Structural and Electrochemical Aspects of LiNi0.8Co0.1Mn0.1O2

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Letter

Structural and Electrochemical Aspects of LiNi0.8Co0.1Mn0.1O2 Cathode Materials Doped by Various Cations Tina Weigel, Florian Schipper, Evan Erickson, Francis Susai, Boris Markovsky, and Doron Aurbach ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b02302 • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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ACS Energy Letters

Structural and Electrochemical Aspects of LiNi0.8Co0.1Mn0.1O2 Cathode Materials Doped by Various Cations Tina Weigel1,2, Florian Schipper1, Evan M. Erickson1, Francis Amalraj Susai1, Boris Markovsky1 & Doron Aurbach1*

1. Department of Chemistry, Faculty of Exact Sciences, Bar-Ilan University, Ramat-Gan, 5290002 Israel 2. Institute of Experimental Physics, Faculty of Chemistry and Physics, TU Bergakademie Freiberg, 09599 Freiberg, Germany * E-mail: [email protected]

ABSTRACT Ni-rich materials of layered structure LiNixCoyMnzO2, x > 0.5 are promising candidates as cathodes in high energy density Li-ion batteries for electric vehicles. The structural and cycling stability of Ni-rich cathodes can be remarkably improved by doping with small amount of extrinsic multivalent cations. In this study, we examine development of fast screening methodology for doping LiNi0.8Co0.1Mn0.1O2 with cations Mg2+, Al3+, Si4+, Ti4+, Zr4+ and Ta5+ by a “top down” approach. The cathode material is coated by a precursor layer that contains the dopant, which then is introduced into the particles by diffusion during heat treatment at elevated temperatures. The methodology described herein can be applied to Ni-rich cathode materials and allows to identify relatively easily and promptly most promising dopants. Then further optimization work can lead to development of high capacity stable cathode materials. The present study marks Ta5+ cations as very promising dopants for Ni-rich NCM cathodes.

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One of the most important challenges of modern electrochemistry is promotion of the electro-mobility revolution,1 for which development of power sources that guarantee long driving ranges of electric vehicles in critically important. Beside H2/O2 fuel cell vehicles,2–4 lithium ion batteries are the most appropriate power sources for electrochemical propulsion, since they may provide the necessary energy density, cycle life, stability and reasonable safety features.5 The limiting factor of energy content in Li ion batteries are the cathodes. In this paper, we have focused on positive electrodes (cathodes) for advanced lithium-ion batteries, comprising layered lithiated oxides of transition metals (TMs) – Ni, Co, Mn with a close packed lattice consisting of alternating layers of lithium and TMs between oxygen slabs. These materials show high specific capacity stored per unit of volume,6,7 especially for Ni-rich oxides of the general formulae LiNiaCobMncO2 (a + b + c = 1; a > 0.5). 8 The nomenclature used for them is NCM 523, NCM 622, NCM 811 etc., where the numbers reflect the atomic contents of Ni, Co, and Mn in the compounds. Nickel-rich layered cathode materials demonstrate higher capacity, rate capability and lower cost than LiCoO2.9 As the content of nickel is higher the specific capacity is higher and can be extracted at charging potentials < 4.3V vs. Li, however the stability

is

lower.10,11

This

instability

is

connected

to

the

nature

of

the

intercalation/deintercalation processes via both reversible and irreversible phase transitions and the phenomenon of “cation mixing”, where cations from the transition metal sites mix with ions from the Li sites, induced by the similar ionic radii of Li+ (76 pm) and Ni2+(69 pm).12 The reversible ones induce stresses that lead to cracks formation that accelerates capacity fading.13 The irreversible layered-to-spinel phase transition that these materials undergo, leads to both capacity and voltage fading.14 These situations which include disordering, stresses and cracking


