LiNi0.5Mn1.5O4 Li-Ion Battery System - ACS Publications

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Tracing the Impact of Hybrid Functional Additives on a HighVoltage (5 V-class) SiOx-C/LiNi0.5Mn1.5O4 Li-ion Battery System Gaojie Xu, Xiao Wang, Jiedong Li, Xuehui Shangguan, Suqi Huang, Di Lu, Bingbing Chen, Jun Ma, Shanmu Dong, Xinhong Zhou, Qingyu Kong, and Guanglei Cui Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03764 • Publication Date (Web): 29 Oct 2018 Downloaded from http://pubs.acs.org on October 30, 2018

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Chemistry of Materials

Tracing the Impact of Hybrid Functional Additives on a HighVoltage (5 V-class) SiOx-C/LiNi0.5Mn1.5O4 Li-ion Battery System Gaojie Xu†,○, Xiao Wang†,○, Jiedong Li†, Xuehui Shangguan†, Suqi Huang†, Di Lu†, Bingbing Chen†, Jun Ma†, Shanmu Dong†, Xinhong Zhou‡, Qingyu Kong§ , Guanglei Cui*,† †Qingdao Industrial

Energy Storage Technology Institute, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, P. R. China ‡College

of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao, 266042, P. R. China. § Société

civile Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin - BP 48, 91192 Gif-sur-Yvette CEDEX, France. ABSTRACT: The development of next generation high energy density lithium ion batteries (LIBs) adopting electrode materials with higher specific capacity or higher working voltage has attracted great interests. In this paper, fluoroethylene carbonate (FEC) and 1,3-propanediolcyclic sulfate (PCS) are unprecedentedly combined as hybrid functional additives to significantly improve the performances of a very challenging next-generation high voltage (5 V-class) SiOx-C/LiNi0.5Mn1.5O4 battery system, where SiOx-C composite shows a high specific capacity of 450 mAh g-1 and LiNi0.5Mn1.5O4 has a high working voltage plateau (~ 4.7 V vs. Li+/Li). Combining in-situ differential electrochemical mass spectrometry (DEMS) technology, theoretical calculations, and conventional ex-situ characterizations, it is revealed that small amounts of lithium-containing species (such as LiF, sulfate species, and organic sulfite species) with excellent electronic-insulating, ionic-conducting and compact properties are derived from prior decomposition of additives and incorporated into the solid electrolyte interface (SEI) layer of SiOx-C electrode, suppressing the reductive decomposition of carbonate solvents as well as the gas generation (C2H4, CO2, and H2). Moreover, hybrid functional additives are beneficial for forming a compact and homogeneous cathode SEI layer, alleviating dissolution of transition metals, structure degradation and loss of active lithium. This manuscript provides a very useful research method for understanding the working mechanism of functional additives, and will also help us to depict the SEI layer formation mechanism more accurately.

INTRODUCTION Despite the commercialized rechargeable lithium-ion batteries (LIBs) have been widely used in consumer electronics, and becoming popular in large-scale applications of electric vehicles and renewable energy-storage systems, their energy density remains insufficient to satisfy the ever-growing requirements.1-2 This promotes the development of next generation high energy density LIBs, adopting advanced electrode materials with higher theoretical specific capacity or higher working voltage.3-5 Recently, silicon (Si) and Si oxide (SiOx), with a high theoretical specific capacity of ~ 4200 mAh g-1 and ~ 2615 mAh g-1, respectively, have been regarded as very promising alternatives for conventional graphite anodes (only ~ 372 mAh g-1).6-11 Unfortunately, some critical

