High Mass Loading and Stable Cycling Micrometer Tin Electrode Material for Sodium-Ion Batteries

High-capacity anode materials are essential for high-energy sodium-ion batteries. Sn anodes with their high theoretical capacity can face significant volume expansion (i.e., up to 420%) during sodiation, thus affecting material stability. Previous strategies for long-cycling micron-sized Sn are low loadings and inadequate for commercial needs. Enhancing cyclic stability under a high mass loading is thus crucial for high-performance sodium-ion batteries. In this paper, we simply mixed Sn with HC to form an HC "fence" that could prevent Sn particle reagglomeration and allow self-evolution. Adding 50% (in mass) HC boosted Sn/HC cyclic stability without shedding or cracking after 100 cycles at 70% (in mass) HC. The optimized μSn₃₀HC₇₀ delivered 1.8 mA·h/cm² at 200 mA/g and retained 93.3% capacity after 200 cycles at 500 mA/g. The μSn₃₀HC₇₀||Na₃V₂(PO₄)₃(NVP)full cell had 186 W·h/kg energy density at 0.5 C and retained 92.11% capacity at 6 C.

A slurry was prepared via mixing Sn/HC, conductive agent (SP), and sodium alginate (SA) in deionized water in a mass ratio of 8:1:1. The mixture was coated onto carbon-coated aluminum foil, dried under vacuum at 80 ℃ for 12 h, and then cut into electrode disks with the diameter of 11 mm. The electrode disks with different μSn_xμHC_(100–x) ratios were obtained, i.e., HC, μSn₂₀HC₈₀, μSn₃₀HC₇₀, μSn₄₀HC₆₀, and μSn₅₀HC₅₀, via adjusting Sn content in the mixed anode (x=0%, 20%, 30%, 40%, and 50%). In an argon-filled glovebox (the volume fractio of H₂O, O₂ ≤ 1×10^–6), 2325-type coin-cell half-cells were assembled with sodium as a counter/reference electrode, Celgard2500 as a separator, and 1 mol/L NaPF₆ in Bis(2-methoxy ethyl)ether as an electrolyte. The full cells of (μSn₃₀HC₇₀||NVP and μSn||NVP) were also constructed with μSn₃₀HC₇₀/μSn as an anode and NVP as a cathode. The calculations of capacity and energy density excluded the masses of binder and conductive agent.

At a loading of 5.5 mg/cm², μSn experiences a rapid capacity decay, losing up to 60% after 200 cycles. However, blending 50% (in mass) HC enhances Sn/HC cyclic stability under the same conditions. The SEM images of the sample after 100 cycles show the best-preserved electrode surface morphology with 70% (in mass) of HC, absent of active material shedding or surface cracking. This indicates that 70% (in mass) of HC achieves a structural stability for Sn anode, exhibiting a promising potential for developing long-term cyclic stability. At a current density of 500 mA/g, this electrode retains 93.3% of its initial capacity after 200 cycles.

The results show that the capacity contribution of HC within the electrode material increases as the HC content increases, via comparing the voltage-capacity profiles of Sn/HC anodes with different HC proportions. The charging platforms of Sn at 0.2 V and 0.31 V merge into a platform, potentially due to the reduced transport resistance facilitated by HC. This can be further clarified via comparing the electrochemical impedance spectroscopy (EIS) data of μSn₃₀HC₇₀ and pure Sn, where the intrinsic impedance and diffusion impedance of Sn/HC both are lower than those of pure Sn.

The ex-situ X-ray Diffraction (XRD) patterns indicate the phase transitions of Sn/HC at different potentials. A peak at 2θ of 35.7° appears and disappears during the charge-discharge process, indicating the high reversibility of Sn material during cycling.

The μSn₃₀HC₇₀||NVP full cell validates Sn/HC practicality in sodium-ion batteries. achieving 186 W·h/kg energy density at 0.5 C and retains 92.11% at 6 C

Mixing HC with Sn effectively could enhance the cyclic stability of micron-sized Sn. This was attributed to the HC fence structure around Sn particles. The optimized μSn₃₀HC₇₀ anode had an areal capacity of 1.8 m·h/cm² at a current density of 200 mA/g and retained 93.3% after 200 cycles at a current density of 500 mA/g. The full cell assembled with NVP maintained a capacity retention of 92.11% after 100 cycles at 0.5 C, further validating the effectiveness of the HC "fence" structure design in improving the stability of Sn anodes under high-mass loading conditions.