Structural Evolution and High-Voltage Structural Stability of Li

Article Cite This: Chem. Mater. 2019, 31, 376−386

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Structural Evolution and High-Voltage Structural Stability of Li(NixMnyCoz)O2 Electrodes

Damian Goonetilleke,† Neeraj Sharma,*,† Wei Kong Pang,‡ Vanessa K. Peterson,‡,§ Remi Petibon,∥ Jing Li,⊥ and J. R. Dahn∥,⊥ †

School of Chemistry, UNSW, Sydney, New South Wales 2052, Australia Institute for Superconducting & Electronic Materials, Faculty of Engineering, University of Wollongong, Wollongong 2522, Australia § Australian Centre for Neutron Scattering, Australian Nuclear Science and Technology Organization, Locked Bag 2001, Kirrawee DC, New South Wales 2232, Australia ∥ Department of Chemistry, Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada ⊥ Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada

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‡

S Supporting Information *

ABSTRACT: Positive electrode materials remain a limiting factor for the energy density of lithium-ion batteries (LIBs). Improving the structural stability of these materials over a wider potential window presents an opportune path to higher energy density LIBs. Herein, operando neutron diffraction is used to elucidate the relationship between the structural evolution and electrochemical behavior for a series of Li-ion pouch cells containing Li(NixMnyCoz)O2 (x + y + z = 1) electrode chemistries. The structural stability of these electrodes during charge and discharge cycling across a wide potential window is found to be influenced by the ratio of transition-metal atoms in the material. Of the electrodes investigated in this study, the Li(Ni0.4Mn0.4Co0.2)O2 composition exhibits the smallest magnitude of structural expansion and contraction during cycling while also providing favorable structural stability at high voltage. Greater structural change was observed in electrodes with a higher Ni content, while decreasing inversely to the Ni and Co content in the positive electrode. The combination of structural and electrochemical characterization of a wide range of NMC compositions provides useful insight for the design and application of ideal electrode compositions for long-term cycling and structural stability during storage at the charged state.

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of the cathode material and also avoid parasitic reactions with the electrolyte.6−9 Although certain electrolytes have been shown to exhibit short-term stability at high voltages exceeding 5 V,10−12 cycling of cells at such high voltages is typically avoided to prevent unfavorable structural transitions.13,14 This effectively means that the maximum achievable capacity of the cells is not being utilized. Working toward increasing the cyclability of LIBs at high voltages hence presents a means by which to improve their energy density. Various high-voltage cathode materials have been reported which offer improved energy density compared to current-generation LIBs.15−17 First-generation LiCoO2-based LIBs are favored for their high specific capacity and reliable cycling characteristics and remain

ithium-ion batteries (LIBs) are a relatively wellestablished and thoroughly developed technology which has been widely adopted in many portable devices requiring compact energy storage systems. Newer applications for LIBs, such as electric vehicles (EVs) and grid energy storage, will continue to increase the demand for this technology. These newer applications, particularly EVs, would benefit greatly from improvements in the energy density of lithium-ion batteries, such as increasing the available capacity while reducing the weight/volume of the energy storage system, and allow for more efficient EVs which offer a greater range.1 Layered lithium transition-metal oxides (e.g., LiMO2, where M = Al, Co, Mn, Ni, etc.) have attracted the most interest as cathode materials for LIBs.2−5 These materials offer theoretical specific capacities in excess of 270 mA h/g; however, in reality, such capacities are not achieved as cells are typically cycled over a safe voltage window to decelerate structural degradation © 2018 American Chemical Society

Received: August 19, 2018 Revised: December 15, 2018 Published: December 17, 2018 376

