31-08-2022 | By Liam Critchey
Different types of batteries are used for various applications, but the overall dominant player in the rechargeable battery space is the Li-ion battery. However, there are applications where they are not the best suited because their energy densities are not as high as many other lithium battery types—they are just much more stable for prolonged use in portable electronics, hence their commercial dominance.
The electrification of the transport sector is one area that is being developed to bring about carbon-neutral transport, but it’s possible that many of these electric vehicles (EVs) will use batteries that are not Li-ion in the future. While many Li-ion batteries are currently employed in EVs because of their adaptability to existing technologies, the range and limited energy density of Li-ion batteries could see other batteries being developed for future use.
It’s unlikely that something will change in the near future because of the existing manufacturing and R&D facilities for Li-ion batteries, but there could be other batteries developed over time that become more commercially feasible for electric transport applications—especially as more and more countries are looking to adopt EVs on a larger scale.
It’s possible that the move may involve looking into other lithium battery types where lithium metal is used in the anode. These batteries have high theoretical capacities that could help to improve the energy density of rechargeable batteries severalfold. However, when lithium metal is used, the anode needs to be really thick, and this often offsets any gains in the specific energy of the battery and increases the cost beyond commercially feasible levels. One of the other options that have been emerging is to use anode-free batteries, and researchers have now created an anode-free battery that uses a lithium sulphide cathode that could theoretically be suitable for transport electrification applications.
Anode Free Lithium Batteries
Anode-free batteries have emerged as an alternative and cost-effective technology in the renewable battery space. In anode-free designs, there is only a lithium-rich cathode that is attached to a metal current collector directly. When the battery is charged, active lithium ions are released from the cathode and deposited onto the metal current collector—which acts as a temporary anode. This allows the lithium discharge to behave in a similar manner (chemically) to Li-ion batteries.
This approach is advantageous as far as maximising the specific energy and energy density is concerned, and the lack of anode allows these battery systems to be produced at a lower cost and with a lower degree of complexity compared to lithium metal anode options. However, there are no extra lithium sources in anode-free batteries, so there is no way to replenish the irreversible loss of lithium ions over time during cycling. Therefore, for anode-free batteries to succeed in any capacity (fundamental research or commercially), the cathode needs to possess as high lithium content as possible so that a high energy density can be maintained for long periods.
The Utilisation of Lithium Sulphide Cathodes
While several different lithium-rich cathodes could be used, one of the more promising candidates is lithium sulphide (Li2S) cathodes because they can possess up to a 66.7% lithium content, so they are seen as one of the best potential candidates. With a full lithiated structure, these cathodes experience a negligible level of volumetric expansion when cycling, so they have the potential to be manufactured into high-load cathode and solid-state batteries.
Studies have shown that these cathodes can deliver a specific capacity of 1166 mAh g-1, and this could potentially create anode-free cells that have specific energies up to 2451 Wh kg-1 and energy densities up to 40668 Wh L-1. Compared to other electrodes, lithium sulphide has no oxygen molecules, so compared to other electrodes with some form of oxygen within their molecular structure, there are fewer side reactions with the organic electrolyte within the battery.
Despite their promise, cathodes composed of purely bulk lithium sulphide have tended to suffer from a high initial activation overpotential and slow charge kinetics because the ionic lattice is insulating and poorly soluble in an organic electrolyte. Ways are being sought to overcome this, including using other materials, such as nanomaterials, to overcome any issues with the cathode itself.
Developing A Quasi-Solid-State Anode-Free Battery Cell
Aside from ensuring that the cathode is loaded with a high concentration of lithium and trying to tackle some of the charge issues, ways are being sought on the electrolyte front, and there are plans to move away from using liquid electrolytes toward quasi-solid-state electrolytes that consist of a liquid electrolyte in a solid matrix—i.e., a polymeric gel material.
The quasi-solid-state anode-free batteries developed by researchers from China have used oxygen-free lithium sulphide cathodes and a non-flammable composite gel polymer electrolyte (CGPE) that has a fast ion transport, good thermal stability, and fire retardancy. Instead of using pure lithium sulphide cathodes, the researchers decided to integrate a high density of MXene material into the cathode.
In principle, anode-free cells follow similar lithium pathways as Li-ion batteries. These pathways involved soluble lithium polysulphide ions as the redox intermediates to convert the lithium sulphide into sulphur and lithium metal. The electrolyte and cathode combination prevents uncontrolled exothermic reactions of reactive oxygen and excess lithium ions from occurring at the cathode surface. This prevents the polysulphide ions from shuttling out of the cathode and creating any irreversible and inactive molecules on the surface of the cathode, and it stops any dendrites from forming within the battery.
The addition of MXenes into the cathode helped alleviate some of the activation and charge issues present within pure lithium sulphide cathodes by utilising the electron-withdrawing sites of the MXene materials. The lithium sulphide-MXene matrix is composed of polar groups (groups with an electric dipole moment), which ensures that the battery has a high redox activity, making the battery charging process more efficient. The batteries developed so far using this method have exhibited an energy density of 1323 Wh L-1 at the pouch cell level.
While the energy density is not yet anywhere near the theoretical potential of lithium sulphide cathode batteries, the results so far have had a good start while simultaneously tackling some of the challenges associated with other anode-free batteries that rely on liquid electrolytes. The battery developed so far is effective at directing smooth lithium plating and stripping to prevent dendrite growth and is also effective in reducing the occurrence of polysulphide shuttling within the battery. The battery has a long life, a good reversibility and has been found to exhibit a low level of self-discharge when it is subjected to abuse tests, so there is a good platform to work from for other quasi-solid-state anode-free batteries.
Qiu J. et al., Development of quasi-solid-state anode-free high-energy lithium sulfide-based batteries, Nature Communications¸ 13, (2020), 4415.