Improvement on the Li-Rich Mn-rich Cathode Performance By Interface Modification Using Anion Receptors

Li-rich, Mn-rich (LMR) layered composites, xLi₂MnO₃ · (1-x) LiMO₂ (M = Ni, Co, Mn), have demonstrated the highest discharge capacity along with cost reduction and safety enhancement.^{1, 2} The advantages of Li₂MnO₃ component and its influence on the structural stability and electrochemical properties of these LMR cathode series have been extensively investigated so far. After activation of Li₂MnO₃ in the initial charge process, a discharge capacity exceeding 250 mAh g^-1 can be achieved for these LMR cathodes. However, oxygen is simultaneously released during Li₂MnO₃ activation, resulting in the damage of electrode surface structure, formation of micro-cracks at the crystal surface and the distortion of crystal periodicity.³ More importantly, the extracted intermediate oxygen species (e.g. O₂^2-, O₂^- or even O₂^Ÿ-/O^Ÿ-) steadily oxidize the organic solvents and form passivation film on the cathode.⁴In addition, high cut-off voltage (4.6-4.8 V) beyond the stability of carbonate-based electrolyte is required for LMR cathode and unavoidably initiates electrolyte decomposition during each charge, further thickening the passivation layer on the cathode surface and deteriorating the electrochemical performances of LMR cathodes.

        In this work, boron-based anion receptor, tris(pentafluorophenyl)borane (TPFPB) was adopted as an electrolyte additive to limit the negative effects from oxygen species in the system. Considering the strong anion coordination effect of TPFPB, the released oxygen species in the form of oxygen anions (O₂^2-/O₂^-) or radicals (O₂^Ÿ-/O^Ÿ-) may be trapped in the vicinity of TPFPB instead of attacking the electrolyte. Based on this hypothesis, a systematic investigation was performed to verify this concept which may provide valuable clues to guide the research and development of electrolyte additives to modify the interfacial reactions.

Fig. 1a shows the initial charge-discharge curves of Li[Li_0.2Ni_0.2Mn_0.6]O₂ in electrolytes with different TPFPB concentrations (2.0-4.7 V). The charge/discharge profiles almost overlap with each other for Li[Li_0.2Ni_0.2Mn_0.6]O₂ electrodes in baseline electrolyte and TPFPB added electrolyte due to the low current density (C/10) used for the first three formation cycles. An irreversible voltage plateau at ca. 4.4 ~ 4.6 V is observed in all three cells caused by the lithium ion removal concomitant with irreversible loss of oxygen from the LMR electrode lattice. After activation, the electrode material delivers similarly high discharge capacity of ca. 245 mAh g^-1 in all electrolytes, indicating that TPFPB additive has good compatibility with both Li[Li_0.2Ni_0.2Mn_0.6]O₂ electrode and the electrolyte during electrochemical processes. In the subsequent cycling at C/3, as presented in Fig. 1b, an initial discharge capacity of ~200 mAh g^-1 was still achieved in all cells. However, fast capacity fading was observed for Li[Li_0.2Ni_0.2Mn_0.6]O₂ in baseline electrolyte upon cycling, which retains only 33.5% of its initial capacity after 500 cycles. In contrast, remarkably improved cycling stability is revealed with the addition of TPFPB. Even after 500 cycles, the discharge capacities were maintained at 145 and 153 mAh g^-1for cathodes tested in electrolytes with 0.1 M and 0.2 M TPFPB, respectively, corresponding to high capacity retentions of 74.2% and 76.8%, which are among of the best performances ever reported for LMR cathode materials.

Fig. 1. (a) Initial charge/discharge profiles at C/10 (25 mA g^-1) and (b) long-term cycling performance of cathode material Li[Li_0.2Ni_0.2Mn_0.6]O₂at C/3 after three formation cycles at C/10.

For LMR cathode, one important reason for the continuous capacity fading is the deterioration of electrode/electrolyte interface, associated with the thick passivation layer formation and the corrosion/fragmentation of LMR cathode bulk structure. If TPFPB in the electrolyte effectively accepts oxygen anions or radicals (highly reactive) before O₂ is generated, the damages to surface structure of LMR cathode electrode should be much less than that without TPFPB. Meanwhile, the passivation film accumulated from various paths on the cathode surface could be largely alleviated, which has been confirmed by the TEM characterization, impedance measurement and electrochemical data analysis. Detailed results on the effect of TPFPB additive on the electrochemical performance of Li[Li_0.2Ni_0.2Mn_0.6]O₂will be presented and discussed during the meeting.

References

1. M. M. Thackeray, C. S. Johnson, J. T. Vaughey, N. Li and S. A. Hackney, J. Mater. Chem., 15, 2257 (2005).

2. M. M. Thackeray, S.-H. Kang, C. S. Johnson, J. T. Vaughey, R. Benedek and S. A. Hackney, J. Mater. Chem., 17, 3112 (2007).

3. J. Zheng, M. Gu, J. Xiao, P. Zuo, C. Wang and J.-G. Zhang, Nano Lett., 13, 3824 (2013).

4. N. Yabuuchi, K. Yoshii, S.-T. Myung, I. Nakai and S. Komaba, J. Am. Chem. Soc., 133, 4404 (2011).