Roadmaps explored – Understanding the battery challenges from chemistry to recycling
A blog post by Jon Regnart.
In this second article of the APC’s Roadmap Explored series, the focus shifts to the Automotive Council’s electrical energy storage roadmap. With battery technology being identified as a high priority for the UK government, how can batteries be optimised for current and future automotive applications?
Research into battery technology has intensified
Over the last decade there has been a flurry of research activity in battery technology. Eager technology start-ups and renowned research institutions are all pursuing the holy grail of higher energy densities at lower cost, with the search showing no signs of abating. As Figure 1 illustrates, in 2006 there were only 1,353 global patent applications filed in the area of automotive electrical energy storage. However, fast forward to 2016 and almost 7,000 global patent applications have been filed so far with patent activity intensifying further since 2014.
Looking at the assignee trend for 2016 (see Figure 2), it’s no surprise that patent applications in battery technology are emerging from both established automotive players and new market entrants. These new companies range from Chinese automotive OEMs, who have experienced tremendous growth over the past few years, to consumer electronics companies (such as BYD, Panasonic and Samsung) who have been manufacturing smaller batteries for many years.
This presents a challenge to incumbent automotive supply chains and has stimulated vast amounts of public and private investment resulting in numerous avenues of research opening up. Innovations such as solid state batteries, lithium metal anodes, sodium ion, lithium sulfur and metal air batteries are just some examples of the breakthroughs making media headlines.
Building on the foundations of the 2013 roadmap
The Automotive Council’s Chemical Energy Storage Roadmap, released at LCV2013, charted the progression of various lithium based cathodes and illustrated the importance of optimising cell chemistries for different vehicle types. The roadmap and accompanying analysis helped lay the foundation for continued government and industry investment in UK battery research and paved the way for initiatives such as the Faraday Challenge.
For the Automotive Council’s updated Electrical Energy Storage Roadmap, the APC’s roadmapping team restructured the roadmap around a set of ambitious medium and long term technical targets for automotive battery packs. It’s crucial that the targets on the roadmap were set at a pack level. This is because vehicle manufacturers and battery pack suppliers take cells and construct them into modules and packs so they can be integrated into the vehicle more effectively. While cell level metrics are informative (and we’ve included them in our analysis slides), they do not communicate the final cost or the required packaging space to the OEM. Therefore showing battery pack targets on the roadmap performs the dual function of reflecting the vehicle manufacturers’ requirements while simultaneously encouraging research in other areas other than just battery cells.
The roadmapping team created many pack and cell level metrics, but the three targets along the top of the roadmap reflect some of the biggest challenges facing vehicle manufacturers: reducing cost ($/kWh), increasing volumetric energy density (Wh/l) and increasing gravimetric power density (kW/kg). Dave Greenwood, Professor of Advanced Propulsion Systems at WMG and the APC’s Electrical Energy Storage Spoke lead, believes the new roadmap targets set a clear challenge for the automotive industry:
For the next one or two vehicle cycles, existing lithium ion batteries are likely to take a dominant position in the market. Incremental improvements in cell chemistry, battery pack management and manufacturing processes will be the main path to satisfy the energy density and cost requirements in the short term. However, in order to reach the more challenging cost, energy and power density targets set for battery packs in 2035, new chemistries and manufacturing processes will have to be developed and commercialised.
But against the backdrop of intense global activity in battery development, how ambitious are the new roadmap targets?
A closer look at the roadmap targets
Looking at Figures 3, 4 and 5, the UK’s targets for cost, energy and power density are relatively aggressive compared to other industry projections and national roadmaps. Battery pack cost trends (Figure 3) show that greater cost reductions will occur in the short term with a gentler trend line occurring in the medium to long term.
Possible routes for achieving the greater costs savings in the shorter term could be through using Li-ion chemistries that contain less or no expensive elements such as cobalt and nickel or optimising existing manufacturing methods for higher volume automotive production. However, to achieve 2035 cost target of $100/kWh, fundamental new chemistries that utilise lower cost, more abundant materials will have to be used. Commercialising these technologies will take significant development but research around the world is already taking place with possible candidate chemistries including sodium ion, lithium sulfur, lithium-air and magnesium ion.
Looking at volumetric energy density, which is important when OEMs want to understand the packaging space needed, the trend is again more aggressive in the short term with a more gradual increase in the medium to long term.
Possible methods the roadmap identifies for increasing energy density in the short term are using higher voltage capable electrolytes, introducing more silicon into the anode and better processes for ageing and cycling the batteries after they’re manufactured. Similar to the strategy for reducing costs, optimising the chemical composition of lithium cathodes can also increase energy density. However, this could require adding elements such as nickel and cobalt which as mentioned before, can increase costs. This illustrates the trade-offs between increasing energy density and reducing cost and shows different OEMs may prioritise different targets depending on their product.
