Synthesis and characterization of integrated layered nanocomposites for lithium ion batteries
© Gim et al; licensee Springer. 2012
Received: 20 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
The series of Li[Ni x M x Li1/3-xMn2/3-x]O2 cathodes, where M is cobalt or chromium with a wide compositional range x from 0 to 0.33, were prepared by hydroxide coprecipitation method with subsequent quenching. The sample structures were investigated using X-ray diffraction results which were indexed completely on the basis of a trigonal structure of space group with monoclinic C2/m phase as expected. The morphologies and electrochemical properties of the samples obtained were compared as the value of x and substituted transition metal. The particle sizes of cobalt-substituted Li[Ni x Co x Li1/3-xMn2/3-x]O2 samples are much smaller than those of the Li[Ni x Cr x Li1/3-xMn2/3-x]O2 system. The electrode containing Li[Ni x Co x Li1/3-xMn2/3-x]O2 with x = 0.10 delivered a discharge capacity of above 200 mAh/g after 10 cycles due to the activation of Li2MnO3.
PACS: 82.47.Aa; 82.47.-a; 82.45.Fk.
The development of rechargeable lithium ion batteries depends critically on the technological advances in electrode materials. Over the years, several compounds such as spinel LiMn2O4, olivine LiFePO4 , and layered LiCoO2 and LiNiO2 have been studied extensively by many researchers as cathode materials for lithium ion batteries. In fact, LiMn2O4 and LiFePO4 have distinct advantages of being cost-effective and environmentally benign. However, LiMn2O4 suffers from capacity fading due to the dissolution of manganese and Jahn-Teller distortion [2, 3], while LiFePO4 delivers insufficient capacity and low electronic conductivity .
Commercially used LiCoO2 cathode has advantages of easy synthesis and excellent lithium ion mobility though challenging issues of stability, achieving practical capacities, and environmental risks need to be addressed . The layer-structured rhombohedral LiMnO2 () attracts interest as a potential cathode due to its cost effectiveness and relatively high capacity, but it exhibits severe capacity fading during extended cycling. More precisely, its discharge behavior during electrochemical cycling needs significant improvement. The strategies to overcome such limitations in rhombohedral LiMnO2 have been focused on metal ion substitution [5, 6]. Due to its higher theoretical capacity, LiNiO2 has also been investigated as an alternative cathode to commercial LiCoO2. However, it is complicated to synthesize a pure-layered structure with a well-ordered phase because of severe cationic disordering between nickel and lithium ions that occurs due to the ionic radii values of Ni2+(0.069 nm) and Li+(0.068 nm) being almost similar. Further, capacity fading occurs during discharge since the electronic state in low spin Ni3+ serves as the satisfactory condition for the Jahn-Teller distortion observed in the spinel LiMn2O4.
In light of the above discussions, many researchers have investigated on the strategies to replace LiCoO2. First, alien transition metal ions such as Ni, Mn, and Cr could be introduced in order to exploit their advantages of stable and high redox-couple properties. Second, by combining stable Li2MnO3 as an inactive frame with layered LiMO2, lithium-saturated solid solutions or nanocomposite x Li2MnO3·(1-x)LiMO2 with prolonged structural integrities have been researched to take advantage of their stable and rigid structure [7–11]. Here, Li2MnO3, which has a layered rock salt structure (space group ) with a monoclinic phase (C2/m), can be represented in layered form as Li[Li1/3Mn2/3]O2. Further, the nanocomposites can be represented by the notation, Li[M1-xLix/3Mn2x/3]O2 with a layered structure [12–14]. Our earlier work was focused on investigating one such nanocomposite electrode namely, 0.4Li2MnO3·0.6LiMO2 (M = Ni1/3Co1/3Mn1/3 and Ni1/3Cr1/3Mn1/3) . The encouraging results obtained from that study led us to investigate the physicochemical properties of the doped nanocomposites with a layered structure over a range of stoichiometric compositions.
Therefore, the present work reports on the synthesis and systematic investigations on the structure, morphology, and electrochemical performances of an integrated layered nanocomposite system, viz Li[Ni x M x Li1/3-xMn2/3-x]O2, where M is cobalt or chromium with a wide compositional range x from 0 to 0.33. Ultimately, it is aimed to arrive at the optimized compositions (x) of Co and Cr in the integrated nanocomposite that exhibit impressive electrochemical properties.
