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Improved Performance of LiNi0.5Mn1.5O4 Cathodes with Electrolytes Containing Dimethylmethylphosphonate (DMMP) Mengqing Xu, Dongsheng Lu, Arnd Garsuch and Brett L. Lucht J. Electrochem. Soc. 2012, Volume 159, Issue 12, Pages A2130-A2134. doi: 10.1149/2.077212jes Email alerting service Receive free email alerts when new articles cite this article - sign up in the box at the top right corner of the article or click here To subscribe to Journal of The Electrochemical Society go to: http://jes.ecsdl.org/subscriptions © 2012 The Electrochemical Society A2130 Journal of The Electrochemical Society, 159 (12) A2130-A2134 (2012) 0013-4651/2012/159(12)/A2130/5/$28.00 © The Electrochemical Society Improved Performance of LiNi0.5 Mn1.5 O4 Cathodes with Electrolytes Containing Dimethylmethylphosphonate (DMMP) Mengqing Xu,a,∗ Dongsheng Lu,a Arnd Garsuch,b and Brett L. Luchta,∗∗,z a Department of Chemistry, University of Rhode Island, Kingston, b BASF SE, GCN/E, Ludwigshafen am Rhein, 67056, Germany Rhode Island 02881, USA The cycling performance of Li/LiNi0.5 Mn1.5 O4 cells with 1.0 M LiPF6 in ethylene carbonate (EC)/ethyl methyl carbonate (EMC) (3:7) with and without added dimethyl methylphosphonate (DMMP) (0.5–1.0%) was investigated. Addition of DMMP resulted in improved capacity retention during cycling to high voltage (4.9 V vs Li). Ex-situ surface analysis of LiNi0.5 Mn1.5 O4 electrodes after cycling via Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS) and Infrared (IR) spectroscopy suggest that addition of DMMP inhibits electrolyte decomposition on the surface of the cathode. Addition of DMMP also inhibits the dissolution of Mn from LiNi0.5 Mn1.5 O4 particles stored in electrolyte at 85 ◦ C. © 2012 The Electrochemical Society. [DOI: 10.1149/2.077212jes] All rights reserved. Manuscript submitted August 13, 2012; revised manuscript received September 10, 2012. Published October 23, 2012. Lithium ion batteries have the best gravimetric and volumetric energy density of commercial rechargeable batteries. The cathodes are typically composed of lithium transition metal oxides or phosphates and the anode is graphite, which provides a cell with a high operating voltage (3 ∼ 4.5 V) and high capacity (150 ∼ 240 mAh/g).1 Due to the high energy density, lithium ion batteries are being extensively pursued for transportation applications, such as hybrid electric vehicles, plug-in hybrid electric vehicles, and fully electric vehicles. However, cost, safety, cycle life, energy and power are some of the major hurdles for implementation of lithium ion batteries into transportation applications.2 In pursuit of higher energy density lithium ion batteries, there is growing interest in cathode materials that operate at high voltages (>4.5 V vs Li) and exhibit higher energy densities. Great progress has been achieved in developing new cathode materials, including LiNi0.5 Mn1.5 O4 (4.9 V)2,3 and Li2 MnO3 -LiMO2 (4.6 V).4,5 However, a major difficulty in utilizing these high-voltage cathodes is the instability of the organic electrolytes in contact with the cathode surface at high operating voltages (>4.5 V).2,6 Significant effort has been directed to inhibit the detrimental reactions of the LiNi0.5 Mn1.5 O4 with electrolyte at high voltage, including surface coating with various inorganic oxides, including Al2 O3 , ZnO, and Bi2 O3 .7–9 The surfacecoated cathodes have superior cyclability compared to the uncoated material, however; the surface-coating method has a negative effect on the discharge capacity of the material. Other methods of improving the performance include doping with other transition metals such as Fe10 and Cr11 and the incorporation of electrolyte additives.12–14 The development of a thorough understanding of the reactions of electrolyte with the surface of high voltage cathode materials and the development of novel electrolytes capable of long term reversible cycling to high voltage is of great current interest. Recently, Xing et al investigated the oxidative decomposition mechanism of PC with and without PF6 − and ClO4 − .15 They found that the presence of PF6 − and ClO4 − anions significantly reduces PC oxidation stability, stabilizes the PC-anion oxidation decomposition products, and changes the order of the oxidation decomposition paths. Interestingly, they also found that HF and PF5 were detected upon the initial step