Structural Exfoliation of Layered Cathode under ... - ACS Publications

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Structural Exfoliation of Layered Cathode under High Voltage and Its Suppression by Interface Film Derived from Electrolyte Additive Yunmin Zhu,† Xueyi Luo,† Huozhen Zhi,† Xuerui Yang,† Lidan Xing,†,‡ Youhao Liao,†,‡ Mengqing Xu,†,‡ and Weishan Li*,†,‡ †

School of Chemistry and Environment, South China Normal University, Guangzhou 510631, China Engineering Research Center of MTEES (Ministry of Education), Research Center of BMET (Guangdong Province), Engineering Laboratory of OFMHEB (Guangdong Province), Key Laboratory of ETESPG (GHEI), and Innovative Platform for ITBMD (Guangzhou Municipality), South China Normal University, Guangzhou 510006, China

‡

S Supporting Information *

ABSTRACT: Layered cathodes for lithium-ion battery, including LiCo1−x−yNixMnyO2 and xLi2MnO3·(1−x)LiMO2 (M = Mn, Ni, and Co), are attractive for large-scale applications such as electric vehicles, because they can deliver additional specific capacity when the end of charge voltage is improved to over 4.2 V. However, operation under a high voltage might cause capacity decaying of layered cathodes during cycling. The failure mechanisms that have been given, up to date, include the electrolyte oxidation decomposition, the Ni, Co, or Mn ion dissolution, and the phase transformation. In this work, we report a new mechanism involving the exfoliation of layered cathodes when the cathodes are performed with deep cycling under 4.5 V in the electrolyte consisting of carbonate solvents and LiPF6 salt. Additionally, an electrolyte additive that can form a cathode interface film is applied to suppress this exfoliation. A representative layered cathode, LiCoO2, and an interface film-forming additive, dimethyl phenylphosphonite (DMPP), are selected to demonstrate the exfoliation and the protection of layered structure. When evaluated in half-cells, LiCoO2 exhibits a capacity retention of 24% after 500 cycles in base electrolyte, but this value is improved to 73% in the DMPP-containing electrolyte. LiCoO2/graphite full cell using DMPP behaves better than the Li/LiCoO2 half-cell, delivering an initial energy density of 700 Wh kg −1 with an energy density retention of 82% after 100 cycles at 0.2 C between 3 and 4.5 V, as compared to 45% for the cell without using DMPP. KEYWORDS: lithium cobalt oxide, exfoliation of layered structure, interface film, anion insertion, electrolyte additive

1. INTRODUCTION Lithium-ion battery has played an important role in the development of electronic industry due to its superior performances as compared to other secondary batteries.1−4 However, the energy density of current lithium-ion battery cannot meet the requirement of long-distance transportation when it is applied in electric vehicles.5−9 Layered cathodes, including LiCo1−x−yNixMnyO2 and xLi2MnO3·(1−x)LiMO2 (M = Mn, Ni, and Co), are attractive for the energy density improvement of lithium-ion battery, because these cathodes can deliver additional specific capacity under voltage higher than 4.2 V, a limitation voltage of the conventional lithium-ion battery.10−14 xLi2MnO3·(1−x)LiMO2 provides large specific capacity, but suffers capacity decaying during cycling under high © 2017 American Chemical Society

voltage due to the transformation of layered structure to a spinel one. Comparatively, LiCo1−x−yNixMnyO2 shows better structural stability and thus has been widely used in practice. 15−18 Because the increased capacity of LiCo1−x−yNixMnyO2 under high voltage results mainly from the contribution of cobalt, LiCoO 2 (x = y = 0, in LiCo1−x−yNixMnyO2) delivers the largest additional capacity when the upper limitation voltage is increased to over 4.2 V.19,20 Additional 30% specific capacity of LiCoO2 can be achieved when the upper limitation voltage is enhanced from Received: January 2, 2017 Accepted: March 20, 2017 Published: March 20, 2017 12021

DOI: 10.1021/acsami.7b00032 ACS Appl. Mater. Interfaces 2017, 9, 12021−12034

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ACS Applied Materials & Interfaces

Figure 1. Cyclic stability of Li/LiCoO2 half-cells cycled in electrolytes with and without 0.5% DMPP at 0.5 C for three initial cycles and at 1 C for the following cycles between 3 and 4.2 V (1 C = 145 mAh g−1) (A) and 3 and 4.5 V (1 C = 185 mAh g−1) (B); Coulombic efficiency of the half-cells cycled between 3 and 4.5 V (C); calculated oxidation potential (V vs Li+/Li) and optimized structures of EC, EMC, DEC, and DMPP (D); and cyclic stability of Li/graphite half-cells at 0.2 C in the voltage range of 0.005−2.5 V in electrolytes with and without 0.5% DMPP (E).

