High Rate Li-Ion Batteries with Cation-Disordered Cathodes

Article

High Rate Li-Ion Batteries with CationDisordered Cathodes Minkyu Kim, Donghoon Kim, Yuren Wen, ..., Hong Li, Lin Gu, Byoungwoo Kang [email protected]

HIGHLIGHTS A new activation mechanism for high-rate cation-disordered cathodes Development of a new type of cation-disordered cathodes Cation-disordered cathodes with high rate capability Controlling the way of disordered cation distribution to activate cation-disordered cathodes

Kim et al. report a new class of cation-disordered material that can achieve reasonable Li diffusion by controlling the way of the cation distribution. The fully activated cation-disordered LiFeSO4F delivers a substantially improved electrochemical performance such as superior rate capability up to 100 C and excellent capacity retention for 2,500 cycles at 5 C charge/20 C discharge rate. These results extend the scope of electrode materials to include cation-disordered materials and provide new opportunities for designing high-performance Li-ion batteries.

Kim et al., Joule 3, 1–16 April 17, 2019 ª 2019 Elsevier Inc. https://doi.org/10.1016/j.joule.2019.01.002

Please cite this article in press as: Kim et al., High Rate Li-Ion Batteries with Cation-Disordered Cathodes, Joule (2019), https://doi.org/10.1016/ j.joule.2019.01.002

Article

High Rate Li-Ion Batteries with Cation-Disordered Cathodes Minkyu Kim,1 Donghoon Kim,1 Yuren Wen,2 Minkyung Kim,1 Hyun Myung Jang,1 Hong Li,2 Lin Gu,2 and Byoungwoo Kang1,3,*

SUMMARY

Context & Scale

Cation disordering in electrode materials for Li-ion batteries is deemed as an undesired structural feature for achieving high electrochemical activity owing to sluggish Li diffusion in the structure. Therefore, extensive efforts have been made to develop electrode materials having a well cation-ordered structure. We report a new class of cation-disordered material that can achieve reasonable Li diffusion by controlling the way of the cation distribution rather than the degree of cation disordering itself. The fully cation-disordered LiFeSO4F, activated by changes in the synthesis temperature and the Li/Fe ratio, delivers a substantially improved electrochemical performance. It exhibits a superior rate capability up to 100 C (36-s discharge) with  60 mAh/g and excellent capacity retention for 2,500 cycles at 5 C charge/20 C discharge rate. These results extend the scope of electrode materials to include cation-disordered materials and provide new opportunities for designing them for high-performance Li-ion batteries.

We report a new class of cationdisordered material that can achieve reasonable Li diffusion by controlling the way of the cation distribution rather than the degree of cation disordering itself. The fully cation-disordered LiFeSO4F, activated by changes in the synthesis temperature and the Li/Fe ratio, delivers a substantially improved electrochemical performance. It exhibits a superior rate capability up to 100 C (36-s discharge) with  60 mAh/g and excellent capacity retention for 2,500 cycles at 5 C charge/20 C discharge rate. These results extend the scope of electrode materials to include cationdisordered materials and provide new opportunities for designing them for high-performance Li-ion batteries.

INTRODUCTION Li-ion batteries (LIBs) are one of the most advanced energy conversion and storage technologies. Since the commercialization of LIBs,1 their use has been widely extended. Recently, they have been utilized for powering large-scale applications such as electric vehicles (EV) and energy storage systems (ESSs). To meet this surge of new applications, developing novel electrode materials that simultaneously exhibit high energy density and excellent structural stability for long cycle life is in great demand.2,3 To meet such strong demands, many efforts have focused on cation-ordered compounds in the design of novel electrode materials,4–19 where Li and transition metals (TMs) occupy specific sites because the ordered cations in these structures can provide a long-range Li-diffusion pathway, resulting in reasonable electrochemical activity. In contrast, materials with full cation disordering show a very limited Li diffusion; thus, they have been discarded as potential electrode materials. For example, fully cation-disordered rock-salt-type structures such as a-LiFeO2 are known to be electrochemically inactive due to severely limited Li diffusion. The materials that have a partial cation disordering are also not considered as preferable electrode materials because they can deliver poor electrochemical activity. In olivine structures, such as LiFePO4 that has a fast 1D Li-diffusion channel, the degree of cation disordering severely degrades the electrochemical performance because TMs in the Li sites block the 1D Li-diffusion pathway.20 Furthermore, in layered Li TM oxides such as Li1-xNi1+xO2, the cation disordering (TM ions in the Li layers) can block the Li diffusion channel; as the degree of cation disordering increases, the electrochemical

