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Layered Lithium-Rich Oxide Nanoparticles Doped with Spinel Phase: Acidic Sucrose-Assistant Synthesis and Excellent Performance as Cathode of Lithium Ion Battery Min Chen, Dongrui Chen, Youhao Liao, Xiaoxin Zhong, Weishan Li, and Yuegang Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10219 • Publication Date (Web): 22 Jan 2016 Downloaded from http://pubs.acs.org on January 25, 2016

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ACS Applied Materials & Interfaces

Layered Lithium-Rich Oxide Nanoparticles Doped with Spinel Phase: Acidic Sucrose-Assistant Synthesis and Excellent Performance as Cathode of Lithium Ion Battery

Min Chena, Dongrui Chena, Youhao Liaoa,b, Xiaoxin Zhonga, Weishan Lia,b*, Yuegang Zhanga

a. School of Chemistry and Environment, South China Normal University, Guangzhou 510631, China b. Engineering Research Center of MTEES (Ministry of Education), Research Center of BMET (Guangdong Province), Engineering Lab. of OFMHEB (Guangdong Province), Key Lab. of ETESPG (GHEI), and Innovative Platform for ITBMD (Guangzhou Municipality), South China Normal University, Guangzhou 510006, China

ABSTRACT Nano-layered lithium-rich oxide doped with spinel phase is synthesized by acidic sucrose-assistant sol-gel combustion and evaluated as cathode of high energy density lithium ion battery. Physical characterizations indicate that the as-synthesized oxide (LR-SN) is composed of uniform and separated nanoparticles of about 200 nm, which are doped with about 7% spinel phase, compared to the large aggregated ones of the

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product (LR) synthesized under the same condition but without any assistance. Charge/discharge demonstrates that LR-SN exhibits excellent rate capability and cyclic stability: delivering an average discharge capacity of 246 mAhg-1 at 0.2 C (1C = 250mA g-1) and earning a capacity retention of 92% after 100 cycles at 4C in the lithium anode-based half cell, compared to the 227 mAg-1 and the 63% of LR, respectively. Even in the graphite anode-based full cell, LR-SN still delivers a capacity of as high as 253 mAh g-1 at 0.1C, corresponding to a specific energy density of 801 Wh kg-1, which are the best among those that have been reported in literature. The separated nanoparticles of the LR-SN provide large sites for charge transfer, while the spinel phase doped in the nanoparticles facilitates lithium ion diffusion and maintains the stability of the layered structure during cycling. KEYWORDS: Layered lithium-rich oxide, Nanoparticles, Spinel phase doping, Cathode, Lithium ion battery.

1. INTRODUCTION Lithium-ion battery has been widely used in mobile electronic devices, because it has many advantages, including long cyclic life, high energy density and environmental friendliness, compared to other secondary batteries1-3. However, the current lithium ion battery cannot meet the increasing demands for high energy density in large-scale applications such as in the field of electrical vehicles4-5. This problem results mainly from the low specific capacity of currently used cathode materials, such as layered LiCoO2 (~140 mAh g-1, upper cut-off voltage of 4.2 V)6,

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spinel LiMn2O4 (~120 mAh g-1)7, olivine LiFePO4 (~170 mAh g-1)8. Layered lithium-rich oxide, xLi2MnO3 · (1−x) LiMO2 (M = Mn, Ni, Co, Fe, Cr, etc.) composed of the intergrowth of LiMO2 R-3m and Li2MnO3 C2/m phases, has attracted much attention because it can work under higher voltage than other layered oxides9,10 and deliver a specific capacity of more than 250 mAh g-110-12. Unfortunately, the poor rate capability and cyclic stability of layered lithium-rich oxide limits its application in practice13,14. The poor rate capability is related to two-dimensional channels of the layered structure, which does not facilitate the lithium ion transportation compared to the three-dimensional channels of spinel structure4. The poor cyclic stability results mainly from the over-lithiation of layered structure, which leads to the structural destruction and the formation of inactive phase 11. In order to improve the rate capability and poor cyclic stability, many approaches have been developed, including making nanoparticles15, doping with other elements 16 and coating with inert compounds such as AlPO4 and carbon17-20. Up to date, however, these approaches have not yet yielded a satisfactory improvement. Recently, it was reported that the rate capability and cyclic stability of layered lithium-rich oxide could be improved by treating oxide in the solutions containing NH4HF2 and AlF321, 22. With these treatments, lithium in Li2MnO3 is leached partially forming fluorite coating, which is accompanied by the formation of active spinel Li[Mn2-xMx]O4 (M = Ni and Co) at high temperature. The active spinel Li[Mn2-xMx]O4 possesses three dimensional channels for lithium ion diffusion and benefits the rate capability. On the other hand, the faster lithium transportation in the three dimensional channels reduces

