V. Focused Fundamental Research - EERE - U.S. Department of ...

Henderson – NCSU
V.D.6 Nonfluorinated (or Partially Fluorinated) Anions for Lithium Salts and Ionic Liquids (NCSU)
Results
Anion Synthesis. Several new anions have been
synthesized. Examples of these are shown in Figure V ­
154. Synthesis procedures are now being optimized for
scaled-up production of the salts. The DCP - and TCP 2­
anions are initially prepared as potassium salts. Ionexchange
and purification of the resulting lithium salts
to electrochemical-grade materials is now underway.
The anions containing nitrile groups are found to
polymerize during thermal degradation (Figure V - 155)―a
feature which may provide a shut-down mechanism for
a battery during thermal abuse. Unfortunately, the TCP 2­
salts have been found to have very poor solubility in
aprotic solvents. Crystal structures for pure LiETAC
(Figure V - 156) and a (THF) 1 :LiETAC solvate have been
determined to gain insight into how this anion
coordinates Li + cations. The electrochemical properties
of the salts in aprotic solvent mixtures are currently
being determined.
F3C -
DCP ­ - -
2­
TCP
ETAC ­
F3C
-
Figure V - 154: Examples of anions synthesized.
Figure V - 155: TGA heating traces of the salts.
Figure V - 156: Ion coordination in the crystal structure of LiETAC (Lipurple,
O-red, N-blue, F-green).
Characterization of Solvent-LiFSI Mixtures.
Lithium bis(fluorosulfonyl)imide is a promising salt for
electrolyte applications due to the very high
conductivity of LiFSI mixtures with aprotic solvents
(comparable to LiPF 6 ). Little is known about this salt at
present. Some work has been conducted to gain insight
into why the FSI - anion results in exceptional electrolyte
properties. In addition, nitrile and dinitrile solvents may
be of interest for electrolytes suitable for use with highvoltage
(> 4.3 V) electrodes and/or low-temperature
applications. A phase diagram for acetonitrile (AN) n -
LiFSI mixtures has therefore been prepared (Figure V -
157). Both 6/1 and 4/1 crystalline solvate phases form
which melt at low temperature, in common with (AN) n -
LiTFSI mixtures. In contrast, (AN) n -LiPF 6 mixtures
form a high melting 5/1 phase which consists of Li +
cations tetrahedrally coordinated to four AN molecules,
­
one uncoordinated AN molecule and uncoordinated PF 6
anions. Certain compositions of the (AN) n -LiFSI
mixtures, in fact, form crystallinity gaps in which the
samples cannot be crystallized. The data suggests that
binary solvent-LiFSI electrolytes may be of practical
utility due to the low T m of the solvates, whereas mixed
solvents are generally required with LiPF 6 electrolytes
due to the high T m of the corresponding solvates.
Characterization of Solvent-BF 4 and LiDFOB
(or BF 2 Ox) Mixtures. Phase diagrams for (AN) n -
LiDFOB and adiponitrile (ADN) n -LiDCTA mixtures
have been prepared (Figure V - 158). Crystal structures
for (AN) 3 :LiDCTA, (AN) 1 :LiDCTA and
(ADN) 1 :LiDCTA solvates have been determined (Figure
V - 159 and Figure V - 160) to provide insight into the
molecular interactions of these solvents with the Li +
cations and DCTA - anions.
Ionic Liquid (IL) Synthesis. ILs with N-alkyl-Nmethylpyrrolidinium
cations and the difluoro(oxalato)­
borate anion (PY 1R DFOB where R is the alkyl chain
length) have been synthesized and characterized.
Despite utilizing various synthesis and purification
procedures, the ILs always contain small amounts of
BF - 4 and bis(oxalato)-borate (LiBOB) impurities. It is
FY 2011 Annual Progress Report 613 Energy Storage R&D