Potapenko, A. V.

Electronic Repository Kyiv National University of Technologies and Design

 Promising Materials and Processes in Technical Electrochemistry. Part 1  UDC 544.6.076.324.4 + 544.016   ELECTROCHEMICAL PROPERTIES OF LiMn2O4  SURFACE-MODIFIED WITH LiNi0.5Mn1.5O4   POTAPENKO A.V Joint Department of Electrochemical Energy Systems, 38а Vernadsky Blvd., Kiev 03680, Ukraine avoloshka@ukr.net  Surface modified samples of LiMn2O4 coated with LiNi0.5Mn1.5O4 have been synthesized by a citric acid aided method and tested as a cathode material in sample lithium ion batteries. It has been been found that the surface layer of LiNi0.5Mn1.5O4 is easily permeable for Li+ ions. The surface modified samples are able to endure current loads of 20C (2940 mAh/g) without degradation and after heavy-duty tests deliver the same specific capacity as before current loading.  The growing need in renewable energy storage is enhancing the research on new materials and the ways of their improvement. Nowadays, substituted lithium manganese spinels are used as cathode materials for rechargeable lithium ion batteries due to their low cost and high potential vs. Li/Li+. These are promising for numerous applications, in spite of capacity fading upon cycling, which hinders their commercialization. In view of this fact, numerous methods of enhancing their properties have been suggested. One of them is based on the surface modification of the electrode material [1]. In this case, the composition of a solid layer formed on the surface of the mother material is changing, and less degradation of the electrode and electrolyte solution occurs.  In our recent papers, two types of lithium manganese spinels, LiMn2O4 and LiNi0.5Mn1.5O4, have been studied in one and the same electrolyte, viz. a 1 mol/L solution of LiPF6 in a mixture of ethylene carbonate/dimethyl carbonate with a mass ratio of 1:1 [2-4]. It has been found that in spite of a considerably higher working potential range of the latter material, it is extremely stable on cycling even at high current loads.  Therefore, the aim of this work was to obtain the lithium manganese spinel coated with LiNi0.5Mn1.5O4. It might be helpful in achieving more efficient cycling of the electrode material due to the presence of a protective “shell” on the surface of the stoichiometric lithium manganese spinel.   33   Promising Materials and Processes in Technical Electrochemistry. Part 1  Research Methodology Materials under investigation were synthesized by the citric acid aided procedure described in Refs. [5,6] using solutions of lithium nitrate, nickel (II) nitrate, manganese (II) nitrate, and citric acid (all of analytical grade) mixed in proper molar ratios. In order to modify their surfaces, a pyrolysed precursor of LiMn2O4 was soaked with the solution containing lithium, nickel, manganese nitrate, and citric acid, so as to obtain a layer of LiNi0.5Mn1.5O4 on its surface after pyrolysis. The mass ratio of LiMn2O4 and LiNi0.5Mn1.5O4 was adjusted to 1:0.1. The temperatures of pyrolysis and further annealing of pyrolysed samples were 400 °C and 700 °C, respectively. For more details of the procedure see our Refs. [2-4]. Electrochemical measurements were performed in model CR2016 coin cells on a home-made automated electrochemical workstation using cyclic voltammetry (CV) and galvanostatic charge/discharge cycling methods. Cells with a lithium metal anode serving as a counter and reference electrode, a cathode made of the material in question, a Celgard 2500 separator membrane, and a 1 mol/L solution of LiPF6 in a mixture of ethylene carbonate/dimethyl carbonate with the mass ratio of 1:1 were assembled in a dry glove box. The working electrodes were made of 82% of the material under consideration, 10% of a conducting additive and 8% of a poly(vinylidene difluoride) binder.   