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Surface Modification of Ni-rich LiNi Co Mn O Cathode Material by Tungsten Oxide Coating for Improved Electrochemical Performance in Lithium Ion Batteries Dina Becker, Markus Börner, Roman Nölle, Marcel Diehl, Sven Klein, Uta C. Rodehorst, Richard Schmuch, Martin Winter, and Tobias Placke ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02889 • Publication Date (Web): 02 May 2019 Downloaded from http://pubs.acs.org on May 2, 2019
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Surface Modification of Ni-rich LiNi0.8Co0.1Mn0.1O2 Cathode Material by Tungsten Oxide Coating for Improved Electrochemical Performance in Lithium Ion Batteries Dina Becker 1, Markus Börner 1, Roman Nölle 1, Marcel Diehl 1, Sven Klein1, Uta Rodehorst 1, Richard Schmuch 1, Martin Winter 1,2, *, Tobias Placke 1, * 1 University
of Münster, MEET Battery Research Center, Institute of Physical Chemistry, Corrensstr. 46, 48149 Münster, Germany
Abstract Ni-rich NCM-based positive electrode materials exhibit appealing properties in terms of high energy density and low cost. However, these materials suffer from different degradation effects, especially at their particle surface. Therefore, in this work, tungsten oxide is evaluated as a protective inorganic coating layer on LiNi0.8Co0.1Mn0.1O2 (NCM-811) positive electrode materials for lithium ion battery (LIB) cells and investigated regarding rate capability and cycling stability under different operation conditions. Using electrochemical impedance spectroscopy (EIS), the interfacial resistance of uncoated and coated NCM-811 electrodes is explored to study the impact of the coating on the lithium ion diffusion. All electrochemical investigations are carried out in LIB full cells with graphite as negative electrode to ensure better comparability with commercial cells. The coated electrodes show an exellent capacity retention for the long-term charge/discharge cycling of NCM-811-based LIB full cells, i.e. 80% state-of-health after more than 800 cycles. Furthermore, the positive influence of the tungsten oxide coating on the thermal and structural stability is demonstrated using post-mortem analysis of aged electrodes. Compared to the uncoated electrodes, the surface modified electrodes show less degradation effects, such as particle cracking on the electrode surface and improvement of the thermal stability of NCM-811 in the presence of electrolyte. KEYWORDS lithium ion batteries; Ni-rich NCM cathode; NCM-811; surface modification, tungsten oxide coating; LIB full cell
1. Introduction Lithium ion batteries (LIBs) are the state-of-the-art battery technology for numerous applications, such as portable consumer electronics (cell phones, tablets, etc.) as well as hybrid (HEV), plug-in hybrid (PHEV) and full electric (EV) vehicles.1-2 Furthermore, a significantly increasing demand for LIBs is forecasted for the next years, which is mainly driven by the announcements of car manufacturers to intensify vehicle electrification and proposing various EV series. Besides high safety and low cost, one of the main challenges for the broad application of LIBs in the electromobility market, is to further increase of the specific energy (Wh kg-1) and energy density (Wh L-1) of the cell, which is directly correlated to the driving range.2-3 On the positive electrode (=cathode) side, layered lithium nickel cobalt manganese oxides LiNixCoyMnzO2 (NCM, x + y + z = 1) are predominant, due to their outstanding properties such as increased capacity and stability as well as lower material cost compared with LiCoO2,4-5 which has been introduced used as cathode material in the first generation of commercialized LIBs and is still used in LIBs for portable electronics.6-7 The current strategy for further increasing the energy density of LIBs, focuses on layered NCM-based cathode materials with high nickel content.8-10 Currently, LiNi0.5Co0.2Mn0.3O2 (NCM-523) and LiNi0.6Co0.2Mn0.2O2 (NCM-622) can be considered as state-of-the-art cathode materials for application in high-energy density lithium ion batteries, including EV batteries.2 In general, an increase in the Ni content leads to an enhanced capacity at the same cut-off potential, however, it is believed that the thermal stability will concurrently decrease in the opposite way and may result in safety issues.10-12 In this respect, Kasnatscheew et al. emphasized the dependency of the structural stability of various LiMO2 positive electrode materials on the respective Li+ extraction ratio, which is related to the specific charge capacity and charge cut-off potential.13 In particular, high Ni-content LiNixCoyMnzO2 materials with x ≥ 0.6 have attracted more attention recently.14 For example LiNi0.8Co0.1Mn0.1O2 (NCM-811)15, with a Ni content of 80% can deliver a discharge capacity of ≈200 mAh g-1 when charged to a potential of 4.3 V vs. Li/Li+. However, the high Ni content causes severe capacity fading of the Ni-rich materials. Due to similar ionic radii of Ni2+ (0.69 Å) and Li+ (0.76 Å), Ni2+ ions can occupy lithium sites, which is called “cation mixing”.16 This cation migration from the transition metal ions into the lithium layer leads to further problems, such as phase transformation and particle cracking.16-17 Amongst others, these challenges are hindering the broad application of Ni-rich materials in commercials cells. One of the challenges for a suitable NCM-811 material lies in the synthesis process of the Ni-rich material. In order to ensure that the highly ordered Ni-rich material is synthesized, an excess of lithium must be ensured to avoid 3 ACS Paragon Plus Environment
