Nickel-rich layered metal oxide LiNi 1−y−z Mn y Co z O 2 (1 − y − z ≥ 0.8) materials are the most promising cathodes for next-generation lithium-ion batteries in electric vehicles. However, they lose more than 10% of their capacity on the first cycle, and interfacial/structural instability causes capacity fading. Coating and substitution are possible direct and effective solutions to solve these challenges. In this Letter, Nb coating and Nb substitution on LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) is easily produced through a scalable wet chemistry method followed by sintering from 400 to 800 °C. A Li-free Nb oxide treatment is found to remove surface impurities forming a LiNbO 3 /Li 3 NbO 4 surface coating, to reduce the first capacity loss and to improve the rate performance. Nb substitution stabilizes the structure, as evidenced by less heat evolution on heating, thus providing better long cycling stability with a 93.2% capacity retention after 250 cycles.

ε-VOPO4 has the potential to be the next high energy density cathode material for lithium-ion batteries due to its high thermal stability and its ability to reversibly intercalate two full Li+, giving a high discharge capacity of 305 mAh g−1. However, vanadyl phosphate materials typically experience poor Li+ kinetics that impedes the high-rate capability at the high voltage plateau. In this work, we applied niobium substitution to improve the high-rate performance of the 4.0 V plateau of ε-VOPO4. Elemental analysis and lattice parameter refinements determined that up to 10% Nb can be substituted into ε-VOPO4 without any impurities. TEM and Synchrotron-based EXAFS confirmed that all the substituted samples have Nb in the ε-VOPO4 structure. Electrochemical tests revealed that 1% Nb substitution can deliver a high discharge capacity of ∼300 mAh g−1 without any degradation in cycling behavior by maintaining its nanosized morphology.

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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 ...

Lithium vanadyl phosphate (LiVOPO4) is a next-generation multielectron battery cathode that can intercalate up to two Li-ions per V-ion through the redox couples of V4+/V3+ and V5+/V4+. However, its rate...