Functionality Selection Principle for High Voltage Lithium-ion Battery

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Functionality Selection Principle for High Voltage Lithium-ion Battery Electrolyte Additives Chi-Cheung Su,†,‡,§ Meinan He,†,‡,△ Cameron Peebles,‡ Li Zeng,∥,# Adam Tornheim,‡ Chen Liao,‡,§ Lu Zhang,‡,§ Jie Wang,⊥ Yan Wang,△ and Zhengcheng Zhang*,‡,§ ‡

Advanced Electrolyte Research Group, Chemical Sciences and Engineering Division, §Joint Center for Energy Storage Research, and ⊥ Electron&X-ray Microscopy, Nanoscience and Technology, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439-4837, United States ∥ Applied Physics Program and #Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States △ Department of Mechanical Engineering, Worcester Polytechnic Institute, 100 Institute Road, Worcester, Massachusetts 01609, United States S Supporting Information *

ABSTRACT: A new class of electrolyte additives based on cyclic fluorinated phosphate esters was rationally designed and identified as being able to stabilize the surface of a LiNi0.5Mn0.3Co0.2O2 (NMC532) cathode when cycled at potentials higher than 4.6 V vs Li+/Li. Cyclic fluorinated phosphates were designed to incorporate functionalities of various existing additives to maximize their utilization. The synthesis and characterization of these new additives are described and their electrochemical performance in a NMC532/graphite cell cycled between 4.6 and 3.0 V are investigated. With 1.0 wt % 2-(2,2,2-trifluoroethoxy)-1,3,2-dioxaphospholane 2-oxide (TFEOP) in the conventional electrolyte the NMC532/graphite cell exhibited much improved capacity retention compared to that without any additive. The additive is believed to form a passivation layer on the surface of the cathode via a sacrificial polymerization reaction as evidenced by X-ray photoelectron spectroscopy (XPS) and nuclear magnetic resonsance (NMR) analysis results. The rational pathway of a cathode-electrolyte-interface formation was proposed for this type of additive. Both experimental results and the mechanism hypothesis suggest the effectiveness of the additive stems from both the polymerizable cyclic ring and the electron-withdrawing fluorinated alkyl group in the phosphate molecular structure. The successful development of cyclic fluorinated phosphate additives demonstrated that this new functionality selection principle, by incorporating useful functionalities of various additives into one molecule, is an effective approach for the development of new additives. KEYWORDS: fluorinated cyclic phosphate, electrolyte additive, functionality selection principle, high voltage electrolyte, post-test analysis, LiNi0.5Mn0.3Co0.2O2 cathode

1. INTRODUCTION Lithium-ion batteries have become widely used power sources for portable electronics and electric vehicles.1−3 To massively commercialize electric vehicles, the development of Li-ion batteries with higher energy and power density is essential.3−8 To this end, extensive research on exploring new cathode materials with an elevated operating voltage (>4.5 V vs Li/Li+) and specific capacity has been carried out.9−12 Because the discovery of lithium nickel manganese cobalt oxide (LiNi0.33Mn0.33Co0.33O2, NMC333) by Ohzuku et al. in 2000,13 tremendous efforts have been paid to the development of Ni-rich layer-structured NMC materials due to its high capacity when charged to potentials higher than 4.3 V vs Li+/Li.14,15 However, the voltage instability of the conventional electrolyte, 1.2 M lithium hexafluorophosphate © 2017 American Chemical Society

(LiPF6) dissolved in a mixture of ethylene carbonate (EC)/ ethyl methyl carbonate (EMC), hinders the extensive application of these high voltage NMC materials.16−18 Electrolyte solvents with intrinsically high voltage stabilities such as fluorinated carbonates, sulfones, dinitriles and diisocyanates have been actively pursued for this purpose.19−25 A more economical and effective approach is the formation of a passivation layer on the cathode particle surface which could be in situ formed by sacrificial reaction of an electrolyte additive26 or atomic layer deposition (ALD) either on the particle level or the electrode level.27 Received: June 21, 2017 Accepted: August 18, 2017 Published: August 18, 2017 30686

DOI: 10.1021/acsami.7b08953 ACS Appl. Mater. Interfaces 2017, 9, 30686−30695

Research Article

ACS Applied Materials & Interfaces

high voltage operation. We report the electrochemical performance improvement of these additives in a NMC532/graphite cell and postcycling analysis by NMR, SEM/EDS and XPS. The formation of a unique cathode-electrolyte-interface was proposed via a ring-opening polymerization initiated by the nucleophilic substitution of phosphorus atom by the surface oxygen sites that are bonded to transition metals.

