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In this work, toward uniform in situ carbon coating on nano-LiFePO 4 (nano-LFP) via a solid-state reaction, we systematically investigated the effects of the heating rate on the characteristics of the product and process with nano-FePO 4 as the template and iron source. It was found that the high reactivity of nano-FePO 4 almost eliminated the effect of the heating rate on the phase transformation of LiFePO 4 (LFP) but aggravated the impacts on the morphology and the distribution of the carbon layer of the final product. The uniform carbon coating on nano-LFP was found to correspond to a moderate diffusion rate of gas molecules containing carbon, as determined by the thermal decomposition characteristics of the carbon source and the heating rate together. This strategy showed promise in the accelerated synthesis of nano-LFP/carbon composites with high electrochemical performance and was verified to be effective when using different carbon sources, sucrose and polyvinyl alcohol, which had distinct differences in thermal decomposition characteristics. As a result, we achieved the preparation of nano-LFP with an excellent rate performance (140 mA•h•g −1 @10 C) using a high heating rate (15 °C/min), low calcination temperature (650 °C), and short calcination time (4 h).

In this work, we focused on the interpretation on a nonclassical crystallization route of Na x Mn­[Fe­(CN)6] y ·nH2O (0 < x < 2, 0 < y < 1, MnHCF) nanocrystal preparation via a bottom-up approach. Combining explosive homogeneous nucleation in the microreactor and subsequent crystal growth in the microtube, we carefully studied the dynamic behaviors of MnHCF crystallization under a high time resolution and found that in the early growth stage, it went through a rapid transition of morphology and crystal structure with the enrichment of the sodium content and the decrease of Fe­(CN)6 defect and lattice water. Meanwhile, reducing the primary particle size, increasing the particle concentration, or adjusting the sodium ion concentration could accelerate the transition process, and the sodium ion also played an important role in the anisotropic aggregation-mediated growth. Based on these cognitions, we proposed the growth mechanism of the nonclassical crystallization process from the interplay of free-energy landscapes and particle reaction dynamics. In addition, the crystal architecture laws of MnHCF via a bottom-up approach were summarized to afford rapid and flexible regulation on morphologies and crystal composites.

In this work, for the performance enhancement of iron hexacyanoferrate, an electrochemically active Mn-doped iron hexacyanoferrate cathode is fabricated via a bottom-up approach. It is found that the pre-treatment of interstitial water and appropriate Mn doping are two keys to achieving higher capacity and higher stability. The interstitial water has a trade-off effect between the alleviation of volume expansion upon Na+ (de)­intercalation and the retardation of Na-ion diffusion. The moisture-tailored iron hexacyanoferrate with appropriate Mn doping exhibits a high initial Coulombic efficiency of 94.8%, enhanced capacity and rate performance, and excellent cycling stability. These results benefit from the fact that the extraction/insertion of Na ions from/into the lattice via a solid-solution mechanism correspond to both the slight volume expansion and fast sodium diffusion rate; otherwise, the removal of interstitial water and a higher Mn content might lead to poor cycling stability due to excessive volume expansion resulting from rhombohedral to cubic phase transformation. Finally, the less demand on the control of air humidity for the fabrication of electrodes and the potential for the full cell coupled with hard carbon are also demonstrated, which shows great potential for practical applications.

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In this work, using electrochemical active Fe as an ion-exchange element (attack side) and the Na x MnFe(CN) 6 slurry with a high solid content (MnHCF) as a template (defensive side), a series of binary hexacyanoferrates are prepared via a simple Mn/Fe ion-exchange process, in which Na x FeFe(CN) 6 (FeHCF) and solid solution Na x (FeMn)Fe(CN) 6 are concentrated on the shell and the core, respectively. The proportions of the two structures are mainly controlled by the competition between the ion-exchange rate in the bulk material and dissolution-reprecipitation rate. Slowing down the attacking rate, such as the use of a chelating agent complexed with the attacker Fe, is advantageous to form a thermodynamically metastable state with homogeneous distribution of elements since the diffusion of Fe 2+ in the solid MnHCF is relatively fast. The shell FeHCF could be adjusted by the dissolution-reprecipitation rate, which is driven by the solubility difference. Adding the chelating agent in the defensive side will promote the dissolution of MnHCF and reprecipitation of FeHCF on the surface. Meanwhile, with the increase of Fe sources, the thickness of the shell FeHCF increases, and correspondingly the content of solid solution decreased due to FeHCF is more stable than solid solutions in thermodynamics. Finally, such a design principle in this case study could also be generalized to other ion-exchange processes. Considering the difference of two components in solubility, the larger difference can make the core/shell structure more clear due to the enhancement of dissolution–reprecipitation route.

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