StructureandElectrochemicalBehaviorofMinorMn-Doped ......demonstrates the TGA curves of two Mn-doped olivines acquired in air at 10 C/min in the temperature range of 25–800 C. e

Research ArticleStructure and Electrochemical Behavior of Minor Mn-DopedOlivine LiMnxFe1−xPO4

LeThanhNguyenHuynh ,1,2 PhamPhuongNamLe,1VietDungTrinh,3HongHuyTran,1

Van Man Tran,1,2 and My Loan Phung Le 1,2

1Applied Physical Chemistry Laboratory (APCLAB), VNUHCM-University of Science, Ho Chi Minh City, Vietnam2Department of Physical Chemistry, Faculty of Chemistry, VNUHCM–University of Science, Ho Chi Minh City, Vietnam3School of Chemical Engineering, Hanoi University of Science and Technology, Hanoi, Vietnam

Correspondence should be addressed to Le �anh Nguyen Huynh; [email protected]

Received 8 September 2019; Revised 20 October 2019; Accepted 24 October 2019; Published 26 December 2019

Guest Editor: Chu Liang

Copyright © 2019 Le �anh Nguyen Huynh et al. �is is an open access article distributed under the Creative CommonsAttribution License, which permits unrestricted use, distribution, and reproduction in anymedium, provided the original work isproperly cited.

In the recent years, olivine LiFePO4 has been considered as a prospective cathode material for lithium-ion batteries. However, lowconductivity is an obstacle to the commercialization of LiFePO4; doping the transition metal such as Mn and Ni is one of thesolutions for this issue. �is work aimed to synthesize the Mn-doped olivines LiMnxFe1− xPO4 at low content of Mn (x� 0.1, 0.2)via the hydrothermal route followed by pyrolyzed carbon coating. �e synthesized olivines were well crystallized in olivinestructure, with larger lattice parameters compared with LiFePO4.�e EXD and TGA results confirmed the coated carbon of 4.14%for LiMn0.1Fe0.9PO4 and 6.86% for LiMn0.2Fe0.8PO4. Both of Mn-doped olivines showed higher diffusion coefficients of Li+

intercalation than those of LiFePO4 that led a good performance in the cycling test. LiMn0.2Fe0.8PO4 exhibited a higher specificcapacity (160mAh/g) than LiMn0.1Fe0.9PO4 (155mAh/g), and the Mn content is beneficial for the cycling performance as well asionic conductivity.

1. Introduction

In the last three decades, the world has witnessed urges forsignificant breakthroughs in the hi-tech industry. Amongthem, the demand for power storage can provide a largeamount of energy with good cycling performance. Lithium-ion batteries (LIBs) have taken those requirements as theiradvantages. A great deal of research studies have delveddeeply into the topic of LIBs in order to develop suitablematerials to improve the performance of batteries.�e use ofcathode materials for LIBs has been an attractive field be-cause they are the determining factor of power density,capacity, lifetime, and cost of the batteries [1, 2]. Besides thepopular layered transition metal oxides, olivine materialsLiMPO4 (M=Mn, Co, Fe, . . .), represented by LiFePO4, arecompetitive candidates to the commercialized LiCoO2. �eyare of low cost and easy to synthesize and nontoxic. Highthermal stability and comparable specific capacity

(170mAh/g) to LiCoO2 (140mAh/g) are the key to theirlarge-scale application in the future [3, 4].

Olivine materials are categorized as orthorhombicstructures with the space group Pnma.�e lattice of LiFePO4contains [FeO6] octahedral sites and [PO4] tetrahedral sitessharing vertices and edges. �e octahedrons are connectedto form a tunnel structure, which facilitates the migration ofLi+ ions [5]. �e alternate arrangement of [FeO6] and [PO4]groups, however, leads to low conductivity (∼10− 9 S/cm atroom temperature), which is the main limitation of LiFePO4[6]. In order to solve this issue, many research projects havefocused on minimizing the particle size to nanoscale with amodern synthetic process. Besides, doping the material withMn or Ni (LiMnxFe1− xPO4 or LiNixFe1− xPO4) is also apromising solution [7]. Nakamura et al. reported that theMn2+ substitution in olivine compounds (prepared by thesolid-state reaction) expended the unit cell along the c-axis,which facilitated the Li immigration in doped olivines, and

