Surface Surgery of the Nickel-Rich Cathode Material LiNi0.815Co0

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Surface Surgery of Ni-rich Cathode Material LiNi0.815Co0.15Al0.035O2: Towards Complete and Ordered Surface Layered Structure and Better Electrochemical Properties Zhongfeng Tang, Junjie Bao, Qingxia Du, Yu Shao, Minghao Gao, Bangkun Zou, and Chunhua Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11431 • Publication Date (Web): 29 Nov 2016 Downloaded from http://pubs.acs.org on November 30, 2016

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ACS Applied Materials & Interfaces

Surface Surgery of Ni-rich Cathode Material LiNi0.815Co0.15Al0.035O2: Towards Complete and Ordered Surface Layered Structure and Better Electrochemical Properties Zhongfeng Tang,a Junjie Bao, a, b Qingxia Du,a Yu Shao,a Minghao Gao, b Bangkun Zou,a Chunhua Chen*a a

CAS Key Laboratory of Materials for Energy Conversions, Department of

Materials Science and Engineering & Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of China, Anhui Hefei 230026, China b

School of Chemistry and Chemical Engineering, Anhui University, Anhui Hefei 230601, China

Abstract: A complete and ordered layered structure on the surface of LiNi0.815Co0.15Al0.035O2 (NCA) has been achieved via a facile surface-oxidation method with Na2S2O8. The FE-TEM images clearly show that the pre-oxidation of the hydroxide precursor can eliminate the crystal defects and convert the Ni(OH)2 into layered β-NiOOH, which leads to a highly ordered crystalline NCA with its (006) planes perpendicular to the surface in the sintering process. X-ray photoelectron spectroscopy and Raman shift results demonstrate that the contents of Ni2+ and Co2+ ions are reduced with the pre-oxidization on the surface of the hydroxide precursor. The level of Li+/Ni2+ disordering in the modified NCA determined by the peak intensity ratio I(003)/I(104) in X-ray diffraction patterns decreases. Thanks to the complete and ordered layered structure on the surface of secondary particles, lithium ions can easily intercalate/extract in the discharging-charging process, leading to greatly improved electrochemical properties. Keywords: Li-ion battery; lithium nickel cobalt aluminum oxide; sodium peroxodisulfate; surface oxidation; crystal defect; cation disorder *

Corresponding author. Tel.: +86 551 63606971; fax: +86-551-63601952. E-mail address: [email protected] (Chun-Hua Chen)

ACS Paragon Plus Environment


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1. Introduction Many kinds of cathode materials, such as LiFePO4, LiMn2O4, LiNi0.5Mn1.5O2 and layer-structured ternary cathode materials have been developed to replace LiCoO2 to seek further improvements in the aspects of toxicity, specific capacity and cost1-6. Among them, LiNiO2-based cathode materials such as LiNi1/3Co1/3Mn1/3O2, LiNi0.5Co0.2Mn0.3O2 and LiNi0.8Co0.15Al0.05O2 (NCA) are widely considered as promising substitutions for LiCoO2 in Li-ion batteries because of their high reversible capacity and low cost. The NCA is particularly attractive for its much higher specific capacity (~200 mAh g-1) than that of LiNi1/3Co1/3Mn1/3O2 (~160 mAh g-1) and LiNi0.5Co0.2Mn0.3O2 (~170 mAh g-1) in the voltage range of 2.8-4.3 V. However, the rapid capacity fading and inferior thermal stability of NCA are hindering its applications in electric vehicles and large energy storage systems. Recent researches on

the deterioration mechanism of

NCA in

the

charge-discharge cycling have been conducted with various in-situ and ex-situ detection methods. Using in-situ hard X-ray absorption spectroscopy (XAS) and soft XAS, Yoon et al. reported that substantial amount of the Ni ions at the surface of NCA particles exist as Ni2+ while most of Ni ions in the bulk are Ni3+ 7. The cation mixing layer caused by migration of Ni2+ and Co2+ into Li+ slabs due to their similar ionic radii is the main reason for the rise of impedance during cycling. By combining the results of high-resolution electron microscopy, selected area electron diffraction and electron energy loss spectroscopy, Hwang et al. confirmed that the reduction of Ni3+ and the loss of oxygen attributed to the surface change from layered structure to disordered spinel structure or rock-salt-like structure lead to a charge imbalance on the surface of NCA particles, and eventually resulting in material degradation8. To address these issues, coating a layer onto the surface of NCA particles has been widely used to suppress the side reactions with electrolyte and surface phase transition. The coating materials can be inactive metal oxides9-11, metal phosphates12-14, metal fluorides15 and even some electrochemically active materials with better cycle stability than NCA such as LiCoO216 and LiNi1/3Co1/3Mn1/3O217. However, such a


