Ultrahigh Precision Coulometry Research Articles

Studies of the Effect of Varying Prop-1-Ene-1,3-Sultone (PES) Concentration on Li-Ion Batteries

Introduction Electrolyte additives are the most effective way to improve the calendar life and cycling performance of lithium-ion batteries. Vinylene carbonate (VC) is perhaps the most well-known additive for Li-ion batteries and has been shown to be useful during the solid electrolyte interphase (SEI) formation [1], which can result in longer calendar life and reduced irreversible capacity (IRC). However, it has been shown that the performance of cells containing VC decreases at high temperatures and high voltages. Sulfur-containing additives have been studied in hopes to overcome the temperature sensitivity of VC. Prop-1-ene-1,3-sultone (PES) was found to have smaller IRC than VC-containing cells and was suggested to be an SEI forming additive [2]. In this study, we show that PES is a useful additive for Li-ion pouch cells and suggest that PES, when used in combination with other additives, may be more effective than VC at achieving long lifetimes and high cycling performance. Experimental Li[Ni1/3Mn1/3Co1/3]O2 (NMC)/graphite pouch cells made with 1M LiPF6in EC:EMC 3:7 (by weight) and varying amounts of PES were studied using ultra high precision coulometry and storage experiments. In addition, gas evolution measurements employing Archimedes principle and electrochemical impedance spectroscopy (EIS) measurements were performed. The PES-containing cells were compared to the same control electrolyte containing 2% VC. Results Figure 1 shows a preliminary set of results from both storage and cycling experiments of PES-containing cells compared with control electrolyte and 2% VC-containing cells. The dotted line in Figures 1a, d, and e indicates the value for 2% VC. Figure 1a shows the gas evolution during formation at 40°C and Figures 1b and c show gas evolution during storage at 60°C and 40°C, respectively. Cells containing 2, 4 and 6% PES show smaller gas during formation than that of 2% VC and all concentrations of PES studied show significantly smaller gas during 40°C and 60°C storage than VC-containing cells. The most prominent advantage of PES compared to VC is the extremely low gas evolution during high temperature storage. Figure 1d shows the voltage drop during storage from 4.2 V at 40°C for 500 hours. Figure 1e shows the average coloumbic inefficiency (CIE) during the last three cycles of cycling between 2.8 V and 4.2 V at C/20 and 40°C. Through the voltage drop and CIE, it is clear that 2% PES appears to be as good as 2% VC. This work suggests that PES is a viable alternative to VC and is superior during high temperature storage. PES may also prove to be beneficial in combination with other additives.

Single-Phase Layered Compositions in the Li-Mn-Ni-O System Which Do Not Significantly Oxidize Electrolyte at 4.6 V Versus Li/Li+

Detailed phase diagrams of the Li-Mn-Ni-O system have been determined to provide a broader understanding of how synthesis conditions affect the phase composition of Li-Mn-Ni-O positive electrode materials.1,2 Figure 1 shows the positive electrode region of the Li-Mn-Ni-O pseudoternary phase diagram for samples quenched from 900oC. The phase diagram is labeled with various individual compositions, single-phase regions, tie-lines, and multi-phase co-existence regions all elucidated through extensive crystallographic analysis,3 introducing many opportunities for characterization of new positive electrode materials. In order to develop Li-ion batteries with high energy densities and long cycle lives, ideal novel materials would not react with carbonate-based electrolytes at high potentials (≥ 4.6 V vs. Li/Li+).In this study, three single-phase layered compositions in the Li-Mn-Ni-O system, labeled as A, B and C in Figure 1, were studied by ICP-OES, XRD, and ultra high precision coulometry (UHPC), a technique which uses precise measurement of coulombic efficiency (CE) and charge endpoint capacity slippage to detect electrolyte oxidation. Sample A was determined to be Li[Li0.157Ni0.122Mn0.650□0.071]O2, a Li-deficient, Mn-rich material containing 3.5% metal site vacancies.4 Sample B was Li[Li0.117Ni0.325Mn0.558]O2, an essentially Ni-rich member of the Li-rich oxide solid solution series (dashed orange line in Figure 1), while sample C was determined to be Li[Li0.148Ni0.480Mn0.471]O2, an approximate Li-rich analogue of Li[Ni0.5Mn0.5]O2.Figure 2 shows the CE, discharge capacity, and normalized charge endpoint capacity for cycle 10 onwards for samples A, B, and C cycled to 4.6 V and 4.8 V, and for Li[Ni1/3Mn1/3Co1/3]O2 cycled to 4.2 V, 4.4 V, and 4.6 V. In general, cycling to 4.6 V yielded better CE and lower slippage of the Li-Mn-Ni-O materials compared to cycling to 4.8 V, which produced more slippage due to electrolyte oxidation. The performance of the Li-deficient Li[Li0.157Ni0.122Mn0.650□0.071]O2 material cycled to 4.6 V was striking, as it maintained a substantially higher CE and a lower charge endpoint capacity slippage per cycle than Li[Li0.117Ni0.325Mn0.558]O2, Li[Li0.148Ni0.480Mn0.471]O2, and industry standard Li[Ni1/3Mn1/3Co1/3]O2 (cycled to only 4.2 V) while maintaining a reversible capacity of 225 mAh/g after 50 cycles. These results highlight the inherent “inertness” of Li[Li0.157Ni0.122Mn0.650□0.071]O2 and its suitability as a thin protective shell in a core-shell particle configuration.

