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Synergistic effects of stabilizing the surface structure and lowering the interface resistance in improving the lowtemperature performances of layered lithium-rich materials Shi Chen, Lai Chen, Yitong Li, Yuefeng Su, Yun Lu, Liying Bao, Jing Wang, Meng Wang, and Feng Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13995 • Publication Date (Web): 21 Feb 2017 Downloaded from http://pubs.acs.org on February 23, 2017

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ACS Applied Materials & Interfaces

Synergistic Effects of Stabilizing the Surface Structure and Lowering the Interface Resistance in Improving the Low-Temperature Performances of Layered Lithium-rich Materials Shi Chen,a,b,c Lai Chen,a,c Yitong Li,a,c Yuefeng Su,a,b,c Yun Lu,*,a,c Liying Bao,a,c Jing Wang,a,b,c Meng Wang,d Feng Wu*,a,b,c a

School of Material Science and Engineering, Beijing Key Laboratory of

Environmental Science and Engineering, Beijing Institute of Technology, Beijing, 100081, China b

Collaborative Innovation Center for Electric Vehicles in Beijing, Beijing, 100081,

China c

National Development Center of High Technology Green Materials, Beijing, 100081,

China d

Advanced Manufacture Technology Center China Academy of Machinery Science

& Technology, Beijing, 100083, China

KEYWORDS: lithium-ion batteries, layered lithium-rich cathode, low-temperature performances, surface coating, interface resistance

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ABSTRACT:

Layered

lithium-rich

cathode

material,

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Li1.2Ni0.2Mn0.6O2

was

successfully synthesized by a sol-gel method followed by coating with different amount of Li3-BO3 (LBO, 1wt %, 3wt %, 5wt %). The effects of LBO coating layer on the structure, morphology and low-temperature (-30 °C) electrochemical properties of these materials are investigated systematically. The morphology, crystal structure and grain size of the Li-rich layered oxide are not essentially changed after surface modification; and according to the TEM results, the Li-B-O coating layer exists as an amorphous layer with a thickness of 5~8 nm when the amount is 3wt %. Electrochemistry tests reveal that 3wt % LBO-coated samples presents the best electrochemical capability at low temperature. At -20 °C, the initial discharge capacity of 3wt % LBO-coated sample could retain 45.7% (131.7/288.0 mAh g−1) of that at 30 °C, while the pristine material could only maintain 22.5% (57.5/256.0 mAh g−1). XPS spectra and EIS results reveal that such an enhancement of low-temperature discharge capacity should be attributed to the proper LBO coating layer, which not only endows the modified materials with more stable surface structure, but also lowers the interface resistance of Li+ diffusion through the interface and charge transfer reaction.

1. INTRODUCTION

Owing to the competitive energy and power density of lithium-ion batteries (LIBs), they have gradually conquered the portable electronics market and show full potential 2 ACS Paragon Plus Environment

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for use in electric vehicles (EVs).1-2 However, the traditional cathode materials, including layered LiCoO2, olivine LiFePO4 and layered LiNixCoyMn1-x-yO2,3-4 all deliver low practical capacities, thus hardly meeting the ever increasing demands for devices requiring high energy and power such as EVs. In this regards, layered lithium-rich oxides xLi2MnO3·(1-x)LiMO2 (M = Ni, Mn, Co.) have drawn much interest because of their high specific capacity (over 250 mAh·g−1) when cycling above 4.5 V,5-7 as well as their lower cost and better environmentally benign.8

Nevertheless, the dramatic capacity decline under low temperature is an important drawback for commercial use of lithium-rich cathodes, while the reported researches concerning their low temperature behavior are not so common.9-11 To find out the causes for malfunction of cathode materials at low temperature, we carefully scrutinized the main processes occurring with lithium ions and electrons transfer during cycling as shown in Figure 1a.12-13. During the charge, the electrons firstly run into current collector from the electrode. Secondly, lithium ions diffuse in the bulk phase of electrode materials. Thirdly, the lithium ions pass through the interface between electrode and electrolyte, and finally run into the electrolyte. The discharge process is just reversed. It has been indicated that the low-temperature performances of cathodes can be effectively enhanced by decreasing the charge-transfer resistance of cathode materials, which means the diffusion of lithium ions through the interface of electrode/electrolyte is sensitive to temperature and can be restricted by low temperatures, resulting in poor low-temperature performances.9-11



