Modifying High-Voltage Olivine-Type LiMnPO4 Cathode via Mg

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Modifying High-Voltage Olivine-Type LiMnPO Cathode via Mg-Substitution in High-Orientation Crystal Liguang Wang, Han Zhang, Qi Liu, Jiajun Wang, Yang Ren, Xiaoyi Zhang, Geping Yin, Jun Wang, and Pengjian Zuo ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00923 • Publication Date (Web): 01 Oct 2018 Downloaded from http://pubs.acs.org on October 2, 2018

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ACS Applied Energy Materials

Modifying High-Voltage Olivine-Type LiMnPO4 Cathode via Mg-Substitution in High-Orientation Crystal Liguang Wang,a,b* Han Zhang,a Qi Liu,c Jiajun Wang,b Yang Ren,d Xiaoyi Zhang,d Geping Yin,a Jun Wang,b* Pengjian Zuo*a

a MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China Email: [email protected]; [email protected] b National Synchrotron Light Source II, Brookhaven National Laboratory, Building 743 Ring Road, Upton, NY 11973, USA Email: [email protected] c Department of Physics, City University of Hong Kong, 83 Tat Chee Ave, Kowloon Tong, Hong Kong, China d X-ray Science Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA

Abstract The LiMnPO4 material with Mg2+ ion substitution in transition metal site is successfully obtained by a facile solvothermal method combining with the subsequent spray drying and pyrolysis. The TEM image shows the controlled crystalline orientation of the primary Mg-doped LiMnPO4 particle, and the three-dimensional (3D) hierarchical micro-nano structure and the electronic

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structure of the Mg-doped material are investigated by the reconstructed nano-tomography and X-ray absorption spectroscopy, respectively. The uniformity of Mg2+ ions substitution at the secondary particle level is confirmed by the transmission X-ray microscopy combined with Xray absorption near edge structure. The prepared material exhibits an excellent electrochemical performance with the capacity of 156 mAh/g at the rate of 0.1 C, and good capacity retention of 100 % after 400 cycles at the rate of 1 C. The in operando synchrotron X-ray diffraction data indicates that the performance improvement of the Mg-doped materials is attributed to the changed electrochemical reaction mechanism from the typical two-phase reaction for pristine LiMnPO4 material to the two-phase reaction combined with the single-phase (solid-solution) reaction process. These findings provide a new approach and understanding on developing highenergy cathode materials for lithium-ion batteries.

Keywords: high-voltage cathode; olivine-type structure; Mg-substitution; high-orientation; lithium-ion battery.

Introduction

As the growing demands of portable electronics and electric vehicles, developing highperformance lithium-ion batteries (LIBs) with high energy density, long cycle life and safety is extremely urgent. Among the cathode materials, the olivine-type LiFePO4 is one of the most promising materials for LIBs due to its high theoretical capacity (170 mAh/g), high stability, low cost, and environmentally friendly.1-3 Therefore, the LiFePO4 cathode has been widely studied in the past two decades, exhibiting an excellent electrochemical performance, especially the rate


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performance and long cycling stability.4-6 However, the low energy density mainly related to the operating voltage of 3.3 V vs. Li+/Li of LiFePO4 impedes its further commercial applications. The isostructural LiMnPO4 (LMP) with the similar theoretical capacity to LiFePO4 has attracted much attention due to its high working voltage of 4.1 V vs. Li+/Li (corresponding to a high energy density of ~700 Wh/kg) within the electrochemical window of carbonate ester-based electrolyte.3, 7-8 However, LMP suffers from more serious intrinsic slow kinetics and the lower electronic conductivity than those of LiFePO4.9

Many efforts have been made to mitigate the problems in LMP by a solid solution substitution of Mn atoms by various cations (Fe, Co, Ni, et al.).10-13 Among these substitution cations, Fe2+ ion doping exhibits the most promising effectiveness in enhancing the electrochemical performance of LMP to a large extent. Previously, we have investigated several solid-solution phases of LiMn1-xFexPO4 (x = 0.1, and 0.2) prepared by the modified solvothermal process.14-16 Our results show the accessibility of Fe2+ on the Mn2+ site in the LiMnPO4 material, and these solid-solution phases can achieve excellent electrochemical properties, especially the rate and long-cycling performance. Recently, Chiang et al. investigated the correlation between the transformation strain of various compositions and the special capacity at high C-rates (20 C, 35 C, 50 C), illustrating that the selection criterion of the proportion of the LiMn1-xFexPO4 solid solutions for minimizing transformation strain during lithiation and de-lithiation processes.17 More recently, Park et al. revealed the effects of the multi-transition-metal electronic structure in the solid-solution LiMn1-xFexPO4 (0 < x < 1) phase on the electrochemical performance.18 They believe that the alleviated Jahn-Teller behavior of Mn3+ in LiMn1-xFexPO4 after lithium ions extraction is essential to achieve good electrochemical performance. Among the isovalent cations,



