Time-Dependent in-Situ Neutron Diffraction Investigation of a Li(Co0

ARTICLE pubs.acs.org/JPCC

Time-Dependent in-Situ Neutron Diffraction Investigation of a Li(Co0.16Mn1.84)O4 Cathode Neeraj Sharma,*,† M. V. Reddy,‡ Guodong Du,§ Stefan Adams,# B. V. R. Chowdari,‡ Zaiping Guo,§ and Vanessa K. Peterson† †

The Bragg Institute, Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, New South Wales 2232, Australia ‡ Department of Physics, National University of Singapore, Singapore 117542, Singapore § School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, Wollongong, New South Wales 2522, Australia # Department of Materials Science and Engineering, National University of Singapore, Singapore 117542, Singapore ABSTRACT: Real-time in-situ neutron diffraction data reveal for the first time that the Li(Co0.16Mn1.84)O4 cathode undergoes current-free discharge. We find that current-free discharge occurs in a partially charged Li(Co0.16Mn1.84)O4 cathode during its first charge cycle over a period of 11 h resulting in a 44(2)% expansion of the crystal lattice. The rate of change in the lattice parameter during the current-free discharge process is half the rate and more linear than for an applied-current discharge of 0.5 mA. The origins of currentfree discharge are discussed along with the implications of nonequilibrium relaxation processes in in-situ neutron and X-ray diffraction studies. We show that the lattice does not return to the predischarge values after either currentapplied or current-free discharge, indicating a limited ability for Li reinsertion (capacity loss) in partially charged Li(Co0.16Mn1.84)O4 batteries.

’ INTRODUCTION Li-ion batteries are used to power personal electronic devices such as mobile phones and laptop computers. Research and development aimed at improving the functional materials in these batteries, for example, anodes and cathodes, may facilitate the widespread use of this technology in hybrid and fully electric vehicles.1,2 Commonly used cathode materials include LiCoO2,3 LiFePO4,4 and LiMn2O4.5,6 These materials are termed intercalation compounds because Li is extracted during charge and reinserted (or intercalated) during discharge.7 The process of insertion and extraction of Li-ions leads to crystal-structure changes in the parent cathode, which can include lattice parameter changes, that is, shrinkage or expansion of the unit cell and/or fully fledged phase transitions.7 The cubic spinel-type Li1 xMn2O4 preserves the spinel-type structure between 0 e x e 1 with charge discharge from 3.8 to 4.4 V.8 LiMn2O4 features lower manufacturing costs and longer cycle-lives in some cases, and is more environmentally friendly than the more widely used LiCoO2 (Mn is more benign than Co1). However, capacity-fade and undesirable temperature-induced structural transformations have limited the development of LiMn2O4 batteries.9,10 Various structural studies have explored the relationship between voltage, Li content, and phase evolution of LiMn2O4. These studies are complicated by the synthesis-dependent crystal chemistry of the samples, the structural phase-transitions, reliance on ex situ experimentation, and the difficulty of Li-content determination (Li content is inferred from voltage or from r 2011 American Chemical Society

equilibrated samples).11 14 In-situ synchrotron X-ray diffraction (XRD) and in-situ neutron diffraction (ND) have been used to follow the phase evolution of LiMn2O4 as a function of charge/ discharge (Li extraction/insertion).8,10,15 In-situ XRD is generally not used to investigate relatively large prismatic or 18650 cylindrical-type16 batteries due to the limited penetration of X-rays into the sample. Neutrons are advantageous to X-rays for studying such batteries due to their higher sample penetration depth relative to X-rays, enabled by the nuclear-scattering mechanism. The bulk-sample measurement capability of ND results in all battery components contributing to the data, which is multiphase and reflection-dense, and consequently often complicated to analyze. Additionally, hydrogenous components such as the separator and liquid electrolyte reduce the signal-to-noise ratio of the ND data by significantly contributing to the background. A consequence of these complications is that the use of ND for in-situ studies in the Li-ion battery research community has been relatively limited.15,17 23 To overcome these difficulties, a custom-designed cell has been developed21 to measure the crystal structures of Li-ion battery components using in-situ ND with continuous charge discharge. This cell has been optimized for use on instruments such as Wombat, the high-intensity powder diffractometer at the Open Pool Australian Light-water Received: March 20, 2011 Revised: September 19, 2011 Published: September 21, 2011 21473

