Al2O3 Coating through a Wet-Chemical Method to

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Tunable LiAlO2/Al2O3 Coating through a Wet-Chemical Method to Improve Cycle Stability of Nano-LiCoO2 Changlong Chen, Weiliang Yao, Qianran He, Maziar Ashuri, James A. Kaduk, Yuzi Liu, and Leon L. Shaw ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b02079 • Publication Date (Web): 04 Apr 2019 Downloaded from http://pubs.acs.org on April 5, 2019

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ACS Applied Energy Materials

Tunable LiAlO2/Al2O3 Coating through a Wet-Chemical Method to Improve Cycle Stability of Nano-LiCoO2 Changlong Chen, 1 Weiliang Yao, 1 Qianran He, 1 Maziar Ashuri, 1 James Kaduk, 2 Yuzi Liu, 3 Leon Shaw 1,* 1

Department of Mechanical, Materials and Aerospace Engineering, Illinois Institute of Technology, Chicago, Illinois 60616

2

Department of Chemistry, Illinois Institute of Technology, Chicago, Illinois 60616 3

Argonne National Laboratory, 9700 S. Cass Avenue, Chicago, Illinois 60439 Abstract

A facile wet-chemical method to coat nano-LiCoO2 particles is investigated and established in this study. In this newly-developed wet-chemical method, Al(NO3)3 is used as the Al source to form Al2O3 and LiAlO2, whereas LiNO3 is used as a sacrificial agent to protect nanoLiCoO2 and at the same time to form LiAlO2 by reacting with Al2O3. Addition of LiNO3 into the Al(NO3)3 coating solution suppresses the unwanted formation of Co3O4 during the coating process and leads to a thin (5–10 nm) and continuous LiAlO2/Al2O3 coating on nano-LiCoO2 particles. 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 exhibits an unusually high initial specific capacity of 225 mA h g-1, while micro-LiCoO2 can only deliver a specific capacity of 145 mA h g-1 even though the charge/discharge voltage windows are the same. Furthermore, 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 offers 100% increase in the specific capacity over pristine nano-LiCoO2 after 5 cycles at 0.1C, 20 cycles at 1C and 20 cycles at 3C. Moreover, 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 is able to charge/discharge for 425 cycles at 3C with only 18% capacity loss and maintain a final specific capacity of 128 mA h g-1. In contrast, the specific capacity of micro-LiCoO2 diminishes to 50 mA h g-1 after 200 cycles at 3C. The unusually high specific capacity and superior capacity retention for long cycle life at high rates of 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 are attributed to its huge electrode/electrolyte interfacial area for charge/discharge with small polarization and the effectiveness of LiAlO2/Al2O3 coating in preventing capacity decay during soaking as well as during cycling. The principle and methodology of this newly-developed wet-chemical coating method are applicable to other 1 ACS Paragon Plus Environment

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layered transition metal oxide cathodes and can open up new opportunities to obtain superior electrochemical properties from these advanced cathodes in the future. Keywords: LiCoO2 cathode, nano-LiCoO2, Al2O3 coating, LiAlO2 coating, Li-ion batteries * Corresponding author: [email protected] ‘Declarations of interest: none’ 1. Introduction Lithium-ion batteries (LIBs), as one of the most advanced energy-storage technologies, have drawn extensive attention in the past few decades.1-3 They are currently powering a wide range of portable electronic devices from cell phones to cameras, laptops, power tools, etc. Further improvements in their properties such as reducing charge time from hours to minutes can open up new applications and expand the market for present ones. A case in point for the benefit of short charge time is the continuous effort of electric vehicle manufacturers such as Tesla, Inc. to develop superchargers that can shorten charge time from several hours to about one hour.4 Short charge time can enable long distance travel and remove a critical barrier to consumer acceptance of electric vehicles.5 The goal of increasing the rate capability can be achieved by using nanomaterials since nanomaterials are able to offer larger surface area for Li ion intercalation/deintercalation and shorter Li-ion diffusion distance. To date, some studies have indeed proven the higher-rate capability obtained through the use of nanomaterials.6-9 However, nanomaterial-based electrodes normally have faster capacity decay over charge/discharge cycles than electrodes made of micrometer-sized materials because many capacity decay mechanisms start at the electrode/electrolyte interface.10-15 It is well known that layered transition metal oxide cathodes suffer from many capacity decay mechanisms including transition metal dissolution, electrolyte decomposition, formation of insulating phases at the particle surface, particle surface transformation, and loss of lattice oxygen.10-18 Since all of the aforementioned decay mechanisms start at the particle surface, nanomaterials having larger surface areas than micrometer-sized materials can be expected to have faster capacity decay behavior. One of the approaches to address the surface-related decay mechanisms is to use surface 2 ACS Paragon Plus Environment

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coating. Indeed, coatings of MgO,19,

20

TiO2,21,

22

ZrO2,22,

23

ZnO,24 Al2O3,25-31 LiAlO2,32

Li2ZrO3,33 SnO2,34 silica,35 phosphates,36-38 multi-component oxides (such as MgAl2O4 and ZrTiO4),39 fluorides,40 and carbon41 have been shown to be capable of improving the electrochemical performance of LiCoO2 in different degrees. However, most of the coating methods developed so far are demonstrated with micrometer-sized LiCoO2 particles.19-31, 34-41 As such, investigation is required to prove that the coating methods demonstrated with micrometersized LiCoO2 particles can be extended to nano-LiCoO2 particles. Otherwise, they may not work for nano-particles. For example, sol-gel coating works well for micrometer-sized LiCoO2,20, 22, 25, 34

but has not been reported for nano-LiCoO2 yet. The challenge for coating nano-particles is that

the coating needs to be “ultrathin”. Otherwise, the overall capacity of coated nano-LiCoO2 will decrease significantly because of the weight of the inactive coating. Apparently, the lack of control over ultrathin thickess and uniformity prohibits sol-gel coating for nano-LiCoO2.42 As a result, atomic layer deposition (ALD), which can provide conformal thin films with atomic thickness,42-45 often becomes the choice of coating method for nano-LiCoO2 material.42 Nevertheless, ALD usually requires expensive equipment and thus simple, inexpensive and scalable coating methods are urgently needed for nanomaterials. In this study, we focus on the strategy of a wet-chemical method to deposit thin and continuous LiAlO2/Al2O3 coating on nano-LiCoO2 particles to improve its electrochemical properties. In this wet-chemical method, Al(NO3)3 is used as the Al source to form Al2O3 and LiAlO2, whereas LiNO3 is used as a sacrificial agent to protect nano-LiCoO2 and at the same time to form LiAlO2 by reacting with Al2O3. It is shown that LiNO3 can prevent the formation of Co3O4 during the coating process and leads to a thin and continuous LiAlO2/Al2O3 coating on nano-LiCoO2 particles. Furthermore, nano-LiCoO2 particles with LiAlO2/Al2O3 coating exhibit significant improvements in the specific capacity and cycle life over pristine nano-LiCoO2. To our best knowledge, this is the first time that LiAlO2/Al2O3 coatings have been investigated. As the first study of such coatings, we have focused on a LiAlO2/Al2O3 coating with a composition of ~70 wt. % LiAlO2 and ~30 wt. % Al2O3 to investigate its effectiveness in improving the electrochemical performance of nano-LiCoO2. This work will provide a solid foundation for future studies of LiAlO2/Al2O3 coatings with tunable compositions of different LiAlO2-to-Al2O3 ratios to identify the best composition that can combine the cycle stability advantage offered by Al2O3 coating 25-31 and the higher specific capacity advantage offered by LiAlO2 because LiAlO2 3 ACS Paragon Plus Environment


can provide Li+ and accept Li+ in the cycling process while Al2O3 cannot.32 2. Methods Synthesis of nano-LiCoO2. Nano-LiCoO2 powder was synthesized using the hydrothermal reaction established in Ref. 46 and specific details of experimental conditions can be found in Ref. 47. Al2O3 and LiAlO2/Al2O3 coatings for nano-LiCoO2. The Al2O3 coating on the assynthesized nano-LiCoO2 powder was carried out using a wet chemical method with Al(NO3)3 as the precursor. Nano-LiCoO2 and Al(NO3)3·9H2O (Alfa Aesar) were mixed in pure ethanol at mass ratios of 1:1, 1:1.5 and 1:2 to produce nano-LiCoO2 with different quantities of Al2O3 coatings. If all the Al elements were retained in the system and reacted with oxygen, then the corresponding Al2O3 quantities in the Al2O3-coated LiCoO2 system would be ~12 wt. %, ~17 wt. % and ~21 wt. %, respectively. The mixture of nano-LiCoO2 powder and Al(NO3)3 solution was stirred using a Thinky Mixer at 250 rpm for 5 min. Then the resulting slurry was dried in a forced air convection oven at 100 °C for 2 h. After drying, the obtained product was heated in a tube furnace up to 600 °C under a compressed air flow and hold at that temperature for 3 h to obtain the desired Al2O3 coating. To avoid the formation of Co3O4 in the Al2O3 coating process, LiNO3 has been added to the Al(NO3)3 ethanol solution in a molar ratio of 3:4 before dispersing nanoLiCoO2 powder in the mixed Al(NO3)3 + LiNO3 solution. The remaining procedure for forming LiAlO2/Al2O3 coating using the mixed Al(NO3)3 + LiNO3 solution is the same as that for forming Al2O3 coating using the pure Al(NO3)3 solution. Characterizations. The crystal structures of nano-LiCoO2 powder before and after Al2O3 coating were examined using X-ray Powder Diffraction (XRD, Bruker D2 Phaser) employing CuKα radiation (1.54056 Å). To provide good determination of the lattice parameters (a and c) of nano-LiCoO2 with and without Al2O3 or LiAlO2/Al2O3 coating, 5 wt. % micrometer-sized Si (micro-Si hereafter) powder was mixed uniformly with the LiCoO2 powder before the XRD experiment to serve as an internal reference. Rietveld refinement of the XRD data was carried out using GSAS-II.47 The lattice parameter for the internal standard of micro-Si was fixed at 5.43094 Å, whereas a scale factor was refined for all other phases and so did the lattice parameters for LiCoO2 and Co3O4. The z-coordinate of the O atom was refined for LiCoO2, but no restraints 4 ACS Paragon Plus Environment

