Potapenko, A. V.

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COMPOSITION OF INTERMEDIATE PHASES SHOWING UP UPON DELITHIATION OF LITHIUM-MANGANESE SPINEL POTAPENKO A.V. Joint Department of Electrochemical Energy Systems, 38A Vernadsky Ave.,03680 Kyiv, Ukraine avoloshka@ukr.net An analysis of voltammetric data for LiMn2O4 has been performed, so as to recognize the possible presence of the intermediate phases. It has been found that in charge/discharge processes, in addition to the well-known Li0.5Mn2O4 phase, Li2/3Mn2O4 is easily identifiable, and its presence may be a key for understanding the properties of LiMn2O4.  Lithium manganese spinel, LiMn2O4 is a cheap, environmentally benign alternative to currently employed LiCoO2 as a cathode in lithium ion batteries. Being able to operate at discharge currents of 14800 A·g-1 [1,2], this material has good prospects for using in high-rate applications. Reversible electrochemical deinsertion of lithium ions from LiMn2O4 takes place within the potential range of 3.0-4.5 V in accord with the following equation  LiMn2O4↔Li1-xMn2O4+xLi++xe-, (1) with the theoretical capacity of 148 mAh·g-1, and leads to MnO2. The structure of LiMn2O4 is cubic ( mFd 3 , Z=8) [3-4] with lithium ions occupying the tetrahedral (8a) positions, manganese ions (Mn3+/Mn4+) in the octahedral positions (16d), and lithium ions inhabiting the tetrahedral (32e) positions and forming the cubic dense packing. Being separated from their neighbors by voids (16c), lithium ions have three-dimensional channels (8a–16c–8a–16c) for possible insertion, deinsertion and transport.  Structure transformations upon lithiation/delithiation are studied in great detail and may serve a key for better understanding the processes occurring in the spinel lattice. Both voltammetric and galvanostatic studies reveal a two-stage charge/discharge process enabling one to suggest the presence of an intermediate compound with x=0.5, Li0.5Mn2O4 [5-8]. Significant efforts have been undertaken so as to describe the structure of the Li0.5Mn2O4 intermediate. As follows from Ref. [8], this phase belongs to the modulated mR3 space group, Z=6, with O2- in the 6c and 18h positions, Mn3+ in the 3b positions, Mn4+ in the 9e positions, and Li+ partially filling 6c sites.  22 It has been found, however, that the aforementioned structures do not cover a wide variety of structural transformations occurring in LiMn2O4 upon delithiation. Studying delithiated LiMn2O4 single crystals by means of X-ray diffraction, Bjork et al. [7] have revealed that several superstructures may coexist in the samples. Continuing such investigations [8] they have concluded that among modulated phases belonging to these superstructures, Li2/3Mn2O4, Li0.5Mn2O4, and Li0.25Mn2O4 can be present, and Li0.25Mn2O4 and Li2/3Mn2O4 structures can be derived as superlattices of the adjacent Li0.5Mn2O4 lattice. At that same time, in electrochemical studies of Li+ extraction from and insertion into LiMn2O4 single crystal, Docco et al [9, 10] have observed a new peak at 3.86 V, which has not been reported previously for polycrystalline samples and possibly corresponds to a new intercalation phase.  In this work, an analysis of voltammetric data for the lithium manganese spinel has been performed, so as to recognize the presence of the intermediate phases described above. To do so, cyclic voltammetric curves have been treated as a set of closely overlap lines, and a method suggested in Ref. [11] for decomposing such lines has been employed. It appears that in charge/discharge processes, Li2/3Mn2O4 is easily identifiable, and its presence may be a key for understanding the properties of LiMn2O4.  Research Methodology The stoichiometric lithium-manganese spinel LiMn2O4 was synthesized by the citric acid route, which was described in previous papers. CR2016 coin cells were made to obtain electrochemical characteristics of the synthesized materials. Cyclic voltammetry (CV) was performed on a home-made automated electrochemical workstation. The “dry” component ratio of the analyzed material, the conductive additive, and the PVdF binder for the cathodes was 82:10:8. The cells were assembled in a dry glove box and consisted of a cathode made from the synthesized material, a lithium metal anode used as a counter and reference electrode, a Celgard 2500 separator membrane, and 1 mol/L solution of LiPF6 in the 1:1 mass ratio mixture of EC and DMC.  