LFP with Acetonitrile Electrolyte

With Asahi Kasei licensing [1] their acetonitrile electrolyte for use in EAS Batteries LFP cells it is worth looking deeper at the technology and benefits. The flyer [2] has the following headlines:

The flyer [2] presents data on cycle life and cold temperature performance for a LFP/Graphite cylindrical cell.

Charge: 0.5 C, 3.6 V (CCCV), Discharge: 1 C, 2.0 V CC

Charge: 0.5 C, 3.6 V (CCCV at 25°C), Discharge: 10 C, 1.0 V CC

The cold temperature performance is down to the high ionic conductivity of acetonitrile. However, the challenge with acetonitrile is that it decomposes in the presence of graphite, severely impacting the lifetime of any cell. The challenge of stabilizing acetonitrile with additives is described in the original Asahi Kasei work that was published by the team lead by Dr. Akira Yoshino [4].

The American Ceramic Society [3] summarise this paper very clearly:

First, they prepared the nonaqueous, acetonitrile-based electrolytes by dissolving various lithium salts in acetonitrile. The ionic conductivity showed a maximum (>15 mS/cm) at a 1.3 mol concentration of the salts in 1 liter of solvent. Then, they added a combination of vinylene carbonate and ethylene sulfite to the electrolyte, which formed a solid electrolyte interface between the anode and electrolyte and effectively suppressed the decomposition reaction.

Degradation during cycling increased once acetonitrile content became greater than 35 vol.%. Conversely, the cycling performance was similar to a reference carbonate-based electrolyte when acetonitrile content was reduced to 10 vol.%. The cycling performance was improved for all acetonitrile concentrations by adding pyridine as a stabilizer.

In the original study, all electrolytes showed a superior performance at −40°C, retaining a capacity between 37 to 54 milliampere-hours per gram mass at 1°C. In contrast, the carbonate-based reference electrolyte showed no remaining capacity at 1°C.

Rate capability of LFP/graphite cells (a) under six different discharge C-rates from 0.1 to 20 C at 25 °C 

This figure from the original paper by Matsuoka et al [4] shows the acetonitrile electrolyte performs significantly better at high C-rates.

The cost improvement is proposed based on a number of factors:

• Acetonitrile is a widely used industrial solvent, and its raw cost can be relatively low.
  • Additives to stabilize it will add to the cost.
  • A full study of handling, recovery, filling equipment and wetting time etc need to be understood to fully develop an overall cost model.
• Improved ionic conductivity opens up the ability to increase the thickness of the active materials and hence improve the overall cell energy density. Hence allowing for an improved package and hence the possibility of lower costs.
• Low and high temperature performance would allow for a reduced cooling/heating system. Both in terms of complexity and energy required to operate.

Perhaps though the performance improvements and the ability to operate at -40°C is a very significant game changer for LFP.

References

1. Asahi Kasei licenses electrolyte technology to EAS Batteries, electrive
2. Breakthrough High Ionic Conductive Electrolyte, Asahi Kasei
3. A game changer: Acetonitrile-based electrolyte improves temperature range of Li-ion batteries, The American Ceramic Society
4. Dr. Naoki Matsuoka, Hirokazu Kamine, Dr. Yutaka Natsume, Dr. Akira Yoshino, Moderately Concentrated Acetonitrile-containingElectrolytes with High Ionic Conductivity for Durability-oriented Lithium-Ion Batteries, ChemElectroChem 2021, 8, 3095 – 3104

See also: Lithium Iron Phosphate