Solid-state electrolytes for lithium-ion batteries

Interest in alternative energy conversion has increased due to ongoing depletion of conventional fossil fuels and the most recent trend of de-carbonization. Lithium-ion batteries (LIBs) have initially been created for the commercial market to power portable electronics by Sony Co. in 1991. Since then, LIBs have undergone a tremendous improvement to become ubiquitous in everyday life, raging from smartwatches to electric cars. The cathode of a LIB supplies Li ions, whereas the anode stores Li ions during charging. The Li ions are carried by an electrolyte between the two electrodes. The safety of LIBs are inherently questioned due to the unstable interaction between the organic electrolyte and the anodes. Moreover, the requirements towards energy density, storage capacity, thermal stability, and manufacturing cost are constantly increasing. Many of these challenges can be addressed with replacing the existing liquid, organic electrolytes with solid electrolytes.

Solid-electrolytes for LIBs are generally distinguished in three main categories: 1. Ceramic (e.g., garnet, LZZO, NASICON, LATP, etc.), 2. Polymer (e.g., PEO), and 3. Composite [e.g., PEO:LLZO]. Solid-state electrolytes have the primary benefit of not being combustible, corroding, or causing internal short circuits. They also act as internal separator between electrodes while resisting dendrite growth. Most of the above stated properties can be found in ceramic solid-state electrolytes. In our group, we focus on synthesizing LLZO, LATP, and LYZP for LIBs via spray-flame synthesis (SFS).

SFS is a well-studied alternative (to solid-state reaction, sol-gel reaction) method to synthesize ceramic/oxide materials as powder/thin films. As a continuous synthesis technique, the important materials properties such as phase composition, particle-size distribution, surface area, stoichiometric ratio, etc. can be kept constant throughout the reaction. As precursors, we use cheap and readily available nitrates. As solvents, we use, e.g., ethanol or non-polar toluene. Depending on the nature of the desired materials the precursors, solvents, and fuels (e.g., methane, oxygen flame), reactor pressure, etc., can be tuned. For example, in our lab we have used lithium nitrate, aluminum nitrate, lanthanum acetate, and zirconium-tetra-propoxide in a propapanol solution to synthesize LLZO. [i]

Although LIBs manage to deliver a lot compared to the current market demand, limited Li source in the earth crust, high cost, etc. propels the community to find other alternative sources such as Na ion battery (SIBs).

[i] M. Y. Ali, H. Orthner, H. Wiggers, Spray-flame synthesis (SFS) of lithium lanthanum zirconate (LLZO) solid electrolyte materials 14 (2021) 3472

Silicon-based anode materials for lithium-ion batteries

With a growing demand for lithium-ion batteries for mobile devices and electromobility, high-energy-density battery materials are of great interest to reduce the overall weight and size and to increase the capacity and long-term stability. Partial or even complete replacement of graphite, the active component on the anode side of lithium-ion batteries, by silicon is the near-term option to meet the industry demands. Partially substituting graphite (specific capacity of 372 mAh/g), which is still the most common anode material in use today, by silicon (3579 mAh/g) can significantly increase the anode capacity. To ensure the required electrochemical and mechanical stability, silicon particle size should be preferably limited to the sub-micron range. On the other side – due to unwanted but unavoidable parasitic surface reactions – the silicon particles should have a low specific surface area to enable high first-cycle Coulombic efficiencies.

In coarse silicon particles, the mechanical stress during charging and discharging cannot be compensated, and the particles crack due to the enormous volume expansion of up to 300 % during lithiation. Cracking causes poor cycling performance as new surfaces are repeatedly generated leading to additional parasitic reactions, and the overall performance and electrode integrity are lost. This can be mitigated by the use of amorphous nanoparticles as opposed to the crystalline ones. Also, size control of silicon nanomaterials for battery applications is crucial. In addition, surface passivation/functionalization is a further way to reduce the unwanted surface reactions.

Hot-wall reactor synthesis:

In the Nanoparticle Synthesis group, various materials are developed to overcome these issues. For example, SiN_x materials form a stable matrix phase during cycling, which enables a greatly increased lifetime of the battery. a-Si:C materials are also investigated, which show a similar behavior for high carbon contents. Furthermore, pure silicon particles or doped materials can also be produced and optimized for battery applications. All these materials can already be produced in the kilogram scale in our labs, and upscaling to industry scale has been shown by a cooperation. Furthermore, these materials are also to be used in solid state batteries.

EMPI-RF Publications

Plasma-reactor synthesis:

The cost per kilogram is a very important property when it comes to materials for applications. In radio-frequency plasmas, silicon nanoparticles can directly be produced from metallurgical silicon. Enabling the usage of this cheap precursor material gives a good opportunity for a cost-effective and scalable synthesis method of a well-defined nanomaterial for battery applications.