Crystal Chemistry and Electrochemistry of LixMn

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Crystal Chemistry and Electrochemistry of LiMn Ni O Solid Solution Cathode Materials x

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Wang Hay Kan, Saravanan Kuppan, Lei Cheng, Marca Doeff, Jagjit Nanda, Ashfia Huq, and Guoying Chen Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b01898 • Publication Date (Web): 19 Jul 2017 Downloaded from http://pubs.acs.org on August 8, 2017

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Chemistry of Materials

Crystal Chemistry and Electrochemistry of LixMn1.5Ni0.5O4 Solid Solution Cathode Materials Wang Hay Kan,a,† Saravanan Kuppan,a,† Lei Cheng,a,b Marca Doeff,a Jagjit Nanda,c Ashfia Huqc and Guoying Chena,* a

Energy Storage and Distributed Resources Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA b

Department of Material Sciences and Engineering, University of California, Berkeley, California 94720, USA c

Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

Abstract For ordered high-voltage spinel LiMn1.5Ni0.5O4 (LMNO) with the P4321 symmetry, the two consecutive two-phase transformations at ∼4.7 V (vs. Li+/Li), involving three cubic phases of LMNO, Li0.5Mn1.5Ni0.5O4 (L0.5MNO) and Mn1.5Ni0.5O4 (MNO), have been well established. Such a mechanism is traditionally associated with poor kinetics due to the slow movement of the phase boundaries and the large mechanical strain resulting from the volume changes among the phases, yet ordered LMNO has been shown to have excellent rate capability. In this study, we show the ability of the phases in dissolving into each other and determine their solubility limit. We characterize the properties of the formed solid solutions and investigate the role of nonequilibrium single-phase redox processes during the charge and discharge of LMNO. By using *

Corresponding author’s email: [email protected] † The authors contributed equally to this work. 1 ACS Paragon Plus Environment


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an array of advanced analytical techniques, such as soft and hard X-ray spectroscopy (XAS), transmission X-ray microscopy (TXM) and neutron/X-ray diffraction (ND/XRD), as well as bond valence sum (BVS) analysis, the present study examines the meta-stable nature of solid solution phases and provides new insights in enabling cathode materials that are thermodynamically unstable.

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1. Introduction The growing market of portable electronics and electric/plug-in hybrid electric vehicles worldwide demands advanced energy storage systems with excellent energy density, cyclability and thermal stability.1-2 Among the reported cathode materials, the high-voltage spinel LiMn1.5Ni0.5O4 (LMNO) is of great interest due to its relatively high energy density (700 Wh kg1

) and 3D conduction pathways for lithium ions.3-5 One of its remarkable properties is the

structure-dependent redox reaction and phase transformation. The disordered polymorph with an Fdm symmetry, in which Mn and Ni ions are randomly distributed in the16d octahedral sites, is often synthesized at 900 oC through a fast cooling process.3-5 Kunduraci and others conducted extensive in-situ powder X-ray diffraction (PXRD) studies and revealed that the disordered spinel has a substantial solid solution region at low states-of-charge (LixMn1.5Ni0.5O4 or LxMNO, 0.5 < x < 1).6-7 The disordered LMNO also possesses higher conductivity largely due to the presence of Mn3+. Both high Mn3+ content and the extensive solid solution involvement are considered to be the main reasons behind its superior rate capability, compared to the ordered phase.8 In the latter case, which has the symmetry of P4321, Mn (12b) and Ni (4a) ions are located in two separate octahedral sites. The ordered LMNO typically has very low Mn3+ content and the Ni cation, whose valence changes from 2+ to 4+ in the two end members, is often the only transition-metal involved in the electrochemical reactions.5 This leads to two-consecutive two-phase redox reactions at ∼4.7 V (vs. Li+/Li) with a much smaller solid solution region, with the highest reported x value LxMNO approaching 0.8.9 This value is also morphology-dependent as recently revealed by Hai et al.10 Introducing dopants into the spinel may also alter the solid solution region.. A recent paper by Xiao et al. showed that the solid solution region in



