The construction of metal phosphate coating layers has been widely recognized as an effective strategy for protecting high-energy cathode materials in lithium-ion batteries (LIBs). However, due to the low solubility of metal phosphates, their precipitation in aqueous solution becomes too fast to control, making it a significant challenge to ensure a heterogeneous growth process towards surface coatings. Herein, we report a solution-based synthetic process to achieve conformal metal phosphate coating through coordination-assisted precipitation, which involved the reaction between phytic acid (PA), urea, and metal ions, particularly Nb⁵⁺, in ethanol solution to achieve a well-tamed coating effect. The use of PA, a plant-derived compound known as inositol hexaphosphoric acid, was facile to form soluble phytate-metal complex, which precipitated with urea to form metal-phosphate-containing coatings with thickness controlled at high precision. This proposed synthetic protocol was applied for the surface coating of high-voltage cathode materials in the form of LiNi_0.5Mn_1.5O₄ (LNMO), leading to significantly enhanced structural and electrochemical stability for its working at 5 V. Notably, after 300 cycles, the modified LNMO was able to achieve capacity retention of 85.1% for its working at 45 °C at a current density of 1 C as compared to only 29.9% of the pristine sample. Our findings highlight the potential of solution-based processes in building conformal coatings for the stabilization of high-energy cathode materials in their LIBs application.

High-capacity Li-rich cathode materials can significantly improve the energy density of lithium-ion batteries, which is the key limitation to miniaturization of electronic devices and further improvement of electrical-vehicle mileage. However, severe voltage decay hinders the further commercialization of these materials. Insights into the relationship between the inherent structural stability and external appearance of the voltage decay in high-energy Li-rich cathode materials are critical to solve this problem. Here, we demonstrate that structural evolution can be significantly inhibited by the intentional introduction of certain adventive cations (such as Ni²⁺) or by premeditated reservation of some of the original Li⁺ ions in the Li slab in the delithiated state. The voltage decay of Li-rich cathode materials over 100 cycles decreased from 500 to 90 or 40 mV upon introducing Ni²⁺ or retaining some Li⁺ ions in the Li slab, respectively. The cations in the Li slab can serve as stabilizers to reduce the repulsion between the two neighboring oxygen layers, leading to improved thermodynamic stability. Meanwhile, the cations also suppress transition metal ion migration into the Li slab, thereby inhibiting structural evolution and mitigating voltage decay. These findings provide insights into the origin of voltage decay in Li-rich cathode materials and set new guidelines for designing these materials for high-energy-density Li-ion batteries.