Electrolyte Additives for Reducing the Irreversible Capacity Loss, Impedance and Polarization of a Doped LiCoPO4 Cathode

High voltage cathode materials show promise for improving the energy density of the state-of-the-art lithium ion battery. One notable cathode material that displays a discharge voltage of ~4.8V, and has the potential to improve the specific energy to ~802Wh/kg, is the olivine-structured LiCoPO₄. One significant issue, however, that has plagued LiCoPO₄-based systems is the irreversible electrolyte decomposition that occurs at higher voltages. The electrolyte decomposition is further perpetuated by the requirement of a constant voltage charging step (trickle charge) that is necessary due to a lower electronic conductivity of the material and increasing interfacial impedance from the unstable electrolyte.

We have previously demonstrated that Fe-doped LiCoPO₄ performs significantly better than neat LiCoPO₄ by stabilizing the cathode structure and improving the electronic conductivity of the material (1). These structural changes, however, do not solve the problem of perpetual electrolyte decomposition that leads to polarization and loss of cycleable lithium.

In order to promote a low resistance passivation layer that prevents electrolyte decomposition, many researchers have incorporated electrolyte additives (2-3). We previously demonstrated that using electrolyte additives improves the performance of doped-LiCoPO₄ half cells by forming a protective passivation layer on the cathode (4). The cathode half-cell performance can be somewhat misleading, however, because it does not take into consideration the loss of cycleable lithium due to irreversible redox reactions on the electrodes.

The present study serves as a next-step for demonstrating the viability of our doped-LiCoPO₄ cathode for the production of high specific energy Li-ion batteries. Full coin cells were fabricated and tested, including a next-generation LiCoPO₄-doped cathode (developed by ARL) and a standard graphite anode. Various electrolyte additives were examined with the new cathode to minimize the first cycle irreversible capacity loss, reduce the interfacial impedance, and promote a long cycle life. These additives include tris(hexafluoro-iso­-propyl)phosphate (HFiP), lithium difluoro(oxalato)borate (LiDFOB), lithium bis(oxalato)borate (LiBOB), fluoroethylene carbonate (FEC), trimethylboroxine (TMB), and other ARL-proprietary additives. These additives significantly improved the performance when compared to the baseline electrolyte.

ACKNOWLEDGEMENT

The authors wish to express their gratitude to the DOE ABR program for partial financial support.

REFERENCES

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