4.2 to 4.5 V.21−23 However, the enhanced upper limitation voltage leads to the poor cyclability of LiCoO2. In general, the mechanisms concerning the failure of the LiCoO2 electrode under high voltage proposed at present are as follows: the formation of a high impedance film on LiCoO2 surface due to electrolyte decomposition;20 and the structural transformation due to the dissolution of transition metal ions and oxygen loss of Li1−xCoO2 after deep lithium extraction.17,24 Electrolyte decomposition is a universal phenomenon when high voltage cathodes are used and is usually ascribed to the thermodynamic instability of carbonate-based electrolytes. Under a voltage higher than 4.2 V, the electrolyte tends to decompose, yielding polymers such as carbonate oligomers, gases such as carbon dioxide, and acids such as HF, which causes the increased interfacial impedance and the transition

metal ion dissolution from cathodes.25−28 As the common lithium salt in carbonate-based electrolytes, LiPF6 is less stable than salts such as LiX (X = TFSI, TDI, FSI, and FTFSI) salts.29 In the presence of a trace amount of water in the electrolyte using LiPF6, HF might be formed from the hydrolysis of LiPF6.30 In addition, HF is also formed during the charge process. Under high voltage, the solvents (carbonates) will be decomposed, which occurs in the form of the complexes of solvents and PF6−, forming HF together with polymer deposits and gases.31 To mitigate the electrolyte decomposition and reduce the transition metal ion dissolution, several approaches have been developed, including partial replacements of transition metal ion,32−35 surface coatings,36−43 and applications of electrolyte additives.19,20,27,44,45 Among these approaches, adding additives 12022


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cells were cycling by charging to 4.5 V at 0.1 C (1 C = 185 mAh g−1) for the initial three cycles and 0.2 C for the subsequent cycles, followed by a constant voltage (4.5 V) up to 10% of applied charging current, and discharging to 3.0 V at the same current. Li/LiCoO2 half-cells were cycled by charging to 4.2 or 4.5 V at 0.5 C (1 C = 145 mAh g−1 for 4.2 V and 1 C = 185 mAh g−1 for 4.5 V) for the first cycle to the third cycle and then under 1 C for the following cycles, followed by a constant voltage (4.2 or 4.5 V) for 10 min, and discharging to 3.0 V at the same current. For the rate capability test, the cells were cycled by charging at 0.2 C and discharging at various current rates between 3.0 and 4.5 V. Li/graphite half-cells were charged/discharged at 0.2 C (1C = 372 mAh g−1) in the voltage range of 0.005−2.5 V. Potentiostatic test was conducted on Solartron-1480 (England). Electrochemical impedance spectroscopy was performed on PGSTAT-30 (Autolab, Switzerland) from 100 kHz to 0.01 Hz with an amplitude of 5 mV. For cobalt dissolution analyses, V-type cells were charged to 4.5 V at 0.1 C and then kept at this potential for 48 h. Physical Characterizations. Electrodes were taken from the cycled cells, rinsed with DMC, and then dried overnight under vacuum in the glovebox mentioned above for physical characterizations. The morphological images were obtained with SEM (JSM-6510, Japan) and TEM (JEM-2100HR, Japan). XRD patterns were obtained on Bruker D8 Advance (Germany). XPS was performed on ESCALAB 250 (USA), and FTIR was performed on Bruker Tensor 27 (Germany). The contents of cobalt deposited on lithium electrodes in the cycled half-cells and dissolved in the electrolyte of the V-type cells were analyzed by ICP-MS on Optima 8300 (U.S.). The content of HF in the V-type cells was analyzed by titration with 0.01 M NaOH solution, and bromothymol blue (0.001 wt %) as indicator was used.

into electrolyte is the most cost-efficient, which is based on a protective interface film derived from the additives that can be oxidized preferentially as compared to the base electrolyte.30,46−51 Several electrolyte additives have been successfully used for LiCoO2 cathode, including trimethylboroxine,19 vinyl ethylene carbonate,21 diphenyl disulfide,48 and aliphatic nitrile.52 It is easy to accept that the product accumulation of electrolyte decomposition would increase the interfacial impedance of LiCoO2/electrolyte. This effect might weaken the cyclability of LiCoO2 but never causes the collapse of layered structure in LiCoO2. Similarly, the cobalt dissolution from LiCoO2 should be insignificant in the limited electrolyte and will not cause the structural collapse. However, fast capacity decay can be observed when LiCoO2 electrode is performed with deep cycling under high voltage. This should be related to the structural collapse of LiCoO2. Unfortunately, less attention has been paid, and no explanation has been given to this collapse. In this work, we reported an interesting finding that the collapse of layered structure happens when LiCoO2 is performed with deep cycling under 4.5 V, and this failure results from the insertion of PF6− anions into the interlayers, which causes the exfoliation of the layered structure. Additionally, dimethyl phenylphosphonite (DMPP), a protective interface film-forming electrolyte additive for maintaining the structural stability of LiMn2O4,51 was used to prevent the collapse of layered structure in LiCoO2.

