A non-aqueous sodium hexafluorophosphate-based electrolyte degradation study: Formation and mitigation of hydrofluoric acid

Barnes, Pete; Smith, Kassiopeia; Parrish, Riley; Jones, Chris D.; Skinner, Paige; Storch, Erik; White, Quinn; Deng, Changjian; Karsann, Devan; Lau, Miu Lun; Dumais, Joseph J.; Dufek, Eric J.; Xiong, Hui

doi:10.1016/j.jpowsour.2019.227363

Cited by 60 publications

(49 citation statements)

References 53 publications

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“…First, water can react with the carbonate solvents, with evidence for the hydrolysis products of EC and EMC, such as LEMC, ethylene glycol, poly-EO based oligomers, and methanol (see Scheme S1 ), detected in the 1 H NMR spectra of cycled electrolytes ( Figure 7 ). EC has been reported to be more susceptible toward hydrolysis than EMC, 70 compounding the instability of EC-containing electrolytes in the presence of an oxygen-releasing cathode. The alcohols formed by solvent hydrolysis (i.e., methanol, ethanol, and ethylene glycol; Scheme S1 ) and/or solvent chemical oxidation (i.e., ethanol; Scheme 1 ) can be chemically or electrochemically oxidized, as shown in Scheme 2 , forming aldehydes (consistent with the 1 H NMR assignments above) and either peroxide or protons.…”

Section: Discussionmentioning

confidence: 99%

Electrolyte Reactivity at the Charged Ni-Rich Cathode Interface and Degradation in Li-Ion Batteries

Dose

Temprano

Allen

et al. 2022

ACS Appl. Mater. Interfaces

112

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The chemical and electrochemical reactions at the positive electrode–electrolyte interface in Li-ion batteries are hugely influential on cycle life and safety. Ni-rich layered transition metal oxides exhibit higher interfacial reactivity than their lower Ni-content analogues, reacting via mechanisms that are poorly understood. Here, we study the pivotal role of the electrolyte solvent, specifically cyclic ethylene carbonate (EC) and linear ethyl methyl carbonate (EMC), in determining the interfacial reactivity at charged LiNi 0.33 Mn 0.33 Co 0.33 O 2 (NMC111) and LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) cathodes by using both single-solvent model electrolytes and the mixed solvents used in commercial cells. While NMC111 exhibits similar parasitic currents with EC-containing and EC-free electrolytes during high voltage holds in NMC/Li 4 Ti 5 O 12 (LTO) cells, this is not the case for NMC811. Online gas analysis reveals that the solvent-dependent reactivity for Ni-rich cathodes is related to the extent of lattice oxygen release and accompanying electrolyte decomposition, which is higher for EC-containing than EC-free electrolytes. Combined findings from electrochemical impedance spectroscopy (EIS), TEM, solution NMR, ICP, and XPS reveal that the electrolyte solvent has a profound impact on the degradation of the Ni-rich cathode and the electrolyte. Higher lattice oxygen release with EC-containing electrolytes is coupled with higher cathode interfacial impedance, a thicker oxygen-deficient rock-salt surface reconstruction layer, more electrolyte solvent and salt breakdown, and higher amounts of transition metal dissolution. These processes are suppressed in the EC-free electrolyte, highlighting the incompatibility between Ni-rich cathodes and conventional electrolyte solvents. Finally, new mechanistic insights into the chemical oxidation pathways of electrolyte solvents and, critically, the knock-on chemical and electrochemical reactions that further degrade the electrolyte and electrodes curtailing battery lifetime are provided.

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Section: Discussionmentioning

confidence: 99%

Electrolyte Reactivity at the Charged Ni-Rich Cathode Interface and Degradation in Li-Ion Batteries

Dose

Temprano

Allen

et al. 2022

ACS Appl. Mater. Interfaces

112

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“…[9,10] While NaPF 6 offers high ionic conductivity, its high susceptibility to undergo hydrolysis is problematic, as toxic HF is formed as well as NaF and POF 3 (which goes on to further react with water). [11,12] The presence of NaF causes solubility difficulties as highlighted in our previous work, [13] whereas the ability to form HF and POF 3 poses significant safety concerns due to their high toxicity; a recent report showing high levels of these gases are produced in LIB fires. [14] Furthermore, the presence of PF 6…”

