A New Strategy to Stabilize Capacity and Insight into the Interface

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A New Strategy to Stabilize Capacity and Insight into the Interface Behavior in Electrochemical Reaction of LiNi0.5Mn1.5O4/Graphite System for High-Voltage Lithium-Ion Batteries Hongqiang Wang,†,§ Xuesong Xie,† Xiaolu Wei,† Xiaohui Zhang,†,§ Jiujun Zhang,‡ Youguo Huang,*,† and Qingyu Li*,†,§ †

School of Chemical and Pharmaceutical Science, Guangxi Normal University, Guilin 541004, China College of Sciences, Shanghai University, Shanghai 200444, China § Guangxi Key Laboratory of Low Carbon Energy Materials, Guangxi Normal University, Guilin 541004, Chain ‡

S Supporting Information *

ABSTRACT: The performance of CEI and SEI configuration and formation mechanism on the cathode and anode side for LiNi0.5Mn1.5O4/natural graphite (LNMO/NG) batteries is investigated, where series permutations of the NG electrodes modified with TEOS species as the anode for the LNMO full cells. It is believed that the excellent long-term cycling performance of LNMO/NG full cells at the high voltage is a result of alleviating the devastated reaction to form the CEI and SEI on the both electrodes with electrolyte, respectively. At a voltage range from 3.4 to 4.8 V for the LNMO full cells, 95.0% capacity retention after 100 cycles is achieved when cycled with TEOS-modifying NG anode. This mechanism may be explained that eliminating the HF and absorbing water impurities in the electrolyte by introducing the TEOS group, which can transform the SiO2 species that react with the acid of HF at the organic solvent environment instead of destroying/forming the anode SEI and attacking the LNMO spinel structure to form the dense and high resistance CEI, meanwhile the SiO2 species will absorb the water molecule and precipitate into the anode surface further stabilizing the SEI configuration during the cycling. KEYWORDS: full cells, LiNi0.5Mn1.5O4 cathode, graphite anode, electrolyte decomposition, CEI and SEI

1. INTRODUCTION The energy density of current lithium secondary batteries is not totally enough to accommodate the expected objective of electrical vehicles with high efficiency and long life, for example, plug-in hybrid electrical vehicles (PHEVs) and fully electric vehicles (EVs), which people have sought.1 To further improve the energy density, one practical and cost-effective way is to elevate the operating voltage. Consequently, the exploration of high-voltage cathode material with excellent electrochemical performance and physical properties is required. Amid all the types of cathode materials, the LiNi0.5Mn1.5O4 (LNMO) cathode is, reasonably, paid much attention for production of the next generation lithium ion materials because of its high operating voltage (∼4.7 V vs Li/Li+), rate capacity, theoretical specific capacity of 148 mAh·g−1, and importantly, the 650 Wh kg−1 theoretical energy density for LNMO/NG full cells, which © 2017 American Chemical Society

is 20% and 30% higher than those of conventional LiCoO2/NG and LiFePO4/graphite cells, respectively.2,3 Considering the manufacturing process, the LNMO cathode materials are easy to prepare, are less environmentally polluting, and potentially and cost-effectively capable of the massive need of EV.4,5 However, as much as the 5 V LNMO has potential to be a widely used as cathode materials, there are still many hurdles to cross, especially for the serious capacity fading in the full-cell system. In a significant fraction of lithium ion battery literature, the LNMO/Li half cells can easy conduct hundreds of cycle times with negligible loss in the capacity,6 even at 55 °C unlike its parent counterpart LiMn2O4 that shows severe capacity Received: June 20, 2017 Accepted: September 7, 2017 Published: September 7, 2017 33274

DOI: 10.1021/acsami.7b08828 ACS Appl. Mater. Interfaces 2017, 9, 33274−33287

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ACS Applied Materials & Interfaces fading at this temperature,7 and those fascinated electrochemical performance as mentioned above could be further improved by the cation-doping,8 morphological control,9 and surface modification,10 while the high-level of capacity fading is still emerged in this full systems,11,12 when the counter electrodes are carbon materials, such as MCMB,13 graphite,14 and so on.15 The reasons that cause the capacity fading in full cells are still a subject of debate. Recent studies, however, point out that the most critical and crucial barrier for the pervasive using in reality is the electrolyte decomposition at high voltages.16 The electrolyte deposition occurs owing to the state-of-art electrolyte (i.e., LiPF6 slat in carbonated solvents) continuously decomposes above 4.5 V vs Li/Li+, which are also inferior for LNMO/NG full cells.17 A amount of the literature results are obtained from the half cells, where the Li metal as the counter or reference electrodes that provide an easy way to understand the LNMO cathode,18−21 while the full cell is unique from the half system in the theory and practice level. It is the comprehensive subject to understand the serious capacity fading in the full cells system. Recently, Xiao et al.22 reported that 5 nm thick alumina coating on the graphite anode side can deliver the best performance of full cell compared to that of the cathode coating and both noncoating samples. Manthiram et al.11 suggested that the inferior performance in the full-cells could originate from metal dissolution migrating to the anode surfaces through the separator and reduce to metallic nanoparticles such as Mn or Mn−Li alloy, which incorporate into the solid electrolyte interphase (SEI) and promote the SEI formation that results in trapping the active lithium ion gradually. In this work, we draw the attention back to the “panic zone”, the interfaces between the electrolyte and the electrodes, in other words, the place where the formation of the CEI on the LNMO electrode surface and the SEI on the anode surface. Herein, after high-voltage cycling, the continuously dense and nonuniform CEI layers, as well as the uneven and scattered SEI layers, were detected for the LNMO full cells when the counter electrode was the pristine NG. While, the LNMO cathodes equipped with NG materials modified with TEOS siliceous organic (tetraethyl orthosilicate) showed the clear surface after cycling at the same current condition. And, the surface of cycled NG with TEOS species was found to be covered with dense SiO2 sphere instead of scatted of SEI layers. It is understood and confirmed that the layers of the surface of electrodes are formed by the decomposition products of the lithium salt and electrolyte solvent.23 The novel strategy stabilizes the capacity significantly by forming the artificial SEI film which mitigates the negative side reaction occurring at the interface between electrode and electrolyte, instead of other strategies of modifying cathode material itself, replacing the new electrolyte system,24 or combining with Li4Ti5O12 anode.16 In addition, the capacity retention exhibits superior to the conventional modifying-graphite methods with incorporating the Li2CO3 inorganic components and doping-Li strategy. The mechanism may be the reason that TEOS species can react with the increasing HF and absorb water in the electrolyte after cycled especially at high voltage, instead of destroying the anode SEI and attacking the LNMO spinel structure. Meanwhile, the SiO2 species will precipitate into the NG surface and further stabilize the SEI construction. As the result of scavenging the HF and water impurities, the CEI layers formed by the electrolyte oxidation can be largely limited.