Results and Discussion Typical cyclic voltammetric curves for LiMn2O4 obtained at various scanning rates are plotted in Fig. 1. A representative example of data fits is shown in Fig. 2. Cathodic and anodic curves have been treated separately. Composite voltammetric curves can be well represented by three severely overlap symmetric profiles, and a two-wave approximation undoubtedly fails. In what follows we enumerate the fitted waves from lower to higher potentials as 1, 2 and 3; in Fig. 2 these are situated at ca. 23 4.00, 4.03 and 4.16 V, respectively. Overlapping signifies that the previous processes still proceed when the next processes already run.  Fig. 1. Cyclic voltammetric curves for LiMn2O4 registered at various scanning rates. Fig. 2. Data fits for the cathodic and anodic waves of LiMn2O4. First of all, it is interesting to check if waves 1, 2 and 3 could correspond to the formation of intermediates predicted for LiMn2O4 by 24 Björk et al. [7,8]. An analysis of capacities stored/delivered on charging/discharging processes shows that the transformation of LiMn2O4 to Li2/3Mn2O4 and backwards requires 0.366Q, Eq. (2) (we remind that Q is the theoretic capacity value), the conversion of Li2/3Mn2O4 to Li0.5Mn2O4 or the reverse reaction need 0.134Q, Eq. (3), and further two-step delithiation of Li0.5Mn2O4 to Li0.25Mn2O4 and then to MnO2 or similar two-step lithiation of manganese dioxide necessitate 2×0.25Q, Eqs. (4), (5). LiMn2O4↔Li2/3Mn2O4+1/3Li++1/3e–, (2) Li2/3Mn2O4↔Li0.5Mn2O4+1/6Li++1/6e–, (3) Li0.5Mn2O4↔Li0.25Mn2O4+0.25Li++0.25e–, (4) Li0.25Mn2O4↔2MnO2+0.25Li++0.25e–. (5) Fig. 3. Distribution of specific capacity between three cathodic peaks for LiMn2O4 at various scanning rates. Respective dependences of capacity fractions (peak areas normalized to experimental specific capacities) on the scanning rate are plotted in Fig. 3. It is well seen that at the zero scanning rate, these dependences tend to the values predicted for Li2/3Mn2O4 and Li0.5Mn2O4, whereas the presence of Li0.25Mn2O4 is indistinguishable suggesting that reactions (4) and (5) occur in one step,  25 Li0.5Mn2O4↔2MnO2+0.5Li++0.5e–, (6) Of course, it is possible to perform calculations in terms of four composite waves, so as to reproduce a missing Li0.25Mn2O4 phase but we prefer to keep the parameterization of our calculations at the lowest possible level. On the other hand, one may safely state that the Li0.25Mn2O4↔MnO2 process, if exists, meets no kinetic limitations, and the Li0.25Mn2O4 phase is indiscernible in voltammetric experiments. Another point to stress is the greater the scanning rate, the greater the departure from Eqs. (2), (3), (6).  Analyzing the dependences of the peak fractions on the scanning rate enables one to draw qualitative conclusions regarding lithiation/delithiation kinetics. For all the three deinsertion waves, two different dependences on the scanning rate converging to a single point can be discerned, one for lower scanning rates and the other for higher ones, see the left hand side of Fig. 3 from bottom to top. It is a remarkable feature of these dependences for the deinsertion process that the first peak fraction reaches the theoretical value from above, while the second and the third ones from below. At lower scanning rates, the amounts of Li2/3Mn2O4 and Li0.5Mn2O4, as follows from respective peak areas, are lower than expected, and an excess of LiMn2O4 remains in the system. This may signify that Li+ extraction from LiMn2O4 meets kinetic hindrances; the transformation of LiMn2O4 to Li2/3Mn2O4 is slow and serves as a bottleneck for the Li2/3Mn2O4→Li0.5Mn2O4 and Li0.5Mn2O4→MnO2 processes, which are of commensurable rates. At higher scanning rates, the amounts of LiMn2O4 and Li0.5Mn2O4 appear lower than expected, and an excess of Li2/3Mn2O4 accumulates in the system. One may guess that in this case, the transformation of Li2/3Mn2O4 to Li0.5Mn2O4 is the slowest. These relations may be described by the following schemes, which may signify that at different currents applied, ionic transport is carried out through different pathways: slow scanning rates: 2420.5422/342 MnOOMnLiOMnLiOLiMn →→ →fastfastslow , fast scanning rates: 2420.5422/342 MnOOMnLiOMnLiOLiMn →→→fastslowfast . Unlike the deinsertion waves, the dependences of peak areas on scanning rate for the insertion waves demonstrate less distinct peculiarities. Examining the right hand side of Fig. 3 from top to bottom one may conclude that upon growing discharge rate, the 3rd peak fraction is greater than expected, and an excess of MnO2 remains in the system. 