Constructing a Protective Interface Film on Layered Lithium-Rich

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Constructing a protective interface film on layered lithium-rich cathode using an electrolyte additive with special molecule structure Xiongwen Zheng, Xianshu Wang, Xia Cai, Lidan Xing, Mengqing Xu, Youhao Liao, Xiaoping Li, and Weishan Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09554 • Publication Date (Web): 18 Oct 2016 Downloaded from http://pubs.acs.org on October 19, 2016

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ACS Applied Materials & Interfaces

Constructing a protective interface film on layered lithium-rich cathode using an electrolyte additive with special molecule structure

Xiongwen Zhenga, Xianshu Wanga, Xia Caia, Lidan Xinga,b, Mengqing Xua,b, Youhao Liaoa,b, Xiaoping Lia,b, Weishan Lia,b*

a. School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China b. Engineering Research Center of MTEES (Ministry of Education), Research Center of BMET (Guangdong Province), Engineering Lab. of OFMHEB (Guangdong Province), Key Lab. of ETESPG (GHEI), and Innovative Platform for ITBMD (Guangzhou Municipality),South China Normal University, Guangzhou 510006, China.

ABSTRACT Phenyl vinyl sulfone (PVS) as a novel electrolyte additive is used to construct a protective interface film on layered lithium-rich cathode. Charge-discharge cycling demonstrates that the capacity retention of Li(Li0.2Mn0.54Ni0.13Co0.13)O2 after 240 cycles at 0.5 C between 2.0 and 4.8 V (vs. Li/Li+) reaches about 80 % by adding 1 wt. % PVS into a standard (STD) electrolyte, 1.0 M LiPF6 in EC/EMC/DEC (3/5/2 in weight). This excellent performance is attributed to the special molecular structure of PVS, compared to the additives that have been reported in literature. The double bond

ACS Paragon Plus Environment


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in the molecule endows PVS with preferential oxidizability, the aromatic ring ensures the chemical stability of the interface film and the sulfur provides the interface film with ionic conductivity. These contributions have been confirmed by further electrochemical measurements, theoretical calculations and detailed physical characterizations.

KEYWORDS: Phenyl vinyl sulfone, Molecular structure, Electrolyte additive, Layered lithium-rich oxide, Cyclic stability.

1. INTRODUCTION For the large-scale application in electric vehicles or renewable energy storage systems, the energy density of lithium ion battery needs to be improved1-4. To meet this requirement, many new electrode materials were developed, including the anodes that possess larger specific capacity than the currently used graphite and the cathodes that possess higher work potential and larger specific capacity than the well-known LiCoO2, LiMn2O4 and LiFePO4

5-7

. Among the potential cathodes, layered

lithium-rich oxides (LLO), Li1+[x/(2+x)]M1-[x/(2+x)]O2 (M=Mn, Ni and Co)7, is the most attractive candidate, because this cathode possesses several advantages including larger specific capacity, higher working potential and most importantly lower cost7-11.

However, there remain issues to be solved before LLO can be put into practical use, including low initial coulombic efficiency, voltage decay and poor rate capability and cyclic stability. Up to date, a number of approaches have been proposed to solve


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these issues. It is found that the voltage decay can be mitigated by increasing Ni-content or doping other metal ions in LLO 12,13, the initial coulombic efficiency of LLO can be improved by surface modifications14-17, and the rate capability of LLO can be modified by introducing electronically conductive materials such as graphene18-20. Nonetheless, these approaches are complicated and not beneficial for improving the cyclic stability of LLO. The poor cyclic stability of LLO originated mainly from the instable interface between LLO and electrolyte under high potential. The carbonate-based electrolyte tends to decompose when the working potential is beyond 4.2 V (vs. Li/Li+), generating HF21-23, which might attack LLO and even corrode the current collector, leading to the poor cyclic stability of LLO24. To improve the cyclic stability of LLO, it is necessary to separate LLO from the direct contact with electrolyte. Coating LLO with inert inorganic compounds or forming protective interface films on LLO by using additives is effective for improving cyclic stability of LLO. Comparatively, forming a protective interface film is more cost-efficient than coating. Various electrolyte additives have been used for improving the cyclic stability of LLO. For example, the capacity retention of Li(Li0.2Mn0.54Ni0.13Co0.13)O2 was enhanced to 80 % after 200 cycles at 0.5 C between 2 V and 4.8 V by using dimethylacetamide (DMAc) 8,

