Influence of Alkyl Sidechain Length on Lithium-Ion Conductivity for Polyether Electrolytes
Authors: Sai Ryansu , Katayama Yu , Tsutsumi Hiromori 
Published: December 2020

Lithium-ion batteries (LIBs) have been used in various energy devices, from small electronic devices to electric vehicles. However, LIBs have safety concerns, mainly due to the flammable organic solvents used for electrolytes. The development of electrolytes based on flame-retardant materials, including polymer, is one of the potential solutions. Although various polymer electrolytes (PEs) were successfully synthesized, the low lithium-ion (Li+) conductivity hinders its practical application.

It is considered that the Li+ conductivity in PEs is determined by two factors; (i) segmental mobility of polymer chains and (ii) Li+ concentration, which is connected to the Li+ mobility and the number of ion carriers, respectively. Most of the PEs failed to meet those requirements, which hinders the improvement of Li+ conductivity. The problem is that the polar groups in the polymer chains tend to actively coordinate with Li+ under high Li+ concentration, which inhibits the mobility of the polymer chains. Also, the Li+-polar group interaction anchors Li+ on the polymer chain, which may also suppress the Li+ conductivity.

Therefore, to establish a polymer design strategy, it is necessary to elucidate the factors which affect the interaction between Li+-polar groups and Li+ transportation. Previous studies have focused mainly on the polar properties (e.g., donor number and dielectric constant) of the polymers to understand the interaction between Li+ and polar groups. In contrast, the effect of non-polar groups was often overlooked. Non-polar groups can affect the mobility and polarity of polymers as well as the Li+-polar group interaction, due to its plasticization effect, polarity, and steric hindrance.

Here, in this study, we selected polyethers having alkyl sidechains with different lengths (–(CH2)m–H, m = 1, 2, 4, 6, 8, and 12), to clarify the contribution of the non-polar groups on the Li+ conductivity in PEs. Electrochemical impedance spectroscopy revealed that the ionic conductivity (σ) at low temperatures was dominated by glass transition temperature, which correlates with the segmental mobility of polymer chains. Furthermore, σ increase with the increase in the length of the alkyl group due to the internal plasticization effect of the long alkyl groups. On the contrary, the σ at high temperature was mostly determined by salt dissociation capacities of polymers, which tended to increase as the alkyl group became shorter. The Li+ transportation number (t
Li+) gradually improved with the extension of the alkyl groups; 0.16 (m = 1) < 0.26 (m = 2) < 0.28 (m = 4) < 0.32 (m = 6) < 0.36 (m = 8) < 0.43 (m = 12). We believe that the gradual changes of t
Li+ are due to the low polarity of long alkyl sidechain, giving low dielectric constant and/or low viscous properties. The activation energy of Li+ transportation at the electrode-electrolyte interface (E
int) showed a large gap between m =1 and others; E
int = 68–82 kJ mol–1 (m = 2, 4, 6, 8, and 12) < E
int = 116 kJ mol–1 (m = 1). The coordination number of ether groups (n
ether) estimated from ν(COC) intensity of infrared spectra showed a similar trend; n
ether = 3.6–4.4 (m ≥ 2) < n
ether = 6.2 (m = 1). The interaction between the ether group and Li+ can be affected by the length of alkyl sidechain, and it effectively suppressed for alkyl sidechain of m > 1, probably because of the steric hindrance. Considering the similar trend in both E
int and n
ether, the amount of ether-Li+ interaction can be a descriptor to determine Li+ transportation at the electrode-electrolyte interface.

In summary, the Li+ conductivity, including ionic conductivity (σ) and Li+ transportation number (t
Li+) strongly influenced by the properties of the alkyl group, such as the internal plasticization effect and low polarity. The Li+-ether group interaction, which was governed by the steric hindrance of alkyl sidechain, can be a dominant factor for the activation energy of Li+ transportation at the electrode-electrolyte interface (E
int). Designing alkyl sidechains is thus a valid strategy to optimize segmental mobility and polarity of the bulk as well as the local interaction at the interface, which leads to the improved Li+ conductivity at bulk and electrode-electrolyte interfaces.