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• W2808026006 abstract "•Subparticle-level chemical heterogeneity exists in delithiated oxide•Short-range ordering is thermodynamically favorable in rock salts•Strong correlation between structural defects and chemical propagation is demonstrated A new class of disordered rock-salt oxides is capable of delivering energy density much higher than those of current lithium (Li)-ion battery cathodes. Theoretical calculation predicted facile Li diffusion enabled by the formation of percolating network in the Li-excess oxides; however, fundamental knowledge regarding how Li diffuses within the network and what affects this process is lacking. Here, we provide experimental evidence of Li-diffusion pathways at multiple length scales and report the dominating effect of local chemistry on Li mobility. We reveal that the existence of local cation ordering redirects Li movement through non-equilibrium pathways and leads to the unusual persistent chemical heterogeneity observed at the subparticle level. The strong correlation between structural defects and the propagation of chemical reaction is clearly demonstrated, providing essential insights needed for further design and development of these cathode materials. We report the observation of persistent chemical gradient on rock-salt Li1.3Nb0.3Mn0.4O2 single crystals transforming through a second-order reaction and reveal the dominating effect of local chemistry on Li diffusion within the percolated network. By using advanced 2D and 3D nanoscale X-ray spectro-microscopy on well-formed crystal samples, our study visualizes the mesoscale chemical distribution as a function of the state of charge at the subparticle level. We further reveal the presence of thermodynamically favorable short-range ordering of Nb-cation-only (Nb6) and Nb-cation-enriched (MnNb5) configurations, which promote non-equilibrium diffusion pathways and the expansive chemical heterogeneity observed on LixNb0.3Mn0.4O2 particles. The present study utilizes large single crystals to eliminate the influence of kinetic factors such as particle-size distribution, crystal facet, grain boundary, and strain, allowing us to clearly demonstrate the strong correlation between a material's structural defects and chemical propagation and its crucial impact on electrode performance and stability. We report the observation of persistent chemical gradient on rock-salt Li1.3Nb0.3Mn0.4O2 single crystals transforming through a second-order reaction and reveal the dominating effect of local chemistry on Li diffusion within the percolated network. By using advanced 2D and 3D nanoscale X-ray spectro-microscopy on well-formed crystal samples, our study visualizes the mesoscale chemical distribution as a function of the state of charge at the subparticle level. We further reveal the presence of thermodynamically favorable short-range ordering of Nb-cation-only (Nb6) and Nb-cation-enriched (MnNb5) configurations, which promote non-equilibrium diffusion pathways and the expansive chemical heterogeneity observed on LixNb0.3Mn0.4O2 particles. The present study utilizes large single crystals to eliminate the influence of kinetic factors such as particle-size distribution, crystal facet, grain boundary, and strain, allowing us to clearly demonstrate the strong correlation between a material's structural defects and chemical propagation and its crucial impact on electrode performance and stability. Recently, lithium (Li)-excess transition-metal (TM) oxides have attracted intense interest because of their ability in utilizing both TM cation and oxygen anion redox reactions to deliver very high capacity.1McCalla E. Abakumov A.M. Saubanere M. Foix D. Berg E.J. Rousse G. Doublet M.-L. Gonbeau D. Novak P. Van Tendeloo G. et al.Visualization of O-O peroxo-like dimers in high-capacity layered oxides for Li-ion batteries.Science. 2015; 350: 1516-1521Crossref PubMed Scopus (550) Google Scholar, 2Pearce P.E. Perez A.J. Rousse G. Saubanere M. Batuk D. Foix D. McCalla E. Abakumov A.M. Van Tendeloo G. Doublet M.-L. Tarascon J.-M. Evidence for anionic redox activity in a tridimensional-ordered Li-rich positive electrode β-Li2IrO3.Nat. Mater. 