3. RESULTS AND DISCUSSION Cyclability of LiCoO2 and Graphite. To understand the effects of the additive on cathode and anode, Li/LiCoO2 and Li/graphite half-cells were performed with cycling tests. The obtained results are presented in Figure 1. Figure 1A shows the cyclic stability of LiCoO2 electrode under 4.2 V, a normal endoff charge voltage in industry. LiCoO2 electrode exhibits excellent cyclability in both electrolytes, with capacity retention of 85% and 88%, respectively, indicating that LiCoO2 works well when it is cycled under 4.2 V and DMPP hardly affects the cyclability of LiCoO2. LiCoO2 delivers initially only 144 mAh g−1 at 0.5 C under 4.2 V, which is far lower than its theoretical capacity (274 mAh g−1). When the end-off charge voltage is enhanced to 4.5 V, the initial discharge capacity of LiCoO2 is increased to 192 mAh g−1, as shown in Figure 1B. However, the enhanced voltage deteriorates the cyclic stability of LiCoO2 in the base electrolyte. As shown in Figure 1B, LiCoO2 electrode cycled in the base electrolyte exhibits a continuous capacity decaying up to 400th cycle, where the capacity decays abruptly. After 500 cycles, the discharge capacity almost disappears, remaining only at 51 mAh g−1, with a capacity retention of only 24%. It is well accepted that, under a charge voltage over 4.2 V, the base electrolyte tends to oxidatively decompose, yielding polymers, gases, and HF.18 The polymers and gases increase the interfacial impedance of LiCoO2/electrolyte, while the HF will cause the dissolution of cobalt from LiCoO2. These effects account for the continuous discharge capacity decaying before the 400th cycle, but cannot explain the abrupt capacity decaying after the 400th cycle. This abrupt capacity decaying may be related to the collapse of layered structure in LiCoO2, but less attention has been paid and no explanation has been given to this structural collapse. When DMPP is used, the cyclability of LiCoO2 is significantly improved, as shown in Figure 1B. After 500 cycles, LiCoO2 retains a discharge capacity of as high as 140 mAh g−1 with a capacity retention of 73%. No abrupt discharge

2. CALCULATIONS AND EXPERIMENTAL SECTION Calculations. Calculations were conducted using the Gaussian 09 package. Equilibrium structure was optimized with the B3LYP in conjunction with the 6−11++G (d) level basis set. At the same level, frequency calculations were also performed to confirm all optimized molecules with a consistent stationary. Oxidation potentials were obtained from the free-energy cycle for the oxidation reaction. Experiment. Dimethyl phenylphosphonite (DMPP) was purchased from Alfa Aesar Technology Co. (>98%) and used without further purification. A base electrolyte, 1.0 M LiPF6 in ethylene ethyl methyl carbonate (EMC)/carbonate (EC)/diethyl carbonate (DEC) (3/5/2 in weight), was provided by Guangzhou Tinci Materials Technology Co. Ltd., China. To obtain the DMPP-containing electrolyte, 0.5 wt % DMPP was applied in the base electrolyte in an argon-filled glovebox (MBraun Unilab, Germany), where the contents of oxygen and moisture were controlled to less than 10 and 0.1 ppm, respectively. LiCoO2 cathode was prepared by mixing 80 wt % LiCoO2 (Hunan Shanshan Advanced Material Co., Ltd., China), 10 wt % acetylene carbon black, and 10 wt % poly(vinylidene fluoride) in N-methyl-pyrrolidone, coating this mixture on Al foil, drying at 80 °C for 1 h and at 120 °C for 12 h under vacuum, and rolling. Graphite anode was composed of artificial graphite (89 wt %, Dongguan Kaijin New Energy Technology Co., Ltd., China), Super-p (2 wt %), Ks6 (4 wt %), and poly(vinylidene fluoride) (5 wt %). The slurry was coated on a Cu foil and dried at 80 °C for 12 h under a circulation oven. CR2025-type LiCoO2/graphite coin cells were assembled with anode capacity in excess of 15%, based on 180 mAh g−1 for LiCoO2 and 350 mAh g−1 for graphite. CR2025-type Li/graphite and Li/LiCoO2 halfcells were assembled with lithium foil as counter electrode. Celgard 2400 microporous membrane was used as the separator. The loaded mass of LiCoO2 is about 2.65−3.54 mg cm−2 in Li/LiCoO2 half-cells, while it is about 13.27−17.70 mg cm−2 in LiCoO2/graphite full cells. Transparent V-type Li/LiCoO2 cells were also assembled as previously reported.31 All of the cell assemblies were conducted in the glovebox mentioned above. Electrochemical Measurements. Cyclic performances were obtained on Land (CT2001A, Wuhan, China). LiCoO2/graphite full 12023


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Figure 2. Charge curves (A−F) at the selected cycles of Figure 1B, evolution of charge capacity at different regions with cycling (G), and corresponding charge/discharge Coulombic efficiencies (H).

