Unravelling the Structure and Electrochemical Performance of LiâCr

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Unravelling the structure and electrochemical performance of Li-Cr-Mn-O cathode: From spinel to layered Xuelei Li, Dan Li, Dawei Song, Xixi Shi, Xu Tang, Hongzhou Zhang, and Lianqi Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18097 • Publication Date (Web): 22 Feb 2018 Downloaded from http://pubs.acs.org on February 23, 2018

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ACS Applied Materials & Interfaces

Unravelling the structure and electrochemical performance of Li-Cr-Mn-O cathode: From spinel to layered Xuelei Li,† Dan Li,† Dawei Song,† Xixi Shi,† Xu Tang,‡ Hongzhou Zhang†,* and Lianqi Zhang†,* †

Tianjin Key Laboratory for Photoelectric Materials and Devices, School of Materials Science and Engineering, Tianjin University of Technology, Tianjin, 300384, China.

‡

Electron Microscopy Laboratory, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China.

KEYWORDS: Lithium-ion batteries, cathode materials, Li-Cr-Mn-O oxides, spinel-layered composite, structure, electrochemical performance

ABSTRACT: To explore new serials of cathode materials with high electrochemical performance, the spinel-layered (1-x)[LiCrMnO4]·x[Li2MnO3·LiCrO2] (x = 0, 0.25, 0.5, 0.75 and 1) composites are synthesized with sol-gel method. XRD, HRTEM, SAED and Raman spectra reveal that the structures of the (1-x)[LiCrMnO4]·x[Li2MnO3·LiCrO2] cathode materials evolve from spinel to hybrid spinel-layered and layered structure with the increase of the Li concentration. Test results reveals that the structure and electrochemical performance of (1x)[LiCrMnO4]·x[Li2MnO3·LiCrO2] (x = 0.25, 0.5 and 0.75) composite have the characteristics of

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both spinel (x = 0) and Li-rich layered phase (x = 1). In particular, x = 0.5 and 0.75 electrodes exhibit relatively high capacity retention and rate capability, which is mainly ascribed to the synergistic effect of the spinel and Li-rich layered phase, the 3D Li-ion diffusion channels of spinel phase and the low charge-transfer resistance (Rct) and Warburg diffusion impedance (Wo).

1. INTRODUCTION The Lithium-ion batteries have been widely used in electric vehicles (EVs). The ever-increasing demands of lithium ion batteries for high energy, high power, high safety, long durability and low cost prompt scientists to explore new cathode materials.1 Layered, spinel and olivine materials are the three major groups of the cathode materials for the commercial lithium-ion batteries at present.2 In principle, manganese-based multiple oxides are promising cathode materials owing to low cost and superior thermal stability of Mn4+.3 In particular, one of the most attractive Mn-based cathode materials is Li-rich layered oxides, which have attracted significant attention due to their high specific capacities (>250 mAh g–1).4, 5 The large initial charge capacity is due to the oxidation of the transition metal ions below 4.5V and the activation of Li2MnO3-like component above 4.5V.6-8 During the subsequent cycles, manganese (Mn3+/Mn4+) valence variation provide large capacity. However, there are still some intrinsic drawbacks of Li-rich layered oxides, such as the large initial irreversible capacity resulting from the irreversible oxidation of O2-,9 the inferior rate capability owing to the poor intrinsic electronic conductivity10, 11

and the phase transformation during long cycling.12 Another promising Mn-based cathode

material is spinel LiMn2O4 in virtue of its high operating voltage and rate capabilities.13-17


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Nevertheless, spinel LiMn2O4 suffers from Mn dissolution into the electrolyte induced by HF acid,18 and the strong Jahn-Teller effect of Mn3+ below 3 V.19, 20 Recently, the hybrid spinel-layered structure has been widely studied on account of the synergistic effects of the spinel and layered structures.21-23 Park et al.24 endorse the strategy to design high capacity electrodes by using integrated ‘composite’ structures in which a spinel component is used to stabilize a layered component at high potentials. Zhang et al.25 design new electrodes via blending high-voltage spinel LiNi0.5Mn1.5O4 with high-capacity layered Li1.5Ni0.25Mn0.75O2.5, which accommodates the needs of high-energy and high-power for lithiumion batteries. There are several advantages of the spinel-layered composite cathode materials: 1) the cubic-close-packed oxygen arrays in spinel and layered phases are structurally compatible, which is beneficial for integrating spinel and layered components to synthesize cathode materials with high electrochemical performances;26 2) Rate capability of the electrode can be improved with the three-dimensional diffusion pathway for Li-ions in the spinel phase;27 3) the columbic efficiency of the cathode materials is expected to be improved because the extracted lithium ions during the activation of the Li2MnO3 component can be accommodated by the spinel phase in the subsequent discharge process.3, 22, 24 In this work, to explore new serials of spinel-layered composite cathode materials with high electrochemical performance, Mn- based (1-x)[LiCrMnO4]·x[Li2MnO3·LiCrO2] (x = 0, 0.25, 0.5, 0.75 and 1) composites are designed, synthesized and evaluated. Cr is introduced into the cathode material because three electrons transferred (Cr3+ ↔ Cr6+) during the Li+ extraction/insertion reactions and therefore are expected to achieve high energy and power densities.28 Moreover, it is reported that Cr substituted spinel29 and layered30 cathode materials exhibit improved 5V capacity, excellent capacity retention and enhanced rate capabilities. It is


