Article Cite This: Chem. Mater. 2018, 30, 1907−1911
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Switch of the Charge Storage Mechanism of LixNi0.80Co0.15Al0.05O2 at Overdischarge Conditions Rosa Robert* and Petr Novák Paul Scherrer Institut, Electrochemistry Laboratory, CH-5232 Villigen PSI, Switzerland ABSTRACT: The specific charge loss during the first cycle of LiNi0.80Co0.15Al0.05O2 (NCA) is due to low Li+-ion diffusion kinetics on discharge down to 3.0 V. Discharge to 2.14 V pushes further reinsertion of Li+ into the layered NCA lattice; thus, part of the specific charge loss can be regained. Additional discharge to even lower potentials switches the charge storage mechanism from a solid solution to a two-phase reaction process. Pushing the NCA to work at potentials lower than 2.14 V vs Li metal irreversibly alters the material’s crystal structure and microstructure. The gradual overall change in the energy storage reaction mechanism, promoted by a repeated deep discharge, causes a decrease in NCA’s performance and lifetime, thus challenging the validity of cycling over a wide potential window in practical cells.
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INTRODUCTION The rechargeable lithium-ion battery (LIB) technology enabled the technological revolution of cell phones, laptop computers, and tablets, thus transforming global communication. Subsequently, the LIB technology demonstrated great potential for electromobility (EVs) and is continuously evolving.1 Furthermore, it is considered a key technology for electricity storage applications.2 Certainly, lithium transition-metal oxides with the layered LiTMO2 (TM = transition metal) structure are by far the most promising cathode materials to replace LiCoO2 in Li-ion batteries, as they can deliver a high reversible specific charge. Within the layered transition metals LiTMO2 family of materials, Li(Ni1−x−yCoxAly)O2 (NCA) and Li(Ni1−x−yMnxCoy)O2 (NMC)3−6 have already reached commercialization. These cathodes operate at high potentials vs Li+/Li and can therefore enable high specific energy values of the cells. These cathodes typically exhibit irreversible specific charge losses after the first cycle. Several studies have been conducted to underpin the causes behind this specific charge loss, attributing it to reasons such as lack of full lithiation of the NCA material when discharged to 3.0 V versus lithium metal,7 formation of inactive domains within the electrode,8 which also lead to long-term specific charge fade,9 and side reactions such as electrolyte oxidation.10,11 Recent experiments by MuellerNeuhaus et al.12 and Kang et al.13 disclosed that layered oxides charged to 4.3 V would completely recover their specific charge loss on discharge if a following deep discharge step to 10 h at a constant potential (CP) and finally washed using DMC. The capillaries were then sealed with wax inside an argon-filled glovebox. Operando XRD measurements were carried with an in-house constructed cell in reflection geometry, cycled galvanostatically between 4.3 and 1.2 V at a rate of 10 mA/g. The diffraction patterns were recorded continuously by step scanning (with a step size of 0.017°) over the 2θ range of 16° ≤ 2θ ≤ 23° and 34° ≤ 2θ ≤ 47°. Xray powder data were treated using the FullProf suite15 and HighScore suite16 software.
Table 1. Structural Data at Room Temperature for Pristine LiNi0.80Co0.15Al0.05O2 and after Charge to Different Upper Cut-Off Potentials and Discharge to 3.0 V vs Li+/Li with a CP > 10 h space group R3̅m a (Å) c (Å) V (Å3)
pristine
4.3−3.0 V
4.3−3.0−4.3−3.0 V
2.86457(9) 14.17903(7) 100.76(3)
2.8652(5) 14.1900(4) 100.88(8)
2.8649(1) 14.1924(7) 100.88(2)
stage of discharge (3.0 V vs Li metal) is consistent with previous research.7 Therefore, in addition to other factors such as irreversible structural modifications undergone during the first charge17 and issues like loss of particle contact after longterm cycling, the specific charge loss on discharge is also caused by a hampering of Li+ reinsertion on discharge. In other words, if complete lithium reinsertion should be possible, it would proceed with a very sluggish kinetics at the end of the process. Recent reports by Mueller-Neuhaus et al.12 and Kang et al.13 have disclosed a possible full recovery of the specific charge loss after the first cycle when NCA is put through a deep discharge cycling protocol. The particular reactions paths by which electrochemical reactions allow further charge transfer bellow 3.0 V are not clearly defined. Here, we identify the amount of the irreversible specific charge loss that is recovered from the layered NCA structure under overdischarge conditions and the impact that the low potential cycling has on the NCA material. Figure 2a shows the electrochemical cycling within a wider working potential window (4.3−1.5 V) over 10 cycles. The
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RESULTS AND DISCUSSION Ex situ XRD measurements were performed on LiNi0.80Co0.15Al0.05O2 electrodes charged to different upper cutoff potentials and discharged to 3.0 V vs Li+/Li with a CP > 10 h. Figure 1 presents the XRD pattern of the cathode discharged to
Figure 1. Ex situ diffraction patterns of NCA pristine material and NCA electrode charge to 4.3 V and discharged to 3.0 V with CP > 10 h.