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the over-lithiation of layered structure and benefits the cyclic stability. Additionally, the active spinel Li[Mn2-xMx]O4 is more stable structurally than layered Li2MnO3 and its existence reduces the direct contact of Li2MnO3 with electrolyte and also benefits the cyclic stability of layered lithium-rich oxide23-31. However, the procedure in these treatments is complicated and the introduction of inert fluorine-containing compounds leads to the decreased capacity of layered lithium-rich oxide. Furthermore, the excess Li[Mn2-xMx]O4 phase causes the capacity loss due to the far lower specific capacity of the spinel Li[Mn2-xMx]O4 than the layered lithium-rich oxide32. Therefore, it is a challenge to synthesize layered lithium-rich oxide with good rate capability and cyclic stability. In this work, we proposed a facile synthesis of layered lithium-rich oxide. A representative layered lithium-rich oxide, Li1.2Mn0.54Ni0.13Co0.13O2, was synthesized by acidic sucrose-assistant sol-gel combustion method. Sucrose and nitric acid were introduced, which coordinate metal transition ions, Mn2+, Ni2+ and Co2+, and disperse these ions at a molecular level in viscous gel. When the gel is heated, sucrose acts as reductant to exhaust oxygen, which is beneficial to the transformation of Li2MnO3 to active Li[Mn2-xMx]O4 (M= Mn, Co, or Ni) under acidic environment, resulting in uniform Li1.2Mn0.54Ni0.13Co0.13O2 nanoparticles with doping active Li[Mn2-xMx]O4 phase in a small amount32. The formation process is illustrated schematically in Scheme 1.

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Scheme 1. Schematic illustration on the formation process of LR-SN.

2. EXPRIMENTAL SECTION Materials synthesis: Li1.2Mn0.54Ni0.13Co0.13O2 was synthesized by sol-gel combustion method, in which sucrose was served as complex and combustion agent, nitric acid was used to adjust the pH value of the medium and also as a complex for the transition metal ions. Typically, 1.35 g lithium acetate (LiCH3COO·2H2O), 1.32 g manganese

acetate

(Mn(CH3COO)2·4H2O),

0.32g

nickel

acetate

(Ni(CH3COO)2·4H2O), 0.32g cobaltous acetate(Co(CH3COO)2·4H2O), and 2 g sucrose (C12H22O11) were dissolved in 100 mL distilled water under constant magnetic stirring. Stoichiometrically, 5wt. % excess lithium source was added to compensate the loss during high temperature calcination treatment. After continuous stirring for 30 min, 20 mL 10 wt % dilute nitric acid was added to the above solution, resulting in reddish brown sol, which was evaporated at 90 ℃ to get viscous gel and then heated to 200 ℃ to get brown foamy mass precursor. Nitric acid was used to adjust pH 5



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value to about 2. Higher concentration nitric acid would react with sucrose, which is not beneficial to the formation of uniform and separated nanoparticles. Lower concentration nitric acid cannot provide an acidic environment. The final product was obtained by presintering at 450 ℃ for 5 h and calcining at 800 ℃ under air atmosphere for 12 h. The obtained sample was marked as LR-SN. For comparison, the sample synthesized without using sucrose and dilute nitric acid was denoted as LR.