Lithium hydroxide monohydrate (98.0% to approximately 102.0%; Junsei Chemical Co., Ltd., Chuo-ku, Tokyo, Japan), manganese acetate tetrahydrate (97%; Yakuri Pure Chemicals Co., Ltd., Kyoto, Japan), nickel acetate tetrahydrate (98.0%, Junsei Chemical Co., Ltd.), Cobalt acetate tetrahydrate (98.5%, Junsei Chemical Co., Ltd.) and Chromium acetate (22% as Cr, Wako Pure Chemical Industries, Ltd., Chuo-ku, Osaka, Japan) were used as precursors for the solution synthetic method. The samples with different stoichiometric compositions in the layered Li[Ni x M x Li1/3-xMn2/3-x]O2 system where x = 0, 0.05, 0.1, 0.17, 0.24, and 0.33 were prepared by coprecipitation method. In brief, the transition metal acetate precursors and lithium hydroxide were dissolved separately in distilled water. The aqueous solution of lithium hydroxide was then slowly dripped into the transition metal solution to facilitate hydroxide coprecipitation at room temperature for 24 h. The precipitated solution was subsequently dried in an oven at 85°C to evaporate residual water, and the dried powders were ground well before heating at 600°C for 3 h to eliminate undesired organic materials that remained. The heated powders were ground completely and then fired at 900°C for 12 h for crystallization. The resultant powders were obtained after quenching the fired powders using two copper plates in air and subsequent grinding. The final products were obtained after washing with distilled water to remove unwanted impurities such as Li2CrO4 and subsequent vacuum drying at 120°C.
Structural and physical characterization
The crystalline nature of the obtained samples in the Li[Ni x M x Li1/3-xMn2/3-x]O2 system were characterized by X-ray diffraction [XRD] using a Shimadzu X-ray diffractometer (Shimadzu Corporation, Nakagyo-ku, Kyoto, Japan) with Ni-filtered Cu-Kα radiation (λ = 1.5406 Å) operating at 40 kV and 30 mA within the scanning range angle from 10° to 80° (2θ). Inductively coupled plasma atomic emission spectrometer [ICP-AES] analysis utilizing PerkinElmer OPTIMA 4300 DV (PerkinElmer, Waltham, MA, USA) was performed to confirm the compositions of the obtained materials. The particle morphologies and sizes were observed by field-emission scanning electron microscopy [FE-SEM] using the HITACHI S-4700 instrument (Hitachi High-Tech, Minato-ku, Tokyo, Japan). The sample surface areas were measured by the Brunauer Emmett and Teller [BET] method using a surface area analyzer (ASAP 2020, Micromeritics Instrument Co., Norcross, GA, USA).
The electrochemical properties of the cathodes fabricated from the samples in the Li[Ni x M x Li1/3-xMn2/3-x]O2 system were evaluated using the NAGANO battery tester system 2004H equipment (NAGANO KEIKI Co., LTD, Ohta-ku, Tokyo, Japan). The fabricated cathode consisted of 72 wt.% active materials, 10 wt.% conductive carbon (Ketjen black), and 18 wt.% polytetrafluoroethylene as binder. The pasted film was then pressed onto a stainless steel mesh with a 2-cm2 area and dried under vacuum at 120°C for 12 h. The electrolyte employed was a 1:1 (v/v) mixture of ethylene carbonate and dimethyl carbonate containing 1 M LiPF6. A 2032 coin-type cell which consists of the cathode and lithium metal anode separated by a polymer membrane was fabricated in an Ar-filled glove box and aged for 12 h. The cells assembled were tested with 0.1 mA/cm2 of current density in the voltage range from 2.0 to 4.8 V.
Results and discussion
The Li[Ni x Co x Li1/3-xMn2/3-x]O2system
The ICP data confirming the stoichiometries of the prepared Co-doped samples and the corresponding BET values.
Measured stoichiometry (Ref:Mn)
x = 0.33
x = 0.24
x = 0.17
x = 0.10
x = 0.05
x = 0
The Li[Ni x Cr x Li1/3-xMn2/3-x]O2system
The ICP data confirming the stoichiometries of the prepared Cr-doped samples and the corresponding BET values.
Measured stoichiometry (Ref:Mn)
x = 0.33
x = 0.24
x = 0.17
x = 0.10
x = 0.05
x = 0
In summary, structurally integrated nanocomposite materials belonging to the system, Li[Ni x M x Li1/3-xMn2/3-x]O2 where M is Co or Cr, were synthesized by hydroxide coprecipitation method and subsequent quenching process. The XRD patterns of all the prepared nanocomposite samples were well indexed to the trigonal (R3m) structure and monoclinic (C2/m) phase. However, obtaining the target stoichiometric composition is not trivial due to the reactivity of lithium at elevated temperatures. The average particle size of the crystallites in the Li[Ni x M x Li1/3-xMn2/3-x]O2 system is dependent on whether the transition metal of M is Co or Cr. In the case of the Co-substituted system, particle sizes were much smaller than those in the Li[Ni x Cr x Li1/3-xMn2/3-x]O2 system. Consequently, impressive electrochemical properties were attained since discharge capacities as high as 200 mAh/g and above were registered after the initial 10 cycles for the sample with x = 0.10 in the Li[Ni x Co x Li1/3-xMn2/3-x]O2 system. Further studies focused not only on the co-existence of R3m and C2/m, but also investigation on the local structure characterization will be required in detail using advanced analysis such as transmission electron microscopy and nuclear magnetic resonance.
This work was supported by the Korea Research Foundation grant (KRF-2007-313-D00950) and by the Basic Research Laboratories Program of National Research Foundation of Korea (NRF). In addition, this research was also supported by the Human Resources Development of Korea Institute of Energy Technology Evaluation and Planning (KETEP) with the grant funded by the Korean government's Ministry of Knowledge Economy (20114010203100).
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