Energy storage mechanism for hybrid battery

Many devices require both high energy and high power density, and lithium ion batteries and super-capacitors cannot separately always meet the requirements. In this work, we study the operating mechanism of a hybrid battery, which combines the best properties of batteries and supercapacitors. We analyze the lithium ion storage mechanism using XRD, Raman, TEM and electrochemical measurements. The model system studied combines a non-intercalating carbon black anode with a LiFePO$_{\mathrm{4}}$ cathode. At 50{\%} state of charge, XRD data for LiFePO$_{\mathrm{4}}$ cathode material shows a mixture of LiFePO$_{\mathrm{4}}$ and FePO$_{\mathrm{4}}$, indicating battery reaction. On the other hand, the activated carbon remains structurally unchanged. We also discuss the impact of a range of activated carbon/ LiFePO$_{\mathrm{4}}$ (AC/LFP) ratios. From cyclic voltammetry and charge/discharge results, the system exhibits battery-domain characteristics when the AC/ LFP ratio is below one, but showing more supercapacitor-domain traits when the ratio is higher. Besides, the systems have higher rate capacity at AC/LFP ratio around four as compared to one. This research is supported by NSF under Award Number 1318202.