• Hollenkamp, Anthony
• Barghamadi, Marzieh
• Best, Adam

Abstract

Removing the wires from trams is becoming an attractive proposition in what is known as “Catenary free” trams. We have designed a long cycle-life high-power energy storage device which could potentially be used in this application. Here, the fast charge capability is focussed on rather than the fast discharge capability, in order to half-charge the devices rapidly at the stations (in a matter of a minute) without excessive degradation in capacity or an increase in device resistance with continued cycling. Due to the power requirement, the energy density is much less than a typical Lithium-ion device, however this leads to a longer cycle life with a target of 100,000 cycles at partial state-of-charge.By incorporating changes to the typical Li-ion device the energy density is greatly reduced from ~200 Wh kg-1 to a projected 42 Wh kg-1 while achieving the above requirements.Firstly, a Lithium Iron Phosphate – Lithium titanate (LFP-LTO) chemistry was selected which lowers the energy density due to the lower specific capacity of the anode and cathode materials and the lower cell voltage. LFP helps with high charging rates and LTO helps with safety and stability as this “zero-strain” material operates at a voltage well removed from lithium plating potentials under over-charge conditions. Next, the electrode loadings were reduced to allow for faster charging which results in further lowering of the energy density. Lowering the active material content to 70%, while increasing the conducting carbon content further lowers the energy density but allows for enhanced charging capability and less increase in resistance with cycling. Also incorporated into the electrode paste are highly conductive carbon additives which assists in charging rate and coherence and adherence of pastes. These high carbon loadings also maximise the conducting pathway to the active material even when parts of the electrode begin to degrade. These measures overall reduce the energy density to about 42 Wh kg-1. Other measures are incorporated to improve the cycle life and power.Herein, CSIRO-Australia has prototyped a lithium battery-based device with an energy density of 42 Wh kg-1 that has high power and long cycle life. The energy density of the device is tailored to allow for high charging power with more than 85% capacity retention at 20C or 3-minute charging, and long cycle life of over 27,000 full State-of-Charge cycles at high rate 15C/5C charge/discharge with little increase in resistance (<10%) concurrent with the capacity fade to 80%. Consequently, we have designed a long-life high-power energy storage device which could potentially be used in catenary-free trams.

Topics

• density
• impedance spectroscopy
• Carbon
• energy density
• laser emission spectroscopy
• Lithium
• iron
• wire
• carbon content