Synthesis and Electrochemical Performance of Layered Li1-xNax[Li0.1Ni0.2Co0.3Mn0.4]O2 Cathode Materials for Lithium Secondary Batteries

Abstract

The presently commercialized lithium-ion batteries use layer structured LiCoO2 which is extremely well-suited for use in cathode. An intensive search for new cathode materials has been investigated over the years due to the high cost and toxicity of cobalt. One of the most attractive cathode materials is the spinel LiMn2O4 and its derivatives due to their low cost, abundance, and nontoxicity [1-2]. However, the spinel LiMn2O4 and its derivatives demonstrate smaller discharge capacity than layer structured materials and a slow capacity loss at elevated temperature [3-4]. Therefore, many research groups have been studied to prepare layered LiMnO2 with the α-NaFeO2 structure as LiCoO2 and LiNiO2. However, the LiMnO2 materials transform to spinel-like phase during cycling only by a minor cationic arrangement, leading to degradation of electrode performance. In order to stabilize the layer structure, many research groups have been studied the Mn-substituted LixMyMn1-yO2 (M= Al, Cr, Ga, Co, Ni) [5-9]. Recently, Lu et al. reported a layer-structure Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O2 (x=1/3, 5/12, and 1/2) based on Li2MnO3 by substitution of Li+ and Mn4+ by Ni2+ while maintaining the remaining Mn atoms in the 4+ oxidation state [10]. In this report, we present pillaring cation effect on Li[Li0.1Ni0.2Co0.3Mn0.4]O2 material by substitution of Li+ in 3a site by Na+. The Li1-xNax[Li0.1Ni0.2Co0.3Mn0.4]O2 (x=0.05, 0.1, and 0.2) materials were prepared by sol-gel method using glycolic acid as a chelating agent. The as-prepared powders were confirmed to have well-defined α-NaFeO2 hexagonal structure except for x=0.2 (Fig. 1). The cycling performances of samples are shown in Fig. 2. For the Li0.95Na0.05[Li0.1Ni0.2Co0.3Mn0.4]O2 sample, it delievers lower initial discharge capacity than unsubstituted Li[Li0.1Ni0.2Co0.3Mn0.4]O2. But it shows excellent capacity retention and higher discharge capacity than Li[Li0.1Ni0.2Co0.3Mn0.4]O2 after 13th cycle. It was due to the stabilized structure as larger pillaring Na+ ions (1.02 A) support the 3b lithium layer during repeated de-/intercalation of lithium. From above result, we believe that Na substitution is very effective to enhance the structural stability and capacity retention.