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A highly homogeneous nanocoating strategy for Li-rich Mn-based layered oxides based on chemical conversion
Journal of Power Sources 277 (2015) 393e402
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A highly homogeneous nanocoating strategy for Li-rich Mn-based layered oxides based on chemical conversion Jin Ma a, Biao Li a, Li An a, Hang Wei a, Xiayan Wang b, Pingrong Yu a, Dingguo Xia a, * a
College of Engineering, Peking University Key Lab of Theory and Technology for Advanced Batteries Materials, No. 5 Yiheyuan Road Haidian District, Beijing, China b Department of Chemistry and Chemical Engineering, Beijing University of Technology Key Laboratory for Green Catalysis and Separation, No.100, Pingleyuan, Chaoyang District, Beijing, China
h i g h l i g h t s Highly homogeneous nanocoating has firstly been prepared by chemical conversion. The thermostability of coated Li-rich material is superior to pristine material. The as-prepared material shows good high rate performance and cycle stability.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 August 2014 Received in revised form 14 November 2014 Accepted 28 November 2014 Available online 2 December 2014
Herein, we report a novel strategy for preparing a highly homogeneous nanocoating for Li-rich Mn-based layered oxides by the elemental Al doping followed by chemical conversion in phosphate buffer solution. X-ray photoelectron spectroscopy, soft X-ray absorption spectroscopy and transmission electron microscopy results exhibit that there exist AlPO4 nanocoating on the surface of particles. The resultant AlPO4-nanocoated Li[Li0.2Ni0.11Co0.11Mn0.54Al0.04]O2 particles exhibit a greatly enhanced reversible capacity with superior thermal stabilities in relative to pristine Li[Li0.2Ni0.13Co0.13Mn0.54]O2. Under a current density of 30 mA g1, the AlPO4-nanocoated Li[Li0.2Ni0.11Co0.11Mn0.54Al0.04]O2 can deliver a specific capacity of 282.1 mAh g1 with capacity retention of 89% after 35 cycles. © 2014 Published by Elsevier B.V.
Keywords: Composite coating Conversion reaction Segregation Cathode materials Lithium ion batteries
1. Introduction New technologies such as portable electronics, hybrid electric vehicles, and renewable energy storage devices rely on the performance of high-energy-density Li-ion batteries (LIBs). The development of low cost alternative cathode materials with high specific capacities is critical for next-generation LIBs [1]. Compared with the current commercial cathode material, LiCoO2, Li-rich Mnbased layered materials, xLi2MnO3(1x)Li[Ni1/3Co1/3Mn1/3]O2, have received particular attention in recent years as promising cathode materials for new high-energy-density energy storage devices due to their extremely high discharge specific capacities of more than 250 mAh g1, low price, low environmental toxicity, and facile processing [2e5]. However, drawbacks such as poor cycle
* Corresponding author. E-mail address: [email protected] (D. Xia). http://dx.doi.org/10.1016/j.jpowsour.2014.11.133 0378-7753/© 2014 Published by Elsevier B.V.
life, low rate performance, and low thermal stability have hindered their practical application in LIBs. To overcome these problems, tremendous efforts have been made to realize electrochemical performance improvement, including partial element doping [6,7] and the coating of active materials [8]. The doping of Li-rich Mnbased layered oxides by metallic elements can modify their cyclability and thermal stability in the highly oxidized state [9,10]. Nevertheless, such dopants usually reduce capacity due to the substitution of inactive for active components. The coating approach using metal oxides [8,11], fluorides [12,13], or phosphates [14,15] has greatly improved the high-rate capability, coulombic efficiency, and cycle stability of Li-rich Mn-based layered oxides [14,16]. However, heterogeneous coatings usually result in compromised functionality of protection [17] in the electrochemical reaction for the materials during practical processing. Therefore, a highly homogeneous nanocoating on the surface of micro- or nanoparticles would be extremely desirable.
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Herein, we report a novel strategy for preparing a highly homogeneous nanocoating for Li-rich Mn-based layered oxides by the elemental Al doping followed by chemical conversion in aqueous phosphate buffer solution (PBS). The conversion reaction method ensures uniform contact between the dissolved reagents and bulk particles at the atomic level [18]. The elemental Al uniformly segregates on the particle surfaces layer and can provide nucleation sites for the chemical conversion treatment. The resultant AlPO4nanocoated Li[Li0.2Ni0.11Co0.11Mn0.54Al0.04]O2 exhibits a greatly enhanced reversible capacity with excellent cycling and superior thermal stabilities relative to the pristine samples.
wrapped with AlPO4 were then subjected to heat treatment at 500 C for 12 h under N2 to obtain a dark powder (LNCM-AlPO4 (500 C in N2)). For comparison, Li[Li0.2Ni0.13Co0.13Mn0.54]O2 in the absence of Al was also prepared and subsequently treated via conversion reaction to form LNCM and LNCM-MPO4, respectively. The formation of phosphorus species on the surface of LNCM and LNCM-Al particles can be described by the equations below:
M3þ þ 3H2 PO 4 ¼ MðH2 PO4 Þ3 2MðH2 PO4 Þ3 ¼ M2 ðHPO4 Þ3 þ 3H3 PO4
2. Experimental section
M2 ðHPO4 Þ3 ¼ 2MPO4 þ H3 PO4
2.1. Preparation methods
M2 O3 þ 6H3 PO4 ¼ 2MðH2 PO4 Þ3 þ 3H2 O
Li[Li0.2Ni0.13Co0.13Mn0.54]O2 (LNCM) and Li[Li0.2Ni0.13xCo0.13xMn0.54Al2x]O2 (LCNM-Al (x ¼ 0.02e0.1)) materials were prepared by the Pechini process [19]. Using LiAc2H2O, Co(Ac)2, Mn(Ac)24H2O, Ni(Ac)24H2O, and Al(NO3)39H2O as metallic element sources. Citric acid monohydrate (C6H8O7H2O) associated with glycol was used as both the complexing and esterification reagent. The mole ratio of all metallic elements, citric acid, and glycol was 1:1:4, and LiAc2H2O was used in 5% stoichiometric excess to offset the loss of Li during calcination. All reagents were fully dissolved in distilled water. Rotary evaporation was used to prepare a homogeneous colloidal precursor. The precursor was dried in a vacuum oven at 150 C and then annealed at 900 C for 15 h. In order to prepare phosphate-buffered-solution (PBS)-treated samples, Na2HPO4eNaH2PO4 was adjusted to pH 5.7. The LNCM-Al (x ¼ 0.04) powder was dispersed in the PBS solution and stirred for 3 days. The suspension was filtered, dried in a vacuum oven overnight, and finally annealed at either 500 C or 700 C in air or N2 for 12 h. Specifically, Scheme 1 illustrates the approach to forming the highly continuous, uniform nanocoating. LNCM particles with richAl layer on their surfaces were synthesized by a typical solegel solid reaction method. The LNCM-Al (x ¼ 0.04) particles were dispersed into a phosphate buffer with mild stirring at room temperature. The phosphorus-based anions were distributed uniformly on the particle surfaces due to their strong affinities toward metals or metal oxides [20e22]. The insolubility of AlPO4 in phosphate buffer solution [22] results in formation of the homogeneous nanocoating by the deposition of AlPO4. The as-obtained particles
2.2. Characterization and electrochemical measurements X-ray diffraction analysis was performed on a diffractometer (Bruker, D8 X, Germany) with Cu Ka radiation at 40 KeV and 40 mA. The morphologies of the materials were characterized by cold field emission scanning electron microscopy (SEM, Hitachi, S-4800, Japan) and transmission electron microscopy (TEM, FEI, TECNAI F20, USA). Fourier-transform infrared spectroscopy (FTIR) was performed using a Thermo Fisher spectrophotometer. XPS measurements were obtained using an X-ray photoelectron spectrometer (Axis Ultra, Kratos Analytical Ltd., England) at the Analytical Instrumentation Center of Peking University. All spectra were calibrated with the C 1s photoemission peak at 284.8 eV. The atomic ratios of the relevant elements were determined from multiplex spectra using the integrated areas with all the satellite contributions included and a Shirley-type background. The relative sensitivity factors for Li 1s, C 1s, O 1s, Ni 2p, Mn 2p, Co 2p, Al 2s, and P 2p photoemission lines were given as 0.025, 0.278, 0.78, 4.044, 2.659, 3.59, 0.193, and 0.486. Quantitative analysis was performed using high-resolution spectra of the photoemission lines using CasaXPS software. Auger electron spectroscopy (AES) depth profiles were measured at the Beijing Electronic Energy Spectrum Center (PHI-700, ULVAC-PHI, Japan). O K-edge spectra were collected via soft X-ray absorption spectroscopy (XAS) in the total electron yield (TEY) mode at beamline 4B9B at the Beijing Synchrotron Radiation Facility (BSRF). Nitrogen adsorptionedesorption isotherms were recorded at 77.25 K by using ASAP 2010. The specific surface area
Scheme 1. Schematic illustration for the formation of nanocoating.