to 73 % after 220 cycles by tris (trimethylsilyl) borate

(TMSB)9, and to 74 % after 225 cycles by tris (trimethylsilyl) phosphite (TMSPi) 22. However, these improvements are not satisfactory. Electrolyte additives have been successfully applied for improving the cyclic stability of other high potential cathodes25-28. It has been known that the molecule



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structure of an electrolyte additive plays an important role in the formation of a protective cathode interface film. To avoid the interference of electrolyte decomposition products, the cathode interface film should be formed at the potential lower than that for electrolyte oxidation decomposition. This oxidation preference is determined by its molecular structure of the additive. For example, the better effectiveness of TMSPi than TMSP is attributed to the easier oxidation of TMSPi than TMSP25, because the oxidation state of phosphorus in TMSPi is lower than that in TMSP. The C=C double bond in a molecule is supposed to be active for the oxidation of the molecule26, so that vinyl ethylene carbonate (VEC) is able to form a protective cathode interface film27,28. Since the interface film mainly consist of the decomposition products from an additive, the molecular structure of the additive will determine the properties of the film, in terms of physical and chemical stability and ionic conductivity. Aromatic compounds have been widely applied as additives for the formation of stable cathode interface films on cathode materials due to the robustness of aromatic ring29-34. Sulfur in the electrolyte additives is found to be beneficial for the ionic conductivity improvement of the resulting cathode interface film35-38. Based on the significance of the molecule structure in electrolyte additives, we proposed a novel electrolyte additive, phenyl vinyl sulfone (PVS), for constructing a protective cathode interface film on LLO. As seen in Scheme 1, PVS possesses three functional groups: C=C double bond, aromatic ring and sulfone. With this special molecule structure, a stable and ionic conductive cathode interface film on LLO was


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constructed, and the cyclic stability of LLO was significantly improved. The contributions of PVS were investigated by theoretical calculations, electrochemical measurements and physical characterizations.

Scheme 1. Molecular structure of phenyl vinyl sulfone (PVS).

2. EXPERIMENTAL AND THEORETICAL METHODS Preparation: The base electrolyte (STD) was provided by Dongguang Kaixin Battery Materials

Technology

Co.

Ltd,

China,

and

contained

1.0

M

lithium

hexafluorophosphate (LiPF6) dissolved in a mixed solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC), EC/EMC/DEC = 3/5/2 in weight. The electrolyte additive, phenyl vinyl sulfone (PVS, >99 %), was purchased from Alfa. Both STD electrolyte and PVS were used without further purification. 0.5 %, 1 % or 2 % PVS in weight was added into STD electrolyte in an argon-filled glove box (Braun, Germany) under manual vibration to obtain additive-containing electrolyte. Water and oxygen in the glove box were controlled to less than 0.1 ppm. Li(Li0.2Mn0.54Ni0.13Co0.13)O2, denoted as LR-NCM, was synthesized through the co-precipitation as described in our previous report39. LR-NCM

electrode

was

prepared

as

follows.


N-methyl

pyrrolidone


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(NMP)-based slurry of LR-NCM powder (80 wt. %, obtained after uniform grinding), 10 wt. % polyvinylidenefluoride (PVDF) as binder and 10 wt. % acetylene black as conductive agent, was coated on aluminum foil, followed by drying and calendaring. 2025-type coin cells were assembled in the glove box mentioned above, with LR-NCM cathode as working electrode and lithium foil anode as counter and reference electrode.