2017; 16: 580-586Crossref PubMed Scopus (241) Google Scholar, 3Sathiya M. Rousse G. Ramesha K. Laisa C.P. Vezin H. Sougrati M.T. Doublet M.L. Foix D. Gonbeau D. Walker W. et al.Reversible anionic redox chemistry in high-capacity layered-oxide electrodes.Nat. Mater. 2013; 12: 827-835Crossref PubMed Scopus (1032) Google Scholar, 4Luo K. Roberts M.R. Guerrini N. Tapia-Ruiz N. Hao R. Massel F. Pickup D.M. Ramos S. Liu Y.-S. Guo J. et al.Anion redox chemistry in the cobalt free 3d transition metal oxide intercalation electrode Li[Li0.2Ni0.2Mn0.6]O2.J. Am. Chem. Soc. 2016; 138: 11211-11218Crossref PubMed Scopus (233) Google Scholar, 5Luo K. Roberts M.R. Hao R. Guerrini N. Pickup D.M. Liu Y.-S. Edstrom K. Guo J. Chadwick A.V. Duda L.C. Bruce P.G. Charge-compensation in 3d-transition-metal-oxide intercalation cathodes through the generation of localized electron holes on oxygen.Nat. Chem. 2016; 8: 684-691Crossref PubMed Scopus (731) Google Scholar, 6Gent W.E. Lim K. Liang Y. Li Q. Barnes T. Ahn S.J. Stone K.H. McIntire M. Hong J. Song J.H. et al.Coupling between oxygen redox and cation migration explains unusual electrochemistry in lithium-rich layered oxides.Nat. Commun. 2017; 8: 2091Crossref PubMed Scopus (366) Google Scholar One class of such materials is the disordered rock-salt oxides. Through density function theory (DFT) calculations, the oxides were shown to have facile Li mobility because of the existence of a percolated Li-conduction network in the structure.7Lee J. Urban A. Li X. Su D. Hautier G. Ceder G. Unlocking the potential of cation-disordered oxides for rechargeable lithium batteries.Science. 2014; 343: 519-522Crossref PubMed Scopus (765) Google Scholar In 2015, Yabuuchi et al. reported a new Li-excess disordered oxide cathode, Li1.3Nb0.3Mn0.4O2 (LNMO), which delivered an impressive discharge capacity of ca. 300 mAh/g at 60°C.8Yabuuchi N. Takeuchi M. Nakayama M. Shiiba H. Ogawa M. Nakayama K. Ohta T. Endo D. Ozaki T. Inamasu T. et al.High-capacity electrode materials for rechargeable lithium batteries: Li3NbO4-based system with cation-disordered rocksalt structure.Proc. Natl. Acad. Sci. USA. 2015; 112: 7650-7655Crossref PubMed Scopus (334) Google Scholar The oxide was found to have a rock-salt crystal structure with all cations randomly located at the 4a sites and all M-O (M = Li, Nb, and Mn) bonds having the same distance of ∼2.096 Å. Our recent studies focusing on bulk structural analysis of LixNb0.3Mn0.4O2 (0 ≤ x ≤ 1.3) revealed a two-phase reaction involving oxygen redox in 0 < x < 0.9 and a single-phase reaction involving Mn3+/Mn4+ cation redox in 0.9 < x < 1.3.9Kan W.H. Chen D. Papp J.K. Shukla A.K. Huq A. Brown C.M. McCloskey B. Chen G. Unravelling solid-state redox chemistry in Li1.3Nb0.3Mn0.4O2 single-crystal cathode material.Chem. Mater. 2017; 30: 1655-1666Crossref Scopus (62) Google Scholar Considering the similarity, it is expected that Li-ion diffusion pathways in LNMO resemble that of the percolated Li1.25−xMn0.5Nb0.25O2 (x = 0, 0.75, and 1) system recently calculated by Lee et al.10Seo D.-H. Lee J. Urban A. Malik R. Kang S.Y. Ceder G. The structural and chemical origin of the oxygen redox activity in layered and cation-disordered Li-excess cathode materials.Nat. Chem. 2016; 8: 692-697Crossref PubMed Scopus (810) Google Scholar With the technological significance of disordered rock-salt oxides as promising cathode materials for high-energy Li-ion batteries (LIBs), it is imperative to gather experimental understanding on how Li ions diffuse within the percolated network as well as the detailed solid-state phase transformation mechanism. For materials transforming though a second-order phase transition or solid-solution reactions, the voltage driving force is expected to lead to chemical homogeneity after sample relaxation, at least at the primary particle level. In real systems, however, some kinetic factors could play dominant roles in the process. As a result, successful imaging of electrode materials with high spatial resolution and chemical sensitivity to directly visualize the coupling of kinetic factors and phase transformation, along with the associated mechanical damage or fracture that could significantly affect the material's performance and stability, is of fundamental and practical importance. Full-field transmission X-ray microscopy combined with X-ray absorption near-edge structure (FF-TXM-XANES) was recently introduced for this purpose and has been broadly used as a novel approach to visualizing electrochemically driven solid-state phase transformation.11Bauer S. Biasi L.D. Glatthaar S. Toukam L. Geßwein H. Baumbach T. In operando study of the high voltage spinel cathode material LiNi0.5Mn1.5O4 using two dimensional full-field spectroscopic imaging of Ni and Mn.Phys. Chem. Chem. Phys. 2015; 17: 16388-16397Crossref PubMed Google Scholar, 12Li L. Chen-Wiegart Y.