DOI: 10.1021/acs.chemmater.8b03525 Chem. Mater. 2019, 31, 376−386

Article

Chemistry of Materials a commonly used commercial LIB system.18 However, the high cost of elemental cobalt has led to the exploration of alternative positive electrode materials which can be produced more economically.19 Additionally, LiCoO2-based cells are typically cycled only up to ≈4.45 V depending on the applied current. At higher potentials, the amount of Li extracted from the cathode can cause an undesired phase transition from an O3-type LixCoO2 to an O6-type LixCoO2 and an O1-type CoO2. These transitions cause instability in the electrode structure, reducing the useful lifetime of batteries.20,21 A promising alternative is the layered Li(NixMnyCoz)O2 (x + y + z = 1) system, commonly abbreviated to NMC, which is cheaper and able to be operated at higher voltages, increasing the energy density of the battery.22−28 NMC, first reported as the LiNi0.33Mn0.33Co0.33O2 (x = 1/3, NMC111) composition, is a layered system which retains the same α-NaFeO2 structure as LiCoO2, shown in Figure 1.4,5 Many isostructural variants of

using X-ray scattering methods,56 and neutrons are able to penetrate through the battery casing as well as active components, allowing the study of commercial cells with little or no modification.57−60 However, the large incoherent scattering from hydrogen from components such as the electrolyte and separator can obscure the diffraction signal from components of interest such as the electrodes. The substitution of hydrogen for deuterium such as in the deuterated forms of carbonate-based electrolytes typically used in Li-ion batteries is sometimes necessary, but can be prohibitively expensive.58 Specially designed in situ cells have previously been used to study the structural changes taking place in the electrodes,61−65 but will not necessarily perform identically to commercial cells. The study of NMC-based batteries using a newly developed neutron-friendly deuterated ethyl acetate (d-EA) electrolyte has been reported.31 The electrolyte is a suitable cost-effective alternative to carbonatebased electrolytes, which exhibits stability at high voltages and does not adversely affect the performance of the battery for tests lasting less than a few months. The previous study revealed the structural evolution of the NMC442 positive electrode containing the newly developed d-EA electrolyte during cell operation. A reversible structural evolution was observed when cycling to 4.7 V; however, electrolyte decomposition occurred when holding the cell at the higher voltage of 4.9 V.66 This study presents an operando study of NMC electrodes of composition Li(Ni0.33Mn0.33Co0.33)O2, Li(Ni0.4Mn0.4Co0.2)O2, Li(Ni0.5Mn0.3Co0.2)O2, Li(Ni0.5Mn0.4Co0.1)O2, and Li(Ni 0.8 Mn 0.1 Co 0.1 )O 2 , known as NMC111, NMC442, NMC532, NMC541, and NMC811, respectively. This set of electrodes allows exploration of the Ni-poor to Ni-rich compositional parameter space. The positive-electrode materials are studied in a pouch cell using the d-EA-based electrolyte. Rietveld analysis was used to study the structural evolution of the NMC lattice as a function of cell voltage, allowing us to investigate structural−electrochemical relationships and structural stability as a function of electrode composition and at high voltage. Understanding the structural changes which occur upon lithium insertion and extraction is fundamental to developing new and improved electrode materials. A comprehensive comparison of the structural evolution which takes place during cycling is presented by studying LIBs containing various ratios of transition metals using operando neutron diffraction.

Figure 1. Refined crystal structure of layered trigonal Li(Ni0.33Mn0.33Co0.33)O2.39 Illustration rendered using VESTA.40

this system have since been reported,29−31 which have been found to have good rate capability and electrochemical performance, in some cases with reversible capacities in excess of 200 mA h/g within a voltage window of 2.5−4.6 V while only exhibiting a 1−2% volume change.32−34 The Ni, Co, and Mn atoms adopt valence states of 2+/3+, 3+, and 4+, respectively,29,35 and electronic structure studies suggest that oxidation of Ni2+ to Ni4+ followed by Co3+ to Co4+ at higher voltage contributes to the considerable reversible capacity of the material. Various compositions of NMC have been synthesized and tested to overcome the drawbacks of LiCoO2 (mentioned above), LiNiO2 (capacity fade and poor safety),30 and LiMn2O4 (capacity fade and low capacity),36 while retaining favorable properties: stability at high current densities (LiCoO2), high capacity (LiNiO2), and improved thermal stability (LiMn2O4).37,38 The grand challenge for the NMC electrode is to tune composition to optimize electrochemical properties that target the energy storage requirements of specific applications. For this to occur, the compositional influence on structure−electrochemistry relationships needs to be established for a range of compositions. Neutron powder diffraction has been established as an excellent in situ or operando technique for investigating the crystallographic transitions which occur in materials inside functional devices, such as batteries.41−55 Neutron scattering cross sections are favorable to allow a greater sensitivity to elements commonly found in battery electrodes than obtained

