ISSN 00201685, Inorganic Materials, 2015, Vol. 51, No. 12, pp. 1264–1269. © Pleiades Publishing, Ltd., 2015. Original Russian Text © E.V. Makhonina, A.E. Medvedeva, V.S. Dubasova, V.S. Pervov, I.L. Eremenko, 2015, published in Neorganicheskie Materialy, 2015, Vol. 51, No. 12, pp. 1361–1366.

LiFePO4–LiMn2O4 Composite Cathode Materials for LithiumIon Batteries E. V. Makhoninaa, A. E. Medvedevaa, V. S. Dubasovab, V. S. Pervova, and I. L. Eremenkoa a

Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119991 Russia bResearch Institute of Electrode Carbon, per. Gorki 1, Elektrougli, Moscow oblast, 142455 Russia email: [email protected] Received April 27, 2015

Abstract—A method has been proposed for the fabrication of cathode materials for lithiumion batteries using composites from electrochemically active phases and ultrasonic processing. We have studied the influ ence of ultrasonic processing medium and intensity on the properties of the materials. The results demon strate that the composites possess better electrochemical performance than do their constituent components. DOI: 10.1134/S0020168515110059

INTRODUCTION Lithiumion batteries (LIBs) hold a leading place among current sources for mobile devices. The use of LIBs in electric and hybrid electric vehicles and standby power supplies is limited by the requirement of higher power energy density. These limitations are determined to a significant degree by the energy capacity and kinetic characteristics of the positive electrode (cathode) of LIBs. At present, one poten tially attractive solution is the development of hybrid batteries having two independent current sources: one with a low cycling rate but high energy density and the other with low energy density but high cycling rate [1–4]. This will make it possible to ensure both a longtime basic load and shortterm consump tion of high power [5–7] but will unavoidably lead to a more complex design and higher cost of the battery. An alternative solution is create a battery using composite cathode materials. As shown earlier [8], a LiCoO2 and LiMn2O4based composite offers better electro chemical performance than do its components. The preparation of composite materials based on several electrochemical phases has been the subject of rather extensive discussion in specialized literature. For example, Whitacre et al. [4] reported the fabrica tion of a composite based on a carboncoated LiFePO4 and an oxide with a layered structure (they examined Li[Li0.17Mn0.58Ni0.25]O2 and LiCoO2). Mixtures of active materials were prepared by three distinct proce dures: mixing of active components, sequential arrangement of the components on a substrate, and arrangement of the components in the form of layers on a substrate. The best electrochemical performance was offered by the material with the sequential arrangement of the components on a substrate. How ever, Whitacre et al. [4] failed to obtain a material

whose electrochemical performance would be better than the additive one. Park et al. [9] produced electrodes based on lithi ated manganese nickel oxides with a constant metal ratio (Mn : Ni = 3 : 1) and various percentages of lith ium, and tested them in lithiumanode cells. Accord ing to their results, the composition of the materials can be represented in the form of a threecomponent system comprising one spinel structure and two lay ered structures: Li[Mn1.5Ni0.5]O4–{Li2MnO3 · Li(Mn0.5Ni0.5)O2}. The use of a wide range of working voltages (4.95 to 2.0 V) allowed them to electrochem ically activate the layered component Li2MnO3 at high voltage and utilize the capacity of the spinel at a volt age below 3 V. As a result, they obtained a reversible capacity above 250 mA h/g at a current density of 0.1 mA/cm2. Stux and SwiderLyons [10] produced a composite based on a homogeneous LiCoO2 + Li2RuO3 mixture. The Li2RuO3 material was chosen as a component of a composite mixture owing to its high theoretical capac ity, the mechanical stability of its structure, and its high ionic and electronic conductivities. An active material was produced by mixing preprepared LiCoO2 and Li2RuO3 materials in molar ratios of 2.3 : 1.0, respectively. Electrochemical tests of the resultant composites in lithiumanode cells demonstrated an increase in cell capacity and power, especially during cycling at high currents. An increase in capacity was observed even when the materials were arranged sequentially in a cell (that is, when chemical interac tion between them was ruled out). Because of its high cost, Li2RuO3 cannot be used as an electrode material for the mass production of batteries, but it can be applied in microdevices where high capacity is needed.