26 This may signify that the transformation of MnO2 to Li0.5Mn2O4 is kinetically restricted. The 2nd peak fraction is smaller than expected, and the transformation of Li0.5Mn2O4 to Li2/3Mn2O4 is fast. The 3rd peak fraction suggests that at slow scanning rates the transformation of Li2/3Mn2O4 to LiMn2O4 is fast, and at fast scanning rates it is slow, slow scanning rates: 42422/3420.52 OLiMnOMnLiOMnLiMnO →→→slowfastslow , fast scanning rates: 42422/3420.52 OLiMnOMnLiOMnLiMnO →→→fastfastslow . These findings are summarized in Table 1. Table 1. Dependences of peak fractions on scanning rate for insertion/ deinsertion processes Delithiation Lithiation slow scan rate fast scan rate slow scan rate fast scan rate LiMn2O4 slow fast - - Li2/3Mn2O4 fast slow slow fast Li0.5Mn2O4 fast fast fast fastMnO2 - - slow slow Conclusions  In addition to the assumption regarding possibly different pathways accessible for ionic transport at different currents applied, one may infer that (i) forward and backward electrochemical reactions do not have the same rates and limiting stages, (ii) both lithiation and delithiation of Li0.5Mn2O4 are fast and therefore (iii) lithiation and delithiation of Li2/3Mn2O4 are the rate-determining step of both charge and discharge processes. Acknowledgement The author is grateful to Dr. M. Gorobets for performing data fits and to Prof. S.A. Kirillov for help. 27 References [1]  Potapenko A. V. Lithium manganese spinel materials for high-rate electrochemical applications / A. V. Potapenko, S. A. Kirillov // J. Energy Chem. 2014. – Vol. 23. – P. 543–558. [2]  Potapenko A. V. A new method of pretreatment of lithium manganese spinels and high-rate electrochemical performance of Li[Li0.033Mn1.967]O4 / A. V. Potapenko, S. I. Chernukhin, S. A. Kirillov // Mater. Renew. Sustain. Energy. 2014. – Vol. 3. – P. 40-47. [3]  Rossouw M.H. Structural aspects of lithium-manganese-oxide electrodes for rechargeable lithium batteries / M.H. Rossouw, A de Kock, L.A de Piciotto, M.M. Thackeray // Mater. Res. Bull. 1990. – Vol. 25. – P. 173-182. [4]  Ohzuku T. Electrochemistry of manganese dioxide in lithium nonaqueous cell III. X‐Ray diffractional study on the reduction of spinel‐related manganese dioxide / T. Ohzuku, M. Kitagawa, T. Hirai // J. Electrochem. Soc. 1990. – Vol. 137. – P. 769-775. [5]   Liu W. Mechanism of the electrochemical insertion of lithium into LiMn2O 4 spinels / W. Liu, K. Kowal, G.C. Farrington // J. Electrochem. Soc. 1998. – Vol. 145. – P. 459–465. [6]  Lee Y.J. 6Li and 7Li magic‐angle spinning nuclear magnetic resonance and in situ X‐Ray diffraction studies of the charging and discharging of LixMn2O4 at 4 V / Y.J. Lee, F. Wang, S. Mukerjee, J. McBreen, C.P. Grey // J. Electrochem. Soc. 2000. – Vol. 147. – P. 803–812. [7]  Björk H. Single-crystal studies of electrochemically delithiated LiMn2O4 / H. Björk, T. Gustafsson, J.O. Thomas // Electrochem. Commun. 2001. – Vol. 3. – P. 187-190. [8]  Björk H. Long-range ordering during delithiation of LiMn2O4 cathode material / H. Björk, T. Gustafsson, J.O. Thomas, S. Lidin, V. Petříček // J. Mater. Chem. 2003. – Vol. 13. – P. 585–589. [9] Dokko K. Electrochemical studies of Li-ion extraction and insertion of LiMn2O4 single crystal / K. Dokko, M. Nishizawa, M. Mohamedi, M. Umeda, I. Uchida, J. Akimoto, Y. Takahashi, Y. Gotoh, S. Mizuta // Electrochem. Solid-State Lett. 2001. – Vol. 4 . – P. A151-A153. [10] Dokko K. Kinetic study of Li-ion extraction and insertion at LiMn2O4 single particle electrodes using potential step and impedance methods / K. Dokko, M. Mohamedi, M. Umeda // J. Electrochem. Soc. 2003. – Vol. 150. – P. A425-A429. [11] Kirillov S.A. Time-correlation functions from band-shape fits without Fourier transform / S.A. Kirillov // Chem. Phys. Letters. 1999. – Vol. 303. – P. 37–42.  28 

Composition of intermediate phases showing up upon delithiation of lithium-manganese spinel

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Composition of intermediate phases showing up upon delithiation of lithium-manganese spinel

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