-c. K. Wang J. Gao P. Ding Q. Yu Y.-S. Wang F. Cabana J. Wang J. Jin S. Visualization of electrochemically driven solid-state phase transformations using operando hard X-ray spectro-imaging.Nat. Commun. 2015; 6: 6883Crossref PubMed Scopus (67) Google Scholar, 13Meirer F. Cabana J. Liu Y. Mehta A. Andrews J.C. Pianetta P. Three-dimensional imaging of chemical phase transformations at the nanoscale with full-field transmission X-ray microscopy.J. Synchrotron Radiat. 2011; 18: 773-781Crossref PubMed Scopus (200) Google Scholar, 14Nowack L. Grolimund D. Samson V. Marone F. Wood V. Rapid mapping of lithiation dynamics in transition metal oxide particles with operando X-ray absorption spectroscopy.Sci. Rep. 2016; 6: 21479Crossref PubMed Scopus (38) Google Scholar The high brightness and the energy tunability of synchrotron-based hard X-ray enable nanoscale spatial resolution at ∼30 nm along with high chemical and elemental sensitivities in a large field of view (FOV; 30 × 30 μm). Recent studies successfully utilized advanced FF-TXM-XANES to reveal the nucleation and growth process of new phases as a function of Li content in cathode particles with the first-order transitions, including LiMn1.5Ni0.5O4 single crystals,15Kuppan S. Xu Y. Liu Y. Chen G. Phase transformation mechanism in lithium manganese nickel oxide revealed by single-crystal hard X-ray microscopy.Nat. Commun. 2017; 8: 14309Crossref PubMed Scopus (93) Google Scholar LiFePO4 (LFP),16Wang J. Chen-Wiegart Y.-C. Wang J. In operando tracking phase transformation evolution of lithium iron phosphate with hard X-ray microscopy.Nat. Commun. 2014; 5: 4570Crossref PubMed Scopus (163) Google Scholar, 17Wang J. Karen Chen-Wiegart Y.-C. Eng C. Shen Q. Wang J. Visualization of anisotropic-isotropic phase transformation dynamics in battery electrode particles.Nat. Commun. 2016; 7: 12372Crossref PubMed Scopus (98) Google Scholar, 18Lim J. Li Y. Alsem D.H. So H. Lee S.C. Bai P. Cogswell D.A. Liu X. Jin N. Yu Y.-S. et al.Origin and hysteresis of lithium compositional spatiodynamics within battery primary particles.Science. 2016; 353: 566-571Crossref PubMed Scopus (280) Google Scholar LiMn2O4 (LMO),19Yu Y.-S. Kim C. Liu Y. van der Ven A. Meng Y.S. Kostecki R. Cabana J. Nonequilibrium pathways during electrochemical phase transformations in single crystals revealed by dynamic chemical imaging at nanoscale resolution.Adv. Energy Mater. 2015; 5https://doi.org/10.1002/aenm.201402040Crossref Scopus (40) Google Scholar and FeF3.12Li L. Chen-Wiegart Y.-c. K. Wang J. Gao P. Ding Q. Yu Y.-S. Wang F. Cabana J. Wang J. Jin S. Visualization of electrochemically driven solid-state phase transformations using operando hard X-ray spectro-imaging.Nat. Commun. 2015; 6: 6883Crossref PubMed Scopus (67) Google Scholar A number of TXM studies have also been performed on cathode secondary particles transforming through a second-order phase transition. For example, Xu et al. observed unusual particle-level chemical inhomogeneity during the delithiation of LiCoO2. In an in situ cell with a charging rate of 0.2 C, they observed the existence of Li-poor and Li-rich domains separated by boundaries with a chemical gradient.20Xu Y. Hu E. Zhang K. Wang X. Borzenets V. Sun Z. Pianetta P. Yu X. Liu Y. Yang X.-Q. Li H. In situ visualization of state-of-charge heterogeneity within a LiCoO2 particle that evolves upon cycling at different rates.ACS Energy Lett. 2017; 2: 1240-1245Crossref Scopus (121) Google Scholar As they increased the C rate, such gradient boundaries were found to migrate, causing increased heterogeneity. Gent et al. studied the chemical distribution on secondary particles of Ni-rich Li nickel manganese cobalt oxides (NMCs) at various states of charge (SOCs). Domains with a large deviation in Li content (x ± 0.09, where x is the average SOC of the sample) were observed even after extensive relaxation of the electrochemical cell (∼170 hr).21Gent W.E. Li Y. Ahn S. Lim J. Liu Y. Wise A.M. Gopal C.B. Mueller D.N. Davis R. Weker J.N. et al.Persistent state-of-charge heterogeneity in relaxed, partially charged Li1-xNi1/3Co1/3Mn1/3O2 secondary particles.Adv. Mater. 