Understanding the Effects of Cationic and Anionic Substitutions in Spinel Cathodes of Lithium-ion Batteries
The global demand for energy has reached epic proportions and is projected by the U.S Energy Information Administration (EIA) to continue to grow in the future. Efficient storage and utilization of electrical energy is critical if we are to exploit renewable energies like solar and wind to their full potential. In this regard, lithium-ion batteries are being intensely pursued as energy storage devices because they provide higher energy and power densities compared to other battery systems such as lead-acid and nickel metal – hydride batteries. In addition, they are also being pursued as a power source for transportation applications. Cost, safety, cycle life, and power capability are important criteria for these large battery applications. LiMn2O4 spinel as a cathode for lithium-ion batteries is appealing in this regard due to its lower cost and fast charge capability. Yet severe capacity fade has plagued the spinel cathode and prevented it from becoming widely commercialized. Many different approaches have been taken to improve the capacity retention of the spinel cathode. Our group has shown that substituting other transition metals for manganese followed by partial substitution of fluorine for oxygen can improve the capacity retention while maintaining moderate capacity values. We present here a systematic investigation of (i) the amount of fluorine that can be substituted for oxygen before impurity phases form or performance degradation occurs and (ii) how the chemical characteristics of the dopants (electronegativity and dopant-oxygen bond dissociation energy) affect the electrochemical performance of the cathode materials.