Graphite at High Voltages with an Electrolyte Additive

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Surfaces, Interfaces, and Applications

Stabilizing LiCoO2/graphite at High Voltage with Electrolyte Additive Suping Wu, Yilong Lin, Lidan Xing, Gengzhi Sun, Hebing Zhou, Kang Xu, Weizhen Fan, Le Yu, and Weishan Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01053 • Publication Date (Web): 16 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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ACS Applied Materials & Interfaces

Stabilizing LiCoO2/graphite at High Voltage with Electrolyte Additive

Suping Wua,†, Yilong Lina,†, Lidan Xinga,b, Gengzhi Suna,b, Hebing Zhou a,b,*, Kang Xuc,*, Weizhen Fand, Le Yud, Weishan Lia,b,*

a.

School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China

b.

National and Local Joint Engineering Research Center of MPTES in High Energy and Safety LIBs, Engineering Research Center of MTEES (Ministry of Education), and Key Lab. of ETESPG(GHEI), South China Normal University, Guangzhou 510006, China

c.

Electrochemistry Branch, Sensor and Electron Devices Directorate, Power and Energy Division, U.S. Army Research Laboratory, Adelphi, MD 20783, USA.

d.

Guangzhou Tinci Material Technology Co., Ltd, Guangzhou 510760, China

ABSTRACT The energy density of commercial Li-ion batteries (LIBs) using LiCoO2 has been subject to the limited access to the Li stored in CoO2 lattice, which is imposed partially by the instability of carbonate-based electrolyte at potential higher than 4.5 V. In this work, we report a novel approach to fully utilizing these extra Li via simultaneously stabilizing anode and cathode interfaces with a designed additive 4-propyl-[1,3,2]dioxathiolane-2,2-dioxide (PDTD), which strongly coordinate with Co ions dissolved in electrolytes while decomposing to form protective interphases on both cathode and anode surfaces. The Co ions present in the electrolyte deposit on anode in the form of the coordinate with PDTD, avoiding the formation of Co metal that will catalyze the reduction decomposition of the additive-free electrolyte. The presence of PDTD in electrolyte enables a higher charging potential of 1

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4.45 V for LiCoO2/graphite cell, which significantly improved energy density and cycling stability of this cathode chemistry that is already extensively used in commercial LIBs.

KEYWORDS: Lithium cobalt oxide/graphite cell; High voltage; Cyclic stability; Cobalt coordination; Electrolyte additive.

1. INTRODUCTION Among all electrochemical energy storage devices, lithium-ion batteries (LIBs) possess advantages of high energy densities and long cycle life.1-4 Since its commercialization, the flagship chemistry of LIB, lithium cobalt oxide (LiCoO2)/graphite battery, has dominated the mobile electronics market, due to the unparalleled tap density and cyclic stability of LiCoO2.5-7 However, LiCoO2 presents a mediocre specific capacity of about 150 mAh g-1 under a normal cutoff voltage of 4.2 V (vs. Li/Li+) instead of its theoretical value of 274 mAh g-1, because only half of the Li stored in CoO2 lattice could be reversibly utilized.8-10 Charging LiCoO2 beyond 4.2 V can access these extra Li, significantly raising the specific capacity of LiCoO2 and eventually the energy density of LIBs. This possibility had attracted intensive interests,11-13 but had also encountered significant difficulties, not only because the layered structure of LiCoO2 becomes destabilized due to over-delithiation, inducing Co dissolution and oxygen generation, but also because at the high potential of delithiating, the carbonate molecules in the electrolyte start irreversible oxidation decomposition, leading to massive generation of gaseous CO2.14,15 Furthermore, the dissolved cobalt will migrate to the anode side and deposit on graphite surface, inducing further electrolyte reduction decomposition.16-19 Much effort has been made up to date to address this issue,20-24 among which, modifying the LiCoO2/graphite interface with an electrolyte additive is the 2