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As for the aforementioned traditional cathode materials, the efforts expended to address this issue, including increasing the low-temperature ionic conductivities of electrolytes,14 controlling particle size/morphology,15 bulk doping and surface modification.16-20 For instance, the discharge capacity retention of LiFePO4 cathode material decorated with carbon-nanotube at the temperature of -25 °C could raise to 71.4%.21 Surface modification has been proved to be an effective way to enhance the conductive properties and stable the electrolyte/electrode interface for electrodes.22, 23 As an example, SnO2 has been reported to enhance the electrochemical performance of lithium-rich materials by increasing their surface electronic conductibility,24 while surface modification with inactive oxides and phosphates can provide a physical barrier to moderate the corrosion from electrolyte.25-29 Nonetheless, these coating layers are difficult to form a uniform thin layer as they are composed of nanoparticles that trend to self-agglomerate.30 In this sense, lithium boron oxide Li2O-2B2O3 (LBO) glass seems to be particularly suitable for coating ceramics because of its good wetting properties, and relatively low viscosity for formation of homogeneous coating on the surface.31,

32

In addition, LBO glass exhibits good ionic conductivity and

remains stable against the high oxidation potentials of the 4-V when coated on the cathodes.32, 33 Moreover, the strategy of B3+ doping is effective to stabilize crystal structure of lithium-rich materials.34

In this work, because of the aforementioned advantages of LBO glass, it was employed to coating on the surface of lithium-rich cathode materials as show in


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Figure 1, and Li1.2Ni0.2Mn0.6O2 was chosen as a host Li-rich material to investigate the effect of LBO coating on the low-temperature performances of Li-rich materials.

2. EXPERIMENTAL SECTION 2.1. Synthesis of LBO-coated Li1.2Ni0.2Mn0.6O2 cathode material. The pristine Li1.2Ni0.2Mn0.6O2 powder was synthesized by a facile sol-gel method followed by high temperature calcination. The citric acid was employed as a chelating agent here, and the mole ratio of citric acid and metal ion is 1:1. The homogeneous and transparent solution

of

mental

ion

was

composed

by

stoichiometric

amounts

of

Li(CH3COO)·2H2O (5wt % excess to offset the lithium evaporative losses), Ni(CH3COO)2·4H2O, and Mn(CH3COO)2·4H2O. During the reaction of these two solutions, the synthetic pH value of the mixed solution was adjusted to ∼7.0 by ammonium hydroxide. And the reaction solution was heated at 80 °C with continuous stirring to form a high viscous gel. After drying at 120 ºC for 12h, the gel was fired at 500 °C for 5h firstly, and then at 900 °C for 12h in air.

To modify the Li1.2Ni0.2Mn0.6O2 material with LBO, LiOH·H2O and H3BO3 with the stoichiometric amount were dissolved in deionized water with a designed amount of LBO (0wt %, 1wt %, 3wt %, 5wt % of Li1.2Ni0.2Mn0.6O2 powder). Then the as-prepared Li1.2Ni0.2Mn0.6O2 sample was slowly poured into the LBO solution. The above-presented solution was constantly stirred at 80°C until the water was evaporated totally. The resulting powder was heated at 500ºC for 3h in air and cooled



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to room temperature. The obtained products were named as LBO-0, LBO-1, LBO-3 and LBO-5 respectively.

2.2. Material Characterization. The crystal structures of the as-prepared samples were characterized using X-ray diffraction (XRD) (Rigaku, Model Ultima IV-185) with a Cu Kα radiation source. The samples were scanned from 2θ = 10°~80° at a scan rate of 8° per minute. The morphology and microstructure of the particles was measured by a field-emission scanning electron microscopy (FESEM) system (FEI QUANTA 250), equipped with energy X-ray (EDX) analysis. Transmission electron microscope was carried out on a Titan-G260-300 instrument. The X-ray photoelectron spectroscopy (XPS) of the sample was performed by X-ray photoelectron spectroscopy (XPS, PHI Quantera II). The chemical compositions of the as-synthesized materials were analyzed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES, PROFILE SPEC).