HindawiJournal of ChemistryVolume 2019, Article ID 5638590, 10 pageshttps://doi.org/10.1155/2019/5638590

mailto:[email protected]://orcid.org/0000-0003-4806-7362https://orcid.org/0000-0003-1055-3672https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://doi.org/10.1155/2019/5638590

Li+ diffusions were found to be in the order of 10− 12 cm2/s[8]. Novikova et al. prepared the olivines LiMnxFe1− xPO4(x� 0–0.4) by the sol-gel method, and the particles varied inthe range of 400 nm to 4 μm. By using Mössbauer spec-troscopy, they proved the low-content Mn doping (x� 0.1,0.2) enhanced the discharge capacity to 142mAh/g (at thecurrent density of 55mA/g) due to the solid solution for-mation under gradual changes in the potential of Mn [9].

Our work aimed to synthesize the Mn-doped olivineLiMnxFe1− xPO4 at low-content of Mn doping (x� 0.1, 0.2)via the hydrothermal route followed by pyrolyzed carboncoating. �e structure and morphology of the synthesizedolivines were evaluated by X-ray diffraction and FE-SEMcoupling with EDX and TGA. We also considered the ki-netics of Li+ intercalation (lithium diffusion coefficient, DLi)determined from cyclic voltammetry results, which are re-lated to the cycling performance.

2. Methods

2.1. Preparation of LiMnxFe1− xPO4. Mn-doped olivinesLiMn0.1Fe0.2PO4 and LiMn0.2Fe0.8PO4 were synthesized viathe hydrothermal method. 150mL of aqueous solution ofMn(CH3COO)2.4H2O and Fe(NO3)3.9H2O (at the molarratio of 1 : 9 or 2 : 8) with a small amount of n,n-dime-thylformamide C3H7NO as the reducing agent was stirred at80°C for 1 hour. �e solution was then mixed withLiOH.H2O, H3PO4, C6H8O6, and HNO3 and stirred thor-oughly at ambient condition for 6 hours using a magneticstirrer. �e mixture was transferred into a hydrothermalsystem and annealed at 180°C for 12 hours.�e outcome wasseparated from the suspension by the centrifuge and low-pressure filter, then washed by deionized water, and finallydried at 60°C overnight. In order to coat carbon on thesurface of the synthesized olivines, the obtained solid wasgrounded with glucose at the mass ratio of 1 :1 and thencalcined at 700°C in N2/Ar atmosphere (95/5, v/v) for3 hours.

2.2. Structural, Morphological, and ElectrochemicalCharacterizations. �e crystalline structure of the synthe-sized olivines was characterized by X-ray diffraction usingD8-ADVANCED (Bruker, CuKα radiation) in the 2θ rangeof 10–70° at the scanning rate of 0.020°C/step. �e mor-phology and elemental composition were evaluated byscanning electron microscopy (SEM) and SEM-EDX using aHitachi FE-SEM S-4800 scanning electron microscope. �eproportion of carbon coating on the particles was quantifiedby thermal analysis from 25°C to 800°C in air at the scanningrate of 10°C/min using a LABSYS evo TG–DSC 1600 system(Setaram, France).

�e cathode slurry was prepared bymixing 90 wt% activematerials, 7.5 wt% graphite, 7.5% acetylene black, and 5 wt%polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) in N-methyl-2-pyrrolidone (NMP). �e slurry thenwas coated on the aluminum foil with a thickness of 50 μmand density of 2mg/cm2. �e olivine electrodes were cutwith a diameter of 10mm and dried at 130°C under vacuum

for 24 hours. �e two-electrode Swagelok-type cells wereassembled in an Ar-filled glovebox, and the cell consisted ofan olivines electrode as the positive electrode, lithium foil asthe negative electrode, three glass Whatman microfibers asthe seperator, and 100mL of 1M LiPF6 in a mixture ofethylene carbonate (EC) and dimethyl carbonate (DMC) at avolumetric ratio of 2 :1 as the electrolyte.