the Li/Ni cation mixing. Therefore, the unreacted Li at the active material surface can react with moisture to form lithium containing residual species, such as LiOH and Li2CO3.11, 18-19 These residues can further react with the non-aqueous electrolyte components to form an insulating LiF layer at the surface, resulting in an increased resistance of the positive electrode.16,
20
Furthermore, the reaction with the electrolyte can lead to the evolution of gaseous species, such as O2, CO2 and CO.21-22 Gas generation is a common phenomenon, leading to negative consequences for the performance and cycle life of LIBs.23-25 In addition, the highly reactive material surface in the delithiated state of the Ni-rich active materials can oxidize the electrolyte, which might result in active material loss and the formation of a thicker surface film,16 the socalled cathode electrolyte interphase (CEI).26 Due to the properties of Ni-rich cathode materials, such as a lower thermal stability in their highly delithiated state27 and high sensitivity towards air and moisture upon storage and processing,18 the surface chemistry and surface modification can have a significant impact on their electrochemical performance.16 Surface modification via a lithium-reactive tungsten oxide coating is a promising strategy to effectively stabilize the interface of the cathode active material with the electrolyte, reducing parasitic side reactions and improving electrochemical performance. The acid-based coating is added to diminish/eliminate the residual Li species (such as LiOH and Li2CO3) at the surface of the cathode material particles. In addition, vaporizable products (NH3 and H2O) are formed during the neutralization reaction.28 After mixing, these vaparizable products are removed during annealing and lithium tungsten oxide is formed on the active material surface. Consequently, Li impurities at the surface of the active material are removed, leading to improved rate capability, thermal stability and reduced internal resistance in LIB cells.22, 28-30 Other studies have shown, that the lithium-reactive coating diminished Li residual species at the active material surface and improved electrochemical performance. For example, Kim et al. used Co3(PO4)2 as a Li-reactive coating to form a LixCoPO4 coating layer on LiNi0.8Co0.16Al0.04O2 during the subsequent annealing procedure. During this coating procedure, Co3(PO4)2 reacted with superficial Li impurities, leading to a reduction of side reactions. The resulting coating layer suppressed transition metal dissolution and structural transformation to the spinel phase during storage at 90 °C.22 Lithium tungsten oxide (Li2WO4) is particularly suitable as an inorganic coating layer for LIB materials due to its beneficial properties such as high lithium ion conductivity, low toxicity and good thermal stability.28 Studies on other tungsten oxide modified (mostly doped) lithium cobalt oxide (LCO)- or different NCM-based cathode materials reported on enhanced diffusion of lithium ions in the modified materials, resulting in a reduction of interfacial resistance, 4 ACS Paragon Plus Environment
improving the electrochemical performance of LIB cells.31-35 In this respect, Hayashi et al. fabricated a lithium tungsten oxide-modified LiCoO2 (LCO) thin film electrode by pulsed laser deposition. They observed that the interfacial Li+ ion transfer resistance decreased by about one third by this surface modification.34 In this work, we studied the effect of the surface modification of NCM-811 via a lithiumreactive tungsten oxide coating on the electrochemical performance. We used the coating method to improve a commercially available cathode material in order to demonstrate the positive effect on a state-of-the-art material with respect to its properties such as particle size, surface area and morphology. Furthermore, the application and effect of this protective coating was purposely investigated in NCM-811║graphite LIB full cells. In contrast to the previous studies, the goal of this investigation was to mimic the technical parameters (material selection, mass loading and porosity of the electrodes) and the operation conditions of commercial LIB cells, by using graphite as anode material instead of Li metal as negative (or counter) electrode. In this case, the inventory of active lithium is