Stepwise Reduction of Electrochemically Lithiated Core-Shell Heterostructures Based on the Prussian Blue Analogue Coordination Polymers K0.1Cu[Fe(CN)6]0.7 .3.5H2O and K0.1Ni[Fe(CN)6]0.7 .4.4H2O

The magnetic properties of a series of core-shell particles based on the Prussian blue analogues K0.1Cu[Fe(CN)(6)](0.7).3.5H(2)O and K0.1Ni[Fe(CN)(6)](0.7).4.4H(2)O (CuFe-PBA@NiFe-PBA) are investigated as a function of electrochemical titration and lithium ion insertion. The particles, with average size 305 +/- 50 nm, are reduced using the galvanostatic intermittent titration technique to prepare 10 samples of Lix(CuFe-PBA@NiFe-PBA) with 0 <= x <= 1.0. Magnetization as a function of temperature for each member of the series reveals the ferromagnetic ordering of the individual components, NiFe-PBA (T-c similar to 24 K) and CuFe-PBA (T-c similar to 18 K). The magnetic ordering of each component is suppressed upon reduction and Li+ incorporation, but in a stepwise fashion with the CuFe-PBA core reduced before the NiFe-PBA shell. The separate reductions of the core and shell are also seen in magnetization vs field measurements at low temperature. By introducing a lattice-gas model, the enthalpy changes (Delta H-i) associated with each redox couple after Li-ion insertion were calculated and applied to the mean field approximation to reproduce the magnetic transition temperatures. The results are significant as the CuFe-PBA@NiFe-PBA particles had previously been shown to exhibit superior performance over the individual components as cathode materials for lithium ion batteries, although the stepwise reduction had not previously been discerned. Furthermore, the report is the first showing the systematic control of magnetism in core-shell coordination polymer heterostructures by electrochemical guest ion insertion.