Lithium-rich materials are strong candidates for the next generation cathode because of their higher theoretical capacity over 300 mAh g-1[1]. It has been reported that the charge compensation mechanism of the lithium-rich cathode is consisted of both 3d and/or 4d transition metal cation and oxide anion redox reaction [1], and the use of anionic redox leads to the large capacity. The electrode performance is profoundly influenced by the electronic state of transition metals and oxide ions [2]. Stabilizing the anionic redox is a promising approach to design and develop the new cathode materials with high energy density.
The previous researches are mainly focused on the effect of transition metals, like 3d-trantision metal Li2MnO3 and 4d-transition Li2RuO3 [3]. The difference of transition metal changes electronic states of oxide ions and cations, greatly affecting the electrode performance.
This work offers different perspectives from previous works, doping another anion to the Li-rich cathode materials to control the electronic structure of anion. The cation-disordered rock salt type Li1.2Ti0.4Mn0.4O2 cathode [4] and new N-doped cathode materials Li1.2Ti0.4Mn0.4O1.93N0.05 and Li1.2Ti0.4Mn0.4O1.85N0.10 were synthesized and the electrochemical performance were measured to estimate the effect of nitrogen-doping to the Li-rich cathode materials.
Experimental
The cathode materials Li1.2Ti0.4Mn0.4O2-0.015xN0.01x (x=0, 5 and 10) were synthesized by dry mechanical ball-milling. The cathode was prepared from a paste by mixing 70 wt% of as-prepared active materials, 20 wt% of acetylene black and 10 wt% of polyvinylidene difluoride binder in 1-methyl-2-pyrrolidone solvent and then this paste was coated on the Al foil. Li foil used as the counter electrode material and 1 mol dm-3 LiPF6 in an acetonitrile solvent was used as an electrolyte. 2032-type coin cells were assembled in the Ar-filled glove box. The galvanostatic charge discharge measurements were operated at 50 °C or room temperature.
Results and Discussion
The XRD pattern confirmed that each cathode materials have single-phase cation-disordered rocksalt structure. The diffraction peaks are shifted to lower angle with increasing the amount of nitrogen doping, indicating the increasing of lattice constant. The 1st charge of Li1.2Ti0.4Mn0.4O2 exhibited a plateau about 4.5 V as previously reported [4], and about 280 mAh g-1 capacity at C/20 rate. In the subsequent discharge process, the cathode material exhibited 220 mAh g-1. On the other hand, the N-doped cathode Li1.2Ti0.4Mn0.4O1.85N0.10 exhibited about 350 mAh g-1 charge capacity and 280 mAh g-1 discharge capacity during 1st cycling. These results suggest that nitrogen doping improves practical capacity and cycle performance in the Li1.2Ti0.4Mn0.4O2 series. The improvement mechanism of the electrochemical performance will be discussed in the presentation.
References
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[2] K.Luo, M R. Roberts, R. Hao, N. Guerrini, D M. Pickup, Y-S. Liu, K. Edstroem, J. Guo, A V. Chadwick, L C. Duda and P G. Bruce, Nat. Chem. 2016, 8, 684.
[3] M, Sathiya, K. Ramesha, G. Rousse, D. Foix, D. Gonbeau, A.S. Prakash, M. L. Doublet, K. Hemalatha and J-M. Tarascon, Chem. Mater., 25, (2013) 1121
[4] N.Yabuuchi, M.Nakayama, M. Takeuchi, S. Komaba, Y. Hashimoto, T. Mukai, H. Shiiba, K. Sato, Y. Kobayashi, A. Nakao, M. Yonemura, K. Yamanaka, K. Mitsuhara and T. Ohta, Nat. Commn. 7 (2016) 13814
Aknowledgement
This work was supported by JSPS Grant-in-Aid for Scientific Research on Innovative Areas, “Mixed-Anion” (Grant Number JP16H6441).