Electrochemical measurements and physical characterizations: Charge-discharge cycling was carried out on Land cell test system (Wuhan, China) at room temperature. The cells were charged to 4.8 V and discharged to 2.0 V at 0.1 C (1 C=250 mAh g-1) for 3 cycles. After initial cycling, the cells were charged-discharged at 0.5 C for evaluating the cyclic stability, or at 1 C, 2 C, 4 C and 5 C successively for the evaluation of rate capability. Voltage range for charge/discharge was between 2.0 V and 4.8 V. And electrochemical impedance spectroscopy was performed on PGSTAT-30 (AutoLab, Metrohm, Netherlands) with the frequencies ranging from 105 to 0.01 Hz and the voltage amplitude of 0.01 V. To ensure a quasi-stable state for impedance measurements, cells were charged to 4.8 V at 0.1 C and then kept at this potential, until the current decreased to 10 % of the applied current. For physical characterizations, LR-NCM electrodes after cycling test were washed with anhydrous DEC and then evacuated overnight under room temperature. The morphologies of the samples were observed with SEM (ZESISS ULTRA55) and TEM (JEM-2100, JOEL, Japan). XPS patterns were obtained on ESCALAB 250 by


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using a focused monochromatized Al Kα radiation (hυ=1486.6 eV) under ultra-high vacuum. The graphite peak (284.8 eV) was used for the final adjustment of energy scale. The experimental results were fitted with XPS peak software (version 4.1) and Lorentzian and Gaussian functions.

Calculation details: Density functional theory (DFT) calculation was performed using the Gaussian 09 package40, to optimize the equilibrium structure of isolated neutral molecules EC, EMC, DEC and PVS, and their corresponding radical cation (EC+, EMC+, DEC+ and PVS+) at B3LYP/6-311++G(d) level. At the same level, frequency calculations were also performed to confirm all optimized molecules with a consistent stationary. The vibration frequency and intrinsic reaction coordinate (IRC) analysis were adopted to confirm each transition state (TS) that connects both product and reactant in the same pathway at the same level. Polarized continuum model (PCM) was used to investigate the role of solvent effect. Acetone (dielectric constant=20.5) wad used as the default solvent for all PCM calculations. Atomic charge distributions were obtained from natural population analysis (NPA) based on natural bond orbital (NBO) theory.

3. RESULTS AND DISCUSSION

Cyclic stability of LR-NCM:

The effect of PVS on the cyclic stability of LR-NCM

was evaluated by charge-discharge cycling. Fig. 1 presents the cyclic stability of LR-NCM in the electrolyte containing various contents of PVS. In the STD electrolyte, LR-NCM exhibits poor cyclic stability: delivering a discharge capacity of



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38 mAh g-1 with a capacity retention of 18 % after 240 cycles under 0.5 C. This poor performance is the main issue that limits the application of LLO in practice. Under the charge potential higher than 4.2 V, the electrolyte tends to decompose generating polymers, gaseous products and HF21-23. The resulting HF might attack the electrode materials and corrode the current collector, leading to the poor cyclic stability of LR-NCM in the STD electrolyte. When PVS is added, the cyclic stability of LR-NCM is significantly improved, especially in the electrolyte containing 1 % PVS. LR-NCM in 1 % PVS-containing electrolyte delivers 166 mAh g-1, with a capacity retention of 80 % after 240 cycles under 0.5 C. These values are the best among those that have been reported in literature for improving the cyclic stability of LLO. It has been shown that TMSB9 and TMSPi24 are good electrolyte additives for LLO, but the capacity retention is less than 80 % under the conditions similar to that for PVS. This effectiveness should be attributed to the special molecular structure of PVS that determines the formation potential and the properties of a protective cathode interface film. It should be noted from Fig. 1 that the capacity retention of LR-NCM is decreased when the content of PVS is less than or higher than 1 %. This phenomenon is universal when an additive is used for forming a protective cathode interface film. Lower content of the additive is insufficient, while higher content of the additive will add adverse effects to the carbonate-based electrolyte, lowering the ionic conductivity of the electrolyte or deteriorating compatibility between cathode and electrolyte. Additionally, the cyclic stability test data is not stable when the content of PVS is


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lower than 0.5 %, as shown in Fig. 1. This phenomenon results from the gaseous products from electrolyte decomposition on the unprotected LR-NCM. Gases such as carbon dioxide will accumulate and evolve on LR-NCM, which might interrupt the lithium insertion/extraction, leading to unstable capacity delivery during cycling. Therefore, 1 % PVS was selected for further investigations.