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known for us that chromium is considered as one of the priority pollutants owing to its toxicity.31 In our opinion, if superior electrochemical performances of the lithium-ion batteries with Crbased cathode material can be achieved, management systems will be established for the recycle of the spent lithium ion batteries, which is similar to that of lead acid batteries.

2. EXPERIMENTAL SECTION 2.1 Material preparation. (1-x)[LiCrMnO4]·x[Li2MnO3·LiCrO2] (x = 0, 0.25, 0.5, 0.75 and 1) cathode materials were synthesized with sol-gel method. Stoichiometric amounts of Cr(NO3)3·9H2O and Mn(CH3COO)2·4H2O, an excess amount (5% in molar ratio) of LiOH·H2O and citric acid (molar ratio of citric acid to transition metal ions was 2:1) were dissolved in deionized water to form an uniform solution. Ammonium hydroxide was added into the solution to adjust the pH value to 9. Then, the solution was evaporated at 80 °C under constantly stirred and dried at 120 °C to get the gel. After initially decomposed at 480 °C for 5 h and ground, the resulting powder was calcined at 850 °C for 10 h in air to obtain the final products. Obviously, when x = 0 and 1, the obtained samples are spinel LiCrMnO4 and Li-rich layered Li[Li0.2Cr0.4Mn0.4]O2, respectively. When x = 0.25, 0.5 and 0.75, the cathode materials (1x)[LiCrMnO4]·x[Li2MnO3·LiCrO2]

are

3/4[LiCrMnO4]·1/4[Li2MnO3·LiCrO2],

1/2[LiCrMnO4]·1/2[Li2MnO3·LiCrO2]

spinel-layered

composites and

1/4[LiCrMnO4]·3/4[Li2MnO3·LiCrO2], respectively. Apparently, the five samples can also be expressed as Li0.5Cr0.5Mn0.5O2, Li0.75Cr0.5Mn0.5O2.125, LiCr0.5Mn0.5O2.25 and Li1.25Cr0.5Mn0.5O2.375 and Li1.5Cr0.5Mn0.5O2.5, respectively, along with the increase of the Li concentration. 2.2 Materials characterization. The crystalline structures of the cathode materials were characterized with an X-ray powder diffractometer (XRD, D/MAX-2500, Rigaku, Cu Kα


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radiation). The morphologies of the cathode materials were observed by a scanning electron microscope (SEM, JMS-6700F, JEOL). High resolution transmission electron microscope (HRTEM) images, selected area electron diffraction (SAED) and X-ray energy dispersive spectroscopy (EDS) of the cathode materials were recorded on a transmission electron microscope (TEM, JEM-2100, JEOL). Raman spectra were collected on a Renishaw Raman spectroscopy system equipped with 512nm laser source. X-ray photoelectron spectroscopy (XPS) measurements of the cathode materials were performed with a ThermoFisher K-alpha Xray photoelectron spectrometer (XPS, ESCALAB 250Xi, Thermo Fisher Scientific). 2.3 Electrochemical testing. The as-prepared cathode material, acetylene black and PVDF (80:10:10 in weight) were mixed in NMP to form homogeneous slurry, which was coated onto Al foil and dried at 120 °C for 12 h. Followed by a roll-pressing, the electrodes (mass loading is about 5.0 mg cm-2) were cut into wafers for assembling 2032 type coin cells. Lithium metal and Celgard 2400 microporous film were used as anode and separator. The electrolyte was 1 mol L-1 LiPF6 in a mixture of ethylene carbonate and diethyl carbonate (1 : 1 in volume). The cells were charged and discharged between 2.0 and 4.95 V at 20 mAh g-1 for the initial five cycles and 40 mAh g-1 for the subsequent cycles, and the rate test was performed at different rates with a LAND CT-2001A instrument at 25 °C. Cyclic voltammograms (CV) were measured by a electrochemical workstation (PMC1000/DC, Princeton) at a scan rate of 0.1 mV s-1 between 2.0 and 4.95 V. Electrochemical impedance spectra (EIS) was investigated using a Zahner IM6ex electrochemical workstation in the frequency range of 100 kHz to 10 mHz. 3. RESULTS AND DISCUSSION 3.1 Physical Properties. XRD patterns of the (1-x)[LiCrMnO4]·x[Li2MnO3·LiCrO2] cathode materials are presented in Figure 1. As is shown in Figure 1a, the diffraction peaks of x = 0


