Low-cost cobalt-free biodegradable rechargeable lithium batteries, particularly for grid-scale storage, are the dream of a research group at York University in Toronto, which might have bought them closer to reality with an organic cathode.

“Organic electrode materials are considered to be extremely promising materials for sustainable batteries with high power capabilities,” said Professor Thomas Baumgartner from the university. “Electrodes made with organic materials can make large‐scale manufacturing, recycling or disposing of these elements more environmentally friendly. The goal is to create sustainable batteries that are stable and have equally as good if not better capacity.”

Made from elements such as carbon, nitrogen, oxygen, phosphorus and sulphur, experimental organic electrodes already exist.

To varying degrees, they all suffer from: cycle-by-cycle deterioration from their own red-ox reactions, gradually dissolving into the electrolyte, low conductivity and low terminal voltage.

The team picked a chemical called a viologen that is a known electron acceptor that can give up its electron without damage. Better than that, it can also do the same with two electrons – so higher storage capacity.

In particular, it is a custom phosphaviologen (called PY2PV), where the phosphoryl part allows it to operate at higher terminal voltages without loosing the capacity to hold two electrons.

Single-walled carbon nano-tubes (SWCNTs), introduced to reduce electrode resistance, make up the rest of the electrode.

The customisation, which is novel part of the research, is to build pyrene into the to phosphaviologen molecule.

Pyrene has a strong affinity to SWCNTs and forms bond with it (by physisorption), forming PY2PV:SWCNT composite. This bonding has the effect of preventing the phosphaviologen from dissolving away into the electrolyte – solving one of the habitual organic electrode problems mentioned above.

Added to this, according to the paper ‘Phosphaviologen‐based pyrene‐carbon nanotube composites for stable battery electrodes‘, published in the journal Batteries & Supercaps, the bonding may also be improving solid-state electron transfer between the viologen molecules and the SWCNTs as they are now intimately coupled rather than just mixed, all without reducing electron storage capacity.

Measurements (working against a lithium metal anode in a coin cell) showed that a 1:2 PY2PV:SWCNT composite cathode can store 48mAh/g (at 1C charge and discharge) – representing 90% of its theoretical two-electron capacity.

Rather than fading, capacity increased slightly (to 53mAh/g) over 500 1C charge-discharge cycles (cycles at 1 C. “Ultimately, the electrodes display high material usage, capacity retention at high C rates, and high cycle stability; all properties we can attribute to the PY2PV:SWCNT composite”, according to the paper. However, capacity over 500 cycles did dicay when discharge was increased to 5C.

Charge and discharge worked at voltages between 3.2 and 2.8 V (vs Li/Li+) – a relatively high voltage for organic materials, according to the team.

“It has a very good voltage, up to the 3.5 volts, which is really where current batteries are now,” said Baumgartner. “It’s an important step forward in making fully organic and sustainable batteries.”

And the disadvantages?

“As for disadvantages, organic batteries are still not quite where current Li-ion batteries are in terms of performance,” Baumgartner told Electronics Weekly. “It continues to be a maturing field. At this point overall battery capacities still lag behind Li-ion types – the organic molecules are quite heavy themselves, which reduces the energy density.”