A Simple Water Treatment Strategy to Optimize the Li2MnO3

Jun 24, 2019 - By the study of gradient activation in the initial charge, we find that the separate activation of platform regional (4.4 V~4.6 V) has ...
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Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Simple Water Treatment Strategy To Optimize the Li2MnO3 Activation of Lithium-Rich Cathode Materials Min-Jun Wang,† Ai-Fen Shao,‡ Fu-Da Yu,*,† Gang Sun,† Da-Ming Gu,† and Zhen-Bo Wang*,† †

MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, No. 92 West-Da Zhi Street, Harbin 150001, China ‡ Shanghai Institute of Space Power-Sources, No. 2965 Dongchuan Road, Minhang District, Shanghai 200245, China

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S Supporting Information *

ABSTRACT: Li-rich layered oxides, despite their superhigh capacity, still suffer from oxygen release and structural transformation during the first charge process, causing low Coulomb efficiency and continuous capacity fading. Preactivation has been widely recognized as a promising strategy to improve electrochemical performance of Li-rich material, though still plagued by the complexity of adopted methods and vulnerability of material structure. Here, we present a novel and gentle water treatment strategy to realize preactivation. Composition and structure analysis reveal that the Li-rich oxides can react with water by the extraction of lithium ion. In situ X-ray diffraction indicates that water treatment optimizes the activation process of Li-rich cathode material by decreasing transition metal ion diffusion. After treatment, the target material displays improved Coulombic efficiency of 86% and high discharge capacity of 279 mAh g−1 and shows prominent cycling stability of 114% after 200 cycles. By studying gradient activation in the initial charge, we find that separate activation of the platform region (4.4−4.6 V) has significant effect on the improvement of material capacity, which may be mainly responsible for the improvement of discharge capacity and Coulombic efficiency of Li-rich materials from preactivation. KEYWORDS: Preactivation, Water treatment, In situ X-ray diffraction, Cycle performance, Gradient activation, Li-rich oxide cathode material

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INTRODUCTION To meet the requirements of long-endurance electric vehicles and large electrical energy storage applications, advanced electrode materials with higher energy and power density are in urgent demand. Li-rich layered oxides, such as a late-model cathode material with capacity of more than 250 mAh g−1 and high operating voltage up to 4.8 V, represents a new milestone in energy density as compared with current commercial cathode materials, such as LiCoO2, LiMn2O4, and LiFePO4.1−3 This class of materials displays a quite intricate integral structure with two diverse local compositions of Li2MnO3 and LiMO2 (M = Ni, Co, Mn). The Li2MnO3 component is transformed from LiMnO2 by substituting a part of Mn by Li. The extra Li ions in this material not only can deliver a higher capacity but also play a vital role in the stability of the crystal structure at high voltage.4 Normally, Li2MnO3 gets activated behind the reduction reaction of Co3+/Co4+ and Ni2+/Ni4+ in the LiMO2 component. The unsteadiness of the LiMO2 component at the lithium-depleted state can be settled by diffusion of Li ion from octahedral sites in the manganese layer of Li2MnO3 to tetrahedral sites in the lithium-depleted layer.5 Therefore, Li2MnO3 stabilizes the LiMO2 component by Liion compensation. When charge potential is above 4.5 V, Li2MnO3 gets activated and brings considerable capacity for Li-rich materials, © XXXX American Chemical Society

but this also leads to endangerment for its crystal structure. The lattice oxygen is released from the Li-rich material surface for charge compensation with extraction of Li ion,6−8 accompanied by undesirable structural rearrangement. The oxygen loss and structural rearrangement in the Li-rich materials induces a series of drawbacks, such as low initial Coulombic efficiency (CE), poor rate performance, and insufficient cycling stability.9−14 Therefore, optimizing the activation process of cathode materials is a promising strategy to improve electrochemical performance of Li-excess materials. Recently, preactivation before electrochemical cycling has been widely investigated to reduce oxygen release and adjust the structural rearrangement during electrochemical activation.15−19 In previous reports, harsh chemical conditions were always adopted to overcome the thermodynamic equilibrium of cathode materials. Acid leaching associated with a H+/Li+ exchange reaction was adopted in an early stage. Bruce et al. reported that Li2MnO3 can be activated by exchange of Li+ by H+ and simultaneous O2− removal.20,21 Kang et al. dealt with Li-rich layered oxides with HNO3, following an annealing treatment.15 In their report, Li ions were taken away from the Received: March 28, 2019 Revised: May 23, 2019 Published: June 24, 2019 A

DOI: 10.1021/acssuschemeng.9b01719 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 1. Schematic illustration of water treatment for preactivation of Li-rich material.

