In-situ 3D Electrode Fabrication of High Capacity Silicon-Carbon Anodes
7.1. Abstract
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Chapter 7
In-situ 3D Electrode Fabrication of
176 7.2. Background and Motivation
US Department of Energy (DOE) year 2022 goals to develop next generation lithium ion batteries that have high energy density (300-400 Wh kg-1) which enable a large market penetration of HEV’s and EV’s [1]. Besides, the LIBs should have reduced cost, improved safety and cycle life. So current research emphasis is given to develop high energy density cathodes, high voltage electrolytes coupled with high capacity silicon anodes for increasing energy density in LIBs [2-8]. As discussed in Chapter 3 and 4 Li and Mn rich TM oxide (LMR-NMC) composite cathodes which has almost double the capacity (372 mAh g-1 for 1.2 Li transfer) of currently available cathodes have been investigated as a promising cathode material for LIBs [4, 9-13]. To enhance the performance of anodes which meet the requirement of the automotive industry, researchers have been investigating materials which form alloys with lithium to generate anodes that have specific capacities an order of magnitude higher than graphite [2-3, 6]. Silicon is an attractive anode material for LIBs mainly because of its very high theoretical charge capacity of 4200 mAh g-1 (Li4.4Si). But silicon has various issues of low electronic and ionic conductivity and most important one is high volume change of ~400% during lithiation and de-lithiation leading to structural degradation (pulverization) followed by capacity fade and reduced cycle life [6,14- 21] need to be addressed before it is considered a potential candidate as anodes for lithium-ion batteries.
As discussed in Chapter 6, there has been an intense research to mitigate volume change during cycling, such as producing Si-NPs [15-21], aligned Si- nanowires/nanotubes [14, 22-24], dispersing silicon into an active (such as carbon) /inactive (eg. SiO2) matrix [25-33], silicon based thin films [34-38], free standing Si- C electrodes and different morphologies of silicon [39-49]. Taking benefit of high conductivity of carbon and high capacity silicon, recent reports demonstrate carbon- silicon nanocomposites can circumvent the issues associated with silicon and improve the overall electrochemical performance of Si-anode for LIBs [25-33]. This is because Si-C nanocomposites can accommodate huge strain with reduced pulverization, provide good electronic contact, and exhibit short diffusion path for lithium ion insertion.
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Another interesting approach of Si-C composite free standing electrodes (binder less and current collector less) which are flexible and are used to create thin and flexible LIBs. Flexible free standing graphene-silicon composite film prepared by in-situ filtration method shows discharge capacity of 708 mAh g−1 beyond 100 cycles [39]. Composites of Si-NPs and graphene accommodated on 3D network of graphite exhibited high Li ion storage capacities of >2200 mAh g−1 after 50 cycles and >1500 mAh g−1 after 200 cycles [40]. Freestanding macroporous silicon film in combination with pyrolyzed polyacrylonitrile composite anode shows discharge capacity of 1260 mAh g−1 for 20 cycles [41]. Light-weight free-standing carbon nanotube-silicon films prepared by sputtering method shows specific charge storage capacity (∼2000 mAh g-1) for 50 cycles [42]. Binder and additive free 3D porous nickel based current collector coated conformally with layers of silicon delivers high capacity of 1650 mAh g-1 after 120 cycles of charge/discharge [42]. There are also several reports such as carbon-coated Silicon nanowires on carbon fabric [43], 3D free-standing carbon nanotubes [44], flexible nanoporous Si-carbon nanotube paper [45-48], hierarchical nano-branched C-Si/SnO2 nanowire free standing electrodes [49] as tested as anodes which delivers reasonable good capacity ~1000 mAh g-1 for 100s of charge-discharge cycles. In overall, Si-C composite free standing electrodes prepared by various synthesis approaches shows capacity between 700-2000 mAh g-
1 for about 100 cycles.
Here, we present a unique organic binder less, additive free 3D electrode architecture of Si-C NPs on CF current collector which replaces usual copper foil current collector [50-51]. P-pitch is used as carbon source which makes a good electrical contact between the CF current collector and Si- active particles at high temperatures >500 oC. CF mat has numerous advantages over copper current collector such as incorporation of large amount of active material into the 3D CF network, provides high interfacial contact of active material to the conductive network. Besides the carbon fibers are more flexible and can accommodate the volume change of silicon.
But the silicon nanoparticles will lose the contact from CF due to absence of binder.
So there is need of a material which supports the volume change, improves conductivity and binds silicon onto CF. P-pitch which is complex mixture of
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polynuclear aromatic hydrocarbons, derived from heat treatment of coal and petroleum tars can be used as a high temperature binder to bind Si-NPs on to CF. Pitch undergoes carbonization above 500 oC to form conducting carbon through mesophase (liquid crystalline state) [50-52]. So annealing mixture of P-pitch and Si-NPs coated on to CF current collector at high temperatures ≥ 500 oC, melts the P-Pitch and allows the conformal coating throughout the silicon and carbon fiber which enables to have a conductive network and enough space for the volume expansion and contraction without undergoing pulverization. In addition, the binding strength of carbonized pitch with silicon and carbon fiber is influenced by temperature. In this work, we have studied effect of different annealing temperatures (700, 900, and 1000 oC) on the electrochemical performance of Si-C 3D electrodes. Electrodes developed by this method provide enough space for silicon surrounded by carbonized pitch on carbon fiber which support the volume expansion and contraction during cycling thus reducing the pulverization of the silicon. In this type of electrode, all the active material is exposed to electrolyte which implies the entire silicon is electrochemically active whereas in conventional composite electrode in which copper foil is used as current collector, interior part of the electrode is electrochemically inactive due to lack of access to electrolyte. Moreover, polyvinylidene fluoride (PVdF) binder in the composite electrode causes impedance rise leading to detrimental in the electrochemical performance. In addition, copper foil adds extra weight to the electrode. But 3D Si-C freestanding CF electrodes does not contain any organic binder, and less weight. This would further enhance energy density.