Lithium is the smallest, lightest metal \u2013 and eager to give up an electron<\/p><\/blockquote>\n
\u2018If you just look at the laws passed in the US and Europe around electric vehicles and emissions, and you do the sums on the size of the market, there is complete certainty that these new batteries are coming,\u2019 says Matthew Hill, a battery chemist at Monash University in Melbourne, Australia. How quickly these next-generation battery concepts finally emerge from the lab is the big question, he adds. \u2018If we nickel and dime along, there\u2019s no known end to this situation \u2013 but if we put in a serious investment, then in a couple of years\u2019 time we\u2019ll start to see products coming.\u2019<\/p>\n
Pedal to the metal<\/strong><\/p>\n
To make a better battery for mobile applications such as phones, laptops or electric vehicles, the defining design parameter is to pack the maximum electrical energy into the smallest, lightest package possible. If you were to go back to the periodic table, and design such a battery from scratch, it is still hard to go past lithium (see Lithium short box below). It\u2019s the smallest, lightest metal \u2013 and eager to give up an electron.<\/p>\n
\u2018In terms of whether there are brand new chemistries that can have the energy density competitive with lithium, at this moment the short answer is no,\u2019 argues Ping Liu, a battery researcher at the University of California, San Diego, US. But that\u2019s not to say we couldn\u2019t make lithium-based batteries with many times the energy density of today\u2019s commercial batteries, he adds.<\/p>\n
Rather than lose the lithium, many beyond-lithium-ion battery concepts aim to eliminate some of the baggage weighing the current generation technology down. Starting with the anode.<\/p>\n
Commercial lithium-ion batteries invariably use graphite as an intercalation anode. The lithium ions nestle between individual layers of the graphite host. \u2018With graphite you build a happy house for lithium,\u2019 Liu says. But the graphite adds bulk and weight to the battery, lowering its energy density.<\/p>\n
\nLithium short<\/strong><\/p>\n
Lithium may remain the metal of choice for mobile applications, but such is the predicted need for batteries as the electricity grid and transportation sectors decarbonise, lithium demand could outstrip supply.<\/p>\n
Aluminium could transfer three electrons for each atom<\/p><\/blockquote>\n
Numerous alternative battery chemistries, employing different metals, are in development. Many, however, are bulkier and heavier than lithium batteries, better suited to fixed grid- or household-level storage rather than mobile applications. \u2018There is a lot of enthusiasm about the sodium-ion battery, for example, but that is a low-energy-density battery suitable for stationary storage,\u2019 says Ping Liu, a battery researcher at the University of California, San Diego, US.<\/p>\n
One possible exception to battery bloat could be aluminium, which counters its extra weight over lithium by carrying more charge. \u2018Aluminium can ideally transfer three electrons for each atom, which theoretically gives it more capacity to store electricity,\u2019 says Xiaodan Huang, a University of Queensland battery materials researcher.<\/p>\n
Aluminium is also much more abundant, and cheaper, than lithium. Plus, it is safer, Huang adds. \u2018Unlike lithium they are not flammable to air or water,\u2019 he says.<\/p>\n
In 2015, Hongjie Dai group at Stanford University in the US showed that \u2013 akin to today\u2019s lithium-ion batteries \u2013 a\u00a0graphite electrode might offer a practical way<\/a>\u00a0\u00a0to get aluminium batteries up and running. The basic architecture of a typical aluminium battery consists of an aluminium metal anode, and a graphite cathode to store aluminium ions between its layered carbon structure.<\/p>\n
Graphite\u2019s quite rigid structure and narrow space between layers limits aluminium storage<\/p><\/blockquote>\n
But the aluminium ion\u2019s triple charge is a significant challenge when it comes to the practicalities of battery performance, notes Liu. In the typical chloride-based electrolytes used in aluminium batteries, aluminium will not exist as the simple Al3+<\/sup>\u00a0\u00a0ion, he says. \u2018The ion is too polarising, it will acquire anions and make a complex, like AlCl4<\/sub>\u2013<\/sup>. You are essentially running a battery based on those ions rather than aluminium itself.\u2019 This complexation causes problems from the mass balance point of view of ion transport, as well as complications in interactions with the cathode.<\/p>\n