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were placed on bond distances or angles. It was not possible to refine the isotropic displacement coefficients, and thus they were fixed at typical values (based on the published LiCoO2 structure, i.e., layered R3m structure) of 0.02 Å2 for Li and O and 0.01 Å2 for Co. Further, with only one data set and the possibility of Li, Co, Al, or vacancies at each of the two cation sites in LiCoO2, it was not possible to obtain a full description of the contents of each site. The number of electrons per the Li site and Co site in nano-LiCoO2 was computed based on the refined site occupancies and the atomic numbers of Li and Co to probe the possibility of Al incorporation into nanoLiCoO2 among various samples with different coating conditions. For the as-synthesized nano-LiCoO2 powder without coating, the peak profiles were found to be dominated by size broadening, and thus a uniaxial model (unique axis 001) was used. The XRD patterns of the coated samples were, however, dominated by strain broadening, and thus the generalized tensor model of Stephens48 was used to describe the profiles. A 6-term Chebyshev polynomial was used to model the background. A specimen displacement term was refined for each sample. In addition, the average sizes of pristine nano-LiCoO2 particles, Al2O3-coated and LiAlO2/Al2O3-coated counterparts perpendicular to a given crystallographic plane (hkl), Dhkl, were estimated using the Scherrer formula 49 and the correction for instrumental broadening was taken into account using the formula describe in Ref. 50. The morphologies and sizes of pristine nano-LiCoO2 were observed using energy-filtered transmission electron microscopy (EFTEM, JEOL JEM2100F TEM in the Center for Nanoscale Materials, Argonne National Laboratory), whereas the morphologies and sizes of Al2O3-coated and LiAlO2/Al2O3-coated nano-LiCoO2 particles were analyzed using scanning transmission electron microscopy (STEM, FEI Tecnai F20ST STEM) equipped with energy dispersive spectroscopy (EDS) in the Electron Microscopy Center, Argonne National Laboratory because EFTEM is not suitable for detecting Al elements. The morphology and sizes of commercially available micrometer-sized LiCoO2 (MTI Corporation) were examined using scanning electron microscopy (SEM). Brunauer, Emmett and Teller (BET) measurement was performed to determine the specific surface area (SSA) of micro- and nano-LiCoO2 with and without coating with a two-channel Nova Quantachrome 2200e surface area & pore size analyzer. Electrochemical measurements. To evaluate the electrochemical performance, 75 wt. % nano-LiCoO2 powders with different Al2O3 or LiAlO2/Al2O3 coatings were mixed with 15 wt. % Super P carbon black (CB) powder and 10 wt. % polyvinylidene fluoride (PVDF) in N-methyl5 ACS Paragon Plus Environment


2-pyrrolidone (NMP). To obtain uniform mixing of all ingradients in the cathode, a multi-step mixing is used. Specifically, PVDF was added to NMP first and then CB was added to the mixture after PVDF was uniformly mixed with NMP. Nano-LiCoO2 with or without coating was added to the mixture in three steps with each step adding one third of the desired nano-LiCoO2 powder. A THINKY machine and sonication were used to improve mixing after each adding step mentioned above. The resulting slurries were cast on Al foils, followed by vacuum drying at 60 °C for 6 h and then 120 °C for 6 h. Disc electrodes were punched out from the dried electrode panel with a controlled mass loading of the active material at about 2 mg cm-2. CR2032-type coin cells were assembled with a Li metal disc (thickness ~1.1 mm) as the anode, 1 M LiPF6 in ethylene carbonate and diethyl carbonate (EC:DEC 1:1 volume ratio) as the electrolyte, and the trilayer membrane (Celgard 2325) as the separator. The entire process of coin cell fabrication was conducted within an argon-filled glovebox with H2O < 0.1 ppm and O2 < 1 ppm. The coin cells were rest for 4 h before the electrochemical tests. Galvanostatic charge-discharge was carried out employing a Neware® battery testing system at room temperature. The cells were charged and discharged at C/10 for 5 cycles, followed by cycling at 1C for 20 cycles, and finally 3C for 20 cycles in the voltage range between 2.8 V and 4.3 V. Cyclic voltammetry (CV) test was performed with Parstat 4000 (Princeton Applied Research) from 2.8 to 4.3 V vs. Li/Li+ at scan rates ranging from 0.1 to 32 mV s-1, while electrochemical impedance spectroscopy (EIS) tests were performed using the same equipment. EIS tests conducted at the open circuit voltage (OCV) corresponded to the fully discharged state. A sinusoidal signal with amplitude of 10 mV was scanned with a frequency ranging from 10 kHz to 1 Hz. To fit the EIS data to an equivalent electronic circuit, ZSimpWin software package was employed. For comparison, commercially available microLiCoO2 powder (MTI Corporation) was also used to fabricate coin cells and tested under the same conditions as nano-LiCoO2 with and without Al2O3 coating. 3. Results and Discussion 3.1 Characteristics of Pristine Nano-LiCoO2 The XRD pattern of pristine nano-LiCoO2 is shown in Figure 1 along with those of Al2O3coated and LiAlO2/Al2O3-coated nano-LiCoO2. As observed, the diffraction peaks of the pristine nano-LiCoO2 match the layered structure of LiCoO2 with space group of R3m well (PDF card: 6 ACS Paragon Plus Environment

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04-013-4373). Moreover, the measured lattice parameters, a and c, are similar to those reported in Ref. 46. In addition, the c/a ratio for the as-synthesized LiCoO2 is close to 5.00 (Table 1), indicating that pristine nano-LiCoO2 has the layered structure with space group of R3m rather than a spinel-type structure (cubic, Fd3m).46, 51 The dimensions of pristine nano-LiCoO2 platelets estimated using the Scherrer formula

49

with the correction of instrumental broadening

50

are

summarized in Table 1 along with the measured lattice parameters. These estimations reveal that pristine nano-LiCoO2 platelets have an average width of ~39 nm and an average thickness of ~24 nm. SEM and TEM analyses indicate that pristine nano-LiCoO2 has a platelet morphology with clear hexagonal, rhombus or trapezoidal shape (see Figures 2(a) and S1 and S2 in the Supporting Information). The platelet width ranges from ~20 to 100 nm, confirming the estimation of the average width of ~39 nm from the XRD analysis, while the thickness of the platelets is determined to be ~23 nm, a number very close to the XRD analysis of ~24 nm. In contrast, the particles of micro-LiCoO2 are very large with sizes from 3 to 15 m and having an equiaxed shape with flat facets in many cases (Figure 2b). The specific surface areas of micro- and nano-LiCoO2 powders determined using the BET method are listed in Table 2. Based on the specific surface area the equivalent particle diameter can be calculated if one assumes spherical particle shape. It is noted that the equivalent particle diameter of micro-LiCoO2 is 3.05 m which is at the lower end of the SEM observation. The equivalent particle diameter of pristine nano-LiCoO2 is found to be 16 nm which is in the same order of magnitude as nano-LiCoO2 platelets determined from both TEM and XRD. Using these equivalent particle diameters and assuming that the density of the coating is the same as that of the LiCoO2 core, one can easily estimate the coating quantity needed to form a 5-nm thick coating. This simple estimation reveals that the weight percent of the coating is only 0.1% of the LiCoO2 core for micro-LiCoO2 particles, whereas the weight percent of the coating is 330% of the LiCoO2 core for nano-LiCoO2. Thus, the quantity of the LiAlO2/Al2O3 coating ranging from ~10 wt. % to ~20 wt. % is investigated in this study. Note that this quantity range is significantly larger than the typical 1 wt. % to 5 wt. % coating reported for micrometer-sized particles.19-32 In spite of the very high coating percentage, it will be shown later that the highest coating quantity, 21 wt. % LiAlO2/Al2O3 coating, offers the best improvement in electrochemical properties for the pristine

