Lithium metal othosilicate (Li2MSiO4, where M = Fe, Mn, Co or Ni) has currently attracted a good deal of interest as a high voltage positive electrode material due to its high theoretical charge and discharge capacities of 330 mAh g-1 and high voltage up to 5.1 V. Such high capacity arises from the ability to extract two lithium ions per one transition metal. However, this class of material has been reported to exhibit low conductivity and poor cycling performance due to structural instabilities with the transition from orthorhombic to monoclinic phase during charge and discharge.[1]
Mn-site doping with transition metal ions (Ni2+, Cr2+, Ti4+, V5+) or the aliovalent ions (Al3+, Ga3+) can induce defects in the lattice and stabilize overall structure. Since the structural instability is generally caused by the collapse of MnO4 coordination structure to MnO6, works have been done to retain the MO4 structure.[2] A study by Cheng, et al. reported that substituting Cr on Mn site partly helps retain M-O coordination of MnO4.[3] In this study, our research focuses on stabilizing an othosilicate-based positive lithium-ion battery electrode material, specifically, lithium manganese silicate (Li2MnSiO4) via partial substitution of Mn with Cr up to 15 mol%. To understand the effect of Cr doping on structural evolution during the charge/discharge process, the in situ XRD is also conducted to understand the correlation between the electrode material’s structural and electrochemical behavior.
EXPERIMENTAL
Li2Mn1-xCrxSiO4 (x = 0 - 0.15) were prepared by with an acetic acid-assisted sol–gel method. The dried materials were fired at 800°C for 6 hours in Ar atmosphere. Structural properties of the synthesized samples were studied by X-ray diffraction (Rigaku TTRAXII) using Cu-Kα (λ=1.5418 Å) radiation. Particle morphologies and size of the resulting compounds were observed using a field emission scanning electron microscope (FE-SEM, Hitachi SU8030). Electrochemical characterizations were performed by in Swagelok® cells. Active material, conductive carbon black and polyvinylidene fluoride (PVDF) were mixed in the ratio of 87:5:8 by weight. Galvanostatic charge-discharge cycling was done between 1.5-4.8 V at 0.05C using a MACCOR Series 4000 at room temperature.
RESULTS AND DISCUSSIONS
Figure 1 shows XRD patterns of the synthesized Li2Mn1-xCrxSiO4 where Cr = 0 - 0.05. Main diffraction peaks could be indexed as an orthorhombic unit cell with a space group Pmnb. Appearance of the evidence of small amount of monoclinic phase, MnO and Li2SiO3. There is no obvious evidence of a phase containing Cr. With an increasing the Cr doping ratio, the unit cell volume slightly increases from 339.52 Å3 to 339.79 Å3 as expected due to larger ionic radii of Cr2+ than Mn2+ indicating the substitution of Cr into the structure of Li2MnSiO4. Electrochemical performance in relations to structural stability of Li2Mn1-xCrxSiO4 will be discussed in the meeting.
REFERENCES
[1] R. Dominko, M. Bele, A. Kokalj, M. Gaberscek, J. Jamnik, Journal of Power Sources, 2007, 174, 457-461.
[2] V.V. Politaev, A.A. Petrenko, V.B. Nalbandyan, B.S. Medvedev, E.S. Shvetsova, The Journal of Solid State Chemistry, 2007, 180, 1045–1050.
[3] H.M. Cheng, S.X. Zhao, X. Wu, J.W. Zhao, L. Wei, Applied Surface Science, 2018, 433, 1067-1074.
Fig. 1 Powder X-ray diffraction patterns of Li2Mn1-xCrxSiO4 (x = 0, 0.025 and 0.05).