2.3. Electrochemical measurements. Electrochemical performances of the samples were tested in CR2025 coin cells with galvanostatic cycling. The positive electrodes of the active materials (80wt %), carbon black (10wt %), and polyvinylidene fluoride (10wt %) were mixed in N-methyl pyrrolidinone (NMP). And the resultant slurry was coated onto Al foil. After drying at 80ºC for 12h, the electrodes were assembled into CR2025 coin cells in a glove box filled with argon. The cells were composed of lithium metal as the anode, 1 M LiPF6 dissolved in a mixture of ethylene carbonate/ethyl methyl carbonate (1:1 volume ratio) as the electrolyte, and Celgard 6 ACS Paragon Plus Environment

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2400 membrane as separator. The cells were cycled galvanostatically between 2.0~4.8 V (vs. Li/Li+) at a desired C rate (1C corresponds to 250 mA·g-1). Electrochemical impedance spectroscopy (EIS) measurements were also carried out with a frequency range from 0.01 Hz to 100 kHz.

3. RESULTS AND DISCUSSION

3.1. Physical characteristics. The XRD patterns of the bare powder and LBO-coated Li1.2Ni0.2Mn0.6O2 cathode materials are presented in Figure 2. All the materials exhibit high crystallinity for the sharp diffraction peaks, and the patterns can be well indexed to a layered α-NaFeO2 type structure (R-3m space group). The weak peaks in the range of 2θ = 20~25° are corresponding to the LiMn6 short-range superlattice ordering of Li+/Mn4+ occurs in the transition metal layers, characterizing the structure of Li2MnO3-like components (C2/m space groups).35, 36 The clear peak separations between the (006)/(012) and (018)/(110) reveal a well-crystallized layered structure. However, the absence of new peaks corresponding to impurity or LBO phase results no difference of the spectra between the bare and LBO-coated samples. This phenomenon suggests two possibilities: the amount of coating material (LBO) is too low to be detected, or the surface coating layer is amorphous. Actually, although the coating amount is increased to 5wt %, there are still no peaks corresponding to LBO phase, indicating that the LBO layer might be amorphous. In addition, the the cell parameters a and c (inset of Figure 2) changes little with the increased contents of LBO. These results imply that the coating layer will not damage the host stucture. 7 ACS Paragon Plus Environment


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FESEM was performed to confirm the morphologies of the bare and LBO-coated samples (Figure 3). As presented in Figure 3a, the bare powders have a morphology as loose aggregates of irregular primary grains with a diameter of 100~200 nm. In Figure 3b, when the amount of LBO is 1wt %, there is no apparent change in morphology and grain size. However, the grain boundary becomes fuzzy with increasing LBO amount (Figure 3c-d). Even so, the morphologies and particle size of coated samples essentially maintain that of the pristine ones. Thus, any low-temperature performance differences in the final products can be directly related to the coating layer, irrespective of particle size/morphology.11 The distribution of chemical elements in the bare and LBO-3 samples were determined by energy dispersive X-ray (EDX) spectroscopy as presented in Figure S1. All the elements disperse uniformly as shown in their EDX mapping results. ICP results in Table 1 shows a comparison of the theoretical stoichiometry values and the calculated molar ratios of Li, Ni, Mn and B for the synthesized materials. And all the samples comply well with the target composition.

More detailed morphology and microstructure of the bare and LBO-coated samples are observed by TEM as shown in Figure 4. In Figure 4a, the bare Li1.2Ni0.2Mn0.6O2 particles are stacked by irregular primary grains. The surface of the bare material is clear and smooth, while that of the LBO-coated particles get blurry and rough (Figure 4 c-f). A closing inspection of the bare sample in Figure 4b reveals a continuous interference fringe spacing corresponding to the (003) plane (~0.479nm) of the R-3m layered structure (or (001) planes of C2/m). As shown in Figure 4c-d, the surface of 8 ACS Paragon Plus Environment

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the Li1.2Ni0.2Mn0.6O2 is almost completely covered by the LBO layer when its amount increased to 3wt %, and the formed coating layer exists as an amorphous layer with a thickness of 5~8nm, which agrees very well with the XRD results. This homogeneous coating layer will protect electrode materials to avoid side reaction with the electrolyte as the surface has been completely covered. Moreover, given that LBO glass can exhibit good ionic conductivity, the Li+ transport across the interface should be accelerated in the LBO coated samples at low temperature. Certainly, the thickness of the coating layer should be optimized. As seen in Figure 4e, the coating layer of LBO-1 seems too thin to encapsulate the host material perfectly, and when the LBO amount reaches 5wt %, the thickness of the LBO layer increases to ~18.9nm, which may instead hinder the transfer of the Li+.

