A Systematic Study of Electrolyte Additives in Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 (NMC)/Graphite Pouch Cells

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NMC532/artificial graphite cells using single crystal NMC532 active material can have excellent long term lifetime at 4.4 V and elevated temperature if appropriate electrolytes are used. However, electrolytes developed earlier for these cells and reported in the literature cannot support even C/2 rates during charging without unwanted lithium plating at room temperature. This work is thus focused on the development of new electrolytes for single crystal NMC532/artificial graphite cells that can yield long lifetime and support higher charging rates. Ex-situ and in-situ gas measurements, ultra-high precision coulometry, isothermal microcalorimetry, lithium plating tests and long term cycling tests were used for the screening of electrolytes. Capacity loss in lithium ion cells can be caused by the loss of lithium inventory to the solid electrolyte interphase (SEI).1-3 Active material loss due to structural degradation and due to electrical disconnection at the particle/electrode level can also lead to capacity loss. Internal impedance or polarization increase is another major contributor to capacity loss under high rate discharge conditions. Moreover, unwanted lithium plating, which can occur during high rate or low temperature charging, can also result in severe capacity fade. At high potentials, accelerated unwanted reactions in the electrolyte such as electrolyte oxidation occur and can hasten capacity loss by causing reconstruction of the positive electrode surface which can lead to impedance growth. [1][2][3][4][5] In addition the oxidized by-products can migrate to the negative electrode surface and be reduced there.6,7 Such reactions can lead to the consumption of lithium ions from the electrolyte (to maintain charge neutrality in the electrolyte), a reduction in lithium inventory, as well as a thickening of the negative electrode SEI which together ultimately cause impedance growth and capacity loss. 8,9 These processes are usually accelerated by higher charging potentials and higher temperatures.Novel electrolyte additives and modifications to positive electrode materials have been developed to improve the lifetime of Li-ion cells operated to high potential. • C, more than 88% of the original capacity was maintained after testing for one year (∼2000 cycles with C/2 rate, CCCV) and more than 82% was maintained after 18 months (3000 cycles with C/2 rate, CCCV). Figure S1 in the supplemental information shows the extended test results for the same cells shown in Figure 12 of Reference 22.Liu et al. found that additives and electrolytes that increase the negative electrode area-specific resistance lead to a decreased onset current for unwanted lithium plating.23 Such additives are often those that also increase lifetime of cells under moderate rate conditions. PES211 electrolyte usually leads to large negative electrode charge transfer resistance and therefore is not suitable for applications requiring high rates during charging. Liu et al. showed that NMC111/graphite cells with PES211 electrolyte could not be...

Section: Resultsmentioning

confidence: 99%

Development of Electrolytes for Single Crystal NMC532/Artificial Graphite Cells with Long Lifetime

Stone

et al. 2018

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“…Systematic studies by Nelson et al 8 showed that 2% PES is the optimum concentration of PES for NMC111/graphite and NMC442/graphite cells. Recently, a major study of the effects of 19 electrolyte additives and their combinations on NMC111/graphite cells by Wang et al 9 showed that selected ternary or quaternary additive combinations, especially 2% PES + 1% methylene methanedisulfonate (MMDS) + 1% tris(trimethylsilyl) phosphite (TTSPi) ("PES-211") gave significant improvements in long-term cycling, CE, and charge end-point capacity slippage.…”

mentioning

confidence: 99%

Improving the High Voltage Cycling of Li[Ni_0.42Mn_0.42Co_0.16]O₂(NMC442)/Graphite Pouch Cells Using Electrolyte Additives

