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Electrochemical performance improvement of Li1.2[Mn0.54Ni0.13Co0.13]O2 cathode material by sulfur incorporation
Accepted Manuscript Title: Electrochemical performance improvement of Li1.2 [Mn0.54 Ni0.13 Co0.13 ]O2 cathode material by sulfur incorporation Author: Ban Liqing Yin Yanping Zhuang Weidong Lu Huaquan Wang Zhong Lu Shigang PII: DOI: Reference:
S0013-4686(15)30281-4 http://dx.doi.org/doi:10.1016/j.electacta.2015.08.031 EA 25499
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
3-4-2015 6-8-2015 7-8-2015
Please cite this article as: B. Liqing, Y. Yanping, Z. Weidong, L. Huaquan, W. Zhong, L. Shigang, Electrochemical performance improvement of Li1.2 [Mn0.54 Ni0.13 Co0.13 ]O2 cathode material by sulfur incorporation, Electrochimica Acta (2015), http://dx.doi.org/10.1016/j.electacta.2015.08.031 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Electrochemical performance improvement of Li1.2[Mn0.54Ni0.13Co0.13]O2
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cathode material by sulfur incorporation Liqing BAN
Yanping YIN
Weidong ZHUANG*
Huaquan LU
Zhong WANG
Shigang LU
ip t
( R& D Center for Vehicle Battery and Energy Storage, General Research Institute for Nonferrous Metals, Beijing 100088, China )
The
enhanced
electrochemical
performance
of
the
lithium-rich
solid
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Abstract:
solution
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Li1.2[Mn0.54Ni0.13Co0.13]O2 (LMNCO) cathode is enhanced by sulfur incorporation. Various sulfur contents are
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introduced by adding (NH4)2SO4 into the raw material. The effects of different sulfur contents on the structure, morphology and electrochemical performance are investigated. The original sample (as-received sample) and
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the sulfur incorporated samples were characterized by X-ray Diffraction (XRD), High Resolution Transmission Electron Microscopy (HRTEM), Electrochemical Impedance Spectroscopy (EIS), X-ray Photoelectron
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Spectroscopy (XPS) and Fourier Transform Infrared Spectroscopy (FTIR). Electrochemical performance such as charge/discharge capacity and rate capability was assessed with lithium ion cells. XRD patterns show that a new phase of Li2SO4 was formed and distributed on the surface of the particle. The electrochemical performance of the sulfur incorporated LMNCO samples is significantly improved due to the formation of the new phase on the surface of the particles. In comparison with the original material, the modified materials show an improved rate performance attributed to the interface between LMNCO and the second phase, which may provide fast diffusion channels for lithium ion.
Key words: Lithium-rich layered oxide; Sulfur incorporation; Rate performance; Lithium-ion batteries.
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1. Introduction The lithium-rich solid solutions of layered Li2MnO3 and LiMO2 (M=Ni, Co, Mn) have drawn much
Corresponding author: Tel: +8613911718416; E-mail addresses: [email protected]
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attention in recent years, and they are among the most potential cathode materials for next generation lithium
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ion batteries, because they can provide much higher capacity (>250 mAh g-1) with significantly reduced cost
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compared to the LiCoO2 materials[1, 2]. However, some major problems, such as poor rate capability, high initial
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cycle irreversibility, and significant decrease in the discharge voltage plateau with successive cycling, still
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remain unresolved[3]. In addition, the ionic conductivity of these materials is quite low, which limits their
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electrochemical performance at higher charge/discharge rate, and the poor rate capability cannot satisfy the
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requirement of batteries for application in electric or hybrid electric vehicles, especially when power
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characteristics is concerned[4].
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Recently, several treatment methods, such as surface modification and ion doping, have been developed to
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enhance the electrochemical properties of the lithium rich cathode materials. Surface modification can prevent
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the direct contact of electrode from electrolyte by coating with stabilizing materials[5]. The coating materials
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including AlF3[5], Al2O3[6, 7], LiNiPO4[8, 9], are likely to diffuse easily into the cathode surface or deposit on the
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surface. Ion doping method is to substitute oxygen or transition elements (Ni/Co/Mn) with other elements[9, 10],
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The substitution of anion for oxygen is effective in reducing impedance and lattice changes during cycling and
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improving cycle life[11] and the substitution of the transition elements is effective in stabilizing materials’ micro
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structures[10, 11]. A novel strategy by blending the lithium-rich layered oxide with other materials can eliminate
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the irreversible capacity[12, 13]. A lot of literatures on surface modification of the Li-rich and Mn-based cathode
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materials such as fluorides and oxides have been published. However, this family of cathode materials after
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modification still has drawbacks such as poor rate capability. Therefore, high ionic or/and electronic conductor
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modified cathode materials have become increasingly attractive methods to improve the electrochemical
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performance of lithium ion batteries[14]. For instance, Li3PO4 polyanion with high Li+ diffusion coefficient have
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been used to modify and improve the rate performance of various battery materials[15, 16]. In fact, polyanion-type
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compounds are a class of materials in which tetrahedral polyanion structure units (XO4)n- and their derivatives
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(XmO4m+1)n- ( X=B, P, Si, S, As and or Mo) express high stability oxygen ions[17-19]. Nevertheless, to our
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knowledge, research and report involving sulfur polyanions incorporation are rarely demonstrated. Therefore, in
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this work, we intend to develop a facile synthesis of the manganese-based Li-rich oxide cathode material which
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is incorporated with sulfur polyanions to improve electrochemical performance. In this work, nano-sized Li1.2[Mn0.54Ni0.13Co0.13]O2 samples with and without sulfur incorporation were
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synthesized (marked as LMNCOS and LMNCO). The structures of LMNCOS and LMNCO were characterized
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by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), X-ray photoelectron
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spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The enhanced electrochemical
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performance was illustrated by galvanostatic charge/discharge test and electrochemical impedance spectroscopy
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(EIS).
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2. Experimental
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2.1 Material preparation and characterization
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Layered LMNCO and LMNCOS materials were synthesized by a simple solid-state method.
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Stoichiometric amounts of lithium carbonate (99.9%, AR), manganese carbonate (99.0%, AR), tricobalt
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tetroxide (98.0%, AR) and nickel oxide (100%, AR) (molar ratio of Ni: Co: Mn=0.13: 0.13: 0.54), were
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diffused in 350ml distilled water with continuous stirring for 24h, and then grinded for 4h. Various amount of
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(NH4)2SO4 were added into the ultrafinely grinded homogenate. The resulting solution was pumped into a spay
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drying instrument(L-117, LaiHeng) to produce homogenous precursor. Then the spray dried precursor was
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initially decomposed at 650◦C for 10h in air, after being ground, the resulting powers were put into a corundum
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crucible and calcined at 900◦C for 36h in air. The original and modified samples were marked as LMNCOS-00,
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LMNCOS-04, LMNCOS-05, LMNCOS-06, and LMNCOS-08 (0.0, 0.2, 0.4, 0.5, 0.6 and 0.8 wt% of pristine
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material LMNCO), respectively. The whole process is shown in Fig. 1.
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Scanning electron microscopic(SEM) investigation of the original and cycled electrodes was performed on
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a S-4800 microscope (HITACHI Japan). X’ Pert PRO XRD patterns were recorded with a Phillips X-ray
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diffractometer with Cu Kα radiation (λ=0.1541 nm) with an accelerating voltage of 40 kV, the 2θ value range is
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10° to 90° with an increments of 0.0017° at room temperature. TEM data were collected with a JEOL-TEM
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equipment to assess the microstructures of the modified samples. XPS data were collected at room temperature
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with Kratos Analytical Spectrometer and monochromatic Al Kα (1486.6 eV) X-ray source to assess the
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chemical state of the surface elements. Multiplex spectra of various photoemission lines were collected at
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medium resolution using an analyzer pass energy of 40eV at 0.1 eV step and an integration interval of 1 s·eV-1.
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Inductively coupled plasma (ICP) technique was used to determine the dissolved amount of elements. Fourier
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transform infrared spectroscopy (FTIR) spectra of the samples were performed on a JASCO 400 FTIR
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spectroscopy. Samples were mixed with KBr and pressed to form pellets for measurement. The materials were
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pressed into pellets of 12 mm diameter and about 0.55 mm thickness, and their conductivity was measured by
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Van der Pauw four-point direct current (dc) method, using a four point probing system (HMS-3000/0.55T)
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2.2 Electrochemical measurements
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The cathode slurry was prepared by dissolving 80wt% active material, 10wt% super-P and 10wt%
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polyvinylidene fluoride (PVDF) binder in N-methylpyrrolidone(NMP), and then the slurry was rolled on a thin
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sheet of aluminum foil, after that, the sheet was cut into circular electrodes with a diameter of 14mm. The mass
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of the active material is 9.6 mg, and the mass loading of the active material is 6.5 mg cm-2 in the electrode. Coin
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cells were assembled with the as-prepared cathodes, lithium foil anode (Φ=15.8 mm×0.5 mm, China Energy
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Lithium Co. Ltd.), 1 mol/L LiPF6 dissolved in ethylene carbonate/diethyl carbonate (EC/DEC, Beijing, AR)
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electrolyte, and Celgard 2500(America) polypropylene separator.