dx.doi.org/10.1021/jp2026237 | J. Phys. Chem. C 2011, 115, 21473–21480

The Journal of Physical Chemistry C (OPAL) reactor facility,24 providing the capability to measure real-time data that is sufficient for Rietveld analysis. Previous work applying in-situ ND to study Li1 xMn2O4 cathodes during electrochemical cycling used relatively large (ca. 3 g) quantities of LiMn2O4.15 The “in-situ” nature of this previous work was with respect only to the study of the cathode within the battery and not to the state-of-charge of the battery, as ND data were collected (for an unreported period of time) on precharged cells (1.0 mA or C/750).15 These in-situ ND experiments used a cell design different from that used here and attributed the observed anomalous lattice expansion at 4.16 V to an experimental mishap or an unresolved phase.8,15 Significant diffraction line-broadening was also noted,15 which may indicate structural changes, as a result of charge equilibration, that are unresolved on the time scale of the measurement. The charging current passed through a LiMn2O4 cathode was shown not to be directly proportional to the Li content.10,15 The difference between the applied current and Li content attributed to the formation of a surface layer on the cathode consuming charge and current,15 similar to the solid electrolyte interface (SEI) layer found on graphitic anodes.7 Furthermore, no twophase region at x > 0.6 is shown with in-situ ND, a finding attributed to the slow charging rate relative to the in-situ XRD and Raman studies.15,25,26 Thus, the collection of real-time in-situ data during electrochemical cycling would enable the effects of charge equilibration and nonequilibrium processes of LiMn2O4 batteries to be better understood and therefore lead to the development of better batteries. In the present work, we study a Co-doped LiMn2O4 cathode material. The main advantage of substituting Co3+ for Mn3+ is to prevent the disproportionation of Mn3+ to Mn2+ and Mn4+, associated with a loss in cathode performance caused by the dissolution of Mn2+ into the electrolyte.10,27 Co3+ substitution increases the average valency of Mn and reduces the quantity of Mn3+ thereby reducing the Jahn Teller effect, and consequently the theoretical capacity of electrode may be obtained.28 In practice, the substitution of cations such as Co for Mn eliminates two-phase behavior, observed during delithiation of Li1 xMn2O4 at x > 0.6,11,14 potentially leading to smaller stresses on the cathode during cycling. More than 2400 publications exist that study LiMn2O4 cathodes, but only ca. 50 studies focus on Codoped LiMn2O4 materials, and none that use in-situ XRD or ND. Here we use in-situ ND to study the spinel-type Li(Co0.16Mn1.84)O4 cathode, a material that does not significantly suffer from Mn dissolution into the electrolyte and exhibits only solidsolution behavior, both of which may facilitate better cathode function. We observe and quantify the occurrence of current-free discharge along with the real-time structural changes of the cathode occurring during charge discharge processes. This work explores the first few charge discharge cycles and measures capacity loss directly from changes to the material’s lattice parameters.

’ EXPERIMENTAL SECTION LiCo0.16Mn1.84O4 was synthesized using stoichiometric amounts of Li2CO3 (Merck, 99% chemical purity), Co(OH)2 (Aldrich, 95% chemical purity), and MnCO3 3 xH2O (Merck, 98% chemical purity), mixed with NaCl (Merck, 99.5% chemical purity) and KCl (Merck, 99.5% chemical purity), to produce a eutectic composition. The molar ratio of metal ions to the eutectic composition was fixed to 1:6. The mixture was ground

ARTICLE

and transferred to an alumina crucible, followed by a heat treatment of 800 °C for 6 h in air. The samples were washed with water to remove the excess Li salts, filtered, and then dried at 70 °C under a vacuum for 12 h to produce black, crystalline powders. Energy-dispersive spectroscopy (EDS) was undertaken on a JEOL 6701F scanning electron microscope. Density determination of pure powder was carried out using an AccuPyc 1330 (Micromeritics, USA) pycnometer. The cathode for use in the electrochemical cell was prepared using Li(Co0.16Mn1.84)O4 mixed with carbon black and poly vinyl difluoride (PVDF) to form a paste. The paste was applied to an aluminum foil, dried under a vacuum at 110 °C overnight and placed into an argon glovebox (0.1 ppm < O2 and 0.1 ppm < H2O). The thickness of the electrode on the aluminum foil was approximately 400 μm, with a mass of ∼643 mg, density of 0.31 g cm 3 and the foil upon which the electrode was deposited on was cut to size (∼14 3.7 cm) to fit into the battery. The area of the electrode was ∼51.8 cm2 resulting in a loading per unit area of 0.012 g cm 2. The roll-over cell21 was constructed in an argon glovebox from the following layers arranged in order: Celgard (insulator), Al-foil coated in the composite cathodepaste, Celgard (separator), and Li metal. Cu wires were placed in contact with both electrodes. The layered arrangement was rolled (using the outer Celgard layer) and inserted into a vanadium can of 9 mm inner diameter. The electrolyte solution was deuterated to mitigate unwanted neutron scattering from hydrogen21,22 and formed from 1 M LiPF6 dissolved in a 1:1 vol % mixture of deuterated ethylene carbonate (CDN, 99% isotopic purity and 99.3% chemical purity) and deuterated dimethyl carbonate (Cambridge Isotopes, 99% isotopic purity and 98% chemical purity). The electrolyte was added dropwise to the cell, which was then sealed using wax. Coin-type cells for offline experiments were constructed as described elsewhere.29 Ex situ ND data of Li(Co0.16Mn1.84)O4 were collected on Echidna, the high-resolution powder diffractometer, at the OPAL reactor facility at the Australian Nuclear Science and Technology Organization (ANSTO).30 Data were collected at λ = 1.6215(1) Å for 4 h in the two-theta (2θ) range 5 e 2θ e 158°. In situ ND data were collected on Wombat, the high-intensity powder diffractometer, at the OPAL reactor facility at ANSTO.24 The electrochemical cell was placed in a neutron beam of λ = 1.5407(1) Å, and data were collected every 5 min for 87 h in the range 22 e 2θ e 142°. Wombat features an area detector covering 120° in 2θ, enabling data to be continuously collected rather than the slower step-scan type acquisition that is used by Echidna and conventional powder diffractometers. The combination of Wombat’s relatively intense neutron beam and area detector makes Wombat an ideal instrument for in-situ ND studies of this type. ND data correction, reduction, and visualization were undertaken using the program LAMP.31 During the insitu ND experiment, the electrochemical cell was cycled in galvanostatic (constant current) mode with applied currents ranging from (1 to 5 mA in the voltage range 2.5 4.16 V using a Neware battery testing device. Rietveld refinements were carried out using the GSAS32 suite of programs with the EXPGUI33 interface.

