Alleviating Surface Degradation of Nickel-Rich Layered Oxide

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Alleviating Surface Degradation of Nickel-Rich Layered Oxide Cathode Material by Encapsulating with Nanoscale Li-Ions/Electrons Superionic Conductors Hybrid Membrane for Advanced Li-Ion Batteries Lingjun Li, Ming Xu, Qi Yao, Zhaoyong Chen, Liubin Song, Zhian Zhang, Chunhui Gao, Peng Wang, Ziyang Yu, and Yanqing Lai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09197 • Publication Date (Web): 24 Oct 2016 Downloaded from http://pubs.acs.org on October 25, 2016

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ACS Applied Materials & Interfaces

Alleviating Surface Degradation of Nickel-Rich Layered Oxide Cathode Material by Encapsulating with Nanoscale Li-Ions/Electrons Superionic Conductors Hybrid Membrane for Advanced Li-Ion Batteries Lingjun Li,†‡ Ming Xu,§‡ Qi Yao, † Zhaoyong Chen, † Liubin Song, ⊥ Zhian Zhang, § Chunhui Gao, § Peng Wang, § Ziyang Yu, § Yanqing Lai*§ AUTHOR ADDRESS †: School of Material Science and Engineering, Changsha University of Science and Technology, Changsha 410004, P. R. China. §: School of Metallurgy and Environment, Central South University, Changsha 410083, P. R. China. ⊥ : School of Chemistry and Biological Engineering, Changsha University of Science and Technology, Changsha 410004, Hunan Province, P. R. China.

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KEYWORDS Hybrid nano-membrane, Lithium residue, Nickel-rich, Cathode material, Lithium-ion batteries

ABSTRACT

Nickel-rich layered oxide cathode materials for advanced lithium-ion batteries have received much attention recently because of their high specific capacities and significant reduction of cost. However, these cathodes are facing a fundamental challenge of loss in performance as a result of surface lithium residue, side reactions with the electrolyte and structure rearrangement upon long-term cycling. Herein, by capturing the lithium residue on the surface of LiNi0.8Co0.1Mn0.1O2 (NCM) cathode material as Li source, we propose a hybrid coating strategy incorporating lithium ions conductor LixAlO2 with superconductor LixTi2O4 to overcome those obstinate issues. By taking full advantages of this unique hybrid nano-membrane coating architecture, both the lithium ion diffusion ability and electronic conductivity of LiNi0.8Co0.1Mn0.1O2 cathode material are improved, resulting in remarkably enhanced electrochemical performances during high voltage operation, including good cycle performance, high reversible capacity, and excellent rate capability. A high initial discharge capacity of 227 mAh g−1 at 4.4 V cut-off voltage with Coulombic efficiency of 87.3 %, and reversible capacity of 200 mAh g−1 with 98 % capacity retention after 100 cycles at a current density of 0.5 C can be attained. The improved electrochemical performance can be attributed to the synergetic contribution from the removal of lithium residues and the unique hybrid nano-membrane coating architecture. Most importantly, this surface modification technique could save some cost, simplify the technical procedure, and show great potential to optimize battery performance, apply in a large scale and extend to all nickel-rich cathode material.


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TEXT 1. INTRODUCTION Nickel-rich layered cathode materials (NCM), namely LiNixCoyMnzO2 (x>0.3), have been intensively investigated as a promising cathode material for lithium-ion batteries (LIBs) due to its high discharge capacity of over 220 mAh g-1.1-13 The energy density of LIBs with NCM cathode materials can approach 350 Wh kg-1, which is favourable for the application in electric vehicles (EVs).1,3,5,14-16 However, the family of NCM cathode materials still suffered from structure instabilities and poor rate capability as a result of the migration of unstable Ni4+ ions to the lithium layer.1,17-24 Such migration will cause a structural degradation from the layered phase to the spinel-like phase

and rock-salt phase

in NCM cathode

materials.1,17-21 Upon cycling, the continual formation of these impurity phases greatly increases the kinetic barrier for Li-ions diffusion and electron transfer, leading to power fade and capacity degradation of cathode materials. Over past decade, most progress in enhancing the electrochemical performance of NCM cathode materials has been made by lattice doping,24-31 surface modification,32-45 tuning the material composition,6,7 and building core-shell or concentration gradient architectures.2-4,8-13 Among these strategies, surface modification is widely adopted as a promising way to mitigate the irreversible side reactions with the electrolyte and suppress the crystal collapse during long-term cycling of LIBs, thus significantly stabilizing the structure of NCM cathode materials. The typical metal oxides (Al2O3,32,33 TiO2,34 V2O535,37), fluorides (AlF336), phosphate40,43, and spinels45 were widely adopted to modify the cathode materials. However, the weak bonding between the coating layer and the host materials makes it possible for part of the coating layer to peel off from the electrode surface during the charge/discharge process, resulting in deterioration


