Sulfide and oxide based electrolytes for all solid state batteries

Lead Research Organisation: University of Oxford
Department Name: Materials


Current lithium ion battery technology faces challenges to improve energy density and battery safety. Implementing lithium metal anodes would be a way of increasing energy density. However, dendrite formation and short circuiting upon cell cycling presently impede its use. Furthermore, the flammability of the employed liquid electrolytes poses a severe safety hazard. Replacement of organic liquid electrolytes by solid electrolytes could pave the way for using lithium metal, simultaneously improving energy density and battery safety.

Large and thin pinhole-free sheets of solid electrolyte are required for high power devices, but these are challenging to make with brittle ceramic materials. Additionally, the interfacial contact with the electrodes must be maintained upon cell cycling, which requires chemical compatibility and a certain degree of flexibility of the system components. Also, volume changes of the electrodes may occur during cell cycling and must be tolerated by the solid electrolyte to avoid fracture.

Solid electrolytes are required to be good lithium ion conductors with low bulk and grain boundary resistances. High density helps achieve optimum conductivity. Good ionic conductivity has been reached with oxide and sulfide glass-ceramic electrolytes. In addition, sulfide based electrolytes have favorable mechanical properties which allow the material to be applied in an all solid state battery without requiring heat treatment during assembly of the cell. Nonetheless, oxide based electrolytes are equally promising, as they are somewhat more tolerant against air and moisture than most sulfide systems. However, their use in all solid state batteries with thick composite electrodes involves high sintering temperatures and remains a challenge.

Certain protective coatings on electrode materials are known to improve cycling behavior. However, the cause for the improvement is still poorly understood at the scale of the interface. This project will investigate the fundamental requirements of maintaining a working interface between the electrode and electrolyte materials, which is critical for a functioning all solid state battery. Solid state characterization of the materials will be critical to this work. Combining state of the art solid state analysis methods, including electron microscopy, XRD, electrochemical impedance spectroscopy and mechanical testing, will be crucial to providing a holistic insight into the mechanisms of solid state battery chemistry. This combined approach provides pathways to an in-depth analysis of the electrolyte-electrode interfaces in order to develop better performance all solid state batteries.

Energy, Engineering, Manufacturing the Future, Physical Sciences


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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509711/1 01/10/2016 30/09/2021
1801932 Studentship EP/N509711/1 01/10/2016 31/03/2020 Gareth Hartley
Description The capacity of lithium ion batteries is currently limited by the innate chemistry of intercalation electrodes. Replacing these electrodes with lithium metal would increase battery capacity by x 10. This could massively increase the range of electric cars and the battery life of electronic devices. However, lithium metal is incompatible with all known liquid electrolytes. The development of solid-state electrolytes could enable these inherent problems to be avoided. However, few solid electrolytes are currently known which have good cycling stability with lithium.
In this project, it was shown that there are two separate mechanisms which contribute to the failure of a solid state batteries employing a lithium metal electrode with a solid-state electrolyte; therefore, the maximum rate at which a battery can be charged differs from the max rate of discharge. This work revealed that pressure has a profound effect on performance and therefore all studies of ASSBs must control pressure for conclusions to be drawn. This is an important result for the field.

Solid-state batteries show mechanical incompatibility with cathodes due to volume changes. Therefore, hybrid batteries using a solid-electrolyte to protect a lithium anode and a liquid electrolyte to avoid volume change induced cathode problems, were developed. This work revealed that a junction between a liquid electrolyte and a solid electrolyte has considerable resistance and that this resistance is in part innate but is also increased due to reactions forming an resistive layer.

Nitrogenous solid-electrolytes have unique interfacial stability against lithium but they have low ionic conductivity so cannot be used for fast charging batteries. Sulfide based solid electrolytes have good ionic conductivity but poor stability against lithium. As such, part of my research was focused on creating an solid-electrolyte with good interfacial stability against lithium metal in addition to high ionic conductivity. The objective of this project was to find a method of producing a nitrogenous sulfide based solid-electrolyte (lithium nitridothiophosphate) to show whether the beneficial properties of both solid-electrolyte classes could be combined.
I showed that the ball milling of Li3N and P2S5 did not produce a nitrogenous solid-electrolyte as previously thought. Instead, I showed that the reaction produced a mixture of known sulphide based solid electrolytes. This demonstrated that solid-electrolytes produced through this reaction would not be good solid-electrolyte for lithium metal batteries.
Exploitation Route Consequently, I met my objective of developing a fundamental understanding of the science but unfortunately this method does not satisfy the overall goal of producing a world beating battery. The outcomes of this project will benefit companies who are investigating the development of new battery technology and will also inform researchers more generally of the reactions of lithium nitride. However, my research indicates that if the lithiumnitridothiophates can be made by a different method, they would be very interesting materials for future battery research.
Sectors Chemicals,Electronics,Energy,Manufacturing, including Industrial Biotechology,Transport

Description My current research is focused on enabling lithium metal to be used as the negative electrode in a battery. Achieving this goal will mark a step change in the battery field and will enable batteries with significantly improved capacity which will in turn increase the range of electric vehicles. Through this award, it was discovered that pressure plays a considerable role in the failure mechanism of solid state batteries employing the lithium electrode. When a battery is discharged, voids form in lithium at the interface with the solid electrolyte. The resulting decreased interfacial contact causes the polarisation of the cell to increase, until either the critical plating current for dendrite formation is reached and a short-circuit failure is induced, or electrochemical degradation of the cell occurs, or the voltage limit of the cell is exceeded. It was found that, increased pressure can somewhat suppress void formation and increase the critical stripping current. This breakthrough in understanding clearly has important implications for the engineering of solid-state batteries for commercial cells and is directing the research activities of the field towards, alloying and temperature investigations. As such, this research was published in Nature Materials.
First Year Of Impact 2019
Sector Electronics,Energy,Environment,Government, Democracy and Justice,Transport
Impact Types Societal,Economic,Policy & public services