Integrated Surface Functionalization of Li-rich Cathode Materials for Li

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Surfaces, Interfaces, and Applications

Integrated Surface Functionalization of Lirich Cathode Materials for Li-ion Batteries Dandan Wang, Tinghua Xu, Yaping Li, Du Pan, Xia Lu, Yong-Sheng Hu, Sheng Dai, and Ying Bai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16319 • Publication Date (Web): 07 Nov 2018 Downloaded from http://pubs.acs.org on November 8, 2018

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ACS Applied Materials & Interfaces

Integrated Surface Functionalization of Li-rich Cathode Materials for Li-ion Batteries Dandan Wang, † Tinghua Xu, † Yaping Li, † Du Pan, † Xia Lu, ‖ Yong-Sheng Hu, § Sheng Dai, ‡ Ying Bai †,* † Key

Laboratory of Photovoltaic Materials of Henan Province and School of Physics & Electronics, Henan University, Kaifeng 475004, P.R China

‡ Chemical

Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

§ Institute

of Physics, Chinese Academy of Sciences, Beijing 100190, P.R. China

‖ School

of materials, Sun Yat-sen University, Guangzhou 510275, P.R. China

*Corresponding author. Tel.: +86-0371-23881602; E-mail address: [email protected] Keywords: Li-rich layered material; F-doped Li2SnO3 coating layer; Electrochemical performance; Mechanism investigation, Integrated design.

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Abstract: As candidates for high-energy density cathodes, lithium-rich (Li-rich) layered materials have attracted wide interests for the next-generation Li-ion batteries. In this work, surface functionalization of a typical Li-rich material Li1.2Mn0.56Ni0.17Co0.07O2 is optimized by fluorine (F) doped Li2SnO3 coating layer, and electrochemical performances are also enhanced accordingly. The results demonstrate that F doped Li2SnO3 modified material exhibits the highest capacity retention (73% after 200 cycles), with approximately 1.2, 1.4, and 1.5 times of discharge capacity for Li2SnO3 surface-modified, F-doped and pristine electrodes, respectively. To reveal the fundamental enhancement mechanism, intensive surface Li+ diffusion kinetics, postmortem structural characteristics, and aging tests are performed for four sample systems. The results show that the coating layer plays an important role in addressing interface compatibility, instead of stabilizing the bulk structure and simultaneously suppressing side reactions, as the most decisive factor in performance controlling for the active electrodes. These findings not only pave the way to commercial application of the Li-rich material, but also shed new light on surface modification in batteries and other energy storage fields.

1. Introduction Advanced lithium ion batteries (LIBs) with high energy densities and considerable reliability are considered as the promising candidates for powering next-generation hybrid electric vehicles (HEVs) and electric vehicles (EVs) to address urgent demands for clean, renewable energy.1 In the way to fabricate better LIBs, various strategies have

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also been proposed to enhance the electrochemical stability of electrodes, including the anode and cathode materials.2-4 And noticeably, in the last decade, special interest has been paid in high capacity cathodes concerning their high energy density and safety features. Among the available high-capacity cathode materials, lithium-rich (Li-rich) layered materials (1-x)LiMO2·xLi2MnO3 (M = Mn, Co, Ni) draw particular attention because of high reversible capacity above 200 mAh g-1. From an electrochemical point of view, two voltage platforms in the initial charging process of Li-rich cathodes, correspond to the oxidation of transition metal (TM) ions and the irreversible oxygen loss from Li2MnO3, respectively. During Li+ insertion/extraction, Li-rich layered materials suffer inevitably poor electrochemical performance and voltage decay because of structural rearrangement, including the migration of Li+ from the original octahedral sites into the metastable tetrahedral sites, the interaction of Li ions with Li vacancies, and Li-TM cation mixing ordering.4,5 Therefore, two main strategies typically are used, i.e. surface coating and elemental doping (cation and anion), to address the aforementioned issues and improve the electrochemical performances of Li-rich materials. For surface coatings, modification materials have expanded from traditional metal oxides,6 metal fluorides7,8 and metal phosphates,4,9

to

novel

fast

Li-ion

transporting

compounds

(such

as

solid

electrolytes),10,11 wherein the enhanced electrochemical performances are generally attributed to the shielding effect and the alleviation of surface side reactions. However, these advantages of the traditional coating strategy play a gradually diminishing role in 3



stabilizing electrochemical performance along with the proceeding of electrochemical cycles. On the other hand, doping with cations and/or anions such as Mn4+,12 Mg2+,13 Al3+,14 Ti4+,15 Na+16 and F-17 are found beneficial for the structural stability and the facilitated Li+ diffusion kinetics, which are mainly responsible for boosting the electrochemical properties of Li-rich compounds. Generally, surface coatings are deposited via traditional physical techniques such as chemical vapor deposition (CVD),18 atomic layer deposition (ALD)19 and pulsed laser deposition (PLD),20 as well as chemical methods including co-precipitation21 and sol-gel.9 A more compact surface layer can be established through physical strategies, but these have little chance of being commercialized due to the highly dependence on expensive apparatus. And although offering important advantages of easy to operate, low in cost and suitable for commercialization, the chemical coating methods are still challenged by poor interface compatibility between active material substrate and surface coating layer. Recent studies have revealed a synergetic contribution of the surface coating layer and the doped elements in improving the electrochemical performance of Li-rich materials. For the analogous ionic radii among the ions in coating and those in the bulk matrix and/or favorable conditions provided via coating process, which is attributed to stabilization of both surfaces and bulk structures.22,23 very recently, the multifunctionality of surface doping and coating was investigated in Li1.2Mn0.54Ni0.13Co0.13O2 cathode, and the optimized electrochemical performance was ascribed to the protective effect of a 4