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EXPERIMENTAL SECTION

Pouch Cells. Dry (no electrolyte) wound 240 mA h Li(NixMnyCoz)O2/graphite pouch-type cells were obtained from LiFUN Technology (Xinma Industry Zone, Golden Dragon Road, Tianyuan District, Zhuzhou City, Hunan Province, PRC, 412000). The external dimension of the pouch cell is 40 mm × 20 mm × 3.5 mm (or canonical size 402 035). The cells were vacuum-sealed in an assembly dry room in China and then shipped to Dalhousie University in Canada. The active-mass ratio of negative to positive electrode materials was balanced so that the cells could be cycled to at least to 4.7 V, depending on the composition, without lithium plating. The negative electrode contained 15−30 μm artificial graphite flakes blended with carbon black as conducting additive and carboxymethyl cellulose−styrene−butadiene rubber as binder, in a weight ratio of 96:2:2. The positive electrode consisted of 7−15 μm NMC particles mixed with carbon black and polyvinylidene fluoride in a weight ratio of 96:2:2. The separator consisted of a porous polypropylene membrane coated with Al2O3 on the positive-electrode side, with a 377


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total thickness of 20 μm. The as-received dry cells were cut open just below the vacuum seal and placed in a vacuum oven at 80 °C overnight (approximately 14 h) to remove residual water. After drying, the cells were transferred to an argon-filled glovebox without exposure to ambient air, where they were filled with 0.85 mL electrolyte. The electrolyte consisted of lithium hexafluorophosphate (LiPF6, BASF, 99.9%), lithium bis(fluorosulfonyl)imide (LiFSi, HSC Corporation, Zhangjiagang, Jiangsu, China, purity > 99.95%, water content < 50 ppm, free chlorine < 50 ppm), and deuterated EA (d8EA, CDN Isotopes, 99 atom % D, 99.5% purity) in a 1:1:2 molar ratio of LiFSi/LiPF6/d8-EA. This electrolyte has been shown previously to remain stable during cycling between 2.8 and 4.7 V for up to 250 h at elevated temperatures without significant adverse effects on capacity retention, polarization growth, or gas evolution.31 The cells were subsequently sealed with a compact vacuum sealer (MSK-115V, MTI Corp.) to ∼94% of full vacuum (−95.2 kPa gauge pressure or 6.1 kPa absolute pressure) with a 6 s sealing time at 165 °C. Formation. All cells were placed in a temperature-controlled box at 40.0 ± 0.1 °C and held at 1.5 V for 24 h to ensure complete wetting of the electrodes with the electrolyte. The cells were then charged at C/20 to 3.5 V using a Maccor series 4000 automated test system (Maccor Inc.), where C/20 is the current required to complete a full charge or discharge in 20 h. The cells were held at 3.5 V until the current reached a value smaller than C/200. The cells were then transferred to an argon-filled glovebox and cut open to release any gas produced. The cells were then vacuum-sealed as previously described. Prior to shipping to the Australian Nuclear Science and Technology Organization (ANSTO) for the NPD experiments, the cells were charged to 3.6 V at C/20. Operando NPD. During the neutron diffraction measurements, the cells were first charged to 4.7 V, discharged to 2.5 V, and then finally charged to 4.7 V and held at 4.7 V potentiostatically. A current of C/10 (24 mA) was used for the charge/discharge steps with an Autolab PGSTAT302N potentiostat. NPD data were simultaneously collected on the high-intensity neutron powder diffractometer WOMBAT67 at the Open Pool Australian Light-Water (OPAL) research reactor facility at ANSTO. WOMBAT features an area detector covering 120° in 2θ, enabling diffraction data to be continuously collected rather than a 2θ-step-scan type acquisition. The battery was placed in the neutron beam and patterns were collected every 5 min for 59 h in the two theta (2θ) range 16° ≤ 2θ ≤ 136°. The sample orientation was optimized to give good data in the 2θ regions of interest. The wavelength of the neutron beam was determined using the NIST La11B6 660b Standard Reference Material.68 The wavelength of the beam was determined to be λ = 2.417(6) Å for NMC111, NMC541, and NMC811, and λ = 2.422(1) Å for NMC442 and NMC532. Collected data were processed using LAMP.69 Rietveld refinements were performed using GSAS-II.70 It should be noted that the NMC442 and NMC532 cells were stored for 1 year before data collection and open circuit voltages (OCPs) recorded prior to data collection are listed in Table 1.

