Operando investigation on the fast two-phase transition kinetics of LiFePO4/C composite cathodes with carbon additives for lithium-ion batteries

Decoupling Electrochemical Kinetics of Li-Rich and Li-Poor Phases in LiFePO4 Cathodes Using Single-Particle Electrochemical Impedance Spectroscopy

LiFePO4 (LFP) undergoes a two-phase transformation during lithium insertion or extraction, forming lithium-rich and lithium-poor phases. Determining the kinetic parameters of these phases is crucial for electrochemical models but remains challenging. In this study, we decouple the reaction and diffusion kinetics of the Li-rich and Li-poor phases in LFP cathodes using single-particle electrochemical impedance spectroscopy (EIS). LFP agglomerates comprising primary particles are fabricated into single-particle microelectrodes. EIS measurements are conducted on single LFP particles at various insertion ratios. A physics-based impedance model is developed for phase-transformation electrodes, and the evolution of the exchange current density (i0) and diffusion coefficient (DLi) for both phases is extracted. In the single-phase region, the Li-poor phase exhibits a steeper change in i0 with varying insertion ratios compared with the Li-rich phase. In the two-phase coexistence region, the Li-poor phase shows a higher i0 than the Li-rich phase. Additionally, DLi for the Li-poor phase is higher than that for the Li-rich phase in both the single-phase and two-phase coexistence regions. We also compare the kinetic parameters of the Li-rich and Li-poor phases in LFP agglomerates of varying particle sizes to clarify the impact of particle size on electrochemical kinetics. The proposed impedance-based approach decouples the electrochemical kinetics of Li-rich and Li-poor phases in LFP cathodes, and the extracted kinetic parameters serve as the basis for developing models considering phase transformation.

Constructing a Homogeneous Medium Layer To Promote the Direct Regeneration of Spent Lithium Iron Phosphate

With the massive application of lithium-ion batteries in electric vehicles, spent lithium iron phosphate (LFP) batteries have accumulated in recent years, inducing an urgent requirement in recycling technology. Direct repair technology has been considered as a promising approach for recycling spent LFP. However, the traditional solid-phase repair technology is limited by the heterogeneous contact between the lithium source and spent LFP. Herein, polyacrylonitrile (PAN) was selected as the additive to facilitate the intimate contact between the lithium source and spent LFP. PAN displays a strong interaction with defective LFP, leading to the effective contact between spent LFP and the lithium replenishment agent. Moreover, PAN is favorable for the diffusion of lithium ions in the spent LFP lattice during the regeneration process. Therefore, the unevenly distributed FeLi defects in spent LFP are well-repaired. As a result, the regenerated LFP exhibits a high capacity of 155 mA h g-1 at 0.1 C and improved cycling stability with a capacity retention of ∼87% after 400 cycles at 1 C.

Robust, High-Temperature-Resistant Polyimide Separators with Vertically Aligned Uniform Nanochannels for High-Performance Lithium-Ion Batteries

Separator is an essential component of lithium-ion batteries (LIBs), playing a pivotal role in battery safety and electrochemical performance. However, conventional polyolefin separators suffer from poor thermal stability and nonuniform pore structures, hindering their effectiveness in preventing thermal shrinkage and inhibiting lithium (Li) dendrites. Herein, we present a robust, high-temperature-resistant polyimide (PI) separator with vertically aligned uniform nanochannels, fabricated via ion track-etching technology. The resultant PI track-etched membranes (PITEMs) effectively homogenize Li-ion distribution, demonstrating enhanced ionic conductivity (0.57 mS cm-1) and a high Li+ transfer number (0.61). PITEMs significantly prolong the cycle life of Li/Li cells to 1200 h at 3 mA cm-2. For Li/LiFePO4 cells, this approach enables a specific capacity of 143 mAh g-1 and retains 83.88% capacity after 300 cycles at room temperature. At 80 °C, the capacity retention remains at 85.92% after 200 cycles. Additionally, graphite/LiFePO4 pouch cells with PITEMs display enhanced cycling stability, retaining 73.25% capacity after 1000 cycles at room temperature and 78.41% after 100 cycles at 80 °C. Finally, PITEMs-based pouch cells can operate at 150 °C. This separator not only addresses the limitations of traditional separators, but also holds promise for mass production via roll-to-roll methods. We expect this work to offer insights into designing and manufacturing of functional separators for high-safety LIBs.

Tartaric Acid Cross-Linking Polyvinyl Alcohol as Degradable Separators for Rechargeable Lithium Ion Batteries

The escalating focus on environmental concerns and the swift advancement of eco-friendly biodegradable batteries raises a pressing demand for enhanced material design in the battery field. The traditional polypropylene (PP) that is monopolistically utilized in the commercial LIBs is hard to recycle. In this work, we prepare a novel water degradable separators via the cross-linking of polyvinyl alcohol (PVA) and dibasic acid (tartaric acid, TA). Through the integration of non-solvent liquid-phase separation, we successfully produced a thermally stable PVA-TA membrane with tunable thickness and a high level of porosity. These specially engineered PVA-TA separators were implemented in LiFePO4 (LFP)|separator|Li cells, resulting in superior multiplicative performance and achieving a capacity of 88 mAh g-1 under 5 C. Additionally, the straightforward small molecule cross-linking technique significantly reduced the crystalline region of the polymer, thereby enhancing ionic conductivity. Notably, after cycling, the PVA-TA separators can be easily dissolved in 95 °C hot water, enabling its reutilization for the production of new PVA-TA separators. Therefore, this work introduces a novel concept to design green and sustainable separators for recyclable lithium batteries.

