Electrolyte Optimization of a Substituted-LiCo1-xFexPO4 Cathode

Advanced high voltage cathode materials can increase the energy density through increased charging voltage but this usually limits the cell’s cycle life and/or efficiency. This tradeoff is due to irreversible electrolyte decomposition or damage to the cathode structure that occurs at higher potentials. Furthermore, materials such as LiCoPO­₄ are naturally electronically insulating and thus may either require a constant voltage charging step or very high voltage constant current cutoff voltage( >5.0V) to ensure a full charge is obtained. This can lead to perpetual electrolyte decomposition on each cycle due to the increased time the cell is held at higher potentials. Many researchers have incorporated various transition metals as dopants to improve the cycle life of these cells while maintaining the same cutoff voltage, and subsequently achieving higher energy density and cycle life. This was demonstrated recently in our lab by Fe-doping LiCoPO₄ (1).  This is perhaps attributed to better electronic conductivity of Fe-doped LiCoPO₄and stabilization of the cathode material.

An alternative approach involves the use of electrolyte additives for improved passivation layer formation (2-3). This approach, however, does not affect any structural changes that may occur within the cathode (i.e., additives are not as effective in stabilizing the cathode structure as doped metals). The use of additives can nevertheless improve the high-temperature performance of the cell and ensure a longer cycle life (4).

This study utilizes a joint cathode/electrolyte research approach. A next-generation LiCoPO₄-doped cathode was used (developed by ARL) that is less insulating than traditional LiCoPO₄, as indicated by a shorter CV step requirement. Various electrolyte additives were examined with the new cathode to further optimize the performance. These additives include tris(hexafluoro-iso­-propyl)phosphate (HFiP), lithium difluoro(oxalato)borate (LiDFOB), lithium bis(oxalato)borate (LiBOB), fluoroethylene carbonate (FEC) and Trimethylboroxine (TMB). While investigating the properties of the new cathode material, the electrolyte mixture was optimized for high-capacity cycling. This study has yielded a high-efficiency cell with a long cycle-life that is stable at room temperature and elevated temperatures.

ACKNOWLEDGEMENT

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

REFERENCES

1. J.L. Allen, T.R. Jow and J. Wolfenstine, J. Power Sources, 196, 8656 (2011).
2. R. Sharabi, E. Markevich, K. Fridman, G. Gershinsky, G. Salitra, D. Aurbach, G. Semrau, M.A. Schmidt, N. Schall and C. Bruenig, Electrochem. Commun., 28, 20 (2013).
3. M. Hu, J. Wei, L. Xing and Z. Zhou, J. Appl. Electrochem., 42, 291 (2012).
4. N.P.W. Pieczonka, L. Yang, M.P. Balogh, B.R. Powell, K. Chemelewski, A. Manthiram, S.A. Krachkovskiy, G.R. Goward, M. Liu and J.-H. Kim, J. Phys. Chem. C, 117, 22603 (2013).