MOFs Help to Form a Safer Horizon for Li-ion Batteries

While lithium ion (Li-ion) batteries are the preferred battery format in today’s marketplace, few people actually realize that they can be a major fire hazard when used in some applications.

Figure: Simulated structures of the MOF PCN-250 being loaded with ionic liquids

  

There have been several notable aircraft and vehicular fires associated to their use.  Due to their high energy densities, low self-discharge rates and low maintenance requirements, other less hazardous rechargeable cells such as Ni-Cad and NiMH batteries have not found a significant following. Dr. Noppadon Sathitsuksanoh at the University of Louisville added novel non-flammable liquid electrolytes to Metal Organic Frameworks, often called MOFs, to demonstrate a new and safer battery design.

We were looking for a porous material that could promote unifom Li-ion transport & suppress the formation of dead Liitium, and we found it with MOFs.

 Dr. Noppadon Sathitsuksanoh of the the University of Louisville

Enter MOFs: Metal Organic Frameworks are crystalline 3D structures with an extraordinary internal surface area that transfers into an incredible ability to absorb molecules.  Dr. Sathitsuksanoh and framergy’s Ray Ozdemir encapsulated ionic liquids into the channels of the MOF PCN-250, commercially available from Strem Chemicals and marketed as AYRSORBTM F250. Here the impregnated MOF serves as membranes to improve lithium ion conductivity and the lithium transference number. 

Li-ion battery safety issues typically relate to the thermal stability of the constituent materials. Li-ion batteries are thermodynamically unstable and the compatibility of the battery components is kinetically attained with the presence of the surface passivation films on the electrode surface.  The decomposition of this solid electrolyte inter-phase layer, resulting from the electrochemically reductive decomposition of the electrolyte on the graphite anode, can initialize exothermic reactions between the lithiated graphite anode and the electrolyte. 

When a cell is heated above a certain temperature, exothermic chemical reactions between the electrodes and the flammable electrolyte can occur and lead to an increase of the internal cell temperature.  The overcharge of Li-ion batteries can lead to chemical and electrochemical reactions between battery components, gas release, and a rapid increase of the cell temperature. Fire hazards occur due to thermal and overcharge abuse.

When PCN-250 with ionic liquids was tested on LiFePO4/Li electrode, the novel assembly showed very promising results with over 95% coulombic efficiency for this solid-state battery at preliminary stage. The charge-discharge and cycle test using coin-type cells at different loadings exhibited excellent rate capability with capacity of 110 mAh/g at 5mA/g current density.

The team also developed a novel membrane technology with high ion-selectivity derived by MOFs and  based on functional layers to prevent unwanted crossover while improving performance in non-aqueous redox flow battery architectures. For this technology and product development effort, MOFs were utilized in two distinct ways to provide a new cell architecture with a highly conductive pathway for lithium ions while restricting the influence of shuttle effect from lithium polyselenides, which irreversibly degrades the battery performance.

This approach to the functional layer on commercial battery membranes was shown to prevent the shuttle effect of lithium selenides in Li-Se battery architectures and improve battery performance. When activated carbon (AC) was used as a control, the [email protected] cathode had a high initial discharge capacity of 800 mAh/g and progressively faded. The high initial capacity of [email protected] is because of a high surface area and pore volume of AC. However, AC does not have metal oxides in the particles to suppress the shuttle effect. Hence, a progressive decrease in capacity was observed.

The University of Louisville and framergy successfully generated preliminary data demonstrating the feasibility of incorporating certain ionic liquids in select MOFs such as framergy’s AYRSORB™ F250 as well as casting functional MOFs on a commercial Celgard battery membranes. The MOF-based cell architecture potentially improves safety and cycle performance of Li-ion batteries by suppressing the Li dendrite formation and shuttle effect.  This and the other breakthrough focused on safety will be needed to help convert the world to electrification.  

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