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obstacles such as huge volume expansion (~300% and ~200%, respectively), unstable solid-electrolyte interphase (SEI) formation, and low initial Coulombic efficiency (ICE) greatly hinder their practical applications.8-11 Meanwhile, the complicated and costly prelithiation processes are always needed to compensate for the severe active lithium loss in the first electrochemical formation cycle of Si-based and SiOx-based full-cells.11-17 For shortterm consideration, Si-C and SiOx-C composites with adjustable specific capacity are always adopted for scale up attempt in commercial LIBs.10-11,18-21 Despite of its lower theoretical specific capacity compared to Si, SiO x has also attracted great interest of the industry due to its relatively smaller volume expansion, fewer parasitic reactions with electrolytes and better cyclability.11,22 To facilitate their practical applications in full-cells, SiOx or SiOx-C composites are paired with various cathode materials including layered oxides (Li0.8Mn0.8Ni0.2O2.2,23 LiCoO2,12,14,24 LiNixCoyMn1-x-yO2,13,24 LiNi0.8Co0.15Al0.05O2,15,25-27 lithium-rich layered oxides28-29), olivine LiFePO4,17,30 and very recently reported spinel LiMn2O4 and LiNi0.5Mn1.5O4.31-32. These full-cells mainly concentrate on material synthesis, prelithiation technology, cell design/performance evaluation, and deterioration mechanism study, rarely considering the impact of electrolyte functional additives from the perspective of SEI layer. It is well known that the use of electrolyte functional additives is one of the most economical, feasible and effective strategy to significantly improve the performances of LIBs.33-34 Presently, commercial LIBs often contain several functional electrolyte additives that, apparently, can work synergistically together to modify and stabilize the SEI layer, which greatly inﬂuences their electrochemical performances and safety.33-38 Fluoroethylene carbonate (FEC) has been proved to be the most effective electrolyte additive for improving the cycling performance of Si-based and SiOx-based electrodes in both half-cells and full-cells.17,20,28-29,39-49 In virtue of its high working voltage plateau (~ 4.7 V vs. Li+/Li), high theoretical capacity of 148 mAh g-1, fast Li+ diffusion kinetics, and potentially low cost, spinel LiNi 0.5Mn1.5O4 is regarded as one of the most promising cathode materials for next generation high energy density LIBs.5,50 Unexpectedly, full-cells using LiNi0.5Mn1.5O4 as cathode always suffer from rapid capacity deterioration (normally > 50% capacity loss after 100 cycles), mismatching its excellent cycling stability in half-cells.50-57 This is mainly ascribed to the Mn/Ni dissolutionmigration-deposition process and the subsequent severe consumption of limited active lithium (especially at anode side). Organic sulfur-containing compounds, such as sultones, sulfites, and sulfates have been proposed to stabilize SEI layers (especially on graphite anode).50,58-59 Due to its ability of suppressing Mn deposition onto graphite anode and modifying SEI layers on both electrodes, 1,3-propanediolcyclic sulfate (PCS) can enhance the electrochemical performances of graphite/LiNi0.5Mn1.5O4 battery system.50,58 Here, FEC and PCS are unprecedentedly combined as hybrid functional additives to significantly improve the performances of a very challenging next-generation high voltage (5 V-class) SiOx-C/LiNi0.5Mn1.5O4 battery system, where SiOx-C composite is a commercial product with an instructional specific capacity of 450 mAh g -1. It is estimated that the volume deformation of LIB at cell level can be controlled below 20% if the reversible specific capacity of SiOx-C composite is in the range of 450 mAh g-1-900 mAh g-1.21 Encouragingly, the ICE of SiOx-C composite can reach the level of commercial graphite anodes (90%-94%) just using functional electrolyte additives, saving the complicated and costly prelithiation processes. Herein, a novel in-situ differential electrochemical mass spectrometry (DEMS) technology is adopted to real-time identify and quantify gas byproducts generated at different potentials of SiOx-C/Li half-cells, investigating the impact of hybrid functional additives on interfacial chemistry by a new perspective. 60-65 The proposed research method of combining in-situ DEMS technology (can also be assisted by isotope-labeling of solvents/additives), theoretical calculations, and conventional ex-situ characterizations, can help us to understand the SEI layer formation mechanism and working mechanisms of additives more accurately. It is believed that this manuscript will be meaningful for the evolution of next generation high energy density LIBs, as well as other emerging energy storage devices.

EXPERIMENTAL SECTION


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Preparation of electrolytes. The baseline electrolyte (BE) was 1.0 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC)/ethyl methyl carbonate (EMC) (1:1:1 by volume), purchased from Suzhou Qianmin Chemistry Co. Ltd. (China). The amount of water in BE was 13.7 ppm. Fluoroethylene carbonate (FEC) was purchased from Shanghai Macklin Biochemical Co., Ltd. (China), and 1,3-propanediolcyclic sulfate (PCS) was purchased from Sigma-Aldrich. All of these reagents were used without further purification. In an argon-filled glovebox (Mikrouna, China) with H2O and O2 less than 1 ppm, electrolytes with different formulations are prepared: BE + 1 wt.% FEC, BE + 1 wt.% PCS, BE + 1 wt.% FEC + 1 wt.% PCS. Preparation of electrodes and fabrication of half-cells and full cells. The anode was consisted of 93 wt.% SiOx-C composite (S450TM, Shenzhen BTR New Energy Material Co., Ltd. (China)), 2 wt.% conductive carbon (Super PTM Li, TIMCAL) and 5 wt.% aqueous binder (LA133TM, Chengdu Indigo Power Sources Co., Ltd. (China)). The cathode was composed of 88 wt.% LiNi0.5Mn1.5O4 (Sichuan Xingneng Group (China)), 6 wt.% conductive carbon (Super PTM Li, TIMCAL) and 6 wt.% polyvinylidene fluoride (PVDF, SolefTM 5130, Solvay) binder. The anode (pure H2O as solvent) and cathode (N-methyl-2-pyrrolidinone (NMP, Sigma-Aldrich) as solvent) slurries were coated on copper foil and aluminum foil, respectively. After solvent drying and pressing (5 MPa), the electrodes were punched into disks followed by additional vacuum drying at 120 °C for 8 h prior to the assembling of both half-cells and full-cells. Coin-type half-cells with as-prepared electrolytes were assembled with SiOx-C composite as working electrode, lithium foil (Tianjin China Energy Lithium Co., Ltd. (China)) as counter electrode, Celgard 2500 polypropylene membrane as separator. Coin-type full-cells with as-prepared electrolytes were fabricated with N/P values of ~1.3 (calculated according to 425 mAh g -1 and 120 mA h g-1 for SiOx-C and LiNi0.5Mn1.5O4, respectively). Electrochemical measurements. The charge-discharge behaviors of assembled half-cells and full-cells were measured using a LAND battery testing system (Wuhan LAND electronics Co., Ltd.(China)). SiOx-C/Li half-cells were cycled at 0.1 C rate (1 C = 450 mA g−1) under a voltage range between 0.005 and 1.5 V. On-line electrochemical impedance spectroscopy (EIS, ranging from 1 MHz to 100 mHz with a voltage amplitude of 10 mV (VMP3, BioLogic Science Instruments, SAS)) monitoring of SiO x-C/Li half-cells was performed at the 1st discharge process with selected potentials at 1.5 V, 1.0 V, 0.6 V, 0.5 V, 0.4 V, 0.3 V, 0.2 V, and 0.1 V, respectively. The cyclability of SiOx-C/LiNi0.5Mn1.5O4 full-cells was evaluated at 0.2 C rate (1 C = 120 mA g−1, determined by LiNi0.5Mn1.5O4 cathode). SiOx-C/LiNi0.5Mn1.5O4 full-cells were charged to 4.9 V, followed by a constant potential of 4.9 V for 5 min, and then discharged to 3.5 V. The electrochemical impedance spectroscopy (EIS) of SiOx-C/LiNi0.5Mn1.5O4 full-cells at fully discharged state was measured over frequencies ranging from 1 MHz to 100 mHz with a voltage amplitude of 10 mV (VMP3, Bio-Logic Science Instruments, SAS). In-situ differential electrochemical mass spectrometry (DEMS).The adopted in-situ cell was a commercial ECC-DEMS cell (EL-CELL GmbH). Herein, porous glass microfiber filter paper (GF/D, Whatman) was used as separator and argon was selected as inert carrier gas. The equipped mass spectrometer in DEMS system was HPR-20 (Hiden Analytical Ltd.). Ex-situ characterizations. The cycled electrodes were carefully dismantled from the discharged SiOxC/LiNi0.5Mn1.5O4 full-cells and charged SiOx-C/Li half-cells, rinsing three times with electronic grade DMC (Shenzhen Capchem Technology Co., Ltd. (China)) to remove the residuals. The surface morphologies and structure of the cycled electrodes were obtained using field emission scanning electron microscopy (FE-SEM, Hitachi S-4800) and high resolution transmission electron microscopy (HRTEM, JEOL). X-ray diffraction (XRD) patterns of the cycled electrodes were recorded (4° min-1, 10° to 90°, with Cu Kα radiation (λ = 1.5406 Å)) on a Ultima IV of Rigaku. To obtain the surface chemical information of cycled electrodes, X-ray photoelectron spectroscopy (XPS) data were acquired using an ESCALab220i-XL spectrometer (VG Scientifc) with Al Kα radiation in twin anodes at 14 kV × 16 mA under ultra-high vacuum conditions. The Mn/Ni dissolution of the fully charged LiNi0.5Mn1.5O4 cathodes (disassembled from SiOx-C/LiNi0.5Mn1.5O4 full-cells cycled at BE and BE +



