NOVELTY - Surface-modified silicon particles are obtained by processing silicon particles by silane coupling agent (I), and forming an organic silane layer on surface of silicon particles.    USE - Surface-modified silicon particles are used for lithium-ion secondary battery for assembled battery used for vehicles (all claimed).    ADVANTAGE - The lithium-ion secondary battery containing the surface-modified silicon particles has excellent charging/discharging characteristics and durability. The negative electrode active material containing the surface-modified silicon particles suppresses drop-off of surface-modified silicon particles from conductive support agent or collector.    DETAILED DESCRIPTION - Surface-modified silicon particles are obtained by processing silicon particles by silane coupling agent of formula: R(4-n)Si(Y')n (I), and forming an organic silane layer on surface of silicon particles.    R=organic group-containing element or organic group having ( pi )-electron or lone pair of electrons;    Y'=monovalent hydrolyzable group;and    n=1-3.    INDEPENDENT CLAIMS are included for the following:    (1) manufacture of surface-modified silicon particles, which involves producing silanol group in surface of silicon particles and processing surface of silicon particles with silane coupling agent. The silanol group production process is carried out before silane coupling agent treatment process or during silane coupling agent treatment process;    (2) lithium-ion secondary battery (10), which contains surface-modified silicon particles as negative electrode material (13); and    (3) vehicle, which is obtained by mounting lithium-ion secondary battery or assembled battery as motor driving power supply.    DESCRIPTION OF DRAWING(S) - The drawing shows the cross-sectional view of lithium-ion secondary battery.    Lithium-ion secondary battery (10)    Negative electrode collector (11)    Negative electrode material (13)    Positive electrode material (15)    Electrolyte layer (17)