Collapse of LiNi1âxâyCoxMnyO2 Lattice at Deep Charge Irrespective

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Collapse of LiNi1-x-yCoxMnyO2 lattice at deep charge irrespective of nickel content in lithium-ion batteries Wangda Li, Hooman Yaghoobnejad Asl, Qiang Xie, and Arumugam Manthiram J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b13798 • Publication Date (Web): 19 Mar 2019 Downloaded from http://pubs.acs.org on March 19, 2019

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Collapse of LiNi1-x-yCoxMnyO2 lattice at deep charge irrespective of nickel content in lithium-ion batteries Wangda Li, Hooman Yaghoobnejad Asl, Qiang Xie, and Arumugam Manthiram* Materials Science and Engineering Program and Texas Materials Institute, the University of Texas at Austin, Austin, Texas 78712, USA Supporting Information ABSTRACT: Volume variation and the associated mechanical fracture of electrode materials upon Li extraction/insertion are a main cause limiting lifetime performance of lithium-ion batteries. For LiNi1-x-yCoxMnyO2 (NCM) cathodes, abrupt anisotropic collapse of the layered lattice structure at deep charge is generally considered characteristic to high Ni content and can be effectively suppressed by elemental substitution. Herein, we demonstrate the lattice collapse is a universal phenomenon almost entirely dependent on Li utilization, and not Ni content, of NCM cathodes upon delithiation. With Li removal nearing 80 mol%, very similar c-axis lattice shrinkage of around 5% occurs concurrently for NCMs synthesized in-house regardless of nickel content (90, 70, 50, or 33 mol%); meanwhile, the a-axis lattice contracts for highNi NCM, but it expands for low-Ni NCM. We further reveal CoMn co-substitution in NCM barely, if at all, affects several key structural aspects governing the lattice distortion upon delithiation. Our results highlight the importance of evaluating true implications of compositional tuning on high-Ni layered oxide cathode materials to maximize their charge-storage capacities for next-generation high-energy Li-ion batteries.

By virtue of high energy content, lithium-ion batteries occupy a privileged position in the energy storage landscape, from everyday portable electronics to emerging domains such as electric vehicles. In spite of this success, energy requirements of the Li-ion battery technology are extremely stringent, especially for automotive applications.1,2 As the cathode is the limiting electrode, there is a need to further enhance the charge-storage capacities of state-of-the-art lithium transition-metal oxides deployed in battery-powered vehicles, most notably layered LiNi13-5 A current push to higher nickel content x-yCoxMn yO2 (NCM). (1-x-y ≥ 0.8) has attracted major interests, which can bring a notable boost in the specific capacity to > 200 mA h g-1. However, high-Ni NCM suffers from limited cyclability for practical applications. Among various reaction mechanisms restricting the operational lifetime, material fracture due to significant volumetric variation under electrochemical cycling is considered as a main culprit.6-11 At deep charge, abrupt anisotropic lattice contraction along the crystallographic c-axis by as much as 5% (i.e., ‘lattice collapse’) has been observed for high-Ni NCM, producing substantial mechanical strains that lead to pulverization of secondary particles and rapid performance deterioration (i.e., capacity and voltage fade) during cycling.7,8,10-14 Collapse of the layered lattice structure of NCM cathodes at highly charged states has been investigated recently, along with attempts to correlate this behavior with chemical composition in the NCM family. It was shown that commercial low-Ni NCMs, such as LiNi1/3Co1/3Mn1/3O2, are largely exempt from this issue;7,8