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nano-LiCoO2, indicating the effectiveness of the simple estimation on the coating quantity based on the size of LiCoO2 particles. 3.2 Formation of Al2O3 or LiAlO2/Al2O3 Coatings with No Side Reactions The XRD patterns of Al2O3-coated samples with and without the addition of LiNO3, obtained with reactions at 600 °C for 3 h, are shown in Figure 1. It can be seen that the XRD pattern of nano-LiCoO2 is present in all samples. However, it is noted that Al2O3-coated samples without the addition of LiNO3 contain the Co3O4 impurity, whereas the formation of Co3O4 is avoided with the addition of LiNO3 (see Table 1). Quantitative analysis of the XRD patterns reveals that the quantity of the Co3O4 impurity increases with increasing the amount of Al(NO)3 added in the coating process. Specifically, the weight ratio of Co3O4 in nano-LiCoO2 samples with 12 wt. %, 17 wt. % and 21 wt. % Al2O3 coating is 1.0:3.0:4.1, respectively. The formation of Co3O4 clearly indicates the presence of side reactions during the coating process. We propose that the side reactions proceed with the following mechanisms. First, Al2O3 reacts with LiCoO2 to form LiAlO2 and Li-deficient LiCoO2, as indicated by Equation (1). Second, the newly formed Li-deficient LiCoO2 is thermally unstable and thus transforms to Co3O4 and LiCoO2,52 as indicated by Equation (2). 1

Al2O3 +yLiCoO2 + 2O2 = 2LiAlO2 +yLi(1 ― 2)CoO2 (y >> 2) y

LixCoO2 =

1―x 3 Co3O4

+xLiCoO2 +

1―x 3 O2

1

(x = 1 ― y and x < 1)

(1)

(2)

In the case of 12 wt. % Al2O3 coating, the quantity of Co3O4 was small because of the limited formation of LiAlO2. However, as the quantity of Al2O3 increases (such as 17 wt. % and 21 wt. % Al2O3 coatings), more LiAlO2 was formed, leading to more Li loss from the fixed amount of the nano-LiCoO2 and thus more formation of Co3O4. It should be pointed out that the proposed reaction (1) is consistent with a recent study12 showing that LiAlO2 is formed between the Al2O3 coating and micro-LiCoO2 particles after 600 and 800 °C annealing, but not at 400 °C. Furthermore, the same study12 shows that no LiAlO2 peaks are observed in XRD patterns because of the use of micro-LiCoO2 particles, but solid-state nuclear magnetic resonance (NMR) clearly 8 ACS Paragon Plus Environment

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reveals the formation of LiAlO2 phase for Al2O3-coated LiCoO2 with 600 and 800 °C annealing. In addition, the formation of Co3O4 is not reported in Ref. 12 because of the use of micro-LiCoO2 particles and thus limited surface areas available to react with Al2O3 to destabilize micro-LiCoO2. Based on the proposed mechanism for the formation of Co3O4, a possible way to suppress the formation of Co3O4 would be to add LiNO3 as a sacrificial agent which can decompose to Li2O, NO2, NO, N2, O2 and other gaseous species at temperature above ~200 °C.53 The decomposition product of Li2O can in turn react with Al2O3 to form LiAlO2, thereby preventing the reaction between Al2O3 and LiCoO2 as shown in Equation (1). The suppression of Reaction (1) also eliminates the possibility of Reaction (2) since stoichiometric LiCoO2 is thermally more stable than the Li-deficient LiCoO2 as shown in Equation (2). Figure 1 confirms that addition of LiNO3 into the Al(NO3)3 solution has indeed prevented the formation of Co3O4 and thus stabilized the nano-LiCoO2. Therefore, LiNO3 can serve as an effective sacrificial agent to avoid the destabilization of nano-LiCoO2 in the wet-chemical Al2O3 coating process. Because of the formation of Co3O4 in Al2O3-coated samples with no addition of LiNO3 in the coating process, it is worthy of examininig whether the nano-LiCoO2 has gone through some changes during the coating process. Furthermore, it will be interesting and important to know what other effects the addition of LiNO3 in the coating process have in addition to preventing the formation of Co3O4. Thus, through detailed Rietveld refinement using GSAS-II, the following trends are observed. First, the c parameter of LiCoO2 in the coated samples varies from sample to sample, depending on the coating conditions. As shown in Table 1, 12 wt. % Al2O3-coated nano-LiCoO2 with no addition of LiNO3 has the lowest c parameter which increases as the quantity of Al2O3 coating increases. We attribute this trend to the competition of two physical processes: (i) partial loss of Li ions from nano-LiCoO2 to form LiAlO2 as indicated by Eq. (1) and (ii) incorporation of Al ions into nano-LiCoO2 to form Al-doped LiCoO2. Based on the bond valence method,54 we expect an octahedral Li-O distance to be 2.13 Å, a Co-O distance to be 1.95 Å, and an Al-O distance to be 1.90 Å. Thus, incorporation of Al ions into LiCoO2 to form Al-doped LiCoO2 would be expected to shrink the c parameter. In contrast, partial delithiation of LiCoO2 leads to slight increase in the c parameter while the a parameter exhibits little change.55 The slight lattice expansion along the c axis (when lithium is partially removed) is due to the weakened coulomb interaction between positively charged Li layers and negatively charged CoO6 layers.55 We believe 9 ACS Paragon Plus Environment


that partial loss of Li ions and Al-ion incorporation take place simultaneously during the coating process. Furthermore, for 12 wt. % Al2O3-coated nano-LiCoO2 the incorporation of Al ions is the dominant event. As such, the c parameter is reduced when compared with that of pristine nanoLiCoO2. However, as the quantity of Al2O3 coating increases, more LiAlO2 is formed, leading to more Li loss from nano-LiCoO2. As a result, the c parameters for both 17 wt. % and 21 wt. % Al2O3-coated nano-LiCoO2 with no addition of LiNO3 are increased when compared to that of pristine nano-LiCoO2. Further, 21 wt. % Al2O3-coated nano-LiCoO2 has the largest c parameter among the three Al2O3-coated samples with no addition of LiNO3 (Table 1) because of the largest amount of LiAlO2 to be expected. Second, the a parameter of LiCoO2 in the coated samples is smaller than that of pristine LiCoO2, as shown in Table 1. This phenomenon is due to Al incorporation into nano-LiCoO2. As mentioned above, the a parameter changes little when Li ions are partially removed from LiCoO2.55 However, reduction in the a parameter would be expected if Al ions are incorporated into the Co site because the bond valence method54 predicts the Co-O distance being 1.95 Å and Al-O distance being 1.90 Å. Therefore, reduction in the a parameter for all coated samples is indicative of the formation of Al-doped LiCoO2 in the coating process. Third, the formation of Al-doped LiCoO2 in the coating process is also supported by examining the lattice microstrain in all samples. As shown in Figure 3, the lattice microstrain is zero for pristine nano-LiCoO2. However, all of the coated samples have discernable lattice microstrain, indicating the presence of elastic distortion in the lattice, which is believed to be caused by Al-ion incorporation into nano-LiCoO2. Furthermore, the lattice microstrains in the coated samples with no addition of LiNO3 are all larger than those in the coated samples with addition of LiNO3, indicating that adding sacrificial LiNO3 during the coating process has minimized Al incorporation into nano-LiCoO2, presumably because of the consumption of Al2O3 in reacting with Li2O derived from LiNO3. Fourth, adding sacrificial LiNO3 in the coating process not only avoids the formation of Co3O4, but also leads to little change in the c parameter for both 17 wt. % and 21 wt. % Al2O3coated samples when compared with pristine nano-LiCoO2 (Table 1). This result suggests that adding LiNO3 has minimized the loss of Li ions from nano-LiCoO2 in the coating process. This is not a surprise since Li can be provided by sacrificial LiNO3 to form LiAlO2 in the coating process. However, adding LiNO3 does not prevent the formation of Al-doped LiCoO2 because the a 10 ACS Paragon Plus Environment

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parameter of both 17 wt. % and 21 wt. % Al2O3-coated nano-LiCoO2 with addition of LiNO3 are smaller than that of pristine nano-LiCoO2 (Table 1). As discussed above, incorporation of Al ions at the Co site results in a smaller a parameter. Additionally, a comparison between 17 wt. % and 21 wt. % Al2O3-coated samples with and without addition of LiNO3 reveals that the a parameter is smaller for 17 wt. % and 21 wt. % Al2O3-coated samples without addition of LiNO3 than those of the counterparts with addition of LiNO3, suggesting that Al2O3-coated samples without addition of LiNO3 have more Al incorporation than the coated samples with addition of LiNO3. This trend is in good agreement with the third trend discussed above. Fifth, it is scientifically interesting and important to know where Al ions are located in Aldoped LiCoO2. To answer this question, we have resorted to analysis of the number of electrons per the Li and Co sites (i.e., at the 3a and 3b sites, respectively, in the layered structure of LiCoO2 with space group of R3m). As shown in Figure 4(a), all of Al2O3-coated samples have lower number of electrons at the Co sites in comparison with pristine nano-LiCoO2. This trend indicates that Al ions are present at the Co sites because Al element has 13 electrons and Co has 27. Partial substitution of Co ions by Al ions will thus lead to reduction in the number of electrons at the Co sites. In addition, it is noted that the number of electrons per the Co site for Al2O3-coated nanoLiCoO2 with addition of LiNO3 (Samples 4 and 6 in Figure 4a) is closer to that of pristine nanoLiCoO2 than the counterparts with no addition of LiNO3 (Samples 2, 3 and 5), indicating that addition of LiNO3 has resulted in less Al occupancy at the Co sites because of less Al incorporation into nano-LiCoO2. This trend is in good accordance with the third and fourth trends discussed above. Finally, it is also noted that the number of electrons per the Co site for pristine nano-LiCoO2 is smaller than 27, suggesting the presence of some Li ions at the Co sites and/or vacant Co sites in pristine nano-LiCoO2. Based on the number of electrons at the Co sites and assuming that the change in the number of electrons at the Co sites is all due to substitution of Co by Al, then one can obtain the Co site occupancy and its site formula as shown in Table 3. Since the Co site occupancy and its site formula are derived from the number of electrons at the Co sites, the trends exhibited by the Co site occupancy and its site formula are thus the same as that discussed above for the number of electrons at the Co sites. Sixth, the number of electrons per the Li site shows a much more complicated trend (Figure 4b). Specifically, the number of electrons per the Li site for 12 wt. % Al2O3-coated sample is higher than that of pristine nano-LiCoO2, indicating that some Al ions are located at the Li sites 11 ACS Paragon Plus Environment


