Enhancing the Thermal and Upper Voltage Performance of Ni-Rich

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Enhancing the Thermal and Upper Voltage Performance of Ni-rich Cathode Material by a Homogeneous and Facile Coating Method: Spray-Drying-Coating Nano-Al2O3 Ke Du, Hongbin Xie, Guorong Hu, Zhongdong Peng, Yanbing Cao, and Fan Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05629 • Publication Date (Web): 22 Jun 2016 Downloaded from http://pubs.acs.org on June 24, 2016

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ACS Applied Materials & Interfaces

Enhancing the Thermal and Upper Voltage Performance of Ni-rich Cathode Material by a Homogeneous and Facile Coating Method: Spray-Drying-Coating Nano-Al2O3 Ke Du, Hongbin Xie*, Guorong Hu*, Zhongdong Peng, Yanbing Cao, Fan Yu *School of Metallurgy and Environment, Central South University, Changsha, 410083, P. R.China *Tel.: +86-15116278438 *E-mail address: [email protected]

(G.H.) [email protected] (H.X.)

Abstract Electrochemical performance of Ni-rich cathode material at high temperature (>50°C) and upper voltage operation (>4.3V), is a challenge for the next generation LIBs due to the rapid capacity degradation over cycling. Here, we report improved performance of LiNi0.8Co0.15Al0.05O2 (NCA) materials via a LiAlO2 coating, which was prepared from a Ni0.80Co0.15Al0.05(OH)2 precursor with Spray-Drying-Coating Nano-Al2O3 on. Investigations from X-ray diffraction (XRD), scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectroscope (EDS), and transmission electron microscopy (TEM) reveal that an Al2O3 layer distribute uniformly on the precursor, and a LiAlO2 layer on the as-prepared cathode material. Such a coating shell acts as a scavenger to protect the cathode material from the HF attacking and serious side reactions, which remarkably enhances the cycle 1 ACS Paragon Plus Environment


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performance at 55°C and upper operating voltage (4.4V and 4.5V). Especially, the sample with 2% Al2O3 coating shows capacity retention of 90.40%, 85.14%, 87.85% and 81.1% after 150 cycles at 1.0C rate at room temperature, 55°C, 4.4V and 4.5V respectively, significant higher than those of the pristine one. This is mainly because the significant improvement of the stable structure lead by the effective coating technique, which could be extended to other cathode materials, further to obtain lithium-ion batteries with enhanced safety and excellent cycling stability. Keywords: Ni-rich Cathode Material, LiNi0.8Co0.15Al0.05O2, Spray-Drying-Coating, Nano-Al2O3, high temperature, upper voltage

1. Introduction Consumer electronics, such as mobile phones, laptops, digital cameras, and electric vehicles (EVs) are advancing rapidly with each passing day, which demand better power sources. Therefore, high energy-density lithium-ion batteries (LIBs) with longer cycle life and better safety are in demand as it is expected to be widely used in device applications1-2. Thus, intensive researches have been implemented on the LIBs to meet the demand with a higher degree of lithium utilization and specific energy density3-5. As cathode materials used in the LIBs are the core to the performance, most research efforts have been focused on them, either to develop alternative cathode materials, or to modify the available ones6-7. Layered lithium Ni-rich compounds LiNi1-xMxO2 (0.14.3V) due to the increasing impedance and structure instability11-12. The thermal instability would even lead to serious safety problems during thermal runaway. One of the major reasons for this is the side reactions with CO2 in the air and the moisture on the surface. Excessive LiOH is necessary in the preparation, and residual lithium will still remain on the cathode materials surface which can react directly with air. The ability of rapid moisture uptaking of Ni-rich cathode materials also result in the formation of impurities, such as LiF, Li2CO3, and LiOH13-14. Spontaneous side reactions also take place on the Ni-rich cathode materials’ surface when contacting directly with electrolytes15. These impurities exist on the cathode materials’ surface would hinder the Li+ ions diffusion during charge-discharge process because of their insulation, leading to the electrochemical performance deterioration16-17. Apart from that, when exposed to air, the reduction of Ni3+ to Ni2+ occurred on the cathode materials’ surface would also lead to LiOH, LiHCO3, Li2CO3 and NiO-like materials forming on the surface. Whatever residue on the surface will worsen the cathode 3 ACS Paragon Plus Environment


