Meaning
Operation of a lithium ion cell above its standard rated potential maximizes the extracted energy density by removing more lithium from the cathode. Energy storage capacity increases during high voltage cycling because the state of charge window is expanded. However, this practice subjects the electrolyte and the active materials to extreme oxidative stress.
Most commercial cells are limited to 4.2 volts to ensure a balance between performance and lifespan.
Material Degradation
Transition metal dissolution occurs more rapidly when the electrode potential exceeds the stability limit of the crystal lattice. During high voltage cycling, the structural oxygen in the cathode may become unstable and release as gas. This loss of oxygen leads to a permanent collapse of the layered structure and a reduction in capacity.
Electrolyte Oxidation
Carbonate solvents frequently undergo electrochemical decomposition when exposed to highly charged surfaces. The products of this high voltage cycling reaction form thick, resistive films on the electrodes that impede ion transport. Specialized additives are required to create a passivation layer capable of withstanding these aggressive potentials.
Design Tradeoff
Choosing to operate at higher voltages allows for a smaller pack size for a given range requirement. Engineers must weigh the benefits of high voltage cycling against the increased cost of thermal management and the shorter warranty period. High performance applications often accept these risks to achieve competitive power to weight ratios.
