Understanding Transition-Metal Dissolution Behavior in LiNi0.5Mn1

Jul 12, 2013 - In addition, surfaces of calendar- or cycle-aged LNMO and graphite electrodes were analyzed by X-ray photoelectron spectroscopy (XPS), ...
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Understanding Transition-Metal Dissolution Behavior in LiNi0.5Mn1.5O4 High-Voltage Spinel for Lithium Ion Batteries Nicholas P. W. Pieczonka,† Zhongyi Liu,‡ Peng Lu,‡ Keith L. Olson,‡ John Moote,§ Bob R. Powell,‡ and Jung-Hyun Kim*,‡ †

Optimal CAE Inc., Plymouth, Michigan 48170, United States Chemical and Materials Systems Laboratory, General Motors Global R & D Center, Warren, Michigan 48092, United States § Engineering Operations, General Motors Global Powertrain, Warren, Michigan 48091, United States ‡

S Supporting Information *

ABSTRACT: The high-voltage LiNi0.5Mn1.5O4 (LNMO) spinel is a promising candidate for a positive electrode in lithium ion batteries, but LNMO/graphite full-cells display severe capacity fading issues due to Mn dissolution. In this study, the dissolution behaviors of Mn and Ni were examined systematically under various conditions such as state of charge (SOC), temperature, storage time, and crystal structure of LNMO. In addition, surfaces of calendar- or cycle-aged LNMO and graphite electrodes were analyzed by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), or time-of-flight secondary ion mass spectrometry (TOF-SIMS). The chemical composition of aged electrolyte was determined by gas chromatography (GC) analysis after storage of LNMO electrodes under different conditions. It was found that the amounts of dissolved Mn and Ni and diethyl ether, a decomposition product of diethyl carbonate (DEC) in electrolyte, increased with SOC, temperature, and storage time. The decomposition of electrolyte can be explained, in part, by the self-discharge behavior of LNMO, which promotes electrolyte oxidation. Additional HF is believed to be generated during the formation of diethyl ether (via dehydration reaction from EtOH, another decomposition product of DEC), which accelerates Mn and Ni dissolution from LNMO. In addition, various reaction products that form as a result of Mn and Ni dissolution, such as LiF, MnF2, NiF2, and polymerized organic species, were found on the surface of LNMO electrodes, which will increase battery-cell impedance.

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INTRODUCTION High-voltage LiNi0.5Mn1.5O4 (LNMO) spinel is a promising candidate as the positive electrode in next-generation Li-ion batteries for electric vehicle applications due to its high operating voltage of ∼4.7 V (vs Li/Li+) and good high-rate performance.1−5 However, several issues have challenged the implementation of the LNMO, which include oxidation of electrolytes at the high operating voltage and the Mn dissolution problem.3,5−7 In particular, the LNMO suffers from capacity fading, similar to what has been encountered in other manganese-based spinel materials when paired with graphite anodes (e.g., LNMO/graphite full-cell). Ideally, LNMO contains only Mn4+, which will not cause severe Mn dissolution issues. For instance, fresh LNMO powder showed only small amounts of Mn and Ni dissolution when compared with measurements from other kinds of positive materials such as LiNi0.5Mn0.5O2, LiNi1/3Co1/3Mn1/3O2, and LiMn2O4.8 In contrast, a recent study revealed that LNMO still exhibited nonnegligible amounts of Mn and Ni dissolution during cycling, which resulted in a severe capacity fading issue for LNMO/graphite full-cells.3,6,7 The amount of Mn reduced on © 2013 American Chemical Society

Li-metal anodes in the LNMO/Li half-cells was analyzed after they were cycled 100 times at 30 and 55 °C. The results show that the amount of Mn dissolution corresponds to, respectively, 3000 and 6500 ppm of total Mn mass concentration in LNMO after 100 cycles at 30 and 55 °C.3 From this result, the capacity fading of LNMO/graphite was explained by the impact of Mn dissolution, and active Li+ loss through continuous solid− electrolyte interface (SEI) formation (electrolyte reduction) prompted by metallic Mn on the graphite surface. Additional issues were brought forth by Aurbach and coworkers,9,10 who analyzed LNMO after prolonged cycling to study the aging mechanism of the LNMO. Their results showed that the aging of the LNMO electrode was facilitated at elevated temperature (i.e., 60 °C) due to the formation of a surface film on the LNMO, which leads to an increase in the interfacial impedance of the LNMO electrode. Yang et al.11 suggested that this film may be composed, in part, of Received: May 24, 2013 Revised: July 9, 2013 Published: July 12, 2013 15947