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was calculated using BrunauereEmmetteTeller (BET) method in the relative pressure (p/p0) ranging from 0.015 to 0.20, from adsorption branch of the isotherm. The electrochemical properties of the materials were evaluated using a charge/discharge test (Neware, China). Electrochemical impedance spectroscopy (EIS, Bio-Logic, France) of the coin cells was performed with glass microfiber separators (Whatman, UK) and Li metal anodes. EIS measurements were carried out after rate performance testing and full charging to 4.8 V. The cathode was fabricated by mixing the synthesized LNCM-based material, polyvinylidene difluoride (PVDF) binder, and Super-P conductive carbon black (80:10:10 by percentage mass) in N-methyl-2-pyrrolidone (NMP) and then coating the slurry onto aluminum foil. The cathode was dried for 2 h in air, and then 12 h in a vacuum oven at 120 C. All the CR2032 coin cells were assembled in an argon-filled glovebox. The electrolyte was purchased from Beijing Institute of Chemical Reagents and the ingredient is in confidential, and Whatman glass microfiber filters were used as the separator. DSC measurements were performed using a Mettler (DSC-1) instrument at a rate of 10 C min1 to 400 C. Samples were prepared by charging the cells to 4.6 V at the second cycle with a current density of 30 mA g1, maintained at a constant voltage for 12 h, and then disassembled in an argon-filled glove box. The cathode material was sealed in an aluminum pan with an additional 1 mL fresh electrolyte before removing from the glove box for measurement.
3. Results and discussion Fig. 1a shows the X-ray diffraction (XRD) patterns of LNCM-Al products as a function of Al content x (where x ¼ 0, 0.02, 0.04, 0.06, 0.08, 0.1). All the diffraction peaks can be indexed as the NaFeO2-type layered structure with space group R3 m. The additional weak peaks around 20 e25 are attributed to the presence of a Li2MnO3-like structure (JCPDS No. 27-1252). The separations between the adjacent peaks of (006)/(102) and (018)/(110) can be clearly observed, implying a typical layered structure. No obvious peaks corresponding to Al-containing phases such as g-LiAlO2 or bLi5AlO4 are found in the XRD patterns. This indicates that the Al atoms have blended into the NaFeO2-type crystal structure or that relatively low proportions of Al-containing compounds are present.
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In addition, X-ray diffraction peaks such as the (11l)-type shift to higher 2q values with increasing aluminum content, as shown in Fig. 1b, similar to the anisotropic broadening of the XRD lines in Croguennec's work [23]. This can be attributed to anisotropic strain and size effects for the (110)-type planes, resulting from the tendency of nickel and aluminum to segregate in the layered lithium nickel oxide structure. This distribution of transition metal ions would provide a good foundation for the formation of a highly uniform coating. Fig. 2a shows the XRD patterns of LNCM-AlPO4. There are two distinguishable peaks that occur near 31 and 37 in comparison with the pattern of LNCM-Al (x ¼ 0.04) (as can be clearly seen in Fig. 2b and c), which are consistent with the positions of the (112) plane of AlPO4 (JCPDS No. 72-1161) and the (311) plane of spinel LixMn2O4 compounds (JCPDS No. 52-1841), respectively. As the heat treatment temperature is increased to 700 C, the two peaks grow more pronounced. However, as shown in Fig. 2def, heat treatment in air causes the new peak appearing at about 28.3 . This peak is consistent with the (111) plane of Mn2P2O7 (JCPDS NO. 77-1343). It means that heat treatment under N2 atmosphere favors the formation of the AlPO4. In addition, the XRD pattern of LNCM-MPO4 does not show an obvious change relative to that of LNCM (Fig. 3). To further determine the surface phase of the as-prepared LNCM, LNCM-MPO4, LNCM-Al (x ¼ 0.04), and LNCM-AlPO4 (500 C in N2) materials, we obtained their O K-edge XAS spectra using the total electron yield (TEY) mode. For comparison, the O Kedges of AlPO4 and LiAlO2 standards were also measured. In Fig. 4a, the spectrum of AlPO4 presents three significant features, peaks A, B, and C. Using the “fingerprint” region of the O K-edge XAS, we can assign the existence of AlPO4, indicating the nanocoating on the surface of LNCM-AlPO4 is AlPO4. The adsorption peak A below 534 eV can be attributed to the bulk LNCM, in which the oxygen 2p character is hybridized with the sharp-structured transition-metal 3d band [24]. Fig. 4b shows he O K-edges of MPO4 species (M ¼ Mn, Co, Ni) standards for comparison. By using “fingerprint” of K-edges of MPO4 species, no obvious noticeable features of MPO4 species can be observed in the K-edges spectra of LNCM-MPO4. As to the origin of the peak at about 534 eV, we can't determine its physical significance in this stage. Indeed, the peak at about 534 eV is absent in other work [24]. Fig. 4c shows that there is not an obvious
Fig. 1. XRD patterns of (a) LNCM, LNCM-Al (x ¼ 0.02), LNCM-Al (x ¼ 0.04), LNCM-Al (x ¼ 0.06), LNCM-Al (x ¼ 0.08), and LNCM-Al (x ¼ 0.1). (b) The expanded region shows the anisotropic change for different peaks.
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Fig. 2. XRD patterns of (a) LNCM-Al (x ¼ 0.04) and LNCM-AlPO4 heat treated at 500 C and 700 C in N2, respectively. (b), (c) Expanded regions of XRD patterns of LNCM, LNCM-Al (x ¼ 0.04), and LNCM-AlPO4 heat treated at 500 C and 700 C in N2, respectively. (d) LNCM-Al (x ¼ 0.04) and LNCM-AlPO4 after heat treatment at 500 C and 700 C in air, The patterns in (e) and (f) are expanded regions from (c).
Fig. 3. XRD patterns of (a) LNCM and LNCM-MPO4 after heat treatment at 500 C in N2; (b) Expanded region which shows no changes in the modified sample.
structural difference between LNCM-Al (x ¼ 0.04) and LiAlO2, implying the incorporation of elemental Al into the bulk material. The morphologies and surface structures of the as-obtained materials were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and highresolution (HR)-TEM. Fig. 5 shows uniform spheroidal particles of 100e150 nm in size. Fig. 6aee presents typical TEM images of the
LNCM and LNCM-Al particles with differing Al contents. As Al is incorporated, the nanosized coating grows on the surface of LNCMAl. With an increase in Al content, the LNCM-Al particles become well coated by a homogeneous nanoscale layer. Depth profile measurements were carried out on LNCM-Al (x ¼ 0.04) by Auger electron spectroscopy. Fig. 7 shows the changes in the atomic concentration of elements as a function of depth. The composition
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Fig. 4. (a), (b) O K-edge XAS spectra of LNCM, LNCM-AlPO4 (500 C in N2), LNCM-MPO4, and MPO4. The latter spectrum is included for comparison. (c) O K-edge XAS spectra of LNCM and LNCM-Al (x ¼ 0.04). The LiAlO2 spectrum is presented as a standard for comparison.