2. EXPERIMENTAL SECTION 2.1. Materials Synthesis. The TEOS-NG electrodes were prepared by the beating process that the ethyl orthosilicate (TEOS) was added into the slurry paste mixed with the natural graphite (BTR, China), conductive agents, and binder (carboxymethyl cellulose sodium, CMC, and polymerized styrene butadiene rubber, SBR) in the deionized water, the various contents of TEOS (5, 10, 13, 16, 19, 22 wt % by weight of graphite) as the experimental group and the 0% content as the control group. The obtained slurry was cast on aluminum foils by doctor-blade deposition, the active loading of the anode electrodes was approximately 9.0 mg/cm2. The LiNi0.5Mn1.5O4 (Xing Neng New Material Co., Ltd.) electrodes were prepared by mixing the 80 wt % active material, 10 wt % polyvinylidene fluoride, and 10 wt % acetylene black, and then slurry-coated on the copper foil, which the active material loading was of 5.66 mg/cm2. One layer of the polypropylene was used as the separator (Celgard 2325) in each cell. The weight ratio of NG/LNMO is 1.8. The electrolyte was 1.0 M LiPF6 dissolved in mixing solution of ethylene carbonate/dimethyl carbonate (EC/DMC, 1:2 by volume, water impurities of less than 10 ppm, HF impurities of less than 20 ppm, Shanghai Xiaoyuan Energy Technology Co., Ltd.). 2.2. Materials Characterization. The surface morphology and microstructure of the electrodes both on the cathode and anode was observed by a field emission scanning electron microscopy (SEM, Philips, FEI Quanta 200FEG). Transmission electron microscopy (TEM) was conducted on a JEM-2100F instrument. The component of carbon materials was measured by X-ray energy dispersive spectroscopy (EDS, EDAX JENSIS60S). The NG anodes with and without TEOS, signed TEOS-NG and NG, were respectively used as counter/reference electrode assembled with same LiNi0.5Mn1.5O4 electrode in CR-2032 coin cells. 2.3. Electrochemical Measurements. The full-cells were cycled between 3.4 and 4.8 V with 0.2 C constant current for first two cycles and 1 C constant current (C = 147 mAh·g−1) for the result of cycles at room temperature, using a LAND battery measurement system (LAND CT2001A, Wuhan, China). Electrochemical impedance spectra (EIS) were conducted by the IM6 electrochemical station performed on an acquired from 10 mHz to 100 kHz with an open circuit at an amplitude of 5 mV after the 1, 5, 10, 20, and 50 cycles discharged state of the samples. The XPS analyses for electrode (both cathode and anode) were conducted by the means of an ESCALAB 250Xi spectrometer (Al X-ray source). The graphite peak at 284.3 eV was used as a reference for the final adjustment of the energy scale in the spectra. And then, the spectra obtained were fitted using the XPSPEAK41 software. For the ICP measurement, the cycled graphite anodes were immersion in 10 mL of 2% HNO3 solution for 2 days with ultrasonics to guarantee the components of electrode materials ultimately dissolve in this solution. The solution was diluted to 100 times for the Inductive Couples Plasma−Mass Spectrometer (ICPMS) analysis, which was conducted by Nexlon 300x instrument. The dissembled LiNi0.5Mn1.5O4 cathode and graphite anode were both rinsed with anhydrous DMC solvent three times to remove EC and LiPF6 salt and followed by vacuum drying overnight at room temperature in an Ar-filled glove box (O2 ≤ 0.1 ppm, H2O ≤ 0.1 ppm).

3. RESULTS AND DISCUSSION 3.1. Physicochemical Characterization. Model samples were designed to study the effect of TEOS on the formation of CEI and SEI layers at the surface of electrode for the full LNMO cells. In particular, the seven permutations of LNMO full-cells combined with NG anode modified with different amounts of TEOS (5, 10, 13, 16, 19, 22% and zero by weight of graphite, signed as LNMO/NG-5, LNMO/NG-10, LNMO/ NG-13, LNMO/NG-16, LNMO/NG-19, LNMO/NG-22, and LNMO/NG-0, respectively) were assembled with the same LNMO cathode. 33275


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Figure 1. Scaning electron microscopy (SEM) and energy-dispersive X-ray mapping for C, O, and Si elements of NG electrode with TEOS(a) and without (b).