hybrid functional additives for 3 cycles, 0.2 C rate) was determined by inductively coupled plasma optical emission spectrometry (ICP-OES, ICPOES730, Agilent Instruments, USA). X-ray absorption spectroscopy (XAS) of pristine and cycled LiNi0.5Mn1.5O4 electrodes was measured at the Mn k-edge and Ni k-edge, at the Société civile Synchrotron SOLEIL.

RESULTS AND DISCUSSION Electrochemical properties of SiOx-C/Li half-cells with and without hybrid functional additives. It is well known that, in the full-cell electrochemical formation process (first few cycles), higher ICE of anode always means less consumption of limited active lithium stored in cathode, guaranteeing the electrochemical performances of full-cells. Herein, the impact of functional additives on the ICE of SiO x-C/Li half-cells was investigated. The average ICE of SiOx-C/Li half-cells using BE, BE + 1 wt.% FEC, BE + 1 wt.% PCS, BE + 1 wt.% FEC + 1 wt.% PCS is 68.4%, 90.6%, 91.6%, 91.0%, respectively (Figure S1). It is encouraging that, just using functional additives, the ICE of this SiOx-C composite (with an instructional specific capacity of 450 mAh g -1) can reach the ICE level (90%-94%) of commercial graphite anodes, saving the complicated and costly prelithiation processes usually used in SiOx-based full-cells. Upon long-term cycling at 0.1 C rate, SiOx-C/Li halfcell using BE + 1 wt.% FEC + 1 wt.% PCS demonstrates superior performances, considering the delivered reversible specific capacity and average Coulombic efficiency (Figure S2-S4). The discharge capacity retention ((discharge specific capacity of 100th cycle)/(discharge specific capacity of 2nd cycle)) of SiOx-C/Li half-cells using BE and BE + 1 wt.% FEC + 1 wt.% PCS is 82.0% (358.7 mAh g -1/467.3 mAh g-1) and 85.4% (393.3 mAh g1/460.7

mAh g-1), respectively (Figure 1a and Figure S2). And the corresponding average Coulombic efficiency

Figure 1. (a) Cycling performance and Coulombic efficiency of SiOx-C/Li half-cells (0.005-1.5 V) at 0.1 C rate with and without hybrid functional additives (1 wt.% FEC + 1 wt.% PCS), and (b) the corresponding discharge-charge voltage curves at the 1st, 50th, 100th cycle. (c) and (d) the on-line electrochemical impedance spectroscopy (EIS) monitoring of SiOx-C/Li half-cells (0.1 C rate, with and without hybrid functional additives) for the 1st discharge process at 1.5 V, 1.0 V, 0.6 V, 0.5 V, 0.4 V, 0.3 V, 0.2 V, and 0.1 V, respectively.