Combinations of Ethylene Sulfite (ES) and Vinylene Carbonate (VC) as Electrolyte Additives in Li(Ni1/3Mn1/3Co1/3)O2/Graphite Pouch Cells

The effectiveness of ethylene sulfite (ES) and/or vinylene carbonate (VC) as electrolyte additives in Li(Ni1/3Mn1/3Co1/3)O2 (NMC)/graphite pouch cells was studied using in situ measurements of gas evolution, ultra high precision coulometry (UHPC), automated storage experiments and electrochemical impedance spectroscopy (EIS). Cells containing only ES produced large amounts of gas during both formation and cycling. UHPC cycling and storage results showed that cells containing only ES had lower coulombic efficiency, larger charge end point capacity slippage rate and larger voltage drop during storage than control cells. Moreover, the impedance of cells containing only ES increased after cycling. These results show that ES is not a competitive additive when used on its own. When ES is combined with VC, there are several benefits to cell performance. Cells with 2% VC + ES (1% or 2%) produced virtually no gas during formation and cycling and showed similar performance in UHPC testing to cells containing only 2% VC. The most interesting characteristic is that the use of both ES and VC greatly reduced (by about 50%) the cell impedance after cycling. EIS studies of positive and negative electrode symmetric cells showed that the introduction of ES (1% or 2%) to 2% VC-containing electrolyte led to a significant decrease in impedance at the negative electrode.

Comparative Study on Prop-1-ene-1,3-sultone and Vinylene Carbonate as Electrolyte Additives for Li(Ni1/3Mn13Co1/3)O2/Graphite Pouch Cells

The effectiveness of Prop-1-ene-1,3-sultone (PES) and Vinylene carbonate (VC) as electrolyte additives in Li(Ni1/3Mn1/3Co1/3)O2 (NMC)/graphite pouch cells was studied using in situ measurements of gas evolution, ultra high precision coulometry (UHPC), automated storage experiments and electrochemical impedance spectroscopy (EIS). Gas measurements show that cells containing PES produce less gas than cells containing 2% VC during formation and much less gas during elevated temperature storage (60°C) at 4.2 V. UHPC cycling and storage results show that cells containing 2% PES have higher coulombic efficiency, lower charge end point capacity slippage and smaller voltage drop during storage than cells containing 2% VC. The combination of VC and PES shows even better cycling performance than the single additives, however, at the expense of much higher impedance. EIS studies of positive and negative electrode symmetric cells show that the introduction of PES or VC to control electrolyte affects the individual electrode impedances in a similar way: both increase the negative electrode impedance and decrease the positive electrode impedance. Based on these experiments we suggest that PES is a very useful additive for NMC/graphite Li-ion pouch cells that may encounter high temperatures during storage and cycling.

Comparative Study on Methylene Methyl Disulfonate (MMDS) and 1,3-Propane Sultone (PS) as Electrolyte Additives for Li-Ion Batteries

The effectiveness of two electrolyte additives, 1,3-propane sultone (PS) and methylene methanedisulfonate (MMDS) either singly or in combination with vinylene carbonate (VC) has been studied. Li(Ni1/3Mn1/3Co1/3)O2 (NMC)/Graphite pouch cells containing these additives were studied using ultra high precision coulometry, automated storage experiments and electrochemical impedance spectroscopy. The two additives have the same SO3 functional group but it seems PS is more widely used by battery manufacturers. When used alone, both additives decrease the charge transfer impedance during cycling while MMDS shows better coulombic efficiency and lower charge end point capacity slippage rate than PS. Compared with 2% VC, MMDS in combination with 2% VC significantly improves the coulombic efficiency and the charge slippage rate, reduces the charge transfer impedance during cycling and decreases the voltage drop during storage while the addition of PS to VC-containing electrolyte does not show this added benefit. Compared to PS, MMDS seems to be a more beneficial additive for improving cell performance.