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activity can be poor partly because of poor Li diffusion.21 As a result, the cation disordering in a material should be minimized to improve a limited Li diffusion, which can lead to reasonable electrochemical activity. However, the minimization of the cation disordering in the layered materials can be not very effective because the extraction and insertion of Li can induce the cation disordering; even though a material has a negligible degree of cation disordering before electrochemical cycling, the extraction of Li leaves vacant sites in Li layers, and thereby, TM ions can move to those empty sites in the Li layers leading to the increase in the cation disordering that can limit a Li diffusion.22 As a result, the cation disordering in a material has a negative effect on the electrochemical activity irrespective of the degree of the cation disordering. However, recent studies report that some rock-salt-type cation-disordered electrode materials can show reasonable electrochemical activity23–28 and further show benefits rather than the cation-ordered compounds. Particularly, adding excess Li24,29,30 can provide possible Li percolation networks for Li diffusion; consequently, the reasonable electrochemical activity can be realized.23–25 Although cation-disordered Li compounds are electrochemically activated by excess Li, they still show very poor kinetic properties.29,30 In this regard, triplite LiFeSO4F is a very intriguing material because it can achieve reasonable electrochemical activity with high rate capability26,31–34 even though it has a full degree of cation disordering in the structure. These unexpected kinetic results have not been seen in previously reported cation-disordered materials such as layered Li TM oxides or rock-salt-type Li TM oxides. Here, we report a new class of cation-disordered materials that exhibit both a high rate capability and long-term cycle stability by controlling cation distributions even at a full degree of the cation disorder. In this class of cation-disordered material, Li diffusion behavior and phase stability strongly depend on the way of the cation distribution rather than the degree of cation disordering. The way of the cation distribution can be controlled by increasing the synthesis temperature and changing the stoichiometry. These strategies enable the cation-disordered triplite LiFeSO4F to be simultaneously activated in terms of Li diffusion and the phase stability of the lithiated and delithiated phases. Consequently, the fully activated triplite phase has a superior rate capability even at 100 C (36 s discharge) with prolonged cycle stability of 2,500 cycles without any significant fading in capacity. These results demonstrate for the first time that this new type of cation-disordered material is a very promising electrode material, unlike the general belief about known cation-disordered compounds. Our results will extend the scope of promising electrode materials to include cation-disordered materials and will provide new avenues for designing electrode materials for high-performance LIBs.

RESULTS AND DISCUSSION The Effects of Cation Disordering on Phase Stability and Li Diffusion Behavior In the triplite structure, there are crystallographically two different octahedral sites, the M1 and M2 sites, and there are 16 MO4F2 (8M1 and 8M2 sites) octahedral sites that are edge-shared along the [101] and [010] directions (Figure 1A). The edgeshared metal octahedral connections are corner-shared with SO4 tetrahedra forming a three-dimensional (3D) framework. In the triplite structure, the possible ways to distribute cations such as Li and Fe ions into the 16 metal octahedral sites depend

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1Department

of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk 790-784, South Korea

2Beijing

National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China

3Lead

Contact

*Correspondence: [email protected] https://doi.org/10.1016/j.joule.2019.01.002

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Figure 1. The Crystal Structure of Cation-Disordered Triplite LiFeSO4F (A) Crystal structure of triplite. Blue polyhedron, M1 site; brown polyhedron, M2 site. (B) Two metal octahedral connection environments in triplite LiFeSO 4 F; disparate edge-shared connection (Li-Fe or Fe-Li) and allied edge-shared connection (Fe-Fe and Li-Li). Four representative structures depending on the amount of allied edge-shared connections: (C) Corner-triplite, (D) Mixed #1, (E) Edge-triplite. Green polyhedron, LiO 4 F 2 octahedral site; dark blue polyhedron, FeO 4 F 2 octahedral site; red sphere, oxygen ion; purple sphere, fluorine ion.

on the degree of cation disordering. In the case of full cation disorder, the cations can be randomly distributed into the 16 octahedral sites such as the M1 sites for 4Li/4Fe and the M2 sites for 4Li/4Fe. However, in the case of a cation order (0% cation disordering), each cation can only occupy either the M1 or M2 sites. Given that triplite LiFeSO4F is almost fully disordered (100%),31–33,35,36 the total number of possible ways to distribute Li and Fe in the triplite unit cell is 4,900 Li/Fe arrangements (8C4 3 8C4 = 4,900). Given that MO4F2 forms edge-shared connections, the distributions of Li/Fe can lead two kinds of edge-shared metal octahedral connections: allied edge-shared connections (Li–Li or Fe–Fe) and disparate edge-shared connections (Li–Fe or Fe–Li), as shown in Figure 1B. Consequently, triplite LiFeSO4F will have two different local edge-shared connection environments depending on the distribution of the Li/Fe cations.

