http://support.spring8.or.jp/Report_JSR/PDF_JSR_31A/2019A1796.pdf

2019A1796

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Aierxiding Abulikemu 1, Toshiyuki Matsunaga 1, Aruto Watanabe 1, Kentaro Yamamoto 1, Tomoki
Uchiyama 1, Shogo Kawaguchi2, Keiichi Osaka2, Yoshiharu Uchimoto 1
1

Graduate School of Human and Environmental Studies, Kyoto University, Sakyo, Kyoto 606-8501, Japan.
2
Japan Synchrotron Radiation Research Institute䠄JASRI䠅, SPring-8, 1-1-1 Kouto, Sayo-cho, Sayo-gun,
Hyogo 679-5198, JapanIn this research, our structural analysis reveals that by doping the Nb into Li2MnO3, the Li2NbxMn1-xO3compounds adopt NaCl-type cubic structure in a wide compositional range, at least between x = 5 at.% andx = 50 at.%. One of these cubic materials, Li2Nb0.15Mn0.85O3 maintains 100% of its structure even after
charge/discharge test up to 50 cycles䠄2.0 – 4.8 V vs. Li/Li+䠅. It implies that Nb plays an important role tostabilize the atomic arrangement of Li and Mn ions in the transition metal layers. And due to its stablestructure as well as 5d electron orbits of Nb can stabilize the oxygen redox䠄which is well known䠅, the cubic
Li2Nb0.15Mn0.85O3 shows highly reversible capacity from the second cycle, its coulombic efficiency remains
stable over 98.5% during the 50 cycles. This indicates Nb doping can substantially modify the structure ofpristine monoclinic Li2MnO3 and thus improve its electrochemical performance, which gives us a promising
pathway to design and optimize the Mn-based lithium rich cathode material.
.H\ZRUG lithium ion battery, lithium rich cathode, rocksalt structure
%DFNJURXQGDQGUHVHDUFKREMHFWLYH With the rapid development of electric vehicles and large-scale energy storage systems in recent years,
Lithium-ion batteries䠄LIBs䠅with higher energy density, better cycle stability and higher safety aredemanded urgently. To meet these requirements, it is crucial to exploit the electrode materials with highelectrochemical performance, expecially the cathode materials. LiMO2䠄M: pure transition metal or mixedtransition metals䠅layered compounds have been extensively used as cathode materials in lithium-ionbatteries. The specific capacities of these compounds are about 150 mAh/g in practical applications.
However, in recent years, the so-called Li-rich layeredcompounds Li2MO3 have been extensivelyinvestigated for use as superior cathode materials for lithium-ion batteries because of their higher specificcapacities than that of LiMO2䠄more than 200 mAh/g䠅, as can be expected from their Li-rich compositions.
Among the various Li2MO3 compounds being explored, Li2MnO3 has been examined intensively. In thismonoclinic crystal with ABC stacked atomic layers, Li/Mn mixing at the transitionmetal layers
topologically forms long Lithium percolation paths in the atomic configuration.In addition, replacement ofMn4+ with other lower-valence transition elements such as Ni2+ decreases the potential energy along thepaths so that Li+ can propagate more readily. This is considered a major contributor to the improved charge
capacity of Li2䠄NixMn1-x䠅O3 compounds. Li2䠄NbxMn1-x䠅O3 is another Li2MO3 compounds, in which Mn issubstituted by Nb; these materials also provide higher charge capacities similar to those of Li2䠄NixMn1-x䠅
O3. And the Li2NbxMn1-xO3 compounds adopt NaCl-type cubic structure in a wide compositional range, atleast between x = 5 at.% and x = 50 at.%. One of these cubic materials, Li2Nb0.15Mn0.85O3 maintains 100%of its structure even after charge/discharge test up to 50 cycles䠄2.0 – 4.8 V vs. Li/Li+䠅. It implies that Nbplays an important role to stabilize the atomic arrangement of Li and Mn ions in the transition metal layers,
