Dopant Effect on Lithiation/Delithiation of Highly Crystalline Silicon Synthesized Using the Czochralski Process

The huge theoretical capacity of 3580 mA h g–1 based on alloying reactions with Li strongly motivates the use of Si as a negative electrode material in the construction of Li-ion batteries with high energy density. However, its poor electrical conductivity and low Li+ diffusion coefficients as well as the significant volume change in Si during lithiation/delithiation are the bottlenecks for practical application. As one typical method for improving their properties, impurity doping into Si has been considered; however, many of them were amorphous or the impurity concentration against depth direction was not homogeneous, which has complicated the understanding of the effect of impurities on the lithiation/delithiation of Si. In this work, we synthesized Si ingots heavily doped with P (2000 ppm) or B (1600 ppm) as a dopant using the Czochralski method, which overcomes the above issues. In galvanostatic charge/discharge conditions using slurry-type electrodes, the electrical conductivity increased by 108 times compared to that of the undoped Si. The P- (0.119 V) and B-doping (0.126 V) allowed the Li-insertion reaction to occur at higher potentials compared to undoped Si (0.113 V). Even when using 5 vol % fluoroethylene carbonate as a film-forming additive and suppressing an excess volume change in Si during lithiation by the limitation of a charge capacity of 1000 mA h g–1, the undoped Si showed poor cyclability. In contrast, the optimized condition maximized the effect of the improved electrical conductivity and the mechanical properties and thereby enabled the P-doped Si to achieve an excellent performance for more than 300 cycles. Considering the softer properties and the greater fracture toughness of the P-doped Si (1514 HV and P–Si: 363.6 kJ mol–1) than those of B-doped Si (1787 HV and B–Si: 317 ± 12 kJ mol–1), the mechanical properties and fracture toughness, rather than electrical conductivity, make a significant contribution.