Page 323 - Polymer-based Nanocomposites for Energy and Environmental Applications

292 Polymer-based Nanocomposites for Energy and Environmental Applications

(5) Lithium triflate, CF 3 SO 3 Li (Li-T f )
(6) Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)
(7) Lithium bis(fluorosulfonyl)imide (LiFSI)
(8) Lithium difluoro(oxalato)borate (LiDFOB)
(9) Lithium bis(oxalato)borate (LiBOB)
(10) Lithium tetrafluoro(oxalato)phosphate (LiF 4 OP)
(11) Lithium 2-trifluoromethyl-4,5-dicyanoimidazole (LiTDI)
Among the cited salts, LiPF 6 has been the dominant lithium source in the electrolyte
formulation for more than two decades; its ionic conductivity in nonaqueous
carbonate-based systems reaches 10 mS cm 1 at 20°C and is less toxic compared with
LiAsF 6 . However, LiPF 6 decomposes at elevated temperatures to form LiF and PF 5 .
The PF 5 can further react with small amount of moisture to form HF and POF 3 . Such
reactions deteriorate the safety of the battery. The most studied salts for PCEs are
LiTFSI, LiClO 4 , and LiSO 3 CF 3 .

10.3 Composites electrodes

The electrodes of batteries consist of active materials, conductive additives, current
collectors, and binder materials. The active materials determine the capacity of the
electrode. The conductive additives improve the rate capability of the electrode by
conducting electrons. The binder is used to glue the active materials and conductive
additives together with the current collector.
Silicon (Si) is a promising anode material for LIBs due to its higher theoretical
specific capacity of 3578 mAh g 1 as compared with traditional graphite
1
+
(>350 mAh g ) [69,70], low working potential (<0.5 V vs Li/Li ), and low cost.
Si has 10 the theoretical gravimetric capacity and 4 the volumetric capacity
of graphite [71], and it will in theory lead to increased energy densities and cycling
stability when coupled with an appropriate cathode. However, Si has yet to be success-
fully commercialized due to the major concerns associated with silicon-based
electrodes, which are (i) the volume expansion (up to 400% at full lithiation) [72]
and (ii) instability toward the electrolyte at potentials of below 1 V versus Li. These
issues result in capacity fading via coating degradation plus the continued formation
and growth of a solid-electrolyte interphase (SEI). In addition, the formed lithiated
silicides react continually with the electrolyte, resulting in the self-discharge of the
battery.
Functional polymer binder is a crucial electrode component in maintaining the
structural integrity of the anodes, although it only occupies 2–5% of the mass in a
commercial electrode configuration. Polymer binder ideally should play multiple
roles such as promoting adhesion, stabilizing the SEI formation, accommodating
volume change, and enhancing the electron and ion transport.
Current LIBs only function when a stable SEI is formed on graphite. Growing a
stable SEI on Si is considerably more difficult because the SEI must be sufficiently
elastic to be able to withstand the Si volume expansion on lithiation. Polymer binder
systems with enhanced interaction with Si plus the ability to retain adhesion and