Similar to the graphite anode, the presence of the passivation layer or cathode-electrolyte-interface (CEI) can suppress parasitic reactions on the cathode/electrolyte interface, providing kinetic stability on the charged cathode during the repeated charge and discharge cycling operation. Several electrolyte additives for NMC cathodes have recently been reported in the literature. In 2013, Zuo et al.28 reported lithium tetrafluoroborate (LiBF4) as an electrolyte additive which could enhance the cycling performance of a LiNi0.5Mn0.3Co0.2O2/graphite cell with 1.0 wt % concentration. Dahn and co-workers29−32 investigated the effectiveness of multiple families of electrolyte additives in improving the performance of NMC materials at high voltages. These additives include vinylene carbonate (VC), trimethylene sulfate (TMS), ethylene sulfate, methylene methanedisulfonate (MMDS), prop-1-ene-1,3-sultone (PES) and tris(trimethylsilyl) phosphate (TTSP). Lee et al.33 reported that a fluorinated linear carbonate methyl (2,2,2-trifluoroethyl) carbonate (FEMC) as an electrolyte additive could enhance the capacity retention of a NMC/Li cell.33 More recently, Pires et al.,34 He et al.35 and Chen et al.36,37 independently studied tris(2,2,2-trifluoroethyl) phosphite (TTFP) as a promising electrolyte additive for a Li-rich NMC cathode based on the fact that P3+ in TTFP tends to be oxidized to phosphate (P5+) on the TM-O surface forming a TM−O-PO(CF3CH2)3 layer. In additoin, phosphate compounds as additives have also been studied. Xiang et al.38 demonstrated the improved capacity retention in the presence of 2-ethoxy-1,3,2-dioxaphospholane 2-oxide (EDP) as additive in NMC/Li cell. Tris(1,1,1,3,3,3-hexafluoropropan-2-yl) phosphate (abbreviated as HFiP) was first reported in a 5-V LiNi0.5Mn1.5O4 cathode by Cresce et al.39 and then in a LMR-NMC composite cathode by Tan et al.40 The random selection of organic compounds as electrolyte additives is risky and cost ineffective. Highly accurate theoretical computation/simulation including DFT study of clusterbased, localized basis calculations on isolated additive and ab initio molecular dynamics (AIMD) with explicit incorporation of additive, solvent, and salt molecules have been widely employed to predict the energetics in solvation, reaction and activation barrier and the interaction of electrolyte/additive with electrode surface.41 From an organic chemist’s point of view, selection of the additive should be centered on molecules that bear certain reactive functional groups that could be initiated chemically or electrochemically on the electrode surface. 1,3,2-Dioxathiolane-2,2-dioxide (DTD) contains a dioxolane-like five-member ring that could be easily ring-opened during the electrochemical process forming an artificial interface and has been proved to be an effective additive.26,30 This concept was further confirmed by a phosphate compound, 2-ethoxy-1,3,2-dioxaphospholane 2-oxide (EDP), which mimics the cyclic structure of DTD on the phosphate skeleton. Utilizing this functionality selection principle, we developed and synthesized a new class of phosphate compounds incorporating the functionalities of various existing additives to maximize their utilization on one single molecule. In this paper, a new class of cyclic fluorinated phosphates including 2-(2,2,2-trifluoroethoxy)-1,3,2-dioxaphospholane 2-oxide (TFEOP), 2-(2,2,3,3,3-pentafluoropropoxy)-1,3,2-dioxaphospholane 2-oxide (PFPOP) and 2-((1,1,1,3,3,3-hexafluoropropan-2-yl)oxy)-1,3,2-dioxaphospholane 2-oxide (HFiPOP) were synthesized for the first time with the hope to tailor an interfacial chemistry on the cathode surface that can protect the bulk electrolyte component from oxidative decomposition during

2. RESULTS AND DISCUSSION 2.1. Synthesis and Characterization. Figure 1 shows the chemical structure of the cyclic fluorinated phosphate TFEOP,

Figure 1. Chemical Structures of reported electrolyte additives DTD, EDP, THFPP and new additive TFEOP, PFPOP, and HFiPOP.