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(ICE was considered in the calculation of average Coulombic efficiency) is 99.02% and 99.64%, respectively (Figure 1a and Figure S3-S4). As it is clearly shown in the discharge-charge voltage curves of 1st cycle (Figure 1b and Figure S4), the ICE improvement of SiOx-C/Li half-cells using functional additives is mainly originated from the suppression of irreversible parasitic reactions happened at the voltage range of 0.7-0.2 V (always representing the reductive decomposition of EC solvent). Because of their lower LUMO (Lowest Unoccupied Molecular Orbital) energy than carbonate solvents, FEC and PCS will be preferentially reduced theoretically to modify the anode SEI layer, which can efficiently inhibit the subsequent reductive decomposition of carbonates at lower potentials (Figure S5). To reveal the synergistic effects of hybrid functional additives on the electrochemical processes occurring at SiOx-C electrode interfaces, on-line electrochemical impedance spectroscopy (EIS) monitoring of SiOx-C/Li halfcells was performed at the 1st discharge process (Figure 1c-1d). The selected potential for on-line EIS monitoring is 1.5 V, 1.0 V, 0.6 V, 0.5 V, 0.4 V, 0.3 V, 0.2 V, and 0.1 V, respectively (Figure 1c). The interfacial resistances are always represented by the high-frequency and medium-frequency semicircles, reflecting the properties of surface film (SEI layer) on electrodes.50 Obviously, at each comparative potential, the interfacial resistances of SiOx-C/Li half-cell using BE + 1 wt.% FEC + 1 wt.% PCS are much lower than counterpart (Figure 1d). It is inferred that the surface species derived from preferential reductive decomposition of hybrid functional additives can facilitate charge transfer (lithium ion transportation) and suppress the electron conduction. As a result, it is much more difficult for EC solvent to get electrons to start the reductive decompositions. From the crosssectional SEM image of SiOx-C electrode disassembled from SiOx-C/Li half-cells at the 1st discharge state, the hybrid functional additives can definitely inhibit the appearance of electrode cracks, keeping the integrity of SiOx-C electrode (Figure S6). Gassing behavior of SiOx-C/Li half-cells with and without hybrid functional additives. It is well known that, the formation of SEI layers in full-cell electrochemical formation process (first few cycles) is always accompanied by gas byproduct generation.60-65 Therefore, clarifying the gas evolution will absolutely be beneficial for understanding the impact of hybrid functional additives on interfacial electrolyte decomposition and SEI layer formation. In-situ differential electrochemical mass spectrometry (DEMS) technology can allow real-time identify and quantify gas byproducts generated at different potentials.60-65 Herein, in-situ DEMS is used to investigate the gas generation of SiOx-C/Li half-cell at different potentials, tracing the impact of hybrid functional additives on interfacial chemistry by a new perspective. Obviously, the ethylene (C2H4) and carbon dioxide (CO2) generation are greatly suppressed by hybrid functional additives during the 1st discharge process (Figure 2). The generation of C2H4 (m/z = 26) is mainly observed at the beginning of discharge process (0.7 V ~ 0.25 V region, reaching a maximum peak at ca. 0.5 V), which can be ascribed to the reductive decomposition of EC solvent according to the following equations: 2C3H4O3 (2EC) + 2Li+ + 2e- → (CH2OCO2Li)2↓+ CH2=CH2↑; C3H4O3 (EC) + 2Li+ + 2e- → Li2CO3↓+ CH2=CH2↑ (Step 2 in Figure 3).58,60 The CO2 (m/z = 44) signals mainly occur within a voltage range of 0.5 V ~ 0.1 V, demonstrating a maximum peak at ca. 0.2 V. The possible reaction paths for CO2 generation: (CH2OCO2Li)2 + H2O → Li2CO3↓+ 2CH3OH + CO2↑; 2ROCO2Li + H2O → Li2CO3 ↓+ 2ROH (alcohols)+ CO2↑; ROCO2Li + HF→ LiF↓+ ROH (alcohols) + CO2↑ (Step 3 in Figure 3); where (CH2OCO2Li)2 originates from the reductive decomposition of EC as aforementioned, the lithium alkyl carbonates (ROCO2Li) are typical byproducts of linear carbonates (such as DEC, EMC) reductive decomposition, HF is produced by trace amounts of water with LiPF6 salt.58,60,66-67 In the voltage range (0.5 V ~ 0.1 V) of CO2 generation, H2 (m/z = 2) evolution shows similar overall behavior, suggesting that there is a correlation between CO2 and H2 generation (Figure S7). H2 evolution is believed to be the reduction of H+ originating from trace amounts of water and alcohols.60 Therefore, it is inferred here that the H2 evolution in this voltage range (0.5 V ~ 0.1 V) is ascribed to the alcohols accompanying with the CO2 generation (Step 4 in Figure 3). Due to the



Figure 2. The 1st discharge curve of SiOx-C/Li half-cells (adopting a commercial in-situ cell EL-CELLTM) with and without hybrid functional additives (1 wt.% FEC + 1 wt.% PCS) and the corresponding DEMS signals for m/z =26 (C2H4) and m/z = 44 (CO2).