’ RESULTS AND DISCUSSION 1. Structural Analysis Using Neutron Diffraction. Rietveld refinement of the Li(Co0.16Mn1.84)O4 structure using ex situ Echidna data collected on the pure powder is shown in Figure 1a 21474

dx.doi.org/10.1021/jp2026237 |J. Phys. Chem. C 2011, 115, 21473–21480

The Journal of Physical Chemistry C

Figure 1. Rietveld-refined fit of the Li(Co0.16Mn1.84)O4 structural model to (a) ex situ Echidna data where the inset highlights the 65 e 2θ e 110° region, (b) in-situ Wombat data of the uncycled cell where data are excluded in the region 82 e 2θ e 89 due to temporary detector problems, and (c) in-situ Wombat data in the region 35 e 2θ e 81. Data are shown as red crosses, the calculated model as a black solid line, the difference between the data and the model calculation as a purple line at the bottom, and vertical lines represent reflection markers for the modeled phases.

and structural details are presented in Table 1. The lattice parameter was a = 8.2204(6) Å, with profile factors of Rp = 5.66% and

ARTICLE

Rwp = 7.24%, and the goodness-of-fit term (χ2) = 1.02, using 20 variables. As Li has a large neutron absorption cross section (63.579 barn at λ = 1.6215(1) Å34) the Lobanov and Alte da Veiga absorption correction was applied to the Rietveld model.32,35 The composition refined to LiCo0.17(2)Mn1.83(2)O4, with a smaller lattice parameter relative to LiMn2O4, induced by substitution of the smaller Co3+ for Mn3+.12,29 The Li content was assumed to be close to stoichiometric, as shown by previous work,29,36 and was not refined due to the significant correlation between the site occupancy factor (SOF) and atomic displacement parameter (ADP) for Li.10 Further verification of the Co: Mn ratio was provided by EDS analysis resulting in 0.18(1):1.82(1), which is in close agreement with both input synthetic stoichiometry and Rietveld refined values. The experimental density of the pure powder was 4.2964 g cm 3, which compares favorably with 4.3242(5) g cm 3 density of the structural model refined using ND data. The crystal structure of Li(Co0.16Mn1.84)O4, derived from the Echidna data, was used as a starting model for the multiphase Rietveld analysis of the in-situ Wombat data. The SOF and ADP for Li were fixed in Rietveld refinements,10 an approach that does not impact significantly on the determination of the parameters extracted from this data, that is, lattice parameters and phase fractions. The multiphase fit of the Rietveld-refined models of Li,37 Cu,38 Al,39 and Li(Co0.16Mn1.84)O4 to the Wombat data of the uncycled battery is shown in Figure 1b,c. The figures of merit of this fit were Rp = 1.86%, Rwp = 2.39%, and χ2 = 4.41, for 18 refinement variables. The lattice parameter of Li(Co0.16Mn1.84)O4 in the custom-designed battery was a = 8.205(1) Å, smaller than the a = 8.2204(6) Å found for the uncycled Li(Co0.16Mn1.84)O4 powder using Echidna. The open-circuit voltage for the battery was 3.2 V, and we hypothesize that the decrease in lattice parameter is related to structural changes, possibly electrochemical in nature, that are induced during battery fabrication and optimization. The difference in the a-lattice parameter determined using Echidna (ex situ) and that determined using Wombat (in-situ) may arise due to effects induced by the battery assembly procedure, including excessive heating of Li(Co0.16Mn1.84)O4 during electrode manufacture and/or equilibration processes after battery manufacture. Both of these effects may in part reduce the lattice parameter. 2. Performance of the Electrochemical Cell. Our customdesigned cell compares favorably to that used by others in the insitu ND study of LiMn2O4,15,23 where the step-scan method was used to collect data and the total acquisition time was not reported. This previous work uses similar charging rates to our study, although the previous battery contains ∼10 times the quantity of cathode material to that of this study.15 Our customdesigned battery also compares favorably with the thin pouchtype battery used in an in-situ synchrotron XRD experiment conducted on anion-doped LiMn2O4.11 The in-situ XRD cell and our cell contain similar quantities of active electrode material (406 mg for the XRD study and ∼500 mg for this study) and use similar charge discharge currents ( 4 e I (mA) e 4). Our cell enables data collection at better than 5 min intervals compared with the 22 min acquisition time (step-scan type) in the synchrotron XRD experiment. This is exceptionally good for an in-situ ND experiment, which is generally slower than synchrotron XRD experiments as a consequence of the lower incident neutron flux and the different scattering mechanism. For real-time in-situ experimentation, an area detector that allows fast data acquisition, such as used by Wombat, is preferred; however, these have 21475



