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Credit: Taro Hitosugi and Tokyo Institute of Technology
Scientists at the Tokyo Institute of Technology have examined the mechanisms behind the resistance at the electrode-electrolyte interface of the batteries entirely in the solid state. Their discoveries will help develop much better Li-ion batteries with very fast charge / discharge rates.
The design and improvement of lithium-ion (Li-ion) batteries is essential to push the limits of modern electronics and electric vehicles, as Li-ion batteries are virtually ubiquitous. Scientists at the Tokyo Institute of Technology (Tokyo Tech), led by Professor Taro Hitosugi, had previously announced a new type of battery entirely in the solid state, also based on lithium ions. , which solved one of the major problems of these batteries: resistance to the interface between electrodes and electrolytes that limits the rapid charge / discharge.
Although the devices produced were very promising and far superior to conventional Li-ion batteries in some respects, the mechanism behind the reduced resistance of the interface was not clear. It was difficult to analyze buried interfaces in fully solid state batteries without damaging their layers. As a result, Hitosugi and its team of researchers again investigated fully solid batteries to shed light on this topic. They suspected that the crystallinity (which indicates how well a solid is ordered and periodic) at the electrode-electrolyte interface played a key role in defining the resistance of the interface.
<a href = "https://3c1703fe8d.site.internapcdn.net/newman/gfx/news/hires/2018/1-makingitcrys.jpg" title = "Batteries were manufactured by stacking thin layers of Au (collector current), LiCoO2 (cathode), Li3PO4 (solid electrolyte) and Li (anode) on an Al2O3 substrate (A) Schematic cross-sectional view and (b) Microscopic photograph of the thin-film battery, Credit: Applied materials and interfaces ACS">
The batteries were fabricated by stacking thin films of Au (current collector), LiCoO2 (cathode), Li3PO4 (solid electrolyte) and Li (anode) on an Al2O3 substrate. (a) schematic sectional view and (b) microscopic photograph of the manufactured thin film cell, Applied materials and interfaces ACS
To prove it, they made two different, fully solid batteries, composed of electrode and electrolyte layers using a pulsed laser deposition technique. One of these batteries probably had high crystallinity at the electrode-electrolyte interface, while the other did not. To confirm this, it was possible to use a new technique called diffusion analysis of X-ray crystal truncation. "X-rays can reach buried interfaces without destroying structures," says Hitosugi.
Based on their results, the team concluded that a highly crystalline electrode-electrolyte interface resulted in low interface resistance, producing a high-performance battery. By analyzing the microscopic structure of the interfaces of their batteries, they proposed a plausible explanation of the increased resistance of batteries with less crystalline interfaces. Lithium ions are blocked at less crystalline interfaces, which affects the conductivity of the ions. "Controlled fabrication of the electrolyte / electrode interface is crucial to achieve low interface resistance," says Hitosugi. The development of theories and simulations to better understand the migration of Li ions will be crucial to finally achieve useful and improved batteries for all types of devices based on electrochemistry.
<a href = "https://3c1703fe8d.site.internapcdn.net/newman/gfx/news/hires/2018/2-makingitcrys.jpg" title = "(a) During the discharge process, Li ions migrate to Through the solid electrolyte at the interface, the LiCoO2 film being oriented (0001), the migration of Li ions in LiCoO2 is hampered by the CoO2 layers aligned parallel to the surface of the substrate, resulting in a lateral migration of Li ions on the surface of LiCoO2., diffuse within the grain boundaries. (b) In the case of the disordered surface of LiCoO2, the diffusion of Li ions along the surface and within the grain boundaries is restricted, which gives a high resistance value to the interface. Applied materials and interfaces ACS">
(a) During the discharge process, Li ions migrate through the solid electrolyte to the interface. Since the LiCoO2 film is oriented (0001), the migration of Li ions into LiCoO2 is hampered by the CoO2 layers aligned parallel to the surface of the substrate. As a result, Li ions migrate laterally to the LiCoO2 surface and eventually diffuse into the grain boundaries. (b) In the case of a disordered LiCoO2 surface, diffusion of Li ions along the surface and within the grain boundary is restricted, resulting in a high interface resistance value. Credit: Applied materials and interfaces ACS
Explore further:
Extend the limits of Li-ion batteries – electrodes for fully solid batteries
More information:
Susumu Shiraki et al., A well-ordered structure at the solid electrolyte and the electrode interface reduces interfacial resistance. Applied materials and interfaces ACS (2018). DOI: 10.1021 / acsami.8b08926
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