PFPOP, HFiPOP, DTD, EDP, and THFPP. These compounds were synthesized by the reaction of their corresponding fluorinated alcohols with 2-chloro-1,3,2-dioxaphospholane-2oxide in THF solvent. Triethylamine was added as a scavenger for the hydrogen chloride generated by the reaction in order to facilitate the formation of the desired products. The precipitated ammonium chloride salts could be easily filtered out and the crude products were vacuum distilled yielding colorless liquid products. All compounds were characterized by NMR and GC-MS. Figure 2 shows the 1H-, 13C-, and 19F-NMR spectra of PFPOP. The proton signal (4H) of −CH2CH2− from the five-membered ring overlaps with that of the methylene protons adjacent to the CF2CF3, resulting in one multiplet in the proton NMR spectrum (4.34−4.56 ppm). Two peaks in 19F-NMR spectrum correspond to the fluorine atom in CF2 and CF3 unit. The coupling 13C NMR spectrum further identifies the chemical structure of PFPOP with the triplet of doublets at 63.3 ppm corresponding to the −CH2− carbon adjacent to the CF2CF3 group, the doublet at 66.3 ppm for the −CH2− carbons on the five-membered ring, the triplet of quartets at 111.6 ppm for the CF2 carbon and the quartet of triplets at 118.3 ppm assigned for the CF3 carbon. The clear peak assignments for the multi-NMR spectra of TFEOP and HFiPOP are provided in the Experimental Section. 2.2. Anodic Stability on NMC532 Cathode. Using the strict criteria for electrolyte stability established in a previous report,42 we evaluated the stability of the electrolyte/additive solutions against oxidative decomposition on the charged surface of a cathode composite, in the form of the lithiated layered metal oxide. In this experiment, NMC532 composite electrode was used as the working electrode, whereas lithium metal was used as both counter and reference electrodes. The scan rate was set to be as low as 0.1 mVs−1 in order for the electrode reaction to reach equilibrium. Figure 3 compares the results for the baseline electrolyte and the 1.0 wt % TFEOP additive, which performs the best in the NMC532/graphite cells. 30687


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Figure 2. NMR spectra of PFPOP in CDCl3. (a) 1H, (b) 19F, and (c) 13C.

ion batteries.43 The problem associated with this class of compounds is that it interferes with the intercalation chemistry of the graphite anode due to its facile reduction reaction at low potentials. However, after substitution by an electron-withdrawing fluorinated alkyl group, the fluorinated cyclic phosphates have no obvious interference with graphite anode. dQ/dV profiles of the TFEOP additive graphite/Li half-cell resembled that of the baseline cell either at the early lithiation stage (inset in Figure 4) or the later stages (Figure 4), indicating the additive does not participate or alter the SEI formation process of the baseline electrolyte on the anode side. The advantage was shown up when tested in a NMC532/ graphite full cell. Figure 5a compares the capacity retention profiles of NMC532/graphite full cells with baseline electrolyte and baseline plus 1.0 wt % of previous reported additive DTD, EDP, and THFPP. All three additive cells showed improvement in capacity retention in 100 cycles especially for EDP cell; however, EDP cell suffers from severely low initial capacity indicating the high resistance of the formed passivation layer. Table 1 summarizes the values for selected discharge capacity and capacity retention. The initial capacity and capacity retention after 100 cycles for Gen 2 electrolyte cell are 201.9 mAh g−1 and 67.5%, respectively. Meanwhile, the cycling performance of the full cell with 1.0 wt % DTD, which also has a polymerizable five-member ring, was significantly improved showing an initial capacity of 204.0 mAh g−1 and capacity retention of 74.0%. While the initial capacity was only 189.2 mAh g−1 for the full cell with 1.0 wt % THFPP, a fluorinated phosphate additive, the capacity retention (77.3%) was significantly higher than that of the baseline cell (67.5%). Although the initial capacity of the cell employing 1.0 wt % EDP, a cyclic phosphate, was as low as 141.6 mAh g−1, but the 100th cycle capacity (155.0 mAh g−1) was the highest among all the cells tested. These results demonstrate that introducing a five-member ring containing a fluorinated substitution group and a phosphate moiety into the additive structure indeed could bring additional benefits to improve the cycling performance.

Figure 3. Comparison of the oxidative stability for baseline electrolyte and Gen 2 + 1.0 wt % TFEOP additive on the surface of NMC532. (Scan rate 0.1 mVs−1, Li as the counter and reference electrodes, and cathode composite on the Al substrate as the working electrode.)

TFEOP additive cell exhibited an onset delithiation peak at 3.94 V, which is 0.1 V higher (right shifted) compared with the baseline cell, indicating the presence of a passivation layer on the surface of NMC formed by the reaction of the additive with cathode surface. Moreover, this passivation layer stabilized the interface of the charged NMC cathode and the electrolyte, thus postponed the oxidative decomposition of the bulk electrolyte to a higher value as illustrated by the inset in Figure 3. Figure S1 displays the linear sweep voltammograms of the baseline electrolyte and the 1.0 wt % TFEOP additive. The fact that no obvious preferential oxidation of TFEOP over the baseline electrolyte was observed and suggests the reaction of TFEOP in forming a protective layer is chemical rather than electrochemical. 2.3. Cell Performance and Functionality Selection Principle. Phosphate compounds have previously been widely studied as flame retardants to improve the safety of lithium 30688


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Figure 5. Capacity retention of NMC532/graphite cells using Gen 2 and Gen 2 with 1.0 wt % additives: (a) containing 1.0 wt % reported additives and (b) containing 1.0 wt % synthesized cyclic fluorinated phosphates.