but the detailed mechanism of such improvements is not clear yet. In this research, we use various
characterization methods to analyze the structural and electrochemical mechanism about this material’sperformance.
([SHULPHQWDOThe cubic Li2Nb0.15Mn0.85O3 was prepared by the facile solid state reaction. The raw material Mn2O3, Nb2O5,
Li2CO3 were well mixed in acetone then dried, followed by sintering at 950°C for 24 hours in Ar atmosphere.
The Li2Nb0.15Mn0.85O3 cathode electrodes were prepared by slurrying the active material, acetylene black andPVDF with a weight ratio of 8:1:1, then coating on Al foil. The 2032-type coin cells were assembled for the䠉㻝㻞㻠䠉electrochemical test. The counter electrode was lithium metal and electrolyte was 1 M LiPF6 in EC and EMC
solvent䠄3:7 by volume䠅. The charge-discharge test was operated in SD-8 between 2.0 and 4.8 V at 25°C.
The cells at various state of charge and discharge were prepared for the ex-situ hard X-ray diffraction
measurements, which were conducted at BL19B2 beamlines䠄SPring-8, Hyogo, Japan䠅.
5HVXOWVDQG'LVFXVVLRQThe Fig. 1a shows the electrochemical performance of Li2Nb0.15Mn0.85O3 at the rate of 22 mAh/g, the firstcharge capacity is about 265 mAh/g, all of which is supposed to be contributed by the oxygen redox, for theMn4+ is inactive during the charge process. Although it suffers from distinct capacity decay at the first cycle
which is probably caused by the initial SEI formation, it shows highly reversible capacity from the secondcycles. During the cycles, the average discharge voltage platform has decreased from 3.35 V to 2.85 V, which
implies the redox ratio of oxygen and manganese has continuously changed. But as shown in the inset of Fig.
1a, the capacity of Li2Nb0.15Mn0.85O3 is gradually increasing till 27 cycles, and its coulombic efficiency
remains stable over 98.5% during the 50 cycles. Such stable electrochemical performance could be attributedby its specific NaCl-type cubic structure
To clarify whether Li2Nb0.15Mn0.85O3 cathode material remains its structure during the cycle, the hard X-ray
diffraction measurements are applied by using SPring-8 synchrotron radiation. The diffraction patterns ofLi2Nb0.15Mn0.85O3 at various state are demonstrated in Fig. 1b, the pristine material is single phase NaCl-typecubic, while during the 1st and 2nd cycle, there are some non-react parts of active material, which cause double
peaks in the patterns. The position of each peaks shows small but regular shift, it can be explained by thelattice constant change during the charge and discharge process. And it is noticeable that Li2Nb0.15Mn0.85O3
cathode material remains stable of its cubic structure during the cycle, which could be the critical factor ofits highly reversible capacity.

)LJ 䠄a䠅50 cycles charge/discharge curves of Li2Nb0.15Mn0.85O3 cathode at 22 mAh/g, the inset is cyclic performance.
䠄b䠅Hard X-ray Diffraction patterns of pristine Li2Nb0.15Mn0.85O3 and its various charged and discharged state atdifferent cycles.

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The first coulombic efficiency of Li2Nb0.15Mn0.85O3 cathode material still needs improvement by restraining
the initial irreversible SEI formation, and the distinct voltage decay which is common in all Mn-based lithiumrich cathode material also needs to be further investigated.
5HIHUHQFH[1] Matsunaga, T. , Shimoda, K. , Okazaki, K. , Ishikawa, Y. , Yonemura, M. , Ukyo, Y. , Fukunaga, T. andMatsubara, E, Phys. Status Solidi B,  1900003䠄2019䠅.
[2] Robertson, Alastair D. , and P. G. Bruce, ChemInform ,  2790-2791䠄2003䠅.
[3] Rui Wang, Xin Li, Lei Liu, Gerbrand Ceder, et al,Electrochem. Commun,  70-73䠄2015䠅.

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