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500 °C for 12 h in air. The resulting powders were reground, pelletized, and heated at 650 °C for 12 h in air. This synthesis was completed with a final heat treatment at 900 °C for 6 h in air. The resulting LNMO had disordering between Ni and Mn. In this paper, the disordered LNMO is simply denoted as LNMO. The Ni and Mn ordered LNMO was obtained by annealing the LNMO further at 700 °C for 48 h in air. Details of Ni/Mn disordering and ordering were previously analyzed by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and transmission electron microscopy (TEM).1−3 Electrochemical Analysis. The LNMO positive electrode consisted of 80:10:10 wt % LNMO, super-P carbon, and poly(vinylidene fluoride) (PVDF, Kynar HSV 900). The graphite negative electrode consisted of 90:10 wt % mesoporous carbon microbeads (MCMB, Osaka Gas Co.) and PVDF. Each electrode formulation was mixed with Nmethylpyrrolidone (NMP) and coated onto Al (for LNMO) or Cu (for graphite) foil via the doctor-blade method. Half- and full-coin cells (Hohsen Co., 2032 model) were assembled in an argon-filled glovebox. For the half-cells, lithium foil was used as the anode. Two layers of separator (Celgard, polypropylene/ polyethylene/polypropylene trilayer), and 1 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:2 volume ratio) electrolyte were used for preparing coin cells. All coin cells were cycled by use of a Maccor 4000 battery testing system. Transition-Metal Dissolution Analysis. The LNMO/Li half-cells were cycled two times in a voltage range of 3.5−4.9 V, and discharged to target SOCs (0%, 25%, 50%, 75%, and 100%) at the third cycle. Each cell was then transferred to the argon-filled glovebox, followed by opening it and recovering the LNMO electrode for each specific SOC condition. Thusobtained LNMO electrodes were gently washed with fresh electrolyte and transferred to fresh electrolyte in pressureresistant Teflon vessels. The amount of electrolyte was controlled to be 1 mL/mg of LNMO in the positive electrodes. The Teflon vessels were stored in thermal chambers at different temperatures (30−80 °C) for different storage times (up to 80 days). After storage, each vessel was open in the glovebox to sample the electrolytes, and resulting electrolytes were analyzed by X-ray fluorescence (XRF) spectrometry with a Perform’X (Thermo Scientific). Bulk and Surface Analyses. X-ray diffraction measurements were carried out for some of the LNMO electrodes after the storage tests with a D8 diffractometer (Bruker) using Cu Kα1 radiation in Bragg−Brentano configuration. The lattice parameters of LNMO were calculated by using Topas software (Bruker). Structural and chemical analyses of LNMO and graphite electrodes were carried out after the storage or battery cycling tests on an aberration-corrected JEOL JEM-2100F microscope operated at 200 kV. This microscope is a transmission/scanning tunneling electron microscope (TEM/ STEM) hybrid for imaging and is equipped with an Oxford instrument for chemical analysis via energy-dispersive X-ray (EDX) microanalysis. TEM examinations were performed on both the LNMO positive and graphite negative electrodes. TEM samples were prepared by scraping powder material off the electrode surface and dusting it onto carbon-supported copper TEM grids. These samples were briefly exposed in air during the transfer from the glovebox to the microscope. Indexing and simulation of electron diffraction pattern along

poly(ethylene carbonate) (PEC), a decomposition product of ethylene carbonate (EC). It was found that PEC formation was caused by oxidation of electrolytes; PEC was not observed until 4.3 V (vs Li) but appeared after charging to 5.0 V (vs Li). These results further indicate that a major challenge for the LNMO positive-electrode system is related to electrolyte stability at high voltages and high temperatures. The performance and stability of electrolyte is strongly dependent on the presence of impurities. For example, it is well-known that the presence of trace amounts of water in the electrolyte can generate HF by the following reactions:12−16 LiPF6 ↔ LiF + PF5

(1)

PF5 + H 2O → OPF3 + 2HF

(2)

OPF3 + 2x Li+ + 2x e− → LixPF3 − x O + x LiF

(3)

OPF3 + 3H 2O → PO4 H3 + 3HF

(4)

The presence of protic impurities (mostly alcohol species) in the carbonate-based electrolytes can be additional sources for the generation of HF. It has been reported that electrolytes may contain parts per milllion (ppm) levels of these protic impurities (mostly alcohol species), though definitive analysis is challenging. The production of protic species is facilitated at high temperature by the presence of water impurities.17 Furthermore, the nature of these protic impurities is dependent on the composition of electrolytes: methanol from dimethyl carbonate (DMC), ethanol from DEC, ethylene glycol from EC, propylene glycol from propylene carbonate (PC). Such protic impurities can be involved in further HF generation in the electrolytes. In addition, it has been proposed that λ-MnO2 (fully charged state of LiMn2O4 spinel) itself can catalyze the decarboxylation of DEC, resulting in EtOH production.18 However, further investigation is needed to properly understand the detailed mechanism and the source of catalysts for the reaction. Hunter19 could obtain λ-MnO2 from LiMn2O4 by acid treatment through the following reaction: 2LiMn2O4 + 4H+ → 2Li+ + 3λ‐MnO2 + Mn 2 + + 2H 2O (5)

In eq 5, a disproportionation reaction of 2Mn → Mn + Mn4+ should occur, followed by dissolution of Li2O and MnO at the particle surface. This dissolution reaction could continue because Li+ and Mn3+ ions in the bulk are able to diffuse to the surface (actually electron hopping through MnO6).19 Despite scattered reports addressing the issue of transitionmetal dissolution from LNMO positive electrodes, there has not been a systematic study to understand the critical factors that influence the dissolution behaviors in the literature. With this perspective, we present here Mn and Ni dissolution behaviors under various conditions [state of charge (SOC), temperature, storage time, and crystal structure of LNMO] and their influence on the electrodes/electrolyte interfaces, LNMO bulk structure, and electrolyte decomposition after storage at elevated temperature (60 °C). 3+

2+

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EXPERIMENTAL SECTION Materials Preparation. LNMO was synthesized via solidstate reaction. Stoichiometric amounts of Li2CO3, NiCO3, and MnCO3 precursors were mixed in a ball mill (Spex 8000D) for 30 min. The mixed precursors were pelletized and heated at 15948