Mg2+ was proved as a promising substitution cation to improve the reaction kinetics of LiMnPO4 without energy density loss.19-20 Even though many efforts have been made for understanding the mechanisms of the performance improvement in multi-transition-metal solid solution olivinetype phase, the specific mechanisms of electrochemical inactive Mg2+ ion substitution on the Mn site in the LMP material is still unclear till now.21-23

Therefore, we focus on the Mg2+ substitution of LMP to facilitate the reaction kinetics and stabilize the crystal structure for enhancing the electrochemical performance. In this work, the primary Mg-doping LMP (Mg-LMP) nanoparticles are orientation-controlled by a facile oleic acid-assisted solvothermal method. The crystal and electronic structure of the prepared materials are characterized by synchrotron X-ray diffraction (SXRD), selected area electron diffraction (SAED), and X-ray absorption spectroscopy (XAS), respectively. The uniformity of Mg-doping was investigated by mapping the two-dimensional phases at the particle level using the transmission X-ray microscopy (TXM) combined with X-ray absorption near edge structure (XANES) technique (TXM-XANES). The structure-evolution mechanism of Mg-substituted LMP during charge-discharge process was revealed using the in operando SXRD.

Experimental Section

Preparation of LMP and Mg-LMP samples Mg-LMP sample was prepared with stoichiometric amounts of LiOH･H2O, H3PO4, MnSO4･H2O, and MgSO4･H2O (3:1:0.96:0.04) in ethanol-water solvent (V(H2O): V(ethanol) = 1:1), using the oleic acid as the additive to control the LMP crystal growing orientation under 200℃ for 12 h in


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the 2 L stainless autoclave. The obtained material was mixed with polyacene (1:0.1, at the ratio of weight) through the ultrasonic dispersion in the mixed solvent of water and ethanol (V(ethanol): V(H2O) = 9:1). Then the spherical Mg-LMP was obtained through a spray drying method. Finally, the composite was annealed at 650℃ for 5 hours under a high purity Ar atmosphere. The pristine LMP sample was prepared in the similar process.

Characterization The SXRD data were collected at beamline 11-ID-C, Advanced Photon Source (APS), Argonne National Laboratory (ANL), using X-ray wavelength of 0.2251 Å. The cell parameters (a, b, c) and the cell volume data of the prepared materials were obtained by the Rietveld refinement method performed with GSAS-II software package. The XANES and extended X-ray absorption fine structure (EXAFS) data were collected at beamline 12-BM-B, APS, ANL. The SEM images were taken using field-emission scanning electron microscope (FESEM, FEI Helios Nanolab600i). The TEM images were recorded using the FEI Tecnai G2 F30 transmission electron microscope. The three-dimensional morphology TXM images were collected at beamline 8-BM-B of APS, ANL, as a transition program of National Synchrotron Light Source II (NSLS II).24 Electrochemical tests were performed on CR2025-type coin cells in two electrodes system by a battery testing system (Neware, BST-5V10 mA). The lithium foil is the counter and also can be treated as a quasi reference electrode. The electrodes consist of 80 wt% active materials, 10 wt% carbon black, and 10 wt% polyvinylidene fluoride (PVDF) binder. The loading of the active material (LMP and Mg-LMP) on each electrode for electrochemical (capacity, rate and cycling) performance tests is ~ 2 mg cm-2. Coin cells were assembled in an argon-filled glovebox with both moisture and oxygen levels less than 0.5 ppm using the 1 M



LiPF6 in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) with 1:1 volume ratio as the electrolyte. All the cells used in this research were tested at room temperature.