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GDC (Gd0.1Ce0.9O2-δ) coating layer and the enhancement of surface oxygen vacancies by local gadolinium (Gd) doping.24 As a matter of fact, it has been reported that electrochemistry is highly dependent on the surface/interface properties of electrode and electrolyte,25,26 while is still lack of systematic insight into the dependent influence on the resulted interface Li+ diffusion behavior from surface/interface contact. It is reported that the electrode/electrolyte interfacial characteristic is clearly associated with the improved electrochemical properties.27 Therefore, it is critical to stabilize the interface of electrode materials during cycles. In our previous work, the Li-rich material Li1.2Mn0.56Ni0.17Co0.07O2 (LMNCO) was modified by the Li2SnO3 (LSO) on the surface, in which LSO with the space group of C2/c was selected not only for its analogous structure to monoclinic Li2MnO3 (space group of C2/m), but also for its desirable fast 3-dimensional (3D) Li+ diffusion channels.28,29 In this context, we proposes an integrated design including surface coating & doping to optimize interfacial compatibility and enhance the surface Li+ diffusion kinetics of Li-rich materials. The obtained LSO modification layer was further treated by fluorine (F) doping since F has been widely applied to convert the insulative metal oxide SnO2 to the conductive F-doped SnO2 (FTO),30,31 which will apparently enhance the surface electric conductivity and thus is beneficial for local mass transfer process. In the present work, F-doped LSO modification layer was deposited on the surface of LMNCO to decorate the surface doping & coating modification system. In addition, stand-alone F surface doping materials were synthesized through the similar LSO coating process for intensive 5



comparison. The physical characteristics, electrochemical performances and kinetics traits of the as-prepared materials were investigated systematically. Moreover, detailed postmortem tests and aging experiments also were performed to elucidate the essential mechanisms of surface functionalities on a Li-rich cathode.

2. Experimental 2.1. Material Synthesis. The Li1.2Mn0.56Ni0.17Co0.07O2 (LMNCO) was synthesized using a typical sol-gel method. Metal acetates CH3COOLi∙2H2O, Mn(CH3COO)2∙4H2O, Co(CH3COO)2∙4H2O, Ni(CH3COO)2∙4H2O and citric acid (C6H8O7) were selected (metal cations: C6H8O7 = 2:3 in the molar ratio) as precursor materials. Ethyl alcohol (C2H6O) was used as the solvent. The detailed preparation has already been discussed in previous literatures.9,29,32,33 The Li2SnO3 (LSO) material was also prepared by a sol-gel method. In a typical synthesis, the precursors SnCl4∙5H2O, CH3COOLi∙2H2O, C6H8O7 were dissolved in ethylene glycol (C2H6O2) solution in sequence with vigorous stirring. Then, the precursor mixture was dried at 160 °C for 24 h, and the obtained product was collected and calcined at 400 °C for 5 h to remove organic components. Finally, the obtained powder was fully ground and calcined at 700 °C for 5 h. To attain LSO coated LMNCO (LSO@LMNCO), the as-prepared LMNCO was firstly dispersed evenly in C2H6O2 and then the aforementioned LSO precursor mixture was slowly added. The target LSO@LMNCO sample was obtained after a similar evaporation and annealing process. 6


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To prepare the F/LSO-doped/coated LMNCO (F-LSO@LMNCO) sample, the SnCl4∙5H2O, CH3COOLi∙2H2O, C6H8O7, and NH4F precursors were separately dispersed/dissolved in C2H6O2 in a stoichiometric ratio. Then all the solutions were thoroughly mixed, followed by the evaporation and annealing at 400 °C for 5 h to prepare the targeted F-LSO@LMNCO sample. A separate F-doped LMNCO material (F-LMNCO) was synthesized for comparison as well. To prepare the F-doped sample, as-prepared LMNCO was dispersed in distilled water, after that an NH4F solution (0.5 M) was added to the solution drop by drop, then the mixture was stirred, evaporated, and finally calcined at 400 °C for 5 h to attain the targeted product. During the synthesis, samples with different coating/doping contents were prepared to optimize the degree of modification. The amount of LSO surface coating in the LSO@LMNCO samples was adjusted to 0.5 and 1 wt. %, respectively. The F-LMNCO samples were labeled as F0.1, F0.2, and F0.3, with the labels representing the nominally molar contents of F, probably occupying oxygen (O) sites in Li-rich materials.34 The integrated F-LSO@LMNCO samples were prepared and marked as F0.1-LSO, F0.2-LSO, and F0.3-LSO, in which the 0.5 wt. % LSO coating content was utilized along with the F doping molar ratios. 2.2. Physical characterization. The crystal structures of the as-prepared materials were characterized by X-ray diffraction (XRD, Bruker D8, Germany), collecting at a

7



scanning rate of 0.03° s-1 between 10° and 80° (2θ). Field emission scanning electron microscopy (FESEM, JEOL 7001F) was used for morphology observation, equipped with electron dispersive spectroscopy (EDS) to detect the elemental composition and distribution. High-resolution transmission electron microscopy (HRTEM) images were collected on an FEI Tecnai F20 to analyze the microstructures. The Raman spectra were collected on a laser Raman spectrometer (Renishaw RM-1000) with a 633 nm He-Ne laser. Inductively coupled plasma spectroscopy (ICP) was conducted on a Perkin Elmer Optima 2100DV. Surface chemical environment of the materials was investigated by X-ray photoelectron spectroscopy (XPS) on a Thermo Electron Corporation spectrometer with Al Κα radiation (1486.60 eV). Differential scanning calorimetry (DSC, TAQ600) was carried out by sealing the charged cathode (4.8 V at galvanostatic 0.1 C) in an aluminum crucible in dry argon and heating it from room temperature to 400 °C at a rate of 5 °C min-1. For the postmortem physical tests, the electrodes were carefully extracted after the cells experienced 20 galvanostatic cycles and equilibrated at 4.8 V for 48 h. Before the postmortem XRD and SEM signals were recorded, the collected electrodes were rinsed with dimethyl carbonate (DMC) for three times and thoroughly dried under vacuum in the transition chamber of the glovebox. 2.3. Electrochemical characterizations. Each electrode was prepared by mixing the active material, acetylene black, and binder (polyvinglidene fluoride (PVDF) dissolved in N-methyl-2-pyrrolidone (NMP)) with a mass ratio of 8:1:1. The mixed slurry was spread