ARTICLE

Table 1. Atomic Positions, ADPs (Uiso) and SOF of LiCo0.17(2)Mn1.83(2)O4 in Fd3m Space Group Symmetry with a = 8.2204(6) Å

a

x

y

z

100 Uiso (Å2)

atom

Wyckoff position

SOF

Li

8a

1/8

1/8

1/8

1

Mn

16d

1/2

1/2

1/2

0.914(8)b

1.23(10)a

b

1.23(10)a

Co

16d

1/2

1/2

1/2

0.086(8)

O

32e

0.26335(9)

0.26335(9)

0.26335(9)

1

2.50(26)

1.86(4)

Values constrained to be equal. b Values constrained to full occupation at this site.

lower angular resolution than can be obtained using the step-scan type method. We note some limitations of our custom-designed battery, including an inability to charge the battery completely and limited useable current rates (i.e., high-current rates result in voltage spikes). In particular, the extended length of the second charge-cycle may arise due to voltage fluctuations caused by insufficient energy to further delithiate the cathode or as a result of electrolyte decomposition. These effects arise from the battery design and may be a result of effects of electrolyte deuteration, which alter the electrochemical kinetics. It is necessary to use deuterated electrolytes in order to collect in-situ ND data of the battery with a signal-to-noise ratio that is sufficient for analysis. The effect of the substitution of conventional electrolyte with deuterated electrolyte on the cell performance was determined through offline coin-cell experiments. The first charge discharge curves of two coin cells, one containing deuterated electrolyte and another containing conventional hydrogenated electrolyte, are shown in Figure 2. The cell containing conventional hydrogenated electrolyte exhibits features during the charge curve that are present as gradual slopes in the regions 3.93 e V e 4.05 V and 4.13 e V e 4.20 V, with a step feature at 4.10 V. Features of the corresponding discharge curve appear as gradual slopes between 3.88 e V e 3.98 V and 4.06 e V e 4.13 V, with a step feature at 4.02 V. There is a shift in both voltage and capacity in the related features of the charge discharge curves of the cell containing deuterated electrolyte at voltages below 4.16 V. The features that appear as gradual slopes in the discharge curve of the cell containing deuterated electrolyte are shifted to 3.80 e V e 3.96 V and 4.04 e V e 4.11 V, and the step feature is shifted to 3.99 V. Similarly, the feature that appears as a gradual slope during the charge curve for the cell containing deuterated electrolyte is shifted to 4.00 e V e 4.16 V. We find that deuteration of the electrolyte causes voltage shifts greater than 0.7 V during charge (up to 4.16 V) and lower than 0.2 V during discharge. The voltage and capacity differences between the batteries containing different electrolytes may arise as a result of a kinetic isotope effect, leading to a different chargetransfer rate through the electrolyte. The deuterated electrolyte is also slightly lower in purity than conventional hydrogenated electrolyte (e1% difference). The result is a higher voltage requirement for the cell containing deuterated electrolyte, relative to the cell containing conventional electrolyte, in order to achieve similar performance. At voltages above 4.16 V a parabolic-type shape of the voltage-capacity curve is observed for the cell containing deuterated electrolyte, which may be indicative of electrolyte oxidation. As a result of this information, the cell containing deuterated electrolyte was cycled below 4.16 V during the in-situ ND experiment. 3. Time-Dependent Fluctuations. The Li(Co0.16Mn1.84)O4 custom-designed battery was cycled and regions of the Wombat data and time voltage plot are shown in Figure 3. Changes in the

Figure 2. First charge discharge curve of Li(Co0.16Mn1.84)O4 coin cells containing 1 M LiPF6 dissolved in either 1:1 vol % ethylene carbonate and dimethyl carbonate (black) or 1 M LiPF6 dissolved in 1:1 vol % deuterated ethylene carbonate and deuterated dimethyl carbonate (red). The voltage range of 2 4.3 V vs Li was used for the cell containing hydrogenated electrolyte and 2 4.5 V vs Li for the cell containing deuterated electrolyte. A current rate of 30 mA/g was used.

