Army

Electrolyte salt for the lithium-ion (Li-ion) batteries

A more conductive, better performing electrolyte expands operation temperature range down to -40 degrees Celsius and increased capacity retention at high temperatures

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The chart illustrates results from five identical test cells using an LiBF3Cl-based electrolyte. The cells maintained excellent capacity retention during prolonged cycling (source: ARL-SEDD)

Rechargeable Li-ion batteries require the use of electrolyte salts to function properly. These salts must meet a variety of critical requirements, such as electrochemical stability with the anode and cathode; passivating cathode current collector materials and forming a stable solid-electrolyte interface (SEI) at the anode, thermal stability at high temperatures, and forming a conductive electrolyte capable of operating at low temperatures.

Among many commercially available lithium salts, only lithium hexafluoro-phosphate (LiPF6) is found to satisfy these requirements. Yet, improving low-temperature performance (below -20 C) of LiPF6 has proved elusive.

LiBF4 is known to be superior to LiPF6 in low-temperature Li-ion batteries. Batteries with LiBF4 salt have lower charge-transfer resistance which has been identified as the major influence on low-temperature operability. Compared with LiPF6, however, LiBF4 has two main drawbacks: inferior ability in forming an SEI at the graphite electrode, which robs charge capacity and rate capability; and low-solubility at lower temperatures, which limits operability (below -20 C).

ARL researchers have created a solution that overcomes these obstacles by a reduction in the stereo-symmetry of the BF4− anion, which alleviates the drawbacks of LiBF4. Researchers synthesized a series of test salts partially substituting fluoride with chlorine.  Tests demonstrated that the new salt, LiBF3Cl, is superior in forming SEI and stabilizing SEI at the graphite anode. It also showed higher solubility in organic solvents at low-temperatures and provided Li-ion batteries with improved low-temperature performance and rate capability.

The electrolyte salt, LiBF3Cl, was evaluated as an alternative to LiBF4 for use in Li-ion batteries. LiBF3Cl, like its established cousin LiBF4, has excellent ability in passivating aluminum cathode current collector material at high potentials. However, unlike LiBF4, LiBF3Cl is more efficient in forming SEI.  More importantly, LiBF3Cl, due to its asymmetric anode structure, has a higher solubility than LiBF4 and is superior in producing an electrolyte that performs well in temperatures down to -40 C.

Those interested in this technology may also be interested in 7,820,323 and 7,824,802.

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