Figure 2. (a) First charge and discharge curves of full cells cycled with NG anode and TEOS-NG anode. (b) The initial Coulombic efficiency and (c) the cycling performance of LNMO full cells with the pristine NG electrode and TEOS-NG electrodes.

cells cycled with the TEOS-NG-16 anode exhibit the highest specific capacity of 108.6 mAh·g−1 at the first 0.2 C current density, while the cells cycled with the pristine NG anode only maintain 91.3 mAh·g−1. For the TEOS-NG-19 sample, the discharge capacity in the first cycle is 106.1 mAh·g−1 and increase to 112.3 mAh·g−1 in the second cycle. The other type TEOS-NG configurations also exhibit the superior discharge specific capacity to the NG sample. It also can be demonstrated that the first average discharge plateau of the pristine NG sample is of 4.4705 V which lower than that of all the TEOSNG samples (4.4900 V for the LNMO/NG-13 sample, the 4.4898 V for the LNMO/NG-16 and 4.4759 V for the LNMO/ NG-19) shown in Table S1. Another interesting point is the initial Coulombic efficiency of LNMO full cells based on the TEOS-NG as shown in Figure 2b. Undoubtedly, cycling at high voltage typically results in low Coulombic efficiency and poor cycling performance due to the oxidative decomposition of electrolyte solvent and the consumption of active Li-ion.26 As shown in Figure 2b, the LNMO/NG-13 full cells deliver the highest Coulombic efficiency of 77.78%, the LNMO/NG-19 exhibits the modest

The Figure 1 illustrates the scanning electron microscopy (SEM) and its corresponding energy-dispersive X-ray spectroscopy (EDS) analysis for the fresh NG and TEOS-modified NG (NG-19) electrodes. Obviously, the C, O, Na species were detected as the common elements that come from the binders (CMC and SBR) and the carbon materials.25 The acute different signs of EDS was the Si peaks from the NG with TEOS modification (TEOS-NG) electrode as shown in the Figure 1a, while there is no corresponding peaks in the normal NG sample shown in the Figure 1b. In addition, the uniform dispersion of Si element also can be validated by the SEM mapping for the TEOS-NG electrode shown in the Figure 1a, while a few of Si element is also observed in the pristine NG sample which may be from the container. The other characterizations will be shown in the following section. 3.2. Electrochemical Characterization. Figure 2 exhibits the electrochemical performance of LNMO full cells cycled with the TEOS-NG and the pristine NG respectively, (a) the first charge−discharge profiles between the 3.4 V to the 4.8 V at the current density of 0.2 C, (b) the Coulombic efficiency, and (c) the cycle performance at 1 C rate. As for Figure 2a, the full 33276


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Practically, lithium ions can pass through the SiO2 particle layer, reach the host of graphite anode during the charge and discharge process,30 where this network largely isolate the anode from electrode acting as the protective layer to limit the irreversible reaction during the first cycles. The cyclic stability of LNMO full cells assembled with TEOS-NG and the pristine NG electrodes were further explored by galvanostatic charge−discharge measurement at 1 C current rate for 50 cycles (the battery formation is 0.2 C rate for the first two cycles) as shown in Figure 2C. According to this curves, it is obvious can be observed that the use of the TEOS-NG significantly improves the cycling performances of the full cell. The cell with the pristine NG a capacity drop from 81.9 to 66.4 mAh·g−1, with the capacity retention of 81.07% after 50 cycles. Although the operating voltage exceeds the stability windows of electrolyte,31 the capacity of cell with the TEOS-NG-19 at 52nd cycle is 105.3 mAh·g−1 and retains 93.77% of its initial capacity at 1 C rate (111.9 mAh·g−1). It is worthy to be mentioned, the capacity of the LNMO/TEOSNG-13 sample at 52 cycle is 91.7 mAh·g−1 and exceeds 100% of it initial capacity (91.1 mAh·g−1) after the first 1 C rate. Figure 3 shows the charge and discharge curves of LNMO full cells cycled with the TEOS-NG anode and the pristine NG anode at the different cycling number (5th, 10th, 20th, and 50th) under a current rate of 1C. From the theoretical electrochemical reaction, there must are two subtle discharge plateaus at around 4.5 V corresponding to the Ni4+/Ni3+ and Ni3+/Ni2+ redox couples.27 As shown in Figure 3, compared to the slippage of discharge plateau for the NG smaple, the TEOSNG sample are of relatively direct line with two subtle plateaus between 4.4 and 4.6 V, indicating that the small polarization for that TEOS-NG sample.32 Additionally, comparing the charge− discharge features of full cells at the same cycle number, it is obviously found that the gaps between the charge and discharge

Coulombic efficiency of 74.0%. While the cell cycled with the pristine NG sample is only 59.91%, indicating the high-level of surface reactions for the electrolyte at the surface of electrode and large number of the consumption of active Li-ion.13 The initial Coulombic efficiency could indicate the detrimental degree of full cells on the internal level at the first cycle.27 Aside from the LNMO/NG-19, the initial Coulombic efficiency of the rest of TEOS-NG samples also shows the superior to that of the pristine NG sample. The improvement of initial Coulombic efficiency in the cell with TEOS modified NG electrode suggests that the SiO2 species that are formed by the TEOS groups validaded by the XRD, XPS for the TEOS-NG electrode before cycling shown in Figure S3 and XPS, TEM characterizations after cycling would cover the active anode site in the beginning and play a role of a protective layer to prevent the indirect oxidation of the electrolyte.28,29 In addition, the SiO2 particles can absorb impurities such as water and trace organic solvent that lead to the instability reaction at interface between the electrolyte and electrodes. The process of SiO2 protective particles is demonstrated as follows: The Si(OH) 4 is formed by the hydrolyzed of the ethyl silicate (TEOS) in water as shown in eq 1 Si(OC2H5)4 + 4H 2O → Si(OH)4 + 4C2H5OH

(1)

Then, a gelatin film from the condensation polymerization of Si(OH)4 shelters the surface of NG anode. After losing H2O molecule at high temperature, the protective network of SiO2 forms on the surface of the NG anode as shown in eq 2.

