A novel anode developed by researchers at Indian Institute of Science Education and Research (IISER) Pune for use in lithium-ion batteries has five times more capacity than carbon-based electrodes and can be fully charged in about 15 minutes. The team led by Satishchandra Ogale from the Department of Physics at IISER Pune used a composite made of phosphorene (few-layer black phosphorus) and silicon nanoparticles to fabricate the anode.
Unlike carbon-based anodes that have a theoretical capacity limit of just 372 mA h per gram, silicon has 10-12 times higher limit (4,200 mA h per gram). Yet, it has proved very difficult to fully harness the potential of silicon to make electrodes for Li-ion batteries. The reason: when lithium forms an alloy with silicon, the alloy undergoes massive volume expansion of up to 300% during charging. The huge and repeated volume expansion and contraction (during discharging) breaks the electrode thus making it highly unstable and hence unfit for use in a battery.
Improved stability
While other teams have used graphene sheets to encapsulate silicon nanoparticles to absorb the stress and prevent the anode from cracking, the improvement in stability has not been satisfactory. While using up to 40% graphene has not solved the problem, IISER Pune researchers used just 8% of phosphorene to dramatically improve the stability of silicon electrode.
Stable electrode
The stability was so good that the electrode made of silicon-phosphorene mixture showed no discernible cracks even after 250 cycles of charging and discharging. In comparison, electrodes made of silicon nanoparticles alone developed cracks soon after 20 cycles.
“Phosphorene has exceptional flexibility due to its low Young’s modulus and so can absorb huge stress that comes when silicon expands. The flexibility comes from the puckered structure of phosphorene where it looks wrinkled or folded,” says Kingshuk Roy from the Department of Chemistry at IISER Pune and first author of a paper published in the journal Sustainable Energy & Fuels.
Elaborating on the choice of phosphorene, Prof. Ogale says: “Unlike graphene that is a sheet-like material which is difficult to deform or mechanically stretch, the folds of phosphorene are like the accordion instrument. This allows phosphorene to absorb the stress when silicon expands and contracts during every cycle of charging and discharging.”
The silicon-phosphorene anode was prepared by physically mixing the dispersions of silicon nanoparticles and chemically exfoliated sheets of few-layer black phosphorus. The silicon-phosphorene mixture was then coated on a copper foil to make it behave as an anode. “The structure and elasticity of phosphorene helped in retaining the integrity of the electrode,” says Prof. Ogale.
What makes the anode made of silicon-phosphorene composite particularly important is that the capacity remained high at 1,600 mA h per gram even after 250 cycles of charging and discharging. “The charge capacity was as high as 1,100 mA h per gram even when we increased the current density to 4 A per gram,” says Dr. Malik Wahid from the Department of Chemistry at IISER Pune and another first author of the paper. “This means, the battery can be fully charged in say 15 minutes.”
However, when the current density is reduced to 0.5 A per gram, the capacity increases to 1,600 mA h per gram. So when the battery is charged slowly for about 60 minutes, the capacity increases to 1,600 mA h per gram.
Capacity retention
After 250 cycles, there was 65% retention of capacity, which is huge for silicon. In comparison, the capacity retention is 70-75% in the case of carbon. “But the reversible capacity of carbon electrodes is five times less compared with silicon,” Roy clarifies.
Under high magnification, the researchers found phosphorene was uniformly covering the silicon nanoparticles. “So whenever silicon expanded by volume during charging, the phosphorene was able to absorb it thus retaining the integrity of the anode,” says Roy.
“We are planning to make a full cell by using silicon-phosphorene composite as the anode and a commercially available cathode. We are also working to innovate on the cathode,” says Prof. Ogale.