preferential reductive decomposition of hybrid functional additives, some protective species are formed to block the pathways for electron conduction, suppressing the EC reductive decomposition and subsequent correlative C2H4 (m/z = 26), CO2 (m/z = 44), and H2 (m/z = 2) generation. To clarify the key protective species derived from preferential reductive decomposition of hybrid functional additives, surface characterization technology of X-ray photoelectron spectroscopy (XPS) is conducted on SiOxC electrode disassembled from SiOx-C/Li half-cell after the 1st cycle. LiF is a well-known byproduct of FEC reductive decomposition.39-44 A representative equation is demonstrated here: C3H3O3F (FEC) + 3Li+ + e- → LiF ↓ + Li2CO3↓+ CH2=CH2↑.39 The intensity of LiF peak centered at ca. 684.9 eV is tremendously enhanced by the assistance of hybrid functional additives, which is then attributed to the reductive decomposition of FEC additive (Figure S8). The observed sulfur (S) 2p signal suggests the SEI layer of SiOx-C electrode is also modified by the reductive decomposition of PCS (Figure S9). The species are inferred to be organic sulfite species (such as ROSO2Li) and sulfate species (such as Li2SO4 and PCS-like species) centered at ca. 168.7 eV and ca. 169.7 eV, respectively. It is reported that these compact and polar S-containing species can passivate the electrode surface and facilitate Li+ diffusion.50 Combining the aforementioned results of in-situ EIS, in-situ DEMS and XPS, when small amounts of these hybrid functional additives derived species with excellent electronic-insulating, ionicconducting and compact properties are incorporated into the SEI layer of SiO x-C electrode, the reductive composition of carbonate solvents (especially EC) on the surface of SiOx-C electrode as well as the gas generation (C2H4, CO2, and H2) will be greatly suppressed (Figure 3).34,50,58-59


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Figure 3. The proposed working mechanism of hybrid functional additives on SiOx electrode by considering LUMO energy, in-situ EIS, in-situ DEMS, cross-sectional SEM, and XPS.

Electrochemical properties of SiOx-C/LiNi0.5Mn1.5O4 full-cells with and without hybrid functional additives. Herein, a challenging SiOx-C/LiNi0.5Mn1.5O4 full-cell system is preliminarily constructed to evaluate the impact of functional additives. The synergistic effectiveness of FEC and PCS combination on specific capacity (calculated based on cathode mass), ICE, cyclability and average Coulombic efficiency is more pronounced in SiOx-C/LiNi0.5Mn1.5O4 full-cells cycled at 0.2 C rate (Figure 4a-b and Figure S10- S11). Specifically, among four samples, SiOx-C/LiNi0.5Mn1.5O4 full-cell using BE + 1 wt.% FEC + 1 wt.% PCS shows highest initial discharge specific capacity, ICE, discharge capacity retention and average Coulombic efficiency of 112.9 mAh g -1, 79.8% (112.9 mAh g-1/141.5 mAh g-1), 100.4% (113.3 mAh g-1/112.9 mAh g-1), and 98.03% (ICE was considered in the calculation of average Coulombic efficiency), respectively. As an obvious contrast, SiOx-C/LiNi0.5Mn1.5O4 full-cell using BE exhibits poor performances of 81.2 mAh g-1, 59.3% (81.2 mAh g-1/137.0 mAh g-1), 43.3% (35.2 mAh g-1/81.2 mAh g-1), and 97.20%, correspondingly. With the help of hybrid functional additives, the charge-discharge voltage curves with stable voltage plateaus (mainly related to Ni2+/Ni3+ and Ni3+/Ni4+ redox couples) of SiOxC/LiNi0.5Mn1.5O4 full-cell are recorded throughout the cycling process at 0.2 C rate (Figure 4b). The first charge specific capacity (first charge curve) of different samples shows no significant difference (Figure S10). Therefore, the large difference of first discharge specific capacity (first discharge curve) is considered to be associated with the active lithium consumption level induced by parasitic reactions happened at SiOx-C anode side (associating with the ICE of SiOx-C in half-cells).To reveal the synergistic effects of hybrid functional additives on electrode interfacial resistances upon cycling, electrochemical impedance spectroscopy (EIS) measurement was conducted on SiOx-C/LiNi0.5Mn1.5O4 full-cells at the 1st, 50th, 100th cycle (Figure 4c-4d and Figure S12). By the assistance of hybrid functional additives, the interfacial resistances increase in SiOx-C/LiNi0.5Mn1.5O4 full-cell are obviously alleviated, suggesting the formation of some protective, conductive and stable species in SEI layers of both electrodes upon cycling. Therefore, upon cycling, the complicated interfacial parasitic reactions (electrolyte decomposition) on both electrodes will be suppressed by hybrid functional additives, guaranteeing the longterm cycling performance of SiOx-C/LiNi0.5Mn1.5O4 full-cell. In the following, understanding the impact of hybrid additives on the interfacial chemistry of both electrodes in complicated SiO x-C/LiNi0.5Mn1.5O4 full-cell system will be an important and challenging topic.



Figure 4. (a) Cycling performance and Coulombic efficiency of SiOx-C/LiNi0.5Mn1.5O4 full-cells with and without hybrid functional additives (1 wt.% FEC + 1 wt.% PCS), and (b) the corresponding charge-discharge voltage curves at the 1st, 50th, 100th cycle. Electrochemical impedance spectroscopy (EIS) of the fully discharged SiOx-C/LiNi0.5Mn1.5O4 full-cells at 1st, 50th, 100th cycle, with (c) BE and (d) BE + hybrid functional additives, respectively. The testing frequencies ranged from 1 MHz to 100 m Hz with a voltage amplitude of 10 mV. All these charge-discharge tests were conducted at 0.2 C rate with a voltage range of 3.5-4.9 V.