LiCoO2 after deep cycling and its suppression by DMPP can be indicated by the Coulombic efficiencies of Figure 1C. As shown in Figure 1C, the initial charge/discharge Coulombic efficiency of LiCoO2 in the DMPP-containing electrolytes (79%) is lower than the 92% in the base electrolyte. This

capacity decaying appears for the LiCoO2 electrode cycled in the electrolyte with DMPP under 4.5 V. Apparently, the application of DMPP can be effective for inhibiting electrolyte decomposition and cobalt dissolution and suppressing the structural collapse of LiCoO2. The structural collapse of 12024


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Figure 3. Concentration of Co (A) before and after charge tests in V-type cells using base and DMPP-containing electrolytes; rate capability (B), chronoamperometric profiles at 4.4 V (C) and 4.6 V (D), and electrochemical impedance spectra at selected cycles of Figure 1B (E and F) for Li/ LiCoO2 half-cells in electrolytes with and without DMPP.

difference suggests that DMPP is oxidized more easily than the base electrolyte, which contributes to the formation of interface film on LiCoO2. In addition, the cell with DMPP has a Coulombic efficiency of about 100% in the subsequent cycles. Differently, the slightly lower and vibrating Coulombic efficiencies after the 400th cycle can be observed for LiCoO2 in the base electrolyte. Apparently, DMPP suppresses the electrolyte decomposition. The oxidation tendency of DMPP can be predicted by theoretical calculations. Figure 1D presents the optimized structures and the calculated oxidation potentials of solvents and DMPP. The calculated oxidation potential of DMPP (4.59 V) is lower than those of EC (7.02 V), EMC (6.85 V), and DEC (6.71 V), suggesting that DMPP might be preferentially

oxidized. The highest occupied molecular orbital (HOMO) energy and the lowest unoccupied molecular orbital (LUMO) energy of DMPP were also calculated with a comparison of solvents EMC, EC, and DEC. The obtained results are presented in Figure S1A. The HOMO energy of DMPP is higher than those of EMC, EC, and DEC, also suggesting that DMPP can be oxidized more likely than solvents. The LUMO energy of DMPP is lower than those of EMC, EC, and DEC, suggesting that DMPP might be reduced more likely than solvents. However, the initial charge/discharge profiles and corresponding differential capacity curves of Li/graphite halfcells in electrolyte with DMPP are similar to those in base electrolyte, as shown in Figure S1B and C, indicating that DMPP is unstable thermodynamically but cannot be reduced 12025


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to LiCoO2. Different from the evolution of the charge capacities at regions II and III in the base electrolyte, the charge capacities at these regions in the DMPP-containing electrolyte maintain stability up to the 500th cycle. This difference suggests that the side reactions including the irreversible ones for electrolyte decomposition and the reversible ones for insertion/extraction of PF6− can be suppressed effectively. These suppressions provide a protection for LiCoO2 from structural collapse. The dissolution of cobalt from LiCoO2 and the protection that DMPP provides can be indicated by analyzing the cobalt contents on the anode and in the electrolyte in a transparent Vtype glass cell. Li/LiCoO2 V-type cells using base and DMPPcontaining electrolytes were charged to 4.5 V at 0.1 C and then kept at this voltage for 48 h. The lithium anodes taken from the V-type cells after charging were rinsed with DMC and dissolved in 25 mL of 4% HNO3 solution for determining the content of cobalt deposited on lithium electrode. Electrolytes (about 2.5 mL) in the V-type cells were nitrated with 68% HNO3 solution and then diluted to 25 mL by 4% HNO3 solution for determining the content of cobalt dissolved in the electrolytes. As shown in Figure 3A, the content of Co deposited on the lithium electrode in the cell using the base electrolyte is 0.604 ppm, far higher than that using the DMPP-containing electrolyte, which is only 0.068 ppm. Especially, the Co content in the electrolyte is 26.4 mg L−1 in base electrolyte but only 1.128 mg L−1 in the DMPP-containing electrolyte. The dissolution of Co should be related to HF formed from the electrolyte decomposition,53,54 which can be suppressed by applying DMPP. The HF in the transparent V-type cells was analyzed by NaOH titration. There is an increase of 0.42 wt % for HF concentration in the V-type cell using electrolyte containing DMPP, which is far lower than that in the cell using base electrolyte (2.68 wt %), demonstrating that HF results from the decomposition of electrolyte and DMPP can suppress the electrolyte decomposition. It was believed that an electrolyte additive might scavenge HF and thus reduce the dissolution of transition metal ions from the cathode. DMPP should have the ability to scavenge HF because the phosphite group in DMPP can coordinate with F− ions.30 Nevertheless, if DMPP only contributes to the scavenging, a small amount of DMPP (only 0.5 wt %) is not sufficient to scavenge HF that is produced continuously from the electrolyte decomposition.54 Therefore, the main contribution of DMPP is in the formation of a protective interface film that suppresses the electrolyte decomposition. Figure 3B presents the rate capability of Li/LiCoO2 half-cells. During the initial cycling where current rate is small, the cell using DMPP-containing electrolyte exhibits slightly lower discharge capacity than that using base electrolyte, suggesting that the interface film formed from DMPP yields an interfacial impedance for lithium insertion. This impedance is detrimental to the rate capability of LiCoO2. However, the cell in the DMPP-containing electrolyte shows far better rate capability than that in the base electrolyte, as shown in Figure 3B. At 15 C, the capacity of the LiCoO2 in the electrolyte containing DMPP is 167 mAh g−1, but only 131 mAh g−1 for that in the base electrolyte. Apparently, the continuous electrolyte decomposition on LiCoO2 in the base electrolyte increases interfacial impedance, which can be effectively suppressed by the interface film formed from DMPP. Therefore, besides the protection for the structural stability of LiCoO2, DMPP is also beneficial to the rate capability of LiCoO2.