Physical characterizations: The structural analyses of as-synthesized samples were performed on a powder diffractometer (Bruker D8 Advanced Diffractometer System, Germany) with Cu-Kα radiation and a Raman microscope (LabRAM Aramis at 532 nm excitation). The diffraction patterns were recorded in the 2θ degree range from 10° to 80° with a scanning step of 5° min-1. Tap density was determined in a small cylindrical centrifuge tube by taping till the material volume maintains unchanged. The morphologies of the obtained samples were observed by emission scanning electron microscopy (SEM, JSM-6380, Japan) and transmission electron microscopy (TEM, JEOL JEM-2100HR). The chemical states of the samples were investigated by X-ray photoelectron spectroscopy (XPS, Gnesis2000). Thermogravimetric analysis (TGA) was carried out on a Perkin-Elmer TGA 7 at a heating rate of 10 ℃ min-1 in flow air from room temperature to 800 ℃. The specific surface area was determined with Bruner–Emmett–Teller (BET, Micromeritics ASAP 2020 M) at the temperature of liquid nitrogen (77 K).

Electrochemical measurements: Electrochemical behaviors of as-synthesized samples

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were determined by using CR2025 type coin cell, which was assembled in a glove box (MBraun, Germany) under an argon atmosphere (H2O, O2 < 1ppm) at room temperature (25℃). The Li1.2Mn0.54Ni0.13Co0.13O2 electrode (12 mm in diameter) was prepared by casting mixed slurry consisting of 80 wt. % active material, 10 wt. % acetylene black and 10 wt. % polyvinylene difluoride (PVDF) in N-methyl pyrrolidone (NMP) on aluminum foil and then drying in a vacuum oven at 120℃ overnight. The areal cathode loading was about 2.2 mg cm-2. Metallic lithium foil was used as the counter electrode in the half cell. The graphite electrode (12.5 mm in diameter) was fabricated by coating a mixture of 89 wt. % graphite, 4 wt. % Timrex ks6, 2 wt. % super-p, and 5 wt. % PVDF in NMP onto a Cu current collector, and used as anode in the full cell. The areal anode loading was about 2.4 mg cm-2, which has an excess of 16 % compared to the cathode. Charge/discharge tests were performed on LAND CT 2001A battery test system (China) between 2.0-4.8 V (vs. Li+/Li) for half cell and 0-4.8 V for full cell. 1 M LiPF6 in a mixed solvent of EC, DEC and EMC (EC:DEC:EMC = 3:2:5 in weight) was used as the electrolyte, and Cellgard 2300 as the separator. Electrochemical impedance spectroscopy (EIS) was measured on Auto lab (PGSTAT-30, Eco Echemie B.V. Company) in the frequency range from 100 kHz to 0.01 Hz.

3. RESULTS AND DISCUSSION

Formation process: Thermogravimetric analysis (TGA) was carried out to understand the formation process of LR-SN. Fig. 1(A, B) presents the TG and DTA curves of the

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precursors under an air flow condition with a heating rate of 10 °C min-1. In the initial heating stage (at the temperature lower than 120 ℃, the precursors of LR and LR-SN behave similar, undergoing a fast weight loss corresponding to the evaporation of water in the precursors. From 120 to 300 °C, the precursor of LR does not show a significant weight loss, while that of LR-SN undergoes continuously fast weight loss, which should be ascribed to the decomposition of sucrose releasing carbon dioxide. Sucrose is important for the formation of uniform and separated nanoparticles32. The effect of sucrose on the particle sizes can be confirmed by the BET analysis. Fig. 1C and D presents the nitrogen adsorption and desorption isotherms of LR-SN and LR. The obtained specific surface area is 7.93 m2 g-1 for LR-SN and 3.26 m2 g-1 for LR, while the tap density is 1.32 g cm-3 for LR-SN and 1.30 g cm-3 for LR, suggesting that the particles in LR-SN is smaller or separated compared to those in LR. At above 300 ℃ in Fig. 1(A, B), similar weight loss can be identified for both precursors, which should be ascribed to the decomposition of acetates. The exothermic peak at around 500 °C could be assigned to the formation of layered lithium-rich oxides 32-35.

