Page 600 - Handbook of Battery Materials

574 17 Liquid Nonaqueous Electrolytes

them promising for applications with an increased requirement of safety such as
electro traction.
Nevertheless, carbonaceous materials are most often used as anodes in
lithium-ion batteries. Unfortunately the electrochemical and thermodynamical
stability of these systems depends on the composition of the electrolyte. Because
of the layered structure and high surface area, irreversible parasitical reactions
proceed on the surface along with reversible intercalation of lithium ions,
including electrolyte decomposition and co-intercalations, for example, of solvent
molecules.
Lithiated carbon reacts with solvent molecules containing polar bonds such
as C–O, C–N, C–S, S–O, water, gases like CO 2 ,N 2 ,SO 2 , and several reactive
compounds. These reactions can have a beneficial effect, that is, the formation
of a passivating layer on the surface due to decomposition, which significantly
increases stability. Three different models have been suggested for the formation
of a passive layer:
1) SEI model [320]: Film consists of inorganic compounds with a solid surface.
2) Polymer electrolyte interface (PEI) model [321]: Passive layer consists of organic
compounds with polymer properties.
3) Two-phase model [322]:
a. Compact stratified layer (CSL) model: The two-layered structure of the
passive film is built up of a solid and a polymer layer.
b. Solid polymer layer (SPL) model: Passive layer consists of a blend of a solid
and polymer electrolyte.
The passive layer prevents the electrolyte from being reduced continuously
and is permeable to free lithium ions, with the result that, apart from kinetic
stability, the long-life stability of the electrode is improved as well. Permeability
just for free lithium ions through the film is a very important aspect. Among
lithium ions, solvated ions (Li + ) can intercalate into the crystalline structure
solv
with the consequence of an extreme volume expansion. This results in cracking
of the graphite structure, also called exfoliation, and a dramatic loss of storage
capability. Furthermore, solvent molecules can be reduced or polymerized and
yield reaction products in the electrolyte and the electrodes [323–328]. Therefore,
different electrolyte solutions show widely varying irreversible capacities for the
first intercalation/deintercalation cycle at carbon [329]. Early formation of an SEI
at the first charge can disable these problems.
Passivating films, which are formed on the surface when lithium or carbon
electrodes are exposed to an electrolyte, determine [330]
• the quality of passivation,
• electrode kinetics of lithium deposition and intercalation,
• dissolution by the resistivity of the lithium/solution interphase,
• the uniformity and thickness of the lithium/solution interphase,
• the roughness of the lithium metal surface,
• and hence the current distribution.