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materials’ machining performance and electrochemical performance18. To solve the above problems, intensive work has been developed during the recent years. Although there are no methods that efficacious forever yet, coating the surfaces with nano-scale layers is regarded as a practicable one19. Since B2O3 oxide coating was reported to modify cathode materials for the first time by Amatucci20, various coating materials and methods were explored to improve the performance, such as Al2O321-23, SiO224, TiO225, MgO26 , ZrO227, V2O518, 28, AlF329-30, Ni3PO431 and LiCoO232. The coating layer is mainly acting as an isolation layer to prevent side reactions. The main coating techniques include sol-gel33, solvothermal method34, chemical deposition35, magnetron sputtering36 and ALD22-23. Although these coating progresses have improved the performance, it is still difficult to ensure the homogeneity of the coating layer and avoid self-nucleation of the coating material. Spray drying is a rapidly drying method by using hot gas to produce dry powders rapidly from a liquid or slurry37. The drying speed by using a spray drier is very fast, as the specific area increases sharply after the liquid droplets atomized, and 95%~98% water vapored instantly in the high temperature air flow, within only several seconds. Specially, products dried with spray drying are consistent, having good uniformity, dispersion, mobility and solubility. Thus it is a preferred drying method applied to produce a lot of thermally-sensitive materials like foods and pharmaceuticals. Meanwhile, this method has been commonly used in the industry and it was also adapted to prepare nano-porous spherical lithium-ion cathode materials, such as LiNi0.8Co0.15Al0.05O238, LiNi0.6Co0.2Mn0.2O239, LiNi0.8Co0.2O240, Co3O441 and so on. 4 ACS Paragon Plus Environment

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However, most researchers have just focused on the synthesis materials by spray drying. Herein, spray drying coating (SDC) nano-Al2O3 on the Ni-rich cathode materials is proposed in this work, and LiNi0.8Co0.15Al0.05O2 is taken as an example. This new approach resulted in an Al2O3 layer distribute uniformly on the precursor, and a LiAlO2 layer on the as-prepared cathode material, which improved the structural and thermal stability under the conditions such as elevated temperature (>50°C) and upper cutoff voltage (>4.3 V).

2. Experimental 2.1 Materials synthesis As shown in Scheme 1, the used precursor Ni0.8Co0.15Al0.05(OH)2 is commercial supplied (Jintian Energy Co. Ltd, China). Ni0.8Co0.15Al0.05(OH)2 and nano-Al2O3 sol-gel (Al2O3 >20%, Xilong Chemical Co., Ltd. China) are used as starting materials with a mass ratio 99:1, 98:2 and 95:5 respectively. Firstly, nano-Al2O3 sol-gel is dissolved in distilled water and agitated for dispersion for 4 hours using a high energy tip sonicator (400W, KQ-400KDE, Kun Shan Ultrasonic Instruments Co., Ltd, China). Then Ni0.8Co0.15Al0.05 (OH)2 precursor is added into the solution and ultrasonically mixed for 1 hour. The resulting solution is dried rapidly via a commercial spray drying process from which a coated dry precursor is obtained. For this, the atomizer used is a two-fluid nozzle. The atomization pressure in the process is set to 0.3 bar, where the mixed solution is turned into a spray of small droplets. Once the droplets are generated, they are flown into an electronic furnace via an air flow in which the 5 ACS Paragon Plus Environment


