Smarter Batteries that think for themselves
Large-scale electrification of the transportation fleet is now a certainty, making the development of new battery cell chemistries a major priority.
With the announcement in July 2017 that the UK government would follow France’s initiative and ban the sale of conventional petrol and diesel cars from 2040, there is a fresh immediacy to the need to develop new generations of battery technologies for use in battery electric vehicles (BEVs), plug-in hybrid electric vehicles (PHEVs) and other hybrids. While various types of lithium ion technology form the mainstay of today’s BEV batteries, this chemistry is limited in terms of cost and performance and is likely to be superseded by more advanced battery technologies beyond the end of the decade.
In June 2017, just prior to the announcement of the combustion engine phase-out, Ricardo completed a joint research project with partners OXIS Energy, Imperial College London and Cranfield University to investigate the potential of a new lithium sulfur (Li-S) technology for use in BEVs. The project was part funded by Innovate UK and was entitled the Revolutionary Electric Vehicle Battery (REVB).
“The research consortium project focused on what is needed to make a step change in battery technology,” explains John Bailey, Ricardo chief engineer for software and systems. “In this case we were looking at a battery technology that is potentially much cheaper and lighter than today’s lithium ion batteries.” The goal of the project was not just to have cells running in laboratory conditions, but to build up a number of cells into a battery module. Much of the early work by OXIS Energy in developing the cell essentially involved steady-state testing in order to baseline the chemistry. The REVB project, by contrast, would aim to expose cells to the sort of fluctuating duty cycles they would see in a real-life automotive environment.
Lithium sulfur versus lithium ion
“Several years of design and optimization have gone into lithium ion control systems, but we’re still trying to understand lithium sulfur – and we have to understand it properly before we can optimize it”
John Bailey, Ricardo chief engineer for software and systems
The new battery cell is the very latest concept in lithium sulfur technology and offers a number of benefits over the equivalent lithium ion technology. Lithium sulfur technology allows greater depth of discharge than lithium ion, a characteristic that is especially useful for BEVs, where range is a headline factor. Despite occupying a larger volume than lithium ion batteries, the cell chemistry is lighter than that of current lithium ion technology – something that is also crucial to a BEV’s potential to achieve a longer range from a single charge.
Today, one of the main barriers to the acceptance of BEVs in the marketplace is cost: invariably, even the more affordable small electrics are more expensive than their conventionally-powered counterparts. Car manufacturers are collaborating with suppliers in a concerted effort to reduce the cost of battery packs to below $100 per kilowatt hour. This is widely believed to be the point at which EVs will be capable of competing directly with conventional cars in the marketplace. Crucially, lithiumsulfur systems are expected to be significantly cheaper to manufacture than the equivalent lithium ion batteries.
The aim of the REVB project was not only to develop a battery module based on the OXIS Energy cell but also an automotive battery management system to control the new breed of battery when used in electric vehicles. Automotive battery packs comprise hundreds or thousands of battery cells arranged in modules; these modules are in turn used to build a complete battery pack to suit the individual requirements of a particular vehicle. Ricardo’s main responsibilities during the project lay in the design of the battery module, and also a new battery management system.
“The task had several dimensions,” explains Bailey, “from the mechanical design and analysis, modelling and simulation of the thermal characteristics of the pack, acquiring all the parts and building it, updating our battery management system electronics to increase the number of functions offered, and increasing its processing power.”
As well as offering practical and operational benefits, lithium sulfur batteries are safer. Today’s lithium ion cells have the potential for thermal runaway, the condition where they ignite through overload or damage and cannot be extinguished, particularly where physical damage to the cells occurs. By contrast, lithium sulfur cells can tolerate significantly higher levels of mechanical damage, and in the case of a fire or of cell rupture, the chemicals released are significantly more benign than would be the case with lithium ion.
Read the full article in RQ Q3 2017