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EIS measurements were conducted with a Solartron 1260A impedance analyzer in the frequency range of
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100 kHz to 50 mHz with an AC voltage amplitude of 10mV. Potentiostatic Intermittent Titration Technique 4
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(PITT) tests were also conducted on this apparatus at room temperature in the voltage range of 2.0-4.8 V. For
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the PITT measurements, the battery cells were charged to a fixed voltage (4.55 V, 4.65 V, 4.75 V) for 200 s.
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The charge-discharge experiments of the Li/LMNCOS cells were performed galvanostatically at 20 mA g-1 with
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a voltage range of 2.0-4.8 V with a battery tester (Land CT2001A, Wuhan) at room temperature. The rate
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capability of the cells was tested by charging and discharging from 20 mA g-1 to 600 mA g-1. Only the active
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mass of the electrodes was considered during the calculation of capacity. All the potentials mentioned
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throughout this work are in reference to a Li/Li+ couple.
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3. Results and discussion
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3.1. Physical properties of LMNCOS cathode materials
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The morphology and particle distribution of the cathode materials are of great importance to battery
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performance. Fig.2 shows SEM images of the synthesized LMNCOS powders with different sulfur additive
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percents. The LMNCOS particles are agglomerated and have polygonal and irregular shapes, due to surface
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reaction during the calcination process at high temperature or the high surface free energy of the nanoparticles.
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The surfaces of LMNCO with different sulfur additive percent are observed with no clear impurity phase and no
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evident diversity in structure. The observed particle size from SEM was in the range of 140-200 nm.
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XPS is an effective elemental analysis method to provide the information of the oxidation states of the
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elements on the surface. XPS analysis of the LMNCOS-05 sample was carried out and the S2P XPS spectrum is
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given in Fig. 3. The bonding interaction is further verfied by XPS (Fig. 3). There is a strong O1s peak at 529.25
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eV, corresponding to the lattice oxygen of the Li-rich layered oxides. The observed binding energies of the S2P3/2
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and S2P1/2 are 168.62 and 169.80 eV, respectively, which are very close to the values of sulfate (168.5 eV)
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reported in Ref and attributed to the presence of sulfate groups in transition metal sulfates. This result shows that
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the sulfur on the material surface is hexavalent. Correspondingly, the peaks of Ni2p3/2, Co2p3/2 and Mn2p3/2 are
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measurably, whose binding energy are located at 854.80, 779.80 and 641.90 eV, respectively. These results are
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consistent with the reported literature[20] To support the relevant conclusion, FTIR spectra are shown to prove the existence of sulfate in the
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LMNCOS materials (Fig. 4). Characteristic peaks at 1110 and 615 cm-1 are observed in all the samples, which
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are due to the asymmetric stretching modes of S-O bonds in SO42- bending modes. Differently, there is only one
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band that can be shown in the LMNCO material before Sulfur incorporation. With the increasing of sulfur
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content, the characteristic peak intensity becomes stronger.
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Fig.5 shows the XRD patterns of the synthesized LMNCOS powders. The XRD peaks indicate the
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formation of a α-NaFeO2-type layered hexagonal structure with space group of R 3 m, which suggests that the
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crystal structure of LMNCO is not affected by the incoming of (NH4)2SO4. The splitting of the (006)/(012) and
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(008)/(110) peaks of the XRD patterns of LMNCO indicates the formation of a well-layered structure [21].
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However, several weak impurity peaks appear in the small angle of 2θ = 22-30°, as shown in Fig. 5 and 6. With
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the increment of sulfur content, impurity peak intensity becomes stronger (Fig. 5). According to the standard
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PDF CARDS (PDF#01-0443), these weak peaks are assigned to the phase of Li2SO4, and it is then proven by
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the determination of sulfur state in the XPS test.
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Fig. 7 shows the mapping images of each of the transition metal elements (Ni, Co, Mn), oxygen and sulfur.
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These elements uniformly distribute across the detected sections. The surface of LMNCOS particles is covered
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with uniformly distributed sulfur as shown in the Fig. 7(f).
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The high resolution transmission electron microscopy (HRTEM) of the LMNCOS-05 sample was shown in
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Fig. 8(a) indicate that the material is highly crystallized in nature, and the distance between two lattice fringes
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of the bulk materials is 0.47 nm by calculation, which is assigned to the (003) plane of LiMO2 structure. From
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HRTEM study, lattice pattern and selected area electron diffraction patterns (SAED) were shown in Fig. 8(b).
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Lattice pattern showed that the material is highly crystalline in nature and indexed with LiMO2 structure. The 6
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lattice pattern was indexed to [1 -1 1] electron beam direction of LiMO2 for the materials prepared by both
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methods. The Li2MnO3 and LiMO2 phases were distinguished from SAED pattern of the layered composite and
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well indexed with different structures[22]. From Fig. 8(c), it can be clearly observed that several tiny particles are
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distributed uniformly well on the surface of the bulk particle. . From the magnifying image (Fig. 8(d)) of a tiny
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particle[23, 24]. The crystallographic distance and direction between the tiny particles and the bulk particle can be
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clearly distinguished. The inner side of the interface shows clear and typical (003) fringe, characteristic of the
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layered structure of a lithium-rich material. Outside of the interface is a crystalline layer approximately 5 nm
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thick. As further characterized in Fig. 8(d), the outer nanostructure clearly presents fine lattice fringes. The
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distance of the lattice fringes are 0.316 nm and 0.349 nm, which are indexed to the (-112) and (111) planes of
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monoclinic structure of Li2SO4, respectively. Consequently, it can be concluded that a new phase of Li2SO4 (the
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second phase) formed on the surface of the bulk particle (bulk material)[13].
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The composition of the synthesized LMNCOS material is estimated based on Energy Dispersive
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Spectroscopy (HRTEM-EDS) data using the EDAX Genesis Analyzer System as shown in Fig. 8(e) and (f)[25].
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EDS was carried out at different areas of the samples in order to determine the presence and percentage of Mn,
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Ni, Co, S, O elements. Atom percent (%) ratio of Mn, Ni, Co at two areas is listed in Table1. The values of the
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atoms indicate the new particles of the first area contain S and small amount of Mn, and S is not detected in the
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second area.
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Considering that Li2SO4 can dissolve in water easily, the LMNCOS-05 material was washed with distilled
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water, and then ICP technique is performed to determine the dissolved ions of the material surface. EDS study
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was carried out at various areas of the samples in order to determine the presence of Mn, Ni, Co，S elements and
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also to determine their mole percentage. The concentration of S and Li elements in the filtrate are 114 and
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61.7μg g-1 respectively, the molar ratio of which is close to 1:2, indicating the dissolved substance on the
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surface of the particle is Li2SO4. The ICP results show that the Ni, Co and Mn elements are barely detected in 7
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the filtrate.
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3.2 Electrochemical performance of LMNCOS cathode material The first charge-discharge profiles of the materials between 2.0-4.8 V at 20 mA g-1 rate are shown in the
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Fig.9 and Table 3. During the initial charge to 4.8 V, the electrochemical reaction occurs in two dominant stages:
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a smoothly sloping voltage profile below 4.5 V vs. Li/Li+ , which is attributed to the removal of lithium from
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the LiMO2 (M=Ni, Co, Mn) accompanied with the oxidation of Ni2+ to Ni4+ and partial oxidation of Co3+ to
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Co4+, and the voltage plateau profile around 4.5 V vs. Li/Li+[26, 27], which could be ascribed to the activation of
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Li2MnO3 component. This activation process has been considered to be associated with the irreversible
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extraction of Li2O from the transition metal oxide composite materials[22, 28]. During the plateau, activation of
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oxygen and formation of Li2O occur. Electrolyte degradation may also occur, generating gases such as CO and
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CO2. Toward the end of the plateau, the unstable and catalyst-like LMNCO material causes the release of O2 gas,
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which readily reacts with the electrolyte to form CO, CO2, and protons (H+) via a catalytic reaction. Li2O will
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react with the proton (H+) to form Li+ and H2O which solvates and migrates to the anode side and forms
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LiOH·H2O at the end of charging[29]. There is no plateau region in the subsequent charging cycles, indicating
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that the oxygen loss process during the initial charge is irreversible. It is also seen from Fig. 9 that the initial
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charge capacity is larger than the discharge capacity.