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of electrochemical performance. Furthermore, although noticeable achievements have been made by the above-mentioned surface modifications, the most coating materials are still insulators or limitations for Li-ions conduction, which limits improving the rate capability.32-35,40,41 In addition, NCM cathode materials have a strong tendency to form surface residual impurities, such as Li2CO3 and LiOH,1,35,46-50 due to the reaction with oxygen, carbon dioxide, and water molecules in the air. These surface residual species will cause a serial of problems for the fabrication of LIBs, such as the insufficient adhesion of cathode active material on current collector. Furthermore, it has been recognized that these surface residual impurities is electrochemically inactive due to their poor electronic conductivity and low Li ions conductivity.1,46,51,52 Therefore, reducing the amounts of Li residual impurities is very important because they block the channels for Li-ions transportation. Then, an interesting and challenging question appears: is it possible to in-situ form the conductor membrane on the surface of NCM cathode materials by capturing these Li residual impurities? Previous report has shown that these lithium impurities can be incorporated by coating material to form LixAyBz material.40,44 Therefore, it is reasonable to integrate the Li residual impurities and coating material into the desired conductive composite. Amongst a variety of conductors, LiTi2O4 is a unique candidate for our coating strategy because it exhibits superconductivity with a critical temperature as high as 13.7 K.53 Furthermore, a substantial number of studies have been performed, which point to electrons transfer capability at 298 K as the primary property for LiTi2O4. It is generally agreed in these studies that the edge-shared TiO6 octahedrons form a conduction path of electrons and the LiO4 tetrahedrons play a role of charge reservoir to accommodate Ti sites with the carriers.54-58 First-principles results also show a strong bond between lithium and oxygen and between titanium and oxygen in LiTi2O4.54,56,57 However, this


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superconductor LiTi2O4 shows limited Li ions diffusivities (10-5 ~ 10-10 cm2 S-1).56-58 In this work, by capturing the lithium residue on the surface of NCM cathode material as Li source, we propose a hybrid coating with both Li ions conductor LixAlO2 and superconductor LixTi2O4 to stabilize the structure of NCM cathode material and enhance the electrochemical performance. With this hybrid nanolayer as “membrane” encapsulating on a LiNi0.8Co0.1Mn0.1O2 cathode material, the layered structure is expected to be stabilized, and most importantly, this hybrid nano-membrane coating layer can be characterized as a “highway” to rapidly transport both electrons and lithium ions between the electrolytes and the layered bulk. Such a hybrid “highway” coating strategy has three major advantages in achieving good electrochemical properties: 1) the fully encapsulated hybrid coating membrane on the surface of LiNi0.8CO0.1Mn0.1O2 cathode material can average the distribution of electrons and lithium ions, thus effectively enhancing the rate capability;62-65 2) the formation of hybrid coating membrane without separately adding any Li source will consume the Li residue on the surface of LiNi0.8CO0.1Mn0.1O2 cathode material, decreasing the charge transfer resistance during deep cycles and improving the cycling performance; 3) the formation of a homogeneous and conformal coating membrane will suppress the transition metal dissolution and stabilize the host structure, leading to an excellent structure stability. 2. EXPERIMENTAL SECTION 2.1 Sample synthesis LiNi0.8Co0.1Mn0.1O2 (NCM) cathode material was synthesized as reported previously.6,9 Ni0.8Co0.1Mn0.1(OH)2 powder was prepared by a co-precipitation method using NiSO4·6H2O, CoSO4·7H2O and MnSO4·5H2O as raw materials. Microspherical Ni0.8Co0.1Mn0.1(OH)2 powder was filtered, washed with deionized water, and dried at 100 °C to remove adsorbed water. A


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mixture of the dehydrate Ni0.8Co0.1Mn0.1(OH)2 powder and LiOH·H2O was preheated to 480 °C for 5 h and then heated at 850 °C for 12 h under oxygen flow to obtain NMC cathode material. 2.2 Surface modification To prepare the hybrid LixAlO2 and LixTi2O4 membrane coated NCM (NCM-LTAO), 2.0 g asprepared NCM powders were added into 60 mL deionized water and dispersed by strong stirring for 2 h. A given amount of titaniumisopropoxide (TIP, 65 mg), which is as the sources of Ti(OH)4 after hydrolysis at certain temperatures, was carefully weighted and added in 80 mL anhydrous ethanol, and dispersed by ultrasonication for 10