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coating layer would inevitably, to some extent, hinder the Li+ diffusion and decrease the specific capacity of the host material. Co-precipitation is one of the most widely used methods to produce ternary cathode materials in industrial scale production18-20. Spherical hydroxide precursors or carbonate precursors are firstly precipitated with precisely controlled particle size and morphology, followed by a high temperature sintering process with Li2CO3 or LiOH as the lithium source. Considering that it is difficult for Ni2+ in a Ni-based hydroxide precursor to be oxidized completely into Ni3+ even at high temperatures with a flowing oxygen atmosphere, many crystal defects are expected to be produced in the final layer-structured ternary cathode materials, especially on the surface of primary particles. Shoichiro et al.21 has confirmed that, due to the unbalanced stress in the particles accompanied by the volume change, micro-cracks are generated at the inter-surface between the primary particles at a deep discharge state. Therefore, it is desirable to stabilize the layered structure by eliminating crystal defects in order to improve the electrochemical properties. Unfortunately, as far as we know, there is no literature focusing on the surface structure change from precursor to cathode material yet. To decrease the degree of cation mixing, Hu et al.22-23 have reported a co-oxidation-controlled crystallization method with Na2S2O8 to synthesize NCA via a high-valence intermediate product Ni0.8Co0.15Al0.05OOH. Nevertheless, they only emphasize on the effect of the Ni valence. In fact, the crystal defects may also strongly affect the degree of cation mixing. According to Maruta’s work24, Ni(OH)2 can be firstly oxidized by Na2S2O8 into β-NiOOH, which is easily converted into pure LiNiO2 via ion exchange reactions even under moderate hydrothermal conditions because of their similar layered structures (Reactions (1) and (2)). Ni(OH)2 + 1/2S2O82-→NiOOH+HSO4-

(1)

NiOOH + LiOH→LiNiO2+H2O

(2)

In their work, the highly oxidative Na2S2O8 plays a critical role in oxidizing Ni2+ into Ni3+. KMnO4 has also been adapted to oxidize the hydroxide precursor Ni0.8Co0.15Al0.05(OH)2 to improve its electrochemical properties11. The results show



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that the improvement of NCA cathode material is mainly attributed to the surface doping of tetravalent Mn. Herein, we for the first time propose the idea that the pre-oxidation of Ni0.815Co0.15Al0.03(OH)2 with Na2S2O8 can not only increase the average valence of the transition metals (Co and Ni), but also induce re-structure of the surface of the hydroxide precursor. In this study, the bare and treated NCA have been fully characterized by various techniques. The electrochemical tests show that the cycling stability and rate capability of the treated sample have been significantly improved compared with the pristine NCA. 2. Experimental section 2.1. Materials preparation. A commercial Ni0.815Co0.15Al0.035(OH)2 precursor was treated by a pre-oxidation process. Firstly, a mixed aqueous solution containing 1.0 mol L-1 Na2S2O8 and 0.5 mol L-1 NaOH was prepared by dissolving 14.29 g Na2S2O8 and 1.20 g NaOH into 60 mL deionized water. Then 6.0 g pristine Ni0.815Co0.15Al0.035(OH)2 precursor powder was added in the mixed solution and dispersed by magnetic stirring for 30 minutes. According to Reaction (1), the molar ratio of Ni(OH)2 and Na2S2O8 is 2:1. Therefore, the oxidant Na2S2O8 (14.29 g, 0.06 mol)