Fig. 5. SEM images of layered (a) LNCM, (b) LNCM-Al (x ¼ 0.02), (c) LNCM-Al (x ¼ 0.04), (d) LNCM-Al (x ¼ 0.06), (e) LNCM-Al (x ¼ 0.08), (f) LNCM-Al (x ¼ 0.1), and (g) LNCM-AlPO4 (500 C in N2).
Fig. 6. TEM images of (a) LNCM, (b) LNCM-Al (x ¼ 0.02), (c) LNCM-Al (x ¼ 0.04), (d) LNCM-Al (x ¼ 0.06), (e) LNCM-Al (x ¼ 0.1), and (f) LCNM-AlPO4 (500 C in N2).
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J. Ma et al. / Journal of Power Sources 277 (2015) 393e402 Table 1 Results of XPS quantitative analysis of overall surface composition of LNCM, LNCMAl, and LNCM-AlPO4. Element
Li Al Mn Co Ni P C O
Fig. 7. Auger electron spectroscopy depth profiles of LNCM-Al (x ¼ 0.04).
of the outer layer is different from that of the bulk. The Al concentration is higher than the Ni and Co concentration in the outer layer, but the Al concentration decreases as the Mn concentration increases sharply from the surface to the interior. The concentration
LNCM
LNCM-Al (x ¼ 0.04)
LNCM-AlPO4 (500 C in N2)
at%
at%
at%
19.5 9.2 3.4 2.9
19 8.1 11.6 3.8 2.6
25.6 39.4
15.2 39.7
13.8 8.3 10.9 3 2.8 0.7 25 35.5
of Al at 35 nm depth is as low as 4% in comparison with the surface. This indicates the segregation of Al on the surface of the LNCM-Al (x ¼ 0.04) particles. Since the Al surface excess can be related to a decrease in surface energy [25], Al segregation is expected to stabilize smaller particles. After subjecting the LNCM-Al (x ¼ 0.04) particles to the conversion reaction, the TEM images show good surface coverage by a highly continuous and uniform coating with a thickness of 4 nm, as shown in Fig. 6f. As we know, the ultra-thin poorly crystalline homogeneous nanocoating can increase the Liþ ion transfer capacity by reducing the anisotropy of the surface [26]. Furthermore, a
Fig. 8. HR-TEM images of (a) LNCM-MPO4 (b) LNCM, (c) LNCM-Al (x ¼ 0.04), (d) LNCM-AlPO4 (500 C in N2).
Fig. 9. XPS spectra of (a) O 1s, (b) Al 2p, and (c) P 2p for LNCM, LNCM-Al (x ¼ 0.04), and LNCM-AlPO4 (500 C in N2). B.E. indicates binding energy.
J. Ma et al. / Journal of Power Sources 277 (2015) 393e402 Table 2 Quantitative Analysis of different O species in LNCM, LNCM-Al and LNCM-MPO4. O element
MeO CO2 3 PO3 4
LNCM
LNCM-Al (x ¼ 0.04)
LNCM-AlPO4 (500 C in N2)
Position (eV)
at%
Position (eV)
at%
Position (eV)
at%
529.42 531.6 e
60.0 39.1 e
529.53 531.39 e
79 21 e
529.25 530.97 532.64
80.2 16.6 3.17
Fig. 10. FTIR spectra of LNCM-Al (x ¼ 0.04) and LNCM-AlPO4 (500 C in N2) after annealing at 500 C in N2.
highly homogeneous coating can enhance the electrochemical properties of the materials relative to an uneven coating. In contrast, the particle surfaces of LNCM-MPO4 do not have the coating (Fig. 8a), because MPO4 (M ¼ Mn, Co, Ni) hydrates are
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soluble in the phosphate buffer solution, whereas AlPO4 are not [27]. Finally, beneath the outer coating, a spinel phase is present in the LNCM-AlPO4 (500 C in N2) sample compared to LNCM and LNCM-Al (Fig. 8b and c), which is consistent with the XRD results and could contribute to the high rate capability in an LIB, as shown in Fig. 8d. XPS is employed to determine the surface compositions and structures of the as-prepared samples (Fig. 9). The surface chemical compositions of the LNCM, LNCM-Al (x ¼ 0.04), and LNCM-AlPO4 (500 C in N2) samples are presented in Table 1. Quantitative analysis of O 1s photoemission line in all samples is used to evaluate the amount of Li2CO3 on the particle exterior [28] resulting from surface reactions with CO2 in air. Table 2 shows analysis results of different O species in LNCM, LNCM-Al (x ¼ 0.04) and LNCMAlPO4 (500 C in N2). Quantitative comparison of the O 1s peaks reveals that the amount of Li2CO3 is reduced by about 45.8% for LNCM-Al (x ¼ 0.04) and 61% for LNCM-AlPO4 (500 C in N2) compared to the LNCM sample. This means LNCM-AlPO4 (500 C in N2) has more stability in air than LNCM-Al and LNCM. To determine the real particle surface composition, the contribution of the O 1s integrated area from Li2CO3, appearing at around 531 eV should be removed. The ratio of Li2CO3 content is (39.4 39.1): (39.7 21): (35.5 16.6) in the surface of LNCM, LNCM-Al (x ¼ 0.04), and LNCM-AlPO4 (500 C in N2). The corresponding stoichiometric amount of Li in Li2CO3 is also removed. The small peak around 532.64 eV can be assigned to PO3 4 . After calculation, the surface composition of LNCM is Li9.2Mn9.2Co3.4Ni2.9O24.0 (alternatively, Li0.76Mn0.76Co0.28Ni0.24O2). For LNCM-Al (x ¼ 0.04) and LNCM-MPO4, the surface composition are determined as Li13.4Mn11.6Co3.8Ni2.6Al8.1O31.3 and Li9.8Mn10.9Co3.0Ni2.8Al8.3P0.7O29.6, respectively. The surface composition of LNCM-Al (x ¼ 0.04) based on the XPS results is Li13.4Mn11.6Co3.8Ni2.6Al8.1O31.3. The surface Al concentration values are higher than the average atomic ratio of Al in the bulk, which further indicates that segregation of Al occurs on the surface during the 900 C heat treatment. The Al 2p peak occurring at 73.2 eV is comparable to that of LiAlO2 (73.4 eV) [29], implying Al blending in the LNCM-Al (x ¼ 0.04). After the conversion reaction and subsequent heat treatment, the Al 2p peak of LNCM-AlPO4 (500 C in N2) could be deconvoluted to a combination of two peaks at 75.0 and 73.4 eV. The former could be assigned to AlPO4 [29]. The P 2p photoemission peak at 133.7 eV is also comparable to that of AlPO4 [30,31]. Furthermore, FTIR spectroscopy (Fig. 10) showed bands at approximately 1050 cm1 corresponding to the antisymmetric stretching vibrations of the phosphate radical [32]. All
Fig. 11. (a) DSC results for LNCM, LNCM-Al (x ¼ 0.04), and LNCM-AlPO4 (500 C in N2) samples. (b) BrunauereEmmetteTeller (BET) profiles of LNCM, LNCM-A (x ¼ 0.04), and LNCMAlPO4 (500 C in N2).
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Table 3 BET Surface areas of LNCM, LNCM-Al (x ¼ 0.04), and LNCM-AlPO4 (500 C in N2).