kinetically. This is to say, DMPP has no negative effect on graphite electrode. Figure 1E compares the cyclic stability of Li/graphite half-cells in electrolytes with and without DMPP. All of the cells exhibit the same cyclic performances, confirming that DMPP hardly affects the graphite anode when it is used in the electrolyte as an electrolyte additive. The mechanism on the structural collapse of LiCoO2 and the contribution of DMPP can be illustrated by analyzing the capacity evolution at different charge stages. Figure 2 presents the charge behaviors of LiCoO2 electrode at the selected cycles of Figure 1B. The charge profiles are presented in Figure 2A−F, which can be divided into three regions: (I) Constant current region at the potential from 3.0 to 4.2 V, where only lithium extraction from LiCoO2 takes place and the corresponding charge capacity would be a reversible one, contributing to a high charge/discharge Coulombic efficiency. (II) Constant current region at the potential from 4.2 to 4.5 V, where side reactions might take place besides the lithium extraction from LiCoO2. If the side reactions are the electrolyte decomposition, the corresponding charge capacity would be irreversible, contributing to a decreased charge/discharge Coulombic efficiency. On the other hand, the corresponding charge capacity would be a reversible side reaction, contributing to a high Coulombic efficiency. (III) Constant potential region at 4.5 V, where side reactions would take place more seriously. The evolution of charge capacity at different regions is presented in Figure 2G. In the base electrolyte, the charge capacity at the region I decreases quickly with cycling, indicating that LiCoO2 decreases significantly with cycling. This capacity loss results from the dissolution of cobalt and the final collapse of layered structure of LiCoO2, both of which originate from the side reaction, that is, the electrolyte decomposition. Usually, the electrolyte decomposition happens more seriously at the regions II and III, which might decrease the charge capacity with cycling. Instead, the charge capacities at the regions II and III increase with cycling up to the 400th cycle. This unusual behavior suggests that a new reversible side reaction happens at these regions, which might be related to the insertion of PF6−. As the cycling proceeds, the cobalt dissolves continuously and the space between the interlayers of LiCoO2 is enlarged, leaving a space large enough for the insertion of PF6−. This insertion dominates the side reactions and takes place more significantly with cycling, resulting in the increasing charge capacity. When the electrode is performed with deep cycling (from the 400th to the 500th cycle), the over-insertion of PF6− causes the structure collapse of the layered LiCoO2, leading to the fast charge capacity decaying. The charge/ discharge Coulombic efficiency in Figure 2H confirms the contribution of the side reactions to the charge capacity in the base electrolyte. The Coulombic efficiency is lower than 90% but maintains stability up to the 300th cycle, indicating that the increasing charge capacity due to the insertion of PF6− matches the decreasing charge capacity due to the cobalt dissolution and electrolyte decomposition. At the final stage (from the 300th to the 500th cycle), the insertion of PF6− cannot happen due to the collapse of the layered structure, leading to a fast decrease of Coulombic efficiency. A charge capacity decreasing at the region I can also be observed in the DMPP-containing electrolyte, as shown in Figure 2G, suggesting that the cobalt dissolution takes place inevitably. However, this decreasing is far slower than that in base electrolyte, indicating that DMPP has yielded a protection 12026


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Figure 4. SEM and TEM images of fresh LiCoO2 electrode (A and B) and the electrodes after cycling in base (C and D) and DMPP-containing (E and F) electrolytes.

electrolyte has a slightly increased interfacial resistance (Figure 3F), to 164 Ω (consisting of 73 Ω for Rf and 92 Ω for Rct) at the 500th cycle. These resistances are obtained by fitting with the equivalent circuit inserted in Figure 3E and F and are indicated in Figure S2. Apparently, a stable interface film has been formed from DMPP on LiCoO2, which suppresses electrolyte decomposition and protects LiCoO2 from structural destruction effectively. Structural Change of LiCoO2 after Cycling. To confirm the collapse of the layered structure of LiCoO2, the cycled LiCoO2 electrodes were examined by physical characterizations. Figure 4 presents the SEM and TEM images of the cycled LiCoO2 electrodes, with a comparison to a fresh LiCoO2. As shown in Figure 4A and B, the fresh LiCoO2 shows the clean layered structure and smooth surface of LiCoO2 particle, on which conductive carbon is dispersed. After cycling in the base electrolyte, however, there appear gaps in LiCoO2 particles (Figure 4C), which are sheet-like and parallel to the layers of layered structure as indicated by the arrows. This phenomenon has not been reported before in the literature. Additionally, lots of deposits can be observed (Figure 4D), which have been known as the electrolyte decomposition products.31 Apparently, LiCoO2 suffers crystal destruction when it is performed with cycling under high voltage in base electrolyte, which involves the exfoliation of layered structure. In the DMPPcontaining electrolyte (Figure 4E), however, LiCoO2 maintains the layered structure of fresh LiCoO2 (Figure 4A). The significant difference in morphological evolution between two cycled electrodes indicates that the application of DMPP provides a protection for LiCoO2 structure integrity. As shown