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most economically viable.25-29 The mechanism mainly involves the interphasial films generated from the preferentially electrochemical oxidations of the additives, which suppress the cobalt dissolution and the electrolyte decomposition.30-33 Apparently cobalt dissolution can be controlled by applying electrolyte additives to some extent, but cannot be entirely avoided because cobalt has been always identified on the anode surface after certain cycles. The dissolved cobalt ions might be reduced as metal metal thereon, which would form catalytic sites to accelerate the electrolyte reduction decomposition and deteriorate the interfacial resistances on anode. In

this

work,

we

report

a

new

strategy

by

designing

an

electrolyte

additive,

4-propyl-[1,3,2]dioxathiolane-2,2-dioxide (PDTD), which can strongly coordinate with cobalt ions in the electrolyte and prevent them from deposition as cobalt metal on anode. The physical and electrochemical characterizations and theoretical calculations reveal that PDTD not only prevents cobalt ions from deposition as cobalt metal, but also modifies the interphases on both cathode and anode via preferential oxidation or reduction over the bulk electrolyte components. These characteristics of PDTD ensure the stability of bulk electrolyte against the charged surfaces of both LiCoO2 and graphite during extended cycling tests with upper limit of 4.45 V. This unique stabilization strategy provides a simple and practical avenue to improve the energy density of the most popular chemistry of LIB.

2. EXPERIMENTAL Sample Preparation. LiCoO2 and graphite were obtained from Hunan Shanshan Advanced Material Co., Ltd., China and BTR Battery Materials Co., Ltd., China, respectively. The cathode is made up of 96 wt.% LiCoO2, 2 wt.% Super-p, and 2 wt.% polyvinylidene fluoride (PVDF) binder. The anode 3



consists of graphite, acetylene black and CMC-SBR binder (graphite/acetylene black/CMC-SBR = 95/1.5/3.5, in weight). The LiCoO2 and graphite electrodes were prepared by coating respective slurries onto Al foil and Cu foil. Then the pouch cells with a designed capacity of 1500 mAh were fabricated using the cathode, anode prepared above and separator. Battery-grade carbonate solvents and lithium salt (LiPF6) were obtained from Guangzhou Tinci Materials Technology Co., Ltd. The blank electrolyte was prepared by mixing 1.0 M LiPF6 in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (1/2, in weight). PDTD was purchased from Hubei Jusheng Technology Co. Ltd, China, as an electrolyte additive that was added into the baseline electrolyte. Electrochemical Measurement. The cyclic voltammetry (CV) was implemented using Solartron-1480 instrument (England). A multichannel battery cycler (CT-3008W, Neware, China) was used to test the charge-discharge behaviors of LiCoO2/graphite pouch full cells. The pouch cells were conducted at C/10 for the three formation cycles and then at C/1 for subsequent cycles between 3 and 4.45 V. In the meantime, on LAND test system (CT2001A, China), the half cells Li/LiCoO2 and Li/graphite in the constant current mode over the range of 3-4.6 V and 3-0.01 V, respectively, were used to elucidate the mechanism of additive. The half cells Li/LiCoO2 were also tested in the constant current mode between the range of 3-4.6 V and 3-4.2 V to contrast the extent of cobalt dissolution. Li/graphite V-type electrolytic cells were used in chronoamperometry (CA) to explore the electrolyte decomposition origin from the catalysis of cobalt deposited on graphite anode. Electrochemical impedance spectroscopy was performed at discharge state (~3.0 V) in a frequency range of 105-0.01 Hz. Physical Characterization. To detect the morphology, structure and surface composition of electrodes, the cycled pouch full cells were disassembled in a glovebox in argon atmosphere. The cycled LiCoO2 and graphite electrodes were collected and washed with dimethyl carbonate three times 4


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to remove electrolyte precipitations followed by drying overnight at room temperature before surface analysis. The surface morphology of LiCoO2 and graphite electrodes was characterized utilizing scanning electron microscope (SEM, JSM-6510, Japan). The crystal structure was analyzed by X-ray diffraction (XRD, Bruker D8 Advance, Germany), and surface composition was analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250, America), which was conducted using a focused monochromatized Al Kα radiation under ultrahigh vacuum conditions. The graphite peak at 284.3 eV was used as a reference. The contents of Co, Li, Al and Cu deposited on graphite anode were obtained by ICP-MS (IRIS Intrepid II XSP, USA). The composition analysis of electrolyte in Li/graphite V-type electrolytic cells after CA tests was conducted by nuclear magnetic resonance (NMR, Varian 400, USA). Raman spectra were carried out on WITec Apyron: automated confocal Raman microscopy (WITec GmbH). The excitation wavelength is 633 nm, achieving a diffraction-limit spot size with 500 nm in diameter and 1 m in depth length with a 100x objective (NA = 0.90). The Raman data analysis is performed with WITec Project plus software. Calculation Methods. The Gaussian 09 package was used for structural optimization and B3LYP functional of density functional theory (DFT) was adopted. The equilibrium structures were optimized at the 6-311++G (d) level for C, H, O, S and lanl2dz for Co. The polarized continuum model (PCM) with a dielectric constant of 20.5 (acetone) was used. Atomic charge distributions were obtained based on natural bond orbital (NBO) theory.

