Solid-state synthesis of transition-metal (TM) oxide materials remains a "black box" involving its input reactants and output products, in which both thermodynamic and kinetic factors govern crystalline evolution [1]. It is well established that changes in enthalpy (ΔH) and entropy (ΔS) dictate chemical reactivity and drive phase transitions based on ΔG = ΔH – TΔS [2]. Nevertheless, uncovering the individual contributions of entropy and enthalpy to the synthesis of inorganic oxides remains a longstanding challenge [3]. This limitation arises from the intricate coupling between dynamics and thermodynamics, which promotes non-equilibrium reactions [4]. From a thermodynamic standpoint, gaseous species inherently possess significantly higher entropy than their solid or liquid counterparts; thus, their presence exerts a decisive influence on reaction pathways in solid-state transformations. In fact, reactive gaseous species such as O₂, CO₂, and H₂O(g) are often used in the synthesis of high-capacity oxide cathodes, but the exact effects of these gases on the direction of reaction processes and structural evolutions in solid-state synthesis are not well understood [5, 6].

Advanced characterization techniques have been employed to reveal the underlying solid-state synthesis mechanism of these TM oxide materials. These achievements have improved the comprehension of the influence of synthetic conditions on the resulting microstructure and composition, but have rarely been able to separate out thermodynamic from kinetic factors. Furthermore, while significant research attention has been devoted to Ni-rich oxide cathodes [7], the synthesis and understanding of Mn-based oxide materials have been less explored. Notably, Mn has emerged as a vital constituent of ideal cathodes, exhibiting a significantly higher crustal abundance (950 ppm) than Ni (84 ppm) and Co (25 ppm). Consequently, Mn-based materials benefit from increasing global annual mining output and lower raw material costs [8]. Moreover, as a 3d TM, Mn displays a broad spectrum of oxidation states, ranging up to +7, with its electrochemically active redox couples typically occurring between +2 and +4. To fill this basic knowledge gap, Zuo et al. [9] recently reported a breakthrough study in Nature Nanotechnology, in which they used carbonate [Ni_1/4Mn_3/4]CO₃ precursor to uncover the contributions of chemical and structural changes to the entropy and enthalpy changes of Mn-based oxides during solid-state synthesis. Operando synchrotron X-ray diffraction (SXRD), transmission X-ray microscopy (TXM), and multiscale modeling were used to understand the microstructural evolution of earth-abundant Mn-based oxide cathodes for advanced Na-ion batteries (NIBs).

To study the structural and chemical changes during solid-state reactions, operando SXRD (Figure 1A) was used to investigate the [Ni_1/4Mn_3/4]CO₃–NaOH mixture (Na/TM = 0.67) heated at 6℃ min⁻¹. This system undergoes a definite series of structural changes, beginning with the generate of [Ni_1/4Mn_3/4](OH)₂, γ-Na₂CO₃ and O-type oxide below the melting temperature of the sodium source, as described in Equations (1–3) (Figure 1B). After further heating, the remaining carbonates break down, and the TM oxides are further evolved to transient rock-salt and/or spinel structures, and finally to P3-type or O3-type oxides (Equations (4–10) in Figure 1B). The synergistic interaction between thermodynamic drives and dynamic constraints in solid-state reactions results in overlapping phase domains and gradual gas evolution from concurrent reaction pathways. In summary, the transformation of carbonate precursor into layered oxides proceeds via three dominant pathways, accompanied by substantial CO₂ release. Subsequently, a cascade of non-equilibrium phase transitions occurs, encompassing O3-type and P3-type structures. These pathways are dynamically interacting and competing during the sintering process, leading to a complex evolution of structure and composition.

Figure 1 Solid-state synthesis mechanism of earth-abundant Mn-based oxide materials. (A) Operando SXRD results of [Ni_1/4Mn_3/4]CO₃–NaOH mixture heating from 26℃ to 750℃; (B) corresponding chemical reactions and equations within solid-state synthesis; (C) schematic diagram of chemical reaction sequences of Path 1 to 3; (D) temperature–time–phase transition diagram; (E) operando heating TXM-XANES results of morphological and electronic evolution of [Ni_1/4Mn_3/4]CO₃–NaOH mixture under slow (2.3℃ min⁻¹) and fast (24.5℃ min⁻¹) heating rates. Reproduced with permission from Ref. [9]. Copyright 2025, Springer Nature

To rationalize the observed phase evolution sequence (Figure 1C), theoretical calculations were performed to determine the ΔG associated with the related solid-state reactions. The results show that both Path 1 and 2 are thermodynamically favorable at all temperatures (0–1,000 K) and that the reaction energies become more negative for both pathways as the temperature increases, indicating that enthalpy is the main driving force. Path 3, on the contrary, shows a strong temperature dependence. Equations (4, 5) (Figure 1B) are thermodynamically spontaneous at temperatures greater than 420 and 750 K, respectively. The temperature-dependent term TΔS is positive for all four species, but is much more temperature sensitive for the gaseous species than for the solid species [4]. The entropy-driven contribution is largest for CO₂, which is more complex with more rotational and vibrational degrees of freedom. Furthermore, the direct carbonate-to-layered transformation is thermodynamically favored by a moderate increase in the Na/TM molar ratio and favors the formation of layered oxides [10, 11].