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The electrochemical performance of the initial charge/discharge profiles for the LMNCOS electrode is
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presented in Fig. 9 and Table 3. The initial discharge capacities are 278.7 and 280.1 mAh g-1 for LMNCOS-04
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and LMNCOS-05, respectively，higher than 260.5 mAh g-1 for pristine material LMNCOS-00. While the sulfur
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content increased to 0.8%, the initial discharge capacity decreases to 258.2 mAh g-1. When the content of
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Li2SO4 is very low, the interface between LMNCO and trace amount Li2SO4 can increase the diffusion rate and
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conductivity of Li+, so the discharge capacity of LMNCO S-04 and LMNCO S-05 is improved. Nevertheless,
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With the amount of formed Li2SO4 increasing, more lithium ions in bulk material (LMNCO) is deprived, which 8
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influence the component of the Li1.2[Mn0.54Ni0.13Co0.13]O2 material (LMNCO) and the charge-discharge capacity
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of LMNCOS-08 materials. Compares the rate capabilities of the pristine LMNCO and the LMNCOS materials ranged from 20 mA g-1
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to 600 mA g-1 rate in the voltage range of 2.0-4.8 V. After three formation cycles at 20 mA g-1, the cells were
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charged at 40 mA g-1, then discharged at 40 mA g-1, 100 mA g-1, 200 mA g-1 and 600 mA g-1. When the current
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rate is at 40 mA g-1, 100 mA g-1, 200 mA g-1 and 600 mA g-1, the discharge capacities of the pristine LMNCO
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are 237.0, 211.6, 183.7 and 142.7 mAh g-1, respectively (Fig. 10). In contrast, LMNCOS-05 delivers the highest
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discharge capacities of 245.2, 234.4, 223.4, 176.7 mAh g-1. It is obvious that the rate capability of LMNCO can
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be remarkably improved by the adding of sulfur. The outstanding rate capability of the LMNCOS electrode
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mainly results from Li2SO4 surface modification layer, which has more Li+ and high electron conductivity.
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However, the improvement is not linearly proportional to the amount of sulfur additive and the LMNCOS-05
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material has the optimized performance in this experiment. In addition, when the current rate returns back to 20
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mA g-1, the discharge capacity of the LMNCOS-05 material has the best capacity retention during the cycling.
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3.3 Electrical conductivity and EIS
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In order to illustrate the mechanism of the performance improvement of the LMNCOS samples, the
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migration of electron and lithium ion of LMNCOS samples are traced by EIS and electrical conductivity
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measurements.
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EIS is an effective technique for analyzing the differences in the polarization behaviors and understanding the
19
differences in rate capability. According to our previous EIS studies on this type of layered oxide cathodes[30], as
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shown in Fig. 11: EIS of all samples were tested prior to electrochemical oxidation and all of them show a
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semicircle in the high frequency region and a slope in the low frequency region. The semicircle in the high
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frequency region is assigned to charge transfer reaction, and the slope in the low frequency region is attributed
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to the lithium ion diffusion in the bulk material. The charge-transfer impedances calculated from the Nyquist 9
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plots were 149.8, 25.20, 90.95, 46.78, 325.8 Ω for LMNCOS-00, LMNCOS-04, LMNCOS-05, LMNCOS-06,
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LMNCOS-08 materials, respectively. The sulfur additive samples may lower charge-transfer impedances. It is
3
commonly considered that the rate behavior of charge/discharge is closely connected with the charge transfer
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occurring at the cathode/electrolyte interface and the Li+ diffusion in the active material. It can be seen that the
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ascending order of the resistance (Rsf+Rct) of the samples is LMNCOS-05<04<06<00<08.In addition, the
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conductivity measuring results indicate conductivity change of materials with different sulfur additive percent,
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and the conductivity of material with 0.5 wt% sulfur additive (LMNCOS-05) is obviously increased compared
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with that of the blank material (LMNCOS-00). Those results indicate that the LMNCOS-05 sample exhibits
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good rate capability by reducing the resistance and increasing the conductivity through sulfur incorporation [31].
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Potentiostatic Intermittent Titration Technique (PITT) test was carried out to evaluate Li+ diffusion in the
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Li-rich layered oxide. The deintercalation process was performed by applying small potential steps of 10 mV
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and recording the current, which decays to very low steady-state values close to zero, as a function of time. The
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typical It1/2 vs. log t curves obtained from PITT in the potential range of 4.55-4.75 V of the LMNCO and
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LMNCOS-05 electrode are shown in Fig. 12(a)-(d). Fig. 12(a)-(b) show the PITT curves of LMNCO and
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LMNCOS-05 during the initial charge-discharged process from 2.5 V to 4.8 V, which in order to develop a
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steady-state behavior. The It1/2 vs. log t curves demonstrates one broad maximum (plateau) in the short time
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domain, in which It1/2 vs. log t is constant, and a declining line. This maximum is attributed to a Cottrellian
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behavior of finite-length diffusion. The maximum values of It1/2 (which are invariant in time) were used for the
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calculation of the diffusion coefficient according to the Eq. (3.1) [32-34]:
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d ln( I ) 4 L2 （3.1） dt π 2
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DLi =
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L is the diffusion length. Based on Eq. (3.1) and PITT measurement, the diffusion coefficients of Li+ at
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varied voltages can be obtained, as shown in Table 5. Both LMNCOS-00 and LMNCOS-05 show ion diffusion 10
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coefficient of about 10-12 cm2 S-1 during the charge process. However, the ion diffusion coefficient of
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LMNCOS-05 is larger than that of LMNCOS-00 at every state. So the new phase of Li2SO4 enhances the basic
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metal ions diffusion in the lithium-rich solid solution cathode material.
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4. Conclusion
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In this work, LMNCOS material is synthesized by high temperature solid phase synthesis, and the
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effects of different amount of sulfur additive on the material structure, morphology and electrochemical
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properties are studied in detail. The XRD and TEM results show that sulfur is not doped into the LMNCO
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phase, but forms Li2SO4 phase. The capacity of LMNCOS-05 material is higher than the other samples,
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especially on the condition of high current rate, which can deliver 223.4 mAh g-1 at 200 mA g-1 and 176.7
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mAh g-1 at 600 mA g-1. The new phase may effect the rate capacity performance by improving its electrical
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conductivity or lithium ion diffusion of the LMNCOS materials
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Acknowledgement
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The authors thank Ms. Y. Meng and her co-workers (from Shougang Technology Research Institute)
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for their helpful assistance in the experimental work. This study was financially supported by the Ministry
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of Science and Technology of China (2012AA110102), and the National Natural Science Foundation of
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China (51302017).
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[33] S. J. Shi, J. P. Tu, Y. Y. Tang, Y. X. Yu, Y. Q. Zhang, X. L. Wang, C. D. Gu, Combustion synthesis and
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Activity as Positive Electrode in Lithium Cells. J. The Electrochem. Soc. 160 (2013) A324-A337.
Ac ce pt e
d
M
an
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cr
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4
16
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List of Tables Table 1 EDS data of Mn, Ni, Co, S and O in the LMNCOS-05 composite powders in atom percentage (%) ratio of elements. (a) The first area; (b) the second area.
ip t
Table 2 The ICP test results of LMNCOS-05 sample filtrate(μg/g) Table 3 Initial charge-discharge capacity, and initial cycle efficiency of LMNCOS (0.1C, 2.0-4.8V)
cr
Table 4 Parameters obtained from simulation of elements in an equivalent circuit model.
us
Table 5 The Li+ diffusion coefficient of LMNCOS-00 and LMNCOS-05 samples at different charge states
Ac ce pt e
d
M
an
by PITT.