intrinsically limited by the positive electrode material, leading to different aging effects compared to the cells with a Li metal negative electrode.36 2. Experimental 2.1 Surface modification of NCM-811 by ammonium tungstate The tungsten oxide coating was prepared via a sol-gel method followed by heat-treatment. For the 1 wt% coating, 20 g of commercial NCM-811 (LiNi0.8Co0.1Mn0.1O2, Shanshan Tech Co., Ltd) was mixed with 0.22 g ammonium tungstate ((NH4)10H2(W2O7)6), Sigma Aldrich, 99.99 % trace metals basis) in isopropanol (EMSURE ACS) and heated to 50 °C for 12 h. The dispersion was dried at 130 °C to remove the isopropanol and annealed for 12 h at 450 °C in argon atmosphere to obtain the tungsten oxide coating layer at the surface of NCM-811. In order to check whether the coating layer or the heat-treatment do affect electrochemical performance37-38, the pure, uncoated active material was calcined under the same conditions (12 h at 450 °C in Ar) for control purposes. 2.2 Material characterization The samples ((1)uncoated, (2) uncoated and calcined and (3) coated NCM-811) were analysed using infrared spectroscopy (IR, Bruker Vertex 70, ATR-Unit Diamond) to investigate the surface changes after the heat treatment and coating process. The chemical composition of the coated NCM-811 powder was determined using inductively coupled plasma - optical emission 5 ACS Paragon Plus Environment
spectrometry (ICP-OES, Spectro ARCOS EOP). The structural characterization was carried out by X-ray diffraction (XRD, Bruker D8 Advance) using Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) of the coated NCM-811 powder was performed on an Axis Ultra DLD (Kratos) using a monochromatic Al Kα radiation (hν = 1486.6 eV) with 10 mA emission current and 12 kV acceleration voltage. The particle morphology and electrode topography as well as elemental composition of the specimen were studied by scanning electron microscopy (SEM, Carl Zeiss AURIGA, Carl Zeiss Microscopy GmbH) coupled with energy dispersive X-ray spectroscopy (EDX, X-Max 80 mm2 detector, Oxford Instruments) analysis at an acceleration voltage of 20 kV. Electrodes were washed with dimethyl carbonate (DMC, BASF, purity: battery grade) and analyzed at an acceleration voltage of 3 kV and 20 kV to investigate the particle surface for demonstration of the degradation effects on the particle structure before and after electrochemical cycling. 2.3 Electrode and cell preparation For the electrode preparation, uncoated and coated NCM-811 powders were mixed with carbon black (C-nergy Super C65, Imerys Graphite & Carbon) as conducting agent and polyvinylidene difluoride (PVdF, Solef 5130, Solvay) as binder in a weight ratio of 92 : 4 : 4. The electrodes had an active mass loading of 12.4 0.4 mg cm-2. The details for the electrode preparation of the positive electrode as well as for the negative electrodes are shown in the supporting information. For all full cell measurements, the capacity ratio of the negative to the positive electrodes (N/P ratio) was set to 1.05. All electrochemical measurements were carried out in two-electrode coin cells (CR2032, Hohsen Corporation) with a NCM-811-based (uncoated or coated) positive electrode (12 mm Ø) and a graphite-based negative electrode (12 mm Ø). The polymer separator (1 layer, 16 mm Ø, Celgard 2500, Celgard) was wetted with 90 µL of the electrolyte (1 M LiPF6 in a mass ratio of 3:7 ethylene carbonate (EC) and ethyl methyl carbonate (EMC), BASF, purity: battery grade). 2.4 Electrochemical characterization All electrochemical investigations were performed using a Maccor Series 4000 battery tester (Maccor, Inc.). The cells were assembled in dry atmosphere, i.e. in a dry room with a dew point of -50 °C. The specific current for a rate of 1C was defined as 180 mA g-1. All cells were cycled using a constant current charge and discharge step. For rate capability studies, the cells were cycled with current rates in a range of 0.1 to 5C (charge/discharge step), each for five cycles, within a cell voltage range of 2.5 to 4.3 V at 20 °C. All long-term cycling experiments consisted 6 ACS Paragon Plus Environment
of three formation cycles at 0.1C charge/discharge rate, followed by cycling at 0.5C for charge/discharge until the state-of-health (SOH) reached a value of 80%. The SOH was defined as the ratio of discharge capacity of the actual cycle to the discharge capacity of the fifth cycle at 0.5C. The investigation of the long-term performance focused on studies at 1) 20 °C and 4.3 V cut-off voltage, 2) 40 °C and 4.3 V cut-off voltage and 3) 20 °C and 4.5 V cut-off voltage. Electrochemical impendance spectroscopy (EIS) was performed using a Maccor Series 4000 battery tester (Maccor, Inc.,) with a Solartron 1260 impedance gain phase analyzer. The cells were cycled in a voltage range of 2.5 to 4.3 V and a constant current rate of 0.5C. EIS measurements were carried out after the 1st, 25th, 50th and 75th cycle at a state-of-charge (SOC) of 50% with a frequency range from 0.1 Hz to 1 MHz and a voltage amplitude of 5 mV. For all electrochemical studies, at least three different cells were studied to ensure a high reproducibility. 