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Figure 2. The Relative DFT Calculated Formation Energies of Triplite Phase (A and B) Relative DFT calculated formation energies of (A) lithiated triplite phase (LiFeSO 4 F) and (B) delithiated triplite phase (FeSO 4 F) with different amounts of the Fe edge-shared configuration. In both phases, the formation energies of the triplite were taken as the reference to the formation energy of the Corner-triplite.

All 4,900 possible ways for the Li/Fe distribution could be classified in terms of the number of Fe–Fe allied edge-shared connections (Fe edge-shared configuration) as compared to similar reported results.34,37,38 Four kinds of representative triplite structures were chosen (Figures 1C and S1) with respect to the proportion of the Fe edge-shared configuration. Corner-triplite, Mixed #1, Mixed #2, and Edgetriplite have different amounts of the Fe edge-shared configuration; 0%, 13.33%, 26.66%, and 100%, respectively. It should be noted that these four representative structures still have fully cation-disordered structure but have a different distribution of the Li/Fe in the two octahedral sites. The Corner-triplite in Figure 1C comprises only disparate edge-shared connections; consequently, all of the Fe (Li) ions are connected by corner-sharing with Fe (Li) ions through an F ion at the corner (Fe or Li corner-shared configuration). In contrast, the Edge-triplite comprises only allied edge-shared connections. All of the Fe (Li) ions are connected by edge-sharing with another Fe (Li) ion (Figure 1E). Both Mixed #1 and Mixed #2 structures are a combination of the Corner-triplite and the Edge-triplite, as shown in Figure 1D. The different local metal edge-shared connection environments that depend on the distribution of cations affect the thermodynamic stability. In both the lithiated and delithiated phases, the density functional theory (DFT) calculation results (Figures 2A and 2B) demonstrate clearly that the amount of Fe edge-shared configuration in the triplite structure significantly affects its phase stability. Even though the lithiated phase of the Mixed #1 has a slightly lower formation energy than the Cornertriplite (Figure 2A), the Fe ions in the Fe edge-shared configuration can form a strong electrostatic repulsive force, and therefore, the phase stability linearly decreases as the amount of the Fe edge-shared configuration increases. It is noted that the above representative four structures only represent the amount of Fe edge-shared configuration and does not represent the way of cation distribution. Therefore, there can be a difference between the cation distribution of each structure in this report and previously reported results38,39 even though each is the same amount of the Fe edge-shared configuration; as a result, there can be a slight difference in the phase stability. However, the tendency of their phase stability depending on the amount of Fe edge-shared configuration in this report is consistent with the reported results38,39; the formation energies of the representative four structures at the lithiated state are not very different (less than 60 meV). The decrease in the phase stability with an increasing amount of the Fe edge-shared configuration is more striking in the delithiated state than in the lithiated state

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Table 1. Activation Energy (EA) of Li Diffusion of Each Segment between Two Different Li Sites in the Corner-Triplite by Using the NEB Method Activation Energy

Activation Energy

Activation Energy

Activation Energy

B-Channel (blue)

Path A4B 492 meV

Path B 4 C 480 meV

Path C 4 D 502 meV

Path A 4 D 474 meV

R-Channel (red)

Path E 4 F 366 meV

Path F 4 G 305 meV

Path G 4 H 355 meV

Path E 4 H 309 meV

P-Channel (purple)

