Negative electrode material for lithium ion secondary batteries, method for manufacturing the same, paste for negative electrode, negative electrode sheet, and lithium ion secondary

1. A negative electrode material for lithium ion secondary batteries, comprising composite material particles containing nanosilicon particles having a 50% particle diameter (D n50 ) of 5 to 100 nm in the number-based cumulative particle size distribution of primary particles, graphite particles and an amorphous carbon material,

the composite material particles containing the nanosilicon particles covered with only amorphous carbon material,

the composite material particles containing the nanosilicon particles at a content of 30 mass % or more and 60 mass % or less, and the amorphous carbon material at a content of 30 mass % or more and 60 mass % or less,

the composite material particles having a 90% particle diameter (D V90 ) in the volume-based cumulative particle size distribution of 10.0 to 40.0 μm,

the composite material particles having a BET specific surface area of 1.0 to 5.0 m 2 /g,

the composite material particles having an exothermic peak temperature in DTA measurement of 830° C. to 950° C., and no exothermic peak temperature in DTA measurement of less than 830° C.

2. The negative electrode material for lithium ion secondary batteries according to claim 1 , wherein the composite material particles have a 50% particle diameter (D V50 ) in the volume-based cumulative particle size distribution of 5.0 to 25.0 μm.

3. The negative electrode material for lithium ion secondary batteries according to claim 1 , wherein the graphite particles have a BET specific surface area of 5.0 to 50.0 m 2 /g.

4. A paste for negative electrodes, comprising the negative electrode material for lithium ion secondary batteries according to claim 1 .

5. A negative electrode sheet comprising the negative electrode material for lithium ion secondary batteries according to claim 1 .

6. A lithium ion secondary battery comprising the negative electrode sheet according to claim 5 .

7. A method for manufacturing a negative electrode material for lithium ion secondary batteries made of composite material particles, comprising the steps of:

pulverizing a mixture obtained by mixing nanosilicon particles having a 50% particle diameter (D n50 ) in a number-based cumulative particle size distribution of primary particles of 5 to 100 nm and a carbon precursor at a softening point or higher temperature of the carbon precursor to obtain nanosilicon-containing particles comprising the nanosilicon particles covered with only amorphous carbon material (step 1),

treating a mixture, obtained by mixing the nanosilicon-containing particles and graphite particles, in an inert gas atmosphere at a temperature of 900° C. or more and 1200° C. or less and then pulverizing the processed mixture to obtain composite material particles (composite material particles 1) (step 2), and

treating a mixture, obtained by further mixing the composite material particles 1 with the nanosilicon-containing particles, in an inert gas atmosphere at a temperature of 900° C. or more and 1200° C. or less and then pulverizing the processed mixture to obtain composite material particles (composite material particles 2) (step 3),

wherein the composite material particles have an exothermic peak temperature in DTA measurement of 830° C. to 890° C. and no exothermic peak temperature in DTA measurement of less than 830° C.

8. The method for manufacturing a negative electrode material for lithium ion secondary batteries according to claim 7 , wherein the carbon precursor is a petroleum pitch or a coal pitch.