Cr-substituted LiCoPO4 core with a conductive carbon layer towards high-voltage lithium-ion batteries

Author’s Accepted Manuscript Cr-Substituted LiCoPO4 Core with a Conductive Carbon Layer towards High-Voltage Lithium-Ion Batteries Yue Wang, Junhong Chen, Jingyi Qiu, Zhongbao Yu, Hai Ming, Meng Li, Songtong Zhang, Yusheng Yang www.elsevier.com/locate/yjssc

PII: DOI: Reference:

S0022-4596(17)30355-9 http://dx.doi.org/10.1016/j.jssc.2017.08.039 YJSSC19929

To appear in: Journal of Solid State Chemistry Received date: 11 August 2017 Revised date: 29 August 2017 Accepted date: 31 August 2017 Cite this article as: Yue Wang, Junhong Chen, Jingyi Qiu, Zhongbao Yu, Hai Ming, Meng Li, Songtong Zhang and Yusheng Yang, Cr-Substituted LiCoPO 4 Core with a Conductive Carbon Layer towards High-Voltage Lithium-Ion B a t t e r i e s , Journal of Solid State Chemistry, http://dx.doi.org/10.1016/j.jssc.2017.08.039 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 galley proof before it is published in its final citable 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.

Cr-Substituted LiCoPO4 Core with a Conductive Carbon Layer towards High-Voltage Lithium-Ion Batteries Yue Wang a,b, Junhong Chen a, Jingyi Qiu b,c*, Zhongbao Yu b,c, Hai Ming b,c*, Meng Li b.c, Songtong Zhang b,c and Yusheng Yang a,b,c a

School of Materials Science and Engineering, University of Science and Technology Beijing,

Beijing 100083, China b

Research Institute of Chemical Defense, Beijing 100191, China

c Beijing key Laboratory of Advanced Chemical Energy Storage Technology and Materials, Research Institute of Chemical Defense, Beijing 100191, China; * Corresponding author E-mail: [email protected]; [email protected].

Abstract Electrical and ionic conductivity are two major limiting factors for LiCoPO4 cathode material. To overcome these shortcomings, a Cr-substituted LiCoPO4 core with a conductive carbon layer cathode material is synthesized using the sol-gel method. The physical chemistry properties of these materials are systematically investigated by using various characterization methods. For instance, the XRD and Rietveld refinement results reveal that Cr successfully substitutes the Co within the LiCoPO4 core to form LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04, 0.06) without changing the olivine structure but exhibits a decrease in the unit cell volume with increasing Cr substitution. SEM and TEM images indicate that Cr substitution does not lead to changes in the basic morphology of LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04, 0.06) material, which is composed of agglomerated nanoparticles with an 8 nm carbon layer on the surface. The EDS and XPS results

confirm that Cr is uniformly distributed on the surface and that the oxidation state of Cr is +3. FTIR spectra indicate that the antisite defect concentration decreases with increasing Cr substitution. Furthermore, Cr substitution significantly improves the electrochemical performances of LiCo1-1.5xCrxPO4/C (x = 0.02, 0.04, 0.06) cathode. Notably, the LiCo0.94Cr0.04PO4/C delivers an initial discharge capacity of 144 mA h g-1 at 0.1C and shows a capacity retention of 71% after 100 cycles between 3.0 and 5.0 V. The CV and EIS results indicate that the polarization is reduced and that the electronic and ionic conductivities are improved by Cr substitution. The good electrochemical performances for Cr-substituted LiCoPO4/C electrodes are attributed to the lower antisite defect concentration, as the reduction of polarization, the improvement of electronic and ion conductivity and the uniform carbon layer. These features will accelerate the commercial application of LiCoPO4 towards the start-art of the high voltage lithium-ion batteries.

Graphical abstract

Cr (III) incorporated into the LiCoPO4 lattice and increased the number of Co-site vacancies, which improved the cyclic performance of LiCoPO4 cathode significantly.

-1

Capacity (mAh g )

150

0.1C 3~5V

120

LiCoPO4/C LiCo0.94Cr0.04PO4/C

90 60

Vacancies

Chromium

30 0 0

20

40

60

80

Cycle number ( N )

100

Key words: Lithium-ion battery; High voltage; Cathode; LiCoPO4; Cr substitution.

