Investigation on the Overlithiation Mechanism of LiCoO₂ Cathode for Lithium Ion Batteries

Abstract

Overlithiation of lithium ion batteries often causes a structural transformation of the electrode and capacity degradation and may even lead to severe safety problems. In this study, the electrode structure, surface morphology and compositions at the different overlithiation depths of LiCoO2 cathode material were investigated in detailed by examining the LCO/Li cells and anode-free cells, combined with post-mortem characterizations. When LiCoO2 is found in a slight overlithiation state, the cycle capacity fades slightly and minority of LiCoO2 reduces to CoO and Li2O. After LiCoO2 undergoes a deep overlithiation reaction, a long flat voltage plateau appears at 1.2 V and the cycle performance is greatly deteriorated. SEM and EIS tests show that the single-crystal particles are fractured into small particles with thick-layer formation at the surface and the shape of the EIS curve changes strongly with increasing overlithiation depth. Additionally, as shown by XPS Ar-ion etching and TEM characterization, the LiCoO2 surface is converted into greater amounts of Li2O, CoO and Co metal associated with the severe electrolyte decomposition, and LiF, Li2CO3 and LixPOyFz are the main electrolyte decomposition products. The detailed investigation of the overlithiation mechanism of LiCoO2 provides comprehensive and thorough guidance for understanding the overlithiation mechanism of cathode materials as well as important reference information for practical all-solid-state battery applications.

Formation of LiF‐rich Cathode‐Electrolyte Interphase by Electrolyte Reduction

“…In this work, a dense LiF-rich CEI was formed on a single-crystalline LiCoO 2 (LCO) cathode after potentiostatic reduction at 1.7 V in 1.0 M LiDFOB-0.2 M LiBF 4 -FEC-DEC electrolyte, since the electrolyte reduction potential of 1.7 V is higher than the reduction potential of LCO cathode. [47,52,[55][56][57][58] LiF-rich CEI restrains the structural damage of LCO, detrimental oxidative decomposition of solvents and dissolution of Co into electrolyte even at the high cut-off voltage of 4.6 V. The LCO with LiF-rich CEI achieved an excellent cyclability with a capacity of 198 mAh g À 1 and retention of 63.5 % over 400 cycles at 0.5C (100 mA g À 1 , 1C rate corresponds to a specific current of 200 mA g À 1 ) as compared to the LiF-free LCO with only 17.4 % of capacity retention. Also, the LCO//graphite full cell with LiF-rich CEI on LCO maintained 85 % of capacity after 500 cycles.…”

Prelithiation Reagents and Strategies on High Energy Lithium‐Ion Batteries

“…However, the lithiation depth of the cathode needs to be moderate, otherwise it may lead to structural degradation. For example, when the over‐discharge reaches 1.2 V, LiCoO 2 begins to decompose into CoO [110] …”

“…For example, when the over-discharge reaches 1.2 V, LiCoO 2 begins to decompose into CoO. [110] Similarly, when the cubic LiMn 2 O 4 is lithiated to the tetragonal Li 2 Mn 2 O 4 , the volume change will cause poor reversibility in long cycling. Aravindan et al designed a cycling route that using Li 1.26 Mn 2 O 4 prelithiated by half-cell to supply the lithium loss of the α-Fe 2 O 3 anode in the 1st cycle.…”

All‐Solid‐State Garnet‐Based Lithium Batteries at Work–In Operando TEM Investigations of Delithiation/Lithiation Process and Capacity Degradation Mechanism

“…The discharge curves, which show a sharp increase of discharging slope at the voltage below 3.8 V versus Li/Li + , are similar to those batteries using liquid electrolyte and LCO cathode. [79][80][81] The similarity in discharging curves indicates that the LCO within the CPE was well maintained in its phase and structure during the sintering process. Initial structural characterization is essential for the investigation of the dynamic delithiation and lithiation effects on SSLB.…”

“…A plateau at ≈1.25 V versus Li/Li + is assigned to the reaction of LCO and Li-ions to form Co 3 O 4 and Li 2 O, that is, Equation (1), while the plateau at ≈0.96 V versus Li/Li + is the reaction of cobalt oxides (Co 3 O 4 and CoO) and Liions to form Li 2 O and metallic Co, that is, Equations ( 2) and ( 3), according to Shu et al [89] It is also noticed that the specific capacity of LCO is much lower for the SSLB (232 mA h g −1 to 0.5 V vs Li/Li + ) than that for using liquid electrolyte cells (≈1000 mA h g −1 ), where the plateau at ≈0.96 V versus Li/Li + only delivers 96 mA h g −1 for the SSLB while that for liquid electrolyte cells is about 800 mA h g −1 . [79,89,101] Furthermore, the discharge plateaus at ≈1.25 and ≈0.96 V versus Li/Li + were no more observed in the 2nd discharge cycle. Unlike liquid electrolytes which can penetrate through the newly formed materials to provide Li-ion conductive paths, the much lower specific capacity and the disappearing of the two discharge plateaus at ≈1.25 and ≈0.96 V versus Li/Li + can be explained by the reduction of Li-ion conductive paths through LCO after LCO was decomposed into cobalt oxides and Li 2 O.…”