Figure 3. Charge−discharge curves of LNMO full cells cycled NG anode and TEOS-NG anodes at the different cycling 5th (a), 10th (b), 20th (c), and 50th (d). 33277


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Figure 4. Electrochemical impedance spectra of full cells cycled with NG electrode (a) and TEOS-NG-16 (b), TEOS-NG-19 (c), and TEOS-NG-22 (d), respectively.

charge transfer between the electrode and electrolyte. The impedance of Rct + Rf for the cell cycled with the TEOS-NG-19 sample is only from 8 Ω at the first cycle to the 98 Ω after 52 cycles as shown in the Figure 4c, while that of the pristine NG sample increases 10 Ω at first cycle to about 820 Ω after 52 cycles. The rest of TEOS-NG configurations also have low resistance, which can be observed in Figure 4b and 4d. This typical increasing resistance of Rct + Rf for LNMO full cells with NG electrode means that the high level of decomposition of electrolyte is always continue in this system.37 In contrast, the TEOS seems to have the power to mitigate the decomposition of electrolyte for high-voltage LNMO full cells on cycling.

plateaus of TEOS-NG samples reduce with the amount of TEOS incorporation, while the pristine NG sample remains the largest gap amount all samples. This means that the NG electrode modifying with TEOS species can actually help to reduce the intrinsic polarization and internal resistance during the charge−discharge reaction.33 A variety of tools and techniques have been used to analyze the surface quality between the electrolyte and electrodes, the electrochemical impedance spectroscopy (EIS) is a nondestructive analysis tool, which provides useful information from a complex electrochemical system probing electrolyte resistance, electrode kinetic, and double-layer capacitance by the virtue of a wide frequency range.34 Therefore, to investigate the effect of TEOS-NG electrode upon the LNMO full cells, the impedance analysis at variety of discharge state (after the first (first), third (3th), fifth (fifth), tenth (10th), and fiftieth (50th)) and the fresh were conducted as shown in Figure 4. Admittedly, two semicircles are observed for cathode materials as well as for the anode in the half cells after cycling, one with a medium-frequency that corresponding to the charge-transfer resistance (Rct) and another corresponding to the surface film resistance (Rf) at the high-frequency.35 However, in some cases just one semicircle in the Nyquist plot may to be found, even the cells charged at high voltage state. The reason may be due to the similar time constant for both the surface film response and the charge transfer reaction.36 The sloping line in lowfrequency region responding to the Warburg resistance (Zw), reflects the Li-ion diffusion in the bulk material. The intercept at the Z′ axis associated with the series resistance (Rs), including the external wire, electrolyte, separator and electrodes. From Figure 4, it is easily to be found that the cells cycled with NG and TEOS-NG electrode show the similar chargetransfer resistance (Rct) before cycling represented in Figure 4, indicating that the incorporation of TEOS did not change the

4. INTERFACE CHARACTERIZATION Undoubtedly, an optimized SEI or CEI layer is expected to have inconsiderable electrical conductivity, low electrolyte solubility, and high level of lithium ion selectivity and permeability to minimize overpotential due to polarization and prevent further active lithium ion consumption.38 In reality, the interface reactions both on the LNMO surface and the graphite surface with electrolyte may simultaneously occur during the cycling, but the properties of interface layers are quite unique. The SEI layer mainly generates in the first charge−discharge cycles due to the carbonates has a reduction limitation approximately 1.0 V vs Li/Li+.39 And this very SEI layer can inhibit the electrolyte decomposition on the anode surface in turn. On the other hand, the oxidative decomposition of electrolyte also takes place as the voltage plateau approaches 4.7 V vs Li/Li+ (or even less) when charging or even by storage extension at this voltage since the carbonates organic solvent (EC, DMC, EMC, PC, etc.) also have oxidation decomposition limitation (highest occupied molecular orbital (HOMO) at about 4.7 V vs Li/Li+).1 In reality, mixed with the lithium salt, the voltage of traditional electrolyte decomposition limitation 33278


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ACS Applied Materials & Interfaces will reduce to the 4.3 V (vs Li/Li+). In detail, as shown in Figure 5, the Fermi energy of two nickel redox couples of Ni3+/

on this surface as shown in Figure 6b, further be confirmed by the TEM image with the thickness in a range of 20−30 nm shown in Figure 6e. This CEI layer can be attributed to the high-level of the oxidative decomposition of the electrolyte and/or other side reactions.43 In contrast, in the case of TEOSNG sample, nonobvious layers can be found on the edges of the LNMO bulk as described in Figure 6c and TEM in Figure 6f. The same results are also illustrated by the Prof. Fan who pointed out that the use of the high-voltage LNMO as a cathode material in Li-ion batteries is inevitably facing a series of challenges since the current electrolyte (as mentioned above) suffers severe oxidative decomposition on LNMO electrode when cycled to high voltage,44 which the decomposed products are of the high impedance feature that block the transmission of lithium ion. Therefore, the absence of dense CEI layers indicates that the TEOS group could mitigate the oxidative decomposition of electrolyte at the surface of LNMO bulk, or other corresponding reactions. However, what key factors can trigger the formation reaction of dense layer at the surface of LNMO bulk after cycling with the pristine NG electrode. To analyze the meticulous effect of surface decomposition components, the XPS characterizations were conducted in the following section. As shown in Figure 7, both the fresh LNMO and cycled LNMO electrodes cycled with the pristine NG and TEOS-NG anodes are tested by XPS characterization. The C 1s spectra of the fresh LNMO electrode and both cycled LNMO electrodes deliver the same signal without any other peaks, while the intensity are very varied. The fresh LNMO electrode delivers the very stronger peaks at approximately 284.1 eV which contributes to the conductive carbon signal. The cycled LNMO electrode equipped with the pristine NG anode shows the lower intensity than that of TEOS-NG sample. This means that the cycled LNMO electrodes equipped with the pristine NG anode will be covered more thickness of CEI layers than that of the TEOSNG sample, which is coincident with the TEM and SEM results. From the literature, the surface products in form of lithium alkyl carbonate or other organic compounds come from the decomposition of the carbonates solvent precipitated with lithium ions.30 Therefore, this provides a directly way to study the decomposition degree of electrolyte by conducting the surface products. First, the peak appearing at about 286 eV is much to be from C−O (C−O−C and C−O−H bonds), which may be attributed to the oxygen-containing polymeric species generated from the solvent decomposition. For the LNMO electrode cycled with the pristine NG anode, the Li2CO3 species validated by the intensive peaks at ∼531.5 eV (CO, in O 1s) combining with the peaks at 55.3 eV (Li 1s) shows the very sharp peaks than that of the TEOS-NG sample, indicating that the more inorganic Li2CO3 species will be covered at the LNMO surface when cycled with the pristine NG anode as opposed to the TEOS-NG anode. Apart from the above surface species, other inorganic species on the surface are also validated by the Li 1s, F 1s and P 2p spectra, particularly LiF,45 LixPFy, and LixPOyFz (LiF + P2O5 or LixPOF3) compounds,46 which are typical decomposition products for the electrolyte with LiPF6 salt. Remarkable differences in Li 1s and F 1s peaks, the Li 1s spectra can be divided into two Gaussian component peaks at 55.3 and 55.91 eV assigned to lithium carbonates (Li2CO3) and lithium fluoride (LiF) respectively as shown in Figure 7.47 Apparently, the LNMO electrode cycled with the pristine NG anode shows the extremely overwhelming intensity