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sample match well with a face-centered cubic symmetry with the Fd 3 m space group.32 No impurity peak exist, indicating pure spinel LiCr0.5Mn0.5O4 is successfully synthesized. For the sample of x = 1, all the diffraction peaks can be indexed as the O3 type layered structure (Li[Li0.2Cr0.4Mn0.4]O2) with a space group of R 3 m except for the weak reflection peaks of 2θ = 20 - 25°, which manifest the presence of monoclinic Li2MnO3-like phase with a space group of C2/m, as well as the superstructure reﬂections originating from the cation ordering of Li, Cr and Mn atoms in the transition metal layer.33, 34 Besides, well-ordered layered structure also can be verified by the apparent splits of (006)/(102) and (108)/(110) doublet peaks, as shown in Figure 1c and d.35, 36 For the samples of x = 0.25, 0.5 and 0.75, as excepted, these three samples exhibit the features of both spinel LiCrMnO4 and layered Li[Li0.2Cr0.4Mn0.4]O2. As shown in Figure 1b, the intensities of (111) peaks of the spinel component become weaken along with x increase, while the intensities of (003) peaks of the layered structure gradually stronger, which indicates that the content of the Li-rich layered component gradually increase along with the increase of x value. The same conclusion can also be drawn from the intensity variations of (311)S, (222)S, (400)S, (101)L, (006)L, (102)L and (104)L in Figure 1b and the variation tendency of (440)S, (531)S, (018)L, (110)L and (113)L peaks in Figure 1c. These phenomena imply the significant influence of the Li content on the crystal structure of the (1-x)[LiCrMnO4]·x[Li2MnO3·LiCrO2] cathode material. Raman spectra are also used to confirm the structure evolution of the (1x)[LiCrMnO4]·x[Li2MnO3·LiCrO2] cathode materials along with the x value. As depicted in Figure S1, Raman bands of 423 cm-1 in the samples of x = 0.75 and 1 are attributed to the monoclinic Li2MnO3-like structure in the Li-rich layered phase.37 Raman band of Mn-O vibrations in MnO6 (spinel LiMn2O4) is located at 631 cm-1,38 which is vanished along with the


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increase of x value. The Raman bands of 471 and 563 cm-1 may be attributed to the Cr-O bonds in the spinel and layered phase, respectively, and consequently, their intensities vary along with the structure evolution of the cathode material. SEM images of the as-prepared cathode materials are shown in Figure 2. Uniform primary particles of 100 - 200 nm can be found for all the samples. The nano-sized particles of those composites shorten the diffusion distance of Li-ions in the bulk particles, which is beneficial for the improvement of the high-rate performance.39 A typical morphology of spinel phase of truncated octahedral grains with a (111) faceted plane can be observed from Figure 2a and b. The structure diagram of truncated octahedral is shown in Figure 2a’. As the increase of x value, it seems that the content of truncated octahedral grains decrease, as shown in Figure 2b-e. This phenomenon implies that the content of the spinel phase is decreased while the layered phase is increased along with the x increase of the (1-x)[LiCrMnO4]·x[Li2MnO3·LiCrO2] composite, which is in accordance with the XRD and Raman analysis. Additionally, EDS results (Figure S2) indicate that mole ratios of Cr and Mn atoms in the final products are close to the stoichiometric proportion (1:1) of the (1-x)[LiCrMnO4]·x[Li2MnO3·LiCrO2] cathode materials. HRTEM and SAED images of x = 0, 0.5 and 1 samples are displayed in Figure 3. The size of the primary grains of the three samples are about 200 nm, in accord with SEM images (Figure 2). When x = 0, the spinel LiCrMnO4 phase is formed, as shown in Figure 3a. The d-spacing of x = 0 sample is 0.48 nm, which is assigned to the (111) plane of the spinel phase.40, 41 Figure 3d exhibits a typical array of electron diffraction spots, which reflects the cubic spinel structure of the LiCrMnO4 phase and can be indexed to the [ 1 22] zone. For the samples of x = 1, Li2MnO3·LiCrO2 (i.e. Li-rich layered Li1.2Cr0.4Mn0.4O2) shows a homogeneous layered structure with the interference fringe spacing of 0.47 nm, which is the interplanar distance of (003)


