(40b) A Novel Lithium Salt Additive for Improving Electrochemical Performance of High-Voltage LiNi0.5Mn1.5O4 Cathode

Recently, with the raising concern on air pollution and greenhouse emission caused by the fossil fuel usage, lithium ion batteries, high energy density storages, become more attractive for transportation purpose like electric vehicles and hybrid electric vehicles [1]. To achieve this goal, the high voltage cathodes are intensively studied in recent years. Among various high voltage electrodes, LiNi_0.5Mn_1.5O₄ is expected to be the most promising one owing to its relatively moderate price and theoretical specific capacity (147mAh g^-1). However, the practical applications are limited because of transition metal dissolution and the severe oxidation decomposition of the commercial electrolyte caused by the unstable interface between the cathode and electrolyte under high voltage condition (>4.5V vs. Li⁺/Li). There are several methods to improve the interface stability, such as coating the electrode surface, searching for more stable lithium salts and solvents, or employing additives. Among these methods, applying additives is the most efficient and economical way [2, 3].

In this short term, lithium fluorozirconate is synthesized [4] and added into the traditional electrolyte (1M LiPF₆ into EC, DEC, DMC which are mixed at the ratio of 1:1:1 by volume). Preliminary cycle performance tests show that adding lithium fluorozirconate can improve the discharge capacity of the LiNi_0.5Mn_1.5O₄/Li half cells and improve the rate properties. Scanning electron microscope (SEM), high resolution transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) are performed to characterize the fresh and cycled electrodes with/without lithium fluorozirconate. SEM indicates that the cycled LiNi_0.5Mn_1.5O₄ electrode with lithium fluorozirconate maintains the clean and sharp morphology. As a comparison, without lithium fluorozirconate, the sharp edge of LiNi_0.5Mn_1.5O₄ electrode almost disappears completely after cycling. Furthermore, the results of TEM and EDS suggest that the cycled LiNi_0.5Mn_1.5O₄ electrode with lithium fluorozirconate displays a thin and uniform SEI layer which mainly consists of element zirconium. From the XPS observation, it can be inferred that the involvement of lithium fluorozirconate facilitates the formation of SEI layer and effectively inhibits the decomposition of both lithium salts and carbonate solvents, which is consistent with the results of TEM. And XPS depth analysis demonstrates that the SEI layer formed with lithium fluorozirconate can significantly suppress the dissolution of Mn, resulting in a better cycling performance of LiNi_0.5Mn_1.5O₄/Li half cells.

References:

[1] Z.N.Wang, Y.J.Cai, Z.H.Wang, S.M.Chen, X.M.Lu, S.J.Zhang, J Solid State Electrochem 2013, 17, 2839–2848.

[2] K. Xu, Chem Rev 2014, 114, 11503-11618.

[3] Q. Li, J. Chen, L. Fan, X. Kong, Y. Lu, Green Energy & Environment 2016, 1, 18-42.

[4] S. K. Shi, [P].CN103227326A. 2013-07-31.