Triphenyl phosphate as an Efficient Electrolyte Additive for Ni-rich NCM Cathode Materials

Abstract

Nickel-rich lithium nickel-cobalt-manganese oxides (NCM) are viewed as promising cathode materials for lithium-ion batteries (LIBs); however, their poor cycling performance at high temperature is a critical hurdle preventing expansion of their applications. We propose the use of a functional electrolyte additive, triphenyl phosphate (TPPa), which can form an effective cathode-electrolyte interphase (CEI) layer on the surface of Ni-rich NCM cathode material by electrochemical reactions. Linear sweep voltammetry confirms that the TPPa additive is electrochemically oxidized at around 4.83 V (vs. Li/ Li +) and it participates in the formation of a CEI layer on the surface of NCM811 cathode material. During high temperature cycling, TPPa greatly improves the cycling performance of NCM811 cathode material, as a cell cycled with TPPa-containing electrolyte exhibits a retention (133.7 mA h g-1) of 63.5%, while a cell cycled with standard electrolyte shows poor cycling retention (51.3%, 108.3 mA h g-1). Further systematic analyses on recovered NCM811 cathodes demonstrate the effectiveness of the TPPa-based CEI layer in the cell, as electrolyte decomposition is suppressed in the cell cycled with TPPa-containing electrolyte. This confirms that TPPa is effective at increasing the surface stability of NCM811 cathode material because the TPPa-initiated PO x-based CEI layer prevents electrolyte decomposition in the cell even at high temperatures.

Stabilizing Li2O-based Cathode/Electrolyte Interfaces through Succinonitrile Addition

“…Lithium-ion batteries (LIBs) are widely used as energy storage devices and are employed in various applications, ranging from small electronics to electric vehicles [1][2][3][4][5][6][7][8]. The demand for high energy densities has driven the development of high-capacity cathode materials for LIBs [9][10][11][12].…”

Organo-Soluble Decanoic Acid-Modified Ni-Rich Cathode Material LiNi0.90Co0.07Mn0.03O2 for Lithium-Ion Batteries

“…However, the higher Ni content in Ni-rich cathode materials poses more serious challenges in terms of surface slurrying, structural deterioration, interfacial parasitic reaction, and mechanical cracking, in which the structural degradation initiating at the surface and extending to the bulk inherently leads to the sluggish lithiumion diffusion and, thereby, the worsened electrochemical performance, causing the rapid decay of capacity and potential. 9,10 Therefore, surface modification is usually used to improve the cycle stability of Ni-rich cathodes. 11−14 On the surface of cathodes, electrolyte decomposition occurs upon cycling because of oxygen evolution of the highly delithiated cathodes to inevitably form a cathode/electrolyte interface (CEI) layer whose properties such as conductivity of Li-ion and stability can influence the strong electrochemical performance of the cathodes.…”

“…Li-ion batteries are the most popular rechargeable power source for portable electronics and electric vehicles due to their relatively high energy density. − To seek further improvement in energy density, Ni-rich layered oxides such as LiNi x Co y Mn z O 2 ( x ≥ 0.6) are considered to be one of the most promising cathode materials for Li-ion batteries in the near future. − Normally, the higher the Ni content of Ni-rich cathode materials, the higher the capacity. However, the higher Ni content in Ni-rich cathode materials poses more serious challenges in terms of surface slurrying, structural deterioration, interfacial parasitic reaction, and mechanical cracking, in which the structural degradation initiating at the surface and extending to the bulk inherently leads to the sluggish lithium-ion diffusion and, thereby, the worsened electrochemical performance, causing the rapid decay of capacity and potential. , Therefore, surface modification is usually used to improve the cycle stability of Ni-rich cathodes. − On the surface of cathodes, electrolyte decomposition occurs upon cycling because of oxygen evolution of the highly delithiated cathodes to inevitably form a cathode/electrolyte interface (CEI) layer whose properties such as conductivity of Li-ion and stability can influence the strong electrochemical performance of the cathodes. − The nonuniform coverage of the CEI leads to transition metal dissolution and surface structural degradation of the Ni-rich cathode materials because of the uneven lithium transport rate across the interface . In addition, hydrofluoric acid resulting from lithium hexafluorophosphate decomposition attacks the surface to leach transition metal out from the oxide structure, accelerating surface reconstruction .…”

Comparable Investigation of Phosphorus-Based Flame Retardant Electrolytes on LiFePO4 Cathodes

“…However, these flame additives are either too expensive or synthesized through complex processes in the laboratory, which cannot meet the market demand of LIBs. In contrast, phosphorus retardant additives represented by cresyl diphenyl phosphate (CDP), 17,18 dimethyl methyl phosphonate (DMMP), 19,20 trimethyl phosphate (TMP) [21][22][23][24] and triphenyl phosphate (TPP) [25][26][27] have been early studied due to their low price and good flame retardant effect. Most of the research focus on the compatibility between phosphorus flame retardants and graphite anode, and pay less attention to the effect of flame retardants on the electrochemical properties of the cathode.…”

“…Li x PO y F z is considered as an effective chemical composition for CEI layer that would suppress electrolyte decomposition. 27 In addition, Fig. 7A presents the highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energies of DMMP, TMP and electrolyte solvents (EC and DEC).…”