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specific zone axes were done by using the Java version of the electron microscopy simulation (JEMS) program.20 A Phi Trift V nano time-of-flight spectrometer (Physical Electronics, Chanhassen, MN) was employed for the time-offlight secondary ion mass spectrometry (TOF-SIMS) analyses. Samples were rinsed with 3 mL of dimethyl carbonate (DMC), dried in argon atmosphere, and then transferred to the instrument in argon with a sealed apparatus. The analysis chamber of the instrument was maintained at a pressure of less than 5 × 10−7 Pa during analysis. A 30 kV Au+ ion source was used for both sputtering (∼0.1 nm/s, calibrated with 100 nm SiO2) and analysis. The analysis area was 50 μm × 50 μm, within a sputter area of 200 μm × 200 μm. Electrolyte Analysis. Gas chromatography (GC) analysis was performed for the aged electrolyte samples from storage tests under various conditions (different SOC, temperatures, and storage times). Fresh electrolyte was analyzed for comparison. The samples were first analyzed by GC with a simple flame ionization detector (FID) and then by GC coupled with mass spectrometry (MS). The corrosive LiPF6 needs to be removed from the electrolyte samples before GC analysis. For this purpose, the electrolyte samples were treated with CaCO3 to precipitate the LiPF6 into various forms of chemicals such as LiF, Li2CO3, Ca(PF6)2, Ca3(PO4)2, CaF2, and Ca(PO2F2)2, followed by filtering to remove them. One milliliter of electrolyte and 1 mL of methanol that contained a small amount of water were placed in a 50 mL plastic centrifuge tube. After 0.1 g of CaCO3 was added to the tube, the centrifuge tube was capped and shaken for 1 min. The liquid was filtered and placed in a sample vial for analysis. The prepared samples were analyzed by GC-FID and GC-MS with a 30 m × 0.53 mm i.d. PoraPlot Q capillary column (www.sigmaaldrich.com/supelco). The column was held at 40 °C for 2 min and then heated to 210 °C at a rate of 7.5 °C/min. The response of the FID was calibrated by analyzing solutions with known concentrations of ethanol and ethyl acetate in methanol. Standard 70 eV electron impact ionization spectra were recorded with a mass spectrometer (Pegasus 4D, LECO). Spectra were recorded to span the mass range from m/z 30 to 300.

Figure 1. (a) Cycle lives of LNMO/Li and graphite/Li cells by applying C/10 rate at 30 °C. The voltage range was, respectively, 3.5− 4.9 V for LNMO/Li and 0.005−2.5 V for graphite/Li cells. (b) Cycle lives of LNMO/graphite cells with different lower cutoff voltages with a constant current density of 12 mA/g at 30 °C.

The reduced metallic Mn will further promote the loss of active Li+ through formation of thick SEI layers, which lead to significant capacity fading of full-cells. Mn dissolution in LMO has been explained by the surface disproportionation reaction (2Mn3+ → Mn2+ + Mn4+).19 The average oxidation state of Mn in LMO is +3.5 (50% Mn3+ and 50% Mn4+), and one of the most critical approaches to mitigate Mn dissolution in LMO is increasing the average oxidation state of Mn by partly substituting Mn with monovalent (Li+) and/or divalent (i.e., Ni2+, Mg2+) cations.23−26 In contrast, ideal LNMO consists of Mn4+, although it may contain small amounts of Mn3+ due to the formation of oxygen vacancies and/or impurity phases during sintering at high temperatures (>800 °C).27 Since the Mn3+/4+ redox reaction occurs at ∼4 V (vs Li) in the spinel structures, most Mn3+ ions present in LNMO will be oxidized to Mn4+ at >4 V (vs Li). This suggests that cycling the LNMO cells at voltages above 4 V (vs Li) will be beneficial to alleviate the Mn dissolution problem, because it allows Mn to remain as Mn4+ during cycling (thus minimizing Mn3+ content in Li1−xNi0.5Mn1.5O4). To verify this, the influence of low cutoff voltage on the cycle lives of LNMO/ graphite full-cells was examined by cycling cells in two different voltage ranges, 4.2−4.8 V and 4.4−4.8 V. Their voltage profiles during first through fifth cycles are illustrated in Figure 2. Contrary to expectation, limiting the lower voltage cutoff did not improve the cycle lives of the LNMO/graphite full-cells as shown in Figure 1b. Instead, all the full-cells demonstrated

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RESULTS AND DISCUSSION LNMO/Graphite Full-Cell Performance. Figure 1a shows the specific capacity of LNMO/Li and graphite/Li half-cells. When half-cells were cycled at 30 °C in the voltage range 3.5− 4.9 V (vs Li) at ∼ C/10 rate, both half-cells delivered good capacity retention for the 100 cycles. However, LNMO/ graphite full-cells cycled at the same temperature and ∼C/10 rate, in the voltage range 3.4−4.8 V, showed severe capacity fade during 100 cycles in Figure 1b: ∼50% capacity loss. Our earlier study revealed that such capacity loss is caused by the loss of active Li+ in the LNMO electrode.3 It was suggested that such Li+ loss in the LNMO/graphite full-cell can be correlated with transition-metal dissolution from LNMO. Mn dissolution has been well acknowledged as a major failure mechanism of batteries using LMO positive electrodes.8,21,22 Dissolved Mn2+ ions are reduced on the surfaces of graphite negative electrodes in full-cells by partly depleting active Li+ by the following reaction.13 Mn 2 + + 2LiC6 → Mn + 2Li+ + graphite

