Effect of Substituting LiBF 4 for LiPF 6 in High Voltage Lithium-Ion Cells Containing Electrolyte Additives

Silicon electrodes were cycled with electrolytes containing different salts to investigate the effect of salt on the electrochemical performance and SEI structure. Comparable capacity retention were observed for the 1.2 M LiPF 6 , LiTFSI and LiClO 4 electrolytes in ethylene carbonate (EC):dimethyl carbonate (DEC), 1:1, but severe fading was observed for the 1.2 M LiBF 4 electrolyte. The differential capacity plots and EIS analysis reveals that failure of the 1.2 M LiBF 4 electrolyte is attributed to large surface resistance and increasing polarization upon cycling. However, when LiBF 4 was added as an electrolyte additive (10% LiBF 4 and 90% LiPF 6 ), the capacity retention and Coulombic efficiency were improved. The SEI was analyzed by FTIR and XPS for each electrolyte. Both spectroscopic methods suggest that the main components of the SEI are lithium ethylene dicarbonate (LEDC) and Li 2 CO 3 in the 1.2 M LiPF 6 , LiTFSI and LiClO 4 electrolytes, while an inorganic-rich SEI, composed of LiF and borates, was generated for both the Silicon negative electrodes for lithium ion batteries have attracted academic and industrial interest, since they provide ∼10 times more specific capacity (3579 mA g −1 ) than graphite (372 mA g −1 ). However, the large volumetric changes during lithiation and delithiation limits commercial application.1 The volume changes result in mechanical stress to individual Si particles and the binder which maintains physical contact between electrode components, thus degenerating the electrode laminate upon repeated lithiation/delithiation. 2,3In particular, it has been demonstrated that the electric contact loss becomes severe during delithiation when the Si particles are contracted. Thus, incomplete delithiation due to contact resistance has been reported as one of dominant failure mechanisms. [4][5][6][7] In addition to the volume contraction, the solid electrolyte interphase (SEI) has been reported to be another factor that impedes the reversibility of lithiation. 5,7 When the SEI on silicon is modified by fluoroethylene carbonate (FEC), the capacity retention, reversibility of lithiation, and suppression of electrolyte decomposition are observed. 5,7,8 Since the improvement of the SEI is critical for improving the electrochemical performance of Si electrodes, great efforts have been devoted to modify the SEI by using electrolyte additives, surface coatings, or concentrated electrolytes. [9][10][11][12][13][14][15][16] Recently, it has been reported that the SEI can be significantly modified by changing the electrolyte concentration. [16][17][18] For instance, propylene carbonate (PC)-based electrolytes do not generate a stable passivation layer on graphite at low salt concentration. However, upon dissolving high concentrations of either LiPF 6 or LiTFSI into PC a LiF rich passivation layer is generated on graphite affording electrochemical reversibility of the graphite. 16,18 The change in salt concentration has been reported to result in a change solution structure of the electrolyte. 16,...

Section: Methodsmentioning

confidence: 97%

Lithium Salt Effects on Silicon Electrode Performance and Solid Electrolyte Interphase (SEI) Structure, Role of Solution Structure on SEI Formation

Yoon

Chapman

Seo

et al. 2017

“…This amount of self-discharge corresponds to less than 1% of the electrode capacity and is twenty times less than the amount of charge lost irreversibly in high voltage storage of the same cell. 34 Figure 8b shows the volume change of pouch bags inflated with C 2 H 4 , containing charged negative electrodes , after one week of storage at 40 or 60…”

Section: Reactions Of Co 2 -mentioning

confidence: 99%

Quantifying, Understanding and Evaluating the Effects of Gas Consumption in Lithium-Ion Cells