Figure 2. SEM images of (a) WT-0, (b) WT-1, (c) WT-2, and (d) WT-3.

cathode material by Li+/H+ exchange and a spinel layer was created on the material surface during annealing treatment, which evidently improves the specific capacity and initial Coulombic efficiency. In recent studies, a lot of reagents, such as Na2S2O8, (NH4)2SO4, and NH4H2PO4, are also selected to realize preactivation.17−19 Though these chemical treatment strategies are effective in achieving preactivation, the crystal structure of Li-rich oxides is easily damaged, which will lead to the deterioration of cycle stability. In addition, previous studies are rare to investigate the influence of preactivation on the initial electrochemical activation process. In this work, we innovatively introduce a novel and extremely simple strategy to activate Li-rich cathode materials. Deionized water is taken as solvent for the first time and proved effective in realizing preactivation. Inductively coupled plasma and transmission electron microscopy tests demonstrate that lithium is extracted from the material surface without a noticeable interruption in the bulk structure, which indicates that this relatively low temperature treatment

(without annealing) has a less undesirable impact on structural stability of Li-rich material. In situ and ex situ X-ray diffraction tests reveal that the extracted Li ion comes from Li2MnO3 component, and water treatment optimizes the activation process of Li-rich cathode material by reducing the transition metal (TM) ion diffusion. After preactivation, treated samples display improved Coulombic efficiency and discharge capacity, as well as prominent cycling stability. Dynamic tests reflect that water treated material exhibits lesser transfer impedance and stronger Li-ion migration coefficient at a low potential of discharge process. By studying the activation process of water treated material, we find that preactivation can be regarded as a specific form of gradient activation which can play a great role in the improvement of electrochemical performance. Our work demonstrates a novel way to achieve the preactivation of Lirich materials and provides new insight into the relationship between good electrochemical properties and preactivation of Li-rich oxide. B

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EXPERIMENTAL SECTION

processing time is helpful to Li-ion extraction. However, the contents of Mn, Ni, and Co are relatively stable, which suggests the transition metals do not participate in the preactivation reaction. In consequence, oxygen should be deducted from cathode materials during lithium extraction for the sake of valence balance.15 This is equivalent to the initial electrochemical activation that Li and O are extracted simultaneously from the cathode material during the first charge. In the water treatment process, the reaction product of lithium and oxygen extraction during preactivation is easily lost in the water and difficult to detect. Therefore, water vapor was used to react with the cathode material to investigate the reaction mechanism of water treatment. A 0.5 g amount of pristine sample was put into a tube furnace and treated at the atmosphere of water vapor at 120 °C for 5 h. Then, the obtained sample was dried at 120 °C for 5 h to wipe off the residual moisture on the surface. XPS is a useful test to characterize the surface features and chemical state of the cathode material. Figure 3a displays the O 1s spectra of pristine and water vapor treated sample. In the pristine sample, the peak at about 531.6 eV corresponds to oxygen species at the material’s surface,22 and the strong peak at about 529.4 eV can be assigned to lattice oxygen of Li-rich layered oxides.23 The lattice oxygen peak is much stronger than the surface oxide peak. However, after water vapor treatment, surface oxide peak is shifted to high binding energy and the peak intensity is much higher than that of pristine LR-NCM. This must be due to the generation of oxides on the cathode surface by the reaction between water vapor and Li-rich materials. From the binding energy, the reaction product is LiOH or Li2CO3 (Li2CO3 come from the reaction between LiOH and CO2).24−26 Therefore, XPS data further indicates lithium and lattice oxygen were extracted by H2O. The reaction equation can be depicted as follows:

Synthesis of Pristine Materials. The pristine Li-rich material was synthesized by a co-precipitation method, and the detailed process is as follows: 2 mol L−1 aqueous solution of MnSO4, CoSO4 and NiSO4 with molar ratio of 4:1:1 was added into a continuously stirred tank reactor (CSTR), and meanwhile, an equimolar resultant solution of sodium carbonate (Na2CO3) was added as precipitant solution. The pH value was maintained at 7.5 by controlling the feeding speed. After reaction for about 12 h, carbonate precursor was collected and mixed with Li2CO3 (5% Li excess) by grinding. The mixture was first annealed at 500 °C for 5 h and then at 850 °C for 12 h, followed by furnace cooling to produce the pristine Li-rich material. Water Treatment. The synthesis of pristine material is by coprecipitation method and the detailed process is displayed in Supporting Information. Figure 1 shows the schematic illustration of the water treatment method. A 5 g amount of pristine material was added into 500 mL of deionized water and then put into a water bath and stirred at 80 °C for a certain time. After that, the products were filtered and washed by deionized water for three times and dried at 120 °C for 24 h. The primary sample was named WT-0, and the treated samples by water for 0.5, 1, and 2 h were named as WT-1, WT-2, and WT-3, respectively.