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droplets are dried. The coated dry precursor particles are gathered to the cyclone walls, while in contrast, the lighter, moist air is separated from the particles and directed away through the exhaust pipes. The temperatures of the inlet and the outlet of the spray dryer in the process are set as 180°C and 110°C, respectively. Conditions such as feeding speed and solution concentration are controlled strictly during the process. The obtained Ni0.8Co0.15Al0.05(OH)2 / Al2O3 composite spheres is grounded thoroughly in an agate mortar with LiOH·H2O with a molar ratio of 1:1.05. After mixing well for 40 minutes, the prepared mixtures are first preheated at 550 °C for 4 h in air, and then annealed them at 750 °C for 12 h in a continuous flow of O2, from which LiAlO2 coated LiNi0.8Co0.15Al0.05O2 powders are obtained. Three coated samples with 1%, 2% and 5% Al2O3 coating are represented as SDCNCA1, SDCNCA2 and SDCNCA5. 2.2 Materials characterization Powder X-ray diffraction (D/max-r A type Cu Kα1, 40 kV, 300 mA, Japan) is employed into identifying the crystalline phases of the synthesized precursor and as-prepared materials, which is gathered in the 2θ from 10° to 80°. A scanning electron microscopy (SEM) with energy disperse X-ray spectrometer (EDS) (SEM, JEOL JSM-6360LV, Japan) is applied to observe the morphology of the particles and the elements distribution on the surface. To find the interior Al distribution interior, cross-sections for SEM analysis are conducted with the samples by embedding the as-coated LiNi0.8Co0.15Al0.05O2 particles in an epoxy, and then the compound are grinded on metallographic sand papers. Transmission electron microscopy (TEM) and 6 ACS Paragon Plus Environment

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high-resolution transmission electron microscopy (HRTEM, Tecnai G12) is applied to observe the microstructures of the SDCNCA samples. The TGA/DSC analysis is conducted by a NETZSCH STA-449C (Germany) at a heating speed of 5 °C min−1 under Ar atmosphere. 2.3 Electrochemical evaluation The cathode is prepared by blending the active material, acetylene black and poly (vinyl difluoride) PVDF binder by a mass ratio of 8:1:1, which is dispersed in N-pyrrolidinone (NMP) in an agate mortar, and then coated onto an Al foil. The as-prepared cathode foil is dried under vacuum at 120 °C for 12 h to eliminate the NMP solvent, which is assembled a CR2025 coin cell with a lithium metal anode, a porous polypropylene film, using 1mol L-1 LiPF6 in EC/EMC/ DMC (1:1:1 in volume) solvent as the electrolyte in an argon-filled glove box. All the electrochemical properties were cycled galvanostatically between 2.8 and 4.3 V (vs. Li+/Li) and the current density of 1C is set as 180 mA g-1. The initial charge-discharge curves are evaluated at 0.1 C rate at room temperature, while the cycle performance is evaluated at room temperature at 1 C rate for 150 cycles. As to the performance at upper cutoff voltage, it is set as 4.4V and 4.5V respectively. The cells are also tested at 55°C. Electrochemical impedance spectroscopy (EIS) is conducted after 100 cycles with a frequency range from 0.01 Hz to 100 kHz.

3. Results and discussion 3.1 Material characterizations 7 ACS Paragon Plus Environment


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Fig. S-1 shows the XRD diffraction profiles of the pristine Ni0.8Co0.15Al0.05(OH)2 precursor and the Al2O3 coated precursors. As seen from it, the diffraction peak positions of all the coated samples are the same as the pristine one, which matches well with that of Ni(OH)2. Typical phase structure of the LiNi0.8Co0.15Al0.05O2 particles and the as-coated samples are analyzed by XRD, as displayed in Fig. S-2. All of the samples display clear diffraction peaks, and the SDCNCA samples exhibit the same diffraction peaks as those of the pristine NCA with no significant difference in the phase structure. These samples are all single phase with an isostructural of LiNiO2, whose diffraction patterns are related to the hexagonal α -NaFeO2 structure with a space group 𝑅3̅𝑚. This indicates that spray-drying-coating Nano-Al2O3 does not cause any noticeable change in the crystal structure. Fig. 1 displays the surface morphology difference of the SDCNCA precursors and the pristine one. Although after being coated, the SDCNCA precursors retain a spherical morphology in its secondary particles as the pristine precursor, there are significant differences on the surface. As shown in Fig. 1(a), it is clearly that the primary particles of the pristine precursor are needle-like shaped and the secondary forms are densely agglomerated. However, after nano-Al2O3 spray coating on the precursor, the needle-like shaped primary particles are covered by a uniform and tight layer. This can be presumed to be the nano- Al2O3 layer and it is uniformly distributed on the surface of the precursor. To further confirm the change of the Al distribution on the surface, EDS is conduced and shown in Fig. S-3 and Fig.S-4. It is obvious that Ni, Co and Al elements are all distributed uniformly on both the pristine NCA precursor 8 ACS Paragon Plus Environment

