Nanostructures can solve the pulverization problem of the Si anode on cycling, and also allow high operation rate. With specific capacities of more than 3,500 mAh/g having been achieved, and full cells composed of a carbon-silicon core-shell nanowire anode and a traditional LiCoO₂ cathode having been demonstrated, practical applications of Si anodes are close. However, they note:

By replacing traditional graphite anodes with Si anodes, the specific energy or energy density of LIBs increases a lot. To further increase the energy, it is desirable to pair Si anodes with high capacity cathodes.

Recently, many cathode materials, such as V₂O₅, MnO₂, FeSe₂, and sulfur have shown the potential to have high Li storage capacity. However, neither Si anode nor any of these cathodes contains lithium. Therefore, for this type of combination, either the cathode or the Si anode needs to be prelithiated.

...different applications (grid storage, electric vehicles, and portable electronic devices) may require different cathodes for pairing with Si anodes, for the balance of power, capacity, cost, and safety. Therefore, instead of developing an individual prelithiation method for every Li-free cathode material, it is strategically preferable to develop one single prelithiation method for the Si anode and pair it with all the Li-free cathodes.

—Liu et al.

The Stanford team is proposing a facile method for prelithiating a Si nanowire (SiNW) anode by a self-discharge mechanism using cheap Li metal foil as the Li source. With a simple 20 min prelithiation, the amount of lithium preloaded into the SiNWs was measured to be ~2000 mAh/g of Si. The degree of prelithiation can be controlled by changing the prelithiation time. More importantly, they noted, the nanostructure of SiNWs is successfully maintained after prelithiation.

Sulfur cell. To prove their concept, they demonstrated a full battery using a prelithiated SiNW anode and a sulfur/mesoporous carbon nanocomposite cathode. Liu et al. said they chose sulfur because sulfur has the highest specific capacity of ~1670 mAh/g among all solid LIB cathodes. It is also, however, Lithium-free—essentially different from most of the commercialized LIB cathodes. To utilize the sulfur cathode in this type of battery, an anode that contains Li is needed.

Several approaches have been taken to this problem, including pairing sulfur with a Li metal anode; replacing sulfur with its Li₂S counterpart (or Li₂S/metal composites) as electrode materials; or converting a sulfur cathode to Li₂S electrode by reacting with n-butyllithium.

In the cell developed by Liu et al., the only lithium source is the prelithiated SiNW anode. They found that the full cell maintained ~80% of its initial capacity after 10 cycles, but it decays with a constant slope throughout cycling. They speculated that the decay of capacity could be due to:

• The full cell has a limited supply of Li. Side reactions at both electrodes consume Li and therefore cause capacity loss.
• High-order polysulfides are soluble in the electrolyte and carry the Li with them into the electrolyte, which causes the loss of the total Li in electrodes.
• The voltage of the cathode and the anode are not separately controlled. One of the two electrodes might be overcharged or deeply discharged, which may harm the cyclability of the full cell.

By directly pairing this prelithiated SiNW anode with a Li-free sulfur cathode, we demonstrated a proof-of-concept sulfur/Li-Si full battery. With further optimization on the prelithiation condition, cathode/anode capacity matching, and the electrolyte, a sulfur/prelithiated SiNW full battery with improved performance is anticipated. This prelithiation method removes the requirement that the cathode must contain lithium in its original state and opens up new avenues of pairing Li-free electrodes for the next generation high-energy lithium ion batteries.

—Liu et al.

Resources

• Nian Liu, Liangbing Hu, Matthew T. McDowell, Ariel Jackson, Yi Cui (2011) Prelithiated Silicon Nanowires as an Anode for Lithium Ion Batteries. ACS Nano Article ASAP doi: 10.1021/nn201716