To indicate the suppression of the electrolyte decomposition by DMPP, chronoamperometry was performed on Li/LiCoO2 half-cells. The cells were charged to 4.4 or 4.6 V at 0.5 C and then kept at these voltages for 5 h. Under 4.4 V, leakage current can be observed apparently from the cell in base electrolyte but is small for the cell in electrolyte containing DMPP, as shown in Figure 3C. This difference becomes more significant when the voltage is enhanced to 4.6 V, suggesting that electrolyte decomposition takes place seriously in the base electrolyte, which can be suppressed by DMPP. Figure 3E and F presents the electrochemical impedance spectra of Li/LiCoO2 half-cells, which are characteristic of a pressed semicircle at high frequency and a slope line at low frequency. These impedance spectra can be fitted by the equivalent circuit inserted in Figure 3E and F. The semicircle at high frequencies represents the impedance of the lithium-ion transportation through interface film and the charge transfer on the interface, involving the interface film resistance (Rf) and the charge transfer resistance (Rct), respectively, while the slope line represents Warburg impedance.55 As shown in Figure 3E, the cell using DMPP shows a slightly larger initial interfacial resistance (107 Ω, consisting of 60 Ω for Rf and 47 Ω for Rct) than that in the base electrolyte (88 Ω, consisting of 52 Ω for Rf and 36 for Rct), indicating that the interface film derived from DMPP yields additional resistance. Because of the accumulation of continuous oxidation decomposition products of the base electrolyte, the interfacial resistance of the electrode in the base electrolyte increases significantly with cycling (Figure 3F), to 1562 Ω (consisting of 726 Ω for Rf and 836 Ω for Rct) at the 500th cycle. Comparatively, that in the DMPP-containing 12027


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Figure 5. XRD patterns of LiCoO2 electrode after 300 cycles (A) and 500 cycles (B) in base and DMPP-containing electrolytes, with comparison to a fresh one.

transformed to spinel structure after 500 cycles (Figure S3C). On the contrary, a typical layered structure of the fresh LiCoO2 remains for the electrode after 500 cycles (Figure S3D), confirming that the interface film formed from DMPP provides a protection for LiCoO2 structure integrity. Recently, an oxidation−reduction assisted exfoliation of LiCoO2 into nanosheets was reported,57 which shows that the exfoliation of layered LiCoO2 resulted from the insertion of tetraethylammonium cations. Li+ cation is extracted during the charge process, which leaves the space for the insertion of tetraethylammonium cation. Similarly, it has been known that the anion (PF6−) in the electrolyte can be inserted into the interlayer space of layered materials. For example, the interlayer space of graphite or even multiwall carbon nanotubes allows the insertion of PF6−, when the carbon materials are used as conductive agents in the cathode of the lithium-ion battery.58,59 When the cathode is charged, Li+ cation will be extracted from cathode materials, and PF6− might be inserted into the interlayer space if the space is large enough. In the case of LiCoO2 that is performed with cycling under high voltage in the base electrolyte, the solvents together with PF6− are

in Figure 4F, the LiCoO2 particle retains the dark image of the fresh one (Figure 4B), and a thin cathode interface film of about 30 nm can be identified on the LiCoO2 particle. DMPP helps build a protective interface film on the LiCoO2 particle, protecting LiCoO2 from structural destruction. The previously reported mechanisms concerning the failure of the LiCoO2 electrode under high voltage involve the increased interfacial impedance due to the electrolyte decomposition,20 structural transformation from layered to spinel due to the dissolution of transition metal ions, and oxygen loss of Li1−xCoO2 after deep lithium extraction.17,24,56 The electrolyte decomposition is inevitable for the electrode cycled in the base electrolyte. The products from electrolyte decomposition deposit on the electrode, as indicated by the arrows in Figure 4D, and will increase the interfacial impedance. The structural transformation can also be observed for the electrode cycled in the base electrolyte. The detailed crystal structures of the cycled electrodes are presented in Figure S3. The layered structure can be clearly identified for the fresh LiCoO2 (Figure S3A), which can be retained when the electrode is performed for 300 cycles (Figure S3B) but is 12028


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Figure 6. Co 2p, C 1s, O 1s, F 1s, and P 2p XPS patterns (A−E) and FTIR spectra (F) of the LiCoO2 electrodes after cycling test of Figure 1B.

pattern of the fresh one, although the peak intensity becomes weaker. This result indicates that LiCoO2 does not suffer structural destruction during initial cycling. The main problem is the electrolyte decomposition during this stage. The polymers formed from the electrolyte decomposition deposit on LiCoO2, weakening the XRD intensity of LiCoO2, as shown in Figure 5A. After 500 cycles in base electrolyte, however, the typical XRD peaks of LiCoO2 almost disappear. This significant change confirms that LiCoO2 suffers serious structural destruction when it is performed with deep cycling in base electrolyte under 4.5 V, which is ascribed to the exfoliation of layered structure of LiCoO2 as observed from SEM. On the contrary, the LiCoO2 cycled in electrolyte with DMPP maintains the XRD pattern of fresh LiCoO2, as shown in Figure 5B, confirming that DMPP provides a protective interface film for LiCoO2 from structural destruction.