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plane.42 Clear electron diffraction spots of hexagonal symmetry pattern can be observed in Figure 3f, which is indexed to the [001] zone of a typical hexagonal lattice structure. When x is between 0 and 1, the (1-x)[LiCrMnO4]·x[Li2MnO3·LiCrO2] cathodes become the spinel-layered composite and exhibit complex microstructure (Figure 3b and e). As displayed in Figure 3e, the layered hexagonal phase (Li-rich layered Li[Li0.2Cr0.4Mn0.4]O2) and the spinel phase (LiCrMnO4) coexist, in which the [1 1 1 ] of the hexagonal phase is parallel to [ 2 1 1] of the spinel phase. Therefore, it is believed that the structure evolution of the (1-x)[LiCrMnO4]·x[Li2MnO3·LiCrO2] cathodes are induced by the lithium content, which agrees well with the XRD, Raman and SEM analysis. In order to investigation the chemical state of Cr and Mn elements in the composites, XPS spectra are performed and the results are shown in Figure 4. The center of Cr 2p3/2 and Cr 2p1/2 peaks locate at 576.7 and 586.8 eV, respectively, indicating Cr3+ exists in all the samples.35 However, a small amount of Cr6+ is detected in samples of x = 0, 0.25 and 0.5, which is verified with the peaks of Cr 2p3/2 (580.2 ev) and Cr 2p1/2 (589.4 eV) of Cr6+.1,

13

This result

demonstrate that Cr6+ can exist in the crystal structure of the composite and thus Cr3+ can be oxidized into Cr6+ in these composites to provide high capacity. In Mn 2p spectra, core levels of Mn 2p3/2 and Mn 2p1/2 appear at 643.3 and 654.9 eV, respectively, in accordance with Mn4+ in spinel and layered compounds.5, 43 3.2 Electrochemical Performances. Figure 5 shows the charge-discharge curves and CV profiles of (1-x)[LiCrMnO4]·x[Li2MnO3·LiCrO2] cathode materials between 2.0 and 4.95 V. For the spinel LiCrMnO4 (x = 0, Figure 5a and b), the two plateaus / redox peaks above 4.0 V (about 4.1 V and 4.7 V) are correlated to the lithium ion intercalation into 8a tetrahedral sites of the cubic spinel structure and the reverse reaction.44 The short plateau at 4.1 V is accompanied by


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the redox of Mn3+/Mn4+, due to the presence of a small amount of Mn3+ in the Fd 3 m spinel phase. The plateau at 4.7 V originates from the oxidation of Cr3+ to Cr4+, and then to Cr6+ for a part of Cr4+. The discharge plateau and the cathodic peaks below 3V correspond to the lithium ion intercalation into 16c octahedral site of the spinel structure, as well as a cubic to tetragonal phase transition and the reduction of Mn4+ to Mn3+, while the charge plateau and the anodic peaks below 3 V are related to the reverse reaction. This process is believed to induce strong JahnTeller Effect, which leads to the structural distortion and capacity fading.44 For the Li-rich layered Li1.2Cr0.4MnO2 (x = 1, Figure 5i and j), during the initial charge process, Li-ions are initially extracted from lithium layers accompanied by the oxidation of Cr3+ to Cr4+ at about 3.8 V, and the plateau at 4.2 V is ascribed to the oxidation of Cr4+ to Cr6+.45, 46 Thereafter at above 4.5 V, lithium and oxygen are removed irreversibly from Li2MnO3-like structure with a loss of Li2O, resulting in the formation of layered MnO2, which is regarded as the activation process of Li2MnO3-like component.47, 48 In the following cathodic process (Figure 5j), the overlap peak L1 is owing to the interference of the reduction process of Cr6+→Cr3+ and Mn4+→Mn3+ (derived from the activation of the Li2MnO3-like structure). In the 2nd cycle, CV profile is different from the initial, in which the strong oxidation peak at 4.8 V disappears, indicating the activation process of Li2MnO3-like structure above 4.5 V is irreversible. The charge and discharge curves of the spinel-layered composite (x = 0.75, 1.0 and 1.25) exhibit both spinel and layered characteristics, as shown in Figure 5(c-h). Obviously, the charge-discharge characteristics of spinel phase become weaker gradually, while those of layered structure become stronger along with the increase of x value. CV profiles of all the samples exhibit the same characteristics with the corresponding charge-discharge curves, as displayed in Figure 5(a-j).