8


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onto an aluminum (Al) foil and dried in a vacuum oven at 120 °C for 12 h. The coin cells (CR2032) were assembled in an argon-filled glovebox, with the Li foil as the counter electrode and Calgary 2400 porous polypropylene film as separator. The electrolyte consisted of 1 M LiPF6 dissolved into ethylene carbonate (EC) and diethyl carbonate (DEC) in a 1:1 volume ratio. The cells were cycled on a NEWARE test system (Shenzhen, CT-2001A, China) in a potential range of 2.0 ~ 4.8 V at room temperature. All the galvanostatic cycling was performed under a current density of 0.1 C. The electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were carried out on a CHI660E (Shanghai Chenhua) electrochemical workstation with a three-electrode system. The EIS was measured at the charged state of 4.8 V with amplitude of 5 mV over a frequency range of 100 kHz ~ 5 mHz. The CV tests were recorded at variable scan rates of 0.1, 0.2, 0.3, and 0.4 mV s-1 between 2.0 ~ 5.0 V. 2.4. Aging analysis. In the chemical aging experiments, the electrode materials (0.1 g pristine, 5 wt. % LSO@LMNCO, F0.2-LMNCO and F0.2-LSO@LMNCO) were thoroughly dried to eliminate possible surface-adsorbed water. Then these materials were respectively immersed in 50 ml electrolytes (the same used in the electrochemical measurements) in polytetrafluoroethylene (PTFE) containers. The mixtures were continuously stirred at 50 °C. After each one week, mixtures were carefully withdrawn in equal volumes, followed by rigorous solid-liquid separation. The obtained liquids were tightly sealed for titration tests, while the precipitated powders were repeatedly washed

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with DMC and then vacuum dried for SEM, XRD, and ICP characterization. As described in our previous work,32 bromthymol blue was selected as an indicator in a typical titration test. When the electrolyte colors changed from yellow to blue, the NaOH volume and content were recorded (with known concentrations). The acidity of the measured electrolyte (calculated as HF) was determined by the equation: Acidity = V(NaOH)×C(NaOH)×M(HF)×106 /Weight(electrolyte) (ppm).

(1)

3. Results and Discussion 3.1 Physical characterization. Figure 1 presents typical FESEM and HRTEM images of as-prepared LMNCO samples. As shown in Figure 1a-d, all samples exhibit analogous morphologies with homogeneous distribution and a uniform particle size of ~ 400 nm. After being surface modified, the sample surfaces generally loss their smoothness, reflecting the changes in the surface chemical environment after modification. The EDS mapping results for these materials are shown in Figures S2-S4, wherein each element is homogeneously distributed on the material surface, indicating the uniform influences on particle surfaces. The microstructures of these samples are compared in Figure 1e-h. As shown in Figure 1e, the lattice fringes could be clearly observed in the pristine material with d-spacing of 0.47 nm, unambiguously corresponding to the (003) plane of the LMNCO hexagonal phase. With respect to the pristine material (LMNCO), the surface-modified materials (LSO@LMNCO, F-LMNCO and F-LSO@LMNCO) manifest nanoscale interface zones with thicknesses of ~ 10 nm, 10


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providing strong evidence for the existence of surface modification layer. In details, a loosely distributed coating layer is seen outside the LMNCO particle in LSO@LMNCO, and it becomes more compact and homogeneous after F doping in F-LSO@LMNCO. Figure 1g shows the HRTEM image and the corresponding fast Fourier transformation (FFT) patterns along the radical direction from regions I to III in F-LMNCO, and a gradual increase in crystallization from the surface to the bulk area could be observed in the F doped LMNCO, which is possibly explained by F doping process. Specifically, an almost amorphous phase is found at the outermost surface with high F doping content (region I), whereas the crystallization is enhanced gradually toward the bulk, thus a relatively distorted lattice fringes and sporadic diffraction spots are observed in transition region II. In region III, no obvious lattice distortion could be observed, and the local structure progressively recovers to the LMNCO lattice, which is supported by the clear hexagonal feature in FFT pattern. These findings provide a strong evidence of gradient doping from exterior surface to bulk interior. The XRD patterns for the pristine and modified LMNCO samples are shown in Figure 2a-c. The diffraction peaks of all samples could be indexed to the hexagonal α -NaFeO2 layered structure with the space group of R 3 m (M = Mn, Ni, Co). The reflections at low angles (20° ~ 25°) are attributed to the Li/Mn ordering in Li2MnO3 with C2/m space group.35,36 Among pristine and modified electrode materials, it could be clearly seen that the diffraction peaks undergo no evident shift, indicating that the modifications have little influence on the host structure of LMNCO. The lattice 11



parameters, I(003)/I(104) and c/a, are calculated as shown in Table S1, the I(003)/I(104) and c/a values reflect the ordered layered structure and cation mixing,32,37 respectively. From this viewpoint, the electrode materials retain ordered layered structures and low cation mixing after modification. Moreover, the unchanged a and c values among the samples again confirm the bulk structure is uninfluenced by the different modification amounts. Figure S1 shows the Raman spectra of the as-prepared samples. All the profiles demonstrate two prominent vibrations at ~ 477 and 599 cm-1, which could be unambiguously assigned to the Eg (bending of Co3+-O) and A1g (stretching of Ni2+-O, Mn3+-O and Mn4+-O) modes for the typical R 3 m space group.38,39 The weak peak at ~ 421 cm-1 is associated with the monoclinic structure of C2/m, which is known to be the vibration of Li2MnO3.37,40 The observed coexistence of C2/m and R 3 m in the Li-rich material is also revealed in the XRD patterns as shown in Figure 2a-c. After numerical fitting, no difference could be found in the samples after modifications either in peak wavenumber or full width at half maximum (FWHM) values, implying that the treated surfaces have little influence on the host structure of Li-rich material. To further study the surface chemistry, XPS characterization of the pristine, LSO@LMNCO, F-LMNCO, and F-LSO@LMNCO samples are performed as shown in Figure 2d-i. The Li 1s peaks locate at 54.90 eV in Figure 2d, the decay in intensity after modifications could be ascribed to the influence from surface modification layers. As expected, a distinct tin (Sn) signal could be seen in the LSO and F-LSO surface modified materials, which is absent in the pristine and F-LMNCO samples (Figure 2e). The 12