2016; 28: 6631-6638Crossref PubMed Scopus (113) Google Scholar They hypothesized that the presence of micro-strain, resulting from the anisotropic expansion and contraction of the layered structure, may have led to the alteration on Li chemical potential and, consequently, local SOC heterogeneity.21Gent W.E. Li Y. Ahn S. Lim J. Liu Y. Wise A.M. Gopal C.B. Mueller D.N. Davis R. Weker J.N. et al.Persistent state-of-charge heterogeneity in relaxed, partially charged Li1-xNi1/3Co1/3Mn1/3O2 secondary particles.Adv. Mater. 2016; 28: 6631-6638Crossref PubMed Scopus (113) Google Scholar In addition, morphology evolution upon long-term cycling was monitored by the TXM evaluation of complex structural changes in NMC secondary particles.22Yang F. Liu Y. Martha S.K. Wu Z. Andrews J.C. Ice G.E. Pianetta P. Nanda J. Nanoscale morphological and chemical changes of high voltage lithium-manganese rich NMC composite cathodes with cycling.Nano Lett. 2014; 14: 4334-4341Crossref PubMed Scopus (142) Google Scholar Here, we report the observation of chemical heterogeneity on well-formed and partially delithiated LixNb0.3Mn0.4O2 large crystals. Both 2D and 3D TXM imaging was performed on LixNb0.3Mn0.4O2 at three SOCs: x = 1.3, 1.1, and 0.5. We prepared the particles by a chemical delithiation method to ensure uniform reaction conditions and then recovered them after full relaxation (they were stored for more than 1 month) for the ex situ measurements. We examined the local Li-ion diffusion network by using selected area electron diffraction (SAED), DFT calculation, and bond valence sum (BVS) mismatch mapping. The calculation on various supercells with a size of 2 × 2 × 2 and 3 × 3 × 3 revealed the existence of short-range cation ordering and its impact on Li-diffusion pathways. Our study provides a thorough understanding on the relationship between the local chemistry and Li transport properties in disordered rock-salt oxides, enabling future design of better-performing cathode materials. We recently reported the synthesis of phase-pure Li1.3Nb0.3Mn0.4O2 cathode material by using a molten-salt method.9Kan W.H. Chen D. Papp J.K. Shukla A.K. Huq A. Brown C.M. McCloskey B. Chen G. Unravelling solid-state redox chemistry in Li1.3Nb0.3Mn0.4O2 single-crystal cathode material.Chem. Mater. 2017; 30: 1655-1666Crossref Scopus (62) Google Scholar Discrete single crystals with uniform spherical shape and a narrow size distribution centered at ∼5 μm were obtained, as shown in Figure 1A. The lack of the typical faceted appearance on the particles suggests that all crystal facets have similar thermodynamic stability and grow at a similar rate under these synthesis conditions. In other words, no dominating facets are present on the Li1.3Nb0.3Mn0.4O2 single-crystal sample. Electrochemical performance of the composite electrodes containing the ball-milled single crystals was previously evaluated.8Yabuuchi N. Takeuchi M. Nakayama M. Shiiba H. Ogawa M. Nakayama K. Ohta T. Endo D. Ozaki T. Inamasu T. et al.High-capacity electrode materials for rechargeable lithium batteries: Li3NbO4-based system with cation-disordered rocksalt structure.Proc. Natl. Acad. Sci. USA. 2015; 112: 7650-7655Crossref PubMed Scopus (334) Google Scholar During the first cycle, a specific discharge energy of 900 Wh/kg (with a total of 290 mAh/g discharge capacity at an average voltage of ∼3.1 V versus Li+/Li) was obtained, representing one of the highest energy densities reported on LIB cathode materials so far. In operando differential electrochemical mass spectroscopy measurements confirmed the contribution of oxygen redox to the total capacity, which occurred at a voltage above 4.3 V. A series of chemically delithiated LixNb0.3Mn0.4O2 (0 ≤ x < 1.3) crystals was prepared by mixing the pristine Li1.3Nb0.3Mn0.4O2 crystals with various amounts of nitronium tetrafluoroborate (NO2BF4) oxidant in acetonitrile. Compared with the electrochemical method, the main advantage of the chemical reaction is that large particles can be used directly without prior milling, therefore preserving particle morphology for