An in-depth analysis detailing the structural and electrochemical properties within Br− modified LiNi0.815Co0.15A0.035O2 (NCA) cathode material

Electrochimica Acta 318 (2019) 362e373

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

An in-depth analysis detailing the structural and electrochemical properties within Br
modified LiNi0.815Co0.15A0.035O2 (NCA) cathode material Shiyu He a, b, 1, Aijia Wei a, c, 1, Wen Li a, *, Xue Bai a, Lihui Zhang a, Xiaohui Li a, Rui He a, Lili Yang b, **, Zhenfa Liu a, c, *** a b c

Institute of Energy Resources, Hebei Academy of Sciences, Shijiazhuang, Hebei Province 050081, PR China College of Chemistry and Material Science, Hebei Normal University, Shijiazhuang, Hebei 050024, PR China School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300000, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 April 2019 Received in revised form 1 June 2019 Accepted 10 June 2019 Available online 15 June 2019

The Br
modified LiNi0.815Co0.15Al0.035O2 (NCA) cathode materials areprepared via a situ doping method for the first time. The structural and electrochemical properties within Br
modified NCA cathode material are in an in-depth analysis. X-ray diffraction (XRD) results show enlarged interslab spacing due to the fact that partial Br
has doped into the bulk particles of NCA to replace O2
in 6c site. Density functional theory (DFT) calculations suggest that Br
doping mainly occurs in the surface layer. Moreover, the Br‒ modified NCA cathode materials present smaller primary particles than pure NCA, which is proved by calculations of particle size from random scanning electron microscopy (SEM) images. X-ray photoelectron spectroscopy (XPS) results certify that the the valence state of Ni3þ is partly reduced to Ni2þ and residual lithium acts as stable LiBr instead of unstable Li2CO3 at the surface of Br‒ modified NCA cathode materials. From the electrochemical tests, 0.2 mol% Br
modified NCA with reduced potential polarization, decreasing value of Rsf þ Rct and increasing Liþ diffusion coefficient, exhibits excellent cycling performance, even at elevated temperature. These results clearly indicate that Br
modification contributes to the remarkable enhancement of structure stability and cycling performance of NCA. © 2019 Elsevier Ltd. All rights reserved.

Keywords: LiNi0.815Co0.15Al0.035O2 Br
Modification Structure Electrochemical performance

1. Introduction Lithium ion batteries (LIBs) with their higher energy density, longer cyclic life, faster charge/discharge rates and higher security qualities, are at the forefront of today's race to create next generation power sources for mobile phones, electric vehicles and notebook computers which translate into long battery life and short charge time for the end consumer of electronic devices [1,2]. Electrochemical performance of LIBs is significantly influenced by the electrochemical property of cathode materials. Offering higher capacity and improvements in structural stability at a lower cost are some reasons why nickel rich cathode material Li(NixM1
x)O2 has

* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (W. Li), [email protected] (L. Yang), [email protected] (Z. Liu). 1 Shiyu He and Aijia Wei contributed equally to this work. https://doi.org/10.1016/j.electacta.2019.06.061 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

been attracting significant attention [3e6]. An abundance of research efforts points towards stabilization of the layered structure of LiNiO2 as a direct result of the co-doping of Co and Al, which increases the nickel-rich cathode material's electrochemical performance [7e9]. Co and Al co-doped nickel-rich cathode material (NCA) with up to 185 mA h g
1 capacity has been successfully designed today, exhibiting great potential for LIBs particularly those used in electric vehicles. Currently, the challenges facing NCA development include: exponential structural degradation, lackluster cycling performance, harmful side reactions on the surface, which are caused by repeated changes in the lattice during lithiation/de-lithiation [10,11]. Cation substitution (e.g. Mn [12,13], Sb [14], Ti [15,16], Na [17], Mg [18e20] and surface modification (e.g. Al2O3 [21], LiAlO2 [22], NCM [23,24], poly(3-hexylthiophene-2,5-diyl) [25], TiP2O7 [26], TiO2 [27e29], carbon nanotubes [30,31], BiPO4 [32], AlPO4 [33], Co3O4 [34,35], LiCoO2 [35,36], LBO [37,38], SnO2 [39], SiO2 [40], Al-rich surface layer [41], V-based coating [42], FePO4 [43], Li2OeZrO2 [44], AlF3 [45], graphene [46], Ni3(PO4)2 [47], LiNi0.5Mn0.5O2 [48], Li4Ti5O12