To attain the 2035 energy density target of 1000Wh/l the only route is to use alternative chemistries. There are numerous candidates for achieving higher energy densities but frontrunner options identified in the roadmap include: solid state batteries, lithium sulfur, metal-air and multi-valent chemistries such as magnesium or aluminium ion.
Finally there are power density targets. These are perhaps the set of targets where the UK really sets a precedent as data sources for kW/kg pack targets are quite scarce. Nonetheless power density targets are important because they are a key metric for applications such as hybrids that require high power.
The trend line shows major increases in the short to medium term with improvements being more incremental in the longer term. Over the last few years, attention has tended to focus on increasing the energy density for longer range BEVs, however, as electrification becomes viable for a broader range of products, higher power chemistries will be required. High power battery cells tend to cost more than lower power cells due to needing more porous and thinner coatings and emitting more heat due to higher currents. Recent approaches focus on optimising current LTO and LFP chemistries for application such as buses and high performance passenger cars, but it’s unlikely these chemistries will attain the 2035 targets of 12kW/kg. Potential approaches for achieving this target include using graphene in the anode to increase power densities or potentially using high power super-capacitors or lithium ion supercapacitors.
Peter Bruce, Professor of Materials at the University of Oxford, believes that in order to successfully commercialise any of these next generation cell chemistries, an understanding of the interactions and trade-offs within the roadmap is crucial:
Transitioning to battery technologies beyond Li-ion presents challenges across research and innovation. It is not simply about swapping one cathode or anode material for another. It will require a fundamental redesign of the cell chemistry and structures, as well as new manufacturing methods. Beyond lithium-ion will need advances in anodes, cathodes, electrolytes, additives and separators. Therefore, in order to hit the roadmap targets, research and innovation must occur across the whole cell.
To illustrate this point, let’s look at the challenges with both sodium ion and lithium air. Sodium ion cannot intercalate into graphite (the most commonly used anode for lithium ion batteries) due to the unfavourable thermodynamics. Therefore new anode materials such as hard carbon, tin, antimony or metal oxides need to be commercialised for sodium ion batteries to operate properly. Similarly for lithium air, the theoretical energy density of lithium air is close to conventional thermal propulsion system based powertrains. But in order to fully meet the requirements of road transport, challenges must be overcome around the degradation of the electrolyte (leading to significant capacity fade) and improving the C rates which are currently too low. Both of these examples illustrate a fundamental step change in both cell chemistry and design is needed, presenting battery researchers with a whole new set of challenges that need to be overcome.
The roadmap looks beyond the cell to look at pack level innovations
The roadmap recognises the importance of electro-chemistry for the next generation of automotive batteries, but the cells, whether they are traditional lithium ion or next generation chemistries, need to be assembled into modules and packs in order to make them usable for OEMs. Intelligent battery management systems, advanced cooling solutions and increasing the battery pack densities can offer just as much improvement as an alteration in the fundamental cell chemistry. Innovations in these areas also tend to be cell agnostics making them an attractive area for the automotive supply chain.
At the pack level, more innovative battery pack designs and concepts can be tested to maximise both the energy and power densities. For applications such as passenger car PHEVs and buses, the roadmap notes the potential to use high power cells (or ultra-capacitors) and high energy density cells that can deliver a blend of high energy and power. This requires careful management of voltages and temperature difference between individual cells and modules, driving the need for more sophisticated battery management systems.
Another pack level technology highlighted in the roadmap is thermal management strategies. Keeping battery packs within a certain temperature range is integral in maintaining the packs performance and mitigating thermal runaway (which can lead to battery failure). In the short term optimisation of current cooling strategies (i.e. air/liquid or tab/surface cooling) remain key as well as integrating battery thermal management systems with vehicle management systems. More longer term, there is the potential to actively store heat using phase change materials, utilising it for cabin heating or warming the battery up to its optimum temperature on cold days – both of which extend the range.
With regards to battery pack integration, in the short term, there is a focus on the ability to manufacture battery packs in higher volume for both new vehicle platforms and where the battery is being integrated into an existing platform (such as EV versions of the VW Golf or BMW Mini). In the longer term if more exotic body materials are used, there’s the opportunity for structural batteries to be utilised. Here the battery transitions from being a piece of hardware into an integral part of the vehicles body structure. This necessitates a change in vehicle design that could facilitate lighter designs and optimised energy and power consumption.
What will happen to batteries after vehicles are scrapped?