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RESULTS AND DISCUSSION Characterizations of Electrode Material. The SEM figures before and after water treatment are displayed in Figure 2. All sample present a spherical structure, and the diameter is about 10 μm. There are no obvious differences between the samples before and after water treatment, which indicates that water treatment does not have side effect on the spherical dense configuration of the active material. ICP-OES was used for inquiry into the metal contents of samples before and after water treatment. From Table 1, the Table 1. Amounts of Nickel, Manganese, Cobalt, and Lithium in the Cathode Materials (ICP Data) sample

Ni

Mn

Co

Li

WT-0 WT-1 WT-2 WT-3

0.135 0.135 0.135 0.135

0.531 0.532 0.531 0.532

0.137 0.136 0.137 0.137

1.196 1.174 1.146 1.097

Li 2O + 2H 2O → 2LiOH

(1)

Figure 3b presents the O 1s spectra of water treated samples. The peaks of treated samples at ∼531.6 eV have lower intensity than that of pristine sample WT-0, indicating the surface oxygen species have been removed during water treatment. The lattice oxygen peak shifts to higher binding energy with the increase of processing time from ∼529.4 eV for WT-0 to 530.1 eV for WT-3. This can be attributed to the extraction of lattice oxygen which decreases the electrostatic repulsion from adjacent oxygen anions and generates peroxo-

content of Li rapidly reduces from 1.196 for WT-0 to 1.174, 1.146, and 1.097 for WT-1, WT-2, and WT-3, respectively. This indicates that Li ion is abstracted from cathode materials during the treatment process, and the prolongation of

Figure 3. (a) XPS spectra of the pristine and water vapor treated sample. XPS spectra of WT-0, WT-1, WT-2, and WT-3; (b) O 1s and (d) Mn 2p core levels. C

DOI: 10.1021/acssuschemeng.9b01719 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 4. HRTEM images of (a−c) WT-0 and (d−f) WT-2. XRD patterns of (g) WT-0 (h) WT-1, (i) WT-2, and (j) WT-3.

like O22− species.27−32 Figure 3c and Supporting Information Figure S1 exhibit XPS spectra of transition metal ions. The peaks of Mn, Ni, and Co for all samples are located at the same position, and the valence states for the transition metal are Mn4+, Co3+, and Ni2+.22,33−37 This reveals that water treatment does not have much effect on transition metal ions. High-resolution TEM (HRTEM) was performed to explore the effect of water treatment on the crystal structure of the material surface. Panels a−f of Figure 4 show the microcosmic morphology and structure of WT-0 and WT-2. As shown in Figure 4c,f, HRTEM images exhibit a distinct layered structure in the subsurface of the cathode materials and the clear lattice fringes suggest their high crystallinity. In the WT-0 sample, the lattice fringes extend from subsurface to the very edge of the surface, which indicates that the material surface also remains

an intact layer structure. But as shown in Figure 4c, the lattice fringes of WT-2 fade away near the cathode surface and a nonuniform surface is detected. This variation must be due to the phase transformation induced by water treatment. The depth of the non-uniform surface is about 5−10 nm, while the bulk still has integrated lattice fringes, which implies that preactivation is realized without a noticeable interruption in the bulk structure through the water treatment. Panels Figure 4g−j display XRD patterns of pristine and water treated samples. All main peaks in the patterns’ diffraction peaks for these samples can be indexed to a hexagonal α-NaFeO2 structure with R3̅ m space group, excepting for some broad peaks between 20 and 25°, which correspond to the Li2MnO3-type structure.34 The notable split of (006)/(012) and (018)/(110) peaks presents highly layered D

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ACS Sustainable Chemistry & Engineering structure.32,38 The GSAS software is used for Rietveld refinement. In Table 2, parameter c is continuously decreased

4.0 V), which indicates that water treatment also has an influence on the delithiation process of LiMO2. Near the potential of 4.5 V, the charge curves display a characteristic long plateau which is associated with the Li 2 MnO 3 activation.6,40 WT-2 shows a shorter potential plateau than WT-0, which must be due to the consumption of Li2MnO3 by preactivation before electrochemical cycling. Although involved with partial reversible anionic redox, the electrochemical activation of Li2MnO3 also comprises lots of lattice oxygen releasing which deteriorates the initial structure integrity and triggers the phase transitions. Therefore, the shorter potential plateau also implies less oxygen release and undesirable structure rearrangements in the material’s surface. Regarding the discharge, WT-2 delivers the initial specific capacity of 259.6 mAh g−1, while WT-0 only yields 241.4 mAh g−1. The Coulombic efficiency improves from 72.1 to 86.3%. The increases of Coulombic efficiency and initial discharge capacity suggest that more anionic redox of (2O2−/(O2)n−) is induced for charge compensation instead of O2−/O2 which always gives rise to O2 release from material lattice and adverse side effects.41 So, water treatment improves the thermodynamic stability of surface lattice oxygen in Li-rich materials and decreases the undesirable structural rearrangement, which is crucial to perfect the electrochemical performance. Figure 5b displays the corresponding differential capacity (dQ/dV) plots of WT-0 and WT-2 in the first cycles. Peak 1 at about 4.0 V can be assigned to the reduction of transition elements (Co 3+ /Co 4+ and Ni 2+ /Ni 4+ ) in the LiMO 2 component. The peak at 4.5 V corresponds to the Li2MnO3 activation, and the peak intensity is always concerned with the amount of oxygen release.3,42,43 The lower peak intensity in

Table 2. Summary of Structural Refinement of WT-0, WT-1, WT-2, and WT-3 by Rietveld Refinement sample

a=b

c

intermixing of cations, %

WT-0 WT-1 WT-2 WT-3

2.8497(1) 2.8495(1) 2.8493(5) 2.8495(9)