Path D 4 E 325 meV

Path A 4 H 323 meV

Path B 4 G 306 meV

Path C 4 F 439 meV

(Figure 2B). When the Edge-triplite is fully delithiated, the oxidized Fe3+ ions in the Fe edge-shared configurations can induce a much stronger electrostatic repulsive force and, therefore, significantly reduce the phase stability, meaning that the delithiated phase of the Edge-triplite could not be stabilized. This result indicates clearly that a structure with a minimized amount of the Fe edge-shared configuration is the best way to improve the phase stability of the cation-disordered triplite in its delithiated phase. This is consistent with previously reported results.37,38 To investigate how the Li diffusion behavior depends on the distribution of cations that can affect the amount of the Fe edge-shared configuration, the Nudged Elastic Band (NEB) method was performed on three representative lithiated structures through DFT calculations on the Corner-triplite, Mixed #1 and Mixed #2. In the Corner-triplite, which does not have any Fe edge-shared configurations, three kinds of Li diffusion channels exist: the B-channel (blue), R-channel (red), and Pchannel (purple). From a structural point of view, the B-channel and R-channel can allow long-range Li diffusion and the P-channel is a short-range interconnected channel between the B- and R-channels. The activation energies (EA) of each segment between two different Li sites in the three channels are in Table 1. Considering the A site as the ground site, the interconnected 3D Li diffusion pathway in Corner-triplite has the range of EA, 400–500 meV (Figure S2). Also, in the certain interconnected Li diffusion pathways, where Li does not diffuse via A or C sites such as the R-channel in Figure S2G, Li can be diffused with lower EA, 200–300 meV. By a combination of three Li diffusion channels, the Cornertriplite can form 3D Li diffusion channels. Even though the Corner-triplite has slightly higher activation energies compared to other polyanion compounds such as LiFePO4 (200–300 meV)40 and LiVPO4F (300 meV),41 the 3D Li diffusion channels, unlike 1D Li diffusion in LiFePO4 or LiVPO4F, can lead to fast Li (de)intercalation in the Corner-triplite. Fast inter-connected Li diffusion via the P-channel plays a crucial role in enabling triplite LiFeSO4F to exhibit electrochemical activity in spite of the full degree of cation disordering (Figures 3B and 3C). When some of the B- or R-channels are blocked by Fe ions in Mixed #1 (Figure 3B), which is a result of the increase in the Fe edge-shared configurations (blue polyhedral), the fast inter-connected P-channel can provide an alternative bypass route for continuous Li diffusion (yellow arrow in Figure 3B). Hence, Li ions can quickly diffuse to nearby non-blocked long-range R-channels, leading to continuous long-range Li diffusion. This unique Li diffusion behavior via fast inter-connected P-channels can explain why the fully cation-disordered triplite can show reasonable electrochemical activity.33,35,36 However, although Li diffusion is still possible via fast bypass routes such as interconnected P channel, the diffusion channel length of Li ions severely decreases as the proportion of Fe edge-shared configurations increase. In Mixed #2 (Figure 3C), large portions of the B- and R-channels are blocked by Fe ions; thus, the length of the

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Figure 3. The Visualization of Li Diffusion Channels and the Expected Li Diffusion Behavior of Triplite with Different Amount of Fe Edge-Shared Configuration (A–C) Visualization of Li diffusion channels and the expected Li diffusion behavior in schematic diagrams of (A) the Corner-triplite, (B) the Mixed #1, and (C) the Mixed #2 structures. Green polyhedron, LiO 4 F 2 octahedra. For simplification, SO 4 tetrahedra are not displayed, and only the edge-shared FeO 4 F 2 polyhedra are displayed as blue polyhedra. Red sphere, oxygen ion; purple sphere, fluorine ion.

Li diffusion channel is severely reduced to a short-range zigzag type. However, in the Corner-triplite (Figure 3A), 3D long-range of Li diffusion channels can be formed. Considering that the four representative structures such as Corner-triplite, Mixed #1, Mixed #2, and Edge-triplite still have the same degree of cation disordering, these results demonstrate that Li diffusion kinetics in this type of fully cation-disordered materials can be controlled and improved by controlling the distribution of cations. Considering that the Li transport in the triplite structures strongly depends on the amount of the Fe edge-shared configuration, the diffusion length of Li can be described by the amount of Fe edge-shared configuration. For example, one configuration of the fully cation-disordered structure, ‘‘Edge-triplite,’’ which is only composed of Fe edge-shared configuration, can have a short-range diffusion length scale of Li ions. The other configuration of the fully cation-disordered structure, ‘‘Corner-triplite,’’ which does not have any Fe edge-shared configuration, can have a long-range diffusion length scale of Li ions. Furthermore, in a fully cation-disordered material, the different distribution of cations can affect not only Li-ion diffusion behavior but also the distance between TM and TM such as Fe-anion (F or O)-Fe. Given that electrons in this material can be conducted by a hopping mechanism, the distance between Fe and Fe can be an important factor affecting the electronic conductivity. We compare the average Fe-anion (F or O)-Fe distance in triplite structures depending on the amount of Fe edge-shared configuration in Figure S3. The Corner-triplite has the shortest Fe-anion (F or O)-Fe distance. With respect to the electronic conductivity, the Corner-triplite can have the highest electronic conductivity among the structures with different amount of Fe edge-shared configuration.

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Figure 4. The Change of Material Properties Depending on the TS (A) Synchrotron X-ray diffraction patterns of LiFeSO 4 F triplite, synthesized at various temperatures. V, FeF 2 ; +, Fe 3 O 4 impurities. @320-triplite: triplite, synthesized at 320  C; @400-triplite: triplite, synthesized at 400  C; @450-triplite: triplite, synthesized at 450  C. (B) Calculated redox potentials of triplite LiFeSO4 F in the four representative structures that have a different amount of the Fe edge-shared configuration. (C) OCV measurements of the samples using the GITT method. (D) The discharge voltage profiles of the three samples prepared at different temperatures (at a 1 C rate).