a (Å)

b (Å)

c (Å)

Rw (%)

OCP (V)

2.809(8) 2.834(3) 2.830(5) 2.831(3) 2.823(4)

2.809(7) 2.834(3) 2.830(5) 2.831(3) 2.823(4)

14.342(34) 14.442(14) 14.423(16) 14.424(15) 14.478(11)

6.31 6.23 3.70 3.50 3.98

3.66 3.75 3.65 3.72 3.76

RESULTS AND DISCUSSION

The conclusions about the structural evolution taking place in these cells are dependent on the quality of the Rietveld analysis. Multiphase sequential Rietveld refinements of structural models were carried out against the operando diffraction data. It is important to note that several constraints were applied to the refinements because of the nature of the diffraction data, which do not have sufficient resolution for precise structural solution. The resolution is limited by the fact that the diffraction data contain strong background contributions from the cell casing and electrolyte and contributions from multiple phases which may or may not be electrochemically active. The positive electrode material was modeled using a Li(Ni0.33Mn0.33Co0.33)O2 phase with space group R3̅m.71 The site occupancies of the Ni, Co, and Mn sites were adjusted to reflect the cathode chemistry for each cell, but their occupancies were not refined. The negative electrode material was refined using LiC6 and LiC12 phases with space groups P6/ mmm and P63/mmc, respectively.72 In addition, reflections corresponding to the copper current collectors and aluminum cell casing components are also identified; see Figure S1. The electrochemically inactive phases identified were Cu (Fm3̅m)73 and Al (Fm3̅m).74 The structural cell parameters of the electrochemically inactive phases were refined only for the first dataset and then fixed as they do not undergo structural changes during electrochemical cycling. An additional broad reflection at 2θ = 34.5° was observed in some of the cells and is also noted to be electrochemically inactive. Other parameters which were fixed after the first dataset include (i) peak profile functions, (ii) atomic parameters such as atomic coordinates, atomic occupancies, and atomic displacement parameters, and (iii) the background which was modeled using a Chebyschev function with 8−19 terms depending on the background contributions in each sample. Parameters allowed to refine throughout cycling include the (i) sample displacement parameter, (ii) scale factor for each phase, and (iii) cell parameters for the electrochemically active phases. Table 1 shows the results of refinements to the first diffraction patterns collected from each of the cells. The associated Rietveldrefined models of these initial datasets are shown in Figure S1, with statistical measures of the quality of fits shown in Table S1. During sequential refinements, the statistics can vary between each dataset. For the different samples, the Rw values were generally found to vary between 4 and 7%. This variability highlights the unsuitability of these data for precise structural solution. Neutron diffraction is highly sensitive toward lighter elements such as Li and can provide sufficient contrast between elements of similar atomic number such as those in transition-metal cathodes.75 Despite this, accurate modeling of the Li content in the electrode or transition-metal layer ordering was not possible in this study because of overlapping contributions to the diffraction pattern from inactive cell components and the background profile. The high penetration of neutron radiation was used here to study an unmodified, commercially relevant cell. More recent studies using specially designed cells have demonstrated that refinement of atomic parameters is possible.76,77 However, the clearly resolved peaks from the active materials allow for accurate modeling of changes in cell parameters as a function of the electrochemical state. Negative Electrode. Figure 2 shows the evolution of the diffraction data as a function of time for the cells tested. The

Table 1. Lattice Parameters as Determined by Rietveld Refinements of the NMC Structural Models39 against the Datasets Shown in Figure S1a−e, with Cells at As-Received OCP Prior to Cyclinga NMC111 NMC442 NMC532 NMC541 NMC811

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The weighted R-profile figure of merit (Rw) for the refinement is also given.

