Electrochemical Activity and Li-Disordering of Li2MnO3 by Coexisting with CuO
Authors: Arachi Yoshinori , Taura Yuu , Akiyama Shinya , Nakata Yoshiyuki 
Published: February 2020

For many years, challenges to develop lithium ion secondary batteries with high energy density have been extensively investigated. Li2MnO3-based positive electrodes such as LiNi1/3Mn1/3Co1/3O2 have attractive cell performances which are suitable for the electrical vehicles using lithium ion batteries. These materials include a large amount of Li which is essential for large capacity as a positive electrode. However, Li2MnO3 itself without Co or Ni has a poor electrochemical activity except for those of prepared at lower temperatures such as 773 K. Recently, we have found out the way to activate Li2MnO3 electrochemically by coexisting with CuO, not by substitution [1]. We try to clarify the reason why the existence of CuO induces the electrochemical activity of Li2MnO3 focusing on the crystal structure by SR-XRD, XAFS and ab-initiocalculation. In addition, a positive material with new composition exhibiting good electrochemical performance will be presented.

EXPERIMENTAL

The samples were prepared by combining coprecipitation and solid state reaction. The compositions of them are x = 1/5, 1/4, 3/8, 1/2 in (1-x)Li2MnO3-xCuO respectively. After dissolving CuSO4 and Mn(CH3COO)2 into the distilled water, the co-precipitates were obtained by changing pH from 10 to 12. Dry precipitates and LiOH·H2O were mixed and after calcination at 743 K, sintered at 973 K for 12hr under O2 flow. Electrochemical testing was carried out using coin-type cells with Li/1M LiPF6 in EC:DMC(3:7)/samples. X-ray diffraction (XRD) measurements using both CuKα radiation and a synchrotron radiation source were performed and for the latter on BL02B2 and BL19B2 at SPring-8. Structural refinements were carried out by Rietveld analysis using the RIETAN-FP program[2]. X-ray absorption measurements of above samples at the Mn and Cu K-edges by transmission method were performed on BL14B2 at SPring8. The first principle calculations were carried out using the WIEN2k and VASP program packages.

RESULTS AND DISCUSSIONS

To clarify the reason why coexisitng with CuO enhances the electrochemical activity of Li2MnO3, at first we fouced on the crystal structure of Li2MnO3 and carried out Rietveld analysis using SR-XRD for all the investigated samples, based on the structural model of monoclinic Li2MnO3, S.G. C2/m. It was found that Li and Mn locate on both 2b and 4g sites of Wyckoff positions. A disordering of Li and Mn in the structure of Li2MnO3 increased by CuO contents. The results of neutron diffraction patterns supports those of XRD. These disordering behaviour probably may correlate closely with the electrochemical activity of Li2MnO3. In addition, a significnat difference of electron diffraction patterns was observed between x = 1/5 which showed discharge capacity of 160 mAhg-1 and x = 1/2 that of 355 mAhg-1. A distinguished streaks along c* axis appeared for x = 1/2 suggesting a stacking fault, wheares some diffraction spots did for x = 1/5. A similar results for Li(Ni, Mn, Co)O2 are rescribed in ref. [3]. The coexsisting with CuO intoroduce a stacking fault into Li2MnO3. Next, we considered an effect of partial substiontion of Cu. The ab-initio electronic structure calculation allows us to know more about the electrical property. Density of States (DOS) of our assumed supercell, Li16(Mn7Cu1)O24 indicated a new sharp band which expect to show higher electrical conductivity than that of Li2MnO3 itself. Hence, the partial replacing Mn by Cu suggests an improvement of the electrochemical property. Next, we focused on CuO coexisting with Li2MnO3. Whereas a pure CuO is an insulator, Li-doped CuO could be a semiconductor. Therefore, it is reasonable that the kinetic barrier of the electrode reaction for Li2MnO3was depressed by coexisting with CuO.
We will present a new way to activate Li2MnO3 electrochemically by coexisting CuO, not by substitution and discuss the mechanism why coexisitng with CuO enhances the electrochemical activity of Li2MnO3based on both experimental and calculation results.

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CKNOWLEDGEMENT

This research was supported by Japan Society for the Promotion of Science (25410255).

REFERENCES

1 Y. Arachi, K. Hinoshita and Y. Nakata, ECS Transactions, 41(29), 1-7(2012).
2 F. Izumi and K. Momma, Solid State Phenom., 130, 15-20(2007).
3 A. Boulineau, L. Croguennec, C. Delmas, F. Weill, Solid State Ionics., 29, 1652 -1659(2010).