17
Page 17 of 37
Table 1 EDS data of Mn, Ni, Co, S and O in the LMNCOS-05 composite powders in atom percentage (%) ratio of elements. (a) The first area; (b) the second area. The first area / at. %
The second area / at. %
O
67.16
61.70
S
12.22
0.00
Mn
20.62
24.37
Co
0.00
7.99
Ni
0.00
5.94
Sum
100
100
Ac ce pt e
d
M
an
us
cr
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Element
18
Page 18 of 37
Table 2 The ICP test results of LMNCOS-05 sample filtrate(μg g-1) Li
Co
Mn
Ni
S
blank LMNCOS-05 filtrate The pristine material The diffused material
0.05 61.7 94910 100675
0.002 0.01 94472 92281
0.03 0.04 310552 324899
0.004 0.007 100707 106635
0.02 114 10143 883
Ac ce pt e
d
M
an
us
cr
ip t
samples
19
Page 19 of 37
Table 3 Initial charge-discharge capacity, and initial cycle efficiency of LMNCOS (0.1C, 2.0-4.8V) Initial cycle Samples
Charge capacity / mAhg
Discharge capacity / mAhg
Initial Cycle efficiency (%)
LMNCOS-00
323.3
258.7
80.0
LMNCOS-04
338.3
274.7
81.2
LMNCOS-05
342.1
280.1
81.9
LMNCOS-06
337.5
260.9
77.3
LMNCOS-08
330.8
253.4
ip t
-1
76.6
Ac ce pt e
d
M
an
us
cr
-1
20
Page 20 of 37
Table 4 Parameters obtained from simulation of elements in an equivalent circuit model. Rs (Ω)
Rsf (Ω)
Rct (Ω)
ZW
LMNCOS-00
2.31
33.84
149.8
0.0043
LMNCOS-04
3.00
110.1
25.20
0.0032
LMNCOS-05
2.60
21.17
90.95
0.0053
LMNCOS-06
9.22
127.8
46.78
0.0026
LMNCOS-08
2.98
46.96
325.8
0.0028
Ac ce pt e
d
M
an
us
cr
ip t
Samples
21
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Table 5 The Li+ diffusion coefficient of LMNCOS-00 and LMNCOS-05 samples at different charge states by PITT. Site
Voltage
a
DLi(cm2 s-1)
L=40 μm LMNCOS-05
4.55
6.95 E-12
9.96 E-12
b
4.65
5.02 E-12
7.06 E-12
c
4.75
2.87 E-12
6.96 E-12
Ac ce pt e
d
M
an
us
cr
ip t
LMNCOS-00
22
Page 22 of 37
List of Figures Fig. 1. Schematic illustration of the preparation process as template.
ip t
Fig. 2. SEM images of (a) LMNCOS-00; (b) LMNCOS-02; (c) LMNCOS-04; (d) LMNCOS-05; (e) LMNCOS-06; (f) LMNCOS-08.
cr
Fig. 3. S2P XPS spectra of the LMNCOS-05 sample. The inset graph is the magnified plots of the bonding
us
energy of Sulfur element.
Fig. 5. XRD patterns of synthesized materials (◆, Li2SO4).
an
Fig. 4 FTIR spectra of the as-prepared samples.
M
Fig. 6 XRD pattern of the LMNCOS-05 sample in the expanded scale (2θ=17-55°). Fig. 7. TEM mapping images of (a) pristine material; (b) manganese ; (c) cobalt; (d) nickel; (e) oxygen and
d
(f) sulfur elements confirm a uniform distribution of the types of elements after the modification .
Ac ce pt e
Fig. 8. HRTEM images and SAED images of LMNCOS-05 sample (a) low magnification(200K) TEM bright field image of the modified LMNCOS-05 particles
(b) selected area electron diffraction (SAED)
parrerns corresponding to (a), revealing two types of phases, i. e., a layered triclinic structure and spinel cubic structure; (c) HRTEM(50K) image showing surface regions of theLMNCOS-05 particle composed of Li2SO4 monoclinic structure and layered structures inside; (d) 800K-HRTEM. (e) The EDS signals of the compositional mapping of the first area (corresponding to (d) which labeled as 1); (f) The EDS signals of the compositional mapping of the second area (corresponding to (d) which labeled as 2). Fig. 9. Compare the initial charge/discharge profiles of the pristine(LMNCO) and the modified materials(LMNCOS). The charge/discharge was 20mA g-1 with a 4.8V cut off. Fig. 10. Discharge curves of half cells at various big rates after charging at 40mA g-1 to 4.8V for the 23
Page 23 of 37
LMNCOS samples. Fig. 11. Comparison of electrochemical impedance spectra (EIS) of LMNCO/LMNCOS. Fig.12. PITT curves of LMNCOS-00/LMNCOS-05 for the initial charge process between 2.0 V and 4.8 V:
ip t
(a) LMNCOS-00 I vs. t; (b) LMNCOS-05 I vs. t; (c) linear behavior of LMNCOS-00 ln(I) vs. t; (d) linear
Ac ce pt e
d
M
an
us
cr
behavior of LMNCOS-05 ln(I) vs. t at different charge states.
24
Page 24 of 37
1. (NH4)2SO4 / (NH4)H2PO4
solution
grinded for 4h
stirring for 24h
an
900◦C for 36h
spray dryer
us
preheated at 600◦C for 10h
calcined at
cr
ip t
suspension(15%)
Ac ce pt e
d
M
Fig. 1. Schematic illustration of the preparation process.
25
Page 25 of 37
M
an
us
cr
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2.
Ac ce pt e
d
Fig. 2. SEM images of (a) LMNCOS-00; (b) LMNCOS-02; (c) LMNCOS-04; (d) LMNCOS-05; (e) LMNCOS-06; (f) LMNCOS-08.
26
Page 26 of 37
us
cr
ip t
3.
Ac ce pt e
d
M
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Fig. 3. S2P XPS spectra of the LMNCOS-05 sample. The inset graph is the magnified plots of the bonding energy of Sulfur element.
27
Page 27 of 37
us
cr
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4.
Ac ce pt e
d
M
an
Fig. 4. FTIR spectra of the as-prepared samples.
28
Page 28 of 37
(018) (110) (113)
(107)
(104)
♦
LMNCOS-08
♦
♦
LMNCOS-06
ip t
♦
LMNCOS-05
LMNCOS-04
cr
Intensity /a.u.
♦⎯Li2SO4
(105)
(101)
(003)
(006) (102)
5
LMNCOS-02
20
30
40
50
2θ/ degree
60
70
80
an
10
us
LMNCOS-00
Ac ce pt e
d
M
Fig. 5. XRD patterns of synthesized materials (◆, Li2SO4).
29
Page 29 of 37
an
us
cr
ip t
6.
Ac ce pt e
d
M
Fig. 6. XRD pattern of the LMNCOS-05 sample in the expanded scale (2θ=17-55°).
30
Page 30 of 37
M
an
us
cr
ip t
7.
Ac ce pt e
d
Fig. 7. TEM mapping images of (a) pristine material; (b) manganese ; (c) cobalt; (d) nickel; (e) oxygen and (f) sulfur elements confirm a uniform distribution of the types of elements after the modification .
31
Page 31 of 37
Ac ce pt e
d
M
an
us
cr
ip t
8.
Fig. 8. HRTEM images and SAED images of LMNCOS-05 sample (a) low magnification(200K) TEM bright field image of the modified LMNCOS-05 particles
(b) selected area electron
diffraction (SAED) parrerns corresponding to (a), revealing two types of phases, i. e., a layered triclinic structure and spinel cubic structure; (c) HRTEM(50K) image showing surface regions of theLMNCOS-05 particle composed of Li2SO4 monoclinic structure and layered structures inside; (d) 800K-HRTEM. (e) The EDS signals of the compositional mapping of the first area 32
Page 32 of 37
(corresponding to (d) which labeled as 1 ); (f) The EDS signals of the compositional mapping of
Ac ce pt e
d
M
an
us
cr
ip t
the second area (corresponding to (d) which labeled as 2 ).
33
Page 33 of 37
an
us
cr
ip t
9.
Ac ce pt e
d
M
Fig. 9. Compare the initial charge/discharge profiles of the pristine(LMNCO) and the modified materials(LMNCOS). The charge/discharge was 20mA g-1C with a 4.8V cut off..
34
Page 34 of 37
an
us
cr
ip t
10.
Ac ce pt e
d
M
Fig. 10. Discharge curves of half cells at various big rates after charging at 40mA g-1 to 4.8V for the LMNCOS samples.
35
Page 35 of 37
us
cr
ip t
11.
Ac ce pt e
d
M
an
Fig. 11. Comparison of electrochemical impedance spectra (EIS) of LMNCO/LMNCOS.
36
Page 36 of 37
d
M
an
us
cr
ip t
12.
Ac ce pt e
Fig.12. PITT curves of LMNCOS-00/LMNCOS-05 for the initial charge process between 2.0 V and 4.8 V: (a) LMNCOS-00 I vs. t; (b) LMNCOS-05 I vs. t; (c) linear behavior of LMNCOS-00 ln(I) vs. t; (d) linear behavior of LMNCOS-05 ln(I) vs. t at different charge states.