because Al has 13 electrons and Li has only 3. However, as the quantity of the Al2O3 coating increases, the number of electrons at the Li site decreases for the coated samples with no addition of LiNO3. This trend is due to the formation of Li vacancies induced by more Li-ion loss from nano-LiCoO2 to form LiAlO2 in the coating when the Al2O3 coating quantity is increased. Note that this trend is consistent with the first trend discussed above, i.e., the incorporation of Al ions is the dominant event for 12 wt. % Al2O3-coated nano-LiCoO2, whereas loss of Li ions becomes the dominant event for 17 wt. % and 21 wt. % Al2O3-coated nano-LiCoO2 with no addition of LiNO3. It is also noted that the number of electrons per the Li site for pristine nano-LiCoO2 is higher than 3, suggesting the presence of some Co ions at the Li sites because of the cation mixing. Assuming there are no vacancies at the Li sites and thus the extra number of electrons higher than 3 is due to the presence of Co ions at the Li sites, one can estimate that only ~0.4% of the Li sites are occupied by Co ions. Seventh, when sacrificial LiNO3 is added in the coating process, the number of electrons per the Li site is similar to that of pristine nano-LiCoO2 (Figure 4b). As discussed in the fourth trend above, addition of LiNO3 has resulted in both limited loss of Li ions from nano-LiCoO2 and reduced Al incorporation into nano-LiCoO2. Thus, the similar number of electrons per the Li site for pristine nano-LiCoO2 and Al2O3-coated nano-LiCoO2 with addition of LiNO3 is consistent with the expectation that little loss of Li ions from nano-LiCoO2 is accompanied by limited incorporation of Al ions into nano-LiCoO2 for Al2O3-coated nano-LiCoO2 with addition of LiNO3. Combining this trend with the fifth trend discussed above, one can deduce that Al ions are preferentially located at the Co sites when the Al concentration in nano-LiCoO2 is low. However, when the Al concentration in nano-LiCoO2 is high (such as 12 wt. % and 17 wt. % Al2O3-coated nano-LiCoO2 with no addition of LiNO3), Al ions not only reside at the Co sites, but also occupy some of the Li sites, as evidenced by the increased number of electrons at the Li sites for these two Al2O3-coated samples. It should be pointed out that 21 wt. % Al2O3-coated nano-LiCoO2 with no addition of LiNO3 also has a similar number of electrons per the Li site as pristine nano-LiCoO2. However, this case is not due to limited loss of Li ions and limited occupancy of Al ions at the Li sites. Instead, it is caused by large decrease in the number of electrons due to the loss of Li ions from nano-LiCoO2 to form LiAlO2, compensated by large increase in the number of electrons due to the large occupancy of Al ions at the Li sites.


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Eighth, the absence of LiAlO2 and Al2O3 diffraction peaks in all Al2O3-coated samples is attributed to the amorphous nature of both LiAlO2 and Al2O3 coatings. Additionally, the crystallite size estimation using the Scherrer formula reveals that the coating process does not change the particle dimensions, D110 and D003, because the estimated values after coating are similar to those before coating (Table 1). In summary, detailed XRD analysis have established the following trends. (i) The wetchemical Al2O3 coating process with no addition of LiNO3 has resulted in the formation of Co3O4, Li-deficient nano-LiCoO2, Al incorporation into nano-LiCoO2, and amorphous LiAlO2/Al2O3 coating, (ii) addition of sacrificial LiNO3 in the coating process can prevent the formation of Co3O4 and minimize the loss of Li from nano-LiCoO2, but cannot avoid Al incorporation into nano-LiCoO2, (iii) the Al ions diffused into nano-LiCoO2 during the coating process are located predominantly at the Co sites when the Al concentration in nano-LiCoO2 is low, but occupy both the Li and Co sites when the Al concentration in nano-LiCoO2 is high, and (vi) the coating process does not change the sizes of nano-LiCoO2 particles. 3.3 Al2O3 and LiAlO2/Al2O3 Coating Characteristics Figures 5 presents the STEM images and elemental mapping of 12 wt. % Al2O3-coated nano-LiCoO2 treated at 600 °C. Note that the elemental mapping in these figures is acquired using STEM pixel by pixel. As a result, the Co map from LiCoO2 particles appears to be composed of dense dots which should be taken as the indication of being continuous because each individual LiCoO2 particle is continuous. With this in mind, it can be concluded that Al distribution is relatively uniform and a very thin LiAlO2/Al2O3 coating (< 5 nm) appears to be present. Figures 6 and 7 depict the STEM images and elemental mapping of 17 wt. % and 21 wt. % Al2O3-coated nano-LiCoO2 with addition of LiNO3 in the coating process. Several interesting phenomena are observed from these figures. First, it is noted that one LiCoO2 nano-plate is in stand-up mode (Figure 6) and shows the thickness of ~20 nm, consistent with the XRD estimation. Second, the LiAlO2/Al2O3 coatings in Figures 6 and 7 are thicker (5 to 10 nm) than that (< 5 nm) in Figure 5, in good accordance with the addition of more Al(NO3)3 in the coating solution for Figures 6 and 7 than that for Figure 5. Third, the LiAlO2/Al2O3 coating is continuous, but it is not very uniform since some regions have thicker coatings than the other regions.



High-resolution TEM analysis has also been performed to further elucidate the characteristics of the coating. Figure 8 shows two HRTEM images from 21 wt. % Al2O3-coated nano-LiCoO2. These HRTEM images confirm the presence of the crystalline LiCoO2 core, as evidenced by the lattice image of LiCoO2 (101) planes (Figure 8a). The coating thickness is found to be 5 to 10 nm, consistent with the conclusion from the STEM analysis. Furthermore, the coating appears to be amorphous because lattice images cannot be found during sample rotation inside the microscope. This conclusion is in good agreement with the previous XRD analysis, i.e., XRD patterns exhibit no crystalline peaks of LiAlO2 or Al2O3 even when Al2O3 loading is at 17 wt. % and 21 wt. % (Figure 1). Since the percentages of LiAlO2 and Al2O3 phases in the coating cannot be derived from the STEM analysis because EDS cannot detect Li element, we have designed a simple experiment to provide information on this topic. This simple experiment constitutes mixing LiNO3 and Al(NO3)3 in a molar ratio of 3 to 4 in an ethanol solution, stirring the solution with the aid of a Thinky Mixer at 250 rpm for 5 min, then drying the solution in a forced air convection oven at 100 °C for 2 h, and finally heating the resulting powder in a tube furnace in air at 600 °C for 3 h or 900 °C for 1 h. Note that the mixing and heating conditions for 600 °C treatment are identical to those in forming LiAlO2/Al2O3 coatings except no addition of nano-LiCoO2 particles in this simple experiment. The powder obtained from this simple experiment is mixed with ~8 wt. % Si as the internal reference and subjected to XRD analysis (shown in Figure S3). Rietveld refinement of the XRD data reveals that the powder heated at 600 °C for 3 h contains 64.4 wt. % gamma-LiAlO2 (01-075-0905), 24.1 wt. % eta-Al2O3 (04-007-2615), 4.0 wt. % hexagonal Al2O3 (04-016-0539), and 7.5 wt. % Si. For the 900 °C treated sample, the phase composition is found to be 64.8 wt. % gamma-LiAlO2 (01-075-0905), 27.9 wt. % eta-Al2O3 (04-007-2615), and 7.4 wt. % Si. These results reveal that there is little or no loss of Li2O during heating of uniformly mixed LiNO3 and Al(NO3)3 because the phase compositions of the powders treated at 600 °C and 900 °C are nearly the same except the change of 4.0 wt. % hexagonal Al2O3 to eta-Al2O3. Therefore, it is reasonable to deduce that the LiAlO2/Al2O3 coatings in this study contain ~70 wt. % LiAlO2 and ~30 wt. % Al2O3, assuming that the presence of nano-LiCoO2 particles will not change the percentages of LiAlO2 and Al2O3. For 12 wt. % Al2O3-coated nano-LiCoO2 (Figure 5), no LiNO3 is added during the coating process. Nevertheless, a small amount of LiAlO2 is formed as discussed in Section 3.2.


