Johannes Betz of the University of Münster identifies the key calculations required to properly evaluate rechargeable battery systems. For more comprehensive information on this issue, check out his paper ‘Theoretical versus Practical Energy: A Plea for More Transparency in the Energy Calculation of Different Rechargeable Battery Systems’ in Advanced Energy Materials.
What metrics are important for the assessment of new battery systems?
There are several key performance indicators which make the assessment of a battery system possible. The specific capacity, which resembles the amount of charge per weight in an active material, is the main focus of most researchers; however, in practice, only the practical energy content matters: the energy stored per mass (specific energy) or volume (energy density) of a cell. Furthermore, the power (energy per time and volume or mass), lifetime, environmental friendliness, safety, and expected costs per stored energy play an important role.
“specific capacity … is the main focus of most researchers; however, in practice, only the practical energy content matters”
What issues exist in the current reporting of battery performance metrics?
First, the mean discharge voltage is often neglected, although it is equally (or even more) important for the energy and power content than the specific capacity. Furthermore, in many cases, the theoretical values of the active material are quoted and then compared to the practical values of the state-of-the-art lithium ion battery (LIB). These values have nothing to do with each other, as the necessary inactive materials and the electrochemistry of the battery materials have to be considered as well. There is no universal factor between theory and practice, although it is often stated. However, if one goes into the details, every cell chemistry needs a different amount of inactive material. For example, the electrolyte of lithium sulfur batteries (LSBs) is part of the cell reaction and, therefore, a larger amount is needed to achieve reasonable capacities.
“mean discharge voltage is often neglected” despite being as important as specific capacity
How can different battery systems be reliably compared?
The comparison of different battery systems has to be fair. The values have to be compared on the same system level (material, electrode or cell level). Naturally, new cell chemistries cannot yet be as perfectly optimized as LIBs. However, taking the amount of necessary active and inactive materials into account, and calculating the energy based on optimistic assumptions, demonstrates which values could be achievable in the end. LSBs will probably never achieve an energy density as high as for energy-optimized LIBs and, thus, will not replace them in electric cars, where the energy density is crucial. However, LSBs could, for example, be more environmentally friendly than LIBs, as sulfur is very abundant and non-toxic.
Interested in energy research and want to enter the discussion on methods and metrics?
– Methods Perspectives: Assessing Battery Interfaces via NMR
– Hui-Ming Cheng Clarifies Supercapacitor Performance Metrics
– Jonathan Coleman Elucidates Capacitance vs Rate Performance