14.2278(3) 14.2231(0) 14.2168(1) 14.2162(9)

2.29 3.56 3.82 4.52

after water treatment from 14.2278 for WT-0 to 14.2231, 14.2168, and 14.2163 for WT-1, WT-2, and WT-3, respectively, indicating the shrinkage of the TM layer. This also can be attributed to the extraction of lattice oxygen which decreases the electrostatic repulsion by the adjacent (003) plane.39 However, parameter a maintains a relatively stable value among the four samples. In addition, the degree of cation mixing also becomes severe after water treatment. This may be ascribed to the structure evolution that TM ions migrate to vacancies generated by Li+ extraction in the Li layer during treatment. This is in accordance with the structure variation during electrochemical activation in the first charge. Electrochemical Performances. Figure 5a shows the first cyclic curves of WT-0 and WT-2 at 0.1 C. Before 4.45 V, the samples present a smoothly sloping voltage profile which corresponds to reduction of Ni2+/4+ and Co3+/4+ redox in the LiMO2 component. Li2MnO3 is unactivated in this stage and contributes zero capacity. However, the charge curve still exhibits a visible difference, especially in the initial stage (about

Figure 5. (a) Typical initial charge−discharge curves. (b) dQ/dV profiles of WT-0 and WT-2 between 2.0 and 4.8 V. (c) Initial three charge− discharge curves of WT-2 between 2.0 and 4.8 V. (d) Cycling performancse of WT-0, WT-1, WT,-2 and WT-3 at 1 C. E

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Figure 6. (a−c) In situ XRD patterns of WT-2 in the initial two cyclic processes. (d) Intensity variation of peaks during two cycles. (e, f) Intensity variation of (003) and (104) peaks.

of Li+ during water treatment does not come from the LiMO2 component. However, when the potential above 4.5 V and Li2MnO3 begins to be activated, 003 peak shifts to a higher angle, which indicates a bigger interplanar spacing and can be attributed to the decrease of electrostatic repulsion from adjacent layers after oxygen release. In the second charge, 003 peak shifts to lower angle in the whole charge process, which suggests that there is no obvious oxygen release from the material crystal in the second charge process. These structure variations all suggest that XRD characteristics of water treated samples in Figure 4 demonstrate that the Li2MnO3 component in Li-rich cathode material becomes activated. Figure 6c exhibits the intensity variation of peaks during the two cycles. The detailed color varies with intensity, and the 3D image can be seen in Figure S2. Both 003 and 104 peaks exhibit obvious variation at high voltage of the first charge, while these peaks display stable intensity in the second charge. The detailed intensity variation is depicted in Figure 6e,f. Normally, the 003 peak intensity reduces when transition element ions transfer out of the transition element layer and this is undesirable for material structure.30 At the initial stage of 0 min < time < 50 min (slope region), peak intensity varies little, which indicates transition metal ions are stable in this period. This reflects that the material structure of Li-rich layered oxides is robust at low potential and preactivation does not influence the stability of

WT-2 is consistent with shorter voltage platform. Regarding the discharge, peak 3 and 4 are relevant to the insertion of Liion into the TM layer, while peak 5 is relevant to Li-ion insertion into the Li layer.44 WT-2 possesses a noticeably stronger intensity of peak 5 than WT-0, which indicates Li ion has faster insertion kinetics into Li layer after water treatment.45 Panels c and d of Figure 5 display the initial three laps of charge and discharge curves for WT-0 and WT-2, respectively. From Figure 5c, WT-0 exhibits relatively stable discharge capacity for the initial charge−discharge cycles, which suggests WT-0 has been fully activated and its highest capacity is 241.4 mAh g−1. However, in Figure 5d, WT-2 demonstrates gradual increases of discharge capacity for three cycles and reaches 278.8 mAh g−1 for the third one. In situ XRD technique was employed to monitor the evolution of the material structure of WT-2 during the initial two charging−discharging processes. As shown in Figure 6b, at the start of the first charge corresponding to reduction of the LiMO2 component, the 003 peak shifts to the left rapidly. This suggests that the interplanar spacing of 003 slabs becomes bigger and can be ascribed to the increase of electrostatic repulsion from adjacent oxygen layers due to the removal of Li+.46 This is opposite of the structure variation of preactivated samples in Figure 4 (XRD), which verifies that the extraction F

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Figure 7. (a) Cycle performance at 1 C and (b) corresponding voltage for WT-0 and WT-2. (c, d) Charge and discharge profiles and (e, f) dQ/dV curves of WT-0 and WT-2 at different cycles.