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As displayed in Figure 5a, the initial coulombic efficiency (ICE) of LiCrMnO4 (x = 0) electrode is 122.7%. This high ICE is due to the excess discharge capacity in 3 V region, based on the reaction LiCrMnO4 + Li+ + e → Li2CrMnO4, which is related to the lithium ion intercalation into 16c octahedral site of the spinel structure, together with a cubic to tetragonal phase transition and the reduction of Mn4+ to Mn3+, as descripted in the CV section. However, for Li-rich layered Li1.2Cr0.4Mn0.4O2 (x = 1), the ICE is low (69.5%), which is mainly ascribe to the activation process of Li2MnO3-like component in the first charge process, as discussed in the CV section. For the composites of x = 0.75, 0.5 and 0.25, the ICE is 77.2, 85.2 and 97.8%, respectively, increase along with the decrease of the x value. It is reported that the Li-ions deintercalated irreversibly from Li2MnO3-like structure in the first charge process can be accommodated by the spinel component (LiCrMnO4) in 3 V region during discharge process with the above reaction equation.3 Therefore, the ICE of the composites increase with the content of the spinel phase. Nevertheless, the 3 V plateau of the spinel phase is believed to induce strong Jahn-Teller Effect, which leads to the structural distortion and capacity fading. Charge and discharge curves of the five samples after 100 cycles at 0.2C are displayed in Figure 5. As a higher charge-discharge rate than the initial 5 cycles is conducted and the capacity decay after 105 cycles, the profiles of the 105th charge-discharge curves are much different from the first 3 cycles (Figure 5a, c, e, g and f). To exhibit the variation trend of the discharge capacity and the average discharge voltage during cycling, the cycle performance of the samples are displayed in Figure 6a and b. The reversible capacities of x = 0, 0.25, 0.5, 0.75 and 1 samples at the 105th cycle are 105.8, 131.2, 167.6, 176.2 and 190.5 mAh g-1, respectively, with a discharge capacity retention of 68.2, 80.5, 93.1, 93.7 and 95.2%, respectively. The low discharge capacity retention of spinel LiCrMnO4 is mainly ascribed to the strong structural distortion (Jahn-Teller effect) of


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Mn3+ in the low voltage (around 3 V).49 The average discharge voltage of the samples are displayed in Figure 6b. As is well known, the Li- and Mn-rich layered cathode materials suffer from the phase transformation from layered to spinel and therefore the voltage decay during cycling.50, 51 For the as-prepared Li-rich layered sample Li1.2Cr0.4Mn0.4O2 (x = 1), the average discharge voltage decrease from 3.50 to 3.12 V after 105 cycles (∆E = 0.38 V), which is the same

as

the

Li-rich

layered

Li-Ni-Mn-O

oxides

(∆E

=

0.38

V

for

the

0.4Li2MnO3·0.6LiNi0.4Co0.1Mn0.5O2 oxide).5 In contrast, for the spinel LiCrMnO4 (x = 0) electrode, the decrease of the average discharge voltage is only 0.20 V after 105 cycles, much lower than other samples. Ni containing spinel-layered Li-Ni-Mn-O composite has been investigated by several groups.37,

49, 52

For comparison, some of the electrochemical data are

summarized in Table S1. As shown in Table S1a and b, The discharge capacities of Li-Cr-Mn-O composite are generally lower than those of the Li-Ni-Mn-O oxides. This phenomenon may be related to the higher catalytic activity of Cr element and therefore the more side reactions between the cathode material and the electrolyte, which can be implied by the lower initial coulombic efficiency of the Li-Cr-Mn-O composite. Nevertheless, for the electrode of x = 0.5 and 0.75, the initial discharge capacities of Li-Cr-Mn-O composites are higher than those of the Li-Ni-Co-Mn-O oxides, which may be owing to the more effective synergy between the spinel and the layered structure for the Li-Cr-Mn-O composites. Figure 6c display the rate capability of (1-x)[LiCrMnO4]·x[Li2MnO3·LiCrO2] electrodes. Obviously, the spinel-layered composite of x = 0.5 and 0.75 exhibit the relatively higher rate capability than other samples, which may be owing to the similar synergistic effect mentioned in Figure 6a. Besides, the nano-size particles discussed in Figure 2 and the less charge transfer resistance (will be mentioned in the later section) may be responsible for the better


