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6ON THE NEW POSITIVE ELECTRODE MATERIALS FOR HIGH ENERGY DENSITY LITHIUM ION BATTERIES
I. Saadoune*, M. Dahbi**, M. Yahya***, A. Almaggoussi****
LP2E2M, Equipe Chimie des Matériaux et de l'Environnement Avenue A. Khattabi, BP 549, 40000 Marrakech, Morocco Tel.: 212 24 43 46 88, Fax: 212 24 43 31 70 *E-mail: [email protected] **E-mail: [email protected]
LP2E2M, Equipe Chimie des Matériaux et de l'Environnement and Equipe Matériaux Optoélectroniques Avenue A. Khattabi, BP 549, 40000 Marrakech, Morocco Tel.: 212 24 43 46 88, Fax: 212 24 43 31 70 ***E-mail: [email protected] ****E-mail: [email protected]
Received: 22 Sept 2007; accepted: 30 Oct 2007
The layered LiCoo.8Nio.1Mno.1O2 and LiNia65Coa25Mna10O2 positive electrode materials were synthesized by the combustion method using sucrose as fuel, and characterized by X-ray diffraction. The powders adopted the a-NaFeO2 structure (R'3m space group). Lithium is located between the transition metal ions slabs made up by edge sharing MO6 octahedra (M: Ni, Co, Mn). X-ray diffraction data refinement by the Rietveld method shows that both samples present bi-dimensional structure with no Li/Ni cation mixing. Cycling tests revealed a great difference in the electrochemical behaviors. Lithium extraction from LiNi0.65Co0.25Mn0.10O2 involves only one redox process with a continuous evolution of the potential with composition, while they are two domains in the potential vs capacity curve of the Li//LiCo0.8Ni0.1Mn0.1O2 electrochemical cell. The first one at 3.6 V corresponds to the Ni4+/Ni2+ redox couple while the second one at 3.9 V corresponds to the Co4+/Co3+. More than 190 mA-h/g could be delivered by LiCo0.8Ni01Mn0.1O2 electrode which could be considered as a good electrochemical performance by comparison with the most commercialized cathode LiCoO2.
Keywords: electric energy storage, structural materials, new structural materials for renewable energy
Organization(s): University Cadi Ayyad, Faculty of Sciences and Technology Marrakesh, Full professor, Director of the Materials and Environmental Chemistry Laboratory.
Education: French PhD (University Bordeaux I, France, 1988-1992), Moroccan PhD (University Cadi Ayyad, Morocco, 1992-1996), Master course (University Casablanca, Morocco, 1984-1988). Experience: University Bordeaux I, Associate professor (1989-1991). University Cadi Ayyad, Full professor, Tokyo University of Science, Invited professor (2002), Leader of 10 research projects. Main range of scientific interests: lithium batteries, solid state chemistry, physical properties. Publications: 32 papers in international scientific journals, 70 communications in international meetings.
Organization(s): University Cadi Ayyad, Faculty of Sciences and Technology Marrakesh. Education: Graduate diploma (2003-2004), Master (2004-2005), PhD (2006-2009). Experience: Member of 4 research projects.
Main range of scientific interests: structural analysis, lithium batteries.
Publications: 2 papers in international scientific journals, 6 communications in international meetings.
Ismael Saadoune
Mohammed Dahbi
Organization(s): University Cadi Ayyad, Faculty of Sciences and Technology Marrakesh.
Education: Graduate diploma (2003-2004), Master (2005-2006), PhD (2007-2010).
Experience: Member of 2 research projects.
Main range of scientific interests: physics, lithium batteries.
Publications: 2 posters.
Mustapha Yahya
International Scientific Journal for Alternative Energy and Ecology № 6 (62) 2008
© Scientific Technical Centre «TATA», 2008
Abdelmajid Almaggoussi
Organization(s): University Cadi Ayyad, Faculty of Sciences and Technology Marrakesh, Full professor, Member of Optoelectronic Materials Team.
Education: French PhD (University Montpellier I, France, 1986-1992), Moroccan PhD (University Cadi Ayyad, Morocco, 1997-2003), Master course (University Marrakech, Morocco, 1981-1985). Experience: University Cadi Ayyad, Full professor, Member of 4 research projects. Main range of scientific interests: transport phenomena in semiconductors, low dimensional systems (Quantum Well, quantum Wire and Quantum dot).
Publications: 10 papers in international scientific journals, 20 communications in international meetings.