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are translated to a poor stability of untreated Ni-rich NCM cathodes. In order to suppress problematic cation mixing that lowers stability and promotes capacity fading, doping these materials with foreign cations or anions can be useful.9,12,15 The effects of doping can be categorized into four points: 1.) substitution of mobile ions, such as Ni2+ and Li+, with structurally and electrochemically, monovalent, static ions15,16 2.) Hindrance of Ni2+ ions migration to the Li layers17,18 3.) Decrease of oxygen release during electrochemical cycling by increasing oxygen and transition metal ions bond strength.19,20 4.) Increase in strength of the transition-metal oxide bond, which limits the contraction of the material at the end of charge when NiIV-O bond becomes highly covalent, reducing O2- repulsion.21,22 It should be noted however, that the dopants concentration needs fine optimization since too high dopants content may lead to capacity and stability decrease.15,20. An alternative method to improve the performance of NCM electrodes is by coatings that mitigate the reactivity between the active mass and the electrolyte solution. Coating by oxides like TiO2, ZrO2, Al2O3 may form appropriate buffer zones that avoid detrimental surface reactions, as demonstrated by the following examples. Schipper et al. observed an enhanced electrochemical performance of NCM811 cathodes by surface coating with ZrO2 which lead to bulk doping during annealing at high temperature.23 Similarly NCM811 cathodes showed improved stability due to coating by TiO2.24 Coating NCM622 electrodes by LiAlO2 demonstrated pronounced stabilization effects even when the cathodes were cycled with high cutoff voltages of 4.5 - 4.7 V, in order to extract high specific capacity.25 Metal oxide coatings on NCM at the nano scale level can be carried out very successfully by atomic layer deposition (ALD) techniques that allow a very rigorous thickness control.26


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It should be noted that stabilizing cathodes comprising NCM materials by doping have already been explored. For instance, doping Ni rich NCM by Zr, Al and Ti multivalent ions have been shown to improve their structural stability and electrochemical properties, likely by preventing the migration of

Ni2+ ions from the transition metal site to the Li sites.23,27–30

Another

candidates for helpful doping are Mg ions, showing stabilization effects, but explanations for that are still under debate.31,32 Zr4+ ions probably occupy the Li sites and suppress the layered to spinel transformation by inhibiting transport of Ni ions into the Li sites.33 Zr ions in the NCM structure are supposed to hinder Ni to Li sites diffusion by increasing the energy barrier for it by a factor of 2.33,34 Other promising dopants are Fe,28,35 Cr36or Ga.37,38 Although a lot of work was devoted so far to stabilization of Ni rich NCM cathodes, including intensive efforts related to doping, this work is novel, original and important because it examines a new top-down methodology in which doping is reached by heat treatment of active mass particles covered by a surface layer that contains the dopants in their necessary oxidation state. Also the work includes a direct comparison among different dopants and their effect on performance, what paves the way for combinatorial studies that enable fast optimization. It is important to note that this letter intends to present only a first stage in full mapping of doping options for optimal Ni-rich NCM cathodes. In this preliminary stage, attractive and useful dopants can be identified by comparative electrochemical testing and elementary structural analysis. Then thorough and deep studies related to each selected dopant, in which the Li-ions diffusion mechanism, cations bulk and surface distribution, uniformity, concentration effects and rigorous structural effects, are being carried out as we demonstrated recently with Zr doped Nirich NCM cathodes.

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These previous studies showed clearly by cross sectioning of doped

particles using STEM and HRTEM, that this top-down approach indeed works and enables to