Table 1. Summary of Additive Performance Data electrolyte

Figure 4. Differential capacity profile of the graphite/Li half cells with the baseline electrolyte and the baseline electrolyte plus additive: (a) 1st charging and (b) 2nd charging (charging is the lithiation process).

Gen 2 Gen 2 + 1.0 wt % DTD Gen 2 + 1.0 wt % THFPP Gen 2 + 1.0 wt % EDP Gen 2 + 1.0 wt % TFEOP Gen 2 + 1.0 wt % PFPOP Gen 2 + 1.0 wt % HFiPOP

In light of the above results, a family of new molecules including TFEOP, PFPOP and HFiPOP were designed by a new functionality selection principle with a combination of a cyclic structure, a fluorinated substituent and a phosphate. NMC532/graphite full cells using Gen 2 with 1.0 wt % cyclic fluorinated phosphates were subjected to the same cycling test mentioned above. The cell performance is shown in Figure 5b with detailed tabular capacity and capacity retention data summarized in Table 1. Two fluorinated phosphate additives, TFEOP and PFPOP, showed improvements in NMC532/ graphite cells but not HFiPOP, which is the hexafluoroisopropyl substituted cyclic phosphate. The initial capacity of the full cell with 1.0 wt % TFEOP was 193.5 mAh g−1, which was lower than the initial capacity of the full cell with 1.0 wt % PEPOP (204.1 mAh g−1). However, the capacity retention and 100th cycle capacity of TFEOP cell were 83.6% and 161.8 mAh g−1, respectively, both of which were significantly higher than those of the PFPOP cell (73.9% and 150.8 mAh g−1, respectively) and the baseline cell without additive (67.5% and 136.2 mAh g−1, respectively). The Coulombic efficiency (CE) of the HFiPOP cell system was significantly lower than that of the other three systems (Gen 2, Gen 2 + 1.0 wt % TFEOP and Gen 2 + 1.0 wt % PFPOP) as presented in Figure 6. The TFEOP and PFPOP cells displayed relatively high CE (>99.6%) with more extended stability over cycling than baseline cell.

initial capacity (mAh/g)

100th cycle capacity (mAh/g)

capacity retention (100 cycles, %)

201.9 204.0

136.2 150.9

67.5 74.0

189.2

146.2

77.3

141.6

155.0

109.5a

193.5

161.8

83.6

204.1

150.8

73.9

203.5

120.9

59.4

a

Because of the low initial capacity and an activation process, the cell with EDP showed abnormal capacity retention.

Based on these comparison experiments, the NMC532/graphite cells with TFEOP additive showed the best performance in terms of the capacity retention and the Coulombic efficiency for the first 100th cycles. Figure S2 summarizes the voltage profiles of the baseline cell and the additive cells for the first, 25th, 50th, 70th, and 100th cycle. The fact that TFEOP cell displayed largely improved cycling performance clearly demonstrates the effectiveness of the functionality selection principle in the development of new additives. 2.4. Electrochemical Impedance Spectroscopy (EIS). EIS was carried out to probe the electrical properties of electrodes and their interfaces. Figure 7 presents the impedance spectra obtained after 100 charge/discharge cycles using Gen 2, 30689


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Table 2. Extracted Values for Interfacial Impedance and Charge Transfer Resistances cell Gen Gen Gen Gen

2 2 2 2

w/o additive + 1.0 wt % TFEOP + 1.0 wt % PFPOP + 1.0 wt % HFiPOP

Rb (Ohm)

Rsei (Ohm)

Rct (Ω)