Fig. 1. Cyclic capability of LR-NCM electrodes in STD electrolytes containing different contents of PVS.

A protective interface film on cathode should be constructed at the potential lower than that for the electrolyte decomposition, because the interference from the electrolyte decomposition products can be mitigated. To confirm the preferential oxidation of PVS, differential treatment was performed on the initial charge curves of LR-NCM electrodes in STD or PVS-containing electrolyte. Fig. 2a presents the initial charge-discharge curves of LR-NCM in STD or PVS-containing electrolyte. Both



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electrodes behave similarly. In the initial charge process, Li+ ions extract from lithium layer of LR-NCM with a slope potential change, which is associated to the oxidation of Ni from Ni2+ to Ni4+, followed by the oxidation of Co from Co3+ to Co4+. In the later charge process, there appears at potential plateau at about 4.5 V, corresponding to the lithium deinsertion from Li2MnO3, which is accompanied with the oxygen releasing and the structure rearranging7,

11, 41-44

. In the discharge process, there is an

irreversible capacity loss compared to the charge process. Accordingly, the coulombic efficiency of STD sample is 78 %, higher than that of PVS-containing sample (74%). This difference indicates that the oxidation decomposition of PVS might take place during initial charge process. A protective interface film on cathode might be constructed from the decomposition products of PVS, which can be confirmed by the fact that the electrode in PVS-containing electrolyte has a slightly larger impedance than that in the STD electrolyte, as shown by the inset of Fig. 2a. Fig. 2b presents the differential charge capacity profile of Fig. 2a. Two distinct oxidation peaks can be identified for both electrodes at around 4.0 V and 4.5 V, which are associated to oxidation of Ni2+ to Ni4+ via Ni3+ and Co3+/Co4+, and oxygen release and Li2MnO3 activation, respectively45, 46. Differently, the initial oxidation takes place at about 3.92 V for LR-NCM in the STD electrolyte, but at a lower potential (3.84 V) for that in PVS-containing electrolyte. Apparently, PVS is oxidized before initial lithium deinsertion, indicative of the preferential oxidation of PVS. The oxidation of carbonate electrolyte usually takes place at about 4.2 V. A protective cathode interface film can be constructed from the oxidation decomposition of PVS, which maintains


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the structure integrity of LR-NCM. The structure integrity of LR-NCM can be indicated by the change in discharge curves with cycling.

The discharge voltage plateau decreases quickly for the

electrode in the STD electrolyte with cycling (Fig. 2c), but retains relatively stable for that in PVS-containing electrolyte (Fig. 2e). Accordingly, there are differential discharge capacity profiles of two electrodes as shown in Fig. 2d and 2f. Three obvious reduction peaks, peak 1 at ~4.2 V, peak 2 at ~3.7 V and peak 3 at the potential lower than 3.5 V,

can be identified, associated to Ni4+/Ni2+, Co4+/Co3+ and

Mn4+/Mn3+, respectively41. All these peaks shift negatively with cycling for LR-NCM in the STD and PVS-containing electrolyte. However, the peak position shift becomes less significant in PVS-containing electrolyte compared to the STD electrolyte after cycling, which can be attributed to the protection that the cathode interface film generated from PVS. In the STD electrolyte (Fig. 2d), the intensity of all three peaks decreases significantly, which should be ascribed to the initial phase transformation from layered to spinel47 and the later dissolution of spinel phase. In the PVS-containing electrolyte (Fig. 2e), Peak 1 and 2 also show their decreasing intensity but the Peak 3 exhibits an increasing intensity. Apparently, the phase transformation is inevitable in the PVS-containing electrolyte, but the dissolution of spinel phase can be avoided by applying PVS.



