Interfacial Chemistry of Thiophene As an Effective Film-Forming Additive on High Voltage Cathode Revealed By Operando Raman Spectroscopy
Authors: Baghernejad Masoud , Pfeiffer Felix 
Published: July 2022

For decades intense research efforts have been aimed at increasing energy densities with new rechargeable lithium battery (RLB) designs to meet the anticipated increasing energy storage demands. These efforts have yielded much higher capacities with novel materials. However, the theoretical capacity of an RLB cell is seeded in the structure of the electrode, while its practical capacity, performance, cycle life, and safety are deep-rooted in the side reactions at the interphase between electrode and electrolyte (interphase is the phase at the electrode-electrolyte interface) 1,2.

Interphases are essential for the operation of the cells and have been widely recognized as the "most important and the least understood" components of RLBs3. While their characteristics determine their performance, they are not part of the design of RLBs, but only form during the cell operation on the anodes and cathodes. They constitute passivation layers with mixed chemistries due to the electrolytes' electrochemical stability window (ESW) being smaller than the operating potential of RLBs. Although interphase formation is of functional importance, its formation imposes a heavy price on the cell's performance. Formation of the interphase consumes the electrolyte and active lithium stored in the cathode and causes a dramatic capacity loss, especially in the initial charge/discharge cycles. Also, the presence of the interphase introduces a high level of impedance to the cell, which interrupts the fast and constant flow of Li-ion between the anode and cathode. Last but not least, interphase is an active and complex medium that changes its chemical composition and transport properties during charge/discharge cycling and storing conditions. This results in a constant evolution of the interphase and capacity fading, which is more prominent in the high cell voltage applications. Achieving high cell voltage by increasing the cut-off voltage of LiNi
x
Co
y
Mn1−x−y
O2 cathode (NMC) materials to 4.5 V is an effective way to enhance the energy density of RLBs4. However, a high level of parasitic and decomposition reactions result in severe capacity fading of the battery cell5.

Many efforts have been made to improve the properties of the cathode interphase at high cut-off voltage by incorporating different strategies. The in vivo approach is one of the more successful attempts to design the cathode interphase by incorporating film-forming additives in electrolytes to suppress parasite reactions at the electrode-electrolyte interface6-8. In an ideal case, this film should be stable with cycling and have excellent conductivity for Li+ ions and low electronic conductivity9.

Thiophene-based molecules are investigated as effective film-forming on NMC at high cut-off voltages of 4.5 V vs. Li/Li+. Electrochemical investigations show that thiophene oxidizes before carbonate-based electrolytes and forms a stable interphase with high conductivity for Li-ion. Also, the electrochemical performance of the cell is enhanced significantly in the presence of thiophene additive with capacity retention of 95% after 100 cycles (75% capacity retention of the benchmark system). To reveal the underlying mechanisms of the interphase formation and thiophene working principle on the surface of NMC cathode, we performed operando near-field Raman spectroscopy. The Raman and electrochemistry results indicate that a poly-thiophene film is formed that covers the surface of the cathode active materials uniformly, protecting further oxidation of the electrolyte and preventing transition metal dissolution and decomposition of the active cathode materials.

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