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electrochemical performance of x = 0.75 electrode. The surface morphologies of the electrodes after 105 cycles are investigated. As shown in Figure S3, similar morphologies with those of the fresh samples (Figure 2) can be observed, indicating that the surface morphology does not affect the electrochemical performance of the electrode very much. To investigate the kinetics of electrode process of (1-x)[LiCrMnO4]·x[Li2MnO3·LiCrO2] electrodes, electrochemical impedance spectra (EIS) are measured before the initial charging and at fully discharged states at 10th and 20th cycles, as shown in Figure 7. The semicircle of Nyquist plots in the high frequency region is related to the charge-transfer process, and the straight line in the low frequency region is attributed to a semi-infinite Warburg diffusion process in the bulk.43 On the foundation of above analysis, the measured EIS data are fitted with the equivalent circuit in Figure 7d and the simulated electrochemical parameters are shown in Table 1. Obviously, The charge-transfer resistance (Rct) and Warburg diffusion impedance of x = 0, 0.5 and 0.75 electrodes are lower than that of the other two samples, which may be mainly ascribed to the three-dimensional Li+ diffusion channel of spinel structure and the synergistic effect of the spinel and the layered phase mentioned above. The 3D Li+ diffusion channels of spinel (LCMO) structure can enhance the diffusion and charge transfer of lithium-ion through the surface layer, and therefore increase the rate performance. The Li+ diffusion coefficient of the cathodes is calculated based on the EIS results according to the following equation 53 ோమ ் మ

‫ܦ‬௅௜ = మ ర ర మ మ ଶ஺ ௡ ி ஼ ஢ where DLi is the diffusion coefficient of Li+. n, A, D, T, F and C refers to the numbers of electron transferred during oxidization, the surface area of the cathode, the diffusion coefficient of lithium ions, the ideal gas constant, the absolute temperature, the Faraday constant and the Li


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concentration in active material, respectively. σ represents the Warburg factor which can be obtained from the equation Z’ = Rs + Rct + σω-1/2, where ω is the angular frequency (ω = 2πf, f is the frequency in Hz).54, 55 Based on the above analysis, DLi value of the samples at different cycle is displayed in Table 1. Obviously, the Li+ diffusion coefficient of the spinel and spinel-layered composite (x = 0, 0.25 and 0.5) are slightly higher than those of the Li-rich layered oxide (x = 1), which is due to the three-dimensional lithium diffusion channels of spinel phase and in accordance with the electrochemical performance of the samples. 4. CONCLUSIONS New series of spinel-layered cathode materials (1-x)[LiCrMnO4]·x[Li2MnO3·LiCrO2] (x = 0, 0.25, 0.5, 0.75 and 1) are prepared via a sol-gel route for the first time. The effect of spinel contents on the crystal structure, particle morphology, and electrochemical properties of the composites is investigated. XRD results confirm the structural transformation from spinel to an integrated spinel-layered structure and layered structure along with the increase of Li concentration in the composite. The subsequent investigations of HRTEM, SAED and Raman on samples further demonstrate the formation of the integrated spinel-layered structure. Electrochemical performance test implies that the spinel phase and Li-rich layered structure plays synergistic effect in the (1-x)[LiCrMnO4]·x[Li2MnO3·LiCrO2] (x = 0.25, 0.5 and 0.75) composite, in which the structure and electrochemical performance have both the features of spinel and Li-rich layered phase. The Li-ions deintercalated from the Li2MnO3-like structure during the activation process can be accommodated by the spinel component. Consequently, x = 0.5 and 0.75 electrodes exhibit the superior cycle and rate performance, as well as low chargetransfer resistance (Rct) and Warburg diffusion impedance (Wo). This work provide new insights


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into designing and preparing cathode materials with high performance for the lithium-ion batteries.


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FIGURES (a)

(107)

(018) (110) (113)

(104)

x=1

(105)

(101) (006) (102)

Intensity (a.u.)

(003)

Li2MnO3-like

x = 0.75 x = 0.5

30

(533) (622)

(531)

(440)

(511)

(331)

34 36 38 42 44 2θ (degree)

80

(113) L

(110) L

Intensity (a.u.)

(104) L (400) S

(101) L (311) S

19.5

(222) S

Intensity (a.u.)

(006) L (102) L

(111) S

18.5 19.0 2θ (degree)

70

(d)

(c) (003) L

18.0

60

46

62

(531)S

(b)

40 50 2θ (degree)

(018) L

20

(222) (400)

x=0

(440)S

10

(311)

(111)

x = 0.25

Intensity (a.u.)