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LixMn1.5Ni0.45Cr0.05O4 with the P4321 symmetry was significantly enlarged with Cr doping, which reached x = ~0.4.11 The lattice parameters of the three cubic phases in ordered LMNO, which are LMNO, L0.5MNO and Mn1.5Ni0.5O4 (MNO), were previously determined to be 8.1687(2), 8.0910(6) and 8.0005(3) Å, respectively.6, 10 Recently, advanced 2D and 3D nano-tomography was successfully employed to investigate its phase transformation mechanism, but the role of the solid solution remains unclear.12 The movement of cations in electrode materials, either driven by temperature or voltage, has been well studied and the results suggest that alternative redox pathways may be promoted through careful manipulation of these movements.13-15 For example, the existence of a non-equilibrium single-phase electrochemical reaction mechanism was demonstrated in olivinetype LiFePO4 (LFP) under certain conditions, such as fast C-rate (~10 C) charge/discharge, small particle sizes (0.7. Aside from the fact that different preparation methods may lead to the formation of different metastable phases, another possible source for this discrepancy is the contributions from side reactions to the coulometry which leads to overestimation of the Li content in the solid solution. For the subsequent (1st) discharge, only 0.09 Li+ was re-inserted into the compound to a lower cut-off voltage of 4.2 V, corresponding to x = 0.72 in ss-LxMNO. This further indicates the significant role of the surface reduced layer on the electrochemical behavior of the sample. No detectable structural changes were observed in the XRD patterns between 4.2-4.8 V (Figure 7b). On the 2nd charge to 4.8 V, about 0.27 Li+ was removed, which corresponds to x = 0.45 in LxMNO. A second set of peaks, labeled (331)’, (511)’ and (531)’ in Figure 7c, appeared when the Li content was below ∼0.64, consistent with the determined solubility limit of ça. 1:3 between L0.5MNO and LMNO. The new set of peaks correspond to the L0.5MNO phase with a 15 ACS Paragon Plus Environment


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lattice parameter of 8.097 Å. Upon discharge to 4.2 V, L0.5MNO disappeared around a Li content of ∼0.62. At the end of discharge at 4.2 V, the spinel solid solution ss-Li0.65MNO with a lattice parameter of 8.167 Å became the only phase. On the 3rd charge to 4.8 V (Figure 7d), L0.5MNO appeared at x = ∼0.6 which was followed by the appearance of fully delithiated MNO at x = ∼0.2 (diffraction peaks labelled (331)”, (511)” and (531)”). Figure 8 shows the voltage profiles, incremental capacity profiles and capacity retention during the first 100 cycles of ss-L0.82MNO between 4.5 and 4.82 V. A gradual increase in capacity with cycling was observed, reaching a value of 70 mAh/g at the 100th cycle (Figure 8a). This is likely due to a reduction in surface polarization and improved material utilization upon continuous cycling. The single phase behavior was only maintained during the first two cycles, as evidenced by the single peaks on the incremental capacity profiles (Figure 8b). Phase separation, signaled by the doublet at 4.75 and 4.78 V in the profile, was observed on the 3nd cycle, where more Li+ was extracted and inserted compared to the previous two cycles. This is consistent with the observation that the solid solution behavior can only be maintained in the lithium content range of ∼0.6 < x < 1. When charge and discharge occur outside this window (starting at the 3rd cycle), the electrochemical behavior is similar to what was observed in LMNO where a single phase behavior dominates in the lithium content range of ∼0.6 < x < 1 followed by the two phase behavior. The details in strucutal evoluation during charge and discahrge of LMNO was reported in a previous publicatioin. 10 The study clearly reveal the dominating effect of LMNO and L0.5MNO solubility in the phase transformation process. Both capacity and coloumbic efficiency were found to increase with cycle number (Figure 8c), consistent with lowered polarization and improved material utilization upon continuous cycling.

Similar behavior was also observed with ss-

L0.71MNO.


