Anodic Decomposition of Trimethylboroxine as Additive for High Voltage Li-Ion Batteries

Abstract

Trimethylboroxine (TMB) is used as an additive in the electrolyte for improving the performance of LiCoPO 4 (LCP) in Li-ion batteries. In this work, the role and behavior of TMB are investigated by cyclic voltammetry (CV), impedance spectroscopy (EIS) and on line electrochemical mass spectroscopy (OEMS). It was found that TMB oxidizes from 4.6 V and a low amount in the electrolyte is necessary to obtain good performance. On one hand, its oxidation produces boron trifluoride (BF 3 ), phosphorylfluoride (POF 3 ) and carbanion (CH 3 -) linked to a huge increase in impedance. Based on these results, a complete oxidation mechanism is proposed. The catalytic effect of the TMB decomposition products on carbonate polymerization could enhance the performance of LCP. On the other hand, an unexplained water and/or HF release was detected. Further experiments need to be done. Rechargeable lithium batteries were first developed with lithium metal as a negative electrode (anode) and several positive electrode (cathode) materials like Li/MnO 2 1 or Li/LiTiS 2 . 2 Due to safety issues related to dendrite formation with lithium metal anode, the first Li-ion batteries were commercialized by Sony in 1991 using graphite anodes.3 Later on, layered-oxide based cathode materials like LiCoO 2 were developed, which have specific capacities and specific energies of ≈170 mAh/g LiCoO2 and ≈600 mWh/g LiCoO2 , respectively. In order to improve battery safety, cost and energy density, recent research has focused on new electrode materials. On the cathode side, numerous spinel structure materials were tested and showed good electrochemical performance. 4 In 1997, phospho-olivine cathode materials of the general formula LiMPO 4 (M = 3d-transition metal) emerged with the discovery of LiFePO 4 (LFP) by Goodenough's group, 5 which generally have more safety characteristics due to the strong P-O bond preventing O 2 release at high potential/temperature (problematic with layered oxides). So far, more than 1200 patents have been filed for phospho-olivines.6 LFP has a theoretical specific capacity of 171 mAh/g LFP and the oxidation from Fe II to Fe III takes place at 3.45 V, resulting in a specific energy of ≈590 mWh/g LFP . While offering improved safety, its specific energy is very similar to LiCoO 2 , so that with respect to energy density, LFP offers no improvement over LiCoO 2 .One approach toward improved energy density required for electric vehicle applications would be the use of LiCoPO 4 (LCP) cathode material, which promises a theoretical specific energy of ≈800 mWh/g LCP based on a charge/discharge voltage of ≈4.85 V vs. Li/Li + and a theoretical specific capacity of 167 mAh/g LCP . 7,8 However, the reported electrochemical performance of LCP, synthesized by many different routes, does not reach the theoretical specific capacity, shows low coulombic efficiency, and exhibits poor cycling stability. For example, Ni et al.9 synthetized carbon-coated LCP material by a sol gel route which gave a specific capacity of 131 mAh/g LCP at 0.1 C in the...

Why is tris(trimethylsilyl) phosphite effective as an additive for high-voltage lithium-ion batteries?

“…The TMSP anion may decompose into (Me 3 SiO) 2 Pc and Me 3 SiO À , and subsequently Me 3 SiO À may attack a surrounding EC solvent molecule to form the TMSOCH 2 CH 2 OLi species and CO 2 gas. 41,52,53 We think that the reaction products would somehow contribute to the SEI lm if they are not soluble in the electrolyte. In contrast, the decomposition of neutral TMSP species is energetically unfavorable, i.e., highly endoergic, as shown in Fig.…”

Carbon Coating Stability on High-Voltage Cathode Materials in H2O-Free and H2O-Containing Electrolyte

“…In a previous publication by our group, 35 we studied the anodic decomposition of trimethylboroxine (TMB), which is a known additive for improving the performance of LiCoPO 4 . Our OEMS results suggested that TMB decomposes at potentials ≥ 4.5 V, producing large amounts of BF 3 , POF 3 , and CO 2 as well as H 2 O and H 2 .…”

Quantification of PF5 and POF3 from Side Reactions of LiPF6 in Li-Ion Batteries

“…Electrochemical oxidation of LiPF 6 electrolyte.-As already mentioned, POF 3 has been observed at high positive potentials on cathode active materials or on carbon electrodes in LiPF 6 -containing electrolytes at room temperature, but its amount has never been quantified. [29][30][31][32] Therefore, we examined the oxidation reactions of an EC + 1.5 M LiPF 6 electrolyte on a carbon black working electrode vs. a lithium counter electrode, focusing on the evolution and quantification of LiPF 6 decomposition species. Figure 6 shows the current density (a) and the gas evolution (b) during a linear potential sweep from OCV to 5.5 V vs. Li + /Li, whereby all signals have been quantified using the calibration factors given in Table II (note that only signals related to POF 3 were observed, so that a quantification of "POF 3 " is possible).…”