Ion exchange is a powerful method to access metastable materials with advanced functionalities for energy storage applications. However, high concentrations and unfavourably large excesses of lithium are always used for synthesizing lithium cathodes from parent sodium material, and the reaction pathways remain elusive. Here, using layered oxides as model materials, we demonstrate that vacancy level and its corresponding lithium preference are critical in determining the accessible and inaccessible ion exchange pathways. Taking advantage of the strong lithium preference at the right vacancy level, we establish predictive compositional and structural evolution at extremely dilute and low excess lithium based on the phase equilibrium between Li_0.94CoO₂ and Na_0.48CoO₂. Such phase separation behaviour is general in both surface reaction-limited and diffusion-limited exchange conditions and is accomplished with the charge redistribution on transition metals. Guided by this understanding, we demonstrate the synthesis of Na_yCoO₂ from the parent Li_xCoO₂ and the synthesis of Li_0.94CoO₂ from Na_yCoO₂ at 1–1,000 Li/Na (molar ratio) with an electrochemical assisted ion exchange method by mitigating the kinetic barriers. Our study opens new opportunities for ion exchange in predictive synthesis and separation applications.

离子交换是一种获取具有先进功能的亚稳态材料的强大方法，可用于储能应用。然而，从母体钠材料合成锂阴极时总是使用高浓度和不利的大量过量的锂，并且反应途径仍然难以捉摸。在这里，使用层状氧化物作为模型材料，我们证明空位水平及其相应的锂偏好对于确定可及和不可及的离子交换途径至关重要。利用正确空位水平下的强烈锂偏好，我们基于Li _0.94 CoO ₂和Na _0.48 CoO ₂之间的相平衡，建立了在极稀和低过量锂下的预测成分和结构演化。这种相分离行为在表面反应限制和扩散限制交换条件下都很常见，并且是通过过渡金属上的电荷重新分布来实现的。在这一认识的指导下，我们演示了利用电化学辅助离子从母体 Li _x CoO ₂合成 Na _y CoO ₂以及从Li/ _{Na （摩尔比）为 1–1,000 的 Na y} CoO ₂合成 Li _0.94 CoO 2通过减轻动力学障碍的交换方法。我们的研究为预测​​合成和分离应用中的离子交换开辟了新的机遇。