BET surface areas (m2 g1)
LNCM LNCM-Al (x ¼ 0.04)
LNCM-AlPO4 (500 C in N2)
5.6374 5.5972
5.6247
of these identify the surface layer of LNCM-AlPO4 (500 C in N2) as AlPO4. Because of the high lithium conductivity and HF tolerance of AlPO4 [14], it can be expected that the AlPO4 nanocoating will greatly promote the electrochemical performance and thermal stability of the LNCM-AlPO4 (500 C in N2) samples. Improving thermal stability is one of the most important challenges for Li-rich Mn-based cathode materials under the high voltage cut-off for LIBs. Fig. 11a shows the differential scanning
calorimetry (DSC) profiles of LNCM, LNCM-Al (x ¼ 0.04), and LNCMAlPO4 (500 C in N2) in the presence of liquid electrolyte when charged to 4.6 V. The LNCM sample shows a large exothermic peak with a heat release of 1086.7 J g1 at 216.4 C, which agrees well with published data for similar materials [12,33]. In comparison, the LNCM-Al (x ¼ 0.04) and LNCM-AlPO4 (500 C in N2) samples released only 469.4 J g1 at 254.2 C and 225 J g1 at 277.3 C, respectively. Based on the minor differences in specific areas for LNCM, LNCM-Al (x ¼ 0.04), and LNCM-AlPO4 (500 C in N2) (Fig. 11b and Table 3), the improved thermal stability is ascribed to the highly homogeneous nanocoating, which not only protects the highly oxidized cathode materials from direct contact with the electrolyte solution but also supplies a rapid Li-ion diffusion path. Fig. 12a shows the discharge capacity profiles as a function of cycle number under a current density of 30 mA g1. It is noticeable that LNCM-Al (x ¼ 0.04) exhibited the highest capacity of
Fig. 12. (a) Discharge capacity profiles of LNCM and LNCM-Al series samples. (b) Discharge capacity profiles for LNCM, LNCM-Al (x ¼ 0.04), and LNCM-AlPO4 (500 C in N2) samples. (c) Discharge capacity profiles of LNCM-AlPO4 and LNCM-MPO4. (d) Rate capabilities of LNCM, LNCM-Al (x ¼ 0.04), and LNCM-AlPO4 (500 C in N2) samples.
Fig. 13. (a) Voltage profiles of the 1st (dark), 10th (red), 20th (green) and 30th (blue). (b) Average discharge voltage versus cycle number. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
J. Ma et al. / Journal of Power Sources 277 (2015) 393e402 Table 4 EIS data based on equivalent circuit fitting.
LNCM LNCM-Al (x ¼ 0.04) LNCM-AlPO4 (500 C in N2)
R1 (ohm)
R2 (ohm)
R3 (ohm)
4.418 3.703 2.779
111.88 75.55 23.88
79.96 75.54 55.37
Fig. 14. Nyquist plots of LNCM, LNCM-Al (x ¼ 0.04), and LNCM-AlPO4 (500 C in N2) samples after rate testing and full charging to 4.8 V.
257.4 mAh g1 after 35 cycles, which was 14% higher than the LNCM sample. Therefore, LNCM-Al (x ¼ 0.04) was selected as the focus for investigation in the present work. As shown in Fig. 12b, LNCMAlPO4 (500 C in N2) exhibits a higher specific capacity and better cycling stability than LNCM-Al (x ¼ 0.04). LNCM-AlPO4 (500 C in N2) retains a discharge capacity of 282.1 mAh g1 with acceptable capacity retention of 89% after 35 cycles under a current density of 30 mA g1. In contrast, LNCM-Al (x ¼ 0.04), LNCM, and LNCM-MPO4 shows lower specific capacities of 257, 225, and 145 mAh g1 (Fig. 12a and c), respectively. In addition, LNCM-AlPO4 obtained in different conditions, other than at 500 C in N2, exhibit poor cycle stability (Fig. 12c). It is relative with the destruction of uniform nanocoating due to the formation of new phase, such as Mn2P2O7, on the surface of particles. Another considerable improvement for LNCM-AlPO4 (500 C in N2) in comparison with the LNCM-Al (x ¼ 0.04) and LNCM samples is its rate capability. As shown in Fig. 12d, the LNCM-AlPO4 sample demonstrates the highest capacity compared to the LNCM-Al (x ¼ 0.04) and LNCM samples under all investigated current densities. Even under a very high current density of 1200 mA g1, the LNCM-AlPO4 (500 C in N2) sample still exhibited a favorable specific capacity of 162.7 mAh g1, while the LNCM-Al (x ¼ 0.04) and LNCM samples had lower specific capacities of 129.9 and 93 mAh g1, respectively. Importantly, after the high-rate measurements, the specific capacity of the electrode material cycled under a current density of 30 mA g1 could be recovered to its initial value for LNCM-AlPO4 (500 C in N2), implying its good reversibility. Potential fading is also one of the important problem associated with the Li-excess cathode material. Herein, the cycle voltage profiles at a current density of 30 mA g1 were employed to evaluate the potential stability, as shown in Fig. 13a and b. It can be found that the average discharge potential drops from 3.51 V to 3.22 V for LNCM sample after 30 cycles, which is 0.29 V lower than
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initial value. In contrast, LNCM-Al (x ¼ 0.04) and LNCM-AlPO4 (500 C in N2) samples show only a decrease of 0.24 V and 0.11 V, respectively, demonstrating a remarkable improvement of potential fading for LNCM-AlPO4. The potential stability enhancement could be relative with the AlPO4 nanocoating facilitating the Liþ migration [34e36]. The promotion of the Li-ion transfer process between the electrolyte and LNCM-AlPO4 (500 C in N2) was confirmed by electrochemical impedance spectroscopy (EIS). The EIS measurements were performed in the fully charged state after the cells were cycled for 35 cycles. In the fitting data (Table 4), which were calculated from the typical equivalent circuit model for this kind of material [37e39] inserted in Fig. 14, R1 denotes the ohmic resistance between the working electrode and the reference electrode, which provides information on the electric conductivity in the electrolyte, separator, and electrodes. R2 implies the charge transfer resistance, also known as Faraday resistance. R3 represents the resistance for lithium ion diffusion in the solidestate interface layer. Other elements in the equivalent circuit are W2, Q1, and Q3, corresponding to the Warburg impedance for lithium ion diffusion in the bulk material, the constant phase element for the capacitance of the double layer, and the constant phase element for the capacitance of the surface layer, respectively. According to the data in Table 4, all the materials exhibit very similar ohmic resistance differences between the working and reference electrodes. The pristine sample has the largest charge transfer resistance, as much as 111.88 U, which is 5 times larger than the modified sample, which means the charge transfer process has the most influence on the electrochemical properties. Thus, the improved rate capacity and cycle stability are mainly attributable to the decrement in charge transfer resistance, and lower lithium ion diffusion resistance more strongly enhances the electrochemical properties of the LNCM-AlPO4 (500 C in N2) sample. Comparing the Nyquist plots (Fig. 14) and fitting data based on equivalent circuit fitting (Table 4) for LNCM, LNCM-Al (x ¼ 0.04), and LNCM-AlPO4 (500 C in N2), the latter showed much lower charge-transfer resistance R2 than LNCM and LNCM-Al (x ¼ 0.04). The smaller R2 for LNCM-AlPO4 (500 C in N2) clearly indicates that the AlPO4 nanocoating can effectively inhibit side reactions on the cathode surface at high voltage and significantly facilitate lithium ion and electron transport, leading to a high rate capacity and excellent cyclability.
4. Conclusions In conclusion, this work reports a novel surface modification strategy for Li-rich cathode materials. The as-prepared Li [Li0.2Ni0.11Co0.11Mn0.54Al0.04]O2 with homogeneous AlPO4 nanocoating exhibits excellent rate capacity and superior thermal stability. The mitigation in voltage fading was also obvious for the AlPO4-nanocoated Li[Li0.2Ni0.11Co0.11Mn0.54Al0.04]O2. The enhanced electrochemical performance can be attributed the high conductivity and chemical stability of AlPO4 nanocoating. This facile surface modification approach can also be extended to the preparation of other electrode materials.