�e electrochemical behaviors of Mn-doped olivineswere evaluated by cyclic voltammetry (CV) and galvano-static cycling test using the apparatus VSP (BioLogic,France). �e cyclic voltammetry (CV) was carried out in apotential range of 2.5–4.5V (versus Li+/Li) in the variousrates from 20 µV/s to 120 µV/s. �e cycling tests wereconducted at rate C/10.

3. Results and Discussion

3.1. Structure and Morphology. �e X-ray diffraction pat-terns of two Mn-doped olivines (Figures 1(a) and 1(b)) canbe indexed in an orthorhombic structure with the spacegroup Pnmb. Results of Rietveld refinement, gathered inTable 1, are in good agreement with the original olivinestructure (JSPDF: 01-083-2092). �e Li atoms are located at4a sites; the Fe, Mn, and P atoms are in 4c sites; and the Oatoms are in 8a sites. �e fractions of Mn : Fe are 0.1 : 0.9 and0.2 : 0.8, which approximately match the EDX results. �eX-ray diffraction patterns indicate the presence of a pure andvery well crystallized single phase without the impurityphase (e.g., MnO2, Li2O, and Fe2O3). It is obvious thatpartially replacing Fe with Mn amplifies the size of theolivines lattice unit, which can be explained by the fact thatthe ionic radius of Mn2+ (80 pm) is superior to that of Fe2+(78 pm). �is result agrees with previous reports about Mn-doped olivine [10–13].

Morphology of two Mn-doped olivines was analyzed byscanning electron microscopy. �e hydrothermal prepara-tion led to a homogeneous distribution. It can be seen(Figure 2) that the particle grains of both samples point outthe polyhedral shapes at submicrometric scale (approxi-mately 200–400 nm). According to EDX data in Figure 3 andTable 2, the elemental composition of Li, Fe, Mn, and P inthe samples corresponds to the stoichiometric relationshipwhich is suitable to the theoretical composition ofLiMn0.1Fe0.9PO4 and LiMn0.2Fe0.8PO4.

Figure 4 shows the TEM images of two Mn-doped ol-ivines, and we observed clearly that the particles’ size was inthe range of 100-200 nm. �e TEM images were coherent toSEM images. �e Raman spectra of two Mn-doped olivinesin Figure 5 show two fingerprint signals of carbon in thehigh-frequency region at 1385 cm− 1 (D-band) and1583 cm− 1 (G-band) that confirm the composites betweenMn-doped olivine and carbon LiMnxFe1− xPO4@C [14–17].

�ermal gravity analysis (TGA) is useful to quantify theproportion of coated carbon in Mn-doped samples. Figure 6demonstrates the TGA curves of two Mn-doped olivinesacquired in air at 10°C/min in the temperature range of25–800°C. �e increase in mass from 300°C to 400°C iscaused by the oxidation of Fe and Mn elements in thesamples to form metal oxides. At a temperature higher than

2 Journal of Chemistry

400°C, the mass of each sample shows a steady decline thatresults from the reaction between carbon and oxygen [18].�e amount of carbon in each sample is the difference inmass percentage measured at 400°C and 800°C. Figure 6infers that the carbon content in LiMn0.1Fe0.9PO4 (6.86%) ishigher than that in LiMn0.2Fe0.8PO4 (4.14%).

3.2. Electrochemical Behaviors. �e Li insertion into theoriginal olivine phase is described by the domino-cascademechanism between two phases LiFePO4 and FePO4 withthe characteristic voltage at 3.45 V (vs. Li+/Li), corre-sponding to the redox couple Fe3+/Fe2+. �e Mn dopinginto olivine can add a signal of redox couple Mn3+/Mn2+around 4V, and the intensity of signal depends on the Mncontent in olivine, in the potential range of 2.5–4.5V with thescan rate s. Figures 7(a) and 7(b) demonstrate the cyclicvoltammograms of LiMn0.1Fe0.9PO4 and LiMn0.2Fe0.8PO4 invarious scan rates from 20 to 120 μV/s.�e strong and shapedpeaks at ∼3.5 V in all CV curves are evidenced as theoriginal redox reaction, while the signals at 3.8–4.0 V can beassigned to the doping element reaction. �e higher thescanning rate, the higher the peak separation and intensityof LiMn0.2Fe0.8PO4; the Mn signal looks more significantthat proposes more capacity contribution in the galvano-static test.