Huang has been exploring more accommodating carbon-based cathodes. \u2018Graphite\u2019s quite rigid structure, and relatively narrow space between layers, limits aluminium storage,\u2019 he says. \u2018We use graphene nanosheets that we make holes in, to allow more ions in.\u2019 Huang mixes the graphene sheets with a polymer that, when heated, breaks down to generate free radicals that cut holes in the graphene sheets.<\/p>\n
The team is now working on a scaled-up prototype, although increasing the size means confronting issues of mass transfer. \u2018But this year we have a breakthrough, a method to press the graphene materials to increase their density and reduce the thickness when they are loaded in the battery.\u2019 The compressed, holed material shortens the ion transfer pathway. \u2018We saved 80% of the volume, which significantly reduced the transport pathway length,\u2019 Huang says. \u2018Hopefully this will lead us to a mature prototype.<\/p>\n
\u2018We think aluminium batteries have the potential to, if not replace lithium, at least provide a competitive technology to lithium batteries in the market,\u2019 Huang adds.<\/p>\n
Alternative models<\/strong><\/p>\n
One route to a higher capacity next-generation battery could be to replace graphite with an alternative anode host material. Graphite has a charge capacity of 372mAh per gram, but silicon, which can soak up lithium ions via an alloying reaction at the anode, has a capacity of up to 2009mAh per gram. Charging the battery causes the silicon to swell up by 400%, however, a volume change 40 times greater than graphite\u2019s. This extreme expansion and contraction fractures the silicon anode, leading to poor battery cyclability.<\/p>\n
Issues associated with silicon have led many researchers to sidestep it and look to the ultimate lithium battery anode material: lithium metal itself. Made of the active charge carrying material, a lithium metal anode has a towering 3860mAh per gram theoretical capacity.<\/p>\n
Source: \u00a9 Zhaohui Wu et al\/Springer Nature Limited 2023. With lithium dendrites one of the key problems in many diffferent battery technologies, lithium fluoride can be a \u2018magical\u2019 solution<\/figcaption><\/figure>\n Stanley Whittingham\u2019s original work to develop the lithium battery in the early 1970s \u2013 for which he won the 2019 chemistry Nobel \u2013 used a lithium metal anode. But so far, a practical lithium metal battery has proven elusive.<\/p>\n
Charging and discharging a lithium metal battery is fundamentally an electrochemical lithium plating and stripping process. But it\u2019s imperfect. \u2018Just like any electroplating, you can\u2019t guarantee to plate a perfectly smooth lithium structure,\u2019 Liu says. \u2018And when you strip the lithium, it doesn\u2019t dissolve uniformly either.\u2019 Within a few recharge cycles, a block of lithium can start to resemble a sponge. \u2018Lithium is very reactive in electrolyte, so the higher the surface area, the higher the reactivity, and you start to lose lithium.\u2019<\/p>\n
Whittingham\u2019s lithium battery concept only became commercial reality once this problem was avoided with the introduction of carbon-based anodes. \u2018When we move from graphite to lithium, we invite a lot of really difficult challenges back,\u2019 Liu says.<\/p>\n
Lithium fluoride has been a magical compound<\/p><\/blockquote>\n
One area of research has focused on modifying the electrolyte so that it binds the lithium less tightly and allows it to plate out more easily. Another direction has been to use external force, such as pressure, to heal the holey electrode. \u2018Lithium is very soft, so even if you don\u2019t plate very well sometimes you can just squeeze it into shape,\u2019 Liu says.<\/p>\n
In Liu\u2019s lab, his recent work has focused on the surface that the lithium plates onto. Lithium plating involves two interfaces: the anode surface and the electrolyte. Lithium\u2019s high reactivity means the anode is always coated with a passivation layer called the solid electrolyte interface (SEI), produced by electrolyte decomposition (see box Eyeing solutions below).<\/p>\n