1. Introduction Over the past several decades, olivine-structured LiMPO4 (M=Fe, Mn, CO, Ni) has attracted extensive research attention because of its high structural and thermal stabilities [1]. Among these, LiFePO4 cathode has been widely used in energy storage system due to its low cost, environmentally friendly properties, good cyclability and high safety [2]. However, the further application of LiFePO4 has been limited by its low energy density, which is derived from its low operating potential (approximately 3.4 V vs. Li/Li+). Compared to LiFePO4, LiCoPO4 has attracted a great deal of attention because of its high theoretical energy density (800 Wh kg-1) and higher operating potential (approximately 4.8 V vs. Li/Li+) [3, 4]. In theory, LiCoPO4 is a good candidate for use as a high-voltage cathode material in lithium-ion batteries that can meet the critical demand for a higher energy density in PC devices and EV/HEV. Unfortunately, LiCoPO4 is rarely used in practical applications due to its poor electronic (<10-9 S cm-1) and low ionic conductivities [5-7]. Moreover, the severe capacity fading upon cycling is another fatal obstacle that precludes the practical use of LiCoPO4 [8-11]. To solve these critical problems, several methods have been employed and achieved considerable results: (i) decreasing the size of LiCoPO4 particles to shorten the Li+ diffusion distance [12-15]; (ii) coating the particles with carbon [7, 16-20] or stable materials [19, 21-23] to stabilize the interface between the cathode and electrolyte; (iii) cationic doping, partial substitution of Co by V [24, 25], Y [26], Mn [27, 28], Fe [6, 29-32], Mg [33, 34], Ca [33, 35], Cr

[5, 29], Cu [5] and Si [29]; and (iv) adding electrolyte additives [36-39] to form a protective film and stabilize the interface. Among these methods, cation doping was deemed to be an effective way to improve the ionic conductivity and stabilize the structure because it could increase the number of Co-site vacancies [2, 24, 29] and provide an additional pathway for Li-ion diffusion [2, 24, 29], thereby improving the electrochemical performance of a LiCoPO4 cathode. For instance, Karl [24] et al. prepared a V(III) ion-doped LiCoPO4 cathode, which delivered a discharge capacity of 97 mA h g-1 and could keep a capacity of 82 mA h g-1 after 20 cycles at 0.1C. Huanhuan [26] et al. reported that Y3+ doping efficiently improved the cycling performance of LiCoPO4 cathode. Lucangelo [33] et al. prepared LiCo0.9M0.1PO4 (M=Ca, Mg, Co) powders and found that Ca2+ doping could improve the electrochemical performance of the LiCoPO4 cathode. According to the above mentioned reports, the improvements caused by V3+, Y3+, Mg2+ and Ca2+ doping were ascribed to increases in the ionic and electronic conductivity of the LiCoPO4 cathode. Wolfenstine [5] found that Cr3+ and Cu2+ doping enhanced the electronic conductivity of LiCoPO4 from10−9 S cm−1 to 10−4 S cm−1. Jan [29] et al. synthesized Cr, Si-LiCo0.9Fe0.1PO4 and demonstrated a capacity of 140 mA h g-1 at C/3 at an average discharge voltage 4.78 V, noting that Cr substitution could increase the lithium ion diffusivity. However, the effect of Cr substitution on the structure, morphology and electrochemical performance of the LiCoPO4/C cathode material was not systematically studied, although such a study is essential for the industrial application of the LiCoPO4 cathode. In this work, the cathode materials of LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04, 0.06) were synthesized by a typical sol-gel method, and the effects of Cr substitution on its structure and electrochemical performances were systematically investigated. The Cr-substituted LiCoPO4/C

cathode material exhibited a lower antisite defect concentration, smaller polarization and improved ionic and electrical conductivities. Therefore, the LiCo0.94Cr0.04PO4/C cathode delivered an initial discharge capacity of 144 mA h g-1 at 0.1C and showed a capacity retention of 71% after 100 cycles between 3.0 and 5.0 V, which is better than the previously reported values [10, 16, 19, 24, 40] and is an encouraging result for the industrial devolvement of LiCoPO4.

2. Experimental 1.1. Synthesis of LiCo1-1.5xCrxPO4/C material LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04, 0.06) materials were synthesized by a sol-gel method. First, Co(NO3)2.6H2O, Cr(NO3)3.9H2O, LiNO3, NH4H2PO4 and citric acid (CA) were dissolved in deionized water at stoichiometric amounts (nLi : nCo : nCr : nP : nCA =1.05 : 1-1.5x : x : 1 : 2). Then, the solution was heated at 80 °C in a water bath until the wet-gel formed. Subsequently, the wet-gel was dried at 120 °C for 24 h. Then, the obtained dry-gel was moved into a rotary furnace and calcined at 400 °C for 3 h with air atmosphere to eliminate the excess carbon. Finally, the products of LiCo1-1.5xCrxPO4/C material were formed at 700 °C for 2 h with the heating rate of 2 °C min-1 under Ar flow (100 ml min-1). 1.2. Material characterization The crystal information was acquired by X-ray powder diffraction (XRD) using a SmartLab/X-ray diffractometer with Cu Kα radiation (λ = 0.154178 nm). Rietveld refinement was carried out using the TOPAS 5 software. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were performed using a BCPCAS-4800 instrument