During the Li intercalation process, Li+ ions migratedfollowing the 1D channels along the (010) plane in the

orthorhombic structure; thus, Li mobility (also called kineticof intercalation process) affects the electrode performance inthe galvanostatic cycling test and habitually determinedthrough the diffusion coefficient (DLi). In our case, kineticsof Li intercalation process into the Mn-doped olivines wasassessed by cyclic voltammetry. �e scattered plot of ca-thodic peak current versus square root of scan rates was builtup based on CV results and presented in Figure 7(c). Dif-fusion coefficients of the Li+ insertion process (DLi) weredetermined by using the Randles–Sevcik equation:

ip � 2.69 × 105n3/2

ACLiD1/2Li v

1/2, (1)

where A is the surface area of the cathode (0.785 cm2); C isthe concentration of Li+ ions in the material (cm3/mol); andn is the number of transferred electron (n� 1). Figure 7(c)shows a linear proportion between ipc and v1/2 with thecorrelation coefficient R2∼1. DLi values calculated from theRandles–Sevcik equation are listed in Table 3. DLi value ofLiMn0.2Fe0.8PO4 (9.53 × 10− 12 cm2/s) is four times higherthan that of LiMn0.1Fe0.9PO4 (2.81 × 10− 12 cm2/s).According to previous publications, diffusion coefficients ofLiFePO4 were obtained between 10− 14 − 10− 13 cm2/s[15, 19, 20]. �erefore, partial Mn substitution in the olivinestructure benefits the ionic conductivity through the soar ofDLi, compared with the original olivine phase.

Cycling performances of two Mn-doped olivines wereexecuted at rate C/10 in the voltage range of 2.5–4.5 V (vs.Li+/Li). Figure 8 compares the first galvanostatic curve oftwo doping olivines which demonstrate two Li insertionregions: (i) well-defined plateau at 3.5 V and (ii) the shortone at 3.7–4.0 V. We observed the Mn-couple capacitycontribution in LiMn0.2Fe0.8PO4 (∼30mAh/g) was higherthan its in LiMn0.1Fe0.9PO4 (∼10mAh/g). �ese results arecoherent with the CV′ results. �e discharged capacities arefound out to be 158mAh/g for LiMn0.1Fe0.9PO4 and160mAh/g for LiMn0.2Fe0.8PO4.

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Figure 1: XRD patterns of LiMn0.1Fe0.9PO4 (a) and LiMn0.2Fe0.8PO4 (b).

Table 1: Lattice parameters of LiMn0.1Fe0.9PO4 andLiMn0.2Fe0.8PO4.

a (Å) b (Å) c (Å) V (Å3)LiMn0.1Fe0.9PO4 6.015(4) 10.346(1) 4.7050 292.82LiMn0.2Fe0.8PO4 6.041(5) 10.384(1) 4.7222 296.24LiFePO4 [4] 6.008(3) 10.334(4) 4.693(1) 291.39LiMnPO4 [11] 10.452(1) 6.106(1) 4.746(1) 302.90

Journal of Chemistry 3

(a) (b)

(c) (d)

Figure 2: SEM images of LiMn0.1Fe0.9PO4 (a, b) and LiMn0.2Fe0.8PO4 (c, d).

20µm

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Figure 3: Continued.


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nKb

(d)

Figure 3: EDX spectra of LiMn0.1Fe0.9PO4 (a, b) and LiMn0.2Fe0.8PO4 (c, d).


As shown in Figure 9(a), the discharge curves ofLiMn0.1Fe0.9PO4 practically superimpose without any hys-teresis, and the capacity is stable at 158mAh/g upon 50cycles. In case of LiMn0.2Fe0.8PO4, the Mn capacity con-tribution is faded cycle by cycle; after 50 cycles, the Mncapacity region at 3.7 V seems to disappear, leading to thedecline of discharge capacity to 140mAh/g. �e fadingcapacity is interpreted by the release of Mn during the

charge-discharge process.�eCoulombic efficiencies of over97% (Figure 9(c)) are retained during the cycling test, whichindicates a reversible Li intercalation into the olivine hosts.