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Fig. 1. TG/DTA curves of LR-SN (A) and LR (B) precursor and nitrogen adsorption and desorption isotherms curves of LR-SN (C) and LR (D).

Crystal structure and morphology: The crystal structure of the obtained samples was determined by XRD analysis. Fig. 2 presents the XRD patterns of LR-SN and LR. It can be seen form Fig. 2 that both samples has similar XRD patterns, characteristic of the main peaks of the hexagonal a-NaFeO2 structure with space group R-3m, and the weak peaks between 20° and 25° corresponding to the monoclinic Li2MnO3 with space group C/2m. Apparently, both samples exhibit a typical layered structure. The intensity ratio of I(003)/I(104) is 1.25 for LR-SN and 1.21 for LR. The hexagonal lattice parameters were refined to be a= 2.8541 Å and c= 14.2201 Å with c/a ratio of 4.98 for LR-SN, and a= 2.8533 Å and c= 14.2168 Å with a c/a ratio of 4.98 for LR13,18. It can be noted that the peaks of LR-SN exhibits slight broader, which are 9



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more clearly identified by the magnified XRD patterns on the right of Fig. 2. These broad peaks can be ascribed to the X-ray reflections for the spinel structure (Fd3m)24,30, indicating that spinel phase has been formed in LR-SN but not in LR. Raman spectroscopy was used to confirm the existence of spinel phase in LR-SN, the obtained results are presented in Fig. S1. The Raman spectra of two samples contain three peaks near 600, 485 and 430 cm-1, corresponding to two general Raman active vibrations Eg and A1g in the layered lithium-rich oxide. A1g represents the symmetrical stretching of M-O and Eg represents symmetrical deformation. It is accepted that the characteristic bands of spinel phase are at about 630-636 cm-1 36,37 and near 600 cm-1 38. Although LR-SN does not exhibit clearly these characteristic bands, possibly because of the small amount of spinel phase, the difference in Raman spectra between LR-SN and LR can be identified from Fig.S1. Compared with LR, the peak near 600 cm-1 becomes broader and the peak near 430 cm-1 shift to the left for LR-SN, indicating that spinel phase is present in LR-SN14.

Fig. 2. XRD patterns of LR-SN and LR.

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The morphologies of LR-SN and LR were observed by SEM. The obtained results are presented in Fig. 3A and B. LR-SN consists of uniform and separated nanoparticles of about 200 nm (Fig.3A), compared to the aggregated particles of as large as 1 µm (Fig. 3B). This observation is in agreement with the analyses from BET, indicating that the introduction of sucrose facilitates the formation of uniform and separated layered lithium-rich oxide nanoparticles. The nanoparticles provide larger sites for charge transfer and thus benefit rate capability of the layered lithium-rich oxide39-43. TEM observation were performed to confirm the existence of the spinel Li[Mn2-xMx]O4 in LR-SN, the obtained results are presented in Fig. 3C-H. Under the low magnification, the separated nanoparticles in LR-SN can be clearly identified, as shown in Fig. 3C. With high-resolution transmission electron microscopy (HRTEM), it can be found that the nanoparticle of LR-SN consists of layered and spinel phases (Fig. 3D). More detailed HRTEM images of spinel phase and layered phase are shown in Fig. 3E and F. The lattice spacing of 0.147 nm and 0.200 nm can be assigned to the planar spacing of (440) and (400) planes in the spinel phase, while the lattice fringes with interlayer spacing of 0.47 nm correspond to the (003) plane in layered phase 34,44-49

. The Fast Fourier Transformation (FFT) images (Fig. 3 G and H) further

confirms the phase composition in LN-SN nanoparticle. Under acidic sucrose medium, Mn4+ in layered LiMnO3 partially changes to Mn3+, forming spinel Li[Mn2-xMx]O4.