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Therefore, all of these coated nano-LiCoO2 will be termed as “LiAlO2/Al2O3-coated nano-LiCoO2” hereafter. The specific surface area of 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 has been determined to be 31.1 cm2 g-1 (Table 2), which is smaller than that of pristine nano-LiCoO2 (74.1 cm2 g-1). We attribute this decrease in the specific surface area to the function of the LiAlO2/Al2O3 coating in gluing several nano-LiCoO2 particles together as revealed in Figures 5, 6 and 8. In spite of this agglomeration, the equivalent particle size of 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 is still very small (38 nm in diameter) and will not alter the Li+ ion intercalation and deintercalation kinetics because the rate-limiting step for Li+ ion intercalation and de-intercalation is the slow intercalation and de-intercalation of Li+ ions at the electrode/electrolyte interface, as will be discussed later. Finally, it should be mentioned that we are surprised at the fact that the simple mixing experiment with no addition of nano-LiCoO2 particles mentioned above has resulted in the formation of crystalline LiAlO2 and Al2O3 phases even at 600 °C. In contrast, both XRD and TEM analyses reveal that the coating is an amorphous phase even for the coated nano-LiCoO2 samples containing 17 wt. % and 21 wt. % Al2O3 and heat treated at 600 °C. It is not clear at this stage what factors have led to such a difference in crystallinity. The possible factors may include the presence of nano-LiCoO2 particles and the nanometer thickness of the coating rather than micrometer-sized particles in the simple mixing experiment. Future studies are required to understand these different behaviors. 3.4 Charge/Discharge Behaviors of Pristine and LiAlO2/Al2O3-Coated Nano-LiCoO2 Figure 9 shows the charge/discharge voltage profiles of pristine nano-LiCoO2 and 600 °C treated nano-LiCoO2 with different amounts of LiAlO2/Al2O3 coatings. These coin cells are charged/discharged with the same protocol, i.e., 5 cycles at 0.1C, then 20 cycles at 1C and finally 20 cycles at 3C (1C = 145 mA g-1). Only selected voltage profiles are shown in Figure 9 to ensure clear presentation of the curves. Two salient features are noted from these curves. First, at a given charge rate and cycle number (such as the 10th cycle at 1C), the length of the voltage plateau at ~3.85 V in discharge is longer for 17 wt. % and 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 than for pristine and 12 wt. % LiAlO2/Al2O3-coated nano-LiCoO2. A previous study56 has established that the Li-ion intercalation rate at the interface between the LiCoO2 electrode and 15 ACS Paragon Plus Environment


liquid electrolyte is very slow and is the rate-limiting step for Li-ion transport during charge and discharge of LiCoO2. This conclusion from the previous study56 can be confirmed by estimating the diffusion time for Li ions to diffuse from the center of a LiCoO2 particle to its surface. Taking the diffusion constant, DLi, of Li ions in LiCoO2 particles as ~10-10 cm2 s-1 reported in Ref. 57, the diffusion time for a Li ion to diffuse from the center to the surface of a nano-LiCoO2 particle (with an average diffusion distance ~39 nm) is estimated to be ~2.7 × 10-2 seconds. However, experiments for charge at 3C, 1C and 0.1C rates took 20 min, 1 h and 10 h, respectively, to reach the full capacities. These charge times are several orders of magnitude longer than the required diffusion times (only 10-2 seconds) for Li ions to transport from the center of a nano-LiCoO2 particle to its surface, indicating that the rate-limiting step in charge is not due to slow diffusion of Li ions inside nano-LiCoO2 particles. Instead, the rate-limiting step in charge is slow delithiation at the electrode/electrolyte interface. Similar analysis can be applied to discharge experiments at 3C, 1C and 0.1C rates, leading to the conclusion that the rate-limiting step in discharge is slow Li intercalation at the electrode/electrolyte interface. Therefore, the length of the voltage plateau at ~3.85 V in discharge is associated with the capability of LiCoO2 in Li-ion intercalation at the electrode/electrolyte interface. The longer the voltage plateau, the more Li ions are intercalated into the LiCoO2 electrode.56 Thus, Figure 9 reveals that pristine and 12 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 have lower capabilities for Li-ion intercalation at the electrode/electrolyte interface during discharge and de-intercalation during charge than 17 wt. % and 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2. We attribute this difference to the surface damage due to the chemical and structural changes at the surfaces of pristine and 12 wt. % LiAlO2/Al2O3-coated nano-LiCoO2. The nature of these surface chemical and structural changes will be discussed below. The second salient feature in Figure 9 is that the specific capacities of pristine and 12 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 decay much faster over charge/discharge cycles than 17 wt. % and 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2. The inferior performance of pristine and 12 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 can be easier to be observed when the data of Figure 9 is re-plotted in the format of Figure 10 where the specific capacity is calculated based on the weight of LiCoO2 only. It is interesting to note that 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 not only has the best capacity retention, but also possesses the highest initial discharge capacity among all the samples. This result indicates unambiguously that damage to nano-LiCoO2 occurs 16 ACS Paragon Plus Environment

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before charge and discharge cycles and a continuous/thin LiAlO2/Al2O3 coating can prevent this from happening. A recent study using total-reflection X-ray absorption spectroscopy (XAS)58 has revealed unequivocally that when LiCoO2 electrode is in contact with the carbonate electrolyte during soaking, surface Co3+ ions are reduced to Co2+ ions by the electrolyte with the formation of a surface reduced layer of ~3 nm with a composition of LixCo1-xO (x < 1). This surface reduced layer is oxidized to become Co3O4 or other oxides in the subsequent charging process and cannot provide sufficient reduction and Li-ion intercalation in the subsequent discharging process. As a result, the specific capacity of LiCoO2 degrades even before the charge/discharge cycle starts. Clearly, 21 wt. % LiAlO2/Al2O3 coating has formed an effective physical barrier to avoid the direct contact between the nano-LiCoO2 and liquid electrolyte during soaking and prevented the reduction of surface Co3+ ions to Co2+ ions, thereby exhibiting the highest initial discharge capacity among all the samples. Figure 10 also unambiguously shows that the high initial discharge capacity of 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 is retained throughout the entire charge/discharge cycle investigated in this study because 21 wt. % LiAlO2/Al2O3 coating also minimizes the capacity decay during charge/discharge cycles. One of the established capacity decay mechanisms is the formation of CoO-like phases and the loss of lattice oxygen at LiCoO2 surface even when charge is carried out to only ~4.1 V vs. Li+/Li.59 Kikkawa, et al.59 have found that the formation of CoOlike phases at LiCoO2 surface (changing gradually from the surface CoO to the sub-surface Co3O4), which is followed by an oxygen-deficient layer with composition of LixCoO2- (0 < x ≤ 0.05, 0 <  ≤ 0.67) before reaching the center of stoichiometric LiCoO2, initiates at 40% charge (namely at ~4.1 V vs. Li+/Li), and the CoO-like phases grow to ~5 nm after the first charge of 60%. Repeated cycling results in the progression of Co3+/Co2+ reduction with loss of lattice oxygen from the surface region to the center of particles.59 Clearly, 21 wt. % LiAlO2/Al2O3 coating has reduced this capacity decay mechanism to minimum and thus offered superior capacity retention for nano-LiCoO2. The LiAlO2/Al2O3 coating is also expected to mitigate other capacity decay mechanisms such as transition metal dissolution, electrolyte decomposition, and possibly formation of insulating phases at the particle surface,10-18 thereby providing superior capacity retention for nano-LiCoO2. Interestingly, Figure 10 also shows that 17 wt. % LiAlO2/Al2O3 coating is not as effective as 21 wt. % LiAlO2/Al2O3 coating in improving the initial discharge capacity and capacity 17 ACS Paragon Plus Environment


retention. This is likely due to the smaller thickness or the lack of complete continuity of 17 wt. % LiAlO2/Al2O3 coating. Indeed, the effectiveness of the coating in preventing side reactions during charge/discharge cycles can be judged directly from Figure 10(b) which reveals that 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 has the highest coulombic efficiency in the first 5 cycles, followed by 17 wt. % LiAlO2/Al2O3-coated nano-LiCoO2, then 12 wt. % Al2O3-coated nano-LiCoO2 and finally pristine nano-LiCoO2. This sequence clearly indicates that as coating continuity improves and coating thickness increases, more side reactions or the degree of side reactions are mitigated. This trend is in good agreement with those reported in the literature, i.e., a thick coating generally produces better protection and capacity retention.15 3.5 Additional Electrochemical Properties of LiAlO2/Al2O3-Coated Nano-LiCoO2 From the specific capacity viewpoint at the electrode level, the specific capacity of a cathode should include the consideration of the weight of the inactive coating. Figure 11 shows such data with the inclusion of the weight of Al2O3 or LiAlO2/Al2O3 coating in the calculation of the usable specific capacity of the coated nano-LiCoO2. It is surprising to note that although pristine nano-LiCoO2 has the highest usable specific capacity for the first two cycles, 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 has the highest usable specific capacity for all of the remaining cycles. This result unequivocally tells us that the improvement in the specific capacity of nano-LiCoO2 provided by LiAlO2/Al2O3 coating is so large that it outweights the penalty of the weight of the inactive coating. Therefore, the LiAlO2/Al2O3 coating produced using the present wet-chemical method is not only scientifically interesting, but also technologically important in offering high usable specific capacity for nano-LiCoO2. Long-term cycle stability is an important criterion for practical applications. Figure 12 presents the performance data of 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 for 5 cycles at 0.1C, then 20 cycles at 1C and finally 425 cycles at 3C. Clearly, the capacity decay over cycles is still present. However, the capacity loss is relatively small (i.e., only 18% over 425 cycles in the 3C-cycle region). Equally important, after 450 cycles 21 wt. % LiAlO2/Al2O3-coated nanoLiCoO2 still possesses a specific capacity of 128 mA h g-1, a value that is reasonable for practical applications. This result represents a significant improvement over uncoated nano-LiCoO2. For example, in a previous study46 LiCoO2 nano-plates with a thickness of 17 nm (similar to our case here) were charged at 1C and then discharged at 10C for 20 cycles. The specific capacity of this 18 ACS Paragon Plus Environment