In situ XRD In situ synchrotron X-ray diffraction experiments were performed at beamline 11-ID-D, APS, ANL, using a traditional 2032-type coin cell with 3 mm diameters Kapton windows. The used Xray wavelength was 0.799898 Å. 50 wt% active materials, 40 wt% carbon black, and 10 wt% PTFE binder were pelleted as a diameter 10 mm electrode. The diffraction patterns were collected at each state of charge, with 3 s exposure time for each pattern. After recorded each set of data, the sample was allowed to rest for 600 s to minimize any negative impact by the X-ray beam.25

Results and Discussion

Figure 1 shows the refinement results of the XRD data for the prepared LMP and Mg-LMP materials. All the peaks of both materials can be clearly indexed to the orthorhombic space group Pnma (JCPDS no. 74-0375), indicating the perfect olivine-type crystalline structure. There is no obvious difference after Mg2+ ion substitution except the diffraction peaks slightly shift to the high angle. To further understand the effects of Mg-doping, we refined both diffraction patterns for comparing and listed the cell parameters in Table 1. All the cell parameters (a, b, c) and the cell volume reduced after Mg2+ ions doping, indicating the successful substitution of Mn2+ ions by Mg2+ ions in the LMP structure. The volume of Mg-LMP decreases after Mg2+ ions doping due to the small ionic radium of Mg2+ (6.6 Å) in comparison with that of Mn2+ (9.7 Å),


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indicating the reduce average M-O (M = Mn and Mg) bond length after Mg substitution. Therefore, the high crystallinity of the Mg-LMP material shows that all the Mg2+ ions occupy the transition metal (Mn) or lithium site in olivine-type LMP structure.19

To further observe the substitution of Mg2+ ions in the LMP structure, we performed the Xray absorption spectroscopy experiments at Mn K-edge on the prepared materials as shown in Figure 2. The XANES spectra of the two samples show very similar shape with the increasing intensity of the peak at ~6551 eV (“white line”). However, the Mn K-edge absorption peak of Mg-LMP shows nearly no shifts (6552.5 eV) compared with LMP, illustrating an oxidation state of +2.18 The peak at around 6540.5 eV is attributed to the transition from Mn 1s to 3d. The feature of this weak peak indicates that the Mn element in both materials only has an oxidation state of +2. Moreover, the intensity of the shoulder peak at ca. 6558.6 eV seems identical in MgLMP and pristine LMP, corresponding to the non-changing defective structure.26 The Fourier transform magnitude of the Mn K-edge EXAFS spectra in both materials is shown in Figure 2b. In the olivine-type LMP materials, the first and second coordination shells are dominated by oxygen and phosphorus, respectively. The Mn K-edge EXAFS data (Figure 2b) have been quantitatively analyzed by fitting the first two coordination shells with the olivine-structured LMP. The first coordination shell, centered at ca. 2.0 Å, corresponds to the length of the Mn-O bond. The peak at around 3 Å represents the second coordination shell, illustrating Mn-P distance.26 All these features agree well with the lithiophilite structure. The fitting results around Mn atoms in both materials are listed in Table 2. The length of the Mn-O bonds is reduced and the length of Mn-P bonds is increased after Mg2+ ion doping, indicating the Mg2+ ions successfully substituted Mn2+ due to the smaller radius of Mg2+ in comparison with Mn2+. The



shortened length of Mn-O and increased length of Mn-P can stabilize the crystal structure of LMP material, resulting in the enhanced electrochemical performance.27 All the results obtained by the quantitative analysis of EXAFS curves are in good agreement with the refinement XRD data.

The morphology of the secondary particles and the primary crystal structure of the prepared LMP and Mg-LMP were analyzed by SEM and TEM as shown in Figure 3. The morphology of the prepared materials was controlled by the spray pyrolysis method. The secondary particles of both materials are as sphere, with the average particle size of ca. 6 µm. The primary plates with the size of around 80 nm as shown in Figure 3c, e were well controlled growing along with the orientation of (010) using the oleic acid as the additive at the solvothermal process. The small nano-sized primary particles shorten the length of lithium ions diffusion in the material, which will improve the battery performance to some extent. The SAED results of the prepared materials (Figure 3d, f) illustrate the high crystallinity of the prepared primary plates through the solvothermal method and confirm the crystal growing orientation (010) of primary particles. The high orientation (010) of the crystalline is beneficial to enhance the electrochemical performance, especially the rate performance because the lithium ion migration in the olivine-structured phosphate lattice is uniaxial along with the (010) or b-direction. There are no other diffraction patterns in the SAED results of Mg-LMP, indicating the high purity of the material after Mg2+ ions doping, which is well consistent with the XRD results. The schematic fine crystal structure of the prepared materials is also illustrated in Figure 3g. Furthermore, the 3D morphology of the secondary Mg-LMP particles was observed by the nano-tomography experiment (Figure 4). In the secondary particles as shown in Figure 4a, the red region corresponds to the primary Mg-