S. He et al. / Electrochimica Acta 318 (2019) 362e373

[49] and carbon [50] et al.) are effective measures taken to restrain phase transformation and side reactions at the interface in an effort to improve structural stability and decelerate the capacity decay of NCA. Another promising method used in the enhancement of the stability of the cycle of electrode materials for LIBs is anion doping. Currently, anions such as F
[51e53], Cl
[54,55], Br
[56], S2
[57],
and PO2
4 [58] are used. Recently, F has been added to the list of anions deployed by the low temperature method to improve NCA performance [52]. F
doping indicates the process acts as a catalyst to improve growth of primary particles. The valence state of surface nickel ions is lowered; the inter-slab spacing distance is enlarged; the impedance increase during cycling is lowered, all of which help to suppress the cathode material's degradation. The cycling performance of F
modified NCA is therefore enhanced at room temperature and at a high temperature of 55  C, with potential for an even higher upper cut-off. In contrast, the NCA modified with Br
modified NCA would be have a more stable structure than the NCA modified with F
because of M(Ni, Co, Al)eBr bonds which, as a result of greater electron affinity of Br than O and F (Br: 342.54 kJ mol
1, F: 328 kJ mol
1, O:141 kJ mol
1), are stronger than M(Ni, Co, Al)eO or the M(Ni, Co, Al) bonds. Additionally, In addition, it is expected that the formation of NCA can be prepared by the Br
doping with the similar mechanism of the F doping. Furthermore, due to the larger ionic radius (0.196 nm) of Br
compared with that (0.133 nm) of F
or that (0.140 nm) of O2
, NCA modified with Br
can possibly elevate Liþ mobility. In this paper, Br
modified NCA were synthesized via in situ modification. The effects of Br
modification on the structure, morphology and electrochemical performance of the Br
modified NCA cathode materials were also investigated systematically. 2. Description of experiment 2.1. Synthesis and characterization Br
modified NCA materials represented by samples NCABr-x (x ¼ 1, 2, 3, 4) were made using a typical synthesis method [52]. Commercially obtained NCA precursors (Ni0.815Co0.15Al0.035), LiOH$H2O (99.995%, Aladdin) and NH4Br (99.998%, Aladdin) were homogeneously mixed, after which the mixture was sintered at 480  C for 300 min and then calcined at 750  C for 900 min under flowing oxygen. The molar ratio of Li: NCA precursor was set at 1.05 : 1 and an additional amount of Li (5%) was included to make up for Li volatilization during calcination. The dosage of Br
was capped at 0.1 mol%, 0.2 mol%, 0.3 mol%, 0.4 mol% and 0.5mol% of the NCA material, as shown by samples NCABr-1, NCABr-2, NCABr-3 and NCABr-4 respectively. For comparison purposes, pure NCA was made under exact conditions without the addition of Br
. Powder X-ray diffraction (XRD, UItima IV, Japan) employing Cu Ka radiation was used in the identification of the various crystalline phases of the synthesized materials in the 2q range of 10 e90 at a scanning speed of 0.02 s
1 with a count time of 2 s. Crystalstructure refinements employing the Rietveld method were employed using analysis software (TOPAS 4.2). The microstructure and energy-dispersive X-ray spectroscopy (EDS) maps of powders were obtained through the scanning of electron microscopy (SEM; HITACHI SU8020). Inductively coupled plasma-atomic emission spectrometry (ICP-AES) (ICPOES730, Agilent, USA) was used to determinate the Br element contents in the Br
modified materials. Scanning transmission electron microscopy and high-angle annular dark-field (STEM-HAADF) imaging was also performed on the STEM, in addition to energy-dispersive X-ray spectroscopy (EDS) (OXFORD X-MaxN 80T, EDAX, USA). X-ray photoelectron spectroscopy (XPS; Thermo Scientific Escalab 250Xi) was executed to work