meanwhile, a progressive increase in Ni fraction is accompanied with a more severe extent of anisotropic lattice changes at a given cell voltage.7,10,11,13 Indeed, lattice collapse is reminiscent of that observed for LiNiO2, one of the earliest cathode candidates explored at the infancy of Li-ion battery technology.15-19 As a result, Ni is believed to be primarily responsible for this phenomenon through a proposed charge-transfer mechanism:20 at high state-of-charge, transfer of negative charge from O to Ni atoms depletes the effective charge on oxygen, drastically reducing repulsion between the oxygen planes and hence the interlayer spacing (Li-O slabs). Efforts to suppress this effect have been pursued lately through the introduction of alien ions in highNi layered oxides (or LiNiO2) besides Co-Mn, such as Al,21 Mg,22 and Zr.23 Collectively, the overarching conclusion from these studies is that the lattice collapse at deep charge is characteristic to high Ni content and can be effectively reduced via compositional tuning (elemental substitution). The less severe anisotropic lattice shrinkage, however, always comes with a capacity penalty at a given cell voltage in the above studies.7,8,10,11,13,21,23 With less Li extracted, naturally less is the lattice distortion. Hence, an essential question remains open on whether elemental substitution (Co-Mn, Al, Zr, etc.) in effect suppresses the lattice changes under a rigorous control of delithiation. Here, we demonstrate that the collapse of the layered NCM lattice on charge is a universal behavior virtually unaffected by Co-Mn co-substitution. By controlling the extent of delithiation, rather than upper cut-off voltages as commonly exercised, a series of in-house high- and low-Ni NCMs, i.e., LiNi0.9Co0.05Mn0.05O2, LiNi0.7Co0.15Mn0.15O2, LiNi0.5Co0.2Mn0.3O2, and LiNi1/3Co1/3Mn1/3O2, manifest very similar lattice collapse at deep charge. This observation is further corroborated through Density Functional Theory (DFT) calculations on LiNiO2 and LiNi1/3Co1/3Mn1/3O2 model systems. The NCM cathodes were prepared through transition-metal coprecipitation described in our earlier study24 (see Table S1 for chemical composition data). Figure 1a-1d present powder X-ray diffraction (XRD) patterns with Rietveld refinement of the asprepared samples, all of which possess the rhombohedral αNaFeO2 structure (R3̅m space group). In Table S2, Rietveld refinement results show a general upward and downward trend, respectively, for lattice parameters a and c with increasing Ni content; meanwhile, Li/Ni mixing (i.e., cation anti-site disorder) decreases as well. This is because of the varying concentration of each transition-metal (TM) ion, Ni(II), Ni(III), Co(III), and Mn(IV), with differing ionic radii (0.69, 0.56, 0.61, and 0.53 Ǻ) 14 and covalency/lengths of the TM-O bonds in the four NCMs. Scanning electron microscopy (SEM) images (Figure 1a-1d inset) show the cathode samples comprise spherical secondary particles packed by an agglomerate of primary particles. The

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FIGURE 1. (a–d) Observed (dots) and calculated (lines) XRD patterns of LiNi0.9Co0.05Mn0.05O2, LiNi0.7Co0.15Mn0.15O2, LiNi0.5Co0.2Mn0.3O2, and LiNi1/3Co1/3Mn1/3O2 along with tick marks indicating the R3̅m Bragg positions. SEM images of one secondary particle are shown as the inset. Galvanostatic charge-discharge profiles of the four NCMs in coin half cells at C/10 based on (e) upper cutoff voltage (4.4 VLi) and (f) extent of delithiation (80 mol%). secondary particle size is similar (~ 12 μm) while size of the primary particles varies from 1 – 2 μm (LiNi1/3Co1/3Mn1/3O2) to 100 – 200 nm (LiNi0.9Co0.05Mn0.05O2). With a consistent control of crystal structure and morphology, we next evaluate electrochemical charge-discharge behaviors of the four NCM cathodes. Two control setups – charging voltages and extent of delithiation – are applied. In Figure 1e, when charged to 4.4 V vs. Li+/Li (VLi), LiNi0.9Co0.05Mn0.05O2, LiNi0.7Co0.15Mn0.15O2, LiNi0.5Co0.2Mn0.3O2 and LiNi1/3Co1/3Mn1/3O2 deliver a specific capacity of, respectively, 218, 195, 176, 163 mA h g-1 under C/10 (cycling data in Figure S1). Higher accessible capacities at a given voltage with increasing Ni content in the NCM family are well-documented.7,8,11 As discussed above, such a large discrepancy on Li utilization should be avoided to examine the true impact of Co-Mn co-substitution on the anisotropic lattice distortion. Here, the four NCMs are charged to, respectively, 4.49, 4.81, 4.94, and 5.01 VLi for the same 80 mol% delithiation, which correspond to specific capacities of 220 – 222 mA h g-1 (calibrated based on their individual formula weight; Figure 1f). Evolution of open-circuit voltage as a function of Li content during charge-discharge can be found in Figure S2. To establish a correlation of lattice parameter variations with Li content, the charging voltage limits in Figure 1f are used to acquire a series of in situ XRD data on the NCM cathodes, which