3. RESULTS AND DISCUSSION LiCoO2/graphite cells were cycled at a constant current of 1 C (1500 mAh) between 3 and 4.45 V at room temperature (Fig.1a), where comparison was made between blank electrolyte and that containing PDTD additive at 0.5% and 1%. A serious capacity loss occurs in the former, with capacity 5



retention of only 10.87% after 150 cycles. This significant fading can be ascribed to the continuous electrolyte decompositions on cathode and anode under high voltage, along with structural destruction of LiCoO2 lattice on cathode32 and Li metal deposition on anode34. However, presence of PDTD improves the capacity retention to 47.39% and 67.78% at 0.5% and 1.0%, respectively (Fig. 1b). The charge/discharge profiles at first cycle and the corresponding dQ/dV prifiles of LiCoO2/graphite pouch cells were also compared between the blank and PDTD-containing electrolytes , with cell cycled at 0.1 C (150 mA) and room temperature between 3 and 4.45 V. The incorporation of PDTD gives rise to the increase of initial discharge capacity as well as the coulombic efficiency, which rises from 83.76 to 88.00% (Fig. 1c). A tiny but noticeable plateau at ~2.7 V (vs. graphite) was observed when the additive exists in electrolyte during the first charging process (Fig. 1d), which should be ascribed to the characteristic reduction of PDTD on anode (~1.25 V, vs. Li/Li+).

Since this process occurs at a higher

potential than carbonate solvents, it is reasonable to assume that it leads to the formation of a protective interphase before the bulk electrolyte experiences reduction. This assumption can be further demonstrated in cyclic voltammograms of Li/LiCoO2 and Li/graphite half cells obtained at a scan rate of 0.1 mV s-1 (Fig. 1e and f). The first oxidation peak occurs at 4.2 V in blank and 4.1 V in presence of PDTD, respectively, during the anodic sweep, while the peak separation between anodic and cathodic scans is much narrower for the latter, indicating a much less polarization when PDTD additive is used. The increase of initial discharge capacity and coulombic efficiency achieved in PDTD-containing electrolyte can be mainly attributed to in the improvement on graphite anode side (Fig. 1f). The presence of PDTD gives rise to a new reduction current peak at around 1.25 V, highly consistent with the dQ/dV curve in Fig. 1d. This cathodic process occurring at higher potential than the blank electrolyte (about 1.1 V) ensures that the interphase on graphite surface would be mainly contributed 6


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by the decomposition product of PDTD instead of EC. The energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of PDTD were computed in comparison with EC and EMC using density functional theory (DFT). The LUMO energy of PDTD (-0.02450 a.u.) is distinctly smaller than that of EC (-0.00759 a.u.) and EMC (-0.00824 a.u.), a clear indication that PDTD is prone to reduction than EC and EMC (Fig. S1) and dominates the chemistry of solid-electrolyte-interphase (SEI). The HOMO energy of PDTD (-0.31834 a.u.) sits between EC (-0.32082 a.u.) and EMC (-0.30922 a.u.), revealing the additive with carbonates could also be decomposed in the anodic process, contributing in a significant extent to the formation of cathode-electrolyte-interphase (CEI). The oxidative and reductive decompositions of PDTD additive are believed to generate more stable and less resistive interphases on both cathode and anode, thus improving the cyclic stability and rate performance in the subsequent cycles. Fig. 1g and h present discharge capacity of LiCoO2/graphite pouch cells under various rate currents at room temperature and 0.5 C at 0 °C, respectively. Significant improvement is observed for the batteries with 1% PDTD containing electrolyte, no matter in terms of capacity retention, rate capability, or low temperature performances. To understand how PDTD works, we compare the cyclic stability of both cathode and anode half-cells Li/LiCoO2 and Li/graphite. Fig. 2 presents the cycling performance of Li/LiCoO2 at 1 C from 3 to 4.6 V and Li/graphite half cells at 0.5 C from 3 to 0.01 V, as well as their corresponding selected charge/discharge profiles in blank and PDTD-containing electrolytes. Slight improvement of cyclic stability occurred in the Li/LiCoO2 with PDTD after 150 cycles (Fig. 2a). The medium discharge potential drop of LiCoO2 electrode during this period is only 0.08 V in PDTD-containing electrolyte (Fig. 2c) but is 0.10 V in blank electrolyte (Fig. 2b). The smaller potential drop with PDTD may stem 7



