Dimethyl acetamide (DMAC) and N-methyl pyrrolidinone (NMP) have been found to be useful as high- temperature- resilience-enhancing additives to a baseline electrolyte used in rechargeable lithium-ion electrochemical cells. The baseline electrolyte, which was previously formulated to improve low-temperature performance, comprises LiPF6 dissolved at a concentration of 1.0 M in a mixture comprising equal volume proportions of ethylene carbonate, diethyl carbonate, and dimethyl carbonate. This and other electrolytes comprising lithium salts dissolved in mixtures of esters (including alkyl carbonates) have been studied in continuing research directed toward extending the lower limits of operating temperatures and, more recently, enhancing the high-temperature resilience of such cells. This research at earlier stages, and the underlying physical and chemical principles, were reported in numerous previous NASA Tech Briefs articles.

Reversible Capacities, after storage at a temperature of 65 °C of cells containing the baseline electrolyteplus various additives were measured at 20 °C at a discharge rate of 25 mA [numerically equal to 1/16 of(the nominal capacity in A·h)] after the cells had been charged to a potential of 4.1 V at a rate of 25 mA.
Although these electrolytes provide excellent performance at low temperatures (typically as low as –40 °C), when the affected Li-ion cells are subjected to high temperatures during storage and cycling, there occur irreversible losses of capacity accompanied by power fade and deterioration of low-temperature performance. The term "high-temperature resilience" signifies, loosely, the ability of a cell to resist such deterioration, retaining as much as possible of its initial charge/discharge capacity during operation or during storage in the fully charged condition at high temperature. For the purposes of the present development, a temperature is considered to be high if it equals or exceeds the upper limit (typically, 30 °C) of the operating-temperature range for which the cells in question are generally designed.

Prior studies focusing on the reactivity of electrolytes like the ones of interest here had established that LiPF6 can thermally decompose to form LiF and PF5, the later product being a strong Lewis acid that further reacts with alkyl carbonates to form a number of byproducts. The present additives — DMAC and NMP — are Lewis bases that act as stabilizing agents in that they reversibly bind with PF5, thereby preventing decomposition of the carbonate solvents at high temperature.

To enable testing of these additives, rechargeable carbon-anode/LiNi0.8 Co0.2O2-cathode cells containing the baseline electrolyte plus various proportions of these additives were assembled. For comparison, cells containing, variously, the baseline electrolyte alone or the baseline electrolyte plus either of two previously known additives [vinylene carbonate (VC) and vinylethylene carbonate (VEC)] were also assembled. The cells were subjected to charge-discharge cycling tests and other electrochemical tests at various temperatures from room temperature (23 °C) down to –20 °C. The cells were also evaluated with respect to high-temperature resilience by measuring the capacities retained after storage for 10 days at a temperature of 55 °C, followed by 10 days at 60 °C, followed by 10 days at 65 °C. The greatest retention of capacity was observed in a cell containing 3 percent of DMAC as the additive. Other cells, in order of decreasing retained capacity, included those containing 1 percent of DMAC, 1.5 percent of VC, 1.5 percent of VEC, 3 percent of NMP, and no additive (see figure).

This work was done by Marshall Smart and Ratnakumar Bugga of Caltech and Brett Lucht of the University of Rhode Island for NASA's Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

Innovative Technology Assets Management

JPL

Mail Stop 202-233

4800 Oak Grove Drive

Pasadena, CA 91109-8099

E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Refer to NPO-44805, volume and number of this NASA Tech Briefs issue, and the page number.

Topics:
Chemicals Electrolytes Lithium Materials handling Materials properties Metals Physical Sciences Research and development Storage Wear
This Brief includes a Technical Support Package (TSP).
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DMAC and NMP as Electrolyte Additives for Li-Ion Cells

(reference NPO-44805) is currently available for download from the TSP library.

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NASA Tech Briefs Magazine

This article first appeared in the October, 2008 issue of NASA Tech Briefs Magazine (Vol. 32 No. 10).

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Overview

The document discusses advancements in lithium-ion battery technology, particularly focusing on the use of electrolyte additives, dimethyl acetamide (DMAC) and N-methyl pyrollidone (NMP), to enhance the performance of batteries in extreme temperature conditions. These developments are crucial for future NASA missions, including those aimed at exploring Mars and the outer planets, where rechargeable batteries must operate effectively in temperatures ranging from -40 to +30 °C.

The primary challenge addressed is the inadequate low-temperature performance and resilience to high-temperature exposure of current lithium-ion systems, which are essential for applications such as landers, rovers, and penetrators. The document highlights that while state-of-the-art lithium-ion batteries meet many requirements, they struggle with maintaining performance over their operational lifespan, particularly due to the continuous reaction of the electrolyte on electrode surfaces, leading to the formation of resistive surface films known as solid electrolyte interfaces (SEI). These films limit both low-temperature capability and power delivery, especially as the battery ages.

The research presented indicates that the incorporation of DMAC and NMP into optimized electrolyte formulations can significantly improve the high-temperature resilience of lithium-ion cells. These Lewis base additives have been shown to reversibly bind with PF5, preventing the decomposition of LiPF6 and carbonate solvents at elevated temperatures. This stabilization is expected to extend the life of the cells and maintain their low-temperature performance throughout their operational life.

Experimental lithium-ion cells, featuring MCMB carbon anodes and LiNi0.8Co0.2O2 cathodes, were fabricated to validate the effectiveness of these additives. The results demonstrated that the addition of DMAC notably slowed the degradation of anode kinetics, while the greatest retention in cathode kinetics was observed with the addition of vinylene carbonate (VC). Although the experimental cells were not hermetically sealed, the trends in stability are anticipated to be similar in more refined prototype cells.

In summary, the document outlines the promising potential of DMAC and NMP as electrolyte additives to enhance the performance and longevity of lithium-ion batteries, addressing critical challenges for future space exploration missions. The ongoing research aims to develop advanced electrolytes that can operate effectively across a wide range of temperatures, ensuring reliable energy storage solutions for NASA's ambitious exploration goals.