Solving the Power/Energy Paradox for High-Power Defense Applications
- Saturday, 01 December 2012
While often used interchangeably, power and energy are not the same. Energy is the capacity to perform and power is the rate at which that capacity is exercised — the rate at which energy is delivered. When measured, energy is an amount; power is a ratio. In relation to energy storage, a common misconception is that the larger the quantity of stored energy, the higher the potential rate of power. In fact, the opposite is true, rendering the following principles wholly counterintuitive to those unfamiliar with the nuanced mechanics of energy storage: when less energy is stored, a system is capable of producing high power and conversely, when more energy is stored, a system is capable only of producing low power. As a result, engineers are presented with a challenge in the manufacture of energy storage systems for highpower mobile applications with the highest energy density possible. In pursuit of this goal, modern research into lithium ion (Li-ion) electrochemistries is proving very promising.
As is widely known, use of Li-ion batteries in the consumer electronics sector is flourishing. This is also the technology of choice for electric vehicle (EV) as well as hybrid/plug-in hybrid electric vehicle (HEV/PHEV) applications, mainly due to its high energy density. By applying the same Li-ion electrochemistry on the industrial level, Saft America, Space and Defense Division (Saft SDD) is making significant headway in the quest to solve the power/energy paradox — a development that has been critical to a variety of defense and space applications that require very high power. This innovation is being achieved by optimizing both the electrochemical part of the cell as well as the cell’s mechanical design.
A battery is an electrochemical energy storage and conversion device, where the inter-conversion between chemical and electrical energy occurs via electron transfer reactions, or oxidation-reduction (redox) reactions. There are several classifications of batteries — primary, secondary rechargeable, mechanical replaceable, reserve and thermal. The Li-ion battery is classified as a secondary rechargeable battery. In Li-ion battery cells, the charge transfer between the positive electrode (cathode) and negative electrode (anode) is carried out by lithium ions in the electrolyte. No metallic Li is used in a Li-ion battery. Some of the commercial production cathode materials include LiCoO2, Li(NiCoAl)O2 (NCA), Li(NiCoMn)O2 (NMC), LiMn2O4 (LMO), and LiFePO4 (LFP). Currently, the most common anode material is graphite. During cell discharge, lithium ions are extracted from the anode and inserted into the cathode. The movement of Li-ion is carried out in reverse during cell charge. The electrolyte for a Li-ion battery is organic, which can be stable at voltage exceeding three volts.
In the constant search for higher power and capacity, researchers have been focusing on groups of electrode materials that can provide higher voltage and/or higher capacity. For the anode, silicon (Si) alloys — which can potentially achieve 6x the capacity of graphite — are actively pursued with the goal of overcoming material stability and cycle life issues. In the field of advanced cathode materials, higher voltage phosphates such as LiMnPO4 and LiCoPO4, as well as high voltage spinel, e.g. LiMnxNi2-xO4, are of research interest for improvement in material stability and cycleability.