An apparatus was built to make accurate and precise in situ measurements of the volumes of gas evolved in Li-ion pouch cells during operation. With a thin film load cell accurately measuring the weight of a cell submerged in a fluid, the volume of a pouch cell can be precisely monitored using Archimedes' Principle. Examples showing the utility and sensitivity of the device have been selected from measurements made during the formation cycle (very first charge and discharge) of Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 /graphite (NMC) Li-ion pouch cells. Gas production occurs at the very beginning of the formation cycle but quickly stops for cells containing a variety of electrolytes. The volume of the pouch cell then decreases with time. The testing of cells with various electrolyte additives indicated that the common additive, vinylene carbonate, is very effective at reducing the amount of gas formed during formation, but the best results among the additives reported here were obtained by using a combination of 2% vinylene carbonate and 2% prop-1-ene 1,3-sultone. The additives vinyl ethylene carbonate and ethylene sulfite were found to delay the onset of gas production during formation.
This preliminary study investigates initial gas formation in Li [Ni 0.4 Mn 0.4 Co 0.2 ]O 2 (NMC442) pouch cells with three different electrolytes: 3:7 ethylene carbonate : ethyl methyl carbonate (EC:EMC) w/ 1 M LiPF 6 as the control, control + 2% prop-1-ene-1,3-sultone (PES) and control + 2% vinylene carbonate (VC). In situ volume measurements reveal three main features of gas evolution, an initial gas step, gas absorption, and a second gas step at higher voltage. Gas chromatography-mass spectrometry is employed to identify the gaseous compounds. This work illustrates the strong dependence of volume change due to gas evolution on additives, charging rate and temperature.
Wound LiCoO 2 /graphite cells with 1 M LiPF 6 EC:EMC electrolyte containing 1 wt%, 2 wt% vinylene carbonate (VC), 0.3 wt% trimethoxyboroxine (TMOBX) and 2 wt% VC + 0.3 wt% TMOBX were subjected to extended storage studies. After storage, the electrodes were studied using the symmetric cell and electrochemical impedance spectroscopy (EIS) approach described by previous workers. This approach allows the impact of an additive on the impedance of the negative and the positive electrode to be distinguished. Compared to the control cells, adding 1 wt% VC reduced the positive electrode impedance and only slightly affected the negative electrode impedance. Adding 2 wt% VC reduced the positive electrode impedance and greatly increased the negative electrode impedance. An addition of 0.3 wt% TMOBX greatly decreased the positive electrode impedance and slightly increased the negative electrode impedance. Compared to the cells with 2% VC only, adding 2% VC + 0.3% TMOBX decreased the positive electrode impedance without affecting the negative electrode impedance leading to a significant reduction in full cell impedance. These results help explain why the combination of VC and TMOBX additives can be effective in LiCoO 2 /graphite cells designed for long life time.Lithium-ion batteries have high gravimetric and volumetric energy densities which make them suitable for portable electronics and electric vehicle applications. However, parasitic reactions between the electrolyte and the electrochemically active material limit their lifetime, especially at elevated temperatures. Electrolyte additives are generally used in commercial batteries to improve capacity retention and calendar life. [1][2][3] Although it is very apparent that electrolyte additives play an important role, the details of how they work are poorly understood. The most-studied additive, vinylene carbonate (VC) has been shown to change the chemistry of the passivation film on the graphite electrode. 4-7 It is not clear whether this changed film is actually a better film, because recent experiment by Xiong 8 show that only at 60 • C are the parasitic reactions with electrolyte reduced in rate in the presence of VC: at lower temperatures, the reactions are accelerated. Burns et al. and Sinha et al.,9,10 have shown that VC strongly reduces the rate of reactions between the electrolyte and the charged positive electrode, and it seems that the major impact of VC may be at the positive electrode.Burns et al. studied electrolyte additives in wound prismatic cells using high precision coulometry and electrochemical impedance spectroscopy (EIS). 11 These methods show how additives affect the cycling performance, coulombic efficiency, charge and discharge end-point capacity slippage rates and potential drop during storage. As a motivation for the work in this paper, Figure 1 reviews some of the earlier work by Burns et al., 11 where cells were first tested for 600 hours at 40 • C on the high precision charger, then impedance spectra were collected and then cells were cycled f...
The capacity fade mechanisms of LiCoO 2 /Si-alloy:graphite pouch cells filled with a 1M LiPF 6 EC:EMC:FEC (27:63:10) electrolyte were studied using galvanostatic cycling, electrochemical impedance spectroscopy on symmetric cells, gas-chromatography and differential voltage analysis. Analysis of the gas generated during the first cycle indicated that FEC reacts at the negative electrode following a 1-electron reduction pathway and other pathways that do not lead to the formation of gaseous products. An analysis of the electrolyte showed that FEC is continuously consumed during the first 80% of the first charge (formation cycle). Typical cells charged and discharged at 40 • C showed a gradual capacity loss for the first 250 cycles followed by a sudden capacity drop associated with a large polarization growth. Analysis of the electrolyte showed that this sudden failure is associated with the depletion of FEC. The capacity loss as well as the consumption of FEC prior to the sudden failure was fitted using a model that includes a time dependence and a cycle number dependence. The time dependence was associated with the thickening solid electrolyte interface at the surface of the negative electrode particles (Si-alloy and graphite) and the cycle number dependence was associated with the solid electrolyte interface repair at the surface of Si-alloy particles during repeated expansion and contraction. Cost reduction and an increase in the volumetric energy density of Li-ion cells can possibly be attained using silicon-based negative electrodes.1-3 While these benefits have been known for many years, commercial implementation remains limited due to silicon's inherent challenges.Si-based electrodes can suffer from particle pulverization upon cycling (e.g. micron sized silicon), 1,4-7 can have loss of electronic contact between particles after repeated expansion and contraction during cycling, 2,8-10 can have high irreversible first cycle coulombic efficiency 11,[12][13][14][15] and can have low coulombic efficiency (compared to graphite electrodes) during cycling. 1,16,17 The root cause of these challenges is the large volume expansion 1,18 of the material during lithiation and subsequent contraction during delithiation. Several issues have been solved through the use of either nanosized Si or nanostructured Si. For instance the use of nanosized Si particles solves some of the pulverization problems encountered by micron sized Si particles. 1,4,11,[19][20][21] However, nanosized Si still suffers from inter-particle contact loss during cycling as well as low coulombic efficiency due to large specific surface area. 1,16In a recent publication, Chevrier et al. 16 presented a rational way to design commercially relevant Si-based negative electrodes. They showed that confining nano-domains of silicon in an alloy matrix suppresses particle pulverization and greatly reduces the surface area of Si exposed to the electrolyte. This approach yields materials with lower first cycle irreversible capacity (IRC) loss, better coulombic e...
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