Acknowledgments This work is supported by the National Natural Science Foundation of China (11179001, 21275014) and the National High Technology Research and Development Program (No.2012AA052201). We would also like to acknowledge the staff of the XAS beamlines (beamline 4B9B) of the Beijing Synchrotron Radiation Facility.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
A highly homogeneous nanocoating strategy for Li-rich Mn-based layered oxides based on chemical conversion Jin Ma a, Biao Li a, Li An a, Hang Wei a, Xiayan Wang b, Pingrong Yu a, Dingguo Xia a, * a
College of Engineering, Peking University Key Lab of Theory and Technology for Advanced Batteries Materials, No. 5 Yiheyuan Road Haidian District, Beijing, China b Department of Chemistry and Chemical Engineering, Beijing University of Technology Key Laboratory for Green Catalysis and Separation, No.100, Pingleyuan, Chaoyang District, Beijing, China
h i g h l i g h t s Highly homogeneous nanocoating has firstly been prepared by chemical conversion. The thermostability of coated Li-rich material is superior to pristine material. The as-prepared material shows good high rate performance and cycle stability.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 August 2014 Received in revised form 14 November 2014 Accepted 28 November 2014 Available online 2 December 2014
Herein, we report a novel strategy for preparing a highly homogeneous nanocoating for Li-rich Mn-based layered oxides by the elemental Al doping followed by chemical conversion in phosphate buffer solution. X-ray photoelectron spectroscopy, soft X-ray absorption spectroscopy and transmission electron microscopy results exhibit that there exist AlPO4 nanocoating on the surface of particles. The resultant AlPO4-nanocoated Li[Li0.2Ni0.11Co0.11Mn0.54Al0.04]O2 particles exhibit a greatly enhanced reversible capacity with superior thermal stabilities in relative to pristine Li[Li0.2Ni0.13Co0.13Mn0.54]O2. Under a current density of 30 mA g1, the AlPO4-nanocoated Li[Li0.2Ni0.11Co0.11Mn0.54Al0.04]O2 can deliver a specific capacity of 282.1 mAh g1 with capacity retention of 89% after 35 cycles. © 2014 Published by Elsevier B.V.
Keywords: Composite coating Conversion reaction Segregation Cathode materials Lithium ion batteries
1. Introduction New technologies such as portable electronics, hybrid electric vehicles, and renewable energy storage devices rely on the performance of high-energy-density Li-ion batteries (LIBs). The development of low cost alternative cathode materials with high specific capacities is critical for next-generation LIBs [1]. Compared with the current commercial cathode material, LiCoO2, Li-rich Mnbased layered materials, xLi2MnO3(1x)Li[Ni1/3Co1/3Mn1/3]O2, have received particular attention in recent years as promising cathode materials for new high-energy-density energy storage devices due to their extremely high discharge specific capacities of more than 250 mAh g1, low price, low environmental toxicity, and facile processing [2e5]. However, drawbacks such as poor cycle
* Corresponding author. E-mail address: [email protected] (D. Xia). http://dx.doi.org/10.1016/j.jpowsour.2014.11.133 0378-7753/© 2014 Published by Elsevier B.V.
life, low rate performance, and low thermal stability have hindered their practical application in LIBs. To overcome these problems, tremendous efforts have been made to realize electrochemical performance improvement, including partial element doping [6,7] and the coating of active materials [8]. The doping of Li-rich Mnbased layered oxides by metallic elements can modify their cyclability and thermal stability in the highly oxidized state [9,10]. Nevertheless, such dopants usually reduce capacity due to the substitution of inactive for active components. The coating approach using metal oxides [8,11], fluorides [12,13], or phosphates [14,15] has greatly improved the high-rate capability, coulombic efficiency, and cycle stability of Li-rich Mn-based layered oxides [14,16]. However, heterogeneous coatings usually result in compromised functionality of protection [17] in the electrochemical reaction for the materials during practical processing. Therefore, a highly homogeneous nanocoating on the surface of micro- or nanoparticles would be extremely desirable.
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Herein, we report a novel strategy for preparing a highly homogeneous nanocoating for Li-rich Mn-based layered oxides by the elemental Al doping followed by chemical conversion in aqueous phosphate buffer solution (PBS). The conversion reaction method ensures uniform contact between the dissolved reagents and bulk particles at the atomic level [18]. The elemental Al uniformly segregates on the particle surfaces layer and can provide nucleation sites for the chemical conversion treatment. The resultant AlPO4nanocoated Li[Li0.2Ni0.11Co0.11Mn0.54Al0.04]O2 exhibits a greatly enhanced reversible capacity with excellent cycling and superior thermal stabilities relative to the pristine samples.
wrapped with AlPO4 were then subjected to heat treatment at 500 C for 12 h under N2 to obtain a dark powder (LNCM-AlPO4 (500 C in N2)). For comparison, Li[Li0.2Ni0.13Co0.13Mn0.54]O2 in the absence of Al was also prepared and subsequently treated via conversion reaction to form LNCM and LNCM-MPO4, respectively. The formation of phosphorus species on the surface of LNCM and LNCM-Al particles can be described by the equations below:
M3þ þ 3H2 PO 4 ¼ MðH2 PO4 Þ3 2MðH2 PO4 Þ3 ¼ M2 ðHPO4 Þ3 þ 3H3 PO4
2. Experimental section
M2 ðHPO4 Þ3 ¼ 2MPO4 þ H3 PO4
2.1. Preparation methods
M2 O3 þ 6H3 PO4 ¼ 2MðH2 PO4 Þ3 þ 3H2 O
Li[Li0.2Ni0.13Co0.13Mn0.54]O2 (LNCM) and Li[Li0.2Ni0.13xCo0.13xMn0.54Al2x]O2 (LCNM-Al (x ¼ 0.02e0.1)) materials were prepared by the Pechini process [19]. Using LiAc2H2O, Co(Ac)2, Mn(Ac)24H2O, Ni(Ac)24H2O, and Al(NO3)39H2O as metallic element sources. Citric acid monohydrate (C6H8O7H2O) associated with glycol was used as both the complexing and esterification reagent. The mole ratio of all metallic elements, citric acid, and glycol was 1:1:4, and LiAc2H2O was used in 5% stoichiometric excess to offset the loss of Li during calcination. All reagents were fully dissolved in distilled water. Rotary evaporation was used to prepare a homogeneous colloidal precursor. The precursor was dried in a vacuum oven at 150 C and then annealed at 900 C for 15 h. In order to prepare phosphate-buffered-solution (PBS)-treated samples, Na2HPO4eNaH2PO4 was adjusted to pH 5.7. The LNCM-Al (x ¼ 0.04) powder was dispersed in the PBS solution and stirred for 3 days. The suspension was filtered, dried in a vacuum oven overnight, and finally annealed at either 500 C or 700 C in air or N2 for 12 h. Specifically, Scheme 1 illustrates the approach to forming the highly continuous, uniform nanocoating. LNCM particles with richAl layer on their surfaces were synthesized by a typical solegel solid reaction method. The LNCM-Al (x ¼ 0.04) particles were dispersed into a phosphate buffer with mild stirring at room temperature. The phosphorus-based anions were distributed uniformly on the particle surfaces due to their strong affinities toward metals or metal oxides [20e22]. The insolubility of AlPO4 in phosphate buffer solution [22] results in formation of the homogeneous nanocoating by the deposition of AlPO4. The as-obtained particles
2.2. Characterization and electrochemical measurements X-ray diffraction analysis was performed on a diffractometer (Bruker, D8 X, Germany) with Cu Ka radiation at 40 KeV and 40 mA. The morphologies of the materials were characterized by cold field emission scanning electron microscopy (SEM, Hitachi, S-4800, Japan) and transmission electron microscopy (TEM, FEI, TECNAI F20, USA). Fourier-transform infrared spectroscopy (FTIR) was performed using a Thermo Fisher spectrophotometer. XPS measurements were obtained using an X-ray photoelectron spectrometer (Axis Ultra, Kratos Analytical Ltd., England) at the Analytical Instrumentation Center of Peking University. All spectra were calibrated with the C 1s photoemission peak at 284.8 eV. The atomic ratios of the relevant elements were determined from multiplex spectra using the integrated areas with all the satellite contributions included and a Shirley-type background. The relative sensitivity factors for Li 1s, C 1s, O 1s, Ni 2p, Mn 2p, Co 2p, Al 2s, and P 2p photoemission lines were given as 0.025, 0.278, 0.78, 4.044, 2.659, 3.59, 0.193, and 0.486. Quantitative analysis was performed using high-resolution spectra of the photoemission lines using CasaXPS software. Auger electron spectroscopy (AES) depth profiles were measured at the Beijing Electronic Energy Spectrum Center (PHI-700, ULVAC-PHI, Japan). O K-edge spectra were collected via soft X-ray absorption spectroscopy (XAS) in the total electron yield (TEY) mode at beamline 4B9B at the Beijing Synchrotron Radiation Facility (BSRF). Nitrogen adsorptionedesorption isotherms were recorded at 77.25 K by using ASAP 2010. The specific surface area
Scheme 1. Schematic illustration for the formation of nanocoating.