were precycled to minimize influences from first-cycle irreversible capacity loss.20 Figure 2a-2b depict the in situ XRD patterns corresponding to the (003) and (110) Bragg reflections, respectively, during one charge-discharge cycle. The former represents lattice changes along the c-direction while the latter along the a-direction. Among the four NCMs upon charge, we notice very similar shifts of the (003) peak, indicative of an initial mild increase followed by a sudden plunge of lattice parameter c. By contrast, the (110) peak shifts are more complex and vary for each composition: while a roughly continuous decline in lattice parameter a is observed for LiNi0.9Co0.05Mn0.05O2, such a trend at deep charge (Li removal ≥ 50 mol%) is reversed for other NCMs, especially LiNi1/3Co1/3Mn1/3O2. These observations are visualized by the calculated lattice parameters as a function of Li content on charge in Figure 2c-2d. Strikingly, our conclusions are the opposite from those in a recent report,11 in which a constant voltage window was applied to a handful of NCMs and the lattice distortion was found to be, with more Ni, notably larger along the c-axis but essentially the same along the a-axis. The evolution of a complete set of diffraction peaks within 10 – 80o for the four NCMs are included in Figure S3-S6. Based on in situ XRD, Co-Mn co-substitution shows little, if at all, effectiveness on inhibiting the anisotropic lattice collapse of NCM cathodes at deep charge: with an extraction of roughly 80


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FIGURE 2. In situ XRD patterns with a sequential offset in intensity of LiNi0.9Co0.05Mn0.05O2, LiNi0.7Co0.15Mn0.15O2, LiNi0.5Co0.2Mn0.3O2, and LiNi1/3Co1/3Mn1/3O2 during one charge-discharge cycle (as in Figure 1f) at C/10, showing shifts of (a) (003) and (b) (110) reflections, respectively. Corresponding calculated lattice parameters along (c) a-axis and (d) c-axis of the four NCMs on charge as a function of Li content. (e) The c/a ratio based on (c) and (d), showing the degree of ‘anisotropy’ of lattice changes. mol% Li, abrupt c-axis lattice contraction is still very severe for LiNi1/3Co1/3Mn1/3O2 (4.9%) relative to LiNi0.9Co0.05Mn0.05O2 (5.7%); worse, for low-Ni NCM, plunge of the interlayer spacing along the c-axis is accompanied by notable lattice expansion along the a-axis. The c/a ratio as a function of Li content, a simple gauge of ‘anisotropy,’ is shown in Figure 2e. Evidently, the lattice changes of NCM cathodes at deep charge, regardless of Ni fraction, are highly anisotropic and will drastically accelerate secondary particle pulverization. Indeed, substantial cracks were observed for LiNi1/3Co1/3Mn1/3O2 charged to 4.7 VLi.25 To provide mechanistic insights into the experimental observations, DFT calculations were conducted on ordered LiNiO2 (i.e., no Li/Ni anti-site mixing) and LiNi1/3Co1/3Mn1/3O2 of different Li content. Figure 3a illustrates the relaxed crystal structures of the two cathodes at 0, 50 and 83 mol% delithiation along with planar-average (along [001]) of the total potential energy in Figure 3b, defined as Vtot = Vionic + VHartree + Vexchangecorrelation. For the c-axis lattice, both the initial moderate expansion and subsequent sudden contraction were shown to be dominated by evolution of the Li-O slabs.8,14 The expansion is generally ascribed to a decreasing screening effect from Li that increases repulsion between adjacent oxygen planes; the contraction occurs when the repulsion diminishes due to combined influences of (i) dissipation of the effective charge on oxygen, and (ii) residual nonlocal dispersion forces (i.e., Van der Waal interactions) with substantial Li vacancies. In Figure 3b, the Li screening effect,

