Stabilizing the Structure of LiCoPO₄ Nanocrystals via Addition of Fe³⁺: Formation of Fe³⁺ Surface Layer, Creation of Diffusion-Enhancing Vacancies, and Enabling High-Voltage Battery Operation

Abstract

Factors affecting the cyclability of the Fesubstituted LiCoPO 4 (LiCo 0.8 Fe 0.2 PO 4 , LCFP) material were elucidated, including both the structural and electrode/ electrolyte stability. Electrochemical characterization of the synthesized LCFP nanoparticles lends clear evidence for improved electrochemical stability of LCP, as well as enhanced rate capability, with Fe 3+ substitution. Surface analysis using X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS) suggest that Fe enrichment on the surface of LCFP occurs through the oxidation of Fe 2+ into Fe 3+ in the synthesis process. The Fe 3+rich phase on the LCP surface enhances the stability of the delithiated phase, preventing oxidative reactions with electrolytes during high-voltage operation. This surface protection persists as long as the electrochemical reduction of Fe 3+ is avoided by ensuring that the full range of operating voltages lie above the Fe 3+ /Fe 2+ redox potential. Our findings may offer new approaches to stabilize the structure of LCP and other high-voltage positive electrodes for use in 5 V-class Li-ion batteries.

Crystal-structure-matched FePO₄ Surface-coating on LiCoPO₄/MWCNT Nanocomposites for Long Lifecycle 5 V Class Lithium Ion Batteries

“…In this work, the addition of Fe dopants to the original mixture was dispensed, instead applying a coating of FePO 4 (FP) to LCP nanoparticles after synthesis. This yielded the same protective Fe 3+rich surface layer as previously reported 11 as confirmed by X-ray photoelectron spectroscopy (XPS) with a lower overall dose of Fe, reducing mass and thereby improving specific capacity. Our updated techniques yielded improvement of the performance relative to that of LCFP (95 mAh g ¹1 ), 11 increasing specific capacity to 106 mAh g ¹1 while retaining the outstanding cycle performance observed previously.…”

“…This yielded the same protective Fe 3+rich surface layer as previously reported 11 as confirmed by X-ray photoelectron spectroscopy (XPS) with a lower overall dose of Fe, reducing mass and thereby improving specific capacity. Our updated techniques yielded improvement of the performance relative to that of LCFP (95 mAh g ¹1 ), 11 increasing specific capacity to 106 mAh g ¹1 while retaining the outstanding cycle performance observed previously. Our findings may suggest new techniques for prolonging cycle-life performance without unnecessarily decreasing capacity.…”

“…In the previous work, 11 the formation of Fe 3+ -rich layer on Fedoped LiCoPO 4 (LiCo 0.8 Fe 0.2 PO 4 , LCFP) particles was achieved via the air annealing process of LCFP/MWCNT composites. In this study, we applied the same annealing process on the LiFePO 4 (LFP, Fe 2+ )-coated LCP/MWCNT in order to yield FePO 4 (FP, Fe 3+ )coated LCP/MWCNT as a final product.…”

“…LCP/MWCNT and LCFP/MWCNT composites were synthesized via ultracentrifugation (UC) treatment as previously reported. 11 In case of FePO 4 (FP) coating, two kinds of solutions (solution A and B) were prepared. The solution A was composed of 0.2500 g of synthesized LCP/MWCNT composite and 0.0183 g of H 3 PO 4 (1.0 eq.)…”

“…In our previous work, 11 this paradigm have been shifted by demonstrating that a combination of novel fabrication techniques, and modified operating parameters was able to yield LCP-based materials, namely Fe-substituted LCP (LCFP), whose performance lied in an entirely different regime from that of previous studies and one much closer to what is required for practical applications: our LCFP cells exhibited cycle life in excess of 5,000 cycles with >85% capacity retention despite requiring no additives of any kind. This dramatic improvement was the product of multiple factors: the formation of Fe 3+ -rich surface layer assisted by the novel fabrication step of ultracentrifugation (UC) that served to stabilize the crystal against lattice expansion during high-voltage operation, the creation of vacancies at M2 sites in the crystal structure that enhances the Li + diffusivity in LCFP crystals and decreases its charge transfer resistance, and the restriction of the operating voltage range to ensure that Fe remained oxidized at valence state 3+, preventing its reduction to Fe 2+ .…”