(Hitachi) with an acceleration voltage of 15 kV. The core@shell and crystalline structure of LiCo1-1.5xCrxPO4/C samples were analyzed by transmission electron microscopy (TEM) using a Tecnai F20 (200 kV) transmission electron microscope (FEI). The carbon content was measured by an Elemental analyzer (EA, Elementar Vario EL Cube). Fourier transform infrared (FTIR) spectra were collected by an IR spectrometer (PerkinElmer, Spectrum One). X-ray Photoelectron Spectroscopy (XPS) was obtained by using a KRATOS Axis Ultra X-ray photoelectron spectrometer with a monochromatized Mg Kα X-ray (hν = 1283.3 eV). The stoichiometry of as-prepared LiCo1-1.5xCrxPO4/C samples was determined by inductively coupled plasma mass spectrometry (ICP-MS) with a PerkinElmer NexlON 350X instrument. 1.3. Electrochemical performance Electrochemical tests were carried out using a 2025-type coin cell assembled in the glove box filled with pure argon, in which the moisture and oxygen were strictly controlled to less than 0.1 ppm. The half-cell is configured with Li metal (−) | Microporous polypropylene separator (Celgard 2400) | electrode (+) filled with the electrolyte of 1.0 mol L-1 LiPF6 in a mixture of dimethyl carbonate (DMC)/ethylene carbonate (EC) (v/v, 1/1), with a weight ratio of tris(trimethylsilyl) borate additive (TMSB, Sigma) of 1 %. The LiCo1-1.5xCrxPO4/C cathode was prepared by casting a mixture of 75 wt. % active material, 15 wt. % acetylene black and 10 wt. % polyvinylidene fluoride (PVDF) dissolved in N-methyl pyrrolidinone (NMP) on an Al foil. The working cathode was dried in a vacuum oven at 120 °C for 12 h and cut into circular sheets (Ø is 10 mm). Then, these sheets were dried in a vacuum oven at 120 °C for 12 h again. The weight

mass of Li metal was approximately 15 mg, and that of the electrode was 2.1 mg, of which the active material accounted for approximately 1.57 mg. Galvanostatic cycling was conducted by the LAND CT2001A unit at different current densities, and cyclic voltammetry (CV) data were collected by a CHI660D electrochemical workstation under the scan rate of 0.02 mV s-1 and within the voltage windows of 3.0 ~ 5.3 V. Electrochemical impedance spectroscopy (EIS) was measured on a Solartron SI 1260 and SI 1287 for the frequency range from 100 kHz and 10 mHz.

3. Results and discussion

3.1. Structure and composition of LiCo1-1.5xCrxPO4/C Typical XRD patterns of LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04 and 0.06) samples are presented in Fig. 1. It is obvious that the diffraction peaks of all four samples can be indexed to the olivine structure of LiCoPO4 with the orthorhombic Pnma space group (JCPDS: 89-6192), indicating that the samples with Cr substitution are well crystallized. No impurity peaks of the Cr-related composites are observed due to the low substitution concentration. In addition, the crystallite amount is calculated, as shown in Fig.S1. Further, Rietveld refinements were performed to explore the details of the structural differences. Fig. 2 displays the Rietveld refinement results for LiCo1-1.5xCrxPO4/C (x=0.02, 0.04 and 0.06), and the structural parameters are listed in Table 1. Noticeably, the a and b parameters decrease with increasing Cr substitution, ultimately resulting in the decrease of the unit cell volume (284.11 Å3, 283.76 Å3, 283.7 Å3 and 283.58 Å3 for x=0, 0.02, 0.04 and 0.06, respectively). Such a variation suggests that Cr (III) substitutes Co (II) and is

incorporated into the LiCoPO4/C lattice. The ionic radius of Cr (III) (61.5 pm) is smaller than that of Co (II) (74.5 pm), leading to a decrease of the structural parameters. The ICP results for these four samples are listed in Table 2. All of the above results indicate that the cationic Cr(III) successfully substitutes Co(II) within the LiCoPO4 to form the LiCo1-1.5xCrxPO4/C composite, and this substitution does not cause any crystal structure change, except for tiny variations in the elemental composition.

The SEM images of LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04 and 0.06) samples are shown in Fig. 3. The basic morphologies of the four samples are similar and are composed of agglomerated nanoparticles with numerous open pores. The average size of the nanoparticles ranges from 200 to 500 nm, suggesting that the morphologies should not be obviously influenced by Cr substitution. To determine the distribution of Cr within the LiCo1-1.5xCrxPO4/C sample, EDS was performed for the LiCo0.94Cr0.04PO4/C sample. As shown in Fig. 4, the Cr element is uniformly distributed in LiCo0.94Cr0.04PO4/C, and the same result is obtained for the Co, P and O elements.

