Letter www.acsami.org
Layered/Spinel Heterostructured and Hierarchical Micro/ Nanostructured Li-Rich Cathode Materials with Enhanced Electrochemical Properties for Li-Ion Batteries Ya-Ping Deng,† Zu-Wei Yin,† Zhen-Guo Wu,†,§ Shao-Jian Zhang,† Fang Fu,‡ Tao Zhang,† Jun-Tao Li,*,† Ling Huang,‡ and Shi-Gang Sun*,†,‡ †
College of Energy and ‡State Key Lab of Physical Chemistry of Solid Surface, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China § School of Chemical Engineering, Sichuan University, Chengdu 610065, China S Supporting Information *
ABSTRACT: Although holding a high capacity, Li-rich materials are far from the demand of practical market because of their inherent drawbacks, such as poor initial efficiency and rate capability. Herein, Li-rich materials of Li1.16Mn0.6Ni0.12Co0.12O2 have been prepared via a one-step solvothermal strategy. The detail characterizations demonstrate that the as-prepared materials present morphology of nanoparticleaggregated hierarchical microspheres and a heterostructure of layered and Li4Mn5O12-type spinel components. Compared to materials of pure-layered structure, layered/spinel heterostructured materials exhibit simultaneously great reversible capacity (302 mAh g−1 at 0.2 C), high initial Coulombic efficiency (94% at 0.2 C) and remarkable rate capability (193 mAh g−1 at 10 C).
KEYWORDS: Li-ion batteries, Li-rich cathode materials, hierarchical micro/nanostructure, layered/spinel heterostructure, solvothermal synthesis
B
has reported that a one-step formation of heterostructured Lirich materials by optimizing synthesis conditions is also beneficial for its rate performance.11−13,16,21 Besides, large improvement of the initial Coulombic efficiency has been achieved by mechanical blending Li-rich materials with Fd3m ̅ spinel Li4Mn5O12, although its rate capability is still inferior due to lack of synthetic effects between the two mechanical-mixed components.8 However, there are few strategies reported so far to synthesize Li-rich materials that combine those advantages simultaneously, including remarkable initial Coulombic efficiency, superhigh capacity and excellent rate capability. With the aim of simultaneously boosting the reversible capacity, the initial Coulombic efficiency and the rate capability of Li-rich cathode materials, we present a one-step solvothermal route to synthesize Li4Mn5O12-based layered/spinel heterostructured Li1.11Mn0.6Ni0.12Co0.12O2 (LS). Such heterostructure could combine the merits of layered and spinel components. Theoretically, the layered structure offers extensive Li + migration to provide high capacity; meanwhile, the spinel Li4Mn5O12 provides empty 16c octahedral sites to decrease initial irreversible capacity loss, and 3D Li+ diffusion channels to
ecause of the rapid development of electric vehicles (EVs) and hybrid electric vehicles (HEVs), the demand on high energy and high power density of Li-ion batteries (LIBs) is ever-increasing.1 The energy density, cyclability, and safety of LIBs greatly depend on electrochemical properties of electrode materials, especially on cathodes.2 Therefore, it is highly demanded to boost electrochemical performance of cathode materials for LIBs. Lithium-rich and manganese-based materials, a solid solution of Li2MnO3 and LiMO2 (M = Mn, Ni, Co), have been considered to be one of the most promising cathode candidates, because of their high discharge capacity (>250 mAh g−1) between 2.0 and 4.8 V (vs Li/Li+), reasonable cost, and low toxicity.3−7 However, considering their low electronic conductivity, 2D Li+ diffusion plane offered by hexagonal layered structure and their irreversible structural changes of Li2MnO3 caused by inevitable oxygen loss, Li-rich materials still suffer from some inherent drawbacks, especially inferior rate performance and poor initial Coulombic efficiency.8−10 Huge efforts have been made recently to address the abovementioned drawbacks. Among them, to modify Li-rich materials to form layered (R3̅m)-layered (C2/m)-spinel (Fd3̅m) heterostructure has been demonstrated as one of the most feasible solutions.4,6,11−21 Specifically, surface postmodification on Li-rich materials to form spinel membraneencapsulated layered particles has illustrated advantages to enhance rate capability and cycling stability.4,6,11,14,15,17,19,20 It © XXXX American Chemical Society
Received: April 4, 2017 Accepted: June 8, 2017 Published: June 8, 2017 A
DOI: 10.1021/acsami.7b04726 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 1. (a) X-ray diffraction (XRD) patterns, (b) Raman spectra, and (c) Mn K-edge X-ray absorption near edge structure (XANES) spectra of PL and LS.