37
Page 37 of 37
S0013-4686(15)30281-4 http://dx.doi.org/doi:10.1016/j.electacta.2015.08.031 EA 25499
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
3-4-2015 6-8-2015 7-8-2015
Please cite this article as: B. Liqing, Y. Yanping, Z. Weidong, L. Huaquan, W. Zhong, L. Shigang, Electrochemical performance improvement of Li1.2 [Mn0.54 Ni0.13 Co0.13 ]O2 cathode material by sulfur incorporation, Electrochimica Acta (2015), http://dx.doi.org/10.1016/j.electacta.2015.08.031 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Electrochemical performance improvement of Li1.2[Mn0.54Ni0.13Co0.13]O2
2
cathode material by sulfur incorporation Liqing BAN
Yanping YIN
Weidong ZHUANG*
Huaquan LU
Zhong WANG
Shigang LU
ip t
( R& D Center for Vehicle Battery and Energy Storage, General Research Institute for Nonferrous Metals, Beijing 100088, China )
The
enhanced
electrochemical
performance
of
the
lithium-rich
solid
cr
Abstract:
solution
us
Li1.2[Mn0.54Ni0.13Co0.13]O2 (LMNCO) cathode is enhanced by sulfur incorporation. Various sulfur contents are
an
introduced by adding (NH4)2SO4 into the raw material. The effects of different sulfur contents on the structure, morphology and electrochemical performance are investigated. The original sample (as-received sample) and
M
the sulfur incorporated samples were characterized by X-ray Diffraction (XRD), High Resolution Transmission Electron Microscopy (HRTEM), Electrochemical Impedance Spectroscopy (EIS), X-ray Photoelectron
Ac ce pt e
d
Spectroscopy (XPS) and Fourier Transform Infrared Spectroscopy (FTIR). Electrochemical performance such as charge/discharge capacity and rate capability was assessed with lithium ion cells. XRD patterns show that a new phase of Li2SO4 was formed and distributed on the surface of the particle. The electrochemical performance of the sulfur incorporated LMNCO samples is significantly improved due to the formation of the new phase on the surface of the particles. In comparison with the original material, the modified materials show an improved rate performance attributed to the interface between LMNCO and the second phase, which may provide fast diffusion channels for lithium ion.
Key words: Lithium-rich layered oxide; Sulfur incorporation; Rate performance; Lithium-ion batteries.
1 2
1. Introduction The lithium-rich solid solutions of layered Li2MnO3 and LiMO2 (M=Ni, Co, Mn) have drawn much
Corresponding author: Tel: +8613911718416; E-mail addresses: [email protected]
1
Page 1 of 37
attention in recent years, and they are among the most potential cathode materials for next generation lithium
2
ion batteries, because they can provide much higher capacity (>250 mAh g-1) with significantly reduced cost
3
compared to the LiCoO2 materials[1, 2]. However, some major problems, such as poor rate capability, high initial
4
cycle irreversibility, and significant decrease in the discharge voltage plateau with successive cycling, still
5
remain unresolved[3]. In addition, the ionic conductivity of these materials is quite low, which limits their
6
electrochemical performance at higher charge/discharge rate, and the poor rate capability cannot satisfy the
7
requirement of batteries for application in electric or hybrid electric vehicles, especially when power
8
characteristics is concerned[4].
us
cr
ip t
1
Recently, several treatment methods, such as surface modification and ion doping, have been developed to
10
enhance the electrochemical properties of the lithium rich cathode materials. Surface modification can prevent
11
the direct contact of electrode from electrolyte by coating with stabilizing materials[5]. The coating materials
12
including AlF3[5], Al2O3[6, 7], LiNiPO4[8, 9], are likely to diffuse easily into the cathode surface or deposit on the
13
surface. Ion doping method is to substitute oxygen or transition elements (Ni/Co/Mn) with other elements[9, 10],
14
The substitution of anion for oxygen is effective in reducing impedance and lattice changes during cycling and
15
improving cycle life[11] and the substitution of the transition elements is effective in stabilizing materials’ micro
16
structures[10, 11]. A novel strategy by blending the lithium-rich layered oxide with other materials can eliminate
17
the irreversible capacity[12, 13]. A lot of literatures on surface modification of the Li-rich and Mn-based cathode
18
materials such as fluorides and oxides have been published. However, this family of cathode materials after
19
modification still has drawbacks such as poor rate capability. Therefore, high ionic or/and electronic conductor
20
modified cathode materials have become increasingly attractive methods to improve the electrochemical
21
performance of lithium ion batteries[14]. For instance, Li3PO4 polyanion with high Li+ diffusion coefficient have
22
been used to modify and improve the rate performance of various battery materials[15, 16]. In fact, polyanion-type
23
compounds are a class of materials in which tetrahedral polyanion structure units (XO4)n- and their derivatives
Ac ce pt e
d
M
an
9
2
Page 2 of 37
1
(XmO4m+1)n- ( X=B, P, Si, S, As and or Mo) express high stability oxygen ions[17-19]. Nevertheless, to our
2
knowledge, research and report involving sulfur polyanions incorporation are rarely demonstrated. Therefore, in
3
this work, we intend to develop a facile synthesis of the manganese-based Li-rich oxide cathode material which
4
is incorporated with sulfur polyanions to improve electrochemical performance. In this work, nano-sized Li1.2[Mn0.54Ni0.13Co0.13]O2 samples with and without sulfur incorporation were
6
synthesized (marked as LMNCOS and LMNCO). The structures of LMNCOS and LMNCO were characterized
7
by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), X-ray photoelectron
8
spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The enhanced electrochemical
9
performance was illustrated by galvanostatic charge/discharge test and electrochemical impedance spectroscopy
an
us
cr
ip t
5
(EIS).
11
2. Experimental
12
2.1 Material preparation and characterization
d
M
10
Layered LMNCO and LMNCOS materials were synthesized by a simple solid-state method.
14
Stoichiometric amounts of lithium carbonate (99.9%, AR), manganese carbonate (99.0%, AR), tricobalt
15
tetroxide (98.0%, AR) and nickel oxide (100%, AR) (molar ratio of Ni: Co: Mn=0.13: 0.13: 0.54), were
16
diffused in 350ml distilled water with continuous stirring for 24h, and then grinded for 4h. Various amount of
17
(NH4)2SO4 were added into the ultrafinely grinded homogenate. The resulting solution was pumped into a spay
18
drying instrument(L-117, LaiHeng) to produce homogenous precursor. Then the spray dried precursor was
19
initially decomposed at 650◦C for 10h in air, after being ground, the resulting powers were put into a corundum
20
crucible and calcined at 900◦C for 36h in air. The original and modified samples were marked as LMNCOS-00,
21
LMNCOS-04, LMNCOS-05, LMNCOS-06, and LMNCOS-08 (0.0, 0.2, 0.4, 0.5, 0.6 and 0.8 wt% of pristine
22
material LMNCO), respectively. The whole process is shown in Fig. 1.
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Scanning electron microscopic(SEM) investigation of the original and cycled electrodes was performed on
2
a S-4800 microscope (HITACHI Japan). X’ Pert PRO XRD patterns were recorded with a Phillips X-ray
3
diffractometer with Cu Kα radiation (λ=0.1541 nm) with an accelerating voltage of 40 kV, the 2θ value range is
4
10° to 90° with an increments of 0.0017° at room temperature. TEM data were collected with a JEOL-TEM
5
equipment to assess the microstructures of the modified samples. XPS data were collected at room temperature
6
with Kratos Analytical Spectrometer and monochromatic Al Kα (1486.6 eV) X-ray source to assess the
7
chemical state of the surface elements. Multiplex spectra of various photoemission lines were collected at
8
medium resolution using an analyzer pass energy of 40eV at 0.1 eV step and an integration interval of 1 s·eV-1.
9
Inductively coupled plasma (ICP) technique was used to determine the dissolved amount of elements. Fourier
10
transform infrared spectroscopy (FTIR) spectra of the samples were performed on a JASCO 400 FTIR
11
spectroscopy. Samples were mixed with KBr and pressed to form pellets for measurement. The materials were
12
pressed into pellets of 12 mm diameter and about 0.55 mm thickness, and their conductivity was measured by
13
Van der Pauw four-point direct current (dc) method, using a four point probing system (HMS-3000/0.55T)
14
2.2 Electrochemical measurements
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The cathode slurry was prepared by dissolving 80wt% active material, 10wt% super-P and 10wt%
16
polyvinylidene fluoride (PVDF) binder in N-methylpyrrolidone(NMP), and then the slurry was rolled on a thin
17
sheet of aluminum foil, after that, the sheet was cut into circular electrodes with a diameter of 14mm. The mass
18
of the active material is 9.6 mg, and the mass loading of the active material is 6.5 mg cm-2 in the electrode. Coin
19
cells were assembled with the as-prepared cathodes, lithium foil anode (Φ=15.8 mm×0.5 mm, China Energy
20
Lithium Co. Ltd.), 1 mol/L LiPF6 dissolved in ethylene carbonate/diethyl carbonate (EC/DEC, Beijing, AR)
21
electrolyte, and Celgard 2500(America) polypropylene separator.
