Differential Thermal Analysis of Li-Ion Cells as an Effective Probe of Liquid Electrolyte Evolution during Aging

Abstract

A technique has been developed to allow for in-situ observation of the state of the liquid electrolyte present in an electrochemical device through exploiting the nature of solid-liquid phase transitions. An apparatus was developed to perform differential thermal analysis (DTA) on complete NMC/graphite lithium-ion pouch cells. The DTA apparatus monitored the temperature of a sample cell alongside that of a thermally inert reference cell during a controlled temperature scan through the melting point of the electrolyte in the sample cell, producing a thermal signature of the sample electrolyte without damage to the cell performance. This signature holds information pertaining to the composition and amount of liquid electrolyte present in the electrochemical device, allowing for detailed study of electrolyte evolution at various points of life and indications of the state-of-health of the device. In recent years, lithium-ion batteries have surpassed other competing technologies as the pre-eminent choice in a wide range of energy storage applications. With expansion beyond use in personal electronics into mainstream automotive applications and grid storage, a more complete understanding of the complex processes evolving in these devices over a decade of life is of utmost importance. A generic lithium-ion cell comprises two electrodes and a liquid electrolyte (itself composed of ever-increasingly complex systems of lithium-based salts, organic solvents and a blend of electrolyte additives intended to stabilize and improve the cell operation and lifetime). Parasitic reactions are known to coincide with regular operation of the device, leading to reduction and oxidation of the liquid electrolyte through development of solid electrolyte interface layers (SEI) and also other reactions at the electrodes.1-3 Depending on the specific electrolyte chemistry and cycling protocol, this may result in substantial changes to the chemistry of the electrolyte compared to that introduced to the cell at time of manufacturing, significant electrolyte consumption, or both.To date, studies of this evolution have been limited to ex situ post mortem studies of the cells. Possible methods include X-ray photoelectron spectroscopy (XPS) 2 and gas chromatography coupled with mass-spectrometry (GC/MS). 4 These studies are limited as XPS studies the SEI directly as opposed to the electrolyte itself and GC/MS is insensitive to the electrolyte salt. Furthermore, all such schemes are inherently destructive, precluding intermittent study of a single cell at arbitrary points of life.A thorough understanding of the evolution and loss of liquid electrolyte are critical to both determining chemistries for study in research programs, as well as establishing the value and remaining lifetime of an used lithium-ion cell. The strict capacity requirements for Li-ion cells in electric vehicles necessitating battery disposal at an intermediate state of life has resulted in the recent emergence of a used-battery market. 5,6 This further underscores the value of...

Quantifying Changes to the Electrolyte and Negative Electrode in Aged NMC532/Graphite Lithium-Ion Cells

“…Since transition metal dissolution has been shown to be driven by HF formation from PF 6 − , 30 it could also be that the difference in the results from this study and the Gilbert study originates from relative amounts of formed PF 5 and HF. This is also consistent with the lack of dimerization products measured in the electrolyte in this study.…”

“…The PTFE vial is then machine shaken in two directions for a total of 30 minutes. Some of the extract is removed and filtered with a syringe filter containing a PTFE membrane with a 0.45 μm pore size into a second PTFE vial containing additional CH 2 Cl 2 and deionized-water for LiPF 6 and HF extraction. 17 The new solution is then shaken for another 5 minutes, centrifuged for 20 minutes at 2200 RPM (800 g) to separate the organic and inorganic layers.…”

“…5 Aged lithium-ion cells with many cycles or after testing in extreme conditions show dramatic electrolyte changes. [6][7][8] Recorded electrolyte changes include oxidation at the positive electrode, 7,9 reduction at the negative electrode, 7,9 dimerization of ethylene carbonate (EC) and linear carbonates, 8 transesterification of the linear carbonates, 10 LiPF 6 break down and subsequent reactions, 7,[11][12][13] and consumption of active Li and electrolyte in solid electrolyte interphase (SEI) formation. 7,14 These reactions may be paired with electrode transformations later in cell life including changes to the positive electrode surface structure from layered to spinel 1,15 and transition metal dissolution from the positive electrode followed by deposition on the negative electrode.…”

Dramatic Effects of Low Salt Concentrations on Li-Ion Cells Containing EC-Free Electrolytes

“…Day et al 46 showed that depletion of LiPF 6 salt occurred during aggressive charge-discharge cycling to 4.5 V in NMC442/graphite cells at 55…”

Improving Linear Alkyl Carbonate Electrolytes with Electrolyte Additives

“…[3][4][5] It was generally believed that the graphite electrode could not be electrochemically lithiated in most common solvents, including ethers, esters, butyrolactones and propylene carbonate, without the sacrificial reaction of EC to form the SEI. 6 While the passivation of graphite is virtually complete during the first few cycles, the remaining EC in the electrolyte can be oxidized at high voltages, causing the performance of cells containing EC to diminish as the upper charge voltage increases above 4.4 V. 7,8 Electrolyte oxidation at the positive electrode at high potentials can lead to salt consumption 9 and gas evolution. 10 Precipitation of electrolyte oxidation by-products onto the surface of the positive and negative electrodes causes polarization and impedance growth, 11,12 which can limit cycle life.…”