Enhanced Rate Performance of Al-Doped Li-Rich Layered Cathode

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Enhanced rate performance of Al-doped Li-rich layered cathode material via nucleation and post-solvothermal method Wenchao Yan, Yu Xie, Jicheng Jiang, Deye Sun, Xiaodi Ma, Zhenggang Lan, and Yongcheng Jin ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03634 • Publication Date (Web): 18 Feb 2018 Downloaded from http://pubs.acs.org on February 20, 2018

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ACS Sustainable Chemistry & Engineering

Enhanced rate performance of Al-doped Li-rich layered cathode material via nucleation and post-solvothermal method Wenchao Yana, Yu Xiea, Jicheng Jianga,b, Deye Suna, Xiaodi Maa, Zhenggang Lan a*

,Yongcheng Jina*

a

Qingdao Industrial Energy Storage Technology Institute, Qingdao Institute of

Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, P. R. China b

University of Chinese Academy of Sciences, 19A Yuquanlu Road, Beijing 100049, P.

R. China

Corresponding Author *E-mail: [email protected]; [email protected] Mailing address: No.189 Songling Road, Laoshan District , Qingdao 266101, China. Fax: +86-532-80662703

ACS Paragon Plus Environment

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Abstract Al-doped layered cathode materials Li1.5-xAlxMn0.675Ni0.1675Co0.1675O2 have been successfully synthesized via rapid nucleation and post-solvothermal method. The surface morphology and crystal structures of Al-doped Li-rich materials are investigated via scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectra and X-ray photoelectron spectroscopy (XPS). After optimization, the Li1.45Al0.05Mn0.675Ni0.1675Co0.1675O2

(Al=0.05)

sample

showed

excellent

electrochemical performance, the discharge capacities are 323.7 and 120 mAh g-1 at rate 0.1 and 20 C respectively., These improvements, based on electrochemical performance evaluation and density functional theory (DFT) calculations, might be ascribed to the increased electron conductivity of layered Li-rich material via Al3+ ions doped into crystal structure.

KEYWORDS: Lithium ion batteries, Li-rich material, Al doping, High rate performance, Density functional theory


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Introduction Lithium ion batteries (LIBs) have been extensively investigated and widely used as rechargeable power sources for application in energy storage fields and electric vehicles.1-3 The continued rapid growth of electrical vehicles requires rechargeable power sources with higher energy and power density, however, current secondary lithium ion batteries cannot meet the requirement. It is reported that positive electrodes restricted the energy stored in LIBs.4 At present, the common positive materials mainly include layered LiCoO2 (cal. 140 mAh g-1) 5, olivine LiFePO4 (cal. 170 mAh g-1) 6, and spinel LiNi0.5Mn1.5O4 (cal. 146 mAh g-1) 7, but these materials still suffer from low specific capacity hindering their utilization as positive material in future

electric

vehicles.

Li-rich

layered

cathode

materials,

written

as

xLiMO2·(1-x)Li2MnO3, have been extensively studied owing to their high specific capacity (ca. 250 mA h g-1) with higher operational window of 4.6-4.8 V.

8-9

At the

same time, high contents of Mn and Ni in these material provide advantage of low cost.

10

The Li-rich layered material with eminent properties has been deemed to be

one of the most promising next generation cathode material as high energy density for LIBs. 11-12 In spite of these merits, however, there is no doubt that Li-rich layered material also suffer from several drawbacks, such as an initial irreversible capacity loss

13-14

,

poor cycling stability 15 and rate capability 16, which could impede their application in future electric vehicles. The reason for poor rate capability is partially due to the electronically insulated Li2MnO3 phase denting the electron conductivity of Li-rich material.

17-18

For improving the rate capability of these material, several different

methods were used, such as minimizing particle size


19-20

, carbon coating

21

and


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element doping 22. Among them, the substitution of metal elements such as V, Sn, Cr, Al, Fe, Mg, Mo

17, 23-29

for transition elements in xLiMO2·(1-x)Li2MnO3 material has

been proved to be effective in improving rate performance. Based on previous research, it has been shown that ionic and electronic conductivities of Li-rich material can be improved after cationic doping.24 Another effective improvement strategy is to enlarge the Li slab distance in the layered

materials,

thus

improving

interaction/deinteraction process.

30

the

Li+

diffusion

coefficient

during

Inspired by this idea, researchers introduced that

large ionic radius of Na+ (0.102 nm) or K+ (0.138 nm) into the Li layer to enlarge the diffusion path and enhance the rate capability of Li-rich materials.

