product of S-O bond breaking (9) is stabilized by a lithium cation to create a 7-membered ring (as for DTD) and any further modification of this rigid structure is difficult, why a



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second-electron reduction (Ered = 3.64 V vs. Li+/Li°) is expected with the formation of Li2SO3 and Li2S end products (as for PS).

Figure 3. LUMOs illustrated using isosurfaces of 0.03 e/bohr3 for: a) DTD, b) PS, c) SPA, and d) PES. Table 2. Experimental and calculated reduction potentials. exp. Ered

calc. Ered / V vs. Li+/Li°

∆Ered exp-calc / V

/Vvs. Li+/Li°

closed ring

open ring

closed ring

open ring

DTD

1.05; 2.20

1.11

2.04

-0.06

0.16

PS

0.74; 1.75

0.44

1.78

0.30

-0.03

SPA

1.23; 2.29

1.21

2.01

0.02

0.28

PES

1.12; -

0.95

2.65

0.17

-

standard deviation

0.14

0.13


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Table 3. Summary of predicted SEI-layer compounds and selected characteristic IR vibrational frequencies. The calculated data have been scaled by 0.96 and those bands easily identifiable experimentally are in bold. Calculated /cm-1

Experimental /cm-1

SEI compound

Additive

Li2SO3

DTD, PS, SPA, 831; 936 PES

Li2S

PS, PES

738; 788

R(OSO3Li)2

DTD

972; 1106; 1225

PS

996; 1092; 1148

1080; 1188

PES

994; 1094; 1153

1075; 1182

PS

978; 1002; 1048

1019; 976

PES

975; 1018; 1044

1027; 977

LiSO2RCO2Li

SPA

915; 953; 1589

1583

R(SO2Li)2

SPA

920; 964; 976

1003

1049; 1279; 1663

1294

R(SO3Li)2

R(OSO2Li)2

LiSO2OC(O)ROSO2Li SPA R(C(O)OSO2Li)2

SPA

904

1194; 1102; 1231

1034; 1042; 1159; 1143 1669



Figure 4 Reaction paths and associated energies [kJ·mol-1] of initial stages of reduction and subsequent SEI compound formation for: a) DTD, b) PS, c) SPA and d) PES.


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Figure 5 Further reactions and associated energies [kJ·mol-1] for: a) DTD, b) PS, c) SPA and d) PES. 3.4. Spectroscopic analyses of the SEI-layers



To verify (or dismiss) the predicted SEI compounds as parts of the SEI-layers resulting from the electrochemical cycling, spectroscopic analyses of the graphite electrodes were performed. All analyses were performed on electrodes after 10 cycles using electrolytes with 5 wt% of additive (initial tests after 100 cycles with 1 wt% of additive produced too weak IR signatures). Starting with the XPS studies, the measured spectra were fitted with peaks at fixed binding energies for the expected species. The C 1s spectra (Figure 6) show binding energies at 284.3 and 284.8 eV corresponding to the electrode itself, sp2 from graphite and sp3 from binder, respectively,41 while the peaks at 287.3 and 286.3 eV can be attributed to carboxylic and chain carbon of lithium ethylene dicarbonate (LEDC) or similar compounds.31 In the O 1s spectra the peaks at ca. 531.5 and ca. 533 eV correspond to Li2CO3 and lithium alkyl carbonates, respectively, or possibly their sulfur analogues.31,42,43 Semi-quantitatively, high contents of Li2CO3 and/or Li2SO3 were found for all samples, but especially for the electrodes cycled with the SPA and PES additives in the electrolyte. The S 2p peaks at ca. 169 eV correspond of Li2SO3 and di-anionic structures,27,34,41,42,44 and while any separation of these broad and closely positioned peaks becomes ambiguous, the main peak position can anyhow be used as a qualitative indicator. Hence, the S 2p spectra were fitted by individual double peaks having non-fixed binding energies. For DTD the peak is shifted toward higher binding energies (ca. 170.0 eV) as compared to PS and PES (both ca. 169.5 eV) possibly indicative of sulfur in a higher oxidation state e.g. in ROSO3Li. The peak for SPA at 168.7 eV is in contrast likely related to sulfur in a lower oxidation state as in e.g. RSO2Li, and hence supports the formation of 20. The minor peaks at 163 eV for both PES and PS are interpreted as Li2S. Moving to the FT-IR spectroscopy data and analysis (Figure 7) more or less give the same overall picture. The bands at 1483, 1405 and 873 cm-1 indicate presence of Li2CO3 for all electrodes.45 Likewise, LEDC is confirmed by bands at 1637 and 1315 cm-1,46 and with lower


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contents for electrodes cycled with the SPA and PES additives in the electrolytes. Thus the FT-IR and XPS analyses agree both qualitatively and semi-quantitatively, despite their different surface penetration. Furthermore, a band at 904 cm-1 assigned to Li2SO3 25 and bands in the regions 1175-1190 and 1075-1105 cm-1, by comparison to the calculated data assignable to RSO3Li and ROSO3Li, were observed for all electrodes cycled with additives. Re-assuring for the interpretation of the SEI composition is that most of the by DFT predicted main vibrational frequencies match the experimentally observed within < 20 cm-1 (Table 3), especially important when an accurate experimental assignment is lacking. For the electrode cycled with the electrolyte containing SPA a few additional bands were obtained; a feature of O-C interaction between SO3 and carbonyl group at ca. 1294 cm-1 and at 1583 and 1003 cm-1 attributable to the stable di-anion 19 and to RSO2Li compounds, respectively, based on the DFT data. The weak signal at ca. 1451 cm-1 corresponds to a C=C stretching vibration from the electrode47,48 and is strongest for DTD and SPA, while it disappears for PES indicating a thicker SEI-layer.