is

sufficient

to

oxidize

the

surface

of

the

hydroxide

precursor

Ni0.815Co0.15Al0.035(OH)2 (6.0 g, about 0.066 mol). It was noted that the green-color precursor immediately turned into black because of the surface oxidation from Ni2+ into Ni3+. Then the powder was filtered, washed with deionized water, and dried at 80°C in an oven overnight. The dried powder was thoroughly mixed with stoichiometric amount of nano-sized Li2CO3 and sintered at 750°C for 12 hours under a flowing oxygen atmosphere. For comparison, the pristine precursor was also reacted with Li2CO3 with the same sintering process to obtain the pristine NCA. For convenience, we denoted the pristine and the surface modified materials as P-NCA and S-NCA, respectively. 2.2. Characterizations. The crystal structures of the hydroxide precursors and as-prepared NCA samples were measured by powder X-ray diffraction (XRD, Rigaku


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TTR-lll) with Cu Kα radiation at a scan rate of 10° min-1 over a 2θ range of 10-70°. Morphological studies were performed using scanning electron microscopy (SEM, JSM-6390LA, JEOL). The surface structures and crystal defects of the precursors and cathode materials were observed using field emission transmission electron microscopy (FE-TEM, JEM-2100F). Raman spectroscopy (Renishaw inVia) was used to analyze the average oxidation state of the transition metals on the surface of NCA particles. Moreover, X-ray photoelectron spectroscopy (XPS, ESCALAB 250, Thermo-VG Scientific) measurements were performed to obtain information on the surface compositions of the particles. 2.3. Electrochemical measurement. The electrochemical properties of the synthesized NCA samples were examined by using CR2032 coin type cells with metallic lithium as negative electrode. The positive electrodes were prepared by mixing the NCA powders (84 wt%), carbon black (8 wt%) and polyvinylidene fluoride (PVDF) binder (8 wt%) dispersed in N-methyl-2-pyrrolidone to make slurries. Then the slurry was coated on an aluminum foil and dried at 80°C in an oven overnight. The laminates were punched into round disks with a diameter of 14 mm. A typical electrode disk contained 6.5 mg NCA. The cells were assembled in an Ar-filled glove box (McBraun Labmaster 130) with an electrolyte of 1 M LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) (EC:DEC = 1:1 V/V) and a separator of Celgard 2400 porous membrane. The cells were cycled on Newware BTS 610 multi-channel battery cycler at 25°C. The cycle performance was measured at 0.1C rate (1C=180 mA g-1) for initial three cycles and then at 1C for 100 cycles in the voltage range of 2.75 - 4.3 V, and, to check the overcharging behaviors, at 0.5C between 2.75 - 4.5 V. For the rate performance test, the cells were cycled at 0.1C, 0.5C, 1C, 2C and 5C for five cycles respectively in the voltage range of 2.75 - 4.3 V. The impedance measurement was conducted on a CHI640 Electrochemical Workstation using electrochemical impedance spectroscopy (EIS) in the frequency range of 0.01Hz-100 kHz. The amplitude of the alternating current (AC) signal was 5 mV. The cyclic voltammograms (CV) were obtained with a scan rate of 0.1 mV s-1 between 2.5-4.7 V.



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3. Results and discussion 3.1. Morphologies and structure. Figure

1

shows

the

SEM

images

of

pristine

and

surface-oxidized

Ni0.815Co0.15Al0.035(OH)2 precursors. Although the color of the hydroxide precursor changes from green (P-precursor, Figure 1a inset) to black (S-precursor, Figure 1b inset) during the surface oxidation process, the particle morphology of the precursor has no obvious change after the treatment. Also, the morphology of the secondary particles of S-NCA (Figure 1d) is almost the same as that of P-NCA (Figure 1c), which implies that there should be no significant change on the tap density and packing density of the NCA electrodes.

Figure 1. SEM images of (a) P-precursor, (b) S-precursor, (c) P-NCA and (d) S-NCA. The two inset photos show the color change of the precursor before and after the treatment.


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Figure 2. FE-TEM images of (a) P-precursor, (b) S-precursor, (c) P-NCA and (d) S-NCA.