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reach uniform doping by heat treatment of coated particles. A parallel thorough work is being carried out with the other doped materials described herein (beyond the scope of this letter). The selection of dopants for this study was based on previous experience. In this work, Zr, Mg, Ti, Al, Si and Ta are used for doping of the Ni-rich LiNi0.8Co0.1Mn0.1O2 (NCM811) cathode material. Data on the oxidation states of atoms and ions size of these dopants is provided in Table S1. Based on our previous experience in similar doping of NCM811 with dopant precursors, we set their concentration to be 1.5 mol.%. The influence of dopants on the structural and electrochemical behavior of NCM811 cathodes was investigated with X-ray diffraction (XRD) on pristine materials, in-situ XRD during the charging process (Li-deintercalation), and post-mortem analysis of cycled electrodes by XRD as well. Electrochemical properties were characterized by long term cycling experiments at 45 C with different rates and impedance spectroscopy. The experimental work is described in details in the supporting information (enclosed SI file). The doping procedures and the preparations of cells for in-situ XRD measurements are described in Figures S1 and S2 respectively. The results of this work show how different cationic dopants can influence the electrochemical behavior of layered lithiated transition metal oxide cathode materials in lithium cells. In light of this study several dopants can be marked as very promising, deserving further thorough studies and optimization. The XRD patterns of the un-doped and doped NCM811 cathode materials before and after cycling are shown in Figures S3 and S4, respectively. The reflections in XRD pattern can be indexed on a basis of a hexagonal lattice with the α-NaFeO2 structure (space group R3̅m).39 Oxygen ions occupy the 6c site, Li ions the 3a and transition metal ions (Ni, Co, Mn) the 3b site, respectively.40 Lithium sites are partially occupied by Ni ions due to their similar ionic radii (Ni2+ 0.69 Å and Li+ 0.76 Å).39 The splitting of the 006/102 and 108/110 reflections and the c/a


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value of about 4.9 are indicators for a well-ordered layered structure.39,40 The R003/104 ratio gives information about the degree of cation mixing and is a measure of the electrochemical reactivity of the cathode material as well.41 If the R003/104 ratio is significantly less than 1.2, then the 108 and 110 reflections are indistinguishable, what indicates contamination with rock salt domains. Such a contamination has an adverse effect on the specific capacity and stability.42 A low R003/104 ratio often indicates poor electrochemical properties.43 In our case, the R003/104 is around 1.2 and the 108 and 110 reflections are clearly split. A comparison with the un-doped material shows that the doped NCM811 materials have slight changes in the lattice parameters. A few changes of the c/a and R003/104 values are observed. This indicates that the doping processes used herein (a minor level of doping) have some influence on the crystal structure. The XRD patterns of all samples were assigned to layered NCM material of R-3m space group. The resulting lattice parameters a and c, the unit cell volume VUC, the c/a ratio, the ratio of the integrated intensities of the reflections 003 and 104 R003/104 as well as χ2 and RBragg from the Rietveld refinement are listed in Table-S2. The distribution of dopants in the particles is being studied further by HRTEM and element analyses (cross sectioning of particles cut by FIB) as we already described in previous publications. The doping processes used herein were found to be very effective for doping NCM cathodes with Zr4+ cations.

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Further parallel studies (beyond the scope of this letter) examine

the distribution of the other dopants. The electrochemical studies show a pronounced influence of all the doping processes that can be assigned to both surface and bulk effects. All our experience with doped Ni-rich NCM materials show that there are segregation phenomena near the particles’ surfaces. The layer near the surface is always slightly richer in dopants compared to the bulk. Such segregation phenomena are supposed to be even more pronounced in the doped materials produced in this work via the top-down processes.


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The electrochemical performance of the doped and undoped materials is discussed in the following section. We focused on the electrochemical characterization at 45 °C similarly to that of NCM811-Zr doped electrodes studied at 30 °C.23 The discharge capacities vs. cycle for the doped and undoped electrodes at 45°C are presented in Figure 1. First, rate-capability tests up to 4C (C defined as 180 mAh g-1), and then cycling measurements at a C/3 rate were performed. In general, all doped materials show a decrease in the discharge capacity in the initial cycles at C/15 – 1C rates. However, at higher rates of 2C and 4C Aluminum and Tantalum-doped electrodes exhibit capacities close to undoped NCM811 cathodes. The results of cycling at elevated temperature (45 C) may indicate that doping increases the electrodes impedance, probably due to the segregation phenomena that occur during doping process forming intrinsic surface layers. 44,45

In turn, all the dopants used in this work improve the electrodes cycling stability during

prolonged cycling at 45 C, compared to the un-doped NCM 811 electrodes. Another important result is the relative stabilization of the hysteresis between average charge and discharge voltages upon cycling due to doping (Figure 1c). It is important to note that in order to exhibit differences among the reference and doped materials in relatively short experiments, we show herein results of cycling tests carried out at 45 0C. Also, the cathodes that were tested during prolonged cycling (at 45 0C) underwent before then experiments at high rates. In such experiments un-doped NCM 811 cathodes exhibit relatively fast capacity fading (Figure 1b, black star), what enables a clear differentiation in the behavior of the electrodes during relatively short experiments (20 cycles at the low rates). When the cycling experiments are conducted from the beginning at low rates, the capacity fading upon cycling is pronouncedly lower. Same trends are observed but they become significant after much longer experiments.


