Polymer electrolytes are critical components in solid-state and flexible battery systems, offering advantages in safety and design versatility. The choice of lithium salt significantly impacts performance, with key considerations including ionic conductivity, electrochemical stability, and interfacial compatibility. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), and emerging alternatives like lithium borohydride (LiBH4) represent distinct classes of salts with differing trade-offs in polymer matrices.

Anion size and dissociation efficiency are primary determinants of ionic conductivity in salt-doped polymer electrolytes. LiTFSI features a large, delocalized anion (TFSI−) with a molecular volume of approximately 0.28 nm³, promoting high dissociation due to weak Coulombic interactions with lithium ions. This results in room-temperature conductivity values in the range of 10⁻⁴ to 10⁻³ S/cm in poly(ethylene oxide) (PEO) systems. However, the bulky anion contributes to increased electrolyte viscosity, which can limit Li⁺ mobility at higher salt concentrations. LiFSI has a smaller anion (FSI−) with a molecular volume near 0.20 nm³, enabling faster ion transport and achieving conductivities up to 1.5 times greater than LiTFSI in comparable systems. The reduced steric hindrance allows for more efficient segmental motion in the polymer host. LiBH4 represents an extreme case with a compact, spherical anion (BH4−) of about 0.05 nm³ volume, theoretically enabling very high lithium transference numbers. However, its strong coordination with lithium ions often leads to incomplete dissociation in polymer hosts, resulting in conductivities typically below 10⁻⁵ S/cm without additional modifications.

Corrosion behavior varies substantially between salt types due to anion chemistry. LiTFSI demonstrates good aluminum current collector stability up to 4.5 V vs Li⁺/Li, attributed to the formation of a passivating fluoride-rich layer. However, trace HF impurities from moisture degradation can induce pitting corrosion over extended cycling. LiFSI shows more aggressive corrosion tendencies, with studies reporting aluminum dissolution above 3.8 V unless specialized coatings are applied. The fluorine-sulfur bonds in both TFSI− and FSI− anions undergo gradual decomposition at high voltages, generating corrosive byproducts that degrade battery components. LiBH4 presents a different challenge—while it lacks corrosive fluorine species, its reductive decomposition at potentials above 3.0 V limits compatibility with high-voltage cathodes. The borohydride anion can also react with moisture to produce hydrogen gas, creating safety concerns in poorly sealed systems.

Transference number measurements reveal fundamental differences in lithium ion mobility. Typical values for LiTFSI-based polymer electrolytes range from 0.2 to 0.3, reflecting significant anion mobility contribution. LiFSI systems show modest improvements (0.25–0.35) due to reduced anion size. LiBH4 theoretically enables transference numbers approaching 0.9, but practical measurements often show lower values (0.4–0.6) due to ion pairing effects. The disparity highlights the challenge of achieving both high dissociation and cation-selective transport in polymer hosts.

Thermal stability follows the trend LiFSI < LiTFSI < LiBH4 in polymer electrolytes. LiFSI begins decomposing near 150°C in PEO, with exothermic reactions peaking at 200°C. LiTFSI demonstrates better stability up to 250°C, though both sulfonimide salts catalyze polymer decomposition at elevated temperatures. LiBH4-containing systems show superior thermal resilience up to 300°C, but require strict moisture exclusion to prevent hydrolysis reactions. Differential scanning calorimetry measurements confirm these degradation thresholds across multiple polymer hosts.

Mechanical properties of the electrolyte films are equally salt-dependent. LiTFSI plasticizes polymer matrices, reducing elastic modulus by 30–50% compared to salt-free films at 20 wt% loading. LiFSI causes even greater softening due to its higher solubility in the polymer phase. LiBH4 shows minimal plasticization effect but often leads to brittle composites unless combined with flexible polymer backbones. The storage modulus G' for typical systems follows the order LiBH4 > LiTFSI > LiFSI at equivalent salt concentrations.

Interfacial stability with lithium metal anodes presents another critical distinction. LiTFSI enables relatively stable solid electrolyte interphase (SEI) formation, with impedance growth rates around 5 Ω/cm² per cycle in symmetric cells. LiFSI forms a more conductive SEI layer, reducing initial impedance but showing faster degradation over time (10 Ω/cm² per cycle). LiBH4 reacts spontaneously with lithium metal to form a mixed boride-oxide interface that initially shows low impedance (<10 Ω/cm²) but may develop inhomogeneities during plating/stripping.

Emerging salt engineering strategies focus on anion modifications to address these limitations. Fluorinated borate salts (e.g., LiBF3CF3) attempt to combine the thermal stability of borates with the dissociation characteristics of fluorinated anions. Sulfonate-amide hybrids seek to balance corrosion resistance and ion mobility. These developments aim to achieve conductivities above 10⁻³ S/cm with transference numbers exceeding 0.6 while maintaining compatibility with existing battery components.

The selection criteria for polymer electrolyte salts depend heavily on application requirements. High-energy-density systems may prioritize LiFSI for its conductivity despite corrosion challenges. Long-duration storage applications could favor LiTFSI for its balance of properties. Solid-state configurations requiring minimal interface reactions might utilize LiBH4 derivatives with stabilizing additives. Ongoing research continues to refine these materials through anion functionalization and polymer-salt coordination control.