Due to safety and efficiency considerations, the world is turning to solid-state batteries (SSB), where the electrolyte — the material between the anode and cathode through which ions shuttle back and forth during charging and discharging — is solid, rather than liquid. As detailed in previous issues of Quantum, SSBs have become the holy grail of battery technology.
Researchers are also working on all-lithium anodes. Negatively charged, the anode ‘donates’ electrons when the battery discharges. Lithium, a metal that has ‘spare’ electrons in the outermost ring of its atoms, is a good anode material but needs to be embedded (‘intercalated’) in some other material such as graphite. It would help to have an all-lithium anode because that would mean more electrons to be had on every charge-discharge cycle.
However, if you have an ‘embedded lithium’ anode and a solid electrolyte, it doesn’t give you the bang you want because the higher weight of the solid electrolyte (compared with liquid) nibbles away some efficiency.
So, the darling battery would be the one that has an all-lithium anode and a solid electrolyte. But there is one challenge yet.
As ions move from the anode to the cathode through the electrolyte (while discharging), the anode material (lithium) gets ‘pulled’ to form filaments called dendrites. If these dendrites touch the cathode, you have a short circuit and, likely, fire.
It was once believed that this dendrite formation would not happen if you use solid electrolytes instead of liquid, especially if the solid electrolyte’s rigidity (shear modulus) is twice that of lithium.
However, research revealed that dendrites grow even through solid electrolytes and, indeed, more than in liquid electrolytes. Researchers from Indian Institute of Science, Bengaluru, brought out a paper on the subject that says, “Lithium growth through solid electrolytes was observed at current densities as low as 100 microamperes per sq cm, much lower than current densities observed in liquid electrolytes.”
In essence, lithium cracking through the electrolyte, whether solid or liquid, is the biggest challenge in lithium-ion electrochemical batteries.
Prof Naga Phani Aetukuri at the solid state and structural chemistry unit of IISc and his student Vikalp Raj have delved into the problem of dendrite formation in solid-state Li-ion batteries and come up with a solution. In their research, they realised that microscopic ‘voids’ were developing in the lithium anode during discharge. The currents concentrated at the edges of these voids were about 10,000 times more than the average currents across the battery cell. This, they deduced, was creating stress on the solid electrolyte, leading to dendrite formation.
Now, the task was simply to prevent the voids. Aetukuri and Raj introduced an ultra-thin layer of a refractory (heat-resistant) metal between the lithium anode and the solid electrolyte. This ‘lithium-phobic interlayer’ delayed dendrite growth.
That is the science. Now, it is up to engineers to adopt it into a battery.