Surface Reduction Stabilizes the Single-Crystalline Ni-Rich Layered Cathode for Li-Ion Batteries

Abstract

The surface of the layered transition metal oxide cathode plays an important role in its function and degradation. Modification of the surface structure and chemistry is often necessary to overcome the debilitating effect of the native surface. Here, we employ a chemical reduction method using CaI2 to modify the native surface of single-crystalline layered transition metal oxide cathode particles. High-resolution transmission electron microscopy shows the 2 formation of a conformal cubic phase at the particle surface, where the outmost layer is enriched with Ca. The modified surface significantly improves the long-term capacity retention at low rates of cycling, yet the rate capability is compromised by the impeded interfacial kinetics at high voltages. The lack of oxygen vacancy generation in the chemically induced surface phase transformation likely results in a dense surface layer that accounts for the improved electrochemical stability and impeded Li-ion diffusion. This work highlights the strong dependence of the electrode's (electro)chemical stability and intercalation kinetics on the surface structure and chemistry, which can be further tailored by the chemical reduction method.Here, we employed a chemical redox reaction to modify the surface of a single-crystalline LiNi0.83Mn0.06Co0.11O2 (SX-NMC). This redox method induces a conversion reaction at the SX-NMC particle surface, which transforms into a dense NiO-like cubic phase. Notwithstanding the apparent similarity with the electrochemically induced surface transformation and the impeded interfacial reaction kinetics, this chemical transformation obviates the generation of the oxygen vacancies and leads to improved capacity retention over long-term cycling. Experimental Methods Preparationof oxidized single-crystalline NMC. The single-crystalline LiNi0.83Mn0.06Co0.11O2 (SX-NMC) was purchased from Ruyuan East Sunshine Magnetic Materials Co., Ltd. The oxidation of SX-NMC was performed by the reaction of SX-NMC powder with liquid bromine in a round-bottom flask. Excess bromine was used for this reaction. The molar ratio between bromine and SX-NMC was 10:1. The mixture was stirred overnight at room temperature to assure complete reaction. After the reaction, the resulting powder was recovered by centrifuge and washed several times with anhydrous acetonitrile until the liquid became clear to assure the complete removal of bromine. The powder was finally dried under vacuum at 65 ℃. This oxidized SX-NMC is denoted as Br-NMC. Preparation of reduced SX-NMC. CaI2 (VWR) was used as the reducing agent to reduce Br-NMC. In a typical experiment, 0.0905 g CaI2 was added to 10 mL anhydrous acetonitrile in a 20 mL vial, which was covered with aluminum foil to avoid the decomposition of CaI2 through light exposure. The mixture was sonicated and stirred for 15 min to ensure the complete dissolution of CaI2. The CaI2 solution was light pink. Then 0.6 g Br-NMC powder was added into the CaI2 solution and stirred at 65 ℃ for two days. The color of the solution started to turn ...

Durable and Adjustable Interfacial Engineering of Polymeric Electrolytes for Both Stable Ni‐Rich Cathodes and High‐Energy Metal Anodes

“…[ 55 ] Examination of XPS results from SIP‐NCM cathodes obtained from cycling at 4.4 V manifests that both characteristic peaks attributed to the adverse reactions are slightly elevated after 50 cycles (Figure S23, Supporting Information), probably catalyzed by high‐valence Ni. [ 56 ] The results show that both oxidative decomposition and Al corrosion reaction reinforce each other, leading to significant deterioration of interfacial compatibility in the TiP case. By contrast, the SIP‐derived coating confers to an inhibition of Al corrosion and EO oxidation simultaneously.…”

“…For comparison, the unpolymerized monomers in the TiP case are susceptible to oxidative decomposition on the NCM cathode surface, which forms an inhomogeneous Li + barrier accompanied by LiTFSI decomposition and the generation of H + . [35,56] Furthermore, the decomposition reaction is aggravated by the presence of OH terminal groups in the TiP-derived SPE. [58] As a result, Al corrosion is induced by H + , which in turn causes the inferior interface to further accelerate the electrolyte decomposition.…”

Exploring the Optical and Energetic Properties of a Co(II)-Based Mixed Ligand MOF

“…However, an in-depth investigation has demonstrated that electrode materials play a crucial role in the performance of lithium-based batteries. Finding appropriate electrode materials with high electronic conductivity, a large specific surface area, high efficiency, and great electrochemical performance is the major challenge toward developing efficient LIB storage systems. − Currently, several materials have been investigated as LIB electrodes, including metal oxide, − graphene, − carbon nanotubes, , and so on. Due to their superior electrical conductivity and large pore volume, these materials are considered as good electrodes or support materials for lithium-based batteries.…”