Introduction
Development of environmentally friendly and renewable energy sources is of great importance for a sustainable future. Fossil fuels and nuclear power are currently the dominating energy sources and will most likely continue to be so for the next generations. However, these power sources have drawbacks; the amount of fossil available is finite and nuclear wast disposal involves well-known problems. Today, as evidence of global warming accumulates [1], researchers from many fields focus on developing alternative energy sources and storage techniques, eg., photoelectrochemical cells [2], fuel cells [3] and batteries. The major advantages of the batteries (especially lithium-ion batteries) is its high energy density compared to others systems. It is therefore, used successively in portable electronics e.g., cellular phones, laptops and camcorders.
с Positive
Li ,.XC
V_
First charge e- w
JL
First discharge
e"
Fig. 1. Schematic representation of the lithium-ion battery
As schematically represented in Fig.1, lithium-ion battery consists of carbon as anode, liquid electrolyte and cathode. During the charge process, lithium ions move from the cathode across the electrolyte to the anode and vice versa as the battery is charged. Choosing the positive electrode material is crucial for the battery's overall performance. The most studied materials are LiCoO2 [4], LiNiÛ2 [5], LiMn2Û4 [6] and LiFePO4 [7].
Since cobalt is expensive, the battery manufacturers are looking for other materials, which can help to reduce the price of the final product without decreased performance.
In the present paper, new positive electrode materials based on Co, Ni and Mn elements were prepared by the combustion method. The electrochemical performances of the studied compounds were discussed in relation with their structural properties.
Experiment
Two compositions for the positive electrode materials were selected: LiCo0.8Ni01Mn01O2 and LiNi0 65Co0.25Mn010O2. Stoichiometric amounts of Li, Co, Ni and Mn nitrates (oxidant) were dissolved simultaneously with the sucrose (fuel) in an aqueous solution. The reaction is extremely violent when using a stoichiometric amount of sucrose. To avoid it, the oxidant/fuel ratio was optimized to 0.67. In this way, the reaction is well controlled. The reagents solution was heated at about 120 °C for 1 h, and then dried; it starts to swell up due to the evaporation of the generated gases leading to a foamy mass. After a few minutes, the mass starts to burn spontaneously without flame. The as prepared material is very light and downy. The final thermal treatments are 900 °C/12 h and 900 °C/1 h for LiCo0.8Ni0.1Mn0.1O2 and LiNi0 65Co0.25Mn010O2 respectively. X-ray diffraction patterns of the powdered samples were obtained with a STOE STADI/P diffractometer (Mo-Ka1 radiation, curved Ge (111) monochromator, transmission mode, data step width 0.02° (29), linear PSD counter). Electrochemical properties measurements were performed in lithium cells containing a lithium foil as negative electrode. Positive electrodes were prepared by spreading a mixture of 85 % active material, 10 % carbon black, and 5 % of PVDF [poly(vinylidene fluoride)] in NMP (1-methyl-2pyrrolidinone) onto an aluminum foil. The electrolyte was 1 M LiPF6 (lithium hexafluorophosphate) dissolved in a 2:1 volume ratio solution of EC (ethylene carbonate) and DEC (diethyl carbonate). Cells were cycled galvanostatically using a multi-channel potensiostat (VMP2/Z; Ametek) battery testing system.
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Fig. LiCo0
2 shows 8Ni0.1Mn0.
(0p3)
Results and discussion
the X-ray diffraction patterns of 1O2 and LiNi0.65Co0.25Mn0.10O2.
LiCo„ Ni„ Mn„ 0„
o observed -calculated
R = 0.04 , R = 0.06; R = 0.02
(104)
(018) (no)
I 1/
I I
II II
(003)
■—^—
(104)
~r 30
40
50
20Mo/°
0 observed — calculated
R =0.05; R =0.06; R = 0.04
(018)l( 110)
I III I I [ II I
II II II
T 20
« 29mo/° 50
Fig. 2.
X-ray diffraction patterns ofLiCo08Ni0jMn0jO2 and LiNio. 65Co0.25Mn0.10O2
All diffraction peaks can be indexed based on the a-NaFeO2-type structure (Space Group R-3m). The relative intensity of (003) to (104) peaks and the splitting of (018)/(110) diffraction lines clearly indicate a good 2D character of the structure. As shown by Fig. 3, Li and transition metal ions (Ni,Co,Mn) occupying alternate layers with an octahedral environment. The structural refinement of the X-ray data, using the Rietveld method [8], was done assuming that Li, M (M: Ni,Co,Mn) and O occupy (001/2); (000) and (00z) atomic coordinates. It gives a good reliability between the calculated and observed profiles as demonstrated by the low values of the Rp, Rwp and RB reliability factors. The hexagonal unit cell parameters for the two compounds are: a = 2,8270(1) A; c = 14,1132(1) A for the cobalt rich phase (LiCo0.8Ni0.1Mn0.1O2) and a = 2,8459(1) A; c = 14,0980(3) A for the nickel rich phase (LiNi0.65Co0.25Mn0.10O2). According to these crystallographic data, we can conclude that within the
(Ni,Mn,Co) sheets, a high covalency exists leading to an easy electronic transfer during the electrochemical process. In the same time, lithium ions establishes only a very weak bonding (Van der Waals type) with the oxygen anions. This also leads to an easy lithium diffusion upon lithium extraction or insertion. We could then expect that this kind of compounds exhibit a convenient features to be used as positive electrode materials in lithium-ion batteries.