2θ value of the indexed reflections for Lix(Co0.16Mn1.84)O4 indicate changes in d (interplanar spacing) that are commensurate with the insertion and extraction of Li from Lix(Co0.16Mn1.84)O4, resulting in lattice expansion and contraction, respectively. Changes in the 2θ value of Lix(Co0.16Mn1.84)O4 reflections are also noted with no applied current. Coin cells of Li(Co0.16Mn1.84)O4 were partially charged to 4.02 and 4.14 V in their first cycle and the voltage was measured for over 11 h (Figure 4a,b) with no applied current. The measured voltage decreased with the removal of applied current, and coincide with the changes in the 2θ positions of Lix(Co0.16Mn1.84)O4 reflections, observed in Figure 3, with no applied current. Voltage time curves for coin cells containing conventional or deuterated electrolyte (Figure 4a,b) reveal voltage relaxation. Voltage relaxation is more pronounced in deuterated-electrolyte containing batteries. The cause of the difference in voltage relaxation between the deuterated and hydrogenated electrolyte batteries is out of the scope of this work; however, we postulate that increased polarization due to a kineticisotope effect is a significant contributing factor, whereby chargetransfer is significantly slower in the deuterated electrolyte relative to conventional electrolyte. Sequential Rietveld refinements were performed and the temporal evolution of the lattice parameter of Lix(Co0.16Mn1.84)O4, measured voltage, and applied current are shown in Figure 5. The figures of merit of the sequential Rietveld refinements were in the range 1.86 e Rp e 2.54%, 2.74 e Rwp e 4.31%, 21476



ARTICLE

Figure 3. Plot of selected regions of the in-situ Wombat data as a function of time. The (222), (622), (731), (553), and (662) reflections of Lix(Co0.16Mn1.84)O4 are labeled and the voltage-profile of the battery is shown. Note that the (731) and (553) reflections are superimposed. Peak intensity is shown by the color scale. Below are the Rietveld plots using in-situ Wombat data at the end of the first charge step 740 min (left) and at the end of the current free discharge step 1400 min (right). Data are red crosses, the calculated model is the black solid line, the difference between the data and the model calculation is the purple line at the bottom, and vertical lines represent reflection markers for the modeled phases. Data are excluded in the region 82 e 2θ e 89 due to detector artifacts.

and 3.40 e χ2 e 4.64. During Rietveld analysis, only the lattice parameter and phase fractions were refined. Refinement of further structural parameters, such as atomic positions, gave no significant improvement in the fit of the refined models to the data, and consequently, was not justified. Importantly, the refinement of these further structural parameters had no significant effect on the determined lattice parameters. We use data

from the extremities of the electrochemical processes, where structural changes would be greatest, to demonstrate this point. Refinements using data from the two end points of currentfree discharge were performed where the Li SOF was fixed at various values, for example, 0.2, 0.5, 0.7, and 1.0. The Rp for these refinements was found to vary by 0.01 0.02%, Rwp by 0.01 0.03%, and χ2 by 0.04 0.11. Free refinement of the Li 21477



Figure 4. Coin cells charged to (a) 4.02 and (b) 4.14 V followed by voltage measurements exceeding 11 h with no applied current. Data from cells containing 1 M LiPF6 dissolved in 1:1 vol % ethylene carbonate and dimethyl carbonate are shown in black, and data from cells containing 1 M LiPF6 dissolved in 1:1 vol % deuterated ethylene carbonate and deuterated dimethyl carbonate are shown in red.

SOF led to values greater than 1 and nonphysical ADPs. In all cases, the refined lattice parameter was not affected. These results show that these in-situ data do not support the refinement of Li SOFs or other structural parameters beyond those already included in the refinement. Nonetheless, the applied current and measured voltage are directly correlated to the Rietveldrefined lattice parameters, which decrease with charging to 4.16 V from a = 8.205(1) to 8.118(2) Å during the first 740 min, from a = 8.165(1) to 8.099(1) Å during the second charging period between 1410 to 4000 min, and from a = 8.185(1) to 8.140(2) Å during the third charging period between 4550 and 4905 min (Figure 5). Both current-free (without applied current) and applied-current discharging occurs. Current-free discharge is evidenced by the increase in lattice parameter with no applied current from a = 8.118(2) to 8.165(1) Å between 740 and 1410 min and from a = 8.159(2) to 8.181(1) Å between 5000 and 5230 min (shaded in blue in Figure 5). The current-free discharge process also results in a drop in voltage from 4.16 to 4.01 V at 740 and 1410 min, respectively. Conventional discharge occurs with an applied current of 0.5 mA, resulting in a lattice