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Imachi et al. [11] described the use of a multilayer structure of LiFePO4/LiCoO2based cathode active mass for improving the overcharge stability of batter ies. The cathode materials were fabricated by three distinct procedures, using both a mixture of the start ing components and different sequences of compo nent layers on aluminum foil: LiFePO4/LiCoO2/foil and LiCoO2/LiFePO4/foil. In electrochemical tests, the latter configuration of the material showed the best overcharge stability in comparison with the others. According to Imachi et al. [11], this was due to the large increase in the Ohmic resistance of a LixFePO4 delithiated layer between the LiCoO2 layers and the current collector, which leads to an increase in cell resistance and shuts down the charging current during overcharging. Shatilo et al. [8] prepared and investigated cathode materials based on LiCoO2 and LiMn2O4 of various compositions. According to their results, the discharge capacities of all the materials studied exceed additive values calculated from the discharge capacity of their constituent components. Pure LiFePO4 is of limited utility as a cathode material in power plants because of its relatively low discharge voltage and low bulk density of nanoparti cles. Nevertheless, its excellent rate performance at voltages of up to 4.2 eV (with a plateau around 3.6 eV) with respect to Li/Li+, in combination with its high stability, makes LiFePO4 a suitable component for the fabrication of composites with active substances pos sessing, for example, a higher specific capacity or higher thermal stability. In this context, lithium man ganese spinel (LiMn2O4) and LiMn2O4based com pounds are of interest because they offer high thermal stability and, when doped with nickel, possess high discharge capacity. In this study, we address the formation and proper ties of LiFePO4 and LiMn2O4based composites. The composites were produced using ultrasonic processing of suspensions of their constituent components.

cation media. The ultrasonic processing effect in eth ylene glycol was stronger owing to its higher viscosity. The processing conditions corresponded to ultrasonic field energy densities from 0.2 to 0.7 W/cm2. After the ultrasonic processing, the resultant suspensions of the components were dried in air at a temperature of 110°C for 12 h.

EXPERIMENTAL The starting materials used were the lithium iron orthophosphate LiFePO4 (HydroQuebec, Canada) and the lithium manganese spinel LiMn2O4 prepared by solidstate reaction. Composite materials of equimolar composition were prepared using ultrasonic processing at various ultrasonic field energy densities in different media. We used a UZG11M commercial ultrasonic generator and a PMS1 magnetostrictive transducer matched with the generator. The resonance frequency of the transducer–waveguide–reactor sys tem was 22.4 ± 0.1 kHz. The effect of the ultrasonic processing on suspensions was controlled by varying the ultrasonic vibration (displacement) amplitude in the waveguide and the ultrasonic processing time. Isooctane and ethylene glycol were used as ultrasoni

Electrochemical tests were performed in dry boxes in laboratory cells, with a cathode mass from a mixture of the cathode material (93.5%), carbon (3.2%), and a binder (polyvinylidene difluoride, 3.3%), which was applied to an aluminum current collector. The refer ence electrode was of lithium, and the electrolyte was a 1 M LiРF6 solution in a 1 : 1 : 1 mixture of ethylene carbonate, ethyl methyl carbonate, and dimethyl car bonate. The water content of the electrolyte was deter mined by the Fischer method. Charge–discharge curves were recorded in galvanostatic mode between 3.0 and 4.5 V at a current density of 6 mA/g. Each sample was tested in several (four to six) parallel cells. During cycling, the applied voltage was raised gradu ally, beginning at 4.2 V, and was brought to 4.5 V by the seventh cycle.

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The phase composition of the materials thus pro duced was determined by Xray diffraction on D8 Advance diffractometers (2θ/θ scan mode, angu lar range 2θ = 4°–80° , copper anode). All calculations in determining and refining unitcell parameters were performed using an application software package [12]. The interplanar spacings and integrated intensities used to index Xray diffraction patterns and refine unitcell parameters were determined from profile analysis data (Rietveld method) for observed Xray diffraction patterns. Xray diffraction patterns were indexed using the SearchMatch program in the ICDD PDF2 powder diffraction database (all of the pro grams in the HighScore Plus pack). The quality (con vergence) of the refinement of unitcell parameters was quantified by Snyder’s FOM. The particle size and morphology were determined by scanning electron microscopy (SEM) in combina tion with Xray microanalysis (Carl Zeiss NVision 40 workstation). The specific surface area of samples was measured using an ATKh06 analyzer of texture characteristics (ZAO Katakon, Novosibirsk). Nitrogen and helium were used as an adsorbate and a carrier gas, respec tively. Determinations were carried out by multipoint BET measurements in conformity with the RF State Standard GOST 2340190 (international standards ASTM D3663, ASTM D4820, and ASTM D1993). The characteristics of the samples were measured automatically in adsorption–desorption mode using the Sorbtometr M program. The relative uncertainty in our measurements was within 6%.