min. Then, 25 mg

aluminiumisopropoxide (AIP) was placed in the TIP dispersion and dissolved by ultrasonication for 1 h. The homogeneous AIP/TIP solution was then transferred to the as-prepared NCM powders dispersion. The mixture of the AIP/TIP solution and NCM powders dispersion was then kept at 80 °C in a water bath to ensure the hydrolysis of AIP/TIP. After continuous stirring for 4 h, the obtained sol was transferred into a 100 mL Teffon-lined stainless steel autoclave. After then the autoclave was sealed and maintained at 80 °C for 36 h to obtain the precursor. The final NCM-LTAO sample was prepared by calcining this precursor at 700 °C for 5 h, and the hybrid coating content corresponds to 1 wt %. For comparison, the coating process of the pristine NCM sample with LixAlO2 (NCM-LAO) or LixTi2O4 (NCM-LTO) was carried out as follows: 62 mg AIP or 65 mg TIP was placed in 80 mL anhydrous ethanol and dispersed by ultrasonication for 1 h. The homogeneous AIP or TIP solution was then transferred to the NCM powders (2.0 g) dispersion. In the subsequent step, the resulting mixture was then kept at 80 °C in a water bath and kept stirring for 4 h to ensure the hydrolysis of AIP or TIP. After then, the as-prepared sol precursors were then transferred into a 100 mL Teffon-lined stainless steel autoclave and maintained at 80 °C for 36 h. After filtering


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and vacuum drying at 100 °C, the NCM-LAO and NCM-LTO were finally obtained by heating at 700 °C for 5 h. The amounts of coated LixAlO2 or LixTi2O4 were ca. 1wt % of the NCM sample. 2.3 Additional experiments To demonstrate possibility for the synthesis of LiAlO2 (LAO), LiTi2O4 (LTO) or hybrid material (LTAO) via the above-mentioned approach, we performed the following experiment: 0.12 g AIP or TIP was placed in 80 mL anhydrous ethanol and dispersed by ultrasonication for 1 h. The homogeneous AIP or TIP solution was then kept at 80 °C in a water bath and kept stirring for 4 h to ensure the hydrolysis of AIP or TIP. After then, the as-prepared sol precursors were transferred into a 100 mL Teffon-lined stainless steel autoclave and maintained at 80 °C for 36 h. After filtering and vacuum drying at 100 °C, the precursors for preparing LiAlO2 or LiTi2O4 were obtained. After then, these precursors were mixed with stoichiometric LiOH and calcined at 700 °C for 5 h to obtain the LiAlO2 (or LiTi2O4). For the LTAO synthesis, the 62 mg AIP or 65 mg TIP was dissolved in 80 mL anhydrous ethanol and dispersed by ultrasonication for 1 h. After then, the hybrid LTAO material can be obtained by following the same procedure as presented above. To investigate the structure collapse of the as-prepared pristine NCM cathode material, the transition metal ions dissolution experiment was carried out in this paper. Accurately weighed 0.5 g NCM cathode material was immersed in 5 mL 1 M LiPF6-EC:DMC=1:1 (v/v) electrolyte for 10 days in a Ar-protected glove box at room temperature. Then, the electrolyte soaked NCM cathode material was rinsed with dimethyl carbonate (DMC) several times and then dried at room temperature. The dried NCM cathode material was applied directly for XRD and SEM characterization.


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2.4 Characterization X-ray diffractometer (XRD, Rigaku D/Max 200PC) was employed to characterize the crystallographic structures of all samples. The morphologies of pristine and surface modified NCM cathode materials were examined by scanning electron microscope (SEM, Nova NanoSEM-230) and transmission electron microscopy (TEM, JEM-2010, 200 kV). Energy dispersive X-ray spectroscopy (EDX) mapping was used for morphological assessment and surface elemental characterization on the surface modified NCM cathode materials. The method and procedure for investigating the microstructure and transition metal dissolution of the cathode materials after cycling were reported everywhere.35-38,40 2.5 Electrochemical measurements The electrochemical measurements were performed in CR2025-type coin cells. Galvanostatic charge/discharge cycling was performed between 2.0 and 4.8 V at room temperature with a LAND CT2001 A battery testing equipment. The working electrodes were prepared by a slurry coating procedure. The slurry consisted of 85 wt % as-synthesized cathode materials, 10 wt % acetylene black, and 5 wt % polyvinylidene difluoride (PVDF) in an N-methyl pyrrolidone (NMP) solvent coated onto aluminum foils. After coating, the cathode electrode films were dried at 120 °C for 12 h to remove any residual NMP and traces of water. Electrode discs of 10-mm diameter with a thickness of 0.025 mm were punched from the cathode electrode film. The typical mass loading of the active material of the cathode electrode was approximately 3.0 mg. Coin cells were assembled with the cathode electrodes as-prepared, metallic lithium foil as counter electrode, Cellgard 2400 monolayer polyethylene membrane as separator, and 1 M LiPF6 dissolved in ethyl carbonate (EC) and dimethyl carbonate (DMC) (1:1 in volume) as electrolyte in an argon-filled MIKROUNA Universal 24401750 glovebox. Electrochemical impedance


