A polymeric material consisting of self-terminated oligomers with hyper branched architecture, also known as STOBA, is used as surface coating of Li(Ni0.4Co0.2Mn0.4)O2 cathode particles, and it is shown to effectively suppress thermal runaway and slow down voltage drop of graphite/Li(Ni0.4Co0.2Mn0.4)O2 18650-battery during nail-penetration test. In situ synchrotron X-ray diffraction study indicates that the STOBA coating does not affect the layer-to-spinel phase transformation, suggesting that the suppressing mechanism is operative below the temperature of oxygen release from the oxide particles. Data of electrochemical and differential scanning calorimetry (DSC) analyses suggest that STOBA coating passivates the surface activity of charged cathode particles so that formation of solid-electrolyte-interphase (SEI) under high potential is suppressed and that heat-generation rate arising from SEI break-down and electrolyte decomposition by the charged cathode surface upon heating is significantly reduced. The combination of the reduced joule-heating and diminished heat generation by the electrolyte-oxide interfacial reaction may contribute to the suppression of thermal runaway as a result of the STOBA coating.