22
EIS measurements were conducted with a Solartron 1260A impedance analyzer in the frequency range of
23
100 kHz to 50 mHz with an AC voltage amplitude of 10mV. Potentiostatic Intermittent Titration Technique 4
Page 4 of 37
(PITT) tests were also conducted on this apparatus at room temperature in the voltage range of 2.0-4.8 V. For
2
the PITT measurements, the battery cells were charged to a fixed voltage (4.55 V, 4.65 V, 4.75 V) for 200 s.
3
The charge-discharge experiments of the Li/LMNCOS cells were performed galvanostatically at 20 mA g-1 with
4
a voltage range of 2.0-4.8 V with a battery tester (Land CT2001A, Wuhan) at room temperature. The rate
5
capability of the cells was tested by charging and discharging from 20 mA g-1 to 600 mA g-1. Only the active
6
mass of the electrodes was considered during the calculation of capacity. All the potentials mentioned
7
throughout this work are in reference to a Li/Li+ couple.
8
3. Results and discussion
9
3.1. Physical properties of LMNCOS cathode materials
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The morphology and particle distribution of the cathode materials are of great importance to battery
11
performance. Fig.2 shows SEM images of the synthesized LMNCOS powders with different sulfur additive
12
percents. The LMNCOS particles are agglomerated and have polygonal and irregular shapes, due to surface
13
reaction during the calcination process at high temperature or the high surface free energy of the nanoparticles.
14
The surfaces of LMNCO with different sulfur additive percent are observed with no clear impurity phase and no
15
evident diversity in structure. The observed particle size from SEM was in the range of 140-200 nm.
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XPS is an effective elemental analysis method to provide the information of the oxidation states of the
17
elements on the surface. XPS analysis of the LMNCOS-05 sample was carried out and the S2P XPS spectrum is
18
given in Fig. 3. The bonding interaction is further verfied by XPS (Fig. 3). There is a strong O1s peak at 529.25
19
eV, corresponding to the lattice oxygen of the Li-rich layered oxides. The observed binding energies of the S2P3/2
20
and S2P1/2 are 168.62 and 169.80 eV, respectively, which are very close to the values of sulfate (168.5 eV)
21
reported in Ref and attributed to the presence of sulfate groups in transition metal sulfates. This result shows that
22
the sulfur on the material surface is hexavalent. Correspondingly, the peaks of Ni2p3/2, Co2p3/2 and Mn2p3/2 are
5
Page 5 of 37
1
measurably, whose binding energy are located at 854.80, 779.80 and 641.90 eV, respectively. These results are
2
consistent with the reported literature[20] To support the relevant conclusion, FTIR spectra are shown to prove the existence of sulfate in the
4
LMNCOS materials (Fig. 4). Characteristic peaks at 1110 and 615 cm-1 are observed in all the samples, which
5
are due to the asymmetric stretching modes of S-O bonds in SO42- bending modes. Differently, there is only one
6
band that can be shown in the LMNCO material before Sulfur incorporation. With the increasing of sulfur
7
content, the characteristic peak intensity becomes stronger.
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Fig.5 shows the XRD patterns of the synthesized LMNCOS powders. The XRD peaks indicate the
9
formation of a α-NaFeO2-type layered hexagonal structure with space group of R 3 m, which suggests that the
10
crystal structure of LMNCO is not affected by the incoming of (NH4)2SO4. The splitting of the (006)/(012) and
11
(008)/(110) peaks of the XRD patterns of LMNCO indicates the formation of a well-layered structure [21].
12
However, several weak impurity peaks appear in the small angle of 2θ = 22-30°, as shown in Fig. 5 and 6. With
13
the increment of sulfur content, impurity peak intensity becomes stronger (Fig. 5). According to the standard
14
PDF CARDS (PDF#01-0443), these weak peaks are assigned to the phase of Li2SO4, and it is then proven by
15
the determination of sulfur state in the XPS test.
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Fig. 7 shows the mapping images of each of the transition metal elements (Ni, Co, Mn), oxygen and sulfur.
17
These elements uniformly distribute across the detected sections. The surface of LMNCOS particles is covered
18
with uniformly distributed sulfur as shown in the Fig. 7(f).
19
The high resolution transmission electron microscopy (HRTEM) of the LMNCOS-05 sample was shown in
20
Fig. 8(a) indicate that the material is highly crystallized in nature, and the distance between two lattice fringes
21
of the bulk materials is 0.47 nm by calculation, which is assigned to the (003) plane of LiMO2 structure. From
22
HRTEM study, lattice pattern and selected area electron diffraction patterns (SAED) were shown in Fig. 8(b).
23
Lattice pattern showed that the material is highly crystalline in nature and indexed with LiMO2 structure. The 6
Page 6 of 37
lattice pattern was indexed to [1 -1 1] electron beam direction of LiMO2 for the materials prepared by both
2
methods. The Li2MnO3 and LiMO2 phases were distinguished from SAED pattern of the layered composite and
3
well indexed with different structures[22]. From Fig. 8(c), it can be clearly observed that several tiny particles are
4
distributed uniformly well on the surface of the bulk particle. . From the magnifying image (Fig. 8(d)) of a tiny
5
particle[23, 24]. The crystallographic distance and direction between the tiny particles and the bulk particle can be
6
clearly distinguished. The inner side of the interface shows clear and typical (003) fringe, characteristic of the
7
layered structure of a lithium-rich material. Outside of the interface is a crystalline layer approximately 5 nm
8
thick. As further characterized in Fig. 8(d), the outer nanostructure clearly presents fine lattice fringes. The
9
distance of the lattice fringes are 0.316 nm and 0.349 nm, which are indexed to the (-112) and (111) planes of
10
monoclinic structure of Li2SO4, respectively. Consequently, it can be concluded that a new phase of Li2SO4 (the
11
second phase) formed on the surface of the bulk particle (bulk material)[13].
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The composition of the synthesized LMNCOS material is estimated based on Energy Dispersive
13
Spectroscopy (HRTEM-EDS) data using the EDAX Genesis Analyzer System as shown in Fig. 8(e) and (f)[25].
14
EDS was carried out at different areas of the samples in order to determine the presence and percentage of Mn,
15
Ni, Co, S, O elements. Atom percent (%) ratio of Mn, Ni, Co at two areas is listed in Table1. The values of the
16
atoms indicate the new particles of the first area contain S and small amount of Mn, and S is not detected in the
17
second area.
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Considering that Li2SO4 can dissolve in water easily, the LMNCOS-05 material was washed with distilled
19
water, and then ICP technique is performed to determine the dissolved ions of the material surface. EDS study
20
was carried out at various areas of the samples in order to determine the presence of Mn, Ni, Co，S elements and
21
also to determine their mole percentage. The concentration of S and Li elements in the filtrate are 114 and
22
61.7μg g-1 respectively, the molar ratio of which is close to 1:2, indicating the dissolved substance on the
23
surface of the particle is Li2SO4. The ICP results show that the Ni, Co and Mn elements are barely detected in 7
Page 7 of 37
1
the filtrate.
2
3.2 Electrochemical performance of LMNCOS cathode material The first charge-discharge profiles of the materials between 2.0-4.8 V at 20 mA g-1 rate are shown in the
4
Fig.9 and Table 3. During the initial charge to 4.8 V, the electrochemical reaction occurs in two dominant stages:
5
a smoothly sloping voltage profile below 4.5 V vs. Li/Li+ , which is attributed to the removal of lithium from
6
the LiMO2 (M=Ni, Co, Mn) accompanied with the oxidation of Ni2+ to Ni4+ and partial oxidation of Co3+ to
7
Co4+, and the voltage plateau profile around 4.5 V vs. Li/Li+[26, 27], which could be ascribed to the activation of
8
Li2MnO3 component. This activation process has been considered to be associated with the irreversible
9
extraction of Li2O from the transition metal oxide composite materials[22, 28]. During the plateau, activation of
10
oxygen and formation of Li2O occur. Electrolyte degradation may also occur, generating gases such as CO and
11
CO2. Toward the end of the plateau, the unstable and catalyst-like LMNCO material causes the release of O2 gas,
12
which readily reacts with the electrolyte to form CO, CO2, and protons (H+) via a catalytic reaction. Li2O will
13
react with the proton (H+) to form Li+ and H2O which solvates and migrates to the anode side and forms
14
LiOH·H2O at the end of charging[29]. There is no plateau region in the subsequent charging cycles, indicating
15
that the oxygen loss process during the initial charge is irreversible. It is also seen from Fig. 9 that the initial
16
charge capacity is larger than the discharge capacity.
