Solid-state batteries with polymer electrolytes and Li metal negative electrodes are promising candidates for their application in electric vehicles. CIC energiGUNE is a research center committed to the development of solid-state batteries with polymer electrolytes with the research work being carried out in the ceramic electrolyte and polymer electrolyte research lines.

Conventional Li-ion batteries present significant risks ought to the flammable nature of the liquid electrolyte. Additionally, Li-ion batteries are close to their practical energy density limit due to the constraints of the carbon-based negative electrode. Therefore, improvements in the electrolyte must follow a complete concept shift approach, rather than only engineering the liquid electrolyte chemistry.

Recent Li-ion electrolyte research has focused on discarding unsafe organic liquid solutions and transitioning towards more inert electrolytes; ideally, solvent-free Li+ conducting electrolytes. Solid polymer electrolytes are inherently safe, significantly decreasing the risk of extensive fires compared to current Li-ion battery liquid electrolytes.

Additionally, solid polymer electrolytes will allow to remove current carbon-based negative electrodes, transitioning toward Li metal and anodeless negative electrodes. This will allow enhancing the energy density of batteries to values which will increase the driving range of electric vehicles up to around 800 km, achieving parity with current internal combustion engines.

There are different polymer chemistries suitable for battery applications as solid polymer electrolytes. Each polymer chemistry offers different advantages, as discussed in our previous blog post. Nevertheless, it is particularly difficult to develop a polymer electrolyte with a sufficiently large energy gap that offers electrochemical stability against Li metal and high-voltage positive electrode active materials simultaneously.

Therefore, CIC energiGUNE has developed a smart and simple strategy combining polymer layers with different properties within the same battery. These layers can be tailored to match the properties required in each battery section. This approach is named Double Layer Polymer Electrolyte (DLPE).

Our research in the last 4 years has focused on developing this Double Layer Polymer Electrolyte technology, where two different polymer electrolytes are used within the same battery: one at the cathode side (catholyte) and one as separator (electrolyte). Each battery section requires different electrochemical and mechanical properties from the polymer; thus, the polymers used in each section may be different. On the one hand, the catholyte requires a high ionic conductivity, stability against oxidative potentials (>4 V vs Li/Li+) and excellent binding properties to maintain all active material particles glued together. On the other hand, the electrolyte requires high ionic conductivity, chemical and electrochemical stability against Li metal negative electrodes, and sufficient mechanical properties to prevent dendrite penetration. Thus, the DLPE approach prevents the undesired degradation of the electrolyte or catholyte that occurs when using a single polymer electrolyte in the whole cell derived from the low energy gap mentioned earlier in the text. Additionally, other properties such as mechanical stability, binding properties and ionic conductivity can be tailored depending on the needs of each battery application.

The main drawback of the Double Layer Polymer Electrolyte strategy is the Li salt interdiffusion between the different polymer layers. To cope with it, CIC energiGUNE has developed a strategy to avoid this spontaneous Li salt interdiffusion between the polymer layers of the battery, which has led to the filing of a patent application in collaboration with battery manufacturing companies such as Saft. This proprietary technology consists of a modification of the Li salt anions, which efficiently prevents the free-movement of the Li atoms and does not add any significant weight of volume to the cell, maintaining high gravimetric and volumetric energy density values.

In the aforementioned development, Poly (ethylene oxide) (PEO) and Poly (propylene carbonate) (PPC) are used as electrolyte and catholyte, respectively. These polymers are composed by different chemistries: PEO is composed by ethylene oxide (EO) units that offer high ionic conductivity and stability against reductive potentials (with Li metal); nevertheless, it suffers from degradation when being under oxidative potentials (>4 V vs Li/Li+). PPC is composed by propylene carbonate units (PC), which offer ionic conductivity, and are highly stable to oxidative potentials (>4 V vs Li/Li+); however, these units are degraded under reductive potentials present in the Li metal side of the cell. Therefore, each polymer fulfills all the requirements for each battery section (electrolyte and catholyte).

Applying the Double Layer Polymer Electrolyte approach, the combination of these polymer electrolytes with different chemistries may allow to withstand Li metal and high-voltage positive electrode active materials simultaneously.

Comparatively, an electrochemical cell comprising a single polymer electrolyte (i.e. PEO) shows degradation of the positive electrode and short battery life. The electrochemical cell developed through the Double Layer Polymer Electrolyte strategy outperforms this state-of-the-art cell, offering higher stability, capacity retention and cycle life.

Currently, CIC energiGUNE develops new polymer electrolytes and Li salts enhancing compatibility with both electrodes. The cell integration through the Double Layer Polymer Electrolyte approach allows for the use of the best performing electrolytes in the same electrochemical device, and enhancing the potential electrochemical stability window of future solid state Li metal batteries based in polymer electrolytes.

Author: Mikel Arrese-Igor, Associate Researcher of the Advanced Electrolyte and Cell Integration Research Group of CIC energiGUNE.

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