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was calculated using BrunauereEmmetteTeller (BET) method in the relative pressure (p/p0) ranging from 0.015 to 0.20, from adsorption branch of the isotherm. The electrochemical properties of the materials were evaluated using a charge/discharge test (Neware, China). Electrochemical impedance spectroscopy (EIS, Bio-Logic, France) of the coin cells was performed with glass microfiber separators (Whatman, UK) and Li metal anodes. EIS measurements were carried out after rate performance testing and full charging to 4.8 V. The cathode was fabricated by mixing the synthesized LNCM-based material, polyvinylidene difluoride (PVDF) binder, and Super-P conductive carbon black (80:10:10 by percentage mass) in N-methyl-2-pyrrolidone (NMP) and then coating the slurry onto aluminum foil. The cathode was dried for 2 h in air, and then 12 h in a vacuum oven at 120 C. All the CR2032 coin cells were assembled in an argon-filled glovebox. The electrolyte was purchased from Beijing Institute of Chemical Reagents and the ingredient is in confidential, and Whatman glass microfiber filters were used as the separator. DSC measurements were performed using a Mettler (DSC-1) instrument at a rate of 10 C min1 to 400 C. Samples were prepared by charging the cells to 4.6 V at the second cycle with a current density of 30 mA g1, maintained at a constant voltage for 12 h, and then disassembled in an argon-filled glove box. The cathode material was sealed in an aluminum pan with an additional 1 mL fresh electrolyte before removing from the glove box for measurement.
3. Results and discussion Fig. 1a shows the X-ray diffraction (XRD) patterns of LNCM-Al products as a function of Al content x (where x ¼ 0, 0.02, 0.04, 0.06, 0.08, 0.1). All the diffraction peaks can be indexed as the NaFeO2-type layered structure with space group R3 m. The additional weak peaks around 20 e25 are attributed to the presence of a Li2MnO3-like structure (JCPDS No. 27-1252). The separations between the adjacent peaks of (006)/(102) and (018)/(110) can be clearly observed, implying a typical layered structure. No obvious peaks corresponding to Al-containing phases such as g-LiAlO2 or bLi5AlO4 are found in the XRD patterns. This indicates that the Al atoms have blended into the NaFeO2-type crystal structure or that relatively low proportions of Al-containing compounds are present.
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In addition, X-ray diffraction peaks such as the (11l)-type shift to higher 2q values with increasing aluminum content, as shown in Fig. 1b, similar to the anisotropic broadening of the XRD lines in Croguennec's work [23]. This can be attributed to anisotropic strain and size effects for the (110)-type planes, resulting from the tendency of nickel and aluminum to segregate in the layered lithium nickel oxide structure. This distribution of transition metal ions would provide a good foundation for the formation of a highly uniform coating. Fig. 2a shows the XRD patterns of LNCM-AlPO4. There are two distinguishable peaks that occur near 31 and 37 in comparison with the pattern of LNCM-Al (x ¼ 0.04) (as can be clearly seen in Fig. 2b and c), which are consistent with the positions of the (112) plane of AlPO4 (JCPDS No. 72-1161) and the (311) plane of spinel LixMn2O4 compounds (JCPDS No. 52-1841), respectively. As the heat treatment temperature is increased to 700 C, the two peaks grow more pronounced. However, as shown in Fig. 2def, heat treatment in air causes the new peak appearing at about 28.3 . This peak is consistent with the (111) plane of Mn2P2O7 (JCPDS NO. 77-1343). It means that heat treatment under N2 atmosphere favors the formation of the AlPO4. In addition, the XRD pattern of LNCM-MPO4 does not show an obvious change relative to that of LNCM (Fig. 3). To further determine the surface phase of the as-prepared LNCM, LNCM-MPO4, LNCM-Al (x ¼ 0.04), and LNCM-AlPO4 (500 C in N2) materials, we obtained their O K-edge XAS spectra using the total electron yield (TEY) mode. For comparison, the O Kedges of AlPO4 and LiAlO2 standards were also measured. In Fig. 4a, the spectrum of AlPO4 presents three significant features, peaks A, B, and C. Using the “fingerprint” region of the O K-edge XAS, we can assign the existence of AlPO4, indicating the nanocoating on the surface of LNCM-AlPO4 is AlPO4. The adsorption peak A below 534 eV can be attributed to the bulk LNCM, in which the oxygen 2p character is hybridized with the sharp-structured transition-metal 3d band [24]. Fig. 4b shows he O K-edges of MPO4 species (M ¼ Mn, Co, Ni) standards for comparison. By using “fingerprint” of K-edges of MPO4 species, no obvious noticeable features of MPO4 species can be observed in the K-edges spectra of LNCM-MPO4. As to the origin of the peak at about 534 eV, we can't determine its physical significance in this stage. Indeed, the peak at about 534 eV is absent in other work [24]. Fig. 4c shows that there is not an obvious
Fig. 1. XRD patterns of (a) LNCM, LNCM-Al (x ¼ 0.02), LNCM-Al (x ¼ 0.04), LNCM-Al (x ¼ 0.06), LNCM-Al (x ¼ 0.08), and LNCM-Al (x ¼ 0.1). (b) The expanded region shows the anisotropic change for different peaks.
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Fig. 2. XRD patterns of (a) LNCM-Al (x ¼ 0.04) and LNCM-AlPO4 heat treated at 500 C and 700 C in N2, respectively. (b), (c) Expanded regions of XRD patterns of LNCM, LNCM-Al (x ¼ 0.04), and LNCM-AlPO4 heat treated at 500 C and 700 C in N2, respectively. (d) LNCM-Al (x ¼ 0.04) and LNCM-AlPO4 after heat treatment at 500 C and 700 C in air, The patterns in (e) and (f) are expanded regions from (c).
Fig. 3. XRD patterns of (a) LNCM and LNCM-MPO4 after heat treatment at 500 C in N2; (b) Expanded region which shows no changes in the modified sample.
structural difference between LNCM-Al (x ¼ 0.04) and LiAlO2, implying the incorporation of elemental Al into the bulk material. The morphologies and surface structures of the as-obtained materials were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and highresolution (HR)-TEM. Fig. 5 shows uniform spheroidal particles of 100e150 nm in size. Fig. 6aee presents typical TEM images of the
LNCM and LNCM-Al particles with differing Al contents. As Al is incorporated, the nanosized coating grows on the surface of LNCMAl. With an increase in Al content, the LNCM-Al particles become well coated by a homogeneous nanoscale layer. Depth profile measurements were carried out on LNCM-Al (x ¼ 0.04) by Auger electron spectroscopy. Fig. 7 shows the changes in the atomic concentration of elements as a function of depth. The composition
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Fig. 4. (a), (b) O K-edge XAS spectra of LNCM, LNCM-AlPO4 (500 C in N2), LNCM-MPO4, and MPO4. The latter spectrum is included for comparison. (c) O K-edge XAS spectra of LNCM and LNCM-Al (x ¼ 0.04). The LiAlO2 spectrum is presented as a standard for comparison.
