In the pursuit of global carbon neutrality and sustainable energy systems, the development of efficient and cost-effective energy storage technologies is paramount. Among various options, sodium-ion batteries have emerged as a promising alternative to lithium-ion batteries due to the abundance and widespread distribution of sodium resources. However, the practical application of sodium-ion batteries, especially in extreme environments such as low temperatures, faces significant challenges related to electrode kinetics and interfacial stability. In this study, we investigate the influence of hard carbon anode kinetics on the cycling durability and low-temperature performance of sodium-ion batteries, employing commercial materials to simulate real-world conditions. By examining the structural and electrochemical properties of different hard carbon sources, we aim to elucidate the mechanisms governing performance degradation at low temperatures and propose strategies for improvement.
The sodium-ion battery system comprises a sodium iron phosphate pyrophosphate (Na4Fe3(PO4)2P2O7) cathode and various hard carbon anodes, assembled into pouch cells with a nominal capacity of 900 mAh. We focus on three types of hard carbon: biomass-derived HC-A, resin-derived HC-B, and biomass-derived HC-C. These materials were characterized using scanning electron microscopy, X-ray diffraction, Raman spectroscopy, and electrochemical techniques to correlate their physical properties with kinetic behavior. The charge-discharge cycling tests were conducted at temperatures ranging from -30°C to 25°C, with varying current rates to assess fast-charging capability and low-temperature resilience.
The structural parameters of the hard carbons, derived from XRD analysis, are summarized in Table 1. The interlayer spacing (d002), crystallite sizes along the c-axis (Lc) and a-axis (La), and the empirical R-value indicate distinct microstructural differences. These parameters are critical as they influence sodium-ion diffusion pathways and storage mechanisms. For instance, a larger d002 generally facilitates easier ion insertion, which is essential for maintaining performance at low temperatures.
| Hard Carbon | d002 (Å) | Lc (nm) | La (nm) | R-value |
|---|---|---|---|---|
| HC-A | 3.88 | 1.17 | 2.23 | 2.87 |
| HC-B | 3.75 | 1.18 | 2.25 | 4.34 |
| HC-C | 3.77 | 1.19 | 2.40 | 3.24 |
The electrochemical kinetics were probed using electrochemical impedance spectroscopy (EIS) and distribution of relaxation times (DRT) analysis. The EIS spectra typically consist of high-frequency semicircles attributed to solid electrolyte interphase (SEI) resistance, mid-frequency semicircles related to charge transfer resistance, and low-frequency Warburg regions indicative of solid-state diffusion. The DRT method deconvolutes these processes by transforming frequency-domain data into time-domain relaxation spectra, allowing for precise identification of individual resistances. The total impedance (Z(ω)) can be expressed as:
$$Z(\omega) = R_0 + \int_0^\infty \frac{\gamma(\tau)}{1 + j\omega\tau} d\tau$$
where $R_0$ is the ohmic resistance, $\gamma(\tau)$ is the distribution function of relaxation times, $\omega$ is the angular frequency, and $\tau$ is the relaxation time. The charge transfer resistance ($R_{ct}$) and SEI resistance ($R_{SEI}$) extracted from DRT analysis for the hard carbons are compared in Table 2. These values directly impact the rate capability and low-temperature performance of sodium-ion batteries.
| Hard Carbon | $R_{SEI}$ (Ω) | $R_{ct}$ (Ω) | Total Impedance (Ω) |
|---|---|---|---|
| HC-A | 12.5 | 25.3 | 45.8 |
| HC-B | 18.7 | 40.1 | 68.9 |
| HC-C | 13.0 | 30.5 | 52.5 |
To further quantify ion transport, galvanostatic intermittent titration technique (GITT) was employed to determine the sodium-ion diffusion coefficient ($D_{Na^+}$) during charge and discharge. The diffusion coefficient can be calculated using the following equation based on Fick’s second law:
$$D_{Na^+} = \frac{4}{\pi\tau} \left( \frac{n_m V_m}{S} \right)^2 \left( \frac{\Delta E_s}{\Delta E_\tau} \right)^2$$
where $\tau$ is the pulse time, $n_m$ is the number of moles, $V_m$ is the molar volume, $S$ is the electrode area, $\Delta E_s$ is the steady-state voltage change, and $\Delta E_\tau$ is the voltage change during the pulse. The $D_{Na^+}$ values in the slope region (above 0.1 V) and plateau region (below 0.1 V) are critical for understanding storage behavior. As shown in Table 3, HC-A exhibits the highest diffusion coefficients, particularly in the plateau region, which correlates with its superior low-temperature performance.
| Hard Carbon | $D_{Na^+}$ in Slope Region (cm²/s) | $D_{Na^+}$ in Plateau Region (cm²/s) |
|---|---|---|
| HC-A | 1.2 × 10-10 | 5.6 × 10-12 |
| HC-B | 8.5 × 10-11 | 1.9 × 10-12 |
| HC-C | 1.1 × 10-10 | 2.3 × 10-12 |
The cycling performance of sodium-ion batteries at room temperature (25°C) under 1C charge-discharge rates revealed similar capacity retention among the three hard carbons, but significant differences in energy efficiency. The energy efficiency ($\eta$) is defined as:
$$\eta = \frac{E_{discharge}}{E_{charge}} \times 100\%$$
where $E_{discharge}$ and $E_{charge}$ are the energy outputs during discharge and inputs during charge, respectively. HC-A achieved an energy efficiency of 94%, compared to 90% for HC-B, highlighting the impact of kinetic properties on overall battery efficiency. At higher charge rates (5C charging), HC-A and HC-C maintained stable capacity retention over 100 cycles, while HC-B showed rapid capacity fade due to sodium plating on the anode surface, as evidenced by post-mortem analysis.
