HIGHLIGHTS
• Solid-state lithium electrolytes utilize a polymer binder to impart structural and mechanical stability.
• Differences in the molecular weight of polyisobutylene, used as a polymer binder, affected the structural integrity, grain boundary resistance, current density and strength of the flexible thin film electrolytes produced.
• A higher molecular weight polymer may enable a solid-state lithium battery to double its energy storage to 500 watt-hours per kilogram.

The vulnerability of liquid electrolytes used in lithium-ion batteries has been well established. Organic solvents are used that are highly flammable when subjected to high temperatures. 

Solid-state lithium batteries are emerging as a viable option due to the electrolyte being non-flammable, leading to much safer operating conditions. As a result, research to commercialize solid-state lithium batteries has accelerated.

“Sulfide solid-state electrolytes match the ionic conductivities of the liquid electrolytes used in lithium-ion batteries. These materials establish conductive paths that facilitate the movement of lithium ions during the charge and discharge cycles,” says Dr. Guang Yang, an R&D associate at Oak Ridge National Laboratory in Oak Ridge, Tenn. “Sulfide solid-state electrolytes resemble sand initially. These are loosely packed, making them less effective as ion conductors and challenging for battery manufacturing. The key hurdle is integrating these loosely packed solid electrolytes, which consist of particles ranging in size from a micron to several tens of micrometers, to efficiently shuttle lithium ions between the cathode and the anode.”

The commonly used approach of compressing the solid-state electrolyte using a hydraulic press results in a more densely packed composition, which is predominantly suitable for lab-scale experiments. Yang notes, “While this method can enhance the performance of solid-state electrolytes, it does not scale effectively for commercial applications limiting its potential utility.”

A better strategy is to utilize a slurry process to incorporate the solid-state electrolyte with a polymer binder in a solvent to form thin film flexible solid-state lithium-ion battery electrolytes through a tape-casting method, which has been widely adopted by the lithium-ion battery industry. The polymer binder acts to impart structural and mechanical stability, which can be provided either by adhesive forces or through entanglements.

Thin films of the solid-state electrolyte can be produced by first slurrying the solid-state electrolyte powder, and polymer binder in a solvent followed by casting using a blade or slot die casting method. Yang explains, “High molecular weight polymer binders behave like long chains of spaghetti, which when highly entangled among each other, can hold embedded electrolyte particles securely within the entanglement matrix. This interaction is akin to meatballs dispersed through spaghetti. Such a structure provides excellent mechanical properties, thereby facilitating the development of flexible sheet-type solid-state electrolytes.”

Figure 5 shows a schematic of a thin film solid-state electrolyte. 


Figure 5. Flexible solid-state battery electrolytes are safer and now exhibit comparable ionic conductivity to that of liquid electrolytes. New research found that the molecular weight of the polymer binder used in the electrolyte is an important factor in affecting solid-state battery performance. Figure courtesy of Oak Ridge National Laboratory.

Polymer molecular weight appears to play a role in the mechanical and chemical interactions that can occur in forming the slurry and in the resulting performance of the solid-state electrolyte film separator. Yang and his colleague conducted a study to assess how changes in the molecular weight of a model polymer binder impact solid-state electrolyte performance. 

Polyisobutylene
The polymer binder chosen in the study by the researchers was polyisobutylene. Yang says, “We selected polyisobutylene because it does not react with the solid-state electrolyte particles, exhibits low polarity and is electronically insulating. In contrast, other polymer types contain side chains functionalities that can react with the sulfide, solid-state electrolyte.”

Slurries were prepared using a sulfide based solid electrolyte that also contains lithium, phosphorus and chlorine and is known as Li6PS5Cl (LPSCl). Polyisobutylene with molecular weights of 400, 850 and 1,270 kilograms per mole were used at 5 wt.%. This polymer loading was used because it enables creation of robust films for all three polyisobutylene molecular weights. 

Differences in molecular weight affected the structural integrity, grain boundary resistance, current density and strength of the resulting flexible thin films produced. For the 1,270 kilogram per mole, molecular weight polymer, the structural integrity of the flexible electrolyte film is higher, the grain boundary resistance is also higher, and critical current density is lower than lower molecular weight polyisobutylenes. Yang says, “Critical current density is the maximum current which the electrolyte is subjected to that does not lead to the formation of lithium dendrites. The latter if formed can cause a battery to short circuit which will cause a fire to be generated.”

Further testing was conducted with a full cell solid-state battery containing LPSCl electrolyte in combination with a NMC811 cathode that contains nickel, cobalt and manganese and a lithium-indium alloy. After 50 cycles, the low molecular weight polyisobutylene displayed significant capacity fade (70% initial discharge capacity) while the two higher molecular weight polyisobutylenes exhibited a gain in discharge capacity (120%). 

The higher molecular weight polyisobutylenes are better able to handle the volumetric changes and cracks detected during the continuous lithiation and delithiation of the NMC particles. In contrast, cracks are produced in the lower molecular weight polymer binder/solid electrolyte system leading to a loss in contact between cathode particles and the solid-state electrolyte resulting in inferior battery performance. 

Yang says, “The molecular weight of the polymer binder appears to be able to influence the ability of the lithium battery to store energy. Using a higher molecular weight polymer may lead to doubling energy storage in the solid-state lithium battery to 500 watt-hours per kilogram. This step will allow battery manufacturers to incorporate more energy into lithium batteries extending battery life between charges.”

Future work will involve gaining a better understanding about how longer polymer chains provide better performance than shorter chains. Yang says, “We also intend to evaluate other polymer architectures such as branching and explore different functionalities in the polymer backbone to advance binder development for both solid-state electrolytes and electrodes.”

Additional information can be found in a recent article¹ or by contacting Yang at yangg@ornl.gov. 

REFERENCES
1. Mills, A., Kalnaus, W., Su, Y., Williams, E., Zheng, X., Vaidyanathan, S., Hallinan, D., Nanda, J. and Yang, G. (2024), “Elucidating polymer binder entanglement in freestanding sulfide solid-state electrolyte membranes,” ACS Energy Letters, 9 (6), pp. 2677-2684.