Lithium-ion batteries are one of the most important energy storage devices today, occupying a leading position in the fields of portable electronic products and electric vehicles (EVs). With the rapid expansion of the EV market, people have higher requirements for the cruising range, which forces development of high-energy-density cells, and as consequence, new cathode and anode materials. In terms of cathodes, lithium-rich layered oxide (LLO) materials have attracted increased attention of many researchers, due to their ultra-high theoretical specific capacity and low costs. Such oxides are considered to be one of the most promising candidates for the next-generation cells [1,2]. However, inherent issues, such as low initial Coulombic efficiency (ICE) and voltage attenuation seriously hinder practical application of lithium-rich materials until now. In this regard, researchers are committed to mitigating the first effect through surface coating and bulk doping. Besides the modification on active material itself, introduction of functional additives into an electrolyte is a low-cost and highly targeted method to optimize the performance of LLO electrodes. In fact, electrolyte additives are found to be crucial for optimizing the properties. They can stabilize the electrolyte, remove HF/H2O, adjust the solvent structure, improve flame retardancy, etc. Special electrolyte additives that inhibit TM dissolution and oxygen release are also critical to the electrochemical performance of LLO materials. Also, the composition and structure of cathode-electrolyte interphase (CEI) can be effectively adjusted through the design and use of such additives. Furthermore, multiple additives can be coupled to exhibit synergistic effects in CEI, thereby bestowing in situ its structure with multifunctionality.

In this work, Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ cathode material was prepared by co-precipitation method, and the influence of various electrolyte additives such as vinylene carbonate (VC), 1,3,2-dioxathiolane-2,2-dioxide (DTD), tris(trimethlysilyl) phosphate (TTSP), tris(trimethlysilyl) phosphite (TTSPi), and lithium 2-trifluoromethyl-4,5-dicyanoimidazole (LiTDi) on the material properties was studied. In order to explore the effect of electrolyte additives on the changes in lattice volume of the electrochemically-active phase during charging and discharging, operando XRD experiments were conducted. A custom-made cell, having a Be window was used for the measurements. The registered charge/discharge curves, obtained using the operando cell, were nearly the same as those obtained using the normal 2032 coin-type half cells. In addition, operando Raman method was used to monitor the vibration of O during the cycling processes of LLO electrodes using different electrolyte additives, demonstrating the role of different additives in inhibiting TM dissolution and oxygen release. Results of XAS measurements at the edge of TM L showed that after 50 cycles at 0.5C the bulk-related signal of the samples with various electrolyte additives did not differ significantly from the initial signal, but there was a significant difference in surface-related signal. Only the samples with TTSP additive exhibited the closest signal to the initial samples, indicating that adding TTSP can improve the surface stability of the materials during the cycling process. The electrochemical performance of the LLO cathodes was characterized using 2032 coin-type half cells, with a Li metal anode and different electrolytes. The cycling tests were performed between 2.0 V and 4.8 V at different current densities. One of the best performing cathodes with TTSPi delivered an initial discharge capacity more than 250 mAh g^-1 at 0.1 C, with a capacity retention of 85% after 100 cycles, indicating largely improved electrochemical characteristics.