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 LiMn_1.5Ni_0.5O₄, 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 LiMn_1.5Ni_0.5O₄, respectively. Furthermore, these SAFs improve the cycling stability and reduce capacity fading of both LiCoO₂ and LiMn_1.5Ni_0.5O₄. 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 LiMn_1.5Ni_0.5O₄ 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.