Lithium ion cathode materials research is one of the most pressing and challenging aspects of lithium ion battery development today, currently holding the most potential for improvement in terms of cell energy density, electrical performance and safety. Lithium ion cells contain many complex material structures and chemical reactions, some wanted and some unwanted, with the most significant bottleneck being the capacity, voltage, stability and cost of the cathode material itself.
The main ingredient that makes a lithium ion cell hazardous and risky of fire or explosion is the highly electronegative cathode material needed for positive electrode function. The cathode consists of a strong oxidizing agent needed to absorb incoming lithium cations from the electrolyte and electrons from the external circuit during discharge. To do this the cathode material contains a high level of oxygen bonded in various structures with metal atoms such as cobalt, nickel, vanadium, chromium, aluminum or manganese or in the form of a metal phosphate such as iron phosphate. The danger of having so many oxygen atoms in the cathode material is that a high temperature failure event could release the oxygen in a vicious fiery reaction or explosive burning with the often volatile hydrocarbon based electrolyte forming a perfect self contained fire triangle consisting of heat, fuel and oxygen!
Another strong oxidizing agent is sulfur which has the same valance configuration as oxygen and can be used in high energy density lithium-sulfur systems as are currently being developed by a few American companies such as Sion Power, Polyplus and Oxis Energy. The main drawback of a sulfur based cathode is that some of the intermediary LiSx compounds are solvent in the electrolyte commonly used in lithium ion cells causing the cathode and electrolyte to deteriorate, plus they often use a high capacity lithium metal anode in order to match the high capacity of the sulfur cathode and must therefore also deal with the uneven plating problems associated with lithium metal anodes. The theoretical energy density of the lithium sulfur system is very high up to 2500 Whr/kg with a practical energy density more in the range of 300-500 Whr/kg.
Synthesis of most common cathode materials are lithium containing in the discharged state and are the only source of lithium within the cell upon construction. After construction the first charge is called the formation charge where the cathode is slowly delithiated to form the Solid Electrolyte Interphase (SEI) layer against the typically carbon anode. The SEI layer is formed from components in the electrolyte and lithium from the cathode. Additional lithium is also irreversibly lost into the carbon anode itself for a total irreversible first charge capacity loss of 20% or more.
Table 1 gives a basic comparison of a number of cathode materials commonly used today. The best cathode materials have both high capacity and high voltage versus lithium to give maximum energy density, as well as low cost, low toxicity and high cycle life.
|Cathode Material||Theoretical Capacity mAhr/g||Practical Capacity mAhr/g||Voltage versus Lithium||Cycle Life||Safety||Toxicity||Cost|
|Layered Mn Oxide||285||160-190||3.8||Very Good||Very Good||Low||Low|
|Iron Phosphate||170||120||3.2||Very Good||Very Good||Low||Low|