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Table 1. Fabrication conditions of the composites Sample

Medium

Vibration amplitude in the waveguide, µm

FM1

Isooctane

2

FM2

Isooctane

4

FM3

Ethylene glycol

4

RESULTS AND DISCUSSION Table 1 summarizes the fabrication conditions of the composites. Figure 1 shows SEM micrographs of the starting materials LiMn2O4 and LiFePO4 and com posites obtained using various ultrasonic processing conditions. The lithium manganese spinel consists of agglomerates of particles 300–400 nm in size (the average agglomerate size is D50 = 10 µm). The lithium iron orthophosphate is not agglomerated and has an average particle size in the range 200–300 nm. It is seen in the micrographs in Fig. 1 that the LiFePO4 and LiMn2O4 particles are well intermixed with each other, which suggests at least partial disintegration of the spinel agglomerates and the formation of new agglomerates, containing both constituent compo

nents. That the particles of the components are uni formly distributed over the composite is also evidenced by the element Xray maps in Fig. 2. Xray diffraction data allowed us to reveal the fol lowing general relationships: The Xray diffraction patterns of the composites show reflections from both constituent components (Fig. 3). The crystallite size of the components varies little from sample to sample. The reflections from the composites are somewhat broadened in comparison with those from their con stituent components. Increasing the ultrasonic pro cessing intensity leads to further broadening of the reflections from the composites, suggesting an increase in defect density in the samples. The specific surface area of the composites (Table 2) differs little from that of their constituent components and exceeds the corresponding additive value by no more than the uncertainty in our measurements. It seems likely that the slight increase in specific surface area is caused by the partial disintegration of the lith ium manganese spinel particles. Figure 4 presents the electrochemical testing results for the composites after ultrasonic processing in comparison with additive values.

200 nm

(a)

2 μm (b)

2 μm

(c)

1 μm (d)

500 nm

Fig. 1. Micrographs of the starting materials (a) LiFePO4 and (b) LiMn2O4 and (c, d) FM2 composites at different magnifica tions. Inset: magnified image of the LiFePO4. INORGANIC MATERIALS

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500 μm

Mn Kα1

C Kα1, 2

O Kα1

P Kα1

Fe Kα1

Fig. 2. Element (Mn, C, O, P, Fe) Xray maps obtained by EDX analysis (FM2 composite).

It is seen in Fig. 4a that the discharge capacities of the composites exceed additive values evaluated from the data obtained for their constituent components in similar tests (with and without ultrasonic processing). Ultrasonic processing at the highest intensity (Fig. 4a, curve 3) reduces the discharge capacity but, in the cycling range studied here, the decrease with an increase in the number of cycles for this sample is somewhat smaller than that for the FM1 and FM2

samples, which were obtained at lower ultrasonic field energy densities (Fig. 4a, curves 1 and 2, respectively). Figure 4b shows the discharge capacity of the com posites as a function of discharge current. The rate performance of the composites is essentially indepen dent of ultrasonic processing intensity and varies little from sample to sample, but all of the composites exhibit better highrate cyclability than do their con stituent components. For comparison, Fig. 4b pre

Table 2. Comparison of the specific surface area of the starting materials and composites Specific surface area, m2/g LiFePO4

LiMn2O4

additive value

FM1

FM2

FM3

9.60

1.37

5.20

5.39

5.06

5.37

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LiFePO4 LiMn2O4

Intensity

(a)

(b)

(c)