of PC-PF6 − oxidation while HClO4 formed during initial oxidation of PC-ClO4 − .16 In addition, it has been reported that the thermal instability of LiPF6 in carbonate solvents leads to the generation of acidic species including PF5 and HF.17 The presence of acidic species in the electrolyte can lead to corrosion of the surface of the metal oxide and dissolution of transition metals.18 This is especially problematic with LiMn2 O4 or LiNi0.5 Mn1.5 O4 .18,19 However, the incorporation of Lewis basic species can inhibit the generation of acidic species and metal oxide surface corrosion.18,20,21 ∗ Electrochemical Society Student Member. Electrochemical Society Active Member. z E-mail: [email protected] ∗∗ As a expansion of our investigation of Lewis basic additives designed to thermally stabilize LiPF6 electrolytes,20,21 we have investigated, dimethyl methylphosphonate (DMMP), as an electrolyte additive to improve the performance of LiNi0.5 Mn1.5 O4 cathode material cycled to high voltage (4.9 V vs Li). DMMP has previously been investigated as a non-flammable co-solvent which is compatible with the electrode materials.22–24 The electrochemical performance of Li/LiNi0.5 Mn1.5 O4 cells with DMMP containing electrolyte has been investigated and the surface of LiNi0.5 Mn1.5 O4 electrodes have been characterized to provide a better understanding of differences in performance. Experimental Battery grade solvents, ethylene carbonate (EC) and ethyl(methyl)-carbonate (EMC), and lithium hexafluorophosphate (LiPF6 ) were provided by Novolyte Technologies (U.S.). Dimethyl methylphosphonate (DMMP) was purchased from Aldrich, followed by distilling and soaking with 4Å molecule sieves before use. 1.0 M LiPF6 EC/EMC (3/7, v/v) electrolyte was prepared as the standard electrolyte (STD). The concentration of DMMP was 0.5% and 1% (wt.), respectively. LiNi0.5 Mn1.5 O4 powder is provided by Hunan Shanshan advanced materials Co. LTD (China). The cathode electrode is composed of 89% LiNi0.5 Mn1.5 O4 , 6% conductive carbon, and 5% PVDF. 2032-type coin cells were assembled with lithium foil anode, LiNi0.5 Mn1.5 O4 cathode, and Celgard 2325 separator. Each cell contains 30 μL electrolyte. Cells were built in triplicate with good reproducibility. The cells were cycled with a constant current-constant voltage charge and constant current discharge between 3.5 to 4.9 V with Arbin BT2000 cycler according to following protocol: 1st cycle at C/20; 2nd and 3rd cycles at C/10; remaining cycles at C/5. All cells were produced in triplicate and representative data is provided. After 50 cycles the cells were opened and the LiNi0.5 Mn1.5 O4 cathodes were extracted, washed with anhydrous DMC three times to remove the residual EC and LiPF6 salt, and vacuum dried overnight at room temperature for XPS, SEM, and IR analysis. The samples for thermal storage were prepared in a high purity argon filled glove-box as described previously.17 The vials were charged with 0.1 g LiNi0.5 Mn1.5 O4 powder followed by addition of 2 mL standard and 1% DMMP electrolyte, respectively. The vials were flame sealed under reduced pressure. Care was given to avoid contamination of the vial walls near the sealing point. The sealed samples were transferred to an oil bath of 85◦ C for 4 days. Samples were weighed before and after thermal storage to confirm seal. After thermal storage, vials were opened in an argon filled glove-box. The solid cathode LiNi0.5 Mn1.5 O4 sample was separated from the electrolyte and then washed with dimethyl carbonate (DMC) three times followed by drying in vacuum. The resulting cathode particles were analyzed by scanning electron microscopy (SEM) and X-ray photoelectron Journal of The Electrochemical Society, 159 (12) A2130-A2134 (2012) STD 0.5% DMMP 1% DMMP 120 98 96 94 100 92 80 STD 0.5% DMMP 1% DMMP 60 Efficiency, % 140 Capacity, mAh/g Results and Discussion 100 160 90 88 0 5 10 15 20 25 30 35 40 45 A2131 50 Cycle Number Figure 1. Cycling performance of Li/LiNi0.5 Mn1.5 O4 cells with standard (STD) and DMMP containing electrolyte. spectroscopy (XPS). The residual electrolyte solutions were analyzed by ICP-MS to determine Mn and Ni content. The XPS spectra were acquired with PHI 5500 system using Al Kα radiation (hν = 1486 eV) under ultrahigh vacuum. Lithium was not monitored due to its low inherent sensitivity and small change of binding energy. Calibration of XPS peak position was made by recording XPS