Li[Li0.2Ni0.16Mn0.56Co0.08]O2 Nanoparticle/Carbon Composite Using Polydopamine Binding Agent for Enhanced Electrochemical Performance
© Lim and Park. 2015
Received: 23 March 2015
Accepted: 17 June 2015
Published: 26 June 2015
Li[Li0.2Ni0.16Mn0.56Co0.08]O2 nanoparticles were composited with carbon (Super P) in order to achieve an enhanced rate capability. A polydopamine pre-coating layer was introduced to facilitate the adhesion between Super P and pristine nanoparticles. The Super P particles were dispersed on the surface of Li[Li0.2Ni0.16Mn0.56Co0.08]O2 powders. The composite samples that were heat-treated in a N2 atmosphere showed increased capacity and enhanced rate capability, which was caused by the improved electronic conductivity owing to the presence of carbon. However, the composite samples that were heat-treated in air did not present these carbon-related effects clearly. The capacity changes observed during the first several cycles may be due to the oxygen deficiency of the structure caused by the heat-treatment process.
Nowadays, lithium-ion batteries are being used in emerging applications such as electric vehicles, smart mobile devices, and energy storage systems [1–9]. In order to meet the demands of such applications, better cathode materials capable of delivering more energy for advanced lithium-ion batteries are required [10–15]. Among the cathode materials reported so far, lithium-rich layered oxides, Li[NixLi1/3-2x/3Mn2/3-x/3]O2, are some of the most attractive candidates because of their high specific capacity and low cost [16–24]. However, these materials suffer from several problems, such as insufficient cyclic performance due to phase instability and low rate capabilities caused by low electronic and ionic conductivities [25, 26]. As an approach to enhance the rate capability, carbon has been coated on the cathode surfaces. In particular, carbon layers prepared by the in situ carbonization of an organic precursor have been successfully applied to olivine iron-based phosphates such as LiFePO4 [27–29]. However, a similar method is difficult to apply to lithium-rich layered oxides because in situ carbonization requires heat treatment in an inert atmosphere, which may deteriorate the structural integrity of the lithium-rich layered oxides owing to loss of oxygen during the heating process.
Herein, we prepared a composite material using existing carbon powders in order to improve the rate capability of lithium-rich layered oxides. Li[Li0.2Ni0.16Mn0.56Co0.08]O2 nanoparticles, which were prepared via a combustion method, were used as lithium-rich layered oxides. Commercially available Super P was adopted as the carbon material. A special point to be considered in this work is that the pristine (uncoated) powders used were characterized by small-sized and differently shaped grains. Moreover, the Super P particles were also extremely small (0.05–0.1 μm), assuring that a homogeneous mixing and composition between cathode powders and Super P is very difficult to achieve. To overcome this problem, a polydopamine pre-coating layer was introduced as a binding agent between the Li[Li0.2Ni0.16Mn0.56Co0.08]O2 nanoparticles and Super P. The polydopamine coating layer significantly promotes secondary surface-mediated reactions [30, 31], which have been successfully applied to the composition process between carbons and oxides [32–34]. Hence, the polydopamine pre-coating layer on the surface of Li[Li0.2Ni0.16Mn0.56Co0.08]O2 nanoparticles may facilitate the homogeneous adhesion of Super P particles. The Li[Li0.2Ni0.16Mn0.56Co0.08]O2/Super P composition is expected to present an enhanced rate capability due to the high electronic conductivity of surface-attached Super P particles.
Li[Li0.2Ni0.16Mn0.56Co0.08]O2 nanoparticles were prepared using a surfactant-modified combustion method. Manganese acetate tetrahydrate [Mn(CH3CO2)2·4H2O (Aldrich, 99+%)], nickel(II) nitrate hexahydrate [Ni(NO3)2·6H2O (Aldrich, 99.99 %)], cobalt(II) nitrate hexahydrate [Co(NO3)2·6H2O (Aldrich, 98 %)], lithium acetate dihydrate [CH3CO2Li·2H2O (Aldrich, 98 %)], and lithium nitrate [LiNO3 (Aldrich)] were used as source materials. Two types of polymeric materials, gelatin (Aldrich) and hydroxypropylcellulose (HPC, Aldrich), were used as surfactants to control the particle sizes of the cathode materials. The source materials were dissolved in a solvent composed of distilled water and acetic acid. For every 10 g of source material, 1 g of surfactant was added to each solution. The solutions were continuously stirred on a hot plate at 90–110 °C. As the solvent evaporated, the mixed solutions turned into viscous gels. These gels were then annealed at 400 °C for 1 h, during which a vigorous decomposition process occurred, resulting in the formation of an ash-like powder. This powder was ground and then annealed in air, first at 500 °C for 4 h and subsequently at 800 °C for 6 h. Next, the powder was quenched at room temperature.
Results and Discussion
Discharge capacity and capacity retention of the Li[Li0.2Ni0.16Mn0.56Co0.08]O2/Super P composite for various current densities. The listed values are obtained from the initial cycle at each current density
44 mA g−1
110 mA g−1
220 mA g−1
660 mA g−1
1320 mA g−1
In summary, a composite consisting of Li[Li0.2Ni0.16Mn0.56Co0.08]O2 nanoparticles and carbon (Super P) was prepared in order to improve the rate capability of pristine cathodes. A polydopamine layer was pre-coated on the surface of Li[Li0.2Ni0.16Mn0.56Co0.08]O2 nanoparticles as a binding agent to promote a homogenous composition with Super P. The Super P nanoparticles were successfully dispersed and attached on the surface of pristine powders. The electrochemical properties of the composites were highly dependent on the heat-treatment atmosphere. The rate capability and discharge capacity were significantly enhanced when the composites were heat-treated in N2, which is attributed to the high conductivity of the composite owing to the presence of carbon. However, the composites that were heat-treated in air did not clearly show an improved electrochemical performance. The impedance values of the composite samples (especially those heat-treated in N2) were smaller than those of pristine samples both before and after cycling. This result can explain the enhanced rate capability of the composite samples.
This research was supported by the National Strategic R&D Program for Industrial Technology (10043868), funded by the Ministry of Trade, Industry and Energy (MOTIE).
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