ELECTROCHEMICAL PROPERTIES OF LIFEPO4 THIN FILMS PREPARED BY RF MAGNETRON SPUTTERING Текст научной статьи по специальности «Физика»

БОРТОВЫЕ АККУМУЛЯТОРЫ ЭНЕРГИИ

ON-BOARD ENERGY ACCUMULATORS

ХИМИЧЕСКИЕ АККУМУЛЯТОРЫ ЭНЕРГИИ

CHEMICAL ENERGY ACCUMULATORS

Статья поступила в редакцию 06.08.12. Ред. рег. № 1388 The article has entered in publishing office 06.08.12. Ed. reg. No. 1388

УДК 544.653; 621.355

ЭЛЕКТРОХИМИЧЕСКИЕ СВОЙСТВА ТОНКИХ ПЛЕНОК LiFePO4, ПОЛУЧЕННЫХ РЧ МАГНЕТРОННЫМ НАПЫЛЕНИЕМ

Г. Кучинскис, Г. Баярс, Я. Клеперис

Институт физики твердого тела при Латвийском университете Латвия, Рига, LV-1063, ул. Кенгарага, д. 8 Тел.: +371 67 187 816; e-mail: gints.kucinskis@gmail.com

Заключение совета рецензентов: 20.08.12 Заключение совета экспертов: 25.08.12 Принято к публикации: 30.08.12

LiFePO4 типа оливин является многообещающим материалом для катодов в литиевых батареях. В статье рассматриваются результаты структурного и электрохимического анализа как для объемных образцов LiFePO4, так и тонких пленок, полученных РЧ магнетронным напылением. Гравиметрическая зарядная мощность достигает 61 мАч/гр для тонких пленок и 135 мАч/гр для объемных образцов. Были исследованы структура и морфология и их влияние на свойства образцов. Также для всех образцов был определен коэффициент диффузии ионов Li+ при различных значениях заряда.

Ключевые слова: тонкие пленки, LiFePO4, катод, литиевые батареи.

ELECTROCHEMICAL PROPERTIES OF LiFePO4 THIN FILMS PREPARED BY RF MAGNETRON SPUTTERING

G. Kucinskis, G. Bajars, J. Kleperis

Institute of Solid State Physics 8 Kengaraga Str., Riga, LV-1063, Latvia Tel.: +371 67 187 816; e-mail: gints.kucinskis@gmail.com

Referred: 20.08.12 Expertise: 25.08.12 Accepted: 30.08.12

LiFePO4 is a promising olivine-type cathode material for lithium ion batteries. Structural and electrochemical analysis has been carried out for LiFePO4 bulk material and thin films deposited by RF magnetron sputtering. Thin films display gravimetric charge capacities of up to 61 mAh/g, whereas bulk material - up to 135 mAh/g. Thin film structure, morphology and their effects on thin film electrochemical properties have been studied. Li+ diffusion coefficients have been determined for LiFePO4 bulk material and thin films in various states of charge.

Keywords: thin films, LiFePO4, cathode, lithium batteries.

Organization(s): Mag. Phys., Research Assistant at Institute of Solid State Physics, University of Latvia.

Education: University of Latvia, Faculty of Physics and Mathematics (2007-2012). Experience: Institute of Solid State Physics, Engineer (2010-2011). Institute of Solid State Physics, Research Assistant (2011-present).

Main range of scientific interests: Material science, solid state ionics, lithium ion batteries, energy, renewable energy, thin films.

Publications: 1. G.Bajars, G.Kucinskis, J.Smits, J.Kleperis (2011) Physical and electrochemical properties of LiFePO4/C thin films deposited by DC and RF magnetron sputtering. Solid State Ionics, vol.188, p.156-159.

2. J.Smits, G.Kucinskis, G.Bajars, J.Kleperis (2011) Structure and electrochemical characteristics Gints Kucinskis of LiFePO4 as cathode material for lithium-ion batteries. Latvian Journal of Physics and Technical

Sciences, vol. 48, Nr. 2, p.27-31.

y2 International Scientific Journal for Alternative Energy and Ecology № 09 (113) 2012 H ffil .SE1! HS CIS

© Scientific Technical Centre «TATA», 2012

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Dr.chem., senior research scientist, Institute of Solid State Physics (ISSP), University of Latvia. He has the Diploma in chemistry from the University of Latvia (1979) and doctoral degree Diploma on PhD thesis „Physical and chemical processes in electrochromic systems with solid electrolytes" was defended in 1987. Guest researcher in the field of solid electrolytes at the Institute of Electrochemistry, Vienna Technical University (1988-1989); Assistant professor (1996-2002), Vice-Rector and Rector in Vidzeme University College (1998-2001), Vice-Rector (2002-2003) in the School of Business Administration Turiba. The main research areas are alternative energy sources, hydrogen technologies, sustainable development and tourism. Participated in local and international projects on material science, ecodesign, energy and environment. He is an author of more than 150 scientific publications, mostly on topic of fast ion transport in solids - synthesis of materials and their properties, applications. Gunars Bajars is the member of International Society of Electrochemistry Gunars Bajars and guest editor of Solid State Ionics journal.

