Solid-state polymer electrolytes are good candidates to replace conventional liquid electrolytes in energy storage devices. Polymers have great advantages such as easy processing, low cost, good electrochemical properties and, in addition, they eliminate the safety issues present in liquid electrolytes. However, polymer electrolytes exhibit much lower ionic conductivities than their liquid counterparts at room temperature.

The development of such electrolytes has been hampered by the search for a good compromise between good mechanical properties and high ionic conductivity. In recent years, different polymer structures have been designed with the aim of improving both properties at the same time. However, amorphous solid-state polymer electrolytes with low glass transition temperature and low crystallinity (essential properties to accelerate ionic transport) can hardly form membranes with good mechanical properties.

Among all the polymers investigated in the solid state, polyethylene oxide (PEO) has received the most attention. However, this type of electrolyte has several drawbacks. On the one hand, it presents low anodic stability (<4.0 V vs. Li⁰│Li⁺). On the other hand, it is a semi-crystalline polymer in which ionic transport occurs mainly in the amorphous phase, leading to low ionic conductivity at temperatures below the melting point (T_m, aprox. 65 ^oC). For this reason, solid-state batteries based on PEO operate at elevated temperatures (70‒90 ^oC).

To try to overcome these drawbacks, in recent years CIC energiGUNE has been working on the design and synthesis of non-crystalline polymeric matrices with low glass transition temperatures, good mechanical properties and higher stability against Li anode to avoid the formation of dendrites

Among these modifications at the molecular level, strategies such as the design of comb-like polymeric structures with a high degree of branching, cross-linking of the polymeric network and block or random polymerization can be highlighted.

Strategy 1: New polymer matrix

Initially, a new type of polymer matrix with a comb-like structure based on imide rings with Jeffamine^® side chains was developed. These side chains are composed of ethylene oxide (EO) and propylene oxide (PO) units. It was shown that the ratio between EO and PO units is a determining factor in obtaining a good ratio between low crystallinity and high ionic conductivity, even at room temperature.

On the other hand, the influence of the molecular weight on the mechanical properties was analyzed, obtaining a higher degree of chain entanglement with increasing molecular weight.

Despite the excellent electrochemical properties presented by these electrolytes, it was decided to carry out some additional modifications to improve the mechanical properties of the polymer matrix in order to enhance the contact between the electrolyte and the electrodes.

Strategy 2: New family of block copolymers

In this context, a new family of block copolymers based on the above-mentioned polymer together with blocks based on polystyrene were synthesized. These new polymers showed a great improvement in mechanical properties with a very small decrease in ionic conductivity. In addition, the incorporation of the polystyrene block was able to improve the electrochemical stability of the electrolyte against lithium.

Strategy 3: Modification of physical properties

Another alternative, instead of chemically modifying the polymer matrix, was to make modifications to the physical properties of the polymer. The suppression of chain entanglement in Jeffamine^® polymers resulted in a flowable polymeric matrix with higher chain flexibility and, as a consequence, lower glass transition temperature values and higher ionic conductivities. In addition, the good adhesive properties presented by this new material allowed a better contact between the lithium electrode and the electrolyte, thus delaying the formation of dendrites.

Due to the poor mechanical properties presented by these new materials, in a first analysis they were used as a protective layer between the lithium electrode and the PEO-based polymer membranes (acting as a separator) to improve the contact and stability of the interface.

It was shown that the stability against lithium electrode could be significantly improved by using this protective layer, and the Li⁰│LFP cell using PEO as electrolyte showed good specific capacity, good stability and high coulombic efficiency at different charge and discharge rates.

Strategy 5: Deposition on PVDF fibers

Finally, in order to provide stiffness to the electrolyte, this new flowable polymer electrolyte was deposited on poly(vinylidene fluoride) (PVDF) fibers. These reinforced electrolytes allowed obtaining membranes with good mechanical properties and presented high ionic conductivity at room temperature. Moreover, the stability against lithium electrode was not affected by the incorporation of these fibers, resulting in good performance of the Li⁰││LFP cells even at room temperature.

CIC energiGUNE is currently working on the design and development of new polymer matrices that represent reliable solutions for the scientific community in the field of lithium metal solid state battery development in terms of both safety and environmental friendliness. In particular, electrolytes with well-balanced ionic conductivity and mechanical properties are excellent candidates to meet the demand for this type of battery.