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Fig. 2. Initial charge-discharge curves together with the impedance spectra after initial three cycles at 0.1 C (a) and corresponding dQ/dV profiles (b) of LR-NCM electrodes in STD and PVS-containing electrolyte at 0.1 C. Discharge curves and corresponding dQ/dV profiles of LR-NCM electrodes in STD (c and d) and PVS-containing (e and f) electrolytes at the selected cycles.

Effect of PVS on surface property of LR-NCM: Fig. 3 presents the SEM and TEM


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images of fresh LR-NCM electrode and the electrodes after cycling test (240 cycles) in STD and 1 % PVS-containing electrolytes of Fig. 1. Independent LR-NCM particles with uniform size and smooth surface can be observed on the fresh LR-NCM electrode. After deep cycling in the STD electrolyte, however, LR-NCM particles become smaller in size and covered with uneven thick deposits. The average particle size is 200 nm for the fresh electrode but becomes 162 nm for that in the STD electrolyte. The oxidation decomposition of the base electrolyte yields deposits including organic polymer and inorganic LiF, gaseous products including carbon monoxide and dioxide, and acidic products such as HF. The polymers deposit on LR-NCM particles unevenly due to the evolution of gaseous products and cannot provide a protection for LR-NCM, leading to the transition metal ion dissolution from LR-NCM and the reduced particle size due to the attack of HF. Differently from the morphological change in STD electrolyte, after deep cycling in the PVS-containing electrolyte, LR-NCM particles maintain the morphology of the fresh ones with a thin and uniform interface film. The average particle size is 211 nm for the cycled LR-NCM in the PVS-containing electrolyte, slightly larger than the fresh one due to the existence of the cathode interface film. These observations confirm that a protective cathode interface film has been constructed on LR-NCM, which suppresses the electrolyte decomposition and prevents LR-NCM from dissolution. The detrimental effect of electrolyte decomposition and the contribution of PVS are illustrated schematically in Fig. 3.



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Fig. 3. SEM and TEM images of fresh LR-NCM electrode (a and b), the cycled LR-NCM electrodes in STD (c and d) and PVS-containing (e and f) electrolytes. The insets schematically illustrate the image evolution of LR-NCM particles.

The surface compositions of the fresh LR-NCM electrode and those after cycling test (240 cycles) in STD and 1 % PVS-containing electrolytes of Fig. 1, were compared by XPS analysis. The XPS analysis results are presented in Fig. 4. For the fresh electrode, the peaks in C 1s, F 1s and O 1s spectra, originated from the conductive carbon, PVDF binder, Li2CO3 on LR-NCM and metal oxide in LR-NCM, can be identified48, respectively. These peaks remain on the cycled electrodes with different intensity, suggesting that the surface property of LR-NCM has been changed after cycling. For the cycled electrodes, the peaks of Li2CO3 (531.5 eV in O 1s), LiF (684.5 eV in F 1s), LixPOyFz (686.5 eV in F 1s, about 134 eV in P 2p) and LixPFy (137 eV in P 2p), which are mainly originated from the electrolyte oxidation