363

out the surface chemical state of materials. 2.2. Computational method Density functional theory (DFT) calculations using the Cambridge Serial Total Energy Package were supplied by Shenzhen HUASUAN Technology co. LTD. The exchange-correlation energy was described using the revised Perdew-Burke-Ernzerhof exchange-correlation density functional (PBESOL) [59] within the generalized-gradient approximation (GGA). A 400 eV plane-wave kinetic energy cutoff was chosen. The Brillouin zone was sampled with 8  8  2 and 4  4  1 MonkhorstePack grids, respectively, for NCA bulk and surfaces calculations. The atomic positions were fully relaxed until a maximum energy difference and residual force on atoms, respectively, converged to 10
5 eV and 0.03 eV Å
1. A 15 Å thick vacuum layer was used to avoid the interaction between top and bottom surfaces. 2.3. Electrochemical measurement Electrochemical properties were tested using a CR2032 cointype half-cell assembled in a glovebox filled with argon. The halfcell comprises a cathode and a Li metal anode set apart by a porous polypropylene film (Celgard 2400, USA). The cathode slurry was a homogeneous mixture of the active material (pure NCA or Br
modified NCA), conductive Super-P (TIMCAL), and a polyvinylidene difluoride binder (PVDF900, ARKEMA) in a mass ratio of 8 : 1: 1 in N-methyl-2-pyrrolidone (NMP, MYJ CHEMICAL). The slurry was then cast onto a piece of aluminum foil. After a period of 300min of drying to eradicate the NMP solvent, the electrode laminate was punched into disks (10 mm in diameter) and further dehydrated overnight in a vacuum oven at 105  C. The mass loading of the active material was nearly 3.0 mg cm
2. The electrolyte used in the experiment was TC-E2823 produced by Guangzhou Tinci Materials Technology Co., Ltd., China. Galvanostatic chargedischarge experiments at different C rates within the range of 2.8 Ve4.3 V were conducted on an automated galvanostatic chargedischarge unit manufactured by Land CT 2001A, Wuhan, China. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests were performed on an electrochemical system (Interface1000, Gamry, USA). For CV measurements, the potential range at a scan rate of 0.1 mV s
1 was found to vary from 2.8 V to 4.5 V. EIS measurements performed over a frequency range from 105 Hz to 10
2 Hz were executed at a fully charged state of 4.3 V. 3. Results and discussion 3.1. Material characterizations Fig. 1 exhibits the XRD patterns of pure NCA and NCABr-x (x ¼ 1, 2, 3, 4) samples. As results indicate, the diffraction peaks of all samples are exactly the same as the hexagonal a-NaFeO2 layered structure with an R3m space group (card No. 42-1467). All patterns demonstrate sharp diffraction peaks which show crystallinity. The obvious splitting of the (006)/(102) and (108)/(110) peaks hints at a structure with satisfactory layers [12e14]. Through the analysis software of TOPAS 4.2, the Rietveld refinement of the XRD patterns using the phase model of rhombohedral LiNiO2 (space group R3m) confirmed the structures of all samples. For the NCA material, Liþ and Ni2þ could occupy the 3a and 3b sites, while Co3þ and Al3þ are found in the 3b sites, and oxygen in the 6c sites [17e19]. The Rietveld refinement patterns are shown in Fig. 2, and structural data obtained by the Rietveld analysis are listed in Table 1. The a in the hexagonal unit cell is a measure of the average M-M (M ¼ Li, Ni, Co, Al) distance in slabs ‒ the layers that are filled predominantly by

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Fig. 1. (a)XRD patterns of pure NCA and NCABr-x (x ¼ 1, 2, 3, 4) samples; Rietveld refinement patterns for NCA (b), NCABr-1(c), NCABr-2(d), NCABr-3 (e) and NCABr-4 (f).

transition metal ions, while c is the thickness of three slabs and three inter-slabs (Li layers) [18e24]. Regarding the NCABr-x (x ¼ 1, 2, 3, 4) samples, the lattice constant of a shows no visible changes when compared to pure NCA. Nevertheless, the NCABr-x (x ¼ 1, 2, 3 and 4) samples reveal an enlarged lattice constant of c and axis ratios of c/a, compared with pure NCA. NCABr-x (x ¼ 1, 2, 3, 4) samples carry evidence to indicate Br
tried to replace O2
in the 6c sites, creating an increase in the value of c as a result of the larger ionic radius of Br
(0.196 nm) when compared against the smaller one of O2
(0.140 nm). The enlargement of c provides irrefutable evidence that a small change in the thickness of LiO2 and MnO2 slabs between the metal ions has the ability to affect the diffusion paths of Liþ and has a greater effect on inherent electrochemical property. The larger values of c for NCABr-x (x ¼ 1, 2, 3, 4) samples is a sign of wider Liþ diffusion channels have been created, which is good for the Liþ diffusion process. It has been well confirmed that the axis ration c/a can be used as a direct measure for the deviation of lattice from a perfect cubic close-packed (ccp) lattice, namely the degree of layeredness of the lattice [21e25]. A ccp lattice is seen as ideal when it has a c/a value of 4.94 [21e25]. For all samples prepared in this paper, the values of c/a are higher than 4.94, suggesting a better hexagonal phase formed in the NCA materials. The increased c/a for all NCABr-x (x ¼ 1, 2, 3 and 4) samples indicates an

