Structural Effect on Electrochemical Performance of 4,4’-Biphenyldicarboxylate Sodium Salts as an Anode for Na-ion Batteries

Abstract

The crystal structures and electrochemical performance of 4,4’-biphenyldicarboxylate sodium salts is first introduced as an anode for Na-ion batteries. The structural modification of 4,4’-biphenyldicarboxylate sodium salts showing different degree of deprotonation and the coordination of a water molecule are deliberately accomplished through various precipitation and solvothermal methods, resulting in the formation of three different crystal structures even though they are composed of the same organic (bpdc) and inorganic (Na+) building blocks. The crystal structures are determined by single-crystal X-ray diffraction. The powder X-ray diffraction patterns showed the good agreement with the corresponding simulated patterns, indicating that the phase pure powders have the same crystal structure as the single crystals. The level of deprotonation in 4,4’-biphenyldicarboxylate sodium salts affects not only electrochemical performance but also reaction mechanisms. The fully deprotonated 4,4’-biphenyldicarboxylate disodium salt (Na2bpdc) exhibits promising electrochemical performance including reversible capacity of 220 mA h g-1 at ca. 0.5 V vs. Na/Na+, negligible capacity fading over 150 cycles, and excellent rate performance delivering about 100 mA h g-1 even at a 20C rate, which is better than monosodium 4,4’-biphenyldicarboxylate (NaHbpdc) that is partially deprotonated. This better rate performance of Na2bpdc salts is definitely attributed to the smaller particle size (short diffusion length) of that compared to NaHbpdc. However, even the dehydrated disodium 4,4’-biphenyldicarboxylate monohydrate (h-Na2bpdc) having similar size to NaHbpdc exhibited better rate performance than NaHbpdc. This means that the rate performance is affected by the degree of deprotonation in 4,4’-biphenyldicarboxylate sodium salts. Carboxylic group causes the large amount of electrolyte decomposition to form thick solid electrolyte interphase (SEI) layers, resulting in the increase of polarization due to large charge-transfer resistance. Also, the de/sodiaiton of Na2bpdc salts proceeds in a two-phase reaction, regardless of the degree of deprotonation. And, the amorphization of Na2bpdc salts occurred during cycling, even though the crystal structure is maintained. However, unlike the fully deprotonated showing reversible phase transition during sodiation and desodiation, the partially deprotonated exhibits irreversible phase transition during cyclings. It seems to be occurred the partially phase transition to Na2bpdc which is fully deprotonated due to the ion-exchange between Na+ and H+.