Designing the Charge Storage Properties of Li-Exchanged Sodium Vanadium Fluorophosphate for Powering Implantable Biomedical Devices

The growing demand for bioelectronics has generated widespread interest in implantable energy storage. These implantable bioelectronic devices, powered by a complementary battery/capacitor system, have faced difficulty in miniaturization without compromising their functionality. This paper reports on the development of a promising high-rate cathode material for implantable power sources based on Li-exchanged Na _1.5 VOPO ₄ F _0.5 anchored on reduced graphene oxide (LNVOPF-rGO). LNVOPF is unique in that it offers dual charge storage mechanisms, which enable it to exhibit mixed battery/capacitor electrochemical behavior. In this work, electrochemical Li-ion exchange of the LNVOPF structure is characterized by operando X-ray diffraction. Through designed nanostructuring, the charge storage kinetics of LNVOPF are improved, as reflected in the stored capacity of 107 mAh g ⁻¹ at 20C. A practical full cell device composed of LNVOPF and T-Nb ₂ O ₅ , which serves as a pseudocapacitive anode, is fabricated to demonstrate not only high energy/power density storage (100 Wh kg ⁻¹ at 4000 W kg ⁻¹ ) but also reliable pulse capability and biocompatibility, a desirable combination for applications in biostimulating devices. This work underscores the potential of miniaturizing biomedical devices by replacing a conventional battery/capacitor couple with a single power source.