In order to improve the safety and performance of lithium- ion cells, especially over a wide operating temperature range, a number of Li-ion electrolytes have been developed that contain flame-retardant additives in conjunction with fluorinated co-solvents to provide a safe, wide-operating-temperature- range system. Previously, fluorinated esters were incorporated into multi-component electrolyte formulations and their performances have been demonstrated over a wide temperature range (–60 to +60 °C). The fluorinated esters co-solvents were employed due to their favorable properties and improved safety characteristics, mainly associated with their low flammability associated with their halogenated nature.
In the present work, the safety characteristics of these electrolytes have been further improved by the addition of flameretardant additives, such as triphenyl phosphate (TPP), tributyl phosphate (TBP), triethyl phosphate (TEP), and bis(2,2,2—trifluoroethyl) methyl phosphonate (BTFEMP). A number of electrolytes based upon these approaches have delivered good performance over a wide temperature, good cycle-life characteristics, and improved safety characteristics; namely, reduced flammability. Thus, the following electrolyte formulations were investigated and demonstrated in experimental MCMB carbon-LiNi0.8Co0.2O2 cells:
- 1.0 M LiPF6 in EC+EMC+TFEB+TPP (20:55:20:5 v/v %)
- 1.0 M LiPF6 in EC+EMC+TFEB+TBP (20:55:20:5 v/v %)
- 1.0 M LiPF6 in EC+EMC+TFEB+TEP (20:55:20:5 v/v %)
- 1.0 M LiPF6 in EC+EMC+TFEB+BTFEMP (20:55:20:5 v/v %)
- 1.0 M LiPF6 in EC+EMC+TPP (20:75:5 v/v %)
- 1.0 M LiPF6 in EC+EMC+TPP (20:75:5 v/v %) + 1.5% VC
- 1.0 M LiPF6 in EC+EMC (20:80 v/v%) + 1.5% VC
- 1.0 M LiPF6 in EC+EMC (20:80 v/v%) (baseline)
Of the electrolytes studied, 1.0 M LiPF6 EC+EMC+TFEB+TPP (20:55:20:5 v/v%) (where TPP = triphe nyl phosphate) was identified as being a promising non-flammable electrolyte, due to reasonable low-temperature performance and superior life characteristics. In addition, the electrolyte consisting of 1.0 M LiPF6 EC+EMC+TPP (20:75:5 v/v%) + 1.5% VC was demonstrated to have even further improved life characteristics due to the incorporation of a solid electrolyte interphase (SEI) promoter (i.e., VC = vinylene carbonate), which appears to inhibit the decomposition of the TPP.
A number of experimental Li-ion cells, consisting of MCMB carbon anodes and LiNi0.8Co0.2O2 cathodes, have been fabricated to study the described technology. These cells serve to verify and demonstrate the reversibility, low-temperature performance, and electrochemical characterization techniques. All cells displayed good reversibility at room temperature and minimal reactivity during the formation cycling. The high coulombic efficiency and comparable irreversible capacity losses are indirectly related to the overall stability of the solutions and the electrode filming characteristics. Reasonable reversibility was observed with the cells containing all the electrolyte variations, when compared after the formation cycling. Some variation in capacity was due to different electrode weights, and not to electrolyte type, so most comparisons were expressed in terms of percentage of initial capacity under ambient temperatures. Triphenyl phosphate displayed the lowest irreversibility capacity losses and the highest coulombic efficiency, suggesting that it is not electrochemically decomposing and participating in the electrode filming process deleteriously.
This work was done by Marshall C. Smart, Kiah A. Smith, and Ratnakumar V. Bugga of Caltech and Surya G. Prakash of the University of Southern California for NASA’s Jet Propulsion Laboratory.
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Refer to NPO-46262.