UC San Diego

A facile and low-cost route based on thermodynamic principles of surface amorphous films (SAFs), intergrainular films (IGFs), and cation surface segregation benefits electrochemical performances of electrode materials for lithium-ion batteries and sodium-ion batteries.

SAFs as a facile and generic surface modification method is utilized to significantly improve the rate performance and cycling stability of cathode materials for lithium-ion batteries. A thermodynamic framework of SAFs is proposed. These nanoscale SAFs form spontaneously and uniformly upon mixing and annealing at a thermodynamic equilibrium, and they exhibit a self-regulating or “equilibrium” thickness due to a balance of attractive and repulsive interfacial interactions acting on the films. Specially, spontaneous formation of nanoscale Li3PO4-based SAFs has been demonstrated in two proof-of-concept systems LiCoO2 and LiNi0.5Mn1.5O4. Furthermore, SAFs introduced by nitridation can also benefit the performance of TiO2 anode material for sodium-ion batteries. The amorphous intergrainular films (IGFs) are found in the system of Sn doped Si anode for lithium-ion batteries. The coexistence of IGFs and porous secondary structure (characterized by FIB/SEM on the cross section) results in an enhanced performance. SAFs and IGFs can be used to guide future experiments of other material systems.

Utilizing anisotropic surface segregation to thermodynamically control the particle morphology and the surface composition is another economic, facile, and effective method to significantly improve the electrochemical performance of battery electrodes. WO3 doping and anisotropic surface segregation can change the facet relative surface energy to tailor the particle Wulff shape of LiMn1.5Ni0.5O4 spinel materials and the surface Mn/Ni ratio and benefits performances. The WO3 surface segregation can also improve Co-free Li-rich layered oxide Li1.13Ni0.3Mn0.57O2 cathode material performance. X-ray photoelectron spectroscopy in conjunction with ion sputtering has shown that W segregates to the particle surfaces and decreases the surface Ni/Mn atomic ratio; high-resolution transmission electron microscopy has further suggested that the segregation of W increases the structural disorder at the particle surfaces, which may also benefit the rate performance.