3.41 3.85 3.07 3.47

10.2 6.18 7.24 7.89

475 133 164 356

growth from the cathode side was confirmed by EIS analysis of assembled Aged (+)/Formed (−) cells. This conclusion is in accord with a recent FIB-SEM series sectioning study on the cycled NMC-based electrode by Liu et al.47 The significantly reduced impedance of the additive-containing cells clearly suggest the new additives can passivate the cathode surface preventing the oxidative decomposition of the bulk electrolyte. 2.5. Post-Test Analysis. 2.5.1. Electrolyte. NMR was used to shed light on the chemical species decomposed from electrolyte breakdown during high voltage cycling. No obvious decomposition products of the electrolyte were observed by NMR for both the cycled baseline and the additive-containing electrolyte, significant transeasterification of the EMC solvent was confirmed by 1H NMR. Figure 8 is the 1H NMR spectra of the electrolyte collected after 100 cycles from the baseline cell and the TFEOP added cell. A noticeable amount of diethyl carbonate (DEC) and dimethyl carbonate (DMC) generated by the transesterification reaction of EMC are present in the cycled baseline electrolyte (Figure 8a). In contrast, hardly any transesterification byproducts could be detected by NMR in the cycled TFEOP-added electrolyte. Because the transestirification reaction is facilitated by the catalytic surface of charged transition metal oxide,35,48 the absence of the byproducts for the additive cell clearly suggests the presence of a passivation layer on the surface of cathode preventing the direct contact of the electrolyte with the cathode. This result supports the improved cycling performance of the TFEOP additive cell as described in the cell performance section. 2.5.2. Harvested Electrode Morphology. The presence of a passivation layer on the surface of NMC formed by the reaction of the additive with the cathode surface was further analyzed by SEM/EDS on the cycled graphite anode. SEM images of a pristine graphite anode, the harvested graphite anodes from the baseline cell and the TFEOP cell were provided in Figure S3a−c. Gen 2 cycled graphite was covered by a layer of nonuniform decomposition products, whereas the morphology of anode cycled with 1% TFEOP resembled the pristine anode. SEM images of the pristine cathode, the cycled cathodes from the baseline cell and the TFEOP added cell are shown in Figure 9a−c, respectively. Compared with the pristine electrode, the morphology of Gen 2 cycled cathode completely changed and heavy decomposition products were observed on the surface (Figure 9b). The surface of the aged cathode was covered with thick deposits (Figure 9b). However, little change in morphology was observed for the cycled cathode with TFEOP additive electrolyte (Figure 9c). The surface resembles the pristine cathode as shown in Figure 9a. As discussed in Figure 4, the TFEOP additive did not participate in the SEI formation on the graphite anode. This result indicates that TFEOP participated the CEI formation on the cathode surface and suppresses the dissolution of transition metals into the electrolyte with subsequent diffusion and deposition on the graphite anode. This process is known to catalyze

Figure 6. Coulombic efficiency of NMC532/graphite cells using Gen 2 and Gen 2 with 1.0 wt % cyclic fluorinated phosphate additives.

Gen 2 with 1.0 wt % TFEOP, 1.0 wt % PFPOP and 1.0 wt % HFiPOP. These impedance spectra consist of two partially formed semicircles, which were fitted by the equivalent circuitry in the inset of Figure 7.44,45 Rb is the bulk resistance of the

Figure 7. Electrochemical impedance spectra for NMC532/graphite cells using Gen 2 and Gen 2 with 1.0 wt % cyclic phosphate additives after 100 cycles (inset: the Rsei area).

battery that represents the electric conductivity of the electrolyte, separator and electrodes. The semicircle at high frequencies is related to Rsei and Csei, which are resistance and capacitance of the solid electrolyte interface on the electrodes, respectively. Rct and Cct are faradic charge-transfer resistance and its relative double-layer capacitance, which correspond to the semicircle at the medium frequencies. The estimated values are summarized in Table 2. Although the Rsei of the baseline cell is slightly higher than that of the other cells with additives, the charge transfer resistance (Rct) of the baseline cell is two times higher than that of the cells with TFEOP and PFPOP, indicating severe electrolyte decomposition on the cathode surface after extensive cycling. Gilbert et al.46 investigated the impedance evolution with aging for the same NMC532/ graphite cell chemistry. They found that the rapid growth of the full cell impedance was observed when cycled at a high charging voltages or aged for an extended period of time. The impedance 30690


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Figure 8. An excerpt of the 1H NMR spectra of the cycled electrolytes for 100 cycles. (a) Gen 2 without additive and (b) Gen 2 + 1.0 wt % TFEOP.

Figure 9. SEM images of (a) pristine cathode, (b) cycled cathodes with Gen 2 electrolyte, and (c) cycled cathode with Gen 2 + 1.0 wt % TFEOP electrolyte.

Figure 10. X-ray photoelectron spectra of cycled LiNi0.5Mn0.3Co0.2O2 electrodes cycled in Gen 2 (baseline) electrolyte (top row) and Gen 2 + 1.0 wt % TFEOP electrolyte (bottom row). (a) C1s, (b) F1s, (c) O1s, and (d) P2p.

A significant Mn peak was detected in the spectrum of the anode cycled with Gen 2 electrolyte (Figure S4a), which did not show up on the EDS spectrum for the anode cycled with TFEOP additive electrolyte (Figure S4b).

electrolyte decomposition on the anode. The transition metal dissolution and deposition was analyzed by the energy-dispersive X-ray spectroscopy (EDS) analysis. Figure S4 shows the EDS spectra of the pristine and cycled anode with two electrolytes. 30691


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Figure 11. Proposed mechanism of the TFEOP polymerization forming the cathode passivation layer.