2.5 Thermal analysis Differential scanning calorimetry (DSC) was performed using a Q2000 from TA Instruments on NCM-811 positive electrodes in the delithiated state. The LIB full-cells were fully charged at a constant voltage of 4.3 V after 10 cycles and disassembled in the charged (delithiated) state in dry atmosphere (dry room). The positive electrodes were washed with dimethyl carbonate (DMC) and dried at room temperature. For the analysis, ≈6 mg of positive electrode material was used with 7 µL fresh electrolyte (1 M LiPF6 in a mass ratio of 3:7 EC and EMC). The samples were heated in a stainless steel pan with gold-plated copper with a heating rate of 5 °C min–1. 3. Results and discussion 3.1 Characterization of tungsten oxide coated NCM-811 Figure 1 shows a schematic illustration of the coating process of NCM-811 with the inorganic tungsten oxide layer. During the mixing of the precursor ((NH4)10H2(W2O7)6) with the active material in anhydrous isopropanol, the residual lithium species (LiOH and Li2CO3) at the NCM particle surface can react with the precursor of the tungsten oxide coating. Through this neutralization reaction, the residual lithium species, i.e., LiOH, were consumed to form Li2WO4 at the active material surface (according to equation (1)).28, 31 The subsequent annealing step leads to the decomposition of the precursor and the generation of a coating layer of Li2WO4/WO3 at the NCM-811 surface (according to equation (2)). However, it has to be 7 ACS Paragon Plus Environment
noted that the surface of the active material is not homogeneously covered by the surface impurities (LiOH and Li2CO3), however, these are present as island-like aggregates. Since the exact structure of the coating cannot be determined (i.e., Li2WO4 or WO3) by the methods applied in this study, it remains not completely clear how tungsten oxide compounds are present at the particle surface. For a precise determination of the composition of coating layer, transmission electron microscopy (TEM) may further help to elucidate the coating structure. (1) 24 LiOH + (NH4)10H2(W2O7)6 → 12 Li2WO4 + 10 NH3 + 18 H2O ∆𝑇 (2) (NH4)10H2(W2O7)6 12 WO3 + 10 NH3 + 6 H2O In order to evaluate the impact of the tungsten oxide coating on the surface functional groups and the interaction of the NCM-811 surface with the coating layer, the active material surfaces were analysed by FT-IR (Figure S1, supporting information). The FTIR results further indicate that Li impurities at the active material surface were at least partially removed by the coating procedure, suggesting that the tungsten oxide coating changes the surface chemistry. In turn, the change of the surface chemistry might reduce parasitic side reactions, i.e., with the electrolyte, as will be discussed below. The coating amount was estimated to be ≈1 wt% through ICP-OES analysis of the tungsten content. Figure 2 shows the XRD patterns of the (a) uncoated and (b) coated NCM-811 (after the calcination step) and the lattice parameters for both materials, determined by Rietveld refinement, are displayed in Table 1. All XRD patterns were indexed as a hexagonal α-NaFeO2 structure (space group R3m) and no obvious diffraction pattern shifts and no significant changes of the lattice parameters were observed. In comparison, Shang et al. recently demonstrated a tungsten-doping for NCM-811, causing a significant extension appearing in the lattice parameters, due to the higher ionic radius of W6+ (0.6 Å) compared to the transition metals of the active material (Ni3+: 0.56 Å; Co3+: 0.55 Å; Mn4+: 0.53 Å).39 In addition, the c/a values are >4.94 for both the uncoated and coated NCM-811 materials, indicating a well layered structure (Table 1). The peak intensity ratio of I(003)/I(104) can be used to indicate the cation mixing/disorder,39 while an increased ratio corresponds to a lower occupation of Ni2+ in the Li layers and, thus less cation mixing. Therefore, the increased ratio of the coated NCM-811 (Table 1) indicates an ordered hexagonal layered structure with lower cation mixing than the uncoated material. This might further indicate that the coating enhances the structural stability of the active material, as discussed below with respect to the electrochemical performance. In order to elaborate whether the coating layer has an amorphous structure or the content of the coating would not suffice for identification, the pure precursor was calcined at the same 8 ACS Paragon Plus Environment
conditions, as performed during of the coating process (Figure 2c). The XRD patterns of the pure coating without active material (precursor after calcination) were indexed as a orthorhombic WO3 structure (Pnma space group). In turn, this indicates that the resulting tungsten oxide coating structure is not amorphous, but the content of the coating is outside of limit of detection.