Fig. 5. SEM images of layered (a) LNCM, (b) LNCM-Al (x ¼ 0.02), (c) LNCM-Al (x ¼ 0.04), (d) LNCM-Al (x ¼ 0.06), (e) LNCM-Al (x ¼ 0.08), (f) LNCM-Al (x ¼ 0.1), and (g) LNCM-AlPO4 (500 C in N2).
Fig. 6. TEM images of (a) LNCM, (b) LNCM-Al (x ¼ 0.02), (c) LNCM-Al (x ¼ 0.04), (d) LNCM-Al (x ¼ 0.06), (e) LNCM-Al (x ¼ 0.1), and (f) LCNM-AlPO4 (500 C in N2).
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J. Ma et al. / Journal of Power Sources 277 (2015) 393e402 Table 1 Results of XPS quantitative analysis of overall surface composition of LNCM, LNCMAl, and LNCM-AlPO4. Element
Li Al Mn Co Ni P C O
Fig. 7. Auger electron spectroscopy depth profiles of LNCM-Al (x ¼ 0.04).
of the outer layer is different from that of the bulk. The Al concentration is higher than the Ni and Co concentration in the outer layer, but the Al concentration decreases as the Mn concentration increases sharply from the surface to the interior. The concentration
LNCM
LNCM-Al (x ¼ 0.04)
LNCM-AlPO4 (500 C in N2)
at%
at%
at%
19.5 9.2 3.4 2.9
19 8.1 11.6 3.8 2.6
25.6 39.4
15.2 39.7
13.8 8.3 10.9 3 2.8 0.7 25 35.5
of Al at 35 nm depth is as low as 4% in comparison with the surface. This indicates the segregation of Al on the surface of the LNCM-Al (x ¼ 0.04) particles. Since the Al surface excess can be related to a decrease in surface energy [25], Al segregation is expected to stabilize smaller particles. After subjecting the LNCM-Al (x ¼ 0.04) particles to the conversion reaction, the TEM images show good surface coverage by a highly continuous and uniform coating with a thickness of 4 nm, as shown in Fig. 6f. As we know, the ultra-thin poorly crystalline homogeneous nanocoating can increase the Liþ ion transfer capacity by reducing the anisotropy of the surface [26]. Furthermore, a
Fig. 8. HR-TEM images of (a) LNCM-MPO4 (b) LNCM, (c) LNCM-Al (x ¼ 0.04), (d) LNCM-AlPO4 (500 C in N2).
Fig. 9. XPS spectra of (a) O 1s, (b) Al 2p, and (c) P 2p for LNCM, LNCM-Al (x ¼ 0.04), and LNCM-AlPO4 (500 C in N2). B.E. indicates binding energy.
J. Ma et al. / Journal of Power Sources 277 (2015) 393e402 Table 2 Quantitative Analysis of different O species in LNCM, LNCM-Al and LNCM-MPO4. O element
MeO CO2 3 PO3 4
LNCM
LNCM-Al (x ¼ 0.04)
LNCM-AlPO4 (500 C in N2)
Position (eV)
at%
Position (eV)
at%
Position (eV)
at%
529.42 531.6 e
60.0 39.1 e
529.53 531.39 e
79 21 e
529.25 530.97 532.64
80.2 16.6 3.17
Fig. 10. FTIR spectra of LNCM-Al (x ¼ 0.04) and LNCM-AlPO4 (500 C in N2) after annealing at 500 C in N2.
highly homogeneous coating can enhance the electrochemical properties of the materials relative to an uneven coating. In contrast, the particle surfaces of LNCM-MPO4 do not have the coating (Fig. 8a), because MPO4 (M ¼ Mn, Co, Ni) hydrates are
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soluble in the phosphate buffer solution, whereas AlPO4 are not [27]. Finally, beneath the outer coating, a spinel phase is present in the LNCM-AlPO4 (500 C in N2) sample compared to LNCM and LNCM-Al (Fig. 8b and c), which is consistent with the XRD results and could contribute to the high rate capability in an LIB, as shown in Fig. 8d. XPS is employed to determine the surface compositions and structures of the as-prepared samples (Fig. 9). The surface chemical compositions of the LNCM, LNCM-Al (x ¼ 0.04), and LNCM-AlPO4 (500 C in N2) samples are presented in Table 1. Quantitative analysis of O 1s photoemission line in all samples is used to evaluate the amount of Li2CO3 on the particle exterior [28] resulting from surface reactions with CO2 in air. Table 2 shows analysis results of different O species in LNCM, LNCM-Al (x ¼ 0.04) and LNCMAlPO4 (500 C in N2). Quantitative comparison of the O 1s peaks reveals that the amount of Li2CO3 is reduced by about 45.8% for LNCM-Al (x ¼ 0.04) and 61% for LNCM-AlPO4 (500 C in N2) compared to the LNCM sample. This means LNCM-AlPO4 (500 C in N2) has more stability in air than LNCM-Al and LNCM. To determine the real particle surface composition, the contribution of the O 1s integrated area from Li2CO3, appearing at around 531 eV should be removed. The ratio of Li2CO3 content is (39.4 39.1): (39.7 21): (35.5 16.6) in the surface of LNCM, LNCM-Al (x ¼ 0.04), and LNCM-AlPO4 (500 C in N2). The corresponding stoichiometric amount of Li in Li2CO3 is also removed. The small peak around 532.64 eV can be assigned to PO3 4 . After calculation, the surface composition of LNCM is Li9.2Mn9.2Co3.4Ni2.9O24.0 (alternatively, Li0.76Mn0.76Co0.28Ni0.24O2). For LNCM-Al (x ¼ 0.04) and LNCM-MPO4, the surface composition are determined as Li13.4Mn11.6Co3.8Ni2.6Al8.1O31.3 and Li9.8Mn10.9Co3.0Ni2.8Al8.3P0.7O29.6, respectively. The surface composition of LNCM-Al (x ¼ 0.04) based on the XPS results is Li13.4Mn11.6Co3.8Ni2.6Al8.1O31.3. The surface Al concentration values are higher than the average atomic ratio of Al in the bulk, which further indicates that segregation of Al occurs on the surface during the 900 C heat treatment. The Al 2p peak occurring at 73.2 eV is comparable to that of LiAlO2 (73.4 eV) [29], implying Al blending in the LNCM-Al (x ¼ 0.04). After the conversion reaction and subsequent heat treatment, the Al 2p peak of LNCM-AlPO4 (500 C in N2) could be deconvoluted to a combination of two peaks at 75.0 and 73.4 eV. The former could be assigned to AlPO4 [29]. The P 2p photoemission peak at 133.7 eV is also comparable to that of AlPO4 [30,31]. Furthermore, FTIR spectroscopy (Fig. 10) showed bands at approximately 1050 cm1 corresponding to the antisymmetric stretching vibrations of the phosphate radical [32]. All
Fig. 11. (a) DSC results for LNCM, LNCM-Al (x ¼ 0.04), and LNCM-AlPO4 (500 C in N2) samples. (b) BrunauereEmmetteTeller (BET) profiles of LNCM, LNCM-A (x ¼ 0.04), and LNCMAlPO4 (500 C in N2).
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Table 3 BET Surface areas of LNCM, LNCM-Al (x ¼ 0.04), and LNCM-AlPO4 (500 C in N2).