spectra for reference compounds, which are present on the electrode surface: LiF, PVDF, and lithium alkyl carbonate. The graphite peak at 284.3 eV was used as a reference for the final adjustment of the energy scale in the spectra. The spectra obtained were fitting using XPS peak software (version 4.1). Linear syntheses of elemental spectra were conducted using Gaussian-Lorentzian (80:20) curve fit with Shirley background subtraction. Scanning electron spectroscopy (SEM) was conducted on a JEOL-5900 SEM. FTIR spectra were acquired on a Bruker (Tensor 27) with attenuated total reflection (ATR) mode with 4 cm−1 resolution and a germanium crystal and 128 scans. Cycling performance of Li/LiNi0.5 Mn1.5 O4 cells.— The cycling performance of Li/LiNi0.5 Mn1.5 O4 cells with standard electrolyte (1.0 M LiPF6 in EC/EMC (3:7)) and electrolyte containing 0.5% and 1% added DMMP after initial formation cycles are depicted in Figure 1. The cells were cycled from 3.5 to 4.9 V vs. Li. The cell with 0.5% DMMP electrolyte shows slightly higher initial discharge capacity than the other cells, 129.1 mAh/g compared to 122.9 mAh/g (standard) and 120.5 mAh/g (1% DMMP). This higher initial discharge capacity is ascribed to the higher coluombic efficiency of the cell cycled with electrolyte containing 0.5% DMMP during formation cycles. Cycling cells at room temperature results in moderate capacity fade for all cells. However, the cells with DMMP added electrolyte have superior capacity retention to the cells containing standard electrolyte after 50 cycles, 76.8%, 70.0%, and 64.8% capacity retention, for 0.5% DMMP, 1% DMMP, and standard electrolyte, respectively. Scanning electron microscopy (SEM) of LiNi0.5 Mn1.5 O4 electrodes.— The SEM images of LiNi0.5 Mn1.5 O4 cathodes fresh and after 50 cycles with standard electrolyte and DMMP containing electrolytes are depicted in Figure 2. Aggregated cathode particles and conductive carbon are present on the fresh electrode. The aggregated particle is composed of primary particles with octahedral structure, ranging from 1–5 μm (Figure 2a). After 50 cycles, the surface of LiNi0.5 Mn1.5 O4 particles are altered. The cathode particles cycled with standard electrolyte have a smooth surface and it is difficult to distinguish the primary particles (Figure 2b). The cells cycled with the DMMP containing electrolytes have surfaces that more resemble the fresh cathode that the cathode cycled with standard electrolyte. X-ray photoelectron spectroscopy (XPS) analysis of LiNi0.5 Mn1.5 O4 electrode.— In order to better understand the changes to the surface of the LiNi0.5 Mn1.5 O4 electrode upon cycling at high voltage, electrodes cycled with standard electrolyte and DMMP containing electrolyte were analyzed by XPS. Table I contains the elemental concentrations of LiNi0.5 Mn1.5 O4 electrode before and after cycling. After 50 cycles, the concentrations of C, F and Mn are decreased; while O and P are increased. The changes to Figure 2. SEM images of LiNi0.5 Mn1.5 O4 electrodes: fresh electrode (a); after 50 cycles with standard electrolyte (STD) (b); after 50 cycles with 0.5% DMMP containing electrolyte (c); and after 50 cycles with 1% DMMP containing electrolyte (d). A2132 Journal of The Electrochemical Society, 159 (12) A2130-A2134 (2012) Table I. Relative elemental concentrations of LiNi0.5 Mn1.5 O4 electrode before and after cycling. fresh STD 0.5% DMMP 1% DMMP C1s% O1s% F1s% 48.2 46.9 47.9 45.1 12.0 18.5 16.8 19.6 28.7 26.4 25.0 24.7 P2p% Mn2p% Ni2p% 2.3 0.7 0.3 7.5 1.5 4.4 5.3 3.5 4.4 5.2 4.9 1% DMMP 0.5% DMMP STD the surface concentrations are consistent with the presence of a thin surface layer. The XPS element spectra are provided in Figure 3. The C 1s XPS spectrum of the fresh LiNi0.5 Mn1.5 O4 electrode contains three peaks. The peak at 284.3 eV is assigned to conductive carbon, and the peaks at 285.7 and 290.4 eV are characteristic of the PVDF binder. The F1s peak of PVDF is also observed at 687.6 eV. The O 1s spectrum of fresh LiNi0.5 Mn1.5 O4 electrode is dominated by metal oxide (529 eV), and contains a very low concentration of Li2 CO3 (531.6 eV), which is frequently present on fresh cathode particles.18 After 50 cycles, new species corresponding to the electrolyte decomposition products are formed on the electrode surface, as observed in C 1s, O 1s, F 1s, and P 2p XPS spectra. Analysis of the XPS spectra reveals significant difference between the electrode