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spectroscopy (EIS) measurements were performed on an Autolab PGSTAT302N electrochemical workstation with requency range of 0.01-100,000 Hz. 3. RESULTS AND DISCUSSION 3.1 Formation of hybrid nano-membrane The schematic illustration in Scheme 1 shows our strategy to in-situ building a “highway” for rapid Li ions and electrons transport in NCM cathode materials. By capturing the surface lithium residual impurities (Li2CO3 and LiOH 46-49) as Li source, a hybrid coating with LixAlO2 and LixTi2O4 membrane was obtained successfully. It is generally accepted that the hydrolysis of AIP and TIP will lead to the formation of Al(OH)3 and Ti(OH)4, respectively. These Al(OH)3 and Ti(OH)4 colloidal particles will adsorb on the NCM microspheres and distributed homogeneously under continuous stirring. During hydrothermal treatment, the Al(OH)3 continues to hydrolyze and the LiAl(OH)4 coating layer is formed via capturing Li-residue on the surface of NCM microspheres, while the condensation of Ti(OH)4 colloidal particles begins to produce amorphous TiO2 which will incorporate in LiAl(OH)4 randomly. After calcinations at 700 °C, a hybrid coating with LixAlO2 and LixTi2O4 is formed. With the ultrathin hybrid nanomembrane encapsulating on the surface of NCM cathode materials, the layered bulk can be effectively stabilized during long-term cycling, and most importantly, this conductive membrane can be characterized as a “highway” for rapid Li ions and electrons transport.53-55 Differing from methods such as atomic layer deposition (ALD), sol-gel, or chemical vapor deposition, the hydrolysis-assisted hydrothermal treatment combined subsequent calcination in our strategy can effectively capture the surface lithium residual impurities and ensure a homogeneous distribution of hybrid LTAO layer on the surface of NCM cathode materials, thus effectively suppressing the transition metal ions dissolution upon cycling.


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3.1.1 Morphology and crystallinity analysis To demonstrate the possibility for the synthesis of LixAlO2 nano-sheet, LixTi2O4 nano-particle and hybrid coatings, we performed additional experiments to synthesize LAO, LTO, and LTAO materials (details can be seen in Experimental Section). As shown in Figure 1, the clear crystallinity demonstrates the successful formation of LAO (JCPDS no. 74-2232), LTO (JCPDS no. 82-2318), and hybrid LTAO materials. Figure 2 shows the SEM images of pristine NCM, NCM-LTAO, NCM-LAO, and NCM-LTO samples, and differences in morphology can be seen obviously. As shown in Figure 2a, the pristine NCM microsphere are assembled by large amounts of primary particles with size of 200-800 nm. The primary particles protruded slightly toward the outside of large microspheres. The surface of NCM microsphere is clean, no any sign of coating layer can be observed. Such exposed surfaces of pristine NCM microspheres without any protective coating layer will suffer from the HF attack and other side-reactions caused by the decomposition of electrolyte upon long-term cycling. For the NCM-LTO sample (Figure 2b), a large amount of LixTi2O4 nano-particles distributed on the surface of NCM microspheres. EDS mapping images (Figure S1, Supporting Information) of various elements in the NCM-LTO shows homogeneous distribution of LixTi2O4 nano-particles coating layer. Although the edgeshared TiO6 octahedrons in LixTi2O4 nano-particles form a conduction path of electrons, the limited Li ions diffusivities will increase interfacial resistance.56-58 Furthermore, the surface lithium residue could not be fully captured by this LixTi2O4 nano-particles coating layer, thereby decreasing battery performance. As shown in Figure 2c and Figure S2 (Supporting Information), the surface of NCM-LAO microspheres is completely covered with LixAlO2 nano-sheets. However, the unavoidable accumulation of LixAlO2 nano-sheets will increase the transportation length for Li+ ions. Significantly, it can be seen in Figure 2d that every single primary particle of


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NCM-LTAO microspheres is homogeneously encapsulated by the hybrid nano-membrane. The EDS mappings show a homogeneous distribution of Ni, Co, Mn, Al, and Ti elements, as shown in Figure 2. Although both the LixAlO2 and LixTi2O4 coatings were obtained in the same approach, the difference in structure and morphology may yield very different effect in alleviating the structure degradation and suppressing the transition metal dissolution for the NCM cathode materials. Comparatively, the hybrid nano-membrane can as HF scavengers to maintain the structural stability of the NCM-LTAO cathode materials. Most importantly, this hybrid nano-membrane coating layer can be characterized as a “highway” to rapidly transport both electrons and lithium ions between the electrolytes and the layered bulk. Figure 3 shows the XRD patterns of pristine NCM and all surface modified cathode materials. Obviously, all samples could be indexed to a well-defined hexagonal α-NaFeO2-type structure with a space group of R-3m. Furthermore, the clear split between the adjacent peaks of (006)/(102) and (018)/(110) suggests a typical layered structure for all samples. The lattice parameters are calculated from the diffraction data and displayed in Table 1. Significantly, it is concluded that the lattice volume of NCM-LTO and NCM-LTAO samples increases by comparing with the pristine one due to the substitution of smaller Mn4+ (0.53 Å) ions by larger Ti4+ (0.605 Å) ions. Meanwhile, the decreased lattice volume is also observed for NCM-LAO sample. The ionic radius of Al3+ is 0.53 Å that is smaller than that of the Ni2+ (0.70 Å) and Co3+ (0.61 Å), thereby implying the formation of LiM1-xAlxO2 (M=Ni, Co, Mn) hierarchical structure. 3.1.2 Identification of hybrid nano-membrane coating layer To further evaluate the structure characteristics of hybrid nano-membrane, LixAlO2 nano-sheet, and LixTi2O4 nano-particle coatings, TEM and HRTEM characterizations were performed and