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(a) 225

Discharge capacity / (mAh g-1)

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C/15

C/10

C/10

C/3 0.8C

200

1C 2C 4C

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NCM811 NCM811 + Al NCM811 + Ti NCM811 + Mg

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NCM811 + Si NCM811 + Zr NCM811 + Ta 10

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Cycle number


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(b) 225

Discharge capacity / (mAh g-1)

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C/3 Ta

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Al Mg

Si

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NCM811

NCM811 NCM811 + Al NCM811 + Ti NCM811 + Mg 40

NCM811 + Si NCM811 + Zr NCM811 + Ta 60

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(C) 0.3

Voltage Hysteresis Uhys (V)

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NCM811 NCM811 + Al NCM811 + Ti NCM811 + Mg

NCM811 + Si NCM811 + Zr NCM811 + Ta

0.2

Al

Ti

Si NCM811 0.1

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Cycle number Figure 1: (a). Discharge capacity obtained during the rate capability tests (C/15 – 4C, as indicated) followed by cycling at a C/3 rate, 45 °C. (b) Discharge capacities of NCM811 electrodes comprising undoped and doped active materials, in coin cells at 45 °C. (c) The voltage hysteresis (Uhys = Vcharge – Vdischarge) for electrodes comprising undoped and doped active cathode materials NCM811 at 45 °C. The results presented in this figure are averaged from several electrochemical cells running in parallel in the same instrument, for statistics purposes. The accuracy of the capacity measurements was around 95% in these experiments.


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It was established that the Ta doped electrodes show the highest initial discharge capacity at 450C as well as the increased capacities and much more stable performance during 100 cycles at a C/3 rate (Figure 1b). The average capacity fading per cycle during prolonged cycling experiments is listed in Table S3. The improvement in capacity retention for all doped materials is clearly evident (e.g. Ta doped NCM811 electrodes exhibited the lowest capacity fading of 0.10% per cycle compared to 0.47% per cycle for the un-doped electrodes). In Figure 1c, we show the evolution of the voltage hysteresis during cycling, calculated from the difference between the mean charge and discharge voltages. As the specific energy of these systems is the product of voltage and specific capacity, as the voltage hysteresis is lower, round turn energy efficiency is higher. Except for Ti cations, the presence of dopants lowers the voltage hysteresis of these electrodes upon cycling. Based on many previous studies as described above, one of the major reasons for the capacity fading and voltage decay of these cathodes is that during the charging process, Li ions are extracted from the lattice and transition metal ions with similar radius (Ni2+) migrate to these positions. During the discharge process, the Li+ ions return to their original positions, but the Li diffusion paths are hindered by transition metal ions remaining in the Li+ sites. Hence, the specific capacity decreases. Additionally, post mortem analyses of electrodes after cycling (not shown herein) indicate that cracks are formed in the particles due to stresses that develop during cycling which propagate during prolonged cycling. The surface area of the active mass thus increases, leading to detrimental side reactions and further loss of capacity. Finally, there are irreversible transformations of the original, desirable layered structure to spinel and rock salt phases (lower the average voltage).