As shown in Figure 5, the existence of LBO on the surface is further verified by X-ray photoelectron spectroscopy measurements (XPS). Figure 5 shows various XPS spectra for Mn2p, Ni2p and O1s of the bare and decorated samples, while B1s spectra is only found in LBO-3 sample (Figure 5c), which implies that the LiBO compound coating layer has been formed. Figure 5a exhibits a typical XPS spectra of Mn 2p3/2 with a binding energy of ~642.5 eV, suggesting that the valence state of element Mn in the both samples are tetravalent.21, 37 Whereas, the Ni (2p3/2) spectrum of the LBO coated sample tend to split into two peaks with the appearance of higher binding energy (856.9 eV, Ni3+) after coating. This change in peak position may result from the

local environment change

originating from

BO33- ions with

higher

electronegativity compared to O2- anions. In Figure 5d, the strong O 1s peak at 9 ACS Paragon Plus Environment


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529.6eV can be attributed to the lattice oxygen of the host material,38, 39 remains unchanged after LBO coating. The another peak at 531.8eV, whose intensity increases obviously after LBO coating, further indicating the formation of Li2O-2B2O3 coating layer. These results imply that the coating layer causes the local environment changes in the surface area of the host material, and benefit from the strong electronegativity of BO33- in LBO coating layer, both manganese and nickel atoms exhibit a higher binding energy, resulting a more stable surface after coating, which was in favor of Li+ transport at low temperature.

3.2. Electrochemical performances. To explore the effect of LBO coating layer on their low-temperature electrochemical properties, the galvanostatic charge/discharge tests at 0.1 C (25 mA·g-1) were carried out at various working temperatures (30 ºC, 20 ºC, 10 ºC, -10 ºC, -20 ºC, -30 ºC). The initial charge-discharge curves of all the samples are exhibited in Figure 6, and the corresponding detail data is shown in Table 2. At 30 ºC, all the samples present typical potential plateau of lithium-rich materials at about 4.5 V regions, and bare Li1.2Ni0.2Mn0.6O2 material delivers reversible capacities of 256.0 mAh·g−1. After coating, the discharge capacities of LBO-1, LBO-3, LBO-5 increase to 263.6, 288.0 and 272.0 mAh·g−1 respectively. Obviously, the LBO-coated samples have a better charge-discharge ability at room temperature, which may benefit from the good ionic conductivity of the LBO layer. Besides, when the coating amount is 3wt % the material show the best performance, which is due to its optimized thickness. According to the FESEM results, the 1wt % coating layer cannot cover the whole surface of the bare material, while the 5wt % coating makes 10 ACS Paragon Plus Environment

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the layer become so thick that lithium ion cannot pass quickly, especially at lower temperature.

In Figure 6 b-f, with lowering temperature, the plateau at 4.5 regions becomes short and even disappeared at -30 ºC. Because of the incomplete activation of Li2MnO3 phase at low temperature, the discharge capacity of the samples decreased dramatically, along with a significant increase of electrode polarization and violent voltage drop (Table 2 and Figure S2). These performances decay may contribute to the increased interface reaction impedance and slow ionic movement at low temperatures.40 As candidate materials to be utilized as solid electrolyte, LBO compositions exhibit good ionic conductivity.32 Thus, LBO-3 samples always exhibit a better discharge performance and higher initial coulombic efficiency (ICE). It has been reported that as for lithium-rich cathodes, the process of the lithium intercalation into the MnO2-like component is sensitive to the operation temperature and further determines the ICE at low temperature.10 According to the results of Table 2, we can conclude that the LBO coating layer can facilitate the diffusion of lithium ions through the interface, thus improving the low-temperature performances of lithium-rich material.

The cycle performances of the bare and LBO-coated Li1.2Ni0.2Mn0.6O2 samples within 2.0~4.8 V at 0.1C and various temperature are illustrated in Figure 7a. The electrochemical performance is greatly dependent on the temperature that there is an obvious decrease in capacity with the decrease in temperature. Even so, LBO-3 11 ACS Paragon Plus Environment


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always leads to a superior capacity with higher capacity retention. To eliminate the influence from LBO layer in improving the capacity for the host materials at room temperature as shown in Figure 6 a-b, we calculated the capacity retention of the bare and LBO-3 material at different temperatures as shown in Figure 7b. The advantage of LBO coated samples in in capacity retention become more evident below 0 ºC, indicating an intrinsically improved low-temperature performance that benefits from LBO coating.