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Figure 3. (a) Annular dark field (ADF) image showing the microstructure of a pristine graphite particle. (b) Typical TEM image showing nanoparticles with different sizes (from a few nanometers to tens of nanometers) deposited on cycled graphite particles. (c) High-resolution TEM image highlighting a Mn nanoparticle (∼18 nm in diameter) deposited on a cycled graphite particle. (d) Energy-dispersive X-ray spectrometry (EDX) showing that a deposited nanoparticle (∼30 nm) (highlighted with a yellow box) contains both Mn and Ni. The Cu signal is from the Cu TEM grid, and the F and P signals may come from SEI or residual electrolyte.

chemical properties of graphite electrodes. They observed that transition-metal reduction on graphite surfaces was detrimental to electrochemical performance in the following order: Mn > Co > Ni. This has been explained by the relatively high reduction potential of Ni2+ and the absence of catalytic activity of Ni metal for electrolyte decomposition compared with that of Mn and Co. In addition, EDX analysis demonstrated that a nanoparticle (highlighted by a yellow box in Figure 3d) consisted of Ni and Mn with an atomic ratio of 1:1 (based on the intensities of Ni and Mn peaks). From the binary phase diagram, this result supports the formation of MnNi alloy on graphite surface.30 The concentration and formation mechanism of Mn−Ni alloy, and its impact on SEI growth compared with that of the Mn metal, need to be investigated in future studies. Surface Analysis of Cycled LNMO Electrodes. The LNMO positive electrode from a 100-cycle LNMO/graphite full-cell at 30 °C was analyzed by X-ray photoelectron spectroscopy (XPS). Figure 4 compares the depth profiles of the Li, Ni, and Mn elements from the surface of the cycled LNMO electrode. The Mn and Ni intensities sharply increased up to 10 nm depth but became stable beyond it. The intensity ratio between Mn and Ni was approximately 3:1 below 10 nm depth, which agrees with the bulk composition of the LNMO. The Mn and Ni deficiency measured at the surface can be understood to occur as a result of their dissolution and subsequent deposition/reduction at the surface of graphite electrode, as evidenced by TEM analysis (see Figure 3). In contrast, Li intensity was high in the top 5 nm surface, but its

Figure 2. Voltage profiles of LNMO/graphite cells with different cutoff voltages at 30 °C: (a) 3.4−4.8 V, (b) 4.2−4.8 V, and (c) 4.4−4.8 V.

similar slopes of capacity fading, which implies that they all experienced Mn and Ni dissolution at similar levels. Surface Analysis of Cycled Graphite Electrodes. The graphite negative electrode from a 100-cycle LNMO/graphite full-cell was analyzed by TEM. Figure 3a,b shows TEM images of pristine and cycled graphite. The results show that there were various sizes of nanoparticles, ranging from a few nanometers to tens of nanometers, on the surface of the cycled graphite particle. By contrast, the pristine graphite revealed a very clean surface (Figure 3a). Figure 3c shows a TEM image of a highlighted particle on a cycled graphite surface. EDX measurements showed that most particles consisted of Mn metals. The Mn particles were randomly distributed on graphite surfaces (Figure 3b). This can be understood by a reduction of Mn2+ to metallic Mn at graphite surface. It has been generally accepted that the Mn metallic particle will lead to a thicker SEI, through the consumption of active Li+ and resulting in capacity fading of LNMO/graphite full-cells.3 Komaba et al.28,29 compared the effect of various transition-metal (Mn, Ni, Co) reductions on the electro15950

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Figure 4. XPS depth profiles of Li, Ni, and Mn from the LNMO electrode after 100 cycles at 30 °C.

content decreased to 10 nm depth. This can be explained by the deposition of LiF as a result of Ni and Mn dissolution.31 Since all the elements showed changes of intensity slopes at ∼5 nm depth, it could be suggested that a decomposition layer was formed as a result of Ni and Mn dissolution after cycling. Transition-Metal Dissolution Behavior in LNMO. The LNMO/graphite full-cells showed capacity fade as a result of Mn and Ni dissolution regardless of lower cutoff voltage (Figure 1b). This result suggests that there exist other critical factors which govern Mn dissolution behavior other than the Mn3+ content in LNMO. To understand this, it is necessary to find critical parameters that influence transition-metal dissolution in the LNMO. In this regard, transition-metal dissolution amount from LNMO was compared by controlling four different conditions: (i) state of charge (SOC), (ii) storage temperature, (iii) storage time, and (iv) crystal structure of LNMO: Ni/Mn disordered (simply denoted here as LNMO) and ordered spinel. First, the influence of SOC on transition-metal dissolution from LNMO was examined. After being cycled two times, the LNMO/Li half-cells were discharged to specific SOC targets (0%, 25%, 50%, 75%, and 100%) and each LNMO electrode was recovered from its coin cell. The resulting LNMO electrodes with different SOCs were stored in fresh electrolyte at 60 °C for 60 days. Chemical analysis of the electrolytes by XRF revealed that the amount of Ni and Mn dissolution increased with SOC in Figure 5a. In accordance with earlier work,8 the LNMO had a small amount of Mn dissolution with negligible Ni dissolution in the fully discharged state (∼3.5 V vs Li, 0% SOC). The amount of Ni and Mn dissolution showed a moderate increase until 50% SOC and then increased abruptly at 75% and 100% SOCs. The Mn dissolution amount increased about 10-fold between 0% and 100% SOC. The amount of Ni dissolution was severe and comparable with that of Mn at 100% SOC. Similarly, the LMO spinel experienced increasing Mn dissolution with SOC. To explain this behavior, Aoshima et al.18 proposed an electrolyte solvent (DEC) decomposition mechanism that produces H2O and HF via catalytic activity of delithiated Mn2O4 (λ-MnO2 phase) at high SOC state. In addition, Wang et al.32 monitored the in situ Mn2+ dissolution behavior from the LMO by using a rotating ring-disk electrode system. The amount of Mn2+ dissolution increased with upper cutoff voltages from 4.3 to 4.5 to 4.8 V. As the LMO becomes fully delithiated (λ-MnO2) at >4.1 V, they suggested that such

