Synthesis of hierarchical mesoporous lithium nickel cobalt manganese oxide spheres with high rate capability for lithium-ion batteries

Highlights

• Hierarchical mesoporous LiNi_1/3Co_1/3Mn_1/3O₂ spheres were synthesized.

• The possible formation mechanism is speculated.

• The samples show good rate performance.

Abstract

Hierarchical mesoporous LiNi_1/3Co_1/3Mn_1/3O₂ spheres have been synthesized by urea-assisted solvothermal method with adding Triton X-100. The structure and morphology of the as-prepared materials were analyzed by X-ray diffraction and electron microscope. The results show that the as-prepared samples can be indexed as hexagonal layered structure with hierarchical architecture, and the possible formation mechanism is speculated. When evaluated as cathode material, the hierarchical mesoporous LiNi_1/3Co_1/3Mn_1/3O₂ spheres show good electrochemical properties with high initial discharge capacity of 129.9 mAh g⁻¹, and remain the discharge capacity of 95.5 mAh g⁻¹ after 160 cycles at 10C. The excellent electrochemical performance of the as-prepared sample can be attributed to its stable hierarchical mesoporous framework in conjunction with large specific surface, low cation mixing and small particle size. They not only provide a large number of reaction sites for surface or interface reaction, but also shorten the diffusion length of Li⁺ ions. Meanwhile, the mesoporous spheres composed of nanoparticles can contribute to high rate ability and buffer volume changes during charge/discharge process.

Introduction

Lithium-ion batteries (LIBs) have attracted great interest as renewable energy sources, due to their higher energy densities, suitable design for portable applications and longer lifetimes than comparable battery technologies [1], [2], [3], [4]. Cathode materials hold the key to making further development for LIBs. As one of cathode materials of LIBs, LiNi_1/3Co_1/3Mn_1/3O₂ has been considered as one of the candidate cathodes owing to its high operating voltage, high capacity, and low cost. However, high rate performance of LiNi_1/3Co_1/3Mn_1/3O₂ has been one of the critical problems in the development of high-power LIBs [5], [6], [7]. Reducing the particle size to nanoscale level has been demonstrated as an approach to improving the rate capability of LIBs, which can shorten Li⁺ ions diffusion and electron transfer distance, leading to enhance electrochemical performances [8], [9]. Therefore, substantial efforts have been made to develop nanoscale materials with high rate capability. Unfortunately, the biggest challenge for nanoscale materials is that they suffer from the electrolyte decomposition on their enhanced specific surface [10]. In addition, nanoscale materials usually have low packing density, leading to low volumetric density, and the spalling nanoparticles from the electrode surface can cross the separator, causing internal shorts triggered [11]. It has been found that microscale level particles can achieve high packing density. However, their kinetics, Li⁺ ions intercalation capacity, and structure stability is limited by the microscale framework [12]. Therefore, hierarchical architecture materials have attracted considerable attention owing to synergy effect of nanometer-sized particles and secondary micro structure [13]. It is noteworthy that mesoporous materials may greatly improve power rate during electrochemical process because of the large surface area, abundant channels for Li⁺ ions. In addition, they also can accommodate the volume expansion during charge/discharge process, and facilitate electrolyte and ions transport [14], [15], [16]. From the point of view, it is expected to utilize the synergistic effect of hierarchical architecture in conjunction with mesoporous structure. Because hierarchical mesoporous materials have some advantages: First, nanoparticles can provide short transport paths both for electron and Li⁺ ions, and particles at micrometer scale can guarantee good stability [17], [18]. Second, when nanoparticles were interconnected and assembled into micrometer-sized secondary particles, a mesoporous structure was formed. Mesoporous and void space within materials can provide extra space to store Li⁺ ions and accommodate volume change during cycling process, which can be beneficial to improve specific capacity [19], [20]. Third, the large surface area can enlarge electrode/electrolyte interfacial contact to afford superior Li⁺ ions insertion/extraction kinetics, leading to a high charge/discharge capacity [21]. Hence, much effort has been devoted to design hierarchical mesoporous materials to enhance electrochemical performances. For example, Fang et al. [22] synthesized hierarchical mesoporous layered-cube Mn₂O₃ with excellent rate capability and cycle stability. Cai et al. [23] fabricated hierarchical mesoporous Li₄Ti₅O₁₂ microspheres with excellent discharge capacity of 92.1 mAh g⁻¹ at 100C. Li et al. [14] designed hierarchical mesoporous Li[Li_0.2Ni_0.2Mn_0.6]O₂ material with high discharge capacity of 152.4 mAh g⁻¹ at 5C.

Based on the above discussion, hierarchical mesoporous LiNi_1/3Co_1/3Mn_1/3O₂ spheres were synthesized by solvothermal method with adding Triton X-100 in this work. The possible formation mechanism is speculated. The as-prepared material shows not only excellent cycle performance, but also better rate performance than other LiNi_1/3Co_1/3Mn_1/3O₂ materials reported previously [24], [25], [26]. In addition, the relation between structure and electrochemical properties of the hierarchical mesoporous LiNi_1/3Co_1/3Mn_1/3O₂ spheres is also discussed in details.