decomposed, yielding polymer deposits, gaseous products, and HF.18 The resulting HF erodes LiCoO2 and causes the dissolution of cobalt from LiCoO2. The cobalt dissolution might leave suitable space for the insertion of PF6−. These processes take place more seriously as the cycling proceeds, leading to the final exfoliation of the layered structure. With the application of DMPP, a protective interface film is formed, which prevents the insertion of PF6− and protects LiCoO2 from structural destruction. The crystal structure of the LiCoO2 cycled in base and DMPP-containing electrolytes was determined by XRD, with a comparison to fresh LiCoO2. Figure 5 presents the obtained results, which contain typical diffraction peaks of layered LiCoO2. The diffraction peaks of the current collector (Al) can be identified on the fresh electrode.60−62 After 300 cycles in base electrolyte, the LiCoO2 electrode maintains the XRD 12029


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Figure 7. Schematic representation showing the decay mechanism of LiCoO2 electrode cycled in base electrolyte at 4.5 V and the contribution of DMPP to the structural integrity maintenance of LiCoO2.

electrolyte. Additionally, the peak intensities of the products from decomposition of electrolyte, including Li2CO3 at 290.08 eV as C 1s and 531.6 eV as O 1s, LiF at 685.5 eV as F 1s, LixPFy at 688.08 eV as F 1s, and LixPOyFz at 134.94 eV as P 2p,64,65 are far stronger in the base electrolyte than in the DMPPcontaining electrolyte. The peak of metal oxide (529.2 eV) in O 1s for the electrode cycled in DMPP-containing electrolyte still dominates.20 This difference accounts for the thick deposits due to the continuous electrolyte oxidation decomposition and the

XPS and FTIR were used to analyze the surface composition of the cycled LiCoO2 electrodes. Figure 6A−E presents the XPS patterns of the cycled electrodes, with a comparison to the fresh one. As shown in Figure 6A−E, the elements of the electrode before cycling, covering Co in LiCoO2 as Co 2p, F in PVDF at 687.6 eV as F 1s, C in PVDF at 290.4 and 286.1 eV as C 1s, and C in acetylene black at 284.8 eV as C 1s,63 still exist in the cycled electrodes, but the peak intensity is weaker in electrolyte without DMPP. Especially, the peak for cobalt in LiCoO2 as Co 2p almost disappears after cycling in the base 12030


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Figure 8. Cyclic stability (A) of LiCoO2/graphite full cells with and without DMPP at 0.1 C for the activation and at 0.2 C for the follow cycles from 3.0−4.5 V, dQ/dV profiles of the initial cycle at 0.1 C (B), and charge/discharge profiles of the selected cycles in electrolytes without (C) and with (D) DMPP.

thin and uniform protective interface film on LiCoO2, as observed from the TEM images (Figure 4). Figure 6F presents the FTIR spectra of the cycled LiCoO2 electrodes, in which the absorption peaks of PVDF are at 1400, 1180, and 690 cm−1.66,67 However, the electrode cycled in electrolyte containing DMPP shows additional new peaks, including those at 1556, 1436, and 875 cm−1, corresponding to the CC bond and C−H bond, those at 1134 and 970, 772 cm−1, corresponding to the P−O−C bond and P−O bond,51 and that at 1035 cm−1, corresponding to the C−O species in DMPP. These species are originated from DMPP. Apparently, DMPP decomposition products have been incorporated into the interface film. The LiCoO2 cycled in electrolyte without DMPP has a stronger absorption at 1640 and 840 cm−1, which corresponds to the polycarbonate and LixPFy,68,69 suggesting that electrolyte decomposition proceeds continuously and the application of DMPP inhibits the electrolyte decomposition. With the above results, the failure of LiCoO2 and the effect of DMPP under 4.5 V can be illustrated in Figure 7. When LiCoO2 electrode is performed with cycling in base electrolyte under 4.5 V, electrolyte decomposes continuously, yielding HF. The resulting HF erodes LiCoO2 and causes the dissolution of cobalt from LiCoO2. As the cycling proceeds, the cobalt dissolution leaves a suitable place in the interlamination of LiCoO2 for the insertion of PF6−, leading to the final exfoliation of the layered structure and failure of LiCoO2 electrode. In the case of DMPP-containing electrolyte, a protective interface film is generated on LiCoO2 from DMPP oxidation during the initial charge process. This film suppresses the electrolyte

