Analyzing the Mechanism of Functional Groups in Phosphate Additives on the Interface of LiNi_0.8Co_0.15Al_0.05O₂ Cathode Materials

Abstract

The design of a functional electrolyte system that is compatible with the LiNi 0.8 Co 0.15 Al 0.05 O 2 (LNCA) cathode is of great importance for advanced lithium-ion batteries (LIBs). In this work, chelated lithium salts of lithium difluoro(bisoxalato) phosphate (LiDFBOP) and lithium tetrafluoro(oxalate) phosphate (LTFOP) are synthesized by a facile and general method. Then, the complexes of LiDFBOP, LTFOP, and lithium difluorophosphate (LiDFP), all of which have a central phosphorus atom, were selected as the salt-type additives for the LiPF 6 -based electrolyte to improve the electrochemical performances of LNCA/Li halfbatteries, respectively. The results of electrochemical tests, quantum chemistry calculations, potential-resolved in situ electrochemical impedance (PRIs-EIS) measurements, and surface analyses show that the interface property and the battery performance are closely associated with molecular structures of phosphorus-centered complex additives. It indicates that LiDFP with the PO bond can significantly reduce the interfacial impedance of LNCA/Li half-batteries due to the increase of Li 3 PO 4 and the decrease of Li 2 CO 3 in the cathode electrolyte interface (CEI). While in LiDFBOP, according to the calculated vertical ionization potential (VIP), the two oxalate-chelated ligands bring about a bidirectional cross-linking reaction, which makes it preferential to be oxidized. This process is self-healing and can form a dense and stretched CEI, which is favorable to the cycle performance at the late stage. In contrast, the polymerization reaction will occur in one direction for LTFOP due to its lone oxalate ligand. Additionally, an unfavorable side reaction between LTFOP and EC has been proposed by the aid of Gibbs free energy calculation. This is a good explanation for the formation of the uneven and unstable CEI, as well as the continuous decomposition of the electrolyte in PRIs-EIS measurement. This work has an extensive applicability and practical significance not only for molecular designing of novel lithium salts, but also for the construction of a functional electrolyte system that is compatible with different electrode materials.

High-Energy-Density LiNi0.8Co0.15Al0.05O2 and Dual-Phase LTO-R-TiO2 Materials via a Microwave-Assisted Reaction: Alleviating the Capacity Fading Mechanism by Nanocoating of Al2O3 and PEDOT

“…Their high oxidizing ability during high-voltage operation forms an ultrathin cathode–electrolyte interface (CEI). − Similarly, this issue can be overcome by stabilizing the SEI using additives such as FEC. Based on several reports, the charged LNCA material undergoes various phase transitions by simultaneously releasing oxygen species at high temperatures. − Eventually, these highly reactive oxygen species react with the electrolytes, leading to thermal runaway and intragranular cracking within primary particles . Conversely, from Figure c,d, we observe that the morphology of the electrodes remained unaffected without any major structural deterioration or cracks over the surface.…”

“…14−16 These surface modifications ameliorated the disintegration of transition metals from LNCA, which suppressed the electrolyte decomposition and charge-transfer resistance. 17,18 However, the surface structure of the oxidecoated cathode weakened the protection layer due to continuous defacing over an extended period of cycling. 19,20 Recently, Liang et al successfully employed NaAlO 2 as a new aluminum source for cation mixing in the preparation of the LNCA cathode via the coprecipitation process that resulted in improved electrochemical performances.…”

“…The lithium-deficient phases in LNCA have made it highly inactive, with propagation of microcracks during the cycling process. , Consequently, various schemas were put forward, including cation substitution of Mg, Co, Al, or Mn at the nickel site. Furthermore, the inorganic coating such as TiO 2 , SiO 2 , Al 2 O 3 , ZrO 2 , AlPO 4 , CoPO 4 , AlF 3 , and Li 2 ZrO 3 lessened the detrimental cationic disorders. − These surface modifications ameliorated the disintegration of transition metals from LNCA, which suppressed the electrolyte decomposition and charge-transfer resistance. , However, the surface structure of the oxide-coated cathode weakened the protection layer due to continuous defacing over an extended period of cycling. , …”

Analysis of Main Factors Limiting the Improvement of Electrochemical Performances of Low‐Concentration Electrolyte

“…Particularly, white implies that d(−Z im )/d(Z re ) equals zero, corresponding to the top point of the semicircles in conventional Nyquist graphs. [ 27 ] It is obvious that the Li + ‐ion transfer impedance of the two electrolyte systems does not change significantly with the gradual increase of the voltage before 3.4 V, indicating that there is no obvious interface reaction at the present stage. In the range of 3.4–3.6 V, an obvious fluctuation process can be observed in the Z w part of the two electrolytes systems, indicating that an obvious interfacial reaction occurred at the LFP/electrolyte interface.…”

Antioxidation Mechanism of Highly Concentrated Electrolytes at High Voltage

“…When the voltage increases to potentials of PO 2 F 2 – re-oxidation, stable LiF, and unstable PO 2 F are formed (eq ). PO 2 F can easily react with Li 2 CO 3 on the electrode surface to form Li 3 PO 4 with better conductivity, thereby reducing the impedance of the CEI film (eq ), which is reflected in the improvement of the rate performance. ,, As a result, the key difference in the chemical composition of CEI films between HCE and dilute electrolyte is mainly the concentrations of LiF, Li x PO y F z , and Li 3 PO 4 , namely, the contribution of soluble products and the PF 6 – anion at high voltage (Scheme ). …”