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decomposition, become much stronger in the STD electrolyte but changes insignificantly in the PVS-containing electrolyte, confirming that electrolyte oxidation decomposition has taken place severely on the cycled electrode in the STD electrolyte, but can be effectively suppressed by applying PVS. The peaks for C–O (533~534 eV) and C=O (532~533 eV) in O 1s spectra49 corresponding to lithium alkyl carbonates ROCO2Li and polycarbonates, which are also originated from electrolyte decomposition, can be detected on both electrodes, suggesting that the electrolyte decomposition products also exist in the cathode interface film formed from PVS50. Since XPS analysis is limited by the escape depth of electrons51, the peak in O 1s spectra for the bulk metal oxides (about 529 eV) becomes weaker for both electrodes. The O 1s spectra of bulk oxides still can be observed for the electrode cycled in the STD electrolyte, because the particles of LR-NCM become smaller due to the transition metal ion dissolution from the bulk oxides but do not disappear. Comparatively, this peak for the cycled LR-NCM in PVS-containing electrolyte is stronger than that in STD electrolyte, confirming that the deposit layer originated from the electrolyte decomposition is thicker than the cathode interface film constructed from PVS. It should be noted that the peaks at 169.2 eV, 168 eV and 163.5 eV in S 2p spectrum, corresponding to Li2SO3, ROSO2Li and C–S–C respectively48, 52, 53, and the peak at 532.2 eV in O 1s spectrum, corresponding to –SO2–53, can be detected on the cycled LR-NCM in PVS-containing electrolyte but cannot on that in the STD electrolyte. These differences suggest that sulphur-containing compounds originated



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from PVS have been incorporated into the cathode interface film. The existence of sulphur in the cathode interface film is beneficial to the ionic conductivity of the cathode interface film54, 55. Since the aromatic ring in PVS is chemically stable, it should also be incorporated into the cathode interface film although no independent XPS signal of aromatic ring is detected, possibly due to the too strong peak of C-C in the fresh electrode. The aromatic ring is beneficial to the chemical stability of a cathode interface film34.

Fig. 4. XPS profiles of the fresh LR-NCM electrode (top) and the ones cycled in STD (middle) and PVS-containing (bottom) electrolytes.

The ionic conductivity of cathode interface film constructed from PVS can be indicated by the improved rate capability of LR-NCM electrode in PVS-containing electrolyte. Fig. 5 presents the discharge capacity variation of LR-NCM electrode at 1 C, 2 C, 4 C, and 5 C. As shown in Fig. 5, LR-NCM in PVS-containing electrolyte delivers larger discharge capacities than that in STD electrolyte, especially under high current rate. At 5 C rate, the average discharge capacity is 100 mAh g-1 for LR-NCM


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in PVS-containing electrolyte, but only 62 mAh g-1 for that in the STD electrolyte. Apparently, the rate capability of LR-NCM electrode is improved by applying PVS.

Fig. 5. Rate capability of LR-NCM electrodes in STD and PVS-containing electrolytes.

Oxidation decomposition mechanism of PVS: Theoretical calculations were performed to understand the oxidation decomposition mechanism of PVS. Fig. 6a presents the optimized geometric structure of the neutral molecule PVS and its cation PVS+ after one electron oxidation, labeled with some special bond length (in Å) and NPA charge distributions in color. It can be found from Fig. 6a that some bond lengths have changed but no significant structural distortion happens after one electron oxidation of PVS.



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Fig. 6. Optimized structure and NPA charge distributions of PVS before and after one electron oxidation (a), optimized structures and adiabatic ionization energy (AIE, kJmol-1) of EC, DEC, EMC and PVS before and after one electron oxidation (b), and optimized structures and adiabatic ionization energy (AIE, kJ mol-1) of PVS-PVS, PVS-solvent and PVS-PF6- complexes before and after one electron oxidation (c). The bond length is in Å, the sum of NPA charge in red circle is one positive charge.

Fig. 6b compares the optimized neutral and cationic structure and the


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corresponding adiabatic ionization energy (AIE) of PVS with three solvent molecules, EC, DEC and EMC. It can be seen from Fig. 6b that the AIE of PVS is lower than those of solvents, indicative of the thermodynamically oxidative activity of PVS compared to these solvents. Since there is an interaction between additive and anions or solvent molecules, which favors the oxidation decomposition of the additive22, 23, -

