Unveiling the Intrinsic Cycle Reversibility of a LiCoO₂ Electrode at 4.8-V Cutoff Voltage through Subtractive Surface Modification for Lithium-Ion Batteries

Abstract

The thermodynamic instability of the LiCoO2 layered structure at >0.5Li extraction has been considered an obstacle for the reversible utilization of its near theoretical capacity at high cutoff voltage (>4.6 V vs Li/Li+) in lithium-ion batteries. Many previous studies have focused on resolving this issue by surface modification of LiCoO2, which has proven to be effective in suppressing phase transformation. To determine the extent to which surface protection of LiCoO2 is effective despite its thermodynamic instability and presumably incomplete reversibility involving the O1 phase, here we verify the intrinsic reversibility of bulk LiCoO2 with extended lithium extraction by ruling out the effect of a surface. Specifically, first, we show that, contrary to conventional belief, electrochemical cycling of LiCoO2 at a cutoff voltage of 4.8 V (vs Li/Li+) results in better cycle stability and lower polarizations than those at 4.6 V. We demonstrate, using an exhaustive suite of characterization tools, that the rapid cycle degradation under high-voltage cycling is mostly caused by the formation of a surface resistive layer; however, these damaged surfaces are leached out faster than they are accumulated above a certain potential, which results in superior cyclability compared with that achieved for less oxidative 4.6-V cycling. This beneficial leaching out of the resistive surface layer serves as a “subtractive” surface modification and plays a role in enhancing the cycle stability and is distinguished from conventional “additive” surface modification such as coating. This approach allows us to decouple factors of the bulk and surface degradations that contribute to the capacity fade and leads to the finding that, in the absence of a resistive surface, the capacity retention of a LiCoO2 electrode with 4.8-V cutoff cycling can be intrinsically high, indicating that the instability of the crystalline Li x CoO2 (x < 0.5) has a limited effect on the cycle stability. Our findings also explain why the strategy of coating foreign materials on the surface of LiCoO2 can improve the high-voltage cycling to some extent despite the expected thermodynamic instability of the highly charged phase.

High‐Voltage‐Driven Surface Structuring and Electrochemical Stabilization of Ni‐Rich Layered Cathode Materials for Li Rechargeable Batteries

“…In contrast, the suppressed lattice contraction and expansion in the electrode cycled at 4.9 V hints at the possible positive high‐voltage effects, as demonstrated in earlier studies on the LCO system. [ 33 ] The structural disorder in the electrode attributed to the atomic disorder or lattice strain is further confirmed by the Williamson–Hall plot in Figure S8, Supporting Information. An increase of the slope, indicating an increase in the amount of microstrain, was observed from the electrode after electrochemical cycling at a cut‐off voltage of 4.25 V (Figure S8a, Supporting Information), and the slope was maintained up to 50 cycles.…”

“…Note that the surface structure of the electrode cycled at 4.9 V remained much more stable despite the voltage being above 4.6 V. We speculate that this stability mainly originates from the dissolution of Ni at a specific high voltage over 4.6 V reported in previous studies. [ 23,33 ] Charging up to 4.6 V induces the transition metal migration, resulting in resistive spinel and rock‐salt formation on the particle surface; however, at certain voltages above 4.6 V, the resistive layer is observed to dissolve, leaving only the original layer.…”

“…[ 17,29 ] A recent study demonstrated the unexpectedly improved cycle performance at high voltage (4.8 V) in the LCO system, where the high voltage induced the dissolution of the resistive layer (spinel, rock‐salt) and its reversal to its pristine form (rhombohedral (R‐3m) layer), signifying the role of high working voltage on surface structuring during electrochemical cycling. [ 33 ] However, even though these studies have shown the effect of high voltage on cathode systems, the formation mechanism of resistive layers has not yet been revealed, and in‐depth interpretation of the role of the amount of Li extracted in Ni‐rich NCM and its correspondence to structural evolution at high voltage remains elusive. Thus, thorough analysis and quantification of structural parameters that affect the structural evolution of the resistive layer with the gradual extraction of Li for high‐voltage electrochemistry is critical for further development of cathode materials.…”

Hard X-ray Photoelectron Spectroscopy Analysis of Surface Chemistry of Spray Pyrolyzed LiNi_0.5Co_0.2Mn_0.3O₂ Positive Electrode Coated with Lithium Boron Oxide

“…5 In addition, it was reported that resistive Co 3 O 4 is electrochemically leached out from the surface of LiCoO 2 during cycling. 36 The generation and dissolution of NiO during cycling between 2.5-4.5 V in the uncoated sample may cause surface damage of the cathode particles due to the structural changes. As a result, it was considered that the surface resistance of the uncoated sample greatly increased and led to the observed capacity decrease.…”

Monitoring the phase evolution in LiCoO2 electrodes during battery cycles using in‐situ neutron diffraction technique

“…Unfortunately, excess delithiation usually leads to the release of undercoordinated oxygen, causing structural instability . Furthermore, the continuous growth of the resistive layer at high voltage is considered the root cause of capacity decay in the range of 4.3–4.9 V, whereas over the range of 3–4.5 V, the root cause of capacity decay is attributed to LCO structural changes . Both X‐ray powder diffraction (XRD) and neutron powder diffraction (NPD), alongside transmission electron microscopy (TEM), have been used to study the LCO structure as a function of lithiation, showing that Li ion ordering in Li x CoO 2 at x = 0.5 induces an irreversible hexagonal to monoclinic symmetry phase transformation, reducing the specific capacity of the entire battery .…”