cycled in standard electrolyte and DMMP added electrolyte. In addition to the peaks present on the fresh electrode, new peaks consistent with C-O (286 eV) and C=O (288 eV) containing species are present in the C 1s spectrum of the LiNi0.5 Mn1.5 O4 electrode cycled with either standard or DMMP containing electrolyte. The corresponding peaks for C=O (532–533 eV) and C-O (533–534 eV) containing species are observed in the O1s spectrum. C-O and C=O species are consistent with the presence of lithium alkyl carbonates and polycarbonates. The intensity of the peak of the metal oxide in the O 1s XPS spectrum is greater for the electrodes cycled with electrolyte containing 0.5% DMMP or 1% DMMP than electrodes cycled with standard electrolyte, suggesting a thinner surface layer formed on the electrode surface with electrolytes containing DMMP. The F 1s spectra contain new peaks at 684.5 eV characteristic of LiF. The intensity of the LiF peak is strongest for the electrode cycled with the standard electrolyte and is much weaker for the electrodes cycled with DMMP containing electrolytes. The P 2p spectra are similar for all cycled electrodes and contain a peak at 2200 C-C F-C 2000 1800 1600 1400 1200 1000 Wavenumbers, cm Figure 4. FTIR-ATR spectra of fresh LiNi0.5 Mn1.5 O4 electrode, cycled with standard electrolyte (STD), and DMMP containing electrolyte. ∼134 eV characteristic of Lix PFy Oz . The similarity of the P2p spectra suggests that the DMMP does not react on the surface of the cathode. The results discussed above suggest that the surface layer formed on the LiNi0.5 Mn1.5 O4 electrode in DMMP containing electrolyte is thinner than that formed with standard electrolyte. The thinner surface layer formed on LiNi0.5 Mn1.5 O4 electrode with DMMP containing electrolyte correlates with the improvement of cycling performance to 4.9 V as shown in Figure 1. FTIR-ATR of LiNi0.5 Mn1.5 O4 electrode.— Ex-situ IR spectra of LiNi0.5 Mn1.5 O4 electrodes are depicted in Figure 4. The IR spectrum of a fresh LiNi0.5 Mn1.5 O4 electrode and electrodes after cycling are dominated by absorptions of the PVDF binder at 1400, 1180, 879 cm−1 . After 50 cycles, additional absorptions are present in the IR spectra at 1768 and 1582 cm−1 corresponding to polycarbonates and lithium alkyl carbonates, respectively,6 which are associated with the oxidative decomposition products of the electrolyte at high voltage and consistent with the XPS data. O 1s P 2p O-M LixPO yFz C=O fresh C-O LixPO yFz C=O 800 -1 F 1s C 1s PVDF fresh LiF LixPFy STD 0.5% DMMP 1% DMMP Figure 3. C 1s, F 1s, O 1s and P 2p XPS spectra of LiNi0.5 Mn1.5 O4 electrode before and after cycled with standard (STD) and DMMP added electrolyte. Journal of The Electrochemical Society, 159 (12) A2130-A2134 (2012) A2133 Figure 5. SEM images of LiNi0.5 Mn1.5 O4 powder before and after thermal storage: fresh powder (a); storage in 1.0 M LiPF6 EC:EMC (3/7) at 85◦ C for 4 days (b); and storage in 1.0 M LiPF6 EC:EMC (3/7) + 1.0% DMMP at 85◦ C for 4 days (c). Scanning electron microscopy (SEM) of LiNi0.5 Mn1.5 O4 particles before and after thermal storage.— In order to better understand the mechanism for the improvement of the cycling performance and inhibition of cathode surface film generation in the presence of DMMP, the reactions of fresh uncycled cathode particles with electrolyte with and without added DMMP at elevated temperature (85◦ C) was conducted. LiPF6 electrolytes are unstable at elevated temperature resulting the generation of PF5 , HF, and other acidic flourophophate species.17 These acidic species can react with the surface of the metal oxide cathode particles resulting in the deposition of electrolyte decomposition products and dissolution of the transition metal ions.18 Lewis basic species, related to DMMP, have been reported to inhibit the thermal decomposition of LiPF6 and slow cathode surface reactions and transition metal dissolution.18,21 Figure 5 shows the SEM images of LiNi0.5 Mn1.5 O4 powder before and after storage at 85◦ C in the presence of electrolyte. The structure of the primary particles is severely damaged upon storage in the presence of standard electrolyte at 85◦ C for 4 days (Figure 5b). The cathode surface appears etched and significant deposition of electrolyte decomposition products has occurred. The etching of the metal oxide is ascribed to the generation of HF, PF5 , and related fluorophosphates at elevated