ARTICLE

parameter increase from a = 8.099(1) to 8.185(1) Å between 4000 and 4550 min and from a = 8.140(2) to 8.159(2) Å between 4905 and 5030 min (shaded in purple in Figure 5). Notably, the current-free discharge process is observed after both the applied-current charge and discharge. The current-free (from 740 to 1410 min) and applied-current (from 4000 to 4550 min) discharges were compared using a linear fit to the time-dependency of the lattice parameters. The rate of lattice parameter increase with current-free discharge is 6.76(7) 10 5 Å min 1, while the rate with an applied current of 0.5 mA is 1.52(2) 10 4 Å min 1. Therefore, an applied-current discharge of 0.5 mA results in a rate of change of the lattice parameter that is 2.25(4) times that for the current-free discharge and less linear. The relatively linear variation of lattice parameters during the current-free discharge may indicate a more direct relationship between changes in Li content of Li(Co0.16Mn1.84)O4 and voltage than for the applied-current process. If the absolute value of the lattice parameter change is considered during the in-situ experiment (0.106(1) Å), the (first) current-free discharge process increases the lattice parameter by 44(2) % during 670 min, while the (first) applied-current discharge process increases the lattice parameter by 81(1)% during 550 min. We observe in real-time the nonlinear behavior of the Lix(Co0.16Mn1.84)O4 lattice parameter with applied current, and we note a more linear relationship during the current-free discharge than for the applied-current discharge. Hence, data that is ex situ with respect to electrochemical activity, that is, collected on an apparent “equilibrium” state of the cell, may be inadvertently measuring the battery during a current-free discharge process. The peak broadening reported in “in-situ” ND15 and XRD11 experiments can be, in part, attributed to unresolved time-dependent relaxation processes, such as the current-free discharge investigated here. Evidence of current-free discharge manifests in both in-situ synchrotron XRD8 and in-situ ND15 as minor abnormalities in the trend of LiMn2O4 lattice parameters during interruptions in the electrochemical cycle where cells were allowed to charge equilibrate. Furthermore, ex situ studies using large-area cells show a noticeable voltage relaxation upon current removal.10 We demonstrate using real-time in-situ ND data that current-free discharge occurs in a Lix(Co0.16Mn1.84)O4 cathode and that this can account for the above-mentioned voltage relaxation and latticeparameter abnormalities reported in the parent LiMn2O4 material.8,10,15 Self-discharge, microshort circuits, and a small resistive load applied by the battery tester can all lead to the current-free discharge process observed in this study. The resistive load of the battery tester without an applied current was 0.6 MΩ, which corresponds to a current of approximately 7 μA at 4 V. The current is less than 2% of the applied current discharge, whereas the rate of change of lattice parameter during the current-free discharge process is 44.5(7)% of the applied current discharge. Thus, microshort circuits and/or self-discharge are likely to contribute to the current-free discharge process. A number of other factors can also influence the current-free discharge characteristics of batteries, including the synthesis method of the cathode material, the battery assembly procedure, localized heating of the cathode during testing, secondary electrochemical reactions such as electrolyte oxidation,11 charge polarization in the cathode layer during charge discharge,18 and any nonequilibrium or quasi-equilibrium states that occur as a result of charge discharge processes. 21478



ARTICLE

Figure 5. Lattice parameter (black) of the Lix(Co0.16Mn1.84)O4 cathode with measured voltage (red) and applied current (blue) as a function of time. Current-free discharge processes are shaded in blue and applied-current discharge processes are shaded in purple. Fluctuations at approximately 4200 min are related to changes in the applied current during discharge. The applied current was altered to ensure that the cell could be discharged at higher rates.

Both current-free and current-induced discharge processes will cause changes to the lattice parameter. Generally, changes associated with applied-current discharge processes will outweigh any changes from current-free processes. However, in the case of only current-free discharge, both the mechanism and the magnitude of the contributing reactions will determine their relative influence; for example, the rate of change of the lattice parameter during current free discharge will depend on factors such as the electrode polarization, the temperature, the external resistive load, self-discharge mechanisms, and any micro shortcircuits that are present. Current-free discharge may arise as a result of multiple processes that include micro short-circuits, selfdischarge, and small resistive loads. The increase in the lattice parameter during the current-free discharge in the in-situ ND data reflects the voltage-relaxation behavior exhibited in our coincell experiments, suggesting that the process we measure is an intrinsic factor of this cathode and not only a short-circuit. Measurements of open-circuit voltage and capacity loss in LiMn2O4 batteries provide evidence of self-discharge.5,27 The self-discharge in LiMn2O4 is proposed to occur via a variety of mechanisms that include the dissolution of Mn or Li into electrolyte (which is indicated as a significant process27), the electrochemical reduction of impurities, and the formation of a passivation layer (SEI layer) on the electrode surface.1,27 Codoped LiMn2O4, as studied here, does not suffer from significant Mn dissolution into the electrolyte as a result of increased crystal stability.11,27,29 The formation of a passivation layer is likely to be occurring in our study as we investigate the first and second charge/discharge cycles where this layer should be in the process of forming as noted for materials such as graphite.7 We propose that any self-discharge of the Lix(Co0.16Mn1.84)O4 examined here may occur as a result of a concentration-gradient of Mn oxidation states, possibly in the SEI layer, that can trap Li during application of a current and release the trapped Li when the current is removed. We note that a concentration-gradient of Mn oxidation states would drive to equilibrate via reduction or

