Gaining traction for emission-free railways

European manufacturers are developing a new generation of battery-powered rolling stock for emission-free railways. 

They will need autonomous onboard power for distances of around 15 km or even longer. Philippe Blanchard, head of Saft's multi-disciplinary lithium-ion energy and mobility product engineering team, explains why the fast-charging capability of lithium titanate oxide (LTO) batteries makes them ideal for this demanding application.

LTO is an important new addition to Saft’s growing portfolio of lithium-ion (Li-ion) chemistries. They feature a lithium titanate-based anode, in contrast to the graphite or carbon-based anode found in other Li-ion batteries. This allows them to achieve very high charge and discharge rates – making them ideal for applications requiring fast-charging. Typically, an LTO battery can receive a full charge in just 10 minutes or less.

The LTO chemistry is also particularly flexible in terms of operations:  as an example, it can be stored at zero percent State Of Charge or even zero volt without any destructive effect.

It has a particularly benign behavior when subjected to abuse such as short circuit or mechanical damage. This high level of inherent safety reduces the need for fire suppression and other safety measures at the system level. 

Overall, LTO batteries offer the potential to create more simple and cost-effective battery systems where fast-charging and safety are paramount and energy density is less important.

Taking EMUs "off-wire"

Saft is working with leading train manufacturers on projects that will enable intercity and urban rail services to enjoy the benefits of emission-free, sustainable electric power. But using batteries to avoid the major capital costs associated with wide-spread electrification of networks. Here, the fast-charging capability of LTO batteries is crucial, as the time available for recharging is measured in minutes rather than hours.

In one scenario, an EMU (electric multiple unit) will be able to go ‘off-wire’, leaving the main electrified route to run on battery power for the last kilometer or so of a non-electrified branch line. For longer range operation, charging stations might be installed at the terminus or possibly along the route.

Another scenario is that the onboard battery will enable a train to run through regular long neutral sections – such as tunnels and bridges – recharging when it reconnects to the main power supply. This would allow for a more cost-effective, discontinuous electrification scheme.

Even when off-wire, the LTO batteries will enable the electric trains to outperform diesel trains. Their faster acceleration will cut journey times, and they will be lighter, far more efficient and significantly cheaper to maintain.

Commercially ready in prismatic format

Saft has been developing LTO batteries for around six years. About two years ago they reached the industrialization stage. Our factory in Jacksonville, US is now shipping the first generation of cells that offers 28 Ah capacity. And in 2021 we will uprate to 31 Ah capacity.

We have adopted the prismatic format for the LTO cells, rather than cylindrical. This offers the best combination of packing density together with the reliability and durability for a service life of at least 10 years and possibly up to 15 years.

LTO - for more than just rail

Although our initial focus is on rail, LTO chemistry is suitable for many other applications requiring batteries that combine fast charging with a long cycle life. One example is the AGVs (automated guided vehicles) used in warehouses, hospitals and other logistics applications. When not in use, these AGVs are able to return to a dock for fast-rate opportunity charging that ensures they never run out of charge.

LTO batteries also offer several advantages for small satellites that operate in LEO (Low Earth Orbit), the main one being reduced mass. Due to their extended operating range (-30°C to +70°C), there is no need for heaters to maintain the optimum temperature for space conditions, thereby simplifying the system and increasing usable energy.

High charge/discharge rates are also very advantageous for radar/agile LEO satellites, where only a short time is available for recharge during sunlight periods.

What comes after LTO?

Saft’s research teams are always looking for the next step forward. They are already investigating a potential successor to LTO batteries that use NTO (Niobium Titanium Oxide) as a next-generation anode material. NTO could offer all the benefits of LTO batteries, while also being able to store 20 percent more energy. 

Philippe.blanchard@saftbatteries.com