Page 599 - Handbook of Battery Materials

17.4 Bulk Properties 573

Another method for suppression of corrosion is the use of solvents with smaller
dipole moments, such as THF, DME [260], GBL [299], or the nonflammable
methyl nonafluorobutyl ether (MFE) [297]. Thus, decomposition products are less
stabilized and the reaction rate of dissolution of Al decreases. Instead, solvents
with a large dipole moment such as EC can promote corrosion [300, 301]. Another
promising solvent is methyl difluoroacetate (MFA), which forms a passivating layer
consisting of organic compounds [302]. In an electrolyte with LiTFSI and MFA
+
as solvent, corrosion of aluminum only starts at 5 V vs Li/Li . Moreover, the
maximum observed corrosion current is about one-fifth lower than in an analogous
electrolyte containing an EC/DEC blend as solvent.
New, interesting alternatives for additives are ILs. 1-Butyl-3-methylimidazolium
tetrafluoroborate (BMIm BF 4 ) shows highly passivating characteristics. XPS and
energy-dispersive X-ray spectroscopy (EDX) reveal a film consisting of AlF 3 and
Al 2 O 3 that even resists highly corrosive salts like LiTFSI [303].

17.4.4
Chemical Stability of Electrolytes with Lithium and Lithiated Carbon

In lithium and lithium-ion batteries, several types of lithium intercalation materials
are investigated. The most efficient anode material would be lithium metal, which
−1
shows by far the highest energy density, theoretically 3860 mAh·g . However, its
main problems are the poor cyclability and safety problems due to lithium dendrite
formation on the surface [304–306]. Metallic lithium dendrites can grow toward
the cathodic side, which causes a short circuit in the cell, a drastic increase in
temperature, uncontrolled reactions, and cell destruction.
To avoid these drawbacks, intercalation compounds are used. Because of low
operational voltages and low costs, coke or various types of carbon such as graphite
(Li x C 6 ,1 ≥ x ≥ 0) are used as active material [307]. The layered structure of carbon
hosts lithium ions by successive intercalation, leading to a maximum theoretical
−1
capacity of 372 mAh·g .
+ −
xLi + xe + 6C Li x C 6 (17.40)
Further developments in anode materials result in different metallic alloys. Lithium
metal alloys with the general formula Li x M y (M = Al, Ni, Mo, Pb, Si, Sn, Ti,.. .) [280,
308–310] show a wide range of combinations. These alloys have higher specific
capacities but exhibit serious structural problems during charge and discharge
processes. Drastic increase of volume leads to cracking of the material, reducing
cyclability and power capability [311]. To avoid this problem, several intermetallic
alloys are used that allow better volume control and increasing battery stability
such as Mg x Ni y [312], Sn/SnSb, Sn/Bi [313], Ag x Sn y [314], CaSi 2 [315], or Ni 3 Sn 4
[316]. Some new types of anode material have spinel structures, for example,
Li 4 Mn 5 O 12 ,Li 2 Mn 4 O 9 [317], and Li 4 Ti 5 O 12 [318], showing very good cyclability
[319]. These compounds exhibit a flat voltage response with a definite voltage
increase near end-of-charge state. Furthermore, no deposition of metallic lithium
during charge takes place because of their chemical structure. These aspects make