The FE-TEM analysis is conducted to investigate the surface structure change of the precursors and the cathode materials synthesized with or without the treatment of Na2S2O8 (Figure 2). According to Yang et al.25, the synthesis of co-precipitated hydroxide precursor includes four stages, i.e. nucleation, concurrent crystal growth, agglomeration and re-crystallization. The primary particles are formed in the nucleation stage. They are usually nano-crystals with different orientations so that many crystal defects and even impurity phases should be present in or between the primary particles at grain boundaries (Figure 2a). Most of the defects inside the particles can be "repaired" in the following roasting process because of the internal pressure at a high temperature, while those near the surface have been inherited (Figure 2c). When soaked in the Na2S2O8 aqueous solution, the nano Ni-Co-Al-(OH)2 crystals on the surface of the particles can be restructured, forming an ordered β-NiOOH layer with a thickness ranging from 4 nm to 11 nm that has a similar layered structure with LiNiO2, while the inside remains the same as the bare precursor (Figure 2b) because the particles are very dense and the reaction time is controlled.



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Note that the particle size of the pristine Ni0.815Co0.15Al0.035(OH)2 is around 11 µm (Figure 1a), implying that only a small degree of oxidation has occurred during a 30 minutes of reaction time. If the reaction time is longer, the degree of oxidation would increase accordingly (see Figure S1 in supporting information). In the restructure process, Ni2+ ions at the surface of Ni(OH)2 are firstly oxidized into Ni3+ ions, followed by the nucleation and crystal growth of β-NiOOH. Simultaneously, most of the crystal defects near the surface can be eliminated. Therefore, a relatively continuous β-NiOOH thin layer forms at the surface of the hydroxide precursor. In the following reaction process with Li2CO3, it can be easily converted into a pure LiNiO2 phase with little crystal defects. Thus we describe this restructure process as “surface surgery”. Moreover, as can be seen in Figure 2d, the (006) planes are observed to be perpendicular to the particle surface. These parallel (006) planes are actually the 2-dimensional pathways for the diffusion of lithium ions. Therefore, such a surface microstructure is beneficial for a favorable rate performance, as confirmed by the electrochemical characterization below.

Figure 3. X-ray diffraction patterns of (a) P-precursor and S-precursor; (b) P-NCA and S-NCA; the insert picture is the magnified (003) peak of P-NCA and S-NCA

Table 1. The lattice parameters and the peak intensity ratio of (003)/(104) of P-NCA and S-NCA

Samples

a

c

I(003)/I(104)

P-NCA

2.866

14.204

1.497

S-NCA

2.863

14.107

1.574


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Figure 3a shows the X-ray diffraction (XRD) patterns of P-precursor and S-precursor. After being oxidized by Na2S2O8, the precursor still keeps mainly the original phase of Ni0.815Co0.15Al0.035(OH)2 but with slight position shifts of the diffraction peaks. However, the peak intensity ratio of (001)/(101) of S-precursor is much higher than that of P-precursor, which can be attributed to the generation of NiOOH on the surface of the precursor particles. It should also be noticed that all of the main peaks of S-precursor have broadened, which may be resulted from the crystal restructuring on the surface of NCA precursor particles and poor crystallinity of NiOOH. Figure 3b gives the XRD patterns of P-NCA and S-NCA. Both samples have good crystallinity and correspond to an α-NaFeO2 structure with R3m space group. The inset figure in Figure 3b shows the magnified (003) peaks, displaying that the peak of S-NCA has a slight shift to higher angle owing to the possible conversion from larger Ni2+ ions to smaller Ni3+ and hence smaller lattice size. As shown in Table 1, the lattice parameters, especially the c-axis, decrease after surface treatment. The relatively large lattice parameters for P-NCA may be attributed to the grain boundaries and cation mixing area in the disordered surface region. For LiNiO2-based cathode materials, the value of the peak intensity ratio I(003)/I(104) reflects the degree of Li+/Ni2+ mixing 26. Generally, the higher the I(003)/I(104) value is, the lower the degree of cation mixing that usually leads to better electrochemical performance. By calculation, S-NCA has a higher I(003)/I(104) value (1.574) than P-NCA (1.497), indicating the lower degree of cation mixing in S-NCA than in P-NCA. 3.2. The valence of the transition metals. Figure 4 shows the Raman spectra of P-NCA and S-NCA. Raman microscopy can be used to investigate the surface structure and chemical composition at a micrometer-scale spatial resolution. Obviously, the broad peak at around 500 cm-1 for P-NCA splits into two major peaks at 475 and 560 cm-1 for S-NCA, which indicates an increase in the average oxidation state of transition metals according to Lei’s study 27

. It demonstrates that the pre-oxidation method can effectively increase the oxidation

state of transition metal elements, particularly Ni ions here, in the samples. This result



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is consistent with the above XRD analysis.