oxidation processes. Our battery is connected to the galvanostat during current-free discharge that applies a small resistive load, providing energy for the reduction process, biasing the re-equilibration. Alternatively, other contributing factors may cause the selfdischarge, such as surface impurities. Only further time-dependent in-situ bulk and surface investigations such as grazingincident ND and atom-selective near-edge X-ray absorption fine structure (NEXAFS) spectroscopy may allow the determination of the cause of the observed current-free discharge. A reduced rate of change of lattice parameter is observed between 3000 and 4000 min (Figure 5), although no changes in the ND patterns correlate with this feature. The reduced rate of change in lattice parameter is associated with two-phase behavior in LiMn2O3.74F0.26; however, our observations support the theory that cation substitution suppresses two-phase behavior.11 Lattice parameters ca. 8.10 Å at 4.1 V are reported,10 commensurate with the smallest lattice parameter measured here, although further reduction in the lattice parameter is known to occur when LiMn2O4 batteries are charged to higher potentials, for example, a ∼ 8.05 Å at 4.2 V forming λ-MnO2.15 Using the reported relationship for LiMn2O3.74F0.26, a (Å) = 8.0112 + 0.266x,11 where x is the Li content and noting in our study that Co is substituted for Mn and there is no F, we calculate the maximum change in Li content during electrochemical cycling to be ca. 0.41. This indicates that our custom-made battery is only partially charged and discharged, a fact that we attribute to the oxidation of some Mn3+ to Mn4+, and in agreement with the voltage range used. The maximum lattice parameter of Lix(Co0.16Mn1.84)O4 during the applied-current discharge is a = 8.185(1) Å (Figure 5), which is lower than both the original in-situ (a = 8.205(1) Å) and ex situ (a = 8.2204(6) Å) values. This corresponds to a loss of 0.24(1)% and 0.43(1)%, relative to the original in-situ and ex situ lattice parameters, respectively, suggesting that Li cannot be fully reinserted into this cathode (capacity loss). Further electrochemical cycling is required to conclusively characterize the Li-insertion limits. 21479



’ CONCLUSIONS Time-dependent in-situ ND data of a custom-made Li(Co0.16Mn1.84)O4 battery reveals current-free discharge from a partially charged state in the first cycle. The lattice parameter of the Li(Co0.16Mn1.84)O4 cathode in the interrupted battery increases by 44(2)% over 11 h at a rate of 6.76(7) 10 5 Å min 1. The rate of change of the lattice parameter during the current-free discharge is more linear and approximately half that of the applied-current discharge at 0.5 mA. The cathode does not recover its initial lattice parameter after undergoing partial charging. This work highlights the importance in understanding the equilibrium state of Li-ion batteries containing spinel-derived LiMn2O4 compounds during experimental measurements aimed at exploring the link between structure and electrochemical behavior. Real-time data that is acquired on a sufficiently short time scale presents a way to follow nonequilibrium states in the battery. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: +61 2 9717 7253. Fax: +61 2 9717 3606.

’ ACKNOWLEDGMENT Part of this project was supported by the Australian Research Council (ARC) through an ARC Discovery project (DP0878611) and Ministry of Education (MOE), Singapore (Grant No. WBS-R284-000-076-112). We thank Ms. Zhao Xuan for help with preparation of Li(Co0.16Mn1.84)O4 and G. V. Subba Rao for fruitful discussions. ’ REFERENCES (1) Goodenough, J. B.; Kim, Y. Chem. Mater. 2009, 22, 587–603. (2) Tarascon, J.-M.; Armand, M. Nature 2001, 414, 359–367. (3) Nagaura, T.; Tozawa, K. Prog. Batteries Solar Cells 1990, 9, 209. (4) Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. J. Electrochem. Soc. 1997, 144, 1188–1194. (5) Guyomard, D.; Tarascon, J.-M. Solid State Ionics 1994, 69, 222–237. (6) Thackeray, M. M.; de Kock, A.; Rossouw, M. H.; Liles, D.; Bittihn, R.; Hoge, D. J. Electrochem. Soc. 1992, 139, 363–366. (7) Winter, M.; Besenhard, J. O.; Spahr, M. E.; Novak, P. Adv. Mater. 1998, 10, 725–763. (8) Baehtz, C.; Buhrmester, T.; Bramnik, N. N.; Nikolowski, K.; Ehrenberg, H. Solid State Ionics 2005, 176, 1647–1652. (9) Wakihara, M. Mat. Sci. Eng. R 2001, 33, 109–134. (10) Berg, H.; Thomas, J. O. Solid State Ionics 1999, 126, 227–234. (11) Palacin, M. R.; Le Cras, F.; Seguin, L.; Anne, M.; Chabre, Y.; Tarascon, J.-M.; Amatucci, G.; Vaughan, G.; Strobel, P. J. Solid State Chem. 1999, 144, 361–371. (12) Rousse, G.; Masquelier, C.; Rodríguez-Carvajal, J.; Hervieu, M. Electrochem. Solid-State Lett. 1999, 2, 6–8. (13) Palacin, M. R.; Chabre, Y.; Dupont, L.; Hervieu, M.; Strobel, P.; Rousse, G.; Masquelier, C.; Anne, M.; Amatucci, G. G.; Tarascon, J.-M. J. Electrochem. Soc. 2000, 147, 845–853. (14) Amatucci, G. G.; Schmutz, C. N.; Blyr, A.; Sigala, C.; Gozdz, A. S.; Larcher, D.; Tarascon, J.-M. J. Power Sources 1997, 69, 11–25. (15) Berg, H.; Rundlov, H.; Thomas, J. O. Solid State Ionics 2001, 144, 65–69. (16) Secondary Lithium Cells and Batteries for Portable Applications. Secondary Lithium Batteries; IEC 61960-2:2001; International Electrotechnical Commission: Geneva, Switzerland, 2001.