imaging studies at the subparticle level. Furthermore, because all particles in the sample are exposed to the oxidizing agent in a homogeneous environment, a more uniform reaction can be expected to lead to higher fidelity in sample analysis on the basis of a limited number of individual particles. The elimination of carbon additive and polyvinylidene fluoride binder used in the conventional composite electrodes further simplifies the post-mortem analysis. As previously reported, detailed structural analysis using synchrotron X-ray and neutron diffraction techniques revealed two regions in LixNb0.3Mn0.4O2 structural transformation: a single-phase region (0.9 < x < 1.3) involving Mn3+/Mn4+ cation redox and a two-phase region (0 < x < 0.9) involving oxygen oxidation. To further understand the mechanism of the solid-state redox reactions, we resorted to particle-level hard X-ray spectro-imaging by using transmission X-ray microscopy coupled with X-ray absorption near-edge structure spectroscopy. The technique allows nanoscale visualization of TM chemical distribution as a function of the SOC. The capability of full-field imaging in a large (tens of micrometers) FOV also allows multiple particles to be examined in the same measurement. The current study focuses on the SOC region where Mn is redox active. This is because the Mn K-edge is within the optimal energy window where the Fresnel zone plate has good efficiency. Detailed studies using other techniques for the O redox active region will be presented in a future report. Figures 1B and 1C show the synchrotron X-ray and neutron diffraction patterns of the pristine and chemically delithiated LixNb0.3Mn0.4O2 (x = 1.3, 1.1, and 0.5) crystals, respectively, which confirm the phase purity of the samples. Previous studies showed that Nb cations remain at 5+ and inactive during the delithiation, whereas Mn cations remain at 4+ in the Li content range of 0 < x ≤ 0.9. The ratio between Mn3+ and Mn4+ in these three samples was then calculated to be 1:0, 1:1, and 0:1, which represent two end members along with an intermediate in the Mn3+/Mn4+ redox process. We performed 2D nanoscale chemical imaging with a nominal spatial resolution of 30 nm on these three samples to investigate the depth-averaged distribution of Mn-oxidation state. To effectively evaluate the subparticle-level chemical heterogeneity, we collected a large number of single-pixel Mn K-edge absorption spectra on the micron-sized crystals. We used the edge energy, determined by the intensity of 0.5 in the normalized spectra, to quantify the local oxidation state of Mn and create the 2D chemical distribution maps, which are shown in Figure 2. For the two end members, Li1.3Nb0.3Mn0.4O2 and Li0.5Nb0.3Mn0.4O2, the chemical maps were dominated by blue (centered at ∼6,547 eV) and green (centered at ∼6,551 eV), respectively (Figures 2A and 2B). The visual appearance of homogeneity among all examined pixels on each examined particle confirms that the Mn-oxidation state is uniform at 3+ and 4+ in Li1.3Nb0.3Mn0.4O2 and Li0.5Nb0.3Mn0.4O2, respectively. The depth-averaged values are consistent with the Mn reference spectra collected in a previous report22Yang F. Liu Y. Martha S.K. Wu Z. Andrews J.C. Ice G.E. Pianetta P. Nanda J. Nanoscale morphological and chemical changes of high voltage lithium-manganese rich NMC composite cathodes with cycling.Nano Lett. 2014; 14: 4334-4341Crossref PubMed Scopus (142) Google Scholar as well as those from the corresponding bulk spectroscopic measurements of the same crystal samples.9Kan W.H. Chen D. Papp J.K. Shukla A.K. Huq A. Brown C.M. McCloskey B. Chen G. Unravelling solid-state redox chemistry in Li1.3Nb0.3Mn0.4O2 single-crystal cathode material.Chem. Mater. 