Syntheses and electrochemical properties of the Na-doped LiNi0.5Mn1.5O4 cathode materials for lithium-ion batteries
Introduction
Due to high energy, high power density, high working voltage and excellent cyclic performance [1], [2], lithium ion battery has become one of the main power supply sources for mobile phones, digital cameras, laptop computers and so on. With the lithium ion battery expansion to electric vehicle (EV), hybrid electric vehicle (HEV), plug-in hybrid electric vehicle (PHEV), communication technology and mobile storage devices field [3], the performance of lithium ion battery, especially the power density and working voltage, is proposed higher and higher requirements to satisfy production and people's demands. Performance of lithium-ion batteries, such as working voltage and power density, is mainly determined by the property of the cathode materials. Therefore, the development of high voltage cathode materials is one important research direction for lithium ion battery with high energy density.
Because of its low cost, little toxicity and good safety performance, spinel LiMn2O4 (LMO) is thought to be an ideal cathode material for lithium ion batteries [4], [5]. Lots of studies have shown that the Fermi energies of the materials are improved and their electrode potentials are raised as a result by doping the spinel LiMn2O4 materials with a certain amount of transition metal elements M to prepare LiMn2-xMxO4 (M = Fe Co Ni Cr etc.) [6]. Of the improved cathode materials with spinel structure, the material LiNi0.5Mn1.5O4 has been shown to have an acceptable performance and stable discharge capacity. It has been reported that there are a high discharge platform existing near 4.7 V and a small platform around 4 V [6], [7]. The platform around 4.7 V is assigned to the oxidation-reduction process of Ni2+/Ni4+, while the 4.0 V platform is attributed to the Mn3+/Mn4+ oxidation-reduction process. The specific weight energy of the LNMO is greatly improved compared to the conventional cathode materials such as LiCoO2, LiMn1/3Ni1/3Co1/3O2 [8], [9], LiFePO4 [10], [11] and so on. The LNMO can not only meet the power demands for personal consumer electronics and electric equipment, but also be compatible with high working voltage anode materials such as Li4Ti5O12 [12], [13] and so on. These advances improve the energy density and safety performance of the battery, making it one of the most high-profile positive electrode material for the next generation advanced lithium ion batteries.
There are two crystallographic structures for LNMO, cation orderly P4332 spinel and disorderly Fd3 m spinel [14], [15]. The electrical conductivity of disordered Fd3 m lattice is higher than that of orderly P4332 crystal structure by 2.5 orders of magnitude [14], [16]. Because of the disordering character of Ni and Mn, two additional electron hopping paths happen as follows compared with the P4332 dot matrix: Ni2+/3+ →Mn4+ → Ni3+/4+ and Ni2+/3+ →Mn4+ ↔ Mn3+ →Ni3+/4+ [17]. The additional electron hopping paths contribute to a better charge transfer ability and relieve the ohmic polarization and electrochemical polarization of materials [18], especially in fast charge-discharge process and at the high temperature state. Studies have found that by doping metal ions such as Ru [19], Cr [20], Co [21], Cu [22], W [23], Nb [24], Mg [25] and so on can increase the disorder extent of nickel and manganese in LNMO, eliminating the formation of the LiyNi1-yO impurity, thus improve cycling performance of LiNi0.5Mn1.5O4. However, these heavy metal dopants are expensive, limited and environment-unfriendly. In contrast, Na element is cheap, abundant and environmental friendly, which is a promising dopant to substitute for the heavy metals.
In this work, we have successfully synthesized the Na-doped LNMO. The doping of Na element can increase the cation disordered degree of nickel and manganese of LNMO. When the doping amount was 5% (nNa/n(Na + Li), mole ratio), the material presents optimal cycle performance and rate performance.
Section snippets
Material preparation
The plate-like [Mn0.75Ni0.25](OH)2 were prepared by a co-precipitation method. An aqueous solution of NiSO4·6H2O and MnSO4·5H2O with a concentration of 2.0mol·L−1 was pumped into a continuously stirred beaker (1L) under an N2 atmosphere. At the same time, a mixed solution of NaOH (4.0mol·L−1) and desired amount of NH4OH solution as a chelating agent were pumped into the reactor. The concentration, pH, temperature and stirring speed of the mixture in the reactor were carefully regulated. The
Results and discussion
Fig. 1(a-f) show the SEM images of manganese sources [Mn0.75Ni0.25](OH)2, 1% Na-LNMO, 3% Na-LNMO, 5% Na-LNMO and 10% Na-LNMO, respectively. It is found that the precursor [Mn0.75Ni0.25](OH)2 is of a flake-like shape with an average thickness of 50 nm. After calcination, the Na doped LNMO materials are 100-300 nm polyhedrons with clean and smooth surface facets, which indicate perfect crystal of all the samples. Interestingly, the size of the doped-LNMO particles decreases with the increase of
Conclusions
In summary, the Na-doped LNMO are synthesized via a solid-state method. The obtained materials are of good single-crystal with a particle size of approximately 100-300 nm. The doping of Na ions increases the disorder degree of nickel and manganese ion in the spinel structure and improves lithium ion diffusion coefficient, and two additional electron hopping paths happened, which contribute to a better charge transfer ability and resultantly relieve the ohmic polarization and electrochemical
Acknowledgements
The authors gratefully acknowledge the financial supports from the National High Technology Research and Development Program of China (2012AA110204) and Key Project of Science and Technology of Xiamen (2013H6022). Weiqing Lin expresses his special thanks to National Found for Fostering Talents of Basic Science (J1310024). The authors also wish to express their thanks to Drs. Peng Zhang, Bo Liu, Jing Yang and Binbin Xu of Xiamen University for their valuable suggestions.
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