An equivalent electrical circuit model based on parameters taken from ac impedance measurements obtained from a Li-ion polymer battery is simulated in a Matlab/Simulink environment. The model representation contains relevant parameters, including ohmic resistance, slow migration of Li-ions through the surface layers, faradaic charge transfer process, solid-state diffusion of Li-ions, and charge accumulation (intercalation capacitance) within the host material. The model also takes into account the non-homogeneous distribution properties (e.g, particle size, pore geometry) of the electrode which account for deviation from the ideal finite space Warburg behavior. The simulated and experimental results are compared and demonstrate that the impedance model can accurately predict the discharge power performance and transient and dynamic behavior of the Li-ion polymer batteries.
A cycle life study was done on commercial lithium-ion polymer batteries to quantify contributions to capacity fade with continuous charge–discharge cycling. The cell consists of graphite (meso-carbon microbeads) as an anode material and lithium cobalt oxide
(LixCoO2)
as a cathode material. Analyses were done using ac impedance spectroscopy, scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. The results show that contributions to capacity fade with continuous charge/discharge cycling included solvent–salt deposition on the anode surface; however, instability and cation disorder in the cathode electrode were identified as the main reasons for capacity fade with continuous charge/discharge cycling.
Olivine structure LiFePO 4 (LFP) was synthesized via solid state processes, using Li 2 CO 3 , NH 4 H 2 PO 4 , and FeC 2 O 4 ⋅H 2 O and C 12 H 22 O 11 as precursor materials. The effects of calendaring are analyzed in terms of electrochemical performance, cycle life, surface morphology, and ac impedance analysis. The resulting LFP electrode was divided into calendared and uncalendared samples. Under electrochemical impedance testing, the calendared and uncalendared electrodes exhibited a charge transfer resistance of 157.8 Ω and 182.4 Ω, respectively. The calendared electrode also exhibited a higher discharge capacity of about 130 mAh/g at 0.1 compared to a discharge capacity of 120 mAh/g at 0.1 for the uncalendared electrode.
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