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nano-LiCoO2 decayed by ~32% in just 20 cycles of mixed 1C charge and 10C discharge.46 Our own data for pristine nano-LiCoO2 also shows similar capacity loss of ~25% over 20 cycles at 1C (Figure 10a), exhibiting much worse behavior than 21 wt. % LiAlO2/Al2O3-coated nanoLiCoO2. Note that 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 also displays similar performance as ALD Al2O3-coated submicron-sized LiCoO2 (~400 nm in the particle diameter) which has little capacity decay and still delivers about 150 mA h g-1 capacity after 200 cycles at 2.8C.42 It is known that without protective coatings the capacity decay increases as the size of nano-LiCoO2 decreases.46 Since the LiCoO2 particle size of the present study (~23 nm) is much smaller than 400 nm in Ref. 42 and the capacity retention of 21 wt. % LiAlO2/Al2O3-coated nanoLiCoO2 is similar to that of ALD Al2O3-coated submicron-LiCoO2, it is reasonable to deduce that the LiAlO2/Al2O3 coating formed via wet-chemical method in this study has the similar protective effect as ALD Al2O3 coatings. Although these comparisons are not carried out with the same particle size and same charge/discharge rates because of the limited data available in the literature, there is no doubt that 21 wt. % LiAlO2/Al2O3 coating has drastically improved the usable specific capacity of nano-LiCoO2 and its cycle stability. To further demonstrate the effectiveness of the LiAlO2/Al2O3 coating in improving the cycle stability of nano-LiCoO2, we have conducted charge/discharge experiments for microLiCoO2 at the 3C rate for comparison. As shown in Figure 13(a), 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 not only possesses a higher specific capacity for the initial discharge, but also has much better capacity retention than micro-LiCoO2. Specifically, micro-LiCoO2 exhibits much lower capacity retention than the coated nano-LiCoO2, i.e., loss by ~67% capacity versus only ~13% loss for the LiAlO2/Al2O3-coated nano-LiCoO2 from the 25th cycle to the 210th cycle at 3C. The superior capacity retention exhibited by 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 is due to the prevention of LiCoO2 surface degradation during soaking, in the first charge process and in the subsequent charge/discharge cycles, as discussed in Section 3.4. In contrast, the significant capacity loss displayed by micro-LiCoO2 is likely due to its poor rate capability, leading to large polarization and thus high overpotential at the cathode. This high overpotential could in turn result in fast formation of CoO-like phases at the surface of micro-LiCoO2 particles59 as well as electrolyte oxidation and formation of insulating phases at the particle surface60 and thus more resistance to Li-ion intercalation/de-intercalation and faster capacity decay.61



Another interesting and important phenomenon displayed in Figure 13(a) is that 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 provides a much higher initial specific capacity than microLiCoO2. It is well known that the typical specific capacity for micro-LiCoO2 is about 145 mA h g-1 when micro-LiCoO2 is charged and discharged at 0.1C with the voltage window set between 2.8 and 4.3 V vs. Li/Li+.16, 19, 20, 25-29 Thus, the specific capacity exhibited by micro-LiCoO2 in Figure 13(a) is consistent with the literature. However, the initial specific capacity exhibited by 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 with the 0.1C rate is ~225 mA h g-1, significantly higher than the expected value based on the prior knowledge derived from micro-LiCoO2.16, 19, 20, 25-29 To identify whether this phenomenon is related to the change in thermodynamic driving force

(i.e., electrochemical potential) or the change in redox kinetics, we have examined the voltage profiles of charge/discharge curves carefully. As shown in Figure 13(b), the discharge plateaus for 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 at 1C and 3C are similar to those for microLiCoO2 (both at ~3.85 V vs. Li/Li+). Similarly, the charge plateaus for both 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 and micro-LiCoO2 at 1C and 3C start at ~4.0 V vs. Li/Li+. In fact, by checking Figure 9 we notice that all nano-LiCoO2 with and without the LiAlO2/Al2O3 coating have similar discharge plateaus (~3.85 V) and similar initial charge plateaus (~4.0 V), indicating that the intercalation potentials of nano-LiCoO2 with and without the LiAlO2/Al2O3 coating remain the same as micro-LiCoO2. Thus, the unusually high initial specific capacities (~200 mA h g-1 for pristine nano-LiCoO2 and ~225 mA h g-1 for 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2) are not caused by a change in the thermodynamic driving force; instead, it is likely due to the improvement in the redox reaction kinetics, as discussed below. We hypothesize that the typical specific capacity of ~145 mA h g-1 exhibited by microLiCoO2 subjected to the 0.1C rate with the upper cutoff voltage at 4.3 V is limited by slow deintercalation of Li ions at the electrode/electrolyte interface. This slow de-intercalation at the interface results in a large polarization and thus a large overpotential at the LiCoO2 cathode and a low charge capacity. A recent density functional theory (DFT) calculation55 has revealed that the intercalation potentials are ~3.78 V and ~3.95 V vs. Li/Li+ when LiCoO2 is charged from LiCoO2 to Li0.5CoO2 and Li0.25CoO2, respectively. Since the specific capacity of 145 mA h g-1 corresponds to extracting and inserting 0.53 Li per LiCoO2, its intercalation potential should be close to 3.78 V if there were no polarization induced by slow extraction at the electrode/electrolyte interface. In reality, experiments typically require a charging potential > 4.0 20 ACS Paragon Plus Environment

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V vs. Li/Li+ to achieve the specific capacity of 145 mA h g-1,16, 19, 20, 25-29 suggesting the presence of significant polarization at micro-LiCoO2 electrodes. For pristine nano-LiCoO2, the initial specific capacity of 200 mA h g-1 observed in this study corresponds to extracting and inserting 0.73 Li per LiCoO2. According to the DFT calculation,55 this specific capacity could be achieved with the intercalation potential at ~3.95 V vs. Li/Li+ if there were no polarization at the electrode. In experiments this specific capacity of 200 mA h g-1 is in fact obtained with the upper cutoff voltage at 4.3 V vs. Li/Li+, suggesting the presence of polarization at the pristine nano-LiCoO2 electrode. However, the polarization at the pristine nano-LiCoO2 electrode is much smaller than micro-LiCoO2 electrodes because micro-LiCoO2 only exhibits ~145 mA h g-1 capacity with the same upper cutoff voltage as nano-LiCoO2. The smaller polarization and higher initial specific capacity displayed by pristine nano-LiCoO2 is due to its much larger electrode/electrolyte interfacial area than micro-LiCoO2. Based on the dimensions of nano-LiCoO2 and micro-LiCoO2 used in this study, one can estimate that the electrode/electrolyte interfacial area for nano-LiCoO2 is at least 166 times larger than that for micro-LiCoO2. This huge interfacial area of nano-LiCoO2 allows lithiation and delithiation to proceed with smaller polarization and thus faster lithiation and delithiation rates and higher initial specific capacity than micro-LiCoO2 because Li-ion intercalation and de-intercalation at the electrode/electrolyte interface are the rate-limiting steps in charge/discharge processes of LiCoO2, as discussed in Section 3.4. A previous study on nanoLiCoO2 46 has also shown that nano-LiCoO2 has a higher specific capacity than micro-LiCoO2 in the first charge process when the upper cutoff voltage is set at 4.2 V vs. Li/Li+ with a charge rate of 1C. The result of this previous study is consistent with our findings here. 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 further improves the initial specific capacity from 200 mA h g-1 for pristine nano-LiCoO2 to ~225 mA h g-1 when the upper cutoff voltage stays the same at 4.3 V (Figure 10). The initial specific capacity of 225 mA h g-1 corresponds to extracting and inserting 0.82 Li per LiCoO2 in the first charge/discharge cycle and is mainly attributed to the protection of the LiAlO2/Al2O3 coating against degradation of the surface region of nano-LiCoO2 during soaking and in the first charge process, as discussed in Section 3.4. We hypothesize that the surface reduced layer, LixCo1-xO (x < 1) created during soaking of LiCoO2 58

and the CoO-like surface layer formed in the first charge process 59 have a high resistance to

Li-ion transport or a high charge transfer resistance at the electrode/electrolyte interface. In contrast, when a LiAlO2/Al2O3 coating is applied, the surface reduced layers are prevented, 21 ACS Paragon Plus Environment