10

20

30

40 50 2θ, deg

60

70

80

data obtained in this study demonstrate that the pro cesses underlying the formation of the composite materials have a complex nature, so they will be the subject of further investigation. At the present stage, it is valid to say that, using ultrasonic processing, we have produced composites from electrochemically active lithium iron orthophosphate and lithium manganese spinel with defect structures (or surface compounds) that are favorable for electrochemical lithium interca lation and deintercalation during battery operation. It seems likely that the increased interfacial defect density in the composites facilitates lithium ion diffu sion. Increased ionic conductivity of a ceramic com posite was first reported by Liang [13]. Various models have been proposed to explain the increased ionic con ductivity of composites, e.g., a space charge layer model [14] and a percolation model [15]. Jiang and Wagner [16] attributed the increased ionic conductivity of com posites to the formation of an interfacial amorphous phase with increased ionic conductivity. In our instance, since the constituent components of the com posites are similar in structural characteristics, the for mation of intergranular amorphous defect structures is highly probable, especially in the course of the ultra sonic processing of homogenized starting mixtures.

Fig. 3. Xray diffraction patterns of the FM3 composite (a) and the starting materials LiMn 2O4 (b) and LiFePO 4 (c).

sents analogous data for LiFePO4 after ultrasonic pro cessing (curve 6). It is worth noting that the potential of the compos ites varies stepwise in the charge–discharge process, but the average discharge voltage of the composites is lower than that of their constituent components and the voltage steps are more closely spaced (Fig. 5). The

120

4

100

5

3 2

1

80 60 0

5

10

(b)

160 Discharge capacity, mA h/g

Discharge capacity, mA h/g

The present results demonstrate that the fabrication of composites from electrochemically active phases using ultrasonic processing is a viable approach to the preparation of cathode materials with improved elec trochemical performance. The composites produced in this study have better cyclability and rate performance than do the starting materials used. We believe that not only does ultrasonic processing ensure homogenization of the constituent components of the composites but it

(a)

140

40

CONCLUSIONS

15 20 Cycle

25

30

140 120 100

6

80 60

3

40

2 1

20 0

0.1

0.2 0.3 0.4 Discharge current, A/g

0.5

Fig. 4. Specific discharge capacity as a function of (a) the number of cycles and (b) discharge current for the composites and start ing materials: (1) FM1, (2) FM2, (3) FM3, (4) additive values for the starting materials, (5) additive values for the starting mate rials after ultrasonic processing, (6) LiFePO4 after ultrasonic processing. INORGANIC MATERIALS

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First cycle

4.2

Voltage, V

4.0 3.8

FM1 LiMn2O4

3.6

LiFePO4

3.4 3.2 3.0 –20

0

20

40 60 80 100 Discharge capacity, mA h/g

120

140

160

Fig. 5. Charge–discharge curves of the FM1 composite and starting materials after the same ultrasonic processing.

is also favorable for the formation of intergranular struc tures with increased ionic conductivity. ACKNOWLEDGMENTS This work was supported by the Federal Agency for Scientific Organizations, state research target no. 01201353365. REFERENCES 1. Lam, L.T. and Louey, R., Development of ultrabattery for hybridelectric vehicle applications, J. Power Sources, 2006, vol. 158, no. 2, pp. 1140–1148. 2. Chandrasekaran, R., Sikha, G., and Popov, B.N., Capacity fade analysis of a battery/super capacitor hybrid and a battery under pulse loads—full cell stud ies, J. Appl. Electrochem., 2005, vol. 35, pp. 1005–1013. 3. Han, J. and Park, E.S., Direct methanol fuelcell com bined with a small backup battery, J. Power Sources, 2002, vol. 112, no. 2, pp. 477–483. 4. Whitacre, J.F., Zaghib, K., West, W.C., and Ratnaku mar, B.V., Dual active material composite cathode structures for Liion batteries, J. Power Sources, 2008, vol. 177, no. 2, pp. 528–536. 5. Verbrugge, M., Frisch, D., and Koch, B., Adaptive energy management of electric and hybrid electric vehicles, J. Electrochem. Soc., 2005, vol. 152, pp. A333–A342. 6. Mayo, R.N. and Ranganathan, P., Energy consump tion in mobile devices: why future systems need requirementsaware energy scaledown, PowerAware Comput. Syst., 2005, vol. 3164, pp. 26–40. 7. Harrison, A.I., The changing world of standby batteries in telecoms applications, J. Power Sources, 2003, vol. 116, nos. 1–2, pp. 232–235. INORGANIC MATERIALS

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Translated by O. Tsarev