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LMP particles. It can be clearly seen that there are many nano-sized pores (black color) between these primary particles. To estimate the average surface area and the size distribution of the pores in the samples, the Brunauer-Emmett-Teller (BET) experiments were performed for the prepared materials (Figure S1). The LMP and Mg-LMP materials exhibit relatively high surface area of 91 and 94 m2/g, respectively. The nanoporous structure (micropores and mesopores) can provide large amount of solid-liquid reaction interfaces for electrochemical reactions. Figure 4b shows the 3D surface rendering nano-tomography of the Mg-LMP secondary particles, which is in good agreement with the SEM images. The Mg-LMP secondary particles show a smooth uniform geometry, which is beneficial to improve the contact with electrolyte combined with nanoporous structure within the particles.28

To further understanding the spatial distribution of Mg2+ ion doping, we performed TXMXANES 2D imaging on the Mg-LMP secondary sphere particles (Figure 4c).25 The Mgsubstitution in the material would increase the intensity of the (‘white line’) peak at Mn K-edge XANES spectra as shown in the X-ray absorption results (Figure 2a). To observe the uniformity of the Mg2+ ion doping over the whole secondary particles, five random areas along with the red arrow across the secondary particle were selected to analyze the spatial distribution of Mg2+ ion doping by the Mn K-edge XANES spectra. The average XANES spectra of the five areas were extracted using the program developed in-house (beamline X8C group, National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (BNL)).29-31 The absorption edge of the five average spectra is nearly the same, which means the invariability of the valence of Mn element in the secondary particle level. However, the intensity of the ‘white line’ peak of the five average XANES spectra are higher than that of LMP (Figure 4d), indicating the Mg2+ ion doping



in all the five random areas. Therefore, based on the TXM-XANES results, the Mg2+ ions have homogenously substituted in the Mn2+ sites of the olivine-type structure of LMP, which will be beneficial to the electrochemical properties of Mg-LMP cathode materials during the lithiationdelithiation processes.

The electrochemical performance of the prepared materials was evaluated in half-cells using lithium metal as the reference electrode, as shown in Figure 5. Figure 5 a and b show the initial charge-discharge profiles of the two materials at the various charge and discharge rates, respectively. The discharge capacity of Mg-LMP (156 mAh/g) at the rate of 0.1 C exhibits an obvious improvement compared with that of LMP (149 mAh/g). In addition to the discharge capacity, the rate performance is also enhanced significantly after Mg2+ ion doping. The MgLMP delivers as high as 101 mAh/g (59% of theoretical capacity) and 75 mAh/g (44% of theoretical capacity) at the high discharge rate of 5 C (12 min discharging) and 20 C (3 min discharging), which are higher than that those of LMP, delivering 81 mAh/g (47% of theoretical capacity) and 52 mAh/g (30% of theoretical capacity), respectively. Furthermore, the voltage polarization during the charge-discharge processes at the rate of 0.1 C reduces dramatically from 188 mV for LMP to 155 mV after Mg-LMP, which indicates the fast diffusion kinetics of lithium ions within the doped material. The enhanced capacity properties can be attributed to the increasing defects in the material after Mg2+ ion doping as proved above and the fast lithium ion diffusion kinetics in the materials.

The rate performance of the Mg-LMP improves significantly at different rates compared with that of LMP (Figure 5c). When the charge-discharge current back to a low rate (0.1 C) again, the


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discharge capacity of Mg-LMP can go back nearly to the initial discharge capacity, exhibiting excellent rate performance. However, the discharge capacity of LMP only remains 110 mAh/g (73.8% of the initial capacity) when the charge-discharge current goes back to 0.1 C. Mg2+ ion doping also dramatically enhances the long-cycling performance. Although both electrodes show the similar coulombic efficiency (Figure S2), the capacity retention increases from 62.7 % for LMP to 100 % for Mg-LMP after 400 cycles at the rate of 1 C. The reason for the significant improvements of the rate and cycling performance after Mg2+ ion doping is the shortened length of the Mn-O bonds and the stabilized structure after Mg2+ ion doping as proved by the synchrotron-based XRD, XANES and EXAFS results.