�e rate capability performance of Mn-doped olivines isdemonstrated in Figure 9(d), in which current density wasset up from C/10 to 5C (1C corresponds to 165mAh/g). �eMn-doped olivines show a stable capacity, for instanceLiMn0.1Fe0.9PO4, 155, 147, 139, 130, 115, and 87mAh/g at C/

Table 2: Elemental composition of LiMn0.1Fe0.9PO4 and LiMn0.2Fe0.8PO4 determined by SEM-EDX.

Element keV Mass% Atom% Ratio Mn : Fe

LiMn0.1Fe0.9PO4

Oxygen 0.525 38.38 63.50

0.09 : 0.9Phosphorous 2.013 19.08 16.31Manganese 5.894 3.58 1.73Ferrous 6.398 38.95 18.46

LiMn0.2Fe0.8PO4

Oxygen 0.525 38.74 63.46

0.18 : 0.8Phosphorous 2.013 20.52 17.36Manganese 5.894 7.44 3.55Ferrous 6.398 33.30 15.63

(a) (b)

Figure 4: TEM images of LiMn0.1Fe0.9PO4 (a) and LiMn0.2Fe0.8PO4 (b).

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nsity

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.)

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(a)

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Figure 5: Raman spectra of LiMn0.1Fe0.9PO4 (a) and LiMn0.2Fe0.8PO4 (b).


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LiMn0.2Fe0.8PO4LiMn0.1Fe0.9PO4

Figure 6: TGA curves of LiMn0.1Fe0.9PO4 and LiMn0.2Fe0.8PO4, acquired in air at 10°C/min in the temperature range of 25–800°C.

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10, C/5, C/2, 1C, 2C, and 5C; an initial capacity of 155mAh/g is remained at C/10 after 30 cycles. �e discharge capacityof LiMn0.2Fe0.8PO4 still seems to be higher, but a slightly

fading capacity is also observed. �e rate capability per-formance compares very well with the previous reports onMn-doped olivines [10, 13].

–0.00015

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y = 8.6825e – 6 – 0.0085133xR = 0.99956


y = –8.1546e – 6 – 0.012148xR = 0.99744 i

(A/c

m2 )

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(c)

Figure 7: CV curves of LiMn0.1Fe0.9PO4 (a); LiMn0.2Fe0.8PO4 (b), and evolution of ipc in function of root of scan rate (c).

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E (V

) (vs

. Li/L

i+)

Figure 8: First galvanostatic curves of LiMn0.1Fe0.9PO4and LiMn0.2Fe0.8PO4 at C/10 rate.

Table 3: Lithium diffusion coefficients of LiMn0.1Fe0.9PO4 and LiMn0.2Fe0.8PO4.

CM (cm3/mol) Slope DLi (cm

2/s)

LiMn0.1Fe0.9PO4 0.0227 0.0085 2.81 × 10− 12LiMn0.2Fe0.8PO4 0.0224 0.0121 9.53 × 10− 12


4. Conclusions

In brief, two olivine-type materials LiMn0.1Fe0.9PO4 andLiMn0.2Fe0.8PO4 were successfully synthesized via thehydrothermal route, which present an orthorhombicstructure, and their lattice parameters acceded to thepreceding reports. Doping Mn can increase the diffusion ofLi+ ions in two orders of magnitude as compared with theoriginal phase LiFePO4. Two Mn-doped olivines showed astable cycling performance upon 50 cycles as well as ratecapability. For the next step, the influence of Mn2+ on the

activation energy of Li intercalation will be deeplyconsidered.

Data Availability

�e data used to support the findings of this study are in-cluded within the article.

Conflicts of Interest

�e authors declare that they have no conflicts of interest.

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Figure 9: Typical galvanostatic curves of LiMn0.1Fe0.9PO4 (a) and LiMn0.2Fe0.8PO4 (b) at C/10 rate; (c) evolution of the specific capacity withcycle number and (d) rate capability performance.


Acknowledgments

�is work was supported by Vietnam National UniversityHo Chi Minh (VNUHCM) through grant number C2018-18-11.

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StructureandElectrochemicalBehaviorofMinorMn-Doped ......demonstrates the TGA curves of two Mn-doped olivines acquired in air at 10 C/min in the temperature range of 25–800 C. e