High Voltage Li-Ion Battery Development

The approach of replacing the state-of-the-art Li-ion battery cathodes such as 4.2 V lithium cobalt oxide LiCoO₂ and lithium nickel cobalt manganese oxide LiNi_1/3Co_1/3Mn_1/3O₂ with high voltage cathodes such as 4.7 V lithium nickel manganese spinel LiMn_1.5Ni_0.5O₄ (LNMO) and 4.8 V lithium cobalt phosphate LiCoPO₄ (LCP) [1], versus Li/Li⁺, for the increase of energy density has encountered problems including lower Coulombic efficiency (CE) [2-4], faster capacity fading [5], and therefore, shorter cycle life than that of the state-of-the-art Li-ion batteries have achieved.  At elevated temperatures, e.g. 55 ^oC, the LNMO/graphite or LCP/graphite cells when cycled in the state-of-the-art Li-ion electrolytes, e.g. 1.2 M LiPF6 in EC:EMC (3:7 w/o), shows CE values can be as low as below 90% [4]. What are the issues? Electrolyte degradation at high voltages is one. The instability of the cathode at high voltage is another one, which may include the instability of cathode structure itself and the high cathode reactivity with the electrolytes.  Additionally, the degradation at the cathode side also plays a role in degrading the anode resulting in poor cell performance. 

The understanding of electrolyte oxidative stability at high voltages through computation [6] and surface analysis of reaction products on the surfaces of both cathode and anode [7] indicates that very much improved electrolytes are urgently needed.  The modification of the electronic structures of LNMO and LCP through substitutions [4,5] also improves capacity retention.  This suggests that the stability of the high voltage cathodes can be improved.

The developments of improved electrolytes including the use of additives and fluorinated solvents for improved electrolyte stability and improved LNMO and LCP cathodes using substitution for stabilizing the cathodes will be reviewed.  Recent advances coupling the improvements in high voltage cathode materials and electrolytes together for better performance will also be presented.

ACKNOWLEDGEMENT

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

REFERENCES

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