Ex-situ characterizations of SiOx-C anode disassembled from SiOx-C/LiNi0.5Mn1.5O4 full-cells upon long-term cycling. For ex-situ characterizations, the SiOx-C/LiNi0.5Mn1.5O4 full-cells with BE and BE + 1 wt.% FEC + 1 wt.% PCS were disassembled at the 100th cycle (discharged state). As clearly shown by field-emission scanning electron microscopy (FESEM), surface morphologies of uncycled pristine SiO x-C electrode and cycled SiOx-C electrodes are of significant difference (Figure 5). On the surface of SiOx-C electrode cycled in BE, there are many homogeneously distributed large-size (0.5 μm ~ 1 μm) deposits, which are significantly suppressed by using hybrid functional additives. The EDX elemental analysis (Insets of Figure 5a-5c and Figure S13- S15) shows that, by the addition of hybrid functional additives, the oxygen (O) element (mainly originating from carbonates) and fluorine (F) element (mainly originating from LiPF6) are obviously reduced, while the carbon (C) element content increases, confirming their inhibiting effect on complicated interfacial parasitic reactions (electrolyte decomposition) upon long-term cycling. Moreover, the deposition of Mn-containing and Ni-containing species is significantly alleviated. In summary, with the help of hybrid functional additives, a thinner and more protective SEI layer on SiOx-C electrode is formed upon long-term cycling.


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Figure 5. Typical FESEM images of (a) uncycled pristine SiOx-C electrode, and cycled SiOx-C electrodes disassembled from SiOx-C/LiNi0.5Mn1.5O4 full-cells with (a) BE and (b) BE + hybrid functional additives (1 wt.% FEC + 1 wt.% PCS) after 100 cycles. Inset of (a), (b), and (c) is the corresponding elemental analysis. All these charge-discharge tests were conducted at 0.2 C rate with a voltage range of 3.5-4.9 V.

To find out key surface-protective species derived from hybrid functional additives on the surface of SiO x-C electrode, XPS characterization was conducted. Both the F 1s spectra of SiO x-C electrodes long-term cycled in BE and BE + hybrid functional additives demonstrate two peaks centered at ca. 684.9 eV and ca. 687.6 eV (Figure 6a). The peak centered at ca. 684.9 eV is a typical representative of LiF, while the peak centered at ca. 687.6 eV is believed to be correlated with P-F species like LixPOyFz and LixPFy.45,50 Normally these F-containing species are formed by the decomposition of LiPF6 salt and positively correlated with each other (LiPF6 (sol.) → LiF (s) + PF5 (sol.); LiPF6 (sol.) + H2O → POF3 (sol.) + LiF (s) + 2HF (sol.); PF5 (sol.) + H2O → POF3 (sol.) + 2HF (sol.); PF5 (sol.) + 2xLi+ + 2xe- → LixPF5-x (s) + xLiF (s); POF3 (sol.) + nH2O + nLi+ → LixPOyFz (s) + nHF (sol.)).50 Interestingly, despite the intensity of peak centered at ca. 687.6 eV is significantly decreased by the addition of



Figure 6. The (a) F 1s, (b) S 2p, (c) C 1s, and (d) O 1s XPS spectra of the cycled SiO x-C electrodes disassembled from SiOx-C/LiNi0.5Mn1.5O4 full-cells with BE and BE + hybrid functional additives (1 wt.% FEC + 1 wt.% PCS) after 100 cycles. All these charge-discharge tests were conducted at 0.2 C rate with a voltage range of 3.5-4.9 V.

hybrid functional additives, the intensity of LiF peak centered at ca. 684.9 eV is enhanced apparently. This abnormal LiF intensity increase can ascribed to the LiF formation induced by reductive decomposition of FEC additive.39-44 This hints the reductive decomposition path of FEC on the surface of SiO x-C electrode are similar in both SiOx-C/Li half-cells and SiOx-C/LiNi0.5Mn1.5O4 full-cells. Definitely, the S 2p spectra of SiOx-C electrode long-term cycled in BE + hybrid functional additives can be distinctly deconvoluted into two peaks at ca. 169.7 eV and ca. 168.7 eV corresponding to sulfate species (such as Li 2SO4 and PCS-like species) and organic sulfite species (such as ROSO2Li), respectively (Figure 6b). This means the reductive decomposition path of PCS on the surface of SiOx-C electrode are also similar in both SiOx-C/Li half-cells and SiOx-C/LiNi0.5Mn1.5O4 full-cells. With the help of binary functional additives, more polar motifs (-CO2- at ca. 288.8 eV in C 1s spectra, Figure 6c; C=O at ca. 531.8 eV in O 1s spectra, Figure 6d) are formed in the SEI layer SiOx-C electrode. It is reported that polar motifs such as -OCO2- and O=C–O can facilitate charge transport through the SEI layer of Si-based anodes.65 In the C 1s spectra, the unique high intensity peak at ca. 283 eV is attributed to the formation of R-Li (C-Li), which may be helpful for facilitating charge transfer.65,68 In summary, very small amounts of sulfate species, organic sulfite species and Li-containing species (such as LiF, ROCO2Li, R-Li) with electronic-insulating, ionicconducting and compact properties in SEI layer can work together to play a key role in protecting SiO x-C electrode and suppressing reductive decomposition of electrolyte, guaranteeing the normal long-term cycling of SiOx-C/LiNi0.5Mn1.5O4 full-cell. Ex-situ characterizations of LiNi0.5Mn1.5O4 cathode disassembled from SiOx-C/LiNi0.5Mn1.5O4 full-cells upon long-term cycling. As observed in EDX elemental analysis (Insets of Figure 5a-5c and Figure S13- S15), the deposition of transition metal-containing species on the surface of SiOx-C electrode is significantly alleviated,