ARTICLE

(17) Rosciano, F.; Holzapfel, M.; Scheifele, W.; Novak, P. J. Appl. Crystallogr. 2008, 41, 690–694. (18) Colin, J.-F.; Godbole, V.; Novak, P. Electrochem. Commun. 2010, 12, 804–807. (19) Rodriguez, M. A.; Ingersoll, D.; Vogel, S. C.; Williams, D. J. Electrochem. Solid-State Lett. 2004, 7, A8–A10. (20) Rodriguez, M. A.; Van Benthem, M. H.; Ingersoll, D.; Vogel, S. C.; Reiche, H. M. Powder Diffr. 2010, 25, 143–148. (21) Sharma, N.; Du, G.; Studer, A. J.; Guo, Z.; Peterson, V. K. Solid State Ionics 2011, 199 200, 37–43. (22) Sharma, N.; Peterson, V. K.; Elcombe, M. M.; Avdeev, M.; Studer, A. J.; Blagojevic, N.; Yusoff, R.; Kamarulzaman, N. J. Power Sources 2010, 195, 8258–8266. (23) Bergstom, O.; Andersson, A. M.; Edstrom, K.; Gustafsson, T. J. Appl. Crystallogr. 1998, 31, 823–825. (24) Studer, A. J.; Hagen, M. E.; Noakes, T. J. Physica B 2006, 385 386, 1013–1015. (25) Huang, W.; Frech, R. J. Power Sources 1999, 81 82, 616–620. (26) Mukerjee, S.; Thurston, T. R.; Jisrawi, N. M.; Yang, X. Q.; McBreen, J.; Daroux, M. L.; Xing, X. K. J. Electrochem. Soc. 1998, 145, 466–472. (27) Blyr, A.; Sigala, C.; Amatucci, G.; Guyomard, D.; Chabre, Y.; Tarascon, J.-M. J. Electrochem. Soc. 1998, 145, 194–209. (28) Berg, H.; Kelder, E. M.; Thomas, J. O. J. Mater. Chem. 1999, 9, 427–429. (29) Sakunthala, A.; Reddy, M. V.; Selvasekarapandian, S.; Chowdari, B. V. R.; Christopher Selvin, P. Electrochim. Acta 2010, 55, 4441–4450. (30) Liss, K.-D.; Hunter, B. A.; Hagen, M. E.; Noakes, T. J.; Kennedy, S. J. Physica B 2006, 385 386, 1010–1012. (31) Richard, D.; Ferrand, M.; Kearley, G. J. J. Neutron Research 1996, 4, 33–39. (32) Larson, A. C.; Von Dreele, R. B. General Structure Analysis System (GSAS), Los Alamos National Laboratory Report LAUR 86-748; Los Alamos National Laboratory: Los Alamos, NM, 1994. (33) Toby, B. H. J. Appl. Crystallogr. 2001, 34, 210–213. (34) Sears, V. F. Neutron News 1992, 3, 29–37. (35) Lobanov, N. N.; Alte da Veiga, L. Sixth European Powder Diffraction Conference, Budapest, Hungary, August 22 25, 1998; Abstract P12 16. (36) Gummow, R. J.; de Kock, A.; Thackeray, M. M. Solid State Ionics 1994, 69, 59–67. (37) Berliner, R.; Fajen, O.; Smith, H. G.; Hitterman, R. L. Phys. Rev. B 1989, 40, 12086–12097. (38) Bragg, W. L. Phys. Z. 1922, 23, 114–120. (39) Otte, H. M.; Montague, W. G.; Welch, D. O. J. Appl. Phys. 1963, 34, 3149–3150.

21480


Time-Dependent in-Situ Neutron Diffraction Investigation of a Li(Co0

Recommend Documents