Figure 5. Typical charge−discharge curves of the LNMO half cells and full cells versus electrolyte decomposition.

Ni2+ and Ni4+/Ni3+ for the LNMO materials deliver the two subtle voltage plateau approximately 4.7 V (vs Li/Li+) and both beyond the thermodynamic stability window for the conventional liquid carbonate electrolyte.2 In the full cells, as shown in Figure 5, the charge−discharge curves of LNMO/NG full cells also exceed the stability window of electrolyte. As this reason, the surface of LNMO cathode must be covered with cathodeelectrolyte interface (CEI) layer to some degree. In fact, the SEI and CEI layers are both gradually growing with the cycling process continuing because of electron exposure to electrolyte or electrolyte diffusion to the electrodes, although the layer thickness growth after a few cycles is not nearly as great as the amount after the first cycle.38,40 As we known, the gradual thickening of layers further consumes the active Li-ions, solvents and salts, and ultimately leads to skyrocketing impedance increase in the full cells. Therefore, it is indispensable for the electrochemical performance of LNMO/NG full cells to understand what role the CEI, as well as the SEI perform. 4.1. Configuration and Properties of CEI. Most of literature have been proved that coating on cathode material surfaces can greatly improve the capacity retention and cycling stability in the LNMO/Li half and LNMO/NG full cells as well, especially at the elevated temperatures.41,42 It was suggested that coating layer reduces the oxidation decomposition of electrolyte and transition-metal dissolution (e.g., Mn and Ni ions) during the cycling by alleviating the interfaces reaction and altering the surface configuration. Therefore, the reaction and configuration of surface layer can greatly influence the electrochemical performance of the cell. The SEM and TEM were conducted to analyze the microstructure changes in terms of the cathode surfaces as well as the anode surface.The XPS and FTIR analysis were characterized to detect the components of layer at the surfaces of electrode. Figure 6 shows the SEM and TEM images of the fresh LNMO electrode (a, SEM and d, TEM), after 100 cycles with the pristine NG (b, SEM and e, TEM) and TEOS-NG anode (c, SEM and f, TEM) in the same electrolyte, respectively. First, as for the fresh LNMO material in Figure 6a, the SEM shows the neat, integral and clean edges of this bulk, which also can be confirmed by the clear edge of TEM image observed in Figure 6d. While, as for the cycled LNMO electrode with the pristine NG, obviously dense and uniform CEI layers are clearly found 33279


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Figure 6. SEM and TEM images of the LNMO electrode before (a, SEM and d, TEM) and after cycles that equipped with the pristine NG anode (b, SEM and e, TEM) and TEOS-NG anode (c, SEM and f, TEM) respectively.

graphite side after cycling. Figure 8 represents the microscale constructions of TEM to represent the changes of SEI at surface between the pristine NG and TEOS-NG electrodes after 100 cycles. As it exhibits, the dense, uneven and scattered SEI layers are easy to be found in Figure 8a and 8c for the pristine NG electrode, validating that the serious reaction must be taking place at the graphite surface. In contrast, acute differences are observed in Figure 8b that there is no obvious SEI structures instead of amount of well-piled spheres and pillars. Figure 8d shows the HRTEM images of those spheres with high magnification. Combining with the XPS analysis and this HRTEM, it can be validated the 0.33 nm interplanar spacing is consistent with the (011) planes of SiO2. In available literature, SEI layers were deemed as the bilayers configuration representative of the inside layers packed some inorganic compounds, such as LiF, Li2CO3, and Li2O, on the graphite side and outside layers consisted of organic species like Li2CO3, ROCO2Li and polymer on the electrolyte side.51 While

for both of LiF and Li2CO3 signals, highly indicate that the CEI surface layer mainly consist of the density of stable LiF and Li2CO3, which is also validated by the high peak of LiF at about 685 eV in the F 1s.48 This above analysis reveals that the large amounts of LiF and Li2CO3 that will gradually block the electrode and suppress the Li diffusion are formed during the cycling when the anode is the pristine NG anode, while the TEOS-NG anode can greatly suppress the formation of that inorganic species. In addition, the C−F bonds at about 688.3 eV in the F 1s spectra as the typical signal of PVDF binder peaks.49 The LixPOyFz and LixPFy species of LNMO electrode cycled with the pristine NG anode validated by the peaks at 133.4 and 136.2 eV are very sharp than that of the TEOS-NG anode,50 indicating that the more those species are formed on the CEI layers at the LNMO surface. 4.2. Configuration and Properties of SEI. On the other hand, the SEI configuration and properties were carried out by HRTEM, XPS, and FTIR measurements direct into the 33280


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Figure 7. XPS analysis of the fresh LNMO electrode and cycled LNMO electrode that equipped with NG and TEOS-NG anodes, respectively.

