A facile and generic method to improve cathode materials for lithium-ion batteries via utilizing nanoscale surface amorphous films of self-regulating thickness.

As a facile and generic surface modification method, a unique class of surface amorphous films (SAFs) is utilized to significantly improve the rate performance and cycling stability of cathode materials for lithium-ion batteries. These nanoscale SAFs form spontaneously and uniformly upon mixing and annealing at a thermodynamic equilibrium, and they exhibit self-regulating or "equilibrium" thickness due to a balance of attractive and repulsive interfacial interactions acting on the films. Especially, spontaneous formation of nanoscale Li3PO4-based SAFs has been demonstrated in two proof-of-concept systems, LiCoO2 and LiMn1.5Ni0.5O4, which have an equilibrium thickness of ∼2.9 nm and ∼2.5 nm, respectively. At a high discharge rate of 25 C, these Li3PO4-based SAFs improve the discharge capacity by ∼130% for LiCoO2 and by ∼40% for LiMn1.5Ni0.5O4, respectively. Furthermore, these SAFs improve the cycling stability and reduce capacity fading of both LiCoO2 and LiMn1.5Ni0.5O4. At an elevated temperature of 55 °C, Li3PO4-based SAFs can help to maintain ∼90 mA h g(-1) discharge capacity of the high-voltage material LiMn1.5Ni0.5O4 after 350 cycles at a relatively high charge-discharge rate of 1 C. Further mechanistic studies suggest that these SAFs reduce the interfacial charge transfer resistance and suppress the growth of the solid-electrolyte interphase. This facile method can be utilized to improve a broad range of cathode and anode materials. A thermodynamic framework is proposed, which can be used to guide future experiments of other material systems.