achieve superior Li+ diffusion kinetics.4 This study is logically follow-up of our previous work, which focus on the influence of different calcination time on formation of layered/spinel heterostructured Li-rich materials.18 Herein, by various structural characterizations, a Li4Mn5O12 spinel component has been confirmed in such heterostructure. The effects of such spinel component on structural and electrochemical properties of layered Li-rich materials has been investigated and discussed systematically. Compared to the pure-layered Li-rich materials (PL), the as-synthesized LS demonstrate obviously superior electrochemical performances. The XRD patterns of synthesized materials are displayed in Figure 1a. Similarly, the main diffraction peaks of the two materials are corresponding to the typical hexagonal α-NaFeO2 layered structure (R3̅m) except the weak superlattice diffraction peaks between 20 and 23°, which reflect the (020) and (110) peaks of monoclinic Li2MnO3 (C2/m).3,22 Such results suggest that both of PL and LS contain the solid solution of LiMO2 and Li2MnO3 phases. Contrastively, some new shoulder peaks marked by “*” have been only discerned in LS. After being indexed carefully, those additional peaks match with the feature diffractions (111), (220), (311), (222), (400), and (511) of spinel structure (Fd3̅m, Figure S1a),4,12,13,18,23 which indicates that a kind of layered (R3̅m)-layered (C2/m)-spinel (Fd3̅m) heterostructure has been formed in LS. The XRD Rietveld refinement data of the two materials are listed in Table S1 and compared in Figure S2. The intensity ratio of I003/I104 is usually considered to represent the Li+/Ni2+ mixing in layered R3̅m structure.24,25 As listed in Table S1, the I003/I104 ratio of LS is much smaller than that of the PL, signifying that the formation of spinel has a great influence on the cation mixing of layered structure. Rietveld refinements illustrate that the Li+/Ni2+ mixing in LS (17%) is higher than that in PL (12%), resulting in the shrinking of layered lattice parameters a and c of the LS in comparison with PL. Furthermore, 18% spinel structure with a cubic parameter a of 8.991 Å has been confirmed in LS per refinement results. The Li+ diffusion kinetics of LS are expected to be accelerated by such spinel 3D diffusion channels.
Raman spectra (Figure 1b) were presented to further verify such heterostructure of LS. Two broad bands at 493 and 604 cm−1, representing the Eg and A1g vibrations of the R3̅m layered structure, can be observed in both LS and PL. Besides, the narrow and weak peaks at about 400 cm−1 attributed to the fingerprint vibration of C2/m space group can be also seen in both spectrum of the two materials. An obvious shoulder band at 650 cm−1, nevertheless, solely appears in the Raman spectrum of LS. Such shoulder band at 650 cm−1 reflects Fd3̅m spinel structure band, which coincides with previous literatures.6,19 Therefore, those structural characterizations confirm that the synthesized LS contains doubtlessly the spinel structure (Fd3̅m). To reveal the specific spinel type in the LS, X-ray absorption near edge structure (XANES) measurements were conducted for both PL and LS. The LiNi0.5Mn1.5O4-type spinel is excluded since the characteristic spinel Ni2+/Ni4+ redox potential plateau at about 4.65 V is absent during electrochemical cycling (Figure 3a and Figure S4), and the Li4Mn5O12-type structure with Mn4+ and LiMn2O4-type structure with average Mn3.5+ spinel is regarded as the possible candidates.6,26 Previous reports have demonstrated that the metal ion valence of Mn, Ni, Co is +4, +2, +3 in layered Li-rich materials.6 Therefore, the spinel type in LS can be determined according to its Mn valence state obtained by the edge energy position of XANES, which is sensitive to the valence state of Mn-ion. The main peak of XANES curves would shift to higher energy when the valence state of Mn-ion increase, and shift to lower energy when the valence state decrease.26 Figure 1c displays the normalized XANES spectra collected at Mn K-edge (6539 eV) for PL and 4+ LS, with the reference spectra of Mn3+ 2 O3 and Mn O2. The energy position of the main peak for LS is almost the same as for PL, and as the energy position of the Mn4+. Such XANES results indicate that the valence state of Mn-ion in LS has not been influenced by the spinel structure, and therefore Mn-ion in the spinel should be +4, which is consistent with the XPS characterizations (Figure S3). According to the above results and discussions, layered/spinel heterostructure with B
DOI: 10.1021/acsami.7b04726 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 2. (a, b) SEM images, (c, d) typical TEM images and corresponding polycrystalline SAED patterns of PL and LS. (e, f, h) HRTEM images and the corresponding (e) selected area electron diffraction (SAED) or (g, i) fast Fourier transform (FFT) patterns of regions i, ii, and iii.