from the decreased polarization during cycling. In sharp contrast, significant improvement of capacity retention (from 62.95% to 84.76%) is observed in Li/graphite half-cell with PDTD-presence (Fig. 2d). Comparing the selected charge/discharge profiles between the cells with blank (Fig. 2e) and PDTD-containing electrolytes (Fig. 2f), one can conclude that, due to the preferential reduction of PDTD in first discharge cycle, SEI on graphite surface should mainly carry the chemical signature of PDTD and apparently provide better efficiency in suppressing bulk electrolyte decomposition. The improved interphasial stabilities can be further confirmed for cathode and anode half-cells through the impedance spectra, which typically consist of two semicircles and one slope line. The first semicircle at high frequency (interfacial layer resistance, Rf) is attributed to the impedance of Li+ migration through the interphase film, and the other at medium frequency (charge transfer resistance, Rct) is attributed to the impedance of charge transfer at the electrode. The slop line at low frequency (the Warburg impedance, Wo) is attributed to solid phase Li+ diffusion in the bulk of the intercalation compound.35 Nyquist diagrams obtained in this work are consistent with these typical impedance spectra (Fig. 3) as represented by the equivalent circuit in Fig. 3 inset, and the fitting results are displayed in Table 1. The impedances corresponding to interphase film and charge transfer are well separated, which ensures accurate fitting. As revealed in Table 1, after 3 cycles, there is little difference in Rf and Rct for both anode and cathode half-cells, regardless of blank or additive-containing electrolyte. However, the difference becomes stark over 150 cycles, and the PDTD-presence apparently stabilizes both anode and cathode interfaces. Surprisingly, such additive effect is not so pronounced in the anode half-cell as in cathode half-cell. Somehow, PDTD modifies cathode surface more effectively than it does to anode surface, which is counter-intuitive if only judged from the CV (Fig. 1) and voltage profiles (Fig. 2) as well as HOMO/LUMO calculations, because SEI-formation on anode is more 8


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apparent than that of CEI. This raises strong suspicion that the SEI formed on graphite is affecting CEI in certain anode-cathode cross-talk.36 Table 1 EIS fitting results of sample with and without PDTD. Sample

Cathode

Anode

After 3 cycles

Electrolyte

After 150 cycles

Rf(Ω)

Rct(Ω)

Rf(Ω)

Rct(Ω)

Blank

27.9

35

58.21

174.3

With PDTD

21.43

33.65

24.57

93.62

Blank

19.14

62.53

56.94

53.68

With PDTD

36.06

60.33

38.89

58.82

Morphologies of the LiCoO2 and graphite electrodes cycled in blank and PDTD-containing electrolytes, respectively, were compared with their corresponding pristine particles (Fig. 4). LiCoO2 is covered with thick and irregular degradation products after cycling in the blank electrolyte (Fig. 4b), indicative of serious electrolyte decomposition. In sharp comparison, smooth and neat surfaces similar to that of the pristine particles can be observed if PDTD is present in electrolyte (Fig. 4c). On anode side, the effect of PDTD is similar (Fig. 4e and Fig. 4f). The observation of electrode morphology is in good agreement with the cyclic performance enhancement through the incorporation of PDTD.

Table 2 Concentration of elements deposited on graphite anodes with and without PDTD. Electrolyte

Element (mg L-1) Cu

Al

Co

Li

Blank

42.27

0.152

4.572

176.3

With PDTD

18.17

0.109

3.397

94.3

To understand how LiCoO2 cathode behaves under different voltages, Li/LiCoO2 half cells were cycled to 4.2 V and 4.6 V, respectively. At 4.6 V LiCoO2 releases most of its Li, delivering nearly 1/3 extra capacity as compared with LiCoO2 charged to 4.2 V. However, this high capacity came at the 9



expense of cycling stability (Fig. S2), as evidenced by the capacity retention of only 60% after 50 cycles. The elemental analysis performed on the cycled LiCoO2 via inductively-coupled plasma (ICP) reveals that a high concentration of cobalt element deposited on anode when LiCoO2 was charged to 4.6 V, which is the result of massive cobalt dissolution followed by cobalt-diffusion across electrolyte and deposition at the anode. Serious cobalt deposition also occurred in LiCoO2/graphite system under high voltage when blank electrolyte was used (Table 2). The presence of PDTD reduces Co population, but Co can also be detected on the anode cycled in PDTD-containing electrolyte, suggesting that the presence of PDTD prevents cobalt deposition as metal. Since Co metal most likely serves as a catalyst for electrolyte reduction decomposition forming detrimental species such as HF, the PDTD-containing electrolyte inhibits such unwelcomed reactions and stabilizes the overall electrochemical performances. This hypothesis is supported by the fact that much less Al and Cu were also found on anode surface, which usually come from the substrate corrosion occurring on both electrodes caused by acidic species including HF, but more direct evidence can be obtained from nuclear magnetic resonance (NMR) analysis on the electrolyte (1M LiPF6 in EC/EMC/DEC (3/5/2, in weight)) that was exposed under CA tests to the graphite anode recovered from a cycled full cell LiCoO2/graphite. Multi-nuclei NMR spectra are displayed in Fig. 5.