mixing process or the heat treatment moment. Second, as for the cycled electrode, the LiF species attributed to the peak at about 685 eV in the F 1s spectra and 58 eV in the Li 1s spectra are both observed for the both anodes.55 While, compare to the intensity of LiF peak, the NG sample presents much sharper LiF peak than that of TEOS-NG sample, indicating LiF-rich SEI layers that due to the more serious side effect are formed for the NG electrode at the surface of graphite. Therefore, the reason for the nature of high resistant nature of LNMO/NG cells as mentioned above can be partly associated with the high impedance of LiF-rich SEI formation. Especially to be mentioned, the peak of Si 2p at ∼104 eV for TEOS-NG sample after cycling is 100 times much sharper compared to the fresh TEOS-NG electrode, while no corresponding signal peak is found for the NG electrode, which demonstrate that a mount of silicon oxides are formed during cycling and spontaneously incorporated into the SEI layer. Finally, the XPS analysis of LNMO cathode surface reflects that on silicon oxide species are found, indicating that the silicon oxide group does not immigrate to the cathode side.

in the same vein, LiF species were also found on the electrolyte side permeated through the whole SEI layer on some other studies.52 Others deemed that LiF species seems to be primarily dominated surface chemistry for the SEI layers. The properties of LiF, as the common SEI layer products, play an important role in the electrochemical performance because it is of high resistance which could gradually insulate the electrode and then entirely block the electrodes over time.53 On the basis of above discussions, it is fundamental to understand the components of SEI at the anode surface. Figure 9 shows the XPS analysis for the surface composition of anode (the fresh NG and TEOS-NG electrodes, and after cycling respectively) in the delithiated state. First, as for the fresh graphite and fresh TEOS-NG electrode, there are of very similar feature and intensity including the C 1s, O 1s, F 1s, and P 2p spectra, indicating the main components of those fresh anode surface are mainly same. While, the fresh TEOS-NG electrode delivers a slightly silicon oxide (SiOx) validated by the 104 eV peak in the Si 2p spectra,54 indicating that TEOS does affect the surface structure of graphite which may transforms into the silicon oxide at graphite surface in the 33281


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Figure 8. HRTEM images of NG electrode (a, c) and TEOS-NG electrode (b, d) cycled with same LNMO electrode after 100 cycles.

deposit on the anode surface, which is corresponding to the TEM, XPS, and FTIR results that have discussed above.

The Fourier transform infrared spectroscopy (FTIR) analysis were also conducted to further identify the composition of SEI layer at the anode surface after cycling as shown in Figure 10. First, the FTIR spectra of the pristine NG electrodes contains strong peaks characteristic of primary lithium alkyl carbonates (ROCO2Li or (CH2OCO2Li)2) at 1630, 1395, 1305, and 1080 cm−1 associated with the CO stretch, −CH3 asymmetric bend, and C−O stretch vibration, respectively,56 while the weak peaks only are found at the same vibrations for the TEOS-NG anode, indicating that no heavy organic species are formed at this anode electrode surface after cycling. The vibrations at 2930, 2856, 1440, and 1110 cm−1 belong to the absorption peaks of lithium carbonate (Li2CO3). Obviously, the intensity of the Li2CO3 peaks for NG electrode is extremely sharper than that of TEOS-NG anode, meaning that more lithium carbonate is generated by the decomposition of electrolyte cycled with the pristine NG anode.57 Finally, it is worthwhile to look at the separator features of the full cells cycled with NG and TEOS-NG electrodes respectively as shown in the inset section of Figure 10. It shows three main components of the configuration of LNMO full cells after cycling, the anode, separator and the cathode from the left to right in the inset section. By comparing the feature of those cycled separator, it is obvious that this very separator cycled with TEOS-NG electrode is more clean and transparent than that of the pristine NG sample, indicating the severe deposition will occur with the pristine NG electrode as opposed to the TEOS-NG electrode and may be more precipitated species will

5. MECHANISM ANALYSIS 5.1. Metal Dissolution. The proposed capacity fading mechanism of this LNMO full cells is, in part, transition metal dissolution of Ni and Mn,2 which is known as the main reasons for the degradation of full cell based on LiMn2O4 cathode. Some recent research demonstrated that the Ni and Mn dissolution behavior in the LNMO-based battery experienced a similar trend in the LiPF6 electrolyte.58 Therefore, the ICP analysis is indispensable to conduct to validate this hypothesis, further analyze the degree of metal element dissolution on the graphite electrode after cycling. Table 1 lists the concentration of Ni and Mn elements for both cycled electrodes with traditional electrolyte. From the knowledge of literature,59,60 when the LNMO conducts at high voltage, the dissolution of Mn2+ and Ni2+ occurs due to the disproportionation (reaction 2, Mn3+ → Mn2+ + Mn4+) and the corrosion of the HF in the half-cell as well as the full-cell.61 The content of Mn element for both anodes is slightly varied with the 5.232 ppm as for the TEOS-NG electrode compared to the 7.137 ppm as for the pristine NG electrode. The content of Ni for the pristine NG electrode is 5 times higher than that of TEOS-NG electrode, indicating that this very anode of TEOSNG electrode can efficiently inhibit the dissolution of Mn and Ni from the bulk material. Therefor, the metal dissolution could 33282


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Figure 9. XPS analysis of the fresh NG electrode, the fresh TEOS-NG electrode, and the cycled NG and TEOS-NG anodes equipped with the same LNMO electrodes.

be one of the reasons that cause the serious capacity fading of full cells. 5.2. Interfaces Reaction and Configuration. Specifically, the mechanism why using the TEOS-NG electrode as anode can be greatly improve the electrochemical performance in full cells. As mentioned above, the surface of LNMO electrode cycled with the pristine NG anode is covered dense and uniform layers consisted of extremely overwhelming some LiF and Li2CO3 inorganic species as well as organic species, which mainly come from the serious interfaces reaction at the interfaces. However, what is the source of the impurity? In fact, the electrolyte solutions after cycling will contain several hundreds of parts per million of water, with 10−15 times more

than the initial solutions, which is mainly caused by the HF attacking to the carbonate species.49 Where the reaction between the electrolyte and inevitable trace of water molecules will generate the HF species and high Lewis acid of PF5,62 the LiF are formed at the same time. As for as the TEOS-NG samples, the HF impurity is elimited due to the following reaction: the SiO2 particles can react with it that both inevitably observed and generated in the case of LiPF6-based electrolyte,63,64 to form the SiF4 and H2O molecule as shown in eq 3. In addition, same effect can stem from the reality that the SiO2-containing separators can actually improve the capacity retention of LiCoPO4 cathode by acted as 33283