the 3.0 V cutoff potential after 1 cycle compared to the XRD pattern of the pristine electrode. The Bragg reflections are assigned based on the rhombohedral crystal system with R3m ̅ space group, which unit cell contains three MO2 sheets (M = Ni, Co, Al) with the Li+ ions occupying the octahedral sites with an oxygen stacking sequence of ...ABCABC... along the caxis. After the first cycle, the cycled NCA electrode presents the same crystal structure as the pristine electrode, although the (003) Bragg reflection is broader compared to the (003) of the pristine and is also shifted to lower 2θ angles. The (003) reflection is representative of the c lattice parameter, very sensitive to the amount of lithium between the MO2 layers; thus, a shift toward lower 2θ angles tells that, despite the long constant potential step, the complete relithiation of the NCA is not attained. An inspection of the lattice parameters of electrodes charged to 4.3 V and discharged to 3.0 V (CP > 10 h) vs Li+/Li compared to the pristine sample (Table 1) shows that, after the first electrochemical cycle, c increases significantly from the original value of 14.17903(7) Å to 14.1900(4) Å. A more significant increase of the c lattice parameter is observable after two full charge−discharge cycles. This observation is indicative of the presence of vacancies in the MO2 slabs after full discharge to 3.0 V. This trend, which increases with the cycle number, is a consequence of a noncomplete Li reinsertion between the MO2 slabs after the full cycle. This lack of full lithiation of the layered NCA at this
Figure 2. (a) Charge−discharge curves and (b) dQ/dV vs potential curves of LiNi0.85Co0.15Al0.05O2 cycled between 1.5 and 4.3 V at a specific current of 10 mA/g.
electrochemical reactions associated with the first discharge profile can be divided into two main processes defined by a sloped line from 4.3 to 3.0 V and a plateau at about 1.8 V vs Li+/Li. The first process corresponds to the solid solution lithium intercalation process with a contribution to the specific charge on discharge of 182 mAh/g (0.66 Li+ per formula unit), achieving a composition of Li0.92Ni0.80Co0.15Al0.05O2. The plateau on the potential-specific charge curve indicates a twophase coexistence region of a phase transition which starts from 1.91 V descending to 1.5 V and contributes to a specific charge on discharge of ∼29 mAh g−1. Therefore, the total specific charge on the first discharge (lithiation) corresponds to 220 mAh/g, which is well over the charge delivered on the first charge (delithiation), about 202 mAh/g (Li0.28Ni0.80Co0.15Al0.05O2), with charge potential ascending from 3.0 to 4.3 V. The following charge (second charge after deep discharge) delivers a specific charge of 207 and 218 mAh/ g corresponds on reduction. After the 9th cycle, the specific charge on charge corresponds to 190 mAh/g (x = 0.68 Li in 1908
DOI: 10.1021/acs.chemmater.7b04784 Chem. Mater. 2018, 30, 1907−1911
Article
Chemistry of Materials
pristine electrode. This intensity ratio values are a good indication of a fairly ordered layered structure after nine deep discharge cycles and also indicate a quite significant amount of cation mixing. Figure 3b displays the Williamson−Hall plots for the XRD patterns plotted in Figure 3a. Williamson−Hall method is used to determine the average volume weighted crystallite size and microstrain of the samples under study and relies on the principle that size broadening βL and strain broadening βe vary quite differently with respect to Bragg angle θ.18 Using the simplified formula:
