Capacity Recovery Effect in Commercial LiFePO4 / Graphite Cells

Journal of Power Sources, 2014

h i g h l i g h t s Quantitative analyses of degradation modes in high-energy and high-power LiFePO 4 cells. High-power cells show rate-independent capacity fade due to loss of Li inventory (LLI). High-energy cells show rate-dependent fades from LLI and loss of active materials. Detailed dQ/dV analysis and simulation can reveal the attributing degradation modes. Electrochemical milling induces rate-dependent capacity increases in early cycle aging.

Journal of Power Sources, 2005

The cycling performance of low-cost LiFePO 4-based high-power lithium-ion cells was investigated and the components were analyzed after cycling to determine capacity fade mechanisms. Pouch type LiFePO 4 /natural graphite cells were assembled and evaluated by constant C/2 cycling, pulse-power and impedance measurements. From post-test electrochemical analysis after cycling, active materials, LiFePO 4 and natural graphite, showed no degradation structurally or electrochemically. The main reasons for the capacity fade of cell were lithium inventory loss by side reaction and possible lithium deposition on the anode.

Journal of Power Sources, 2003

LiFePO 4 /gel/natural graphite (NG) cells have been prepared and cycled under a fixed protocol for cycle and calendar life determination. Cell compression of 68 kPa was found to represent an optimal balance between cell impedance and the first cycle losses on the individual electrodes with the gel electrolyte. Cells with a Li anode showed capacities of 160 and 78 mAh/g LiFePO 4 for C/25 and 2C discharge rates, respectively. Rapid capacity and power fade were observed in the LiFePO 4 /gel/NG cells during cycling and calendar life studies. Diagnostic evaluations point to the consumption of cycleable Li though a side reaction as the reason for performance fade with minimal degradation of the individual electrodes. Published by Elsevier Science B.V.

Batteries, 2019

Herein, the post-mortem study on 16 Ah graphite//LiFePO4 pouch cells is reported. Aiming to understand their failure mechanism, taking place when cycling at low temperature, the analysis of the cell components taken from different portions of the stacks and from different positions in the electrodes, is performed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoemission spectroscopy (XPS). Also, the recovered electrodes are used to reassemble half-cells for further cycle tests. The combination of the several techniques detects an inhomogeneous ageing of the electrodes along the stack and from the center to the edge of the electrode, most probably due to differences in the pressure experienced by the electrodes. Interestingly, XPS reveals that more electrolyte decomposition took place at the edge of the electrodes and at the outer part of the cell stack independently of the ageing conditions. Finally, the use of high cycling currents buffers the low tempe...

Journal of Power Sources, 2017

Lithium plating is considered one of the most detrimental phenomenon in lithium ion batteries (LIBs), as it increases cell degradation and might lead to safety issues. Plating induced LIB failure presents a major concern for emerging applications in transportation and electrical energy storage. Hence, the necessity to operando monitor, detect and analyze lithium plating becomes critical for safe and reliable usage of LIB systems. Here, we report in situ lithium plating analyses for a commercial graphite||LiFePO 4 cell cycled under dynamic stress test (DST) driving schedule. We designed a framework based on incremental capacity (IC) analysis and mechanistic model simulations to quantify degradation modes, relate their effects to lithium plating occurrence and assess cell degradation. The results show that lithium plating was induced by large loss of active material on the negative electrode that eventually led the electrode to over-lithiate. Moreover, when lithium plating emerged, we quantified that the loss of lithium inventory pace was increased by a factor of four. This study illustrates the benefits of the proposed framework to improve lithium plating analysis. It also discloses the symptoms of lithium plating formation, which prove valuable for novel, online strategies on early lithium plating detection.

Journal of Applied Electrochemistry, 2018

A pseudo two-dimensional model (P2D) was presented to describe the electrochemical behaviour of a commercial 18650 cylindrical cell composed of graphite and LiFePO 4 (LFP) electrodes. Simulations were conducted by COMSOL MUL-TIPHYSICS 5.2. The model validation was done with experimental data taken from Hydro-Québec for a full range of Crates (currents). A mosaic model based on a Crate dependent particle radius in positive and negative electrodes was assumed. The reaction kinetics and diffusion in a solid phase were recognized as cell performance limiting factors in the flat area and in the steep area at the end of discharge of the cell voltage-capacity curve, respectively. Since the diffusion polarization in a solid phase played an important role in the steep area at the end of discharge of the cell voltage-capacity curve, a concentration dependent diffusion coefficient in LFP positive electrode was considered. Based on the fact that activation overpotential was a major polarization in the flat area of the cell voltage-capacity curve in addition to decreasing the particle radius at higher Crates , a contact resistance between the surface of the particles and the solid matrix was predicted. This contact resistance on the surface of active materials in the positive electrode described the feature of low electronic conductivity in LFP. There was a good agreement between the simulated results with experimental discharge data in a full range of Crates .