2017; 30: 1655-1666Crossref Scopus (62) Google Scholar More complex heterogeneous distribution was observed on the chemical maps of the Li1.1Nb0.3Mn0.4O2 crystal sample. As shown in Figure 2C, a range of colors spanning from blue to green was observed on all particles in the FOV, suggesting a wide distribution in Mn-oxidation state between 3+ and 4+. A common feature observed on the investigated particles is a surface layer with higher Mn edge energy signaled by the green-yellow color, which typically has a thickness of ca. 100–200 nm (Figure 2C). This is a clear indication that Li extraction began on the particle surface and the oxidation front moved toward the core as the reaction progressed. The appearance of green color with yellow domains distributed within the shell suggests that Mn4+ is the dominating species but that some Mn3+ species also exist. On the other hand, expansive color variation was observed in the core region. Five particles, referred to as P1–P5, were selected for further study of the distribution of domains with similar edge energy and the chemical gradient among them. For P1–P3, domains enriched in Mn3+ cations (blue) were observed adjacent to the green-colored shell mainly containing Mn4+ cations, whereas for P4 and P5 non-uniformity in the core region was less dramatic and only a small gradient in edge energy was seen. Further statistical analysis on the probability distribution of Mn K-edge energy over the entire particle of P1–P5 revealed that P1–P3 were enriched in Mn3+ cations, whereas P4 and P5 contained a nearly equal amount of Mn3+ and Mn4+ species (Figure 2D). The results also suggest that P4 and P5 were somewhat more oxidized (higher SOC) than P1–P3. We performed further analysis to obtain the depth profile of Mn K-edge energy distribution on two representative particles, P1 and P5 (Figure 3A). Because of the slight variation in particle size, we normalized the radius (r) of both particles to 1 and used the relative surface to center distance of 0–1. For P1, an abrupt change in edge energy was observed, revealing a large chemical gradient from a Mn4+ enriched shell (r = 0) to the subsurface (r = 0.2) where Mn3+ cations dominate. Little change was observed from the subsurface (r = 0.2) to the center of the particle (r = 1). For P5, on the other hand, the change in Mn K-edge energy from the shell (r = 0) to deep into the core (at r = 0.8) was nearly continuous, suggesting a small chemical gradient in this region. No significant changes were observed in the region from r = 0.8 to r = 1 (center of the particle). The distance for transitioning from Mn4+ to Mn3+ species is nearly 4-fold shorter in the P1 particle than in P5. The consequence of a large chemical gradient buildup is significant. In electrode materials transforming through a solid-solution reaction (e.g., layered LiMO2 ↔ layered MO2 + Li+ + e−), Li-ion movement is often driven by this gradient. The phenomenon, in fact, can be used as the basis to reveal Li-ion diffusion pathways in partially transformed particles. An algorithm was developed here to identify the optimal chemical gradient in each pixel, assuming no influence from kinetic factors and that Li ions are able to move spontaneously between neighboring pixels as long as a difference in optimal chemical potential (in this case Mn valence state) exists within a reasonable range (defined as within 6 eV/μm here), which is determined by analyzing the relative probability distribution of the chemical gradient shown in Figure S1. This methodology was further extended to progressively follow Li movement pixel by pixel and map out the global percolation network within the studied particle (see Figure S2 for details of the approach). Figures 3B and 3C show the results obtained on P1 and P5, respectively. The black lines follow the Li-ion diffusion pathway, and the arrows point to the direction of such movement. In this case, the direction corresponds to the gradient of Mn-oxidation state from low to high. It is clear that Li-ion movement in P1 and P5 are distinctively different. In P1, it initializes from the most Mn3+-rich subsurface region near the top of the particle, propagates gradually through the core of the particle, and then moves toward the bottom surface, avoiding the geometrically optimal pathway of moving toward the top surface of the particle. In P5, on the other hand, Li ions were driven entirely from the Mn3+-rich core region toward the shell region. Because the latter scenario is expected in a typical core-shell type reaction, our results show that large chemical gradients between the domains with dominating Mn3+ and Mn4+ species could act as a physical barrier for Li transport. The drastic change in Mn-oxidation state over such a short distance (∼100 nm) in the area near the top edge of P1 prevented Li ions in the blue subsurface region from diffusing upward through the top surface, which