13C

NMR detects typical chemical shifts

assigned to the carbonate esters including EC, EMC and DEC, and the weak peaks are identified as their decomposition products. Compared to the graphite anode cycled with blank electrolyte, much stronger carbonate signals have been detected as result of additive presence, because PDTD suppressed Co population and prevented Co deposition as metal on anode surface. 1H NMR and 19F NMR spectra basically make the same statement with

13C

NMR spectrum, as the HF signal only arises in cycled

graphite recovered from blank electrolyte.37-39 Accordingly, signals of PF6- and its decomposition 10


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products like PO3F2- and PO2F2- were present in both

19F

and

31P

NMR spectra.35,38 Thus, there is a

clear correlation between the Co-presence on graphite anode surface and the electrolyte decomposition. Surface chemical analysis was performed on LiCoO2 electrode using X-ray photoelectron spectroscopy (XPS). The obtained results are presented in Fig. 6. The C 1s spectrum of pristine LiCoO2 cathode includes three peaks, C-C peak at 284.0 eV assigned to the conductor carbon, C-H (285.6 eV) and C-F (290.4 eV) that is characteristic of the PVDF binder.41 The O 1s spectrum is dominant by large quantity of metal oxide (M-O, 529.5 eV), and incorporates low amount of Li2CO3 (531.5 eV). A peak at 687.6 eV in the F 1s fresh spectrum is clearly attributed to PVDF. After 150 cycles, both LiCoO2 electrodes cycled with or without PDTD display new species in C 1s, O 1s, F 1s, P 2p, and S 2p spectra, which arise from different decomposition products deposited on the surface. In the C 1s spectra, the C-C and PVDF (C-F and C-H) of the cycled LiCoO2 electrode are weaker in blank electrolyte than in PDTD containing electrolyte, and a new peak at 289.2 eV appears consistent with the -C=O or -(C=O)-O- in semi-carbonates.42 Due to the protection by PDTD against electrolyte decomposition, such signals become much weaker. In the O 1s spectra, the films covered on the cycled electrodes hinder the detection of M-O peak, but a new peak at ∼533 eV can be observed that represents -(C=O)-O- in polycarbonates.43 Another broader peak at about 532 eV for the electrode cycled in the electrolyte with additive, indicating more lithium alkyl carbonate, R-O-(C=O)-OLi, or R-O-(O=S=O)-OLi (see discussion below), or Li2CO3, on the cathode surface.44 Despite that the electrode was rinsed with DMC three times before XPS analysis, F 1s spectra still reveal precipitation of LiPF6 (687 eV), along with a broad peak that can be assigned as LixPOyFz (686.2 eV). Similarly, LixPOyFz and LixPFy can be found at 134 eV and 136.7 eV in P 2p spectra, respectively.45 On the other hand, LiF at 684.5 eV can be caught in F 1s spectra on cycled LiCoO2 11



recovered from blank electrolyte. Notably, when PDTD is present, a weak peak in S 2p spectrum consistent with R-O-(O=S=O)-O-Li indicates the participation of this additive in interphase formation. It is believed that the additive may have been oxidized along with carbonate solvents during delithiation of LiCoO2, based on the absence of oxidation signal before delithiation (Fig. 1e) and the similar HOMO levels of PDTD and solvents (Fig. S1). The examination of Co 2p spectra further proves the above argument that more electrolyte decomposition products from the blank electrolyte shroud the cathode surface, while a much thinner interphase formed on LiCoO2 electrode due to PDTD participation in the new CEI chemistry. XPS analysis was also performed on graphite anode (Fig. 7). The C 1s spectra reveals that a strong peak at 284.3 eV on pristine anode that corresponds to graphite or elemental carbon, while a very weak peak at 286.7 eV can be attributed to the small quantity of CMC in anode.46,47 However, significant decrease in this graphite peak occurred for electrodes recovered from both blank and PDTD-containing electrolytes after 150 cycles. A new peak at ~289 eV can be clearly observed in the electrode with blank electrolyte, which is consistent with lithium alkyl carbonates or polycarbonates on the anode and impedes the detection of graphite material.48 The O 1s spectra of pristine anode include two main peaks corresponding to the oxygen atoms of the CMC binder.49 After 150 cycles, strong peaks characteristic of C-O (533.7 eV) and C=O (532.5 eV) arise, along with a new peak at 531.5 eV assigned to Li2CO3.50 Comparison in the peak abundances between electrodes recovered from blank and PDTD-containing electrolytes basically convey the same message that additive results in a much thinner interphase and much less decomposition of the bulk electrolyte.51 The F 1s spectra of the anode cycled with blank electrolyte reveal a much stronger peak at 684.5 12