efficiency of 80%. The Li4Mn5O12-like spinel structure in LS is regarded as the critical factor for such enhancement. In the initial charge process, two obvious plateaus could be easily observed for both PL and LS, representing Li+ extraction from the LiMO2 at 4.0 V with oxidation of Ni2+ → Ni4+ and Co3+ → Co3.6+, as well as from the electrochemical activation of Li2MnO3 with inevitable oxygen loss above 4.5 V.7 Comparatively, the initial charge capacity of LS (321 mAh·g−1) is lower than that of PL (348 mAh g−1), because spinel Mn4+ in Li4Mn5O12 cannot be further oxidized. However, upon the initial discharge, besides the potential plateau of Li+ insertion into layered component, a new plateau marked at 2.7 V is solely appeared in the curve of LS and attributed to the characteristic Mn3+/Mn4+ reduction in spinel component,18,19 in which excess Li+ can insert into the 16c empty sites of spinel Li4Mn5O12 (Figure S1b). The dQ/dV curves (Figure S4) also support the new observation of the appearance of spinel Mn3+/ Mn4+ redox peaks of LS. As a result, the initial irreversible capacity loss of Li-rich materials has been effectively reduced from 70 to 19 mAh g−1, i.e., a great boosting of the initial Coulombic efficiency from 80 to 94%. When cycling at high rate of 0.5, 1, and 2 C (Figure 3b), all the initial discharge curves of LS present obvious spinel Mn3+/ Mn4+ potential plateau at 2.7 V, which is always absent for PL. As illustrated in Table S2, excellent initial performance, including superhigh discharge capacity and efficiency, can be achieved in the electrochemical tests of LS at various rates. It performs respectively discharge capacities of 299, 292, and 259 mAh g−1 with initial Coulombic efficiencies of 93, 90, and 88% at 0.5, 1, and 2 C, which are all about 15% higher than those of PL (Figure 3f). Additionally, the LS demonstrates remarkable rate capability. When cycling at 1, 2, 5, and 10 C (Figure 3c, d), it offers the maximal capacities as high as 292, 267, 227, and 193 mAh g−1, respectively. To the best of our knowledge, the obtained performance is much superior to peer reports so far as compared in Table S3. Upon cycling at high rates, the discharge capacity rise to its maximal value within initial few cycles and then stay at a stable level, which is regarded as the activation of the cell.25 In comparison, the PL gives far inferior rate performance, offering the maximal capacities of 199, 174, 118, and 55 mAh·g−1 at each rate (Figure 3c, d). Especially at superhigh charge/discharge rate of 10 C, the stable capacity of
Li4Mn5O12-type characteristic spinel has been formed in LS during the preparation process. The morphological features of PL and LS were characterized by SEM. As shown in Figure 2a, b, although both PL and LS materials present nanoparticle-aggregated microspheres, the obvious differences are found in size of their primary nanoparticles, which are greatly influenced by different calcination conditions. Compared to PL, LS has smaller nanoparticles of 20−30 nm in size, which is beneficial for electrolyte penetration and shortening Li+ diffusion distance.5,18 The structure and morphology of both materials were further studied via transmission electron microscopy (TEM) and selected area electron diffraction (SAED) on single microsphere (Figure 2c, d). In the SAED pattern of PL, these polycrystalline electron diffraction rings, except a (020) plane belongs to C2/m structure, can be indexed to the corresponding planes of R3̅m structure. The single-crystal SAED (Figure 2e) of PL coincidentally exhibit two arrays of symmetry patterns. As for the LS, besides the Li-rich layered characteristics, two emerged diffraction rings could be only indexed to the (220) and (442) planes of the Fd3̅m spinel phase.17,18 Thus, as illustrated in Figure 2f, h, the formation of layered (R3̅m)-layered (C2/m)-spinel (Fd3̅m) heterostructure for LS has been verified by SAEDs. High-resolution TEM (HRTEM) images of corresponding marked regions reflect two sets of clear lattice fringes with different d-spacing. Specifically, a d-spacing of 0.47 nm corresponding to (0003)Hex plane with hexagonal symmetry pattern defined by fast Fourier transform (FFT, Figure 2g) illustrates that the nanoparticle in region ii exhibit layered features, whereas the region iii (Figure 2h) appears a lattice spacing of 0.27 nm that could be only coincided with the interplanar distance of (220)Cub plane of the spinel. The spinel characteristics is also supported by FFT in Figure 2i. These above results indicate that the Li4Mn5O12-like spinel nanoparticles are scattered in microspheres of LS, which would boost its electrochemical performance thanks to the synergic effect between the layered and spinel nanoparticles. Figure 3a and Figure S4 display the initial charge−discharge curves at 0.2 C and corresponding dQ/dV curves. Without any postmodifications, LS delivers a discharge capacity of 302 mAh g−1 together with a Coulombic efficiency as high as 94%. By contrast, PL exhibits inferior initial performances, including a smaller capacity of 278 mAh·g−1 and a lower Coulombic C
DOI: 10.1021/acsami.7b04726 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 3. (a, b) Initial charge−discharge curves and (c, d) cycle performance at different current densities for PL and LS. (e) The comparative cycle life test at 10 C between 2.0 and 4.8 V; (f) the competition of initial efficiencies at various rates and (g) ln DLi at the initial discharge process.