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The electrochemical performance of the initial charge/discharge profiles for the LMNCOS electrode is
18
presented in Fig. 9 and Table 3. The initial discharge capacities are 278.7 and 280.1 mAh g-1 for LMNCOS-04
19
and LMNCOS-05, respectively，higher than 260.5 mAh g-1 for pristine material LMNCOS-00. While the sulfur
20
content increased to 0.8%, the initial discharge capacity decreases to 258.2 mAh g-1. When the content of
21
Li2SO4 is very low, the interface between LMNCO and trace amount Li2SO4 can increase the diffusion rate and
22
conductivity of Li+, so the discharge capacity of LMNCO S-04 and LMNCO S-05 is improved. Nevertheless,
23
With the amount of formed Li2SO4 increasing, more lithium ions in bulk material (LMNCO) is deprived, which 8
Page 8 of 37
1
influence the component of the Li1.2[Mn0.54Ni0.13Co0.13]O2 material (LMNCO) and the charge-discharge capacity
2
of LMNCOS-08 materials. Compares the rate capabilities of the pristine LMNCO and the LMNCOS materials ranged from 20 mA g-1
4
to 600 mA g-1 rate in the voltage range of 2.0-4.8 V. After three formation cycles at 20 mA g-1, the cells were
5
charged at 40 mA g-1, then discharged at 40 mA g-1, 100 mA g-1, 200 mA g-1 and 600 mA g-1. When the current
6
rate is at 40 mA g-1, 100 mA g-1, 200 mA g-1 and 600 mA g-1, the discharge capacities of the pristine LMNCO
7
are 237.0, 211.6, 183.7 and 142.7 mAh g-1, respectively (Fig. 10). In contrast, LMNCOS-05 delivers the highest
8
discharge capacities of 245.2, 234.4, 223.4, 176.7 mAh g-1. It is obvious that the rate capability of LMNCO can
9
be remarkably improved by the adding of sulfur. The outstanding rate capability of the LMNCOS electrode
10
mainly results from Li2SO4 surface modification layer, which has more Li+ and high electron conductivity.
11
However, the improvement is not linearly proportional to the amount of sulfur additive and the LMNCOS-05
12
material has the optimized performance in this experiment. In addition, when the current rate returns back to 20
13
mA g-1, the discharge capacity of the LMNCOS-05 material has the best capacity retention during the cycling.
14
3.3 Electrical conductivity and EIS
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In order to illustrate the mechanism of the performance improvement of the LMNCOS samples, the
16
migration of electron and lithium ion of LMNCOS samples are traced by EIS and electrical conductivity
17
measurements.
18
EIS is an effective technique for analyzing the differences in the polarization behaviors and understanding the
19
differences in rate capability. According to our previous EIS studies on this type of layered oxide cathodes[30], as
20
shown in Fig. 11: EIS of all samples were tested prior to electrochemical oxidation and all of them show a
21
semicircle in the high frequency region and a slope in the low frequency region. The semicircle in the high
22
frequency region is assigned to charge transfer reaction, and the slope in the low frequency region is attributed
23
to the lithium ion diffusion in the bulk material. The charge-transfer impedances calculated from the Nyquist 9
Page 9 of 37
plots were 149.8, 25.20, 90.95, 46.78, 325.8 Ω for LMNCOS-00, LMNCOS-04, LMNCOS-05, LMNCOS-06,
2
LMNCOS-08 materials, respectively. The sulfur additive samples may lower charge-transfer impedances. It is
3
commonly considered that the rate behavior of charge/discharge is closely connected with the charge transfer
4
occurring at the cathode/electrolyte interface and the Li+ diffusion in the active material. It can be seen that the
5
ascending order of the resistance (Rsf+Rct) of the samples is LMNCOS-05<04<06<00<08.In addition, the
6
conductivity measuring results indicate conductivity change of materials with different sulfur additive percent,
7
and the conductivity of material with 0.5 wt% sulfur additive (LMNCOS-05) is obviously increased compared
8
with that of the blank material (LMNCOS-00). Those results indicate that the LMNCOS-05 sample exhibits
9
good rate capability by reducing the resistance and increasing the conductivity through sulfur incorporation [31].
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Potentiostatic Intermittent Titration Technique (PITT) test was carried out to evaluate Li+ diffusion in the
11
Li-rich layered oxide. The deintercalation process was performed by applying small potential steps of 10 mV
12
and recording the current, which decays to very low steady-state values close to zero, as a function of time. The
13
typical It1/2 vs. log t curves obtained from PITT in the potential range of 4.55-4.75 V of the LMNCO and
14
LMNCOS-05 electrode are shown in Fig. 12(a)-(d). Fig. 12(a)-(b) show the PITT curves of LMNCO and
15
LMNCOS-05 during the initial charge-discharged process from 2.5 V to 4.8 V, which in order to develop a
16
steady-state behavior. The It1/2 vs. log t curves demonstrates one broad maximum (plateau) in the short time
17
domain, in which It1/2 vs. log t is constant, and a declining line. This maximum is attributed to a Cottrellian
18
behavior of finite-length diffusion. The maximum values of It1/2 (which are invariant in time) were used for the
19
calculation of the diffusion coefficient according to the Eq. (3.1) [32-34]:
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d ln( I ) 4 L2 （3.1） dt π 2
20
DLi =
21
L is the diffusion length. Based on Eq. (3.1) and PITT measurement, the diffusion coefficients of Li+ at
22
varied voltages can be obtained, as shown in Table 5. Both LMNCOS-00 and LMNCOS-05 show ion diffusion 10
Page 10 of 37
coefficient of about 10-12 cm2 S-1 during the charge process. However, the ion diffusion coefficient of
2
LMNCOS-05 is larger than that of LMNCOS-00 at every state. So the new phase of Li2SO4 enhances the basic
3
metal ions diffusion in the lithium-rich solid solution cathode material.
4
4. Conclusion
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In this work, LMNCOS material is synthesized by high temperature solid phase synthesis, and the
6
effects of different amount of sulfur additive on the material structure, morphology and electrochemical
7
properties are studied in detail. The XRD and TEM results show that sulfur is not doped into the LMNCO
8
phase, but forms Li2SO4 phase. The capacity of LMNCOS-05 material is higher than the other samples,
9
especially on the condition of high current rate, which can deliver 223.4 mAh g-1 at 200 mA g-1 and 176.7
10
mAh g-1 at 600 mA g-1. The new phase may effect the rate capacity performance by improving its electrical
11
conductivity or lithium ion diffusion of the LMNCOS materials
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Acknowledgement
13
The authors thank Ms. Y. Meng and her co-workers (from Shougang Technology Research Institute)
14
for their helpful assistance in the experimental work. This study was financially supported by the Ministry
15
of Science and Technology of China (2012AA110102), and the National Natural Science Foundation of
16
China (51302017).
17
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[29] Sunny Hy, Felix Felix, John Rick, Wei-Nien Su, Bing Joe Hwang, Direct In situ Observation of Li2O
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Evolution on Li-Rich High-Capacity Cathode Material, Li[NixLi(1−2x)/3Mn(2−x)/3]O2 (0≤x≤0.5) J.
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Am. Chem. Soc. 136(2014) 999−1007.
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[30] Z. Wang, E. Liu, C. He, C. Shi, J. Li, N. Zhao, Effect of amorphous FePO4 coating on structure and
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electrochemical performance of Li1.2Ni0.13Co0.13Mn0.54O2 as cathode material for Li-ion batteries. J.
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Power Sources 236 (2013) 25-32.
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13
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[31] C. Yu, H. Wang, X. Guan, J. Zheng, L. Li, Conductivity and electrochemical performance of cathode
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xLi2MnO3-(1-x)LiMn1/3Ni1/3Co1/3O2 (x=0.1,0.2,0.3,0.4) at different temperatures. J. Alloy. compd.
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546 (2013) 239-245.
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[32] Y. S. Jung, A. S. Cavanagh, Y. Yan, S. M. George, A. Manthiram, Effects of Atomic Layer Deposition
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of Al2O3 on the Li[Li0.20Mn0.54Ni0.13Co0.13]O2 Cathode for Lithium-Ion Batteries. J. Electrochem. Soc.
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158 (2011) A1298-A1302.
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[33] S. J. Shi, J. P. Tu, Y. Y. Tang, Y. X. Yu, Y. Q. Zhang, X. L. Wang, C. D. Gu, Combustion synthesis and
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electrochemical performance of Li[Li0.20Mn0.54Ni0.13Co0.13]O2 with improved rate capability. J.
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Power Sources 228 (2013) 14-23. [34] F. Amalraj, M. Talianker, B. Markovsky, D. Sharon, L. Burlaka, S. Dobrick, G. Shafir, E. Zinigrad, O.