Figure 4. Raman spectra of the P-NCA and S-NCA

Figure 5. X-ray photoelectron spectra of Ni2p for (a) P-NCA and (b) S-NCA, and Co2p for (c)


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P-NCA and (d) S-NCA

The XPS spectra of Ni 2p3/2 and Co 2p3/2 peaks of P-NCA and S-NCA are shown in Figure 5. The results can be semi-quantitatively analyzed by XPSPEAK software, showing the relative content of Ni2+/Ni3+ and Co2+/Co3+ based on the integral area of each peak. It clearly shows that the contents of Ni3+ and Co3+ increase from 50.2 at% and 62.0 at% to 60.3 at% and 87.2 at%, respectively. These results further confirm that the average oxidation states of transition metals have increased after being treated by Na2S2O8, thus the degree of cation mixing on the surface of NCA particles can be significantly decreased. For the binding energy of Co2+, the peak of S-NCA (Figure 5d) has a little shift to high energy region, which may be attributed to the slight change of transition metals layer. 3.3. Electrochemical results and discussion. The electrochemical properties of P-NCA and S-NCA are presented in Figure 6. The initial and the second charge-discharge curves at 0.1 C are illustrated in Figure 6a, from which we can see the discharge capacity of S-NCA (195.4 mAh g-1) is quite similar to that of the P-NCA (196.5 mAh g-1). Figure 6b shows the cycle performance of P-NCA and S-NCA at 1C in the voltage range of 2.75-4.3 V. The cells were first activated at 0.1C for three cycles and then cycled at 1C for subsequent cycles. Clearly, the S-NCA exhibits a much better cycle stability than that of P-NCA, with capacity retention of 89.2% and 61.3% after 100 cycles respectively. Figure 6c and 6d are the selected charge-discharge curves during cycling of P-NCA and S-NCA, showing the polarization of electrodes in the charge and discharge process. For the P-NCA electrode, the working voltages decrease rapidly during cycling because of the increasing polarization caused by cation disorder and crystal defects. However, the S-NCA electrode has a more stable working voltage benefited from the complete and ordered surface structure, leading to a much better cycle stability than P-NCA.



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Figure 6. (a) Initial and the second charge-discharge curves of P-NCA and S-NCA at 0.1C; (b) cycle performance at 1C between 2.75-4.3V; and selected charge-discharge curves during cycling of (c, e) P-NCA and (d, f) S-NCA between 2.75-4.3V and 2.75-4.5V, respectively.

Figure 6e and 6f presents the cycle stability of P-NCA and S-NCA at 0.5C between 2.75V and 4.5V to study the overcharging behaviors. The S-NCA delivers a discharge capacity of 161.5 mAh g-1 with capacity retention of 80.3% after 100 cycles, while that of P-NCA is only 135.2 mAh g-1 with capacity retention of 67.4% at same conditions. At the end of charging to 4.5 V, most of the Ni2+/Ni3+ and Co2+/Co3+ ions have already been oxidized to highly oxidative Ni4+ and Co4+ ions, leading to the decomposition of electrolyte and formation of a surface film on the cathode material particles. For P-NCA, as can be seen in Figure 6e, the polarization during cycling


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increases more rapidly than that of S-NCA mainly due to the serious side reactions of electrolyte on the surface of cathode particles, leading to the distortion of layered-structure from surface to bulk, and eventually resulting in the capacity fading.

Figure 7. Rate capabilities of (a) P-NCA and (b) S-NCA

Table 2. A comparison of NCA cathode material prepared with different oxidants/methods. Capacity decay rate / cycle

Rate capability (mAh g-1)

Ref.