10,12,14

Therefore, the results of this

work show that doping of the NCM811 material in the proposed way (relatively simple top-down approach), limits to some extent these capacity fading mechanisms, thus leading to improved


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cycling stability (through both surface and bulk effects). Voltage profiles for the various NCM811 cathodes (with and without dopants) are presented in Figure S5 Differential capacity (dQ/dV) plots for all electrodes are depicted in Figure 2. These plots are important indicators for tracking of retention of phases throughout cycling, as electrochemistry is inherently more sensitive than structural characterization techniques such as XRD at detecting changes in electroactive phases. However, assignment of phase changes and redox centers to specific dQ/dV peaks can be difficult, often requiring in-situ structural and electronic techniques such as XRD, neutron diffraction or X-ray absorption spectroscopy. Considering the case of the undoped NCM811 electrode in Figure 2a all four peaks in the differential capacity plots decrease throughout cycling. This indicates that the fading mechanisms for NCM811 involve general processes such as transition metal ions inactivation or various types of impedance growth. Aluminum doping of NCM (Figure 2b) seems to improve retention of all of these differential capacity peaks, and so may limit one of these fading mechanisms. Since discharge processes (Li intercalation) for cathode materials are concomitant with reduction processes of transition metal ions involving Li+ reintercalation into the NCM lattice, these dopants impede, to some extent lithium re-insertion, possibly due to their possible incorporation in the lithium sites and formation of surface layers (see further Discussion). While this may cause a decrease in initial capacities, the cycling stability of the material can be improved despite these complications. This is the case of the Ta-doped NCM811, as shown in Figure 2g. It seems that for Si doped and Ta doped materials the cathodic peaks around 3.5 – 3.75 V are not split probably due to the sluggish electrochemical kinetics of these electrodes (see the high charge-transfer resistance RCT in Figure 3b). The effect of doping as presented in Figure


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2 proves that doping by the top-down approach described herein has a clear bulk effect (i.e. the dopants diffuse into the particles by the heat treatment).


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Figure 2: The dQ/dV vs. V plots for NCM811 electrodes comprising undoped (a) and doped active NCM811 materials with dopants Al (b), Ti (c), Mg (d), Si (e), Zr (f) and Ta (g), as indicated. Cycles 25, 50, 75 and 100 are shown. Cells comprising NCM811 undoped and doped electrodes underwent impedance measurements. The cells impedance is obviously dominated by the response of the cathodes, thereby, we can discuss these results in terms of cathodes behavior. As an example (single potential), Nyquist plots depicting the results of the impedance spectroscopy measurements at 4.2 V after 101 cycles are shown in Figure 3. Two semicircles are observed in impedance spectra, a small one related to the surface film resistance, Rsf (at high frequencies), and another one (bigger) ascribed to the overall charge-transfer resistance at the electrode-electrolyte interface, the so called “chargetransfer resistance” Rct (at high-to-medium frequencies), respectively.46 Tails in Nyquist plots at very low frequencies (5 – 10 mHz) are referred to as the Warburg impedance or resistance of the


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solid-state Li-ions diffusion.33 We conclude that Ta doped material shows the lowest impedance of surface film and charge transfer resistance, followed by Al, Zr and Mg doped electrode samples. Similar results were observed for un-doped NCM811 and for Ti and Si doped electrodes.


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250

NCM811 NCM811 + Al NCM811 + Ti NCM811 + Mg NCM811 + Si NCM811 + Zr NCM811 + Ta

200

-Z'' ()

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(a)

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Figure 3: (a) Impedance spectra (Nyquist plots) measured at 4.2 V after 101 galvanostatic cycles of NCM811 undoped and doped electrodes, as indicated. Potential dependence of (b) charge-transfer resistance and (c) surface film resistance, derived from the high-to-medium frequencies and high-frequencies semi-circles, respectively, in Nyquist plots. Impedance spectra of cycled electrodes were measured at several equilibrium potentials. The potential-dependent overall charge-transfer resistance (from the medium frequencies features) and surface film resistance (from the high frequency features), RCT and RSF, derived from impedance measurements are depicted in Figure 3b and 3c, respectively. Both RCT and RSF are calculated using Zview (from Scribner Associates) using an electrical circuit model with two RC time-constants. From the results obtained the charge-transfer resistances are much larger than the surface film resistances; indeed, the rate capability measurements in Figure 1a mainly correlate with the charge transfer resistances, e.g. the RCT is the smallest for Ta < Al ~ Zr < Mg < undoped