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leading to faster Li ion de-intercalation and intercalation and thus higher initial specific capacities even though the upper cutoff voltage remains the same. In addition, with little or no loss of the active material during soaking and in the first charge process, 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 is expected to have higher initial specific capacities than pristine nano-LiCoO2 when the specific capacity is computed based on the weight of LiCoO2 only (as shown in Figure 10). To further understand the effect of the LiAlO2/Al2O3 coating, cyclic voltammetry (CV) has been conducted. Figure 14 compares the CV curves of pristine nano-LiCoO2 and 21 wt. % LiAlO2/Al2O3 coated nano-LiCoO2. Two distinct phenomena are noted from this figure. First, the CV curves of 21 wt. % LiAlO2/Al2O3 coated nano-LiCoO2 are relatively stable with little change as the CV scan cycle increases from 1 to 5 (Figure 14a). In contrast, the CV curves of pristine nano-LiCoO2 exhibit a clear trend (Figure 14b), that is, the peak currents of both oxidation and reduction peaks decrease as the number of the CV scan cycle increases. The two different trends exhibited by nano-LiCoO2 with and without coating reveal that the LiAlO2/Al2O3 coating has improved the electrode stability and is consistent with the charge/discharge cycle stability shown in Figures 10, 11 and 12. The second distinct phenomenon is the peak broadening and shifting of both oxidation and reduction peaks after the application of the LiAlO2/Al2O3 coating. The slight peak shifting may be related to the voltage drop of the “insulating” LiAlO2/Al2O3 coating. However, it is not very clear what physical process(es) result in peak broadening. Additional studies are needed in the future to clarify this. In spite of the peak broadening, the cathodic peak of 21 wt. % LiAlO2/Al2O3 coated nanoLiCoO2 remains to be clearly distinguishable. Thus, the 21 wt. % LiAlO2/Al2O3 coated nanoLiCoO2 half cell has been subjected to CV scans with different scan rates. As expected, the peak of cathodic current, Ip, increases and the peak potential decreases with increasing the scan rate (Figure 15a). When Ip is plotted as a function of v1/2 (where v is the scan rate), the peak current exhibits a linear dependence on v1/2 (Figure 15b), revealing that the cathodic reaction of 21 wt. % LiAlO2/Al2O3 coated nano-LiCoO2 is diffusion-controlled. According to the Randles-Sevcik equation, the peak current, Ip (mA), for a reversible redox reaction is given by 62 𝐼𝑝 = 2.69 × 105𝑛3/2𝐴𝐶𝐷1/2𝑣1/2

(3)


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and for an irreversible redox reaction, the peak current is given by 𝐼𝑝 = 2.99 × 105𝑛3/2𝛼1/2𝐴𝐶𝐷1/2𝑣1/2

(4)

where n is the number of electron transfer in the redox reaction, A the area of the working electrode, C the primary concentration of Li ions, D the diffusion coefficient of Li ions, and a the transfer coefficient (0.5). Since the reaction is quasi-reversible, the real value of D should be between the values obtained from Eqs. (3) and (4). We have found DLi = 3.68 × 10-10 cm2 sec-1 using Eq. (3) and DLi = 5.94 × 10-10 cm2 sec-1 using Eq. (4). These values are comparable with the diffusion coefficients of Li ions (10-10 – 10-9 cm2 sec-1) reported for micro-LiCoO2 without coating by multiple researchers,35, 57, 63-65 but significantly lower than 10-12 – 10-18 cm2 sec-1 of micro-LiCoO2 reported by other researchers.66-68 No CV scans with different scan rates were conducted for pristine nano-LiCoO2 because its peak current decreases with the number of CV scan cycles (Figure 14). Nevertheless, it is reasonable to conclude that the diffusion coefficient of Li ions in 21 wt. % LiAlO2/Al2O3 coated nano-LiCoO2 is comparable to or higher than that in micro-LiCoO2. To provide a direct comparison in the kinetics of electrochemical processes between pristine nano-LiCoO2 and 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2, EIS tests were performed on both coin cells before cycling. As shown in Figure 16, Nyquist plots of the pristine and coated nano-LiCoO2 coin cells after 48-h resting at ambient temperature but before charge/discharge cycles have one depressed semicircle followed by a straight line at the low frequency region. It is generally agreed that the intercept of the semicircle with the real axis at the highest frequency, denoted as Rohm, represents the sum of ohmic resistances of the electrolyte and the other resistive components such as cell connectors, current collectors, and separator.69,70 Further, it is known that the semicircle is related to Li-ion diffusion resistance through any surface films on the LiCoO2 surface, Rf, and the charge transfer resistance at the electrode/electrolyte interface, Rct.69,70 This semicircle is separated into two semicircles in some cases and thus Rf and Rct can be separated readily.70,71 However, only one depressed semicircle is present in this study and thus we have used R’ct to stand for the combined values of Rf and Rct. Using the equivalent circuit R(QR)W (the insert in Figure 16), it is found that the Rohm of pristine nano-LiCoO2 after soaking but before cycling is 5.22 Ω. The corresponding value for 21 23 ACS Paragon Plus Environment


wt. % LiAlO2/Al2O3-coated nano-LiCoO2 is 11.20 Ω, slightly higher than that of pristine nanoLiCoO2. However, the R’ct of 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 is only 114.00 Ω which is significantly lower than that (294.30 Ω) of pristine nano-LiCoO2. These results show unequivocally that the replacement of the surface reduced layer, LixCo1-xO (x < 1), formed during soaking of LiCoO2 58 by the LiAlO2/Al2O3 coating has drastically decreased the resistance of Liion diffusion through the surface region of LiCoO2 particles and/or the charge transfer resistance at the electrode/electrolyte interface. It is this drastic decrease that has made possible for 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 to exhibit higher initial discharge capacities than pristine nano-LiCoO2 (Figure 10) because the rate-limiting step in discharge of nano-LiCoO2 is slow Li intercalation at the electrode/electrolyte interface. Before closing, it should be emphasized that the wet-chemical method developed in this study is also applicable to other layered transition metal oxides such as Li(NixMnyCoz)O2 (NMCs where x + y + z = 1) and Li(NixCoyAlz)O2 (NCAs where x + y + z = 1), particularly for nanoNMCs and nano-NCAs which require much more accurate control in unwanted reactions during the coating process than their micrometer-sized counterparts. It has been shown that some lithium is extracted from NMC532 particles during the annealing process in forming Al2O3 coating.12 Such a problem can be readily solved via our method of adding an appropriate amount of LiNO3 as a sacrificial agent in the coating solution. In addition, our approach leads to the formation of LiAlO2/Al2O3 coating which is better than Al2O3 coating because LiAlO2 is a better lithium ion conductor than Al2O3.12, 72 Another problem encountered in forming Al2O3 coating on NMC811 is the disappearance of Al2O3 coating which dissolves into NMC811 during the annealing process.31 The change from coating to doping reduces the improvement in electrochemical performance because doping is not as effective as coating in enhancing electrochemical properties.31 Our method of adding LiNO3 as a sacrificial agent is also expected to impede the coating dissolution process because of the competition between the Al2O3 coating dissolution and forming LiAlO2 compound with Li2O. In short, the wet-chemical method developed in this study has the potential to open up new opportunities in offering better coatings to enhance the specific capacity, cycle life and rate capability of layered transition metal oxide cathodes simultaneously for next-generation Li-ion batteries in the future.


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4. Conclusion This study has successfully developed a facile wet-chemical method to coat nano-LiCoO2 and enhance its specific capacity and capacity retention for long cycle life simultaneously. The major conclusions derived from this study can be summarized below. 1. In this newly-developed wet-chemical method, Al(NO3)3 is used as the Al source to form Al2O3 and LiAlO2, whereas LiNO3 is used as a sacrificial agent to protect nano-LiCoO2 and at the same time to form LiAlO2 by reacting with Al2O3. 2. In the absence of LiNO3, the Al2O3 coating process leads to the formation of Co3O4 because of the reaction of Al2O3 with the lithium from nano-LiCoO2 to form LiAlO2 and Li-deficient LiCoO2 which is thermally unstable and transforms to Co3O4 and LiCoO2 during the coating process. 3. Addtion of LiNO3 into the Al(NO3)3 solution suppresses the unwanted formation of Co3O4 during the coating process and leads to a thin (5–10 nm) and continuous LiAlO2/Al2O3 coating on nano-LiCoO2 particles. 4. Addition of LiNO3 into the Al(NO3)3 solution also makes the LiAlO2/Al2O3 coating thickness tunable without causing damage to the nano-LiCoO2 core. 5. Nano-LiCoO2 is found to have a much higher initial specific capacity (200 mA h g-1) than micro-LiCoO2 (145 mA h g-1) even though the charge/discharge voltage windows are the same. This significant improvement is attributed to the facts that (i) slow Li intercalation and de-intercalation at the electrode/electrolyte interface are the rate-limiting steps in discharge and charge processes and (ii) huge electrode/electrolyte interfacial area possessed by nanoLiCoO2 allows lithiation and de-lithiation to proceed at the interface with small polarization for a given current density and fixed voltage window. 6. 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 shows additional improvement in the initial specific capacity over pristine nano-LiCoO2 because of the protection of the LiAlO2/Al2O3 coating against degradation of the surface region of nano-LiCoO2 during soaking and in the first charge process. 7. The replacement of the surface reduced layer formed during soaking of LiCoO2 by the LiAlO2/Al2O3 coating has drastically decreased the resistance of Li-ion diffusion through the surface region of LiCoO2 particles and/or the charge transfer resistance at the 25 ACS Paragon Plus Environment