Methods/Oxidants

Possible mechanism

Pre-oxidation method (Na2S2O8)

Ordered surface layered structure

0.10%@1C

168@2C

This work

Co-oxidation-controlled crystallization method (Na2S2O8)

Decreased cation mixing

0.08%@0.2C

153@2C

[23]

Oxidizing-coating method (KMnO4)

Surface doping of Mn4+

0.12%@2C

160@2C

[11]

Figure 7 shows the rate capabilities of P-NCA and S-NCA samples in the voltage range of 2.75-4.3 V. The discharge capacities of both samples are almost same at relatively low current densities. However, S-NCA delivers much higher capacities at relatively high current densities than P-NCA. In particular, S-NCA shows a discharge capacity of 148 mAh g-1 at 5C rate, while P-NCA only has 100 mAh g-1. It should also be noted that the charging process, including a constant current charging process from



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2.75-4.3 V and a constant voltage charging process for 30 minutes at 4.3V, of P-NCA differs considerably from that of S-NCA at high rates. And the discharging voltage of P-NCA has a sudden drop with the increasing C rates. This excellent rate capability of S-NCA can also be attributed to the complete and ordered surface structure, while the grain boundaries and other crystal defects on the surface of the P-NCA particles hinder lithium ions diffusion. Table 2 compares the electrochemical properties of the NCA cathode material prepared with different oxidants (Na2S2O8 and KMnO4). Clearly, the NCA with ordered surface structure shows the best rate capability at 2C.

Figure 8. CV curves obtained with scan rate of 0.1mVs-1 for (a) P-NCA and (b) S-NCA

Figure 8 shows the CV curves of P-NCA and S-NCA samples between 2.5-4.7 V with a scan rate of 0.1 mV s-1. In the first cycle, only two peaks can be observed on the oxidations curves, which may correspond to the oxidation of Ni3+ to Ni4+ (about 4.0 V) and Co3+ to Co4+ (about 4.2 V), respectively. However, the oxidation peaks cannot correspond exactly to the reduction peaks, which is mainly caused by the irreversible phase transition in the initial charge process, leading to the relatively low initial coulombic efficiency for Ni-based materials

28

. In the following cycles, there

are three main peaks (3.72 V, 4.03 V, and 4.24 V) can be observed in the oxidation curves (Figure 8b). According to Huang et al.

29

and Han et al.

30

, these three peaks

are attributed to three phase transitions from hexagonal to monoclinic (H1 to M), monoclinic to hexagonal (M to H2) and hexagonal to hexagonal (H2 to H3), respectively. A small peak around 3.8 V indicates the existence of a single phase


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region of M

29-30

. For the P-NCA sample, no obvious peaks can be observed in the

second and the third curves, indicating the larger polarization in the charge-discharge process. And these results are consistent with above results and analysis.

Figure 9. A schematic illustration of the Na2S2O8 treatment and the possible mechanism of the improvement of NCA with pre-oxidation.

These remarkably improved electrochemical properties result from the complete and ordered layered structure and the decreased degree of cation mixing at the surface of NCA particles. The mechanism is described schematically in Figure 9. It should be noted that the degree of oxidation also influences the electrochemical performance (Figures S1, S2 and S3) with the 30-min oxidation as the optimal condition. It needs a further study to clarify the exact reason. As analyzed before, the disordered surface layer initially formed in the sintering process for the P-NCA would inevitably increase the cell impedance, which will be discussed in the following part.



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Figure 10. The electrochemical impedance spectra of half cells containing Li/NCA after the 3rd, 50th and 100th cycles (a) P-NCA and (b) S-NCA