Introduction

Cathode is the heaviest component of the currently used secondary lithium-ion batteries [1], therefore it has recently received increased attention from the scientific community. As electric vehicles are predicted to become the largest lithium-ion battery market in 2015 [2] increased attention will be devoted to researching cathode materials with not only high charge capacity and good cyclability, but also low production costs. LiFePO4 is widely being considered an environmentally benign cathode material with high gravimetric charge capacity, superior cyclability and, with Fe being one of the most abundant elements in earth's crust, it also has a relatively low production cost. Therefore LiFePO4 is fulfilling all the above-mentioned requirements to become one of the most popular cathode materials for lithium ion batteries next to currently more widespread LiMn2O4 and LiCoO2.

As flexible electronics and small electronic devices (such as smart cards and RFID cards) become more popular, the demand for thin-film secondary batteries should also increase. The use of thin films also provides an opportunity to study surface effects of the material and increases surface/mass ratio which is essential for materials with low electronic conductivity (such as LiFePO4). It is also essential to compare thin films electrochemical properties with those obtained for bulk material. Such an analysis is provided in the current work together with studies of structure and morphology of thin films and bulk material.

Experimental part

Both thin film and bulk material structure was studied via x-ray diffraction spectroscopy (XRD, Philips X'Pert Pro MPD diffractometer, Cu Ka radiation). Surface morphology was studied with a scanning electron microscope (SEM, Hitachi S4800). Additionally, energy-dispersive x-ray analysis (EDX) was performed in order to determine the thin film composition.

Thin films with a thickness of 200 to 1000 nm were deposited on a stainless steel and monocrystalic silicon

substrates by using RF magnetron sputtering (Sidrabe). Targets were prepared by pressing LiFePO4 (Linyi Gelon New Battery Materials) under a weight of 7 tons. The diameter of the target used was 15 cm, target thickness -3 mm, distance between target and substrate - 15 cm. The initial pressure of 10-4 Torr was obtained, pressure during sputtering - 1.5-10-3 Torr (argon gas flow - 150 sccm), magnetron power - 300 W. Thin films were annealed in N2 at 600 °C for 1 h.

Bulk material was prepared for electrochemical measurements by mixing 10% acetylene black and 10% polyvinylidene fluoride (both reactants from Sigma-Aldrich, purity >98.0%) and 80% LiFePO4 in 1-methyl-2-pirolidinone (Sigma-Aldrich, purity >98.0%) and dispensing the paste on aluminum substrate. Substrates were then dried for at least 24 h at 30 °C in air and 2 h in nitrogen at 140 °C. Afterwards samples were pressed under 5 ton weight (surface area - 0.50 cm2) in order to improve the adhesion.

Electrochemical measurements for both thin films and bulk material were performed with a metallic lithium counter electrode in a 2 electrode electrochemical cell using 1 M LiPF6 EC-DMC (1:1) electrolyte (all reactants from Sigma-Aldrich, purity >98.0%) and Whatman GF/F glass microfiber separator (average pore diameter - 0.7 ^m). Electrochemical cells were assembled in an argon-filled glove box and electrochemical measurements (voltammetry, chronopotentiometry, electrochemical impedance spectroscopy) were performed using Voltalab PGZ-301 potentiostat.

Results and discussion

The XRD patterns of both LiFePO4 bulk material and thin films are shown in Fig. 1. All observed diffraction peaks can be indexed with the orthorhombic olivine-type structure (space group Pnmb) in agreement with the well-crystalline single phase LiFePO4. Also present are the peaks corresponding to silicon monocrystal. It is worth mentioning that no peaks were present for non-annealed LiFePO4 thin films, indicating that they might be amorphous.