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and the SDCNCA2 precursor. However, after coating, the content of Al on the precursor surface become larger, which is found by the color variation of the Al on the surface displayed in Fig. S-4. This proves that Al2O3 is successfully coated on the precursor surface. The morphology variation of NCA and SDCNCA cathode materials are also characterized by SEM measurements. As shown in Fig.2, no obvious change of the overall morphology is observed for all the cathode particles, which means that the nano-Al2O3 spray coating would not damage the general morphology of the NCA particles. As seen in Fig. 2(a), the primary particles of the pristine NCA samples are extremely obvious with a flake-like shape. Specifically, it even can be found that there are edges between the primary particles which have different crystallographic orientation, slip planes and anisotropic lattice. Nevertheless, the SDCNCA particles deliver a slight difference on the surface which can be seen from Fig. 2 (b-d). Specifically, the edges between the grains blur as if the particles are covered with a layer of gel. The phenomenon is more apparently with the content of Al 2O3 higher. Meanwhile, SEM images as well as the linear sweep signals of energy dispersive spectrometer images on the cross-section of a single particle are carried out to examine the composition change in the SDCNCA2 sample. The atomic ratio of the three elements, Ni, Co, and Al is obtained along the diameter of around 9 um, which is shown in Fig .3. As seen from it, to the SDCNCA2 sample, the concentration of Al in the surface become a slightly higher than in the core, with a corresponding decrease of Ni and Co. This confirms that the SDCNCA2 sample is a material with an 9 ACS Paragon Plus Environment


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aluminium rich surface. TEM observation are further carried out to investigate the micro-structure, and the TEM images of the bare and SDCNCA2 sample are displayed in Fig. 4. Fig.4 (a), (b) reveals that the NCA material is relatively well covered with a coating layer, whose thickness is about 10-40 nm. As shown in Fig. 4(d), the HRTEM image of SDCNCA2, it is obviously that the planar distance of outer space is different from the inner part, which are 0.2401 nm and 0.2449nm respectively. They display a very well developed crystal lattice fringe which are arranged well along the (101) axis of NCA and LiAlO2 respectively, as show in table S-1. It is completely different from the pristine sample (Fig.4(c)). Therefore, all these material characterizations reveal that NCA precursor coated with Al2O3 by the homogeneous and facile coating method has been successfully synthesized. Meanwhile, this spray-drying-coating method confirms a LiAlO2 layer distribute uniformly on the as-prepared cathode material. 3.2 Electrochemical characterizations Electrochemical performance is initially subjected to preconditioning cycles at 0.1C rate from 2.8 V to 4.3V, which is shown in Fig. 5(a). All the SDCNCA samples and the pristine one exhibit similar smooth and monotonous charge/discharge profile. However, the initial discharge capacity is a slightly different for these cathode materials. In specific, the cell based on the pristine active material initially delivered a discharge capacity of 188.4 mAh g-1. It is a little higher than that of the SDCNCA samples, which is 186.6, 184.1 and 174.3 mAh g-1 for SDCNCA1, SDCNCA2 and 10 ACS Paragon Plus Environment

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SDCNCA5 respectively. This can mainly be ascribed to the decreased amount of electrochemical active material which is replaced by an insulator alumina coating layer. After activated two preconditioning cycles at 0.1C rate, the cycle performance of all the samples is tested at 1C rate at room temperature, which is shown in Fig. 5 (b). The initial discharge capacity at 1C rate of the pristine sample delivered is 171.1 mAh g-1, compared with the SDCNCA samples having 167.2, 165.7 and 154.2 mAh g-1 for SDCNCA1, SDCNCA2 and SDCNCA5 respectively. Whereas, the capacity retention of SDCNCA samples is remarkably enhanced, which deliver a retention of 87.56%， 90.40% and 93.58% after 150 cycles for SDCNCA1, SDCNCA2 and SDCNCA5 respectively, while the pristine electrode suffers from 30.9 mAh g-1capacity loss and only a 81.98% retention after 150 cycles. The improvement of the capacity retention shows a superiority to those reported the coated method, which are mostly displayed a retention of

Enhancing the Thermal and Upper Voltage Performance of Ni-Rich

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