Figure 3 shows the SEM images of (a) uncoated and (b) tungsten oxide coated NCM-811 particles. Overall, no significant differences between the surface morphology of uncoated and coated particles can be observed. The EDX mapping of the coated NCM-811 particles shows the expected homogeneous distribution of characteristic transition metals Ni, Mn and Co as well as W from the coating layer (Figure 3c-f). All detected elements are homogeneously distributed on the particle. To further confirm the presence of the coating layer at the surface of the modified NCM-811 sample, the surface of the active material particle was examined by XPS. Figure 4 shows the XPS data of the (a) tungsten oxide coated NCM-811 active material powder and (b) the pure powder of Li2WO4 as reference material. The XPS spectrum of the coated NCM-811 sample exibits two characteristic peaks of W 4f7/2 (≈35 eV) and 4f5/2 (≈37.5 eV), which are consistent with the peaks of pure Li2WO4. These two peaks are well separated, which indicates that W atoms are in the +6 oxidization state.40 However, it is not possible to detect, whether the resulting coating layer consists of Li2WO4 or the corresponding metal oxide WO3. In this respect, Ho et al. investigated the difference between WO42- and WO3 by using XPS analysis. They observed no detectable changes of binding energies of between WO3, Li2WO4 and Li2W2O7.41-42 The differences in peak intensity of coated NCM sample and pure Li2WO4 powder indicate that the coating layer (Li2WO4/WO3) is located only at the surface of the active material. 3.2 Electrochemical characterization All electrochemical investigations were carried out in NCM-811║graphite full cells. A N/P ratio of 1.05 was chosen in order to avoid any detrimental Li metal plating at the negative electrode surface.43 The specific capacity of the cells with tungsten oxide coated NCM electrodes was calculated based on the amount of the active material, and the coating (≈1 wt%) was regarded as active material. Figure 5a shows the rate capability investigations of the two different materials and cells at varying C-rates from 0.1C to 5C at 20 °C within a voltage range 9 ACS Paragon Plus Environment
of 2.5-4.3 V. The rate capabilities of both cells with uncoated and coated electrodes exhibit a very similar behavior at charge/discharge rates up to 2C. Only at 5C, the cells with the tungsten oxide coated NCM-811 electrodes show a slightly enhanced discharge capacity, however, the difference is within the standard deviation. This indicates that the surface modification has no significant negative impact on the rate performance of NCM-811║graphite full cells and, simultaneously, the coating layer possesses good ionic conductivity. It has to be noted that the rate capability typically depends on various factors, including e.g. the type of the coating layer, the coating amount and thickness, as well as the cell setup in which the cathode material is studied. Even though there are reports showing an improved rate capability for the coated or doped cathode materials35, 44, there also also various reports showing a similar or even worse rate performance of the coated cathode material in comparison to an uncoated material.39, 45-46 In order to get a better idea of the impact of the coating layer on the rate capability, additional studies in dependence of the coating amount are needed. In addition, it has to be kept in mind that the graphite negative electrode also shows a relatively poor rate performance. Therefore, our results obtained in NCM-811║graphite cells might also be influenced by a limited rate capability of the negative electrode. To investigate the impact of surface modification on electrochemical cycling stability, the cells with uncoated, uncoated-calcined and coated NCM-811 electrodes were cycled at 20 °C in a voltage range of 2.5 and 4.3 V vs. graphite (Figure 5b). In the first discharge, all cells show a similar capacity. The calcined control sample (≈192 mAh g-1) delivers a slightly higher discharge capacity than the uncoated (≈188 mAh g-1) and tungsten oxide coated electrodes (≈184 mAh g-1). The calcination step leads to a reduction of the amount of alkaline residues (such as LiOH and Li2CO3), thus, a “cleaning” of the NCM-811 surface, which results in an improved discharge capacity.28, 30 Despite the slightly higher initial capacity of the cells with heat-treated NCM-811, these cells exhibit nearly the same capacity fading as the cells with uncoated NCM-811 electrodes. The cells with the coated materials show a strongly improved cycling performance and decreased capacity fading compared to cells with uncoated and calcined materials. While the cells with uncoated and calcined NCM-811 samples achieve a SOH=80% after 465 cycles and 488 cycles, respectively, the cells with the tungsten oxide coated material reach 865 cycles (SOH=80%; Figure 5b). This result implies that the tungsten oxide coating remarkably improves the electrochemical performance of NCM-811 electrodes, while the calcined material shows no improvement with regard to cycle life. To further demonstrate the beneficial effect of the tungsten oxide coating for stabilization of the long-term performance of the NCM-811, the cells were also cycled at a higher cut-off 10 ACS Paragon Plus Environment
voltage and higher operating temperature. Therefore, Figure 5c shows the electrochemical performance at 40 °C and a cut-off voltage of 4.3 V and Figure 5d at 20 °C and 4.5 V. In general, an increase of the cut-off voltage47-49 and/or the operating temperature49-50 leads to a rapid capacity fading of the cells, due to more parasitic reactions. In both cases, the presence of the coating improves the cycling performance of the NCM-811║graphite full cells, i.e., the cycle life of the coated material was improved by ≈82% compared to the uncoated and calcined materials. The Coulombic efficiency (Ceff) is an important parameter for the comparison of the electrochemical performance of different materials and cells, as it reflects parasitic side reactions within the LIB cell, such as reductive electrolyte decomposition by formation of the solid electrolyte interphase (SEI)51, oxidative electrolyte decomposition52, transition metal dissolution53 or loss of active lithium54. For better visualization of the differences in Ceff, the inefficiencies of the respective cycles were accumulated and plotted against the cycle number, as described in work of Besenhard et al.55 