These results demonstrate clearly that compared with conventional cation-disordered materials, such as layered Li TM oxides and rock-salt-type Li TM oxides, the triplite LiFeSO4F has a different type of cation disordering.23,24,29,30 Contrary to previous reports on the conventional cation-disordered compounds in which the degree of cation disordering should be minimized for reasonable Li diffusion, the distribution of cations in the disordered sites can be a key parameter to control the phase stability and Li diffusion in the triplite phase. Furthermore, in this compound, a high phase stability and a fast Li diffusion environment can be simultaneously achieved when the triplite phase has a certain distribution of Li/Fe in which the amount of the Fe edge-shared configuration has been minimized, even though it still has a full degree of cation disordering. Considering that cation disordering in layered Li TM oxides and rock-salt-type Li TM oxides can be thermodynamically driven and significantly restricts Li diffusion, the cation-disordered triplite phase can have a large potential for fully activating the cation disordering in it. Suppression of the Fe Edge-Shared Configuration by Increasing the Synthesis Temperature Considering that the synthesis temperature (TS) can affect the distribution of cations in the compounds when it formed,42 TS was controlled to change the ways to distribute cations, and then to suppress the formation of the Fe edge-shared configuration in the triplite phase. Three samples were prepared at different TS; @320-triplite (TS = 320 C), @400-triplite (TS = 400 C), and @450-triplite (TS = 450 C). The synchrotron X-ray diffraction (XRD) patterns in Figure 4A show clearly that the three samples mainly have the

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Figure 5. STEM Images (A–E) STEM Images of the @400-Triplite (A) simulated ED patterns projected along the [100] direction depending on the amount of the Fe edge-shared configuration, (B) diffraction pattern (DP) image, (C) fast Fourier transform (FFT) patterns of the @400-triplite sample projected along the [100] direction, (D) FFT patterns, and (E) HAADF-STEM image and intensity profile of the A-B line projected along the [001] direction. (F–H) STEM images of the @320-triplite (F) FFT patterns, (G) HAADF-STEM image, and (H) intensity profile of the A-B line projected along the [100] direction.

triplite structure. Even though some impurity phases were formed (i.e., FeF2 in the @400-triplite, Fe3O4 from the decomposition of the triplite in @450-triplite), the three samples have quite similar lattice parameters (Figure S4; Table S1), indicating that the triplite phase in the three samples does not have a long range of structural difference. Even though the XRD results suggest that the triplite phases in the three samples are quite similar, triplite phases with different cation distributions can have different open-circuit voltages (OCVs) and electrochemical properties.26 The changes in the phase stability depend on the amount of the Fe edge-shared configuration (Figure 2) and thereby can lead to changes in the redox potential (Figure 4B). The redox potentials linearly increase with the amount of the Fe edge-shared configuration. Hence, the dependence of the redox potential of the triplite structure on the amount of the Fe edge-shared configuration can be used as an indicator for evaluating the amount of Fe edge-shared configuration in the samples prepared at different TS. The OCVs of the three samples were measured by a galvanostatic intermittent titration technique (GITT) (Figure 4C). The @450-triplite shows the lowest OCV that is close to that of the Corner-triplite, whereas the @320-triplite sample has the highest OCV. The OCV of the @400-triplite is in between the @450-triplite and the @320-triplite values. Consequently, these results indicate that the TS strongly affects the amount of the Fe edge-shared configuration by changing the ways to distribute cations; the higher the TS is, the lower the amount of the Fe edge-shared connection there is. Therefore, the @450-triplite sample has the lowest amount of the Fe edge-shared configuration among the three samples. The change in the way the cations are distributed further significantly affects the Li diffusion kinetics, as predicted (Figure 3) and according to previously reported results.26 To investigate the dependence of Li diffusion kinetics on TS, the rate capability test was performed on the three samples because it can indirectly estimate the Li diffusion kinetics. The rate capability of the samples improved as the TS increases (Figure 4D). At the 1 C rate, the @320-triplite achieved a capacity of only 30 mAh/g, but the @400-triplite achieved 77 mAh/g and the @450-triplite achieved 90 mAh/g. Considering that the decrease in the amount of the Fe edge-shared configuration can facilitate fast Li diffusion as shown in the NEB calculations (Figure 3), these results also corroborate that the increase in TS can lead to the decrease in the amount of the Fe edge-shared configuration by changing the way the cations are distributed. To more locally investigate the change in the cation distribution in triplite LiFeSO4F formed at different TS, scanning transmission electron microscopy (STEM) measurements were conducted on two samples, the @320-triplite and the @400-triplite. Differences in the distributions of the cations can cause differences in the electron diffraction (ED) patterns of STEM (Figure 5A). The simulated ED patterns show that an increase in the Fe edge-shared configuration forms additional reflection spots or forbidden reflection spots. Corner-triplite, which does not have any Fe edge-shared configurations, does not show any additional reflection spots, whereas

