Cathode Materials for Lithium-Ion Batteries

Conclusion:

Rheological measurements provide researchers with a reliable analytical tool for developing new formulations with improved performance and manufacturability. Understanding and controlling the slurry rheology helps in not only choosing an appropriate manufacturing process (roll-to-roll coating, slot-die coating, etc.) but also maximizes the production output to produce consistent, defect-free films of uniform coat weight and good contact with the electrode. These measurements can be used both in R&D and manufacturing settings owing to the DHR’s highly intuitive user-interface that reduces operator training times and increases productivity.

While there are remaining questions about the thermal runaway process in batteries, current understanding suggests that it is initiated by the following series of events. The exothermic reactions leading to thermal runaway interact destructively with every inner component of a Lithium-Ion Battery (LIB) as the battery’s temperature continues to rise; some elements are early casualties while most directly speed up heat accumulation as they fail.

The first component to begin breaking down is the Solid-Electrolyte Interphase (SEI), which generally starts around 80-120°C (176-248°F). At this point, Thermal Runaway can be slowed, but is no longer reversible once the anode is exposed to the electrolyte. Exothermic reactions occurring at the reactive anode surface add more heat into the system until it reaches the next critical temperatures.

The separator is the next component affected, and it fails in two stages. Around 120-150°C (248-302°F) the separator begins to melt and causes a small short-circuit, followed by a more serious internal short circuit when the separator breaks up near 220-250°C (428-482°F).

The following reactions happen quickly and directly following the previous temperature range; the cathode material, binder, and electrolyte all begin decomposition, which drastically raises the temperature of the battery cell to temperatures about 800°C (1472°F). These reactions have gas products which increase the pressure within the LIB.

Aside from quick heat generation, the cathodic reactions have a disastrous byproduct of oxygen which is flammable. Depending on the exact conditions, the immediate result is either “Heat + Oxygen = Fire” or “Heat + Gas = Rupture/Explosion”. Of course, not all materials are made equal and may fall high or low on these ranges – or even outside of these temperatures in the future – so it is essential to make the safest possible choice of materials for a given battery with proper testing.

TGA Thermogram Highlighting Thermal Instability of Graphite Anode Material

To avoid thermal runaway and select battery materials with optimal heat tolerances, battery researchers turn to Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA):

DSC: DSC measures the heat flow into or out of a material as a function of temperature or time. Phase changes interrupt the heat capacity relationship between temperature change and heat absorbed or released and are visible on graph output. It allows for testing in a variety of conditions ranging from safe operating temperature to thermal abuse.

TGA: TGA measures mass of a sample as a function of temperature or time. Generally speaking, a more thermally stable material can reach a higher temperature before any change in mass occurs.

Answer the following questions with results from your DSC:

• The material’s melting temperature, Tm
• The material’s glass transition temperature, Tg
• The lowest phase change temperature of various materials that comprise the battery.
  

Answer the following questions with results from your TGA:

• The temperature at which a material begins decomposition.
• The amount of sample mass lost to thermal or oxidative decomposition at a given temperature.
• The rate of decomposition reactions (both oxidative and thermally induced) at a given temperature.
• The maximum thermally stable temperature of the various materials that comprise the battery.

During the creation of a Lithium-Ion Battery (LIB), preparing the electrode involves the creation of an “electrode slurry”: a suspension of solid conductive particles in a solvent medium, along with polymer binders and active components. The properties of this slurry are essential to the quality of a life, affecting performance and lifetime. The primary conflict of creating an effective electrode slurry is that its viscosity and viscoelasticity must be in a narrow range to be high enough for some steps and low enough for others, with the end goal of a homogenous product.

During initial mixing, the viscosity must be low enough to be evenly blended. After mixing, the viscosity must be high enough for the conductive solids to remain evenly suspended and avoid settling. Additionally, the slurry must be able to coat evenly, remain level during drying, and have sufficient adhesion to avoid delamination. Non-Newtonian fluids are the best solution, as they do not have a single constant measurement for viscosity.

Slurry formulations can be optimized effectively when researchers are able to predict and measure their flow behavior across relevant conditions, ranging from the high shear rates used during the coating process to the very low shear rates experienced at rest. Comparing two experimental anode formulations using only a viscometer suggests their flow behavior is equivalent, but the Core Rheometer’s extended range of measurement shows Anode Slurry 2 is more shear thinning, and its higher viscosity at low shear is advantageous to stabilize the dispersion against settling, indicating the active materials are uniformly distributed.

Discovery Core Rheometer Benefits:

• Measure viscosity under process-relevant conditions: coating, mixing, leveling, storage
• Robust stainless-steel geometries suitable for NMP-based slurries
• Concentric cylinder configuration for easy loading of low viscosity slurries, available with disposable cups for high throughput
• Streamlined, guided operation with rapid analysis results to keep up with fast-paced production needs
• Air-cooled Peltier temperature control eliminates the need for a fluid circulator, ideal for operation in a dry room

Battery materials scientists face a fundamental challenge when developing anode or cathode slurries: maintaining a uniform distribution of the dense solid particles making up the active and conductive materials. Suspension stability is dependent on the slurry’s yield stress, the resistance to flow when the material is at rest.  Multiple rheological techniques may be used to quantify yield stress, such as the example shown above in which shear rate is decreased until the shear stress reaches a plateau.  The Core Rheometer’s torque sensitivity enables precise measurements of the internal microstructure impacting a slurry’s stability.

Discovery Core Rheometer Benefits:

• Measure viscosity under process-relevant conditions: coating, mixing, leveling, storage
• Robust stainless-steel geometries suitable for NMP-based slurries
• Concentric cylinder configuration for easy loading of low viscosity slurries, available with disposable cups for high throughput
• Streamlined, guided operation with rapid analysis results to keep up with fast-paced production needs
• Air-cooled Peltier temperature control eliminates the need for a fluid circulator, ideal for operation in a dry room