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binding energy of Sn 3d5/2 is locating at 486.39 eV, consistent with Sn4+ in Li2SnO3,41 confirming the existence of metastable Li2SnO3 on the surface of LSO@LMNCO and F-LSO@LMNCO samples. The O 1s spectra in Figure 2f correspond to the O2- ion in the as-prepared samples, in which the peak located at 529.50 eV is generally attributed to the lattice O and another peak at 531.50 eV is identified as the surface O.42 Interestingly, a significant impact on LMNCO could be observed in the F-doped sample (F-LMNCO), not only in the increased binding energy (owing to partial substitution of F for O on the surface), but also in the broadened O line as reflected by the FWHM values (due to the concentration gradient of F doping in surface lattice). On the other hand, as shown in Figure S5, the fitted results of the O signals (indicated as the ratios of lattice O and surface O, IL/S) generally experience degradation after different kinds of treatment, which could be attributed to the influence of surface modification. As presented in Figure S6, the F signal is easily detected at 685.70 eV, which could be assigned to the F-M species (M = metal ions)43,44 in the samples with higher doping contents of 0.5 and 1 at. % (F0.5-LMNCO and F1-LMNCO), explaining the absence of F peak in the compared samples (not shown in Figure 2). Finally, the XPS spectra of Mn 2p, Ni 2p and Co 2p as shown in Figure 2g-i, demonstrate the existence of Mn4+, Ni2+ and Co3+ ions in the LMNCO samples, consistent with previous reports.45 Note that the detected metal signals drop slightly after surface modifications, which could be ascribed to the shielding effect of the coating layers as shown in HRTEM images in Figure 1e-h.

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3.2 Electrochemical properties. The cycling performances and rate capabilities of all electrode materials are presented in Figure 3. For the LSO@LMNCO samples, it could be clearly seen that 0.5 wt. % LSO@LMNCO exhibits the best cycling stability. The initial charge-discharge curves of the pristine and LSO modified LMNCO are shown in Figure S7a, in which all samples exhibit similar profiles, suggesting little influence on the bulk structure from the LSO surface modification. Besides, the initial Coulombic efficiency (ICE) of 0.5 wt. % LSO@LMNCO is enhanced to 78% compared with that of the pristine electrode (73%), which might be attributed to the suppressed side reactions in the presence of LSO. The cycling performances of electrodes with various doping levels (F-LMNCO and F-LSO@LMNCO) are also compared. It is clear that the F doped systems manifest higher capacity retentions of 57% for the optimized F-LMNCO electrode and 73% for the F-LSO@LMNCO sample after 200 cycles. Likewise, as shown in Figure S7b, c, there is little discrepancy in the initial charge-discharge curves of these two systems compared with that of the pristine sample. Remarkably, higher initial discharge capacities and ICEs could be obtained from the optimized F-containing materials (80% and 87% for F-LMNCO and F-LSO@LMNCO, respectively), which could be ascribed to the influence of F surface doping.34 Notably, as shown in Figure 3, the cycling performance and rate capability of the optimized F0.2-LMNCO are mostly limited, and deliver slightly higher specific capacities than the pristine material. In comparison, the integrated material with the F0.2-LSO modification layer exhibits the greatest discharge capacities. The specific electrochemical properties of different 14


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LMNCO electrodes are summarized in Table 1, in which it could be seen that surface modification plays an important role in enhancing the ICE and stabilizing the cyclability of the electrochemical cycles. On the basis of the obtained electrochemical results, the optimized F0.2-LSO@LMNCO electrode delivers the best electrochemical properties of all the available electrodes, which highlights the advantages of the reversibility and stability of capacity due to the incorporated surface doping/coating method. Surface modifications are generally found to suppress voltage decay in Li-rich materials, which actually results from the phase transition from layered to spinel structure upon cycling.46,47 The discharge midpoint voltages along with the repeated cycles for all electrodes are shown in Figure S8. Severe voltage decay could be observed for the pristine material, with a huge drop of 1.300 V after 200 cycles, whereas the voltages of the

optimized

electrodes

(0.5

wt.%

LSO@LMNCO,

F0.2-LMNCO,

and

F0.2-LSO@LMNCO) decrease by ~ 0.666, 1.020 and 0.704 V, respectively. This result suggests that the phase transition from layered to spinel structure is likely alleviated by surface modification. The rate capabilities of all electrode materials are shown in Figure 3b. The discharge capacities of the pristine material rapidly decay to 45 mAh g-1 at 2 C, only remaining 22% of the capacity under 0.1 C. However, the modified materials demonstrate improved electrochemical performance at a high current density, delivering higher discharge capacities of 82, 51 and 96 mAh g-1 for LSO@LMNCO, F-LMNCO and F-LSO@LMNCO

electrodes,

respectively.

Moreover,

15


the

modified

materials


demonstrate higher reversibility when the current density recovers to 0.1 C, indicating that structural stability could be improved under high current cycling after modification. The improvement of rate performance for LSO@LMNCO could be partly ascribed to the intrinsic 3D Li+ diffusion channel of LSO as established by previous literature,29 whereas the enhancement in rate capability for F-LMNCO could be attributed to the gradient F doping, resulting in local facilitation of Li+ migration. The F-LSO@LMNCO possesses a more compact artificial layer after F doping as shown in Figure 1h, which might lead to higher compatibility between the surface modification layer and the bulk LMNCO. This would further explain the facilitated kinetics behavior due to the integrated design of surface pinning/coating. CV analysis is widely used to investigate Li ion diffusion behavior in LIB electrode materials. The initial two CV curves of all the optimized electrodes in this work are shown in Figure S9. Similar oxidization/reduction processes could be observed for the four electrodes, revealing that surface modification did not substantially influence the bulk structure of LMNCO electrode. In the initial charge profiles, two oxidation reactions occur at ~ 4.0 and 4.5 V, corresponding to the oxidation of TMs (Ni2+ and Co3+) and the oxygen loss in Li2MnO3, respectively.48 During the initial discharge process, two cathodic peaks are identified at ~ 3.60 and ~ 3.25 V, in accordance with the reduction of Ni4+, Co4+, Mn4+ ions in LMNCO sample.49 The values of FWHM and potential differences for the corresponding oxidation/reduction reactions (O1 and O2) in the initial CV profiles are compared in Figure S9 and Table S2. It is clear that the O1 redox peak of 16