EIS experiments were performed to better understand the kinetic behavior of the P-NCA and S-NCA electrodes. Figure 10 shows Nyquist plots for both samples in a 50% charge state controlled by the median charge voltage of 3.7 V after 3rd, 50th and 100th cycles at 0.5C rate between 2.75V and 4.3V. Generally, each of the impedance spectra exhibits two semicircles and a line inclined at a constant angle to the abscissa axis. According to previous studies 31-32, the semicircle at high to medium frequencies is attributed to the resistance for Li+ ions migrating through the film covered on the surface of cathode materials and film capacitance; the semicircle located in a medium to low frequencies range reflects the charge transfer resistance; and the inclined line at low frequencies is Warburg impedance which is related to Li+ diffusion through solid state electrode. As can be seen in Figure 10, for both samples, the film resistance increases during cycling, suggesting the formation of increasing thicker film due to side reactions with electrolyte on the surface of cathode materials. However, for P-NCA, both film resistance and charge transfer resistance increase more rapidly than that of S-NCA, which results from a relatively thicker cation mixing layer that is full of crystal defects. According to these results and analysis, it can be concluded that the pre-oxidation method can effectively eliminate the crystal defects on the surface of cathode materials and form a complete and ordered layer, bringing about remarkable improvements of electrochemical properties of NCA cathode materials.

4. Conclusions In summary, a facile pre-oxidization method with Na2S2O8 aqueous solution can effectively eliminate the crystal defects on the surface of cathode materials and form a complete and ordered surface layer, which is greatly beneficial to the lithium ions diffusion. Also, the average valence of transition metals increases and the degree of cation mixing decreases for the modified NCA. The S-NCA sample maintains about 89.2% of the initial specific capacity, while that of the P-NCA is only 61.3% after 100


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cycles at 1C. The cycle stability under overcharging condition and rate capabilities of S-NCA sample have also been significantly improved. This pre-oxidation method may also be applicable to other Ni-rich cathode materials. Acknowledgements This study was supported by National Science Foundation of China (grant no. 51577175), NSAF (grant no. U1630106), Hefei Center of Materials Science and Technology

(2014FXZY006),

and

Education

Ministry

of

Anhui

Province

(KJ2014ZD36). We are also grateful to Elementec Ltd in Suzhou.

ASSOCIATED CONTENT Supporting Information Available: XRD patterns of NCA precursors and NCA cathode materials treated with Na2S2O8 for different time, the cycle performance of NCA treated with Na2S2O8 for different time. This material is available free of charge via the Internet at http://pubs.acs.org.



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Abstract Graphic


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Abstract Graphic 303x205mm (96 x 96 DPI)



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Figure 1. SEM images of (a) P-precursor, (b) S-precursor, (c) P-NCA and (d) S-NCA. The two inset photos show the color change of the precursor before and after the treatment.


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Figure 2. FE-TEM images of (a) P-precursor, (b) S-precursor, (c) P-NCA and (d) S-NCA. 945x631mm (72 x 72 DPI)



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Figure 3. X-ray diffraction patterns of (a) P-precursor and S-precursor; (b) P-NCA and S-NCA; the insert picture is the magnified (003) peak of P-NCA and S-NCA 2257x842mm (72 x 72 DPI)


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Figure 4. Raman spectra of the P-NCA and S-NCA 289x202mm (150 x 150 DPI)



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Figure 5 X-ray photoelectron spectra of 2413x2002mm (72 x 72 DPI)


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Figure 6. (a) Initial and the second charge-discharge curves of P-NCA and S-NCA at 0.1C; (b) cycle performance at 1C between 2.75-4.3V; and selected charge-discharge curves during cycling of (c, e) P-NCA and (d, f) S-NCA between 2.75-4.3V and 2.75-4.5V, respectively. 2010x2257mm (72 x 72 DPI)



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Figure 7. Rate capabilities of (a) P-NCA and (b) S-NCA 2257x977mm (72 x 72 DPI)


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Figure 8. CV curves obtained with scan rate of 0.1mVs-1 for (a) P-NCA and (b) S-NCA 2257x952mm (72 x 72 DPI)



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Figure 9. A schematic illustration of the Na2S2O8 treatment and the possible mechanism of the improvement of NCA with pre-oxidation. 308x159mm (96 x 96 DPI)


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Figure 10. The electrochemical impedance spectra of half cells containing Li/NCA after the 3rd, 50th and 100th cycles (a) P-NCA and (b) S-NCA 1206x421mm (72 x 72 DPI)


Surface Surgery of the Nickel-Rich Cathode Material LiNi0.815Co0

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