31-32

At the same

time, Li-rich materials with Li deficiency and layered/spinel heterostructure could significantly improve the rate capability, in which the spinel structure can offer 3D Li+ channels and then improve the Li+ diffusion coefficient.33-35 It is reported that severe Li deficiency in Li-rich materials may migrate the Mn4+ in Li-rich materials and layered-to-spinel transition can be restrained during charge/discharge processes. Based on this method, Xin et al reported that in the Li layer Ti4+ substituting Li+ restricts the phase transition, the reason may be due to the fact that doping Ti4+ enhanced the activation energy of Mn migration, which hindering the Mn migration during cycle.36 In

this

paper,

we

prepared

Al-doped

Li-rich

materials

Li1.5-xAlxMn0.675Ni0.1675Co0.1675O2 via rapid nucleation and post-solvothermal method. After introducing of Al ions, the local environment of elements in the Li-rich material have been changed and electrochemical performance also been enhanced. To study the effect of Al3+ doping on Li+ site in Li-rich materials, the density functional theory (DFT) calculations are also used.


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Experimental Material preparation Li-rich layered material Li1.5-xAlxMn0.675Ni0.1675Co0.1675O2 (x=0, 0.025, 0.05, 0.1, 0.2, denoted as Al=0, Al=0.025, Al=0.05, Al=0.1, Al=0.2 respectively) were synthesized by rapid nucleation and post-solvothermal method. Stoichiometric amount of NiSO4·6H2O, MnSO4·H2O, CoSO4·7H2O and Al(NO3)3·9H2O were dissolved in mixture solution (solution A). And then (NH4)2CO3 solution was poured into the solution A rapidly. The suspension solution was transferred into a poly(tetrafluoroethylene) (Teflon) container and treated at 180 °C for 12 h to form precursors with spherical morphology. Then, these precursors were washed and dried in air at 60 °C for 12 h.

37

To prepare the final cathode material, a desired amount of

Li2CO3 was thoroughly mixed with the precursor. After being ground, the resulting mixture was calcined at 850 °C for 12 h in air. Characterizations A X-ray diffractometer (Bruker AXS D8) equipped Cu Kα radiation has been employed to obtain the X-ray diffraction (XRD) patterns. The morphology of as-prepared materials was analyzed by field emission scanning electron microscope (Hitachi S-4800). Thermo DXR FT-Raman spectrometer with a Nd-line laser source (λ= 532 nm) to collect Raman scattering of samples. X-ray photoelectron spectroscopy (XPS) spectra were investigated by the equipment PHI-5400 (PE, USA).



Electrochemical measurements Electrochemical performance of as-prepared samples was evaluated by two electrodes cells in the 2032 coin-cell-type. The composite cathode materials were prepared as follows: the synthesized samples, polyvinylidene fluoride (PVDF) binder and conductive carbon (super-p) mixed in a wt.% ratio of 80:10:10 on Al foil, as described in previous literature.

37

The cells were assembled in Ar filled glove box,

using 1 mol dm-3 LiPF6 in EC:DMC (1:1 in volume) as electrolyte, lithium foil as the anode and celgard 2500 as separator. LANHE battery testers were used to measure the electrochemical performance in the voltage range between 2.0 and 4.8 V at 25 °C. Computational Details The electronic structure calculations were performed with the spin-polarized density functional theory (DFT) method using the Vienna Ab-initio Simulation Package (VASP).

38

Pseudopotentials used in the calculations were established by the

projector-augmented wave (PAW) method and the Perdew-Burke-Ernzerh (PBE).39-40 Considering the strong-correlative d electrons of transition metal Mn, the effective Hubbard-U parameter (Ueff = U-J = 4.9 eV) was employed. 41 The plane-wave basis was cutoff by an energy of 500 eV. A 2 × 2 × 1 supercell (16 formula units) of Li2MnO3 with the space-group symmetry C2/m was taken to build the configurations for the 6.25% doped Li2MnO3. The configurations were relaxed with a 2 × 2 × 2 centered k-mesh A denser k-mesh (4 × 4 × 4) was used to obtain the density of states (DOS) of these relaxed configurations. Results and Discussion


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Fig. 1 exhibits the XRD patterns of samples. Except the weak superlattice reflection peaks arising between 20 and 25° (Fig. 1(a)), most peaks of samples can be assigned to the diffraction of NaFeO2 structure (R-3m).

13

These weak superlattice

peaks were indexed to the monoclinic Li2MnO3 component corresponding to C2/m space group.42-43 The more Al3+ ions doped into bulk lattice of layered structure, the more Li vacancies were formed because the electrical neutralization, at the same time, the lattice of material was expanded owing to the electrostatic repulsion, resulting in the (003) diffraction shifts from 18.695° in Al=0 to 18.676° in Al=0.2 sample as shown in Fig. 1(b). However, minor impurity peaks regarding spinel phase can be observed in the patterns of Al=0.1 and Al=0.2 samples (Fig. 1(b)), this reason for that mainly is the doping of Al3+ ions in the Li sites generating Li vacancies.36 Fig 2 shows the Rama spectrum of as-prepared samples. As can be seen, the Raman bands of pristine sample at 372 and 422 cm-1 belonging to the monoclinic Li2MnO3 component, other obvious peaks at 477 and 588 cm-1 associated with the bending Eg and stretching A1g modes, respectively. 44-46 With the contents of Al increased from 0 to 0.2, Raman peaks of 372 and 422 cm-1 have disappeared and 472 and 588 cm-1 were blueshifted toward a high wave-number, this can be associated with an irreversible manganese migration into the interlayer lithium sites that results in the formation of a spinel-type cation ordering.47-48 Meanwhile, we can also found that their intensities decreased substantially, the reason may be that after Al doping the electronic conductivity of the material increase.