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LSO@LMNCO shows a larger potential difference than the pristine sample, which may be attributed to the voltage hysteresis due to the phase transformation in the discharge process.50 However, the other modified materials exhibit lower polarizations, indicating that the structure transition is alleviated by the F doping to some extent

3,51

As shown in

Table S2, the FWHM values of oxidation peaks O1 and O2 demonstrate a remarkable decrease after surface modification, indicating that surface modifications are beneficial to the kinetics behavior, which partly explains the improved electrochemical reversibility for the modified samples. The Li ion apparent diffusion coefficients of the optimized electrodes were calculated by CV and EIS. The CV curves at different scanning rates between 2.0 ~ 5.0 V are shown in Figure 4a-d. As the scanning rate increases, the anodic peaks shift to higher potentials, while the corresponding cathodic curves appear at lower voltages, contributing to a larger polarization in this process. Each redox peak current (Ip) manifests a linear relationship with the square root of the scanning rate (ν1/2) (Figure 4e), as described in the following equation: Ip = (2.69 × 105) n3/2 A DLi+1/2 CLi+ ν1/2,

(2)

where Ip is the peak current (A), n corresponds to the charge-transfer number, A is the contact area, CLi+ is calculated as the concentration of Li+ in the cathode, and ν is the potential scanning rate (V s-1).52,53 Based on Equation (2) and the fitted slopes in Figure 4e, the apparent diffusion coefficients (DLi+) of the different electrodes are calculated and listed in Table 2. Apparently, the boosted Li+ diffusion coefficients are obtained in the 17



optimized samples of LSO@LMNCO and F-LSO@LMNCO, accounting for the enhanced rate capabilities shown in Figure 3b. However, for the F-LMNCO sample, the diffusion coefficient is almost half that of the pristine electrode, which should be ascribed to the changes in the local surface by the gradient F doping. As investigated by Zheng et al.,54 F bulk doping is an effective way of improving the electrochemical performance of Li-rich materials, which is ascribed to the structure stability/integrity optimization due to the facilitation of Li+ diffusion on the interface. In this case, F doping unexpectedly induces a decline in the ensemble kinetics parameter, again verifying that the F doping is only limited the local region near the particle surface. This result is not only coinciding with the microstructure observation in Figure 1g, but also corroborates the limited improvement of cycling performance in Figure 3. EIS measurements of all optimized electrode materials were performed after equilibrium at a charged state of 4.8 V from 1st to 20th cycles. As shown in Figure S10, two semicircles and one sloped line appear in the Nyquist plots. These are further fitted according to the equivalent circuit (Figure S10), in which the circuit elements of Rs, Rsf, Rct and W1 represent the internal resistance of the electrolyte, the resistance of the solid electrolyte interface (SEI) film, the charge transfer resistance, and the Warburg impedance, respectively.32,33 The fitted resistances at different cycles are listed in Table S3, wherein the increased values of Rsf and Rct upon cycling are believed to be detrimental to the electrode performance.54 Figure 4f further shows the normalized resistances (based on electrochemical data for the 1st cycle), in which the increasing rates 18


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(slopes) of Rsf and Rct reflect the augmented surface side reactions and more toughened Li+ mass transfer process between the interface layer and bulk material, respectively. The LSO@LMNCO electrode presents the mildest accumulated SEI resistances, indicating that surface side reactions could be effectively suppressed via LSO surface modification. However, the F doped LMNCO electrode exhibits a more rapid increase of Rsf in the same electrochemical process, which could be resulted from the severe reaction between the LiPF6-based electrolyte and active material. As compared in Figure 4f, it could be clearly noticed that the augmented Rct values are significantly alleviated for the F-LMNCO electrode upon cycling, while it is fast accumulated for the LSO modified material, reflecting the fast degradation of interface compatibility for this sample. Then, the integrated electrode of F-LSO@LMNCO displays the suppressed growth in resistance, suggesting the synergetic effect of LSO coating and F doping in the incorporated sample, which would explain the enhanced electrochemical properties as shown in Figure 3. Guo et al.17 attributed the enhanced electrochemical properties of F doped Li-rich material to suppressed resistance owning to the F substitution, which benefits for Li+ diffusion but without distinguishing the specific stages in the interface layer (Rsf) and charge transfer process (Rct). In this work, as an effective physical barrier, the surface modified LSO optimizes the Li+ diffusion process in the SEI layer (Rsf) by inhibiting its continuous growth. Simultaneously, F doping significantly contributes to the ameliorated charge transfer resistance (Rct). Moreover, the Li+ apparent diffusion coefficient could be calculated as follows: 19



R 2T 2 D= 2 4 4 4 2 , 2S n F C δ 1

Zimg = S-W 2 ,

(3) (4)

where R is the gas constant, T is the absolute temperature, S is the surface area of the cathode, n is the number of electrons involved in the reaction, F is the Faraday constant, C is the concentration of Li ions, and δ is the Warburg factor. Based on Equations (3) and (4), the Li+ apparent diffusion coefficients of all electrodes are determined as shown in Table 2. The Li+ apparent diffusion coefficient for the integrated F-LSO@LMNCO is 1.396 × 10-11 cm2 S-1, higher than those of the other electrodes, which is in accordance with the CV results as shown in Figure 4a-e. 3.3 Postmortem analysis. To investigate the structural stability of the cycled materials, all optimized electrodes are evaluated before and after 20 cycles by XRD. As shown in Figure 5a, the diffraction peaks of all samples exhibit no distinction before cycling (as discussed in Figure 2a-c), but a clear shift of the (003) peak with line broadening could be observed after 20 cycles in the magnified drawing on the right, indicating the structural degradation of bulk LMNCO upon cycling. Especially, the F-LMNCO material presents a more serious damage than LSO-contained electrodes, implying the advantage of bulk stabilization of LMNCO through the LSO surface modification over pure element doping. The cell parameters, I(003)/I(104) and c/a for the XRD plots before and after 20 cycles are shown in Table S4, further indicate the improved structural stabilities for LSO@LMNCO and F-LSO@LMNCO after 20