Electrochemical impedance spectra (EIS) are further measured to get insight into the origin of the improvement in the electrochemical performance of LBO-coated Li1.2Ni0.2Mn0.6O2 samples. Figure 8 presents Nyquist plots measured at a charge state of ~3 V after one cycle. The equivalent circuit model used for simulating the experimental data is shown in inset of Figure 8. The measurements of four samples were carried out at the temperature of 30 ºC and -30 ºC. Generally, the small interrupt corresponds to the impedance of solution (RΩ). Rs is the resistance of ion diffusion in the region of the surface layer of particle (including SEI layer and surface modification layer),41 which is ascribed to the first semicircle in high frequency. The semicircle later emerged in intermediate-frequency is associated with the charge transfer resistance (Rct) between the electrode and electrolyte. And a sloping tail at low frequency is related to the diffusion of lithium ion in the solid electrode (Zw).42

As shown in Figure 8a and b, at -30 ºC, the Rs and Rct obviously become larger than that of 30 ºC (Table S1), which means the speed of charge transfer through the 12 ACS Paragon Plus Environment

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interface region is reduced at low temperature. While, the Rct of LBO-3 sample is lower than that of the other three samples, suggesting an enhancement in the kinetics of Li+ diffusion through the interface and charge transfer reaction after proper LBO coating and a consequent increase in low-temperature performance. Note that the thickness of the coating layer should be optimized to wrap up the host material evenly while avoid hindering the Li+ transport. These results also confirm that lower the interface resistance is an effective way to enhance the low-temperature performance of Li-rich materials, which can be realized by a simple LBO coating modification.

4. CONCLUSION

In this paper, the layered lithium-rich Li1.2Ni0.2Mn0.6O2 oxide was successfully synthesized by a sol-gel method, and immediately followed by coating with different amount of LBO glass (1wt %, 3wt %, 5wt %). The effects of LBO glass coating on low-temperature electrochemical performances of Li1.2Ni0.2Mn0.6O2 cathode materials were investigated and the optimum amount of coating is 3wt. %. Electrochemistry tests reveal that the LBO-3 sample exhibits the superior discharge capacity and higher cyclic retention than that of bare one at low temperature. XPS results have confirmed the stronger electronegativity of BO33- compared to O2-, thus improving the binding energy of manganese and nickel atoms at surface, resulting a more stable interface. EIS results show that the coating layer can lower the interface resistance of Li+ diffusion through the interface and facilitate the charge transfer reaction. These results imply that promoting the kinetics of Li+ transport at the interface between electrode 13 ACS Paragon Plus Environment


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and electrolyte, that can be realized by surface modification, is an effective way to improve the low-temperature electrochemical performance of Li-rich layered oxides. The simple and versatile strategy adopted in this work could be expected to apply in other layered materials.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:. EDX spectra of bare and LBO-3 samples; the evolution trend of average voltage of all samples at various temperatures; and the Rct value at 30 and -30 ºC calculated from the results of EIS test. AUTHOR INFORMATION Corresponding Author *E-mail addresses: [email protected]; [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was funded by National Natural Science Foundation of China (51472032, 21573017), National Key Research and Development Program(2016YFB0100301), 14 ACS Paragon Plus Environment

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Program for New Century Excellent Talents in University (NCET-13–0044) and the Major achievements Transformation Project for Central University in Beijing.

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Armand, M.; Tarascon, J. M. Building Better Batteries. Nature 2008, 451, 652-657. 2. Choi, N.; Chen, Z.; Freunberger, S. A.; Ji, X.; Sun, Y.; Amine, K.; Yushin, G.; Nazar, L. F.; Cho, J.; Bruce, P. G. Challenges Facing Lithium Batteries and Electrical Double ‐ Layer Capacitors. Angew. Chem. Int. Edit. 2012, 51, 9994-10024. 3. Mizushima, K.; Jones, P. C.; Wiseman, P. J.; Goodenough, J. B. LixCoO2 (0

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