Рис. 1. Результаты ДРА для тонких пленок LiFePO4/C с 5 масс% сажи, напыленных на кремниевую основу Fig. 1. XRD pattern for LiFePO4/C thin films with 5 wt% carbon black content sputtered on a silicon base

SEM images of both bulk material and thin films are shown in Fig. 2. The grain size of LiFePO4 bulk material is about 50-90 nm, which is confirmed not only by XRD (using Scherrer equation) but also by Fig. 2, a. The surface of thin films is dense and smooth. It is observed that annealing increases surface area (Fig. 2, b and c) by increasing the amount of irregularities on the surface. It can be observed in Fig. 2, d that the grain size in LiFePO4 thin films is somewhat smaller than in bulk material. EDX shows Fe, P and O content of stoichiometric relation of 1:1:4 and a small peak corresponding to C.

Electrochemical measurements show an equilibrium cell potential of about 3.42 V. It can be observed that in the case of both annealed and non-annealed thin films the initial cell open circuit potential (OCP) is about 3.25 V, indicating possibly larger initial lithium content in the material. However, after several cycles the OCP increases to about 3.4 V.

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Рис. 2. Снимки СЭМ для LiFePO4: а - объемный материал; b - непрокаленный материал; c и d- прокаленный материал Fig. 2. SEM images of LiFePO4: a - bulk material; b - non-annealed thin film; c and d - annealed thin film

International Scientific Journal for Alternative Energy and Ecology № 09 (113) 2012

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Voltammograms for bulk material and thin films are shown in Fig. 3 and Fig. 4. An equilibrium potential of 3.42 V and distinct lithium intercalation and de-intercalation peaks can be observed along with good electrochemical reaction reversibility.

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Рис. 3. Цикличная вольт-амперная характеристика объемного LiFePO4 Fig. 3. Cyclic voltammetry of LiFePO4 bulk material

200

100 -

¡

и -100

-200

Cell potential, V

Рис. 4. Цикличная вольт-амперная характеристика непрокаленных и прокаленных тонких пленок LiFePO4

(частота сканирования 1 мВ/с). Fig. 4. Cyclic voltammetry of annealed and non-annealed LiFePO4 thin film (scan rate 1 mV/s)

-annealed non-annealed J — —---г/ К

6 2,8 3,6 3,8

LiFePO4 gravimetric charge capacity of 170 mAh/g. It is, however, common for thin film gravimetric charge capacity to be lower than the one measured for bulk materials [3, 4], mainly due to thin films having smaller surface area, lower electronic conductivity and possibly more impurities.

Рис. 5. Кривые заряжения и разряжения для объемного LiFePO4 Fig. 5. Charge-discharge curves of LiFePO4 bulk material

Рис. 6. Кривые заряжения и разряжения для прокаленных тонких пленок LiFePO4 Fig. 6. Charge-discharge curves of annealed LiFePO4 thin film

It can also be observed that thin film charge capacity decreases faster with increasing charge and discharge current (see Fig. 7) indicating lower rate capability. This observation is consistent with previously published findings [2] and can also be due to due to long solid-state diffusion path lengths, thin film impurities, smaller surface area and possibly lower carbon content.

Charge and discharge curves for bulk material can be seen in Fig. 5, charge and discharge curves for annealed LiFePO4 thin films - in Fig. 6. Gravimetric charge capacity of bulk material is determined to be up to 135 mA h/g at discharge rate C/10, thin film gravimetric charge capacity - up to 61 mAh/g at the same C/10 discharge rate (C = 170 mA/g). Thin film gravimetric charge capacity was calculated, assuming that the thin film density is 3.6 g/cm3 and knowing its thickness to be 1 ^m. It can be seen that thin film gravimetric charge capacity is 45% of that measured for bulk material. Bulk material charge capacity consists 79% of theoretical

Рис. 7. Гравиметрическая зарядная емкость при различных скоростях разряжения Fig. 7. Gravimetric charge capacity at various discharge rates

Electrochemical impedance spectroscopy (EIS) was performed for bulk material and thin films at various states of charge. The Niquist plots can be seen in Fig. 8 and 9. Lithium ion diffusion coefficients in LiFePO4 bulk material and thin films were calculated by using the equation:

d=2

Vl.

FS a

dE_ dx

where Vm is the molar volume; S - surface area of the electrode; F - Faraday constant; dE/dx is the slope of the electrode potential E vs. lithium content x (Lii-xFePO4) evaluated from Fig. 5 and 6; c - Warburg factor (obtained from the slope of Z' vs. ra-1/2).