The surface morphologies of LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04 and 0.06) samples are further analyzed by TEM, and the images are shown in Fig. 5. Only one diffraction fringe with lattice distances of 0.4302 nm, 0.4277 nm, 0.4270 nm and 0.4258 nm is observed in LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04 and 0.06), respectively, matching well with the fringe distance of the (011) diffraction (0.427 nm) plane for LiCoPO4. The weak difference in the diffraction fringe is mainly caused by the Cr substitution, which reduces the fringe distance with increasing Cr substitution, consistent with the refinement results. Meanwhile, an 8 nm thick amorphous

carbon layer can be observed on the side region of the LiCoPO4 particle in the HRTEM images. The results of EA testing confirm the existence of carbon, and the obtained contents are listed in Table 2. The average carbon content within the LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04 and 0.06) is approximately 1.47%, 1.45%, 1.48% and 1.52%, respectively. Such a uniform and continuous carbon layer is beneficial for the electrochemical performance because it could effectively restrain the agglomeration particles [16, 19, 41] and improve the electrical conductivity of LiCoPO4 material [16, 19, 42]. The morphologies of LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04 and 0.06) samples are similar, so the differences in the electrochemical performance of the LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04 and 0.06) cathode are mainly ascribed to the structure difference that caused by Cr (III) substitution.

The FTIR spectra of LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04 and 0.06) samples are presented in Fig. 6. The infrared absorption peaks in the range of 900 to 1200 cm-1 are ascribed to bond stretching of the PO4−3 anions, and the peaks situated at 400 to 700 cm-1 are due to the bending modes [12, 24, 43]. Specifically, the absorption peak at 985 cm-1 belongs to the symmetric stretching υ1 mode, the three absorption peaks in the 1020-1170 cm-1 range are ascribed to the antisymmetric stretching υ3 mode and the four absorption peaks ranging from 400 to 700 cm-1 are attributed to the bending modes υ2 and υ4 [12, 24, 43]. Generally, with increasing Cr substitution, the three υ3 peaks show slight changes in position; however, the υ1 peak position shifts to lower wave numbers by approximately 10 cm-1 from 985 cm-1 (x=0) to 975 cm-1 (x=0.06) . The shift of the υ1 peak position suggests the decrease in the antisite defect concentration [12, 24]. Therefore, it can be concluded that the antisite defect concentration is decreasing with increasing Cr

substitution, implying a reduction of antisite disorder on the Li site.

The XPS spectra of LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04 and 0.06) samples are shown in Fig. 7. From the wide XPS spectra in Fig. 7(a) it can be found that Cr-substituted samples show patterns similar to that of the LiCoPO4/C sample, with the binding energies of Co2p, P2p, O1s and C1s in agreement with the previously reported LiCoPO4 spectra [37]. The Cr2p spectra with the typical spin orbital splitting between 2p1/2 (586.4 eV) and 2p3/2 (576.4 eV) for Cr-substituted samples are shown in Fig. 7(b). The Cr2p spectra confirm the presence of Cr in the LiCo1-1.5xCrxPO4/C (x= 0.02, 0.04 and 0.06) samples and the oxidation state of Cr is +3. Fig. 7(c) shows the Co2p spectra of the four samples, which are in good agreement with the reported binding energy for the Co2+ ion [12, 21]. Furthermore, the binding energy for Li1s in Fig. 7(d) shows a ~0.1 eV downshift from LiCoPO4/C to LiCo0.94Cr0.04PO4/C and a ~0.2 eV downshift for LiCo0.91Cr0.06PO4/C, implying that the Li-O interaction is weakened by the Cr substitution in LiCoPO4/C.

3.2. Electrochemical performance The electrochemical performance of LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04 and 0.06) electrodes in the lithium-ion battery was evaluated in the half-cell test. The CV curves of the first cycle for the four electrodes at the scan rate of 0.02 mV s-1 in the voltage range of 3~5.3 V are presented in Fig. 8. The oxidation peak at approximately 4.3 V is ascribed to the electrolyte oxidation reaction. All four samples exhibit two oxidation peaks and two reduction peaks, corresponding to the two steps of Li+ extraction and intercalation (𝐿𝑖𝐶𝑜𝑃𝑂4 ↔ 𝐿𝑖0.7 𝐶𝑜𝑃𝑂4 ↔ 𝐶𝑜𝑃𝑂4) [9, 44, 45]. Some

differences including the oxidation /reduction reaction potential and the polarization potential can be found in the CV curves and the results are displayed in Table 3. With increasing Cr substitution, the reduction potential increases from 4.685 V (x=0) to 4.748 V (x=0.06), and the polarization potential decreases from 0.255 V (x=0) to 0.152 V (x=0.06). The decrease in the polarization potential suggests that it is easier for Li+ to insert and extract from the Cr-substituted samples than from the unsubstituted sample. The highest peak current density and lowest polarization potential observed in LiCo0.91Cr0.06PO4 indicate the fastest kinetics in four samples. These CV results indicate that the polarization of LiCoPO4 cathode is decreased and that the lithium-ion diffusion coefficient is improved through Cr substitution. The increase in lithium-ion diffusion coefficient could be ascribed to increased Co−O bond hybridization and to reduced Li-O bond in the Cr-substituted samples. Additionally, the reduction peak at ~4.78 V is strengthened with increasing Cr substitution. The change may indicate that the two steps behavior of Li+ extraction and intercalation from LiCoPO4 is reinforced by Cr substitution.