Li1−xNi0.80Co0.15Al0.05O2) and to 192 mAh/g on discharge. Thus, during first charge/discharge cycles, we observe more reduction than oxidation, which may be caused by irreversible parasitic processes such as electrolyte decomposition by reduction and to irreversible structural transformations of the NCA lattice. In the dQ/dV curve of the second charge, shown in Figure 2b, between 3.0 and 4.3 V there are four oxidation peaks at 3.6, 3.74, 3.99, and 4.18 V, which can be assigned to the crystal structure rearrangements17 that the NCA undergoes during (de)intercalation within this potential range and the oxidation of Ni3+ into Ni4+. These peaks are reversible on reduction. The second charge after deep discharge is characterized by a first oxidative process at 2.4 V (dQ/dV curve shown in Figure 2b). The process at low potential is reversible and characterized by a relatively large hysteresis of 0.62 V with respect to the second discharge curve. In the subsequent cycles, the charge and discharge curves show similar profiles, but it is clearly appreciable that the solid-solution process is diminished at expense of the low potential plateau process, i.e. at discharge cycle 4, the delithiation on the solidsolution process starts at 0.4 V more negative potential compared to the previous charge, although the other transitions do not shift (Figure 2b). At cycle nine we see that, as the plateau processes increase and keep the same potential, the solid-solution process is clearly affected by the continuous overdischarge cycles so that the 4 oxidation/reduction couple peaks between ∼3.5 and 4.3 V are no longer visibly defined, implying that there is a serious crystal framework modification. Potential profiles and dQ/dV curves evidence a loss of contribution of the solid solution process and an overall switch of the energy storage reaction mechanism in favor of the plateau process. Figure 3a displays the ex situ XRD plots of three different 2θ regions that comprise the (003), (101), (006), (012), (104),
βtot cos Θ = Cesin Θ +
Kλ L
(1)
by plotting βtotcos θ versus sin θ, the strain component is extracted from the slope (Ce) and the size component from the intercept (Kλ/L). The Williamson−Hall method has many assumptions, but it is quite useful to identify trends when comparing different electrodes that have been cycled under different cycling conditions, giving thus a qualitative indication of sample microstructure. Williamson−Hall plot results in Figure 3b indicate a gradual increase of the crystallographic strain of the NCA electrode after each cycle. These results can be related to microstructural modifications in the samples so that repetitive cycling down to 1.5 V would lead to particle fracturing and disconnection as a result of volume expansion/ contraction, which would lead to compositional inhomogeneities within the electrode. Particle fracturing and disconnection have been previously observed.19 To untangle the causes of the irreversible specific charge loss on the first discharge with respect to the first charge and to gain insights into the nature and degree of reversibility of the processes occurring at deep discharge, we used operando X-ray powder diffraction and operando electrochemical cell. Figure 4a shows the potential profile of the first charge to 4.3 V and first deep discharge down to 1.2 V vs lithium metal. Figures 4b−d show selected 2θ regions of the in situ XRD patterns for the first discharge to 1.2 V vs Li+/Li. The XRD patterns, as the
Figure 3. (a) Ex situ XRD plots of three different 2θ regions of the pristine electrode and electrode samples cycled to 3.0 V after 1, 4, and 10 full charge/deep discharge cycles and (b) Williamson−Hall plots of the patterns from panel a according to the index of its reflection.
