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Nanoscale 3D Printing of a Lithium Ion Battery: Rethinking the Fabrication Concept for a Revolution in Energy Storage

Periodic Reporting for period 2 - NANO-3D-LION (Nanoscale 3D Printing of a Lithium Ion Battery: Rethinking the Fabrication Concept for a Revolution in Energy Storage)

Reporting period: 2022-08-01 to 2024-01-31

The current global need in new rechargeable batteries that can combine both high storage capacity and high power characteristics (possibility to charge very fast and/or deliver high power when needed) is an enormous challenge. A battery that can hold sufficient amount of energy but could be charged within a matter of a few minutes or even seconds is very attractive for a range of different applications, from electronics to automotive, where battery charging time remains a major bottleneck. Despite the tremendous progress in materials engineering it is fundamentally difficult to achieve both of these characteristics, namely high energy density and high rate characteristics. This project aims to approach this challenge by re-engineering the battery design: instead of traditionally planar battery arrangement, the overarching idea in NANO-3D-LION is to develop a battery with three-dimensional battery electrodes. If this 3D battery is fabricated with features with dimensions at low microns scale or below, there is an enormous gain in the available surface area and significant drop of resistance between battery electrodes, allowing to achieve almost the same energy density as in a traditional battery design but with a significantly improved rate characteristics. Until now, there has been simply no technology to test this hypothesis, since fabrication of such battery has been virtually impossible. The current project is aimed to overcome this challengy and is now taking advantage of high resolution electrochemical 3D printing technology that we develop to fabricate structures of both the cathode and the anode for a lithium-ion battery and test this concept of a 3D battery. There is a range of challenges to be resolved. First, the electrochemical 3D printing technology has to be brought to a sufficient performance in terms of resolution and print speed. Second, 3D printing battery materials, and especially at the small scale (from micrometers and down to nanometers) is intrinsically difficult. Third, it is even more challenging to fabricate simultaneously both, battery cathode and anode, given the requirements to avoid short circuit between electrodes. Finally, this new concept of the 3D battery has to be tested and this 3D design optimised. NANO-3D-LION project aims to approach all these challenges and (i) develop new 3D printing technologies, with yet unattained characteristics in terms of resolution and print speed; (ii) develop printing approaches for fabricating battery materials and test how these materials perform; (iii) find ways to fabricate a complete battery and test the validity of this new battery engineering concept.
The following work has been performed from the beginning of the project.

Work package 1. Nanoelectrochemical 3D printing platform
Objective: to build an advanced nanoelectrochemical platform for 3D printing
The goals of the WP are completed and the group now own 5 custom-built 3D printing instruments, including:
1 instrument built inside Ar glovebox
1 instrument built on top of an inverted optical microscope
3 instruments with slightly different capabilities in terms of speed, resolution and print head movement range.
Acquisition of 3 more instruments than planned initially was possible due to the possibility to transfer/re-purchase equipment from the previous employer of the ERC StG grantee.

Work package 2. Pushing the boundaries of 3D printing techniques towards nanoscale
Objective: to develop new and advance existing approaches for electrochemical 3D printing at the nanoscale
Task 2.1 Advancing meniscus-confined electrochemical printing to unprecedented resolution
The objective of the Task 2.1 are fully completed. We developed a new meniscus-confined electrodeposition (MCED) feedback that allows to automate the print process by breaking down the deposition process into small steps, each with a duration of about 2-5 ms. This allows printing of only small layer of material (typically, a few nm thick), after which the nozzle is retracted and reapproached to the same location. By repeating the process multiple times voxels with dimensions of a few hundred nanometers, down to 60 nanometers could be fabricated. We also tested fabrication of tilted structures and overhangs. For the latter, the print process is changed and the retraction occurs in horizontal direction and not vertical, allowing to print nanoscale structures reliably. Finally, we used nozzles with openings as small as 0.9 nm. These are so small that only a single hydrated copper ion can pass through this opening at a time. Our printing results now indicate that voxels as small as 25 nm could be printed by this technology. We also performed FIB cuts on nanofabricated structures to ensure that they are fully dense and contain only a minimal amount of defects and analyzed the chemical purity of the produced metallic structures using EDX. These results indicated that the structures contain no other impurities and the purity of the fabricated metallic features depends almost exclusively on the purity of the electrolyte solution used for electrochemical printing. The results of this work are now published as a peer-reviewed article.

