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Cu–Ni–Mn-based ternary P2-type NaxCu0.15Ni0.20Mn0.65O2(x= 0.50, 0.67, and 0.75) cathodes for sodium-ion batteries (SIBs) are synthesized by a co-precipitation method. We find that Na content plays a key role on the structure, morphology, and the charge–discharge performances of these materials. For x= 0.67 and 0.75, superstructure from Na+-vacancy ordering is observed, while it is absent in the x= 0.50 sample. Despite the same synthesis conditions, materials with x= 0.67 and 0.75 show smaller particle sizes compared to that of the x= 0.50 sample. In addition, redox potentials of the materials differ significantly even though they have the same transition metal ratios. These differences are attributed to the changes in local structures of the as-prepared materials arising from the different amount of Na and possibly oxygen in the lattice. Materials with x= 0.67 and 0.75 show excellent rate performance and cycle stability when tested as cathode material of SIBs. Average discharge potential is as high as 3.41 V versus Na–Na+with capacity of 87 mAh g–1at 20 mA g–1. Excellent capacity and cycle stability are maintained even when they are tested with higher current rates. For instance, a capacity of 62.3 mAh g–1is obtained from the x= 0.67 sample at 1000 mA g–1after 1000 cycles between 3.0 and 4.2 V without any decrease in capacity.

A novel 3.3 V copper–lithium battery using a copper foil as the cathode is a potential candidate for next-generation energy storage system due to its simple manufacturing process. However, the cross-over of copper ions from the cathode to the anode limits the reversibility of the battery. Herein, we suppress self-discharge and migration of copper ions in the cell using a commercial polypropylene separator with a coating of polyacrylic acid (PAA), a chelating polymer. Fourier transform infrared spectroscopy confirms that the PAA layer traps the copper ions and prevents them from passing through. The addition of barium titanate nanoparticles into the PAA layer further enhances ionic transfer through the separator and reduces polarization of the cell at high current rates during charge and discharge. The use of a chelating agent with an inorganic filler as a coating layer on the separator is a cost-effective way to improve reversibility and round-trip efficiency of copper–lithium batteries.

Silicon monoxide is a potential high-capacity anode material for lithium-ion batteries. However, its low initial Coulombic efficiency hinders its adoption in commercial batteries. Here, we use an electrochemical approach, depth of discharge test, to study the origin of the irreversibility in SiO electrodes. We find two contributions to the irreversible capacity of SiO, depending on the reaction products during lithiation. Carbon-coating the material improves the reversible capacity as it increases its electrical conductivity. Disproportionation is an effective way to decrease irreversibility in the initial cycle with the formation of large cluster of SiO2which is electrochemically inactive. Though, the reversible capacity is reduced.

Silicon electrodes can give high capacity as anodes for lithium-ion batteries. However, there has not been much work quantifying the different contributions to the reversible and irreversible capacities. Here, we report the use of an electrochemical approach – depth of discharge test – to separate the charge-discharge capacities of crystalline silicon electrodes into four contributions: (1) SEI formation, (2) lithium accommodation in carbon and binder, (3) lithiation and delithiation of the Si active material, and (4) capacity loss associated with particle cracking and isolation. We find that the intrinsic coulombic efficiency for the crystalline-to-amorphous transition of Si during initial cycle is about 90%, which is independent of particle size. SEI formation is estimated to be about 10 mAh per square meters of active material surface and scales with BET surface area. Mechanical issues and particle isolation are observed in fully discharged electrode when the amount of binder is less than 20%. Capacity limitation prolongs lifetime of Si electrode, but the overall performance is governed by the coulombic efficiency (CE) during cycle. Low CE is due to continual SEI formation with cycling, increase utilization of Si upon cycling, trapping of Li in the material and mechanical failure of the electrode.