and the (110) and (018) diffraction peaks of the pristine electrode and electrode samples cycled to 3.0 V after 1, 4, and 10 full charge/deep discharge (down to 1.5 V) cycles and after an additional CP step of >10 h. Through a qualitative comparison of pristine NCA XRD pattern collected prior to the cell’s initial charge with that of cycle 1, 4, and 10 full charge/deep discharge cycles after cycling down to 3.0 V, one can see that some of the crystallinity of the powder is lost and the diffraction lines are considerably broadened, and after nine full cycles, the (003) and (104) diffraction lines shift to higher 2θ angles. Nevertheless, no new phases or peaks can be observed. Furthermore, the integrated intensity ratio I(003)/I(104) of cycle 9 is I(003)/I(104) = 0.96 and I(003)/I(104) = 1.3 for the
Figure 4. (a) Galvanostatic curves for the first cycle of an NCA electrode cycled vs Li at 10 mA/g at room temperature: the dots displaying specific potentials on the galvanostatic profile corresponding to the X-ray diffractograms that are shown in the following graphs. (b−d) In situ X-ray diffraction patterns at selected 2θ regions for NCA with Bragg peaks according to space group R3̅m and P3̅m1. 1909
DOI: 10.1021/acs.chemmater.7b04784 Chem. Mater. 2018, 30, 1907−1911
Article
Chemistry of Materials NCA is reduced, exhibit a growing new weak peak at about 17.7° 2θ (see Figure 4b). Following the evolution of the (003) Bragg peak in Figure 4c that is related to the variation of the chex axis and is intensely affected by the amount of lithium in the lattice, from the spectra taken at 3.3 V to the spectra taken at 1.2 V, two main processes (stages I and II) are mainly observed. Until a potential of 2.14 V (stage I) is reached, the (003) line shifts to higher angles, providing conclusive evidence that there is further lithiation of the NCA below 3 V. From this potential down to 1.2 V (stage II), no clear shift toward higher 2θ angles is observed, and the intensity of this reflection diminishes gradually while its peak width broadens. At the same time, the (101) reflection suffers a severe intensity reduction and peak width broadening, consistent with the electrochemical onset of a structural change to an amorphous-like or more disordered *R3̅m structure. At this stage II of two-phase transition reaction, clearly defined by the electrochemical potential plateau seen in Figure 2a, the visualization of a new Bragg reflection appearing at about 17.7° 2θ (Figure 2b) is a strong evidence of an additional phase transformation. This new reflection can be assigned to the (001) reflection of the P3m ̅ 1 space group for Li1+xNi0.80Co0.15Al0.05O2 phase, which has been already observed in other layered materials.20,21 The observation of this phase indicates that, in some electrode regions, the NCA composition is such that further lithiation allows the accommodation of an additional Li into the host lattice and promotes the R3̅m LiMO2 to P3̅m1 Li2MO2 phase transformation (see Figure 5). The latter phase has hexagonal
Figure 6. (a) Galvanostatic curves for the first cycle and second charge (in blue) of an NCA electrode cycled vs Li at 10 mA/g at room temperature: the dots displaying specific potentials on the galvanostatic profile corresponding to the X-ray diffractograms that are shown in the following graphs. (b−d) In situ X-ray diffraction patterns at selected 2θ regions for NCA with Bragg peaks according to space group R3̅m and P3̅m1.
selected 2θ regions of the in situ XRD patterns for the second discharge to 1.2 V vs Li+/Li that comprise the (001) diffraction peak of the P3̅m1 phase and the (003) diffraction peak of the rombohedral phase. XRD results, while charging the cell, confirm the reversibility of the P3̅m1 to R3̅m two-phase transition reaction as the (001) P3m ̅ 1 line disappears gradually while the electrode is charged back to 3.0 V (Figure 4b). The (003) R3̅m reflection (Figure 6c) initially shifts to higher 2θ angles and gains intensity to get back to the NCA values at 3.0 V. Later on, as lithium is being deinserted from the NCA lattice from 3.0 to 3.68 V, the (003) line shifts to lower 2θ angles, i.e. the c lattice parameter expands, as is typically observed for layered materials.7 On this second charge after deep discharge, we observe how the two rhombohedral phases, named R3m ̅ and *R3̅m, clearly seen in Figure 6d as well, delithiate via a solid solution mechanism up to a potential of 4.3 V. The two rhombohedral phases delithiate at different rates, which may also indicate slower lithiation kinetics for the new low-potential *R3̅m NCA phase. The distinctly different reaction kinetics observed during the second charge after deep discharge can be then attributed to crystal phase inhomogeneities within the NCA electrode attained at 1.2 V. Although the overlithiation reaction, accompanied by a structural change from R3̅m to P3̅m1 symmetry, initially occurred at some regions of the NCA electrode, as the electrode is further cycled within this operating potential window, the two phase reaction becomes more predominant. In turn, the (de)lithiation of the NCA becomes less reversible, as visible in Figure 7a and b. Repetitive cycling down to the twophase region leads on one hand to the amorphization of the layered NCA and on the other hand to particle fracturing due to the ∼20% crystal volume expansion associated with the R3̅m to P3̅m1 two-phase reaction. These two effects hinder galvanostatic reversibility of the (de)lithiation of the NCA, as the initial layered structure is lost and eventually the particle− particle contacts, which in turn questions the usefulness of cycling over a wide potential window in practical cells.
