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

Abstract

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.

Lithiation/delithiation of silicon heavily doped with boron synthesized using the Czochralski process

“…30 In our previous study, we synthesized highly crystalline P-doped Si (n-type, 2000 ppm) and B-doped Si (p-type, 1600 ppm) by the Czochralski method used in the production processes of semiconductor wafers. 37 Using the single-crystal powder obtained by pulverizing the ingots, we have also demonstrated on a slurry-based electrode that the B-doped Si not only provides Li insertion reaction at a higher potential than that of the P-doped Si, but also exhibits a greater Li + diffusion coefficient. Among p-type semiconductors, especially when focusing on the B-doped Si with powder-based slurry electrodes, many of the studies have relied on ball milling for the synthesis of B-doped Si.…”

“…Inductively coupled plasma (ICP; ICPE-9800, Shimadzu) and time-of-flight secondary ion mass spectrometry (TOF-SIMS; IONTOF GmbH) were used for the analysis of the quantitative determination and the distribution of dopant in the respective Si samples. 37 In the TOF-SIMS measurements, a Bi nanoprobe was the standard primary ion source (Bi 3 2+ , 30 keV, 100 Â 100 mm 2 ) and pulsed to get good mass resolution and a low energy sputter beam of O 2 (1 keV, 300 Â 300 mm 2 ) was employed for the depth analysis. Structural changes in the B-doped Si were tracked using X-ray diffractometry (XRD, SmartLab, Rigaku) and Raman spectroscopy (LabRAM HR Evolution; HORIBA, Ltd) with the 532 nm line of a diode-pumped solid-state laser at room temperature.…”

“…As a typical method, impurity doping into Si has been addressed in many cases. [23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] Regarding the lithiation through two crystallographic faces of Si (100) and (111) using wafers, Greeley and Gewirth et al experimentally found that B doping (p-type) reduces the Li insertion capacity, but the lithiation is energetically as well as kinetically favorable, and the onset potential of lithiation is at a higher potential than that of the P doping (n-type). 30 In our previous study, we synthesized highly crystalline P-doped Si (n-type, 2000 ppm) and B-doped Si (p-type, 1600 ppm) by the Czochralski method used in the production processes of semiconductor wafers.…”

Vital roles of fluoroethylene carbonate in electrochemical energy storage devices: a review

“…8 In this case, three main strategies have been developed to resolve these problems as shown in Fig. 5(a), namely, (a) the utilization of nanoscale particles to better accommodate large strain without cracking by controlling the size and morphology of Si, such as thin films with different thicknesses, 8,44,104,105 nanowires, 41,106,107 nanoparticles with different sizes, 108–112 amorphous columnar silicon, 113,114 porous silicon powder, 115 Si-flake powder, 116–118 single crystal Si(100) (110) (111), 119,120 Si nanosheets, 121 and P-doped Si; 122 (b) the use of carbon material-based composites to accommodate and protect the volume change in Li–Si alloys (Fig. 5(b)), such as Si/derived-carbon, 123 Si/graphite (Fig.…”

Impact of Low Temperatures on the Lithiation and Delithiation Properties of Si-Based Electrodes in Ionic Liquid Electrolytes

“…Silicon (Si) is an up-and-coming material for negative electrodes in next-generation LIBs because its theoretical capacity (3580 mA h g –1 for Li 3.75 Si) is higher than that of currently used graphite (372 mA h g –1 for Li 0.17 C). − Nevertheless, the poor cycling performance of Si-based electrodes is an obstacle to their practical use, which can be attributed to following: the significant volume change during lithiation and delithiation which generates considerable stress and high strain in active materials; high electrical resistivity; and a low Li + diffusion coefficient. − Various attempts have been made to address such issues, for example, synthesizing nanosized Si materials to prevent the occurrence of surface cracking and fracture; ,, reducing the electrical resistivity of Si by coating it with carbon materials; , fabricating composite electrodes to cover the shortcomings of Si; − doping Si with impurities, such as phosphorus (P), boron, antimony, or arsenic, to adjust its properties, including its electrical resistivity, Li distribution, crystallinity, and morphology; − preparing silicides to give ductility and electronic conductivity specific to metals; − and the prelithiation of Si to increase the initial Coulombic efficiency. − …”