56-58

, the complex structures of PVS-PVS, PVS-PF6 and PVS-solvent (EC, DEC or

EMC) were also optimized. The AIE of these complexes was also calculated and -

presented in Fig. 6c. Among these complexes, PVS-PF6 has the lowest AIE (701.5 kJ mol-1), suggesting that this complex is most active for oxidation. Therefore, PVS is oxidized preferentially compared with the solvents. Based on the optimized structure in Fig. 6a, there are two possible pathways for the kinetic decomposition of PVS+. As shown in Fig. 7, the longest C–C bond (1.425 Å) in the phenyl group with one positive charge might be broken (Path 1), or the S–C (C from the vinyl group) bond might be broken (Path 2), involving transition states (TS1 and TS2) and intermediate products (M1 and M2). The structure of each transition state has one imaginary frequency, and the connection of the transition state with the relevant reactant and product has been confirmed by IRC analysis. In Path 1, a new ring with five carbons is formed in M1 from ring opening of phenyl group via TS1. The energy barrier for opening the aromatic ring is about 417.8 kJ mol-1, which is so high that this process cannot happen and confirms that the aromatic ring is stable chemically. In Path 2, acetylene and a positive radical are formed via TS2 with a lower energy barrier (ca. 111.2 kJ mol-1), indicating that Path 2 is the most possible



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pathway for the decomposition of PVS and that the preferential oxidizability of PVS is endowed by the C=C double bond. The intrinsic reaction processes in Path 2 are presented in Fig. 8, which involve S–C breaking, transition state, slight structural relaxation of transition state, H transfer starting, H transfer progress, and H transfer completion with structural relaxation. In the H transfer progress, the oxidation state of C in •CHCH2 increases from -3 to -2, which is accompanied by a drastic decline of energy, suggesting that this process proceeds preferentially.

Fig. 7. Optimized structures in two possible reaction pathways for the oxidation decomposition of PVS+ and relative Gibbs free energy of all stationary points. Sum of NPA charge in red circle is zero (neutral).


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Fig. 8. Energy profile of intrinsic reaction process of Path 2 in Fig. 7.

With the radical available, several final products might be formed, as shown in Scheme 2. The radical (A) might combine lithium ion and the alkyl radical or the RO• radical that is generated from the electrolyte decomposition, forming alkyl sulfinate (B), alkyl sulfonate and benzene (C), or Li2SO3 and alkyl aromatic compound (D). All these sulphur-containing products have been detected in S 2p of Fig. 4, confirming that PVS oxidation decomposition takes place in Path 2 of Fig. 7. Since the aromatic ring always exists together with sulphur-containing products, it should be also incorporated into the cathode interface film. With the special molecule structure of PVS, therefore, a stable and ionic conductive cathode interface film can be constructed, contributing to the excellent cyclic stability of LR-NCM together with its improved rate capability.



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Scheme 2. Possible reaction paths of the intermediate generating from the one electron oxidation of PVS.

4. CONCLUSION Phenyl vinyl sulfone (PVS) with its special molecular structure can be applied as an electrolyte additive to construct a protective cathode interface film on layered lithium-rich oxides and consequently improve cyclic stability of this oxide in carbonate-based electrolyte. The double bond in the molecule endows PVS with preferential oxidizability, the aromatic ring ensures the chemical stability of the interface film and the sulfur provides the interface film with ionic conductivity. With these features of PVS, the resulting cathode interface film provides layered lithium-rich oxide with excellent cyclic stability together with improved rate capability. To our knowledge, PVS is the best candidate among the electrolyte additives that have been reported in literature for improving cyclic stability of layered lithium-rich oxide.


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AUTHOR INFORMATION Corresponding Author *[email protected] Present Address School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China. Author Contribution The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGEMENTS This work is supported by the National Natural Science Foundation of China (Grant No. 21573080), the Natural Science Foundation of Guangdong Province (Grant No. 2014A030313424), the Joint Project of the National Natural Science Foundation of China and the Natural Science Foundation of Guangdong (No. U1401248), the key project of Science and Technology in Guangdong Province (Grant No. 2016B010114001), Guangzhou City Project for Cooperation among Industries, Universities and Institutes (Grant No. 201509030005), and the scientific research project of Department of Education of Guangdong Province (Grant No. 2013CXZDA013).

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Constructing a Protective Interface Film on Layered Lithium-Rich

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