(104) peak intensity is decreased in the charge, but it restores to the initial value when the discharge finished. This further confirms the reversibility of the material structure in the second charging−discharging process and the additional discharge capacity obtained (must be from Li 2 MnO 3 activation) in the second cycle does not deteriorate the crystal structure of WT-2. According to the above analysis, we confirm that water treatment optimizes the activation process of Li-rich cathode material by preactivation of the Li2MnO3 component. The cycle performance was evaluated at 1 C between 2.0 and 4.8 V after an initial activation for three cycles at 0.1 C. From Figure 7a, compared with WT-0, which presents the initial capacity of 174.8 mAh g−1 at 1 C and 145.6 mAh g−1 after 200 cycles (83.3% capacity retention), water treated samples display higher initial capacity and outstanding cycling performance, simultaneously presenting an abnormal increase of discharge capacity during the cycling process. Among the three treated samples, WT-2 presents a prime performance and exhibits an initial capacity of 189.0 mAh g−1 at 1 C and 215.2 mAh g−1 after 200 cycles (113.7% capacity retention). The rate capabilities of WT-0 and WT-2 are depicted in Figure S3. Due to the increase of capacity upon cycling, rate tests are implemented after cycling for 50 cycles at 1 C. After cycling, WT-2 displays a much higher rate capability with

the LiMO2 component. In addition, this is also inconsistent with the increase of cation mixing of treated sample exhibited in Table 2. However, in the high-potential region of 50 min < time < 110 min, the intensity of (003) peak fastly decreases, indicating the transference of transition element ions out of the original position to stabilize the material structure. This is in accordance with the characteristic of Li2MnO3 activation as previously reported, and the unidirectional intensity variation suggests that this is an irreversible process. But in subsequent charge and discharge, despite considerable increase of discharge capacity obtained in the second cycle as shown in Figure 4d, the (003) peak intensity is stable and invertible all along, implying posterior capacity obtained without obvious TM ions migration and structure transformation. This is also in accordance with the obtained consequence that no obvious oxygen releasing from material crystal in the second charge process (Figure 6b). For the 104 peak, the intensity decreases in the whole charge process. This can be ascribed to the extraction of Li+ and mitigation of TM ions in the initial charge (the atomic configuration of 104 planes can be seen in Figure S2c). In the first discharge process, although the intensity increases, it cannot return to the initial value, indicating the irreversible TM mitigation and structure transformation during initial Li2MnO3 activation. However, in the second cycle, the G

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Figure 8. Successive charge and discharge of WT-0 (a) between 2.0−4.6 and 2.0−4.8 for one cycle and (b) between 2.0−4.4, 2.0−4.6, and 2.0−4.8 for one cycle. (c) Schematic illustration of preactivation as special gradient activation.Charge and discharge curves between 2.0 and 4.8 V after different activation program. (d) Cycling performance between 2.0 and 4.8 V after different activation program.

277.4, 239.8, 213.6, and 153.1 mAh g−1 at 0.1, 0.5, 1, and 3 C, respectively, whereas WT-0 delivers the discharge capacities of 231.4, 189.7, 158.1, and 92.3 mA h g−1 at the corresponding rates. Figure 7b shows the variation of the average potential during cycling. WT-0 owns a steady voltage decay rate in all 200 cycles. In contrast, the voltage of WT-2 drops sharply during the first 30 circles and then starts to drop slowly in subsequent cycles. For WT-2, the voltage drop is 0.91 mV/cycle in the cyclic process from the 30th to the 200th, and meanwhile, it is 1.87 mV/cycle for WT-0. Therefore, except for the initial stage, the voltage drop of WT-2 is significantly lower than that of WT-0. Panels c and d of Figure 7 are the normalized charge and discharge profiles of WT-0 and WT-2 upon cycling, respectively. WT-0 displays significant variation of curves no matter whether at the charge or the discharge process (showed by the red arrows). Furthermore, WT-0 displays rapid capacity decay (black arrows), while WT-2 shows increased capacity in low-voltage areas (below 3.0 V). The increase of capacity in low potential will decrease the average voltage of WT-2, which can be responsible for the high voltage drop in the initial 30 cycles. Panels e and f of Figure 7 display the differential capacity (dQ/dV) plots of charge and discharge profiles upon cycling. In the charge process, peaks 1 and 1′ both shift to high potential due to the influence of polarization or impedance, and the smaller offset in WT-2 means that the water treated sample is less affected by cycling, which can be ascribed to its stable surface structure. In the discharge process, the curves of WT-0 and WT-2 exhibit similar variation in high voltage (above 3.5 V). In the low-voltage section, there is only one peak in WT-0 which gradually deviates toward low voltage and its intensity becomes stronger. This is due to the severe structure transformation from layer to spinel, which leads to