improved layered structure. The ratio I003/I104 is known to react to the cation spread in the lattice and to the degree of cation mixing of materials [24e28]. Lower ratios show undesirable cation mixing between Liþ and Ni2þ. The ratios I003/I104 of NCABr-1, NCABr-2, NCABr-3 and NCABr-4 are 1.19, 1.21, 1.20 and 1.21 respectively, which are lower by a small degree than that of pure NCA (1.23). These results show a rise in the degree of cation mixing for the Br
modified NCA sample. This could be a result of the partial substitution of Br
for O2
, which contributes to the transition of a certain amount of Ni ions from Ni3þ to Ni2þ as charge compensation. Assisted by structural refinements, Liþ/Ni2þ cation mixing is calculated to be 1.65% for pure NCA, 2.03% for NCABr-1, 1.89% for NCABr-2, 1.89% for NCABr-3 and 1.90% for NCABr-4. Clearly, the Liþ/ Ni2þ cation mixing has caused tiny changes. SEM images of the pure NCA and NCABr-x (x ¼ 1, 2, 3, 4) samples are displayed in Fig. 2a
e. Fig. 2a reveals pure NCA particles come with smooth surfaces, while NCABr-1 particles exhibited in Fig. 3b show no visible differences, which indicates that a tiny amount (0.1 mol %) of Br
brought into the modification process has no distinct effect on morphology and microstructure. In comparison, as the Br
amount is increased further, the NCABr-x (x ¼ 2, 3, 4) particles exhibit a rougher surface. Upon the level of Br
increasing to 0.3 mol % and 0.4 mol%, many minuscule island-like particles will

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Fig. 2. SEM images of primary particles of pure NCA (a), and NCABr-1 (b), NCABr-2 (c), NCABr-3 (d), NCABr-4 (e); TEM images of pure NCA (f), NCABr-2 (g) and NCABr-4 (h).

Table 1 Refinement parameters of pure NCA and NCABr-x (x ¼ 1, 2, 3, 4) samples obtained by Rietveld Refinement. Sample

NCA NCABr-1 NCABr-2 NCABr-3 NCABr-4

Lattice parameter a (Å)

c (Å)

c/a

V(Å3)

2.86588 2.86589 2.86589 2.86589 2.86589

14.17650 14.17877 14.17992 14.18177 14.18225

4.94664 4.94742 4.94782 4.94847 4.94864

116.4353 116.4549 116.4643 116.4795 116.4834

I(003)/I(104)

Rwp/%

1.24536 1.19251 1.22891 1.21456 1.21346

6.92 6.06 6.56 6.98 7.37

have shown up on the majority of NCABr-3 and NCABr-4 particles. Higher content of Br
possibly exerts a larger effect on the morphology of the bulk NCA particles as demonstrated by these results. Tiny surface particles might exist in an amorphous phase as evidenced by a lack of crystallization of any other impurities in XRD results. Fig. 2f
h shows the TEM images of pure NCA, NCABr-2 and NCABr-4. Fig. 2f indicates pure NCA particles exhibit smooth edges and no other tiny particles. However, there are some tiny and amorphous particles on the surface of NCABr-2 particles shown in Fig. 2g. As the amount of Br-increases, more tiny particles form on the surface of NCABr-4 particles which is shown in Fig. 2h. These results are in accordance with the SEM results revealed in Fig. 2a
e.

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Fig. 3. Primary particle size (PPS) distribution estimated from 200 particles for each sample by SEM of pure NCA(a), NCABr-1(b), NCA Br-2(c), NCABr-3(d), NCABr-4(e).

Additionally, NCABr-x (x ¼ 1, 2, 3, 4) particles interestingly decreased in size when compared with pure NCA particles. Thus, it prompted a calculation of particle size from random SEM images ‒ 200 particles for each sample shown in Fig. S1
S2 and primary particle size (PPS) results are displayed in Figs. 3 and S3, Table 2 and Table S2. The mean PPSs of the NCABr-x (x ¼ 1, 2, 3, 4) particles fell by over 100e200 nm, thereby illustrating the fact: introduction of Br
can also restrain the growth of primary particles of NCA. The smaller particles thus form a short pathway for Liþ diffusion, favoring an improvement in the rate capability of the NCABr-x (x ¼ 1, 2, 3, 4) samples. Fig. 4a is a collection of EDS mapping of cross-sections of NCABr-

Table 2 Primary particle size distribution maximum, minimum, mean and STDEV (Unit: nm). Sample