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the results were shown in Figure 4. Insets in Figure 4a present a smooth surface of the pristine NCM particle. However, as shown in HRTEM image, a fuzzy layer with thickness of ca. 2.0 nm can be observed. It is reported that this fuzzy layer can be identified as the Li-residue layer which mainly formed during calcinations with excess LiOH.46-50 Fortunately, by taking full advantages of our hydrolysis-assisted coating strategy, the Li residue on the surface of pristine NCM particles can be utilized as Li source to obtain hybrid nano-membrane, LixAlO2 nano-sheets, and LixTi2O4 nano-particles coatings. As shown in Figure 4a, a nano-membrane coating with a thickness of ca. 20 nm can be observed clearly. Remarkably, this nano-embrane coating is homogeneous and completely encapsulate the NCM-LTAO particles. HRTEM image in Figure 4b based on the marked area in Figure 4a confirms that the hybrid nano-membrane coating layer is well-crystallized, and some lattice fringes are clearly legible, with interplanar spacings of ca. 0.236 nm and ca. 0.485 nm. These fringes are not only included in the spinel LixTi2O4 structure but quite indexed with a mixed phase with LixAlO2 and LixTi2O4 structure, as shown in Figure 4b. It is interesting to find that the spinel LixTi2O4 domains are incorporated with LixAlO2, which could be characterized as “highway” for rapid Li ions and electrons transport. As for the NCMLAO sample, the NCM particles were covered by LixAlO2 nano-sheets (Figure 4c). The clear lattice fringes and corresponding fast Fourier transform (FFT) result shown in Figure 4d demonstrate that the nanosheets on the surface of the NCM particles can be indexed as (003) facet of LiAlO2 (JCPDS no. 74-2232). Figure 4e shows the TEM image of NCM-LTO particle, it displayed a distinguishable LixTi2O4 nano-particles coating on the surface of NCM-LTO particles. The magnified view is shown in Figure 4f, further confirming the existence of the LixTi2O4 nanoparticles coating. The corresponding FFT result demonstrates the (200) facet of LiTi2O4 (JCPDS no. 82-2318).


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3.2 Electrochemical Performance 3.2.1 Initial Coloumbic efficiency and discharge capacity The NCM-LAO, NCM-LTO and NCM-LTAO samples were characterized as cathode materials for LIBs to evaluate their electrochemical performance. Figure 5a shows the initial charge/discharge voltage profiles of the NCM-LAO, NCM-LTO and NCM-LTAO samples measured between 2.7 V and 4.4 V at 0.1 C at room temperature. All the charge/discharge profiles show a typical potential plateau at 3.75 V. Note that the Coloumbic efficiency of the NCM-LTAO was higher (87.3 %) when compared with the pristine NCM (83.4 %), NCM-LAO (84.7 %), and NCM-LTO (81.2 %) samples owing to a well-developed hybrid nano-membrane on the surface of microspheres that facilitate Li ions diffusion, and thus high lithium utilization. And not only that, the NCM-LTAO sample shows the weakest polarization, indicating that a proper surface morphology (conformal, homogeneous, and uniform) of coated cathode materials is important to keep a balance between surface protection and electrochemical reaction. The pristine NCM sample shows an initial discharge capacity of 219 mAh g-1, while the NCMLAO, NCM-LTO, and NCM-LTAO samples deliver the initial discharge capacities of 208 mAh g-1, 202 mAh g-1, and 227 mAh g-1 respectively. For comparison, the lower initial discharge capacities are observed for the NCM-LAO and NCM-LTO samples compared with the pristine NCM sample, which presumably because of the increased contact resistance and the electrochemically inactive nature of the LixAlO2 nano-sheets and LixTi2O4 nanoparticles coatings within this voltage range. In comparison with the capacity between the pristine NCM, NCMLAO, and NCM-LTO samples, it is interesting to conclude that the initial capacity of NCMLTAO sample is largely enhanced by the hybrid nano-membrane coating. As we discussed