2.5.3. Solid Electrolyte Interface. Figure 10 presents C1s, F1s, O1s and P2p XPS spectra of the cycled cathodes with two electrolyte systems. Figure 10a is the expanded C1s region. In both cycled cathodes, the C1s peak at 284.8 eV originates from the C−C bond in carbon black, whereas the peaks at 286.5 and 288.0 eV arise from the C−O and CO bonds, respectively, and the peak at 291.5 eV corresponds to −CF2− groups from the PVDF binder.49 More importantly, an obvious peak at 293.0 eV exists for the electrode cycled with 1.0 wt % TFEOP electrolyte. This peak originates from −CF3 groups suggesting the presence of a TFEOP-derived decomposition product bound to the cathode surface. The presence of −CF3 group in the surface layer was further confirmed by the extra peak at 689.2 eV from F1s spectra shown in Figure 10b. It should be noted that the F peak from LiF is much higher in content when cycled in Gen 2 electrolyte than that cycled in additive cell indicating a different solid electrolyte interface formation mechanism. Figure 10c presents the O1s spectra. The peak at 530.1 eV is attributed to lattice transition metal oxide of the layered LiNi0.5Mn0.3Co0.2O250 which was significantly suppressed for the additive cell. The peak at 533.3 eV is attributed to the species containing a (fluoro)phosphate group whose intensity becomes much higher compared with that at 530.1 eV providing further evidence for the formation of the passivation layer. In the P2p region, the cycled electrodes shows strong peaks at 133.6, 135.0, and 136.4 eV, which can be attributed to the species containing a (fluoro)phosphate group.51 This group also originates from the decomposition of TFEOP additive and/or LiPF6. 2.6. Proposed Mechanism. Cell performance data and the post-test analysis results clearly indicate the presence of the protective passivation layer on the NMC cathode surface. Herein we speculate the decomposition pathway of the additive on the explicit cathode surface and how the resulting decomposition structure could lead to an optimal passivation layer. Because of the presence of the cyclic 5-member ring in the additive structure, it is rational to assume that ring-opening polymerization occurs catalyzed by the nucleophilic attack of a surface hydroxyl group on the surface transition metal centers.52 The detailed reaction pathway is depicted in Figure 11.53,54 Presumably, the phosphorus center of the cyclic fluorinated phosphates can be attacked by the nucleophilic oxygen of the TM-O rich surface with subsequent proton transfer forming a protonated intermediate. This step could be facilitated by the presence of an acid (or a lewis acid) (originally present in the

Gen 2 electrolyte or generated by the oxidative decomposition of solvents) rendering the cyclic phosphates more prone to attack by the nucleophilic hydroxyl functional group on the NMC particle surface. This unstable intermediate undergoes another proton transfer step leading to the polymerization of the cyclic phosphates via a ring-opening reaction. The polymerization reaction leads to the formation of polyphosphoesters on the surface of NMC particles which are known to exhibit good ionic conductivity.55 It should be noted that the electronwithdrawing −CF3 substituent in the cyclic fluorinated phosphate makes the phosphorus center of the PO bond even more prone to nucleophilic addition compared to the regular alkyl substituent of nonfluorinated cyclic phosphate EDP. Theoretically, the nonfluorinated counterpart EDP could undergo the same reaction; however, without the presence of the fluorinated group the passivation layer becomes more resistive causing lower initial capacity as shown in Figure 5. Therefore, the protective layer formed readily with cyclic fluorinated phosphate was optimized and unequivocally effective in suppressing the oxidative decomposition of the bulk of electrolyte and thus provides superior cell performance. This proposed mechanism agrees with the XPS data, which shows less LiF formation, more LiPxOyFz formation and less TM-O species. In addition, as discussed in a previous report, the passivation of TM-O leads to the suppression of the TM dissolution, especially for Mn, which could cause parasitic reactions on the graphite electrode resulting in loss of active lithium and more rapid capacity fade.35

3. CONCLUSIONS In summary, cyclic fluorinated phosphates bearing a fivemember ring, fluorinated substituents, and phosphate functional groups were designed to incorporate useful functionalities of various existing additives and were successfully synthesized. Their electrochemical performance in a high voltage NMC532/ graphite cell was been studied. The improved cell cycling performance at higher voltages when 1.0 wt % TFEOP or 1.0 wt % PFPOP of additive was added to the cells and EIS and LSV data both suggest the formation of a protective layer on the NMC532 surface which suppresses the electrolyte oxidative decomposition. A cathode surface passivation mechanism was proposed by which the phosphorus in the PO bond in the additive and the oxygen center of the TM-O surface are bound by a nucleophilic substitution reaction. Subsequent ring-opening polymerization of the cyclic group on the additive effectively passivates the TM-O surface which inhibits the direct contact of the electrolyte solvent and the charged cathode 30692


