With rapid development of lithium ion batteries (LIB) and sodium ion batteries (SIB), hard carbon (HC) as new anode material has earned much attention. Besides its rich precursor sources and low cost, HC has higher Li⁺ storage capacity and better rate performance than graphite for LIB. Furthermore, it is also recognized as the most commercially potential anode material for SIB. However, low initial Coulombic efficiency is a common issue for HC. In addition, it is believed that the specific capacity can be further improved with the clarification of the Li/Na ion storage mechanism. In recent years, many researches on electrochemical mechanism have been conducted with some model assumptions proposed for better understanding the mechanism. This review introduced the structures and preparation approaches of HC as well as its application in LIB and SIB. The advantages, especially in fast charging, coating and other subdivision were discussed, and the different modification strategies such as pore structure design, doping, optimizing interface between electrode and electrolyte were summarized, aiming at the increase of capacity and the improvement of Coulombic efficiency of batteries.

Silicon sludge, the photovoltaic cutting silicon waste, has become one of the expected raw materials for the key silicon carbon anode materials used in high energy density batteries above 300 Wh·kg^-1 due to its low cost, two-dimensional lamellar structure and ultrahigh specific capacity (4200 mAh·g^-1). However, silicon sludge requires systematic modification because of its challenges such as complex composition, large particle size, poor electrical conductivity, low stability and poor electrochemical performance. This paper systematically reviews the application status and research progress of silicon sludge in lithium-ion batteries. Firstly, the important effects of metal and non-metal impurities on battery performance are summarized, in which metal impurities are normally removed by magnetic separation and acid pickling, and non-metallic impurities are removed by liquid-liquid extraction and heat treatment. Secondly, detailed elucidation about the initial performance and modification methods of the silicon sludge is provided. Concretely, silicon sludge can be nano-sized to reduce expansion by grinding, etching, electrothermal shock, and alloy dealloying, enhance electrical conductivity through doping the intrinsic silicon and doping the carbon layer on the silicon surface, improve stability through the construction of inert layer, conductive layer and functional group, and obtain mechanical support and protection through silicon-carbon composite. Finally, the challenges, development directions and future prospects of silicon-based anode based on silicon sludge are put forward, aiming to provide a reference for converting silicon sludge into treasure and promote the rapid development of high energy density lithium-ion batteries.

FeF₃∙0.33H₂O possesses the characteristics of high theoretical capacity and high voltage, but its electrochemical cycling performance is unsatisfactory due to its poor conductivity and serious volume change during redox reaction, resulting in limited application. In this study, by using the strategies of dopamine self-assembly coating, carbonization, HCl etching and HF fluorination, the yolk-shell structured composite FeF₃∙0.33H₂O@carbon nanoboxes (FeF₃∙0.33H₂O@CNBs) composed of N-doped graphene shell and nanocube FeF₃∙0.33H₂O core was synthesized. Its particle size is about 250 nm and thickness of carbon shell is 30-40 nm. FeF₃∙0.33H₂O@CNBs displays an initial charge-discharge capacity of 208 mAh·g^-1 at a current density of 0.2C(1C=237 mA·g^-1). After 50 cycles, the capacity remains 173 mAh·g^-1, and the capacity attenuation rate per cycle is only 0.3%. In comparison, the initial capacity of bare FeF₃∙0.33H₂O is 112 mAh·g^-1, and after 50 cycles, only 95 mAh·g^-1 reserves, indicating superior cycle performance of FeF₃·0.33H₂O@CNBs. Furthermore, charging and discharging results at 0.1C-1C show that the rate performance is also significantly better than bare FeF₃∙0.33H₂O. It’s due to that N-doped graphene shell prepared by this strategy provides good electron/ion transport performance. At the same time, the carbon shell can not only buffer and inhibit the volume change of the core FeF₃∙0.33H₂O, but also shorten the ion migration distance and improve the Li⁺ migration rate on the electrolyte storage and retention performance of the electrolyte. As a result, the electrochemical performances are better than those of previous literature.

