Engineering topics

Zamani, Somayeh

Publications and source records attributed to Zamani, Somayeh.

3 records

DOE OSTI2020

Synergistic Effect of High Sulfur Loading Layered Cathode, Ceramic Separator and Gel Electrolyte

Lithium sulfur (Li-S) batteries are promising candidates for next-generation energy storage devices as they are capable of providing higher energy density and lower cost in comparison to the state of the art Li-ion batteries. To gain these characteristics, there are challenges such as polysulfide crossover, large volume changes during cycling, and deposition of insulating lithium sulfide that need to be alleviated. A popular approach to address these issues is to control the structure and properties of the carbon cathode in order to influence the transport of polysulfides, surface area for reaction, and lithium sulfide deposition. Ceramic separators on the other hand can drastically affect ion diffusion and overall battery performance. Safety concerns regarding the flammable liquid electrolyte can be reduced by deploying gel electrolyte. Towards this direction, we developed a layer-on-layer cathode structure with sulfur-impregnated activated carbon alternating with graphene layer, which was fabricated by Air-assisted electrospray. A much higher sulfur loading compared to slurry cast cathodes can be provided in this method. Combining the new cathode with gel electrolyte can offer higher safety of the battery and capacity retention. Ceramic separator would compensate the reduced ion conductivity of the gel electrolyte and the combination of these variables will be very promising in improvement of battery performance and safety.

25 ENERGY STORAGE↗

DOE OSTI2020

Polysulfide entrapment and retardation in gel electrolyte Li–S batteries: experiments and modeling

Gelling the electrolyte in lithium–sulfur (Li–S) batteries provides advantages in terms of safety and mitigating polysulfide shuttling leading to reduced side reactions and dendrite growth on the anode side. In the current study, organic and ceramic crosslinkers have been used to improve the entrapment of polysulfides and prevent them from shuttling to the anode side. However, the performance of gel electrolytes is still far from being on par with liquid electrolytes in terms of raw capacity. The discharge curves exhibit an extended first plateau for gel electrolyte batteries compared to liquid electrolytes, showing that polysulfides are better trapped in the gel system. Yet, in the conversion of soluble polysulfides into insoluble polysulfides, the second plateau was delayed. Qualitative and quantitative analysis of the polysulfide diffusion in the liquid electrolyte and gel electrolyte systems were also conducted to predict the relative diffusivity ratio between the liquid electrolyte and gel electrolyte for simulation. To probe what happens at the end of the discharge, where the mass transport in the gel electrolyte appears to be slow, a modified discharge procedure was used where the cell is first paused after discharge and then discharged a second time before recharging normally. Coin cell tests for two gel electrolytes in addition to liquid electrolyte show up to 7% capacity recovery after the pause, which indicates that the diffusion of species during the pause is responsible for the failure recovery. Additionally, the recovery is higher for gel electrolytes compared to liquid, for higher C-rates, and for longer pause times. To better understand this behavior, a Li–S numerical model with mass transport limited reactions was used to examine the polysulfide diffusion coefficients expected in different electrolyte systems. The model is able to reproduce the trends seen in experiments and yields a higher recovery for smaller Li 2 S 4 diffusion coefficients, suggesting that insufficient Li 2 S 4 mass transport is responsible for failure at the end of discharge. By identifying the effect that gel electrolytes have on the state of the battery during discharge using both experimental and simulation methods, this work is a step towards leveraging the advantages of gel electrolytes and mitigating their weaknesses.

25 ENERGY STORAGE↗

DOE OSTI2019

Investigation of Gel Polymer Electrolyte Failure Mechanism in Lithium-Sulfur Batteries

Lithium sulfur (Li-S) batteries are a promising candidate for next-generation energy storage due to their high theoretical energy density. However, well-known challenges such as polysulfide crossover, large volume changes during cycling, lithium dendrite growth, and passivation of reaction sites limit the capacity and capacity retention of Li-S batteries. One approach to address these issues is to use gel polymer electrolyte (GPE) in place of the typical liquid electrolyte. It is believed that GPE can reduce the dissolution and diffusion of the intermediate polysulfides and enhance the capacity retention in Li-S batteries, but additional research is needed. In particular, the difference in failure mechanisms between GPE and liquid electrolyte cells is not well understood, which is important knowledge for the rational design of high performance GPE cells. We have investigated the effects of gel polymer electrolyte on lithium-sulfur coin cell failure mechanisms. In particular, we used a discharge test with pauses to probe how the choice of electrolyte affected the amount of capacity recovered due to the pause. After discharging normally until failure, the cell was paused for some time, after which discharge was restarted until the second and final failure. Gel polymer electrolyte was found to have a higher percentage capacity recovery after pausing than liquid electrolyte, and that recovery improved at higher C-rates. Large percentage capacities recoveries suggest that mass transport limitations are leading to cell failure rather than cathode passivation as is frequently proposed. To help explain this behavior, we use a 1-dimension + time numerical continuum model for the Li-S full cell that includes the pausing step. The model includes the electrochemical reactions and precipitation of sulfur and intermediate polysulfide species, transport of species through the battery, and the effect of mass transport limitations on cell failure. The simulations show that variations in the bulk polysulfide diffusion coefficients can explain many of the differences between GPE and liquid electrolyte experimental results, and further confirm that mass transport limitations are a critical obstacle in improving the performance of GPE Li-S batteries.

25 ENERGY STORAGE↗