BET surface areas (m2 g1)
LNCM LNCM-Al (x ¼ 0.04)
LNCM-AlPO4 (500 C in N2)
5.6374 5.5972
5.6247
of these identify the surface layer of LNCM-AlPO4 (500 C in N2) as AlPO4. Because of the high lithium conductivity and HF tolerance of AlPO4 [14], it can be expected that the AlPO4 nanocoating will greatly promote the electrochemical performance and thermal stability of the LNCM-AlPO4 (500 C in N2) samples. Improving thermal stability is one of the most important challenges for Li-rich Mn-based cathode materials under the high voltage cut-off for LIBs. Fig. 11a shows the differential scanning
calorimetry (DSC) profiles of LNCM, LNCM-Al (x ¼ 0.04), and LNCMAlPO4 (500 C in N2) in the presence of liquid electrolyte when charged to 4.6 V. The LNCM sample shows a large exothermic peak with a heat release of 1086.7 J g1 at 216.4 C, which agrees well with published data for similar materials [12,33]. In comparison, the LNCM-Al (x ¼ 0.04) and LNCM-AlPO4 (500 C in N2) samples released only 469.4 J g1 at 254.2 C and 225 J g1 at 277.3 C, respectively. Based on the minor differences in specific areas for LNCM, LNCM-Al (x ¼ 0.04), and LNCM-AlPO4 (500 C in N2) (Fig. 11b and Table 3), the improved thermal stability is ascribed to the highly homogeneous nanocoating, which not only protects the highly oxidized cathode materials from direct contact with the electrolyte solution but also supplies a rapid Li-ion diffusion path. Fig. 12a shows the discharge capacity profiles as a function of cycle number under a current density of 30 mA g1. It is noticeable that LNCM-Al (x ¼ 0.04) exhibited the highest capacity of
Fig. 12. (a) Discharge capacity profiles of LNCM and LNCM-Al series samples. (b) Discharge capacity profiles for LNCM, LNCM-Al (x ¼ 0.04), and LNCM-AlPO4 (500 C in N2) samples. (c) Discharge capacity profiles of LNCM-AlPO4 and LNCM-MPO4. (d) Rate capabilities of LNCM, LNCM-Al (x ¼ 0.04), and LNCM-AlPO4 (500 C in N2) samples.
Fig. 13. (a) Voltage profiles of the 1st (dark), 10th (red), 20th (green) and 30th (blue). (b) Average discharge voltage versus cycle number. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
J. Ma et al. / Journal of Power Sources 277 (2015) 393e402 Table 4 EIS data based on equivalent circuit fitting.
LNCM LNCM-Al (x ¼ 0.04) LNCM-AlPO4 (500 C in N2)
R1 (ohm)
R2 (ohm)
R3 (ohm)
4.418 3.703 2.779
111.88 75.55 23.88
79.96 75.54 55.37
Fig. 14. Nyquist plots of LNCM, LNCM-Al (x ¼ 0.04), and LNCM-AlPO4 (500 C in N2) samples after rate testing and full charging to 4.8 V.
257.4 mAh g1 after 35 cycles, which was 14% higher than the LNCM sample. Therefore, LNCM-Al (x ¼ 0.04) was selected as the focus for investigation in the present work. As shown in Fig. 12b, LNCMAlPO4 (500 C in N2) exhibits a higher specific capacity and better cycling stability than LNCM-Al (x ¼ 0.04). LNCM-AlPO4 (500 C in N2) retains a discharge capacity of 282.1 mAh g1 with acceptable capacity retention of 89% after 35 cycles under a current density of 30 mA g1. In contrast, LNCM-Al (x ¼ 0.04), LNCM, and LNCM-MPO4 shows lower specific capacities of 257, 225, and 145 mAh g1 (Fig. 12a and c), respectively. In addition, LNCM-AlPO4 obtained in different conditions, other than at 500 C in N2, exhibit poor cycle stability (Fig. 12c). It is relative with the destruction of uniform nanocoating due to the formation of new phase, such as Mn2P2O7, on the surface of particles. Another considerable improvement for LNCM-AlPO4 (500 C in N2) in comparison with the LNCM-Al (x ¼ 0.04) and LNCM samples is its rate capability. As shown in Fig. 12d, the LNCM-AlPO4 sample demonstrates the highest capacity compared to the LNCM-Al (x ¼ 0.04) and LNCM samples under all investigated current densities. Even under a very high current density of 1200 mA g1, the LNCM-AlPO4 (500 C in N2) sample still exhibited a favorable specific capacity of 162.7 mAh g1, while the LNCM-Al (x ¼ 0.04) and LNCM samples had lower specific capacities of 129.9 and 93 mAh g1, respectively. Importantly, after the high-rate measurements, the specific capacity of the electrode material cycled under a current density of 30 mA g1 could be recovered to its initial value for LNCM-AlPO4 (500 C in N2), implying its good reversibility. Potential fading is also one of the important problem associated with the Li-excess cathode material. Herein, the cycle voltage profiles at a current density of 30 mA g1 were employed to evaluate the potential stability, as shown in Fig. 13a and b. It can be found that the average discharge potential drops from 3.51 V to 3.22 V for LNCM sample after 30 cycles, which is 0.29 V lower than
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initial value. In contrast, LNCM-Al (x ¼ 0.04) and LNCM-AlPO4 (500 C in N2) samples show only a decrease of 0.24 V and 0.11 V, respectively, demonstrating a remarkable improvement of potential fading for LNCM-AlPO4. The potential stability enhancement could be relative with the AlPO4 nanocoating facilitating the Liþ migration [34e36]. The promotion of the Li-ion transfer process between the electrolyte and LNCM-AlPO4 (500 C in N2) was confirmed by electrochemical impedance spectroscopy (EIS). The EIS measurements were performed in the fully charged state after the cells were cycled for 35 cycles. In the fitting data (Table 4), which were calculated from the typical equivalent circuit model for this kind of material [37e39] inserted in Fig. 14, R1 denotes the ohmic resistance between the working electrode and the reference electrode, which provides information on the electric conductivity in the electrolyte, separator, and electrodes. R2 implies the charge transfer resistance, also known as Faraday resistance. R3 represents the resistance for lithium ion diffusion in the solidestate interface layer. Other elements in the equivalent circuit are W2, Q1, and Q3, corresponding to the Warburg impedance for lithium ion diffusion in the bulk material, the constant phase element for the capacitance of the double layer, and the constant phase element for the capacitance of the surface layer, respectively. According to the data in Table 4, all the materials exhibit very similar ohmic resistance differences between the working and reference electrodes. The pristine sample has the largest charge transfer resistance, as much as 111.88 U, which is 5 times larger than the modified sample, which means the charge transfer process has the most influence on the electrochemical properties. Thus, the improved rate capacity and cycle stability are mainly attributable to the decrement in charge transfer resistance, and lower lithium ion diffusion resistance more strongly enhances the electrochemical properties of the LNCM-AlPO4 (500 C in N2) sample. Comparing the Nyquist plots (Fig. 14) and fitting data based on equivalent circuit fitting (Table 4) for LNCM, LNCM-Al (x ¼ 0.04), and LNCM-AlPO4 (500 C in N2), the latter showed much lower charge-transfer resistance R2 than LNCM and LNCM-Al (x ¼ 0.04). The smaller R2 for LNCM-AlPO4 (500 C in N2) clearly indicates that the AlPO4 nanocoating can effectively inhibit side reactions on the cathode surface at high voltage and significantly facilitate lithium ion and electron transport, leading to a high rate capacity and excellent cyclability.
4. Conclusions In conclusion, this work reports a novel surface modification strategy for Li-rich cathode materials. The as-prepared Li [Li0.2Ni0.11Co0.11Mn0.54Al0.04]O2 with homogeneous AlPO4 nanocoating exhibits excellent rate capacity and superior thermal stability. The mitigation in voltage fading was also obvious for the AlPO4-nanocoated Li[Li0.2Ni0.11Co0.11Mn0.54Al0.04]O2. The enhanced electrochemical performance can be attributed the high conductivity and chemical stability of AlPO4 nanocoating. This facile surface modification approach can also be extended to the preparation of other electrode materials.
Acknowledgments This work is supported by the National Natural Science Foundation of China (11179001, 21275014) and the National High Technology Research and Development Program (No.2012AA052201). We would also like to acknowledge the staff of the XAS beamlines (beamline 4B9B) of the Beijing Synchrotron Radiation Facility.
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