Figure 6 The S 2p, O 1s and C 1s spectra obtained by XPS for graphite electrodes cycled in graphite/1 M LiPF6 EC:DMC/Li half-cells without and with 5 wt% of additive. Black dots are measured spectra and full lines are fits of peaks.


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Figure 7 FTIR spectra of the surfaces of the graphite electrodes cycled in graphite/1 M LiPF6 EC:DMC /Li half-cells without and with 5 wt% of additive. Spectra shifted vertically for clarity of presentation. 3.5. Full cell battery performance tests Finally, to check the ability of the SEI-formers to stabilize the battery capacity, cycling of full cells with electrolytes containing 1 wt% of additive were performed (Figure 8). All additives had a possitive impact on the capacity retention. The best results – 85.7% retention of capacity after 100 cycles – were obtained for SPA, much better than for the base electrolyte – 51.5%. Only a little worse stabilization was observed for DTD, and more moderate for PES and PS: 79.7%, 69.3 % and 63.7%, respectively. Reduction of additives during the battery tests take place predominantly during the initial charging of the graphite electrode and hence causes a lower coulombic efficiency for the first cycle – the highest values, 91% and 90%, were recorded for DTD and PES, whereas for SPA and PS they were much closer to base



electrolyte: 87%, 86% and 86%, respectively. The coulombic efficiency for the subsequent cycles were for all cells 98-99%, proving the formation of the SEI-layers to predominantly take place during the initial cycle. A slow growth of the SEI-layer in subsequent cycles, causing decrease in capacity, was observed as increased interphase resistance by means of EIS (Figure 9, S2 and Table S2). The increase was faster for the base electrolyte, implying the presence of additives to modify the interphase and lower the resistance. For the DTD and SPA containing electrolytes the layers formed seem to be stable after 100 cycles, as any further increases are very small and these cells also have the lowest resistances at ca. 170 Ω and ca. 125 Ω, respectively.

Figure 8 Cycling performance at C/10 rate of graphite/1 M LiPF6 EC:DMC /LFP full cells without and with 1 wt% of additive. Two separate cells were tested for each electrolyte, solid and dashed lines, and × indicates cell failure.


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Figure 9. Resistance of the interphase of graphite/1 M LiPF6 EC:DMC/LFP full cells without and with 1 wt% of additive before and during cycling. 4. Conclusions Sulfur-containing analogues of organic carbonates can successfully be used as SEI-formers with the behaviour of the resulting functional electrolytes being highly dependent on small changes in their chemical structure. Presence of electronegative atoms or double bonds eases the initiation of the reduction reaction by increasing the reduction potential. The structures of both DTD and SPA advantageously lead to reduction paths creating large amounts of dimeric sulfur-based species – which macroscopically are seen to contribute to create an advantegeous SEI by the better full cell capacity retention obtained. On the other hand, the PS and PES additives have energetic barriers slowing down their reduction processes and partially block the paths, why a second reduction can take place creating inorganic species, mainly Li2SO3, leading to worse capacity retention (but still these electrolytes perform better than without any additive present). Based on these initial studies we strongly suggest implementation of the SPA additive in LIB electrolytes for further tests incl. full cells with high-voltage cathodes.



The origin to the advantageous properties of SPA is indicated to mainly be due to the presence of oxygen atoms and groups that stabilizes its reduced form by electron delocalization – a notion useful as a guideline to further develop other, even better, SEIforming additives.

5. Acknowledgements The authors wish to acknowledge Dr. Carmen Cavallo for assistance with the preparation of samples for the SEM measurements. All calculations were carried out at the Wrocław Centre for Networking and Supercomputing (www.wcss.pl), grant no. 346. Support from the National Science Center of Poland (grant no. 2015/17/N/ST4/03867), ALISTORE-ERI (doctoral studies), and Chalmers Area of Advance Energy (travel scholarship to Piotr Jankowski) are all gratefully acknowledged. Patrik Johansson acknowledges both the Swedish Energy Agency for a basic research grant via the Swedish Research Council, as well as the continuous support provided by many of Chalmers Areas of Advance: Energy, Materials Science, and Transport.

6. Associated Content Supporting Information Available: Half-cells charge-discharge data, full cell EIS data, SEM images of electrodes after cycling.

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The Global Market for Lithium Ion Batteries for Vehicles Is Expected to Total $221 Billion from 2015 to 2024 https://www.navigantresearch.com/newsroom/the-global-


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Impact of sulfur-containing additives on lithium-ion battery performance

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