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

V.D.5 Advanced Electrolyte and Electrolyte Additives (ANL)
Amine – ANL
and oxidation potentials using our quantum chemical
model. In addition, the candidates are further down
selected on the basis of reaction pathways for
decomposition. These calculations are being used to
suggest additives for protective film formation on anodes
and cathodes as well as for redox shuttles for overcharge
protection.
Lithium tetrafluoro(oxalate) phosphate additive.
Among the promising candidates based on reduction
potential screening are lithium tetrafluoro(oxalate)
phosphate (LTFOP) and lithium tris(oxalato) phosphate
(LTOP). These two compounds were investigated as
additives for MCMB/Li 1.1 [Ni1 /3 Co 1/3 Mn 1/3 ] 0.9 O 2 (NCM)
cells. The reduction potentials were determined by
charging a Li/MCMB half cell. Reduction peaks shown in
Figure V - 151 at 1.7V and 2.1V are observed for LiTFOP and
LTOP, respectively, indicating a new SEI formation prior
to EC decomposition. The measured reduction peaks are in
good agreement with those predicted from our quantum
chemical model of 1.52 and 1.88 eV, for LTFOP and
LTOP, respectively. Based on cell testing including
cycling performance and impedance measurements, the
LTFOP additive is found to perform significantly better
than LTOP. Our density functional calculations suggest
that the presence of the fluorines promotes 2-dimensional
SEI film growth as opposed to 3-dimensional structures for
LiTOP as an additive.
Figure V - 151: Differential capacity profiles of Li/MCMB with 1.2M LiPF6 EC/EMC 3/7+2% additive.
Reaction pathways for ethylene carbonate
decomposition. A computational study of the reaction
pathways involving one and two-electron transfer for
decomposition of ethylene carbonate upon reduction has
been carried out to provide evidence for possible products
that may be important in forming an effective SEI needed
for good performance of Li ion batteries. Understanding
the electrolyte reaction mechanisms for SEI components is
necessary for our screening of new electrolyte additives. In
addition, the knowledge will be used in development of
potentials for simulations of the growth of the SEI layer by
our collaborator, Grant Smith of the University of Utah
We carried out a density functional study of the
different possible reaction mechanisms that will result in
the two alkyl carbonates mentioned above. In this study we
have considered the effect of spin curve crossing in the
reaction pathways for the first time because of the
possibility of radical reactions. Consideration of this effect
is necessary to obtain accurate information on the
pathways. Five distinct pathways were investigated. Of
these five reactions, the only viable path to lithium
ethylene dicarbonate involves two electron reduction. The
other pathways are found to be not favorable due to large
activation barriers. These calculations provide a reaction
pathway for formation of the lithium ethylene dicarbonate
consistent with experimental observation.
1,3,5-triallyl-[1,3,5]triazinane-2,4,6-trione additive.
We have used density functional theory to investigate
1,3,5-triallyl-[1,3,5]triazinane-2,4,6-trione (TTT) as an SEI
additive. The results show that when this molecule is
reduced, an allyl radical detaches as shown in Figure V - 152
without a barrier via an apparent charge-spin segregation
mechanism. This allyl radical can react with other species
in solution to form an SEI layer. The pores of this SEI
layer are likely to be large enough to allow Li + transport.
Energy Storage R &D 610 FY 2011 Annual Progress Report