The first three and the 100th charge/discharge curves of LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04 and 0.06) cathodes between 3 and 5 V at 0.1 C (1 C=170 mA h g-1) are displayed in Fig. 9. All four samples exhibit two charge plateaus at approximately 4.8~4.9 V and two discharge plateaus at approximately 4.7~4.8 V, corresponding to the two-step reaction of Li+ extraction and intercalation, which is consistent with the CV results. At the first charge/discharge process, all four samples exhibit an oxidation plateau at approximately 4.3 V and a long charge process until 4.8 V (ascribed to the electrolyte oxidation reaction). This side reaction process is the main reason for the low initial coulombic efficiency. It is noteworthy that at the 2nd and 3rd charge/discharge cycles,

this phenomenon (side reaction process) disappears because the steady SEI film that formed in the first cycle prevents the continuous oxidation of the electrolyte. The initial discharge capacities of LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04 and 0.06) cathodes are nearly the same, at 143, 142, 144 and 146 mA h g-1, respectively, However, with an increasing cycle number, the difference in the capacity retention can be observed. After 100 cycles, discharge capacities of 70, 89, 102 and 95 mA h g-1, respectively, were obtained. Clearly, the cycle performance is enhanced with Cr substitution.

The cyclic and rate performances of LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04 and 0.06) cathode between 3.0 and 5 V are presented in Fig. 10. Fig. 10(a) shows the cyclic performance of four cathodes at 0.1 C. It is obvious that with Cr substitution, the cyclic performance is improved and LiCo0.94Cr0.04PO4/C cathode shows the best performance. After 100 cycles, approximately 71% of the initial discharge capacity can be retained for the LiCo0.94Cr0.04PO4/C cathode, but this value is only 49% for the unsubstituted LiCoPO4/C cathode. The severe capacity fading is mainly attributed to the continuous decomposition of electrolyte [8, 11] and the antisite defects caused by the exchange between lithium and cobalt atoms during the cycling process [10]. Cr-substituted cathodes have lower antisite defect concentration, so the cycling performance is improved. Table 4 lists the cycling performance comparison of LiCo0.94Cr0.04PO4/C cathode with others reported. It is easy to find that the Cr substitution significantly improve the cycle stability of the LiCoPO4 cathode. For the rate performance, as shown in Fig. 10(b), as expected, the LiCo0.94Cr0.04PO4/C sample also presents the best rate capability and delivers the average discharge capacities of 136, 122, 107 and 91 mA h g-1 at 0.1C, 0.2C, 0.5C and 1C rates, respectively. Finally, when the

discharge rate returns to 0.1C, the discharge capacity recovers to 126 mA h g-1, suggesting the good stability of the electrode. In contrast, for the unsubstituted LiCoPO4/C cathode, the corresponding average capacities are 120, 93, 73 and 56 mA h g-1 at the 0.1C, 0.2C, 0.5C and 1C rates, respectively. The improvement in the rate capacity is due to the enhancement of the ion conductivity resulting from the Cr substitution. A similar improvement in the rate capacity of the Cr-doped LiFePO4/C was reported by Shin et al.[46], which the authors ascribed to the facilitated phase transformation caused by Cr doping. Table 5 lists the rate performance comparison of LiCo1-1.5xCrxPO4/C (x=0 and 0.04) cathode with others reported. It is easy to find that the Cr substitution significantly improve the rate performance of the LiCoPO4/C cathode. But compares with other synthesis methods (like Solvothermal route [47]), the rate performance for LiCo0.94Cr0.04PO4/C cathode synthesis with sol-gel method is little lower for its submicron particles size.

EIS spectra of LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04 and 0.06) cathodes before cycling and the corresponding equivalent circuits are presented in Fig. 11. All four EIS profiles display a semicircle in the high-middle frequency region and a straight line in the low-frequency region. Generally [16, 19], Re represents the resistance through the electrolyte; Rct is related to the cathode/electrolyte interface resistance; and Wo is associated with Li+ diffusion through LiCo1-1.5xCrxPO4/C. The simulation result is presented in Table 6. It is observed that with Cr substitution, there is no obvious difference in Re among the four samples (1.6 Ω, 1.7 Ω, 1.5 Ω and 1.7 Ω for x=0, 0.02, 0.04 and 0.06, respectively) due to their similar basic morphology. However, Rct is obviously decreased (121.6 Ω, 77.9 Ω, 68.1 Ω and 32.3 Ω for x=0, 0.02, 0.04 and 0.06,

respectively), which means that the electronic conductivity is improved by the Cr substitution. The Li+ diffusion coefficient is calculated as [16, 19] 𝐷𝐿𝑖 + =