Journal of Visualized Experiments

The effect of charging and discharging lithium iron phosphate-graphite cells at different temperatures on their degradation is evaluated systematically. The degradation of the cells is assessed by using 10 charging and discharging temperature permutations ranging from-20°C to 30 °C. This allows an analysis of the effect of charge and discharge temperatures on aging, and their associations. A total of 100 charge/discharge cycles were carried out. Every 25 cycles a reference cycle was performed to assess the reversible and irreversible capacity degradation. A multi-factor analysis of variance was used, and the experimental results were fitted showing: i) a quadratic relationship between the rate of degradation and the temperature of charge, ii) a linear relationship with the temperature of discharge, and iii) a correlation between the temperature of charge and discharge. It was found that the temperature combination for charging at +30 °C and discharging at-5 °C led to the highest rate of degradation. On the other hand, the cycling in a temperature range from-20 °C to 15 °C (with various combinations of temperatures of charge and discharge), led to a much lower degradation. Additionally, when the temperature of charge is 15 °C, it was found that the degradation rate is nondependent on the temperature of discharge. Video Link The video component of this article can be found at https://www.jove.com/video/57501/ 60 °C). Other authors have described a square-root of time relationship with fade in capacity 10,30,31,32,33,34. This is supposed to represent the irreversible capacity loss attributed to the growth of SEI 30,31 where active lithium is consumed. Capacity degradation also may have a share of

Electrochimica Acta, 2004

Undoped lithium iron phosphate (LiFePO 4) was prepared and characterized by scanning electron microscope (SEM) and X-ray diffraction (XRD) analysis. The material has a single crystal globular structure with grain-sizes ca. 100-150 nm. It was used to prepare composite electrodes containing different amounts of carbon (10, 15 and 20 wt.%, respectively) used as cathodes in non-aqueous lithium cells. By increasing the carbon content, an increase in the overall electrochemical performance was observed. Impedance spectroscopy was used to investigate the ohmic and kinetic contributions to the cell overvoltage. It was found that increasing the carbon content leads to a reduction of the cell impedance as a consequence of the reduction of the charge transfer resistance. The poor performance exhibited at very high discharge rates is a direct consequence of the high value of the charge transfer resistance. A further decrease of the charge transfer resistance in high carbon content cathodes (20 wt.% carbon) was obtained by improving the powder mixing procedure. The cell performance of well mixed, high carbon content electrodes was better than our previously obtained results in terms of higher capacity retention both for different discharge rates and repeated cycling. For currents larger than a 3 C rate, a severe capacity fade affected the electrodes. It was concluded that the electronic contact at the LiFePO 4 /carbon interface plays a decisive role in material utilization at different discharge rates which affects the capacity fade upon cycling.

Journal of The Electrochemical Society

An advanced model is proposed, describing the capacity losses of C 6 /LiFePO 4 batteries under storage and cycling conditions. These capacity losses are attributed to the growth of a Solid Electrolyte Interface (SEI) at the surface of graphite particles in the negative electrode. The model assumes the existence of an inner and outer SEI layer. The rate determining step is considered to be electron tunneling through the inner SEI layer. The inner SEI layer grows much slower than the outer SEI layer. Another contribution to the degradation process is the exfoliation of SEI near the edges of graphite particles during discharging and the formation of new SEI induced by the volumetric changes during the subsequent charging. The model has been validated by storage and cycling experiments. The simulation results show that the capacity losses are dependent on the State-of-Charge (SoC), the storage time, cycle number and graphite particle size. The model can be used to predict both the calen...

Electrochimica Acta, 2004

The aging and performance of natural graphite/PEO-based gel electrolyte/LiFePO 4 cells are reported. The gel polymer electrolytes were produced by electron-beam irradiation and then soaked in a liquid electrolyte. The natural graphite anode in gel electrolyte containing LiBF4-EC/GBL exhibited high reversible capacity (345 mAh/g) and high coulombic efficiency (91%). The LiFePO 4 cathode in the same gel-polymer exhibited a reversible capacity of 160 mAh/g and 93% coulombic efficiency. Better performance was obtained at high-rate discharge with 6% carbon additive in the cathode, however the graphite anode performance suffers at high rate. The Li-ion gel polymer battery shows a capacity fade of 13% after 180 cycles and has poor performance at low temperature due to low diffusion of the lithium to the graphite in the GBL system. The LiFePO 4 /gel/Li system has an excellent rate capacity. LiFePO 4 cathode material is suitable for HEV application.