Low-temperature performance is a critical metric for sodium-ion batteries intended for use in diverse climates. We evaluated the cells at 0°C with 0.5C charging and 1C discharging. HC-A and HC-C demonstrated stable cycling with capacity retention above 97% after 50 cycles, whereas HC-B failed to support even 15°C operation at 0.5C charging. At -10°C with 0.2C charging, HC-A exhibited robust performance, but HC-C suffered from severe sodium plating, leading to accumulation of by-products on the separator and anode. The discharge capacity ratio at -30°C (0.5C discharge capacity relative to room temperature capacity) further emphasized the kinetic advantages: HC-A achieved 87.5%, HC-C 86.7%, and HC-B only 83.7%. The discharge voltage plateau also varied with hard carbon kinetics, affecting energy output at low temperatures.
To address the poor low-temperature performance of HC-B, we explored the effect of adjusting the negative-to-positive capacity ratio (N/P ratio) in cell design. The N/P ratio is defined as:
$$\text{N/P ratio} = \frac{\text{Area-specific capacity of anode}}{\text{Area-specific capacity of cathode}}$$
By increasing the N/P ratio from 1.1 to 1.2, we effectively raised the anode potential during charging, thereby reducing the risk of sodium plating. As summarized in Table 4, the cell with an N/P ratio of 1.2 maintained approximately 104% capacity retention after 100 cycles at -10°C with 0.1C charging, whereas the cell with an N/P ratio of 1.1 degraded rapidly. This strategy demonstrates that optimizing cell design parameters can mitigate kinetic limitations of hard carbon anodes in sodium-ion batteries.
| N/P Ratio | Capacity Retention after 100 Cycles (%) | Coulombic Efficiency (%) | Energy Efficiency (%) |
|---|---|---|---|
| 1.1 | 95.7 (after 3 cycles only) | 93.7 | 68.4 |
| 1.2 | ~104 | ~99 | ~78 |
The relationship between hard carbon structure and kinetic properties can be modeled using an Arrhenius-type equation for temperature-dependent processes:
$$k = A \exp\left(-\frac{E_a}{RT}\right)$$
where $k$ is the rate constant (e.g., for charge transfer or diffusion), $A$ is the pre-exponential factor, $E_a$ is the activation energy, $R$ is the gas constant, and $T$ is the temperature. Hard carbons with larger interlayer spacing and lower crystallinity, such as HC-A, tend to have lower activation energies for sodium-ion diffusion, enabling better performance at low temperatures. This is consistent with our observation that HC-A’s diffusion coefficient in the plateau region is nearly three times that of HC-B.
Furthermore, the formation and stability of the SEI layer play a crucial role in low-temperature operation. At reduced temperatures, ion transport slows down, and SEI resistance increases, leading to higher polarization. The polarization voltage ($\Delta V$) can be expressed as:
$$\Delta V = I \cdot (R_{SEI} + R_{ct} + R_{diff})$$
where $I$ is the current, and $R_{diff}$ is the diffusion-related resistance. Hard carbons with lower $R_{SEI}$ and $R_{ct}$, as seen in HC-A, minimize polarization, thus preserving capacity and efficiency. In contrast, HC-B’s high resistances result in significant polarization, triggering sodium plating and capacity fade.
In conclusion, our study systematically investigates the impact of hard carbon anode kinetics on the low-temperature performance of sodium-ion batteries. We demonstrate that hard carbons with superior kinetic properties, characterized by larger interlayer spacing, lower crystallinity, and higher diffusion coefficients, enable excellent cycling stability and discharge capacity at temperatures as low as -30°C. Conversely, hard carbons with poor kinetics suffer from sodium plating and rapid degradation. Adjusting the N/P ratio in cell design proves to be an effective strategy to enhance low-temperature resilience by modulating anode potential. These findings provide valuable insights for the optimization of sodium-ion batteries, particularly for applications in cold climates and fast-charging scenarios. Future work should focus on tailoring hard carbon microstructures and electrolyte formulations to further push the boundaries of low-temperature sodium-ion battery technology.
The advancement of sodium-ion battery technology hinges on continuous improvement in material kinetics and cell engineering. As we strive for broader adoption in energy storage systems, understanding and addressing low-temperature challenges will be pivotal. Our research underscores the importance of anode material selection and design parameters in achieving reliable performance across a wide temperature range, paving the way for more resilient and efficient sodium-ion batteries.