serious voltage attenuation in WT-0. The intensity of the WT2 peak at low voltage also increases, and this peak gradually splits into two parts. Peak 3′ at about 2.75 V corresponds to peak 3 in WT-0. The intensity and position of peak 3′ have no significant variation, indicating the phase transformation from layer to spinel in WT-2 is not obvious. Peak 4′ is absent in WT-0, and its intensity increases quickly in the initial cyclic stage, which corresponds to the increase of capacity upon cycling. The generation of peak 4′ must be related to the optimizing function of preactivation on structural evolution, which can be beneficial to rate performance in the cyclic process and induces more capacity from preactivated material at a high-rate state.32 The position of peak 4′ is lower than the average voltage; thus the increase of peak 4′ will reduce the average voltage value. This can be responsible for the rapid voltage drop of WT-2 in the initial 30 cycles, and when the capacity increase slows down, the voltage attenuation mitigates quickly. In view of the increased capacity lowering the average voltage, the voltage drop of WT-2 is still much lower than that of WT-0 in the cyclic process from the 30th to the 200th; therefore phase transformation from layer to spinel is significantly restrained by water treatment. It is noted that the structural transformation from layer to spinel is related to lattice oxygen release, transition metal migration, and other structural changes that happened in the Li2MnO3 activation process. Therefore, the optimizing function of water treatment on Li2MnO3 activation decreases these undesirable structural changes and inhibits the phase transformation from layer to spinel at the source. The preactivation of cathode materials realized by water treatment shows a distinctly positive effect on the electrochemical properties of Li-rich cathode materials. In order to understand the specific mechanism of preactivation on promotion of material performance, we explore the electrochemical activation process with gradient charge and discharge H

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Figure 9. (a−d) Charge and (e−h) discharge Nyquist plots of WT-0 and WT-2 at different open-circuit voltagse. (i, j) Nyquist plots of WT-0 and WT-2 after (i) 3 cycles and (j) 50 cycles. (k) Equivalent circuit.

preactivation of Li2MnO3 in the plateau region plays a crucial role in determining the discharge capacity of Li-rich materials. This may be because the gradient activation relieves the undesirable structure deterioration during the initial Li2MnO3 activation. As shown in Figure 8c, the water treated sample displays a shorter voltage platform than that of the pristine sample, which indicates that preactivated Li2MnO3 corresponds to the component in the plateau region. Thus, ignoring the activation of LiMO2 (M = Mn, Co, Ni), we can regard the preactivated process as a specific form of gradient activation (section α becomes activated first, and then section β becomes activated). The gradient activation is beneficial for the electrochemical performance (especially capacity and rate capability); therefore, to a certain extent, the improvement of electrochemical performance in a preactivated sample can be ascribed to the role of this specific gradient activation. This is also in accordance with the voltage position of the increased capacity. No matter the sample with water treated or with gradient activation, they both have their additional capacity in low potential (2.0−3.2 V) compared with a normal sample. Figure 8d presents the cycling stability of samples after activation in a different procedure at 1 C. The samples with gradient activation have higher initial discharge capacity (about 201 mAh g−1) than a pristine sample (175 mAh g−1). Therefore, the gradient activation not only increases the capacity at low rate performance but also improves the high

process. Panels a and b of Figure 8 display the cycle profiles of pristine WT-0 under different activation process at 0.1 C. In Figure 8a, the button cell is preactivated between 2.0 and 4.6 V for a cycle before testing at 2.0−4.8 V. In this procedure, the discharge capacity is 226.5 mAh g−1 for 2.0−4.6 V (first cycle) and appreciably increases to 264.2 mAh g−1 for 2−4.8 V (second cycle), which is much higher than the capacity of 241.4 mAh g−1 for the normal activation procedure (charge to 4.8 V directly and depicted by imaginary line in Figure 8a). Therefore, the electrochemical preactivation before 4.6 V is significant for the improvement of discharge capacity. This is an interesting and meaningful phenomenon for optimizing the activation process, and few reports have been found to focus on this aspect. Figure 8b demonstrates the initial three charge−discharge curves according to the activation potentials of 2.0−4.4, 2.0−4.6, and 2.0−4.8 V, respectively. After charge and discharge between 2.0 and 4.4 V in the first cycle, the button cell exhibits a discharge capacity of 223.7 mAh g−1 for the second cycle (2.0−4.6 V) and 266.3 mAh g−1 for the third cycle (2.0−4.8 V). Figure S4 shows the charge and discharge curves at different activation procedure, the electrochemical preactivation before 4.4 V has scarcely an effect on the cathode performance, and the voltage range between 4.4 and 4.6 V is pivotal for capacity increase. The potential between 4.4 and 4.6 V also is the long-plateau region during the initial charge process and corresponds to the activation of Li2MnO3. So, the I

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Figure 10. GITT curves of (a) WT-0 and (b) WT-2 in the first cycle. Li-ion diffusion coefficient of (c) charging and (d) discharging processs.