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~ Ti < Si. These results show that the RCT can be significantly reduced by doping, with the exception of Si, which shows RCT very similar to that of undoped electrodes. Doping may increase the surface film resistances of the materials to some degree likely due to segregation and formation of surface layers which are resistive. Doping using top-down approach may promote segregation and formation of somewhat resistive surface layer that increases the high frequency impedance of the electrodes (what is termed here as RSF). Hence, while the surface related impedance (high frequencies) is higher for the doped electrodes (as can be expected) the overall charge-transfer impedance which dominates the overall cells impedance (by roughly an order of magnitude compared to the surface films related impedance) is lower for the doped electrodes. Hence, these results obtained by EIS, are in line with the other electrochemical data, showing how doping by the approach we promote indeed improve the performance of commercial NCM811 cathode materials. We carried out comparative in-situ XRD measurements of doped and un-doped NCM811 cathodes, in order to follow changes in their crystallographic parameters during delithiation processes. These studies substantiate the conclusion that the doping processes improve performance by bulk effects, as discussed above. The XRD patterns of the undoped and Ta doped NCM811 electrodes during delithiation process is shown in Figure S6. The changes of the unit cell volume VUC, the lattice parameters a and c as a function of the Li-content (x) are shown in Figure 4.


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Figure 4: The unit cell volume VUC (a), a (b) and c (c) lattice parameters as a function of the lithium content x in LixNi0.8Co0.1Mn0.1O2 calculated from in-situ XRD measurements of NCM811 electrodes comprising undoped and doped active materials, as indicated. The in-situ XRD derived data in Figure 4 show the decrease of VUC with lower Li content ending in a strong volume contraction. This contraction is induced due to the removal of most of the Li+ from the lattice, leaving unshielded oxide slabs, between which Ni2+ cations migrate.47 Our results are in agreement with those obtained for electrode materials with an especially high Ni content that exhibit similar behavior.48 The a parameter decreases during charging from 3.6 V to 4.3 V as the Li-content decreases, due to the smaller ionic radii of the Ni4+ vs. Ni3+ or Ni2+ in the transition metal layer. Considering the c parameter, it initially increases because of the deintercalation of Li from the Li-layer which results in higher repulsive forces between the O2-


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layers. Furthermore, the deintercalation induces stronger covalent bonding between the transition metals and O2-, leading to less localized electron density on the O2- ions and a decreased c parameter.49 The results show pronounced effect of doping on the above parameters, thus strengthening the claims re: the bulk effect of doping. Upon doping with Ta, Al and Mg the specific volume decreases pronouncedly at the end of charging. However, doping with these elements improves the electrochemical performance. Hence, the phenomenon of specific volume decrease is not correlated to a detrimental capacity fading. As seen in Figure 1, the doped cathodes have lower initial discharge capacity. This lower initial specific capacity may limit irreversible structural changes, caused by the change in the c-parameter associated with formation of the hexagonal H3 phase near the end of charging. The relatively low initial specific discharge capacity observed with the tetravalent dopants Si, Zr and Ti (compared to un-doped cathodes, Figure 1a) can be correlated with relatively little changes in VUC (Figure 4a). Interestingly, Si doped NCM811 electrodes show a splitting of the structure in two hexagonal phases: one Li-rich phase (H1) and one with lower Li content (H2).48–52 This two-phase splitting has been investigated previously mainly with synchrotron radiation experiments showing that dopants with a significantly larger or lower ionic radii than Li facilitate the formation of two phases. 48–51 It should be noted that the structural changes of NCM 811 materials in the course of their lithiation/delithiation processes are not fully resolved. While the electrochemical data may indicate that the processes involve phase transitions (especially around 4.25V), there are reports that conclude that the processes involve mainly solid solutions formation and changes in the oxidation state of Ni and Co ions. 53 However, resolution of these points is not important for the main theme of this screening study.