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the Mixed #1, Mixed #2, or the Edge-triplite, which all have a certain amount of the Fe edge-shared configurations, show additional reflection spots. Compared to the simulated ED patterns, the two samples show quite different local cation arrangements. The diffraction pattern (DP) image (Figure 5B) and fast Fourier transform (FFT) pattern (Figure 5C) of the @400-triplite sample, which is projected along the [100] direction, do not show any additional or forbidden reflection spots. Therefore, it closely matches the simulated ED patterns of the Corner-triplite shown in Figure 5A. In another zone axis [001], the FFT patterns of the @400-triplite (Figure 5D) are also well matched with the simulated ED patterns of the Corner-triplite in Figure S5. This observation supports the idea that the @400-triplite has a similar local arrangement of cations with the Corner-triplite. In addition, the STEM-high angle annular dark field (HAADF) image and the intensity profile of the @400-triplite sample, which is projected along the [001] direction, show that the cations are hexagonally arranged (Figure 5E) and are uniformly distributed at 0.129 nm separation. Considering that various Fe arrangements have been reported to have a range of this distance,34 the uniformly arranged cations (Figure 5E) further support a @400-triplite structure that predominantly contains one specific Fe arrangement that is close to that of Corner-triplite. In contrast, the @320-triplite sample shows quite a different cation arrangement indicating a different amount of the Fe edge-shared configuration. Compared to the patterns of the @400-triplite (Figure 5C), the FFT pattern of the @320-triplite sample projected along the [100] direction clearly shows additional or forbidden reflection spots (Figure 5F); this is quite similar to the simulated ED patterns of Mixed #1 or Mixed #2 structures (Figure 5A). This similarity indicates that a certain amount of the Fe edge-shared configuration can be partially formed in the @320-triplite. It should be noted that the forbidden reflection spots in the @320-triplite are slightly different from the simulated ED patterns of Mixed #1 or #2. This could be from a different degree of distortion of the Fe arrangements in the @320-triplite that could have different amounts of the Fe edge-shared configuration from Mixed #1 or Mixed #2. Furthermore, its intensity profile along the A-B line shows a range of distances between the cations (Figure 5H), indicating that various Fe arrangements can coexist in the @320-triplite. The Mossbauer spectroscopy measurement in Figure S6 further supports that the increase in TS can lead to the decrease in the amount of Fe edge-shared configuration in triplite. Controlling the Li/Fe Stoichiometry for Further Improving Electrochemical Activity Given that triplite LiFeSO4F starts to decompose at 400 C,31,32 raising the TS to further decrease the amount of Fe edge-shared configuration in the triplite cannot be fully exploited. Consequently, the TS should be set at 400 C or below. Instead of increasing the TS, the ratio of Fe to Li in the triplite LiFeSO4F was controlled to further decrease the amount of the Fe edge-shared configuration in the triplite structure. In the ideal stoichiometry (LiFeSO4F), the amount of Fe edge-shared configuration should be the same as that of the Li one due to a constant ratio of Li to Fe. Therefore, the ratio of the Li edge-shared configuration to the Fe one in the triplite is set by the ratio of Li to Fe. In this regard, when the amount of Li in the triplite increases but the amount of Fe decreases, the Li edge-shared configuration increases but the Fe

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Figure 6. Schematic Diagram of the Strategies for Improving the Electrochemical Performance in a Cation-Disordered Triplite LiFeSO4F

edge-shared configuration decreases. Therefore, controlling the ratio of Li/Fe can selectively decrease the amount of the Fe edge-shared configuration even though the total amount of allied edge-shared connections (Fe/Li edge-shared configuration) does not change (Figure 6). It should be noted that the Li edge-shared configuration does not seriously affect the phase stability and Li diffusion. A sample with nominal composition, Li1.05Fe0.95(SO4)0.975F, Off-triplite, was prepared using the same synthesis process as the @400-triplite sample. The charge neutrality was satisfied by the deficiency of sulfate (0.025 mol of SO4). The Off-triplite does not show any impurity phases (Figure 7A), and the refined structural parameters and particle size (Figures S7 and S8; Table S2) are quite similar to the @400-triplite, indicating the formation of a single triplite phase. The ICP measurements of the @400triplite and the Off-triplite also support the change of stoichiometry in the Off-triplite (Li: 3.21%, Fe: 26.87%), compared to the @400-triplite (Li: 3.02%, Fe: 27.86%). To confirm the further decrease in the amount of Fe edge-shared configurations in the Off-triplite, OCVs of the @400-triplite and the Off-triplite were compared. The OCV of the Off-triplite is distinctly lower than that of the @400-triplite (Figure 7B). Therefore, this result indicates that the amount of the Fe edge-shared configuration further decreases in the Off-triplite by just controlling the stoichiometry, not by changing the TS. As expected, a further decrease in the Fe edge-shared configuration leads to enhanced Li diffusion kinetics in the Off-triplite. The diffusion coefficients of the Off-triplite evaluated by GITT are higher than that of the @400-triplite (Figure 7C). In the middle state-of-discharge range, the Off-triplite shows diffusion coefficients about two orders higher than those of the @400-triplite. This increase in diffusion coefficient can enable the remarkably high rate capability of the Off-triplite (Figures 7D and S9A). It achieved a capacity of 118 mAh/g (80% of the theoretical capacity, the theoretical capacity of Off-triplite = 147.3 mAh/g) at 10 C rate (6 min discharge) and 80.7 mAh/g (54.8% of theoretical capacity) at 50 C rate, (72 s discharge). When the current was further increased to 100 C discharge rate (36 s discharge), the Off-triplite still delivered 60 mAh/g of capacity (41% of theoretical capacity). The high rate capability of the Off-triplite can be ascribed to the decrease in the amount of the Fe edge-shared configuration that can lead to the formation of 3D long-range Li diffusion (Figure 3). Also, the nano-sized particles of the Off-triplite that can shorten Li diffusion length in particles (Figure S7) can help