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4.3. Materials and Methods. The cathode was made of 90 wt % LiNi0.5Mn0.3Co0.2O2, 5 wt % carbon black (C45), and 5 wt % polyvinylidene fluoride (PVDF) (Solvay 5130) binder coated on aluminum foil with the active material loading 9.15 mgcm−2. The graphite anode consisted of 90 wt % graphite (ConocoPhillips CGPA12), 4 wt % carbon black (Super P−Li), and 6 wt % Kureha 9300 PVDF binder coated on copper foil with the active material loading 5.3 mgcm−2. Celgard 2325 was used as the separator. The effective diameters of the cathode, anode, and separator are 14, 15, and 16 mm, respectively. Gen 2 electrolyte containing 1.2 M LiPF6 ethylene carbonate (EC)/ethyl methyl carbonate (EMC) (3/7) was used as baseline electrolyte. DTD, TTFP, and THFPP was purchased from Sigma-Aldrich and purified prior use. EDP was synthesized in our lab following the reported procedure.36 4.4. Electrochemical Measurements. Galvanostatic charge/ discharge cycling was performed using 2032 coin cells at a C/3 rate with a cutoff voltage between 4.6 and 3.0 V following two formation cycles at a C/10 rate. Cell voltage profiles and capacity were recorded using a MACCOR Electrochemical Analyzer (MIMSclient). The impedance spectra were obtained using a Solartron analyzer operated between 0.01 Hz and 1 MHz with the amplitude of 10 mV. Linear sweep voltammetry was performed using a Bio-Logic VMP3 station in a three-electrode configuration with NMC532 (100 μm in diameter) as a working electrode and lithium metal as counter and reference electrodes; the scan rate was 0.1 mVs−1. 4.5. Post-Test Analysis. For post-test analysis, wee disassembled the cycled cells in an argon-filled glovebox, and the cycled electrolyte was collected by dipping the electrodes and the separator in small amount of chloroform-d and for NMR analysis. The morphology of the harvested electrodes were examined by scanning electron microscopy (SEM) using a Hitachi S-4700-II microscope at the Center for Nanoscale Materials, Argonne National Laboratory. X-ray photoelectron spectroscopy (XPS) measurements were carried out by using a Thermo Scientific ESCALAB 250Xi with Al Kα source. A low-energy electron flood gun was used to compensate for X-ray beam induced surface charging. C1s peak (284.8 eV) was used for energy calibrations. NMR spectroscopy were measured using a Bruker Avance III HD 300 MHz spectrometer. The chemical shifts in parts per million (ppm) are referenced to a tetramethylsilane standard.

surface. Post-test analyses including NMR, SEM, XPS, and EDS were performed to probe the nature of the electrodes and electrolyte system and validaed our passivation hypothesis. The transesterification side reaction, catalyzed by the surface TM-O species, was significantly suppressed because of the formation of the protective layer by the polymerization of TFEOP additive as evidenced from the 1H NMR results. All results of the post-test analyses are also consistent with the formation of a protective layer which mitigates electrolyte oxidation and transition metal dissolution on the cathode surface. SEM/EDS and XPS analysis results also demonstrated the positive effect of the TFEOP additive. This new functionality selection principle, through the incorporation of useful functionalities of various additives into one single additive, was proven to be an effective approach for the development of new additives.