Figure 6a shows the accumulated Coulombic inefficiency (ACIE) for the long-term performace at 20 °C and a cut-off voltage of 4.3 V. After the formation cycles, both cells with uncoated and coated NCM-811 materials show comparably high ACIEs (≈20%) due to electrolyte and Li consumption for the formation of the SEI layer at the graphite anode surface. With ongoing charge/discharge cycling, uncoated and coated NCM-811 electrodes display an increasing difference in the slope of ACIE, while the surface-coated sample exhibits a reduced ACIE. This is a strong indication for reduced parasitic side reactions. After 400 cycles, the ACIE of cells with uncoated NCM-811 electrode amounts to ≈50%, while the ACIE of the cells with coated electrodes only adds up to ≈37%. The ACIE data for long-term cycling at 20 °C and a cut-off voltage of 4.5 V as well as at 40 °C and a cut-off voltage of 4.3 V are shown in the supporting information (Figure S2). Unlike the performance at 20 °C and a cut-off voltage of 4.3 V, a slightly higher average ACIE of the coated NCM-811 (after 100 cycles: ≈37%) compared to uncoated electrodes (after 100 cycles: ≈35%; Figure S2a) is observed at 40 °C and 4.3 V. Uncoated NCM-811 exhibits a slightly higher discharge capacity (+8 mAh g-1) compared to coated electrodes (Figure 5c), which is most notable within the initial 50 cycles. Despite higher discharge capacity of uncoated NCM-811, its capacity fading is much more pronounced. An increase of the operating temperature to 40 °C leads to a higher discharge capacity and improved rate performance, however, negatively affects cycle life. The kinetics of cells with the coated sample are less affected by the temperature rise. Figure S2b shows the ACIE of uncoated and coated NCM-811 11 ACS Paragon Plus Environment
electrodes for long-term cycling at 20 °C and a cut-off voltage of 4.5 V. Similar to the performance at 20 °C (cut-off voltage: 4.3 V), coated NCM-811 shows an improved Ceff and a more stable long-term cycling performance than uncoated NCM-811. In comparison to cycling at 4.3 V, the ACIE values obtained at 4.5 V (for both samples: uncoated and coated) show higher inefficiencies. The enhanced upper cut-off voltage most likely leads to stronger aging effects, such as enhanced electrolyte decomposition or transition metal dissolution.47, 49, 56-57 Figure 6b shows a comparison of the capacity retention of uncoated and coated NCM-811║graphite cells at different cycling conditions until the SOH of 80% is reached. The calculation of the SOH=80% is based on the capacity of the 5th cycle after formation. In summary, all cells using the tungsten oxide coated cathode materials show a strongly improved capacity retention during charge/discharge cycling compared to the uncoated electrodes. Figure 6c represents the first cycle charge/discharge voltage curves of uncoated and coated materials in the NCM-811║graphite full cells at 0.1C. The tungsten oxide coated material displays a very similar voltage profile compared to the uncoated sample. This indicates that the tungsten oxide coating does not cause additional internal resistance and the Li+ transfer is not hindered. The initial Ceff of ≈86.2% is very similar for both samples. Figure S3 (supporting information) shows the corresponding charge/discharge voltage profiles of (a) uncoated and (b) coated NCM-811║graphite cells of selected cycles (5th, 50th, 100th, 200th and 400th) for 4.3 V as cut-off and at 20 °C. Their specific capacities and Ceffs are also shown in the supporting information (Error! Reference source not found.). Furthermore, Figure S4 depicts the charge/discharge voltage profiles of uncoated and coated NCM-811║graphite cells for 4.3 V as cut-off at 40 °C ((a) and (b)) and for 4.5 V as cut-off and at 20 °C((c) and (d)). Overall, with ongoing cycling the tungsten oxide coated material demonstrates a more stable voltage profile, that is reflected by the strong improvement of the capacity retention. Another important aspect is the effect of the tungsten oxide coating on the overall impedance of the cell, which was studied using electrochemical impendance spectroscopy (EIS). The analysis by EIS enables the identification and investigation of different processes, such as the charge carrier transfer through the electrolyte and resistance of the current collectors in the high frequency range, Li+ ion migration at the electrode surfaces, electrochemical double layer formation in the range of medium-to-low frequencies and the Li+ ion diffusion in the solid phase of the electrodes in the low frequency range.58-60 Figure 7 shows Nyquist plots of (a) uncoated and (b) coated NCM-811 electrodes at SOC=50% after the 1st, 25th, 50th and 75th cycle. Both 12 ACS Paragon Plus Environment
samples, show a relatively similar impedance. After the 1st cycle, both uncoated and coated NCM-811 show a higher impedance compared to the subsequent cycles. An increased impedance in the 1st cycle can be explained by the formation of the interphase layers between both electrodes and the electrolyte (SEI, CEI).26 Within the first 25 cycles, the impedance of the cells is remarkably decreased due to the transformation of already formed interphases to more effective interphases with an improved Li+ ion migration and decreased charge transfer resistance.61 After formation of effective interphases, the impedance of the cells constantly rises with increasing cycle number (cycle 25-75). After 75 cycles, one can observe a very similar impedance for both cells. Therefore, these results reveal that the tungsten oxide coating has no negative impact on the impedance of the cell and the charge transfer of Li+ ions is not impaired, explaining the beneficial cycling performance as discussed above. 