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Fig. 3. SEM images of LR-SN (A) and LR (B); TEM (C) and HRTEM (D) images, detailed crystal planes (E, F), and corresponding Fast Fourier Transform Algorithm pattern (G, H) of LR-SN. X-ray photoelectron spectroscopy was carried out to determine the chemical states of the products. Fig. 4 shows the detailed XPS spectra of LR-SN (Fig. 4A-C, G) and LR (Fig. 4D-F, H). The bonding energy of 780.21 eV for LR-SN or 780.18 eV for LR is close to reported band energy of Co3+, two weak peaks near 790 eV and 805 eV in the Co 2p spectra represent Co3+ ion in an octahedral environment3, 5, 23. The band energy of 855.02 eV for LR-SN or 855.11 eV for LR in Ni 2p3/2 is attributed to the Ni2+ in layered compound23,44. As shown in Fig. 4 A, B, D, and E, there is no significant difference in the spectra of Co3+ and Ni2+. As for Mn 2p3/2 spectra (Fig. 4 C

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and F), the bonding energy is 642.65 eV for LR, which fits well with Mn4+ ions in manganese-based layered compounds 5,44, but shifts to a lower energy of 642.34 eV for LR-SN, suggesting that Mn4+ in layered LiMnO3 partially turns to Mn3+. Mn 3s spectra were obtained to evaluate the content of Mn3+ in the products, as showing in Fig. 4G and H. The energy difference of splitting peaks is 5.5 eV for Mn3+ and 4.6 eV for Mn4+ 45. Based on the fitting by Gaussian-Lorentzian, content of Mn3+ is 9.7% for LR-SN and 2.9% for LR, indicating that there is a transformation of about 7% Mn4+ to Mn3+ due to the contribution of sucrose. With this transformation, it can be estimated that there is about 7% Li[Mn2-xMx]O4 doped in layered structure in LR-SN. As indicated by the HRTEM image (Fig. 3D), the doping takes place on the surface of nanoparticle and among the layered phases.

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Fig. 4. Typical XPS profiles of Co 2p, Ni 2p, Mn 2p and Mn 3s for LR-SN(A, B, C, G) and LR(D, E, F, H).

Electrochemical performance: The electrochemical performances of LR-SN and LR were

first

evaluated

in

lithium

anode-based

half

cell

by

galvanostatic

charging/discharging test in a potential window of 2.0-4.8 V at room temperature. The low potential limitation of 2.0 V was set to understand the electrochemical performance under low potential. Fig. 5 presents the first charge/discharge curves and the corresponding differential profiles of LR-SN and LR at a current density of 50 mA

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g-1. It can be seen from Fig. 5 that LR-SN and LR has a similar lithium insertion/extraction behavior. There are a slope region blow 4.5V and a long plateau region around 4.5V in the charge curves (Fig. 5 A and B), which is a typical feature of layered lithium-rich oxide. The former is attributed to Li+ extraction from the lithium layer, which is accompanied by the oxidation of Ni2+ to Ni3+/Ni4+ and Co3+ to Co4+, while the latter corresponds to Li+ extraction from Li2MnO3, which is accompanied with the irreversible oxygen release47-48. These processes can be more clearly identified by the differential charge and curves (Fig. 5 C and D): the first oxidation peak at around 4.0 V can be ascribed to oxidation of Ni2+ and Co3+, while that at around 4.5 V can be assigned to the irreversible oxygen release49. Accordingly, three peaks appear in discharge profiles for two samples: Re1 at around 4.5 V is related to Li occupation in tetrahedral sites, Re2 at around 3.5 V can be ascribed to Li occupation in octahedral sites corresponding to the reduction of Ni4+ and Co4+ and Re3 at lower than 3.5 V can be attributed to the Li occupation in octahedral sites. LR-SN delivers a discharge capacity of 246 mAh g-1 with a coulombic efficiency of 75%, but these values decrease to 227 mAh g-1 and 67% for LR, respectively. Apparently, LR-SN has a small irreversible capacity loss (81 mAh g-1) than LR (108 mAh g-1). This performance is better than that reported in literature34. It is generally accepted that smaller particle size and high BET are not beneficial to gain high initial coulombic efficiency for electrode materials. The higher initial coulombic efficiency of LR-SN than LR should be related to the spinel phase that maintains the structural integrity of the layered oxide. It can be noted that there is additional reduction peak