represented by dips in the Vtot curves at the center of interslabs (grey arrows), is essentially identical upon Li removal from LiNiO2 and LiNi1/3Co1/3Mn1/3O2. In the meantime, the dissipation of negative-charge on oxygen is also very similar, as demonstrated by electron-density contour plots for the two cathodes (Figure 3a). From 0 to 50 and 83 mol% delithiation, a similar redistribution of electron density (white arrows) occurs from localized O atoms towards the center of TM-O bonds (i.e., increasing covalency; except for Mn in LiNi1/3Co1/3Mn1/3O2). Based on the above results and with further inclusion of the Van der Waals interactions, DFT predicts a similar extent of the c-axis lattice distortion up to 83 mol% Li extraction between LiNiO2 and LiNi1/3Co1/3Mn1/3O2 (Figure 3c). In Figure 3c, DFT also shows evolution of the a-axis lattice on charge, consistent with the observed trends with different nickel contents in Figure 2c. Due to the Jahn-Teller active Ni(III) (t2g6 eg1)27, Ni atoms undergo ordered → disordered → ordered (+2 → +3 → +4) in LiNi1/3Co1/3Mn1/3O2 during delithiation, whereas the trend follows disordered → ordered (+3 → +4) for those in LiNiO2. According to the Bader charge analysis26 (Figure S7), Ni in Li(1x)Ni1/3Co1/3Mn1/3O2 maintains a lower oxidation state than Li(1x)NiO2. Thus, upon 83 mol% delithiation, the Ni octahedral environment in LiNi1/3Co1/3Mn1/3O2 is statistically more disordered than that in LiNiO2. This explains the slight increase of lattice parameter a of low-Ni NCM at deep charge (Figure 2c; more details in Figure S7 caption).



LiNi1-x-yCoxMnyO2 (NCM) at deep charge. Besides the lattice collapse, many other property measures deemed intrinsic of highNi layered oxide cathode materials, such as surface reactivity with electrolytes (excluding influences from residual Li compounds28) and poor thermal-abuse tolerance, are also significantly contributed by higher Li utilization at a given voltage compared to their low-Ni counterparts. For instance, electrolyte oxidation on high- and low-Ni NCMs was recently revealed to not be driven by cell voltages and to initiate identically along with singlet oxygen release from the cathode at ~ 80 mol% delithiation irrespective of Ni fraction.29,30 Thermal stability was shown to consistently deteriorate with higher Ni content among NCMs, but such results are routinely obtained under the same charging voltages with broadly varying Li utilization. Under identical delithiation, the difference becomes much less dramatic. The striking disparity between our results and those reported lately testifies to the complex implications of compositional tuning and warrants a reassessment of our notion on certain ‘intrinsic’ problems of highNi layered oxide cathodes for practical Li-based automotive batteries.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Methods, additional experimental and DFT results

AUTHOR INFORMATION Corresponding Author *[email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy through the Advanced Battery Materials Research (BMR) Program (Battery500 Consortium) award number DE-EE0007762 and the Welch Foundation grant F-1254. The authors also acknowledge the Texas Advanced Computing Center (TACC) at The University of Texas at Austin for providing HPC resources that have contributed to the research results reported within this paper. URL: http://www.tacc.utexas.edu.

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FIGURE 3. (a) Relaxed crystal structures of ordered LiNiO2 and LiNi1/3Co1/3Mn1/3O2 upon 0, 50, and 83 mol% Li extraction superimposed with electron-density contour plots calculated from DFT. (b) Corresponding planar-average (along [001]) of the total potential energy (Vtot = Vionic + VHartree + Vexchange-correlation). (c) Calculated lattice distortion of the two NCMs on charge.

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In conclusion, by combining in situ XRD analysis and DFT calculations, we demonstrate that the abrupt anisotropic lattice collapse is a universal phenomenon critically dependent on Li utilization, and not Ni content as commonly believed, of layered

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