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eV corresponding to LiF than that with PDTD-containing electrolyte. As discussed above, this passivation layer effectively suppresses LiF-generation. The other two inorganic products in the F 1s spectra are LixPOyFz (685.9 eV) and LixPFy (686.8 eV), which originate from the decomposition products of LiPF6 and the precipitation of LiPF6.52 Similarly, LixPFy and LixPOyFz also can be detected at 136.9 eV and 133.7 eV in P 2p spectra, respectively, but again the intensity of LixPOyFz is much lower when the electrolyte contains additive PDTD. The differences between the element analyses on the surface of cycled graphite electrodes can be well explained by the S 2p spectrum which was only detected from the PDTD-containing electrolyte. The peaks can be assigned to ROSO2- (170.5 eV) and SO3 (169.0 eV), which should serve as Li ion-conductive component of SEI film.53,54 In the Co 2p spectrum (Fig. 7), much Co deposit can be identified on graphite in the blank electrolyte than in PDTD-containing electrolyte, which is consistent with the ICP test in Table 2. It can be noted from the Co 2p spectrum that Co metal is present on graphite anode in the blank electrolyte but not in the PDTD-containing electrolyte. This analysis confirms that PDTD prevents the deposition of Co ions dissolved in the electrolyte as Co metal on graphite, which would catalyze the reduction decomposition of main electrolyte components. The above discussions have established the correlation between the excellent cell performance and the reduced the detrimental effect of cobalt by PDTD. However, the mechanism of how additive affects Co dissolution and deposition remains little understood. The structure optimizations of species, Co3+, EC, EMC, or PDTD, and their resultant complexes were conducted via theoretical calculations (Fig. 8). The complex Co3+ … PDTD apparently shows larger binding energy (1986.66 kJ/mol) than Co3+ … EC (1582.85 kJ/mol) and Co3+ … EMC (1623.37 kJ/mol). This difference indicates that the association between Co3+ and PDTD is much stronger than those of Co3+ with either solvent, which 13



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prevents the Co deposition on graphite anode as Co metal and further optimizes anode/electrolyte interface properties. Fig. S3 further presents the atomic charge density of Co3+…PDTD complex before and after reduction. It can be found from Fig. S3 that the charge density of Co3+ keeps unchanged when the complex Co3+ …PDTD is performed with one electron reduction, suggesting that the reduction of Co ions into metal is prevented by their coordination with PDTD. This calculation confirms the XPS identification (Fig.7) and the contribution of PDTD.

The XRD patterns (a) and Raman spectra (b) of both pristine and recovered graphite electrodes from either blank or PDTD electrolytes are shown in Fig. S4. After 150 cycles, the cycled graphite with blank electrolyte obviously has lost part of crystallinity as evidenced by weaker diffraction peaks, and the main peak has significantly decreased intensities with shift to higher angles. It most likely results from the serious electrolyte decomposition on graphite after deep cycling, which derived from the catalysis of transition metal deposition. In Raman spectroscopy, a sharp bond at 1580 cm-1 (identified as G bond) has been assigned to in-plane symmetric C-C stretches, which represents high crystalline graphite. A low bond at about 1350 cm-1 referred to as D bond is related to the polycrystalline graphite or disordered and defective carbons.55,56 It has been reported that the average in-plane crystallite domain size La (Å) can be evaluated according Eq. (1) as: 𝐼𝐷 ―1

()

𝐿a = 44

(1)

𝐼𝐺

It is known that the continuous structural disordering generates new graphitic edges and fragments of graphite planes, which in turn catalyze reductive decomposition of the electrolyte, and bring about gradual transfer of lithium from the cathode to the anode SEI and eventually degrading cell capacity.57 According to Eq. (1), it can be immediately calculated that the fresh graphite electrode has a high average in-plane crystallite domain size of 1257.1 Å. However, this domain size has been significantly 14