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


64 66 68 2θ (degree)

70

Figure 1. (a) XRD patterns of (1-x)[LiCrMnO4]·x[Li2MnO3·LiCrO2] cathode materials. (b), (c) and (d) are local magnified areas of (a). The letters L and S behind the Miller indexes refer to the layered and spinel component, respectively.


15


Page 16 of 32

Figure 2. SEM images of (1-x)[LiCrMnO4]·x[Li2MnO3·LiCrO2] cathode materials. (a) x = 0, (b) x = 0.25, (c) x = 0.5, (d) x = 0.75 and (e) x = 1. (a’) is the structure diagram of the typical spinel phase (truncated octahedron).


16

Page 17 of 32 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 3. TEM and HRTEM images of (1-x)[LiCrMnO4]·x[Li2MnO3·LiCrO2] cathode materials. (a) x = 0, (b) x = 0,5 and (c) x = 1. (d), (e) and (f) are the corresponding SAED (selected area electron diffraction) images of (a), (b) and (c), respectively. The letters L and S behind the Miller indexes refer to the layered and spinel component, respectively.


17


Cr3+ 2P3/2

Cr 2p Cr3+ 2P 1/2 Intensity (a.u.)

(e) (d) (c) (b)

Cr6+

Cr6+

(a) 590

585

580

575

570

Binding Energy (ev)

Mn4+ 2P3/2

Mn 2p Mn4+ 2P1/2

(e)

Intensity (a.u.)

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 18 of 32

(d) (c) (b) (a) 655

650

645

640

Binding Energy (ev)

Figure 4. XPS spectra of Cr 2p and Mn 2p for the (1-x)[LiCrMnO4]·x[Li2MnO3·LiCrO2] cathode materials. (a) x = 0, (b) x = 0.25, (c) x = 0.5, (d) x = 0.75 and (e) x = 1.


18

Page 19 of 32

(a) 5.0

(b)

1.5

1st 2nd 3th 105th

4.0 3.5 3.0

1st 2nd 3nd

1.0

Current (mA)

Potential (V)

4.5

0.5

SP

SP

0.0 -0.5

2.5

0

50

100

150

2.0

200

Capacity (mAh g-1)

SP

SP

-1.0

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Potential (V)

(c) 5.0

(d)

1.5

1st 2nd 3rd 105th

4.0 3.5 3.0

1st 2nd 3nd

1.0

Current (mA)

Potential (V)

4.5

0.5

SP L1+SP

L2 SP

L1

0.0

L1+SP

L1

-0.5

2.5

SP

SP -1.0

2.0 0

50

100

150

Capacity (mAh g-1)

200

2.0

250

(f)

3.0

3.5 3.0

1st 2nd 3rd

1.0

Current (mA)

1st 2nd 3th 105th

4.0

0.5

100

150

200

Capacity (mAh g-1)

4.5

5.0

250

L1+SP L2 SP

L1

0.0

L1 2.0

300

L1+SP

SP

4.0

5.0

SP

-1.0

2.0 50

4.0

SP

-0.5

2.5

0

3.5

1.5

4.5

Potential (V)

2.5

Potential (V)

(e) 5.0

2.5

3.0

3.5

4.5

Potential (V)

(g) 5.0

(h)

L1+SP

1.5

1st 2nd 3rd 105th

4.0 3.5 3.0

Current (mA)

Potential (V)

4.5 1.0 0.5

L1 SP

L2 SP

0.0

SP SP

L1+SP L1

-1.0

2.0 0

(i)

1st 2nd 3rd

-0.5

2.5

50

100

150

200

Capacity (mAh g-1)

250

2.0

300

2.5

3.0

3.5

4.0

4.5

5.0

Potential (V)

5.0

( j)

1.5

1st 2nd 3rd 105th

4.0 3.5 3.0

Current (mA)

4.5

Potential (V)

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


0.5

L2

L1

1st 2nd 3rd

1.0

0.0

-0.5

2.5

L1 -1.0

2.0 0

50

100

150

200

-1

250

300

Capacity (mAh g )

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Potential (V)

-1

Figure 5. Charge-discharge curves of the initial three cycles (20 mA g ), the 105th cycle (40 mA g-1) and CV profiles (0.1 mV s-1) of the (1-x)[LiCrMnO4]·x[Li2MnO3·LiCrO2] cathode materials between 2.0 and 4.95 V. (a), (b) x = 0; (c), (d) x = 0.25; (e), (f) x = 0.5; (g), (h) x = 0.75 and (i), (j) x = 1. The CV peaks of L1, L2 and SP indicate contributions from LiCr0.5Mn0.5O2, Li2MnO3like (L2) and spinel LiCrMnO4 (SP), respectively.