electrode/electrolyte interface. It is this drastic decrease that has made possible for 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 to exhibit higher initial discharge capacities than pristine nano-LiCoO2 because the rate-limiting step in discharge of nano-LiCoO2 is slow Li intercalation at the electrode/electrolyte interface. 8. 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 exhibits 100% increase in the specific capacity over pristine nano-LiCoO2 after 5 cycles at 0.1C, 20 cycles at 1C and 20 cycles at 3C when only the weight of LiCoO2 is considered. When the weight of the inactive coating is included in the specific capacity calculation, 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 still exhibits 58% increase in the usable specific capacity over pristine nano-LiCoO2, unambiguously proving the effectivenss of LiAlO2/Al2O3 coating in enhancing the specific capacity and cycle stability simultaneously for nano-LiCoO2. 9. The significant improvements in the specific capacity and cycle stability are due to the effectiveness of LiAlO2/Al2O3 coating in preventing capacity decay during soaking as well as during cycling. 10. In the composition range investigated in this study, 21 wt. % LiAlO2/Al2O3 coating has the best enhancement in the electrochemical properties of nano-LiCoO2, followed by 17 wt. % LiAlO2/Al2O3 coating and finally 12 wt. % LiAlO2/Al2O3 coating. This trend is due to the improvement in the coating continuity and thickness as the loading of LiAlO2/Al2O3 coating increases. 11. 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 is able to charge/discharge for 425 cycles at 3C with only 18% capacity loss. Furthermore, after 450 cycles 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 still possesses a specific capacity of 128 mA h g-1. In contrast, the specific capacity of micro-LiCoO2 diminishes to 50 mA h g-1 after only 200 cycles at 3C. 12. The results above have clearly demonstrated that 21 wt. % LiAlO2/Al2O3-coated nanoLiCoO2 possesses high specific capacity and superior capacity retention for long cycle life at high rates with potential for real-world applications. Acknowledgments - This work is supported by Rowe Family Endowment Fund for Sustainable Energy at Illionis Institute of Technology. The use of the Center for Nanoscale Materials (CNM) was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. 26 ACS Paragon Plus Environment

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ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxxxxxxx. **[TEM and SEM images of pristine nano-LiCoO2, XRD patterns of LiAlO2/Al2O3 coating at different temperatures] **

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Figures and Tables Table 1. Powder characteristics of pristine and 600 °C treated, Al2O3-coated nano-LiCoO2 with and without the addition of LiNO3 in the coating process Co3O4

Sample ID

a (Å)

c (Å)

c/a

D110 (nm)

D003 (nm)

1: Pristine nano-LiCoO2

2.8180±0.0003

14.0796±0.0014

4.9963

54.0

25.1

No

2: 12 wt. % Al2O3-coated LiCoO2 w/o LiNO3

2.8145±0.0004

14.0719±0.0016

4.9998

50.4

33.8

Yes


2.8124±0.0003

14.1078±0.0014

5.0163

45.0

28.0

Yes

4: 17 wt. % Al2O3-coated LiCoO2 w/ LiNO3

2.8152±0.0002

14.0756±0.0008

4.9999

65.6

37.5

No


2.8128±0.0004

14.1095±0.0016

5.0162

39.7

21.5

Yes

6: 21 wt. % Al2O3-coated LiCoO2 w/ LiNO3

2.8145±0.0002

14.0813±0.0009

5.0031

56.5

38.1

No

formation

Table 2. BET surface area measurement of micro- and nano-LiCoO2 particles as well as 21 wt. % Al2O3-coated nano-LiCoO2 * Sample

Micro-LiCoO2

Pristine Nano-LiCoO2

21 wt. % Al2O3-coated LiCoO2 w/ LiNO3

Specific Surface Area (cm2/g)

0.389

74.142

31.095

Equivalent Particle Diameter (nm)

3050

16

38

* The equivalent particle diameters were computed with the assumption of spherical particles.


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Table 3. The number of electrons at the Co sites, Co site occupancy, and Co site formula derived from Rietveld refinement* Sample ID

Number of electrons at Co sites

Co site occupancy

Co site formula

1

24.3(5)

0.90(2)

Co0.9

2

22.1(5)

0.82(2)

Al0.16Co0.74

3

21.3(5)

0.79(2)

Al0.21Co0.69

4

23.2(3)

0.86(1)

Al0.08Co0.82

5

22.1(5)

0.82(2)

Al0.16Co0.74

6

22.4(3)

0.83(1)

Al0.14Co0.76

* The Co site occupancy and its site formula were obtained by assuming that the change in the number of electrons at the Co sites is all due to substitution of Co by Al.



Figure 1. XRD patterns of pristine and Al2O3-coated nano-LiCoO2 in various conditions. All samples contain 5 wt.% Si as the internal standard. The standard XRD patterns of LiCoO2 (04013-4373) and Co3O4 (04-005-4386) are also included for comparison.


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Figure 2. (a) TEM image of pristine nano-LiCoO2 and (b) SEM image of micro-LiCoO2.

0.030 0.025

Microstrain

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.020 0.015 0.010 0.005 0.000 1

2

3

4

5

6

Sample ID Figure 3. Lattice microstrain in nano-LiCoO2 with and without coatings. The coating condition of each sample can be found in Table 1 using the sample ID.


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27.0

(a) 26.0 25.0 24.0 23.0 22.0 21.0 20.0 1

2

3

4

5

6

5

6

Sample ID Number of electrons per the Li site

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Number of electrons per the Co site


4.5

(b) 4.0

3.5

3.0

2.5

2.0 1

2

3

4

Sample ID Figure 4. Number of electrons per (a) the Li sites and (b) the Co sites of nano-LiCoO2 with and without coatings. The coating condition of each sample can be found in Table 1 using the sample ID.


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(a)

(b)

(c)

(d)

Figure 5. STEM images of 600oC treated nano-LiCoO2 with 12 wt.% Al2O3 coating: (a) STEM high-angle annular dark-field (HAADF) image of several particles, (b) EDS mapping of Co element, (c) EDS mapping of Al element, and (d) combined mapping of Al and Co elements.

(a)

(b)

(c)

(d)

Figure 6. STEM images of 600oC treated nano-LiCoO2 with 17 wt. % Al2O3 coating and addition of LiNO3 in the coating process: (a) STEM high-angle annular dark-field (HAADF) image of several particles, (b) EDS mapping of Co element, (c) EDS mapping of Al element, and (d) combined mapping of Al and Co elements.

(a)

(b)

(c)

(d)

Figure 7. TEM images of 600oC treated nano-LiCoO2 with 21 wt. % Al2O3 coating and addition of LiNO3 in the coating process: (a) STEM high-angle annular dark-field (HAADF) image of one particle, (b) EDS mapping of Co element, (c) EDS mapping of Al element, and (d) combined mapping of Al and Co elements.



(a) 7 nm d = 0.24nm LiCoO2 (101)

(b) 5-10 nm

Figure 8. HRTEM images of 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2: (a) showing the lattice image of LiCoO2 (101) planes and the coating on the surface of the crystalline LiCoO2 particle and (b) another LiAlO2/Al2O3-coated nano-LiCoO2 image with the coating thickness of 5 to 10 nm.


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Figure 9. Cell voltage versus discharge capacity: (a) pristine nano-LiCoO2, (b) nano-LiCoO2 with 12 wt. % LiAlO2/Al2O3 coating, (c) nano-LiCoO2 with 17 wt. % LiAlO2/Al2O3 coating, and (d) nano-LiCoO2 with 21 wt. % LiAlO2/Al2O3 coating.



Figure 10. Comparison between samples with various LiAlO2/Al2O3 contents (pristine, 12 wt. % LiAlO2/Al2O3, 17 wt. % LiAlO2/Al2O3, and 21 wt. % LiAlO2/Al2O3): (a) discharge capacity versus cycle number, and (b) coulombic efficiency versus cycle number. The specific capacity is calculated based on the weight of LiCoO2 only.


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Figure 11. Comparison in the discharge capacity versus cycle number between samples with various LiAlO2/Al2O3 contents (pristine, 12 wt. % LiAlO2/Al2O3, 17 wt. % LiAlO2/Al2O3, and 21 wt. % LiAlO2/Al2O3). The specific capacity is calculated based on the weight of LiCoO2 plus LiAlO2/Al2O3 coating.

Figure 12. Discharge capacity and coulombic efficiency of 21 wt. % LiAlO2/Al2O3-coated nanoLiCoO2 versus cycle number. The cell was charged/discharged for 5 cycles at 0.1C, then 20 cycles at 1C, and finally 425 cycles at 3C.



(a)

(b)

Figure 13: (a) Comparison in discharge capacity as a function of the cycle number between 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 and micro-LiCoO2. The cells were charged/discharged for 5 cycles at 0.1C, then 20 cycles at 1C, and finally 185 cycles at 3C. (b) Comparison of the selected voltage profiles in galvanostatic charge/discharge processes between 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 and micro-LiCoO2.


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Figure 14. Cyclic voltammogram curve of (a) 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 between 2.8 and 4.3 V at scan rate of 1 mV s-1 and (b) pristine nano-LiCoO2 between 3.0 and 4.5 V at scan rate of 0.1 mV s-1.

Figure 15. (a) Cyclic voltammogram curves of 21 wt. % LiAlO2/Al2O3-coated nano-LiCoO2 for different scan rates, v and (b) cathodic peak current (Ip) vs. v1/2.



Figure 16. Comparison of EIS curves between pristine nano-LiCoO2 and 21 wt. % LiAlO2/Al2O3coated nano-LiCoO2. The measurements were done before charge/discharge cycles in the frequency range of 10 kHz – 1 Hz. The insert is the R(QR)W equivalent circuit.


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Table of Contents

LiAlO2/Al2O3 coating has improved the specific capacity and cycle stability of nano-LiCoO2


Al2O3 Coating through a Wet-Chemical Method to

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