A typical two-phase reaction mechanism during the lithiation and delithiation process for the pristine LiMnPO4 material has been proved according to the structure evolution of the material during cycling.6, 32 To ravel the structure changes of Mg-LMP during cycling as well as the performance improving mechanism of Mg2+ ion substitution, the in operando XRD experiment was conducted for the Mg-LMP cathode material during the first lithiation and delithiation process. The in operando XRD results of Mg-LMP as shown in Figure 6 demonstrate a typical two-phase transformation mechanism from the lithium-rich or pristine phase to the lithium-poor phase during the charging process. More interestingly, the (002) diffraction peak of the lithiumpoor phase exhibits a continued shift (indicated by the arc-shaped arrow) to high angle companying with further lithium ion extraction from the Mg-LMP material, which means a solid-solution reaction process during the charging process at high voltage region of over 4.30 V. 33

The evolution of the (121) reflection peak position upon the first cycling is extracted from the

in operando XRD results for quantification analysis (see Figure 7). It can be seen that the (121)



peak position does not change until x = 0.20 (x in LixMn0.96Mg0.04PO4) at the charging voltage of about 4.30 V, where the phase transformation reaction from pristine phase to lithium-poor phase occurs.9 The lithium-poor phase is emerging when charged to 4.35 V (x = 0.15). The solidsolution reaction process (indicated by the arc-shaped arrow) at the initial discharging stage can also be observed. At the end of the first discharge, the peak position of lithium-rich phase goes back to that of pristine Mg-LMP material, exhibiting an excellent structural reversibility. Therefore, in comparison with the pristine olivine-structured phosphate cathode materials (LFP and LMP), the Mg-LMP material complies with a combined reaction mechanism of typical twophase transformation and single-phase solid-solution reaction process. The origin of the combined reaction mechanism in Mg-LMP is usually considered to be related to the nonequilibrium Li+ ion distribution in the material during the electrochemical reaction or the presence of the solid-state solution reaction process.32 Here, the micro-sized secondary Mg-LMP particles composing of the nano-sized primary crystal, which has been observed by the 3D nanotomography technique, shows the homogeneous Li+ ion distribution in the material, especially at the low rate of 0.1 C. Therefore, the in operando XRD results indicate that the Mg2+ substitution in the Mn2+ site improves the electrochemical performance, which is mainly attributed to the combined reaction mechanism of two-phase and solid-solution reaction process during cycling for the Mg-LMP cathode material.

Conclusions

In summary, the Mg-LMP material with the homogenously Mg2+ substitution in the Mn2+ site was prepared successfully via combining the orientation-controlled solvothermal method with


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the subsequent spray drying and pyrolysis process, showing a hierarchical micro-nano 3D nanoporous structure. The Mg-LMP material exhibits a high initial capacity of 156 mAh/g (91.7 % of theoretical capacity) at the rate of 0.1 C, and shows an excellent cycling performance with the capacity retention of 100 % after 400 cycles at the rate of 1 C. The improved cycling performance is contributed to the stabilized structure due to the reduced length of M-O bonds by Mg2+ substitution. The performance improving of the Mg2+ ions doping is mainly related to the changing electrochemical reaction mechanisms from the typical two-phase reaction to the singlephase (solid-solution) reaction mechanisms. Therefore, the few inactive Mg2+ ion doping can provide a new insight for developing other high-energy cathode materials in lithium-ion batteries.

Associated Content Supporting information available: Nitrogen sorption isotherm and the pore size distribution of the prepared materials. The coulombic efficiency of the two prepared materials during the cycling at the rate of 1 C.

Acknowledgement

This work was partially supported by the National Natural Science Foundation of China (no. 51772068). This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Use of beamline 8BM at APS is partially supported by the National Synchrotron Light Source II, Brookhaven National



Laboratory, under DOE Contract No. DE-SC0012704. L. Wang acknowledges the scholarship from the China Scholarship Council No. 201506120263.