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earlier, the morphology and intrinsic properties of surface coating layer are key factors that are found to play a vital role in alleviating polarization and capacity loss for NCM cathode material. Furthermore, it is also demonstrated that only the homogeneously distributed hybrid nanomembrane coating can to the maximum extent enhance the overall electrochemical performance of NCM cathode materials, which should be associated with the surface morphology of the NCM-LTAO sample as shown in Figure 2d. And not only that, the conformal hybrid nanomembrane coating on NCM-LTAO microspheres can average distribution of electrons and Li ions on the whole surface of NCM-LTAO microspheres, as schematically presented in Figure 5b. Hence, electrochemical polarization from electron conglomeration and concentration gradient of Li ions can be significantly reduced in the cathode, especially under high charge/discharge rates.62-65 3.2.2 Rate capability and cycling performance Figure 6 compares the discharge capacities of the pristine NCM, NCM-LAO, NCM-LTO and NCM-LTAO samples with the C rates increasing from 0.1 C to 3.0 C every five cycles. When discharged at 0.2 C rate, the pristine NCM sample delivered a reversible capacity of 208 mAh g1; the NCM-LAO sample, 204 mAh g-1; the NCM-LTO sample, 197 mAh g-1; the NCM-LTAO sample, 213 mAh g-1; these samples do not show much of advantages over the pristine NCM at low current rate. However, when discharge at the 3.0 C rate, the NCM-LTAO sample delivered the highest reversible capacity, showing a great advantage over the pristine NCM, NCM-LAO, and NCM-LTO sample. We believe that the high rate capability of the NCM-LTAO sample has a strong correlation with the coating layer, originating from the well-crystallized and uniform hybrid nano-membrane coating that could be characterized as “highway” for rapid Li ions and electrons transport.


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To compare the capacity retention of the samples at room temperature, the cycling performance with charge and discharge rates at 0.5 C and 1.0 C for 100 cycles of the pristine NCM and surface coated samples was shown in Figure 7. As we can see in Figure 7, the pristine NCM sample shows dramatic capacity decrease after only 20 cycles. It is known that capacity degradation of NCM cathode materials becomes more severe as Ni-content increases due to the chemical decomposition of both the surface of the cathode material and the electrolyte.1,2,17,20,22 More detailed investigating by electrolyte immerse and morphology characterization led to the same conclusion (Figure S3, Supporting Information). Comparing to the pristine NCM cathode material, after 100 cycles, 175 mAh g-1 and 164 mAh g-1 remained for the NCM-LAO and NCMLTO cathode materials at 0.5 C rate with capacity retention of 92 % and 88 %, respectively. Meanwhile, when the current rate increase to 1 C, capacity retention of 90 % and 85 % can be obtained after 100 cycles. As for the NCM-LTAO sample, it displays great enhancement in cycling performance. As shown in Figure 7a and 7b, the NCM-LTAO sample yields a maximal capacity of 202 mAh g-1 and 196 mAh g-1 at 0.5 C and 1 C rates, and the capacity retained 199 mAh g-1 and 187 mAh g-1 after 100 cycles, with excellent capacity retention of 98 % and 96 %. Figure 8 presents the discharge voltage profiles evolution upon cycling and corresponding dQ/dV plots. In contrast to the pristine NCM sample (Figure 8a), the NCM-LAO (Figure 8b), NCM-LTO (Figure 8c), and NCM-LTAO (Figure 8d) samples show much less voltage fade during the long-term cycling test. In contrast, the NCM-LTAO sample exhibits a more superior reproducibility of voltage plateaus and discharge profiles than that performed by the NCM-LAO and NCM-LTO samples. One of the major reasons for the voltage decay of NCM cathode materials upon deep cycling is that contaminating species are usually found on the electrode surface and these surface residual species can react with electrolyte forming insulating layer.1,22-


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In addition, the parasitic side reactions between pristine NCM and electrolyte causes sever

transition metal ions dissolution, thus resulting in the collapse of the surface lattice structure.17-21 Therefore, the main mechanism of improvement after LixAlO2 nano-sheet, LixTi2O4 nanoparticle and hybrid nano-membrane coatings is principally assigned to minimizing harmful side reactions by forming a protective layer at the interface between active material and electrolyte. Meanwhile, combined with the SEM and TEM results, it is reasonable to conclude that the electrochemical performance of coated NCM cathode materials is very sensitive to the surface morphology. The remarkable cycling stability of NCM-LTAO sample indicates that the hybrid nano-membrane is facile for Li ions diffusion and can effectively alleviate decomposition of electrolyte and reduce electrochemical polarization on the surface of NCM-LTAO particles. As the electrochemical performance is greatly enhanced by surface modification with the wellcrystallized and uniform hybrid nano-membrane coating, the EIS of both fresh and cycled electrodes for the pristine NCM and NCM-LTAO smaples are performed and shown in Figure 9. The physical meaning and reasons for variation of Rsf, Rct, Rb, and CPEsf are clearly explained in previous reports.59-61 These Nyquist plots are fitted with the equivalent circuits as shown in Figure 9 and corresponding resistance values can be seen in Table S1 (Supporting Information). It is noted that the Rsf+ct value of fresh electrodes is 180 Ω and 410 Ω for the NCM and NCMLTAO samples, respectively. After 100 cycles, the Rsf+ct and Rb values of NCM-LTAO electrode are much lower than those of the NCM electrode. Furthermore, the lithium ion diffusion coefficient was calculated via a widely accepted method,40,59 and it was 5.4×10-11 cm2 S-1 and 7.1×10-13 cm2 S-1 for the NCM-LTAO and pristine NCM samples after 100 cycles. The high lithium ion diffusion coefficient after cycling should be attributed to both the removal of Li