For efficient battery charging, an SEI rich in lithium fluoride has been shown to be beneficial. \u2018That has been a magical compound,\u2019 Liu says. \u2018People say it is because lithium has very weak interaction with lithium fluoride, so the passivation layer doesn\u2019t slow it down.\u2019 Rapid movement of the lithium atom allows the formation of a smooth, flat anode surface, rather than one covered in branching dendrite spikes.<\/p>\n
But according to conventional wisdom, the surface onto which the lithium is plating should be highly lithophilic, so that the lithium likes to stick. \u2018Somehow we have imposed a different requirement on the two interfaces involved in lithium plating,\u2019 Liu says. The lithium should move fast through the repellent lithium-fluoride-rich SEI, but then meet a substrate it simply wants to stick to.<\/p>\n
Having the incoming lithium ions able to skate about on the surface might be a better route to a smooth, flat lithium surface, Liu suspected. \u2018If lithium fluoride is such a magic compound, why don\u2019t we also have it on the substrate side?\u2019 he wondered. His team\u00a0created a lithium fluoride surface<\/a>\u00a0studded with conductive iron fluoride islands as lithium nucleation sites. \u2018Once lithium nucleates on iron, its surface diffusion is very rapid and we got these beautiful crystals. So the whole idea that lithium fluoride is magical is true.\u2019 The team now aims to incorporate the concept into a practical battery.<\/p>\n
Eyeing solutions<\/strong><\/p>\n
Despite decades of intensive research, certain fundamental questions remain about next-generation lithium battery technologies, hampering their adoption. But new characterisation techniques are increasingly offering new ways to get to grips with these issues.<\/p>\n
Source: \u00a9 Yuzhang Li et al. CryoEM allowed lithium dendrites to be examined as they grew<\/figcaption><\/figure>\n \n\u2018You could argue that the progress we have made with the lithium metal battery is largely due to the use of powerful new tools,\u2019 says Ping Liu, a battery researcher at the University of California, San Diego, US. \u2018Using synchrotron x-ray diffraction, we can put the whole battery in and still see through because the beam is so powerful. We can do in operando experiments: while we are charging\/discharging we are observing in real time what is happening. These techniques are critical for us to make progress.\u2019<\/p>\n
But perhaps the biggest step forward has been the adoption of cryo-electron microscopy (cryoEM) to battery materials research, Liu says. One fundamental question cryoEM is being used to address is the nature of the solid electrolyte interface (SEI) passivation layer that covers the lithium metal anode surface due to a reaction between lithium and the electrolyte.<\/p>\n
SEI formation, and the associated lithium loss, is a key issue for battery longevity, says Yuzhuang Li, a researcher at University of California, Los Angeles, US. \u2018Right now, the state of the art in lithium metal research labs and start-ups, we can get around 99.5% efficiency, which means we get back 99.5% of the lithium we store each recharge cycle\u2019 says Li. But by losing 0.5% of the lithium to the SEI each cycle, the lithium losses soon mount.<\/p>\n
\u2018If we can understand the fundamental mechanism of SEI formation, and how lithium metal grows in its absence, we can maybe get to 99.9%,\u2019 Li says. In 2017, Li was one of the\u00a0team that adopted cryoEM<\/a>\u00a0\u2013 which had proven so powerful in structural biology \u2013 for battery analysis. The researchers showed that they could electrochemically deposit lithium battery materials onto the conductive copper \u2018grid\u2019 used in cryoEM analysis, while keeping everything under an inert atmosphere.<\/p>\n
Using the technique, the team was able to see the SEI structure for the first time. \u2018It allows us to look at the interface, and determine the structure of the SEI film, to lend us an understanding of how lithium ions are transported through the layer and lithium metal is growing,\u2019 Li says. \u2018Our understanding can potentially help us to minimise SEI formation.\u2019<\/p>\n<\/div>\n
Thinking positive<\/strong><\/p>\n
In the commercial lithium-ion battery, electrode compromises continue at the cathode side of the cell. Again, a nice, layered home is provided for lithium ions to nestle into as the battery is discharged, in the form of a nickel cobalt manganese oxide material. But the metal oxide material adds a certain weight, as well as cost.<\/p>\n
