The world we live in today runs on batteries. But the lithium ion batteries that dominate the market are expensive and environmentally demanding to extract. The raw materials for lithium ion batteries are scarce and concentrated in a few geographical regions. This places continued pressure on supply chains.
Sodium-ion batteries are a promising alternative because they use abundant materials. But sodium has shortcomings that have blocked it from being used as a replacement for lithium.
In work carried out at the University of Limerick's Bernal Institute , my team has now produced a battery that combines the strengths of sodium and lithium. This could lead to more sustainable batteries that reduce the supply chain pressures associated with lithium. The results have been published in the journal Nano Energy.
Sodium-ion batteries lag behind lithium ones in their energy density . Energy density is the amount of energy stored in a battery relative to its weight or size. Lower battery energy densities have an impact on the devices and machines they power.
If electric vehicles used battery modules with lower energy densities, it would limit the distance they could travel before needing to be recharged. Lowering the energy densities of batteries would also make tablet devices and laptops heavier.
As an energy storage researcher, this paradox gnawed at me. How could we harness sodium's sustainability without sacrificing performance? The tension feels like the ancient philosophical concept of yin and yang . This idea describes how seemingly opposing forces are actually complementary and connected.
In this case, sodium is abundant but weak, while lithium is powerful but scarce. Inspired by this dichotomy, I wondered whether the two technologies could work in harmony rather than competing.
This led us to produce the first full cell battery with two electrodes - one positive, one negative - that uses two charged atoms or molecules (ions). In this case the charged atoms are sodium and lithium. Batteries that use different positively charged ions to store and transfer energy are known as dual cation batteries.
Why sodium ions fall short
A standard battery is made up of one or more cells. The cell converts chemical energy into electrical energy . In the cell are two electrodes, or terminals: a positive terminal called a cathode and a negative terminal called the anode.
When the battery is used to power an electronic device, negatively charged electrons flow through the circuit and reach the battery's positive terminal. The chemical medium between the anode and cathode is called the electrolyte.
I decided to combine lithium and sodium in a half cell, which has one electrode immersed in an electrolyte rather than two. Just a modest amount of lithium salt added to a sodium-dominant electrolyte radically changed the way the battery behaved.
It roughly doubled the storage capacity of our half cell compared with an equivalent state-of-the-art sodium based battery. It was also stable up to 1,000 charge-discharge cycles at higher charging currents. Charge-discharge cycles measure how many times a battery can drain from 100% to 0% and recharge to 100% before its capacity degrades.
For someone who had previously watched sodium-ion batteries fade after a few dozen cycles, these results felt like witnessing a miracle.
Behind the scenes, a fascinating chemical ballet between lithium and sodium was taking place. Lithium ions are smaller than sodium ions, so they can move more easily through the anode material. Their movement helps open smoother pathways for sodium, lowering the "diffusion barrier" - resistance at the anode that normally slows sodium batteries down. This allowed more ions to enter the anode, allowing it to store more charge.
Just as importantly, sodium helped prevent lithium from getting trapped inside the material after discharge. This back-and-forth exchange kept the reaction reversible, giving the battery both higher capacity and better cycle stability. In this yin-yang interplay, neither ion dominated; instead, they worked in harmony.
Powering clean energy
Half-cell tests are the first step towards real world applications. For the next step, I demonstrated how a mixture of lithium and sodium worked in a full battery cell.
Battery capacity retention measures the percentage of the energy originally stored in a battery that remains available after a given period of usage. The full cell delivered a battery capacity retention of 70% after 200 cycles. This is far better than the sodium-only electrolyte, which started to fail after about 50 cycles.
The full cell performance was particularly satisfying, as sodium remains the dominant charge carrier. This ensures that the battery is still fundamentally a sodium-ion system.
This breakthrough could help power the world's clean-energy transition by reducing reliance on cobalt- and nickel-rich cathodes, which are common, but expensive, supply-constrained and linked to environmental concerns. Our design keeps sodium as the main working ion and pairs it with a more sustainable iron sulphide cathode. Because sodium and iron are more abundant than many conventional battery metals, the chemistry could be cheaper and easier to scale.
The small amount of lithium acts mainly as a performance booster rather than the main resource. That makes the battery both higher performing and potentially less dependent on costly critical materials. Furthermore, it represents a new route to storing renewable energy on the grid, which can help communities and industries transition to a greener future.
Despite the success of our prototype, much work remains to be done. The anode in our cell and half-cell was made of germanium, which is expensive. The next challenge is to replace germanium with cheaper anode materials. One candidate is silicon, which can reversibly host both lithium and sodium ions during charging and discharging, but also provide the battery with a higher storage capacity.
This boost would increase the energy density of sodium-dominated batteries. We also need to pair the anode with a cathode capable of producing higher voltages than we currently have.
I have already been exploring alternative and sustainable pairings of different ions, such as lithium-magnesium and potassium-sodium. I am also experimenting with new electrolyte formulations.
My team's research shows that by embracing the yin-yang of lithium and sodium, we can move towards batteries that show both high performance and sustainability. This raises the prospect of a world where your phone, car and even the grid will draw power from cheap, abundant sodium ions - gently assisted by a whisper of lithium.
Syed Abdul Ahad receives funding from Research Ireland Postdoctoral Fellowship supported by government of Ireland. While Syed Abdul Ahad led the conceptual design and experimentation, these findings would not have been possible without the support of his mentor, Professsor Hugh Geaney, and collaborators within the University of Limerick and the University of Birmingham.