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residue on the surface of NCM particles and hybrid nano-membrane coating for rapid Li ions and electrons transport. 3.2.3 Alleviated surface degradation by surface coating Figure 10 shows the SEM images and corresponding EDAX results of the separators from disassembled cells based on the pristine NCM, NCM-LTO, NCM-LTAO, and NCM-LTAO samples. It can be seen clearly that the separators exhibit very different morphology characteristics. A large amount of nanoparticles can be observed on the separator base on the pristine NCM cathode material (Figure 10a). The corresponding EDAX results show that the nanoparticles were composed of Ni, Co, Mn, F and P elements, demonstrating transition metal dissolution during a long-term cycling test. Table 2 presents the atomic ration detected by EDAX. It is widely accepted that the LiPF6-based electrolyte always contains a small amount of water as impurity. It is generally agreed that the LiPF6 salt in typical electrolytes is chemically unstable when exposed even a small amount of water, which generates hydrofluoric acid (HF).51,52 The directly exposed NCM cathode materials in electrolyte upon extensive cycling suffer from the dissolution of transition metals, which causes severe capacity decay. Consequently, the Ni, Co and Mn ingredients produce byproducts on the surface of the separator by HF attack during cycling. With surface modification, the coating layer could alleviate the HF attack, and thereby suppressing transition metal dissolution. As for the NCM-LAO (Figure 10b) and NCM-LTO (Figure 10c) samples, the corresponding results show that HF attack could not alleviated effectively by LixAlO2 nano-sheet and LixTi2O4 nano-particle coatings. In contrast, as we can observe in Figure 10d and corresponding EDAX result, very weak signals of Ni, Co and Mn were detected for the separator based on the NCM-LTAO sample, demonstrating that the homogeneous hybrid nano-membrane coating layer scavenged the acidic HF species from the


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electrolyte. Another visible evidence of structure degradation for the pristine NCM sample and effective alleviation of crystal collapse for NCM-LTAO sample is the XRD pattern displayed in Figure 11. As shown in Figure 11, the diffraction peaks at 20~25 are attributed to the carbon black in the electrodes. It is noted that the (018) peak of the pristine NCM sample (Figure 11a) diminished after 100 cycles. In comparison with the NCM-LAO (Figure 11b) and NCM-LTO (Figure 11c), the reflections corresponding to the layered phase remain basically unchanged for the NCM-LTAO sample (Figure 11d). Consequently, when the NCM-LTAO cathode material was attacked by the HF from the electrolyte, the fully wrapped and homogeneous coating layer gradually transforms to metal fluoride layers by scavenging F- from the HF. By obtaining F- from HF on the surface of the coating layer, the concentration of acidic species in the electrolyte becomes significantly lower, leading to less degradation of cathode material by the acidic species during cycling. The surface degradation, lattice break-up, and TM ions dissolution are striking observations, which are the primary factors contributing to the capacity fading and poor rate performance of NCM cathode materials. In the present work, by capturing the lithium residue on the surface of LiNi0.8Co0.1Mn0.1O2 (NCM) cathode material as Li source, we propose a hybrid coating strategy incorporating lithium ions conductor LixAlO2 into superconductor LixTi2O4 to overcome those obstinate issues. The greatly enhanced electrochemical performance indicate that this coating strategy can lead to the development of a wide range of functional nickel-rich cathode materials with better rate capability, better cycle life characteristics. 4. CONCLUSION In summary, we have synthesized a high capacity LiNi0.8Co0.1Mn0.1O2 (NCM) cathode material via co-precipitation method. By capturing the lithium residue on the surface of nickel-rich cathode material as Li source, we have successfully obtained a hybrid nano-membrane with


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LixAlO2 and LixTi2O4 coated NCM cathode material in a hydrolysis-assisted coating process. Comparatively, LixAlO2 nano-sheets and LixTi2O4 nano-particles coated NCM cathode materials were also obtained via the same approach to evaluate their performance. Consequently, the hybrid nano-membrane coated NCM cathode material shows excellent rate capability and capacity retention at 0.5 C, 1 C, and 3 C rates, indicating the effectiveness of surface coating for protecting cathode materials from HF attack. The XRD and SEM results after long-term cycling test demonstrate less structure degradation in the hybrid nano-membrane coated NCM cathode material. Therefore, as a highly Li-ions and electrons conductive coating material, the hybrid nano-membrane coating shows great potential in surface modification and expected to improve performance for other functional cathode materials of LIBs.


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FIGURES Figure 1. XRD patterns of LAO, LTO, and LTAO materials.


