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.
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© 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
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