exhibits larger initial RSEI and Rct than those of WT-0 at the voltage of 4.4 V, which must be due to the after-effect of preactivation. However, with the elevation of charge potential, the gap of Rct between WT-0 and WT-2 is decreased. In the voltage of 4.5 V, the semicircles of Rct are almost the same. When first charging completes (4.8 V), WT-2 exhibits a smaller Rct than that of WT-0, which reflects that the SEI film forms more seriously in a pristine sample. This can be ascribed to more Li2MnO3 being activated and more severe side reactions being generated in WT-0. In the discharging process, WT-0 and WT-2 display approximate Rct at high voltages of 4.2 and 3.8 V. But at low potential, WT-2 presents significantly smaller impedance than WT-0, especially at 3.0 V, which indicates that water treated sample possesses higher electrochemical reaction dynamics at low potential of discharge process. Panels i and j of Figure 9 display the Nyquist plots of WT-0 and WT-2 after 3 and 50 cycles. From Table S1, WT-2 not only presents smaller RSEI and Rct in the initial cycle, but also demonstrates smaller impedance change after 50 cycles. This suggests that water treated sample possesses greater diffusion velocity through the SEI film and stronger reaction dynamic at the material surface in the initial, and also owns minor sensitivity of impedance to the cyclic process. Therefore, the optimization effect of preactivation on the cathode impedance is a permanent effect, not only in the initial state, and preactivation has a positive effect to restrain impedance growth. Figure 10 displays the GITT test which is a utility method to explore Li-ion diffusivity of the cathode materials. The calculation process is depicted in Supporting Information. Panels a and b of Figure 10 are the test data of WT-0 and WT2, and panels c and d of Figure 10 are the calculated results. In first charging process, WT-2 shows a relatively low diffusivity value of 1.86 × 10−14 cm2 s−1, at initial state, and increases to 4.73 × 10−14 cm2 s−1, at 4.2 V. However, Li-ion diffusivity of

rate capacity, which is in accordance with the performance of a water treated sample. The capacity retention for gradient activation is about 82%, which is similar to the normal activation sample, indicating the gradient activation plays a minor role in the cycling stability. This implies that preactivation achieved by water treatment is more preferable than electrochemical gradient activation. This difference may be induced by the alteration of Li+ ion extraction order after preactivation. Normally, the order of Li+ extraction during electrochemical charge is from LiMO2 first, followed by the activation of Li2MnO3. In this process, Li2MnO3 always acts as a reservoir of surplus lithium and the lithium ion diffuses to the LiMO2 component for structural stability.5 Thus, Li2MnO3 becomes a lean lithium state at high potential, which is undesirable for activation and easily leads to serious structural transformation. However, water treatment alters the settled activation order of electrochemical charge and makes a portion of Li2MnO3 preferentially activated before LiMO2. Therefore, in the water treatment process, preactivation of Li2MnO3 at the lithium-rich state can decrease undesirable structural transformation and benefits the cycling stability. In summary, water treatment not only makes the Li-rich material be activated stepwise but also alters the order of Li+-ion extraction, which increases the discharge capacity and cyclic stability simultaneously. In order to further explore the influence of water treatment on the initial charge and discharge processes, EIS and GITT tests are employed. Panels a−h of Figure 9 show the Nyquist plots of WT-0 and WT-2 at different open-circuit voltages during the first cycle. The impedance spectra display the same features, including a high-frequency semicircle corresponding to Li-ion migration through the solid electrolyte interface (SEI) film (RSEI), a mid-frequency semicircle which is assigned to the surface charge-transfer process (Rct), and a sloped straight line in the low-frequency region, which is related to the bulk diffusion process.47,48 In the charging process, WT-2 J

DOI: 10.1021/acssuschemeng.9b01719 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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WT-0 is relatively stable between 3.8 and 4.2 V. This difference demonstrates that although water treatment process mainly works on the Li2MnO3 and TM ions are insensitive to the treatment, preactivation still has effect on the delithiation process of LiMO2 during electrochemical charge process. This may be due to the diffusion of lithium from LiMO2 component to Li2MnO3 during water treatment. Near the voltage 4.5 V, the Li-ion diffusion coefficient decreases notably in both samples, due to severe kinetic barrier for Li-ion diffusion in the initiate of Li2MnO3 activation and subsequent structure rearrangement. The Li-ion diffusivity of WT-2 reaches a minimum value at higher voltage than pristine sample, which is in accordance with the initial charge curve in Figure 5a. This may indicate that preactivated Li2MnO3 belongs to the front part of the platform area in the first charge curve, and it is the most susceptible component that can be activated in Li-rich material. Therefore, the treated sample exhibits higher activation inertness. Regarding the discharge, the curves display similar shape and diffusivity value at high voltage. But at the potential below 3.4 V, WT-2 displays much higher diffusivity value of 5.65 × 10−15 cm2 s−1 compared with that of 1.65 × 10−15 cm2 s−1 for WT-0. This is also in accordance with the initial discharge curve in Figure 5a and indicates that the higher initial discharge capacity presented by WT-2 must be related with the greater Li-ion migration power at low potential.

Zhen-Bo Wang: 0000-0001-9388-1481 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the National Natural Science Foundation of China (Grant Nos. 21273058, 21673064, and 51802059), the China Postdoctoral Science Foundation (Grant Nos. 2017M621285 and 2018T110292), and Harbin Technological Achievements Transformation Projects (Grant No. 2016DB4AG023) for their financial support and Fundamental Research Funds for the Central Universities (Grant Nos. HIT. NSRIF. 2019040 and 201904).