the LS is 138 mAh·g−1 higher than that of the PL. Regarding the voltage fading during charge and discharge processes, the normalized discharge capacity in 50th cycles at 1 C is presented in Figure S5. An obvious downward shift of discharge voltage can be easily seen in PL, while much slower decline tendency was observed in LS. Such result coincides with previous report about the “swallow effects” of Li4Mn5O12-based heterostructured Li-rich materials during cycling.15 Briefly, considering the low energy barrier between the two 16d sites of Li4Mn5O12 and LiMn2O4, partial Li(16d) ion can easily migrate from Li4Mn5O12 into LiMn2O4, and then the 16d vacancies of Li4Mn5O12 can be inserted again by Li+ during the following discharges.15 Consequently, the slower voltage fading is obtained due to the higher discharge voltage of Li4Mn5O12 than LiMn2O4. To determine how spinel component boosts the Li+ diffusion kinetics, we carried the potentiostatic intermittent titration technique (PITT) experiments in half-cells using, respectively, PL and LS during the initial discharge. Although it is hardly to quantify precisely Li+ diffusion coefficient in such heterostruc-
tured cathodes, qualitative analysis and comparison of Li+ diffusion ability between PL and LS is meaningful.27,28 Figure 3g illustrates the Li-ion chemical diffusion coefficient (DLi, cm2 s−1) calculated upon the chronoamperometry and ln I versus t plots in Figure S5. In the initial discharge process, the LS presents always much higher DLi than PL does, which supports the electrochemical analysis. The comparative Electrochemical Impedance Spectroscopy (EIS, Figure S7) of PL and LS also exhibit coincident results. Such superior rate capability of LS is ascribed to its hierarchical micro/nanostructure and layered/ spinel heterostructure. The former structural feature is beneficial for electrolyte penetration and shortening the diffusion distance of Li+; and the latter one could offer 3D channels to greatly boost Li+ diffusion ability. From the discussions above, through the structural modification by adjusting synthesis route, the as-prepared Li-rich materials can share the merits of great reversible capacity, advanced initial performance, and remarkable rate capability. D
DOI: 10.1021/acsami.7b04726 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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In summary, hierarchical micro/nanostructured and layered/ spinel heterostructured Li-rich materials have been successfully prepared in this study by one-step solvothermal route. In this kind of materials, the nanoparticle-aggregated microspheres allow electrolyte penetration and shorten Li+ diffusion distance, and also the Li4Mn5O12-type spinel component offers empty 16c octahedral site and 3D Li+ diffusion channels. With those advantages, when compared to pure layered Li-rich material, the layered/spinel heterostructured Li-rich materials exhibit simultaneously greater reversible capacity, higher initial Coulombic efficiency and superior rate capability. The current study has thrown new insights into the design and synthesis of advanced cathode materials for next-generation LIBs.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b04726. Experimental details, XRD patterns and electrochemical properties of synthesized pure Li4Mn5O12, figures and table of Rietveld refined results, XPS spectra, dQ/dV curves, performance comparison, normalized discharge curves upon cycling, plots of chronoamperometry and EIS plots (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (S.S.). *E-mail:
[email protected] (J.L.). ORCID
Jun-Tao Li: 0000-0002-9650-6385 Ling Huang: 0000-0003-1092-5974 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported financially by National Natural Science Foundation of China (21373008), and National Key Research and Development of China (2016YFB0100202)
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REFERENCES
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