Section: Introductionmentioning

confidence: 85%

Sodium Borates: Expanding the Electrolyte Selection for Sodium‐Ion Batteries

et al. 2022

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Sodium-ion batteries (SIBs) are a promising grid-level storage technology due to the abundance and low cost of sodium. The development of new electrolytes for SIBs is imperative since it impacts battery life and capacity. Currently, sodium hexafluorophosphate (NaPF 6 ) is used as the benchmark salt, but is highly hygroscopic and generates toxic HF. This work describes the synthesis of a series of sodium borate salts, with electrochemical studies revealing that Na[B-(hfip) 4 ]•DME (hfip = hexafluoroisopropyloxy, O i Pr F ) and Na[B(pp) 2 ] (pp = perfluorinated pinacolato, O 2 C 2 -(CF 3 ) 4 ) have excellent electrochemical performance.The [B(pp) 2 ] À anion also exhibits a high tolerance to air and water. Both electrolytes give more stable electrodeelectrolyte interfaces than conventionally used NaPF 6 , as demonstrated by impedance spectroscopy and cyclic voltammetry. Furthermore, they give greater cycling stability and comparable capacity to NaPF 6 for SIBs, as shown in commercial pouch cells.

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Section: Discussionmentioning

confidence: 99%

“…The heightened susceptibility for EC vs. EMC towards hydrolysis driven degradation 62 will likely also play a role in the overall TM dissolution/deposition due to acidification of the electrolyte (Scheme S1 and Scheme 2). Further, the relative stabilizing influence of the carbonate solvent and/or electrolyte degradation products in solvating TMs in solution may also contribute to this effect.…”

Section: Discussionmentioning

confidence: 99%

Electrolyte reactivity at the charged Ni-rich cathode interface and degradation in Li-ion batteries

Dose

Temprano

Allen

et al. 2021

Preprint

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The chemical and electrochemical reactions at the positive electrode-electrolyte interface in Li-ion batteries are hugely influential on cycle life and safety. Ni-rich layered transition metal oxides exhibit higher interfacial reactivity than their lower Ni-content analogues, reacting via poorly understood mechanisms. Here, we study the role of the electrolyte solvent, specifically cyclic ethylene carbonate (EC) and linear ethyl methyl carbonate (EMC), in determining the interfacial reactivity at LiNi0.33Mn0.33Co0.33O2 (NMC111) and LiNi0.8Mn0.1Co0.1O2 (NMC811). Parasitic currents are measured during high voltage holds in NMC/Li4Ti5O12 (LTO) cells, LTO avoiding parasitic currents related to anode-cathode reduction species cross-over, and are found to be higher for EC-containing vs. EC-free electrolytes with NMC811. No difference between electrolytes are observed with NMC111. On-line gas analysis reveals this to be related to lattice oxygen release, and accompanying electrolyte decomposition, which increases substantially with greater Ni content, and for EC-containing electrolytes with NMC811. This is corroborated by electrochemical impedance spectroscopy (EIS) and transmission electron microscopy (TEM) of NMC811 after the voltage hold, which show a higher interfacial impedance and a thicker oxygen-deficient rock-salt surface reconstruction layer, respectively. Combined findings from solution NMR, ICP (of electrolytes) and XPS analysis (of electrodes) reveal that higher lattice oxygen release from NMC811 in EC-containing electrolytes is coupled with more electrolyte breakdown and higher amounts of transition metal dissolution compared to EC-free electrolyte. Finally, new mechanistic insights for the chemical oxidation pathways of electrolyte solvents and, critically, the knock-on chemical and electrochemical reactions that further degrade the electrolyte and electrodes curtailing battery lifetime are provided.

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A non-aqueous sodium hexafluorophosphate-based electrolyte degradation study: Formation and mitigation of hydrofluoric acid

Cited by 60 publications

References 53 publications

Electrolyte Reactivity at the Charged Ni-Rich Cathode Interface and Degradation in Li-Ion Batteries

Electrolyte Reactivity at the Charged Ni-Rich Cathode Interface and Degradation in Li-Ion Batteries

Sodium Borates: Expanding the Electrolyte Selection for Sodium‐Ion Batteries

Electrolyte reactivity at the charged Ni-rich cathode interface and degradation in Li-ion batteries

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