Maximum

Minimum

Mean

STDEV

NCA NCABr-1 NCABr-2 NCABr-3 NCABr-4

880.6 809.2 714.0 642.6 666.4

261.8 190.4 166.6 156.8 123.4

540.26 390.08 358.9 323.63 328.34

168.31 115.51 102.13 100.32 108.97

2 secondary particles. Br element is found to be homogeneously spread across in the cross-sections of NCABr-2 secondary particles comprising main elements (Ni, Co, Al). This ensures Br
exist at the center, boundary, and outer surfaces of the NCABr-2 secondary particles. Fig. 4b shows EDS mappings of cross-sections of NCABr-2 primary particles. Br element is also uniformly distributed in the cross-sections of NCABr-2 primary particles comprising main elements (Ni, Co, Al). This demonstrates to a greater extent the incorporation of Br
into the bulk phase of the primary particle. To examine further the embedding of Br
into the lattice, STEMHAADF imaging with EDS mapping was executed on the NCABr-2 primary particles. The results are shown in Fig. 4c, indicating a conclusion that Br element is also observed and uniformly distributed in the NCABr-2 primary particles comprising main elements (Ni, Co, Al). Moreover, XPS spectra analysis was later performed to find out the surface chemical state of pure NCA, NCABr-2 and NCABr-4, and the results are shown in Fig. 5. XPS result indicate that the binding energy of Ni2p3/2, Co2p3/2 and Al 2p presents subtle change, compared with pure NCA (shown in Table 3)， thereby proving Br
has doped into the bulk NCA. The Ni 2p3/2 peak of NCABr-2 and NCABr-4 are at 854.81 eV and 854.84 eV, respectively, which are slightly lower than that of pure NCA (854.95 eV). Transferring to

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Fig. 4. EDS mappings of a cross-section of second particle (a) and primary particle (b) for NCABr-2; (c) STEM-HAADF imaging with EDS mapping of primary particle for NCABr-2.

lower energy indicates a decrease of the covalence of nickel-oxygen and a decrease of the valence of the transition metal nickel, that is, more Ni2þ appear on the surface of NCABr-2 and NCABr-4 than pure NCA. An estimation of the ionic valence content can be gauged from the area under the curve, and the ratios of Ni3þ 2p3/2 to Ni2þ 2p3/2 for NCA, NCABr-2 and NCABr-4 are 2.83, 2.72 and 2.63, respectively. And more Ni2þ ions on the surface of the particles have been demonstrated to help improve the structural stability of the Ni-rich materials [60e62]. In addition, the Co 2p3/2 and Al 2p peaks of NCABr-2 and NCABr-4 shift to a slightly higher bonding energy, due to the Br doping. The Br 3d5/2 bonding energy for LiBr is 69.2 eV. However, the Br 3d5/2 peaks of NCABr-2 and NCABr-4 are located at 68.68 and 68.48 eV, deviating from 69.2 eV, which indicates that Br not only acts as LiBr, but also M(Ni,Co,Al)eBr bonds. As the amount of Br
increases, the intensity of the Br 3d5/2 peaks slowly elevates as well, indicating an increase in bromide metal with increasing x for NCABr-x (x ¼ 2, 4) samples. In comparison to NCA, as x increases, the Ni 2p and Co 2p peaks of NCABr-x samples exhibit a

monotonous degradation in intensity due to more bromide metal forming on the surface of the NCABr-x (x ¼ 2, 4) particles. Fig. 5e shows that the C 1s spectra can be divided into two peaks at 289.4 eV and 285 eV, which are linked to carbon and residual carbon containing lithium impurities such as Li2CO3. The ratio of Li2CO3 to carbon for NCA, NCABr-2 and NCABr-4 are 0.68, 0.54 and 0.56 respectively. Compared against pure NCA, the content of residual lithium impurities in the NCABr-x (x ¼ 2, 4) samples decreases due to the fact that residual lithium acts as LiBr. Br
modification is effective in lowering the average oxidation state of Ni and it is also able to result in a fall of the content of residual lithium impurities. To better understand the mechanism of Br
doping, DFT calculations were performed to access the energy of doping Br
to the first, second and third NCA layers, respectively, and the results are shown in Fig. 6. From the DFT results, it can be concluded that Br
doping mainly occurs in the surface layer, thereby changing the structure of NCA.

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Fig. 5. (a) XPS spectra; (b) Ni 2p spectra, (c) Co 2p spectra, (d) Br 3d spectra, (e) Al 2p and (f) C1s spectra of pure NCA and NCABr-x (x ¼ 2, 4) samples.