3.3 Post-mortem analysis To investigate the effect of the tungsten oxide coating as protective layer on the structural stability of the electrodes, electrochemically aged electrodes were analysed via SEM. Figure 8 shows the SEM images of the surface at different magnifications of uncoated ((b, e) surface and (h) cross section) and coated ((c, f) surface and (i) cross section) electrodes. For a fair comparison, the different cells were cycled to a SOH of 80%. To better analyze the changes in the electrode structure during electrochemical charge/discharge cycling, the surface of the fresh uncoated NCM-811 electrodes is shown in Figure 8a, d (surface) and g (cross section). In the course of electrode compression, secondary particles of the active material can be deformed by the applied pressure, leading to cracking of the particle perimeter (Figure 8d). Compared to the mechanically induced deformation of the secondary particle, the particle cracking, which is induced by electrochemical stress (volume change during the delithiation/lithiation, formation of new phases with larger lattice parameters) takes place inside the particles (Figure 8e).62-63 The electrochemically aged NCM-811 electrode shows some irregularly distributed particle cracking on its surface after 465 cycles (Figure 8b). This may lead to loss of the electronic contact between the particles and enhanced parasitic side reactions due to formation of new contact surfaces between electrolyte and active material. The tungsten oxide coating is able to act as a protective layer between the active material surface and the electrolyte and suppresses particle cracking. As a result, we observed less particle cracking on the surface of coated
electrodes (Figure 8c and f) and more structural stability of the active material, which results in an improvement of cycling performance. Further analysis of electrochemically aged electrodes (each at a SOH of 80%) by XPS could not reveal any differences in the surface chemistry of uncoated and tungsten oxide coated materials. The practical application of LIBs relies on a high operating safety and, thus, a high thermal stability of the used active materials, which depends on the active material composition and morphology.63-64 The main thermal decomposition of layered transition metal oxide materials typically starts in a temperature range of 150-250 °C and is caused by structural degradation and oxygen release.27, 65-66 Surface modification via ceramic coatings has proven to be an effective method to stabilize the active material surface. Especially metal oxides such as Al2O344, 67, TiO268, SiO245-46, ZrO269, ZnO70, have been successfully applied as protective layer to restrain side reactions at the particle/electrolyte interface. As result, the direct contact between the highly reactive, delithiated active material and electrolyte is reduced and the release of oxygen is most likely suppressed. Figure 9 shows the DSC analyses of delithiated uncoated and coated NCM-811 charged to 4.3 V after the 10th cycle, which is in direct contact with fresh electrolyte. Uncoated delithiated NCM-811 shows two well-resolved exothermic peaks at ≈210 °C and ≈228 °C with specific heat generation of 1955 J g-1. In contrast, one can observe a shift of the exothermic peak to ≈225 °C (only one peak) and a reduced heat generation of 1722 J g-1 for the surface modified NCM-811. This observation indicates that the coating is able to form a protective layer, reducing the direct contact between active material and electrolyte solution and, thus, increasing the thermal stability and the safety properties of the NCM-811 material. Furthermore, the coating effectively prevents the degradation of the active material during electrochemical cycling as shown above. Nevertheless, it also has to be kept in mind that the thermal stability of the uncoated and coated materials may change over charge/discharge cycling, i.e., degradation effects such as phase changes and particle cracking can lead to a reduced thermal stability. Hence, differences in the thermal stability are expected to become even more pronounced during cycling due to the enhanced degradation of the uncoated material. However, further in-depth safety studies are needed to verify the improved safety over longterm charge/discharge cycling.
4. Conclusion We report on the effect of a protective tungsten oxide coating for the Ni-rich layered cathode material NCM-811 on the electrochemical performance in NCM-811║graphite LIB full cells. The tungsten oxide coating (≈1 wt%) was achieved via a simple sol-gel method, which leads to a smooth and homogeneous coating layer. Using EDX analysis, the presence of the coating could be validated by the homogeneous distribution of the tungsten signal at the particle surface. Despite the presence of the inorganic tungsten oxide coating layer, the C-rate performance was not impaired and, further, no negative impact on the impedance of the cells was observed. Furthermore, the long-term performance of NCM-811║graphite full cells was investigated at different operation conditions, such as different cut-off voltages and at an increased operating temperature of 40 °C. Overall, cells with surface-coated NCM-811 show a significantly improved electrochemical performance with respect to capacity retention compared to uncoated NCM-811. Especially, the cycling performance at 20 °C and a cut-off voltage of 4.3 V is significantly improved, as the cells with tungsten oxide coated NCM-811 reached the end of life criterium (at SOH=80%) after 865 cycles, whereas the uncoated electrodes reached it already after 465 cycles. A further comparison of the tungsten oxide coated and simply calcined control sample provides additional evidence that the coating is responsible for the improvement of the long-term performance and not the simple heat-treatment of the cathode active material. Post-mortem analysis of electrochemically aged uncoated NCM-811 material shows degradation of the secondary NCM-811 particles in the form of particle cracking at the electrode surface, as observed by SEM measurements of the electrode surface and particle cross-section. In contrast, the aged coated materials show significantly less degradation effects at the electrode surface. Furthermore, we could show that the tungsten oxide coated NCM-811 electrodes show an increased thermal stability as the coating delays the oxygen release from NCM-811 leading to a considerably reduced ractivity between the active material and electrolyte compared to the uncoated NCM-811 electrodes (both cycled for 10 cycles). Overall, the improved electrochemical performance of tungsten oxide coated NCM-811 can be attributed to the coating layer acting as a protective shield against undesirable side reactions. As a consequence, the direct contact between active material and electrolyte is reduced, the degradation of the active material can largely be inhibited, as observed by the greatly improved long-term charge/discharge cycling performance. In summary, the tungsten oxide coating is a promising surface modification method to enable a safer and more performant application of NCM-811 in LIB cells.