𝑅2 𝑇 2 2 2 2𝐴2 𝑛4 𝐹 4 𝐶𝐿𝑖 + 𝜎

R: gas constant (8.314 Jmol-1K-1). T: absolute temperature (298K). A: cathode surface area (0.78 cm2). n: the number of electrons per molecule during oxidization (n=1). F: the Faraday constant (96 486 C mol-1). C: the concentration of Li+ and σ: Warburg factor and is related to Z’ according to 𝑍 ′ = 𝑅𝑠 + 𝑅𝑓 + 𝑅𝑐𝑡 𝜎 𝜔 −1/2 ω: angular frequency. σ: the Warburg factor. Fig. 11(b) presents the linear relationship between Z’ and ω-1/2 in the four samples. The σ and D Li+ results are also displayed in Table 6. With increasing Cr substitution, the D Li+ is improved and LiCo0.94Cr0.06PO4/C shows the largest value of 3.59x10-14 cm2 s-1, whereas D Li+ of the unsubstituted LiCoPO4/C cathode is only 8.53x10-16 cm2 s-1. The EIS results indicate that electronic and ion conductivities are improved by Cr substitution, which is an intrinsic but a competitive change for LiCoPO4/C. This improvement is mainly due to the decreasing of the antisite defect concentration on the Li-site and the increasing of the Co-site vacancy that provides an additional pathway for Li+ diffusion [24, 29]. The good electrochemical performance of LiCo1-1.5xCrxPO4/C cathode can be mainly attributed to its enhanced internal electronic and ion conductivities through doping. The as-prepared LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04 and 0.06) composites are well crystallized and consist of agglomerates of nanoparticles with open pores that provide numerous pathways for Li-ion diffusion. The uniform carbon layer improves the electrical conductivity of LiCoPO4

cathode and stabilizes the interface between the cathode and electrolyte. Cr substitution is adopted to improve the internal electronic and ion conductivities. All of the factors mentioned above enable the significantly improved electrochemical performance of the LiCo0.94Cr0.04PO4/C cathode material.

4. Conclusions Cr-substituted LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04 and 0.06) cathode materials are synthesized and the effects of Cr substitution on the structure and electrochemical performances are systematically investigated. Cr substitution obviously reduces the polarization and enhances the electronic and ion conductivities of the LiCoO4/C cathode material. As a cathode material for LIBs, the cyclic and rate performances of LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04 and 0.06) cathode are significantly improved. Notably, the LiCo0.94Cr0.04PO4/C cathode delivers the initial discharge capacity of 144 mA h g-1, with a capacity retention of 71% after 100 cycles at 0.1C between 3.0 and 5.0 V and shows an average capacity of 91 mA h g-1 at 1 C rate. In addition to the uniform size distribution with uniform carbon coating, the lower antisite defect concentration, the reduction of polarization and the improvement of electronic and ion conductivity that benefits from the Cr substitution can ensure a superior electrochemical performance for the LiCoPO4. Such a concept would be highly influential in energy storage, environmental and materials science due to the practical application and easy extension of the synthetic strategy, novel structure and intriguing properties of LiCo1-1.5xCrxPO4/C, especially for the concept of using high-voltage cathode in lithium-ion batteries.

Acknowledgements

Tianci is gratefully acknowledged for supplying the electrolyte. The authors would like to thank Dr Li Bin for performing Rietveld refinement.

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Figure captions Fig.1 XRD patterns for LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04 and 0.06) samples. Fig.2 Refinement results for LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04 and 0.06) samples. Fig.3 SEM images of LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04 and 0.06) samples. Fig.4 EDS images of LiCo0.94Cr0.04PO4/C sample. Fig.5 TEM images of LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04 and 0.06) samples. Fig.6 FTIR spectra of LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04 and 0.06) samples. Fig.7 XPS spectrum of LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04and 0.06) samples. Fig.8 CV curves of LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04 and 0.06) cathodes. Fig.9 The first three cycles and the 100th cycle charge/discharge curves of

LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04 and 0.06) cathodes. Fig.10 Cyclic (a) and rate (b) performances of LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04 and 0.06) cathodes. Fig.11 EIS spectra of LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04 and 0.06) cathodes (a) and the linear relationship between Z’ and ω-1/2 in four cathodes (b).