Table 3 The binding energy of Ni2p3/2, Co2p3/2, Al2p and Br 3d5/2 for pure NCA, NCABr-2 and NCABr-4 (Unit: eV). Samples

Ni 2p3/2

Co 2p3/2

Al 2p

Br 3d5/2

NCA NCABr-2 NCABr-4

854.95 854.81 854.84

780.08 780.28 780.28

67.98 68.58 68.58

/ 68.68 68.48

A comprehensive study of structural characterization results shows that partial Br
dopes into the bulk particles of NCA, and the residual Br
forms LiBr on the surface of NCA particles after Br
modification process.

3.2. Electrochemical characterizations Galvanostatic charge-discharge experiments were deployed to characterize the rate capability of pure NCA and NCABr-x (x ¼ 1, 2, 3, 4) samples. Fig. 7a shows the initial charge-discharge curves of pure NCA and NCABr-x (x ¼ 1, 2, 3, 4) samples at a constant current density of 0.1 C (1 C ¼ 185 mA h g
1) between 2.8 and 4.3 V at 25  C. The initial charge/discharge capacity of pure NCA, NCABr-1, NCABr2, NCABr-3 and NCABr-4 is 213.3/179.4, 212.9/191.9, 215.3/195.6, 217.7/184.5 and 196.7/173.3 mA h g
1 respectively. Corresponding

coulomb efficiency is 84.1%, 90.1%, 91.9%, 84.7% and 88.1% respectively. NCABr-x (x ¼ 1, 2, 3, 4) samples clearly shows greater initial capacity and corresponding coulomb than pure NCA. Furthermore, the discharge capacity of NCABr-x (x ¼ 1, 2, 3, 4) samples first increased, followed by a decrease with increasing x. Adding more amounts of Br
show more bromide metal which lacks the ability to provide capacity will form, thereby lowering initial discharging capacity. Fig. 7b and Table 4 show the discharge capacity of all samples at different C-rates between 2.8 V and 4.3 V at 25  C. NCABr-x (x ¼ 1, 2, 3, 4) samples exhibit better rate capability than pure NCA. NCABr-2 in particular presents the best capability when compared to all other samples. Pure NCA has a discharge capacity of 185.5, 175.0, 172.0, 165.9, 156.2, 152.0, 146.6 and 142.6 mA h g
1 at rates of 0.1, 0.2, 0.5, 1, 2, 3, 4 and 5 respectively. The corresponding discharge capacities of NCABr-2 are 195.7, 191.8, 179.9, 184.3, 180.5, 174.7, 172.4 and 169.1 mA h g
1 respectively. Improvements at rates of 5.4%, 9.6%, 7.2%, 8.8%, 11.8%, 13.4%, 16.4% and 18.1% were clearly illustrated. The enhanced rate capability of NCABr-2 might be a result of enlarged Liþ diffusion channels and shorter Liþ diffusion paths which have been proven by XRD and SEM results. The chargedischarge voltage profiles of pure NCA and NCABr-2 at different C rates in the range 2.8e4.3 V are shown in Fig. 7c and d. NCABr-2 has a potentially smaller difference between discharge and charge potential plateaus when compared to pure NCA especially at high C

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Fig. 6. DFT calculation results.

Fig. 7. (a) Galvanostatic charge-discharge curves at 0.1 C rate for the 1st cycle, (b) discharge capacity as a function of C rate of pure NCA and NCABr-x (x ¼ 1, 2, 3, 4) samples (1 C ¼ 185 mA h g
1); galvanostatic charge-discharge curves at different C-rates of pure NCA (c) and NCABr-2; CV curves of pure NCA (e) and NCABr-2 (f) at a scan rate of 0.1 mV s
1.

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Table 4 Discharge capacity of both materials at different C rates (Unit: mAh g
1). Sample

0.1C

0.2C

0.5C

1C

2C

3C

4C

5C

0.1C

NCA NCABr-1 NCABr-2 NCABr-3 NCABr-4

185.5 190.7 195.6 185.6 179.0

175.0 184.5 191.8 182.5 177.3

172.0 179.5 184.3 178.9 172.7

165.9 175.1 180.5 171.4 165.4

156.2 169.4 174.7 165.0 160.4

152.0 167.7 172.4 161.7 155.0

146.6 165.9 170.7 155.8 151.2

142.6 163.7 169.1 153.6 148.3

179.9 190.1 194.2 180.8 169.4

Table 5 Potential difference between the anode and cathode peaks for pure NCA and NCABr2 (Unit: V). Cycle