a

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Figure S1. It should be noted that the shape of the background profile varies between the samples, which can be attributed to the orientation of the batteries with respect to the neutron beam and detector.57 The 2θ position of the labeled NMC reflections, Figure 2b, shift to higher angles as the voltage of the battery is increased. This corresponds to a shrinkage of the c lattice parameter as the Li content in the positive electrode is reduced. The 2θ position of the 003 reflection appears to reach a local maximum (minimum in the c lattice parameter) as the cell reaches its minimum voltage (2.5 V) before decreasing until the cell reaches 4 V, and then increasing again as the cell is charged to 4.7 V. Figure 3 shows the evolution of the NMC111 lattice parameters throughout a single charge/discharge cycle. The a

Figure 2. Contour plots with intensity shown increasing from darkest to lightest of the diffraction patterns as a function of time. (a) NMC111 and (b) NMC541. The voltage of the cell during the data collection is shown alongside.

batteries’ voltage as a function of time is shown alongside these data. During the charging processes, a new lithiated graphite phase LiC6, shown in pink in Figure 2a, is formed as the lithium concentration in the graphite negative electrode increases toward the charged state. During discharge, the intensity of the LiC6 001 reflection reduces, while the intensity of the LiC12 002 reflection increases. As the battery is discharged further, the lithiated graphite 002 reflection shifts to a higher 2θ value, the c lattice parameter of lithiated graphite reduces as more lithium is removed from the negative electrode during discharge. The phase staging that occurs in graphite as a negative electrode material for LIBs has been studied previously,78−80 and the higher current rate observations are consistent with the expected evolution of the graphite negative electrode in these cells. It should be noted that at the current rate used, the transition from LiC12 to approximately graphite is akin to a solid solution reaction, but lower currents are likely to show the multiple solid solution and two-phase regions in the process. Positive Electrode. In Figure 2b, some of the prominent positive electrode reflections have been labeled. Lattice parameters derived from the Rietveld refinement of operando diffraction data with the cell at OCP are shown in Table 1, with the corresponding Rietveld refinement profiles shown in

Figure 3. Change in cell parameters as a function of time during electrochemical cycling for NMC111.

lattice parameter is observed to expand and contract in an approximately opposite manner to the c lattice parameter. This behavior is typical of layered positive electrode materials and has been previously observed during studies of NMC electrodes; see Table 2. As lithium is extracted from the positive electrode and transition-metal ions are oxidized to species with smaller ionic radii, Ni2+ (0.69 Å)and Co3+ (0.65 Å) undergo oxidation to Ni3+/Ni4+ (0.56 Å/0.48 Å) and Co4+ (0.545 Å), respectively, to compensate for loss of Li+ in the structure.81,82 Thus a decrease in the a lattice parameter is observed because of the decrease in the average distance between the transition-metal atoms. Figure 4a−e shows the change in the lattice parameters as a function of cell voltage during a single charge and discharge cycle. From 2.5 to ≈3 V, the cell parameters appear to remain relatively constant as a 379


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Table 2. Previously Reported Lattice Parameters Reported in Papers Regarding Operando Studies of Commercial LIBs Using NMC Electrodes Cycled between 3 and 4.2 Va composition

amin (Å)

amax (Å)

cmin (Å)

cmax (Å)

refs

Lix(Ni0.5Co0.25Mn0.25)O2 LixNiyCozMn(1−y−z)O2 Lix(Ni0.5Mn0.3Co0.2)O2 Lix(Ni0.42Mn0.42Co0.16)O2 Lix(Ni0.52Mn0.24Co0.24)O2

2.811 2.82* 2.81880(5) 2.822 2.8182(8)

2.864 2.85* 2.86601(5) 2.876(2) 2.844(1)

14.2004 14.22* 14.28635(45) 14.12(1) 14.41(1)

14.48* 14.40* 14.44935(64) 14.32(1) 14.467(5)

89 90 72 66 59

Values marked with * have been extrapolated from figures.

a

Figure 4. Lattice parameters of (a) NMC111, (b) NMC442, (c) NMC532, (d) NMC541, and (e) NMC811 as a function of cell voltage (V) during a single charge/discharge cycle. Cell volume as a function of cell voltage for the different NMC compositions as determined by Rietveld refinement. (f) Comparison of change in cell volume as a function of cell voltage for the different NMC compositions.