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Figure 7. XRD patterns of uncycled pristine LiNi0.5Mn1.5O4 electrode, and cycled LiNi0.5Mn1.5O4 electrodes disassembled from SiOx-C/LiNi0.5Mn1.5O4 full-cells with BE and BE + hybrid functional additives (1 wt.% FEC + 1 wt. % PCS) after 100 cycles. Inset is the enlarged (111) diffraction peak of LiNi0.5Mn1.5O4 electrodes. All these charge-discharge tests were conducted at 0.2 C rate with a voltage range of 3.5-4.9 V.

suggesting that the crystal phase of LiNi0.5Mn1.5O4 electrode can be well preserved by the cathode SEI layer formed with the assistance of hybrid functional additives. The X-ray diffraction (XRD) patterns of uncycled pristine LiNi0.5Mn1.5O4 electrode, and cycled LiNi0.5Mn1.5O4 electrodes are demonstrated to confirm this inference (Figure 7). Different from the typical cubic spinel phase of LiNi0.5Mn1.5O4 (JCPDS No. 80-2162), an obvious new crystal phase (possibly Li0.115MnO2, JCPDS No. 82-2168) is generated after long-term cycling without any additive. As expected, this serious crystal structure degradation of LiNi0.5Mn1.5O4 is effectively suppressed by the hybrid functional additives modified cathode SEI layer. This indicates that crystal phase destruction of LiNi 0.5Mn1.5O4 inner core can be definitely alleviated by the cathode SEI layer protection on outer shell of LiNi 0.5Mn1.5O4. The surface of uncycled pristine LiNi0.5Mn1.5O4 particle is clean and smooth (Figure 8a), and shows excellent crystallinity (Figure 8d). After long-term cycling in BE, there are some significant trace of exfoliation (as enclosed by yellow rectangles in Figure 8b) on the surface of LiNi0.5Mn1.5O4 particle accompanying with the appearance of large amounts granulate deposits. In addition, the formation of compact and homogeneous cathode SEI layer is not observed in the high resolution transmission electron microscopy (HRTEM) image, and the severe surface crystal phase damage depth is at least 10 nm (Figure 8e and Figure S16). As a striking contrast, the hybrid functional additives can help to suppress LiNi0.5Mn1.5O4 surface exfoliation as well as inhibit the surface formation of granulate deposits (Figure 8c). More importantly, a compact and homogeneous cathode SEI layer with a thickness of ca. 5 nm is formed with the addition of hybrid functional additives, and the surface crystal



Figure 8. Typical FESEM images of (a) uncycled pristine LiNi0.5Mn1.5O4 electrode, and cycled LiNi0.5Mn1.5O4 electrodes disassembled from SiOx-C/LiNi0.5Mn1.5O4 full-cells with (b) BE and (c) BE + hybrid functional additives (1 wt.% FEC + 1 wt.% PCS) after 100 cycles. (d), (e), and (f) is the corresponding high resolution transmission electron microscopy (HRTEM) image. The corresponding (g) F 1s and (h) S 2p XPS spectra of the cycled LiNi0.5Mn1.5O4 electrodes. (i) A comparison of the Mn/Ni dissolution of the fully charged LiNi0.5Mn1.5O4 cathodes. All these charge-discharge tests were conducted at 0.2 C rate with a voltage range of 3.5-4.9 V.

phase damage depth is less than 5 nm (Figure 8f and Figure S17). It is believed that this cathode SEI layer plays a key role in blocking the dissolution of transition metal and electrolyte oxidation, protecting the crystal phase of LiNi0.5Mn1.5O4 from damage upon long-term cycling. The F 1s spectra suggests that the formation of LiF and P-F species (ca. 686.4 eV, ca. 687.5 eV) in cathode SEI layer is enhanced by prescribing hybrid functional additives (Figure 8g). Similar to SiOx-C electrode, the S 2p signals of sulfate species (such as Li2SO4 and PCS-like species) and organic sulfite species (such as ROSO2Li) are also observed at LiNi0.5Mn1.5O4 electrode, suggesting that PCS is a bifunctional additive for modifying the SEI layers of anode and cathode in full-cells (Figure 8h). Inductively coupled plasma (ICP)-optical emission spectrometry (OES) was used to investigate the Mn/Ni dissolution behavior of the fully charged LiNi0.5Mn1.5O4 electrode (disassembled from SiOx-C/LiNi0.5Mn1.5O4 full-cells cycled at BE and BE + hybrid functional additives for 3 cycles) soaked in BE and stored at 50 °C for 12 h. At such an extreme testing condition, the hybrid functional additives can obviously alleviate the dissolution of Mn and Ni ions from LiNi0.5Mn1.5O4 by participating in the formation of a cathode SEI layer (Figure 8i). Hard X-ray absorption near edge structure (XANES) spectra of transition metals (Ni and Mn K edges) are helpful for evaluating their valence states in electrode materials at different state of charge.69-70 In LiNi0.5Mn1.5O4 material, the valence state of Ni and Mn is supposed to be 2+ and 4+, respectively. The specific capacity of LiNi0.5Mn1.5O4 material is contributed by Ni2+/Ni3+ and Ni3+/Ni4+ redox couples and Mn4+ remains stable during charging/discharging processes. Obviously, the Ni K-edge XANES spectra of pristine LiNi0.5Mn1.5O4 electrode