Figure 5. Schematic representation of the LiMO2 with R3̅m to Li2MO2 with P3m ̅ 1 phase transformation.
symmetry in which the oxygen ions are arranged in a hexagonally close-packed (hcp) array; the metal ions are located in all the octahedral sites of one layer, and the Li ions are located in all the tetrahedral sites of adjacent layers.20 At such very low potential, a displacement reaction, in which the transition elements are reduced to their metallic state, can likely occur. For the NCA, however, there is XRD evidence at 1.2 V of neither Ni nor Co reduction to the metal in the low-voltage two-phase region and at the end of discharge. Using X-ray powder diffraction within an operando cell provides conclusive evidence that, from the total irreversible charge loss on first discharge to 3.0 V (∼10 mAh/g), about 5 mAh/g can be associated with the kinetically hindered but still possible relithiation process of NCA when cycled down to 2.14 V, while the additional 5−6 mAh/g of specific charge loss on the first cycle may be related to irreversible phase changes, parasitic reactions at the surface,22 and particle disconnection9 that will lead to inactive material within the electrode. Figure 6a shows the potential profile of the first cycle and second charge to 4.3 V vs lithium metal. Figures 6b−d show 1910
DOI: 10.1021/acs.chemmater.7b04784 Chem. Mater. 2018, 30, 1907−1911
Article
Chemistry of Materials
(6) Ellis, B. L.; Lee, K. T.; Nazar, L. F. Positive Electrode Materials for Li-Ion and Li-Batteries. Chem. Mater. 2010, 22, 691−714. (7) Robert, R.; Bünzli, C.; Berg, E. J.; Novák, P. Activation Mechanism of LiNi0.80Co0.15Al0.05O2: Surface and Bulk Operando Electrochemical, Differential Electrochemical Mass Spectrometry, and X-ray Diffraction Analyses. Chem. Mater. 2015, 27, 526−536. (8) Arai, H.; Okada, S.; Sakurai, Y.; Yamaki, J.-I. Reversibility of LiNiO2 Cathode. Solid State Ionics 1997, 95, 275−282. (9) Liu, H.; Wolf, M.; Karki, K.; Yu, Y.-S.; Stach, E. A.; Cabana, J.; Chapman, K. W.; Chupas, P. J. Intergranular Cracking as a Major Cause of Long-Term Capacity Fading of Layered Cathodes. Nano Lett. 2017, 17, 3452−3457. (10) Freunberger, S. A.; Chen, Y.; Peng, Z.; Griffin, J. M.; Hardwick, L. J.; Bardé, F.; Novák, P.; Bruce, P. G. Reactions in the Rechargeable Lithium−O2 Battery with Alkyl Carbonate Electrolytes. J. Am. Chem. Soc. 2011, 133, 8040−8047. (11) Guéguen, A.; Streich, D.; He, M.; Mendez, M.; Chesneau, F. F.; Novák, P.; Berg, E. J. Decomposition of LiPF6 in High Energy Lithium-Ion Batteries Studied with Online Electrochemical Mass Spectrometry. J. Electrochem. Soc. 2016, 163, A1095−A1100. (12) Mueller-Neuhaus, J. R.; Dunlap, R. A.; Dahn, J. R. Understanding Irreversible Capacity in LixNi12yFeyO2 Cathode Materials. J. Electrochem. Soc. 2000, 147, 3598−3605. (13) Kang, S.-H.; Yoon, W.-S.; Nam, K.-W.; Yang, X.-Q.; Abraham, D. P. Investigating the First-cycle Irreversibility of Lithium Metal Oxide Cathodes for Li Batteries. J. Mater. Sci. 2008, 43, 4701−4706. (14) Kondo, H.; Takeuchi, Y.; Sasaki, T.; Kawauchi, S.; Itou, Y.; Hiruta, O.; Yonemura, M.; Kamiyama, T.; Ukyo, Y. Effects of Mgsubstitution in Li(Ni,Co,Al)O2 Positive Electrode Materials on the Crystal Structure and Battery Performance. J. Power Sources 2007, 174, 1131. (15) Rodríguez-Carvajal, J. FullProf computer program; https://www. ill.eu/sites/fullprof/, 1998. (16) Degen, T.; Sadki, M.; Bron, E.; König, U.; Nénert, G. The HighScore suite. Powder Diffr. 