Figure 5. Mn and Ni dissolution amount in aged electrolyte samples after storage of LNMO electrodes under various conditions. (a) SOC (of LNMO) dependence of Mn and Ni dissolution under a given condition (60 °C for 60 days). (b) Temperature dependence of Mn and Ni dissolution under a given condition (100% SOC of LNMO for 60 days). (c) Time dependence of Mn and Ni dissolution under a given condition (100% SOC of LNMO at 60 °C).

increase in Mn2+ dissolution from 4.3 to 4.8 V would be related to instability of λ-MnO2 and/or electrolyte oxidation. Second, the temperature dependence of transition-metal dissolution from LNMO electrodes was investigated. The fully delithiated LNMO electrodes (100% SOC) were stored in fresh electrolyte at 30, 45, 60, and 80 °C for 60 days. Figure 5b shows that the amount of Mn and Ni dissolution from the LNMO electrodes increased with temperature. On the basis of earlier reports, this trend can be explained by hydrolysis of LiPF6 in the presence of trace amounts of water and subsequent generation of HF by eqs 2−4. From differential scanning 15951

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calorimetry (DSC) measurement, the above hydrolysis reaction was reported to occur at around 60 °C and the generated HF will cause further Mn dissolution as seen in Figure 5b. Both the hydrolysis reaction of LiPF6 and Mn17 and Ni dissolution by HF will be accelerated at elevated temperatures. Third, the amount of transition-metal dissolution with storage time was measured for fully delithiated LNMO electrodes (100% SOC) at 60 °C. In Figure 5c, the Mn and Ni dissolution amounts increased rapidly in the first 10 days, followed by decreasing slope, and finally they became steady after 45 days. To understand this behavior, surface and bulk structure analyses were performed for the LNMO electrodes (100% SOC) after the storage tests. These results will be discussed in later sections. Finally, transition-metal dissolution behavior between Ni/ Mn disordered and ordered LNMO electrodes (100% SOC) were compared. It should be recalled that simple LNMO in this paper refers to the disordered structure. Figure 6 compares the

Figure 6. Comparison of Mn and Ni dissolution amount in aged electrolytes samples between LNMO (disordered) and ordered LNMO electrodes with 100% SOC after storage at different temperatures for 60 days.

amount of Mn and Ni dissolution at different temperatures. At any given temperature, the ordered LNMO had slightly larger amounts of Mn and Ni dissolution compared with those of the LNMO (disordered). However, the difference was not severe, and the trend of dissolution was similar between the Ni/Mn disordered and ordered LNMO. Surface and Bulk Analysis of Aged LNMO Electrodes. The surfaces of delithiated LNMO electrodes (100% SOC) were analyzed by TOF-SIMS and TEM after storage in electrolyte at 60 °C for 60 days for the Mn dissolution experiments. TOF-SIMS depth profile of the LNMO electrode (100% SOC) surface after storage is shown in Figure 7. In Figure 7a, C2H−, CH−, and PO3− fragmentation ion intensities were highest at the LNMO surface and decreased to background level after approximately 3−5 nm. These fragments were likely generated from the surface species with carbon backbones or POx structures, and the decreasing trend indicates that decomposition products of a similar organic nature were present mostly on the LNMO electrode top surface. In Figure 7b, 6Li+, Ni+, and Mn+ ions, which represent cumulative signals from various Li/Ni/Mn-containing species (i.e., metal oxides and fluorides), showed different trends from

Figure 7. TOF-SIMS depth profiles of various chemical species from the surface of aged LNMO electrode with 100% SOC after storage in the electrolyte at 60 °C for 60 days.

the surface to the bulk of LNMO. Unlike Li+ and Ni+ profiles, Mn+ intensity was lower at the LNMO surface but increased to a constant level after ∼10 nm. The low Mn intensity at the surface can be understood to occur as the result of Mn dissolution from LNMO. Similarly, an earlier XPS study reported the Mn deficiency at the surface of aged LiMn2O4 electrodes after storage at 60 °C for 300 h.33 The Ni+ intensities were similar through the depth measured. However, given that the sensitivity of TOF-SIMS to Ni+ is less than to Mn+, this trend may not be meaningful. XPS data from a cycled LNMO electrode sample clearly showed Ni deficiency at the LNMO surface (Figure 4). The Mn deficiency at the surface of LNMO 15952

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Figure 8. (a) High-resolution TEM image showing a nanoparticle (indicated by a dotted white circle, about 5 nm in diameter) deposited on the surface of the aged LNMO electrode with 100% SOC after storage in the electrolyte at 60 °C for 60 days. (b) Fast Fourier transform (FFT) patterns from region 1 (highlighted particle in red box 1). (c) FFT patterns from region 2 (red box 2). (d) Calculated diffraction pattern of MnF2 along [101] zone axis.

