Lithium Tetrafluoro Oxalato Phosphate as Electrolyte Additive for Lithium-Ion Cells

Abstract

Lithium tetrafluoro oxalato phosphate ͑LTFOP͒ was investigated as an electrolyte additive to improve the life of mesocarbon microbead ͑MCMB͒/Li 1.1 ͓Ni 1/3 Co 1/3 Mn 1/3 ͔ 0.9 O 2 ͑NCM͒ cells for high power applications. With the addition of 1-3 wt % LTFOP to MCMB/NCM cells, the capacity retention after 200 cycles at 55°C significantly improved. Electrochemical impedance spectroscopy showed that the LTFOP addition in the electrolyte increased the initial impedance but lowered the impedance growth rate during cycling. Aging tests at 55°C indicated that the capacity retention of the negative electrode significantly benefited as a result of the LTFOP addition. Differential scanning calorimetry showed that the safety of the lithiated MCMB is significantly improved with the LTFOP addition. Lithium-ion batteries are being developed as the power source for hybrid and plug-in hybrid electric vehicles, 1-4 which generally require a battery with 15 years of life, improved safety, and low cost. This requirement is a significant challenge for the current technology of lithium-ion batteries, and a great deal of effort has been devoted to extend their calendar and cycle life. A promising approach is to develop functional electrolyte additives that limit the degradation of electrode materials during cycling by stabilizing the electrode/electrolyte interface. The state-of-the-art nonaqueous electrolyte for lithium-ion batteries is LiPF 6 dissolved in carbonates; however, LiPF 6 is sensitive to a trace amount of moisture, which can trigger the decomposition of LiPF 6 to produce PF 5 , LiF, POF 3 , and HF.5 HF, in turn, attacks the cathode materials to release metal ions. Amine et al. 1 reported that a trace amount of reaction by-products, such as HF, LiF, POF 3 , and Mn 2+ , can attack the electrodes ͑in particular, the negative electrode͒ and dramatically shorten the life of the battery. To overcome this problem, researchers have developed several passivation additives that can polymerize and form a stable passivation film at the electrode surface during the formation cycles. Examples of these additives are vinyl ethylene carbonate, [6][7][8] vinylene carbonate, 7,9-11 lithium bis͑oxalato͒borate ͑LiBOB͒, 12,13 and vinyl pyridine.14 It has been reported that the bis͑oxalato͒borate anion ͑BOB − ͒ of LiBOB can be reduced at 1.7 V vs Li + /Li and form an artificial and stable solid electrolyte interface ͑SEI͒ layer that can improve the capacity retention of graphite negative electrodes. 12,13 Although not yet confirmed, BOB − is believed to be first reduced at 1.7 V vs Li + /Li, after which an oxalato ring forms ͓1͔The free end of the opened oxalato group would then attack another BOB − to initiate a polymerization reaction and form a protective film on the surface of the graphite. Equation 1 also shows that the other oxalato ring of BOB − can be electrochemically activated and trigger a cross-linking reaction between different BOB-based polymer chains. This cross-linking reaction can then lead to the significant growth of the SEI ...

New class of nonaqueous electrolytes for long-life and safe lithium-ion batteries

“…With the presence of a Lewis acid such as PF 5 , LiBOB has a tendency to react with LiPF 6 to generate lithium difluoro(oxalato)borate (LiDFOB) and lithium tetrafluoro (oxalato)phosphate (LiTFOP) 16 ; both of these electrolyte additives have been reported to protect graphitic anodes 17,18 . We believe that a high concentration of Lewis acid (PFPTFBB 19 ) generates more LiDFOB or LiTFOP to maximize the protection for the graphitic anode.…”

Review of Electrolyte Additives for Ternary Cathode Lithium-ion Battery

“…LiODFB 阻抗更小, 一个原因是: LiBOB 中一个草酸根被还原后, 另一个草酸根开环与 其它 BOB -发生电化学聚合, 引发交联反应, 使得阻抗 增大; 而 LiODFB 只有一个草酸根, 不会发生类似交联 反应 [61,62] . 深入研究发现, LiBOB 和 LiODFB 在被还原 过程中分别释放草酸根和 F -, 促使形成了 SEI 膜内部无 机层 [63] .…”

High Performance Lithium-Ion Batteries Using Fluorinated Compounds

“…Of particular interest here are two fluorinated additives, lithium difluoro[oxalato]borate (LiDFOB) [20,[37][38][39] and lithium tetrafluoro[oxalato] phosphate (LiTFOP) [40] (see Figure 1.6(b) and (c) for detailed molecular structures). Both additives can form a thermally more stable SEI layer on the surface of graphite to extend the life of graphite-based lithium-ion cells, but without adding extra interfacial impedance to the cells.…”

“…During the initial charging of the cell without additive, two small peaks appeared in the voltage range between 2.5 and 3.0 V, both of which were assigned to the formation of SEI on the graphite surface in the LiPF 6 -based electrolyte. When LiTFOP was added as an electrolyte additive, an extra peak appeared at about 1.65 V, which is attributed to the formation of an artificial SEI layer [40]. capacity retention of almost 100%, even after more than 80 days of aging.…”