most significant expansion is observed in NMC811, which exhibits a change in c and a lattice parameters of 0.72(2) and 0.0529(3) Å, respectively, between 2.5 and 4.7 V, or 5.25 and 1.89%. The minimum a and c lattice parameters observed for NMC811, 13.73(2) and 2.802(3) Å, respectively, are also significantly lower than for the other NMC compositions tested, suggesting the higher nickel content may lead to greater contraction of the stacking axis as lithium is extracted. As Li is intercalated/extracted, the valence of the MO6 (M = Ni, Mn, Co) octahedra changes progressively, which is accompanied by changes in ionic radii.81 Considering the stacking axis only, NMC811 is followed by NMC111 (Δ 0.66(2) Å), NMC532

function of cell voltage which is consistent with the limited low capacity delivered in this voltage window; see Figure 5. A common feature observed in all the compositions is the initial increase in the c lattice parameter until ≈4 V due to increasing electrostatic repulsion between the transition-metal octahedra as the occupancy of the lithium layer is reduced. Beyond this voltage, the c lattice parameter decreases rapidly, which is consistent with previous studies of layered intercalation compounds.83−86 Larger changes in the lattice parameter are generally unfavorable as the larger amount of contraction/ expansion results in greater repeated mechanical stress on the material with cycling.87,88 From Table 3, it is evident that the 380


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smallest change in unit cell volume during cycling (3.36%). To identify any trends between composition and the change in cell parameters, Figure 6 shows the percentage change in each of the lattice parameters as a function of atomic composition. The minimum and maximum values of the cell parameters do not show strong correlation with atomic composition. However, the change in cell volume during cycling tends to be higher in cells with a positive electrode containing a higher Ni content and decreases inversely to the Mn and Co content. Previous studies suggest that the Ni and Co redox couples are electrochemically active and have a significant effect on the cell capacity.27 The Mn4+, however, does not participate in the electrochemical reactions but instead helps to stabilize the structure.17,91 Of the compositions tested, those with the highest Mn content (NMC442 and NMC541) exhibit the smallest change in c lattice expansion in the voltage range explored (2.5−4.7 V), while the largest change in lattice parameters is observed for the Ni-rich NM811 composition. Voltage profiles during charge and discharge are shown in Figure 5. Subtle differences in the shape of the voltage curves are evident. Closer observation of the electrochemical curve for Ni-rich NMC811 composition shows an extra feature in the 4−4.5 V region, which is not present in the other samples. Comparing NMC541 and NMC532 with the same Ni content but different Mn and Co, the lattice evolution as a function of voltage is similar, and these compositions also exhibit a similar change in the c lattice parameter of 0.44(3) Å in NMC532 compared to 0.41(1) Å in NMC541 (see Table 3). The two compositions with the same Mn content, NMC442 and NMC541, exhibit changes in the c lattice parameter of 0.29(3) and 0.41(1) Å, respectively, or a relative change of 2.09 and 2.9%, respectively. This is in agreement with previous highvoltage structural studies of NMC442.66 Comparing the same Co content in NMC541 and NMC811, here, the higher Ni content dominates generating larger a and c lattice changes. Generating trends from the different compositions and the structural data is difficult without a wider range of compositions; however, some interesting inferences can be made. In moving from NMC111 to NMC532 or NMC442, the Co content is lowered and the Ni content increased, but in one case, the Mn is slightly lowered, while in the other it is slightly increased. The combination of Ni and Mn content increase reduces the change in the c lattice parameter. Moving from NMC111 to NMC541, the combination of lowering Co content and increasing Ni content the most, NMC541, results in reduced c lattice change. Going from NMC111 to NMC811, a significant increase in Ni content, while reducing both Mn content and Co content, produces the most unfavorable expansion/contraction during cycling. To investigate the structural stability of the different positive electrode materials at high voltage, the cells were subject to a potentiostatic hold at 4.7 V after the second charge step, reflecting storage of the batteries at the charged state for an

Figure 5. Capacity vs potential plots for charge (solid-line) and discharge (dashed-line) processes of the different NMC compositions.