nanoparticle, which can be indexed as MnF2 along the [101] zone axis. The MnF2 phase was further confirmed by calculating the electron diffraction pattern of MnF2 [101] zone axis (Figure 8d), which is in agreement with the experimental diffractogram (Figure 8b). From this analysis as well as the above TOF-SIMS results, it can be concluded that MnF2 phase is present at the surface of LNMO electrode. In addition to the surface analysis, bulk structures of aged LNMO electrodes were analyzed after storage in electrolyte at 60 °C for 60 days. The structures of aged LNMO electrodes with initial SOC values of 25%, 50%, 75%, and 100% were determined by XRD, and their representative patterns are shown in Figure 9. The lattice parameters from the aged LNMO electrodes were determined to be in the range a = 8.158−8.162 Å, which is very close to that of 0% SOC (8.177 Å). Regarding the initial lattice parameters of LNMO electrodes at each SOC [i.e., between 8.177 Å (0% SOC) and 7.990 Å (100% SOC)], our results suggest that partly or fully delithiated LNMO electrodes were relithiated during storage in electrolyte at 60 °C. Since there was no electrical circuit connected to the electrode, it can be understood to occur at the expense of electrolyte oxidation to compensate charge neutrality as follows:

can be seen as a result of Mn dissolution from the LNMO to electrolyte. As shown in Figure 7c, LiF, MnF2, and NiF2 species were found to have the largest intensity within ca. 5 nm of the surface, between the organic decomposition products (Figure 7a) and LNMO. As the electrode surface was analyzed, these intensities need to be considered as average values and may include particle-to-particle variation. MnF3−, NiF3−, and LiF2− are ionization products from MnF2, NiF2, and LiF, respectively, which are generated during the SIMS analysis. Benedek and Thackeray31 proposed a Mn dissolution mechanism based on the presence of HF. Similarly, in the LiNi0.5Mn1.5O4 spinel, the Mn and Ni dissolution reaction in the presence of HF can be proposed as follows: 2LiNi 0.5Mn1.5O4 + 4H+ + 4F− → 3Ni 0.25Mn 0.75O2 + 0.25NiF2 + 0.75MnF2 + 2LiF + 2H 2O

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TEM analysis was performed for the lithiated LNMO electrode (100% SOC) surface after storage in electrolyte at 60 °C for 60 days. The high-resolution TEM image (Figure 8a) shows the atomic planes with an interspacing of 2.46 Å (also see diffractogram in Figure 8c), which belongs to the {311} planes of LNMO phase. Also shown in this image (Figure 8a) is a nanoparticle (approximately 5 nm in diameter) overlapping with the LNMO phase. Figure 8b is a diffractogram of this

Ni 0.5Mn1.5O4 + x Li+ + x electrolyte → LixNi 0.5Mn1.5O4 + x electrolyte+ 15953

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al.11 investigated the effect of electrochemical potential on the oxidation of EC at the LNMO electrode. Their results showed that poly(ethylene carbonate) (PEC) was formed on the electrode surface when the voltage increased above 4.7 V (vs Li).11 It was also reported that PF5 from the LiPF6, a strong Lewis acid, catalyzes ring opening of EC, followed by its polymerization into poly(ethylene oxide) (PEO)-like products.14,37 Although the relationship between thermal and electrochemical decomposition of the electrolyte is not yet fully understood, Sloop et al.14 reported a similar collection of decomposition products. From the literature, the following EC decomposition reaction may occur: −CO2

EC ⎯⎯⎯⎯→ PEC ⎯⎯⎯⎯⎯→ PEO LiPF6

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The chemical compositions of aged electrolytes were determined by GC-MS after storage with LNMO electrodes under different conditions: state of charge (SOC), temperature, and storage time. Representative chromatograms are illustrated in Figure 10. The chromatograms were reconstructed by

Figure 9. Representative XRD patterns of aged LNMO electrodes (25% and 100% SOC) after storage in the electrolyte at 60 °C for 60 days. The XRD pattern of fresh LNMO powder is illustrated for comparison.

A recent work also indicated the occurrence of self-discharge of LiNi0.4Mn1.6O4 high-voltage spinel.34 In addition, selfdischarge has been reported to occur in other types of positive electrode materials such as LiMn2O4 and LiNi0.8Co0.2O2.35,36 Therefore, it can be regarded that for a higher SOC in LNMO, a larger amount of electrolyte oxidation can be expected on the basis of our results. The oxidation of either EC or DEC (or both) solvent in the presence of LiPF6 was simply denoted here as electrolyte+ because the resulting chemical species and formation mechanism is not clearly understood in the literature. However, it will be discussed further, based on our results, in a following section. In Figure 5c, the amount of Mn and Ni dissolution from LNMO electrode became steady after storage for 40 days at 60 °C. At the given storage time (60 days) and temperature (60 °C), the amount of Mn and Ni dissolution decreased significantly with decreasing SOC; 1 order of magnitude decrease of Mn and negligible dissolution of Ni at 0% SOC in comparison with those of 100% SOC. The XRD data (Figure 9) show that lithiated LNMO (25−100% SOC) experienced relithiation (by self-discharge) until it came close to 0% SOC during storage in electrolyte at 60 °C. These results suggest that changes in the chemical composition of LixNi0.5Mn1.5O4 (0 ≤ x ≤ 1) influence the electrolyte decomposition reaction and consequent amount of HF, and so too the amount of Mn and Ni dissolution as observed in Figure 5a,c. In addition, SIMS and TEM analyses found that the surface of LNMO became rich in metal fluorides (LiF, MnF2, NiF2) as side reaction products from eq 6 after storage in electrolytes. Therefore, the amount of metal fluoride production will increase as more Mn and Ni dissolution reactions progress. Since these metal fluorides are stable against HF attack, they can act as coated barriers at the LNMO/electrolyte interphase and retard the dissolution reactions. It is assumed that the above two reactions may occur in parallel during the storage tests. Electrolyte Analysis. The thermal and electrochemical decomposition of electrolytes has been investigated for the past decade due to its severe impact on Li-ion battery performance. Recent activities have focused on high-voltage electrolytes because the highest occupied molecular orbital (HOMO) of current standard electrolytes (ca. ∼4.5 V vs Li) cannot meet the stability requirement for the LNMO (ca. ∼5 V vs Li).16 Yang et