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(1) Xu, X.; Lee, S.; Jeong, S.; Kim, Y.; Cho, J. Recent Progress on Nanostructured 4V Cathode Materials for Li-Ion Batteries for Mobile Electronics. Mater. Today 2013, 16 (12), 487−495. (2) Wang, R.; He, X.; He, L.; Wang, F.; Xiao, R.; Gu, L.; Li, H.; Chen, L. Atomic Structure of Li2MnO3 after Partial Delithiation and Re-Lithiation. Adv. Energy Mater. 2013, 3 (10), 1358−1367. (3) Boulineau, A.; Simonin, L.; Colin, J. F.; Bourbon, C.; Patoux, S. First Evidence of Manganese-Nickel Segregation and Densification Upon Cycling in Li-Rich Layered Oxides for Lithium Batteries. Nano Lett. 2013, 13 (8), 3857−3863. (4) Liu, J.; Hou, M.; Yi, J.; Guo, S.; Wang, C.; Xia, Y. Improving the Electrochemical Performance of Layered Lithium-Rich TransitionMetal Oxides by Controlling the Structural Defects. Energy Environ. Sci. 2014, 7 (2), 705−714. (5) Thackeray, M. M.; Kang, S.-H.; Johnson, C. S.; Vaughey, J. T.; Benedek, R.; Hackney, S. A. Li2MnO3-Stabilized LiMO2 (M = Mn, Ni, Co) Electrodes for Lithium-Ion Batteries. J. Mater. Chem. 2007, 17 (30), 3112−3125. (6) Arunkumar, T. A.; Wu, Y.; Manthiram, A. Factors Influencing the Irreversible Oxygen Loss and Reversible Capacity in Layered Li[Li1/3Mn2/3]O2-Li[M]O2(M = Mn0.5‑yNi0.5‑yCo2y and Ni1‑yCoy) Solid Solutions. Chem. Mater. 2007, 19, 3067−3073. (7) Tran, N.; Croguennec, L.; Menetrier, M.; Weill, F.; Biensan, P.; Jordy, C.; Delmas, C. Mechanisms Associated with the “Plateau” Observed at High Voltage for the Overlithiated Li1.12(Ni0.425Mn0.425Co0.15)0.88O2 System. Chem. Mater. 2008, 20 (15), 4815−4825. (8) Shunmugasundaram, R.; Senthil Arumugam, R.; Dahn, J. R. High Capacity Li-Rich Positive Electrode Materials with Reduced First-Cycle Irreversible Capacity Loss. Chem. Mater. 2015, 27 (3), 757−767. (9) Yabuuchi, N.; Yoshii, K.; Myung, S. T.; Nakai, I.; Komaba, S. Detailed Studies of a High-Capacity Electrode Material for Rechargeable Batteries, Li2MnO3-LiCo1/3Ni1/3Mn1/3O2. J. Am. Chem. Soc. 2011, 133 (12), 4404−4419. (10) Qian, D.; Xu, B.; Chi, M.; Meng, Y. S. Uncovering the Roles of Oxygen Vacancies in Vation Migration in Lithium Excess Layered Oxides. Phys. Chem. Chem. Phys. 2014, 16 (28), 14665−14668. (11) Armstrong, A. R.; Holzapfel, M.; Novak, P.; Johnson, C. S.; Kang, S. H.; Thackeray, M. M.; Bruce, P. G. Demonstrating Oxygen Loss and Associated Structural Reorganization in the Lithium Battery Cathode Li[Ni0.2Li0.2Mn0.6]O2. J. Am. Chem. Soc. 2006, 128, 8694− 8698. (12) Fell, C. R.; Qian, D.; Carroll, K. J.; Chi, M.; Jones, J. L.; Meng, Y. S. Correlation between Oxygen Vacancy, Microstrain, and Cation Distribution in Lithium-Excess Layered Oxides During the First Electrochemical Cycle. Chem. Mater. 2013, 25 (9), 1621−1629. (13) Dogan, F.; Long, B. R.; Croy, J. R.; Gallagher, K. G.; Iddir, H.; Russell, J. T.; Balasubramanian, M.; Key, B. Re-Entrant Lithium Local Environments and Defect Driven Electrochemistry of Li- and Mn-

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CONCLUSIONS In this work, preactivation has been realized by using a simple treatment strategy with deionized water as reaction solvent. From the results of ICP and XPS measurements, we confirm that lithium and oxygen can be extracted from cathode materials during water treatment and transform into lithium hydroxide. The preactivated sample not only presents improved discharge capacity and Coulombic efficiency but also displays striking cycling performance. The dynamic tests demonstrate that Li-ion diffusion coefficient and electrochemical reaction dynamics are promoted after preactivation. The study of gradient activation gives us new insight into the relationship between good electrochemical properties and preactivation of Li-rich oxide.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b01719. Materials characterization, electrochemical measurements, XPS spectra of pristine and water treated samples, atomic configuration of 003 and 104 planes, rate capabilities of WT-0 and WT-2, charge and discharge curves between 2 and 4.8 V after different activation program, simulated impedance parameters of WT-0 and WT-2 after cycling, and calculation method of Li-ion diffusion coefficient (PDF)

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REFERENCES

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*Tel.: +86-451-86417853. Fax: +86-451-86418616. E-mail: [email protected] (F.-D.Y.). *Tel.: +86-451-86417853. Fax: +86-451-86418616. E-mail: [email protected] (Z.-B.W.). K

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