Structural Changes and Reversibility Upon Deintercalation of Li from LiCoPO₄ Derivatives

Abstract

In an effort to improve the cycle life and rate capability of olivine LiCoPO4, Cr, Fe, and Si were added to produce nominal Li1.025Co0.84Fe0.10Cr0.05Si0.01(PO4)1.025. This cathode material has an energy density comparable to LiCoPO4, with markedly improved electrochemical performance. Here, we apply operando X-ray diffraction to gain an understanding of the crystallographic delithiation mechanism of this new substituted electrode material, compared to both LiCo0.75Fe0.25PO4 and LiCo0.75Fe0.25PO4. Throughout charging, the extent of solid-solution domains was significantly increased in Li1.025Co0.84Fe0.10Cr0.05Si0.01(PO4)1.025 and LiCo0.75Fe0.25PO4 compared to LiCoPO4. These domains reduce the mechanical strain during electrode function, providing a clear explanation for the high durability with Co substitution. Li1.025Co0.84Fe0.10Cr0.05Si0.01(PO4)1.025 operated at notably higher average potential than LiCo0.75Fe0.25PO4, which would increase the energy density of the cell. Ex situ measurements reveal the persistence of structural irreversibilities in the substituted phase after the first cycle, identifying avenues for further improvement in durability. This finding sheds light on the strategies for judicious cation substitution in LiCoPO4 electrodes to maximize the cycle life while preserving high energy density, especially compared to LiFePO4.

High-Voltage Lithium-Ion Battery Using Substituted LiCoPO4: Electrochemical and Safety Performance of 1.2 Ah Pouch Cell

“…This suggests that the Py 13 FSI-based electrolyte did not decompose because of its wide chemical stability window [ 14 ]. Figure 1 a shows the voltage profiles of the 1.2 Ah graphite/LiFSI-Py 13 FSI/LCP pouch cell cycled at 0.2 C rate in the voltage range of 3.5–4.9 V. As can be seen, a sloping region centered around 4.65 V followed by a flat plateau at ~4.75 V appeared during charging, while a sloping voltage profile was obtained upon discharge; this indicates the presence of a solid-solution and the two-phase behavior of the Co–Fe–Si-substituted LiCoPO 4 cathode [ 15 ]. In addition, a reversible capacity of 1.19 Ah (specific capacity 120 mAh g −1 ) was obtained with the LiFSI-Py 13 FSI electrolyte, which implies that the Py 13 FSI–based electrolyte can be used not only for a 3.5 V olivine LiFePO 4 (LFP) cathode [ 16 ], but also for a 5 V olivine LCP cathode.…”

Phosphate Polyanion Materials as High-Voltage Lithium-Ion Battery Cathode: A Review

“…Further extraction of Li + to x < 0.5 would reduce the redox energy of Co 3+ /Co 4+ lower than that of O 2– , eventually trapping the holes in O 2– at the surface, inducing oxidation of an oxygen anion, followed by evolution of gaseous O 2 . , Generally, the μ c of LiCoO 2 will be pinned at the top of the O 2p-orbitals, where beyond x > 0.55 severe capacity deterioration would occur due to hexagonal structural phase transformations. , This phenomenon explained the inability to complete delithiation ( x = 0) for Li x CoO 2 , which is usually delithiated to Li 0.5 CoO 2 , and offered C SP of ∼150 mAh·g –1 , even though it has a theoretical C SP of >274 mAh·g –1 . However, such interpretation is insufficient in explaining the capability of a few TMO cathodes to function at high working potentials (>4.5 V vs Li/Li + ), ,, especially spinel LiCoMnO 4 and olivine LiCoPO 4 with voltages of approximately 5 V vs Li/Li + . − …”

Preliminary Study of LiCoPO4 cathode material synthesis for LiCoPO4/Graphite Cell

“…LiCoPO4 is isostructural to LiFePO4. It has an olivine-type structure with considerably good safety aspects and thermal stability due to a strong P-O bond [5]. Theoretically, it has a similar specific coulombic capacity to LiFePO 4 but a significantly higher operating voltage of >4.8 V. This can result in a high energy density Li-ion battery.…”