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To investigate structural changes induced due to long-term cycling, the discharged electrodes (OCV around 3 V) after 101 cycles at 45 °C were investigated by powder XRD analysis as shown in Figure S4. The XRD patterns show no additional reflections in the cycled materials suggesting no structural changes occur during cycling and indicate the materials high structural stability. Minor additional reflections originate from aluminum foil current collector (on which the NCM811 was coated) and the carbon black and graphite additives used for the electrode fabrication. More information can be found in Supporting Information, as well as the lattice parameters of cycled electrodes calculated by the Rietveld refinement, which are listed in Table S4. In Summary, this work included a comprehensive study of NCM811 cathode materials with a variety of cationic dopants using electrochemical and structural measurements. Some of the dopants studied in this work improve pronouncedly the electrochemical properties of NCM electrodes in lithium cells. We can highlight doping with Ta that increases the discharge capacity of the NCM811 cathodes at 450C, their long term cycling stability, decreases their charge transfer resistance and lower their voltage hysteresis compared to the un-doped material. We can also conclude that the higher c-parameter of the Ta-doped cycled electrodes (compared to un-doped NCM811) indicates the obvious dopant incorporation into the lattice. Higher resistance of the surface films measured for the doped materials, assigned to impeding surface layers, correlates well with lower rate capability of the cathodes comprising doped materials. In turn, lower charge-transfer impedance of the doped cathodes also correlates with a better stability and decrease in the c-parameter contraction at the end of electrodes’ charging to 4.3 V. The Zr and Mg doped materials demonstrate enhanced cycling performance as well.


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These dopants worsen the rate capability of the material but improve cycling stability. The stability improvement can be correlated to a lower lattice contraction at the end of charging. Lower volumetric changes in the active mass upon lithiation/de-lithiation processes mean lower stresses and thereby a lower level of cracking. The electrochemical cycling characteristics of the Al, Si and Ti doped NCM811 are enhanced compared to the undoped material, but the improvement is considerably lower in comparison to Ta, Zr and Mg doping. In general, dopants with higher oxidation states and/or ionic radii similar to Li ions improve the structural and electrochemical properties of the NCM811 cathode material. Doping with ions of similar radius to the Li+ ion (0.76 Å) increases the inter-layer distance (higher c parameters of the doped material) and reduce the cation mixing. This enables faster electrochemical processes and higher rate capabilities due to enhanced ionic conductivity.54,55 Dopants with high oxidations states inhibit the Ni2+ migration and reduce the number of Jahn–Teller active Ni3+ ions.33 It is important to emphasize that cause for improvement conferred by dopants is complex due to multiple structural and surface effects, including 1) limitation of the sharp c-parameter contraction observed at the end of charging, 2) increased Li+ intercalation kinetics, 3) decreased cation mixing and 4) surface stabilization resulting in lowered impedance growth throughout cycling. Isolating the contributions of each of these issues is especially difficult considering their possible interrelation; for example, the electrode active surface area (and cell impedance) can be increased throughout cycling due to particle cracking, a phenomenon exacerbated by the large structural lattice contraction at the end of electrode charging. This letter presents only a first screening stage. Obvious further studies should concentrate on the most effective dopants (as presented herein), exploring deeply electrochemical, surface, and structural behavior by surface and bulk sensitive techniques, like X-ray photoelectron and time-


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ACS Energy Letters

of-flight secondary ions spectroscopies, transmission electron microscopy and nano-beam electron diffraction. Such studies will enable a judicious optimization, after which it will be possible to demonstrate improved performance in full cells.

Supporting Information Additional data and figures and a detailed description of the experimental methods (PDF)

AUTHOR INFORMATION Corresponding Author: *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT

T. Weigel thanks Freiberger Compound Materials GmbH and Federmann Enterprises Ltd for financial support with the Federmann-Scholarship. Partial support for the work discussed herein was provided by BASF, the Israeli Prime Minister’s Office, and the Israeli Committee for Higher Education within the framework of the INREP project.


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Mizushima, K.; Jones, P. C.; Wiseman, P. J.; Goodenough, J. B. LixCoO2 (0

Structural and Electrochemical Aspects of LiNi0.8Co0.1Mn0.1O2

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