Effect of Mixtures of Lithium Hexafluorophosphate (LiPF₆) and Lithium Bis(fluorosulfonyl)imide (LiFSI) as Salts in Li[Ni_1/3Mn_1/3Co_1/3]O₂/Graphite Pouch Cells

Abstract

The effect of mixtures of lithium hexafluorophosphate (LiPF 6 ) and lithium bis(fluorosulfonyl)imide (LiFSI) with different molar ratios in the electrolyte of Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 /graphite pouch cells was studied using the ultra-high precision charger (UHPC) at Dalhousie University, an automated storage system, electrochemical impedance spectroscopy (EIS) and gas evolution measurements. Ethylene carbonate: ethyl methyl carbonate (EC:EMC, 3:7 wt.% ratio) solvent was used as the base solvent in these studies. Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 /graphite pouch cells containing both LiPF 6 and LiFSI (with a total salt content of 1 M) with or without 2% VC showed smaller or similar self-discharge, lower charge transfer resistance (R ct ), and smaller amounts of gas evolution during formation and during storage at high temperature, compared to cells containing only 1 M LiPF 6 with and without 2% VC, respectively. For cells without 2% VC, cells with 0.3 M LiPF 6 + 0.7 M LiFSI showed the smallest self-discharge and the lowest R ct after 40 • C storage. For cells with 2% VC, cells with 0.5 M LiPF 6 + 0.5 M LiFSI + 2% VC showed the lowest voltage drop and the lowest R ct after 40 • C storage. The UPHC cycling data showed that cells containing LiPF 6 :LiFSI mixtures with 2% VC showed similar coulombic efficiency (CE), and similar charge end-point capacity slippage compared to cells with 1 M LiPF 6 + 2% VC. The combination of LiFSI and LiPF 6 in electrolytes that contain 2% VC can bring benefits of improved storage properties and reduced gas evolution at high temperature while maintaining all other properties of the cells in experiments limited to 4.2 V. However, preliminary experiments at voltages up to 4.45 V suggest that LiFSI may lead to increased transition metal dissolution (Ni, Mn and Co) compared to lithium bistrifluoromethane sulfonyl imide (LiTSFI), another salt additive used to improve storage properties and reduce gassing. The use of electrolyte additives is one of the most economical and effective ways to improve the performance of Li-ion cells.1,2 Vinylene carbonate (VC) has been shown to be an effective electrolyte additive for the improvement of cell performance.3,4 Burns et al. 5 showed that LiCoO 2 /graphite cells with 2% VC demonstrated improved life-time with a higher coulombic efficiency (CE) and lower charge and discharge end-point capacity slippage rates, compared to cells without VC. Lithium hexafluorophosphate (LiPF 6 ) is the most commonly used salt in Li-ion cells, due to balance of properties, such as high dissociation constant, high conductivity and good electrochemical stability against Al corrosion.1,2,6,7Recent studies [8][9][10] have shown that lithium bis(fluorosulfonyl)imide (LiFSI) can be thermally stable up to 200• C. However, Al current collector corrosion was observed in Li-ion cells with LiFSI-based electrolytes.10-14 The Al corrosion problem for LiFSI-based electrolytes can be readily solved by the addition of LiPF 6 , which can be decomposed to form a protective film of AlF 3 . ...

Study of the Failure Mechanisms of LiNi0.8Mn0.1Co0.1O2Cathode Material for Lithium Ion Batteries

“…[2][3][4][5] The rate of the parasitic reactions is related to both the catalytic role of the cathode surface which depends on its composition and surface area, 3,6 as well as on the stability of the electrolyte. [2][3][4][5] Methods such as the use of electrolyte additives [7][8][9][10][11] and core-shell positive electrode materials [12][13][14] have been developed and studied to reduce the rate and extent of parasitic reactions, and hence increase capacity retention and lifetime of high-voltage Li-ion cells. The layered lithium Ni-Mn-Co oxides Li 1+x (Ni y Mn z Co (1-y-z) ) 1-x O 2 (NMC) are considered to be promising positive electrode materials.…”

Enabling High Performance Potassium‐Based Dual‐Graphite Battery Cells by Highly Concentrated Electrolytes

“…The electrolyte used in this work does not contain EC and thus, only the LUMOs of EMC and FSI − are left to take up an electron during reduction. As FSI‐based salts are also used as additive to form a more effective and better conducting SEI layer in LIBs, they are likely to be reduced earlier even in diluted electrolytes …”

Investigation of Dimethyl Carbonate and Propylene Carbonate Mixtures for LiNi0.6Mn0.2Co0.2O2‐Li4Ti5O12 Cells

“…An observed voltage drop during storage must therefore indicate insertion of lithium into the NMC electrode. Lithium can be inserted into the NMC material if oxidation side reactions take place at the NMC electrode, and voltage fading in charged cells has indeed often been ascribed to this phenomenon . Figure shows that the voltage fading is much more apparent in the cell containing pure DMC electrolyte than the cells with EC and PC containing electrolytes.…”