1st 2nd 3rd 4th

NCA

NCABr-2

4pa

4pc

DV

4pa

4pc

DV

3.991 3.812 3.785 3.767

3.639 3.673 3.662 3.658

0.352 0.139 0.123 0.109

3.921 3.705 3.699 3.685

3.673 3.675 3.677 3.678

0.248 0.030 0.022 0.007

rates, which verifies Br
modification is conducive in bringing down the level of electrode polarization. An understanding of the electrochemical reaction of the electrode materials can be obtained from the CV curves of pure NCA and NCABr-2 within the voltage range of 2.8 Ve4.5 V at a scan rate of 0.1 mV s
1 as shown in Fig. 7e and f. The CV curves of NCABr-2 look similar to that of pure NCA, indicating Br
modification has not changed the electrochemical reaction of NCA. Moreover, oxidation peaks of both samples detected at a high voltage during the first cycle exhibits obvious change when exposed to a lower voltage during the following cycle, indicating irreversible electrochemical reactions had taken place during the initial cycle. The potential differences (DV) of oxidation peaks at different cycles for both materials are listed in Table 5. Compared to pure NCA, NCABr-2 shows a smaller value of DV during initial cycles, suggesting that irreversible capacity loss of NCABr-2 is less for it to form the solid electrolyte interphase (SEI) film and result in higher coulombic efficiency during the first cycle [51]. Furthermore, the value of DV is usually used to represent electrode polarization [51,52]. The values

Fig. 8. Cycling stability of pure NCA (a) and NCABr-2 at a rate of 0.5C; (b) the corresponding 1st, 50th, and 100th galvanostatic charge-discharge curves of pure NCA at 25  C (c) and 55  C (d); the corresponding 1st, 50th, and 100th galvanostatic charge-discharge curves of NCABr-2 at 25  C (e) and 55  C (f).

S. He et al. / Electrochimica Acta 318 (2019) 362e373 Table 6 Discharge capacity retention for pure NCA and NCABr-2. Sample Discharge capacity at 25  C (mAh g
1) 1st cycle

100th cycle

NCA 172.8 NCABr- 182.4 2

110 134.5

Retention (%)

63.7 73.7

Discharge capacity at 55  C (mAh g
1) 1st cycle

100th cycle

201.4 201.2

83.5 152.3

Retention (%)

41.5 75.7

of NCABr-2 at following cycles are a bit less than those of pure NCA, which displays how Br
modification could impede potential polarization. The cycle stability of pure NCA and NCABr-2 was looked at in greater depth through cycling tests for 100 cycles at 0.5 C at 25  C and 55  C respectively. The results are displayed in Fig. 8 and Table 6. As shown in Fig. 8a, pure NCA exhibits the discharge capacity of 172.8 mA h g
1 at the first cycle at 25  C, with a capacity retention of 63.7% after 100 cycles. In contrast, NCABr-2 delivers 128.6 mA h g
1 at first cycle and 124.5 mA h g
1 at the 100th cycle, with a capacity retention of 73.7%. Furthermore, Fig. 8b shows the initial discharge capacities of pure NCA and NCABr-2 are 201.4 and 201.2 mA h g
1 at 55  C respectively, which are higher than those obtained at 25  C because high temperature speeds up Liþ diffusion

371

[63]. After 100 consecutive cycles, NCABr-2 can still demonstrate a high capacity retention value of 75.7%, which is higher than that of pure NCA at 41.5%. These results further show Br-modification can effectively improve the cyclability of NCA cathode materials at elevated temperature. The enhanced cycling stability could be attributed to the stronger M-Br bonds, lower valence state of Ni ions and less unstable Li2CO3 on the surface of NCA. Fig. 7cef shows the charge-discharge voltage profiles of pure NCA and NCABr-2 at 25  C and 55  C respectively. One can see that NCABr-2 exhibits a potentially smaller difference between discharge and charge potential plateaus when compared to pure NCA after cycling tests, further indicating that Br
modification could reduce the electrode polarization. EIS tests which were performed on pure NCA and NCABr-2 after the 1st, 50th and 100th cycle operated at a fully charged state of 4.3 V unmasked the reason for the enhancement of the cycling performance of NCABr-2. Nyquist plots for the samples after various cycles are exhibited in Fig. 9a and b. Typical plots are fitted with a simple modified Randles-Ershler equivalent circuit displayed in Fig. 9c and the correlating fitted values of the parameters are listed in Table 7. Two semicircles appear, representing surface film resistance (Rsf) and charge transfer resistance (Rct) [63,64]. The value of Rsf þ Rct for pure NCA is nearly the same as that of NCABr-2 after the first cycle at a rate of 0.5 C. After the 100th cycle, pure NCA displays a significantly bigger value of Rsf þ Rct (145.7 U) than that of

ω

Fig. 9. EIS curves after 1st, 50th, 100th cycles for pure NCA (a) and NCABr-2 (b) at full charge states; (c) equivalent circuit model used for fitting the EIS curves; (d) Zre vs u
0.5 plots in the low-frequency region obtained from EIS measurements.