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decreased after cycling in blank electrolyte (La = 70.5 Å). The presence of PDTD additive, however, maintains this domain size from excessive degradation (La = 218.9 Å). Thus, the structural disordering at the graphite surface can be well inhibited through the incorporation of PDTD upon prolonged cycling for the LiCoO2/graphite pouch cells. With the results available, the contribution of PDTD to the improved cyclic stability of LiCoO2/graphite battery under high voltage can be illustrated in Fig. 9. As shown by the TEM images in Fig. 9, obviously rough and thick deposits form on both cathode and the anode surfaces when cycled in blank electrolyte, which originated from the serious electrolyte oxidation decomposition under high voltage. The corrosive HF generated from the electrolyte oxidation decomposition attacks LiCoO2 and causes the dissolution of Co ions. The dissolved Co ions are reduced on graphite as Co metal, which catalyzes the electrolyte reduction decomposition on anode. Significant improvement occurs with the addition of PDTD at low concentration, leading to superior interphase chemistries consisting of sulfur-containing species. Cathode electrolyte interphase (CEI) derived from the preferential oxidation of PDTD inhibits cobalt dissolution from cathode and the electrolyte oxidation decomposition. Simultaneously, the anode SEI derived from the preferential reduction of PDTD prevents cobalt deposition as metal at anode and suppresses the electrolyte reduction decomposition. On the other hand, PDTD serves as a coordinate for the dissolved Co ions in electrolyte due to the stronger PDTD… Co3+ complex than solvent…Co3+(Fig. 8). The PDTD becomes more reducible when it coordinates to Co3+ (Figs. S5 and 6) and is reduced without changing the valence of Co3+ (Fig. S3), which might be present in the form of stable organic sulfates rather than Co metal on anode, as detected by ICP (Table 2) and XPS (Fig. 7). Therefore, the catalyzed electrolyte reduction decomposition can be effectively suppressed by applying PDTD. The superior interphase chemistries on both electrodes promote Li+ 15



migration because of their lower interfacial resistance, which further improve the rate capability and cycling performance of the LiCoO2/graphite cells in low temperature.

4. CONCLUSION The incorporation of PDTD into electrolyte remarkably improves the electrochemical performance of LiCoO2/graphite pouch cells at room and low temperature. The mechanism underneath the improvements has been thoroughly investigated by electrochemical, spectroscopic and computational means. The results unequivocally establish that on the anode side the additive contributes to suppress electrolyte decomposition while protecting graphitic structure by forming new SEI, and on cathode side stabilizes LiCoO2 at high potential via the formation of new CEI. Most importantly, PDTD can coordinate with Co ions dissolved in electrolytes, preventing the formation of Co metal that will catalyze the reduction decomposition of the additive-free electrolyte. PDTD proves to be a promising electrolyte additive that can significantly improve the available energy density of LiCoO2 in a simple and practical manner.

ASSOCIATED CONTENT Supporting Information. Additional figures including HOMO, LUMO energy of molecules, EC, EMC and PDTD, cyclic performance of Li/LiCoO2 cells, charge density of Co3+ … PDTD complex, XRD patterns and Raman spectra of graphite, cyclic voltammetry of Li/graphite half cells, and electron affinity energy of PDTD and Co3+…PDTD.

AUTHOR INFORMATION Corresponding Authors *

E-mail: [email protected]

*


*


Author Contributions 16


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All authors have given approval to the final version of the manuscript.

† These authors contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is financially supported by the National Natural Science Foundation of China (Grant No. 21872058) and the Key Project of Science and Technology in Guangdong Province (Grant No. 2017A010106006).

