An effective lithium incorporation strategy to boost the charge-storage capacity of bimetallic metal–organic frameworks with theoretical insights and solid-state lithium-ion capacitors

Abstract

In this work, a strategy to incorporate lithium into the bimetallic NiCo MOF (Li–NiCo MOF) is proposed by a simple hydrothermal process to architect the structure and tune the electronic properties of the electrodes. The electrochemical results show that the reversibility of Li-ion storage in the Li–NiCo MOF half-cell composite is significantly higher than that of NiCo MOF, and it shows excellent rate performance, charging capability, and stable long cycle life. Moreover, the effective process of lithium incorporation by the hydrothermal method (in situ) improves the charge-storage properties and is also compared with the lithium incorporation achieved by the sonochemical method. The experimental findings are correlated with the incorporation of Li into the O–Co–O and M₂O₄ sites of the NiCo MOF by density functional theory calculations. The results confirm the reduction of band energy and, consequently, an increase in the electronic conductivity and charge transport. Furthermore, Li incorporation at O–Co–O sites shows better Li diffusion properties confirmed by the reduction of diffusion barrier potential, compared to the pristine NiCo MOF. A solid-state lithium-ion capacitor (Li–NiCo MOF//AC) is fabricated, which achieves a high energy density of 145 W h kg⁻¹ while maintaining a power density of 400 W kg⁻¹ and depicts a high power density of 4000 W kg⁻¹ at 40.2 W h kg⁻¹ of energy density. The capacitor device exhibits excellent electrochemical structural stability, and can still maintain an original capacity of 85% after 3000 charging–discharging cycles. Thus, lithium incorporation into the MOF structures could be of great significance for improving lithium storage and LIC device performance.