Xia

Dahn

2014

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Li [Ni 0.42 Mn 0.42 Co 0.16 ]O 2 (NMC442)/graphite pouch cells with 1 M LiPF 6 in ethylene carbonate (EC): ethyl methyl carbonate (EMC) (3:7 by weight) electrolyte containing 2 wt% vinylene carbonate (VC), 2 wt% prop-1-ene-1,3 sultone (PES), or 2 wt% PES + 1 wt% tris(trimethylsilyl) phosphite (TTSPi) + 1 wt% methylene methanedisulfonate (MMDS) ("PES-211") were subjected to cycling tests at various upper cut-off voltages from 4.2 to 4.7 V. After every 10 cycles, the pouch cells were charged or discharged to 3.8 V and evaluated using electrochemical impedance spectroscopy (EIS). This approach allows the impact of electrolyte additives on the impedance of cells cycled to different cut-off voltages to be studied systematically. The addition of 2% PES or "PES-211" proved to be beneficial in suppressing impedance growth when cells are cycled up to and above 4.4 V. Long-term cycling tests between 3.0 V and 4.4 V at 45 • C showed that "PES-211" had the best capacity retention (∼85%) and the least impedance growth after 500 cycles. The results suggest the use of "PES-211" could lead to longer lived and higher power NMC/graphite Li-ion cells with better tolerance to high voltage (4.4 V) which will improve energy density. Lithium-ion cells are now widely used in various applications ranging from portable electronics to electrified vehicles.1 However, they still suffer from limited life-time due to the parasitic reactions between charged electrode materials and electrolytes during cycling or storage.2 The addition of electrolyte additives is one of the most economical and effective ways to improve the performance (e.g. calendar life, impedance, safety etc.) of lithium-ion cells. 3,4The most famous additive, vinylene carbonate (VC) has been shown to be able to reduce charge end-point capacity slippage and increase coulombic efficiency (CE) showed that prop-1-ene-1,3 sultone (PES), a novel sulfur-containing electrolyte additive, could also increase CE, decrease charge end-point capacity slippage and even suppress gas production of NMC111/graphite pouch cells with a charge cut-off voltage of 4.2 V, which made PES more attractive than VC. Systematic studies by Nelson et al. 8 showed that 2% PES is the optimum concentration of PES for NMC111/graphite and NMC442/graphite cells. Recently, a major study of the effects of 19 electrolyte additives and their combinations on NMC111/graphite cells by Wang et al. 9 showed that selected ternary or quaternary additive combinations, especially 2% PES + 1% methylene methanedisulfonate (MMDS) + 1% tris(trimethylsilyl) phosphite (TTSPi) ("PES-211") gave significant improvements in long-term cycling, CE, and charge end-point capacity slippage. NMC442/graphite pouch cells can, in principle, be operated up to 4.7 V, which will access more positive electrode capacity and increase energy density. However, high voltage operation has proven to be difficult because many electrolyte solvents and additives are unstable at higher voltage, which can cause a dramatic decrease in the lifetimes of Li-...

“…Some promising VC-based and PES-based additive combinations 9 were selected for evaluation during long-term cycle testing at 55…”

Section: Resultsmentioning

confidence: 99%

Ternary Electrolyte Additive Mixtures for Li-Ion Cells that Promote Long Lifetime and Less Reactivity with Charged Electrodes at Elevated Temperatures

Xia

Dahn

2015

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(NMC111)/graphite and Li [Ni 0.42 Mn 0.42 Co 0.16 ]O 2 (NMC442)/graphite pouch cells demonstrate superb performance when ternary blends of electrolyte additives are added to the cells. These ternary blends contain one of vinylene carbonate (VC) or prop-1-ene-1,3-sultone (PES) plus a sulfur containing molecule and tris(trimethylsilyl) phosphite (TTSPi). Here, the excellent charge-discharge capacity retention of cells containing these ternary blends of additives is demonstrated at both 45 • C and 55 • C by comparison to cells with only VC or PES additives alone. The impact of a number of sulfur-and phosphorous-containing electrolyte additives on the reactivity of charged electrode materials with electrolyte at elevated temperatures was studied using accelerating rate calorimetry (ARC). The electrolyte additives, methylene methanedisulfonate (MMDS) and TTSPi, dramatically reduce the reactivity between lithiated (charged) graphite and electrolyte. Lithium-ion cells used in electrified vehicles require longer calendar and cycle lifetime than those used in portable electronics (e.g. laptops and cell phones). In addition, cell used in electrified vehicles can reach high temperatures, so improved tolerance of Li-ion cells to high temperature exposure is also desired. One method of extending lifetime and temperature tolerance of Li-ion cells is to use electrolyte additives in order to suppress parasitic reactions between charged electrodes and electrolyte.1,2 Vinylene carbonate (VC), a popular electrolyte additive, has been shown to improve Li-ion cell performance. 3,4 Recently, some novel sulfur-containing and phosphorouscontaining electrolyte additives have also been shown to be able to increase lifetime, control impedance growth and suppress gas production, etc. for Li-ion cells. Xia et al. 5 showed that the addition of ethylene sulfate (DTD) or trimethylene sulfate (TMS) to Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 (NMC111)/graphite pouch cells could increase coulombic efficiency (CE) and decrease impedance after cycling compared to cells without additives. Methylene methanedisulfonate (MMDS) has been shown to be able to greatly decrease the impedance of NMC111/graphite cells after cycling and slow the rate of parasitic reactions at the Li x MO 2 electrode according to storage results.6 Xia et al.7 also showed that prop-1-ene-1,3-sultone (PES) could increase CE, decrease charge end point capacity slippage and suppress gas production during cycling for NMC111/graphite pouch cells. Sinha et al. 8 showed that the CE of NMC111/graphite pouch cells was improved and impedance was reduced when tris(trimethylsilyl) phosphate (TTSP) or tris(trimethylsilyl) phosphite (TTSPi) was added to the electrolyte. In a massive study of over 110 electrolyte additive combinations tested in NMC111/graphite pouch cells, Wang et al. showed that ternary electrolyte additive blends containing one or more of VC or PES, one or more of DTD, MMDS, ES or TMS and one or more of TTSP or TTSPi led to cells with excellent performance. Electrolyte additives improve...