The oxide particles obtained (heated at 950℃) have two different morphologies: (i) compact spheroids with smooth surface and (ii) clustered spheroids with irregular surface and texture. A time–temperature–transition diagram (Figure 1D) is depicted to better understand the mechanistic origin of these morphological differences. The reaction pathway changes from enthalpy-driven (Path 1) to more entropy-driven pathways (Path 3) as temperature increases. Thus, higher heating rates induce enthalpy- and entropy-driven reaction paths, resulting in a more open and more clustered architecture of the particles. To test this hypothesis, the heating rate was deliberately varied from 0.5℃ min⁻¹ to 20℃ min⁻¹ to see how the relative contributions of entropy and enthalpy driven within solid-state reactions changed. Scanning electron microscopy (SEM) images clearly showed the dependence of imaging results on the ramp rate: the spheroidal particles were grouped together, and the proportion of such particles increased with the ramp rate, while compact spheroids with smooth surfaces became predominant at lower ramp rates (0.5℃ min⁻¹ and 1℃ min⁻¹).

To elucidate the effect of heating rate on regulating the material's defect evolution, operando TXM combined with X-ray near-edge spectroscopy (XANES) of [Ni_1/4Mn_3/4]CO₃–NaOH mixture at two heating rates (2.3℃ min⁻¹ and 24.5℃ min⁻¹) was analyzed. As shown in Figure 1E, pore change and Ni white-line (WL) energy as a function of heating temperature were extracted, and it was found that inner pores start to generate at a lower temperature, about 150℃, at the lower heating rate of 2.3℃ min⁻¹, which corresponds to the initial reactions of Path 1 (Figure 1D). Compared with their counterparts at low heating rates, the pores at a high heating rate (24.5℃ min⁻¹) are significantly smaller at 450℃ and 750℃, and the pore ratio is also much smaller, but the pores begin to grow at an elevated heating rate between 750℃ and 950℃, and at the highest heating rate, the pore ratio has reached an increased value of 18.6%.

The electrochemical properties of the as-prepared P2-type cathodes were investigated in the voltage range between 1.5 and 4.0 V (vs. Na/Na⁺). The P2-type Na_2/3Ni_1/4Mn_3/4O₂ electrode made at 20℃ min⁻¹ shows a reversible capacity of 152 mAh g⁻¹, which is higher than other P2-type cathodes prepared by slower heating rates (0.5–10℃ min⁻¹). The P2-type Na_2/3Ni_1/4Mn_3/4O₂ (20℃ min⁻¹) electrode still has more than ten times higher capacity than the P2-type Na_2/3Ni_1/4Mn_3/4O₂ (0.5℃ min⁻¹) counterpart at a high current density of 1,500 mA g⁻¹. Furthermore, the P2-type Na_2/3Ni_1/4Mn_3/4O₂ (20℃ min⁻¹) exhibits good long-term cycling stability. After 200 cycles at 150 mA g⁻¹, the P2-type cathodes synthesized at heating rates of 0.5℃ min⁻¹, 1℃ min⁻¹, 5℃ min⁻¹, 10℃ min⁻¹, and 20℃ min⁻¹ retain 88%, 89%, 89%, 92%, and 94% of their initial capacities, respectively.

To conclude, this pioneering work shows that the gas evolution of CO₂ from the Mn-based carbonate precursor acts as a thermodynamic switch to provide an entropy-driven synthesis route to high-performance P2-type oxides. This study successfully separates the effects of enthalpy and entropy on determining the pathways of both reactions and the structural evolution of layered TM oxides during the calcination process, thereby providing fundamental insights into how thermodynamic factors influence the electro-chemo-mechanical properties and overall electrode performance. To prevent the formation of undesirable defect nucleation and growth, such as chemical heterogeneity, residual lattice strain and closed-pore generation, a rationally designed fast-sintering strategy is proposed. Given the common crystallographic framework and analogous degradation mechanisms of these layered TM oxides [12, 13], the rapid-sintering strategy, originally developed for P2-type cathodes, can be readily extended to P3- and O3-type counterparts to improve their electrochemical and mechanical performance and advance the development of low-cost NIBs. Altogether, this work provides mechanistic insights and synthetic strategies for tuning thermodynamic and kinetic parameters in a synthetic approach to maximize the performance of earth-abundant Mn-based oxide cathode materials in advanced batteries.