Technology Advances
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TECHNOLOGY ADVANCES
Lithium Batteries Improved by Use of a Nonflammable Electrolyte plus CNT A major drawback to the widespread adoption of lithium rechargeable batteries, particularly for electric vehicles, is their flammability. A rechargeable lithium battery system that couples high-energy density with a safe, nonflammable and nonvolatile electrolyte is under development at Phoenix Innovation Inc. It is based on phosphorus (specifically, polyphosphonate) chemistry and does not use any carbonates, the conventional solvents of choice with lithium systems. In addition to its nonflammability, the electrolyte is stable up to 5 V. The polyphosphonate electrolyte family investigated includes a number of possible chemistries that allow the electrolyte’s properties to be tailored over a wide range of conductivity and power density, depending on the desired application. Figure 1 shows a typical calorimetry scan of a polyphosphonate electrolyte compared with a standard carbonate electrolyte. These scans were run in the presence of lithium metal, which melts at ~180°C. Figure 1 also shows that the reaction between the carbonate mixture and the molten lithium is highly exothermic, while the phosphonate is much less reactive. By modifying the substituents attached to the phosphorus and/or the chemical group between the phosphonate moieties in the backbone of the polymer chain (see Figure 2), the properties can be tailored to give improved battery performance, including stability and flame retardation. A primary goal has been to achieve both cost-effectiveness and improved performance; only one synthetic step is required, and the product yield is very high. The second objective of the rechargeable lithium battery research, which is synergistic with the nonflammable electrolyte work, is the development of carbon nanotube (CNT)-based electrodes with very high capacities. These materials can be easily and economically processed into flexible papers that can be fashioned into virtually any size, shape, or dimension. Depending on the conditions under which electrodes are prepared, a wide range of properties can be realized. After synthesis, the nanotubes are heatprocessed at various temperatures and under an inert or oxidizing atmosphere. Depending on the processing conditions
Figure 1. Differential scanning calorimetry results (with molten lithium present) of a polyphosphonate electrolyte and of a standard ethylene carbonate/ethyl methyl carbonate electrolyte mixture.
O O
P
OCH 2 CH 2
R
n R = e.g., aryl, alkyl, ether, etc.; n is an integer Figure 2. The structure of a typical polyphosphonate monomer. Phosphonates have two P–O linkages and one P–C linkage, as well as one P=O linkage.
(i.e., duration of heating, temperature, and gas composition of the treatment atmosphere), the electrochemical properties of the product can vary substantially; for example, in an oxidizing atmosphere of CO2 or O2, the capacities of the nanotubes can change markedly. The company is working to optimize the conditions to obtain optimal performanc
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