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electrochemical cycling. Figure 5b presents the DSC curves of the corresponding electrodes. Clearly, the exothermal peaks of all the modified electrodes shift to higher temperatures with weakened intensities (heat releases) than those of the pristine material, indicating that the thermal safety of surface modification materials is enhanced significantly. For the modified materials, the exothermal peaks are seen at 311, 312 and 322 °C for LSO@LMNCO, F-LMNCO, and F-LSO@LMNCO, respectively, significantly higher than that of the pristine electrode (283 °C). Besides, the normalized heat releases of the LSO-contained electrodes are 54.46 and 53.35 J g-1, lower than that of the pristine material (61.45 J g-1). It could also be noticed that the heat release is the least in F-LMNCO (38.18 J g-1) among of all the electrodes, which could be ascribed to the partial F substitution for O in the LMNCO lattice and suppression of the O release at delithiated state of the electrochemical cycling.55 Figure S11 compares the surface morphologies of four typical electrodes after 20 repeated cycles. Apparently, the particles of LSO-contained materials remain more integrated in the electrochemical environment, revealing enhanced structural stability, which is coinciding with the discussion in Figure 5a. An obscure surface layer could be observed in cycled F-LMNCO electrode, which coarsens the clear surface of LMNCO to some extent. The layer corresponds to the generation of an SEI film, is coinciding with the sharply increased Rsf in EIS analysis.

21



3.4 Chemical aging tests. As reported in our previous work,32 the LMNCO electrode experiences similar changes under both electrochemical and chemical treatments. Aging experiments for the selected electrode materials were conducted to further investigate the enhancement mechanism for the modified samples. Figure 6a compares the color changes of soaked electrolytes after aging for each week. In the four systems, it could be clearly observed that the color of separated electrolytes exhibits evident differences with the extension of time. The colors of the pristine and F-LMNCO materials convert from colorless to tan and hazel after aging for 4 weeks. While comparatively, the LSO-contained materials exhibit little distinction in color changes after soakage. To determine the fundamentals of the color evolution, pure LSO material was also soaked under the same conditions (not shown). Throughout the same aging experiment, no color difference was observed, therefore the influence of LSO itself could be excluded. Next, NaOH titration is used to test the acidity of the residual electrolyte after aging for one month, and those values are recorded in Table S5. Clearly, the pristine LMNCO soaked electrolyte demonstrates the highest acidity of all the samples, and relative lower acidities are obtained from the LSO@LMNCO and F-LMNCO systems. The LSO surface coating & F doping integrated LMNCO soaked electrolyte exhibits the least acidity. It is interesting to note that the measured values of acidity are in accordance with the color difference: the higher the acidity, the deeper the color for the corresponding electrolyte. The increase in acidity is remarkable, especially for pristine and F doped LMNCO soaked electrolytes, which will inevitably corrode the 22


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material surface and accelerate the dissolution of metal ions from bulk LMNCO. Moreover, quantitative ICP testing of residual solid powders after soakage is performed, and the histograms of metal ion contents with aging time are shown in Figure 6b. The comparison shows that the dissolution of metal ions in the soaked pristine and F-LMNCO powders are more prominent than in the LSO-contained materials. This result clearly confirms the robust protective effect of LSO surface modification on suppressing the ion dissolution and stabilizing the bulk structure of LMNCO, in alignment with the XRD results for the electrochemical process (Figure 5a) and the enhanced electrochemical stabilities (Figure 3). Figure S12 compares XRD results for the structures of the pristine and modified electrodes before and after soaking for one week. A new diffraction at ~ 18.2° could be observed for the pristine, F doped and F-LSO electrodes, which could be assigned to the spinel phase Li-M-O (Fd 3 m space group) accompanied with the electrochemical structure degradation.56,57 The absence of this characteristic spinel peak in LSO@LMNCO provides strong evidence for the structure of the active bulk is mostly stabilized by the LSO coating, whereas the pristine and F-LMNCO samples display more obvious structural degradation, in agreement with the previous ICP analysis (Figure 6b). In contrast, the F surface doping further deteriorates the structural stability of LMNCO lattice, which could be attributed to the enhanced side reactions in this case, corresponding to the EIS results as shown in Figure 4f. This finding also explains the slightly decayed structure of the F-LSO@LMNCO sample in contrast with the 23



LSO@LMNCO. Figure S13 compares the morphology evolution of different electrodes after soakage. It could be seen that the particles in all the samples continuously lose their smooth surfaces with aging, confirming the accumulated SEI layers from spontaneous side reactions in the chemical environment. Under the same conditions, the least deposition among the four electrodes could be observed in the surface of the LSO@LMNCO, which coincides with significantly restrained surface side reactions and the resultant interface resistances. On the contrary, a remarkable deposition layer could be found on the surface of F-LMNCO, in agreement with the sharply increased Rsf values as shown in Figure 4f. The morphology degradation revealed on that sample is in associated with the ICP and XRD analyses in chemical aging process, proving again the enhanced structural stability is gained through LSO surface modification. The intrinsic functions of LSO surface coating and F doping are illustrated in Figure 7. The LSO surface coating layer (marked in purple) prevents the electrolyte from attacking the active materials, which consequently alleviates the surface side reactions (Rsf) and thus contributes to the superior electrochemical and structural stabilities. Besides, F surface doping is introduced in the presence of LSO surface modification, which plays a crucial role in facilitating the charge transfer (Rct) process. Functional surface modification is thus optimized through the synergic effect of LSO coating (adding a protective layer) and F surface pinning (using a compatibilizer). The integrated modification strategy proposed in this work provides great potential to stabilize the bulk 24


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structure and ameliorate interface contact of electrode materials with solid/liquid electrolytes. As a promising anode candidate for next generation LIBs, silicon (Si) has attracted wide attention because of its high Li storage capacity. Nevertheless, it is still challenged by a sharp deterioration in electrochemical performance resulting from high volume deformation and poor interface contact.58 Therefore, a rational electrode design such as Si-based doping and coating, has been intensively investigated by previous researchers.59,60 The integrated modification strategy proposed in this work provides great potential to stabilize the bulk structure of Si and ameliorate the interface contact issues at the same time. In addition, solid-state LIBs and sodium-ion (Na+) batteries (NIBs) also are attracting the growing interest as energy storage systems owing to their low cost and excellent thermal/electrochemical stability.61,62 Recently, some researches demonstrated that the decomposition products at the interfaces between the solid electrolyte and electrodes may cause high interfacial resistance and sluggish kinetics in all-solid-state LIBs and NIBs.63-66 To enhance the interface kinetics of Li+, Na+ and the structural stability of active material, the incorporated surface doping & coating route as described in this paper could be utilized to fabricate an intermediary layer between active material and solid electrolyte.

