Structural Effect on Electrochemical Performance of 4,4’-Biphenyldicarboxylate Soduim Salts As an Anode for Na-Ion Batteries
Authors: Choi Aram , Kim Yun Kyeong , Moon Hoi Ri , Lee Kyu Tae 
Published: February 2020

Sodium ion batteries have recently gained recognition as a promising candidate for next-generation battery systems on the basis of their potential cost advantages due to the natural abundance of Na resources [1-6]. In this presentation, we first introduce the crystal structures and electrochemical performance of 4,4’-biphenyldicarboxylate sodium salts 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. This results 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, and the powder X-ray diffraction patterns showed the good agreement with the corresponding simulated patterns. This indicates that the phase pure powders have the same crystal structure as the single crystals.
The level of deprotonation in 4,4’-biphenyldicarboxylate sodium salts affected not only electrochemical performance but also reaction mechanisms. The fully deprotonated 4,4’-biphenyldicarboxylate disodium salt (Na2bpdc) exhibited 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. However, unlike the fully deprotonated showing reversible phase transition during sodiation and desodiation, the partially deprotonated exhibited irreversible phase transition during cyclings. It seems to be attributed to the partial phase transition of NaHbpdc to Na2bpdc due to the ion-exchange between Na+ and H+.
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