Acknowledgement The authors would like to thank the German Federal Ministry of Education and Research (BMBF) for funding this work in the project “BenchBatt” (03XP0047A). We also thank the project partners (Group of Prof. Dr. Kwade, Institute for Particle Technology (iPat), Technische Universität Braunschweig) for providing graphite negative electrodes within the BenchBatt project. The authors also acknowledge Debbie Berghus for differential scanning calorimetry (DSC) measurements and Andre Bar for his graphical support.
40 60 80 2-Theta / degree Figure 2: X-ray diffraction patterns of (a) Uncoated NCM-811, (b) Coated NCM-811 after calcination at 450 °C in Ar flow for 12 h, (c) Calcined (NH4)10H2(W2O7)6 at 450 °C in Ar flow for 12 h and (d) reference XRD pattern for WO3 (PDF 04-007-0880). Table 1: Rietveld refinement results of the XRD data for uncoated and tungsten oxide coated NCM-811.
Figure 3: SEM images of (a) uncoated NCM-811 and (b) tungsten oxide coated NCM-811 powder and EDX elemental mappings of tungsten oxide coated NCM-811 powder corresponding to (c) W, (d) Ni, (e) Mn and (f) Co at the same magnification 4kx as in part b).
Figure 5: Comparison of the electrochemical performance of uncoated (red dot) and tungsten oxide coated (green triangle) NCM-811 positive electrode materials. All experiments were carried out in coin cells using graphite negative electrodes (1C = 180 mA g-1 for NCM-811, error bars: standard deviation of three cells). (a) Rate capability at different C-rates from 0.1C to 5C at 20 °C within a voltage range of 2.5-4.3 V. (b), (c) and (d) Longterm cycling performance at 0.1C for three cycles and then at a rate of 0.5C. (b) At 20 °C and a cut-off voltage of 4.3 V. (c) At 20 °C and a cut-off voltage of 4.5 V. (d) At 40°C and a cut-off voltage of 4.3 V.
Figure 6: (a) Accumulated Coulombic inefficiencies vs. cycle number of uncoated and tungsten oxide coated NCM-811║graphite full cells. The cells were cycled at 20 °C and a cut-off voltage of 4.3 V with three formation cycles at 0.1C, followed by 0.5C (1C=180 mA g-1). (b) Achieved cycle numbers for different cycling condition at SOH=80%. The calculation of the SOH is based on the capacity of the 5th cycle. (c) Voltage profiles upon the first charge/discharge cycle (0.1C/0.1C) of uncoated and tungsten oxide coated NCM-811 at 20 °C within a voltage range of 2.5-4.3 V vs. graphite negative electrodes.
Figure 7: Nyquist plots of the electrochemical impedance measurements of (a) uncoated and (b) tungsten oxide coated NCM-811 at SOC=50% after the 1st, 25th, 50th and 75th cycle. The cells were cycled at 20 °C and a rate of 0.5C within a voltage range of 2.5–4.3 V vs. graphite negative electrodes.
Figure 8: SEM images of (a,d) surface and (g) cross-section of the fresh uncoated NCM-811 electrode; (b,e) surface and (h) cross-section of electrochemically aged uncoated NCM-811 electrode (after 465 cycles; SOH=80%); (c,f) surface and (i) cross-section of electrochemically aged tungsten oxide coated NCM-811 electrode (after 867 cycles; SOH=80%) at three different magnifications: (a,b,c) 1kx, (d,e,f) 5kx, (g,h,i) 6.5kx.
Figure 9: DSC analyses of delithiated uncoated and tungsten oxide coated NCM-811 electrodes charged to 4.3 V after 10 cycles. Measurements were carried out in contact with fresh electrolyte (1 M LiPF6 in EC:EMC = 3:7 w/w).
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