Cellulose fibers, cellulose fiber-containing composition, cellulose fiber dispersion, and method for producing cellulose fibers

Cellulose fibers having a fiber width of 1000 nm or less and having phosphoric acid groups or phosphoric acid group-derived substituents, wherein

• the content of the phosphoric acid groups or phosphoric acid group-derived substituents is 0.5 mmol/g or more, and
• the supernatant yield measured by the following measurement method (a) is 70% or less;
• measurement method (a):
• an aqueous dispersion A of the cellulose fibers having a solid concentration of 0.5% by mass is prepared, a defibration treatment is then performed on the aqueous dispersion A using a defibration treatment device under conditions of 21500 rotations/minute for 30 minutes to obtain an aqueous dispersion B;
• subsequently, the aqueous dispersion B is converted to an aqueous dispersion C having a solid concentration of 0.2% by mass, which is then subjected to a centrifugation treatment under conditions of 12000 G and 15° C. for 10 minutes, and then, a supernatant is recovered;
• and the solid concentration in the recovered supernatant is measured, and a supernatant yield is then calculated according to the following equation:
• Supernatant yield (%)=solid concentration (% by mass) in supernatant/0.2 (% by mass)×100.