Results and Discussion In Fig. 1, CV curves for neat LiMn2O4, LiMn2O4 modified with LiNi0.5Mn1.5O4, and neat LiNi0.5Mn1.5O4 are compared. The data for neat spinels well correspond to those published [2,3]. The surface modified material perfectly reproduces the features of the neat LiMn2O4. The presence of LiNi0.5Mn1.5O4 on the surface of the lithium manganese spinel is recognizable as reversible waves at potentials around 4.4-4.9 V.  Galvanostatic charge/discharge cycling data within the 3.5-4.5 V voltage range reveal that the surface modified samples behave like the samples of the neat LiMn2O4. This means that the surface layer of LiNi0.5Mn1.5O4 is easily permeable for Li+ ions. Clearly, much work should be done in order to determine the role of the modifier and changes in the resistance of LiMn2O4 after covering with LiNi0.5Mn1.5O4. However, the dependence of capacity retention on the current load presented in Fig. 2 reveals that the surface modified samples are able to endure the current loads of 20C (2940 mAh/g) without degradation and after heavy-duty tests deliver the same specific capacity as before current loading.  34   Promising Materials and Processes in Technical Electrochemistry. Part 1  3.6 3.8 4.0 4.2 4.4 4.6 4.8-1500150300U, V-200-1000100200I, mAh/g-300-1500150300LiMn2O4/LiNi0.5Mn1.5O4 LiMn2O4  LiNi0.5Mn1.5O4  Figure 1. Cycling voltammograms for LiMn2O4, LiNi0.5Mn1.5O4, and surface modified sample LiMn2O4/LiNi0.5Mn1.5O4. 0 10 20 30 40 50 6030405060708090100  1C control20C15C12C10C8C6C4C2C1CQ, mAh·g-1Number of cycle0.5C Figure 2. Dependence of discharge capacity on cycle number for the surface modified LiMn2O4/LiNi0.5Mn1.5O4 sample. Numbers in the figure mean current loads in C units (1C=147 mAh/g) 35   Promising Materials and Processes in Technical Electrochemistry. Part 1  Conclusions The surface modified samples of LiMn2O4 coated with LiNi0.5Mn1.5O4 have been synthesized by a citric acid aided method. Electrochemical tests reveal that the surface layer of LiNi0.5Mn1.5O4 is easily permeable for Li+ ions. The surface modified samples are able to endure the current loads of 20C (2940 mAh/g) without degradation and after heavy-duty tests deliver the same specific capacity as before current loading.  Acknowledgments The author is grateful to Dr. S.A. Kirillov for discussions, advice and comments.  References [1] Z. Chen, Y. Qin, K. Amine, Y.-K. Sun, Role of surface coating on cathode materials for lithium-ion batteries, Journal of Materials Chemistry, 20, 7606–7612 (2010). [2] A.V. Potapenko, S.I. Chernukhin, I.V. Romanova, K.Sh. Rabadanov, M.M. Gafurov, S.A. Kirillov, Citric acid aided synthesis, characterization, and high-rate electrochemical performance of LiNi0.5Mn1.5O4, Electrochimica Acta, 134, 444-449 (2013). [3] A.V. Potapenko, S.I. Chernukhin, S.A. Kirillov, A new method of pretreatment of lithium manganese spinels and high-rate electrochemical performance of Li[Li0.033Mn1.967]O4. Materials for Renewable and Sustained Energy 3, 40-47 (2014). [4] A.V. Potapenko, S.I. Chernukhin, I.V. Romanova, K.Sh. Rabadanov, M.M. Gafurov, S.A. Kirillov, High-rate Electrochemical Performance of Nanosized LiNi0.5Mn1.5O4, International conference «Nanomaterials: Application and Properties (NAP-2013)», Sumi, Ukraine, 2, 04NEA06 (4 pp) (2013). [5] I.A. Farbun, I.V. Romanova, T.E. Terikovskaya, D.I. Dzanashvili, S. A. Kirillov, Complex formation in the course of synthesis of zinc oxide from citrate solutions, Russian Journal of Applied Chemistry, 80, 1773-1778 (2007). [6] I.A. Farbun, I.V. Romanova, S.A. Kirillov,  Optimal design of powdered nanosized oxides of high surface area and porosity using a citric acid aided route, with special reference to ZnO, Journal of Sol-Gel Science and Technology, 68, 411-422 (2013).        36  

Electrochemical properties of LiMn2O4 surface-modified with LiNi0.5Mn1.5O4

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Electrochemical properties of LiMn2O4 surface-modified with LiNi0.5Mn1.5O4

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