(Δ 0.44(3) Å), and NMC541 (Δ 0.41(1), and the smallest contraction is observed in NMC442 (Δ 0.29(3) Å). The maximum lattice volume observed for each of the NMC compositions occurs at varying states of charge, at ≈3 V for NMC442, and as high as 3.31 V for NMC541; see Table 4. A comparison of the changes in cell volume as a function of voltage is shown in Figure 4f. For all the NMC compositions, the minimum lattice volume is observed as the cell approaches the charged state, as the lithium content in the positive electrode diminishes. NMC811 exhibits the largest change in unit cell volume during cycling (6.97%) and also the smallest volume at the charged state (≈93.4 Å3). NMC442 shows the

Table 3. Maximum and Minimum Parameters Observed during a Single Charge/Discharge Cycle composition

cmin (Å)

cmax (Å)

Δc (Å)

amin (Å)

amax (Å)

Δa (Å)

NMC111 NMC442 NMC532 NMC541 NMC811

13.92(1) 14.10(2) 13.96(2) 14.10(1) 13.71(2)

14.57(1) 14.39(2) 14.39(2) 14.51(1) 14.43(1)

0.66(2) 0.29(3) 0.44(3) 0.41(1) 0.72(2)

2.820(2) 2.826(3) 2.820(3) 2.822(2) 2.802(3)

2.852(2) 2.857(2) 2.86(4) 2.867(1) 2.855(1)

0.0316(3) 0.031(4) 0.042(5) 0.045(1) 0.0529(3)

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Chemistry of Materials Table 4. Maximum and Minimum Cell Volumes Observed during a Charge/Discharge Cycle NMC111 NMC442 NMC532 NMC541 NMC811

voltage/V

volume max/Å3

voltage/V

volume min/Å3

Δvolume/Å3

3.28 3.00 3.24 3.31 3.24

101.12(9) 100.93(10) 101.11(21) 101.68(6) 100.41(7)

4.586 4.544 4.72 4.57 4.578

96.21(9) 97.54(12) 96.34(16) 97.36(7) 93.41(10)

4.91(13) 3.389(2) 4.77(26) 4.33(1) 7.00(12)

Figure 6. Structural response (% change) as a function of atomic composition. (a) Change in cell volume. (b) Change in the c lattice parameter. (c) Change in the a lattice parameter. Structural response was calculated by finding the difference between the maximum and minimum cell parameters observed during a complete discharge and charge cycle between 2.5 and 4.7 V. The size of each data point is proportional to the magnitude of the response to provide a visual comparison as a function of atomic composition.

results of fitting are shown in Table S3. The c lattice parameter relaxes in an approximately exponential fashion during the voltage hold step. Comparison of the fitting parameters shows that the exponential coefficient (A1) is highest for NMC811 and lowest for NMC532. The relaxation time parameter (t1) reflects the rate of decay of the lattice parameter. It is found to be significantly higher for NMC442 compared to the other compositions and lowest for NMC111. To more easily visualize trends in the structural relaxation between the different electrode chemistries, Figure 9 shows the relative change in the c lattice parameter during the voltage hold step as a function of atomic composition. In the long term, smaller changes in cell volume are favorable as this reduces the amount of cyclic mechanical stress on the electrode and interface materials. NMC442 displayed the lowest magnitude of structural evolution during cycling and onlyappears to show marginally larger changes in the c lattice parameter at the 4.7 V hold step.

