Organic materials are potential substitutes for the costly transition-metal oxides used in battery electrodes, but their stability is often poor. A polymer design that uses intermolecular interactions solves this problem.
The growing market for electric vehicles and energy-storage systems has triggered a rapid increase in the demand for rechargeable batteries that are reliable, affordable and sustainable. However, there is mounting concern that conventional batteries, such as the lithium-ion batteries currently found in smartphones and many electric vehicles, might not fulfil future demand — both because of the limited availability of the metals needed to make them, and because of unpredictable costs and environmental issues associated with their use. Writing in Energy & Environmental Science, Kolek et al.1 report a battery electrode based on an organic polymer that could help to overcome these challenges and provide long-lasting, affordable energy storage.
There have been increasing global efforts to generate electricity using renewable resources (such as solar and wind power) and to promote the widespread adoption of electric vehicles, to reduce the current dependency on fossil fuels. Rechargeable batteries are a key technology in these efforts, because the energy storage they provide helps to smooth out spatial and temporal fluctuations in energy supply and demand, and extend the distance that electric cars can travel before needing to recharge. A substantial rise in battery production is thus considered inevitable2. For example, full-size electric vehicles require batteries that can store at least 8,000 times more energy than smartphones3,4. If electric-vehicle numbers were to grow to make up 10% of the worldwide car market, this alone would necessitate a fourfold increase in global battery production (see go.nature.com/2y19erv).
Battery electrodes — the components that hold charge — conventionally consist of transition-metal oxides, such as cobalt or nickel oxide. This presents a serious problem, because the availability of these transition metals is limited, and concentrated in certain (sometimes conflict-prone) countries, making production costs high5 and unstable. In addition, the use of large amounts of transition metals is neither sustainable nor environmentally benign, because both their production and recycling generate substantial carbon dioxide emissions. Finally, electrodes based on transition-metal oxides have already almost reached the theoretical limit of their charge capacity6,7 (the amount of charge available per unit weight), leaving little room for further improvement.
Organic materials have been studied as alternatives to transition-metal oxides for battery electrodes. These materials are potentially affordable, lightweight and sustainable, and they have a smaller CO2-emission footprint than transition-metal oxides2. However, batteries whose electrodes contain organic materials have short cycle lives (the number of charge–discharge cycles a battery can undergo while retaining at least 80% of its initial capacity) and low power capabilities, which make charging slow. Other intrinsic problems of organic electrodes include the fact that they dissolve in electrolytes (the ionic media that allow charge flow in batteries), are chemically unstable when in a charged state and have poor electrical conductivity6,7.
Kolek et al. report an organic electrode material called poly(3-vinyl-N-methylphenothiazine) (PVMPT; Fig. 1) that might overcome these problems. The design of this polymer stemmed from the authors' idea that the stability of organic materials can be remarkably improved by designing molecular structures that generate strong intermolecular forces, particularly when in a charge-storing state. In typical organic batteries, electrons and guest ions (such as lithium ions in lithium-ion batteries) are passed between electrodes during charge and discharge; the electroactive organic materials store the extra charge that accumulates. However, the additional charge tends to localize to certain parts (functional groups) of the organic molecule. In high-capacity batteries that involve the transfer of great numbers of electrons, this charge localization causes free radicals to form, making the molecule unstable and vulnerable to side reactions8.
But in PVMPT, a strong intermolecular force (known as a π–π interaction) forms between the polymer's stacked aromatic rings upon the introduction of extra charge. This force effectively delocalizes the accumulated charges, stabilizing the whole molecule and preventing unwanted side reactions. Moreover, the electrical conductivity of the polymer is enhanced by this intermolecular association, leading to faster battery charging and discharging.
Remarkably, Kolek and colleagues found that a PVMPT-containing battery cell could operate stably for 10,000 charge–discharge cycles with quick battery charging (148 seconds per charge). This cycle life is one of the longest reported among batteries that use organic electrodes — which typically have lives of fewer than 500 cycles9 — and even exceeds the cycle lives of conventional transition-metal-oxide electrodes (about 1,000 cycles5,10). The polymer also enables an operation voltage of 3.55 volts, which is comparable with that of conventional electrodes5, but higher than that of typical organic electrodes, which produce voltages7 of about 1–3 V. The relatively high voltage of PVMPT implies that a large amount of energy can be stored for a given amount of charge.
But PVMPT is not without faults. Its charge capacity is only 50 milliamp-hours per gram — less than one-third of that of conventionally used transition-metal oxides5,10 — meaning that batteries incorporating the polymer would be much heavier and larger than those made using the oxides. And even though the conductivity of PVMPT is improved by introducing strong intermolecular forces, it is still less than one-hundredth of that of conventional transition-metal oxides1,11,12. A large amount of a conducting agent, such as carbon, would therefore need to be added to PVMPT for practical applications, further reducing the charge capacity. Nevertheless, the chemical diversity of organic materials is expected to provide great opportunities in the search for a more optimal high-capacity polymer electrode.
Kolek and colleagues' work shows that a molecular design that exploits the charged state of a polymer can open up pathways for stabilizing organic electrodes, thereby enabling steady cycle performance. This could be an important step towards long-lasting, affordable plastic batteries that can store energy from renewable sources. Given that organic materials are often relatively soft, the findings might also open the way to flexible or stretchable plastic batteries, which, in turn, would enable the development of devices that can be worn or used as skin patches.