Tris(trimethylsilyl) phosphite effective as an additive for high-voltage lithium-ion battery

High-voltage lithium-ion batteries(LIBs) have been the subject of intensive research efforts owing to their potential utility as high-energy density batteries for electric vehicles and hybrid electric vehicles, which require battery with long cycle lives and high capacity retention. Now, a number of cathode materials with a high operating potential have been proposed, such as LNMO, NCM et al.

     Toward the above high operating voltage cathode materials, the corresponding more stable electrolyte in order to prevent extensive decomposition of the electrolyte solvent on the cathode surface is also needed. Two solutions are present which include replacing carbonate solvents of conventional electrolytes with sulfones, nitriles and ionic liquids and developing various functional additives, such as tris(trimethylsilyl) borate(TMSB) and tris(trimethylsilyl) phosphite(TMSP). The two additives have been used to form a protective film that stabilizes the surface of the charged cathode, allowing for reversible Li ion intercalation chemistry in the coveted 5V region.

It is known that additive TMSP react directly with the HF molecules in the electrolyte. TMSP is a promising high-voltage additive that has garnered attention from researchers. They found that TMSP improved the cycling performance at high voltages or elevated temperatures. As an additive, TMSP results in excellent high-voltage cycling stability and rate capability by forming a cathode film. And it was also found that TMSP is a better additive than tris(trimethylsilyl)phosphate when added to VC-containing electrolytes for NCM333/graphite cells.

Despite the importance of TMSP as a high-voltage additive for LIBs, the mechanism of its positive effect on electrochemical performance is not elucidated. In Han et al work, they explain the mechanism of TMSP for its outstanding performance as a high-voltage electrolyte additive through density functional theory(DFT) calculations and experiments.

 At first, DFT calculation showed that TMSP has a lower oxidation potential than EC which oxidized more readily than EC to form a protective film at the cathode surface. And the experimental results showed that the cell cycled with TMSP demonstrated stable cycle performance that the cell without TMSP. Furthermore, the cell cycled with TMSP afforded a much higher coulombic efficiency with 99.7% average coulombic efficiency, whereas the cell controlled without TMSP suffered from relatively low average couloubic efficiency. The SEM analyses showed that the electrodes controlled with TMSP has relatively clean surfaces even after 50 cycles, which is virtually identical to the pristine electrode case. This means that TMSP minimized unfavorable chemical/electrochemical reactions in the cell and therefore stabilizes the interfacial chemistry of each electrode.

In conclusion, TMSP exhibits outstanding performance as an LIB electrolyte additive on the strength of its distinct molecular properties. It has a lower oxidation potential that that of EC, a relatively high reactivity with HF. Combining the experimental and calculation results, it is surmised that TMSP removes the HF molecules from the electrolyte, forms a protective film on the cathode surface, and improves cell performance without side reactions at the anode surface.

References:

Y. Han, J. Yoo, T. Yim, DOI: 10.1039/c5ta01253h

 Edited by Suzhou Yacoo Science Co., Ltd.