Work package 3. (Electro)chemical conversion of printed features into functional electrode materials
Objective: fabrication of lithium ion battery through conversion of 3D printed nanoscale metal features into active battery materials
Task 3.1. Electrochemical conversion of printed metal features into battery electrodes
Our efforts so far have been focused mainly on LIB anodes, in particular, Sn, since this material can be relatively easily electrodeposited but also reveals very high performance in terms of energy density. At first, we attempted to fabricate Sn features using MCED with a citrate-containing and methanesulfonate-based electroplating solutions. In either case, printing, however, appears to be difficult due to either instabilities of the meniscus or due to the ingrowth of the Sn into the nozzles causing improper printing feedback response. This issue is causing inconsistent diameters of the printed Sn nanowires with dimensions significantly smaller than those expected otherwise.
To overcome this limitation, the team is now developing a new technique, where printing occurs in liquid electrolyte. In this case, an electrochemical cell consists of 3 electrodes: one quasi-reference counter electrode (QRCE) in the nozzle, one QRCE in the solution bath, and a working electrode (substrate). The nozzle is connected also to a pressure controller capable to deliver constant overpressure value from ca. 1 mbar onwards. The nozzle in this case contains the electrolyte with precursor metal ions, whereas the electrolyte outside the nozzle is metal-ion free. By controlling the overpressure using the pressure controller the user can vary the rate of delivery of the metal ion precursor solution towards the biased substrate, where electrodeposition occurs, whereas the growth is monitored by measuring the magnitude of the ionic current between the QRCEs: the growing voxel causes a drop in this ionic current when the growing metal feature approaches the nozzle opening.
By using this approach we already managed to achieve printing using Cu as a proof-of-concept material and with the addition of EDTA as a scavenger ions to the bath electrolyte it becomes possible to bring the resolution of this technology down to 40 nm. This is achieved by using very small nozzles (ca. 30 nm opening). Although the resolution is not as high as in case of MCED, this new approach allows almost complete design freedom, which is unattainable for MCED. The developed approach has also extensive support by COMSOL simulations, which allow fine tuning of the process and investigation of the influence of a multitude of process parameters on the resulting feature geometry and fabrication speed. We are currently preparing a publication on this work. The developed approach is also very relevant to the techniques described in Work Package 4 (task 4.1) although the delivery of the precursor ions in this case is driven by both pressure driven flow and electromigration of ions, and the interplay between these two processes strongly depends on nozzle size. With smaller nozzle openings, pressure driven flow becomes inefficient (as a consequence of Poiseuille law) and the ions are delivered by electrophoretic flux induced by the applied potential difference between QRCEs.
A particular challenge with this technology was related to the fact that at these dimensions (with voxels <180 nm in diameter), the structures can easily get damaged when they are taken out of the electrolyte solution, with the main issue being a strong surface tension at the air-water interface. To overcome this limitation, we developed an approach, where the solution is gradually changed from salt-containing electrolyte to ultrapure water and then slowly changed to an organic solvent, such as isopropanol, with significantly smaller surface tension. This allowed us to avoid the aforementioned damage to the structures.
With these developed approaches we are now experimenting with Sn deposition. So far this allowed us to grow Sn nanostructures but at the moment Sn deposition suffers from irreproducibility in print rates, requiring to reconsider and optimise the electrolyte composition.
Progress beyond the state of the art
To the moment the group has made two major breakthroughs in 3D printing of metals at a small scale. One achievement is the 25 nm resolution mark for printing Cu nanostructures, which is more than an order of magnitude improvement over existing techniques and is a current world record. Another, is a 40 nm resolution achieved with new printing technology inside the liquid electrolyte. The latter, although slightly less powerful method in terms of resolution, is a very powerful approach to fabricate structures of almost unlimited complexity, which we demonstrated by printing lattice structures with nanoscale critical dimensions.

Expected results until the end of the project
We plan to publish the results of our latest developments in technology and resolution already in the near future. The other major direction in the technology development is to develop a multi-nozzle printing process in order to achieve drastic increase in the printing throughput.
On the other hand, there is a significant interest in the current project to develop LIB materials. For example, the current development in printing Sn anodes for LIBs is very promising and we plan to fabricate Sn nanostructures and test their performance as LIB anodes.
In parallel, our future work on printing cathode materials will be focussed on printing Fe micro- or nanostructures, converting the outer layer of Fe into FePO4 and then testing the performance of this cathode material.