45

In-depth analysis of the relationship between

doped ions and electrical conductivity of as-prepared materials, a potentiostatic



polarization method was used, the current dependence of polarization time was performed in Fig S1, the result exhibited that the Al=0.2 sample has highest electronic conductivity. In addition, the spectrum of Al-doped samples demonstrated a significant increase in the peak width of the Raman band at 599-611 cm-1, this could be an indication of structural and lattice strain effects. Herein, we can draw a conclusion that Al3+ ion doped into the bulk lattice of layered Li-rich material and changed the local coordination structure of Li2MnO3 component. Fig. 3 exhibits the SEM images of Al=0 and Al=0.05 samples. All sample exhibits a typical spherical morphology, and the average sizes of spheres is about 3-6 µm, as shown in Fig. 3(a) and (c). In Fig. 3(b) and (d), we can find that some mesopores existed on the surface of the Al=0 sample, however, primary sizes of the Al=0.05 sample were tightly bonded together. With the contents of Al increase, the spherical morphology of samples were well preserved and primary sizes were connected more closely as shown in Fig. S2(a)-(d). Moreover, obvious change in primary size and morphology occurs when doping ratio reach 0.2, Al=0.2 sample exhibiting broken fragments and larger particle size with obvious stratification, as shown in Fig. S2(e) and (f). These results may indirectly testify that the primary size and surface morphology can be affected by addition of Al. Furthermore, the reason for primary size of Al=0.2 with obvious stratification is unclear at present, it may be caused by excessive Al doping may expand the Li slab. The SEM image and corresponding EDXS maps of the Mn, Ni, Co, Al and O elements in the Al=0.05 sample as shown in Fig. 4. As can be seen, Al element was


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distributed uniformly in the Al=0.05 material. The molar ratios in these areas are listed in Fig. S3, the atomic molar ratio of Mn, Ni, Co and Al was 0.675:0.166:0.163:0.057, approaching to the theoretical value of Al=0.05 sample. To determine the oxidation states of each element and structures of as-prepared samples, the XPS spectrums were exhibited in Fig 5. In the O 1s spectra, the peak located at 529.4 eV correspond to the lattice oxygen in the Li-rich material 23 and the peak around 531.4 eV can be assigned to the impurity Li-oxides existed on the surface of the compounds.49 As can be seen from Fig. 5(a), with the increase of Al doping, the gradually reduced peak intensity at 531.4 ev, which is assigned to the impurity oxides such as Li2O or Li2CO3, indicating that doped materials have more stability in air than pristine material. Concerning the Al 2p peak located at 73.13 eV is correspond to that of LiAlO2 (73.4 eV) in the doped samples.50 As our previous work shown, the oxidation states of Ni, Co, Mn and O are +2, +3, +4 and -2 in the Al=0 sample.

37

What we can see in Fig. 5 and Fig. S4 is that the peak positions of Mn, Co, Ni and O all shift to higher energy positions with increasing the doping content of Al, which demonstrate that the chemical environment of these atoms are changed. At the same time, the additional peaks around at 640.38 eV are observed in spectrum of Al=0.1 and Al=0.2, indicating the formation of Mn3+. Amount of Al3+ ion doped into the Li sites, some of transition metal may reduce the valence to maintain the electrical neutralization of as-prepared materials, thus minor Mn3+ may exist in the spectrum. Herein, we can draw a conclusion that Al3+ can dope into the crystal structure and change the environment of transition metal and oxygen atoms in the doped materials.



To evaluate the influence of Al-doping on electrochemical performance of as-prepared samples, the first charge-discharge profiles of samples were conducted using a rate at 0.1 C, and the result were shown in Fig. 6(a). As normal Li-rich layered oxides, all electrodes show a sloping region and a long plateau, the former is attributed to Li+ de-intercalation from LiMn1/3Ni1/3Co1/3O2 component and the later is corresponding to Li+ and O2- extracted from the Li2MnO3 phase and structural rearrangement in the initial charge process. First charge capacities of 117.4 mAh g-1, when voltage blew 4.5 V, is observed in the Al=0 sample, this value is nearly the same with the excepted values based on the Ni2+/4+ and Co3+/4+ redox reaction (approximately 120 mAh g-1 ).51 With the increase of doped Al3+ content, charge profiles of as-prepared electrodes deliver higher capacities except the Al=0.2 electrode, this result indicate that the addition of Al have influence on the crystal structure in the LiMn1/3Ni1/3Co1/3O2 component, the reason for the higher charge capacities (

Enhanced Rate Performance of Al-Doped Li-Rich Layered Cathode

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