19


Discharge capacity (mAh g-1)

(a) 250

0.1 C

200 150

0.2 C

100

x=1 x = 0.75 x = 0.5

50 0 0

x = 0.25 x=0

20

40

60

80

100

80

100

Cycle number

Average voltage (V)

(b)

4.0

0.1 C

3.5 3.0 2.5

0.2 C 2.0

x=1 x = 0.75 x = 0.5

1.5 1.0

0

20

x = 0.25 x=0 40

60

Cycle number

(c) 250 Discharge capability (mAh g-1)

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 20 of 32

200

0.5 C 1C

150

0.1 C

2C

0.2 C

0.1 C 5C

100 50

x=1 x = 0.75 x = 0.5

0 0

5

10

x = 0.25 x=0 15

20 25 30 Cycle number

35

40

45

Figure 6. Cycle performance of (1-x)[LiCrMnO4]·x[Li2MnO3·LiCrO2] cathode materials between 2.0 and 4.95 V (1C = 200 mA g-1). (a) Discharge capacity and (b) average discharge voltage vs. cycle number. (c) Rate capabilities of the as-prepared cathode materials between 2.0 and 4.95 V.


20

(b)

180

Fresh cell x=1 x = 0.75 x = 0.5 x = 0.25 x=0

150 120 90

180 150

10th x=1 x = 0.75 x = 0.5 x = 0.25 x=0

-Z''/Ω

(a)

120 90 60

60

30 15

0

0

0

90

Z'/Ω

180 150 120 90

-Z''/Ω

x=1 x = 0.75 x = 0.5 x = 0.25 x=0

30

0

20th

60

90

Z'/Ω

12

0 15

30 0

0 12

(c)

60

30

0

60 30 12

0 15

0

0

90

60

30

0

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


-Z''/Ω

Page 21 of 32

Z'/Ω

Figure 7. Electrochemical impedance spectra of (1-x)[LiCrMnO4]·x[Li2MnO3·LiCrO2] cathode materials before the initial charging and fully discharged states at 10th and 20th cycles (a - c) and the equivalent circuit used to fit the experimental data (d). Rs is solution resistance, Rct is charge-transfer resistance, CPE1 is constant phase element and Wo is assigned to the finite Nernst semi-infinite Warburg diffusion impedance in the bulk.


21


Page 22 of 32

TABLES.

Table 1. The simulated results for the EIS spectra and the Li+ diffusion coefficient (DLi) of the (1-x)[LiCrMnO4]·x[Li2MnO3·LiCrO2] samples. Composites

x=0

x = 0.25

x = 0.5

x = 0.75

x=1

Cycle

Rs (Ω)

Rct (Ω)

Wo (Ω)

DLi

0

1.47

88.5

479

1.6×10-14

10

5.53

87. 6

853

5.1×10-12

20

4.34

26.2

809

1.8×10-12

0

1.74

124

512

2.6×10-14

10

8.45

58.1

1283

1.3×10-12

20

5.86

29.6

1315

2.7×10-12

0

3.72

123

403

7.7×10-14

10

9.39

34.5

1426

1.2×10-12

20

7.63

30.0

1607

1.8×10-12

0

1.92

126

691

1.9×10-14

10

10.3

43.8

396

4.8×10-13

20

5.17

24.9

1089

1.1×10-13

0

3.31

118

656

5.1×10-14

10

8.59

71.9

905

1.8×10-13

20

9.14

86.2

1753

1.1×10-14


22

Page 23 of 32 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


ASSOCIATED CONTENT

Information.

Supporting

Raman spectra, EDS results of (1-x)[LiCrMnO4]·x[Li2MnO3·LiCrO2] cathode and SEM images of (1-x)[LiCrMnO4]·x[Li2MnO3·LiCrO2] electrode after 105 cycles, and the comparison of the electrochemical data between the spinel-layered Li-Ni-Co-Mn-O and Li-Cr-Mn-O composites. AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] (Hongzhou Zhang) *E-mail: [email protected] (Lianqi Zhang)

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGMENT This work was financially supported partly by National Key R&D Program of China (2017YFB0102000), NSFC (21503148) and Tianjin Sci. & Tech. Program (16JCQNJC03300, 15ZCZDGX00660, 16YFZCGX00250). REFERENCES


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