4. EXPERIMENTAL SECTION 4.1. Synthesis of Cyclic Fluorinated Phosphates. 4.1.1. 2-(2,2,2-Trifluoroethoxy)-1,3,2-dioxaphospholane 2-oxide (TFEOP). 2-Chloro-2-oxo-1,3,2-dioxaphospholane (10.0 g, 0.0702 mol) dissolved in 20 mL of anhydrous tetrahydrofuran (THF) was added dropwise to a solution of 2,2,2-trifluoroethanol (8.78 g, 0.0877 mol) and triethylamine (12.2 mL, 0.0875 mol) in 160 mL of anhydrous THF at 0 °C. The reaction mixture was allowed to warm up to room temperature and stirred overnight. After the triethylammonium chloride salt was removed by vacuum filtration, the solution was subject to rotary evaporation. 6.94 g of pure product was obtained after vacuum distillation (bp 97 °C, 4 mmHg) with a yield of 48%. The synthesized TFEOP is a colorless liquid. 1H NMR (CDCl3, 300 MHz): δ 4.52−4.32 (m, 6H); 13 C NMR (CDCl3, 75 MHz): δ 122.4 (quartet of doublets (qd), J = 276, 8.2 Hz) 66.3 (d, J = 2.8 Hz), 66.1 (qd, J = 37.8, 4.6 Hz); 19F NMR (CDCl3, 282 MHz): δ −75.7; MS-EI m/z: 205.0 [M-H]−: 186.0, 161.0, 137.0, 107.0, 81.0, 64.0. 4.1.2. Synthesis of 2-(2,2,3,3,3-Pentafluoropropoxy)-1,3,2-dioxaphospholane 2-oxide (PFPOP). Similar synthetic procedure was employed as described in 4.1.1 for the synthesis of PFPOP using the following reagents. 2-Chloro-2-oxo-1,3,2-dioxaphospholane (10.0 g, 0.0702 mol), 2,2,3,3,3-pentafluoro-1-propanol (13.17 g, 0.0878 mol), trimethylamine (12.2 mL, 0.0875 mol). 9.13g colorless PFPOP liquid was obtained after vacuum distillation (bp 107 °C, 4 mmHg) with a yield of 51.0 wt %. 1H NMR (CDCl3, 300 MHz): δ 4.56−4.34 (m, 6H); 13C NMR (CDCl3, 75 MHz): δ 118.3 (quartet of triplets (qt), J = 286, 35 Hz), 111.6 (triplet of quartets (tq) 254, 37.5 Hz), 66.3 (d, J = 2.6 Hz), 63.3 (triplet of doublets (td), J = 28.4, 5.0 Hz); 19 F NMR (CDCl3, 282 MHz): δ −83.7, −125.0; MS-EI m/z: 257.0 [M + H]+: 236.0, 211.0, 137.0, 107.0, 81.0, 64.0. 4.1.3. Synthesis of 2-((1,1,1,3,3,3-Hexafluoropropan-2-yl)oxy)1,3,2-dioxaphospholane 2-oxide (HFiPOP). Similar synthetic procedure was employed as described in 4.1.1 for the synthesis of HFiPOP using 2-chloro-2-oxo-1,3,2-dioxaphospholane (15.0 g, 0.105 mol), 1,1,1,3,3,3-hexafluoro-2-propanol (22.9 g, 0.137 mol), and trimethylamine (19.1 mL, 0.137 mol). The final product (bp 93 °C, 4 mmHg) is a colorless liquid (13.04 g, 0.0476 mmol) with a yield of 45%. 1H NMR (CDCl3, 300 MHz): δ 5.30−5.11 (m, 1H), 4.57−4.44 (m, 4H); 13 C NMR (CDCl3, 75 MHz): δ 120.1 (q, J = 278 Hz) 72.1 (septet of doublets (sd), J = 35.3, 4.5 Hz), 66.8 (d, J = 3.4 Hz); 19F NMR (CDCl3, 282 MHz): δ −74.6; MS-EI m/z: 274.1 [M]+: 254.0, 228.9, 144.9, 106.9, 80.9, 68.9. 4.2. Chemical Structure Characterization. Synthesized cyclic fluorinated phosphates were characterized by both NMR spectroscopy and gas chromatography−mass spectrometry (GC-MS) to identify the chemical structure and purity. All NMR spectra were acquired on a 300 MHz Bruker spectrometer. 1H chemical shifts were referenced to chloroform-d (CDCl3) at 7.27 ppm and 13C chemical shifts were referenced to 77.0 ppm. GC spectra were acquired on an Agilent Technologies 7890B GC System and MS spectra were acquired on an Agilent Technologies 5977A MSD System.

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ASSOCIATED CONTENT

S Supporting Information *

Voltage profiles, SEM/EDX of cycled anodes and LSV of electrolytes with Pt working electrode. This material is free of charge via the Internet at The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acsami.7b08953. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +1-630-252-7868. ORCID

Chi-Cheung Su: 0000-0002-0758-8324 Cameron Peebles: 0000-0002-0062-8645 Li Zeng: 0000-0001-6390-0370 Author Contributions †

C.-C.S. and M.H. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is majorly supported by the Vehicle Technologies Office, U.S. Department of Energy. Argonne National Laboratory is operated by UChicago Argonne, LLC, for the U.S. 30693


Research Article


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Department of Energy under contract DE-AC02-06CH11357. We thank the Cell Analysis, Modeling, and Prototyping (CAMP) Facility of Argonne’s Chemical Sciences and Engineering Division for providing the electrode materials. This work was performed, in part, at the Center for Nanoscale Materials, a U.S. Department of Energy Office of Science User Facility under Contract DE-AC02-06CH11357.

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Functionality Selection Principle for High Voltage Lithium-ion Battery

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