2022 marks the 110th anniversary of X-ray diffraction (XRD), which is a powerful technique used to find out the nature of materials. Rietveld refinement method, as an important means of extracting material structure information, plays a significant role in establishing the relationship between structure and performance of materials. Cathode materials are a vital part of lithium-ion batteries (LIBs). In-depth understanding of their crystal structure and atomic distribution is extremely helpful to promote the development of cathode materials for LIBs. Cathode materials for LIBs are generally the hosts of lithium. Studies on lithium occupation and transfer are inseparable from a deep understanding of its structural characteristics. This review summarizes XRD Rietveld structure refinement and its application in cathode materials for LIBs. XRD Rietveld structure refinement in synthesis, degradation, and structural modification of cathode materials are analyzed by using several types of typical cathode materials as examples. XRD Rietveld method could provide useful structural information of the cathode materials, including phase ratio in composite and crystallographic parameters (e.g., cell parameters, key atomic occupation, and atomic coordinates). Therefore, exploring structure of cathode materials assisted with XRD Rietveld refinement method is of great significance for the development of high-performance cathode materials for LIBs. Finally, the opportunities and challenges in the field of X-ray diffraction technology in detecting structure of cathode materials for LIBs are prospected.

Poly(ethylene oxide) (PEO)-based solid electrolytes are deemed as the most promising alternatives for solid-state lithium batteries on account of their low cost, good stability against Li metal, and easy large-scale production. However, the instability of PEO against high-voltage cathodes severely limits its application in the fields needing high energy density. In this work, a discontinuous cyclized polyacrylonitrile (cPAN) nanolayer, served as an electron-conducting shell, is partially coated on LiNi_0.6Co_0.2Mn_0.2O₂ (NCM) cathode particles, while an ionic liquid acted as ion-conducting pathway is introduced at NCM/PEO interface, enabling the high compatibility of PEO against high-voltage NCM cathode. The cPAN layer not only physically isolates the direct contact of PEO electrolyte from NCM cathode, but also contributes to the electronic transfer inside the cathode due to the formation of delocalized sp² π bond during coating process. Meanwhile, the mobile ionic liquid with good ionic conductivity fully wets cathodic interface, followed by decomposition into cathode-electrolyte interphase (CEI) of LiF and Li₃N, further restricting the oxidation-failure of PEO electrolyte. By taking the combined strategy, the corresponding solid-state NCM/Li battery delivers an excellent electrochemical performance with a capacity retention of 85.3% after 100 cycles at rate of 0.1C (1C=0.18 A·g^-1) under a cutoff voltage of 4.30 V. This work opens up a new direction to address the interfacial stability issues of PEO-based electrolyte against high-voltage cathodes through surface coating and interface modification.

Si anodes hold immense potential in developing high-energy Li-ion batteries. But fast failure due to huge volume change upon Li uptake impedes their application. This work reports a facile yet low-toxic gas fluorination way for yielding F-doped carbon-coated nano-Si anode materials. Coating of nano-Si with F-doped carbon containing high defects can effectively protect Si from huge volume change upon Li storage while facilitating Li⁺ transport and formation of stable LiF-rich solid electrolyte interphase (SEI). This anode exhibits high capacities of 1540-580 mAh·g^-1 at various current rates of 0.2-5.0 A·g^-1, while retaining >75% capacity after 200 cycles. This method also addresses the issues of high cost and toxicity of traditional fluorination techniques that use fluorine sources such as XeF₂ and F₂.

Bentonite is an abundant, cheap and readily available natural clay mineral, with montmorillonite (MMT) as its main mineral composition. MMT possesses excellent ion exchange, adsorption and ion transport properties due to its unique two-dimensional layered nanostructure, abundant pore structure, and high specific surface area. Moreover, it also possesses excellent thermal, chemical and mechanical stabilities. In recent years, MMT has attracted extensive attention in the field of electrochemical energy storage owing to the above excellent characteristics, especially the inherent fast ion (Li⁺, Na⁺, Zn²⁺, etc.) transport properties. Thus, the bentonite-based functional materials have been widely applied to the key components (i.e., electrodes, polymer electrolytes, and separators) of electrochemical energy storage devices and show good application prospects. In this review, the structure and physicochemical properties of bentonite are firstly introduced, and then the research progress of bentonite-based functional materials in the field of electrochemical energy storage, mainly including metal anodes, lithium-sulfur battery cathodes, solid/gel polymer electrolytes, and polymer separators, is comprehensively summarized. On the basis of these facts, the ion transport promotion mechanism of bentonite-based functional materials during the process of electrochemical energy storage is elaborated. Finally, the current problems and challenges faced by application of bentonite-based materials in electrochemical energy storage devices are pondered, and the possible future research directions are prospected. This review provides useful guidance for the design and development of bentonite-based electrochemical energy storage functional materials.