Electrochemical Activity and Li-Disordering of Li2MnO3 by Coexisting with CuO

For many years, challenges to develop lithium ion secondary batteries with high energy density have been extensively investigated.  Li₂MnO₃-based positive electrodes such as LiNi_1/3Mn_1/3Co_1/3O₂ 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. 

ACKNOWLEDGEMENT

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).