Structural Changes in Li₂CoPO₄F during Lithium-Ion Battery Reactions

Abstract

The cobalt-based fluorophosphate Li 2 CoPO 4 F positive electrode has the potential to obtain high energy density in a lithium ion battery since its theoretical capacity is 287 mAh·g −1 when two electrons can react reversibly. This material promises to charge/discharge with an extremely high redox-couple voltage of over 4.8 V vs Li/Li + . Bulk structural analyses including X-ray diffraction, Co K-edge X-ray absorption near-edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) reveal that an orthorhombic Li β CoPO 4 F phase is produced from pristine Li 2 CoPO 4 F by a combination of solid-solution and two-phase reaction manners during the first charging process, and these phases reversibly transform during charge−discharge cycling. The results of 7 Li MAS NMR and classical molecular dynamics simulations suggest that Li ions located at Li(1) sites intercalate/deintercalate through a 1D diffusion path along the b axis, whereas those located at Li(2) and Li(3) sites are fixed. The aforementioned analyses were successfully performed with the enhancement of electrochemical properties by use of a fluoroethylene carbonatebased electrolyte instead of an ethylene carbonate-based one and reducing its volume. Further enhancement was achieved by adding SiO 2 nanoparticles into the electrode slurry. The electrochemical results encourage the possibility of the intercalation/ deintercalation of more than one Li ion from/into Li 2 CoPO 4 F during electrochemical cycling.

Enhancing the energy density of safer Li-ion batteries by combining high-voltage lithium cobalt fluorophosphate cathodes and nanostructured titania anodes

“…However, recent studies have indicated that Li 2 CoPO 4 F can reach a maximum reversible capacity of 150 mA h g −1 , with an outstanding high-voltage operation of ~5 V vs. Li +  /Li111527282930. Because of the high inefficiency from the first to the second cycle observable in Li 2 CoPO 4 F, these electrodes were subjected to activation cycles before being used in the complete lithium-ion cell (see Supplementary Fig.…”

A 4.8 V Reversible Li2CoPO4F/Graphite Battery Enabled by Concentrated Electrolytes and Optimized Cell Design

“…[43][44][45] Finally, tetraethyl orthosilicate (TEOS) was added to the cathode slurry to obtain 5 wt% SiO 2 -coated cathode, which can suppress the oxidation of electrolyte on the cathode at high potentials. [46] The cyclic voltammograms of Li 2 CoPO 4 F in the standard and optimized cells with commercial electrolytes are shown in Figure 4. The peaks of Li 2 CoPO 4 F represent the extraction/insertion of Li + from several Li sites in its crystal lattice with changing valance states of Co. [46] So far, only two oxidation peaks (Li + de-intercalation) with one broad reduction peak (Li + intercalation) have been reported experimentally.…”

“…[46] The cyclic voltammograms of Li 2 CoPO 4 F in the standard and optimized cells with commercial electrolytes are shown in Figure 4. The peaks of Li 2 CoPO 4 F represent the extraction/insertion of Li + from several Li sites in its crystal lattice with changing valance states of Co. [46] So far, only two oxidation peaks (Li + de-intercalation) with one broad reduction peak (Li + intercalation) have been reported experimentally. [47][48][49][50] However, our optimized cell design allows us to observe four sharp redox pairs at 4.8/4.7, 4.9/4.8, 5.1/5.0, and 5.3/5.1 V (vs. Li/ Li + ), for the first time.…”

“…First, the optimized cathode with LiPAA binder, quart separator, and SiO 2 coating, contributes to not only blocking the direct contact between the electrode and electrolyte, but also preventing the HF generation. [41,42,46,47] Thus, the surface damage of the cathode material at high potentials could be mitigated. Moreover, two distinct features of the concentrated LiBF 4 /PC : FEC help to prevent the transition metal dissolution from the cathode material.…”

“…Li 2 CoPO 4 F/C composites were synthesized via a modified multistep synthesis process based on the previous literature. [46] All chemicals were purchased from WAKO and used without any further purification. .…”