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Figure 7. The Fully Activated Electrochemical Performance of Off-Triplite (A) Synchrotron X-ray diffraction patterns of the @400-triplite and Off-triplite. (B) OCV of @400-triplite and Off-triplite, measured by using GITT method. (C) Comparison of the diffusion coefficients of @400-triplite and Off-triplite in the 1 st discharge process, measured by using GITT method. (D) The rate capability of the Off-triplite. Charge rate was 0.2 C, and the voltage window was from 2.0 to 4.5 V. (E) Capacity retention (black, left y axis) and Coulombic efficiency (blue, right y axis) at 5 C charge and 20 C discharge rates over 2,500 cycles. (F) The 1 st , 500 th , 1,000 th, 1,500 th , 2,000 th , and 2,500 th discharge curves at 20 C discharge rate in the voltage range 2.0–4.5 V.

to achieve high rate capability. Considering that triplite phase has full cation disordering, this high rate capability up to the 100 C rate is a remarkable result, and therefore, this suggests that the cation disordering in the triplite phase can be quite different from the reported one in conventional cation-disordered compounds. Consequently, the full degree of cation disordering in the triplite can be electrochemically activated by controlling the distribution of cations rather than minimizing the degree of the cation disordering. Furthermore, the Off-triplite also shows remarkably improved structural stability. Compared to the @400-triplite, the Off-triplite shows a reduced volumetric

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difference (DV/V) between the lithiated (712.146 A˚3 in Table S2) and the delithiated phase (709.741 A˚3 in Table S3; Figure S10). The @400-triplite shows 0.6%36 volumetric change whereas the Off-triplite shows only 0.33% of the volumetric difference. The decrease in the amount of the Fe edge-shared configuration can make the delithiated phase more stable and, consequently, lead to fewer structural changes during charge/discharge leading to a reduced volumetric change. Therefore, the Off-triplite with high structural stability and high electrochemical activity can achieve a very high capacity retention over 2,500 cycles at a 5 C charge/20 C discharge rate without any significant capacity fading (Figures 7E and 7F), and over 1,500 cycles with a capacity of 75mAh/g at a 10 C charge/10 C discharge rate (Figure S9). At a 5 C charge/20 C discharge rate, the Off-triplite shows excellent capacity retention of almost 91% at the 2,500th cycle (Figure 7F). The increase in capacity, compared to that of the initial cycle, can be explained by the activation process in the 1st Li insertion and extraction, as reported previously.26 In this study, we for the first time report on a new class of cation-disordered compound by using triplite LiFeSO4F as a model compound and performing DFT calculations. In this type of cation-disordered compound, the activation of the cation disordering is quite different from conventional cation-disordered compounds such as cation-disordered Li-layered oxides and rock-salt-type Li metal oxides. Here, the cation disordering is fully activated by controlling the cation distributions rather than suppressing the degree of the cation disordering. Distributing the cations can be controlled by changing the synthesis temperature or the composition and thereby can significantly improve both the long-range Li diffusion and the phase stability simultaneously. Consequently, the fully activated triplite phase can achieve very high rate capability, which is unlikely in conventional cation-disordered compounds that cannot intrinsically achieve fast kinetics. Furthermore, the cation disordering in this type of compound results in negligible volume changes during the charge/discharge processes26 and leads to prolonged long-term cycle stability. Negligible volume changes with high electrochemical activity during cycles make this new type of cation-disordered compound a promising candidate for all solidstate batteries where a volume change strongly affects performance.43 We emphasize that our findings fully unlock the potential of the cation disordering in electrode materials. Contrary to the general belief that a cation disordering is an undesired structural feature for battery electrode materials, our results demonstrate that cation disordering can be fully exploited to provide promising electrochemical properties with respect to rate capability and long-term cycle-ability. Our findings of a new class of cation-disordered compounds can extend the limited choice of electrode materials to include cation-disordered compounds and open up new avenues for the design of promising electrode materials for use in high-performance Li-ion batteries.