represents the geometrically optimal pathway. This could have led to the overall lower SOC in the P1 particle than in P5, as shown in Figure 2D. Because 2D transmission measurements lack the depth resolution, we then performed nano-tomography to illustrate the chemical heterogeneity in 3D. As shown in Figures 4A and 4B , the 3D renderings revealed a large but thin region enriched with Mn4+ species (green-yellow color) adjacent to a blue-colored region dominated by Mn3+ species, further confirming the existence of chemical heterogeneity at the subparticle level. Consistent with the observation from 2D maps, the chemical gradient varied significantly, such that it showed a sharp change in Mn-oxidation state similar to the P1 scenario in the top particle and a relatively smooth transformation similar to the P5 scenario in the bottom particle (Figure 4C). The use of the depth-resolved 3D technique further confirmed the lack of uniform coverage of the Mn4+-rich domains on the particle surface, even though the sample was prepared by chemical delithiation that completely submerged all the particles in the same oxidant medium. Our previous structural analysis using synchrotron X-ray diffraction (XRD) and neutron patterns showed that redox reaction in LixNb0.3Mn0.4O2 occurs in a single-phase transformation in the Li content region of 0.9 < x < 1.3, involving Mn3+/Mn4+ cation redox only. Under thermodynamic conditions, solid-solution transformation leads to homogeneous intra-particle chemical distribution. The slopping voltage profile observed on LixNb0.3Mn0.4O2 (0.9 < x < 1.3) supports the existence of significant driving force in minimizing Mn chemical gradient within a single grain. Therefore, the non-uniform initiation and progression of the redox reaction and the presence of such a large chemical gradient at a short length scale evidenced by both 2D and 3D nano-tomography studies reveal the existence of non-equilibrium Li-diffusion pathways in the disordered oxide cathode, which warrants careful investigation of possible origins. Some particle-level chemical heterogeneity was previously reported on materials transforming through a single-phase redox reaction.15Kuppan S. Xu Y. Liu Y. Chen G. Phase transformation mechanism in lithium manganese nickel oxide revealed by single-crystal hard X-ray microscopy.Nat. Commun. 2017; 8: 14309Crossref PubMed Scopus (93) Google Scholar The non-equilibrium states were typically caused by a variety of kinetic factors, such as particle size, crystal facet, grain boundary, strain, and reaction rate. For example, it was reported that anisotropic expansion in the layered LiMO2 led to accumulation of large micro-strain that alters the chemical potential of the entire system and results in non-homogeneous chemical distribution.20Xu Y. Hu E. Zhang K. Wang X. Borzenets V. Sun Z. Pianetta P. Yu X. Liu Y. Yang X.-Q. Li H. In situ visualization of state-of-charge heterogeneity within a LiCoO2 particle that evolves upon cycling at different rates.ACS Energy Lett. 2017; 2: 1240-1245Crossref Scopus (121) Google Scholar, 21Gent W.E. Li Y. Ahn S. Lim J. Liu Y. Wise A.M. Gopal C.B. Mueller D.N. Davis R. Weker J.N. et al.Persistent state-of-charge heterogeneity in relaxed, partially charged Li1-xNi1/3Co1/3Mn1/3O2 secondary particles.Adv. Mater. 2016; 28: 6631-6638Crossref PubMed Scopus (113) Google Scholar In our case, the 3D Li-ion percolation network in the rock-salt structure is isotropic in nature. As the sample was made in spherical single crystals, the effects of particle size and surface facet are also negligible. Our previous studies determined a total volume change of merely 0.2% when x was reduced from 1.3 to 0.9, which suggests that mechanical strain in LixNb0.3Mn0.4O2 is most likely insignificant.21Gent W.E. Li Y. Ahn S. Lim J. Liu Y. Wise A.M. Gopal C.B. Mueller D.N. Davis R. Weker J.N. et al.Persistent state-of-charge heterogeneity in relaxed, partially charged Li1-xNi1/3Co1/3Mn1/3O2" @default.
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• W2808026006 date "2018-09-01" @default.
• W2808026006 modified "2023-10-10" @default.
• W2808026006 title "Understanding the Effect of Local Short-Range Ordering on Lithium Diffusion in Li1.3Nb0.3Mn0.4O2 Single-Crystal Cathode" @default.
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