Ellis

Allen

Thompson

et al. 2017

Self Cite

114

Lithium-ion cells produce a considerable amount of gas in their first cycle. If the gases are not removed in a degassing step, most are consumed by the cell over time. This phenomenon has never been investigated explicitly in the literature. In this paper, the evolution and subsequent consumption of gas in typical lithium-ion cells are measured by Archimedes' principle and gas chromatography. It is found that all evolved gases are subsequently consumed to some degree, except for saturated hydrocarbons. The consumption of gas occurs predominantly at the negative electrode, where the gases are reduced to form part of the solid-electrolyte interphase (SEI). Changes to the negative electrode SEI upon gas consumption are investigated using X-ray photoelectron spectroscopy. The effect of gas consumption on cell performance is studied with ultra-high precision charging and high voltage storage experiments. It is found that gas consumption does not result in measurable adverse effects to cell performance. Lithium-ion cells can produce a significant amount of gas during the first charge (in the formation cycle), as electrolyte and additives react at the surfaces of the charging electrodes to form passivating films. If lithium-ion cells are packaged in a flexible casing, these gases are normally removed by the manufacturer in a degassing step, to prevent deformation of the cell and to ensure uniform stack pressure on the electrodes. If the degassing step is omitted, a large portion of the gas evolved is consumed over time.1 The reactions that consume gas are presumably prevalent in hard-cased cylindrical cells, such as 18650 s, which are often hermetically sealed before the first charge, and therefore cannot be easily degassed. The reactions that consume gas are presumably less prevalent in pouch-type cells, which are degassed.Several authors have speculated about the fates of gases in lithiumion cells.2-5 There has been no work explicitly dedicated to understanding the phenomenon of gas consumption. There is no consensus as to whether the effects of gas consumption are beneficial or harmful to cell performance. For example, it has been argued by some that the consumption of CO 2 is beneficial to cells, as it reacts to form a passivating film on the negative electrode. 3,4,6 However it has also been argued that the consumption of CO 2 is detrimental to cells, as it may reduce at the negative electrode to form Li 2 C 2 O 4 , which causes continual self-discharge at high voltage. 2It is important for both scientists and manufacturers of lithium-ion cells to understand the causes and the effects of gas consumption. If gas consumption is quick, benign, or even beneficial to cell performance, then the time-consuming degassing step for lithium-ion pouch cells might be skipped. 7 The gases evolved in lithium-ion pouch cells could be left for consumption within the cell, perhaps leaving the pouch cell flat and rigid after several hours if all the gases were consumed. If gas consumption in a cell produces undesirable effects, such...

“…However, LiPF 6 is known to have poor thermal and hydrolytic stability. 10,11 Decomposition of LiPF 6 results in the formation of reactive species, notably HF and PF 5 , which are thought to cause a cascade of undesirable side-reactions inside the cell. [10][11][12] LiBF 4 has often been studied as a substitute for LiPF 6 in lithium-ion cells because of its improved thermal and hydrolytic stability.…”

mentioning

confidence: 99%

“…3,[12][13][14][15][16][17] LiBF 4 has demonstrated several other important advantages over LiPF 6 : improved performance at sub-zero temperatures, improved passivation of the Al current collector, and improved performance at the positive electrode. 1,[3][4][5][6][7][8][9][12][13][14]17 The disadvantages of LiBF 4 are lower conductivity compared to LiPF 6 and poor performance at the negative electrode. 13,16 Our previous work explored the use of LiBF 4 in machine-made, high voltage, lithium-ion pouch cells.…”

mentioning

confidence: 99%

Synergistic Effect of LiPF₆and LiBF₄as Electrolyte Salts in Lithium-Ion Cells

Ellis

Hill

Gering

et al. 2017

Self Cite

Recent work shows promise for LiBF 4 as an electrolyte salt for high voltage lithium-ion batteries. LiBF 4 is more hydrolytically stable than LiPF 6 , and in some cases has proved to be more stable at high voltage. This work shows that 1.0 M electrolytes consisting of equal parts LiBF 4 and LiPF 6 have better performance in certain high voltage (4.35 V) lithium-ion cells than electrolytes with LiPF 6 or LiBF 4 alone. This is shown using long-term cycling and ultra-high precision coulometry on lithium-ion cells, with and without electrolyte additives. Ultra-high precision coulometry showed that electrolyte with 1:1 LiPF 6 :LiBF 4 and no other additives performed as well as electrolyte with LiPF 6 and state-of-the-art additive blends in these cells. The synergism between LiPF 6 and LiBF 4 was explored using XPS, symmetric cells and EIS analysis. These show that LiBF 4 reduces impedance growth at the positive electrode, while LiPF 6 improves passivation at the negative electrode.