The ocean accounts for 97% of the total water resources on earth, covering over 70% of the map's surface area. With the continuous consumption of non-renewable energy sources such as fossil fuels and the rapid development of renewable energy, humans are increasingly paying attention to the utilization of ocean resources. Ocean energy includes tidal energy, wave energy, temperature difference energy, and salinity gradient energy. Salinity gradient energy is the energy generated by the interaction of seawater and fresh water, which is the ocean energy existing in the form of chemical energy. This energy is mostly generated in estuaries. The osmotic pressure generated by mixing water with different salinity can be converted into electrical energy driven by potential differences or ion gradients. Salinity gradient energy, as a new renewable energy source, has received widespread global attention and research in recent years, making rapid progress. The utilization of salinity gradient energy provides a renewable and sustainable alternative to the recent surge in global energy consumption.

At present, pressure delay osmosis technology, reverse electrodialysis technology and capacitive mixing technology are three main technologies for extracting salinity gradient energy. In this work, we built a new type of salt difference cell based on capacitive mixing technology, using molybdenum disulfide (MoS₂) and multiwalled carbon nanotubes (MoS₂/MWCNTs) composite electrode as the anode and an activated carbon (AC) as the cathode.

We composited two materials with different ion storage mechanisms together. MoS₂ has a layered structure like graphene, with an interlayer spacing of about twice that of graphene. It is a battery electrode material that can undergo intercalation reaction with Na⁺. MWCNTs have a typical double electric layer effect. When discharging, while adsorbing Na⁺ on its surface, it can help Na⁺ enter the interlayer of MoS₂ more quickly, accelerating the ion transport efficiency and the extraction efficiency of salt differential energy. We conducted physical and electrochemical characterizations of MoS₂/MWCNTs composite material, and tested its salt difference energy extraction ability on a salt difference battery composed of it and AC electrode. We found that the concentration response voltage reached 150 mV, and the energy density of the extracted salt difference energy after a complete four-step cycle reached up to 6.96 J·g^-1. The advantages of low raw material price of the device and without using ion membranes make it more environmentally friendly, providing a new approach for the study of extracting salinity gradient energy.

Nowadays, the development of high-voltage LiCoO₂ (lithium cobalt oxide, LCO) cathodes has attracted the widespread attention from both the academic and industry fields. Among the multiple concerns, researches on the surface issues would provide the most effective performance optimization pathway for the synthesis of high-voltage LCO. In this work, the issues of high-voltage LCO, including the phase transitions and crack formation, the oxygen redox related issues and side reactions, as well as the surface structure degradation, have been systematically reviewed. Then, we further clarify the surface modulations, and the interplay between the surface modulation and electrolyte tuning. Finally, we propose our prospects for developing the more advanced LCO cathodes, including the low-cost and high-quality manufacturing, designing suitable LCO cathodes in some extreme conditions (such as high-temperature, high-rate charging, low temperature, etc.), and achieving stabilized capacity release of about 220 mAh·g^-1 of LCO, etc. We hope that this work can serve as a reference to promote the development and application of high-voltage LCO in future.

Developing in situ spectroelectrochemistry methods, which can provide detailed information about species transformation during electrochemical reactions, is very important for studying electrode reaction mechanisms and improving battery performance. Studying real-time changes in the surface of electrode materials during normal operation can be an effective way to assess and optimize the practical performance of electrode materials, thus, in situ and in operando characterization techniques are particularly important. However, batteries are hard to be studied by in situ characterization measurements due to their hermetically sealed shells, and there is still much room for battery characterizations. In this work, a specially designed battery based on the structure of coin cells, whose upper cover was transparent, was constructed. With such a device, acquisition of diffuse reflectance spectra of electrode materials during charging and discharging was realized. This not only provided a simple measurement accessory for diffuse reflectance spectroscopy (DRS), but also complemented in situ characterization techniques for batteries. Taking commonly used cathode materials in lithium-ion batteries (LIBs), including LiFePO₄ (LFP), NCM811 and LiCoO₂ (LCO) as examples, we managed to find out the response relationships of different electrode materials to visible light of different wavelengths under ordinary reflectance illumination conditions. Heterogeneity of different cathode materials on interaction relationships with the lights of different wavelengths was also revealed. This work demonstrated the capability of guiding wavelength selection for different materials and assessing electrochemical performances of in situ diffuse reflectance spectroelectrochemistry. By combining electrochemistry with diffuse reflectance spectroscopy, this work made an effective complementary for spectroelectrochemistry.