Alternate cathode chemistries are under investigation, not least sulfur. \u2018We are removing the nickel, cobalt and manganese, which have cost, supply and ethical issues associated with them, and replacing that with sulfur which is basically a waste product available all over the world,\u2019 says Hill (for more on global resource issues, see\u00a0The lithium rush<\/a>). \u2018That gives us lots of sustainability improvements, but also the performance of these batteries is potentially a lot better.\u2019<\/p>\n
For a solid, sulfur is a very light anionic element. \u2018They are so light you get much more performance per unit mass \u2013 around five times higher on paper,\u2019 says Hill. \u2018It\u2019s really attractive in situations where weight is important, and also where cost is important \u2013 which is everywhere.\u2019<\/p>\n
We\u2019ve tried to use MOFs as a model separator material<\/p><\/blockquote>\n
One downside of sulfur is that its discharge voltage, at around 2V, is lower than today\u2019s lithium ion technology. As a result, it doesn\u2019t pair well with a graphite anode. \u2018Because the voltage is low, you need to store more charge, which would mean you need more graphite,\u2019 Liu says. \u2018That\u2019s a losing proposition.\u2019 A lithium metal anode, in comparison, has no such compromise. \u2018Lithium metal opens the door to sulfur,\u2019 Liu says.<\/p>\n
Technical challenges on the sulfur side remain. One issue is side reactions between sulfur and lithium that form polysulfides. \u2018If polysulfides find their way over to the anode, they start to form dendrites on the lithium surface,\u2019 says Hill. \u2018That\u2019s the path towards a short-circuit, and the end of your battery.\u2019<\/p>\n
To supress this issue, Hill has been working on improved battery separators, the nanoporous membranes placed between the anode and cathode to keep conflicting chemical species apart. \u2018We want lithium to go across the separator really fast, and we want essentially nothing else to go through,\u2019 Hill says.<\/p>\n
Hill\u2019s research background is in metal\u2013organic frameworks (MOFs). \u2018We\u2019ve tried to use MOFs as a model separator material, to understand the property and behaviour, and then translate that into cheap, very scalable, readily available materials,\u2019 he says. The team has developed low-cost carbon-based separator materials where, by controlling the pore size and charge, they permit easy transit for little positive lithium ions while blocking the larger, negatively charged polysulfides.<\/p>\n
An additional complication with lithium sulfur batteries is that, when the sulfur absorbs lithium during battery discharge, it swells by about 80%. A slab of sulfur would disintegrate after a few cycles; a more elastic material is required. \u2018We have developed a sort of spiderweb network where we use flexible binders to hold nanoparticles of sulfur together, and then the cathode could swell up and contract without falling apart,\u2019 Hill says. \u2018The latest one we just reported<\/a>, working really well, is using cellulose.\u2019 The material proved to have the added benefit of absorbing certain polysulfides, helping to keep them sequestered away from the anode. The team is now incorporating these developments into larger batteries, a little bigger than an smartphone battery, Hill adds. \u2018We used the battery to power a drone, to show we can do something practical at scale.\u2019<\/p>\n
Ten years ago, we got excited when we made a cell we could charge and discharge 10 times<\/p><\/blockquote>\n
In Liu\u2019s lab, flexible sulfur cathodes are also under development. \u2018One approach we have taken is anchoring sulfur to a conjugated polymer backbone,\u2019 Liu says. \u2018To make it, we just heat up a mixture of polyacrylonitrile, an industrial textile precursor, with elemental sulfur.\u2019 At around 350\u00b0C, the inexpensive starting materials react when the sulfur dehydrogenates the acrylonitrile to form a sulfur-containing, electronically conducting polymer with a polypyridine-type backbone.<\/p>\n
\u2018The material works beautifully, but the structure is extremely complex,\u2019 Liu says. The team is working on characterising it to further optimise its performance. \u2018A lot of people in the community are trying to understand this structure because it could be the key to a sustainable cathode material,\u2019 he says.<\/p>\n
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