Abstract
Performance of LiNi0.5Mn1.5O4/graphite cells cycled to 4.8 V at 55°C with the 1.2 M LiPF6 in EC/EMC (3/7, STD electrolyte) with and without added lithium catechol dimethyl borate (LiCDMB) has been investigated. The incorporation of 0.5 wt% LiCDMB to the STD electrolyte results in an improved capacity retention and coulombic efficiency upon cycling at 55°C. Ex-situ analysis of the electrode surfaces via a combination of SEM, TEM, and XPS reveals that oxidation of LiCDMB at high potential results in the deposition of a passivation layer on the electrode surface, preventing transition metal ion dissolution from the cathode and subsequent deposition on the anode. NMR investigations of the bulk electrolyte stored at 85°C reveals that added LiCDMB prevents the thermal decomposition of LiPF6.
Highlights
One effective method for improving the performance of high voltage cathodes involves the incorporation of SEI and cathode electrolyte interface (CEI) forming electrolyte additives that are sacrificially oxidized on the surface of electrodes to generate a passivation film which inhibits transition metal dissolution and further electrolyte oxidation
The related additive, lithium difluorooxalato borate (LiDFOB)[27,28,29,30,31] has been reported to improve the properties of Li1.2Ni0.15Mn0.55Co0.1O2 cathodes cycled to high potential[19] In addition to the lithium oxalato borates,[26,32] we have recently reported on the beneficial effect of the incorporation of lithium tetralkylborates as Additives for Designed
Characterization of lithium catechol dimethyl borate (LiCDMB).—The as-synthesized product is purified via crystallizations, and characterized by nuclear magnetic resonance (NMR) spectroscopy in D2O (1H, 11B)
Summary
Incorporation of LiCDMB into a standard lithium ion battery electrolyte improves the electrochemical performance of LiNi0.5Mn1.5O4/Graphite cells cycle to high potential (4.8 V vs LiC6/C6) (Figure 1). The O 1s spectrum of the fresh cathode is dominated by the metal oxide at 529.0 eV.[39,40] The cathode cycled with STD electrolyte contains the same O-M (M = Mn, Ni) peak at 529.0 eV, along with new peaks at higher binding energy which correspond to electrolyte decomposition products on the cathode surface characteristic of C=O, C-O, and O-C=O bonds respectively at 531.2 eV, 532.5 eV, and 533.0 eV.[39,40,44,45,46] The differences are greater for the cathode cycled with LiCDMB electrolyte (Figure 10).
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