temperature due to the thermal dissociation of LiPF6 . The SEM images of the LiNi0.5 Mn1.5 O4 particles stored at elevated temperature in the presence of electrolyte containing DMMP electrolyte reveal much less etching of the cathode surface and deposition of electrolyte decomposition products (Figure 5c). Addition of DMMP inhibits the thermal decomposition of LiPF6 /carbonate electrolytes which likely protects the cathode surface from damage during storage at elevated temperature.22 C 1s C-O fresh O 1s C-H C=O Table II. Relative concentrations of elements LiNi0.5 Mn1.5 O4 powder before and after thermal storage. C1s% O1s% 16.1 31.6 10.7 45.9 31.0 31.3 fresh STD 1% DMMP F1s% 18.8 29.1 P2p% Mn2p% Ni2p% 2.1 1.0 31.6 10.4 18.3 6.4 6.1 9.6 XPS spectra of LiNi0.5 Mn1.5 O4 powder before and after thermal storage.— The XPS spectra of fresh uncharged LiNi0.5 Mn1.5 O4 powder before and after storage at 85◦ C for 4 days with standard electrolyte and electrolyte containing 1% DMMP are provided in Figure 6. The C 1s spectrum of the fresh sample contains the universal C-H contamination peak at 284.8 eV, and some C-O and C=O containing species. The O 1s spectrum of the fresh sample is dominated by the metal oxide peak at 529.5 eV, but also contains low concentrations of C-O and C=O containing species, consistent with the C1s spectra and the presence of Li2 CO3 a commonly found surface species on lithiated metal oxides.18 After storage at elevated temperature, the LiNi0.5 Mn1.5 O4 powder is covered by electrolyte decomposition products, as shown in Figure 6, including C-O and C=O species characteristic of lithium alkyl carbonates from the decomposition of the carbonate solvents and LiF and Lix POy Fz from the thermal decomposition of LiPF6 . The relative elemental concentrations presented in Table II also suggest that the surface of LiNi0.5 Mn1.5 O4 powder is covered with electrolyte decomposition products. The concentration of F is significantly increased; while the concentrations of O and Mn are P 2p F 1s O-M Li2CO3 O-C LixPFy LixPOyFz LiF STD 1% DMMP 292 290 288 286 284 282 280 Binding Energy (eV) 540 538 536 534 532 530 528 526 Binding Energy (eV) 696 694 692 690 688 686 684 682 Binding Energy (eV) 142 140 138 136 134 132 130 Binding Energy (eV) Figure 6. XPS spectra of LiNi0.5 Mn1.5 O4 powder before and after thermal storage, fresh powder (top), storage in 1.0 M LiPF6 EC:EMC electrolyte (middle), and storage in 1.0 M LiPF6 EC:EMC + 1% DMMP electrolyte (bottom) at 85◦ C for 4 days. A2134 Journal of The Electrochemical Society, 159 (12) A2130-A2134 (2012) Table III. Concentration of Mn and Ni dissolution in the electrolyte after thermal storage. STD 1% DMMP Mn dissolution (%) Ni dissolution (%) 0.9 0.7 0.32 0.21 decreased. The metal oxide peaks of the sample after thermal storage are still present, suggesting that the surface layer is thin, less than the depth of electron penetration (<5 nm). ICP-MS results.— After thermal storage, the electrolyte was analyzed with ICP-MS to monitor the concentration of Mn and Ni in the electrolyte. The percentage of Mn and Ni dissolved in the standard electrolyte is slightly higher than those in the 1% DMMP containing electrolyte as shown in Table III, which is consistent with the results discussed above, suggesting that DMMP can inhibit the dissolution of Mn and Ni at elevated temperature in LiPF6 -based electrolyte. Conclusions The effect of the addition of a novel Lewis base, dimethyl methylphosphonate (DMMP), to LiPF6 /carbonate electrolytes on the cycling performance of Li/LiNi0.5 Mn1.5 O4 cells cycled to 4.9 V vs Li has been investigated. Addition of DMMP improves the capacity retention upon cycling. Ex-situ surface analysis of the cycled electrodes suggests that the addition of DMMP inhibits the decomposition of electrolyte on the cathode surface and Mn dissolution from the cathode particles. The improved performance of the electrolyte is attributed to better thermal stability of the LiPF6 electrolyte and the inhibition of the generation of acidic decomposition products in the presence of DMMP.22 DMMP is a promising additive for improving the performance of high voltage spinel cathodes. Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, Subcontract No 6879235 under the Batteries for Advanced Transportation Technologies (BATT) Program. We also thank BASF SE for support of this work. M. 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