oxidation decomposition and prevents LiCoO2 from the attack of HF and the insertion of PF6−, maintaining the structure integrity of LiCoO2 and contributing to the improved cyclic stability of LiCoO2 under 4.5 V. Application of DMPP in LiCoO2/Graphite Full Cell. Because the cobalt dissolution will deposit on anode, which might deteriorate the performance of graphite anode, DMPP should also provide a LiCoO2/graphite full cell with improved performances through suppressing the cobalt dissolution. This effect was evaluated by comparing the charge/discharge behaviors of LiCoO2/graphite full cells using electrolytes with and without DMPP. Figure 8A presents the cyclic stability of LiCoO2/graphite full cells, with an indication of the energy density of the cells based on LiCoO2. The cells were performed with cycling at 0.1 C for the initial three cycles and 0.2 C for the subsequent cycles. Both cells deliver 700 Wh kg−1 and 2400 Wh L−1 based on LiCoO2 with a tap density of 3.2 g cm−3, showing that a high voltage cathode provides lithium-ion battery with high energy density. Similar to the behavior of the half-cell (Figure 1B), the full cell using the base electrolyte suffers a fast capacity decaying: from initial 182 to 82 mAh g−1 at the 100th cycle, with a discharge capacity loss of 55% under 0.2 C. The decay of the LiCoO2/graphite full cells using the electrolyte without additive involves the LiCoO2 destructions and the negative effect of deposited cobalt on graphite anode. Surprisingly, a significant improvement in the cyclic stability is observed for the cell using DMPP. This cell delivers its initial capacity of 181 mAh g−1 and retains 149 mAh g−1 at the 100th cycle, with a capacity retention of 82% at 0.2 C. After 100 12031


Research Article

ACS Applied Materials & Interfaces cycles, the energy density is reduced to 125 Wh kg−1 for the cell in the base electrolyte, but retains 409 Wh kg−1 for that containing DMPP, indicating the failure of the full cell in the base electrolyte and the contribution of DMPP to the performance improvement. Figure S4 presents the XPS patterns of graphite electrodes taken from the cycled LiCoO2/graphite full cells, with a comparison to the fresh one. It can be noted from Figure S4 that the peak of Co 2p for the electrode cycled in base electrolyte is more intensive than that in electrolyte containing DMPP, suggesting that the dissolution of Co from LiCoO2 cathode takes place more seriously in base electrolyte, which could be effectively suppressed by applying DMPP. Figure 8B presents the initial differential capacity profiles of LiCoO2 electrodes in the base and the DMPP-containing electrolytes during the initial charge process at 0.1 C. Differing from the cell without additive, there is a new oxidation peak seated at 3.76 V in the cell using DMPP, confirming that DMPP has been oxidized during the initial charge process. A protective interface film is generated from the oxidation of DMPP, which suppresses the continuous electrolyte oxidation decomposition, the cobalt dissolution, and the PF6− insertion of interlamination during cycling, and thus provides the full cell with improved performances. Figure 8C and D presents the charge/discharge curves of LiCoO2/ graphite cells at the selected cycles of Figure 8A. There appears a voltage plateau at about 4.25 V during the first charge process for the electrode cycled in the base electrolyte, which should be ascribed to the electrolyte decomposition reaction. The plateau is observed at a slightly lower voltage for the electrode cycled in the DMPP-containing electrolyte, indicating that the DMPP is oxidized preferentially as compared to the base electrolyte. The cell using DMPP exhibits a lower initial Coulombic efficiency (85%) than that using the base electrolyte (87%), confirming that DMPP has participated in the electrolyte decomposition during the initial charge process.70 Besides the faster energy density decaying with cycling, the full cell using the base electrolyte also shows larger polarization (larger gaps between charge and discharge profiles) than that using DMPP, confirming that the interfacial resistance is increased due to electrolyte decomposition, which can be mitigated by applying DMPP.

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AUTHOR INFORMATION

Corresponding Author

*Tel./fax: 86-20-39310256. E-mail: [email protected]. ORCID

Weishan Li: 0000-0002-1495-4441 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by a joint project of the National Natural Science Foundation of China and the Natural Science Foundation of Guangdong Province (Grant no. U1401248), the key project of Science and Technology in Guangdong Province (Grant no. 2016B010114001), the Guangzhou City Project for Cooperation among Industries, Universities and Institutes (Grant no. 201509030005), and the scientific research project of the Department of Education of Guangdong Province (Grant no. 2013CXZDA013).

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REFERENCES

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4. CONCLUSIONS LiCoO2 suffers severe structural destruction and exhibits a poor cyclic stability when it is cycled in a LiPF6-based carbonate electrolyte under 4.5 V. Under such a high voltage, the electrolyte decomposes yielding HF, which erodes the electrode and causes the dissolution of cobalt from LiCoO2. As the cycling proceeds, the cobalt dissolution leaves the interlamination of the layered LiCoO2 large enough for the insertion of PF6−, leading to the final exfoliation of layered structure in LiCoO2. DMPP is effective for the cyclic stability improvement of LiCoO2 half-cell and LiCoO2/graphite full cell. A protective interface film can be generated on LiCoO2 from DMPP, suppressing the electrolyte decomposition and protecting the layered structure of LiCoO2 from exfoliation.

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Optimized structure, HOMO, LUMO energy of molecules, EMC, EC, DEC, and DMPP; initial discharge/charge curves and corresponding differential capacity curves of the Li/graphite half-cells with and without DMPP; resistance of the electrodes cycled in the base and DMPP-containing electrolytes; HRTEM images of LiCoO2 before and after cycling; and XPS patterns of graphite electrodes taken from the cycled LiCoO2/ graphite cells (PDF)

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b00032. 12032


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