EXPERIMENTAL PROCEDURES Synthesis To prepare a mixture of precursors, LiF, FeC2O4$2H2O, and (NH4) 2SO4 (mole ratio: 1: 1: 1 or 1.05: 0.95: 0.975 for Off-triplite) were ball milled with various diameter zirconia balls for 48 h in acetone. The mixture of precursors was dried and then formed into pellets (5 mm radius) under 5 metric tons of pressure. Then, the pellet was annealed at various conditions in a covered alumina crucible. The annealing time of @400-triplite and @450-triplite was fixed at 1 h. To achieve highly pure triplite LiFeSO4F at 320 C, the annealing time of @320-triplite was 250 h in a dried autoclave.

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To prepare a mixture of precursors with polyvinylidene fluoride (PVDF), PVDF was separately settled in a covered alumina crucible with the pellets of the precursor mixtures. Because the electrical conductivity of PVDF is low and it is also used as a binder material for electrodes, it is fatal to mix it with the precursors. The schematic diagram is provided in Figure S11. The pellet was annealed at various experimental conditions in a covered alumina crucible. Synchrotron XRD Pattern Synchrotron XRD patterns were collected on the 9B beamline at the Pohang Accelerator Laboratory (PAL). The scan range was 10 –130.5 in increments of 0.02 . The data were collected at room temperature. Electrochemical Characterization Electrochemical tests were performed in a Swagelok-type half-cell with a Li metal anode. Electrodes were prepared by mixing the active material, carbon (Super P, Timcal), and binder (PVDF, KUREHA, KF7208) in a weight ratio of 65:30:5. The cells were assembled in an Ar-filled glove box and tested on a Maccor Series 4000 operating in the galvanostatic mode using a non-aqueous electrolyte (1M-LiPF6 in EC:DEC [1:1] from Panaxetec) and Celgard 2400 as a separator. All the cells were tested at room temperature. The loading density of the electrode was about 1–2 mg/cm2. DFT Calculations DFT calculations were performed using the plane-wave basis VASP code44,45 with an energy cutoff of 500 eV. The projector-augmented-wave (PAW) method46 was adopted to describe the potential from ionic cores. Structural relaxation was performed until the maximum Hellmann-Feynman forces were less than 0.01 eV A˚ 1 using the Monkhorst-Pack k-point sampling47 of 2 3 4 3 2. We employed the generalized gradient approximation (GGA) plus the Hubbard U method48,49 (Ueff of 4 eV for Fe 3d orbital) with the Perdew-Burke-Ernzerhof (PBE) exchangecorrelation functional.50 STEM The atomic structures of @320-triplite and @400-triplite were characterized using an ARM-200CF (JEOL, Tokyo, Japan) transmission electron microscope operated at 200 kV and equipped with double spherical aberration (Cs) correctors. ABF and HAADF images were acquired at collection angles of 11–22 and 90–250 mrad, respectively.

SUPPLEMENTAL INFORMATION Supplemental Information includes 11 figures and 3 tables and can be found with this article online at https://doi.org/10.1016/j.joule.2019.01.002.

ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) and was funded by the Ministry of Science, ICT & Future Planning (NRF-2015M2A2A6A01044985, NRF2017M3A7B8065394). This work was partially supported by the Fundamental R&D Program for Technology of World Premier Materials (WPM) funded by the Ministry of Knowledge Economy (grant no. 10037918) and by the Center of Futuristic Material-system of the Brain Korea 21 Project. We thank Professor J.M. Tarascon for his insightful discussions. We also thank Dr. Maxim Avdeev for measurements at the Neutron diffraction in Australian Nuclear Science and Technology Organization

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(ANSTO) and Dr. Docheon Ahn for providing beam time on the 9B beamline at the Pohang Accelerator Laboratory (PAL) for synchrotron diffraction X-ray measurements. We also thank Professor Chul Sung Kim in the Department of Physics at Kookmin University for measurements of Mossbauer.

AUTHOR CONTRIBUTIONS Minkyu Kim and B.K. designed the project, and B.K. supervised the project. Y.W. and L.G. contributed with STEM measurement and D.K. and H.M.J. contributed with the DFT calculation. Minkyung Kim contributed analyses of synchrotron diffraction patterns. H.L. suggested his insightful opinion for discussion. Minkyu Kim performed electrochemical tests, preparation of samples, etc. Minkyu Kim and B.K. wrote the manuscript.

DECLARATION OF INTERESTS The authors declare no competing interests. Received: September 19, 2018 Revised: November 26, 2018 Accepted: January 8, 2019 Published: February 5, 2019

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