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3.3. Meta-stability in ss-LxMNO These combined thermal and electrochemical studies have shown that LixMn1.5Ni0.5O4 solid solutions can exist in the Li content range of ∼0.65 < x < 1. We previously showed that the solid solution appeared as a transient phase during electrochemical charge/discharge of LMNO and phase separates upon the removal of current.20 The thermally driven solid solution, however, can be isolated and maintained at room temperature for a long period of time. Recently, Kishida et al. performed a thermodynamic analysis of LxMNO (0 ≤ x ≤ 1) and their calculations showed that x = 0, 0.5, and 1 are the most stable phases in the system at 0 K, consistent with the experimental detection of the three phases during Li extraction and insertion of LMNO.36 However, their decomposition energy was comparable to that of other compositions, with a difference as small as ∼0.4 eV calculated. This indicates that less stable LxMNO phases may easily form under external stimulus. The successful synthesis of ss- LxMNO solid solutions at elevated temperatures clearly supports this conclusion. Figure 9a and 9b show the distribution of Ni oxidation states in the as-prepared L0.71MNO and ss-L0.71MNO, respectively, constructed from single-pixel TXM-XANES spectra (at a spatial resolution of 30 nm) collected at SSRL. While phase segregation is clearly seen in the as-prepared sample, a homogeneous Ni oxidation state in the entire particle of ss-L0.71MNO suggests that thermally driven Li movement occurs at the particle level. Although ss-LixMNO bears much similarity to LixFePO4 solid solutions, it is significantly more stable. Considering that the Li mobility in LMNO is generally higher, mostly due to the presence of 3D Li diffusion pathways in the cubic lattice, we speculate that the enhanced stability is related to the small enthalpy difference among the phases in the LMNO system. The presence of the surface reduction layer may also play a role in preventing phase separation in the formed solid solutions. As shown in the SEM images in Figure 9c, pitting and 17 ACS Paragon Plus Environment


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other surface roughing features were broadly observed in the solid solution particles, suggesting that during the thermal treatment, mass changes such as oxygen loss may have occurred along with the reduction of Ni on the surface. The presence of this reduced layer also contributes to the observed polarization during the cycling of ss-LixMNO electrodes. Detailed properties of the surface layer are currently being investigated and will be reported in the future.

4. Conclusions The role of non-equilibrium single-phase redox reaction in the performance of intercalation electrode materials was revealed through the studies performed on a series of LixMNO (0.71 < x < 1) solid solution phases in the ordered high-voltage spinel. Formation of solid solution is observed in the Li content range of ∼0.65 < x < 1 which is promoted either through thermal treatment or electrochemical Li insertion/extraction of pre-synthesized ssLixMNO. The observation of phase segregation outside of this window reveals the solubility limit of ça. 1:3 between L0.5MNO and LMNO. Detailed analysis on crystal structure and valence state of the transition metal cations in ss-LxMNO are presented for the first time. While XAS and BVS studies confirmed that Ni is the only redox active transition metal, ND patterns revealed a substantial amount of lithium-ion vacancies in the solid solution phases, a key feature that enables the single-phase redox process at low state of charge evidenced by in situ XRD studies. BVS mapping also reveals low energy 3D-topological pathways for lithium ion transport, supporting the observed higher conductivity in ss-LxMNO. Our findings provide insights and guidance on the synthesis of difficult metastable phases and their investigation as potential electrode materials for rechargeable lithium or sodium batteries.


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Acknowledgements We thank Drs. Young-Sang Yu and Dennis Nordlund for helping with the experiments carried out at SSRL. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. We thank Dr. Chetan Dhital for help with the neutron diffraction measurements. The POWGEN beamline at the Oak Ridge National Laboratory’s (ORNL) Spallation Neutron Source (SNS) is sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of FreedomCAR and Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.



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Figures

Figure 1. (Top) variable temperature powder XRD patterns of L0.71MNO and (bottom) a schematic diagram shows the merge of two phases (LMNO and L0.5MNO) into a single-phase ssLxMNO (0.71< x

Crystal Chemistry and Electrochemistry of LixMn

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