Рис. 8. График Найквиста для объемного LiFePO4 (с шагом 0.1 В, кривые поднимаются на 0.1 k^/см2) Fig. 8. Niquist plots for LiFePO4 bulk material (step 0.1 V; curves are moved up sequentially by 0.1 k^/cm2)

Рис. 9. График Найквиста для прокаленных тонких пленок LiFePO4 при различных потенциалах ячейки Fig. 9. Niquist plots for LiFePO4 annealed thin films at various cell potentials

EIS results were also fitted by using Randles equivalent circuit (see Fig. 10) consisting of two resistances (ohmic resistance RS and charge transfer resistance Rct), constant phase element and Warburg element. Main results are given in Table. Charge transfer resistance has a minimum value when cell potential is closer to the equilibrium potential. This is consistent with the fact that exchange current has a maximum at the equilibrium potential. It must also be noted that charge transfer resistance of thin films is noticeably larger than that of the bulk material.

Рис. 10. Эквивалентная схема, использованная для анализа результатов ЭИС Fig. 10. Equivalent circuit used for fitting EIS results

Сопротивление переносу заряда и коэффициент диффузии Li+ в объемном материале и тонких пленках LiFePO4 Charge transfer resistance and Li+ diffusion coefficients in LiFePO4 bulk material and thin films

Cell potential, V Bulk material Thin films

Rct, fl/cm2 DLi, cm2/s Rct, kfl/cm2 DLi, cm2/s

2.9 223 2.5-10-11 -

3.0 228 1.6-10-11

3.1 225 8.7-10-13

3.2 209 2.6-10-13 24 2.4T0-13

3.3 144 1.010-13 -

3.4 111 5.9-10-14 5.3 9.7T0-14

3.45 - 9.7 2.1 • 10-14

3.5 57 1.310-13 15 7.4T0-14

3.6 69 2.6-10-13 26 4.2-10-13

3.7 90 5.5-10-13 28 9.4-10-13

3.8 94 8.7-10-12

The ohmic resistance of the cell stays almost constant at all cell potentials and is about 5 Q/cm2 for cells containing bulk material cathode and 20 Q/cm2 for cells containing LiFePO4 thin films. The electrical double-layer capacity of the bulk material is observed to be in range from 40 to 60 ^F/cm2, electrical double-layer capacity of thin film - electrolyte interface - 2 to 3 ^F/cm2. The differences between bulk material and thin film results can most likely be explained by variations in effective electrode surfaces.

International Scientific Journal for Alternative Energy and Ecology № 09 (113) 2012

© Scientific Technical Centre «TATA», 2012

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1 I

I - - Bulk material i

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Рис. 11. Коэффициент диффузии лития для объемного

материала и тонких пленок LiFePO4 Fig. 11. Lithium ion diffusion coefficient in LiFePO4 thin films and bulk material

The determined lithium ion diffusion coefficients in LiFePO4 thin films and bulk material can be seen in Fig. 11. Although DLl is a little lower for thin films, lithium ion diffusion coefficients in LiFePO4 bulk material and thin films are very close. The small differences are likely to be caused by impurities in LiFePO4 thin films. The determined DLi are significantly lower than the theoretically determined 10-7 cm2/s [5] but coincide with DLi values obtained experimentally (ranging from 2.0-10-18 to 1.8-10-12 cm2/s [6-9]).

Conclusions

LiFePO4 thin films have been acquired via RF magnetron sputtering. Thin films have lower effective surface area than bulk material samples due to their surface being smoother. Both bulk material and thin film electrochemical properties have been evaluated by cyclic voltammetry, chronopotentiometry and EIS. Bulk material gravimetric charge capacity was evaluated to be 135 mAh/g, thin film charge capacity corresponds to 45% of that determined for bulk material, and is 61 mAh/g. Lithium ion diffusion coefficients in LiFePO4 bulk material and thin films have been determined at various states of charge and are observed to be lowest in equilibrium potential. The determined DLi at cell equilibrium potential are 5.9-10-14 and 9.7-10-14 cm2/s for bulk material and thin films accordingly.

Acknowledgements

Authors acknowledge Taiwan - Latvia - Lithuania cooperation project "Materials and processing development for advanced Li ion batteries" and National Program in Energetic for financial support. The technical assistance and consultations of Sidrabe Inc. are acknowledged. Authors also acknowledge the help from Institute of Chemical Physics, University of Latvia.

References

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8. Prosini P.P., Lisi M., Zane D., Pasquali M. Determination of the chemical diffusion coefficient of lithium in LiFePO4 // Solid State Ionics. 2002. Vol. 148, No. 1-2. P. 45-51.

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