4. Conclusions

25



In this work, the surface modified LMNCO samples (LSO@LMNCO, F-LMNCO and F-LSO@LMNCO) are prepared by a facile method, and the electrochemical tests demonstrate that the surface modified LMNCO samples present better electrochemical and thermal stabilities than the pristine material. The enhanced electrochemical performances are attributed to the improved surface/bulk structure and diffusion kinetics behavior, which is not only determined only by surface interface resistance (Rsf), but also is highly influenced by the interface compatibility (Rct). It is found that the incorporated sample of F-LSO@LMNCO manifests the optimized electrochemical performance due to the synergetic effect of LSO surface coating and F doping. The integrated approach of functional surface construction sheds new light on performance optimization for electrode materials, and provides guidance in designing novel energy devices for future applications.

Acknowledgments This work was supported by the National Natural Science Foundation of China (50902044, 51672069, 10674041), the 863 Program of China (2015AA034201), the Program for Science and Technology Innovation Talents in Universities of Henan Province (16HASTIT042) and the International Cooperation Project of Science and Technology Department of Henan Province (162102410014). This work was also supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.

26


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Supporting Information Raman, XRD, EDX, SEM and XPS characterizations of the obtained samples; electrochemical results (CV curves, EIS curves, charge-discharge curves, discharge midpoint voltage), the calculated XRD results, fitted EIS values acidity values after soakage of all electrode materials.

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Electrochim. Acta 2013, 105, 200-208. (55) Kapylou, A.; Song, J.H.; Missiul, A.; Ham, D. J.; Kim, D.H.; Moon, S.; Park, J.H. Improved Thermal Stability of Lithium-rich Layered Oxide by Fluorine Doping. ChemPhysChem 2018, 19, 116-122. (56) Takamatsu, D.; Koyama, Y.; Orikasa, Y.; Mori, S.; Nakatsutsumi, T.; Hirano, T.; Tanida, H.; Arai, H.; Uchimoto, Y.; Ogumi, Z. First in Situ Observation of the LiCoO2 Electrode/electrolyte Interface by Total-reflection X-ray Absorption Spectroscopy. Angew. Chem. Int. Ed. 2012, 51, 11597-11601. (57) Kawaura, H.; Takamatsu, D.; Mori, S.; Orikasa, Y.; Sugaya, H.; Murayama, H.; Nakanishi, K.; Tanida, H.; Koyama, Y.; Arai, H.; Uchimoto, Y.; Ogumi, Z. High Potential Durability of LiNi0.5Mn1.5O4 Electrodes Studied by Surface Sensitive X-ray Absorption Spectroscopy. J. Power Sources 2014, 245, 816-821. (58) Attia, E.N.; Hassan, F.M.; Li, M.; Batmaz, R.; Elkamel, A.; Chen, Z. Tailoring the Chemistry of Blend Copolymers Boosting the Electrochemical Performance of Si-based Anodes for Lithium Ion Batteries. J. Mater. Chem. A 2017, 5, 24159-24167. (59) Luo, L.; Yang, H.; Yan, P.; Travis, J.J.; Lee, Y.; Liu, N.; Piper, D.M.; Lee, S.H.; Zhao, P.; George, S.M.; Zhang, J.G.; Cui, Y.; Zhang, S.; Ban, C.; Wang, C.M. Surface-coating Regulated Lithiation Kinetics and Degradation in Silicon Nanowires for Lithium Ion Battery. ACS Nano 2015, 5 (9), 5559-5566. (60) Lin, H.Y.; Li, C.H.; Wang, D.Y.; Chen, C.C. Chemical Doping of a Core-shell Silicon

Nanoparticles@polyaniline

Nanocomposite 36


for

the

Performance

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Enhancement of a Lithium Ion Battery Anode. Nanoscale 2016, 8, 1280-1287. (61) Manthiram, A.; Yu, X.; Wang, S. Lithium Battery Chemistries Enabled by Solid-state Electrolytes. Nat. Rev. Mater. 2017, 2(4), 16103. (62) Yang, X.; Zhang, R.Y.; Zhao, J.; Wei, Z.X.; Wang, D.X.; Bie, X.F.; Gao, Y.; Wang, J.; Du, F.; Chen, G. Amorphous Tin-based Composite Oxide: a High-rate and Ultralong-life Sodium-ion-storage Material. Adv. Energy Mater. 2018, 8, 1701827. (63) Tu, Z.; Choudhury, S.; Zachman, M.J.; Wei, S.; Zhang, K.; Kourkoutis, L.F.; Archer, L.A. Designing Artificial Solid-electrolyte Interphases for Single-ion and High-efficiency Transport in Batteries. Joule 2017, 1, 394-406. (64) Zhang, W.; Nie, J.; Li, F.; Wang, Z.L.; Sun, C. A Durable and Safe Solid-state Lithium Battery with a Hybrid Electrolyte Membrane. Nano Energy 2018, 45, 413-419. (65) Luo, W.; Gong, Y.; Zhu, Y.; Li, Y.; Yao, Y.; Zhang, Y.; Fu, K.; Pastel, G.; Lin, C.F.; Mo, Y.; Wachsman, E.D.; Hu, L. Reducing Interfacial Resistance between Garnet-structured Solid-state Electrolyte and Li-metal Anode by a Germanium Layer. Adv. Mater. 2017, 29, 1606042. (66) Wan, H.; Mwizerwa, J.P.; Qi, X.; Liu, X.; Xu, X.; Li, H.; Hu, Y.S.; Yao, X. Core-shell Fe1–[email protected] Nanorods for Room Temperature All-solid-state Sodium Batteries with High Energy Density. ACS Nano 2018, 12, 2809-2817.