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(Re4) at around 2.8 V in LR-SN, which is involved with the transformation from spinel Li[Mn2-xMx]O4 to rock salt Li2[Mn2-xMx]O419,34,50, but not in LR. This difference indicates that the synthesis with assistance of sucrose and nitric acid suppresses the irreversible oxygen release and induces the formation of spinel Li[Mn2-xMx]O4 in the product, leading to the improved performance of LR-SN.

Fig. 5. Initial charge/discharge curves of LR-SN (A) and LR (B) at a current density of 50 mA g-1 between 2.0 - 4.8V and the corresponding dQ/dV profiles of LR-SN (C) and LR (D).

To estimate the amount of spinel Li[Mn2-xMx]O4 in LR-SN, the quantitative correlation of the charge/discharge capacities with the lithium extraction/insertion processes is established, as shown in Fig. 6. During charging, LR-SN has a larger 16


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charge capacity of about 128 mAh g-1 under 4.5V than LR (119 mAh g-1), indicating that more transition metal ions that can be oxidized are available in LR-SN than in LR, which can be ascribed to the oxidation of Mn3+. At the potential close to 4.5 V, the charge capacity is mainly attributed to lithium extraction from Li2MnO3, which is accompanied with oxygen evolution. LR-SN has a smaller capacity than LR at this stage, explaining the formation of spinel phase in LR-SN. During discharge, LR-SN delivers a total capacity of 246 mAh g -1, larger than LR (227 mAh g -1). This capacity includes the reduction of Ni4+ and Co4+ at the potentials from 4.8 V to 3.3 V and that of Mn4+ from 3.3 V to 2.0 V. LR-SN has a capacity of 92 mAh g -1 for the reduction of Mn4+, 10 mAh g -1 larger than LR (82 mAh g -1). This excess is from the spinel phase. Assuming the specific capacity of spinel phase is 147 mAh g -1, the estimated content of spinel phase in LR-SN is 7%, which is in agreement with the XPS analysis.

Fig. 6. Quantitative correlation of the charge/discharge capacities with the 17



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lithium extraction/insertion processes. Fig. 7 present the rate capabilities of LR-SN and LR from 25 to 1000 mA g-1 between 2.0 - 4.8 V (vs. Li+/Li) at room temperature. It can be seen from Fig. 7A that the discharge capacities at different current densities decrease with increasing the applied current density for two samples, which could be explained by the increased polarization at high current density. However, the magnitude in capacity decrease is different, from 266 mAh g-1 at 25 mA g-1 at to 128 mAh g-1 at 1000 mA g-1 for LR-SN, but from 257 mAh g-1 to 89 mAh g-1 for LR. This difference indicates that LR-SN exhibits excellent rate capability, which should be attributed to the nanoparticle size and the spinel phase doping. The nanoparticles provides large sites for lithium insertion/extraction29,34,51-52.

One

other

hand,

the

spinel

phase

provides

three-dimensional channels for lithium diffusion, as shown in Scheme 1. The combination of these factors leads to the accelerated lithium insertion/extraction kinetics and the increased rate capability of LR-SN compared to LR that consists of aggregated particles without spinel phase doping.

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Fig. 7. Dependence of discharge capacity and capacity retention of LR-SN and LR on different current rates (A, B) and electrochemical impedance spectra (C) and Warburg impedance (D) of LR-SN and LR.