Table 1 Structural parameters of LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04 and 0.06) samples. samples

a (Å)

b (Å)

c (Å)

V (Å3)

LiCoPO4 (89-6192)

10.2021

5.9227

4.7003

284.01

LiCoPO4

10.2031

5.9237

4.7007

LiCo0.97Cr0.02PO4

10.1969

5.9221

LiCo0.94Cr0.04PO4

10.1947

LiCo0.91Cr0.06PO4

10.1929

Rwp

Rp

GOF

284.11

4.03

2.84

1.84

4.6991

283.76

3.89

2.79

1.75

5.9218

4.6992

283.70

3.69

2.72

1.74

5.9206

4.6991

283.58

3.68

2.72

1.71

Table 2 ICP and EA results of LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04 and 0.06) samples. ICP results

EA results

samples Li/P

Co/P

LiCoPO4

1.03

0.98

LiCo0.97Cr0.02PO4

1.02

0.96

LiCo0.94Cr0.04PO4

1.02

LiCo0.91Cr0.06PO4

1.01

Cr/P

C (%)

H (%)

N (%)

1.47

0.16

0.25

0.016

1.45

0.17

0.26

0.93

0.035

1.48

0.18

0.27

0.91

0.053

1.52

0.17

0.26

Table 3 The oxidation potential, reduction potential and polarization potential for LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04 and 0.06) samples. Oxidation potential

Reduction potential

Polarization potential

(V)

(V)

(V)

Samples

LiCoPO4/C

4.94

4.82

4.71

4.685

0.255

LiCo0.97Cr0.02PO4/C

4.90

4.79

4.78

4.724

0.176

LiCo0.94Cr0.04PO4/C

4.90

4.79

4.78

4.731

0.169

LiCo0.91Cr0.06PO4/C

4.90

4.80

4.768

4.748

0.152

Table 4 The comparison of cycling performance for LiCo1-1.5xCrxPO4/C (x=0.04) cathode with others reported.

Initial discharge Sample

Capacity

Rate -1

capacity (mAh g )

retention (%)

Cycles

Method

Our work

0.1C

144

71

100

Cr-Substituted

Ref. [24]

0.1C

97

85

20

V-Substituted

Ref. [26]

0.1C

153

21

30

Y-Substituted

Ref. [28]

0.1C

126

88

30

Mn-Substituted

Ref. [34]

0.1C

88

22

20

Mg-Substituted

Ref. [35]

0.1C

104

38

10

Ca-Substituted

Table 5 The comparison of rate performance for LiCo1-1.5xCrxPO4/C (x=0.04) cathodes with others reported. Discharge capacity Sample

Rate

Particles

Method

-1

(mAh g ) 1C

91

Sol-gel with Cr- Substituted

1C

56

Ref. [13]

1C

72

Nano crystals

Supercritical fluid process

Ref. [16]

1C

125

Nano particles

Hydrothermal process

Ref. [20]

1.7C

76

Nano particles

Deposition technique

Ref. [12]

2C

84.9

Submicron crystals

Solvothermal method

Ref. [47]

2C

120

Nano particles

Solvothermal route

Our work

Submicron particles Sol-gel without substitution

Table 6 Impedance parameters of LiCo1-1.5xCrxPO4/C (x=0, 0.02, 0.04 and 0.06) cathodes. Samples

Re (Ω)

Rct (Ω)

σ

D Li+(cm2 s-1)

LiCoPO4

1.6

121.6

357

8.53x10-16

LiCo0.97Cr0.02PO4

1.7

77.9

114

8.36x10-15

LiCo0.94Cr0.04PO4

1.5

68.1

90

1.34x10-14

LiCo0.91Cr0.06PO4

1.7

32.3

55

3.59x10-14

Fig.1

Intensity (A.U.)

LiCo0.91Cr0.06PO4/C LiCo0.94Cr0.04PO4/C LiCo0.97Cr0.02PO4/C

LiCoPO4/C

20

30

40

50

60

70

(a)

10

20

30

40

50

2 (degree)

60

70

10

Intensity (A.U.)

Obs Calc Diff

30

40

50

2 (degree)

60

Obs Calc Diff

20

30

40

50

2 (degree)

(d)

LiCo0.94Cr0.04PO4/C

20

LiCo0.97Cr0.02PO4/C

Intensity (A.U.)

Obs Calc Diff

(c)

10

(b)

LiCoPO4/C

70

10

Fig.3

60

70

LiCo0.91Cr0.06PO4/C Obs Calc Diff

Intensity (A.U.)

Intensity (A.U.)

Fig.2

20

30

40

50

2 (degree)

60

70

Fig.4

Fig.5 Carbon layer

(011) 0.4302 nm (011) 0.4277 nm

(011) 0.4270 nm

(011) 0.4258 nm

Absorbance (a.u.)

Fig.6 474 552 646 522 580

450

975

LiCo0.91Cr0.06PO4/C

LiCo0.94Cr0.04PO4/C

1062 1105 1149

980

982

LiCo0.97Cr0.02PO4/C LiCoPO4/C

600

750

900

985

1050 -1

Wavenumber (cm )

1200

Fig.7

LiCo0.97Cr0.02PO4/C

800

600

Li1s

C1s

400

P2p P2s

LiCoPO4/C

LiCo0.94Cr0.04PO4/C

LiCo0.97Cr0.02PO4/C

200

590

LiCo0.97Cr0.02PO4/C

810

2p1/2

800

2p3/2 sat

790

LiCoPO4/C

780

Binding energy (eV)

770

66

585

580

575

570

Binding energy (eV)

Li 1s

Intensity (a.u.)