Figure 10. Representative chromatograms for fresh and aged electrolyte samples after storage with LNMO electrodes with 100% SOC at 60 °C. Chromatograms were constructed from total ion current GC-MS data.

plotting the total ion current from each recorded spectrum. The samples contained DEC, EC (from electrolyte), and MeOH (employed during sample preparation) as major components. The presence of EtOH and diethyl ether was confirmed by the mass spectral data (see Supporting Information). For example, the recorded mass spectrum matched very well with the library reference spectrum for diethyl ether (Et2O). In addition, trace amounts of dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) were found in all electrolyte samples. The recorded mass spectra for DMC and EMC matched very well with library reference spectra (see Supporting Information). It is assumed that DMC and EMC were impurities in the pristine electrolyte. Here, components labeled as A−G were analyzed by mass spectrometry, and detailed analysis results are discussed in the Supporting Information. In general, components A−G appear to be artifacts from the chemical analysis procedure or the electrolyte manufacturing process because the organosilicon (peaks A, B, and G) and organophosphorus (peaks C, D, and F) compounds are found in both fresh and aged electrolyte samples. 15954

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Table 1 lists the relative amounts (as weight percentages) of EC and DEC in aged electrolytes, determined from the GC-MS

Table 2. Contents of EtOH, Et2O, Mn, and Ni Dissolution in Aged Electrolyte Samplesa

Table 1. Solvent Compositions of Aged Electrolyte Samples Determined by GC-MSa temp (°C)

SOC (%)

storage time (days)

EC (wt %)

fresh electrolyte

40.4

50 100

60 60

100 100 100

30 60 80

100 100

60 60

Variable State of Charge 60 39.5 60 43.0 Variable Temperature 60 40.2 60 43.0 60 42.1 Variable Storage Time 8 42.1 60 43.0

SOC (%)

temp (°C)

storage time (days)

60 60

60 60

DEC (wt wt ratio EC/ %) DEC 59.6

0.68

50 100

60.5 57.0

0.65 0.75

100 100 100

30 60 80

60 60 60

59.8 57.0 57.9

0.67 0.75 0.73

100 100 100

60 60 60

8 15 60

content of EtOH (ppm)

content of Et2O (ppm)

Variable State of Charge b b 110 160 Variable Temperature b b 110 160 10 300 Variable Storage Time 10 50 120 90 110 160

Mn dissolution (ppm)

Ni dissolution (ppm)

700 3000

250 1675

850 3000 3750

400 1675 2150

500 1700 3000

300 850 1675

a

57.9 57.0

Determined after storage of LNMO electrodes (50% and 100% SOCs) under different conditions. bBelow the detection limit of GCMS.

0.73 0.75

a

Determined after storage of LNMO electrodes (50% and 100% SOCs) under different conditions. Formulation of fresh electrolyte was EC/DEC = 1/2 by volume.

negligible amounts of EtOH and Et2O with less DEC decomposition in Tables 1 and 2. In contrast, the 100% sample contained much larger amounts of EtOH and Et2O with severe DEC decomposition. This result can be partly understood by the oxidation of electrolyte and the resulting self-discharge of LNMO in eq 8. It should be noted that the LNMO electrode needs to accept free electrons from the electrolyte for the oxidation process. From this consideration, our data suggest that LixNi0.5Mn1.5O4 acts as an electron acceptor, reducing Ni4+ and Mn4+ via spontaneous lithiation (self-discharge) process. For example, particles of Ni0.5Mn1.5O4 (100% SOC) will experience more electrolyte oxidation compared with those of Li0.5Ni0.5Mn1.5O4 (50% SOC) through the self-discharge process. However, it is difficult to confirm the relationship between self-discharge and catalytic activity of delithiated spinels. Furthermore, the influence of lithiation of Mn2O4 (λ-MnO2) or Ni0.5Mn1.5O4 (isostructural with λ-MnO2) on their catalytic activities for electrolyte decomposition and/or EtOH oxidation is yet unknown. It is possible that such lithiation will weaken the catalytic activity of the λ-MnO2 as observed from the LMO system.18 Another possible reaction for Et2O generation is via an alkyl−oxygen cleavage reaction as follows:38

data. The relative amounts of EC and DEC consumption via decomposition can be derived from their weight ratio. The EC mainly decomposed at 30 °C, while DEC decomposition becomes severe at 60 and 80 °C (100% SOC in LNMO, 60 days). This is because the decomposition rate of DEC is slow compared with that of EC at lower temperatures but increases sharply with temperature and becomes nonnegligible at elevated temperatures.14 At 60 °C, the amount of DEC decomposition increased moderately. It is interesting to note that 100% SOC in LNMO led to more decomposition of DEC than at 50% SOC under the same conditions (60 °C for 60 days). The generation of EtOH has been reported as a decomposition product of DEC.17,38 Aoshima et al.18 suggested a decomposition mechanism of DEC at the presence of λMnO2 acting as a catalyst: [Mn]