The accelerated lithium insertion/extraction kinetics of LR-SN can be confirmed by electrochemical impedance measurements, which were performed at fully lithium inserted state (2V). Fig. 7C presents the electrochemical impedance spectra of LR-SN and LR after 100 cycles at 50 mA g-1. As seen from Fig. 7C , the impedance spectra are characteristic of an arc at high frequencies, which reflects interface impedance between materials and electrolyte, and a slope line at low frequencies, which reflects the Warburg impedance for lithium ion diffusion34. The interface resistance can be estimated from the diameter of the arc at high frequencies51, while the lithium diffusion coefficient can be evaluated by the slope (σ) of linear relation of the

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impedance with the frequency in Fig. 7D based on Eq(1)2,18, 53: DLi+ = R2T2/2n4A2F4C2σ

(1)

where R is the ideal gas constant, T is the absolute temperature, n is the number of electron(s) per molecule oxidized, A is the surface area of the electrode (BET specific surface area was used in this work), F is the Faraday constant, C is the concentration of Li+ in active material that is determined by its lithium-inserted state, and σ is the Warburg factor. The obtained interfacial resistance and lithium diffusion coefficient are 239 Ω and 8.7 × 10-14 cm2 s-1 for LR-SN and 504 Ω and 5.4 × 10-14 cm2 s-1 for LR, respectively, indicative of the accelerated lithium insertion/extraction kinetics of LR-SN compared to LR. The electrochemical impedance spectra of the electrodes at different potential were also obtained. Fig. S2 A and B presents the spectra obtained at 3.5, 4.0, 4.5, and 4.8V after initial charge process at a current density of 25 mA g-1. Two samples show similar impedance spectra. These spectra can be fitted with the equivalent circuit of Fig. S2 C, in Re is the Ohmic resistance, Rf and Cf represent the SEI film, Rct and Cdl involve the charge transfer, and W is the Warburg impedance53,54. When the values of Cf and Cdl are close, two semicircles will be merged to one pressed semicircle, as shown in Fig. 7C. The sum Rtotal of Rf and Rct is the interfacial resistance. The fitting results are presented in Table S1. At any potential LR-SN exhibits smaller interfacial resistance than LR, confirming the accelerated lithium insertion/extraction kinetics of LR-SN. Galvanostatic intermittent titration technique (GITT) was also carried out to determine Li+ diffusion coefficients (DLi+), Fig. S3 (A to D) shows the GITT curves of

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LR-SN and LR in the first charge process, including the slope region below 4.45V and a plateau at around 4.5V. Obviously, LR-SN has smaller polarization than LR. Fig. S3E shows a typical t versus E profile for a selected titration. According to the GITT measurement, the chemical diffusion coefficient of Li+ can be calculated by the following Eq(2)52: ∆ ≪ ) ( ) ( ) ( ⁄)

DLi+ = (

(2)

where VM is the molar volume of samples, 20.14 cm3 mol-1 for LR-SN and 19.91 cm3 mol-1 for LR which is deduced from the crystallographic data, M and m are the molecular weight and the mass of the active material, A is generally the specific surface area of active material, and L is the thickness of the electrode53,55,56. Since E versusτ1/2 is linear over the entire time period of current flux (Fig. S3F), then Eq (2) can be further simplified as Eq.(3): ∆ ) ( ∆ )

DLi+ = (

(3)

The obtained DLi+ values at different potentials are presented in Fig. S3G. The diffusion coefficients strongly depended on the potential, at the level of 10-14 cm2s-1 in the slope region but decreased to 10-19 cm2s-1 in the middle of the plateau, and slightly increased to 10-17 cm2 s-1 at the end of charge process. This difference results from the slow Li+ diffusion in Li2MnO3 than in LiMO2(M= Mn, Ni, Co). The extremely small DLi+ values in the plateau region result from the structural rearrangement due to the oxygen loss. At any potential, LR-SN exhibits higher DLi+ values than LR, which can be assigned to LR-SN with spinel phase. Fig. 8 further compares the cyclic stability of LR-SN and LR at 1000 mA g-1. It

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can be seen from Fig. 8 that LR-SN exhibits better cyclic stability and higher coulombic efficiency, especially at high current rate. The capacity retention is 92% for LR-SN but only 67% for LR after 100 cycles at 1000 mA g-1. At a small current rate (50 mA g-1), LR-SN also shows better cyclic stability than LR, as shown in Fig. S4. The better cyclic stability of LR-SN than LR should be related to the improved structural stability of LR-SN. The faster lithium transportation in the three dimensional channels reduces the over-lithiation of layered structure. Besides, the active spinel Li[Mn2-xMx]O4 is more stable structurally than layered Li2MnO3 and its existence reduces the direct contact of Li2MnO3. These factors contribute to the improved cyclic stability of LR-SN.