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passive film between the electrolyte and electrode that causes largely irreversible capacity losing. To solve its drawback, a number of approaches have been conducted including to form a protective layers at graphite surface, adulterate the inorganic groups and so on. According to a previously reported hypothesis,69 the incorporation of Li2CO3 at the surface of graphite anode can be greatly improve the first Coulombic efficiency and further enhance the stability of SEI between the electrode and organic electrolyte. Other important way is to infiltrate Li species into the graphite surface in advance so as to reduce the irreversible capacity, signed as the Li-doped graphite anode.70 Therefore, this study also conducts those similar experiences to prepare Li2CO3-coating and Li-doped graphite materials, and then equipped with same LNMO cathode to compare the effect on the high-voltage LNMO full system. The details of the Experimental Section are shown in the Supporting Information. The addition of Li2CO3 content is 0.5% followed by the literature due to this low level of irreversibility in formation of SEI.69 And, the addition of Li content is 0.34% which is followed by the literature so as to stabilize the graphite structure better.70 Figure 11a shows that Coulombic efficiency of LNMO full cells with different modifying-graphite anodes, while the full cell cycled with TEOS-NG (amount 10%) signs as the LNMO/ TEOS-NG, the Li-doped electrode signed as LNMO/NG-Li and the Li2CO3-coating signed as LNMO/NG-Li2CO3. Not surprisingly, the full cells cycled with modifying anodes present the superior initial Coulombic efficiency with the LNNO/NGLi2CO3 sample is of 77.36% and the 70.15% for the LNMO/ NG-Li sample as opposed to the pristine LNMO/NG sample is of only 59.91% as shown in Figure 11a. The improvement of initial Coulombic efficiency indicates that the modification of graphite is an effective way to stabilize the interfaces construction and alleviate the electrolyte decomposition at the interface between the electrode and electrolyte during the cycling. Figure 11b exhibits the cycling performance of LNMO/NG full cells cycled with different anodes respectively at 1 C rate between the 3.4 and 4.8 V. Obviously, it is can be found that the LNMO/TEOS-NG shows the best capacity retention among of those modifications with the capacity retention is of 95% after 100 cycles, while others also still suffer serious capacity fading as the pristine NG anode with 86% capacity retention for LNMO/NG-Li sample and 77% for the LNMO/ NG-Li2CO3, indicating that the TEOS-NG has highly power to mitigate the capacity fading compared to the traditional

Figure 10. FT-IR spectra of the cycled NG and TEOS-NG anodes equipped with the same LNMO electrodes. The inset shows the main three component images of the full cells after cycles.

Table 1. ICP Results of Concentration of Mn and Ni Deposited on the Pristine NG Electrode and TEOS-NG (19%) Electrode samples

Mn (ppm)

Ni (ppm)

pristine NG TEOS-NG

7.137 5.232

107.675 23.441

an HF scavenger to prevent the detrimental reaction at the electrode/electrolyte interfaces.65 SiO2 + 4HF → SiF4 + 2H 2O

(3)

Furthermore, the water molecule will be caught by the SiO2 nanoparticles.66,67 Meanwhile, in this type of the SiO2 species could precipitate with decomposition products of electrolyte spontaneously into the graphite surface, further stabilize the SEI construction and minimize the side reaction of LNMO cathode with electrolyte by consuming the HF group and water impurity.

6. COMPARED STUDIES Graphite materials have been predominantly used as anode materials for lithium secondary batteries, especially for the commercialization because of its high reversible capacity, notoxic products, and low cost.68 However, the drawback of this graphitic anode is that can easily promote the electrolyte decomposition at the graphite surface and subsequently form a

Figure 11. Coulombic efficiency of LNMO full cells with different modifying-graphite anode (a) and cycling performances (b). 33284


ACS Applied Materials & Interfaces methods. And further study shows that this very method can easily apply into other high voltage lithium secondary batteries, such as high voltage LiCoO2 at a voltage range of 3.0 to 4.5 V as shown in Figure S1.

