Battery technology: Looking beyond the obvious

How LTO technology can reduce the total cost of ownership in applications like AGV

Author: Volker Schumann, General Manager Battery Sales, Toshiba Electronics Europe GmbH

Battery technology is rapidly evolving, and at first glance the goals may seem obvious: simply squeeze the most capacity into the least amount of space and cost. While in some applications, such as electric road vehicles, this may be true, other applications require a different way of thinking, and alternative approaches can achieve even higher performance at lower cost.

In this white paper, Toshiba will look at LTO battery technology and consider how it can bring significant benefits in terms of both performance and cost in heavy-duty applications where batteries are frequently charged and discharged. Automatic Guided Vehicles (AGVs) are used as an example to show how LTO technology can help optimize battery life and lower total cost of ownership.

As environmental pressures and declining fossil fuel reserves push the auto industry to replace the internal combustion engine (ICE) with electric propulsion, the battery pack becomes the defining element in terms of vehicle range. vehicle. Designers are driven to make batteries that offer more and more capacity per unit volume while continuing to reduce the cost, measured in euros/kWh.

In fact, that's exactly the right approach for vehicles that have to travel long distances, but are charged relatively infrequently. However, for other applications, such as heavy rail, marine or off-road vehicles, a careful examination of other technologies can reduce costs and increase performance. Rather than maximizing battery capacity at the lowest cost, it is often more beneficial to optimize battery capacity through periodic fast recharges, such as those enabled by the use of LTO technology.

Lithium Titanium Oxide (LTO) battery technology

LTO technology has a fundamentally different chemical structure than other batteries, making it the most powerful and robust Lithium-Ion (Li-Ion) technology available. In LTO batteries, the anode is made of nanocrystals of lithium titanate (Li₄Ti₅O₁₂) instead of the more common graphite powder.

Figure 1: Li ions can easily enter the holes in the spinel structure of LTO

The surface of this material is thirty times greater than that of carbon. It avoids the challenge of rapid and reversible intercalation of lithium ions on carbon. Instead, ions can easily fill the voids in the crystalline structure during charging, giving an LTO battery a very low internal resistance that can handle higher currents.

The spinel structure of an LTO anode is considered to be "zero voltage" as it shows little (if any) volume changes upon insertion and removal of lithium ions. This provides the battery with excellent cyclic stability. Even after 8.000 continuous charge and discharge cycles at 5C from 10 to 90% of the full SOC range, Toshiba's latest high power LTO battery maintained nearly 100% of its rated capacity and showed no discernible degradation.

LTO offers a lower cell voltage of 2,3V compared to the 3,6V found in other Lithium Ion cells. This translates to lower specific energy, although LTO batteries are still capable of exceeding 100 Wh/kg, albeit less than a comparable next-generation NMC or LFP cell.

However, the positive impact of this reduction in cell voltage is that it provides a safety margin that eliminates the risk of Li metallization. As a result, LTO batteries are extremely safe and do not form Li dendrites, even when rapidly charged at low temperatures. In the unlikely event of an internal short circuit, LTO batteries will discharge much more slowly than carbon anode batteries. The slower chemical reaction means less heat is generated, so the risk of thermal runaway, or thermal spread, is much lower than with other types of lithium-ion batteries. This is of vital importance in applications such as marine ones.

LTO cells in AGV applications

As companies seek to become more efficient, automation is spreading across factories. One area of ​​special interest is AGVs, small electric vehicles used to move raw materials and merchandise around the factory and warehouse. AGVs often operate 24 hours a day, sometimes in demanding environments such as refrigerated warehouses or clean rooms.

By considering the typical work profile of a small AGV, we can begin to understand the benefits of LTO battery technology in these applications. About 75% of the time is spent driving with low energy consumption.

Raising the load consumes most of the energy, while lowering it recovers it and returns it to the battery. In a typical 20-hour day, an AGV of this type will consume about 4,8 kWh of energy, assuming about 1.200 races.

There are two options when considering charging strategy and selecting the optimal battery. The AGV can run throughout the workday and drain a large battery that can be recharged in about an hour, or periodic recharges can be done to sustain a much smaller battery throughout the day. In terms of working time, the two scenarios are the same, with 60 minutes of each day dedicated to charging.

The first daily charging option requires a battery with a capacity of 165 Ah. Therefore, even using NMC technology with a very high energy density of 200 Wh/kg, the battery would still weigh almost 40 kg. In the second option, a battery with a much smaller capacity, about 16,5 Ah, is recharged during operation for six minutes, ten times a day. The challenge is that to work in this way, a much faster charge is required: the relative charging power is ten times higher (6C). However, this is entirely within the capabilities of LTO technology, which can recharge faster (even at low temperatures) without the risk of the lithium metal being coated. As a result, despite the lower energy density, this solution weighs less than 10 kg and, assuming a factor of two for the euro/kWh, the cost of the cells would still only be one fifth.

Apart from the lower initial cost of the battery, its smaller size and weight will facilitate the design and reduce the cost of the AGV. Operation will be much more efficient and new use cases will be possible, such as shuttles connected to warehouse racks.

From the point of view of robustness and resistance, the LTO solution is the best. Not only is the risk of fire minimal, LTO batteries do not require a hot environment to charge and over their long life, two or three sets of NMC batteries are likely to be required.

Toshiba SCiB Battery Pack Solution

Toshiba's Super Charge Ion Batteries (SCiB) range includes a 24V / 22Ah LTO battery designed specifically for industrial applications such as AGVs. The pack can work from -30ºC to +45ºC and is capable of supplying up to 125 A for 200 seconds.

With dimensions of 247 x 188 x 165mm and weighing only 8kg, SCiB batteries can be connected in parallel or in series (for 48V operation). Status and diagnostic data is provided via the CAN bus.

Summary

AGVs are becoming more common and to be successful they must be small, manoeuvrable, reliable, and low cost to own and operate. Selecting the right battery is key to achieving this and while the mantra for batteries is often “more capacity at less cost”, choosing an LTO battery can dramatically reduce costs while providing a safer and more robust solution. .