Recently, NaKFePO4F, a layered iron-based fluorophosphate, has been proposed as a promising cathode material for both sodium-ion (SIBs) and potassium-ion batteries (KIBs), with an ion-exchange strategy significantly enhancing its capacity and addressing its low electronic conductivity. However, the atomic-scale mechanisms driving these improvements have yet to be fully explained. For this reason, density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations were systematically employed to assess the electrochemical feasibility of NaKFePO4F as a novel cathode material for these batteries. Analysis of energetically stable configurations reveals that a 50 % exchange of Na with K stabilizes and activates the previously inert sites in the pristine Na2FePO4F material. Notably, NaKFePO4F exhibits enhanced thermodynamic stability and electronic conductivity, with a reduced band gap of 2.40 eV compared to 3.18 eV in the pristine material. Moreover, NaKFePO4F was found to exhibit a low activation energy barrier of 0.42 eV for K ions, as determined by climbing image nudged elastic band (CI-NEB) computations. AIMD predictions also indicate that this material can sustain elevated temperatures from 300 K to 800 K, with ion diffusivity described accordingly. Ultimately, NaKFePO4F achieved an average discharge voltage of 3.67 V and an energy density of 426 Wh/kg for KIBs, surpassing the 3.49 V discharge voltage and 405 Wh/kg energy density of SIBs. Given these predicted results, NaKFePO4F is expected to be a promising cathode material for post-lithium-ion battery technology.

中文翻译：

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NaKFePO4F 氟磷酸盐作为 Na/K 离子电池高性能正极材料的计算研究


最近，NaKFePO4F 是一种层状铁基氟磷酸盐，已被提议作为钠离子 （SIB） 和钾离子电池 （KIB） 的有前途的正极材料，其离子交换策略显着提高了其容量并解决了其低电子导电性问题。然而，驱动这些改进的原子级机制尚未得到充分解释。为此，系统地采用密度泛函理论 （DFT） 和从头计算分子动力学 （AIMD） 模拟来评估 NaKFePO4F 作为这些电池的新型正极材料的电化学可行性。对能量稳定构型的分析表明，50 % 的 Na 与 K 交换可稳定并激活原始 Na2FePO4F 材料中先前的惰性位点。值得注意的是，NaKFePO4F 表现出增强的热力学稳定性和电子导电性，与原始材料的 3.18 eV 相比，带隙降低了 2.40 eV。此外，通过攀升图像轻推弹性带 （CI-NEB） 计算确定，发现 NaKFePO4F 对 K 离子表现出 0.42 eV 的低活化能屏障。AIMD 预测还表明，这种材料可以承受 300 K 到 800 K 的高温，离子扩散率也相应描述。最终，NaKFePO4F 实现了 KIB 的平均放电电压为 3.67 V，能量密度为 426 Wh/kg，超过了 SIB 的 3.49 V 放电电压和 405 Wh/kg 能量密度。鉴于这些预测结果，NaKFePO4F 有望成为后锂离子电池技术的一种有前途的正极材料。