Understanding volume change and conductivity in Si nanostructures for Li-ion anodes
Silicon is a promising next-generation anode material for high-energy lithium-ion batteries due to its high specific capacity, which is theoretically 10 times greater than graphite. However, its cycle life is limited due to volume expansion and fracture during lithium reaction. This degradation of the Si results in loss of electrical connection and pulverization of the electrode. Several fundamental studies still need to be conducted to develop viable Si electrodes for batteries. Yi Cui’s group at Stanford University is working on understanding the properties of various Si nanostructures and is designing new ones based on particles and wires that target improving Si cyclability.
The key strategy for improving cyclability is to dissociate volume expansion from fracture by designing free volume around Si nanostructures. To this effect, hollow Si nanospheres were designed as shown in Figure 1. The hollow nature of the structure results in a maximum tensile stress that is ∼5 times lower than that in a solid sphere with an equal volume of Si. These lower stress values allow the hollow Si nanospheres that are ~400 nm in diameter to undergo isotropic volume expansion and remain intact after full lithiation. In contrast, Si nanowires that have a solid core will fracture when the diameter is larger than 300 nm. Ex situ transmission electron microscopy (TEM) is used for observation of morphological changes in the same nanostructure before and after lithiation and cycling, making it an indispensable tool for studying volume changes in Si nanostructures. The TEM results indicate that Si anodes made from hollow spheres can withstand volume expansion and could be an effective platform for enabling long cycle life. Indeed, the Si hollow sphere electrode has a capacity of 1420 mAh/g even after 700 cycles, with a coulombic efficiency of 99.5% in the later cycles .
To further understand how volume expansion can depend on nanomaterial structure, TEM was also used to investigate the impacts of size and surface oxide on volume expansion. It was found that oxide-free nanowires expand to an approximately constant volume ratio for all sizes. On the other hand, nanowires with native SiO2 exhibit diameter-dependent volume expansion in which those with smaller diameters expand to a lesser degree than larger ones, with expansion being mostly suppressed for those with diameters less than ∼50 nm. Finite element modeling indicated that the strong Li-O and Si-O bonds in the lithiated oxide shell can induce compressive stress in the weaker Li-Si bonded core and that this stress increases in magnitude with decreasing nanowire size. Therefore, the oxide shell mechanically limits the volume expansion and reduces the extent of lithiation, resulting in a lower capacity than the oxide-free nanowires . This finding is consistent with results from another BATT research group headed by Gao Liu at LBNL that indicate that Si nanoparticles without surface oxide exhibit a greater reversible capacity .
At first glance, it seems that having an oxide layer on Si is undesirable due to the lower capacity; however, the suppression of volume expansion can be used to an advantage, i.e., reducing the amount of exposed surface during cycling. A major issue associated with cyclability is the destruction and reformation of the solid electrolyte interface (SEI) layer on expanding and contracting Si surfaces. The unstable SEI layer results in irreversible lithium consumption during cycling, which reduces the capacity over time. Therefore, in order to access the full capacity of Si, volume expansion needs to be accommodated yet suppressed so as to form a stable SEI.
Another issue being addressed in the Cui group besides volume change is the conductivity of Si electrodes. The electrical conductivity of Si is a major factor in determining the power and energy capabilities of an electrode that does not contain inactive materials such as conductive additives and binder. High electrical resistance in the electrode can lead to incomplete lithium reaction and to relatively high overpotentials that reduce the energy and power density. In a study using single Si nanowire measurements, lithiated nanowires exhibited conductivities two to three orders of magnitude higher than those in the delithiated state. Therefore, the capacity of Si-nanowire based electrodes should be kept above 1500 mAh/g to maintain high electrical conductivity .
Since the electrical conductivity of unlithiated Si is relatively low, slow electronic transport could limit the kinetics of the lithiation reaction early on, making pre-lithiation of Si important. The Cui group has developed a simple prelithiation process in which Si nanowires were lithiated to ∼50% of their theoretical capacity within 20 min., which is equivalent to electrochemical lithiation at close to a 2C rate. Although longer prelithiation times result in higher loading of Li, shorter prelithiation times result in better cyclability of the Si nanowires .
Future work in the Cui group will focus on designing new Si structures and pre-lithiation methods that are amenable to scale up so that large quantities of this anode material can be made at a low cost. Fundamental questions such as the best morphology for electrode packing, the type of surface coating for improving cyclability, and the optimal state of charge for these electrodes still need to be answered.
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