Evaluation of Electrolyte Oxidation Stability on Charged LiNi_0.5Co_0.2Mn_0.3O₂Cathode Surface through Potentiostatic Holds

Abstract

The electrolyte oxidation rate was measured at 4.6 V vs. Li + /Li for a NCM/LTO full cell at temperatures ranging from room temperature to 55 • C. A large (∼2.5) N/P ratio was used in electrode fabrication to enable a stable reference in the Li 4 Ti 5 O 12 electrode for both cycling and the potentiostatic hold regimes. LTO was used to prevent electrolyte reduction from contributing to the signal, as well as providing a stable impedance. For the alkyl carbonate electrolyte used, oxidation rate at room temperature to 45 • C approximately ranged from 1-2 μA, as measured during the terminal 4.2 hours of a 60 hour potentiostatic hold. Many cells aged at 55 • C indicated increasing, unstable currents. Upon the completion of the hold, the cells were cycled to evaluate the effect of the hold and analyzed with AC impedance spectroscopy. Higher temperatures during the potentiostatic hold led to lower capacities during post-aging cycling, as well as higher impedance as measured with EIS.In an effort to increase the energy density of batteries, high-voltage cathodes (V > 4.5 vs. Li/Li + ) are being explored as alternatives to the stable 4-V lithium manganese oxide spinel chemistry, including high voltage spinels, 1-3 nickel-cobalt-manganese layered oxides, 4-6 and olivines. 7,8 As the conventional carbonate electrolytes are traditionally thought to be stable up to 4.5 V vs. Li/Li + , improved electrolytes (either through passivating additives or intrinsically stable electrolytes) will have to be implemented to enable the higher performance of the high-voltage cathodes. 9,10 In the field of electrolyte research, there are many publications that report improved cycling performance with the addition of small amounts of novel molecules designed to 'passivate' the surface and inhibit electrolyte oxidation. 11-16 While these results are promising, the interfacial parameters are poorly constrained in these experiments, making a fundamental understanding of electrolyte anodic stability difficult to characterize. For additives that reportedly passivated the surface, there are additional explanations as to the mechanism of improvement -including HF scavenging and prevention of metal ion dissolution, which would lead to better capacity retention. 17,18 Additionally, there are pursuits of intrinsically stable electrolytes that will experience a lower decomposition rate due to a lower thermodynamic or kinetic driving force for the oxidation reaction. This pursuit has led to a variety of fluorinated solvents that show improved performance in cycling tests. 19,20 In order to enable high voltage battery chemistries, a more stable electrolyte system is necessary, and the comparison of new electrolyte compositions against a baseline anodic stability is critical to improving the high voltage limits of electrolyte systems. In this work, potentiostatic holds are performed on full cells with a lithium titanate (Li 4 Ti 5 O 12 ) anode to shed some light on the reaction rate of the conventional carbonate-based electrolyte at the charged cathode sur...

Chemical “Pickling” of Phosphite Additives Mitigates Impedance Rise in Li Ion Batteries

“…With a single phase cathode and a lithium titanate (Li 4 Ti 5 O 12 ) anode with a plateau voltage profile, the electric current flowing through the cell is only sensitive to oxidation reactions occurring at the cathode. 52,53 Figure S6 shows the current as a function of hold time. In each trace, there is an initial rapid decay (<5 h) due to polarization relaxation in the electrolyte and cathode material.…”

“…To assess the effect of chemical aging on parasitic oxidation of the electrolyte at the cathode interface, the potentiostatic hold method was used. With a single phase cathode and a lithium titanate (Li 4 Ti 5 O 12 ) anode with a plateau voltage profile, the electric current flowing through the cell is only sensitive to oxidation reactions occurring at the cathode. , …”

“…In the potentiostatic hold experiments, − we used the coin cells with the anodes composed of 87 wt % Li 4 Ti 5 O 12 (NEI), 8 wt % PVdF (Solvay) binder, and 5 wt % carbon on an Al current collector (26.1 mg/cm 2 coating) and a cathode with a lower loading of 4.08 mg/cm 2 . These cells were cycled at 27 mA/g oxide for one cycle and 85 mA/g oxide for 10 cycles from 2.85 to 1.45 V before being charged at 85 mA/g oxide to a terminal voltage of 3.05 V (which corresponds to 4.6 V vs Li/Li + ).…”

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

“…The LTO anode and the separators were removed and 4.0 mL of ~2 % nitric acid (diluted from concentrated nitric acid; 67-69 %, trace metal grade, Fischer Chemical) was added to the extracted electrolyte (34-70 μL) before analysis. The LTO coating was scraped from the current collector (19)(20)(21)(22)(23)(24)(25)(26) and soaked in 248 μL of 5.1 M nitric acid for 5 days before being diluted to 4.0 mL with 3.75 mL of water for analysis. The LTO coating scraped from uncycled electrodes was measured as a baseline.…”

“…23 As mentioned above, the latter is directly related to the chemical oxidation of the electrolyte. A NMC/Li4Ti5O12 (LTO) cell pairing is used since at 1.55 V vs. Li/Li + there is expected to be negligible continuous electrochemical electrolyte reduction at the LTO electrode, 24,25 thereby avoiding cross-over of electrolyte reduction products formed at the anode to the cathode.…”