One of the biggest challenges in establishing a sustainable battery supply chain is reducing the life cycle impacts of automotive battery manufacturing. A key part of this is ensuring that when electrified vehicles reach the end of their life, the battery packs can be effectively reused. A key risk identified in the roadmap is the current absence of a sustainable high volume solution for end of life batteries. This section articulates two possibilities that have emerged for end of life automotive batteries.
The first option is using automotive batteries for 2nd life applications such as home energy storage. Over the last few years interest in repurposing batteries has increased with a number of companies beginning to use battery packs from electric vehicles for use in other applications. For example, UK company Connected Energy are using batteries from Renault’s electric vehicles to deliver grid load management solutions while Nissan, in partnership with Eaton, are also using batteries from first generation Nissan LEAF’s to provide energy storage solutions under the name of xStorage.
Figure 6 provides data from Bloomberg New Energy Finance stating that an estimated 95GWh of batteries will come out of cars between now and 2025. This factors in the age of vehicles, how many will come off the road, expected number of car crashes and battery failures. Of the 95GWh, BNEF forecast there could be 26GWh of used car batteries suitable for stationary storage from 2016 to 2025.
Recognising this could be a potential area for growth, the roadmap identifies key areas of research required in order to fully exploit this market. Promising areas include: enhanced understanding of battery degradation causes through improved modelling; designing and integrating battery packs into vehicles that make use for 2nd life applications easier and accessing standardised state of health data to inform manufacturers which cells and modules are appropriate for 2nd life.
The second option for the automotive industry is recycling xEV battery packs and reusing the materials in the development of new batteries. Irrespective of whether older automotive batteries are successfully commercialised for 2nd life applications, the repurposed batteries will eventually need to be recycled. Adam Chase, Director of E4tech, believes that scaling up current recycling processes is crucial for batteries to compete with thermal propulsion systems and fuel cells:
For batteries to be a sustainable automotive solution, high volume recycling facilities are required from the mid-2020s to handle EV batteries rolling off the production line today. These processes also need to be cost effective as OEMs cannot afford for recycling to eat into EV margins. The inclusion of recycling in the new roadmap is positive as it demonstrates the automotive industry is responding to a challenge which could otherwise become a threat to EV acceptance.
Current cost estimates of lithium ion recycling are between $25-$60/kWh which is a significant chunk of the 2035 battery pack cost targets. So what are the possible recycling routes? There are broadly two approaches to battery recycling: early stage standardisation leading to volume driven recovery or dedicated processes for differing battery chemistries. Volume driven recovery has minimal initial sorting with higher volumes driving down costs. The ideal scenario is that metals are refined so they can be converted into new battery materials, closing the materials loop. However, this approach requires battery chemistries being homogenised and OEMs agreeing to a limited number.
Current market trends suggest that OEM’s are using numerous variations of lithium ion and will continue to do so in order to differentiate their products. Therefore if OEM’s continue to use a wide variety of lithium ion chemistries, it may necessitate dedicated recycling processes. This could lead to intensive sorting and mechanical preconditioning due to the physical separation of components such as plastics, steel, foils and electrolytes. In an ideal scenario, dedicated processes would be able to economically extract valuable compounds such as the electrolyte, separator and active cathode materials that OEMs are willing to use for new batteries. But this approach tends to require more complicated hydrometallurgical processes which use a range of chemicals that have high global warming potential.
In short, both strategies have their strengths and drawbacks and work is underway around the world to develop economically viable processes for automotive battery recycling. Encouragingly for the UK, the recent Innovate UK Faraday Challenge funding includes numerous end of life projects led by companies such as Johnson Matthey and HSSMI.
A flurry of activity that shows no sign of abating
There is no doubt that batteries will be a key solution going forward for the automotive industry. The roadmap aims to capture the breadth and depth of potential innovations in electrical energy storage and illustrates there is still a wealth of battery research required for mass market EV acceptance. The first wave of funding awarded by the Faraday Challenge demonstrates that the UK’s research community is eager to work collaboratively with industry and government to commercialise current and next generation battery technologies. The challenges with automotive batteries, such as relatively lower energy densities compared to diesel and their embedded environmental impacts, are widely reported in the mainstream media. But these challenges also provide numerous opportunities for UK based companies both in the automotive sector and adjacent sectors. We hope the updated roadmap will help facilitate discussions between interested companies and aid those organisations in adjacent sectors to navigate the automotive battery landscape.
The Advanced Propulsion Centre would like to thank all the participants who attended the Electrical Energy Storage workshop. Your input at the workshop was valuable in delivering the updated Electrical Energy Storage Roadmap. Special thanks also go to members of the steering committee who ensured the APC represented the challenges facing the community.