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Figure 9. (a) Ni K-edge X-ray absorption near edge structure (XANES) spectra of NiO reference, uncycled pristine LiNi0.5Mn1.5O4 electrode, and cycled LiNi0.5Mn1.5O4 electrodes disassembled from SiOx-C/LiNi0.5Mn1.5O4 full-cells with BE and BE + hybrid functional additives (1 wt.% FEC + 1 wt.% PCS) after 100 cycles. (b) is the corresponding Mn K-edge XANES spectra of LiNi0.5Mn1.5O4 electrodes.

shift to higher energy than that of NiO, suggesting the averaged valence state of Ni is slightly higher than 2+ and the averaged valence state of Mn is slightly lower than 4+ due to residual Mn3+ in pristine LiNi0.5Mn1.5O4 electrode (Figure 9a). The Ni K-edge XANES of the long-term cycled LiNi0.5Mn1.5O4 electrode (fully discharged state) in BE shows a tendency of shifting to higher energy than pristine LiNi0.5Mn1.5O4 electrode (the increase of Ni averaged valence state), suggesting the poor reversibility of active redox couples and loss of active lithium (Figure 9a). The increase of Ni averaged valence state is accompanied by the decrease of Mn averaged valence state, which is confirmed by Mn K-edge XANES spectra (Figure 9b). The disproportionation reaction of low valence state Mn3+ is believed to be a very significant reason for Mn dissolution. Thus, the decrease of Mn averaged valence state is also responsible for the performance deterioration SiOx-C/LiNi0.5Mn1.5O4 battery. Interestingly, because of the formation of LiNi0.5Mn1.5O4 cathode SEI layer is assisted by hybrid functional additives, its XANES spectrum at Ni K-edge shifts to lower energy compared to pristine LiNi0.5Mn1.5O4 electrode (the decrease of Ni averaged valence state) while the XANES spectrum shifts to higher energy than pristine LiNi0.5Mn1.5O4 electrode at Mn Kedge (the increase of Mn averaged valence state). This illustrates that this cathode SEI layer can help to improve the reversibility of active redox couples and suppress loss of active lithium and Mn dissolution, ensuring the excellent long-term cycling performance of SiOx-C/LiNi0.5Mn1.5O4 full-cell.



CONCLUSIONS In summary, the ICE and cycling performance of SiOx-C electrode are significantly improved by using hybrid functional additives (1 wt.% FEC + 1 wt.% PCS). The synergistic effects of hybrid functional additives on the electrochemical processes occurring at SiOx-C electrode interfaces are mainly traced by conducting in-situ DEMS, theoretical calculation, and conventional ex-situ characterizations. It is revealed that small amounts of hybrid functional additives derived species (such as LiF, sulfate species, and organic sulfite species) with excellent electronic-insulating, ionic-conducting and compact properties are incorporated into the SEI layer of SiOx-C electrode, suppressing the reductive decomposition of carbonate solvents (especially EC) as well as the gas generation (C2H4, CO2, and H2). Then, a challenging SiOx-C/LiNi0.5Mn1.5O4 full-cell system is successfully constructed with the help of hybrid functional additives. It is demonstrated that the reductive decomposition of hybrid functional additives on SiO xC electrode is similar in both half-cells and full-cells. Moreover, a compact and homogeneous cathode SEI layer is formed by assistance of hybrid functional additives. With the help of this cathode SEI layer, the dissolution of transition metals from LiNi0.5Mn1.5O4 cathode can be greatly inhibited, alleviating structure degradation and loss of active lithium. The proposed research method of combining in-situ DEMS technology (can also be assisted by isotope-labeling of solvents/additives), theoretical calculations, and conventional ex-situ characterizations, can help us to understand the SEI layer formation mechanism and working mechanism of additives more accurately. Therefore, this paper is meaningful for evolution of next generation high energy density LIBs, as well as other emerging energy storage devices.

ASSOCIATED CONTENT Supporting Information Available: Electrochemical properties, gassing behavior, and interface analysis of SiOx-C/Li half-cells with and without hybrid functional additives (including theoretical calculation of electrolyte components). Electrochemical properties and interface analysis of SiO x-C/LiNi0.5Mn1.5O4 full-cells with and without hybrid functional additives (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] Author Contributions ○Gaojie Xu and ○Xiao Wang contributed equally to this work. Guanglei Cui and Gaojie Xu proposed the concepts.

Gaojie Xu designed the experiments. Jiedong Li, Shanmu Dong, and Xiao Wang conducted in-situ DEMS experiments. Bingbing Chen calculated out the LUMO energy of electrolyte components. Qingyu Kong performed characterization of X-ray absorption spectroscopy (XAS). Xiao Wang and Gaojie Xu carried out the other experiments and performed the analysis. Gaojie Xu and Xiao Wang wrote the manuscript with help from all co-authors and all authors contributed to interpretation of the data. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This original research was supported by the Think-Tank Mutual Fund of Qingdao Energy Storage Industry Scientific Research, the National Natural Science Foundation for Distinguished Young Scholars of China (No.


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51625204), National Natural Science Foundation of China (No. 51502319), Youth Innovation Promotion Association CAS (No. 2017253), Shandong Provincial Natural Science Foundation (No.ZR2016BQ18).

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