Intensity (a.u.)

LiCo0.94Cr0.04PO4/C

sat

(d)

LiCo0.91Cr0.06PO4/C

Co 2p

2p3/2

2p1/2

Binding energy (eV)

(c)

LiCo0.91Cr0.06PO4/C

Cr 2p

Intensity (a.u.)

Cr2p Cr2p

LiCo0.94Cr0.04PO4/C

Cr2p

1000

(b)

LiCo0.91Cr0.06PO4/C

O1s

Co2p

OKLL

Intensity (a.u.)

(a)

0.2eV

LiCo0.91Cr0.06PO4/C LiCo0.94Cr0.04PO4/C

0.1eV

LiCo0.97Cr0.02PO4/C LiCoPO4/C

64

62

60

58

56

54

Binding energy (eV)

52

Fig.8 LiCoPO4/C

0.00

-1

Side Reaction

-0.02

-1

Current (A g )

0.04 0.02

0.255V

4.2

4.5

4.8

5.1

Potential (V)

Side Reaction

-0.02 0.169V

4.2

4.5

4.8

5.1

Potential (V)

5.4

LiCo0.97Cr0.02PO4/C

0.00 Side Reaction

-0.02

0.04

LiCo0.94Cr0.04PO4/C

(b)

0.176V

4.2

4.5

4.8

5.1

5.4

Potential (V)

(c)

0.00

-0.04

0.02

-0.04

5.4

-1

-0.04

Current (A g )

0.02

0.04

(a)

Current (A g )

-1

Current (A g )

0.04

0.02

(d) LiCo0.91Cr0.06PO4/C

0.00 Side Reaction

-0.02 -0.04

0.152V

4.2

4.5

4.8

5.1

Potential (V)

5.4

Fig.9 5.0

4.5

(a)

4.0

LiCoPO4/C

Side Reaction

1st cycle 2nd cycle 3rd cycle 100th cycle

3.5 3.0

0

50

Voltage ( V )

Voltage ( V )

5.0

100

150

200

-1

250

4.5 4.0

0

5.0

100

150

200

-1

250

300

5.0

(c)

4.5

LiCo0.96Cr0.04PO4/C

4.0

1st cycle 2nd cycle 3rd cycle 100th cycle

Side Reaction

3.5

0

50

100

150

200

250

-1

Capacity (mAh g )

300

Voltage ( V )

Voltage ( V )

50

Capacity (mAh g )

Capacity (mAh g )

3.0

LiCo0.98Cr0.02PO4/C 1st cycle 2nd cycle 3rd cycle 100th cycle

Side Reaction

3.5 3.0

300

(b)

(d)

4.5

LiCo0.91Cr0.06PO4/C

4.0

Side Reaction

1st cycle 2nd cycle 3rd cycle 100th cycle

3.5 3.0

0

50

100

150

200

-1

250

Capacity (mAh g )

300

Fig.10 0.1C 3~5V

120 90

(a)

60

LiCoPO4/C

30

LiCo0.94Cr0.04PO4/C

LiCo0.97Cr0.02PO4/C

0 0

150

LiCo0.91Cr0.06PO4/C

20

40

60

80

Cycle number ( N )

0.1C 0.2C 0.5C 1C

100

0.1C

-1

Capacity (mAh g )

-1

Capacity (mAh g )

150

120 90

(b)

60

LiCoPO4/C

30

LiCo0.94Cr0.04PO4/C

LiCo0.97Cr0.02PO4/C LiCo0.91Cr0.06PO4/C

0

10

20

30

40

Cycle number ( N )

50

Fig.11 500

(a) LiCoPO4/C

400

LiCo0.97Cr0.02PO4/C

-Z" (ohm)

LiCo0.94Cr0.04PO4/C LiCo0.91Cr0.06PO4/C

300 200

CPE

Re

100 W

0 0

Rct

100

200

300

Wo

400

500

0.8

0.9

Z' (ohm) 400

(b) LiCoPO4/C

-Z" (ohm)

300

LiCo0.97Cr0.02PO4/C LiCo0.94Cr0.04PO4/C LiCo0.91Cr0.06PO4/C

200

100

0 0.4

0.5

0.6 -1/2



0.7 -1

(rad s )

Highlights 1.

Novel composite of LiCo1-1.5xCrxPO4/C was prepared.

2.

The composite and structure of LiCo1-1.5xCrxPO4/C were investigated.

3.

Cr-substituted into LiCoPO4/C does not change its morphology.

4.

Cr-substitution improved electrochemical performance of LiCoPO4 significantly.

5.

The relation between the amounts of Cr and performance of was LiCoPO4 discussed.