Unveiling high-power and high-safety lithium-ion battery separator based on interlayer of ZIF-67/cellulose nanofiber with electrospun poly(vinyl alcohol)/melamine nonwoven membranes

Due to the poor thermal stability of conventional separators, lithium-ion batteries require a suitable separator to maintain system safety for long-term cycling performance. It must have high porosity, superior electrolyte uptake ability, and good ion-conducting properties even at high temperatures. In this work, we demonstrate a novel composite membrane based on sandwiching of zeolitic imidazole frameworks-67 decorated cellulose acetate nanofibers (ZIF-67@CA) with electrospun poly(vinyl alcohol)/melamine (denoted as PVAM) nonwoven membranes. The as-prepared sandwich-type membranes are called PVAM/x%ZIF-67@CA/PVAM. The middle layer of composite membranes is primarily filled with different weight percentages of ZIF-67 nanoparticles (x = 5, 15, and 25 wt%), which both reduces the non-uniform porous structure of CA and increases its thermal stability. Therefore, our sandwich-type PVAM/x%ZIF-67@CA/PVAM membrane exhibits a higher thermal shrinkage effect at 200 °C than the commercial polyethylene (PE) separator. Due to its high electrolyte uptake (646.8%) and porosity (85.2%), PVAM/15%ZIF-67@CA/PVAM membrane achieved high ionic conductivity of 1.46 × 10-3 S cm-1 at 70 °C, as compared to the commercial PE separator (ca. 6.01 × 10-4 S cm-1 at 70 °C). Besides, the cell with PVAM/15%ZIF-67@CA/PVAM membrane shows an excellent discharge capacity of about 167.5 mAh g-1after 100 cycles at a 1C rate with a capacity retention of 90.3%. The ZIF-67 fillers in our sandwich-type composite membrane strongly attract anions (PF6-) through Lewis' acid-base interaction, allowing uniform Li+ ion transport and suppressing Li dendrites. As a result, we found that the PVAM/15%ZIF-67@CA/PVAM composite nonwoven membrane is applicable to high-power, high-safety lithium-ion battery systems that can be used in electric vehicles (EVs).

Enabling high-performance lithium iron phosphate cathodes through an interconnected carbon network for practical and high-energy lithium-ion batteries

The olivine lithium iron phosphate (LFP) cathode has gained significant utilization in commercial lithium-ion batteries (LIBs) with graphite anodes. However, the actual capacity and rate performance of LFP still require further enhancement when combined with high-capacity anodes, such as silicon (Si) anodes, to achieve high-energy LIBs. In this study, we introduce a gelatin-derived carbon network into a nanosized LFP cathode without the need for additional binding and conductive agents, employing a simple and cost-effective method. The resulting cathode exhibits an extremely high LFP content (∼92.3 wt%), enabling it to show a high real capacity of 159.7 mAh/g at 0.2 C in half cells. Additionally, the interconnected carbon network effectively facilitates electron and Li+ transport, providing rapid pathways within the LFP nanoparticles. Consequently, the cathode exhibits superior rate capability (107.3 mAh/g at 10 C) and good cycling performance (with a capacity retention of ∼ 80 % after 500 cycles). To further assess its practical viability, the LFP cathode is assembled into a full cell utilizing a Si-based anode with a N/P ratio of 1.1. The resulting full cell delivers a significantly high energy density of 419.7 Wh kg-1, coupled with prolonged cycle life, highlighting its promising prospects for practical applications.

Reduced Graphene Oxide Coating LiFePO4 Composite Cathodes for Advanced Lithium-Ion Battery Applications

Recently, the application of LiFePO4 (LFP) batteries in electric vehicles has attracted extensive attention from researchers. This work presents a composite of LFP particles trapped in reduced graphene oxide (rGO) nanosheets obtained through the high-temperature reduction strategy. The obtained LiFePO4/rGO composites indicate spherical morphology and uniform particles. As to the structure mode of the composite, LFP distributes in the interlayer structure of rGO, and the rGO evenly covers the surface of the particles. The LFP/rGO cathodes demonstrate a reversible specific capacity of 165 mA h g-1 and high coulombic efficiency at 0.2 C, excellent rate capacity (up to 10 C), outstanding long-term cycling stability (98%) after 1000 cycles at 5 C. The combined high electron conductivity of the layered rGO coating and uniform LFP particles contribute to the remarkable electrochemical performance of the LFP/rGO composite. The unique LFP/rGO cathode provides a potential application in high-power lithium-ion batteries.

Electrochemical performances of a LiFePO4-based heat-treated activated carbon electrode

Activated carbon was heat-treated to investigate the effect of heat-treating activated carbon on the power and long-term reliability characteristics of LiFePO₄-based electrodes. As the heat-treatment temperature of the activated carbon increased, the surface area and total pore volume were decreased. In addition, oxygen functional groups were decomposed and the O/C ratio on the pore surface was reduced. The power and long-term reliability characteristics of the composite electrodes were improved by the use of heat-treated activated carbon, which probably resulted from an increase in the electrical conductivity of the electrodes as the bulk resistance and surface resistance of the heat-treated activated carbon decreased. The diffusion coefficient of the LFP/AC electrode was considerably increased due to the pores of activated carbon.