Fig. 3. Structure of the LiCo0.8Ni0.1Mn01O2 and LiNi0.65Co0.25Mn0.¡0O2 oxides showing lithium ions between the (Ni,Co,Mn)O6 edge sharing octahedra
The two materials have been used separately as cathode materials in lithium batteries in order to study their electrochemical behavior. The voltage vs capacity profiles during the first charge are plotted in Fig. 4. For the cobalt rich phase LiCo0.8Ni0.iMn0.iO2, they are two voltage domains separated by a potential plateau at 3.85 V: the first domain is from 3.6 to 3.8 V, the second from 3.85 to 4.5 V. While for the nickel rich phase LiNi0.65Co0.25Mn0.10O2, a continuous evolution of the potential was evidenced, showing that only one electrochemical process is involved during the lithium extraction from this phase. For the cobalt rich phase, the charge process of the Li//LiCo0.8Ni01Mn01O2 battery starts by the oxidation of Ni3+ to Ni4+ ions at about 3.6 V. The oxidation of all 0.1 nickel ion present in the starting phase corresponds to the extraction of 0.1 lithium ions (about 55 mA-h/g). After this redox process, Co3+ ions start its oxidation process which corresponds to about 3.9 V. The preferential oxidation of Ni3+ compared to Co3+ ions was already demonstrated by studying the physical properties (Li NMR spectroscopy, magnetism) of the LixNi1-yCoyO2 system [9-12].
International Scientific Journal for Alternative Energy and Ecology № 6 (62) 2008
© Scientific Technical Centre «TATA», 2008
Specific capacity / mA-hg-1
Specific capacity / mA-hg"1
Fig. 4. The voltage vs capacity profile of the Li// LiCoa8Nio.1MnalO2 andLi//LiNi065Co025Mn010O2 electrochemical cells cycled in the same regime C/20 which corresponds to the extraction of one Li ion in 20 hours
Furthermore, the electrochemical cycling of these two positive electrode materials occurs without structural modifications and with a small variation of the unit cell parameters. This result, which will be published soon, is important from the applications point of view. Indeed, deterioration of the electrochemical performances (long cycle life, cyclability...) is strongly related to the structural stress in the cathode material resulting from the structural changes in the symmetry or/and in the unit cell volume that occur during the cycling [13]. The sample with larger amount of cobalt delivers a charge capacity of 196 mAh/g, which is 35% greater compared to that delivered by the Li//LiNi0.65Coo.25MnaioO2 electrochemical cell. Furthermore, the electrochemical cycling (lithium extraction/insertion) of the Li//LiCo08Ni0.1Mn01O2 occurs without significant modifications in the structural features while lithium extraction from LixCoO2 system (the commercialized cathode) involves many structural changes as reported elsewhere [4]. Thus, LiCo08Ni0.1Mn01O2 could be considered as much more competitive cathode than LiNi0.65Co0.25Mn0.10O2 for application in the real batteries.
Conclusions
LiCoo.8Nio.iMno.iÜ2 and LiNi0.65Co0.25Mn0.10O2 were prepared by the simple combustion method using sucrose as fuel. From the results of X-ray diffraction, these two samples were characterized by an a-NaFeO2 type structure with alternating lithium layers with transition metal slabs. In the two-electrode electrochemical cells, LiNi065Co025Mn010O2 shows a smooth charge curve with continuous evolution of the potential versus capacity which suggests less structural transformations with intercalation of lithium. For the cobalt rich phase LiNi0.65Co0.25Mn0.i0O2, two electrochemical processes are involved during lithium extraction: the first one corresponds to the oxidation of the Ni ions while the second one corresponds to the Co4+/Co3+ redox couple. For this last phase, about 196 mAh/g can be delivered during the electrochemical cycling which makes this positive electrode material competitive from applications point of view.
Acknowledgements
The authors would like to thank the French Ministry of Foreign Affairs, Agence Française de Développement AFD and Institut de Recherche pour le développement IRD for the financial support under the CORUS program (project No. 02 211 121).
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