ACKNOWLEDGMENTS

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REFERENCES

(1) Goodenough, J. B.; Kim, Y. J. Challenges for Rechargeable Li Batteries. Chem. Mater. 2010, 22, 587−603. (2) Ma, J.; Hu, P.; Cui, G.; Chen, Li. Surface and Interface Issues in Spinel LiNi0.5Mn1.5O4: insights into a Potential Cathode Material for High Energy Density Lithium Ion Batteries. Chem. Mater. 2016, 28, 3578−3606. (3) Hong, D.; Guo, Y.; Wang, H.; Zhou, J.; Fang, H.-T. Mechanism for Improving the Cycle Performance of LiNi0.5Mn1.5O4 by RuO2 Surface Modification and Increasing Discharge Cut-Off Potentials. J. Mater. Chem. A 2015, 3, 15457−15465. (4) Rong, H.; Xu, M.; Xing, L.; Li, W. Enhanced Cyclability of LiNi0.5Mn1.5O4 Cathode in Carbonate Based Electrolyte with Incorporation of Tris(trimethylsilyl)phosphate (TMSP). J. Power Sources 2014, 261, 148−155. (5) Michalak, B.; Berkes, B. B.; Sommer, H.; Bergfeldt, T.; Brezesinski, T.; Janek, J. Gas Evolution in LiNi0.5Mn1.5O4/Graphite Cells Studied In Operando by a Combination of Differential Electrochemical Mass Spectrometry, Neutron Imaging, and Pressure Measurements. Anal. Chem. 2016, 88, 2877−2883. (6) Cho, J. H.; Park, J. H.; Lee, M. H.; Song, H. K.; Lee, S. Y. A Polymer Electrolyte-Skinned Active Material Strategy toward HighVoltage Lithium Ion Batteries: a Polyimide-Coated LiNi0.5Mn1.5O4 Spinel Cathode Material Case. Energy Environ. Sci. 2012, 5, 7124− 7131. (7) Cho, M. Y.; Roh, K. C.; Park, S. M.; Lee, J. W. Effects of CeO2 Coating Uniformity on High Temperature Cycle Life Performance of LiMn2O4. Mater. Lett. 2011, 65, 2011−2014. (8) Liu, J.; Sun, Z.; Xie, J.; Chen, H.; Wu, N.; Wu, B. Synthesis and Electrochemical Properties of LiNi0.5‑xCuxMn1.5‑yAlyO4 (x= 0, 0.05, y= 0, 0.05) as 5V Spinel Materials. J. Power Sources 2013, 240, 95−100. (9) Cho, H. M.; Chen, M. V.; MacRae, A. C.; Meng, Y. S. Effect of Surface Modification on Nano-Structured LiNi0.5Mn1.5O4 Spinel Materials. ACS Appl. Mater. Interfaces 2015, 7, 16231−16239. (10) Wu, H. M.; Belharouak, I.; Abouimrane, A.; Sun, Y.-K.; Amine, K. Surface Modification of LiNi0.5Mn1.5O4 by ZrP2O7 and ZrO2 for Lithium-Ion Batteries. J. Power Sources 2010, 195, 2909−2913. (11) Manthiram, A.; Chemelewski, K.; Lee, E. S. A Perspective on the High-Voltage LiMn1.5Ni0.5O4 Spinel Cathode for Lithium-Ion Batteries. Energy Environ. Sci. 2014, 7, 1339−1350. (12) Chong, J.; Xun, S.; Song, X.; Liu, G.; Battaglia, V. S. Surface Stabilized LiNi0.5Mn1.5O4 Cathode Materials with High-Rate Capability and Long Cycle Life for Lithium Ion Batteries. Nano Energy 2013, 2, 283−293. (13) Kim, J. H.; Pieczonka, N. P. W.; Li, Z.; Wu, Y.; Harris, S.; Powell, B. R. Understanding the Capacity Fading Mechanism in LiNi0.5Mn1.5O4/Graphite Li-Ion Batteries. Electrochim. Acta 2013, 90, 556−562. (14) Lu, D. S.; Yuan, L. B.; Li, J. L.; Huang, R. Q.; Guo, J. H.; Cai, Y. P. Understanding the Capacity Fading Mechanism in LiNi0.5Mn1.5O4/ Graphite Li-ion Batteries. J. Electroanal. Chem. 2015, 758, 33−38. (15) Chong, J.; Xun, S.; Zhang, J.; Song, X.; Xie, H.; Battaglia, V.; Wang, R. Li3PO4-coated LiNi0.5Mn1.5O4: a Stable High-Voltage Cathode Material for Lithium-Ion Batteries. Chem. - Eur. J. 2014, 20, 7479−7485. (16) Kim, J. H.; Pieczonka, N. P.; Yang, L. Challenges and Approaches for High-Voltage Spinel Lithium-Ion Batteries. ChemPhysChem 2014, 15, 1940−1954. (17) Zhang, Z.; Hu, L.; Wu, H.; Weng, W.; Koh, M.; Redfern, P. C.; Curtiss, L. A.; Amine, K. Fluorinated Electrolytes for 5 V Lithium-Ion Battery Chemistry. Energy Environ. Sci. 2013, 6, 1806−1810.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b08828. Experimental details of compared studies, first charge− discharge and cycling curves of LiCoO2/NG full cells cycled with TEOS-NG and NG electrode, comparison of the specific capacity of first cycle and capacity retention of the TEOS-NG sample to that shown in the reported literatures, and XRD chart of the TEOS-NG (19%) and pristine NG anode before cycling (PDF)

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The authors gratefully acknowledge funding from the National Natural Science Foundation of China (U1401246, 51364004, 51064004, 51474110, and 51474077) and the Province Natural Science Foundation of Guangxi (2016GXNSFDA380023).

7. CONCLUSIONS To understand the serious capacity fading of the high-voltage LNMO full cells with the traditional electrolyte, we characterize the interfaces properties on the full cells, both the CEI and SEI formation and configuration after cycling. It concludes that the capacity fading are tightly related to the electrolyte serious reaction at that interfaces, while the dense, nonuniform CEI layers on the cathode side and the uneven, scattered SEI layers are respectively formed when cycled with the pristine NG electrode at a high voltage range from 3.4 to 4.8 V. In this Article, the new strategy stabilizes the capacity significantly by forming the artificial silicon organic (TEOS) SEI film which mitigates the negative side reaction occurring at the interface between electrode and electrolyte. In addition, we also compared to the conventional modifying-graphite methods with incorporating the Li2CO3 inorganic component and doping-Li strategy, it is concluded that the LNMO full cells equipped with TEOS-modifying graphite anode can maintain an excellent electrochemical performance. The TEM, XPS, and FTIR results combining with electrochemical characterization validate that the improvement of capacity retention, as well as other electrochemical performance are due to eliminate the high-level of the reaction between the electrolyte and the electrodes, the formation of CEI and SEI layers. The mechanism of improvement of LNMO full cells may be explained that eliminating the HF and water impurities in the electrolyte by introducing the TEOS reactive group which can transform the SiO2 species that react with the acid of HF at the organic solvent environment instead of destroying/forming the anode SEI and attacking the LNMO spinel structure to form the dense and high resistance CEI, meanwhile the SiO2 species will absorb the water molecules and precipitate into the anode surface further stabilizing the SEI configuration during the cycling.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Tel: +86-773-5856104. ORCID

Jiujun Zhang: 0000-0002-6858-4060 Qingyu Li: 0000-0003-4638-4401 Notes

The authors declare no competing financial interest. 33285


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A New Strategy to Stabilize Capacity and Insight into the Interface

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