Metal borate Li-ion batteries

Salt composition that offers significant improvements in Li-ion cell charge/discharge performance and cycling efficiency at both ends of the operating temperature range


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This ARL invention can be used to increase the capabilities of Li-ion arrays for hybrid electric vehicles. (Department of Energy)

Rechargeable Li-ion batteries have been commercially available for about three decades, and the electrolyte salt LiPF6 has been commonly employed in these power cells. However, LiPF6 has limited applicability in future Li-ion systems owing primarily to its lack of thermal stability. This instability not only impedes safe handling but it can also lead to physical degradation of internal battery components, performance penalties, and a shortened service life.

In recent years, considerable effort has been made to create alternatives to LiPF6. While some promising formulations have been developed, collectively they suffer many flaws: cannot passivate aluminum current collector; challenging to produce and purify; add to production costs; poor solubility in conventional organic solvents; low ionic conductivity; and the process is inherently dangerous and inefficient.

The ARL solution to these shortcomings is LiODFB.  This promising metal borate combines the most favorable attributes of two other alternative salts, lithium bis(oxalato) borate (LiBOB) and lithium tetrafluoroborate (LiBF4), to create a cost-effective electrolyte for high-performing Li-ion batteries that operate well in harsh operating environments.

LiBOB and LiBF4 have been studied extensively as electrolyte salts and have shown significant merit and yet some notable limitations. LiBOB can create a stable solid electrolyte interface (SEI) on graphite anodes, which minimizes capacity loss.  It also has excellent overcharge tolerance. LiBOB works well at higher temperatures, but its limited solubility in common solvents and relatively high viscosity of such solutions reduces power and rate capability at low temperatures. Cells with LiBF4-based electrolytes exhibit better low-temperature performance; however, such electrolytes are relatively inefficient in forming SEI, resulting in a high capacity loss.

The salt produced by the novel ARL synthesis process, LiDOBF, has the combined advantages of LiBOB and LiBF4 due to the similarity of its chemical structure. Tests demonstrate cells with a LiDOBF-based electrolyte operate well at high and low temperatures (-30 C to 60 C), form a conductive SEI promoting excellent charge/discharge rates and cycling efficiencies and offer improved overcharge protection. For example, a LiDOBF-based cell suffered only about 10% capacity loss after 200 cycles at 60 C while maintaining nearly 100 percent charge efficiency.

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