Modern life, to a large extent, relies on applications powered by lithium-ion batteries. By 2030 the number of electric vehicles is expected to increase drastically. And then there is also green electricity from wind turbines and solar panels which need large batteries that can store electricity for when it is needed and smooth out peaks and troughs in demand. There may come a point where lithium becomes too scarce or expensive to be the key material in this development. Therefore, scientists are relentlessly looking for cheaper and more efficient alternatives to lithium for fast-charging batteries.
Now (2021), scientists at Argonne National Laboratory have analysed a material that looks geometrically similar to rock salt and might be a promising candidate for lithium battery anodes that could be used in fast-charging applications. Known as a lithium rocksalt, it (Li3V2O5) consists of lithium, vanadium and oxygen atoms arranged in the same way as table salt, but with more disorder in its crystal structure. To discover the structure and dynamics of the lithium rocksalt, they used neutron diffraction to determine the atomic structure of the material. They also performed high resolution microscopic studies to analyse the structural changes in electrolytes after lithium insertion. Afterwards, they conducted X-ray diffraction and X-ray absorption studies to analyse the crystal structural change and charge compensation mechanisms of the material during charging and discharging. Also, the scientists used ab initio calculations to look into the low voltage and high-rate capability of disordered rock salt Li3V2O5 and found that a redistributive lithium intercalation mechanism with low energy barriers might be responsible for them. They believe that this low-potential, high-rate intercalation reaction could be used to identify other metal oxide anodes for fast-charging, long-life lithium-ion batteries.
Their findings are based on earlier research analysing the nature of lithium batteries. In 2014, scientists took the ribbon structure Li2W2O7 and synthesised the lithium-rich phase Li5W2O7 with an ordered rock-salt-type structure through a topotactic irreversible reaction, employing both electrochemistry and soft chemistry. Other than Li2W2O7, the lithium-rich oxide Li5W2O7 exhibited reversible deintercalation properties of two lithium molecules per formula unit: a stable reversible capacity of 110 mAh/g at 1.70 V was achieved after 10 cycles. The exploration of the lithium mobility in this system demonstrated that Li2W2O7 was a cationic conductor with σ = 4.10 (-4) S/cm at 400 °C and Ea = 0.5 eV.
In 2017, scientists discovered a Li3IrO4 compound (O/M = 4) that could take up and release 3.5 electrons per Ir and had the highest capacity ever reported for any positive insertion electrode. By quantitatively monitoring the oxidation process, they demonstrated the instability of the material against O2 release on removal of all Li. Their results showed that the O/M parameter delineated the boundary between the maximum capacity of the material and its stability, hence providing valuable insights for further development of high-capacity materials.
The new lithium rocksalt anode offers numerous advantages over conventional anodes: some anodes, such as graphite, are too unstable during fast charging, while others, like lithium titanate, cannot store enough energy to be fully effective. The increased potential compared to graphite reduces the risk of lithium metal plating if according charging controls are installed and eliminates a major safety concern (short-circuiting related to Li dendrite growth). In addition, a lithium-ion battery with a disordered rock salt Li3V2O5 anode yields a higher cell voltage than a battery using a commercial fast-charging lithium titanate anode or other intercalation anode candidates. Further, disordered rock salt Li3V2O5 can go through over 1,000 charge–discharge cycles with little capacity decay and exhibits exceptional rate capability, delivering over 40% of its capacity in 20 seconds.
These findings represent an important step forward in battery science as the lithium rocksalt allows batteries to hold a lot of energy and charge quickly. It has a more negative chemical potential and therefore much improved energy density over current commercialized fast charging lithium-titanate anodes. This new material gives the opportunity to explore its potential in new applications and does not have as many drawbacks as previous technologies.