37



Page 38 of 47

Table 1. Electrochemical data of all optimized materials. Initial discharge capacity (mAh g-1)

Initial Coulombic Efficiency (ICE)

Discharge capacity after 200 cycles (mAh g-1)

Capacity retention after 200 cycles

Pristine

205

73%

115

56%

0.5wt.%LSO@LMNCO

211

78%

138

65%

F0.2-LMNCO

223

80%

127

57%

F0.2-LSO@LMNCO

230

87%

167

73%

Sample

38


Page 39 of 47 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Apparent diffusion coefficients of optimized electrodes in CV and EIS measurements.

CV

EIS

Sample

DLi+ (10-12) /cm2 S-1

Pristine

LSO

F

F-LSO

Pristine

LSO

F

F-LSO

5.54

12.70

2.74

31.70

2.11

10.60

1.01

13.96

39



Figure 1. Surface morphologies and microstructures of pristine (a,e); LSO@LMNCO (b,f); F-LMNCO (c,g); and F-LSO@LMNCO (d,h) materials. The corresponding fast Fourier transform (FFT) results for different vertical regions of F-LMNCO are shown in the insets of Figure 1g.

40


Page 40 of 47

Page 41 of 47 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. XRD patterns of as-prepared samples: LSO@LMNCO (a), F-LMNCO (b), and F-LSO@LMNCO (c). XPS spectra of Li 1s (d), Sn 3d5/2 (e), O 2p (f), Mn 2p (g), Ni 2p (h), and Co 2p (i) for the pristine and modified electrodes.

41



b

200

200 Pristine F0.1-LSO@LMNCO F0.2-LSO@LMNCO F0.3-LSO@LMNCO

100 0 200

Pristine F0.1-LMNCO F0.2-LMNCO F0.3-LMNCO

100 0

Pristine 0.5 wt.% LSO@LMNCO F0.2-LMNCO F0.2-LSO@LMNCO

0

-1

-1

0

200 100

Pristine 0.5 wt.% LSO@LMNCO F0.2-LMNCO F0.2-LSO@LMNCO

100

Capacity / mAh g

a Capacity / mAh g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-100 200 100

Pristine F0.1-LSO@LMNCO F0.2-LSO@LMNCO F0.3-LSO@LMNCO

0 -100 200 100

Pristine F0.1-LMNCO F0.2-LMNCO F0.3-LMNCO

0 -100 200

200

0.1 C

100

100

Pristine 0.5 wt.% LSO@LMNCO 1 wt.% LSO@LMNCO

0 0

50

100

-100

200

0.2 C

0.1 C 0.5 C

Pristine 1 0.5 wt.% LSO@LMNCO 1 wt.% LSO@LMNCO

0

150

Page 42 of 47

0

5

10

15

C

2C

20

Cycle number

Cycle number

Figure 3. Cycling performances (a) and rate capabilities (b) of all electrodes.

42


25

30

Page 43 of 47

Current

0.0002 0.0001

b

Pristine 0.1 mV/s 0.2 mV/s 0.3 mV/s 0.4 mV/s

0.00010

0.0000 -0.0001

-0.00005 2.0 2.5 3.0 3.5 4.0 4.5 5.0

0.0001

d

F-LMNCO 0.1 mV/s 0.2 mV/s 0.3 mV/s 0.4 mV/s

0.0000

0.1 mV/s 0.2 mV/s 0.3 mV/s 0.4 mV/s

0.0001 0.0000

-0.0002

2.0 2.5 3.0 3.5 4.0 4.5 5.0

Voltage / V

f

Discharge

Pristine

LSO

F-LMNCO

F-LSO

Charge 0.3

F-LSO@LMNCO

0.0002

-0.0001

-0.0001 -0.0002

0.0003

0.4

0.5

2.0 2.5 3.0 3.5 4.0 4.5 5.0

Voltage / V

Normalized resistance

0.0003

Current

Current

Voltage / V

Voltage / V

0.0002

e

0.1 mV/s 0.2 mV/s 0.3 mV/s 0.4 mV/s

0.00000

2.0 2.5 3.0 3.5 4.0 4.5 5.0

c

LSO@LMNCO

0.00005

Current

a

0.0003

Ip / A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Pristine LSO@LMNCO F-LMNCO F-LSO

Rct

Pristine LSO@LMNCO F-LMNCO F-LSO

Rsf

0

0.6

1/2

(V/s)

10

20

Cycle number

Figure 4. CV curves at different scanning speeds for all electrodes: pristine (a), LSO@LMNCO (b), F-LMNCO (c), and F-LSO@LMNCO (d). Linear relationship of the peak current (Ip) and the square root of scanning rate (ν1/2) (e). Evolution of the normalized Rsf and Rct values in different cycles (f). 43



a

After cycled

b

F-LSO

2

F-LMNCO

Pristine

Before cycled

F-LSO F-LMNCO

30

40

50

60

Pristine

003

20

104

003

LSO

Heat flow / W g -1

283 °C 311 °C

LSO

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 47

70

80 18.4 19.2

322 °C 1

312 °C 0

-1

2 Theta / degree

Pristine 0.5 wt.% LSO@LMNCO F-LMNCO F-LSO@LMNCO

240

300

360

Temperature / °C

Figure 5. XRD patterns of optimized electrodes before and after 20 cycles (a) and DSC profiles of the electrodes after 20 cycles (b).

44


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


b Co Element content

Page 45 of 47

Pristine F-LMNCO

LSO F-LSO

Ni

Pristine F-LMNCO

LSO F-LSO

Mn

Pristine F-LMNCO

LSO F-LSO

1

2

3

4

Soak time / week

Figure 6. Color evolution of electrolytes after aging for pristine and modified materials (a) and ICP histogram results for metal ions in solid powders after soakage (b).

45



a b c

Figure 7. Schematic diagrams of LSO@LMNCO (a), F-LSO@LMNCO (b) and integrated surface design (c).

46


Page 46 of 47

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Table of contents Discharge capacity / mAh g -1

Page 47 of 47

200 100 0

Pristine F0.2-LMNCO

0

200

50

0.5 wt.% LSO F0.2-LSO@LMNCO

100

150

0.1 C 0.2 C

100

0.1 C 0.5 C

0 0

5

200

10

15

1C

20

2C 25

Cycle number

47


30

Integrated Surface Functionalization of Li-rich Cathode Materials for Li

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