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Figure 2. SEM images of a) pristine NCM; b) NCM-LTO; c) NCM-LAO; d) e) NCM-LTAO; EDS mapping images of f) Ni, g) Co, h) Mn, i) Al, and j) Ti; k) EDAX pattern based on (d). Scale bars refer to 2 µm.


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Figure 3. XRD patterns of pristine NCM, NCM-LAO, NCM-LTO, NCM-LTAO cathode materials.


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Figure 4. a) TEM image (insets refer to pristine NCM sample); b) HRTEM image of the NCMLTAO sample (based on the marked area in (a)); c) TEM image; d) HRTEM image of the NCMLAO sample (inset refers to FFT based on marked area); e) and f) TEM images of the NCMLTO sample (insets in (f) refer to HRTEM image and corresponding FFT).


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Figure 5. a) Initial charge/discharge voltage profiles of pristine NCM, NCM-LAO, NCM-LTO, NCM-LTAO cathode materials tested at 0.1 C between 2.7 V-4.4 V at room temperature; b) Schematic view of the conformal hybrid nano-membrane.


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Figure 6. Rate capability of pristine NCM, NCM-LAO, NCM-LTO, NCM-LTAO cathode materials tested from 0.1 C-3.0 C rate.


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Figure 7. Cycling performance of pristine NCM, NCM-LAO, NCM-LTO, NCM-LTAO cathode materials tested at a) 0.5 C and b) 1.0 C rate at room temperature.


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Figure 8. Discharge voltage profiles evolution of a) pristine NCM, b) NCM-LAO, c) NCM-LTO, and d) NCM-LTAO samples upon cycling at 1 C rate (insets refer to corresponding dQ/dV plots).


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Figure 9. EIS plots for fresh and cycled electrodes of the pristine NCM and NCM-LTAO samples.


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Figure 10. SEM image of the separators from disassembled cells based on a) pristine NCM, b) NCM-LAO, c) NCM-LTO, and d) NCM-LTAO samples; e)-h) corresponding EDAX patterns. Scale bars in a)-d) refer to 1 µm.


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Figure 11. XRD patterns of a) pristine NCM, b) NCM-LAO, c) NCM-LTO, and d) NCM-LTAO electrodes after 100 cycles at 1 C rate.


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SCHEMES Scheme 1. Schematic diagram for the design and synthesis of hybrid nano-membrane encapsulated NCM cathode material.


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TABLES Table 1. Lattice parameter a, c, v, ratios of c/a, error values Rp and I(003)/I(104) for pristine NCM, NCM-LAO, NCM-LTO, and NCM-LTAO samples calculated from the refined data.

Lattice parameters Samples 3

Rp

I(003)/I(104)

a/Å

c/Å

v/Å

c/a

Pristine NCM

2.8648

14.1739

100.74

4.9476

8.29 %

1.4132

NCM-LAO

2.8633

14.1726

100.62

4.9497

9.12 %

1.4257

NCM-LTO

2.8667

14.1825

100.94

4.9473

8.46 %

1.4161

NCM-LTAO

2.8649

14.1761

100.77

4.9482

7.47 %

1.4348


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Table 2. Atomic ratio based on the EDAX patterns of separators from disassembled cells. Samples

Atomic ratio Ni

Co

Mn

Pristine NCM

0.36

0.08

0.02

NCM-LAO

0.13

0.04

0.01

NCM-LTO

0.25

0.16

0.02

NCM-LTAO

0.06

0.05

0.01


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ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Yanqing Lai, E-mail: [email protected] Author Contributions ‡L. J. Li and M. Xu contributed equally to this work and should be considered as co-first authors. M. Xu, L. J. Li, and Y. Q. Lai conceived the idea. M. Xu and L. J. Li prepared all materials. Q. Yao conducted SEM experiments. P. Wang, Q. Yao and C. H. Gao conducted XRD experiments. M. Xu, L. J. Li and Q. Yao analyzed the data. M. Xu wrote the manuscript and Z. A. Zhang, Z. Y. Chen, Z. Y. Yu, L. B. Song commented on it. Y. Q. Lai supervised the implementation of the project. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors thank the financial support of National Natural Science Foundation of China (Contract Nos. 51304031). the Project of Innovation-driven Plan in Central South University (2015CXS018) (2015CX001) and State Key Laboratory of Powder Metallurgy, Central South University, Changsha, China.


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Table of Contents By capturing the Li residue on the surface of LiNi0.8Co0.1Mn0.1O2 as lithium source, a hybrid nano-membrane coating layer incorporating lithium ions conductor LixAlO2 into superconductor LixTi2O4 is synthesized through a hydrolysis-assisted method and characterized as a “highway” for rapid Li-ions and electrons transportation. This hybrid nano-membrane modified LiNi0.8Co0.1Mn0.1O2 cathode material shows excellent structure stability, remarkable rate capability, as well as great cycling performance.


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Alleviating Surface Degradation of Nickel-Rich Layered Oxide

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