DEC ⎯⎯⎯⎯→ EtOH + CO2 + C2H4 LiPF6

(10)

It has been reported that various manganese oxides show catalytic activity for oxidation of various organic species, including alcohols such as EtOH and MeOH. The catalytic activity of MnOx-based catalysts (including λ-MnO2) is attributed to the ability of manganese to form oxides with different oxidation states and their high oxygen storage capacity.39,40 Therefore, a dehydration reaction of EtOH can explain the production of Et2O in the presence of HF and Ni0.5Mn1.5O4 as a possible reaction catalyst:41,42

−CO2

DEC + −OEt → EtOCOO− + Et 2O ⎯⎯⎯⎯⎯→ −OEt + Et 2O (12)

In this case, the formation of Et2O accompanies CO2 gas generation and regenerates the equivalent amount of ethoxide, which is reactive toward further DEC decomposition. On the other hand, Kawamura et al.43 proposed LiPF6/DEC electrolyte decomposition reactions. Among the possible reaction mechanisms, the following two reactions involve PF5 attack on the carbonyl oxygen in DEC or C2H5OCOOPF4 and subsequent production of HF:

Ni 0.5Mn1.5O4

2EtOH ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Et 2O + H 2O HF

(11)

The water produced will react with LiPF6 and generate additional HF in the aged electrolyte (see eqs 1, 2, and 4). It is noteworthy that the concentrations of EC, DEC, EtOH, and Et2O in the aged electrolyte samples were strongly dependent on the SOC of LNMO; see Tables 1 and 2. The potential dependence (thus SOC dependence) of EC decomposition on LNMO has been suggested in literature.11 On the basis of our results, eqs 10 and 11 suggest that DEC decomposition may also depend on the SOC in LNMO electrode. The 50% SOC electrolyte sample contained

DEC + PF5 → C2H5OCOOPF4 + C2H4 + HF

(13)

C2H5OCOOPF4 → PF3O + CO2 + C2H4 + HF

(14)

These reactions may occur in parallel with those in eqs 11 and 12. At a given SOC (100%) and storage time (60 days), both the EtOH and Et2O content increased from 30 to 60 °C in Table 2. However, further increases in temperature up to 80 °C 15955

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The Journal of Physical Chemistry C dramatically reduced the EtOH content with increasing Et2O. This can be explained by the fact that the dehydration reaction of EtOH accelerates at elevated temperatures (see eq 11).41 At a given SOC (100%) and temperature (60 °C), the EtOH content increased rapidly from 8 to 15 days but again slightly decreased after 60 days. During this period, Et2O content kept increasing with storage time (from 8 to 60 days) although the rate of increase became slower, similar to the Mn and Ni dissolution behavior in Figure 5c. This may be explained by the slow reaction rates of eq 11 when compared with a DEC decomposition reaction (eq 10) at 60 °C. In general, Table 2 revealed that Et2O content increased with SOC, temperature, and storage time, following the same trend of Mn and Ni dissolution. Since the generation of Et2O results in the production of H2O via eq 11, it will increase the HF concentration (eqs 2 and 4) and subsequent dissolution of Mn and Ni.

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ACKNOWLEDGMENTS

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REFERENCES

We thank Andrew Zabik in Engineering Operations, GM Global Powertrain, and Misle Tessema in the Chemical & Materials Systems Laboratory for XRF and XRD analyses, respectively. We also thank Brian J. Koch, Jeffrey R. Hanba, and Qian Lin in Global Vehicle Engineering and Li Yang and Eric W. Schneider in GM R&D for many helpful discussions.

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CONCLUSION 1. The LNMO/graphite full-cells suffer from poor cycle life due to the Mn and Ni dissolution problem. Metallic Mn and MnNi particles reduced on the graphite surface (as evidenced by TEM) will form thicker SEI by consuming active Li+, which results in rapid capacity fading of the full-cells. 2. The amount of Mn and Ni dissolution from LNMO increased with SOC, temperature, and storage time. Under a given condition, the dissolution amount accelerated with SOC, while it decelerated and finally became steady-state with storage time. 3. As a result of dissolution, aged LNMO showed Mn and Ni deficiencies and decomposition products (LiF, MnF2, and NiF2) on its surface after storage in electrolyte at elevated temperatures. In addition, the aged LNMO electrodes experienced self-discharge during storage in electrolyte, as evidenced by XRD data. 4. Chemical analysis results showed that concentrations of EC, DEC, EtOH, and Et2O in the aged electrolyte samples were strongly dependent on temperature, SOC, and storage time. The DEC decomposition into EtOH and Et2O was prominent at high temperature and SOC. 5. The catalytic activity of delithiated LNMO for electrolyte decomposition may be partly related to the self-discharge process that accompanies electrolyte oxidation. The Mn and Ni dissolution behavior in LNMO showed similar trends as that of the Et2O generation. This can be explained by HF production through dehydration of EtOH, which in turn influences the Mn and Ni dissolution.

ASSOCIATED CONTENT

S Supporting Information *

Additional text and four figures with detailed discussion about GC and GC-MS data of electrolyte samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest. 15956

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