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Haik, D. Aurbach, J. Lampert, M. S. Dobrick, A. Garsuch. Study of the Lithium-Rich Integrated
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Compound xLi2MO 3·(1-x)LiMnO2 (x around 0.5; M = Mn, Ni, Co; 2:2:1) and Its Electrochemical
7
Activity as Positive Electrode in Lithium Cells. J. The Electrochem. Soc. 160 (2013) A324-A337.
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List of Tables Table 1 EDS data of Mn, Ni, Co, S and O in the LMNCOS-05 composite powders in atom percentage (%) ratio of elements. (a) The first area; (b) the second area.
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Table 2 The ICP test results of LMNCOS-05 sample filtrate(μg/g) Table 3 Initial charge-discharge capacity, and initial cycle efficiency of LMNCOS (0.1C, 2.0-4.8V)
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Table 4 Parameters obtained from simulation of elements in an equivalent circuit model.
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Table 5 The Li+ diffusion coefficient of LMNCOS-00 and LMNCOS-05 samples at different charge states
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by PITT.
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Table 1 EDS data of Mn, Ni, Co, S and O in the LMNCOS-05 composite powders in atom percentage (%) ratio of elements. (a) The first area; (b) the second area. The first area / at. %
The second area / at. %
O
67.16
61.70
S
12.22
0.00
Mn
20.62
24.37
Co
0.00
7.99
Ni
0.00
5.94
Sum
100
100
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Element
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Table 2 The ICP test results of LMNCOS-05 sample filtrate(μg g-1) Li
Co
Mn
Ni
S
blank LMNCOS-05 filtrate The pristine material The diffused material
0.05 61.7 94910 100675
0.002 0.01 94472 92281
0.03 0.04 310552 324899
0.004 0.007 100707 106635
0.02 114 10143 883
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Table 3 Initial charge-discharge capacity, and initial cycle efficiency of LMNCOS (0.1C, 2.0-4.8V) Initial cycle Samples
Charge capacity / mAhg
Discharge capacity / mAhg
Initial Cycle efficiency (%)
LMNCOS-00
323.3
258.7
80.0
LMNCOS-04
338.3
274.7
81.2
LMNCOS-05
342.1
280.1
81.9
LMNCOS-06
337.5
260.9
77.3
LMNCOS-08
330.8
253.4
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-1
76.6
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Table 4 Parameters obtained from simulation of elements in an equivalent circuit model. Rs (Ω)
Rsf (Ω)
Rct (Ω)
ZW
LMNCOS-00
2.31
33.84
149.8
0.0043
LMNCOS-04
3.00
110.1
25.20
0.0032
LMNCOS-05
2.60
21.17
90.95
0.0053
LMNCOS-06
9.22
127.8
46.78
0.0026
LMNCOS-08
2.98
46.96
325.8
0.0028
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Samples
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Table 5 The Li+ diffusion coefficient of LMNCOS-00 and LMNCOS-05 samples at different charge states by PITT. Site
Voltage
a
DLi(cm2 s-1)
L=40 μm LMNCOS-05
4.55
6.95 E-12
9.96 E-12
b
4.65
5.02 E-12
7.06 E-12
c
4.75
2.87 E-12
6.96 E-12
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LMNCOS-00
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List of Figures Fig. 1. Schematic illustration of the preparation process as template.
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Fig. 2. SEM images of (a) LMNCOS-00; (b) LMNCOS-02; (c) LMNCOS-04; (d) LMNCOS-05; (e) LMNCOS-06; (f) LMNCOS-08.
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Fig. 3. S2P XPS spectra of the LMNCOS-05 sample. The inset graph is the magnified plots of the bonding
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energy of Sulfur element.
Fig. 5. XRD patterns of synthesized materials (◆, Li2SO4).
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Fig. 4 FTIR spectra of the as-prepared samples.
M
Fig. 6 XRD pattern of the LMNCOS-05 sample in the expanded scale (2θ=17-55°). Fig. 7. TEM mapping images of (a) pristine material; (b) manganese ; (c) cobalt; (d) nickel; (e) oxygen and
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(f) sulfur elements confirm a uniform distribution of the types of elements after the modification .
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Fig. 8. HRTEM images and SAED images of LMNCOS-05 sample (a) low magnification(200K) TEM bright field image of the modified LMNCOS-05 particles
(b) selected area electron diffraction (SAED)
parrerns corresponding to (a), revealing two types of phases, i. e., a layered triclinic structure and spinel cubic structure; (c) HRTEM(50K) image showing surface regions of theLMNCOS-05 particle composed of Li2SO4 monoclinic structure and layered structures inside; (d) 800K-HRTEM. (e) The EDS signals of the compositional mapping of the first area (corresponding to (d) which labeled as 1); (f) The EDS signals of the compositional mapping of the second area (corresponding to (d) which labeled as 2). Fig. 9. Compare the initial charge/discharge profiles of the pristine(LMNCO) and the modified materials(LMNCOS). The charge/discharge was 20mA g-1 with a 4.8V cut off. Fig. 10. Discharge curves of half cells at various big rates after charging at 40mA g-1 to 4.8V for the 23
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LMNCOS samples. Fig. 11. Comparison of electrochemical impedance spectra (EIS) of LMNCO/LMNCOS. Fig.12. PITT curves of LMNCOS-00/LMNCOS-05 for the initial charge process between 2.0 V and 4.8 V:
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(a) LMNCOS-00 I vs. t; (b) LMNCOS-05 I vs. t; (c) linear behavior of LMNCOS-00 ln(I) vs. t; (d) linear
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behavior of LMNCOS-05 ln(I) vs. t at different charge states.
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1. (NH4)2SO4 / (NH4)H2PO4
solution
grinded for 4h
stirring for 24h
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900◦C for 36h
spray dryer
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preheated at 600◦C for 10h
calcined at
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suspension(15%)
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Fig. 1. Schematic illustration of the preparation process.
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Fig. 2. SEM images of (a) LMNCOS-00; (b) LMNCOS-02; (c) LMNCOS-04; (d) LMNCOS-05; (e) LMNCOS-06; (f) LMNCOS-08.
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Fig. 3. S2P XPS spectra of the LMNCOS-05 sample. The inset graph is the magnified plots of the bonding energy of Sulfur element.
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Fig. 4. FTIR spectra of the as-prepared samples.
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(018) (110) (113)
(107)
(104)
♦
LMNCOS-08
♦
♦
LMNCOS-06
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♦
LMNCOS-05
LMNCOS-04
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Intensity /a.u.
♦⎯Li2SO4
(105)
(101)
(003)
(006) (102)
5
LMNCOS-02
20
30
40
50
2θ/ degree
60
70
80
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LMNCOS-00
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Fig. 5. XRD patterns of synthesized materials (◆, Li2SO4).
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6.
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Fig. 6. XRD pattern of the LMNCOS-05 sample in the expanded scale (2θ=17-55°).
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M
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Fig. 7. TEM mapping images of (a) pristine material; (b) manganese ; (c) cobalt; (d) nickel; (e) oxygen and (f) sulfur elements confirm a uniform distribution of the types of elements after the modification .
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8.
Fig. 8. HRTEM images and SAED images of LMNCOS-05 sample (a) low magnification(200K) TEM bright field image of the modified LMNCOS-05 particles
(b) selected area electron
diffraction (SAED) parrerns corresponding to (a), revealing two types of phases, i. e., a layered triclinic structure and spinel cubic structure; (c) HRTEM(50K) image showing surface regions of theLMNCOS-05 particle composed of Li2SO4 monoclinic structure and layered structures inside; (d) 800K-HRTEM. (e) The EDS signals of the compositional mapping of the first area 32
Page 32 of 37
(corresponding to (d) which labeled as 1 ); (f) The EDS signals of the compositional mapping of
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the second area (corresponding to (d) which labeled as 2 ).
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9.
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Fig. 9. Compare the initial charge/discharge profiles of the pristine(LMNCO) and the modified materials(LMNCOS). The charge/discharge was 20mA g-1C with a 4.8V cut off..
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Fig. 10. Discharge curves of half cells at various big rates after charging at 40mA g-1 to 4.8V for the LMNCOS samples.
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Fig. 11. Comparison of electrochemical impedance spectra (EIS) of LMNCO/LMNCOS.
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12.
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Fig.12. PITT curves of LMNCOS-00/LMNCOS-05 for the initial charge process between 2.0 V and 4.8 V: (a) LMNCOS-00 I vs. t; (b) LMNCOS-05 I vs. t; (c) linear behavior of LMNCOS-00 ln(I) vs. t; (d) linear behavior of LMNCOS-05 ln(I) vs. t at different charge states.
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