2014, 29, S13−S18. (17) Robert, R.; Novák, P. Structural Changes and Microstrain Generated on LiNi0.80Co0.15Al0.05O2 during Cycling: Effects on the Electrochemical Performance. J. Electrochem. Soc. 2015, 162, A1823− A1828. (18) Williamson, G. K.; Hall, W. H. X-ray Line Broadening from Filed Aluminium and Wolfram. Acta Metall. 1953, 1, 22−31. (19) Miller, D. J.; Proff, C.; Wen, J. G.; Abraham, D. P.; Bareno, J. Observation of Microstructural Evolution in Li Battery Cathode Oxide Particles by In Situ Electron Microscopy. Adv. Energy Mater. 2013, 3, 1098−1103. (20) Johnson, C. S.; Kim, J.-S.; Kropf, A. J.; Kahaian, A. J.; Vaughey, J. T.; Fransson, L. M. L.; Edström, K.; Thackeray, M. M. Structural Characterization of Layered LixNi0.5Mn0.5O2 (0 < x ≤ 2) Oxide Electrodes for Li Batteries. Chem. Mater. 2003, 15, 2313−2322. (21) Armstrong, A. R.; Lyness, C.; Panchmatia, P. M.; Islam, M. S.; Bruce, P. G. The Lithium Intercalation Process in the Low-voltage Lithium Battery Anode Li1+xV1−xO2. Nat. Mater. 2011, 10, 223−229. (22) Kasnatscheew, J.; Evertz, M.; Streipert, B.; Wagner, R.; Klöpsch, R.; Vortmann, B.; Hahn, H.; Nowak, S.; Amereller, M.; Gentschev, A.C.; Lamp, P.; Winter, M. The Truth About the 1st Cycle Coulombic Efficiency of LiNi1/3Co1/3Mn1/3O2 (NCM) cathodes. Phys. Chem. Chem. Phys. 2016, 18, 3956−3965.
Figure 7. (a) Charge−discharge curves in the potential window between 4.3 and 1.2 V and (b) dQ/dV vs potential curves of LiNi0.80Co0.15Al0.05O2 at a specific current of 10 mA/g.
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CONCLUSIONS The particular reactions paths by which electrochemical reactions allow further charge transfer of the layered NCA material bellow 3.0 V vs Li+/Li were identified and divided in two stages. During stage I, between 3.0 and 2.14 V vs Li+/Li, the NCA’s specific charge loss of the first cycle is partially recovered by a solid solution mechanism. Below 2.14 V and throughout the plateau, a large specific charge on discharge is attained. The lithium storage reaction mechanism at this stage is identified as two-phase reaction mechanism which involves the R3̅m to P3̅m1 phase transition and the formation of a new *R3̅m phase. Indeed, this clarification points to an additional lithiation process where the specific charge is not recovered by the original standard charge storage mechanism of LiNi0.80Co0.15Al0.05O2. Under such working conditions, the material’s crystal structure and microstructure are irreversibly altered. Deep discharge empowers crystal structure heterogeneities of the NCA material that promote heterogeneous electrochemical reactions throughout the electrode on charge and discharge. The changes in the energy storage reaction mechanism, promoted by continuous deep discharge protocol, cause a decrease in the NCA’s performance and lifetime.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Rosa Robert: 0000-0002-4071-6711 Notes
The authors declare no competing financial interest.
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REFERENCES
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DOI: 10.1021/acs.chemmater.7b04784 Chem. Mater. 2018, 30, 1907−1911