Fig. 8.

Cyclic stability of LR-SN and LR at 1000 mA g-1 in coin-type half cells

between 2.0 - 4.8 V.

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The application of LR-SN in practice was preliminarily evaluated in a graphite anode-based full cell where the active mass of cathode material was about 2.5 mg with and anode excess of 16%. The obtained result is presented in Fig. 9. As shown in Fig. 9A, the full cell exhibits similar charge/discharge performances to the lithium anode-based half cell (Fig. 5A), with higher discharge voltage plateaus and large discharge capacity. The full cell delivers an initial discharge capacity of as high as 253 mAh g-1, which is equal to a specific energy density of 801 Wh kg-1. Based on the tap density of LR-SN (1.32 g cm3), the full cell has a volumetric energy of 1057 Wh L-1. When the full cell is performed with a low current of 25 mA g-1 between 0 and 4.8 V, it also exhibits good cyclic stability, as shown in Fig. 9 B and C. These performances are better than those that have been reported in literature57,58, indicating that our product LR-SN is a promising cathode candidate for high energy density lithium ion battery.

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Fig. 9. Charge/discharge curves at selected cycles (A), discharge capacity(B) and energy density (C) of in LR-SN/graphite full cell at 25 mA g-1 between 0 - 4.8 V.

4. CONCLUSION A novel layered lithium-rich oxide has been successfully synthesized by acidic sucrose-assistant combustion method. Under the assistance of acidic sucrose, the resulting oxide possesses a unique configuration, consisting of uniform and separated nanoparticles that are doped with small amount of spinel phase Li[Mn2-xMx]O4, and thus exhibits excellent rate capability and cyclic stability. The separated nanoparticles of the LR-SN provide large sites for charge transfer, while the spinel phase doped in the nanoparticles facilitate lithium ion diffusion and maintain the stability of the layered structure during cycling. When it is evaluated in the graphite anode-based full 24


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cell, the resulting oxide delivers a capacity of as high as 253 mAh g-1 at 0.1C, corresponding to a specific energy density of 801 Wh kg-1. These performances are better than those that have been reported in literature, indicating that the resulting oxide is a promising cathode candidate for high energy density lithium ion battery.

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ASSOCIATED CONTENT Supporting Information Available: Raman spectra, electrochemical impedance spectra at different charge states, GITT curves and cyclic stability at low current density. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author

*[email protected] Present Address School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China. Author Contribution The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGEMENTS This work is financially supported from the National Natural Science Foundation (Grant No. 21273084), the joint project of National Natural Science Foundation of China and Natural Science Foundation of Guangdong Province (Grant No. U1134002), the key project of Science and Technology in Guangdong Province (Grant No. 2012A010702003), and the scientific research project of Department of Education of Guangdong Province (Grant No. 2013CXZDA013).

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a xLi2MnO3·(1 -x)LiMn0.4Ni0.4Co0.2O2 Cathode and a Hard Carbon Anode. Electrochim. Acta. 2011, 56, 7392-7396. (58) Oh, P.; Ko, M.; Myeong, S.; Kim, Y.; Cho, J. A Novel Surface Treatment Method and New Insight into Discharge Voltage Deterioration for High-Performance 0.4Li2MnO3·0.6LiNi1/3Co1/3Mn1/3O2 Cathode Materials. Adv. Energy Mater. 2014, 4, 1400631.

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Layered Lithium-Rich Oxide Nanoparticles Doped ... - ACS Publications

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