extended period. The evolution of the c lattice parameters, representing expansion/contraction of the stacking axis, of the NMC phases for the different compositions during the voltage hold step is shown in Figure 7a−e. During the voltage hold, some of the pouch cells became swollen because of the evolution of gas within the cell; however, there was no visual evidence of sample displacement relative to the scattering axis of the instrument during the voltage hold at 4.7 V, which would be indicated by a shift in the position of reflections corresponding to electrochemically inactive components. It should be noted that the structural evolution at 4.7 V may also be influenced by cell resistance which may vary with different cells, especially if gasses are generated. The current and capacity measured during the voltage hold step are shown in Figure 8. A gradual decrease in c lattice parameter during voltage hold has been previously observed for Li-rich NMC materials,86 and the same behavior is seen in the cells used in this study. Previous studies attribute the change in the lattice parameter during voltage hold to cation migration to fill vacancies created by extraction of lithium during charge. NMC111 and NMC811 exhibit an initial sharp decrease in the lattice parameter before decreasing more gradually, while the other compositions show smoother structural relaxation. Another important consideration is the absolute magnitude of the contraction. Table 5 shows the change in the c lattice parameter throughout the constant voltage hold. The initial lattice parameter was recorded at the beginning of the voltage hold step (t = 0), indicated by the dashed lines in Figure 7. The c lattice parameter of NMC541 undergoes a decrease from 14.11(2) to 13.91(1) Å (or 1.42%) after being held at high voltage for 14 h, the largest change of the compositions studied. NMC442 exhibits the next largest change in the c lattice parameter. Interestingly, the Ni-rich NMC811 exhibits the smallest structural relaxation, with a decrease in the c lattice parameter of 0.044(24) Å after 14 h at 4.7 V. An exponential decay function was fitted to the data in Figure 7 to model the behavior of the structure during the voltage hold step. The

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CONCLUSIONS Operando neutron diffraction was used to investigate the structure−electrochemistry relationships in a series of Li-ion cells containing NMC positive electrodes. The ratio of transition-metal atoms can be controlled to influence the magnitude of structural evolution during cycling, the structural stability at high voltage, as well as the electrochemical performance of the electrode. The change in lattice parameters of the positive electrode material between the charged and discharged state was found to be greatest for the Ni-rich NMC811 composition and lowest for the NMC442 and NMC541 compositions with moderate Mn content. During the 4.7 V potentiostatic hold, NMC541 showed the highest change in the c lattice parameter, while NMC811 and NMC111 showed the smallest changes. Of the compositions tested, NMC442 appears to offer the best structural stability when all cells are cycled within the same voltage window. It should be noted that as the different compositions will exhibit 382


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Chemistry of Materials Table 5. Change in the c Lattice Parameter during Potentiostatic Hold at 4.7 V c/Å

t=0

t = 14 h

Δ/Å

NMC111 NMC442 NMC532 NMC541 NMC811

13.89(2) 14.08(2) 13.96(2) 14.11(2) 13.75(2)

13.81(2) 13.99(3) 13.87(2) 13.91(1) 13.71(1)

−0.080(27) −0.101(35) −0.089(28) −0.20(2) −0.044(24)

Figure 9. Change in the c lattice parameter (Δc %) as a function of atomic composition after potentiostatic hold at 4.7 V for 14 h. The size of each data point is proportional to the magnitude of the relaxation to provide a visual comparison as a function of atomic composition.

electrodes influences the electrochemical performance. The composition of the transition-metal layers must be optimized to achieve favorable properties.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 7. NMC c lattice parameters as a function of time during highvoltage hold step. (a) NMC111, (b) NMC442, (c) NMC532, (d) NMC541, and (e) NMC811.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b03525. Rietveld refinement profiles and statistics of the initial datasets, electrochemical data collected during operando diffraction studies, and results of exponential decay function fitting to data shown in Figure 7 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +61 2 9385 4714. Fax: +61 2 9385 6141. ORCID

Neeraj Sharma: 0000-0003-1197-6343 Wei Kong Pang: 0000-0002-5118-3885 Vanessa K. Peterson: 0000-0002-5442-0591 Jing Li: 0000-0003-3698-7102 J. R. Dahn: 0000-0002-6997-2436

Figure 8. Measured (a) capacity and (b) current during the voltage hold step.

Funding

N.S. would like to thank the Australian Research Council for funding through grants DE160100237 and DP170100269 and W.K.P. through FT160100251. D.G. would like to thank the Research Training Program for PhD funds.

different capacities over a particular voltage window, it may be interesting to perform a similar study comparing the structural evolution with all compositions restricted to deliver the same capacity. This work clearly shows that it is also critical to consider how managing the structural evolution of the

Notes

The authors declare no competing financial interest. 383


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ACKNOWLEDGMENTS Authors wish to recognize the staff at the Australian Centre for Neutron Scattering for assistance during the experiment.

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