Voltage hysteresis loop as a fingerprint of slow kinetics Co2+-to-Co3+ transition in layered NaxCox/2Ti1−x/2O2 cathodes for sodium batteries,Journal of Materials Chemistry A

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Voltage hysteresis loop as a fingerprint of slow kinetics Co2+-to-Co3+ transition in layered NaxCox/2Ti1−x/2O2 cathodes for sodium batteries
Journal of Materials Chemistry A ( IF 10.7 ) Pub Date : 2022-11-21 , DOI: 10.1039/d2ta07972k
Daria Mikhailova ₁ , Mikhail V. Gorbunov ₁ , Hoang Bao An Nguyen ₁ , Björn Pohle ₁ , Sebastian Maletti ₂ , Christian Heubner ₃

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Sodium transition metal oxides are one of the most promising cathode materials for future sodium ion batteries. Chemical flexibility of layered Na-oxides including cobalt enables its partial substitution by other redox-active or non-active metals, often leading to structural stabilization. Sharing the same structural positions with other transition metals in layered oxides, Co can be double- or triple-charged, and as Co³⁺ can adopt a low-spin (LS), intermediate-spin (IS), high-spin (HS) state, or a combination of them. Using Ti⁴⁺ in the structure together with Co²⁺ results in a reduced number of phase transformations compared to Ti-free compositions. However, a large potential hysteresis of about 1.5–2.5 V between battery charge and discharge is observed, pointing a first-order cooperative phase transition. Based on several examples, we found that Na extraction from Na_xCo_x/2Ti_1−x/2O₂ materials with high-spin HS-Co²⁺, crystallizing in the P2 or O3 structure, mostly results in valence and spin-state transition of Co, leading to the formation of a second phase with a low-spin LS-Co³⁺, and a much smaller unit cell volume. We elucidated a kinetic origin of the potential hysteresis, which can be minimized by increasing temperature or reduction of the current density during battery cycling with P2- and O3-Na_0.67Co_0.33Ti_0.67O₂ materials. The slow kinetics of the structural phase transition, especially upon Na-insertion, hampers the application of classical methods of electrochemical thermodynamics, such as determining the entropic potential dE/dT. We showed that the entropic potential depends only on the Na-content in Na_xCo_0.33Ti_0.67O₂ during battery charge or discharge, what additionally confirms a kinetic nature of the potential hysteresis.

中文翻译：

电压滞后环作为钠电池层状 NaxCox/2Ti1−x/2O2 阴极中缓慢动力学 Co2+ 到 Co3+ 转变的指纹

钠过渡金属氧化物是未来钠离子电池最有前途的正极材料之一。包括钴在内的层状钠氧化物的化学灵活性使其能够被其他氧化还原活性或非活性金属部分取代，通常会导致结构稳定。与层状氧化物中的其他过渡金属共享相同的结构位置，Co 可以是双电荷或三电荷，Co ³⁺可以采用低自旋 (LS)、中自旋 (IS)、高自旋 (HS) ) 状态，或它们的组合。在结构中使用 Ti ^{4+和 Co}²⁺与不含 Ti 的组合物相比，相变次数减少。然而，观察到电池充电和放电之间存在约 1.5-2.5 V 的大潜在滞后，表明存在一级协同相变。基于几个实例，我们发现从具有高自旋HS-Co ^2+的Na_x Co _{x /2} Ti _{1− x /2} O ₂材料中提取Na ，在P2或O3结构中结晶，主要导致价态和自旋-Co 的状态转变，导致形成具有低自旋 LS-Co ^{3+的第二相}，以及更小的晶胞体积。我们阐明了潜在滞后的动力学起源，可以通过在使用 P2- 和 O3-Na _0.67 Co _0.33 Ti _0.67 O ₂材料的电池循环过程中提高温度或降低电流密度来最小化。结构相变的缓慢动力学，尤其是在插入 Na 时，阻碍了电化学热力学经典方法的应用，例如确定熵势 d E /d T。我们表明熵势仅取决于 Na _x Co _0.33 Ti _0.67 O _{2中的 Na 含量}在电池充电或放电期间，还证实了潜在滞后的动力学性质。

更新日期：2022-11-25

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