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(17) Ravnsbaek, D. B.; Xiang, K.; Xing, W.; Borkiewicz, O. J.; Wiaderek, K. M.; Gionet, P.; Chapman, K. W.; Chupas, P. J.; Tang, M.; Chiang, Y. M. Engineering the Transformation Strain in LiMnyFe1-yPO4 Olivines for Ultrahigh Rate Battery Cathodes. Nano Lett. 2016, 16, 2375-2380. (18) Iadecola, A.; Perea, A.; Aldon, L.; Aquilanti, G.; Stievano, L. Li Deinsertion Mechanism and Jahn–Teller Distortion in LiFe0.75Mn0.25PO4: Anoperando X-ray Absorption Spectroscopy Investigation. J. Phys. D: Appl. Phys. 2017, 50, 144004. (19) Lu, Q.; Hutchings, G. S.; Zhou, Y.; Xin, H. L.; Zheng, H.; Jiao, F. Nanostructured Flexible Mg-Modified LiMnPO4 Matrix as High-Rate Cathode Materials for Li-ion Batteries. J. Mater. Chem. A 2014, 2, 6368-6373. (20) Omenya, F.; Wen, B.; Fang, J.; Zhang, R.; Wang, Q.; Chernova, N. A.; Schneider-Haefner, J.; Cosandey, F.; Whittingham, M. S. Mg Substitution Clarifies the Reaction Mechanism of Olivine LiFePO4. Adv. Energy Mater. 2015, 5, 1401204. (21) Deng, Y.; Yang, C.; Zou, K.; Qin, X.; Zhao, Z.; Chen, G. Recent Advances of Mn-Rich LiFe1-yMnyPO4 (0.5 ≤ y < 1.0) Cathode Materials for High Energy Density Lithium Ion Batteries. Adv. Energy Mater. 2017, 7, 1601958. (22) Kisu, K.; Iwama, E.; Onishi, W.; Nakashima, S.; Naoi, W.; Naoi, K. Ultrafast NanoSpherical Single-Crystalline LiMn0.792Fe0.198Mg0.010PO4 Solid-Solution Confined Among Unbundled Interstices of SGCNTs. J. Mater. Chem. A 2014, 2, 20789-20798. (23) Gutierrez, A.; Qiao, R.; Wang, L.; Yang, W.; Wang, F.; Manthiram, A. High-Capacity, Aliovalently Doped Olivine LiMn1–3x/2Vx□x/2PO4 Cathodes without Carbon Coating. Chem. Mater. 2014, 26, 3018-3026.



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Figure and captions

Figure 1. Phase structure analysis of the LMP and Mg-LMP materials. The refinement results of the prepared materials (LMP and Mg-LMP) performed on the SXRD.


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Figure 2. (a) The XANES spectrums of the prepared LMP and Mg-LMP materials. (b) The magnitude of the Fourier transform of LMP and Mg-LMP.



Figure 3. Morphology and crystal structure of the LMP and Mg-LMP materials. The SEM images of (a) LMP and (b) Mg-LMP. The TEM images (c) LMP and (e) Mg-LMP and corresponding to selected area electron diffraction (d) LMP and (f) Mg-LMP. (g) the crystal structure of the olivine LMP and Mg-LMP.


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Figure 4. (a) The three-dimensional morphology and (b) surface rendering of the prepared MgLMP secondary particles. (c) TXM-XANES 2D image of Mg-LMP secondary particles. The red dash arrow is along with the diameter of the secondary particle and five random areas along with the red dash arrow. (d) The average Mn K-edge XANES spectra of the selected areas.



Figure 5. Electrochemical performance. The first cycle charge-discharge curves at different current of (a) LMP and (b) Mg-LMP. (c) Comparison of the rate performance of the LMP and Mg-LMP materials. (d) The long-term cycling performance of LMP and Mg-LMP materials at the rate of 1 C.


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Figure 6. In operando synchrotron-based XRD results. The phase transformation of the MgLMP material during the first cycle at the rate of 0.1 C.



Figure 7. The evolution of the (121) reflection peak position upon the first cycle.


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Table 1. The structural parameters of prepared materials obtained by the SXRD refinement. Sample

a/Å

b/Å

c/Å

V/Å3

Rwp/%

x2/%

LMP

10.447(1)

6.096(5)

4.751(3)

302.4

4.8

4.5

Mg-LMP

10.430(2)

6.090(8)

4.738(2)

301.0

3.7

2.5



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Table 2. The fitting results for the Mn K-edge EXAFS of the pristine LMP and Mg-LMP materials. Sample

Mn-O3(1)

Mn-O1(1)

Mn-O3(2)

Mn-P(1)

Mn-P(2)

LMP

2.0868(4)

2.1759(4)

2.2495(4)

2.8453(1)

3.2962(1)

Mg-LMP

2.0635(9)

2.1526(9)

2.2262(9)

2.8577(0)

3.3086(0)


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Table of Contents (TOC) graphic


Modifying High-Voltage Olivine-Type LiMnPO4 Cathode via Mg

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