Table 7 Resistance values obtained from equivalent circuit fitting of experimental data for NCA and NCABr-2. Sample

NCA NCABr-2

Rsf (U cm2)

Rct (U cm2)

s(U cm2 s
0.5)

Rsf þ Rct (U cm2)

1st cycle

100th cycle

1st cycle

100th cycle

1st cycle

100th cycle

1st cycle

100th cycle

28.57 47.72

26.37 54.18

62.4 36.99

119.33 42.4

90.97 84.71

145.7 96.58

2.79 3.06

5.57 3.20

372

S. He et al. / Electrochimica Acta 318 (2019) 362e373

NCABr-2 at 96.58 U. The Liþ diffusion coefficient (D) can be worked out through the low frequency plots using the equation below:

D¼

R2 T 2 2A2 n4 F 4 C 2 s2

The surface area of the electrode is A. The number of the electrons per molecule attending the electronic transfer reaction is n. The Faraday constant is F. C represents the concentration of lithium ion in the electrode. R stands for the gas constant. T is the working temperature, which is room temperature in our experiment. s is the gradient of the line Z’ ~ u
0.5, which can be obtained from the line of Z’ ~ u
0.5 (shown in Fig. 9d). Based on the above equation, the smaller value of s is an indication of a larger Liþ diffusion coefficient. The value of s for pure NCA is similar to that of NCABr-2 after the first cycle. However, after the 100th cycle, the value of s for pure NCA increased to 5.57 U cm2 s
0.5, which is quite significantly larger than that for NCABr-2 (3.20 U cm2 s
0.5). This implies NCABr-2 has larger Liþ diffusion coefficient than pure NCA. The smaller values of Rsf þ Rct and larger Liþ diffusion coefficient testify that an appropriate amount of Br modification can alter the movability of cathode materials. 4. Conclusion After the Br
modification process, partial Br
was successfully substitute for O2
in 6c site, and the residual Br
will form tiny island like particles of bromide metal on the surface of NCA particles. As a result, Br
modified NCA presented larger interslab spacing distance and smaller primary particles than pure NCA, providing wider channels and shorter paths for Liþ. Therefore, the Br
modified NCA exhibited better rate capability than pure NCA, especially NCABr-2. The reduced potential polarization, decreasing value of Rsf þ Rct and increasing Liþ diffusion coefficient for NCABr-2 were proved by CV and EIS tests. Moreover, NCABr-2 demonstrated enhanced cycling stability at room temperature, even at high temperature (55  C) due to the stronger M-Br bonds, lower valence state of Ni ions and less unstable Li2CO3 on the surface of NCA. Acknowledgements This work was financially supported by Hebei Province Applied Basic Research ProgrameKey Basic Research Project (17964407D), Science and Technology Program of Hebei Province (18214404D), Project of Hebei Academy of Science (181604) and Project of Hebei Academy of Science (181605). The characterization results were supported by Beijing Zhongkebaice Technology Service Co., Ltd. We acknowledge critical and quantity of testing work supported by Beijing Zhongkebaice Technology Service Co.,Ltd. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.06.061. References [1] H. Ni, J. Liu, L.-Z. Fan, Carbon-coated LiFePO4-porous carbon composites as cathode materials for lithium ion batteries, Nanoscale 5 (2013) 2164e2168. [2] Y.S. Jung, A.S. Cavanagh, A.C. Dillon, M.D. Groner, S.M. George, S.-H. Lee, Enhanced stability of LiCoO2 cathodes in lithium-ion batteries using surface modification by atomic layer deposition, J. Electrochem. Soc. 157 (2010) A75eA81. [3] W. Lv, Z. Wang, H. Cao, S. Yong, Z. Yi, S. Zhi, A critical review and analysis on the recycling of spent lithium-ion batteries, ACS Sustain. Chem. Eng. 6 (2018) 1504e1521. [4] D. Choi, J. Kang, B. Han, Unexpectedly high energy density of a Li-ion battery by oxygen redox in LiNiO2 cathode: first-principles study, Electrochim. Acta

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