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Figure captions Fig. 1 (a) Cyclic stability of LiCoO2/graphite pouch cells at 1 C after the initial three cycles at 0.1 C at room temperature and (b) corresponding coulombic efficiency; (c) initial charge/discharge profiles at 0.1 C rate and (d) corresponding the dQ/dV profiles in charge process; cyclic voltammograms at a scan rate of 0.1 mV s−1 of Li/LiCoO2 (e) and Li/graphite (f) ; (g) rate capability of LiCoO2/graphite pouch cells at room temperature and (h) cyclic stability of LiCoO2/graphite pouch cells at 0.5 C after initial three cycles at 0.1 C under 0 °C. Fig. 2 (a) Cyclic stability of Li/LiCoO2 cells at 1 C after initial three cycles at 0.2 C under room temperature; selected charge/discharge profiles of Li/LiCoO2 cells cycled in blank (b) and PDTD containing (c) electrolyte; (d) cyclic stability of Li/graphite cells at 0.5 C after initial three cycles at 0.1 C under room temperature; selected charge/discharge profiles of Li/graphite cells cycled in blank (e) and PDTD containing (f) electrolyte. Fig. 3 Impedance spectra of discharge Li/LiCoO2 (a, b) and Li/graphite (c, d) cells in blank (a, c) and PDTD-containing (b, d) electrolyte. Fig. 4 SEM images of LiCoO2 (a, b, c) and graphite (d, e, f) electrode: fresh electrode (a, d) and cycled electrodes in blank (b, e) and PDTD-containing (c, f) electrolyte. Fig. 5

13C, 1H, 19F

and

31P

NMR spectra of the electrolytes (1M LiPF6 in EC/EMC/DEC (3/5/2, in

weight)) after CA tests in Li/graphite V-type electrolytic cells. The graphite electrodes were made from the cycled LiCoO2/graphite pouch cells in blank and PDTD-containing electrolyte, and the V-type cells (inset of 19F NMR) were performed with LSV at 0.2mV/s from OCP to 0.001V and then with CA at 0.001V for 16h. Fig. 6 XPS spectra of fresh and cycled (in blank and PDTD containing electrolyte) LiCoO2 electrodes. Fig. 7 XPS spectra of fresh and cycled (in blank and PDTD containing electrolyte) graphite electrodes. Fig. 8 Optimized structures and comparison of binding energy between Co3+ and EC, EMC or PDTD. Fig. 9 Schematic illustration on the contribution of PDTD to stabilizing LiCoO2/electrolyte and graphite/electrolyte interfaces.

25



Fig. 1 (a) Cyclic stability of LiCoO2/graphite pouch cells at 1 C after the initial three cycles at 0.1 C at room temperature and (b) corresponding coulombic efficiency; (c) initial charge/discharge profiles at 0.1 C rate and (d) corresponding the dQ/dV profiles in charge process; cyclic voltammograms at a scan rate of 0.1 mV s−1 of Li/LiCoO2 (e) and Li/graphite (f) ; (g) rate capability of LiCoO2/graphite pouch cells at room temperature and (h) cyclic stability of LiCoO2/graphite pouch cells at 0.5 C after initial three cycles at 0.1 C under 0 °C. 26


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Fig. 2 (a) Cyclic stability of Li/LiCoO2 cells at 1 C after initial three cycles at 0.2 C under room temperature; selected charge/discharge profiles of Li/LiCoO2 cells cycled in blank (b) and PDTD-containing (c) electrolyte; (d) cyclic stability of Li/graphite cells at 0.5 C after initial three cycles at 0.1 C under room temperature; selected charge/discharge profiles of Li/graphite cells cycled in blank (e) and PDTD containing (f) electrolyte.

27



Fig. 3 Impedance spectra of discharge Li/LiCoO2 (a, b) and Li/graphite (c, d) cells in blank (a, c) and PDTD-containing (b, d) electrolytes.

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Fig. 4 SEM images of LiCoO2 (a, b, c) and graphite (d, e, f) electrodes: fresh electrode (a, d) and cycled electrodes in blank (b, e) and PDTD-containing (c, f) electrolyte.

29



Fig. 5

13C, 1H, 19F

and

31P

NMR spectra of the electrolytes (1M LiPF6 in EC/EMC/DEC (3/5/2, in

weight)) after CA tests in Li/graphite V-type electrolytic cells. The graphite electrodes were made from the cycled LiCoO2/graphite pouch cells in blank and PDTD-containing electrolyte, and the V-type cells (inset of 19F NMR) were performed with LSV at 0.2mV/s from OCP to 0.001V and then with CA at 0.001V for 16h.

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Fig. 6 XPS spectra of fresh and cycled (in blank and PDTD containing electrolyte) LiCoO2 electrodes.

31



Fig. 7 XPS spectra of fresh and cycled (in blank and PDTD containing electrolyte) graphite electrodes.

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Fig. 8 Optimized structures and binding energy between Co3+ and EC, EMC or PDTD.

33



Fig. 9 Schematic illustration on the contribution of PDTD to stabilizing LiCoO2/electrolyte and graphite/electrolyte interfaces.

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Graphical abstract

35


Graphite at High Voltages with an Electrolyte Additive

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